Identification of Rat and Human Cytochrome P450 Forms Involved in the Metabolism of the Thromboxane A2 Receptor Antagonist (1)-s-145

(1)-S-145 {5-(1)-(Z)-7-[(1R, 2S, 3S, 4S)-3-phenylsulfonylaminobicyclo[2.2.1]hept-2-yl]-heptenoic acid} and its b-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 position of the bicyclo ring by microsomal monooxygenases. We investigated the oxidative metabolism of (1)-S-145 and its b-oxidized metabolites with liver microsomes from rats and humans to identify which cytochrome P450 (P450) forms are involved in these reactions. In rats, phenobarbital or dexamethasone treatment significantly increased 5and 6-hydroxylation activities toward (1)-S-145 and its b-oxidized metabolites, suggesting the involvement of P4503A forms. Immunoinhibition studies suggested that P4503A2 was mainly responsible for the 5-hydroxylation of (1)-S-145, bisnor, and DH-bisnor and the 6-hydroxylation of bisnor and tetranor. Furthermore, P4502C6, a phenobarbital-inducible 2C form in the rat, was involved in the 6-hydroxylation of (1)-S-145, bisnor, and DH-bisnor. P4502C11, the major constitutive form (male rats), was partly involved in the 5-hydroxylation of DH-bisnor and the 6-hydroxylation of bisnor and DH-bisnor. Reconstitution studies with purified human enzymes and immunoinhibition studies suggest that P4503A4 is primarily involved in the 5-hydroxylation of (1)-S-145 and bisnor and the 6-hydroxylation of tetranor; P4502C9/10 mainly catalyzed the 5-hydroxylation of tetranor and the 6-hydroxylation of (1)-S145. Results of the present study indicated that the same subfamily P450 forms are responsible for the oxidative metabolism of (1)-S145 in rats and humans. P4503A enzymes were shown to be involved in the formation of 6-hydroxy tetranor, the main metabolite