Resistance problems with caspofungin, an echinocandin inhibitor of fungal cell wall glucan synthesis, have been rare. We noted paradoxical turbid growth of Candida albicans isolates in broth in some high (supra-MIC) concentrations. Among isolates submitted for susceptibility testing and screened at drug concentrations up to 12.5 g/ml, the frequency was 16%. Analysis of the turbid growth indicated slowing of growth in the presence of drug but with numbers of CFU up to 72% those of drug-free controls. Clearing of growth again by the highest drug concentrations produced a quadriphasic pattern in a tube dilution series. Cells growing at high drug concentrations were not resistant on retesting but showed the paradoxical effect of the parent. Among a selected series of isolates tested at concentrations up to 50 g/ml, an additional 53% showed a "mini-paradoxical effect": no turbid growth but incomplete killing at high concentrations (supra-minimum fungicidal concentration). These effects were reproducible; medium dependent in extent; noted in macro-and microdilution, in the presence or absence of serum, and on agar containing drug (but not when drug concentrations were not constant, as in agar diffusion); not seen with other echinocandins and less commonly in other Candida species; and not due to destruction of drug in tubes showing the effect. Cooperative enhancement of inhibition by a second drug could eradicate the effect. We postulate that high drug concentrations derepress or activate resistance mechanisms. The abilities of subpopulations to survive at high drug concentrations could have in vivo consequences.Caspofungin is an antifungal drug of the echinocandin class. Agents of this class can be produced by several fungi. This newly introduced agent has as its mechanism of action noncompetitive inhibition of synthesis of (1,3)-␤-D-glucan, a principal constituent of fungal cell walls (4). It has been shown to be fungicidal in vitro against Candida species and efficacious in animal models of candidiasis and in clinical trials (4). Development of resistance is rare, even after prolonged treatment (4). We recently observed resistance to growth inhibition and killing among some clinical Candida isolates, but only at high concentrations of caspofungin, and here we report those observations in detail.(This paper was presented in part at the 14th European Congress of Clinical Microbiology and Infectious Diseases, Prague, Czech Republic, 1 to 4 May 2004.) MATERIALS AND METHODSIsolates. Isolates were identified to species level by standard methods reported previously (12, 17), with molecular genotyping performed for a subset of the isolates (22).Susceptibility testing. Broth macrodilution and microdilution testing (13) utilizing an 80% turbidity endpoint (6), determination of the minimum fungicidal concentration (MFC) (20), incorporation of drug into agar (2), assays of drug diffusion into agar containing organisms (16), and macrodilution checkerboardtype drug interaction studies (3) were performed as detailed elsewhere. Th...

Central nervous system (CNS) aspergillosis is

Paradoxical growth of some Candida albicans isolates in the presence of caspofungin (CAS) in vitro has been demonstrated previously. We sought to determine whether a similar phenomenon occurred in vivo. A systemic model of candidiasis was studied in CD-1 mice by intravenous inoculation of different isolates of C. albicans. Infected animals were treated with CAS at various dosages (0.01 to 20 mg/kg) and CFU remaining in the kidneys determined. Four clinical isolates that showed paradoxical growth in vitro and one that did not were tested. Recovery of CFU from the kidneys showed that dosages of CAS at 0.1 mg/kg and above were efficacious in the reduction of C. albicans, but were not curative. Against isolates that show paradoxical growth in vitro, CAS was efficacious, but lacked dose responsiveness above 0.5 mg/kg against three of the four. One isolate, 95-68, showed paradoxical growth in vivo with significantly higher CFU recovered from mice given CAS at 20 mg/kg than those given CAS at 5 mg/kg, but the effect was not reproducible in a subsequent experiment. When CAS was given prophylactically and therapeutically, improved efficacy and cure rate were observed. Overall, these data indicate that CAS is highly efficacious against systemic murine candidiasis and a paradoxical effect was not reproducibly demonstrated in vivo.

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