Potent synergistic effect of doxycycline with fluconazole against Candida albicans is mediated by interference with iron homeostasis

Doxycycline was found to act synergistically with the antifungal fluconazole against Candida albicans. Combination with doxycycline converts fluconazole from fungistatic to fungicidal, prevents the onset of drug resistance, and is also effective against a clinical isolate characterized by elevated resistance to fluconazole. Investigation on the interactions between the two drugs by way of checkerboard assays indicated that doxycycline had an influence on the MIC for fluconazole, as defined by CLSI standards, only at high concentrations (200 μg/ml). However, lower concentrations were effective in eliminating residual cell growth at supra-MIC concentrations of fluconazole. Using MIC-0, defined as a drug combination resulting in optically clear wells, as an endpoint, doxycycline was found to be synergistic with fluconazole at a concentration as low as 25 μg/ml, with a fractional inhibitory concentration index < 0.5.Doxycycline-mediated growth inhibition can be reversed by externally added iron, indicating that iron depletion may account for the synergism. Consistently, we confirmed old literature data about iron-chelating activity of doxycycline. Synergism of fluconazole with doxycycline does not appear to be mediated by Calcineurin, since doxycycline further aggravates the susceptibility to fluconazole of mutants lacking the catalytic or the regulatory subunits of Calcineurin.Growth in the presence of fluconazole and doxycycline is restored by elevated dosage of ERG11 in Saccharomyces cerevisiae but not in C. albicans, despite full competence of the pathogen's protein to act as a suppressor in baker's yeast.We advise the C. albicans research community to keep doxycycline-mediated iron chelation under close scrutiny when performing experiments.status: publishe