Investigating the susceptibility profiles and in vitro combinations of caspofungin, itraconazole, fluconazole, voriconazole, clotrimazole, and amphotericin B against clinical isolates causing fungal keratitis

Fungal keratitis is a global cause of blindness, highlighting the need to assess antifungal treatment efficacy before clinical use. This study evaluates the synergistic and antagonistic effects of antifungal agents on fungal keratitis isolates. Susceptibility testing of 35 corneal isolates including Aspergillus spp., Neocosmospora spp., Candida albicans, Penicillium chrysogenum was performed with broth microdilution and checkerboard methods to test six antifungal agents (caspofungin, itraconazole, fluconazole, voriconazole, amphotericin B, clotrimazole). Among Aspergillus spp., the MIC ranges were reported as 0.01–1, 0.5–16, 32–64, 0.06–16, 0.125–8, and 0.25–4 µg/mL for caspofungin, amphotericin B, fluconazole, itraconazole, voriconazole, and clotrimazole, respectively. For Fusarium/Neocosmospora spp., these ranges varied to 0.01–8, 0.125–4, 64, 0.03–16, 0.25–8, and 0.5–8 µg/mL for the same drugs. In Candida spp., the MIC ranges were 0.01–0.03, 0.25, 64, ≥16, ≥16, and 0.5–1 µg/mL for the same antifungal agents. Notably, the combination of caspofungin and amphotericin B exhibited 100% synergistic effects against Aspergillus spp., while combinations of clotrimazole, fluconazole, or voriconazole with amphotericin B showed a 50% synergistic effect against Fusarium/Neocosmospora spp. Furthermore, fluconazole combined with voriconazole or itraconazole, as well as voriconazole and itraconazole, demonstrated 100% synergism against Candida spp. The results indicate that caspofungin is the most effective antifungal agent, while fluconazole exhibited the highest rates of resistance. The combination of caspofungin and amphotericin B proved most effective against Aspergillus spp., azole compounds combined with amphotericin B were most effective against Fusarium/ Neocosmospora spp., and combinations of fluconazole with voriconazole or itraconazole and voriconazole and itraconazole demonstrated synergistic effects against Candida spp.