RP-64477 - A POTENT INHIBITOR OF ACYL-COENZYME-A - CHOLESTEROL O-ACYLTRANSFERASE WITH LOW SYSTEMIC BIOAVAILABILITY

RP 64477 l-3-(p-decyloxybenzamido)-4-(methylthio)benzamide) has been s hown to be a potent inhibitor of the cholesterol esterifying enzyme Ac yl coenzyme A:cholesterol O-acyltransferase (EC 2.3.1.26; ACAT) in int estinal, hepatic, adrenal, and arterial tissue preparations obtained f rom a range of animal species. Drug concentrations producing 50% inhib ition of enzyme activity (IC50 values) ranged from 14-283 nM. Inhibiti on by RP 64477 in a rabbit intestinal enzyme preparation was shown to be non competitive with respect to the substrate oleoyl-CoA. In whole cell assays using human intestinal (CaCo-2), hepatic (HepG2) and monoc ytic (THP-1) cell lines, RP 64477 inhibited ACAT activity with IC(50)s of 113, 503, and 180 nM, respectively. RP 64477 (0.03% w/w by diet) r educed significantly cholesterol absorption in cholesterol/cholic acid -fed rats from 94 +/- 8% to 65 +/- 4%. In cholesterol fed rabbits, cho lesterol absorption was reduced from 72 +/- 5% to 50 +/- 5% and 44 +/- 5% at dose levels of 10 and 30 mg kg(-1) b.i.d., respectively. Plasma cholesterol levels were reduced dose-dependently in both cholesterol/ cholic acid-fed rats and cholesterol-fed rabbits. Neither cholesterol absorption nor plasma cholesterol levels were reduced significantly in animals maintained on standard laboratory diets. Pharmacokinetic stud ies indicated that RP 64477 was very poorly absorbed following oral ad ministration to rats. Plasma levels of drug were <2 ng mL(-1) followin g a dose of 2000 mg kg(-1) p.o.. When radiolabelled RP 64477 was admin istered orally, limited absorption was indicated by the overwhelming e limination of radioactivity in the faeces (96.4% of administered mater ial) coupled with low renal clearance (0.6% of dose) and biliary excre tion (0.05% of dose). In conclusion, this work shows that RP 64477 is a potent inhibitor of ACAT obtained from a range of animal species and man. Inhibition of cholesterol absorption and hypocholesterolaemic ac tivity has been demonstrated in rats and rabbits maintained on diets s upplemented with cholesterol. Pharmacokinetic studies indicate low sys temic exposure to RP 64477 as a result of limited absorption of this d rug.