Serum Differentially Alters the Antifungal Properties of Echinocandin Drugs

Abstract

Antifungal efficacies of the echinocandin drugs caspofungin, micafungin, and anidulafungin were reduced significantly in the presence of 50% human serum, which yielded nearly equivalent MICs or minimum effective concentrations against diverse Candida spp. and Aspergillus spp. Consistent with a direct drug interaction, serum decreased the sensitivity of glucan synthase to echinocandin drugs.The echinocandin drugs caspofungin, micafungin, and anidulafungin inhibit the fungal ␤-1,3-glucan synthase enzyme, which blocks the formation of glucan polymers, thereby disrupting fungal cell wall integrity (2). Animal and human studies indicate that echinocandin drugs are extensively bound to serum proteins (1,4,5,15), and serum was shown to reduce the antifungal properties of micafungin with some Candida spp. (3), yet little is known about the influence of serum on antifungal efficacy with the different echinocandin drugs.Echinocandin susceptibility in the presence or absence of 50% human serum (Sigma) was evaluated with a diverse collection of clinical isolates, laboratory strains, and reference strains of Candida spp. and Aspergillus spp., according to the guidelines in CLSI documents M27-A2 (9) and M38-A (8), respectively. Abnormal colony morphology was used to establish a minimum effective concentration (MEC) for Aspergillus spp. after 48 h of incubation at 35°C (6). Glucan synthase (GS) isolation and 50% inhibitory concentration (IC 50 ) inhibition kinetics were performed as described previously (11). A murine candidiasis model utilizing female BALB/c mice (age, 10 to 12 weeks; weight, 20 to 25 g) was used to assess the relative in vivo efficacies of echinocandin drugs (13).Serum increased caspofungin MICs an average of 2-fold, with a range of 1-to 16-fold, while it had a more pronounced effect on the other drugs, increasing the MIC an average of 16-fold with a range of 8-to 256-fold for anidulafungin and an average of 64-fold with a range of 32-to 128-fold for micafungin (Table 1). The effects of serum on MICs were assessed for other non-Candida albicans spp. The largest MIC shift for caspofungin (eightfold) was with Candida krusei, while Candida tropicalis strains showed the most significant shifts (128-fold) for both micafungin and anidulafungin. These drugs consistently showed pronounced shifts, which reflected their greater antifungal potencies in the absence of serum. These differences disappeared in the presence of 50% serum, where all three drugs showed comparable MICs.Similar serum-induced effects were observed with a collection of Aspergillus spp., where microscopically observed MECs for the three drugs shifted higher in the presence of serum. Micafungin and anidulafungin again showed the most pronounced antifungal shifts, 32-to 133-fold and 16-to 32-fold, respectively, reflecting their more active behavior in the absence of serum ( Table 2). The three drugs displayed nearly equivalent MECs in the presence of 50% serum. The reduced antifungal properties of echinocandin drugs in the presence of serum suggested...

Paradoxical Effect of Caspofungin against Candida Bloodstream Isolates Is Mediated by Multiple Pathways but Eliminated in Human Serum

“…To date, the latter study remains the only one to suggest paradoxical growth of Candida in vivo, since other mouse models of C. tropicalis and C. dubliniensis infections have yielded negative results (3,16). Of further note, our findings were consistent with earlier reports demonstrating an increase in MICs in the presence of serum (11,12,18,19). The issue of incorporating serum in routine susceptibility testing has been discussed extensively in other studies, and our study design does not allow us to address the impact of this practice.…”

Determination of Echinocandin MICs for Candida Species in Less than 8 Hours: Comparison of the Rapid Susceptibility Assay with the Clinical and Laboratory Standards Institute's Broth Microdilution Assay

“…The RSA worked well for all three FDA-approved echinocandins and also yielded lower MICs with Am3-0.2G as the test medium than with RPMI-0.1G. The presence of human serum caused RSA MICs to increase, as has been reported previously for the M27-A method (7). When the inoculum age and culture density were tested, MICs obtained with the RSA and the M27-A3 method were similar.…”

Breakthrough Invasive Candidiasis in Patients on Micafungin

“…Interestingly, the micafungin and anidulafungin MICs of those same mutants were lower (range, 0.16 to 4 g/ml); most were in the "susceptible" range. However, these differences in drug potency were neutralized and in vitro echinocandin crossresistance became apparent when the culture medium was mixed with 50% human serum, a finding which has been replicated elsewhere (34,36). In our case series, caspofungin testing by current CLSI guidelines, which does not include the addition of serum to the test medium, detected all 7 isolates with hot-spot mutations, while anidulafungin and micafungin would have missed 1 and 2 of the C. tropicalis isolates with heterozygous FKS mutations, respectively.…”