Recent developments in the risk assessment of potentially genotoxic impurities in pharmaceutical drug substances.

Controlling the quality of medicines is just as important as demonstrating efficacy. The International Conference on Harmonisation has published general guidance on the quality and safety assessment of impurities in pharmaceutical drug substances and drug products. More recently, the European Medicines Evaluation Agency has published a guideline focusing on limits for genotoxic impurities. This is based on a Threshold of Toxicological Concern (TTC) derived from animal carcinogenicity data using multiple worst case assumptions to estimate a daily dose (1.5 microg/day) associated with a lifetime cancer risk of 1 in 100,000, a risk level considered acceptable for genotoxic impurities in human medicines. Based on these assumptions, presentation of the TTC as a single figure infers an unwarranted level of precision and supports the adoption of a more flexible approach by regulatory authorities when evaluating new drug products; a range within fivefold of the TTC limit would seem sensible. Furthermore, the limit is based on 70 years continuous daily exposure, a scenario that is uncommon for most medicines and irrelevant to the preregistration clinical development phase. To address this latter point, a staged TTC has been developed that proposes limits based on shorter durations of treatment, e.g., up to 1 year. Based on recent history, this approach has been acceptable to some authorities but not to others, and it is imperative that steps are taken to reach a common agreement between the pharmaceutical industry and regulatory authorities globally in order that new medicines can continue to be developed and delivered to benefit patients in a safe and timely manner.

A rationale for determining, testing, and controlling specific impurities in pharmaceuticals that possess potential for genotoxicity.

The synthesis of pharmaceutical products frequently involves the use of reactive reagents and the formation of intermediates and by-products. Low levels of some of these may be present in the final drug substance and drug product as impurities. Such chemically reactive impurities may have at the same time the potential for unwanted toxicities including genotoxicity and carcinogenicity and hence can have an impact on product risk assessment. This paper outlines a procedure for testing, classification, qualification, toxicological risk assessment, and control of impurities possessing genotoxic potential in pharmaceutical products. Referencing accepted principles of cancer risk assessment, this document proposes a staged threshold of toxicological concern (TTC) approach for the intake of genotoxic impurities over various periods of exposure. This staged TTC is based on knowledge about tumorigenic potency of a wide range of genotoxic carcinogens and can be used for genotoxic compounds, for which cancer data are limited or not available. The delineated acceptable daily intake values of between approximately 1.5 microg/day for approximately lifetime intake and approximately 120 microg/day for < or = 1 month are virtually safe doses. Based on sound scientific reasoning, these virtually safe intake values do not pose an unacceptable risk to either human volunteers or patients at any stage of clinical development and marketing of a pharmaceutical product. The intake levels are estimated to give an excess cancer risk of 1 in 100,000 to 1 in a million over a lifetime, and are extremely conservative given the current lifetime cancer risk in the population of over 1 in 4 (http://seer.cancer.gov/statfacts/html.all.html). The proposals in this document apply to all clinical routes of administration and to compounds at all stages of clinical development. It is important to note that certain types of products, such as those for life-threatening indications for which there are no safer alternatives, allow for special considerations using adaptations of the principles outlined in this paper.