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

It is well recognized that nonspecific binding of a drug within an in vitro assay (f u) 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 the mouse, dog, monkey, and human were within 2-fold of rat values for f u in microsomes and hepatocytes, respectively. One compound, amiodarone, exhibited unique species-dependent binding where the f u was approximately 10-fold higher in human microsomes and 20-fold higher in human hepatocytes compared with the average of the other species tested. Overall, these data indicate that using a single species (rat) f u as a surrogate for other major species, including humans, is a means to increase the throughput of measuring nonspecific binding in vitro.