Regulatory forum opinion piece: obligatory microscopic examination of intermediate dose groups in non-rodent toxicity studies--is it necessary?

In non-rodent toxicity studies supporting pharmaceutical or chemical product registration, the value of histopathology evaluation of all tissues from all animals from all dose groups is an ongoing discussion topic among pathologists. This manuscript documents an examination of this topic through a retrospective review of internal nonclinical study data from non-rodent toxicity studies performed at three pharmaceutical companies (Abbott Laboratories, Eli Lilly, and Pfizer, Inc.) and an informal survey of the current practices within the toxicological pathology community. The retrospective review of 325 non-rodent studies in which all organs in all dose groups were examined revealed no evidence that risk assessment would have changed if only the control and high-dose animals and target organs only in intermediate dose groups had been examined. One study had target tissues in a lower-dose group that were not identified in the high-dose group; however, there was no impact on the overall study interpretation. The recently revised European Medicines Agency guideline regarding repeated-dose toxicity studies encourages the examination of all tissues at all dose levels in non-rodent studies. In conclusion, the evaluation of all tissues from all animals may not be justified as a routine practice; however, regulatory guidance with input from toxicologic pathologists will influence these policy decisions.