Metabolic acetal splitting of budesonide. A novel inactivation pathway for topical glucocorticoids.

Topical glucocorticoids usually have a high intrinsic glucocorticoid potency and may, after systemic uptake, induce side effects. The systemic inactivation of budesonide is rapid due to extensive liver biotransformation. The major metabolic pathway, 16 alpha, 17 alpha-acetal splitting, is unique for budesonide within this group of compounds. This biotransformation is catalyzed by microsomal monooxygenases and proceeds via hydroxylation and subsequent rearrangement to an intermediary ester. The ester is cleaved by hydrolysis to 16 alpha-hydroxyprednisolone and butyric acid. The hydrolysis product 16 alpha-hydroxyprednisolone has strongly reduced glucocorticoid activity.