Inhibitory and fungicidal effects of antifungal drugs against Aspergillus species in the presence of serum

Abstract

Given the high protein binding rates of antifungal drugs and the effect of serum proteins on Aspergillus growth, we investigated the in vitro pharmacodynamics of amphotericin B, voriconazole, and three echinocandins in the presence of human serum, assessing both inhibitory and fungicidal effects. In vitro inhibitory (IC) and fungicidal (FC) concentrations against 5 isolates of Aspergillus fumigatus, Aspergillus flavus, and Aspergillus terreus were determined with a CLSI M38-A2-based microdilution method using the XTT methodology after 48 h of incubation at 35 °C with a medium supplemented with 50% human serum. In the presence of serum, the IC and FC of amphotericin B and the IC of echinocandins were increased (1.21- to 13.44-fold), whereas voriconazole IC and FC were decreased (0.22- to 0.90-fold). The amphotericin B and voriconazole FC/IC ratios did not change significantly (0.59- to 2.33-fold) in the presence of serum, indicating that the FC increase was due to the IC increase. At echinocandin concentrations above the minimum effective concentration (MEC), fungal growth was reduced by 10 to 50% in the presence of human serum, resulting in complete inhibition of growth for some isolates. Thus, the in vitro activities of amphotericin B and echinocandins were reduced, whereas that of voriconazole was enhanced, in the presence of serum. These changes could not be predicted by the percentage of protein binding, indicating that other factors and/or secondary mechanisms may account for the observed in vitro activities of antifungal drugs against Aspergillus species in the presence of serum.

Fig 1

Concentration-effect curves of amphotericin B (A), voriconazole (B), and anidulafungin (C) with (open symbols and dashed regression lines) and without (closed symbols and solid regression lines) 50% serum based on inhibitory (circles and black lines) and fungicidal (squares and gray lines) activities against an Aspergillus flavus isolate. Error bars represent standard errors of the means, and lines are the regression lines of the E_max model. No fungicidal activity was found for anidulafungin. Horizontal dotted lines represent the level of 10% growth. Horizontal black and gray arrows represent the shift of inhibitory and fungicidal concentration-effect curves, respectively, whereas the vertical dotted arrow in panel C represents the reduction of the minimal growth E_min at supra-MEC concentrations.