An evaluation of the sensitivity of the Ames assay to discern low-level mutagenic impurities

Low level impurities often reside in active pharmaceutical ingredients (API). Some of these impurities are potentially genotoxic since reactive intermediates are used in the synthetic route for the production of API. Routine mutagenicity testing is conducted in support of clinical trials with the intent to identify genotoxic hazards associated with API. Depending on the amount of impurity present in the API tested, the potency of the impurities and the relative sensitivity of the Ames assay, it is possible that mutagenicity associated with the presence of genotoxic impurities could also be detected while testing API. Therefore, we evaluated published data and generated new information to understand the sensitivity of the Ames assay. Based on a literature survey of approximately 450 mutagens, it was estimated that 85% of mutagens are detected at concentrations of 250 μg/plate or less. Based on this estimate, most mutagens should be detected in an Ames assay testing API concentrations up to 5000 μg/plate if present at a 5% or greater concentration. Data from experiments where several direct and indirect-acting mutagens were spiked into representative API further support the literature-based evaluation. Some limitations of this approach, including toxicity of API and competing metabolism are discussed.