Mechanism for the oxidative defluorination of flunisolide

Flunisolide (6 alpha-fluoro-11 beta, 16 alpha, 17 alpha, 21-tetrahydroxypregna-1,4-diene-3,20-dione-16,17-acetonide) was converted to 6 beta,- 11 beta, 16 alpha, 21-pentahydroxypregna-1,4-diene-3,20-dione-16,17-acetonide (6 beta-OH metabolite) by mouse liver microsomes, but no activity was observed with mouse lung, intestine or kidney microsomes. Two additional metabolites of flunisolide also formed by mouse hepatic microsomes were identified by mass spectral analysis to be 11 beta, 16 alpha, 17 alpha,21-tetrahydroxypregna-1,4-diene-3,6,20-trione-16,17-acetonide (6-keto metabolite) and delta 6-flunisolide. The formation of all three metabolites required NADPH, was inhibited by carbon monoxide and was stimulated by pretreating mice with phenobarbital. A time-couse study suggested the 6-keto metabolite was an intermediate in the formation of the 6 beta-OH metabolite. When added to microsomes, the 6-keto metabolite was converted to the 6 beta-OH metabolite by a carbon monoxide-insensitive enzyme. Our results suggest the conversion of flunisolide to the 6 beta-OH metabolite is catalyzed by a multi-enzyme pathway via a stable intermediate, the 6-keto metabolite. The initial reaction which leads to the formation of the 6-keto metabolite is catalyzed by a cytochrome P-450-mediated microsomal monoxygenase(s), but the reduction of the 6-keto metabolite to the 6 beta-OH metabolite is cytochrome P-450-independent.