Effect of media composition and in vitro activity of new antifungal agents against zygomycetes

Abstract number: 1134_02_355

Gil-Lamaignere C., Hess R., Salvenmoser S., Heyn K., Kappe R., Müller F.-M.

Objectives:

Despite antifungal therapy, mortality in disseminated zygomycosis is still too high. We analysed the in vitro activity of Posaconazole (POS), Voriconazole (VRC) and Caspofungin (CAS) against 59 strains of the genera Rhizopus, Mucor, Absidia, and Cunninghamella in different media compositions.

Methods:

The following five media compositions were compared: RPMI 1640 ± 2% glucose, AM3 ± 2 % glucose and HR-Medium. MICs were determined by microdilution method following NCCLS guidelines with minor modifications. Each well was read visually, the growth in each well was compared with the growth control. Two endpoints were evaluated for each drug: an inhibition of growth >90% war recorded as MIC1 and an inhibition of growth >50% was recorded as MIC 2. The final concentrations of the antifungal agents were 0.015–8 mg/l for POS and VRC, and 0.03–16 mg/l for CAS.

Results:

POS was significantly more active than CAS and VOR, both in R + G and in HR media (P < 0.05 when comparing the MIC1 of POS in HR medium to that of VRC; P < 0.001 for all other comparisons). Growth in RPMI and AM3 media supplemented with glucose was more robust than in the corresponding media lacking glucose. Glucose had little influence on MIC values. The agreement when comparing the MIC1 evaluated in AM3 ± glucose was > 85%, while the use of MIC2 endpoint yielded 100% agreement for all genera. When comparing the data obtained in RPMI ± glucose to that in HR, the agreement was good. The percentage agreements using the MIC2 and MIC1 endpoints were 100% and > 70%, In contrast, the agreement between AM3 ± glucose and the other media was generally poor. Moreover, the average MICs obtained in AM3 medium were lower than those obtained in either HR or RPMI which was due to a difference among genera.

Conclusions:

Our results suggest the following: a) POS is active in vitro against zygomycetes at clinically relevant concentrations. b) within zygomycetes, there are differences between genera in terms of their antifungal susceptibilies. c) growth medium is an important variable for MIC determination in zygomycetes, and the more relevant medium appears to be RPMI supplemented with 2% glucose.