In vitro susceptibility of filamentous fungal isolates from a corneal ulcer clinical trial

Abstract

Purpose: To describe the minimum inhibitory concentration (MIC) of fungal isolates to natamycin and voriconazole, and to compare these MICs to previous ocular susceptibility studies.

Design: Experimental laboratory study using isolates from a randomized clinical trial.

Methods: The Mycotic Ulcer Treatment Trial I was a randomized, double-masked, multicenter trial comparing topical natamycin and voriconazole for fungal keratitis treatment. Susceptibility testing to natamycin and voriconazole were performed according to Clinical and Laboratory Standards Institute methods. The relationship between organism and MIC was assessed. A literature review was performed to compare results to previous ocular susceptibility studies.

Results: Of the 323 patients enrolled in the trial, MICs were available for 221 (68%). Fusarium (n = 126) and Aspergillus species (n = 52) were the most commonly isolated organisms. MICs to natamycin and voriconazole were significantly different across all genera (P < .001). The MIC median (MIC50) and 90th percentile (MIC90) for natamycin were equal to or higher than voriconazole for all organisms except Curvularia species. Compared to other organisms, Fusarium species isolates had the highest MICs to voriconazole and Aspergillus flavus isolates had the highest MICs to natamycin. Our results were similar to previous reports except that the voriconazole MIC90 against Aspergillus species was 2-fold higher and the natamycin MIC90 against Aspergillus fumigatus was 4-fold higher in our study.

Conclusion: In this large susceptibility study, Fusarium isolates were least susceptible to voriconazole and A flavus isolates were least susceptible to natamycin when compared to other filamentous fungi. In the future, susceptibility testing may help guide therapy if performed in a timely manner.

FIGURE. Percentage of Fungal Isolates Inhibited at Different Drug Concentrations in a Fungal Keratitis Clinical Trial

Percentage of fungal isolates inhibited at increasing concentrations of natamycin (Top) or voriconazole (Bottom). Organisms include Fusarium species (N=126), Aspergillus flavus (N=32), Aspergillus fumigatus (N=10), Curvularia species (N=17), and all other fungal species (N=36). Horizontal black lines represent the threshold for the minimum inhibitory concentration median (MIC₅₀) and 90^th percentile (MIC₉₀). *Vertical grey dashed line (Bottom) represents the proposed epidemiological cut-off value (ECV) of 1μg/mL using Clinical and Laboratory Standards Institute methodology for A. flavus and A. fumigatus against voriconazole.^, The ECV distinguishes wild-type strains (exhibit no acquired resistance to the drug in question) from non-wild-type strains. Non-wild-type strains have MICs greater than the ECV and may have acquired resistance mechanisms.

FIGURE. Percentage of Fungal Isolates Inhibited at Different Drug Concentrations in a Fungal Keratitis Clinical Trial

Percentage of fungal isolates inhibited at increasing concentrations of natamycin (Top) or voriconazole (Bottom). Organisms include Fusarium species (N=126), Aspergillus flavus (N=32), Aspergillus fumigatus (N=10), Curvularia species (N=17), and all other fungal species (N=36). Horizontal black lines represent the threshold for the minimum inhibitory concentration median (MIC₅₀) and 90^th percentile (MIC₉₀). *Vertical grey dashed line (Bottom) represents the proposed epidemiological cut-off value (ECV) of 1μg/mL using Clinical and Laboratory Standards Institute methodology for A. flavus and A. fumigatus against voriconazole.^, The ECV distinguishes wild-type strains (exhibit no acquired resistance to the drug in question) from non-wild-type strains. Non-wild-type strains have MICs greater than the ECV and may have acquired resistance mechanisms.