Evaluation of the bile acid transporter in enhancing intestinal permeability to renin-inhibitory peptides

To evaluate the bile acid transporter as a means of enhancing the ability of renin-inhibitory peptides (RIPs) to penetrate the intestinal mucosa, two RIP-cholic acid conjugates and an RIP-taurocholic acid conjugate were synthesized. Conjugation was through the N-terminus of an RIP and the 3-position of the bile acid, via a six-carbon spacer. An RIP derivative containing the spacer without the bile acid moiety was also synthesized. The bile acid-RIP conjugates and the RIP derivative were shown to be potent inhibitors of human renin in vivo and to have in vivo hypotensive activity equivalent to that of the parent RIP (ditekiren) in a human renin-infused rat model. The ability of these RIP derivatives to bind to the bile acid transporter and be transported across an epithelial cell monolayer was evaluated in an in vitro model of the intestinal mucosa consisting of Caco-2 cell monolayers grown on microporous membranes. One of the RIP-cholic acid conjugate (KI = 60 +/- 10 microM) and the RIP-taurocholic acid conjugate (KI = 19 +/- 5 microM), but not the RIP derivative, were shown to be potent inhibitors of the apical (AP) to basolateral (BL) transport of [14C]-taurocholic acid ([14C]-TA). At concentrations up to 250 microM these RIP-bile acid conjugates had no effect on the diffusion of [3H]-PEG (800-1000), which is a marker of the paracellular pathway. The permeability coefficients of the RIP-bile acid conjugates, determined using Caco-2 cell monolayers, were shown to be six times less than that of [3H]-PEG (800-1000). In addition, the transport of one of the RIP-cholic acid conjugates was investigated in perfused rat ileum in which the mesenteric vein was cannulated. The conjugate was not detected in blood samples taken from the mesenteric vein, while its concentration in intestinal perfusate remained almost constant during the perfusion experiment. These results suggest that while the peptide-bile acid conjugates retain binding affinity for the intestinal bile acid transporter, the molecules are not themselves transported.