Evaluating Photochemical Interactions Between Rose Bengal Photodynamic Antimicrobial Therapy and Topical Antifungals in Fungal Keratitis Isolates

Purpose: The purpose of this study was to evaluate the in vitro effects of combining rose bengal (RB) photodynamic antimicrobial therapy (PDAT) with conventional topical antifungals (amphotericin B, natamycin, and voriconazole) on clinical Fusarium and Aspergillus isolates and to determine the antifungal stability under experimental conditions. Methods: Clinical isolates of Fusarium spp. (n = 3) and Aspergillus spp. (n = 3) were cultured using standard microbiology techniques. Agar plates were prepared with three concentrations of amphotericin B, natamycin, or voriconazole along with plates containing no antifungals. The following groups were tested in both dark and light conditions: (1) antifungal monotherapy (antifungal only), (2) RB monotherapy (RB only), and (3) antifungal and RB combination therapy. Plates were incubated for 7 days and percent growth inhibition was quantified. UV-visible spectroscopy and mass spectrometry was performed to assess antifungal stability. Results: Overall, Fusarium isolates were more susceptible to RB-PDAT than Aspergillus isolates. RB-PDAT combination therapy with natamycin reduced growth inhibition, whereas combination therapy with voriconazole increased growth inhibition compared to antifungal monotherapy. UV-visible spectroscopy demonstrated 70% degradation of amphotericin B and full degradation of natamycin when combined with RB-PDAT, whereas voriconazole remained stable. Conclusions: Across both Fusarium and Aspergillus isolates, treatment responses to antifungals with or without RB-PDAT demonstrated strain-specific effects. The increased fungal inhibition and maintained stability of voriconazole when combined with RB-PDAT highlight its potential compatibility with RB-PDAT. In contrast, the reduced inhibition and degradation of natamycin and amphotericin B with RB-PDAT suggests that drug stability may be a factor to consider when integrating RB-PDAT into fungal keratitis management.