Involvement of cytochrome P-450IIIA in metabolism of potassium canrenoate to an epoxide: mechanism of inhibition of the epoxide formation by spironolactone and its sulfur-containing metabolite.

In vitro metabolism studies of potassium canrenoate (PC) were conducted to examine whether spironolactone (SP) and/or its sulfur-containing metabolites inhibit the PC metabolic pathways to mutagenic metabolites and to elucidate the mechanism for any observed inhibitory effect. The mechanistic study was conducted using liver microsomes prepared from male and female rats with and without pretreatment of a cytochrome (Cyt) P-450IIIA inducer [pregnenolone-16 alpha-carbonitrile (PCN) or dexamethasone (DEX)] and with and without a Cyt P-450IIIA inhibitor, triacetyloleandomycin (TAO). The present study demonstrates that SP and its sulfur-containing metabolite 7 alpha-thio-spirolactone substantially inhibited the formation of promutagen 6 beta, 7 beta-epoxycanrenone (6 beta, 7 beta-epoxy-CAN) from PC. The sulfur-containing metabolite of SP that inhibit promutagen formation were not formed from PC, although a glutathione conjugate of PC was formed. The formation rate of 6 beta, 7 beta-epoxy-CAN was greater in liver microsomes prepared from rats pretreated with a Cyt P-450IIIA inducer (PCN or DEX) than in liver microsomes prepared from the untreated rats. The formation rate of the epoxide metabolite was lower after in vitro addition of TAO. Pretreatment of animals with TAO 4 hr before sacrifice produced similar results. Erythromycin, which is N-demethylated by Cyt P-450IIIA, also reduced the formation rate of 6 beta, 7 beta-epoxy-CAN. Inhibition of PC metabolism to 6 beta, 7 beta-epoxy-CAN by TAO and erythromycin, and its induction by DEX and PCN, suggest involvement of Cyt P-450IIIA, which is in turn inhibited by SP and 7 alpha-thio-spirolactone.