Genotoxicity Testing of API
Fortunately the frequency of mutagenic drug candidates recognized in early drug development is very low because positive Ames bacterial mutagenicity test results usually have severe ramifications with APIs dropped from further development. While negative results in the appropriate follow-up tests (e.g., mouse carcinogenicity studies) could enable progression of API development, pragmatically the cost and long duration required for these tests means that most pharmaceutical companies drop mutagenic APIs from development and quickly move on to another drug candidate.
Positive in vitro mammalian genotoxicity tests are far more frequently encountered but most times do not prevent further development of an API. When positive in vitro mammalian genotoxicity test results are obtained, follow-up studies in complementary in vitro or in vivo assays are quickly conducted to understand the biological relevance of the initial positive response. In the majority of cases the additional testing allows the sponsor to demonstrate lack of biological relevance or establish a safety threshold (generally ≥10×) based on exposures in animals at the no observed genotoxic effect level (NOGEL) relative to highest clinical exposure. For all these reasons, follow-up testing in response to an in vitro mammalian cell positive test is generally pursued and often results in continued development of the API.
This chapter focuses on genotoxicity testing strategies and includes case studies where follow-up studies were used to effectively de-risk positive in vitro genotoxicity test results. The regulatory guidelines (e.g., ICH S2(R1)) dictating which tests are required and which follow-up tests are acceptable are also presented. Because the regulatory guidelines list acceptable follow-up tests with little to no guidance on how to select them, this chapter provides recommendations on how to design the most appropriate follow-up testing strategy.
KeywordsGenotoxicity Ames bacterial mutagenicity test DNA damage Clastogenicity ICH S2(R1) No-observed genotoxic effect level (NOGEL)
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