Inhibitor and Substrate Requirements of Sodium Taurocholate Cotransporting Polypeptide and Its Application to Liver Targeting

Abstract

Human sodium taurocholate cotransporting polypeptide (NTCP) is the bile acid transporter that is also involved in hepatitis virus infection, drug disposition and prodrug targeting. Identification of NTCP inhibitors and substrates may help to treat hepatitis B, reduce NTCP mediate drug interaction and develop prodrugs to achieve liver specific drug delivery. However the understanding of structure-activity relationship of NTCP is very limited. One objective of the work in this dissertation is to fill this gap by exploring the inhibitor and substrate requirements of human NTCP. The other objective is to utilize NTCP to achieve liver targeting of ribavirin in order to reduce its off-target side effects. A common feature pharmacophore, a quantitative pharmacophore and a Bayesian model were developed and validated using FDA approved drugs to elucidate the inhibitor requirements of human NTCP. All these in silico models were able to predict NTCP inhibitors. Twenty seven novel NTCP inhibitors were identified which cover variety of therapeutic classes. The substrate requirements of NTCP were studied using native bile acids and bile acid analogs, suggesting a role of hydroxyl pattern and steric interaction in NTCP binding and translocation. One common feature pharmacophore was developed for NTCP substrates, which was used to search a database of FDA approved drugs. Among the retrieved drugs, irbesartan and losartan were identified as novel NTCP substrates, indicating a potential role of NTCP in drug disposition. In order to reduce ribavirin off-target side effects, ribavirin-L-Val-GCDCA was developed as a prodrug to target NTCP. In vitro uptake and metabolic studies indicated that the prodrug was taken up by NTCP, released ribavirin in the mouse live S9 fraction and reduced ribavirin accumulation in red blood cells (RBC). An in vivo study in mice showed that ribavirin-L-Val-GCDCA provided almost the same ribavirin exposure in the liver as ribavirin administration, but with about 2-fold less exposure of ribavirin in RBC, plasma, and kidney, suggesting that ribavirin-L-Val-GCDCA has the potential to achieve greater liver specific delivery of ribavirin. Overall, the work carried out in this dissertation will aid to identify human NTCP inhibitors and substrates, as well as a prodrug design for liver targeting.