IDENTIFICATION OF RAT AND HUMAN CYTOCHROME-P450 FORMS INVOLVED IN THEMETABOLISM OF THE THROMBOXANE A(2) RECEPTOR ANTAGONIST (-S-145())

(+)-S-145 {5-(+)-(Z)-7-[(1R, 2S, 35, nylsulfonylaminobicyclo[2.2.1]hep t-2-yl]-heptenoic acid} and its beta-oxidized metabolites {two [bisnor or dihydro (DH)-bisnor] or four (tetranor) carbon-shortened products at the carboxyl side chain} are hydroxylated at the C-5 or C-6 positio n of the bicycle ring by microsomal monooxygenases. We investigated th e oxidative metabolism of (+)-S-145 and its beta-oxidized metabolites with liver microsomes from rats and humans to identify which cytochrom e P450 (P450) forms are involved in these reactions. In rats, phenobar bital or dexamethasone treatment significantly increased 5- and 6-hydr oxylation activities toward (+)-S-145 and its beta-oxidized metabolite s, suggesting the involvement of P4503A forms. Immunoinhibition studie s suggested that P4503A2 was mainly responsible for the 5-hydroxylatio n of (+)-S-145, bisnor, and DH-bisnor and the 6-hydroxylation of bisno r and tetranor, Furthermore, P4502C6, a phenobarbital-inducible 2C for m in the rat, was involved in the 6-hydroxylation of (+)-S-145, bisnor , and DH-bisnor. P4502C11, the major constitutive form (male rats), wa s partly involved in the 5-hydroxylation of DH-bisnor and the 6-hydrox yiation of bisnor and DH-bisnor. Reconstitution studies with purified human enzymes and immunoinhibition studies suggest that P4503A4 is pri marily involved in the 5-hydroxylation of (+)-S-145 and bisnor and the 6-hydroxylation of tetranor; P4502C9/10 mainly catalyzed the 5-hydrox ylation of tetranor and the 6-hydroxylation of (+)-S-145. Results of t he present study indicated that the same subfamily P450 forms are resp onsible for the oxidative metabolism of (+)-S-145 in rats and humans. P4503A enzymes were shown to be involved in the formation of 6-hydroxy tetranor, the main metabolite of S-1452 in vivo.