Aqueous Humor Kinetics of Erythromycin After a Single-Dose Infusion: A Strategy to Improve Ocular Bioavailability by Modulating Efflux With Corticosteroids

Abstract

Purpose:: Macrolides like Erythromycin are used to treat bacterial keratitis, but their ocular bio-availability is limited by the presence of functionally active P-glycoprotein (P-gp) on rabbit corneal epithelium. Hence, the objective was to study the aqueous humor kinetics of Erythromycin and its interaction with P-gp by co-administering with corticosteroids in a rabbit model utilizing anterior chamber microdialysis.

Methods:: Male New Zealand albino rabbits were anaesthetized and linear microdialysis probes were implanted into anterior chamber of the eye and perfused with isotonic phosphate buffer saline at 2µl/min. A well was placed over the cornea using bonding cement as a surgical adhesive agent. Ten µCi/ml solution of [¹⁴C]Erythromycin with a volume of 150µl was placed in the well for 2hrs. After 2hrs the solution and the well were removed. Microdialysis samples were collected every 20mins and analyzed using liquid scintillation counter. Experiments were repeated with topical infusions of [¹⁴C]Erythromycin in presence of Cyclosporine A (20µM, positive control) and corticosteroids like Methyl Prednisolone, Prednisolone and Prednisone (each at 500µM) which act as P-gp inhibitors. Cytotoxicity studies for these inhibitors were conducted using cultured rabbit primary corneal epithelial cells.

Results:: Pharmacokinetic parameters like area under concentration-time curves (AUC_0-), C_max and C_last of [¹⁴C]Erythromycin after a single-dose infusion were 140.5±53 µg*min/ml, 0.55±0.15 µg/ml and 0.11±0.06 µg/ml respectively. Maximum inhibition of P-gp mediated efflux was observed with 500µM Methyl Prednisolone with approximately 4 times higher AUC_0- compared with control. Cmax and Clast were almost 5 and 3 times higher respectively for Erythromycin with 500 µM Methyl Prednisolone when compared to control. Pharmacokinetic parameters for Erythromycin with 500 µM Prednisolone and Prednisone showed more than 2 times higher (AUC_0-), C_max and C_last when compared to control. Rate of eliminations (k₁₀) for erythromycin and those with inhibitors were not found to be statistically different. Cytotoxicity studies showed that all inhibitors were non-toxic at the concentration used.

Conclusions:: Ocular bioavailability of Erythromycin could be improved by co-administering with corticosteroids which are anti-inflammatory agents and P-gp inhibitors. Since bacterial infections are invariably associated with inflammation, co-administration of Erythromycin with corticosteroids will impart an efficient therapeutic response.

Keywords: cornea: epithelium • keratitis • anterior segment