Structural Requirements of the ASBT by 3D-QSAR Analysis Using Aminopyridine Conjugates of Chenodeoxycholic Acid

Abstract

The human apical sodium dependent bile acid transporter (ASBT) is a validated drug target and can be employed to increase oral bioavailability of various drug conjugates. The aim of the present study was to investigate the chemical space around the 24-position of bile acids that influences both inhibition and uptake by the transporter. A series of 27 aminopyridine and aminophenol conjugates of glutamyl-chenodeoxycholate were synthesized and their ASBT inhibition and transport kinetics (parameterized as Ki, Kt, and Jmax) measured using stably transfected ASBT-MDCK cells. All conjugates were potent ASBT inhibitors. Monoanionic conjugates exhibited higher inhibition potency than neutral conjugates. However, neutral conjugates and chloro-substituted monoanionic conjugates were not substrates, or at least not apparent substrates. Kinetic analysis of substrates indicated that similar values for Ki and Kt implicate substrate binding to ASBT as the rate-limiting step. Using 3D-QSAR, four inhibition models and one transport efficiency model were developed. Steric fields dominated in CoMFA models, whereas hydrophobic fields dominated CoMSIA models. The inhibition models showed that a hydrophobic or bulky substitute on 2 or 6 position of a 3-aminopyridine ring enhanced activity, while a hydrophobic group on 5 position was detrimental. Overall, steric and hydrophobic features around the 24 position of the sterol nucleus strongly influenced bile acid conjugates’ interaction with ASBT. The relative location of the pyridine nitrogen and substituent groups also modulated binding.

Structural requirements of bile acid transporters: C-3 and C-7 modifications of steroidal hydroxyl groups

“…Prior reports investigating the ileal and hepatic bile acid transporters were limited to the structural changes in C-24 region or C-3 regions (Balakrishnan et al, 2006a; Bhat et al, 2005; Gonzalez et al, 2009; Kramer et al, 1992; Kramer et al, 1994; Rais et al, 2010a; Rais et al, 2010b; Swaan et al, 1997; Zheng et al, 2010). No reports examined the effect of structural modifications in the C-7 chemical space on compound interactions with either ASBT or NTCP.…”

“…However, a single negative charge in the C-24 region promotes translocation across the transporter (Balakrishnan et al, 2006a). ASBT also accommodates various drug-like single ring scaffolds with different substituents attached to the bile acid (Gonzalez et al, 2009; Rais et al, 2010a; Rais et al, 2010b; Zheng et al, 2010). Using C-24 bile acid linkage chemistry, bile acid-based prodrugs of gabapentin, ketoprofen, and niacin have been shown to be ASBT substrates (Rais et al, 2011; Zheng and Polli, 2010).…”

Mechanistic interpretation of conventional Michaelis–Menten parameters in a transporter system

“…ASBT Ki vs Km values of 50 compounds from previous studies (9, 10, 11, 13). Each compound is an ASBT substrate and inhibitor.…”

“…Km and Vmax) in order to elucidate microrate constant values that underpin four observed cases, representing four previously observed combinations of high and low Km and Vmax values. The four comparisons considered experimental data from four compounds (9, 10, 11), illustrated in Table 1. Briefly, the four compounds are bile acid derivatives (see Fig.…”

“…To determine if the model-supported conclusion Km = Ki is also supported by observed data, parameters of 50 previously tested non-native bile acids were analyzed (9, 10, 11, 13). In addition to examining previous data, Matlab error simulations were performed to evaluate the extent that error explains observed differences between Km and Ki.…”