BILE-ACID DERIVED HMG-COA REDUCTASE INHIBITORS

The target organ for HMG-CoA reductase inhibitors to decrease choleste rol biosynthesis in hypercholesterolemic patients is the liver. Since bile acids undergo an enterohepatic circulation showing a strict organ otropism for the liver and the small intestine, the structural element s of an inhibitor for HMG-CoA reductase were combined with those for s pecific molecular recognition of a bile acid molecule for selective up take by hepatocytes. Either, the HMG-CoA reductase inhibitors HR 780 a nd mevinolin were covalently attached to 3 xi-(omega-aminoalkoxy)-7 al pha, 12 alpha-dihydroxy-5 beta-cholan-24-oic acids to obtain bile acid prodrugs, or the side chain of bile acids at C-17 was replaced by 3,5 -dihydroxy-heptanoic acid - a structural element essential for inhibit ion of HMG-CoA reductase - to obtain hybrid bile acid: HMG-CoA reducta se inhibitors. The prodrugs could, as expected, not inhibit rat liver HMG-CoA reductase to a significant extent, whereas the hybrid inhibito rs showed a stereospecific inhibition of HMG-CoA reductase from rat li ver microsomes with an IC50-value of 0.7 mu M for the most potent comp ound S 2467 and 6 mu M for its diastereomere S 2468. Uptake measuremen ts with isolated rat hepatocytes and ileal brush-border membrane vesic les from rabbit small intestine revealed a specific interaction,of bot h classes of bile acid-derived HMG-CoA reductase inhibitors with the h epatocyte and ileocyte bile acid uptake systems. Photoaffinity labelin g studies using 3-azi- or 7-azi-derivatives of taurocholate with fresh ly isolated rat hepatocytes or rabbit ileal brush-border membrane vesi cles revealed a specific interaction of bile acid derived HMG-CoA redu ctase inhibitors with the respective putative bile acid transporters i n the liver and the ileum demonstrating the bile acid character of the se derivatives, both for the prodrugs and the hybrids. Cholesterol bio synthesis in Hep G2 cells was inhibited by the bile acid prodrugs with IC50-values in the range of 68 nM to 600 nM compared to 13 nM for HR 780 and 130 nM for mevinolin. Among the hybrid inhibitors, S 2467 was the most active compound with an IC50-value of 16 mu M compared to 55 mu M for its diastereomere S 2468. Preliminary in vivo experiments sho wed an inhibition of hepatic cholesterol biosynthesis after oral dosag e only with prodrugs such as S 3554, whereas the hybrid molecules were inactive after oral application. Measurement of inhibition of cholest erol biosynthesis by incorporation of [C-14]octanoate or [H-3]H2O into the digitonin-precipitable sterol fraction in the liver and extrahepa tic organs demonstrated a significant increase of liver selectivity of HMG-CoA reductase inhibitors such as HR 780 by coupling to bile acids . Taken together, these studies demonstrate that the liver selectivity of an HMG-CoA reductase inhibitor can be increased by combining with bile acid structural elements and making use of the specific bile acid transport pathways of the liver.