The mRNA expression levels of BCRP and other isoforms of MRPs: MRP1 and MRP3 have been reported in the human cornea, though functional activity and localization still remains to be assessed (32). inhibit efflux pumps on the cornea. Such inhibitors could significantly elevate the cellular concentration of the drug in the cornea as well as the aqueous humor. However, specific efflux modulators may cause significant toxicity at doses needed to cause efflux modulation and are not therapeutically acceptable (21). Hence, a dual advantage could be achieved if efflux inhibitors had a therapeutic effect which is relevant in the treatment regimen, in addition to their primary role of modulating efflux. Erythromycin, a broad spectrum antibiotic used to treat superficial bacterial infections of the cornea and conjunctiva (brand name: Ilotycin?) was selected as the drug substrate for our study (22,23). Bacterial infections are invariably associated with inflammation of the eye. For this reason, corticosteroids were chosen as inhibitors for our study. In addition to modulation of efflux, these compounds can also elicit anti-inflammatory action in a relevant anti-bacterial treatment regimen. functional activity of P-gp has already been reported with testosterone as a model inhibitor (20). Though the functional activity of MRP2 on human and rabbit corneal epithelial cells has been reported, its ability to modulate drug concentrations across cornea has not been established in an setting. Moreover, it is essential to NNC0640 determine if therapeutically relevant corticosteroids when co-administered with erythromycin can inhibit both P-gp and MRP mediated efflux in the corneal epithelium. These objectives require determining the pharmacokinetics of erythromycin following topical co-administration with MK571 (a specific MRP inhibitor) and steroids. Unfortunately, there are few drawbacks associated while determining the disposition of drugs applied topically. Several pharmacokinetic models have been proposed to predict absorption and disposition of drugs applied topically to the eye, but all involve NNC0640 varying complexities with regard to numerical analyses (24C26). Another major constraint is the inaccessibility of aqueous humor for serial sampling. Conventional pharmacokinetic studies require sacrificing at least six animals per time point and as such the numbers would drastically increase depending on the number of time points required to develop a complete pharmacokinetic profile. To simplify the approach and to estimate ocular disposition of topically NNC0640 applied drugs, we conceptualized CPB2 the combination of a topical well infusion model and aqueous humor microdialysis sampling. In the topical well infusion model, a constant level of drug is maintained over the cornea with the help of a plastic cylindrical well such that the effect of tear dynamics is minimized and simpler equations can be applied independent of compartmental modeling (27). Absorption through tissues such as conjunctiva NNC0640 and lacrimal glands could be eliminated which helps estimating the corneal absorption rate constant, precisely. Problems involved with serial sampling of ocular fluids could be overcome by utilizing microdialysis which is superior over conventional sampling techniques in determining ocular pharmacokinetics by both reducing the number of subjects and providing statistically robust data (28). Therefore, the objectives of this study were (i) to determine if steroids could inhibit both P-gp and MRP2 mediated efflux of erythromycin, (ii) to evaluate the role of MRP2 in modulating corneal drug absorption and (iii) to assess the role of steroids as potential co-administering agents to enhance corneal drug absorption of actively effluxed drugs, such as erythromycin. Materials And Methods Materials MPL, PL, PS and cyclosporine A (CsA) were purchased from Sigma-Aldrich (St. Louis, MO). MK-571, a specific inhibitor of MRP was procured from Biomol International (Plymouth Meeting, PA). GF120918 was a generous gift from GlaxoSmithKline Ltd. [14C] Erythromycin (specific activity 48.8 mCi/mmol) was obtained from PerkinElmer Life and Analytical Sciences (Boston, MA). Stock solutions of steroids (20 mg/ml), CsA (1 mg/ml), GF120918 (1 mg/ml) and MK-571 (25 mg/ml) were.