ETHOSUXIMIDE IS PRIMARILY METABOLIZED BY CYP3A WHEN INCUBATED WITH ISOLATED RAT-LIVER MICROSOMES

The cytochrome P450 (CYP) subfamily responsible for ethosuximide metab olism was investigated by HPLC assay of ethosuximide incubations with isolated rat liver microsomes from control rats and from rats treated with inducing agents to enrich hepatic microsomes in selected CYP isof orms. Inducing agents included beta-naphthoflavone (BNF, CYP1A inducer ), phenobarbital (PB, CYP2B/2C/3A), isoniazid (INH, CYP2E1), clotrimaz ole (CTZ, CYP3A), clofibrate (CLO, CYP4A), and an imidazole CTZ-analog known as CDD3543 (CYP3A), Incubations with BNF, INH, CTZ, and control microsomes showed significantly (p<0.05) more metabolite produced by CTZ microsomes vs. BNF, INH, and control microsomes at 10, 30, 60, and 120 min incubation, Ethosuximide metabolite levels generated by CTZ m icrosomes at 120 min were 36.5 times those of control microsomes, Corr espondingly, ethosuximide concentrations were significantly (p<0.05) l ower for incubations with the CTZ microsomes compared with BNF, INH, a nd control microsomes at 60 and 120 min. Sixty-minute incubations with all microsome groups exhibited significantly (p<0.05) higher metaboli te formation rates (nmol/nmol CYP/min) for CTZ (11.8x control) and PB (9.6x control) microsomes vs. all other groups. Antibody inhibition ex periments demonstrated ethosuximide metabolite levels for PB microsome s were not affected by CYP2B1 antibodies, whereas CYP3A2 antibodies re duced metabolite levels for both PB and CTZ microsomes by over 80%. Th ese results indicate CYP3A is primarily responsible for ethosuximide m etabolism in rats.