IN-VITRO METABOLISM OF L-696,229, AN HIV-1 REVERSE-TRANSCRIPTASE INHIBITOR IN RATS AND HUMANS - HEPATIC AND EXTRAHEPATIC METABOLISM AND IDENTIFICATION OF ENZYMES INVOLVED IN THE HEPATIC-METABOLISM

The metabolism of L-696,229, zol-2-yl)ethyl]-5-ethyl-6-methylpyridin-2 (1H)-one, a potent human immunodeficiency virus-type 1 reverse transcr iptase inhibitor, by rat liver, lung, gut, and kidney microsomes has b een studied. L-696,229 was metabolized by rat liver microsomes to seve ral products: the 5 alpha-hydroxyethyl (M1); 5,6-dihydrodiol (M2); 6'- hydroxy (M3); 6-hydroxymethyl (M4); and 5-vinyl (M5) metabolites. For these pathways, liver was the most active metabolizing organ, whereas lung was the major extrahepatic organ in the drug metabolism. In all t issues tested, M1 was the major metabolite. With the exception of M3, gender differences in the hepatic formation of all metabolites were ob served. Enzymes responsible for the hepatic metabolism of L-696,229 in rats were also investigated using various enzyme inducers and polyclo nal antibodies to rat P-450. Treatment of male rats with dexamethasone (DX) or phenobarbital (PB) caused significant increases in the hepati c formation of the gender-dependent metabolites. Methylcholanthrene (3 -MC) greatly enhanced the hepatic formation of M1, M3, and M4. Immunoi nhibition studies suggested that CYP2B1/2 and 2E1 were not involved in L-696,229 metabolism, whereas CYP1A was partly responsible for the fo rmation of M1 in untreated rats. CYP3A played an important role in the formation of M1, M2, M4, and M5 in untreated and DX-treated rats. In PB-treated rats, CYP2B1/2 was involved in the increased formation of M 1 and M4, whereas CYP3A was partly involved in the enhanced M2 and M4 formation, and primarily responsible for the increased M5 formation. I n 3-MC-treated rats, CYP1A played a major role in the induction of M1, M3, and M4 formation, whereas CYP3A was an important enzyme for M5 fo rmation. The formation of M1 was stereoselective, with CYP1A preferent ially catalyzed the formation of the (-)-enantiomer, whereas CYP3A and CYP2B1/2 favored the formation of the (+)-enantiomer. The metabolism of L-696,229 by human liver microsomes was qualitatively and quantitat ively similar to that observed by rat liver microsomes. As previously found with rat microsomes, antirat CYP3A significantly inhibited the f ormation of all metabolites in human microsomes, with the exception of M3. A microsomal product prepared from human lymphoblast cells transf ected with a cDNA encoding CYP3A4 showed significant formation of thes e diminished metabolites, further supporting the important role of CYP 3A subfamily in L-696,229 metabolism in humans.