Renin inhibitor: relationship between molecular structure and oral absorption.

Common problems in developing renin inhibitors are low solubility, insufficient oral absorption, and fast hepatic clearance. We focused on the molecular structure of renin inhibitors to overcome these problems. Cyclodextrins (CD) improved the low solubility of renin inhibitors, with β-CD showing the best ability to dissolve renin inhibitors. The intestinal absorption of renin inhibitors varied with both their solubility and molecular structure. Coadministration of β-CD improved the intestinal absorption of some renin inhibitors with low solubility as measured by transport into the mesenteric vein in the absorption experiment using the rat intestinal loop. Substitutions at both the N and C terminals was essential for absorption from the small intestine. A naphthyl group at the N-terminal further improved intestinal absorption. A carrier system appeared to be involved in the intestinal absorption of some renin inhibitors. N-methylation at the amide bond of thiazolylalanine suppressed the high hepatic clearance of one of the test compounds 18 which was well absorbed from the small intestine and it improved its oral bioavailability.