Development of interpretive breakpoints for antifungal susceptibility testing: conceptual framework and analysis of in vitro-in vivo correlation data for fluconazole, itraconazole, and candida infections. Subcommittee on Antifungal Susceptibility Testing of the National Committee for Clinical Laboratory Standards

High-dose therapy with fluconazole > or = 800 mg day-1

Fluconazole dosages greater than 800 mg day-1 have been reported in about 900 patients treated for candidemia, oropharyngeal candidiasis and cryptococcal meningitis in HIV-infected patients, and for initial therapy of endemic mycoses. In patients with life-threatening infections caused by Candida spp., Cryptococcus neoformans and Coccidioides immitis, results of a limited number of dose-finding trials with non-neutropenic and HIV-infected patients show dose-dependent responses. These study results indicate that higher daily doses of fluconazole than are currently approved for these indications are well tolerated and tend to provide better clinical efficacy in selected patient populations. An excellent safety profile of dosages up to 2000 mg day-1 and linear predictable pharmacokinetics up to 1600 mg day-1 appear to justify further clinical investigations to better determine the optimum dosage and duration of treatment.

Antifungal susceptibility testing of yeast isolates from blood cultures by microbroth dilution and the E test

The results of microbroth dilution were compared with those of the E test for 169 yeast isolates tested for their susceptibility to antifungal agents. All isolates were tested by both methods against amphotericin B, ketoconazole, fluconazole, and itraconazole. The E test results generally correlated well with those obtained by the reference method. There was at least 80% agreement of minimum inhibitory concentration results within two dilutions for all yeast species and agents tested, except for Cryptococcus neoformans tested with fluconazole (8% agreement). The E test appears to be a suitable alternative antifungal susceptibility test method for yeasts, although improvements are required for testing Cryptococcus neoformans against fluconazole.

Candidemia in a Canadian tertiary care hospital from 1976 to 1996

The incidence of candidemia was reviewed at the Health Sciences Centre in Winnipeg, Canada, over a 21-year period (1976 to 1996). Candida species were identified as blood-stream isolates in significantly (p < 0.05) higher numbers from 1991 to 1996 than in the previous 15 years. Antifungal susceptibilities remained unchanged with Candida albicans isolates tested from 1985 to 1996. Retrospective chart reviews revealed that all patients with candidemia possessed at least two risk factors. The main risk factors identified were recent or concurrent antibiotic therapy (95% of patients), presence of a central line (93% of patients), and immunosuppression (88% of patients). Treatment generally involved amphotericin B therapy (81% of patients), and death occurred in 52% of the patients. Mortality directly attributable to Candida species could be established in 23% of patients.

Update on antifungal resistance

Overuse of antifungal agents has resulted in the selection of naturally resistant Candida species, as well as expression of resistance from previously susceptible species resulting from genetic mutations and/or selection of resistant subpopulations. Strategies for the appropriate use of antifungal agents need to be developed to prevent further development of resistance.

In vitro evaluation of voriconazole against some clinically important fungi

Voriconazole was compared to amphotericin B, fluconazole, and itraconazole by using an in vitro macrobroth dilution test based upon current National Committee for Clinical Laboratory Standards tentative standards against the dimorphic fungi and several opportunistic molds and yeasts. In all instances, the voriconazole MICs were lower than those of fluconazole. In most instances, the MICs were lower than the recorded MICs of amphotericin B and itraconazole.

The search for new triazole antifungal agents

The first generation antifungal agent triazoles, fluconazole and itraconazole, have revolutionised the treatment of serious fungal infections such as mucosal and invasive candidiasis and cryptococcal meningitis. However, the treatment of some fungal infections, particularly aspergillosis, is still far from satisfactory and thus there is an important requirement for new broad-spectrum antifungal agents. The new second generation triazoles voriconazole and SCH-56592 show considerable promise in achieving this goal in the near future.

Increased mRNA levels of ERG16, CDR, and MDR1 correlate with increases in azole resistance in Candida albicans isolates from a patient infected with human immunodeficiency virus

Resistance to antifungal drugs, specifically azoles such as fluconazole, in the opportunistic yeast Candida albicans has become an increasing problem in human immunodeficiency virus (HIV)-infected individuals. The molecular mechanisms responsible for this resistance have only recently become apparent and can include alterations in the target enzyme of the azole drugs (lanosterol 14alpha demethylase [14DM]), or in various efflux pumps from both the ABC transporter and major facilitator gene families. To determine which of these possible mechanisms was associated with the development of drug resistance in a particular case, mRNA levels have been studied in a series of 17 clinical isolates taken from a single HIV-infected patient over 2 years, during which time the levels of fluconazole resistance of the strain increased over 200-fold. Using Northern blot analysis of steady-state levels of total RNA from these isolates, we observed increased mRNA levels of ERG16 (the 14DM-encoding gene), CDR1 (an ABC transporter), and MDR1 (a major facilitator) in this series. The timing of the increase in mRNA levels of each of these genes correlated with increases in fluconazole resistance of the isolates. Increased mRNA levels were not observed for three other ABC transporters, two other genes in the ergosterol biosynthetic pathway, or the NADPH-cytochrome P-450 oxidoreductase gene that transfers electrons from NADPH to 14DM. Increases in mRNA levels of ERG16 and CDR1 correlated with increased cross-resistance to ketoconazole and itraconazole but not to amphotericin B. A compilation of the genetic alterations identified in this series suggests that resistance develops gradually and is the sum of several different changes, all of which contribute to the final resistant phenotype.

Antifungal pharmacodynamic characteristics of fluconazole and amphotericin B tested against Candida albicans

Time-kill curves were determined for three isolates of Candida albicans tested against fluconazole and amphotericin B at multiples of the MIC. Fluconazole produced fungistatic activity, with concentration-related growth effects observed over a narrow range of concentrations. Amphotericin B exhibited fungicidal activity, with enhancement of activity over a broader range of concentrations.

Azole resistance in Candida

Resistance of Candida to azoles is an increasing problem. Susceptibility testing of Candida against fluconazole and ketoconazole is now feasible and desirable. Good correlation of resistance in vitro with clinical failure of fluconazole therapy has now been shown in mucosal candidiasis. The relationship, if any, between resistance and clinical failure in the context of invasive candidiasis is not clear at present and additional correlative work needs to be done. Monitoring of resistance trends in Candida is clearly important now.

Plasma and tissue concentrations of fluconazole and their correlation to breakpoints

The prediction of clinical outcome during antifungal therapy is an issue of paramount importance in clinical research, because no consistently reliable parameters are available. Minimum inhibitory concentration (MIC) values and breakpoint interpretation may serve as surrogate criteria until standardized in vitro antifungal susceptibility testing is developed, especially for fluconazole. With reproducible susceptibility testing methods for Candida species now available, tentative fluconazole interpretive breakpoints derived from MIC values determined by the National Committee on Clinical Laboratory Standards (NCCLS) M27-T broth macrodilution method are open for public comment. Besides the in vitro susceptibility of the fungus, clinical response to antifungal therapy with fluconazole depends to a great extent on the daily dose and corresponding plasma and tissue concentrations. Promising results from a few dose-finding studies in non-neutropenic patients show that fluconazole doses of up to 1000 mg day-1 result in higher clinical response rates than lower dosages. Therapeutic success depends substantially on achieving fluconazole plasma and tissue levels that are sufficiently higher than MIC values indicated by in vitro testing. However, this simplified concept must be related to the clinical situation and it is essential to consider the immunological status and underlying disease of the patient. Misinterpretation of MIC values may result in selection of an antifungal agent for life-threatening infections that does not provide optimal efficacy or toleration.

[Plasma and tissue concentrations of fluconazole: discussion of the breakpoint problem]

The reliable prediction of the clinical outcome during antifungal therapy is an issue of paramount priority in clinical research, since there are no appropriate parameters available. MIC values and in the future breakpoints may serve as surrogate criteria if further standardization of in vitro antifungal susceptibility testing especially for fluconazole leading to improved interlaboratory reproducibility of the test results can be provided. With reproducible susceptibility testing methods for Candida species now being available, tentative fluconazole interpretative breakpoints derived from MICs determined by the NCCLS M27-T broth macrodilution methodology are now open for public commentary. Besides the proven susceptibility of the fungus, clinical response to antifungal therapy with fluconazole also depends to a great extent on the daily dosage and the corresponding plasma and tissue concentrations as well as the immunological status and the underlying disease of the patient. The promising results of a relatively small number of dose finding studies with fluconazole in non-neutropenic patients indicate that with daily doses up to 1000 mg/die higher clinical response rates compared to those under lower dosages can be achieved. In view of future valid breakpoints, higher corresponding plasma and tissue levels of fluconazole should be achieved on which the therapeutic success depends substantially. However, this simplified concept needs several adjustments. Misinterpretation of MIC values and breakpoints may have as a consequence that patients with often life-threatening fungal infections may not be treated with an efficacious and better tolerated agent.