Exploring the potential of UVC in treating superficial corneal infections

Abstract

Microbial Keratitis (MK) is an acute, sight-threatening corneal infection. First line treatment for bacterial keratitis (BK) comprises topical antibiotics, either a dual therapy of fortified cephalosporin and aminoglycoside, or single fluoroquinolone therapy. Surveys describe increasing bacterial resistance to antibiotics, necessitating exploration of alternative treatments. Ultraviolet C (UVC) light possesses potent broad spectrum antimicrobial activity against prokaryotic cells; bacteria, fungi, protozoa, and viruses, without demonstrating microbial resistance. Eukaryotic (mammalian) cells have an innate resistance to UVC through more evolved DNA repair mechanisms, but fear of carcinogenicity stemming from observed effects of cumulative doses of broad spectrum light (primarily UVB), has limited evaluations of UVC as an option for treating corneal infection. This research first explored the local MK landscape, to provide context for the experimental work. Secondly, while balancing the risk of UVC against the threat of corneal scarring secondary to the infection / inflammation process, the research sought to determine a safe and effective UVC dose for managing corneal infection, through a series of in vitro and ex vivo experiments, and ultimately in vivo experiments, using a developed mouse model of keratitis. Methods: Demographic, clinical and microbiological characteristics of hospitalised MK cases were evaluated over two 2- year periods; 2006/2007 and 2013/2014, and compared with published findings from 1999/2000. In UVC efficacy experiments, the lowest effective antimicrobial dose of 265 nm wavelength UVC, in in vitro and ex vivo settings, was determined by quantifying, with an In Vivo Imaging System, the effect of controlled UVC exposure to a virulent strain of bioluminescent Pseudomonas aeruginosa. In safety assessments, cultured ex vivo human corneal epithelial cells exposed to UVC at this dose underwent immunocytochemical analysis, to quantify DNA damage in the form of Cyclobutane Pyrimidine Dimer (CPD) formation. In the in vivo keratitis model, daily UVC exposure was assessed for treatment efficacy as well as safety, with regard to keratocyte number, inflammation and CPD formation. UVC effects were compared to ciprofloxacin as an established therapy, a combined UVC and ciprofloxacin therapy, and no treatment as control. Results: In Auckland, MK incidence rose consistently over the assessed time period, by approximately 25-30% every seven years. Recent results suggest that over time post-treatment visual outcomes are improving, less time is spent in hospital but the prevalence of P. aeruginosa is rising. Antibiotic efficacy patterns remained generally stable except for cefuroxime which demonstrated only 37% efficacy. In the in vivo keratitis model, daily application of 15 s UVC (28.95 mJ/cm2) for two consecutive days (2 doses) effectively managed an established corneal infection. UVC was, however, unable to control severe infections with once daily exposure. UVC treatment, at this dose, was deemed safe in terms of CPD formation and inflammation but effected a 12% reduction in keratocyte number.