Fraction Unbound for Liver Microsome and Hepatocyte Incubations for All Major Species Can Be Approximated Using a Single-Species Surrogate

Abstract

It is well-recognized that nonspecific binding of a drug within an in vitro assay (fu) can have a large impact on in vitro to in vivo correlations of intrinsic clearance. Typically, this value is determined experimentally across multiple species in the drug discovery stage. Herein we examine the feasibility of using a single species (rat) as a surrogate for other species using a panel of small molecules representing highly diverse structures and physiochemical classes. The study demonstrated that 86% and 92% of the tested compounds measured in mouse, dog, monkey, and human were within two-fold of rat values for fu in microsomes and hepatocytes, respectively. One compound, amiodarone, exhibited unique species-dependent binding where the fu was approximately 10-fold higher in human microsomes and 20-fold higher in human hepatocytes compared to the average of the other species tested. Overall, these data indicate that using a single species (rat) fu as a surrogate for other major species, including human, is a means to increase the throughput of measuring non-specific binding in vitro.