Affiliation:
1. Immunocompromised Host Section, Pediatric Oncology Branch, National Cancer Institute, Bethesda, Maryland
2. Third Department of Pediatrics, Aristotle University, Hippokration Hospital, Thessaloniki, Greece
Abstract
ABSTRACT
The minimum effective concentration (MEC) used to assess the in vitro antifungal activity of caspofungin against
Aspergillus
spp. is a qualitative endpoint requiring microscopic examination of hyphae. We therefore developed a tool for the quantitative assessment of caspofungin activity against
Aspergillus
spp. at clinically applicable concentrations. Susceptibility to caspofungin (0.008 to 8 μg/ml) was studied for 9
A. fumigatus
, 8
A. flavus
, and 12
A. terreus
isolates based on the Clinical and Laboratory Standards Institute M38-A protocol. After 48 h of incubation, the MEC was defined microscopically, and metabolic activity assessed with a modified XTT assay, using 100 μg of the tetrazolium salt XTT/ml and 6.25 μM menadione. A significant reduction in metabolic activity was demonstrated at the MEC (0.25 to 0.5 μg/ml) for all
Aspergillus
spp. and was more pronounced for
A. flavus
(median metabolic activity, 25% of control) compared to
A. fumigatus
and
A. terreus
(median metabolism, 42 and 53%, respectively), allowing determination of MEC with the XTT assay (93 to 100% agreement with microscopic MEC). Fungal metabolism tended to reach the lowest levels (median, 17 to 38% of control) one to two dilutions higher than the MEC, at the minimum metabolic activity concentration (MMC). For 5 of 9
A. fumigatus
isolates, 6 of 12
A. terreus
isolates, and 1 of 8
A. flavus
isolates, a paradoxical increase in metabolism was observed at concentrations greater than the MMC. Sigmoid (
E
max
) or bell-shaped models described accurately (median
R
2
= 0.97) the concentration-dependent metabolic changes in the absence or presence, respectively, of paradoxical response. Assessment of metabolic activity may provide useful quantitative endpoints for in vitro studies of caspofungin against
Aspergillus
spp.
Publisher
American Society for Microbiology
Subject
Infectious Diseases,Pharmacology (medical),Pharmacology
Cited by
49 articles.
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