Resolution of Historically Discordant Ames Test Negative / Rodent Carcinogenicity Positive N-nitrosamines using a Sensitive, OECD-aligned Design

The in vitro Bacterial Reverse Mutation (Ames) Test is crucial for evaluating the mutagenicity of pharmaceutical impurities. For N-nitrosamines (NAs) historical data indicated that for certain members of this chemical class the outcomes of the Ames Test did not correlate with their associated rodent carcinogenicity outcomes. This has resulted in negative outcomes in an OECD aligned Ames Test alone (standard or enhanced) no longer being considered sufficient by regulatory authorities to assess potential carcinogenic risk of NAs if present as impurities in drug products. Consequently, extensive follow-up in vivo testing can be required to characterise the potential mutagenicity and genotoxic carcinogenicity of NA impurities (i.e., beyond that defined in the ICH M7 guideline for non-NA impurities). We previously demonstrated that the mutagenicity of alkyl-nitrosamines can be detected by the appropriately designed, OECD aligned Ames Test and identified those conditions that contributed most to assay sensitivity. This OECD aligned Ames Test design was used to assess seven NAs, i.e. (methyl(neopentyl)nitrosamine, N-methyl-N-nitroso-2-propanamine, N-nitrosodiisopropylamine, bis(2-methoxyethyl)nitrosoamine, N-nitroso-N-methyl-4-fluoroaniline, dinitrosoethambutol, (R,R)- and mononitrosocaffeidine) that were reported to be negative in historical Ames Tests but positive in rodent carcinogenicity studies. All seven of the NAs were demonstrated to be mutagenic in the OECD aligned Ames test and therefore these compounds should no longer be considered as discordant (false negatives) with respect to the correlation of the Ames Test and rodent carcinogenicity. These results confirm the sensitivity of the OECD aligned Ames Test for the detection of NA mutagenicity and provides further support of its pivotal placement within the ICH M7 framework for the assessment of mutagenic impurities in pharmaceuticals to limit potential carcinogenic risk. In addition, we present data for 1-cyclopentyl-4-nitrosopiperazine, that indicates it could serve as a suitable positive control to provide further confidence in the sensitivity of the Ames Test for the NA chemical class.