Exploring the Definition of “Similar Toxicities”: Case Studies Illustrating Industry and Regulatory Interpretation of ICH S6(R1) for Long-Term Toxicity Studies in One or Two Species

ICH S6 (R1) states that safety evaluation of biotherapeutics should normally include 2 relevant species when available (i.e., a rodent and non-rodent species in which the test material is pharmacologically active), at least for short-term toxicology studies (generally supporting Phase I trials). For subsequent long-term toxicology studies (e.g., chronic studies up to 6 months dosing duration), there are options to reduce to only one species when justified, including when the mechanism of action of the biologic is well-understood or the toxicity findings in the short-term studies are “similar” in both the rodent and non-rodent species. Across the industry, around 25 to 33% of biologics assess multiple species within short-term toxicity studies but it is often unclear how different companies and regulators are applying the ICH S6 (R1) principles of “similar toxicity profiles” to progress with either 1 or 2 species in the long-term studies, in particular whether the absence of toxicities is considered within this definition. Sponsors may potentially continue to use 2 species to avoid regulatory risk and potential delays in development timelines, representing missed opportunities for reducing animal use, particularly of non-human primates, during drug development. This article summarizes presentations from a symposium at the 41(st) Annual meeting of the American College of Toxicology (ACT) in November 2020, in which industry case studies and regulatory perspectives addressed considerations and decisions for using 1 or 2 species for long-term toxicity studies, highlighting any common themes or experience that could be applicable for use in future decision-making.