Analysis of Nonlinear Pharmacokinetics of a Highly Albumin-Bound Compound: Contribution of Albumin-Mediated Hepatic Uptake Mechanism

The cause of nonlinear pharmacokinetics (PK) (more than dose-proportional increase in exposure) of a urea derivative under development (compound A: anionic compound [pKa: 4.4]; LogP: 6.5; and plasma protein binding: 99.95%) observed in a clinical trial was investigated. Compound A was metabolized by CYP3A4, UGT1A1, and UGT1A3 with unbound K-m of 3.3-17.8 mu mol/L. OATP1B3-mediated uptake of compound A determined in the presence of human serum albumin (HSA) showed that unbound K-m and V-max decreased with increased HSA concentration. A greater decrease in unbound K-m than in V-max resulted in increased uptake clearance (V-max/unbound K-m) with increased HSA concentration, the socalled albumin-mediated uptake. At 2% HSA concentration, unbound K-m was 0.00657 mu mol/L. A physiologically based PK model assuming saturable hepatic uptake nearly replicated clinical PK of compound A. Unbound K-m for hepatic uptake estimated from the model was 0.000767 mu mol/L, lower than the in vitro unbound K-m at 2% HSA concentration, whereas decreased K-m with increased concentration of HSA in vitro indicated lower K-m at physiological HSA concentration (4%-5%). In addition, unbound K-m values for metabolizing enzymes were much higher than unbound K-m for OATP1B3, indicating that the nonlinear PK of compound A is primarily attributed to saturated OATP1B3-mediated hepatic uptake of compound A. (C) 2017 American Pharmacists Association (R). Published by Elsevier Inc. All rights reserved.