In vitro covalent binding of 14C-mibolerone to rat liver microsomes

Mibolerone (17-Hydroxy-7,17-dimethylestr-4-en-3-one; 7 alpha-17 alpha dimethyl-19-nortestosterone) is being marketed by The Upjohn Company for the inhibition of estrus in bitches. The aim of this study was to determine the extent of covalent binding of mibolerone to rat liver microsomes. Liver microsomes were obtained from Control and phenobarbitol-treated female Fisher rats, and were incubated with 14C-mibolerone at 37 degrees C for 10 minutes. No covalent binding to macromolecules was observed when 14C-mibolerone was incubated with rat liver microsomes. Under identical conditions, 14C-estradiol was covalently bound to macromolecules. Slightly higher covalent binding of estradiol was observed with microsomes from phenobarbitol-treated rats. Ascorbic acid and glutathione inhibited covalent binding of estradiol to macromolecules in the in vitro microsomal system.