New techniques for studying airway drug pharmacokinetics for asthma therapeutics

KEYWORDS:

• Asthma
• clinical pharmacology
• inhalation
• lung disposition
• airway pharmacodynamics

Clinical pharmacokinetics is instrumental in characterizing the fate of therapeutics. An early understanding of disposition, dose linearity, and accumulation of a new candidate drug ensures the safe testing in larger patient trial settings, and ultimately its use in a variety of therapeutic situations, where, e.g., drug interactions, liver or kidney malfunction, dietary restraints, or weight changes may come into play. Pharmacokinetic data can be considered a master blood biomarker for down-stream quantitative effects on target and adverse effect organs, but the mechanism by which peripheral blood concentrations translates to a therapeutic or adverse effect is generally a ‘black box,’ where the pathology together with factors like protein binding, active transport, receptor interaction, receptor expression, and sensitivity also contributes. Many of these factors are—at best—little understood. Yet, with clinical documentation together with a pharmacokinetic/pharmacodynamic (PK/PD) understanding, the prescribing physician will be aware of what blood levels to expect and can be rest assured that a certain blood level in most cases will result in the aspired effect.

For asthma therapeutics inhaled via the airways and acting locally in the lung, certain complexities are introduced, which does not come into play with pills or drug injections:

1. Lung deposition and retention: patient inhalation technique and the inhaler device (whether a nebulizer, pressurized metered dose inhaler (pMDI), or dry powder inhaler (DPI)) affect the variability in drug dosing; lung retention differs when a drug is inhaled vs. other routes of administration, e.g. through slow dissolution or lung metabolism such as reversible esterification [1].

2. Bioavailability: for drugs not metabolized in the lung, the efficiency of dose delivery via inhalation can generally be determined by the lung availability, but pulmonary metabolism and mucociliary clearance may reduce availability. Also, drug waste in the inhaler mouthpiece or by ingestion should be kept to a minimum for locally acting drugs where adverse systemic effects can be anticipated, and/or for low-potency drugs or inhaled biologics where cost-of-goods (COGs) may be high.

3. Disease state and patient age: while airway constriction and bronchial hyper-reactivity, common features of asthma, often affects the central parts of the lungs, inflammation may be more peripherally distributed. Accordingly, different pathologies may require different inhaler formulation properties. Also, as disease progress and treatment continues, receptor access and expression may change; Pediatric and geriatric asthma present particular challenges associated with inhalation and dose coordination.

A full understanding of an asthma drug’s mode of therapeutic action requires knowledge of PK/PD relationships. For an inhaled and locally acting drug, a major challenge is associated with the fact that local concentrations are so difficult to assess, particularly in a clinical setting. Commonly, a new inhaled drug candidate is first tested in receptor and other biological in vitro assays, followed by intratracheal administration or dosing in inhalation chambers in vivo, the latter in which immobilized animals are dosed with a drug solution, suspension or powder formulation through tidal nose breathing in an aerosol chamber. Quantitative whole body autoradiography (QWBA) is required to study the systemic disposition of the drug candidate but rarely through lung dosing because of the complexity associated with inhaled tracer administration. Lung tissue can be resected and homogenized [2], or, in terms of autoradiography, punch samples from the cryo-preserved slices taken for subsequent chromatography and drug quantification [3]. Once efficacy and tolerability has been shown in selected animal systems, data needs to be generated on ‘drugability’, i.e. whether a stable and affordable formulation can be developed which fulfills market needs and regulatory agency scrutiny, and where a benefit of local, as opposed to systemic, dosing can be claimed. Accurate extrapolation of animal and pharmaceutical data to a clinical patient setting is now critical but often associated with poor predictability. This makes subsequent clinical pharmacokinetic assessment all so much more important.

Currently accepted tools (by regulatory bodies and other stakeholders) for prediction and generation of clinical pharmacokinetics and clinical PK/PD data of an inhaled asthma therapeutic include the following:

1. Pharmacokinetics using the to-be-marketed formulation and inhaler device. Here, lung bioavailability can be quantified using charcoal block techniques or by quantifying uptake of both oral and orally inhaled drug vs. intravenous dosing [4]. Potential buccal absorption and local drug metabolism need to be considered and, if present, corrected for. Also, as only small doses (in the order of a few milligrams at most) can realistically be inhaled, drug potency needs to be high and, as a consequence, local and systemic concentrations will be low making assay sensitivity and accuracy critical.

2. Lung deposition. In vitro data to characterize the aerodynamic properties and particle size distribution by impactor experiments will set the scene for lung deposition predictions. The recently developed and validated in vitro techniques, using human throat models and replay of patient inhalation profiles [5] together with appropriate lung modeling [6], have shown excellent correlation with in vivo data and have probably brought the in vitro lung deposition predictions as far as it can get. Radioactive imaging has been utilized for decades to look at total and regional deposition in the airways [7], but suffer from various shortcomings such as the need to manipulate the formulation with an adherent tracer (generally Technetium-99m) rather than using the intact drug molecule. Positron emission tomography was used with some success for inhaled F18-triamcinolone acetonide [8], but examples with covalently bound tracers are few and not yet proven to be worth the extra effort vs. Tc-labeled formulations.

3. Pharmacodynamic data. Ultimately, the above PK tools will need to be put in context of PD data to generate PK/PD relationships and provide evidence that the inhaled medication is effective and has therapeutic advantage vs. a pill or injection. A primary reason to select the inhaled over any other route of administration is commonly to improve the local to systemic effect ratio of a locally acting drug, often a bronchodilator or a corticosteroid. The PK/PD relationships of inhaled bronchodilators are well documented and understood because of the relative ease of measurement of relevant outcome parameters—bronchodilataton or bronchoprotection. However, there are still significant areas remaining to be explored, e.g. β-receptor tolerance development [9]. For inhaled anti-inflammatories, local PK/PD relationships have been more difficult to tease out as there is still lack of sensitive and selective biomarkers of local inflammation in the lung, and as therapeutic benefit is slow to develop. Inflammatory biomarkers in sputum and/or lung biopsies are often used to assess inflammation and drug effects, but are cumbersome for patients and investigators and require significant sampling skill and sample handling to generate conclusive data.

4. Generic drug development. With many inhaled asthma drugs getting out of patent, pharmaceutical companies are increasingly chasing a share of the generic market. However, inhaled generics are much more complex to develop than pills, and the margins and revenues thus less obvious. Detailed guidelines are available in EU [10] and US [11] but approaches are different: EU requires a stepwise bioequivalence (BE) approach, where a large battery of in vitro BE requirements (including, e.g., airflow resistance, delivered dose, and particle size distribution data) need to be fulfilled. If BE cannot be declared based on the stipulated in vitro criteria, human bioavalability studies using oral charcoal (to eliminate gut uptake) need to show BE. If these does not fulfill BE criteria, carefully designed human pharmacodynamic studies to evaluate efficacy and safety are required. In US, a less structured ‘weight of evidence’ approach is utilized, where the sponsor needs to show sufficient evidence for BE in efficacy and safety to be allowed to market a generic formulation. This includes both in vitro and in vivo tests. The BE strategy approach is generally more flexible, but needs to be justified and agreed with the agency upfront. Approval of an inhaled generic in US has historically been more challenging than in EU, but can on the other hand involve formulations and devices which may differ technically from those of the reference products.

1. Future prospects

1.1. Tissue disposition

Techniques are now available to complement QWBA, microautoradiography (MARG) and fluorescence resonance energy transfer (FRET) assessment of tracer-labeled new drug candidates. With mass spectrometric imaging (MSI) using matrix-assisted laser desorption/ionization (MALDI), localization of non-manipulated molecular entities and endogenous markers can be studied concomitantly on histologically defined tissue samples. Spatial visualization can be made of drug molecules and metabolites as well as endogenous proteins, peptides and lipids at good resolution. Resolution can be further improved by secondary ion mass spectrometric imaging (SIMS-MSI), but images are smaller and analysis locally destructive. Also, molecular fragmentation is generally greater and by that the analysis of larger biomolecules made more difficult. However, although cumbersome, temporal information from in vivo experiments can be collected and, by this, drug-tissue interactions be described in more detail [12]. Several successful attempts have been made in the lung, where the tissue localization of an inhaled anticholinergic was determined in a rodent inhalation model [13,14], as well as in a clinical study in COPD patients inhaling therapeutic doses [12,15]. The above mentioned in situ techniques are explained in an excellent review paper by Solon et al. [16].

1.2. Personalized medicine approaches

In order to minimize patient risk and health-care cost, authorities and health-care providers are putting increased weight on personalized health care (PHC) and companion diagnostics approaches. In airway disease in general, and COPD in particular—where the underlying pathology is less known than in asthma—there is a large unmet need in terms of relevant biomarkers for the diagnosis, outcome, and severity classification, as a guide to adjust and optimize treatment [17]. For example, a majority of exacerbations in asthma and COPD are triggered by infectious insults [18], and is often treated with antibiotics in conjunction with increased doses of bronchodilators and anti-inflammatories. However, as the triggers in most cases are virus infection, and as many asthma patients are immunocompromized, a more appropriate treatment would have been an anti-viral or immunostimulating agent like inhaled interferon beta [19]. Accurate early diagnosis will increase chances of informed and appropriate medical decisions resulting in optimal treatment outcome in the individual patient.

While there are modern technologies to develop sensitive and specific biomarker assays and multiplex fingerprint arrays, progress is slow: The pace by which US FDA approves proteome biomarkers has hitherto been appallingly low (1.5 biomarker per year for the last 15 years) [20]—but which hopefully will increase in the years to come. It also appears that the testing and validation of assays and proteomic fingerprints, rather than technical shortcomings, is the true bottleneck of PHC standards development. Standard protocols need to be developed and linked to clinical data [21], assay linearity, and specificity to be established and the technology approved by authorities—and this needs to be prioritized by academy, drug developers, and drug authorities.

1.3. Inhaled biologics

Biologics are increasingly used in asthma, one approved (omalizumab [22], and several in late-phase development [23]. Examples of inhaled biologics are rare, including inhaled insulin (e.g. Exubera®) against diabetes and inhaled dornase alfa (Pulmozyme®) against cystic fibrosis. As yet only one inhaled biologic is in development for asthma—inhaled interferon beta [19]. With the emerging molecular understanding of asthma pathology, this is a field of research expected to grow fast. For this to happen, drug developers will need further insights into the local biology and pharmacokinetics of the new treatment modalities to optimize drug delivery from a benefit/risk, patient convenience, and COG perspective. Methods to circumvent local proteolytic degradation and enhance absorption will also need to be considered.

Patient convenience and/or reduced cost of goods can be commercial drivers for novel inhaled biologics, and then also for targets beyond the lung. For Exubera, convenience was not the critical factor Pfizer originally believed it to be, but for asthma patients—if dosing frequency can be reduced from twice daily bronchodilator/steroid combination plus prn reliever therapy to once monthly or less of a biologic—this will be an attractive option. Cost-of-goods is likely to become a more important driver in the future with the introduction of costly antibodies and other biologics against respiratory disorders. If inhaled, the dose will theoretically be more efficiently exposed to the target organ and lower doses can then be used. Clearly, available analytical tools need to be refined to assess the lower exposures expected, and to characterize the complex processes of biologics absorption and disposition in order to provide the pharmacokinetic documentation required by regulatory bodies,

2. Concluding remarks

The more the lung pharmacology is understood and new drug candidates—small molecules as well as biologics—are being considered for the local treatment of asthma, the greater the urge to simplify and further improve the technologies available for pharmacokinetic and PK/PD understanding of inhaled pharmaceutics. The shortcomings of available in vivo imaging techniques (including radioactive exposure, limited spatial resolution, and lack of metabolite information) make the new in vitro/in situ techniques of MARG, MALDI-MSI, and SIMS-MSI highly interesting complements to better understand the cellular and subcellular disposition and metabolism of new drug candidates, as well as endogenous protein, peptide, and lipid biomarkers. Drug developers should join with regulatory bodies in leading this effort as a means to facilitate drug development and improve the lives of patients suffering from asthma and COPD.

Declaration of interest

The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Additional information

Funding

This paper was not funded.