Abstract

Incubational binding or the fraction of drug unbound in an in vitro incubation, fu_inc, is an important parameter to predict or measure in the pursuit of accurate clearance predictions from in vitro data. Here we describe a method for fu_inc determination directly in the hepatocyte intrinsic clearance (CL_int) assay with emphasis on compounds that are actively transported into hepatocytes, hypothesizing that for such compounds the typical protocol of 1 million hepatocytes/ml systematically underestimates the maximum attainable unbound intracellular drug concentration. Using the transporter substrate atorvastatin as a test compound, incubations were performed and a mathematical model applied to describe metabolism, distribution, and binding at different hepatocyte concentrations. From these investigations it was evident that, since binding is more extensive intracellularly than in the medium, increased partitioning into the cellular volume, due to active uptake, increases the total amount of atorvastatin bound in the incubation. Consequently, a significant lowering of the hepatocyte concentration impacts the free drug concentration in the incubation and increases the observed rate of metabolism and therefore observed CL_int (that is, when viewed from the media drug concentration). The applicability of the findings was tested for a series of 11 actively transported zwitterions for which standard rat hepatocyte metabolic CL_int data (1 million cells/ml incubation) poorly predicted in vivo clearance (average fold error of 5.4). Using metabolic CL_int determined at a lower hepatocyte concentration (0.125 million cells/ml) considerably improved clearance predictions (average fold error of 2.3).