Pharmacokinetic Features of Difluprednate Ophthalmic Emulsion in Rabbits as Determined by Glucocorticoid Receptor-Binding Bioassay

Abstract

Difluprednate (6α,9-difluoro-11β,17,21-trihydroxy-1,4-pregnadiene-3,20-dione 21-acetate 17-butyrate, DFBA) has long been used as an anti-inflammatory dermatological agent. The main objectives of the current study were to evaluate the pharmacokinetic and pharmacodynamic features of DFBA when used as an ophthalmic agent, and to compare these features with those of other common ophthalmic agents, to determine which has the highest activity. A glucocorticoid (GC) receptor-binding test was performed to evaluate GC receptor-binding activity (GCRBA, the index of pharmacological effect). Using this information, we calculated dose-response curves, IC(50) values, and K(d) values to evaluate each drug's K(i) value. Finally, we performed studies in live rabbits to compare the activity of 4 formulations [0.002%, 0.01%, or 0.05% DFBA, or an ophthalmic solution of 0.1% betamethasone sodium phosphate (BMP)] at 4 time points (0.5, 1, 2, 4 h). At each time point, blood and eye samples were taken so that C(max) (the maximum equivalent concentration of the active DFBA metabolite, DFB), T(max) (the time at which C(max) was measured), and the area under the concentration-time curve could be compared across the 4 formulations. BMP had the highest K(i) value (8.4 × 10(-8) nmol/L), whereas DFB had the lowest (6.1 × 10(-11) nmol/L). The GCRBA of DFBA was intermediate to these 2 values (7.8 × 10(-10) nmol/L). Instillation of the DFBA ophthalmic emulsion in the eyes of rabbits led to dose-dependent increases in GCRBA, which was mostly attributable to the activity of DFB. The 0.05% DFBA ophthalmic emulsion elicited the greatest response in both aqueous humor and iris/ciliary body tissues, though there were no significant dose-dependent differences in GCRBA in plasma samples. The 0.05% DFBA ophthalmic emulsion appears to be an effective and safe anti-inflammatory treatment in ocular tissues. It is comparable, and possibly even superior, to the 0.1% BMP solution, and may be particularly useful in cases of severe disease where treatment with BMP solution alone is insufficient.