Importance of Hepatic Transporters, Including Basolateral Efflux Proteins, in Drug Disposition: Impact of Phospholipidosis and Non-Alcoholic Steatohepatitis

Abstract

The objective of this dissertation project was to develop preclinical and clinical tools to assess the impact of liver pathology on transporter-mediated systemic and hepatic exposure to medications. A translational approach utilizing established pre-clinical hepatic transport systems, mathematical modeling, and a pivotal in vivo human study was employed. A novel application of rat sandwich-cultured hepatocytes (SCH) was developed to evaluate the impact of drug-induced phospholipidosis on the vectorial transport of probe substrates for hepatic basolateral and canalicular transport proteins. Results indicated that rat SCH treated with prototypical hepatic phospholipidosis inducers are a sensitive and selective model for drug-induced phospholipidosis; both organic anion transporting polypeptide-mediated uptake and bile salt export pump-mediated biliary excretion were reduced after the onset of phospholipidosis. Enalapril currently is being investigated for its anti-fibrotic effects in the treatment of patients with non-alcoholic steatohepatitis (NASH). Although the basolateral uptake transporters responsible for enalapril entry into the hepatocyte are well characterized, less is known about translocation of enalaprilat (the active metabolite) across the basolateral membrane into the systemic circulation. Studies were conducted using membrane vesicles prepared from transporter-expressing cells and a novel human SCH efflux protocol to assess the involvement of two hepatic basolateral efflux transporters known to be up-regulated in inflammatory liver disease [i.e., multidrug resistance-associated protein (MRP)3 and MRP4]; enalaprilat is transported across the basolateral membrane, in part, by MRP4. A novel clinical protocol was developed using morphine glucuronides as a probe for MRP3 function to determine the impact of altered hepatic transporter expression on drug disposition in patients with NASH. A physiologically-based mathematical model was constructed to inform study design. Systemic concentrations of morphine glucuronides were increased significantly in patients with NASH compared to healthy subjects, as predicted by the model. This work provides a mechanistic understanding of the impact that hepatic transporter function has on the disposition of drugs and endogenous compounds under normal conditions and in response to altered liver function due to drug-induced phospholipidosis or liver disease, specifically non-alcoholic steatohepatitis. This knowledge is fundamental to safe and efficacious dosing of medications that depend on these transporters for disposition and/or elimination.Doctor of Philosoph