Brimonidine concentrations in the eye tissues of pigmented rabbits were determined following single ocular administration of 0.1% brimonidine tartrate ophthalmic solution at pH 7.3. Binding affinity of brimonidine to melanin and melanin content in the retina/choroid of pigmented rabbits was also examined. The concentration of free brimonidine which did not bind to melanin in the retina/choroid was calculated using the binding parameters to melanin.
Topically applied brimonidine rapidly distributed to intraocular tissues. The elimination rate from melanin-containing tissues such as the iris/ciliary body and retina/choroid was slower than the aqueous humor and vitreous body in pigmented rabbits. In both the anterior and posterior retina/choroid, the free brimonidine concentrations were over 100-fold lower than the total concentrations. The concentrations in the vitreous body closely matched to the free concentrations in the posterior retina/choroid. Simulated free concentrations in the posterior retina/choroid were gradually increased when 0.1% solution was instilled twice daily.
The present data indicated that the brimonidine concentration in the vitreous body was comparable to the free concentration in the posterior retina/choroid. This suggests that the vitreous concentration can be a surrogate indicator of the free brimonidine concentration in the posterior retina/choroid. From the present findings, it is expected that multiple instillation of brimonidine tartrate ophthalmic solution may produce the sufficient free concentration for activation of the α2-adrenergic receptor in the retina/choroid in human.