Difference in metabolic profile of potassium canrenoate and spironolactone in the rat: mutagenic metabolites unique to potassium canrenoate.

The metabolic fates of potassium canrenoate (PC) and spironolactone (SP) were compared for the rat in vivo and in vitro. Approximately 18% of an in vivo dose of SP was metabolized to canrenone (CAN) and related compounds in the rat. In vitro, 20-30% of SP was dethioacetylated to CAN and its metabolites by rat liver 9000 g supernatant (S9). Thus, the major route of SP metabolism is via pathways that retain the sulfur moiety in the molecule. PC was metabolized by rat hepatic S9 to 6 alpha, 7 alpha- and 6 beta, 7 beta-epoxy-CAN. The beta-epoxide was further metabolized to its 3 alpha- and 3 beta-hydroxy derivatives as well as its glutathione (GSH) conjugate. Both 3 alpha- and 3 beta-hydroxy-6 beta, 7 beta-epoxy-CAN were shown to be direct acting mutagens in the mouse lymphoma assay, whereas 6 alpha, 7 alpha- and 6 beta, 7 beta-epoxy-CAN were not. These mutagenic metabolites, their precursor epoxides and their GSH conjugates were not formed from SP under identical conditions. The above findings appear to be due to inhibition of metabolism of CAN formed from SP by SP and/or its S-containing metabolites, since the in vitro metabolism of PC by rat hepatic microsomes was appreciably reduced in the presence of SP. The hypothesized mechanism(s) for this inhibition is that SP and its S-containing metabolites specifically inhibit an isozyme of hepatic cytochrome P-450 or SP is a preferred substrate over PC/CAN for the metabolizing enzymes. Absence of the CAN epoxide pathway in the metabolism of SP provides a possible explanation for the observed differences in the toxicological profiles of the two compounds.