Assessment of menstrual cycle length in cynomolgus monkeys as a female fertility endpoint of a biopharmaceutical in a 6Â month toxicity study

- Menstrual cycle assessment can be included in chronic toxicity.
- Sources of background variability are identified.
- Strategies to minimize impact are discussed.
- Optimal ways to collect, present and analyze menstrual cycle data are discussed.
- Key control procedures minimize cycle variability.

According to ICH S6(R1), mating studies are not practical for assessing effects on female fertility of biopharmaceuticals that are pharmacologically active only in non-human primates (NHPs). Instead, fertility should be assessed by evaluating histopathology and organ weights of the reproductive tract in studies of at least 3 months dosing duration using sexually mature NHPs. An assessment of the menstrual cycle in females can be included if there is cause for concern based on pharmacological mode of action or relevant findings in previous studies. However, many factors unrelated to the molecule under evaluation can impact cycle length and thus affect data interpretation. Assessment of a monoclonal antibody in a 6 month repeat dose toxicity study is used as an example in this manuscript to review potential sources of background variability, identify strategies to minimize its impact and recommend optimal ways to collect, present and analyze menstrual cycle data. Experimental variables include the amount of time required for menses to normalize following the transport of animals to the testing facility, stress-related effects on the cycle length due to socialization issues with new cagemates, and the normal background irregularity of cycle length in NHPs. Study related procedures (i.e., animal handling for dosing, blood draws, body weights, ECGs, etc.) did not affect cycle lengths in this study. We show that introducing a number of key experimental control procedures to minimize cycle variability can enable a robust assessment of the effects of a biotherapeutic on menstrual cycling within a chronic toxicity study in NHPs.