0090-9556/97/2501-0075-0080$02.00/0
DRUG METABOLISM AND DISPOSITION
Copyright © 1997 by The American Society for Pharmacology and Experimental Therapeutics
Vol. 25, No. 1

IDENTIFICATION OF RAT AND HUMAN CYTOCHROME P450 FORMS INVOLVED IN THE METABOLISM OF THE THROMBOXANE A₂ RECEPTOR ANTAGONIST (+)-S-145

Yoshitaka Yamaguchi, Shirou Kirita, Takahiko Baba, Junko Aoyama, Akira Touchi, Robert H. Tukey, F. Peter Guengerich, and Takashi Matsubara

Developmental Research Laboratories (Y.Y., S.K., T.B., J.A., A.T., T.M.), Shionogi & Co., Ltd.; Department of Pharmacology (R.H.T.), University of California at San Diego; and Department of Biochemistry and Center in Molecular Toxicology (F.P.G.), Vanderbilt University School of Medicine

(+)-S-145 {5-(+)-(Z)-7-[(1R, 2S, 3S, 4S)-3-phenylsulfonylaminobicyclo[2.2.1]hept-2-yl]-heptenoic acid} and its -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 (+)-S-145 and its -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 5- and 6-hydroxylation activities toward (+)-S-145 and its -oxidized metabolites, suggesting the involvement of P4503A forms. Immunoinhibition studies suggested that P4503A2 was mainly responsible for the 5-hydroxylation of (+)-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 (+)-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 (+)-S-145 and bisnor and the 6-hydroxylation of tetranor; P4502C9/10 mainly catalyzed the 5-hydroxylation of tetranor and the 6-hydroxylation of (+)-S-145. Results of the present study indicated that the same subfamily P450 forms are responsible 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.