Mechanism of Action
Fluconazole is a highly selective inhibitor of fungal cytochrome P450 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.
Drug Resistance
Fluconazole resistance may arise from a modification in the quality or quantity of the target enzyme (lanosterol 14-α-demethylase), reduced access to the drug target, or some combination of these mechanisms.
Point mutations in the gene (ERG11) encoding for the target enzyme lead to an altered target with decreased affinity for azoles. Overexpression of ERG11 results in the production of high concentrations of the target enzyme, creating the need for higher intracellular drug concentrations to inhibit all of the enzyme molecules in the cell.
The second major mechanism of drug resistance involves active efflux of fluconazole out of the cell through the activation of two types of multidrug efflux transporters; the major facilitators (encoded by MDR genes) and those of the ATP-binding cassette superfamily (encoded by CDR genes). Upregulation of the MDR gene leads to fluconazole resistance, whereas, upregulation of CDR genes may lead to resistance to multiple azoles.
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 mcg/mL), the highest fluconazole dose is recommended.
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.
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 (≥16 mcg/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 mcg/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 mcg/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 mcg 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 against Candida species
* 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. ** The intermediate category is sometimes called Susceptible-Dose Dependent (SDD) and both categories are equivalent for fluconazole.
|
| Broth Dilution at 48 hours (MIC in mcg/mL)
| Disk Diffusion at 24 hours (Zone Diameters in mm)
|
Antifungal agent
| Susceptible (S)
| Intermediate (I)**
| Resistant (R)
| Susceptible (S)
| Intermediate (I)**
| Resistant (R)
|
Fluconazole*
| ≤8
| 16-32
| ≥64
| ≥19
| 15-18
| ≤14
|
A report of Susceptible (S) indicates that the antimicrobial drug is likely to inhibit growth of the microorganism if the antimicrobial drug reaches the concentration usually achievable at the site of infection.
A report of Intermediate (I) indicates 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.
A report of Resistant (R) indicates that the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial drug reaches the concentrations usually achievable at the infection site; other therapy should be selected.
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 mcg disks should provide the following range of values noted in Table 2. 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 mcg/mL) @ 48 hours
| Microdilution (MIC in mcg/mL) @ 48 hours
| Disk Diffusion (Zone Diameter in mm) @ 24 hours
|
Candida parapsilosis ATCC 22019
| 2-8
| 1-4
| 22-33
|
Candida krusei ATCC 6258
| 16-64
| 16-128
| ---*
|
Candida albicans ATCC 90028
| ---*
| ---*
| 28-39
|
Candida tropicalis ATCC 750
| ---*
| ---*
| 26-37
|
Single Dose
The convenience and efficacy of the single dose oral tablet of fluconazole regimen for the treatment of vaginal yeast infections should be weighed against the acceptability of a higher incidence of drug related adverse events with fluconazole (26%) versus intravaginal agents (16%) in U.S. comparative clinical studies. (See ADVERSE REACTIONS and CLINICAL STUDIES.)
Post-marketing Experience
In addition, the following adverse events have occurred during post-marketing experience.
Immunologic: In rare cases, anaphylaxis (including angioedema, face edema and pruritus) has been reported.
Body as a Whole: Asthenia, fatigue, fever, malaise.
Cardiovascular: QT prolongation, torsade de pointes. (See PRECAUTIONS.)
Central Nervous System: Seizures, dizziness.
Hematopoietic and Lymphatic: Leukopenia, including neutropenia and agranulocytosis, thrombocytopenia.
Metabolic: Hypercholesterolemia, hypertriglyceridemia, hypokalemia.
Gastrointestinal: Cholestasis, dry mouth, hepatocellular damage, dyspepsia, vomiting.
Other Senses: Taste perversion.
Musculoskeletal System: myalgia.
Nervous System: Insomnia, paresthesia, somnolence, tremor, vertigo.
Skin and Appendages: Acute generalized exanthematous-pustulosis, drug eruption, increased sweating, exfoliative skin disorders including Stevens-Johnson syndrome and toxic epidermal necrolysis (see WARNINGS), alopecia.
Single Dose
Vaginal candidiasis: The recommended dosage of fluconazole for vaginal candidiasis is 150 mg as a single oral dose.
Multiple Dose
SINCE ORAL ABSORPTION IS RAPID AND ALMOST COMPLETE, THE DAILY DOSE OF FLUCONAZOLE IS THE SAME FOR ORAL AND INTRAVENOUS ADMINISTRATION. In general, a loading dose of twice the daily dose is recommended on the first day of therapy to result in plasma concentrations close to steady-state by the second day of therapy.
The daily dose of fluconazole for the treatment of infections other than vaginal candidiasis should be based on the infecting organism and the patient’s response to therapy. Treatment should be continued until clinical parameters or laboratory tests indicate that active fungal infection has subsided. An inadequate period of treatment may lead to recurrence of active infection. Patients with AIDS and cryptococcal meningitis or recurrent oropharyngeal candidiasis usually require maintenance therapy to prevent relapse.
Oropharyngeal candidiasis: The recommended dosage of fluconazole for oropharyngeal candidiasis is 200 mg on the first day, followed by 100 mg once daily. Clinical evidence of oropharyngeal candidiasis generally resolves within several days, but treatment should be continued for at least 2 weeks to decrease the likelihood of relapse.
Esophageal candidiasis: The recommended dosage of fluconazole for esophageal candidiasis is 200 mg on the first day, followed by 100 mg once daily. Doses up to 400 mg/day may be used, based on medical judgment of the patient’s response to therapy. Patients with esophageal candidiasis should be treated for a minimum of three weeks and for at least two weeks following resolution of symptoms.
Systemic Candida infections: For systemic Candida infections including candidemia, disseminated candidiasis, and pneumonia, optimal therapeutic dosage and duration of therapy have not been established. In open, noncomparative studies of small numbers of patients, doses of up to 400 mg daily have been used.
Urinary tract infections and peritonitis: For the treatment of Candida urinary tract infections and peritonitis, daily doses of 50 to 200 mg have been used in open, noncomparative studies of small numbers of patients.
Cryptococcal meningitis: The recommended dosage for treatment of acute cryptococcal meningitis is 400 mg on the first day, followed by 200 mg once daily. A dosage of 400 mg once daily may be used, based on medical judgment of the patient’s response to therapy. The recommended duration of treatment for initial therapy of cryptococcal meningitis is 10 to 12 weeks after the cerebrospinal fluid becomes culture negative. The recommended dosage of fluconazole for suppression of relapse of cryptococcal meningitis in patients with AIDS is 200 mg once daily.
Prophylaxis in patients undergoing bone marrow transplantation: The recommended fluconazole daily dosage for the prevention of candidiasis in patients undergoing bone marrow transplantation is 400 mg, once daily. Patients who are anticipated to have severe granulocytopenia (less than 500 neutrophils per cu mm) should start fluconazole prophylaxis several days before the anticipated onset of neutropenia, and continue for 7 days after the neutrophil count rises above 1000 cells per cu mm.