Abstract

Transporters responsible for hepatic uptake and biliary clearance (CL_Bile) of RSV have been well-characterized. However, the contribution of basolateral efflux clearance (CL_BL) to hepatic and systemic exposure of RSV is unknown. Additionally, the appropriate design of in vitro hepatocyte efflux experiments to estimate CL_Bile versus CL_BL remains to be established. A novel uptake and efflux protocol was developed in sandwich-cultured hepatocytes (SCH) to achieve desired tight junction modulation while maintaining cell viability. Subsequently, studies were conducted to determine the role of CL_BL in the hepatic disposition of RSV using SCH from wild-type (WT) and multidrug resistance-associated protein 2 (Mrp2)-deficient (TR^-) rats, in the absence and presence of the P-glycoprotein and breast cancer resistance protein (Bcrp) inhibitor, GF120918. RSV CL_Bile was nearly ablated by GF120918 in TR^- SCH, confirming that Mrp2 and Bcrp are responsible for the majority of RSV CL_Bile. Pharmacokinetic modeling revealed that CL_BL and CL_Bile represent alternative elimination routes with quantitatively similar contributions to the overall hepatocellular excretion of RSV in rat SCH under baseline conditions (WT SCH in the absence of GF120918), and also in human SCH. Membrane vesicle experiments revealed that RSV is a substrate of MRP4 (K_m=21±7 μM, V_max=1140 ±210 pmol/min/mg protein). Alterations in MRP4-mediated RSV CL_BL due to drug-drug interactions, genetic polymorphisms and/or disease states may lead to changes in hepatic and systemic exposure of RSV, with implications for safety and efficacy of this commonly-used medication.