In vitro susceptibility studies of Cryptococcus neoformans isolated from patients with no clinical response to amphotericin B therapy

Abstract

The in vitro activities of three antifungal drugs alone and in combination were evaluated against five isolates of Cryptococcus neoformans using time-kill curves (TKC). The isolates were from AIDS patients who had either died or had failed to show a clinical response during amphotericin B (AMB) treatment. AMB, fluconazole (FCZ) and flucytosine (5FC), and combinations of the drugs (AMB plus 5FC, AMB plus rifampicin (RIF) and FCZ plus 5FC), were evaluated. With all five isolates AMB did not show fungicidal activity; instead, a persistent or tolerant effect was observed. Combinations of AMB plus 5FC and AMB plus RIF showed a clear synergic effect, except for one isolate tested with AMB plus RIF. In contrast, the FCZ plus 5FC combination did not inhibit growth of any isolate.

Results Obtained with Various Antifungal Susceptibility Testing Methods Do Not Predict Early Clinical Outcome in Patients with Cryptococcosis

“…Determination of minimum fungicidal concentrations and results from time-kill curves have been shown to be better predictors of the clinical outcome than MICs (20,21). In the present study, although a large number of patients were included, the amphotericin B MICs determined by Etest or by the other techniques were not predictive of the early clinical outcome.…”

“…Although the in vitro "resistance" of the infecting C. neoformans strain has been linked to clinical failure during the course of the disease in some case reports (18), only a limited number of studies have assessed the potential relationship between the in vitro susceptibility results obtained at the time of diagnosis and the clinical outcome of cryptococcosis (1,11,16,20,21,30). Moreover, most of these studies have evaluated the clinical efficacy after 10 weeks of therapy, and it is still largely unknown if the MICs of the major antifungals used during the initial treatment of cryptococcosis could be used as predictors of the early clinical outcome.…”

Antifungal Susceptibility Testing: Practical Aspects and Current Challenges

“…The M27 method includes details for testing C. neoformans but does not specify any interpretive breakpoints for agents against this fungus. Although an early report suggested that recurrent cryptococcal meningitis in HIV-infected patients was mostly a function of immune status (28), many subsequent reports have shown the potential for development of true microbiological resistance (1,10,39,167,185,227). The basis of resistance is under investigation (150).…”

“…Use of Antibiotic Medium 3 broth provided good separation between resistant and susceptible isolates, but the best results were obtained with Etest on either RPMI 1640 or Antibiotic Medium 3 agar. Isolates from five patients with cryptococcal meningitis who failed to respond to initial therapy with amphotericin B were shown to be tolerant (i.e., loss of the usual fungicidal effect) to amphotericin B in a study by Rodero et al (185). The same group subsequently reported that MFCs might yield the best correlation with therapeutic response (186); however, correlation was incomplete, and further work is clearly required.…”

“…The need extends beyond testing Candida spp. With resistance demonstrated among such diverse fungi as Cryptococcus neoformans (1,150,167,185,227), Aspergillus fumigatus (40,47,96,104,130), Aspergillus terreus (207,216), Histoplasma capsulatum (224), Pseudallescheria boydii (Scedosporium apiospermum) (219), and Trichosporon spp. (217,218), it is clear that the need for meaningful susceptibility test results is as great for the fungi as it is for the bacteria.…”