Binding affinities of rimexolone (ORG 6216), flunisolide and their putative metabolites for the glucocorticoid receptor of human synovial tissue.

The relative binding affinities (RBA) of two locally used glucocorticoids, rimexolone and flunisolide have been measured for the glucocorticoid receptor of human synovial tissue. The non-fluorinated derivative rimexolone exhibited a binding affinity (RBA of 130) somewhat higher than that of dexamethasone (RBA of 100) but lower than that of flunisolide (RBA 190). Potential metabolites of rimexolone hydroxolated at the C17 side-chain, showed decreased binding affinities, while the 6-hydroxy metabolite of rimexolone and flunisolide (its main metabolite) and the 4,5-dihydro metabolites of rimexolone hardly bound at all. These results support previous pharmacological findings that the high ratio of local to systemic effects of both compounds are due to a pronounced receptor affinity of the parent compounds and the fast systemic metabolism to derivatives with low pharmacodynamic activity.