Organic Anion Transporter 2 Mediates Hepatic Uptake of Tolbutamide, a CYP2C9 Probe Drug

Abstract

Tolbutamide is primarily metabolized by CYP2C9, and, thus, is frequently applied as a clinical probe substrate for CYP2C9 activity. However, there is a marked discrepancy in the in vitro-in vivo extrapolation of its metabolic clearance, implying a potential for additional clearance mechanisms. The goal of this study was to evaluate the role of hepatic uptake transport in the pharmacokinetics of tolbutamide and to identify the molecular mechanism thereof. Transport studies using singly transfected cells expressing six major hepatic uptake transporters showed that tolbutamide is a substrate to organic anion transporter 2 (OAT2) alone with transporter affinity [Michaelis-Menten constant (K_m)] of 19.5 ± 4.3 µM. Additionally, OAT2-specific transport was inhibited by ketoprofen (an OAT2 inhibitor) and 1 mM rifamycin SV (pan inhibitor), but not by cyclosporine and rifampicin (OAT polypeptides/Na⁺-taurocholate cotransporting polypeptide inhibitors). Uptake studies in primary human hepatocytes confirmed the predominant role of OAT2 in the active uptake with significant inhibition by rifamycin SV and ketoprofen, but not by the other inhibitors. Concentration-dependent uptake was noted in human hepatocytes with active transport characterized by K_m and V_max values of 39.3 ± 6.6 µM and 426 ± 30 pmol/min per milligram protein, respectively. Bottom-up physiologically based pharmacokinetic modeling was employed to verify the proposed role of OAT2-mediated hepatic uptake. In contrast to the rapid equilibrium (CYP2C9-only) model, the permeability-limited (OAT2-CYP2C9 interplay) model better described the plasma concentration-time profiles of tolbutamide. Additionally, the latter well described tolbutamide pharmacokinetics in carriers of CYP2C9 genetic variants and quantitatively rationalized its known drug-drug interactions. Our results provide first-line evidence for the role of OAT2-mediated hepatic uptake in the pharmacokinetics of tolbutamide, and imply the need for additional clinical studies in this direction.