Safety of therapeutic ultraviolet C light application at the limbus

Abstract

Purpose : Preclinical studies have confirmed the safety and efficacy of low-intensity ultraviolet C (UVC) light in managing murine bacterial keratitis (15s of 265 nm UVC with 1.93 mW/cm² intensity, dose = intensity x time). Concerns surrounding the perceived risk to limbus from UVC light exposure, on account of the limbal tissue housing the corneal stem cells, have yet to be addressed through scientific study. The current study sought to evaluate the depth and density of UVC-mediated DNA damage within the cornea following exposure to a range of UVC doses to establish the safety profile of UVC as a potential anti-infective treatment.

Methods : Full-thickness porcine corneas were exposed to fixed-intensity UVC light for 5-60 mins or remained unexposed for the equivalent duration (control) (each, n=6). The tissue was then frozen, dissected, and analysed using immunohistochemistry. Primary and secondary antibodies were used to locate cyclobutane pyrimidine dimers (CPDs) as indicators of UVC-mediated DNA damage, and confocal microscopy was performed. Images were analysed using ImageJ software. Outcome measures included 1) the depth of CPD distribution in the cornea and 2) the proportion of epithelial cells exhibiting CPD defects. Validation of the findings in human corneal buttons (n=3) was sought following a similar study protocol.

Results : In the UVC group, CPDs were distributed superficially only within the corneal epithelium, irrespective of the UVC dose applied (total corneal depth vs depth of penetration, p < 0.001 across all doses). The total thickness of the corneal epithelium (mean ± SD) in the UVC group was 38.9±18.9 µm, and the maximum depth of CPD distribution was 11.4±7.3 µm, 12.9±4.4 µm, 10.1±5.2 µm, 13.5±5.2 µm, and 16.4±11.8 µm for 5, 10, 15, 30, and 60 min UVC exposure, respectively. The percentage CPD (mean ± SD) within the corneal epithelium did not differ across the range of UVC doses, at 47.8% ± 25.6, 58.5 ± 16.2, 39.9 ± 26.4, 41.3 ± 27.3, and 38.9 ± 28.3, respectively (p>0.05). The distribution of CPD in the human cornea was similarly limited to the superficial epithelium.

Conclusions : UVC-mediated DNA defects were limited to the superficial cornea and spared deeper tissue, where stem cells are located, even after one hour of UVC exposure. These data lend further evidence of the safety of low-intensity UVC for ocular therapeutic applications.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.