Abstract
Purpose::
Fluorometholone (FM), an ophthalmic steroid drug, does not have marked effect on intraocular pressure (IOP) compared with other ophthalmic steroids. This may be due to a rapid formation of inactive metabolites, 20α and 20ß-dihydrofluoromethorone (20α and 20ß-DHFM), in cornea. It has been reported that the metabolism proceeds stereoselectively in a species-specific manner. The metabolite in human aqueous humor is 20ß-DHFM, whereas rabbits generate 20α-DHFM. However, the each absolute configuration of the 20α and 20ß-DHFM reference standards has not been fully determined. Therefore, we synthesized DHFMs and elucidated their configurations using X-ray crystallography (X-ray) analysis. Next, we investigated the stereoselectivity of 20-DHFM formation in rabbit and cynomolgus monkey cornea as preclinical species. The metabolism in human immortalized corneal cell line was also investigated.
Results::
X-ray and nuclear magnetic resonance (NMR) analyses revealed that the absolute configuration of 20-DHFMs reported in the previous paper (Iqbal et al. J Pharm Biomed Anal. 1993;11:1115) was determined as opposite configurations. Therefore, the metabolite in human in vivo should be 20α-DHFM, which was also formed selectively in both rabbit and cynomolgus cornea incubations. Unexpectedly, the incubation with human corneal cell line yielded 20ß-DHFM stereoselectively. The inhibition rate of 20α-DHFM against the binding of [3H] dexamethasone to glucocorticoid receptor (GCr) was determined as 82% at 100 nM.
Conclusions::
There was no difference in the corneal metabolism of FM between human, cynomolgus and rabbit, which generate 20α-DHFM steroselectively. The rapid formation of 20α-DHFM, which demonstrates about 100-fold less GC receptor binding than FM (IC50 = 0.31 nM), may reduce the incidence of elevation of IOP. Since corneal metabolism of 20-ketosteroids does not show significant species differences, cynomolgus and rabbit are appropriate for the animal models to evaluate the efficacy and toxicity of these compounds. The human immortalized corneal cell line is inadequate for the prediction of metabolite in human in vivo.
Keywords: metabolism • cornea: basic science • inflammation