Correlating Echinocandin MIC and Kinetic Inhibition of fks1 Mutant Glucan Synthases for Candida albicans : Implications for Interpretive Breakpoints

A detailed kinetic characterization of echinocandin inhibition was performed for mutant 1,3-β-d -glucan synthase enzymes from clinical isolates ofCandida albicans with nine differentFKS1 mutations resulting in high MICs. Among 14 mutant Fks1p enzymes studied, the kinetic parameters 50% inhibitory concentration andK i increased 50-fold to several thousandfold relative to those for the wild type. Enzymes with mutations at Ser645 (S645P, S645Y, and S645F) within hot spot 1 showed the most prominent decrease in sensitivity, while those with mutations at the N- and C-terminal ends of hot spot 1 generally retained greater sensitivity to all three drugs. Kinetic inhibitions by caspofungin, micafungin, and anidulafungin were comparable among thefks1 mutant enzymes, although absolute values did vary with specific mutations. Amino acid substitutions in Fks1p did not alterK m values, although some mutations decreased theV max . Given the association ofFKS1 mutations with clinical resistance, an evaluation of the kinetic parameters for the inhibition of mutant 1,3-β-d -glucan synthase as a function of the MIC enabled an independent evaluation of the recently adopted susceptibility breakpoint for echinocandin drugs. Overall, a breakpoint MIC of ≥2 μg/ml for caspofungin captured nearly 100% offks1 C. albicans strains when a kinetic inhibition rise threshold of ≤50-fold for theK i was used as a measure of susceptibility. A similar MIC breakpoint for micafungin and anidulafungin was less inclusive, and a projected MIC of ≥0.5 μg/ml was required for >95% coverage of clinical isolates. However, when MIC determinations were performed in the presence of 50% serum, allfks1 mutants showed MIC values of ≥2 μg/ml for the three echinocandin drugs. The 1,3-β-d -glucan synthase kinetic inhibition data support the proposed susceptibility breakpoint for caspofungin inC. albicans , but a lower susceptibility breakpoint (≤0.5 μg/ml) may be more appropriate for anidulafungin and micafungin. Overall, the data indicate that MIC testing with caspofungin may serve as a surrogate marker for resistance among the class of echinocandin drugs.