Evaluation of Mycograb®, amphotericin B, caspofungin, and fluconazole in combination against Cryptococcus neoformans by checkerboard and time-kill methodologies

At an end point of ≥50% inhibition, the MIC-2s varied from 16 to 128 μg/mL for Mycograb® and from 0.125 to 16 μg/mL for fluconazole. The fractional inhibitory concentration index classified the combination as indifferent for 5 isolates, additive for 3 more isolates, and synergistic in a single isolate. Time-kill analysis on 2 isolates (F/7844 and F/10156), which had synergistic and additive results with amphotericin B, respectively, on checkerboard was performed with 4-16 μg/mL of Mycograb®, 2-8 μg/mL of fluconazole, and 0.0625-2 μg/mL of amphotericin B. This demonstrated an increasingly static effect with augmenting concentrations of fluconazole and an initial static effect with amphotericin B at lower concentrations, which became fungicidal as the level of drug increased. The addition of either 4 or 8 μg/mL of Mycograb® to 0.5 μg/mL of amphotericin B with C. neoformans F/7844 changed a static effect to a fungicidal effect at 8 h with an increased killing of 1.2 logs at 48 h. With C. neoformans F/10156, the addition of 16 μg/mL of Mycograb® to 0.25 μg/mL of amphotericin B produced a difference in killing from 1 logarithm after 4 h to 1.5 logarithms after 48 h. These data suggest that the combination of amphotericin B and Mycograb® would be worth exploring in the treatment of infection due to C. neoformans.