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S.B. Kaye, T. Neal, J. Szkurlat, S. Anderson, S. Bamber, M. Batterbury, G. Smith; Biological assay ofantimicrobial activity across the cornea. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):4929.
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Purpose:To evaluate a biological assay to measure corneal penetration of topically applied antimicrobials Methods:30 patients undergoing penetrating keratoplasty were included. Patients received either topical ciprofloxacin 0.3% or teicoplanin 1%, ½ hourly for 6 hours prior to surgery. Tear samples were collected from both eyes 15’, 30’ and 60’ minutes after the last drop. The central corneal disc was divided into 4 quadrants. Two of the quadrants were placed on isosensitest agar plates inoculated with a lawn of Staphylococcus aureus (NCTC 6571); one epithelium side down and the other endothelium side down. After overnight incubation zones of inhibition were measured and compared to zone diameters from standard concentrations of antimicrobials. The remaining quarters and a sample of aqueous were stored for HPLC analysis of the antimicrobial concentration and histological analysis. Results:Apart from controls, zones of inhibition were observed in all samples. Zones size diameters decreased with time from last instillation. Antimicrobial concentrations reduced in the tear film from last instillation (teicoplanin: 2.2mg/L to 0.8mg/L) but showed good retention at the epithelial surface particularly for ciprofloxacin. For teicoplanin and ciprofloxacin, the ratio of antibacterial effects of the intact epithelium to that of the tear sample at surgery was 0.92 (1.32) and 1.85 (2.52), respectively. The ratio of the antibacterial effect of teicoplanin and ciprofloxacin across the epithelium to that across the endothelium was 0.76 (0.37) and 1.02 (0.58), respectively. Conclusions:The biological method described allows determination of the antimicrobial activity of topically antibimicrobials across the cornea. The model used to biologically assay topical antimicrobial penetration across the cornea, showed both good penetration and retention of ciprofloxacin across the cornea, as reflected in the ratios of lysis zones produced by antimicrobials at the endothelial and epithelial surfaces and tear film. Although penetration across the cornea occurred with teicoplanin, it crossed the intact cornea less well than ciprofloxacin. This is most likely attributable to the larger size of the teicoplanin molecule.
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