Elucidating structural requirements of bile acid transporters: prodrug design for oral delivery

Abstract

Drug transporters play a key role in drug disposition and hold promise for improving pharmacotheapeutic outcomes. Inhibition studies are routinely performed to characterize transporters and identifying type of inhibition. Studies were performed to identify methods which can correctly determine inhibition type. For conventional inhibition data, nonlinear regression performed poorly, whereas Dixon-type data yielded moderately better results. Interestingly, a new approach denoted "nonconventional inhibition data" performed well. Apical sodium dependent bile acid transporter (ASBT) and sodium-taurocholate cotransporting polypeptide (NTCP) are potential prodrug targets. Successful prodrug design is impeded by an incomplete understanding of structural requirements for these transporters. To evaluate the effect of C-3 and C-7 substitution on bile acid interaction with these transporters, 19 bile acid analogues were tested against ASBT and NTCP for binding and translocation. Results indicated that ASBT and NTCP accommodated a wide range of substituents for binding, but all major C-7 modifications resulted in analogues that did not demonstrate active uptake by either ASBT or NTCP. A C-3 modification that was not tolerated at C-7 still afforded translocation via ASBT and NTCP, confirming the relative unacceptability of C-7 modification. Results suggest that drug conjugation to the C-3 hydroxyl group, rather than C-7, has potential for successful prodrug targeting of ASBT and NTCP. Based on these results a prodrug of 5-aminosalicylic acid (5-ASA), first-line therapy for treatment of ulcerative colitis, was designed that conjugated 5-ASA with glycoursodeoxycholic acid via an azo bond. In vitro studies demonstrated that the prodrug had a low permeability and it was selectively released 5-ASA in the colon. Although bile acids are endogenous molecules and are considered safe, they have some limitations to use as pro-moities. Studies were performed to identify small molecules as pro-moities to target ASBT. This successfully identified two non-steroidal molecules that were substrates of ASBT. Results indicated, for the first time, that the bile acid steroid backbone and a negative charge are not required for ASBT translocation. Overall, this work may lead to successful design of prodrugs for better delivery of orally administered drugs.