FUNCTIONAL IMPACT OF GENETIC VARIANTS, MEDICATIONS, AND DISEASE ON HEPATIC BILE ACID TRANSPORTERS

Abstract

Hepatic transporters mediate the movement of substrates including medications and endogenous compounds (e.g., bile acids) across membranes. Transporters are important determinants of pharmacokinetics, and altered transporter function can lead to clinically significant changes in drug disposition and safety. The objective of this dissertation was to characterize changes in hepatic transporter function due to patient specific factors (e.g., genetic variants, concomitant medications, and disease). The association of cholestatic drug-induced liver injury and a common variant (p.V444A) in the bile salt export pump (BSEP) was evaluated to identify underlying mechanisms. The kinetics of glycocholate transport mediated by reference and variant BSEP, and transport inhibition by cholestatic medications, were tested. No differences were found, raising important questions about the mechanistic explanation for the increased risk of cholestasis reported with this BSEP variant. A framework to improve the efficiency of drug-transporter inhibition screenings was developed and applied to identify novel inhibitors of multidrug resistance-associated protein (MRP) 3. A computational model was used to conduct a virtual screen to predict likely inhibitors, these predictions were tested in vitro, and three strong MRP3 inhibitors were identified. This approach can be applied to conduct efficient inhibitor screenings for additional transporters. The impact of nonalcoholic steatohepatitis (NASH), an increasingly prevalent liver disease characterized by steatosis and inflammation, on transporter function was evaluated. Patients with NASH exhibited reduced hepatic uptake clearance mediated by organic anion transporting polypeptide (OATP) 1B1 and OATP1B3, and decreased MRP2-mediated biliary clearance of 99mTechnetium-mebrofenin, an imaging probe. The disposition of medications that are substrates of these transporters could be altered in NASH. Additionally, patients with NASH exhibited increased total serum bile acid concentrations and altered bile acid profiles. Genetic variants in transporters and demographics were important sources of variability that need to be considered when examining specific bile acids as potential biomarkers. Methods and translational approaches utilized in these experiments can be used to improve our understanding of transporter function. This dissertation research underscores the importance of assessing transporter function in the context of patient-specific factors, which will help improve our ability to predict drug disposition and safety in the clinic.Doctor of Philosoph