Fluconazole (Page 3 of 9)


Mechanism of Action

Fluconazole is a highly selective inhibitor of fungal cytochrome P-450 dependent enzyme lanosterol 14-α-demethylase. This enzyme functions to convert lanosterol to ergosterol. The subsequent loss of normal sterols correlates with the accumulation of 14-α-methyl sterols in fungi and may be responsible for the fungistatic activity of fluconazole. Mammalian cell demethylation is much less sensitive to fluconazole inhibition.

Activity In Vitro and In Clinical Infections

Fluconazole has been shown to be active against most strains of the following microorganisms both in vitro and in clinical infections.

Candida albicans

Candida glabrata (Many strains are intermediately susceptible)*

Candida parapsilosis

Candida tropicalis

Cryptococcus neoformans

* In a majority of the studies, fluconazole MIC90 values against C. glabrata were above the susceptible breakpoint (≥16μg/ml). Resistance in Candida glabrata usually includes upregulation of CDR genes resulting in resistance to multiple azoles. For an isolate where the MIC is categorized as intermediate (16 to 32 μg/ml, see Table 1), the highest dose is recommended (see Dosage and Administration). For resistant isolates alternative therapy is recommended.

The following in vitro data are available, but their clinical significance is unknown.

Fluconazole exhibits in vitro minimum inhibitory concentrations (MIC values) of 8 μg/mL or less against most (≥90%) strains of the following microorganisms, however, the safety and effectiveness of fluconazole in treating clinical infections due to these microorganisms have not been established in adequate and well controlled trials.

Candida dubliniensis

Candida guilliermondii

Candida kefyr

Candida lusitaniae

Candida krusei should be considered to be resistant to fluconazole. Resistance in C. krusei appears to be mediated by reduced sensitivity of the target enzyme to inhibition by the agent.

There have been reports of cases of superinfection with Candida species other than C. albicans , which are often inherently not susceptible to fluconazole (e.g., Candida krusei). Such cases may require alternative antifungal therapy.

Susceptibility Testing Methods

Cryptococcus neoformans and filamentous fungi:

No interpretive criteria have been established for Cryptococcus neoformans and filamentous fungi.

Candida species:

Broth Dilution Techniques: Quantitative methods are used to determine antifungal minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of Candida spp. to antifungal agents. MICs should be determined using a standardized procedure. Standardized procedures are based on a dilution method (broth)1 with standardized inoculum concentrations of fluconazole powder. The MIC values should be interpreted according to the criteria provided in Table 1.

Diffusion Techniques: Qualitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of Candida spp. to an antifungal agent. One such standardized procedure2 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 25 μg of fluconazole to test the susceptibility of yeasts to fluconazole. Disk diffusion interpretive criteria are also provided in Table 1.

Table 1: Susceptibility Interpretive Criteria for Fluconazole
The intermediate category is sometimes called Susceptible-Dose Dependent (SDD) and both categories are equivalent for fluconazole.
Isolates of C. krusei are assumed to be intrinsically resistant to fluconazole and their MICs and/or zone diameters should not be interpreted using this scale.

Broth Dilution at 48 hours

Disk Diffusion at 24 hours

Antifungal agent Susceptible (S) Intermediate (I)* Resistant (R) Susceptible (S) Intermediate (I)* Resistant (R)
Fluconazole ≤ 8.0 16-32 ≥ 64 ≥ 19 15-18 ≤ 14

The susceptible category implies that isolates are inhibited by the usually achievable concentrations of antifungal agent tested when the recommended dosage is used. The intermediate category implies that an infection due to the isolate may be appropriately treated in body sites where the drugs are physiologically concentrated or when a high dosage of drug is used. The resistant category implies that isolates are not inhibited by the usually achievable concentrations of the agent with normal dosage schedules and clinical efficacy of the agent against the isolate has not been reliably shown in treatment studies.

Quality Control

Standardized susceptibility test procedures require the use of quality control organisms to control the technical aspects of the test procedures. Standardized fluconazole powder and 25 μg disks should provide the following range of values noted in Table 1. NOTE: Quality control microorganisms are specific strains of organisms with intrinsic biological properties relating to resistance mechanisms and their genetic expression within fungi; the specific strains used for microbiological control are not clinically significant.

Table 2: Acceptable Quality Control Ranges for Fluconazole to be Used in Validation of Susceptibility Test Results
Quality control ranges have not been established for this strain/antifungal agent combination due to their extensive interlaboratory variation during initial quality control studies.
QC Strain

Macrodilution (MIC in μg/mL)


(MIC in μg/mL)

Disk Diffusion

Candida parapsilosis ATCC 22019 2.0-8.0 1.0-4.0 22-33
Candida krusei ATCC 6258 16-64 16-128 *
Candida albicans ATCC 90028 * * 28-39
Candida tropicalis ATCC 750 * * 26-37

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