| To lower cholesterol-related cardiovascular risk, an alternative to inhibition of cholesterol synthesis with statin drugs, is enhancement of cholesterol catabolism by blocking bile acid reabsorption. Hypothesis that a small-molecule competitive inhibitor of ileal bile acid transporter (IBAT) could be a hypocholesterolemic agent, was tested with novel 1,4-benzothiazepine 264W94. Methods. Used were in vitro models of rodent and primate IBAT; in vivo rodent models of bile acid absorption, bile acid synthesis, diet-induced hypercholesterolemia. Results. In vitro, 264W94 dose-dependently inhibited sodium-dependent uptake of 10muM 3H-taurocholic acid (TC) by rat and monkey ileal brush border membrane vesicles (BBMV) with IC50's of 0.24muM and 0.41muM, and had competitive profile with Ki of 0.2muM in CHO-cells expressing human IBAT (CHO-hIBAT). IC 50 for 264W94 against 5muM 3H-TC in CHO-hIBAT was 0.25muM. From ileum in situ, 1--10muM 264W94 decreased uptake of 3mM TC by 24--39%, corresponding to biliary recovery. In rats and mice in vivo, oral 264W94 decreased absorption of TC analog 75SeHCAT with ED 30 0.02 mg/kg bid. Peak (97%) inhibition of 75SeHCAT absorption by distal ileum occurred 4 hours after dose of 264W94 (0.1 mg/kg). IBAT inhibition by 264W94 in rats was associated with same-day, 4-fold induction of Cyp7a1, exhibiting normal diurnal fluctuation. In diet-induced hypercholesterolemic rats, 264W94 (0.03--1.0 mg/kg bid) dose-dependently reduced serum LDL+VLDL cholesterol to 61%. 2169W94, a metabolite of 264W94, inhibited uptake of 10mum 3H-TC by rat BBMV and 5muM 3H-TC by CHO-hIBAT with IC50's of 0.23muM and 0.18muM. In vivo, 2169W94 decreased rat 75SeHCAT absorption with ED 30 0.004 mg/kg bid. Conclusion. 264W94 is a hypocholesterolemic agent that acts by competitive inhibition of IBAT and exhibits activity in a human model. Hypocholesterolemic activity of 264W94 may be augmented by 5-fold more potent metabolite 2169W94 in biliary recirculation, which could explain greater in vivo potency of 264W94 vs. prototype 2164U90 in rats. Public information supports hypothesis in that 264W94 lowers cholesterol in humans. Other benzothiazepine-IBAT inhibitors result in atherosclerosis regression and synergy with statins in animals. Species differences between rodent and human IBAT protein sequences, and nuclear receptor (FXR and LXR) regulation of lipid metabolism may explain more modest effect of 264W94 in clinic. |
