OBJECTIVES: Patients suffering from invasive mycoses often receive concomitant antifungal therapy and antibacterial agents. Ciprofloxacin, a carboxyfluoroquinolone, was previously observed to demonstrate the pharmacodynamic interactions with antifungal agents by altering their growth inhibitory activity against Candida albicans and Aspergillus fumigatus. However, little is known about the interaction between other extended-spectrum fluoroquinolones, such as levofloxacin and moxifloxacin, and antifungal agents against C. albicans and A. fumigatus.METHODS: Using a microdilution chequerboard technique, we employed isobolographic analysis adapted to incorporate a non-active agent in order to analyse the potential in vitro interaction between ciprofloxacin, levofloxacin or moxifloxacin and the following representative antifungal agents: amphotericin B, fluconazole or voriconazole and caspofungin.RESULTS: Synergistic interactions [interaction indices (Iis) 0.69-0.83, P < 0.05] were observed between amphotericin B (0.07-0.31 mg/L) and either ciprofloxacin (0.19-7.65 mg/L) or levofloxacin (0.41-32.88 mg/L) against C. albicans and A. fumigatus. Synergy (Iis 0.56-0.87, P < 0.05) also was found between voriconazole (0.09-0.14 mg/L) and ciprofloxacin (0.22-11.41 mg/L) as well as between caspofungin (8.94-22.07 mg/L) and levofloxacin (0.14-5.17 mg/L) against A. fumigatus. Some antagonistic (Iis 1.16-1.29, P < 0.05) interactions were observed between fluoroquinolones and fluconazole against C. albicans. In general, ciprofloxacin enhanced the activity of antifungal agents more than moxifloxacin and levofloxacin against both C. albicans and A. fumigatus.CONCLUSIONS: The knowledge of the pharmacodynamic interactions between fluoroquinolones and antifungal agents may guide selection and potentially improve the outcome of immunosuppressed patients with concurrent bacterial and fungal infections.