Impact of plasma protein binding in drug clearance prediction: a database analysis of published studies and implications for in vitro in vivo extrapolation

Plasma protein mediated uptake (PMU) and its effect on clearance (CL) prediction has been studied in various formats; however, a comprehensive analysis of the overall impact of PMU on CL parameters from hepatocyte assays (routinely used for IVIVE) has not previously been performed. The following work collated data, reflecting the effect of PMU for 26 compounds with a wide variety of physico-chemical, drug and in vivo CL properties. PMU enhanced the unbound intrinsic clearance in vitro (CLint,u in vitro), beyond that conventionally calculated using fu and was correlated with the unbound fraction of drug in vitro and in plasma (fup), and absolute unbound intrinsic clearance in vivo (CLint,u in vivo), in both rat and human hepatocytes. PMU appeared to be more important for highly bound (fup <0.1) and high CLint,u in vivo drugs. These trends were independent of species, assay conditions, ionisation and ECCS group, although the type of plasma protein used in in vitro assays may require further investigation. Such generalised trends (spanning fup 0.0008 to 0.99) may suggest a generic mechanism behind PMU, however multiple drug-dependent mechanisms are also possible. Using the identified relationship between the impact of PMU on CLint,u in vitro and fup, PMU-enhanced predictions of CLint,u in vivo were calculated for both transporter substrates and metabolically cleared drugs. PMU was accurately predicted, and incorporation of predicted PMU improved the IVIVE of hepatic CL, with an AFE of 1.17, and >50% of compounds predicted within a 2-fold error, for both rat and human datasets (n ≥100).