Ocular and systemic pharmacokinetics of brimonidine and brinzolamide after topical administration in rabbits: comparison between fixed-combination and single-drug formulations

Aim The aim of this study was to compare the ocular and systemic absorption of brimonidine (BMD) and brinzolamide (BZM) in rabbits after the topical administration of a fixed-combination ophthalmic suspension of 0.1% BMD tartrate and 1% BZM (FCBB) with that after the administration of the respective single-drug formulations. Materials and Methods Ocular and systemic drug absorption was estimated by determining BMD and BZM concentrations in the aqueous humor, retina/choroid, vitreous body, and blood/plasma by liquid chromatography/tandem mass spectrometry after the administration of FCBB, 0.1% BMD tartrate ophthalmic solution (0.1% BMD), or 1% BZM ophthalmic suspension (1% BZM) to rabbits. Results In concomitant administration, instilling 0.1% BMD and 1% BZM successively without interval lowered aqueous humor concentrations of both drugs compared to those observed with a 5-min interval. After FCBB administration, BMD and BZM concentrations in the aqueous humor were comparable with those observed after the administration of 0.1% BMD and 1% BZM, whereas BMD concentrations in posterior ocular tissues were equal to or higher than those observed after 0.1% BMD. Plasma BMD concentrations following the administration of FCBB were 0.8-fold lower than those after 0.1% BMD; no remarkable differences were observed in blood BZM concentrations for both formulations. Conclusions FCBB achieved drug distribution in the aqueous humor and systemic exposure that were comparable to those for the single-drug formulations. The viscosity of FCBB may increase BMD distribution in the retina/choroid. The administration interval affects ocular drug absorption with the concomitant administration of 0.1% BMD and 1% BZM, which can be overcome by using the fixed-combination of both drugs.