Potentially mutagenic impurities: analysis of structural classes and carcinogenic potencies of chemical intermediates in pharmaceutical syntheses supports alternative methods to the default TTC for calculating safe levels of impurities

Abstract

Potentially mutagenic impurities in new pharmaceuticals are controlled to levels with negligible risk, the TTC (threshold of toxicological concern, 1.5 μg/day for a lifetime). The TTC was based on the more potent rodent carcinogens, excluding the highly potent "cohort of concern" (COC; for mutagenic carcinogens these are N-nitroso, Aflatoxin-like, and azoxy structures). We compared molecules with DEREK "structural alerts" for mutagenicity used in drug syntheses with the mutagenic carcinogens in the Gold Carcinogenicity Potency Database. Data from 108 diverse synthetic routes from 13 companies confirm that many "alerting" or mutagenic chemicals are in structural classes with lower carcinogenic potency than those used to derive the TTC. Acceptable daily intakes can be established that are higher than the default TTC for many structural classes (e.g., mono-functional alkyl halides and certain aromatic amines). Examples of ADIs for lifetime and shorter-term exposure are given for chemicals of various potencies. The percentage of chemicals with DEREK alerts that proved mutagenic in the Ames test ranged from 36% to 83%, depending on structural class, demonstrating that such SAR analysis to "flag" potential mutagens is conservative. We also note that aromatic azoxy compounds need not be classed as COC, which was based on alkyl azoxy chemicals.