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ANTIFUNGAL AGENTS
ANTIFUNGAL AGENTS
Antifungal agents• Increase of human fungal infections mainly
due to advances in surgery, cancer treatment, critical care, increase in the use of broad spectrum antibiotics and HIV
• Increased number of patients at risk• Antifungals available : systemic drugs, oral
drugs for mucocutaneous infections and topical drugs
Evolution of antifungal agents
POLYENE ANTIFUNGALSAMPHOTERICIN BNYSTATIN• Fungicidal against both filamentous and yeastlike
fungi, Histoplasma, Blastomyces, Coccidioides, Cryptococcus, Cnadida, Aspergillus and Sporotrichum
• In vitro activity against some protozoa• Generally acts on sterols in the cytoplasmic membrane
of fungi leading to rapid leakage and fungal death
AMPHOTERICIN B: CHEMISTRY• Produced by Streptomyces nodosus • Polyene macrolide • Water insoluble, prepared as a colloidal
suspension or in a lipid associated delivery system
AMPHOTERICIN B: PHARMACOKINETICS• Poor GI absorption• Oral administration is effective for fungal infections on
the lumen of the GI tract• >90% protein bound• Serum t1/2 15 days• Large Vd but CSF concentrations is only 2-3% of plasma
concentrations• Poor CSF penetration, may require intrathecal
administration in cases of meningitis• Fugicidal and fungistatic
AMPHOTERICIN B: MECHANISM OF ACTION• Binds to Ergosterol and alters cell permeability• Forms pores in the cell which allows leakage
of intracellular ions and macromolecules• Binds to human membrane sterols,
accounting for toxicity• Resistance if ergosterol binding is impaired
either by increasing ergosterol concentration or modifying the sterol of the target molecule
AMPHOTERICIN B: LIPID FORMULATIONS• Therapy is limited by toxicity• Lipid binding of the drug causes less binding to
mammalian membranes permitting the use of effective doses of the drug.
• Lipid vehicle serves as a reservoir
AMPHOTERICIN B: ADR• Infusion related toxicity: fever, chills, muscle spasms, vomiting,
headache , hypotension• Ameliorated by slow IV infusion or decreasing the dose• Premedications with antihistamines• Start with a test dose• Slower toxicity: • Azotemia is variable but can be serious enough to necessitate
dialysis• Renal toxicity commonly presents with RTA ( renal tubular acidosis
with severe K and MG wasting)• Attenuated by preloading with saline• After intrathecal administration: seizures, chemical arachoidits
AMPHOTERICIN B: Antifungal activity• BROADEST SPECTRUM OF ACTIVITY• Candida albicans• Cryptococcus neoformans• Histoplasma capsulatum• Blastomyces dermatidis• Coccidioides imimtis• Aspergillus fumigatus• Candida lusitaniae and Pseudallescheria boydii
are resistant
AMPHOTERICIN B: Clinical Use• Drug of choice for nearly all life threathening
mycotic infections• Initial induction therapy for serious fungal
infections and is concomitantly replaced by azoles
• Fungal pneumonia, cryptococcal meningitis, sepsis, systemic fungal disease
• Local application: Fungal keratitis, fungal arthritis, bladder irrigation in Candiduria
NYSTATIN
• More soluble than Amphotericin B• Used primarily as a topical preparation• Active against most Candida species• Not absorbed from skin or GI tract• No parenteral administration due to toxicity
AZOLES
• Synthetic compounds– Imidazoles: Ketoconazole, Miconazole,
Clotrimazole– Triazoles: Itraconazole, Fluconazole, Vorioconazole
AZOLES: Mechanism of Action• Reduction of ergosterol synthesis by ionhibition
of fungal cytochrome P450 enzymes
AZOLES: Clinical use• Candida species• Cryptococus neoformasns• B;astomyces• Coccidiomycosis• Histoplasmosis• Dermatophytes• Aspergillus for Itraconazole and Voriconazole• Pseudallscheria boydii
AZOLES: ADRs• Relatively non toxic• Minor GI upset• Abnormalities in liver enzymes
Ketoconazole• Systemic use discontinued because of its great
propensity to inhibit mammalian cytochrome P450 enzymes
• Used in dermatologic infections
Itraconazole• Available in oral and IV formulations• Drug absorption is increased by food or low gastric pH• Reduced bioavailability when taken with Rifampicin,
Rifabutin, Rifapentine• Poor CSF penetration• Drug of choice for HIstoplasma, Blastomyces and
Sporotrix infections• Used extensively in the treatment of dematophytoses
and onychomycosis
Fluconazole• Highly water soluble and high CSF penetration• High oral bioavailability• Better GI tolerance, fewer hepatic enzyme
interactions: widest therapeutic index• Azole of choice in the treatment and secondary
prophylaxis of Cryptococcal infections• Equivalent to Amphotericin B in the treatment of
Candidemia• Reduce fungal disease in bone marrow transplant and
AIDS patients
Voriconazole• Good oral bioavailability• Low propensity for mammalian cytochrome P
450 inhibition• Causes Blurring of vision and altered color
perceptions• Excellent activity against Candida, effective in
the treatment of invasive Aspergillosis
MICONAZOLE/ CLOTRIMAZOLE• Topically active• Fungicidal when administered topically• Poor CSF penetration• Used in ringworm infections and vulvovaginal
candidiasis
FLUCYTOSINE
• Water soluble pyrimidine analog related to 5 FU
• 09% absorbed with peak serum concentrations 1-2 hours after oral administration
• Poor protein binding, penetrates well into all body fluid compartments including CSF
• Eliminated by glomerular filtration, levels rise rapidly in renally impaired patients
FLUCYTOSINE: Mechanism of Action• Taken up by fungal cells via cytosine permease• Converted to 5FU and the to 5
fluorodeoxyuridine 5 F-dUMP and fluorouidine 3 PO4
• Inhiobits DNA and RNA synthesis• Resistance seen in monotherapy• Acts synergystically with Amphotericin B.
FLUCYTOSINE: ADRs• Toxicity related to the formation of 5 FU• Anemia, leukopenia, thrombocytopoenia• Narrow therapeutic window• Used in Cryptococcal infections
Griseofulvin
• Used in the systemic treatment of dermatophytosis, Epidermophyton, Microsporum, Trichophyton,
• Binds to keratin and is deposited in newly forming skin
Terbinafine
• Used in the treatment of dermatophytosis, specifically onychiomycosis
• Like Griseofulvin, it is Keratophyllic• Does not seem to affect cytochrome P450
enzymes
Echinocandins/ Capsofungin
• Newest class of antifungals • Cyclic peptides linked to a long chain fatty acid• Acts at the level of the fungal wall by
inhibiting the synthesis of beta 1-3 glucan, resulting in cell wall disruption
• Used in Invasive Aspergillosis who have failed to respond to Amphotericin B.
To determine the susceptibility of fungi to antifungal agents
• Concentration that inhibits the growth of fungi = Minimum inhibitory concentration (expressed as µg/ml) Several methods can be used to define the MIC MIC reading = depends on the method used(EUCAST, CLSI, E-test,…)
MICs defined via E-test
Growth of fungus
MIC= zone of inhibition
–MICs help, but hard to standardize –Correlations appear possible based on individual isolates.
Broad correlations based on multiple isolates are still lacking–Understanding this helps a lot when trying to correlate
outcome with MIC- Some patients get better despite MICs - Some patients just don’t get better despite MICs
–No rule when it correlates/not correlates
AZOLE RESISTANT ASPERGILLUS FUMIGATUS
Antifungal susceptibility testing
Macrodilution
Microdilution
Disk diffusion
E test
Agar dilution
Based on NCCLS M27-A document
Variables: inoculum, medium, PH, incubation & temperature, MIC
Azole cross-resistance
• Mechanisms of resistance to drug action – Modification of the drug itself– Modification in quantity or quality of the drug
target– Reduced access to the target
• The resistance may result from a combination of these mechanisms
Factors in resistance – Modifications of the azoles have not yet been
documented as a factor– Differing binding affinities of azoles• Some species of Candida
– Efflux of fluconazole • development of fluconazole resistance in some Candida
species• achieved by increased expression of the multidrug
resistance transporter proteins (especially MDR1)
• Factors in resistance– Overexpression of 14-α-demethylase • an azole-resistant strain of C glabrata• the mechanism of cross-resistance exhibited with
itraconazole and fluconazole – Altered membrane sterol composition • methylated sterols, such as methylfecosterol replacing
ergosterol• an azole-resistant and polyene-resistant C albicans
mutant