Identification of Brimonidine as a Novel Substrate of Organic Cation Transporters OCT2 and MATE1 Expressed in Human Eye

Purpose: Brimonidine, a selective α2-adrenergic agonist, is widely used in topical therapy for ocular hypertension or primary open-angle glaucoma. However, effective delivery is hindered by multiple ocular surface barriers. Because brimonidine is a cationic drug, we hypothesized that organic cation transporters (OCTs) and multidrug and toxin extrusion proteins (MATEs) are involved in brimonidine uptake in the human eye. This study aimed to determine if brimonidine is transported by OCT1, OCT2, OCT3, MATE1, and MATE2K and if the identified transporters are localized in anterior eye structures. Methods: Uptake studies were performed using HEK293 cells stably expressing OCT1, OCT2, OCT3, MATE1, and MATE2K. Intracellular brimonidine accumulation was analyzed by mass spectrometry. Immunohistochemistry of glaucomatous human eyes was used to localize relevant transporters in anterior ocular structures. Results: Brimonidine was transported by OCT2 and MATE1 but not by OCT1, OCT3, or MATE2K. Uptake was time- and concentration-dependent. Both OCT2 and MATE1 were expressed in the cornea, the conjunctiva, and the ciliary body. Conclusions: These results imply that OCT2 and MATE1 may play a role in brimonidine uptake into the human eye and may contribute to the interindividual variability of brimonidine concentrations and effects.