Structural determinants for transport across the intestinal bile acid transporter using C-24 bile acid conjugates

The human apical sodium dependent bile acid transporter (hASBT) reabsorbs gram quantities of bile acid daily and is a potential prodrug target to increase oral drug absorption. In the absence of a high resolution hASBT crystal structure, 3D-QSAR modeling may prove beneficial in designing prodrug targets to hASBT. The objective was to derive a conformationally sampled pharmacophore 3D-QSAR (CSP-SAR) model for the uptake of bile acid conjugates by hASBT. A series of bile acid conjugates of glutamyl chenodeoxycholate were evaluated in terms of K_m and normalized V_max (normV_max) using hASBT-MDCK cells. All monoanionic conjugates were potent substrates. Dianions, cations and zwitterions, which bound with a high affinity, were not substrates. CSP-SAR models were derived using structural and physicochemical descriptors, and evaluated via cross validation. The best CSP-SAR model for K_m included two structural and two physiochemical descriptors, where substrate hydrophobicity enhanced affinity. A best CSP-SAR model for K _m/normV_max employed one structural and three physicochemical descriptors, also indicating hydrophobicity enhanced efficiency. Overall, the bile acid C-24 region accommodated a range of substituted anilines, provided a single negative charge was present near C-24. In comparing uptake findings to prior inhibition results, increased hydrophobicity enhanced activity, with dianions and zwitterions hindering activity.