Abstract
Developmental neurobehavioral outcomes attributed to exposure to chlorpyrifos (CPF) obtained from epidemiologic and animal studies published before June 2010 were reviewed for risk assessment purposes. For epidemiological studies, this review considered (1) overall strength of study design, (2) specificity of CPF exposure biomarkers, (3) potential for bias, and (4) Hill guidelines for causal inference. In the case of animal studies, this review focused on evaluating the consistency of outcomes for developmental neurobehavioral endpoints from in vivo mammalian studies that exposed dams and/or offspring to CPF prior to weaning. Developmental neuropharmacologic and neuropathologic outcomes were also evaluated. Experimental design and methods were examined as part of the weight of evidence. There was insufficient evidence that human developmental exposures to CPF produce adverse neurobehavioral effects in infants and children across different cohort studies that may be relevant to CPF exposure. In animals, few behavioral parameters were affected following gestational exposures to 1 mg/kg-d but were not consistently reported by different laboratories. For postnatal exposures, behavioral effects found in more than one study at 1 mg/kg-d were decreased errors on a radial arm maze in female rats and increased errors in males dosed subcutaneously from postnatal day (PND) 1 to 4. A similar finding was seen in rats exposed orally from PND 1 to 21 with incremental dose levels of 1, 2, and 4 mg/kg-d, but not in rats dosed with constant dose level of 1 mg/kg-d. Neurodevelopmental behavioral, pharmacological, and morphologic effects occurred at doses that produced significant brain or red blood cell acetylcholinesterase inhibition in dams or offspring.
Over the past 10 years, three cohort studies of pregnant women and their children were conducted investigating associations between levels of chlorpyrifos (CPF) or metabolites in maternal urine or umbilical cord blood and developmental neurobehavioral outcomes (). In addition, numerous animal studies on CPF-induced developmental neurotoxicity (DNT) effects were published in the peer-reviewed literature (see Tables 5–12). In view of these observations, the objective of this review was to evaluate the results of the CPF-associated developmental neurobehavioral studies from epidemiologic and in vivo animal data within a risk assessment framework relevant to current uses of CPF in the United States.
TABLE 1. Summary of Study Characteristics
Approaches to evaluating epidemiologic studies have become increasingly important to regulatory agencies, as evidenced by the U.S. Environmental Protection Agency (U.S. EPA) draft proposed Framework for Incorporating Human Epidemiologic and Incidence Data in Health Risk Assessment (CitationU.S. EPA 2010). It is also important that regulatory agencies consider published animal studies that evaluate neurodevelopmental endpoints, although investigations may not have followed CitationU.S. EPA (1998a) DNT guideline requirements for sample size (10 males and 10 females from 20 litters; 1 male or female/litter), number of dose levels (>2), or relevant route of exposure (preferably oral). Furthermore, the animal studies can contribute to understanding the biological plausibility of outcomes that have been associated with human exposures in epidemiologic studies.
Previous reviews of CPF written within a risk assessment context include two expert panel reports by CitationClegg and van Gemert (1999a; Citation1999b), and a more recent expert panel review (CitationEaton et al. 2008). Other reviews also reported developmental outcomes associated with various classes of pesticides (CitationEskenazi et al. 1999; Citation2008; CitationWeselak et al. 2007), but with limited discussion specific to potential CPF associations from epidemiologic studies. The CitationEaton et al. (2008) review summarized results of epidemiologic and toxicology studies, including in vivo and in vitro neurodevelopmental studies, critically evaluated human exposure studies, and estimated dermal, oral, and inhalation exposures to children and/or adults in the general population based on currently approved uses. A primary focus of the CitationEaton et al. (2008) review was to compare dose levels producing acetylcholinesterase activity (AChE) inhibition with concentrations at which in vitro or in vivo neurodevelopmental findings were reported. These levels were also compared with exposure concentration to which the general population may be exposed based upon currently approved uses for CPF. While comprehensive, the CitationEaton et al. (2008) review provided narrative summaries of findings from the epidemiologic cohort studies, rather than presenting the measures of association and corresponding confidence intervals, thus making it difficult to evaluate the precision of effect estimates and the consistency of the direction and magnitude of associations for similar measures at similar ages across studies.
The present review encompasses previous reviews and further contributes new analyses by providing (1) in-depth analyses of the analytical epidemiologic studies focused on neurobehavioral outcomes in infants and young children with comparisons of methodologies (including exposure measurement) and quantitative results, consideration of the potential role of systematic error (bias), and application of guidelines recommended by CitationHill (1965) and others for evaluating causality, (2) systematic critical analyses of the in vivo developmental neurobehavioral and neuropharmacology studies that includes assessment of methods and evaluation of pattern of negative and positive findings, and (3) integrative evaluation of the infancy/early childhood neurobehavioral findings with the animal data. The studies evaluated in this review included (1) epidemiologic studies of pregnant women that examined associations of CPF exposure with neurobehavioral measures in their infants and young children published before June 2010 () and (2) in vivo DNT animal studies with emphasis on neurobehavioral, neuropharmacological and neuropathology endpoints at lower dose levels (<10 mg/kg-d) (see Tables 5–12).
APPROACH TO EVALUATION OF CPF STUDIES FOR HUMAN HEALTH RISK ASSESSMENT
The following steps were undertaken in our systematic review of published CPF animal and epidemiologic developmental neurobehavioral studies.
| 1. | Identify uses and exposure patterns (including frequency, concentration, duration, and routes of exposure) in humans for CPF such that relevance of animal studies can be determined. | ||||||||||||||||
| 2. | Assess evidence for possible modes of action (MOA). In the case of CPF, risk assessments are currently based upon an established MOA that is acetylcholinesterase (AChE) inhibition, and this needs to be compared to other reported endpoints. | ||||||||||||||||
| 3. | Incorporate knowledge of the metabolism and pharmacokinetics of CPF in the evaluation of exposure measures in humans (e.g., studies evaluating outcomes associated with organophosphate [OP] metabolites that include metabolites unrelated to CPF need to be excluded or be given lower weight in the assessment). | ||||||||||||||||
| 4. | Define approach and standards for review of human and animal studies.
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| 5. | Extract and summarize all analyses conducted (both significant and non-significant findings), key methodological features, conditions of exposure (e.g., duration, timing and concentration), and direction and magnitude of any associations in the human and animal studies so that conclusions about the weight of evidence will be specific. | ||||||||||||||||
USES AND HUMAN EXPOSURE
Chlorpyrifos (CPF) is a widely used OP insecticide, primarily approved for uses in agricultural pest management (crop protection) in the United States today. Most nonagricultural uses such as residential control of insect pests such as cockroaches and termites, and animal use including flea and tick control were phased out in the United States in 2001 (CitationU.S. EPA 2002) and in the European Union in 2005 (CitationOfficial Journal of the European Union Legislation 2007). Hence, exposures of pregnant women, infants, and children in the general population to CPF occur primarily through the diet and are estimated to be less than 1 × 10−2 μg/kg-d in children and 4–6 × 10−3 μg/kg-d in adults (CitationEaton et al. 2008). Therefore, it is unlikely that inhalation, dermal, or secondary oral (nondietary) CPF exposures contribute significantly in the general population today (CitationEaton et al. 2008).
Exposures to agricultural applicators including women of childbearing age have been estimated by the CitationU.S. Environmental Protection Agency (1999) to be 1.1–9.8 μg/kg-d based on biomonitoring data for mixers, loaders, or applicators using tractors or airplanes to apply liquid or granular formulations. Higher daily doses of 3–23 μg/kg-d were estimated by the U.S. EPA for manual solo applicators using backpack or hand wand sprayers.
CitationAlexander et al.(2006) reported the 90th percentile for CPF exposure from a single application to be 7.4 μg/kg for applicators and 2.2–2.8 μg/kg for children living on the farm. Maximal levels of CPF exposure were estimated to be 16.3, 4.1, 6.3, and 2.5 μg/kg, respectively, for applicators, spouses, children older than 12 yr, and children younger than 12 yr. These estimates were based on levels of 3,5,6-trichloro-2-pyridinol (TCPy), a specific metabolite of CPF, in urine collected continuously over a 4-d period immediately after CPF was applied. Although this study estimated exposure after only one d of application, it provides an estimate of the order of magnitude of exposure to the farm family members who were not applicators.
Populations living in agriculturally intensive areas have potential exposures to outdoor air concentrations ranging from 15–100 ng/m3 (CitationEaton et al. 2008), which results in approximate estimates of daily doses of 6 × 10−3 to 4 × 10−2 μg/kg-d assuming exposure of 4 × 10−4 and 3 × 10−4 μg/kg-d per 1 ng/m3 for children and adults, respectively. Using conservative assumptions, the absorbed dermal exposures of 2.5 to 4 × 10−3 μg/kg-d were estimated based on measures in homes that recently used CPF for pest control, or in agricultural homes (CitationEaton et al. 2008). Nondietary oral intake rate for children are roughly estimated to be 1.4 × 10−3 μg/kg-d in homes of farm workers and applicators based on an assumption that a 20-kg child consumes 50 mg of dust/soil containing 700 ng CPF/g house dust (CitationEaton et al. 2008).
Although most residential uses are no longer permitted today, epidemiologic studies have been published involving previously allowed residential exposures. CitationLowe et al. (2009) estimated CPF residential exposures to pregnant women to be 0.15 μg/kg-d based on physiologically based pharmacokinetic (PBPK)/pharmacodynamic modeling and using umbilical cord blood levels reported by CitationWhyatt et al. (2005) for mothers living in New York City during a period when residential exposures to CPF were allowed. These estimates are consistent with postapplication residential air monitoring for CPF that indicated levels of approximately 50–400 ng/m3 (CitationEaton et al. 2008), which are approximated to result in daily doses of 0.02–0.16 μg/kg-d for children based on an assumption of 4 × 10−4 μg/kg-d exposure to CPF per 1 ng/m3 CPF air concentration (CitationEaton et al. 2008). Exposure to adults would be lower based on the assumption that adults are exposed to 3 × 10−4 μg/kg-d per 1 ng/m3 CPF (CitationEaton et al. 2008).
In summary, exposure to the general population including children and women of childbearing age today is primarily through the diet and in the 10−2 to 10−3 μg/kg-d dose range. Applicators and their families may be potentially exposed immediately after agricultural application in the range of 100 and 101 μg/kg. Other estimates for exposure to women and children living in agriculturally intensive areas are in the 10−2 μg/kg-d dose range. Although residential uses are no longer permitted, estimates for daily exposure directly to children and pregnant women when residential uses were allowed are in the 10−1 to 10−3 μg/kg-d dose range following dermal, inhalation, and/or nondietary oral exposures. Several of these estimates are based on a number of assumptions that may require further examination. However, for the purposes of this review, these estimates provide an initial basis for comparison of doses used in animal studies with possible human CPF exposure levels as recommended by CitationMaurissen (2010).
MODE OF ACTION (MOA) AS CURRENT BASIS FOR RISK ASSESSMENT
The best characterized MOA for acute neurotoxic effects of CPF is inhibition of acetylcholinesterase (AChE) activity. In order to become an effective AChE inhibitor, CPF is converted to chlorpyrifos oxon (CPO) through oxidative metabolism, which then binds to and inhibits AChE. This binding produces inhibition of AChE leading to increased acetylcholine (ACh) levels within synaptic clefts, hyperstimulation of the cholinergic system, and adaptive decreases in muscarinic acetylcholine receptor (mAChR) binding (CitationEaton et al. 2008).
In addition to AChE inhibition in the brain, spinal cord, and peripheral nervous system, CPO binds to and inhibits red blood cell (RBC) AChE, and plasma butyrylcholinesterase (BuChE). Although the physiological role of BuChE and RBC AChE is not clear, levels of inhibition of the activity of these enzymes are currently used in the U.S. as critical toxicity endpoints for human health risk assessment, with RBC AChE inhibition considered more relevant than BuChE inhibition.
Age-related differences between adults and offspring in magnitude, time of onset, and/or recovery of brain and RBC AChE inhibition were reported following acute dermal, oral, or subcutaneous (sc) administration (CitationAbu-Quare et al. 2001; CitationMoser et al. 1998; CitationMoser and Padilla 1998; CitationPope and Chakraborti 1992). Although neonatal pups were more sensitive than adults to higher acute CPF doses (>10 mg/kg), there was less or absence of differential sensitivity to CPF induced inhibition of AChE at lower (<10 mg/kg-d) repeated exposures (CitationZheng et al. 2000; CitationLiu et al. 1999). Following repeated gestational exposures, maternal brain and RBC AChE inhibition was greater in dams than fetus (CitationMattsson et al. 2000; CitationLassiter et al. 1998 Citation1999). As discussed later in reviewing the animal literature, cholinergic and other noncholinergic MOA were proposed for adult and developmental neurotoxic effects ( CitationAldridge et al. 2003; Citation2004; CitationBetancourt et al. 2006; Citation2007; CitationBetancourt and Carr 2004; CitationLiu and Pope 1998; Citation1999; CitationPope 1999; CitationRichardson and Chambers 2005; CitationSlotkin et al. 2006; Citation2007a; CitationZamora et al. 2008).
Although it appears that animals exposed to CPF during gestation recover from brain AChE inhibition more rapidly than adults due to increased synthesis of AChE, there may be long-lasting effects on cholinergic and noncholinergic components (CitationEaton et al. 2008; CitationLassiter et al. 1998; Citation1999; CitationRichardson and Chambers 2004; Citation2005; CitationSlotkin et al. 2001; Citation2002; Citation2004; Citation2006). Therefore, this in-depth review of the developmental neurobehavioral animal studies compared dose levels that produced behavioral and neuropharmacological effects in offspring, with CPF concentrations inducing brain or AChE inhibition RBC or plasma BuChE inhibition in offspring or dams. In our view, from a risk assessment perspective, this dose comparison serves to determine whether regulations based on AChE or BuChE inhibition are protective of adverse effects that may be due to other undefined MOA.
CPF METABOLISM AND OP BIOMARKERS OF EXPOSURE
An understanding of the relative specificity of OP metabolites as potential biomarkers of CPF is critical in weighing the evidence associating CPF exposure with neurobehavioral outcomes in the human studies. The metabolism and pharmacokinetics of CPF were described in detail elsewhere (CitationEaton et al. 2008; CitationTimchalk et al. 2002; 2006). Briefly, CPF is metabolized to a number of metabolites including chlorpyrifos-oxon (CPO), the primary toxic active metabolite, TCPy, and diethylphosphate (DEP) and diethylthiophosphate (DETP). These metabolites or their glucuronic or sulfate conjugates are excreted in the urine (CitationBarr and Angerer 2006). Detoxification of the oxon is brought about by several enzymes including paraoxonases and carboxylesterases. Paraoxonase-1 (PON1) status based on PON1 levels or activity and polymorphism may modulate toxicity of CPF and CPO, but “at lower-level exposures to CPF [<0.5 mg/kg] other esterase detoxification pathways would be capable of compensating for the inter-individual differences in CPOase activity due to PON1Q192R polymorphism” (Cole et al. 2005).
The degree of specificity of the different biomarkers measured in human studies to CPF exposure is as follows (CitationBarr and Angerer 2006; CitationBravo et al. 2004):
| • | CPF and CPO are measured in blood and are biomarkers of highest specificity for CPF. | ||||
| • | TCPy is the most common urinary biomarker of CPF exposure, with important limitations. For example, TCPy is also a metabolite of CPF-methyl and triclopyr (CitationBarr and Angerer 2006; CitationWhyatt et al. 2009). In addition, TCPy itself can be present in food, the environment, or homes, as a result of breakdown products from an application of CPF or chlorpyrifos-methyl (CitationBarr and Angerer 2006; CitationEaton et al. 2008; CitationWhyatt et al. 2009). Finally, significant intraindividual variability in repeat urine samples from the same individual was noted (CitationWhyatt et al. 2009). | ||||
| • | DEPs represent a broad class of OP metabolites of CPF and other OP containing ethyl groups. DEPs include DEP, DETP, and diethyldithiophosphate (DEDTP). Only DEP and DETP are metabolites of CPF. As a class, DEPs are relatively nonspecific as exposure markers for CPF in urine because other OP can also be degraded into DEPs. | ||||
| • | DMPs are a broad class of urinary dimethylphosphate metabolites that are not metabolites of CPF. Although DMPs are useful as potential biomarkers for several methyl OPs including malathion and CPF-methyl (a pesticide distinct from CPF), these are not biomarkers for CPF. Dialkylphosphates (DAPs) are a broad class of OP urinary metabolites, which include both DEPs and DMPs. Therefore, DAPs are not specific biomarkers for CPF because they include metabolites that cannot be formed from CPF. | ||||
Biomarkers measured in blood provide the highest sensitivity for measuring exposure to the parent compound (CitationBarr and Angerer 2006). Since this analytic method was only recently introduced, and obtaining urine samples is more feasible and affordable, measuring metabolite levels in urine is a common practice. Urinary metabolite levels are sensitive to the timing of collection, which may lead to misclassification of exposure. For example, CitationScher et al. (2007) found that results from the first morning void often overpredicted pesticide concentrations based on 24-h urine samples, which may be associated with the pharmacokinetics of the biomarker in the urine.
In summary, the use of urinary metabolites (TCPy, DEPs, DAPs) as indicators of CPF exposure needs to be considered with caution, because they may indicate exposure to metabolite residues and not necessarily exposure to the parent compound of concern ( CitationBarr and Angerer 2006; CitationEaton et al. 2008; CitationLu et al. 2005). Observed associations between levels of nonspecific metabolites and human health outcomes need to be interpreted cautiously with respect to the ability to make inferences regarding CPF specifically (CitationBravo et al. 2004).
ANALYSES OF NEUROBEHAVIORAL OUTCOMES DERIVED FROM EPIDEMIOLOGIC STUDIES
Scope of the Review and Literature Search
Our review includes peer-reviewed epidemiologic studies published in English through May 31, 2010, using the following search terms: “chlorpyrifos,” “TCPy,” “organophosphate,” “organophosphorus,” “neuro,” “neurotoxicity syndromes,” “neurotoxic,” “neurotoxins,” “neurotoxicity,” “neurologic,” “neurological,” “nervous system,” “neurobehavior,” “neurobehavioral,” “behavior,” “motor skills,” “psychomotor” “cognitive,” “cognition,” “cognitive development,” or “impaired cognitive function,” “motor development,” “intelligence,” or “autism,” “parental,” “parent,” “neonate,” “neonatal,” “prenatal,” “pregnancy,” “pregnant,” “fetus,” “fetal,” “maternal,” “developmental,” “child,” “children,” “teen,” “adolescent,” “utero.” In addition, the reference lists were cross-checked in recent reviews on CPF to identify any relevant papers that might have been missed by our search terms.
Studies that were included in this review investigated potential associations between exposure to CPF during critical periods of brain development and neurodevelopmental outcomes in neonates, infants, or young children. Studies based upon self-reported exposure to CPF, measured exposure in the home environment (e.g., air, soil, dust), or biomarkers of exposure were eligible, provided that exposure was evaluated in terms of its potential association with a neurobehavioral measure in infants or young children.
Population Parameters
Studies with populations potentially exposed to CPF during critical periods of brain development, such as in utero, infancy, and early childhood, were included. Only studies that were conducted on neonates, infants, toddlers, or young children were also included.
Exposure Measurements
The CPF biomarkers reported in this review were measured and quantified from biological specimens obtained from women during pregnancy or from their child after delivery. This included measuring CPF in umbilical cord blood, as well as measuring TCPy, DEPs, and DAPs in maternal or child urine (). In addition, two of the cohort studies also reported results for DMPs (CitationEngel et al. 2007; CitationEskenazi et al. 2007; CitationYoung et al. 2005). It is important to note that DMPs are not biomarkers of CPF. Therefore, the results associated with DMPs are not systematically reported and tabulated, but are discussed when relevant to understanding results for DAPs. The use of DAPs, which include both DEPs and DMPs, may not be reflective of exposure to CPF.
The cohort studies utilized a questionnaire to obtain demographic personal and exposure information on study participants including personal habits (i.e., smoking, alcohol consumption, and drug use), maternal medical history, and environmental exposures (including pesticides) among the study participants (). The extent to which information ascertained from the questionnaires was analyzed or included (e.g., as covariates) in final multivariate models is summarized in .
Studies that inferred CPF exposure but did not directly quantify CPF levels were excluded. This included studies that evaluated CPF exposure based solely on the residential location of study participants or that estimated exposures from ambient air on a group or community level. Studies that reported CPF poisoning were excluded, as were any that reported toxic exposure levels above the approved levels of standard use. Studies that described CPF biomonitoring but did not report specific health outcomes were evaluated but not included in our final review.
Outcome Measurements
Studies reporting neurobehavioral outcomes including measures of psychomotor and mental development and assessment of potential behavioral problems were evaluated. The epidemiologic studies with respect to CPF that assessed the neurobehavioral outcomes used the following tools:
Bayley Scales of Infant Development II: Mental Development Index (BSID:MDI)
The BSID:MDI assesses general cognitive development and higher order mental processing, with 178 individual items that measure memory, habituation, generalization, classification, vocalizations, visual preference, visual acuity skills, problem solving, early number concepts, language, and social skills and development (CitationBlack and Matula 2000; CitationSattler 2001; CitationStrauss et al. 2006). Standardized index scores have a mean of 100 and a standard deviation of 15, and range from 50 to 150 (mean score ± standard deviations). Infants with standardized scores between 70 and 84 on the BSID:MDI are classified as having “mildly delayed performance” (CitationBlack and Matula 2000). Infants with scores <70 are classified as having “significantly delayed performance.” The Mental Scale has 22 item sets, designated by age, and each set has about 27 items (range: 20–36 items).
Bayley Scales of Infant Development II: Psychomotor Development Index (BSID:PDI)
The BSID:PDI assesses overall motor development and contains 111 items that measure quality of movement, sensory integration, motor planning, fine and gross motor skills (e.g., rolling, crawling, creeping, sitting, standing, walking, running, jumping, prehension [act of holding, seizing, or grasping], use of writing implements, imitation of hand movements), and perceptual–motor integration (CitationBlack and Matula 2000; CitationStrauss et al. 2006). Standardized scores have a mean of 100 and a standard deviation of 15, and range from 50 to 150 (mean score ± standard deviations). Classifications for “mildly delayed performance” and “significantly delayed performance” are similar to the MDI. The Psychomotor Scale has 22 item sets, designated by age, and each set has about 17 items (range: 14–21 items).
Brazelton Neonatal Behavioral Assessment Scale (BNBAS)
The BNBAS groups the measurement of behavioral abilities and reflexes into the following seven domains: habituation, orientation, motor performance, range of state, regulation of state, autonomic stability, number of abnormal reflexes, and type of abnormal reflexes (CitationBrazelton and Nugent 1995). It has been used extensively as a research instrument; however, it was originally designed to be a clinical instrument, and there are separate guidelines for research versus clinical uses of the BNBAS. The test is suitable for infants up to 2 mo of age and was initially developed to help parents and child care workers understand the language of newborn infants, including coping capacities and adaptive strategies. This test is also referred to as the Neonatal Behavioral Assessment Scale (NBAS).
Child Behavior Checklist (CBCL)
The CBCL 1.5–5 is a paper-and-pencil instrument administered to caregivers, usually parents, of children ages 1.5 to 5 yr of age to measure emotional and behavioral functioning as indicated by the frequency of certain behaviors during the previous 2 mo (CitationAchenbach and Rescorla 2000; CitationRescorla 2005). CBCL items are scored on a 3-point scale as follows: 0 = not true, 1 = somewhat or sometimes true, and 2 = very true or often true, within the past 2 mo (CitationRescorla 2005). The CBCL's seven empirically derived syndrome scales (emotionally reactive, anxious/depressed, somatic complaints, withdrawn, sleep problems, attention problems, aggressive behavior) were developed through factor analysis of data from the general pediatric population (CitationRescorla 2005). In addition, five DSM-oriented scales (affective problems, anxiety problems, pervasive developmental problems, attention deficit/hyperactivity problems, oppositional defiant problems) were developed to be relevant to the commonly used Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) (CitationAmerican Psychiatric Association 2000) diagnostic categories, although they are not directly equivalent to a DSM diagnosis (CitationRescorla 2005). For each syndrome and DSM-oriented scale, a child's score is obtained by summing the ratings for the items that compose the syndrome and comparing to the normative sample as follows: <93rd percentile = normal range, 93rd to 97th percentile = borderline clinical range, >97th percentile = clinical range. The following three scales were evaluated by studies included in our review: Attention Problems syndrome scale (empirically derived), the DSM-oriented Attention-Deficit/Hyperactivity Disorder (ADHD) scale, and the DSM-oriented Pervasive Developmental Disorder (PDD) scale.
CHARACTERISTICS OF EPIDEMIOLOGIC STUDIES
Four publications from three cohort studies evaluated potential associations between CPF exposure and neurodevelopmental outcomes (CitationEngel et al. 2007; CitationEskenazi et al. 2007; CitationRauh et al. 2006; CitationYoung et al. 2005; see ).
Center for Health Assessment of Mothers and Children of Salinas
The research objective of this center (CHAMACOS; CitationEskenazi et al., 2007; CitationYoung et al. 2005) is to evaluate health effects (e.g., growth and development) of exposure to pesticides and other factors in the environment among pregnant women and their offspring who reside in Salinas Valley, California, an agricultural community. Study participants are predominantly low-income Latina women who enrolled in the study between October 1999 and October 2000 during their first trimester of pregnancy. Approximately 531 pregnant women were followed to birth; however, fewer were available for analysis (i.e., <400), depending on the neurobehavioral outcome, age being evaluated, and the metabolite measurement. Information was not available in either paper in order to calculate overall participation rates. Urine samples were collected from mothers twice during pregnancy and thrice from children. Analyses relevant to our review from this center evaluated associations between urinary metabolites TCPy, DEPs, and DAPs, and the BSID:MDI, BSID:PDI, CBCL, and BNBAS. DEPs and DAPs were modeled as continuous variables. TCPy levels were categorized into three groups: less than limit of detection (<LOD), below the median detectable level, and above the median detectable level (CitationEskenazi et al. 2007).
Mount Sinai Center for Children's Environmental Health and Disease Prevention Research
The enrolled cohort (CitationEngel et al. 2007) includes mothers recruited during pregnancy who are multi-ethnic (about 51% Latina, 27% black, 20% white), inner-city (New York) residents, and who gave birth between May 1998 and July 2001. Maternal blood and urine samples were collected once during the third trimester. CitationEngel et al. (2007) reported that 33% of eligible women approached agreed to participate; 404 women had available birth data and 285 of these women had usable data on DAPs and DEPs. CitationEngel et al. (2007) evaluated the association between DEPs and DAPs in maternal urine and the BNBAS for mother–infant pairs. The metabolites were modeled as continuous variables. CitationEngel et al. (2007) also analyzed the potential interaction among PON1 activity, DEPs or DAPs, and abnormal reflexes (one of the seven BNBAS clusters of behaviors). One route of inactivation of CPO is through hydrolysis by PON1 to form TCPy.
Columbia Center for Children's Environmental Health
Study participants (CCCEH; CitationRauh et al., 2006) are nonsmoking, inner-city (New York) mothers and their infants born between February 1998 and May 2002, who self-identified as black or Dominican. CitationRauh et al. (2006) evaluated associations between CPF in umbilical-cord blood and the BSID:MDI, BSID:PDI, and CBCL in up to 254 mother–infant pairs in which the child had reached age 36 mo (out of 536 “active participants”). Overall participation rates could not be calculated based on the information provided in the paper.
CitationRauh et al. (2006) reported in their methods section that there was “no indication of either a linear or nonlinear dose-response relationship between CPF and developmental outcomes” in preliminary analyses. The study implemented a cutoff point of CPF exposure at 6.17 pg/g, and CPF cord blood levels were dichotomized above or below that value. There were 204 participants in the <6.17 pg/g exposure group and 50 in the >6.17 pg/g exposure group (). The level of this threshold was obtained by first categorizing the CPF levels as undetectable or detectable. The CPF levels above detection were then categorized into tertiles, and the lower bound of the highest tertile was 6.17 pg/g. CitationRauh et al. (2006) justified the decision to implement this threshold in their analysis based on three factors. First, their previous results showed a statistically significant association between the highest tertile of CPF exposure and birth weight (CitationPerera et al. 2003; CitationWhyatt et al. 2004). Second, preliminary analyses showed that there were statistically significant differences between the PDI scores of children in the highest tertile of CPF exposure and scores for each of the other exposure groups. Finally, preliminary analysis also showed statistically significant differences between the MDI scores of children in the highest tertile of CPF exposure and scores of those in the lowest tertile, but not compared to the undetectable group.
Cognitive and Motor Endpoints (BSID II: MDI and PDI)
summarizes the reported associations between CPF, TCPy, DEPs, and DAPs, and results of the BSID:MDI and BSID:PDI for the two studies that used this assessment tool (CitationEskenazi et al. 2007; CitationRauh et al. 2006).
TABLE 2. Summary of results from the Bayley Scales of Infant Development II (BSID II)
The CCCEH study, reported by CitationRauh et al. (2006), was the only study that evaluated the parent compound of CPF in umbilical-cord blood, and approximately 20% of the study population had a CPF level in the upper exposure category ( >6.17 pg/g). Mean scores (± SD) on the BSID:MDI for the total study group were 94.0 (±9.8), 85.1 (±12.4), and 89.6 (±11.4) for ages 12, 24, and 36 mo, respectively. Corresponding mean scores (± SD) for the PDI by age group were 96.2 (±12.2), 97.4 (±11.5), and 100.5 (±13). There was no statistically significant association between CPF exposure (>6.17 pg/g compared to <6.17 pg/g) and MDI scores at 12, 24, or 36 mo or PDI scores at 12 or 24 mo of age (). A statistically significant inverse association was observed for the PDI at 36 mo, with children in the higher exposure category scoring lower than children in the lower exposure category (β = −6.46, SE = 2.18) (CitationRauh et al. 2006).
CitationRauh et al. (2006) also created a two-level categorical variable to characterize performance on the BSID. An assessment of mental and psychomotor delay, defined by the authors as BSID scores ≤85, concluded no marked association between CPF exposure and these specific delays at 12 or 24 mo. However, at 36 mo, the odds of “mental delay” and “psychomotor delay” were statistically significantly higher among the children in the higher exposure group (n = 50) compared to children in the lower exposure group (n = 204), which included the lower 2 tertiles and undetectable groups (odds ratio [OR] = 2.37, 95% confidence interval [CI]: 1.08–5.19 and OR = 4.52, 95% CI: 1.61–12.7, respectively) (CitationRauh et al. 2006).
CitationRauh et al. (2006) compared the distribution of CPF umbilical cord blood levels (at birth) and 36-mo BSID scores for children born during “preban” (before January 2000), “midban” (or “phase-out”) (January 2000 to December 2000), and “postban” (January 2001 and later) periods. CPF levels as mean log-transformed CPF levels at delivery were 0.92 pg/g, 0.81 pg/g, and 0.9 pg/g for the preban, midban, and postban periods, respectively. The corresponding mean 36-mo BSID:MDI scores were 87.1, 91.7, and 89.5, respectively. The increase in mean MDI scores from preban to midban periods was statistically significant, whereas the subsequent decrease was not. Comparisons of PDI scores yielded a similar pattern (albeit statistically nonsignificant), with mean scores of 97.3, 101.8, and 99.4, for preban, midban, and postban periods, respectively. These observed decreases in cord blood and personal air sample CPF levels coupled with increases in 36-mo MDI and PDI scores when comparing preban with midban time periods may suggest a crude dose-response pattern. However, changes were smaller and were statistically nonsignificant when comparing preban to postban levels (CitationRauh et al. 2006). This analysis was based on different groups of participants that were tested at different time periods and did not control statistically for factors that may have changed over time and may be associated with these variables (including, but not limited to, maternal IQ and HOME score). Thus, correlations of CPF levels with changing BSID scores before and after the ban need to be interpreted with caution.
CitationEskenazi et al. (2007) also used the BSID:MDI and BSID:PDI assessment tools in the CHAMACOS study; however, potential exposure to CPF can only be estimated indirectly from metabolites TCPy, DEPs, and DAPs in maternal and child urine. In the CHAMACOS study, there were no statistically significant associations between TCPy and BSID:MDI or BSID:PDI at 6, 12, or 24 mo. The only statistically significant finding for DEPs indicated a positive association (“better” performance) between child DEPs and the MDI at age 12 mo (β = 1.89, 95% CI: 0.21–3.58). There was a statistically significant positive association between child DAPs and BSID:MDI at 24 mo, whereas a statistically significant inverse association was reported between maternal DAPs and BSID:MDI at 24 mo (β = −3.54, 95% CI: −6.59 to −0.49). In addition, maternal DMPs was associated inversely (β = −3.64, 95% CI: −6.36 −0.91) and child DMPs was associated positively (β = 2.01, 95% CI: 0.24 - 3.78) with MDI at 24 mo. Thus, the observed associations between DAPs and performance on the BSID:MDI are not likely to be a result primarily of exposure to CPF (CitationEskenazi et al. 2007).
Both the CCCEH (cord blood CPF; CitationRauh et al. 2006) and the CHAMACOS (maternal TCPy and DEPs; CitationEskenazi et al. 2007) studies administered the BSID II at ages 12 and 24 mo, and showed no statistically significant associations. The CCCEH study reported associations between CPF and indications of poorer performance on the MDI and PDI at age 36 mo. The inverse association between CPF and the BSID:MDI in the CCCEH study was statistically significant only in the analysis that modeled both the exposure and the outcome as dichotomous variables (). The CHAMACOS study did not evaluate children at 36 mo, preventing direct comparison of study findings. The decision to use a dichotomous exposure variable in the CCCEH study was based in part on results from previous analyses, including analyses of CPF and birth weight (CitationWhyatt et al. 2004). CitationRauh et al. (2006) stated, “The most highly exposed group and the undetectable group had lower mean MDI and PDI scores than did the middle levels.” CitationGreenland (1998) offers the following advice regarding the categorization of exposure data: “Ideal categories would be such that any important differences in risk will exist between them but not within them.” Thus, it may not have been appropriate to combine the “undetectable” group with the lower two tertiles in the analyses. Furthermore, one cannot assume a monotonic dose-response association based on statistical evaluation of a dichotomous variable. Therefore, the findings presented by the CCCEH study investigators do not necessarily support a dose-response association between increasing CPF and decreasing performance on the BSID MDI or PDI.
The CCCEH study (CitationRauh et al. 2006) was the only study that presented results for dichotomized BSID outcomes, in addition to using a continuous variable. The cutoff point of 85 was based on one standard deviation below the standardized mean score (standardized mean = 100; SD = 15) (CitationBlack and Matula 2000). In a normal distribution, one would expect approximately 16% of scores to be more than one standard deviation below the mean—that is, below 85. In the CCCEH study, 15.7, 49.3, and 32.9% had MDI scores below 85 (“mild or significant mental delay”) at ages 12, 24, and 36 mo, respectively, and 14, 13.2, and 10.5% had PDI scores below 85 (“mild or significant psychomotor delay”) at the same ages, respectively. Thus, the distribution of PDI and MDI scores appear to differ in this cohort. As described previously, mean MDI scores tended to be lower than mean PDI scores. Further evaluation of categorical associations of CPF with BSID scores distinguishing “mildly delayed” from “significantly delayed” performance on the MDI may be informative.
Behavioral Endpoints From the CBCL
Both the CHAMACOS and the CCCEH cohort studies evaluated potential associations between cord-blood CPF (CitationRauh et al. 2006) or maternal and child urinary metabolites (CitationEskenazi et al. 2007) with outcomes measured by the CBCL, albeit at different ages (). In the CCCEH cohort study, CitationRauh et al. (2006) reported a statistically significant association among children in the higher CPF exposure group (n = 50) compared to children in the lower exposure group (n = 204) and results from the CBCL assessed at age 36 mo. This includes increased OR for attention problems (OR = 11.26, 95% CI: 1.79–70.99), ADHD (OR = 6.5, 95% CI: 1.09–38.69), and PDD (OR = 5.39, 95% CI: 1.21–24.11) (CitationRauh et al. 2006). Although relatively high, the estimated OR for these outcomes are imprecise, as indicated by the broad 95% CI.
TABLE 3. Summary of results from the Child Behavior Checklist (CBCL)
In the CHAMACOS study, neither maternal TCPy nor maternal DEPs was statistically significantly associated with any of the CBCL outcomes measured at 24 mo (CitationEskenazi et al. 2007). However, child metabolite levels of DEPs and DAPs were associated with a statistically significant, approximately 70%, increase in risk of maternal-reported PDD (OR = 1.72, 95% CI: 1.12–2.64 and OR = 1.71, 95% CI: 1.02–2.87, respectively) (CitationEskenazi et al. 2007). CitationEskenazi et al. (2007) reported positive significant (p ≤ .05) association between maternal levels of DAPs and PDD (OR = 2.25, 95% CI: 0.99–5.16). However, because CitationEskenazi et al. (2007) noted statistically significant associations with maternal DAPs and DMPs (OR = 2.19; 95% CI:1.05–4.58), but not with DEPs or TCPy, evidence indicated that exposures to pesticides other than just CPF may be involved.
“Pervasive Developmental Disorder” (PDD) on the CBCL includes affirmative responses to items such as “avoids eye contact,” “unresponsive to affection,” and “rocks head, body” (CitationEskenazi et al. 2007). PDD is one of the “DSM-IV oriented scales” on the CBCL, and is consistent with though not equivalent to autism disorder and Asperger's disorder (CitationRescorla 2005; CitationEskenazi et al. 2007). In the CHAMACOS study, CitationEskenazi et al. (2007) combined children with scores in the clinical range (>97th percentile) with those in the borderline clinical range (>93rd percentile) for the CBCL outcomes. For PDD, 29.6% (n = 105) of the children were categorized as being within the clinical or borderline PDD range. Of these, 51 (14.4% overall) were in the clinical range for PDD.
In the CCCEH study, the classification of PDD used by CitationRauh et al. (2006) appears to be a 98th percentile cutoff point, thereby restricting their definition of PDD to “clinical” only. CitationRauh et al. (2006) reported that 4.7% of the 228 children included in the CBCL analyses were categorized as having “PDD problems.” This corresponds to 11 children overall, with 4 children in the higher exposure group and 7 in the lower exposure group (numbers estimated based on reported percentages). Despite these relatively small numbers for analysis, the proportion of children with maternal-reported PDD is relatively high, particularly in the CHAMACOS study. The extent to which maternal-reported PDD in these studies may correspond to a clinical diagnosis of PDD, or to other related behavioral diagnoses, is unknown and is an important consideration as these cohorts continue to be followed. Preliminary data from the CHARGE case-control study of genetic and environmental risk factors for autism suggest that misclassification of PDD in epidemiologic cohort studies is likely (CitationHertz-Picciotto et al. 2006).
Interpretation of the CBCL data from the CCCEH and CHAMACOS study should bear in mind that assessment was conducted at different ages, analyses were based on different cutoff points for defining CBCL problems, and small numbers of outcomes produced imprecise effect estimates. Misclassification of the CBCL outcomes is also of concern.
Neonatal Behavioral Outcomes on BNBAS
Two studies—the CHAMACOS and Mt. Sinai study—reported associations between maternal urinary metabolites and the BNBAS (). The BNBAS was administered to newborns in the CHAMACOS study within 2 mo of delivery (CitationYoung et al. 2005). The median age at administration of the BNBAS was 3 d, with an interquartile range (IQR) between 1 and 26 d postdelivery. CitationYoung et al. (2005) provided results for the total sample and also stratified the infants based on the timing of their assessment (≥ or <3 d after delivery). In the Mount Sinai study, the BNBAS was administered to newborns within 5 d of delivery (CitationEngel et al. 2007). Therefore, the data reported by CitationEngel et al. (2007) may be most comparable with data <3 d after delivery reported in CitationYoung et al. (2005).
TABLE 4. Summary of results from the Brazelton Neonatal Behavioral Assessment Scale (BNBAS)
There was no meaningful or statistical association between maternal DEPs and DAPs and number of abnormal reflexes in analyses of newborns tested within 3 d of birth. However, maternal DEPs were associated inversely and significantly with autonomic stability among infants tested within 3 d of birth, including tremor, startling, and the color of the infant's skin (CitationYoung et al. 2005). DEPs and DAPs were associated positively and significantly with number of abnormal reflexes in analyses of the full CHAMACOS study sample, and analyses of infants evaluated 3 d or more after birth (). Results for the remaining BNBAS domains were variable and statistically nonsignificant, regardless of age at testing.
In the Mt. Sinai cohort study, CitationEngel et al. (2007) reported a statistically significant positive association between maternal levels of DEPs, but not DAPs, and number of abnormal reflexes (). Results for the remaining domains were variable and statistically nonsignificant. CitationEngel et al. (2007) reported that the OR for number of abnormal reflexes did not increase monotonically and were not statistically significant for the highest versus lowest category comparisons based on analyses of quartiles of DEPs and DAPs.
CitationEngel et al. (2007) evaluated the interaction between PON1 activity and urinary metabolites on number of abnormal reflexes. The investigators considered lower tertiles of PON1 activity to be “slower OP metabolizers.” Therefore, the implicit assumption is that slower OP metabolizers might have higher levels of CPO and other oxons. CitationEngel et al. (2007) noted a statistically significant interaction between levels of DAPs and PON1 activity on the relative risk of abnormal reflexes. CitationEngel et al. (2007) reported that for DAPs and DMPs, infants in the lowest tertile of PON1 activity displayed a significantly increased relative risk of abnormal reflexes (DAP RR = 2.38, 95% CI: 1.37–4.15; DMP RR = 1.96, 95% CI: 1.27–3.03). A statistically significant interaction was also observed between DMPs and the middle tertile of PON1 activity. In contrast, there was no statistically significant interaction between DEP metabolite level and PON1 activity. Therefore, the interaction between DAPs and PON1 activity is not likely to reflect consequences of CPF exposure.
The BNBAS findings that are more relevant to CPF are the results from the analyses of DEPs. Both studies with data reported one or more statistically significant associations between maternal DEPs and number of abnormal reflexes. However, the implication of these findings is not clear. Neither study reported statistically significant findings for any of the other six BNBAS measures, including motor performance. The association in the CHAMACOS study was approximately null and statistically nonsignificant among infants examined within 3 d of birth. Thus, results from the two studies are not consistent when infants of comparable ages of examination were analyzed. Testing conditions varied between the two studies (), particularly test location, with all infants in the Mt. Sinai study evaluated prior to discharge, compared with only 30% of infants in the CHAMACOS study. Finally, both studies evaluated DEPs and DAPs, which are not specific markers of CPF. Neither study analyzed associations between TCPy, a more specific urinary biomarker for CPF, and BNBAS outcomes. In a later publication from the CHAMACOS study, CitationEskenazi et al. (2007) reported relatively low correlations between DEPs and TCPy (r = .1 to .2).
Interpretation of Epidemiologic Studies
Review of the three epidemiologic cohort studies with data potentially relevant to CPF exposure and neurobehavioral outcomes indicated relatively few statistically significant findings in the direction of adverse effects, and no consistent patterns of adverse association across studies. One exception may be the positive association between DEPs and number of abnormal reflexes observed in both the Mt. Sinai study (CitationEngel et al. 2007) and the CHAMACOS (full sample) study (CitationYoung et al. 2005). However, the lack of association between DEPs and number of abnormal reflexes among infants evaluated within 3 d of birth in the CHAMACOS study (beta = 0.08) is inconsistent with the statistically significant positive association reported by the Mt. Sinai study for infants evaluated within 5 d of delivery (RR = 1.49). Furthermore, DEPs reflect exposure to several pesticides besides CPF.
Interpretation of this group of epidemiologic studies needs to take into consideration study similarities and differences, and strengths and limitations. There are insufficient data to address the specific research question regarding CPF and neurobehavioral outcomes in infants and young children. Isolating and quantifying the potential effect of CPF is challenging for several reasons. As shown in , the results were from a number of regression models, which varied by factors including the biomarker/metabolite measured, the age of the infant/toddler, and the neurobehavioral test instrument. Thus, it is also important to consider the probability that some statistically significant findings may have occurred by chance. In addition, noncausal associations may also be produced as a result of nonrandom error (bias), including measurement error and confounding.
Measurement Error
All studies used quantitative biomarkers of exposure, which is clearly desirable because they are more objective measures compared to questionnaires. Only one study, the CCCEH study, actually measured the parent compound in blood. This study was limited, however, by measuring CPF at one point in time (i.e., in umbilical cord blood at the time of delivery). The other two cohort studies used urinary metabolites of CPF that also reflect exposures to other OP. CitationEskenazi et al. (2007) examined TCPy in addition to DAPs and DEPs, but observed no statistically significant associations with this metabolite. Although TCPy is more specific than the DAPs, TCPy may reflect other exposures (e.g., chlorpyrifos-methyl, TCPy) and may not represent all routes of exposure equally well (CitationNeedham 2005).
All three cohort studies relied on one or two specimen collections and it cannot be assumed that these data will accurately represent exposure(s) during pregnancy. Eskenazi and colleagues (2004; 2007) reported high within-person variability for urinary metabolites measured at different times in both mothers and their children. In addition, CitationEskenazi et al. (2007) found that the proportion of CHAMACOS study participants with biomarker levels above the limit of detection was 71% at the baseline pregnancy measurement and 82% at the second measurement. The average of these two measures was reported as 91%, which ultimately becomes the reference group for future analysis in this publication. However, using an average limit of detection is likely to lead to misclassification of the exposure and also is likely to mask the exposure level during the different times of brain development throughout the gestational process.
Confounding
Confounding might produce spurious findings or mask “real” effects. It is important to consider the extent to which any factors that predict performance on the BSID-II, or a mother's assessment of her child on the CBCL, or an infant's behavior during evaluation on the BNBAS, are related to factors associated with use of CPF or to factors that determine the degree of exposure to CPF. Such factors may include personal characteristics of the mother, including her age, education level, depressive symptoms, etc. If these factors are not measured and controlled in the analyses, the results may be confounded. In considering factors that are associated with levels of CPF in umbilical cord blood, it is important to evaluate the conditions that may be related to the reasons for being exposed to CPF or other OP (e.g., cockroach infestation associated with poor housing maintenance, hygiene, and care environment; extent of participation in agricultural work), as well as factors related to higher versus lower exposure (e.g., following instruction labels regarding proper use). These factors could also be related to the learning and developmental environment, which may affect the test scores in the child, or may influence the mother's interpretation or reporting of her child's behavior.
It is also important to consider the constellation of established predictors of performance on the BSID, scores on the CBCL, or scores on the BNBAS, and to evaluate these as potential confounders, particularly those that could conceivably be associated with CPF levels. For example, in their consideration of potential confounders, CitationEskenazi et al. (2007) stated that selection of covariates for the multivariate BSID-II models was based on whether the variables were “related to conditions of testing, related to neurodevelopment in the literature and associated (p < 0.1) with most outcomes … or consistently related to neurodevelopment in the literature even if not in our data.” In the CCCEH study, CitationRauh et al. (2006) stated that “Covariates were included in models as possible confounders if they (1) had a significant association with level of pesticide exposure and any measure of developmental outcomes in this sample, (2) altered the estimate of CPF effect by ≥ 10%, or (3) had been identified as confounders in comparable studies.”
There were several differences in the factors that were adjusted for between CitationRauh et al. (2006) and CitationEskenazi et al. (2007). For example, CitationRauh et al. (2006) did not appear to measure or adjust for factors related to the BSID-II testing environment, such as the examiner or the exact age at examination. These factors are known to influence test scores (CitationBlack and Matula 2000) and were measured and adjusted for by CitationEskenazi et al. (2007). As another example, maternal depression, a significant factor in the CitationEskenazi et al. (2007) CBCL models, was not evaluated by CitationRauh et al. (2006). Thus, there are concerns about potential uncontrolled or residual confounding.
Causal Inference
Distinguishing causal from noncausal associations in epidemiology is challenging because of the observational (i.e., nonrandomized) study designs and the inevitable role of bias. Hill proposed several viewpoints to consider when evaluating the evidence from a body of epidemiologic literature, including strength of the association, consistency, dose-response, and biological plausibility (CitationHill 1965). The associations (OR) between CPF and maternal-reported CBCL outcomes were relatively strong in the CCCEH study by CitationRauh et al. (2006), but the imprecision indicated by the accompanying CI should not be ignored. As pointed out by CitationPoole (2001), inference and decision making should prefer estimates with narrow confidence intervals over small p values, “which are least vulnerable to the play of chance. These are the results for which, by virtue of intentional or accidental features of our research methods, our studies provide the most evidence.” In contrast to the results from the CCCEH study, the association between maternal DEPs and PDD was inverse and statistically nonsignificant in the CHAMACOS study (CitationEskenazi et al. 2007).
For the most part, other observed associations were not particularly strong. There was no clear evidence for consistency, but given the limited number of studies, different types of exposure (home use versus agricultural), variability in exposure measure, timing of exposure measure, timing of outcome measure, and modeling of exposure and outcome for analysis, only a limited degree of consistency would be possible to establish. While there were some statistically significant results for analyses of continuous exposure and outcome variables, it was not always clearly indicated whether the presence of nonlinear associations was also explored. CitationRauh et al. (2006) and CitationEngel et al. (2007) did describe data that indicated that some associations were not monotonic. Use of dichotomous exposure variables in the CCCEH study precluded the ability to assess exposure-response associations in that cohort (CitationRauh et al. 2006). Temporality and an argument for biological plausibility were supported in the three cohorts.
Summary
Therefore, data from these three cohorts did not support a causal association between CPF and adverse neurobehavioral outcomes in infants or young children. Additional research in this area is needed in order to assess causality with more certainty. Challenges to future research include limited exposure among most individuals, difficulties in estimating exposure during critical periods of development, and consistency of timing and outcomes measured across multiple studies to allow comparisons. Nevertheless, additional analyses from the existing cohorts to fully examine dose response, repeated measures of outcomes, assessment of test–retest reliability with outcome measures, correlations among all available exposure measures (self-report, biomarker, air monitoring data), and additional follow-up time and testing of the children will provide helpful information.
ANALYSES OF DEVELOPMENTAL NEUROBEHAVIORAL ANIMAL STUDIES
Scope of the Review and Literature Search
PubMed was searched using the terms “chlorpyrifos” and (“offspring” or “neonatal” OR “maternal” OR “in utero” OR “development” OR “developmental” OR “pregnancy” OR “pregnant” OR “gestational” OR “newborn” OR “prenatal” OR “perinatal” OR “teratology” OR “fetus” OR “fetal” OR “maternal” OR “age-dependent” or “age dependent” or “age sensitivity”) AND (“brain” OR “neuron” OR “nervous” OR “neurotoxic*” OR “neurolog*” OR “neurobehavior*” OR “neurodevelopment” OR “developmental neurotoxic*” OR “motor” OR “cognition” OR “cognitive” OR “behavior” OR “receptor” OR “neurotransmitter” OR “cholinesterase” OR “cerebellum” OR “hippocampus” OR “striatum” OR “cortex”). In addition, the reference list for CitationEaton et al. (2008) was cross-checked for additional papers meeting our inclusion criteria.
Our review focused on repeated-dose in vivo DNT animal studies in which CPF alone (i.e., not mixtures, not in sequence with other pesticides, and not as a formulation) was administered in a clearly defined vehicle to pregnant dams, lactating dams, and/or pups prior to weaning. Studies using oral, inhalation, dermal, and sc routes of exposure to CPF were included, but not those using intravenous or direct injections into the brain.
The primary purpose of this review is to provide an in-depth weight-of-evidence evaluation of the absence and presence of effects and direction of change reported for neurobehavioral endpoints. Neuropharmacologic and morphologic endpoints were evaluated for evidence of MOA directly linking effects of these endpoints with neurobehavioral outcomes, and relative sensitivity compared to brain or RBC AChE inhibition.
This review focused on evaluating effects at lower dose levels (i.e., less than 10 mg/kg). Studies conducted with doses near the maximum tolerated dose level, producing significant systemic toxicity or lethality, or designed to compare LD50s were not included. Standard developmental and reproduction studies were not included unless AChE activity inhibition or neurobehavioral, neuropharmacologic, and neuropathologic/morphologic endpoints were evaluated.
Sixty-one research papers measured effects of repeated exposures of CPF during development prior to weaning on behavioral, neuropharmacologic (AChE inhibition, neurotransmitter receptor binding, neurotransmitter levels), and/or brain pathology endpoints. Twenty-three papers included behavioral outcomes, 4 of which were excluded because the test material was a CPF formulation (CitationMuto et al. 1992), test subjects were mutant mice (Laviola et al. 2006), or dose levels were greater than 10 mg/kg-d (CitationChanda and Pope 1996; CitationChakraborti et al. 1993). Of the remaining 19 behavioral studies, there were 7 oral studies (CitationBraquenier et al. 2010; CitationCarr et al. 2001; CitationJohnson et al. 2009; CitationMaurissen et al. 2000; CitationVenerosi et al. 2006; 2009; 2010), 1 dermal study (CitationAbou-Donia et al. 2006), 3 sc studies with peanut oil as vehicle (CitationJett et al. 2001; CitationRicceri et al. 2006 [oral for gestation, sc for postnatal]; CitationVenerosi et al. 2008), and 8 sc studies with dimethyl sulfoxide (DMSO) as vehicle (CitationAldridge et al. 2005a; CitationDam et al. 2000; CitationBillauer-Haimovitch et al. 2009; CitationHaviland et al. 2010; CitationIcenogle et al. 2004; CitationLevin et al. 2001; Citation2002; CitationRicceri et al. 2003). Seven research papers included histopathology or morphometry evaluations of brain areas, one of which was excluded because CPF exposure was 40 mg/kg-d (CitationAmira et al. 2005). Forty-four papers evaluated neuropharmacologic outcomes following developmental exposures to repeated CPF doses less than 10 mg/kg-d.
Comparison of Experimental Design and Statistical Analyses
Using the standards and approaches described earlier in the section entitled “Approach to Evaluation of CPF Studies,” the experimental design and statistical analyses were evaluated. Key studies representative of groups of investigators with similar experimental design and dosing regimens are discussed next for laboratories evaluating multiple behavioral outcomes.
The oral DNT rat study by CitationMaurissen et al. (2000) was the most robust study in terms of number of dose levels tested (3 dose groups and control), number of pups (1 male or female/litter) and litters (20 litters/dose group) tested, and relevant route of exposure. In this study, dams were dosed from gestational day 6 to postnatal day 11 (GD 6–PND 11). Data for each behavioral endpoint from both sexes were pooled into a single analysis of variance (ANOVA). This study was designed so that the litter was the unit of analysis, the time of testing was balanced across sex and dose level, and experimental bias of subjective measures was controlled (e.g., blind observations for learning and memory test). A related study by CitationMattsson et al. (2000) measured AChE or BuChE inhibition in 10 litters/dose group (1 male or female/litter) from brain, RBC, or plasma samples collected 2–4 h after dosing.
Three oral neurobehavioral studies dosed mouse or rat pups directly with three CPF dose levels or vehicle (CitationBraquenier et al. 2010; CitationJohnson et al. 2009; CitationCarr et al. 2001). CitationBraquenier et al. (2010) tested 8–10 female mice/dose group (1 female/litter). Of the three behavioral tests scored by observers (locomotor activity, light/dark box test, elevated plus maze), only the elevated plus maze was scored blind to treatment level (CitationBraquenier et al. 2010). For the two oral rat studies, CitationCarr et al. (2001) and CitationJohnson et al. (2009) dosed the low-dose group with a constant dose level, and exposed the mid- and high-dose groups to incremental dose regimens. CitationCarr et al. (2001) dosed 2 pups/sex/litter from 5 litters/dose group every other day, and included litter as a factor in the statistical analysis. Observations of the open field activity were conducted blind to treatment level. CitationJohnson et al. (2009) used a within litter design, in which all seven exposures (corn oil vehicle, three CPF doses, and three methylparathion doses) were represented within each litter, to the extent possible. The potential for cross-contamination was not addressed. The sample size for each dose group was 9–14 rats/sex/dose level. Statistical analysis included the litter as the experimental unit of analyses, and day or week of testing was included as a repeated-measures factor. There was no indication of whether observers for the radial arm maze were blind to treatment (CitationJohnson et al. 2009).
Oral neuropharmacologic studies by CitationGuo-Ross et al. (2007) and CitationRichardson and Chambers (2003; Citation2004; Citation2005) included two or three dose levels other than the control, and litter was the individual unit of analysis. AChE inhibition was measured in brain samples collected 4–6 h after dosing (CitationGuo-Ross et al. 2007; CitationRichardson et al. 2005). The selection of pups from litters and the number of litters represented was not always clearly indicated (CitationGuo-Ross et al. 2007; CitationRichardson and Chambers 2003 Citation2004; CitationTang et al. 1999). The sample size of three to four or four to six pups per dose group was small even for mechanistic biochemical endpoints. However, the use of multiple dose levels and/or times of sacrifice allowed data to be evaluated for consistency in dose-response and temporal patterns. Statistical analyses for these neuropharmacologic studies did not appear to take into consideration repeated measures, and a correction for multiple comparisons was not included in the analyses. An oral neuropharmacologic study by Eels and Brown (2009) evaluated dopamine neurochemistry in one oral CPF dose group receiving an incremental dosing regimen (1.5 to 6 mg/kg, PND 1–21). This study was considered inadequate because it did not report sample size for pups or litters.
There was one dermal neurobehavioral study that tested only one CPF exposure level. The litter was not the experimental unit (10 pups/sex from 5 litters; CitationAbou-Donia et al. 2006). Observers were blind to treatment level.
CitationSlotkin et al. (2001; Citation2002; Citation2004; Citation2006) and collaborators (CitationAldridge et al. 2003; Citation2004; Citation2005a; Citation2005b; Citation2005c; CitationIcenogle et al. 2004; CitationLevin et al. 2001; Citation2002; CitationQiao et al. 2002; Citation2003; Citation2004; CitationRaines et al. 2001; CitationRoy et al. 2004; Citation2005; CitationSlotkin and Seidler 2005; Citation2007a; Citation2007b; CitationSong et al. 1997) dosed rats sc with CPF in DMSO. For 4 of the 5 neurobehavioral studies, 1 offspring/sex/dose group from 8–10 litters was tested (CitationAldridge et al. 2005a; CitationIcenogle et al. 2004; CitationLevin et al. 2001; Citation2002). For one of the neurobehavioral studies, the pup was the individual unit of analyses with 5–7, 23–24, and 40–46 pups/sex/dose group (2 pups/sex/litter) for locomotor skills, reflex righting, and negative geotaxis, respectively (CitationDam et al. 2000). The litter of origin was confounded in studies that exposed dams during gestation because pups were randomized at birth to create newly constituted litters within a treatment level. In addition, male and female offspring from the same reconstituted litter (1 pup/sex/litter) were pooled together in a multivariate statistical analysis that did not include litter as a factor. Dams were repeatedly rotated every few days to control for differences in maternal care. Aldridge et al. (2005) and CitationDam et al. (2000) indicated that the observers for behavioral tests were blind to treatment level, but CitationLevin et al. (2001; Citation2002) and CitationIcenogle et al. (2004) did not.
The oral mouse study by CitationBraquenier et al. (2010) only tested female offspring, and included 3 CPF-treated groups plus control and 8–10 pups tested per dose group. Behaviors on the elevated plus-maze, but not the light/dark box test, were recorded by observers blind to treatment.
Oral and sc mouse studies conducted by CitationRicceri et al. (2003; Citation2006) and CitationVenerosi et al. (2006; 2008; 2009; 2010) dosed approximately 10 pups/dose level. With the exception of CitationRicceri et al. (2006), these studies indicated that observers were blind to treatment level for at least one of the behavioral measures. Mice were exposed to only one CPF dose level in 2 oral studies (6 mg/kg-d; CitationVenerosi et al. 2009; 2010) and 1 sc study (3 mg/kg-d in peanut oil; CitationVenerosi et al. 2008). CitationRicceri et al. (2003) dosed mice postnatally with two CPF dose levels sc in DMSO. For each oral group of dams dosed during pregnancy (vehicle, 3 mg/kg-d, or 6 mg/kg-d), CitationRicceri et al. (2006) and CitationVenerosi et al. (2006) dosed the pups (vehicle, 1 mg/kg-d, or 3 mg/kg-d) sc and orally, respectively, using a within-litter design such that in total nine different treatment groups were established. The litter was the experimental unit of analyses for most studies, although not always for behavioral endpoints examined after weaning (CitationRicceri et al. 2003). The decision tree for the statistical analyses was unclear because results of post hoc tests were reported even though main effects were not significant (CitationRicceri et al. 2003; Citation2006; CitationVenerosi et al. 2009). Frequency, duration, and latency for many behavioral parameters were analyzed. Corrections for the multiple comparisons were reported in two papers (CitationRicceri et al. 2003; Citation2006).
Relevance of Route, Vehicle, and Age of Exposure to Risk Assessment
The route and age of exposure, as well as the vehicle used, need to be taken into account in the interpretation and application of the results in animals for human health risk assessment. The relevance of different routes of exposure depends on the specific population of concern and uses of CPF. In the United States today, the primary route of exposure for the general population is dietary. The majority of the animal studies on CPF can be divided into the following periods of exposure: (1) gestational exposures to the dam, (2) gestational and postnatal exposure to the dam, so that pups are potentially exposed via lactation, (3) gestational exposures followed by direct exposures to the pups, or (4) postnatal exposures directly to the pup without any maternal exposures. Our review evaluates outcomes following gestational exposures separately from those following direct exposure to pups for each outcome because the developmental period of exposure is an important consideration when evaluating the results.
Although different routes of exposure or vehicles may be used in mechanistic studies, the relevance of the methods must be evaluated when these data are considered for setting acceptable exposure levels for human risk assessment. Oral administration of test material is relevant to dietary exposures, which is the primary route of exposure for CPF to the general population. However, bolus gavage dose of CPF in corn oil may result in peak blood and brain levels that may not be similar to those measured following dietary exposures, the primary route of exposure to children in the United States where residential uses are not allowed. The sc route of exposure is not a human route of exposure. One rationale for using the sc route of exposure in animal studies is that it “avoids the potential confounds of differential rates of gastrointestinal absorption between compounds or ages and first-pass effects on bioavailability” (CitationSlotkin et al. 2006). However, when evaluating data for purposes of human health risk assessment, first-pass effects on bioavailability are important considerations.
CitationCarr and Nail (2008) studied the effects of route, vehicle, and volume of exposure on brain AChE inhibition 4 h after CPF oral and sc exposure to 5 mg/kg to pups from PND 10 to 16. Overall, sc DMSO injections of CPF increased regional brain AChE inhibition by 16–29% compared to levels in pups dosed orally. CitationCarr and Nail (2008) reported statistically significant decreases in body weight after d 4 of sc injections but not after gavage dosing. Thus, sc DMSO injections of CPF increased toxicity compared to corn oil gavage doses as measured by AChE inhibition and body weight decreases. CitationCarr et al. (2001) also pointed out that sc injection could mimic dermal exposure, but “it does not appropriately consider the disposition and metabolism … when penetration through the skin is a determining factor.”
CitationMarty et al. (2007) measured the blood PK for CPF and its major metabolite TCPy following gavage and sc dosing to pups on PND 5. Significant (56 ± 36%) radiolabeled CPF remained at the site of sc injection at the time to maximum concentration (Tmax = 2 h). CitationSmith et al. (2009) compared CPF PK using oral and sc exposures and different vehicles in adult male rats. Orally administered CPF underwent much more extensive metabolism than CPF administered sc, consistent with hypothesis that oral CPF is rapidly absorbed and extensively metabolized in the gut and liver, resulting in greater first-pass metabolism. In addition, PBPK model simulation in adults show that sc administration in corn oil and DMSO is predicted to have “greatly prolonged inhibition of ChE activity in the brain relative to oral exposure,” due to “slow release of CPF from the injection site depot and localized brain CPF metabolism to CPF-oxon” (CitationSmith et al. 2009).
DMSO, by itself, perturbs behavior, neuropathology, or neuropharmacology at volumes administered as a vehicle. Specifically 20-μl injections of DMSO in PND 4 rats produced endocrine and neurochemical changes similar to or additive with chlordecone (CitationRosecrans et al. 1984; CitationUphouse et al. 1982). DMSO exacerbated the neuropathological effects of soman in adult rats at 1-ml/kg doses (CitationBallough et al. 2008), which is the equivalent volume used in CPF sc rat studies (CitationAldridge et al. 2003; CitationLevin et al. 2002). These studies indicated that use of DMSO as a vehicle is a potential confounder for determination of cellular and pharmacologic effects produced by chemicals.
In summary, the relevance of sc injections of CPF in DMSO to dietary exposures for risk assessment is called into question due to (1) differences in PK resulting from a depot of test material at the site of the local sc injection including bypassing first pass metabolism, (2) the possible impact of DMSO itself on neurotoxicity, and (3) resultant effects on toxicology endpoints that may not be representative of human dietary exposures to CPF.
Neurobehavioral Domains
This section evaluates consistency of effects across different laboratories for similar behavioral domains. summarize neurobehavioral effects of CPF exposures on selected behaviors for which there were data from different laboratories. An exception was made for spontaneous alternation, measured only by one laboratory (), because this behavior was considered to be related to other measures associated with learning and memory and/or activity in a novel environment. The major methodological issues of concern were discussed in the previous section and were taken into consideration in the overall assessment.
TABLE 5A. Developmental landmarks/neurodevelopmental reflex in the RAT (oral and sc)
TABLE 5B. Developmental landmarks/neurodevelopmental reflex in the MOUSE (oral and sc)
TABLE 5C. Neonatal behaviors in the MOUSE (oral and sc)
TABLE 6A. Motor activity in the RAT (oral corn oil)
TABLE 6B. Motor activity in the RAT (sc DMSO)
TABLE 6C. Motor activity in the MOUSE (oral or sc)
TABLE 7A. Novelty/plus maze in the MOUSE (oral corn oil)
TABLE 7B. Novelty/plus maze in the RAT (sc DMSO)
TABLE 7C. Novelty and plus maze in the MOUSE
TABLE 8. Spontaneous alternation in the RAT (s.c. DMSO)
TABLE 9A. Learning and memory (rat oral corn oil)
TABLE 9B. Learning and memory (rat sc DMSO or peanut oil)
TABLE 9C. Learning and memory (mouse sc DMSO)
TABLE 10A. Pharmacologic challenge in the MOUSE (oral peanut oil)
TABLE 10B. Pharmacologic challenge in the RAT (sc DMSO)
Developmental Landmarks, Neurodevelopmental Reflex, and Ultrasonic Vocalizations Prior to Weaning
Overall, CPF exposures did not produce a consistent pattern of adverse effects on developmental landmarks, neurodevelopmental reflex and ultrasonic vocalization (USV) measured prior to weaning at doses below 5 mg/kg-d ().
Gestational exposures
CitationMaurissen et al. (2000) reported effects on developmental landmarks in rats only at the highest dose level of 5 mg/kg, which was maternally toxic and produced 25% pup mortality and decreased pup body weight before culling on PND 4. CitationBillauer-Haimovitch et al. (2009) noted no marked effects in mice at doses from 1 to 10 mg/kg-d following gestational sc exposures. The sample size for the latter study was not reported and half the pups were cross-fostered.
In mice, 6-mg/kg CPF exposure from GD 14 to 17 exerted little impact on pup growth and sensorimotor development (; CitationVenerosi et al., 2009). There was a numerical trend toward delayed appearance of hind-limb grasping and decreased total daily qualitative score (comprising all seven reflex measures including hind limb grasping) (CitationVenerosi et al. 2009). CitationVenerosi et al. (2009) concluded that “a definitive conclusion about a retarding CPF effect cannot be drawn” based on this data. There were statistically significant decreases in pivoting frequency and duration and increases in duration of “immobility” at PND 12, but no significant effect on frequency or duration of crossing, head moving and wall climbing (). The significance of the decreases in pivoting to overall motor abilities is uncertain given the low control values (approximately 3–4 frequency and 2–3 s). Thus far, the evidence for adverse effects on sensorimotor development is not compelling.
Exposures to 6 mg/kg from GD 14 to 17 decreased USV and higher peak frequency of calls at PND 10 but not PND 4 and 7 in male mouse pups isolated from the dam for 4 min (; CitationVenerosi et al., 2009). The CPF-treated mice had 80 and 15 USV calls/min on PND 7 and 10, respectively. The controls had 70 and 40 USV calls/min, respectively. It is premature to conclude that the fall in USV at 6 mg/kg-d (the only dose level tested) following gestational exposures is an adverse effect based on a study testing only one dose level. In addition, the overall pattern of rapid decrease in USV in mice at 6 mg/kg CitationVenerosi et al. (2009) is consistent with the ontogenic profile for control mice in two other studies from this laboratory. Specifically, CitationRicceri et al. (2003) observed a rapid decline in USV calls in control pups (approximately 125, 80, and 20 USV calls/min on PND 5, 8, 11, respectively). CitationCalamandrei et al. (1999) reported USV decreasing from approximately 70 to 20 USV/min on PND 7 and PND 11, respectively.
Postnatal exposures
CitationJohnson et al. (2009) noted no significant effects of oral low (1 mg/kg-d), mid (incremental 1, 2, and 4 mg/kg-d) and high (incremental 1.5, 3, and 6 mg/kg-d) doses of CPF from PND 1 to 21 on 5 physical development parameters (pinna detachment, downy fur development, hair growth, incisor eruption and eye opening) and 5 neurodevelopmental reflex parameters (surface righting, negative geotaxis, cliff avoidance, free fall righting, and acoustic startle) (). This was a robust study measuring effects of direct dosing to pups on physical and reflex development at three dose levels that produced statistically significant hippocampal AChE inhibition (14–53%) on PND 20.
CitationRicceri et al. (2003) reported no significant effects of 1 and 3 mg/kg-d CPF (sc DMSO) from PND 1 to 4 on USV or homing behavior in mice (). CitationDam et al. (2000) showed that 1 mg/kg-d CPF (sc DMSO) injected in rats from PND 1 to 4 increased surface righting reflex time and decreased the number of animals meeting the criterion for negative geotaxis (180° turn on an inclined surface) in female mice but not male mice (). Brain AChE inhibition was 75% at 2 h after dosing on PND 1 (CitationDam et al. 2000). The study by CitationDam et al. (2000) was limited because only one CPF dose level in DMSO was tested, and the litter was not the experimental unit for data analysis.
Motor Activity
There were mixed results (increases, decreases, and no effect) on motor activity (), as measured in the open field or automated devices. Some studies reported a tendency for increased activity as measured by increased total session activity, decreased intersession habituation, and/or reduced linear trend (slope) of the habituation curve. However, as discussed next, the weight of evidence does not support a consistent effect at lower exposure levels (i.e., <5 mg/kg) following gestational, lactational, or direct postnatal exposures to the pup.
Gestational exposures with or without postnatal exposures
CitationMaurissen et al. (2000) reported a decrease in automated motor activity at PND 13 and increases at PND 17, 21, and 60 in rats following gestational and lactational exposures to 5 mg/kg, but not at 1 or 0.3 mg/kg (). None of these effects at 5 mg/kg were statistically significant. The lack of effect at lower doses is consistent with two oral mouse studies, in which no marked effects on observer-measured open-field activity (number of line crossing) were demonstrated following in utero exposure to 0.2–6 mg/kg (CitationBraquenier et al. 2010; CitationVenerosi et al. 2009) ().
In contrast, CitationRicceri et al. (2006) found increases in frequency of crossing in an open field at PND 70 in offspring of dams dosed orally with CPF during gestation. In this study, mice were dosed with nine possible combinations of gestational gavage exposures to dams (0, 3, or 6 mg/kg-d) and postnatal sc (peanut oil) injection to pups (0, 1, or 3 mg/kg-d) were tested (). Specifically, prenatal exposure to 3 mg/kg-d in combination with postnatal sc exposure to 3 mg/kg, but not 1 mg/kg or saline, produced an elevation in activity during the first 5 min of the 20-min session (: 3/3 dose group). In addition, elevations in activity were observed in mice exposed in utero to 6 mg/kg-d and postnatally to vehicle or 1 mg/kg-d (: 6/0 and 6/1 dose group), but not in mice exposed in utero to 6 mg/kg-d and postnatally to 3 mg/kg-d (: 6/3 dose group). CitationRicceri et al. (2006) postulated that the higher postnatal dose of 6 mg/kg-d offset the effects of prenatal doses.
CitationIcenogle et al. (2004) and CitationLevin et al. (2002) dosed rat dams with 1 or 5 mg/kg CPF sc in DMSO during early (GD 9–12) or later (GD 17–20) gestation, and reported effects in opposite directions (). CitationIcenogle et al. (2004) noted greater habituation at 5 but not at 1 mg/kg-d, and significant decrease in activity in 2 of 12 time blocks toward the end of the session. This resulted in a higher linear trend (faster rate of decrease) in males and females at 5 mg/kg-d. In contrast, CitationLevin et al. (2002) showed less habituation at 1 and 5 mg/kg, as measured by a lower linear trend (slower rate of decrease) in females only.
Caution is needed in evaluating data converted into linear trend values, which is defined in the figure legends as the “slope of decrease in activity over consecutive 5-min blocks” (CitationIcenogle et al. 2004; CitationLevin et al. 2002). The linear trend analysis is based on fitting linear functions to nonlinear habituation curves with insufficient information provided on goodness of fit to indicate how well they represent the actual data. Based on evaluation of the actual habituation data, there were no marked effects on habituation at 1 mg/kg-d following GD 9–12 exposures (CitationIcenogle et al. 2004, Figure 2), nor at 1 or 5 mg/kg-d following GD 17–20 exposures (CitationLevin et al. 2002, Figure 2). The linear trend values for the CPF groups were approximately 1.2 (GD 17–20, 1 and 5 mg/kg) and 1.6 (GD 9–12, 5 mg/kg). These values are within control linear trend values ranging from 1.2 to 1.6 for three studies published by the same laboratory (CitationLevin et al. 2001 2002; CitationIcenogle et al. 2004).
One dermal study conducted by the CitationAbou-Donia et al. (2006) measured effects of CPF on subjective measures of motor strength and coordination that are related, but not directly comparable to, tests of motor activity. This study exposed dams (5/dose group) dermally to 1 dose level of 1 mg/kg-d CPF in 70% ethanol and measured effects in 1 offspring/sex/litter when they were adults (PND 90) on subjective motor function tests that were conducted blind to treatment level. There were no significant effects on beam-walk time or beam walking ability. However, there was a reduction in rat's fore-paw grip time (gripping of a 5-mm-diameter wood dowel held horizontally) in both males and females, and a decrease in females but not males in the angle at which the animal began to slip backward as the incline plane was raised from an initial horizontal position. Significant increases in female brain AChE were measured at PND 90. AChE was not measured at or soon after the time of exposure. This study is limited by testing only one dose level with a low number of animals. Therefore, these results need to be considered preliminary results from a pilot study that needs to be repeated using a larger sample size, more dose levels, and more objective measures of motor function and strength.
In summary, the overall weight of evidence suggests that there are no marked effects on motor activity following oral or sc gestational exposure to 1 mg/kg-d in rats or mice. Effects reported at 5 or 6 mg/kg include both increases and decreases in activity (). One dermal study that did not meet our standards for sample size (<6 litters) and number of dose levels (<2) reported effects on motor function at 1 mg/kg-d CPF. As described in detail earlier, these differences may be related to differences in species, route of exposure, vehicle, and duration and type of activity measures.
Postnatal exposures
CitationCarr et al. (2001) dosed pups every other day from PND 1 to 21 in a small-scale gavage study (10 males and 10 females from only 5 litters per dose level; ). Although the sample size of five litters did not meet our standards, this study included three dose levels and measured activity at several ages, allowing for evaluation of dose-response and temporal patterns. There was a consistent dose-related decrease in open-field activity in offspring on d 25 and 30, but not at earlier ages. There were no marked effects on motor activity at 3 mg/kg, but decreases in activity following staggered dose regimen (every other day administration) of 3 increasing to 6, and 3 to 6 to 12 mg/kg-d.
In contrast, with sc CPF doses in DMSO to pups, CitationLevin et al. (2001) reported a statistically significant decrease in linear trend of the habituation data at 5 mg/kg (PND 11–14) but not at 1 mg/kg (PND 1–4), which was interpreted as a reduction in habituation consistent with an increase in activity (). However, there was no statistically significant main effect of CPF, and the statistics for the CPF × Block interaction and post hoc tests for each time block were not reported. In comparing the CPF and control habituation curves, the means and standard errors overlapped for all but 2 (3rd and 12th) of twelve 5-min intersession time blocks. Thus, there was no marked effect of CPF on the pattern of habituation, and the statistically significant decrease in linear slope function used to represent the scalloped-shaped habituation curves is not considered an adverse effect.
Studies by CitationRicceri et al. (2003; Citation2006) suggest that CPF increases activity in mice following postnatal sc exposures (). CitationRicceri et al. (2003) reported a rise in total distance traveled at PND 25 following sc injections of 1 and 3 mg/kg-d CPF in DMSO at PND 11–14 but not PND 1–4 exposures. The statistical significance was based on analyses that considered the individual pup and not the litter as the experimental unit, and was based on “post hoc” t-tests that were conducted despite lack of significant main effect on the ANOVA (CitationRicceri et al. 2003). In a subsequent study, CitationRicceri et al. (2006) injected mice from PND 11 to 14 with 1 and 3 mg/kg-d CPF in peanut oil. There was greater frequency of crossing on PND 70 following PND 11–14 exposure to 3 but not 1 mg/kg-d (litter was considered the experimental unit of analysis). Thus, the increase in activity following sc injections of 1 mg/kg-d in DMSO was not replicated, although this may be due to differences in vehicle, age of testing, activity device, and statistical methods.
In summary, postnatal exposures to CPF dose levels above 3 mg/kg-d produced increases or decreases in activity depending on the species, method of measuring activity, and route of exposure. There were no consistent patterns of treatment-related effects on motor activity following postnatal doses below 3 mg/kg-d.
Novelty-Induced Activity, Plus Maze and Chocolate Milk Consumption
Overall, there were no consistent patterns of effects on novelty-induced activity () and behavior, or on activity in open arms and center area, which is considered by some investigators to be correlated with effects on “anxiety”-like or “depressive” behavior.
Gestational exposures with or without postnatal exposures
CitationBraquenier et al. (2010) reported an increase in “anxiety” in female mouse offspring (male mice were not tested) after oral exposures of dams during gestation and lactation to 1 mg/kg, but not to 0.2 or 5 mg/kg (), indicating lack of dose response relationship. Specifically, oral doses of 1 mg/kg decreased the time spent in the open arm of the elevated plus maze, without effects on total number of arm entries (considered by authors to reflect locomotor activity). In addition, oral exposure of CPF decreased time in the center of the light side of the dark/light box at 1 but not 5 mg/kg-d (CitationBraquenier et al. 2010).
In contrast, CitationRicceri et al. (2006) found no marked effects on time in open or closed arm in male and female mice dosed orally in utero to 3 or 6 mg/kg (), and found a decrease in the number of head-dips in males only at 3 but not 6 mg/kg-d. CitationIcenogle et al. (2004) also demonstrated no significant change in the time spent in the open arm versus the closed arm in rat offspring exposed in utero from GD 9 to 12 to sc injections of 1 or 5 mg/kg-d CPF in DMSO ().
The conflicting results may be due to differences in duration of exposure and age of testing or species. Nevertheless, these studies do not support a consistent dose-related pattern of effect on activity level on plus maze or on behaviors potentially related to “anxiety.”
Postnatal exposures
CitationRicceri et al. (2006) reported a statistically significant increase in time spent in the open arms in female mice injected sc with 3, but not 1 mg/kg-d (). There were no marked effects in male mice, and there were no effects on number of center crosses following sc injections of 1 or 3 mg/kg from PND 11 to 14 (). CitationRicceri et al. (2006) considered these findings as a reduction in “anxiety responses” consistent with sex-selective effects reported in rats by Aldridge et al. (2005).
CitationAldridge et al. (2005a) reported that PND 1–4 sc exposure to CPF increased time in open arms and number of center crosses in male but not female rats at 1 mg/kg (), the only dose level tested. Although “sex-selective,” the effects were in the opposite sex from that reported by CitationRicceri et al. (2006). Furthermore, the males in the 1-mg/kg-d CPF treated group from the CitationAldridge et al. (2005a) study spent approximately 17% of time in open arms, which is comparable to control animals from 2 other studies reported by this laboratory using identical procedures and age of testing (PND 50–56). Specifically, CitationRoegge et al. (2008) found that control males and females spent approximately 21 and 23% time in the open field, respectively. CitationTimofeeva et al. (2008) observed approximately 16% of time in open arms for males and females combined. In contrast, CitationAldridge et al. (2005a) reported that the concurrent control males and females spent, respectively, 3 and 20% of time in open arms. The control data from all three studies indicate that there are no consistent sex-specific differences between male and female control rats, and a wide range of control male values.
In general, the statistically significant measures of automated novelty test resulted in no marked effects on novelty preference following sc injections of 3 mg/kg (1 mg/kg not tested) for PND 1–4 or PND 11–14 (CitationRicceri et al. 2003; ). The only effects were statistically significant increases in activity at 3 mg/kg-d (the only dose level tested) during one of the 5-min time bins after the mice were exposed to the novel environment. However, similar changes in motor activity were not seen, especially during the beginning of the sessions, when the activity device is a novel environment to the animals ().
CitationAldridge et al. (2005a) reported effects of PND 1–4 sc injection of 1 mg/kg-d CPF on the consumption of chocolate milk compared to water in both male and female rats tested at PND 54. The control male and female rats in this study clearly demonstrated a preference for chocolate milk compared to water with chocolate milk to water consumption ratios of approximately 4.5 and 5.3, respectively. The CPF-treated male and female rats had chocolate milk–water preference ratios of approximately 3.5, which was lower than for the concurrent control but higher than for controls from other studies by this laboratory (all non-CPF studies) reporting milk–water preference ratios of approximately 1.2–1.75 (CitationRoegge et al. 2008; CitationTimofeeva et al. 2008). The lack of dose-response data and wide range of control values indicate that this finding is not appropriate for risk assessment purposes.
The apparent CPF effect on chocolate milk consumption and increased time spent in the open arm was described as “changes in 5HT-related behaviors that resemble animal models of depression,” citing papers using an olfactory bulbectomized (OB) animal model of depression (CitationAldridge et al. 2005a). However, CitationKelly et al. (1997) and CitationMar et al. (2000) caution that the data generated using the OB model cannot be attributed to any particular neurotransmitter system. In addition, CitationSlotkin et al. (1999) previously found no effects on chocolate milk consumption in young adult OB rats (13 wk old), an age commonly used for this model of depression (CitationKelly et al. 1997).
In summary, there are no consistent patterns of adverse effect on novelty-induced activity, plus maze (), and chocolate milk consumption at any dose level, especially at 1 mg/kg-d.
Spontaneous Alternation
Spontaneous alternation () was measured by CitationLevin et al. (2001; Citation2002) and CitationIcenogle et al. (2004) using a T-maze without any positive or negative reinforcement. There was a consistent lack of effect on spontaneous alternation, the primary endpoint for this test, following sc (DMSO) gestational exposures to the dam or postnatal exposures to the pups. CitationLevin et al. (2002) found a 3- to 5-s decrease in latency in the first of 5 trials following gestational exposures to 1 and 5 mg/kg-d CPF. This effect was considered evidence of hyperactivity. However, there was no evidence of increased activity in the Figure-8 maze motor activity tests, as discussed previously. The biological significance of this decrease in 3- to 5-s latency is uncertain, and there was no consequence to the animals for the time it took to perform the test.
Learning and Memory
Gestation exposures with or without postnatal exposure
CitationMaurissen et al. (2000), CitationIcenogle et al. (2004), and CitationLevin et al. (2002) evaluated the effects of CPF gestational exposures on learning and memory of offspring (). CitationMaurissen et al. (2000) dosed rat dams by gavage with corn oil throughout gestation and early lactation, and tested the same 1 pup/litter from 18 litters/dose group on a T-maze delayed spatial alternation test at weaning and as adults (). There were no significant effects on learning or retention at dose levels of 0.3, 1, or 5 mg/kg-d at PND 22–24 or on long-term memory and retention at PND 61–90. CitationIcenogle et al. (2004) and CitationLevin et al. (2002) injected dams with 1 and 5 mg CPF/kg-d sc in DMSO and tested the offspring using a radial arm maze (RAM), with 12 of 16 arms baited to assess working memory and 4 arms unbaited to test reference memory ().
CitationIcenogle et al. (2004) reported increased errors at 5 but not 1 mg/kg-d following exposures to dams from GD 9 to 12 (). The increased number of errors at 5 mg/kg-d occurred in the first and third block of trials out of 6 for working memory, and only in the first block for reference memory. In contrast, CitationLevin et al. (2002) found that GD 17–20 exposures to dams resulted in increased working and reference memory errors in female offspring at 1 but not 5 mg/kg-d, and considered these findings to be “sex-selective effects on behavioral development.” CitationLevin et al. (2002) speculated that, at a higher dose level of 5 mg/kg, “cholinergic actions may serve to offset adverse, non-cholinergic effects on brain development.”
Another possibility not discussed by CitationLevin et al. (2002) is that the concurrent control females could have an unusually low number of errors based on comparison with two other studies from the same laboratory (CitationAldridge et al. 2005a; CitationLevin et al. 2001). In these two other studies, CitationLevin et al. (2001) and CitationAldridge et al. (2005a) reported that control females normally have higher errors than males, and that in these studies CPF disrupted the normal sex differences in activity so that males and females had the same activity level. In fact, the higher number of female errors compared to male errors at the 1 mg/kg-d dose level in CitationLevin et al. (2002) is consistent with the sex differences in control animals observed in these two other studies from the same laboratory (CitationLevin et al. 2001; CitationAldridge et al. 2005a). Thus, the non-monotonic rise in error in the 1-mg/kg-d GD 17–20 females in CitationLevin et al. (2002) may be more a reflection of variability in control female behavior than an effect of CPF.
CitationBillauer-Haimovitch et al. (2009) used a Morris Water Maze to study effects of sc injection of CPF to mice during GD 9–18 (). Statistical significant increases in latency were measured at 1 and 3 mg/kg-d but not at 5 or 10 mg/kg-d. CitationBilliauer-Haimovitch et al. (2009) indicated that there was an overall significant CPF effect, based on ANOVA, which was “individually significant at 1 and 3 mg/kg-d.” However, the results of the Tukey tests for post hoc analysis mentioned in the methods section were not reported. Based on inspection of the figure, there was overlap of the learning curves for the controls, 1, 5, and 10 mg/kg-d for the first 3 d, with the largest difference between the controls and the 1-mg/kg-d group on the fourth and last day estimated to be 3–4 s. Thus, this study does not provide evidence of a treatment-related effect of CPF.
CitationHaviland et al. (2010) tested mice on an 8-arm (6 baited, 2 unbaited) RAM and a “novel” foraging food maze test (modification of RAM) following sc injection of CPF at 1 or 5 mg/kg-d in DMSO from GD 17 to 20 (). CPF did not affect learning on the traditional RAM, but these data may be unreliable because the controls did not learn on this maze. Based on the results of the “novel” maze test, CPF diminished foraging food recognition learning in females (dose-dependent effects at 1 and 5 mg/kg/d) and enhanced foraging learning in males (1 but not 5 mg/kg-d) (). CitationHaviland et al. (2010) provided inadequate information on methods (). The statistical analysis of the behavior was described as a “mixed procedure accounting for autocorrelation of repeated measures.” If this refers to the repeated-measures ANOVA as recommended by CitationHolson et al. (2008), the statistical significances for the main effect and interaction were not provided. Given the limitations of this study and the novelty of the test procedure, this study provides preliminary evidence of effect of in utero exposure on learning in female but not male offspring.
In summary, there are mixed results on learning and memory tests following CPF exposures during gestation. The differences could be due to differences in species, route of exposure, vehicle, age of exposure, time of testing, and the behavioral test. However, based on detailed evaluation of the methods and results, examination of dose-response relationships, and consideration of historical control data, the overall weight of evidence does not support a consistent pattern of effects on learning and memory across studies at 1 mg/kg-d.
Postnatal exposures
CitationAldridge et al. (2005a) and CitationLevin et al. (2001) reported that performance on the RAM in females improved, and males exerted either no effect or a deficit in performance following sc injection of 1 mg/kg CPF in DMSO to pups from PND 1–4 (). Thus, a decrease in female error rate (improved performance) was replicated in two separate studies by the same laboratory and was considered to be evidence that CPF disrupts sexually dimorphic behaviors by improving female performance to be comparable to that of the males (CitationAldridge et al. 2005a; CitationLevin et al. 2001). No marked effects were reported following sc injection of 5 mg/kg CPF in DMSO from PND 11 to 14 (CitationLevin et al. 2001).
CitationJohnson et al. (2009) dosed CPF daily with incrementally increasing oral doses from PND1 to 21 (). The lowest dose level was 1 mg/kg from PND1 to 21, the mid-dose level incrementally elevated doses from 1 to 2 to 4 mg/kg, and the highest dose level incrementally rising doses from 1.5 to 3 to 6 mg/kg. Beginning on PND 36, working and reference memory were tested using a radial maze. In females, decreased errors (improved performance) were observed in reference memory at the mid- and high-dose levels, but there were no marked effects on working memory. In males, there were increased overall errors in working and reference memory at the highest dose. CitationJohnson et al. (2009) reported statistically significant increases in working memory errors during the fourth week but not in the first 3 wk of testing at the low and mid doses. These effects do not appear to be adverse effects because of the small differences in number of errors (3.2 [low] to 3.3 [mid] errors vs. 2.7 [control] errors), and the lack of a consistent dose-response pattern over the 4-wk period. Statistically significant hippocampal AChE inhibition was measured in all treatment groups at PND 20. AChE inhibition persisted in the mid- and high-dose groups for up to 19 d following exposure.
An increase in escape latency (deficit in performance) was measured in rats (males and females combined) tested from PND 24 to 28 by CitationJett et al. (2001) on the Morris Water Maze test following sc doses of 0.3 (d 1 only) and 7 mg/kg in peanut oil on PND 7, 11, and 15 (). Although not meeting our inclusion criteria for studies because animals were dosed after weaning, a significant increase in latency was also measured from PND 24 to 28 following sc doses of 0.3 and 7 mg/kg on PND 22 and 26. This study was limited because pups from only two to four reconstituted litters/dose level were used, with all pups from the same litter dosed with the same dose level. Finally, only 60% of control animals were able to meet the criterion on this test. It is notable that AChE levels were measured 3 h after injections, but no brain AChE inhibition was measured at 7 mg/kg, a dose level that should have resulted in significant AChE inhibition. Taking all these factors into consideration, this study is not sufficiently reliable for risk assessment purposes.
CitationRicceri et al. (2003) tested effects of postnatal sc injections of CPF in DMSO to mice during PND 1–4 or PND 11–14 (), and found no significant effects on passive avoidance at 1 or 3 mg/kg-d (data were not shown).
In summary, a decrease in errors in females (improvement in learning and memory) and a tendency toward increased errors in males were noted in two postnatal exposure studies following sc injection of 1 mg/kg-d CPF (only dose tested) (CitationAldridge et al. 2005a; CitationLevin et al. 2001). CitationJohnson et al. (2009) measured similar effects at higher oral dose levels (incremental oral doses spanning from 1 to 4 or 1 to 6 mg/kg-d) inducing 50% AChE inhibition in the hippocampus, but did not observe effects at 1 mg/kg.
Pharmacologic Challenge
Pharmaco-logic challenge studies () were conducted to determine whether CPF changes the involvement of underlying neurotransmitter systems that support different behaviors. Many of the studies were conducted in rats injected sc with CPF (). CPF attenuated the effects of scopolamine (muscarinic antagonist) to elevate the number of errors on the RAM following gestational (CitationIcenogle et al. 2004; CitationLevin et al. 2002) or PND 11–14 exposures (CitationLevin et al. 2001), but not PND 1–4 exposures (; CitationLevin et al. 2001). In comparing the effects of scopolamine across the different studies, it appears that the variability in the data is high, and scopolamine did not consistently produce dose-response increase in number of errors in controls (CitationLevin et al. 2001, Fig 4; 2002, Figures 5, 6, and 7). Ketanserin elevated the number of errors on the RAM with increasing doses of ketanserin in animals treated with 1 mg/kg-d of CPF from PND 1–4 (CitationAldridge et al. 2005a). These same doses of ketanserin exerted no marked effect on saline-treated animals, similar to that observed by Levin et al. (2005). Ketanserin is a potent 5HT2 receptor antagonist, but also has effects on other 5HT, α-adrenergic, and histamine receptors (Levin et al. 2005). These data support hypotheses requiring further testing that CPF produces alterations in cholinergic, serotonergic, and/or other neuropharmacologic systems that are unmasked by pharmacologic challenge.
One oral study was conducted by CitationVenerosi et al. (2010) in mice gavaged with a single dose level (6 mg/kg-d) of CPF during gestation (). Attenuation of the effect of a single 40-mg/kg ip dose of fluvoxamine (5HT uptake inhibitor) to increase swimming duration or frequency (i.e., number of swimming intervals) on a forced swim test was measured following 6-mg/kg CPF exposure during gestation. Enhanced swimming duration or frequency in controls was interpreted as less “depressive-like” behavior in controls. Fluvoxamine also reduced duration of maternal aggression while in the nest. The baseline behavior (i.e., response following acute vehicle for fluvoxamine) of CPF and control animals was different for several forced swimming and maternal aggression parameters, making it more challenging to interpret the fluvoxamine effects. CitationVenerosi et al. (2010) speculated that the acute effects of fluvoxamine are due to either “changes in constitutive 5HT levels or overactivation/ hypoactivation of specific 5-HT receptor families,” yet no neuropharmacologic evaluations were conducted.
In summary, these data form the basis for developing hypotheses for further testing. Further testing needs to include concurrent measurements of neuropharmacologic endpoints based on clearly defined hypothesis. The pharmacologic challenge data need to be replicated using multiple doses of CPF, using an oral route of exposure, and avoiding DMSO as the vehicle.
Social and Agonistic Behaviors
Four studies measured multiple parameters for many social agonistic or maternal behaviors in adolescent or adult mice exposed during gestation and/or prior to weaning (CitationRicceri et al. 2003,2006; CitationVenerosi et al. 2006, 2008).
CitationVenerosi et al. (2006) measured USV and frequency and duration of different social recognition behaviors in female offspring. This study exposed pups prenatally (GD 15–18; 0, 3, or 6 mg/kg) and postnatally (PND 11–14, 0, 1, or 3 mg/kg), resulting in nine different groups, each dosed with one combination of gestational and postnatal oral doses. Of 9 possible dose combinations, the only statistically significant effect was (1) an increase in USV by female adult mouse during social interaction with another female mouse and (2) a rise in the frequency of social investigation in mice exposed gestationally to 6 mg/kg and postnatally to vehicle. The lack of effects in mice exposed gestationally to 6 mg/kg and postnatally to 1 or 3 mg/kg indicates a lack of dose-response relationship. The biological function of the USV emission by the female mouse during social interaction with another female is “poorly understood” (CitationVenerosi et al. 2006). Thus, further research and replication of findings are needed before these data may be considered relevant for human health risk assessment.
A similar study by CitationRicceri et al. (2006) also involved nine combinations of the same doses, except that postnatal sc (not oral) injections were used. CitationRicceri et al. (2006) found that 1 and 3 mg/kg (PND 11–14) CPF enhanced maternal-like behaviors in “virgin females” exposed to pups from different mothers. This is an unreliable effect, given the artificial conditions of the test and the fact that there were no effects on latency, frequency, and duration of nest building and number of pups retrieved to the nest. The relevance of these data to human health risk assessment is questionable.
CitationVenerosi et al. (2008) exposed mice from PND 11 to 14 to a single sc dose of 3 mg/kg-d in peanut oil. Measures of sociability, nest-building activity, and maternal behavior were analyzed in dams. The main finding was that females showed alterations in different aspects of the maternal behavior repertoire following artificial conditions of repeated daily removal of nest and pups. These were alterations on individual parameters, and a large number of key maternal behaviors were unaffected (CitationVenerosi et al. 2008). For example, latency to first licking episode on postpartum day 1 and latency to start nest building was lower at 3 mg/kg. However, there was no significant effect on quantitative or qualitative nest features, or on licking durations and frequencies. Decreased combined time crouching and nursing (an apparent a posteriori analysis) was measured, but no marked differences were found in the frequencies, latencies, or durations of the other pup-directed behaviors such as retrieving, sniffing, and nest building. There were also no significant effects on social behavior and social preferences at adolescence. Taken together, there appears to be a low level of concern for the patterns of alterations measured in mice dosed postnatally with 3 mg/kg-d.
CitationRicceri et al. (2003) studied the effects of postnatal injections of 1 and 3 mg/kg-d CPF sc in DMSO. Both doses were used for PND 1–4 and PND 11–14 exposure periods. For PND 1–4 exposures, frequency of aggressive grooming increased at 3 but not 1 mg/kg-day, although the main effect of treatment was not statistically significant. For PND 11–14, male agonistic responses appeared significantly enhanced throughout the test period after both doses of CPF. The agonistic responses included aggressive, defensive, and submissive behaviors. Therefore, conclusions regarding aggressive behaviors specifically require further analyses of data that were not presented. However, these results are consistent with enhanced agonistic behaviors reported by CitationRicceri et al. (2006) at 3 mg/kg-d CPF sc in peanut oil, as described next.
In the CitationRicceri et al. (2006) study, the frequency and duration of six male agonistic behavior parameters were analyzed following GD 15–18 gavage and/or PND 11–14 sc injections of CPF in peanut oil. Following prenatal injections, a significant increase in offensive posture frequency at 6 but not 3 mg/kg/d was found. For postnatal injections, a significant rise in frequency and duration of attack duration at 3 but not 1 mg/kg-d. However, this significant elevation in frequency was based on post-hoc comparisons conducted in the absence of significant main effect on the ANOVA. The duration and incidence of these findings at 3 mg/kg-d was, respectively, 6 versus 2.5 (treated vs. control) s of attack response per 5 min and 3.5 versus 2 attacks per 5 min. It is noteworthy that no effects on aggressive or agonistic behaviors were reported at 1 mg/kg-d when peanut oil was used instead of DMSO as the vehicle (CitationRicceri et al. 2006 compare with CitationRicceri et al. 2003).
In summary, multiple social behavior parameters were measured, and the frequency and duration for each of these parameters were statistically analyzed. The guidance from the U.S. EPA Neurotoxicity Risk Assessment Guidelines to risk assessors on similar type of observational data involving a large number of endpoints (i.e., functional observational battery) applies when considering these data for risk assessment purposes (CitationU.S. EPA 1998b). The relevance of statistically significant test results needs to be evaluated according to “the number of signs affected, the dose(s) at which effects are observed, and the nature, severity and persistence of the effects and their incidence in relation to control animals” (CitationU.S. EPA 1998b).
Taking into account the overall patterns of perturbations of behavioral parameters related to social behaviors, the magnitude of change or low incidence of the behaviors, and the number of multiple comparisons statistically analyzed, the biological significance of these alterations in social behaviors to humans is uncertain. Based on the data thus far, 1 mg/kg-day appears to be a no-observed-adverse-effect level (NOAEL) for perturbations in mouse social, agonistic and maternal behaviors.
Ricceri et al. (2006) measured brain and serum cholinesterase (ChE) activity in offspring at PND 15 after oral exposure from GD 15–18 and/or sc injections from PND 11–14. The mice were sacrificed 24 hr after postnatal exposures, and more than 15 d after last CPF injection for mice receiving only prenatal CPF exposures. There was a 20–50% decrease in serum BuChE activity in all groups dosed postnatally with either 1 or 3 mg/kg-d. As expected, there was no brain or serum ChE inhibition in PND 15 pups dosed prenatally with vehicle, 3 or 6 mg/kg-d, and postnatally with vehicle. CitationRicceri et al. (2003) reported significant pup brain AChE inhibition 1 hr following PND 1–4 (but not PND 11–14) sc doses of 1 and 3 mg/kg-d. In contrast to the large number of rat studies conducted by multiple laboratories at more optimal sacrifice times (2–6 hr) after exposure, RBC and brain AChE inhibition has not been adequately characterized in mice following CPF exposures. From a risk assessment standpoint, 1 mg/kg-d is an effect level for RBC or brain AChE inhibition or plasma BuChE following postnatal exposures to mice or rats. Thus, behavioral perturbations reported in mice exposed to 1–6 mg/kg-d occurred at dose levels considered to be effect levels for CPF based on serum, RBC or brain ChE inhibition.
Neuropharmacologic Endpoints
Many studies evaluated the effects of gestational and/or postnatal exposures of CPF on the cholinergic and other neurotransmitter systems. Although the emphasis of this review is on neurobehavioral effects, understanding potential MOA is important in determining whether the most sensitive endpoints are being used for risk assessment and in understanding the weight of evidence relevant to neurobehavioral outcomes.
Cholinergic Effects
The cholinergic system plays an important role in brain development beginning during the embryonic period and continuing into the postnatal period (CitationBrimijoin and Koenigsberger 1999; CitationLauder and Schambra 1999). Therefore, inhibition of AChE during development by CPF exposure may have the potential to produce alterations in neuronal development. Alterations in cholinergic endpoints such as choline acetyl transferase (ChAT), mAChR, high-affinity presynaptic choline transporter (HAChT), vesicular Ach transporter (VAChT), and choline transporters were found (CitationChakraborti et al. 1993; Chambers 2003; 2004; 2005; CitationQiao et al. 2002; 2003; Citation2004; Richardson and CitationSlotkin et al. 2001; CitationTang et al. 1999; CitationZhang et al. 2002). Although alterations in some of these cholinergic parameters were more persistent than AChE inhibition, the significance of these neuropharmacologic alterations (some less than 20% at lower doses) in terms of causal relationship with functional effects has not been established (CitationCarr et al. 2001; CitationGuo-Ross et al. 2007; CitationRichardson and Chambers 2005). Nevertheless, decreases in mAChR binding (as measured by maximal binding sites Bmax) and HAChT (as measured by 3H-hemicholonium-3 binding) were considered to be among the more sensitive mechanistic endpoints that may potentially result in alterations in neurologic functions.
CitationRichardson and Chambers (2003; Citation2004; Citation2005) measured effects of CPF on cholinergic neurochemistry, including brain AChE inhibition, mAChR, HAChT, VAChT, and ChAT. These studies indicated that gestational exposure to 7 but not 3 or 1 mg/kg-d CPF results in long-term (e.g., 1 mo) alterations of presynaptic cholinergic endpoints at a time when brain AChE inhibition had returned to control levels. CitationRichardson and Chambers (2005) dosed pups from PND 1 to 21 by gavage (corn oil) with a low dose, 1.5 mg/kg-d, and high dose, 1.5 mg/kg, increasing to 3 and then 6 mg/kg. Both dosage levels resulted in persistent inhibition of AChE and alterations in mAChR density and reductions in presynaptic cholinergic endpoints.
One dermal study by CitationAbdel-Rahman et al., (2004) found no marked effects on ligand-binding densities of nicotinic and mAChRs in cortex at PND 60 in offspring of dams exposed dermally to 0.1 mg/kg-d CPF in ethanol. Increases in AChE activity (not inhibition) were found in midbrain regions in males and brainstem regions in females, while other brain regions in both sexes showed no significant effect. AChE activity was not measured during or soon after the period of exposure.
CitationSlotkin et al. (2001) and CitationQiao et al. (2002; Citation2003; Citation2004) measured ability of CPF to alter cholinergic endpoints such as ChAT, HAChT and mAChR following exposures to dams during GD 9–12 or GD 17–20, or directly to pups on PND 1–4 or PND 11–14. A large number of measures were analyzed in males and females at different ages and multiple brain areas. There were some alterations (mostly decreases) in HAChT binding, ChAT activity, and mAChR binding at doses of 1 or 5 mg/kg-d following gestational and/or postnatal exposures. CitationSong et al. (1997) and CitationQiao et al. (2002) measured rat pup brain AChE inhibition 24 h after the last sc injections of CPF in DMSO. Brain AChE inhibition was measured in pups from dams exposed for GD 17–20 to 5 mg/kg-d (>40%), 2 mg/kg-d (around 20%), but not 1 mg/kg-d (CitationQiao et al. 2002). Brain AChE inhibition (25%) was measured in pups directly injected PND 1–4 with 1 mg/kg-d CPF (CitationSong et al. 1997). Even greater AChE inhibition (75%) was found in pups sacrificed 2 h after the first sc injection of 1 mg/kg CPF on PND 1 (CitationDam et al. 2000).
In general, brain AChE inhibition from oral or sc in dams during gestation or pups dosed postnatally is an equally or more sensitive endpoint compared to other cholinergic alterations measured in rat and mouse pups. At present, the relationship between CPF effects on cholinergic measures and behavioral effects is not clearly understood.
Noncholinergic Neuropharmacologic Effects
Several studies reported that CPF produced long-lasting alterations of serotonergic, dopaminergic, and possibly noradrenergic systems after gestational or postnatal sc injections of 1 or 5 mg/kg-d (CitationAldridge et al. 2003; Citation2004; Citation2005b; Citation2005c; CitationDam et al. 1999; CitationRaines et al. 2001; CitationSlotkin et al. 2002; CitationSlotkin and Seidler 2005; Citation2007a; Citation2007b [referred to as Slotkin et al. lab]). These studies measured a number of serotonergic endpoints, including 5HT1a and 5HT2 receptors, as well as 5HT transporter (5HTT) binding in multiple brain regions at different ages. Noradrenergic and dopaminergic endpoints were also measured, but in fewer studies. Alterations occurred in different directions and magnitudes and include increasing, decreasing or flat dose-response relationships depending on the specific parameter, age of exposure, age of sacrifice, dose, and brain regions (Slotkin et al. lab).
In general, these studies used 1 pup/sex/litter from 6 litters, 2 dose levels (1 and 5 mg/kg in DMSO, sc) for the gestational exposures (CitationAldridge et al. 2004; 2003; CitationSlotkin and Seidler 2007b 2005b), and only 1 dose level postnatally (1 mg/kg for PND 1–4, PND 2–5, and 5 mg/kg for PND 11–14; CitationDam et al. 1999; CitationRaines et al. 2001; Seidler 2005; 2007a; Slotkin and CitationAldridge et al. 2005b; Citation2005c; CitationSlotkin et al. 2002). Limitations regarding the sc route of exposure, DMSO as the vehicle, and lack of dose-response data need to be considered in evaluation of results for risk assessment purposes. In addition, caution is needed in interpreting the biological significance of these noncholinergic data due to methodological issues described next.
Biological significance of statistical significant pooled data require further evaluation of specific measures
A major challenge in evaluating consistency of results within and across studies is that the statistical analyses were based on data pooled together across multiple brain regions, exposure durations, ages, and neuropharmacologic endpoints (Slotkin et al. lab). Lower order analyses (e.g., group comparisons) were conducted depending on which interactions were statistically significant. For example, CitationAldridge et al. (2004) concluded that exposure to a low dose of CPF (GD 9–12) “elicited a significant overall elevation of 5HT1a, 5HT2 and 5HTT ligand binding without statistical distinction by region, measure or sex.” In other words the statistical significance was based on statistical analysis that pooled together data for multiple 5HT receptor ligands and brain regions and both sexes. However, inspection of the graphed data indicates that many of the changes in individual measures in this and other studies (Slotkin et al. lab) for each sex, period of exposure, neuropharmacologic measure, and/or brain region may be of questionable biological significance. Some of these individual effects that were statistically significant in the pooled analyses are <10–20% alterations from control and/or have large standard errors within 10% of the x axis representing 0 % change from control.
Biological significance of percent changes require evaluation of control values
Most of the neuropharmacologic effects are reported as percent changes. The statistical analysis was based appropriately on the actual data, not on percent differences from control (Slotkin et al. lab). Control data were reported in some but not all papers, or in nonstandard units of measure (sometimes not corrected for protein level or wet weight of the brain region), making it more difficult to assess the reliability of measures.
If control levels are too small to measure reliably, this may result in apparent large percent differences from control. For example, CitationAldridge et al., (2005b) reported a statistically significant 40–50% decrease in dopamine levels in the hippocampus at PND 60 at 1 and 5 mg/kg-d, with no changes in the striatum, cortex, midbrain, or brainstem following gestational exposure to CPF. However, the small dopamine (DA) values for hippocampus in controls raise doubts about the biological significance of the 40–50% changes from control. Indeed, in later publications, CitationSlotkin et al. (2002) state that DA levels were not measured in the hippocampus, because they were considered too low to be reliable. Similarly, in control animals, CitationDam et al. (1999) and CitationAldridge et al. (2003) demonstrated that levels of several cholinergic, noradrenergic, and DA endpoints were low from GD 17 to PND 5 and then rapidly rose (depending on brain area) from PND 5 to PND 20. Thus, evaluation of biological significance of percent changes from control measured during these early periods should take into consideration low control values and normal control variation (e.g., historical control values) during this period of rapid change.
Receptor binding assays use subsaturating ligand concentrations
The serotonergic binding assays and the majority of those for cholinergic binding for CPF were made with single ligand concentrations that are at or slightly above the Kd value and below full saturation (CitationAldridge et al. 2005c). This is in contrast to approaches used by CitationRichardson and Chambers (2004) for cholinergic endpoints, in which a single saturating concentration was used based on preliminary kinetic studies conducted in the laboratory under identical experimental conditions, or by CitationTang et al. (2003), in which different concentrations of ligands were used to calculate Kd and Bmax.
Although this method of using subsaturating concentrations might be appropriate as a screen in preliminary-hypothesis-generating experiments, it is important to rigorously confirm that these alterations are biologically meaningful by conducting a full kinetic examination or employing a sufficient single concentration (i.e., 10 × Kd) that will allow for an accurate estimation of Bmax. Thus, further confirmation is needed to ensure that reported differences from control (<10–25%) are not artifacts of the screening methods used.
Data are inadequate to establish MOA, and risk assessments based on RBC AChE inhibition are protective of non-AChE inhibition effects
At present, data support hypotheses requiring further testing that there may be alternative MOA for developmental neurotoxic effects other than brain AChE inhibition. For the reasons outlined earlier, data are insufficient to consider serotonergic or other neuropharmacologic as a MOA for behavioral or other developmental outcomes. In addition, many of the neuropharmacologic changes studies did not measure AChE inhibition concurrently in the experiments. However, CitationSong et al. (1997) and CitationQiao et al. (2002) reported significant offspring brain AChE inhibition following postnatal exposures to 1 mg/kg CPF (). CitationMaurissen et al. (2000) and CitationMattsson et al. (2000) also found significant inhibition of brain and RBC AChE inhibition and of plasma BuChE in dams following oral exposure to 0.3 or 1 mg/kg CPF from GD 6 to 20. Therefore, all effects reported at 1 mg/kg following gestational or postnatal sc exposures occur at dose levels that would be considered in risk assessments to produce brain or RBC AChE or plasma BuChE inhibition in pregnant dams or offspring. Thus, it cannot be ruled out that these “non-AChE inhibition” effects were a result of AChE inhibition that occurred daily during the period of dosing.
TABLE 11. CPF effects on brain or RBC AChE inhibition in rats
This review focuses on neuro- pharmacologic (e.g., serotonergic, noradrenergic, dopaminergic) findings. Other mechanistic in vitro and in vivo studies were comprehensively reviewed by CitationEaton et al. (2008). CitationEaton et al. (2008) conducted an expert panel review of the in vitro and in vivo mechanistic studies and concluded that “the weight of evidence from animal studies and in vitro mechanistic studies suggests that many of the neurodevelopmental effects of CPF are secondary to inhibition of AChE in target tissues such as the developing brain, although plausible alternative mechanisms have been proposed, based on in vitro studies” (102). The CitationU.S. EPA (2011) recent preliminary human health risk assessment of CPF stated that “although multiple mechanisms have been proposed, a coherent mode of action with supportable key events, particularly with regard to dose-response and temporal concordance, has not yet been elucidated.”
From a risk assessment perspective, data thus far indicate that these “non-AChE inhibition” effects occur at doses that produced RBC or brain AChE inhibition and/or plasma BuChE inhibition in adults, dams, or offspring. Although there is insufficient evidence to determine whether neuropharmacologic effects represent different MOA, risk assessments based on AChE inhibition are likely to be protective of these potentially different cholinergic or noncholinergic MOA.
Neuropathologic and Morphometric Measurements
Although neuropathologic and morphometric measurements are not a focus of this review, the main findings are briefly discussed because they may provide additional weight of evidence. There were one oral gestation/lactation study (CitationMaurissen et al., 2000), three dermal gestational studies (CitationAbdel-Rahman et al., 2003; Citation2004; CitationAbou-Donia et al. 2006); and two sc (DMSO) postnatal studies (CitationRoy et al., 2004; Citation2005) evaluating effects of CPF on histopathology or morphometric measures in the brain. With the exception of the oral study by CitationMaurissen et al. (2000), all other studies tested only one dose level.
Briefly, alterations in morphometric measurements and other quantitative measures (e.g., length, thickness, neuron and glial cell counts) were found at 5 mg/kg-d following either oral GD 6–PND 10 (CitationMaurissen et al. 2000) or sc PND 11–14 exposures (CitationRoy et al. 2004 Citation2005). These effects occurred at doses that are known to produce significant brain or RBC AChE and plasma BuChE inhibition in the dams during gestation and >65% brain AChE inhibition in pups following direct dosing to pups (). In addition, CitationMaurissen et al. (2000) reported that 5 mg/kg-d produced maternal toxicity such as decreased body-weight gain and clinical signs of AChE inhibition such as tremors. In the pups, this was accompanied by an increase in mortality between PND 1 and 4 (1% in controls vs. 25% at 5 mg/kg-d), and decreases in body and brain weight.
Since this article is focused on neurobehavioral endpoints, it is of interest that CitationRoy et al. (2005) reported a number of “subtle morphologic changes in the hippocampus” including number of neurons and glia, and neuronal cell diameter. The hippocampus is related to learning and memory function. These morphologic changes included decreased layer thickness and neuron and glial cell count in the hippocampus following PND 11–14 sc injections of 5 mg/kg-d CPF. CitationLevin et al. (2001) also dosed rats from PND 11–14 with sc injections of 5 mg/kg-d CPF, and found no effect on RAM working or reference memory, but did not report any histopathology examination of the hippocampus. Similarly, CitationMaurissen et al. (2000) observed no effects on a delayed spatial alternation test of learning and memory following GD 6–PND 10 maternal doses of 0.3 to 5 mg/kg-d CPF, and no morphometric effects on the hippocampus. The differences in morphometric measurements may be related to differences in the types of brain measurements made and the route and period of exposures. At present, neuropathology data cannot be directly linked to any behavioral effects.
Decreases (approximately 12%) in number of surviving Purkinje cells and increases (approximately 8%) in glial fibrillary acidic protein (GFAP) immunostaining in cerebellum were also noted in offspring at 1 mg/kg-d (CitationAbou-Donia et al. 2006) but not 0.1 mg/kg-d dermal exposures during gestation by the same laboratory (CitationAbdel-Rahman et al. 2003; Citation2004). The GFAP measures were based on methods of approximating areas with targeted levels of pixels from digitized images of GFAP immunostained sections. In contrast, CitationGarcia et al. (2002) found no effects on GFAP levels measured from brain homogenates following doses of 1–10 mg/kg from GD 17 to 20. These conflicting results may also be due to differences in duration and route of exposure, age of sacrifice, and method of measuring GFAP levels. The functional relevance of the findings from CitationAbdel-Rahman et al. (2003; Citation2004) remains unknown.
EVALUATION OF EPIDEMIOLOGY AND ANIMAL NEUROBEHAVIORAL DATA FOR RISK ASSESSMENT
The purpose of this review was to provide in-depth analyses of CPF exposure and developmental neurobehavioral outcomes from the published human and animal in vivo peer-reviewed papers within the context of human health risk assessment. Previous approaches provided a summary of significant findings appropriate for comparisons of dose levels at which potential adverse effects were reported. In contrast, this review concentrated on providing a critical analysis of the neurobehavioral studies for risk assessment purposes. An important aspect of the approach used in reviewing the data is that methods and results (absence and presence of findings) were systematically tabulated by dose level and period of exposure. In addition, historical control data was considered as an additional perspective.
In the animal studies, the lowest dose level at which alterations in developmental neurobehavioral, neuropharmacologic, and neuropathologic/morphometric endpoints have been reported more frequently is 1 mg/kg-d (1000 μg/kg-d). The weight of evidence for neurobehavioral effects at 1 mg/kg-d is not compelling when taking into consideration dose response, study methodology, pattern of effects, and consideration of both the absence and presence of findings from different studies. There were a greater number of findings reported following exposure to 3–6 mg/kg-d. Significant RBC or brain AChE inhibition either in dams or in pups was produced at and below these dose levels. Therefore, risk assessments based on AChE inhibition are likely to be protective of these behavioral effects. As discussed in the introduction of this review, exposures to the general population today are primarily dietary in the 10−2 to 10−3 μg/kg-d dose level, which is more than 5 orders of magnitude below the animal dose levels. Estimates for daily exposure to pregnant women and children following previously allowed residential uses are in the 10−1 to 10−3 μg/kg-d dose range (CitationLowe et al. 2009; CitationEaton et al. 2008). Estimates for agricultural worker exposures are in the 101 μg/kg-d dose range.
Epidemiology studies are also considered for use in risk assessment. However, the dose-response data for CPF are not reliable for human health risk assessment, largely because of the paucity of exposure data available from each study. Further, in the studies with multiple sources of data (e.g., biomarker data and air monitoring samples), correlations among these measures were weak (r ∼ .2) (CitationWhyatt et al. 2005). Furthermore, while data from the biomarkers of CPF at a single time point provide some indication of short-term exposure, understanding long-term patterns of use could add valuable information regarding exposures within and across critical stages of development in utero and after birth. In addition, although measures of nonspecific metabolites may be more objective quantitative measures of exposure to OP in general as compared to self-reported exposure information, such measures cannot form the basis for causal conclusions about CPF or other specific chemicals. At present, the epidemiology studies are useful to consider for hazard characterization.
Based on the epidemiologic studies reviewed thus far, the evidence for causality is weak. As new studies are published, there are four important principles in evaluating the CPF literature that are important to consider: (1) relative strengths and limitations of the studies, particularly in their ability to provide informative data about associations between CPF exposure and neurobehavioral and other relevant outcomes, taking into account potential bias and with consideration of the evidence for and against causality; (2) differences in exposure settings across the studies (CitationNeedham 2005); (3) measurement and characterization of exposure, keeping in mind that different biomarkers have different degrees of specificity with respect to CPF, and thus, a statistically significant result for DAPs and DMPs in the absence of a similar finding for DEPs is not likely to reflect exposure to CPF; and (4) weight of evidence for biological plausibility needing to be considered, rather than selecting individual findings from human studies and trying to find a “match” with animal data.
The conclusion of this review is that there is insufficient evidence that human neurodevelopmental exposures to CPF result in adverse neurobehavioral effects in infants and children based on studies that estimated CPF exposure using measures derived from maternal or child urine, umbilical cord blood, or personal air monitoring samples. In looking for consistent patterns across studies, and certainly across species, one needs to guard against simply accumulating a list of positive, statistically significant, yet potentially random findings. With this caveat in mind, two examples are discussed.
An increasing number of abnormal reflex in newborn infants was the only one of seven BNBAS domains that was found to be statistically significant in two epidemiologic studies evaluating DEPs and DAPs in maternal urine (CitationEngel et al. 2007; CitationYoung et al. 2005). CitationYoung et al. (2005) and CitationEngel et al. (2007) relate these outcomes to animal results such as significant alterations in cliff avoidance and righting reflex at postnatal days 1 and 3 following sc gestational exposures to 25 mg/kg-d reported by CitationChanda and Pope (1996). The route of exposure and dose level are not relevant to humans. Dose levels of 5 and 25 mg/kg-d produced greater than 60% brain AChE inhibition (CitationChanda and Pope 1996; CitationMattsson et al. 2000; CitationMaurissen et al. 2000) and maternal toxicity (CitationMaurissen et al. 2000, CitationQiao et al. 2002). The overall weight of evidence indicates that CPF does not exert effects on neurodevelopmental landmarks and reflexes in animals tested prior to weaning at doses below 5 mg/kg-d (). Therefore, the conclusion that these studies provide biological plausibility for abnormal reflexes in newborn humans is not appropriate.
As a second example, one of the epidemiology studies reported a statistically significant association (with wide 95% confidence interval) between cord blood CPF and maternal-reported PDD problems at age 36 mo (CitationRauh et al. 2006). Animal models can evaluate only certain aspects of heterogeneous complex neurologic disorders. At present, there are no definitive animal behavioral models for PDD or autism, and it is not appropriate to make direct comparisons between specific behavioral effects of CPF in animals with autism. Furthermore, none of the epidemiologic studies in this review were able to evaluate outcome misclassification by obtaining clinical confirmation information regarding PDD, ADHD, or other behavioral problems or disorders measured by maternal reports on the CBCL.
CONCLUSIONS
This review evaluated the published human epidemiology and animal literature that described potential associations between CPF exposure and developmental neurobehavioral outcomes, emphasizing their role in informing risk assessment. The epidemiologic studies do not support a causal association between CPF exposure to mothers and adverse neurobehavioral outcomes in infants or young children. Only one study evaluated associations between potential child (postnatal) CPF based on urine metabolites and neurobehavioral outcomes and similarly did not provide strong evidence in support of causality. These cohorts are being followed into early and middle childhood, permitting further evaluation.
Taking into consideration both oral and sc animal behavioral studies, data indicate that most of the alterations of neurobehavioral, neuropharmacologic, or morphologic parameters occur at exposure levels that also produce brain or RBC AChE or plasma BuChE inhibition in adults or pups. Based on animal studies reviewed in this article, the no-observed-effect level (NOEL) for RBC AChE inhibition is <0.3 mg/kg-d following gestational exposures to pregnant rats. The U.S. EPA estimated a BMDL10 of 0.03 mg/kg-d for RBC AChE inhibition (CitationU.S. EPA 2011). Therefore, the most sensitive endpoint for CPF is RBC AChE inhibition. Taking into consideration consistency of outcomes across studies, and strength of experimental design and methods for risk assessment purposes, the NOAEL for behavioral effects is 1 mg/kg-d. There is strong evidence from the animal literature that AChE inhibition (RBC or brain from adult or offspring) is a sensitive endpoint that is protective of neurobehavioral, neuropharmacologic, and morphologic alterations that were measured following gestational, lactational, and/or early postnatal exposures to 1 to 6 mg/kg-d.
Acknowledgments
The authors thank Rebecca Edwards for excellent editorial support and Drs. Jim Lamb, Michael Kelsh, Jack Mandel, and Jason Richardson for critical review of the article. The authors are grateful to Dow AgroSciences LLC for funding support. The analyses, conclusions, and opinions expressed in this article are solely those of the authors.
Related Research Data
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