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Abstract
A 1999 California state agency cancer potency (CP) evaluation of methyl tert-butyl ether (MTBE) assumed linear risk extrapolations from tumor data were plausible because of limited evidence that MTBE or its metabolites could damage DNA, and based such extrapolations on data from rat gavage and rat and mouse inhalation studies indicating elevated tumor rates in male rat kidney, male rat Leydig interstitial cells, and female rat leukemia/lymphomas. More recent data bearing on MTBE cancer potency include a rodent cancer bioassay of MTBE in drinking water; several new studies of MTBE genotoxicity; several similar evaluations of MTBE metabolites, formaldehyde, and tert-butyl alcohol or TBA; and updated evaluations of carcinogenic mode(s) of action (MOAs) of MTBE and MTBE metabolite's. The lymphoma/leukemia data used in the California assessment were recently declared unreliable by the U.S. Environmental Protection Agency (EPA). Updated characterizations of MTBE CP, and its uncertainty, are currently needed to address a variety of decision goals concerning historical and current MTBE contamination. To this end, an extensive review of data sets bearing on MTBE and metabolite genotoxicity, cytotoxicity, and tumorigenicity was applied to reassess MTBE CP and related uncertainty in view of MOA considerations. Adopting the traditional approach that cytotoxicity-driven cancer MOAs are inoperative at very low, non-cytotoxic dose levels, it was determined that MTBE most likely does not increase cancer risk unless chronic exposures induce target-tissue toxicity, including in sensitive individuals. However, the corresponding expected (or plausible upper bound) CP for MTBE conditional on a hypothetical linear (e.g., genotoxic) MOA was estimated to be ∼2 × 10–5 (or 0.003) per mg MTBE per kg body weight per day for adults exposed chronically over a lifetime. Based on this conservative estimate of CP, if MTBE is carcinogenic to humans, it is among the weakest 10% of chemical carcinogens evaluated by EPA.
Acknowledgments
The authors are grateful for suggestions offered by anonymous reviewers, and for assistance at Exponent by Wendy Hillwalker and Ryan Weidling (technical) and by Richard Nelson (editorial).
Declaration of interest
The employment affiliation of the authors is as shown on the cover page. The authors prepared the paper during the normal course of their employment by Exponent (Health Sciences), which is a consulting firm that, among other services, provides advice on toxicological and risk analysis issues to private and public clients. Research and evaluations pertaining to MTBE CP that provided the basis for this paper were funded in part by the Carroll Independent Fuel Company, which was involved in litigation with regard to MTBE and other oxygenates in fuels. One of the authors (Dr. Bogen) served as an expert witness in that litigation. Formulation of scientific questions addressed, review of the literature, synthesis and integration of scientific information, and conclusions drawn in the paper are the exclusive professional product of the authors, and are not necessarily those of Exponent or any of its clients. Only Exponent reviewed the submitted paper and funded its preparation.
Supplementary material available online
Supplementary Appendices A–E to be found online at http://dx.doi.org/10.3109/10408444.2015.1052367