Identification of multidrug resistance associated proteins in human cornea---evaluation of the role of efflux transporters in ocular drug resistance

Drugs employed to treat the ocular diseases of anterior segment of the eye are administered topically. Cornea, sclera and conjunctiva are the primary sites for drug absorption. Drugs absorbed through sclera and conjunctiva are eliminated through blood circulation and drugs penetrating across the cornea directly reach the anterior segment of the eye. Hence cornea is considered a major barrier for ocular drug delivery. Tight junctions, lipophilicity and efflux transporter (P-glycoprotein (P-gp)) were earlier considered responsible for poor permeation of drugs across cornea. Apart from P-gp, ABCC family of transporters includes multidrug resistance associated proteins (MRP's), breast cancer resistant protein (BCRP) and lung resistant protein (LRP).;Recently, we have identified the presence of MRP's on human and rabbit corneal epithelial cells. The MRP's identified have been shown to cause significant resistance to the absorption of drugs across cornea. In-vitro uptake and efflux studies using rabbit corneal epithelial cells, human corneal epithelial cells, transfected cells over expressing MRP's, ex-vivo transport study using excised rabbit cornea and in-vivo ocular microdialysis technique on male New Zealand white rabbits were employed to measure the role of individual transporters in ocular drug resistance.;14C-Erythromycin, 3H-Adefovir dipivoxil and 3H-Acyclovir were employed to screen the role of individual transporters. Use of specific inhibitors for MRPs like MK571 and non-specific inhibitors like sulphinpyrazone and indomethacin have resulted in significant inhibition of MRPs and increase in ocular bioavailability. Glaucoma treating drugs, bimatoprost and latanoprost were found to be rapidly effluxed by MRPs. Breast cancer resistance protein (BCRP) was also found to be functionally active in transfected human corneal epithelial cells and resulted in resistance to the absorption of mitoxantrone across the cells.;In conclusion, use of specific and non specific MRP inhibitors has resulted in significant enhancement in permeability of drugs across corneal epithelium. For the first time we have established the molecular presence and functional expression of MRPs on the human corneal epithelium. In light of our new findings, in future, various inhibitors might be employed in ocular formulations to modulate various efflux transporters and improve anterior segment drug delivery.