In vitro transcorneal penetration of metronidazole and its potential use as adjunct therapy in acanthamoeba keratitis

Abstract

To investigate the in vitro permeation of metronidazole through rabbit and human corneas in the presence and absence of 0.01% benzalkonium chloride and to suggest its use as adjunct therapy in the treatment of Acanthamoeba keratitis.Metronidazole permeation through rabbit and human corneas, the latter being unsuitable for transplantation, were used for all permeability experiments. Flux rates for metronidazole from 0.5% and 1.0% aqueous solutions in the presence and absence of 0.01% benzalkonium chloride (BZCL) were determined. ANOVA and the Duncan multiple range test were used to test for steady state and an unpaired t test with the Welch correction was used to test for differences between the mean flux values at each time point. A significance level of 5% was used for all the statistical tests. In the clinical cases described, 0.5% aqueous solution was used.Steady-state flux rates for metronidazole from 0.5% and 1.0% solutions across both rabbit and human corneas were achieved after 6 and 4 hours, respectively. No statistically significant differences were obtained in the presence and absence of 0.01% BZCl (P < 0.05) between the steady state flux values at both concentrations of metronidazole of human and rabbit corneas, except for 0.5% metronidazole across rabbit corneas. Flux rates of metronidazole across human corneas were 12%-33% higher than those across rabbit corneas. For both rabbit and human corneas, flux rates of metronidazole from 0.5% and 1.0% solutions were reduced by between 4% and 11%, respectively, in the presence of 0.01% benzalkonium chloride.Although statistically significant differences in flux values were obtained between human and rabbit corneas, the study supports the suitability of the in vitro rabbit cornea as a model for investigating permeation of drugs through human corneas. However, direct extrapolation of animal data to humans must be approached cautiously. The metronidazole from a 1% solution had a steady-state flux rate approximately double that from the 0.5% solution. Higher concentrations of up to 1% may be considered for clinical use for treating Acanthamoeba keratitis infections. It would appear to be prudent to omit benzalkonium chloride as a preservative from preparations of metronidazole formulated for topical ophthalmologic use. Early clinical experience with the topical solution as adjunct therapy in the treatment of Acanthamoeba keratitis is encouraging.