Kinetic determinants of hepatic clearance: plasma protein binding and hepatic uptake

Hepatic clearance prediction, using scaled data obtained from hepatocytes and microsomes, often under-predicts the eventual observed clearance. This occurs commonly where the compound is highly bound and/or a sinusoidal transporter substrate. The authors' own laboratory observations and those reported in the literature indicate that consideration of transporter effects in vitro is not sufficient to provide a direct, quantitative estimate of hepatic clearance in vivo. The physiology of contributing processes has been reviewed and the processes were compiled into a kinetic model of compound disposition for a hepatocyte compartment. The model has variables describing the kinetic effects of plasma protein binding, sinusoidal uptake, passive permeability, and cellular disposition. Parameters were determined experimentally requiring, in some instances, assays less familiar or routine to Drug metabolism and pharmacokinetics (DMPK) laboratories. The model accurately fitted data for the hepatic disposition of a UCB-proprietary compound that did not undergo metabolism but was a substrate for protein binding and sinusoidal uptake. On addition of bovine serum albumin to the assay, the uptake kinetics approximated neither those for the free fraction nor the total concentration. However, the model accurately predicted the intermediate observed kinetics and illustrated the kinetic effects of plasma protein binding and the complex interplay between various competing and collaborating processes. Through the use of simulations the model illustrated the influence of different combinations and influences of hepatic kinetic processes. Possible causes behind the extraction of highly bound compounds were identified as (1) low compound permeability facilitating active uptake and (2) high permeability facilitating the role of intracellular metabolism. In almost all circumstances k(off) is not low enough (<1 s(-1)) to limit the extraction of plasma protein bound compounds; however, k(on) still exerts an effect through competition with the uptake process. Crucially the model indicated the complex nature of the combined processes, whose broad range of effects on hepatic disposition could only be accurately determined with a kinetic model.