THE ORALLY-ACTIVE RENIN INHIBITOR A-74273 - IN-VIVO AND IN-VITRO MORPHOLINE RING METABOLISM IN RATS, DOGS, AND HUMANS

The metabolism and disposition of [C-14]A-74273-a potent, orally activ e renin inhibitor-were investigated in beagle dogs and Sprague-Dawley rats. Two male and two female dogs received a single 10 mg/kg oral or 1 mg/kg intravenous dose in a cross-over experiment and urine and fece s were collected for 5 days. After both intravenous and oral dosing, > 92% of the dose was recovered in the feces and <3% was recovered in th e urine. The predominance of hepatobiliary elimination in the clearanc e of A-74273 was verified in a bile-exteriorized dog, where 79.8% of a 1 mg/kg intravenous dose was excreted in the bile within 6 hr after a dministration. Similarly, administration of a 1 mg/kg intravenous dose to a bile-exteriorized rat resulted in biliary excretion of 60.5% of the dose within 6 hr. Radio-HPLC analysis of bile and feces from both species indicated extensive metabolism of A-74273 to three major morph oline ring-opened metabolites; the ethanolamine A-78242, the amine A-7 8030, and the carboxylic acid A-81307. These three metabolites each co ntributed 12.0-20.2% of the biliary radioactivity after intravenous do sing, while unchanged A-74273 contributed 5-17%. Incubation of [C-14]A -74273 with rat, dog, and human liver microsomes afforded nearly equal amounts of the three in vivo metabolites for all three species, sugge sting that the in vitro system was representative of A-74273 in vivo m etabolism and that humans should also convert A-74273 to the morpholin e ring-opened metabolites in vivo. The observation that, once formed i n vitro, the relative ratios of the three metabolites did not change s uggested that the metabolites were all primary metabolites and were no t formed sequentially. This conclusion was supported by incubation of A-78242 and A-78030 with dog and human liver microsomes, where these c ompounds were converted at significantly slower rates than A-74273 and did not afford the expected amine and carboxylic acid products. Inhib ition studies with troleandomycin and erythromycin suggested that the biotransformation of A-74273 to the three metabolites was mediated, at least in part, by cytochrome P4503A. A mechanistic explanation for th e formation of the three primary metabolites of morpholine ring oxidat ion by cytochrome P4503A is proposed.