Inhibition and Substrate Requirements of human Apical Sodium-dependent Bile Acid Transporter (ASBT) and Its Potential as a Prodrug Target

Abstract

The human apical sodium-dependent bile acid transporter (ASBT; SLC10A2) is an important mechanism for intestinal bile acid reabsorption and plays a critical role in bile acid and cholesterol homeostasis. Its physiological role impacts human health and disease. Furthermore, it is a potential candidate for prodrug targeting due to its high transporter capacity and efficiency. However, the understanding of ASBT's structural determinate of binding and translocation is limited. The work in this dissertation was carried out to study the inhibition and substrate requirement of ASBT, and subsequently to optimize the inhibition assay condition. In particular, work aimed to 1) identify FDA-approved drugs that inhibit ASBT and to derive computational models for ASBT inhibition; 2) evaluate the structural requirements of ASBT by 3D-QSAR analysis using aminopyridine and aminophenol conjugates of chenodeoxycholic acid; 3) synthesis and evaluate in vitro the potential of prolonged release prodrugs via targeting ASBT; 4) identify inhibitor concentrations to efficiently screen and measure inhibition constant Ki values against solute carrier transporters; 5) assess compound cytotoxicity on in vitro apparent transporter inhibition. Many FDA-approved drugs from diverse classes, such as the dihydropyridine calcium channel blockers and HMG CoA-reductase inhibitors were found to be ASBT inhibitors. A 3D-QSAR and a Bayesian model were developed using 38 molecules. 3D-QSAR models also were developed using C-24 conjugates. The models concluded that steric and hydrophobic features strongly influenced conjugate interaction with ASBT, and that the relative location of the pyridine nitrogen and substituent groups also modulated binding. Similar values for Ki and Kt indicated that substrate binding to ASBT was the rate-limiting step. In vitro results showed that the bile acid conjugates are potential prolonged release prodrugs with binding affinity for ASBT. Experimental conditions for Ki screening are suggested to use 10-fold the substrate affinity Kt for potent inhibitors and 100-fold Kt for nonpotent inhibitors; for Ki measurement, the inhibitor concentration range should use 0 to estimated Ki via five different inhibitor concentrations, where a low range of inhibitor concentrations can be used. For some drugs, their cytotoxicites contributed to or were associated with apparent transporter inhibition, where cytotoxicity differed between MDCK and HEK cells; cytotoxicity is suggested for future studies. Overall, the work carried out in this dissertation will aid in advancement in future prodrug design that exploits ASBT and made recommendations for the efficiency and quality of transporter inhibition assays in general.