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ANTIFUNGAL DRUGSModes of Action
Mechanisms of ResistanceSevtap Arikan, MD
Hacettepe University Medical SchoolAnkara Turkey
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MOST COMMON FUNGAL
PATHOGENS• Dermatophytes
• Candida
• Aspergillus
• Cryptococcus
• Rhizopus
• ...
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MODES of ACTION
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ANTIFUNGAL DRUGS--by mode of action
•
Membrane disruptingagents
Amphotericin B, nystatin
•Ergosterol synthesisinhibitors
Azoles, allylamines,morpholine
• Nucleic acid inhibitorFlucytosine
• Anti-mitotic (spindledisruption)
Griseofulvin
• Glucan synthesis
inhibitors
Echinocandins
• Chitin synthesis
inhibitor
Nikkomycin
• Protein synthesisinhibitors
Sordarins, azasordarins
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TARGETS
for antifungal activity• Ergosterol (Cell membrane) Drug-ergosterol interaction
Inhibition of ergosterol synthesis
• RNA/EF3 (Nucleic acid/protein synthesis)Incorporation of 5-FU in RNA
Inhibition of EF3
• Glucan/Chitin (Cell wall)
Inhibition of glucan/chitin synthesis
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TERB
Ergosterol synthesis
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SORDARINS,
AZASORDARINS• EF3: A target in protein synthesis machineryunique to FUNGI
• GM 237354... (sordarins)
GW 471558... (azasordarins)
• Yet investigational
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TARGETS
for antifungal activity• Ergosterol (Cell membrane)Drug-ergosterol interactionInhibition of ergosterol synthesis
• RNA/EF3 (Nucleic acid/protein synthesis)Incorporation of 5-FU into RNA
Inhibition of EF3
• Glucan/Chitin (Cell wall) Inhibition of glucan / chitin synthesis
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NIKKOMYCIN
• Competitiveinhibition of chitin
synthase
• Yet investigational
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RESISTANCE is..
IN VITRO
CLINICAL
MOLECULAR
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A resistant strain may be
present due to:• Intrinsic resistance
• Replacement with a more resistant species
• Replacement with a more resistant strain• Transient gene expressions that cause
temporary resistance (epigenetic resistance)
•Alterations in cell type (?)• Genomic instability within a single strain(population bottleneck)
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Mechanisms of
Amphotericin B Resistance• Reduced ergosterol content (defective ERG2or ERG3 genes)
• Alterations in sterol content (fecosterol,episterol: reduced affinity)
• Alterations in sterol to phospholipid ratio• Reorientation or masking of ergosterol
• Stationary growth phase• Previous exposure to azoles• (?)
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Mechanisms of Resistance toAzoles
• Alteration of lanosterol (14-alpha) demethylase
• Overexpression of lanosterol demethylase
• Energy-dependent efflux systems
a. Major facilitator superfamily (MFS) proteins(BENr =MDR1 of Candida ...)
b. ATP-binding cassette (ABC) superfamilyproteins (MDR, CDR of Candida )
• Changes in sterol and/or phospholipid composition of
fungal cell membrane (decreased permeability)
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Azole Resistance
Molecular Aspects• Single point mutation of ERG11 geneAltered lanosterol demethylase
• Overexpression of ERG11 geneIncreased production of lanosterol demethylase
• Alterations in ERG3 or ERG5 genesProduction of low affinity sterols
• Increase in mRNA levels of CDR1 or MDR1 genesDecreased accumulation of the azole in fungal cell
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If it is azole-resistant..
Clin Microbiol Rev 1998; 11: 382
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Secondary Resistance inC. albicans
to Fluconazole
CID 1997; 25: 908-910
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Resistance to Flucytosine
• PRIMARY non-albicans Candida C. neoformans
Aspergillus (highest)
• SECONDARY C. albicans
C. neoformans
Secondary resistance develops followingflucytosine MONOtherapy.
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Mechanisms of Resistance to
Flucytosine• Loss of permease activity
• Loss of cytosine deaminase activity
• Decrease in the activity of UPRTase
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Flucytosine Resistance
Molecular Aspects• FCY genes (FCY1, FCY2) encode for UPRTase
FCY/FCY homozygotes possess high UPRTase activity
FCY/fcy heterozygotes possess low UPRTase activity
fcy/fcy homozygotes possess barely detectableUPRTase activity
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Resistance to Echinocandins
PRIMARY C. neoformans
Fusarium spp. SECONDARY (?)
The only licensed member is caspofungin (Jan
2001, USA). Resistant mutants due to therapyare not available.
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Echinocandin Resistance
Molecular Aspects• FKS1 encodes glucan synthase• GNS1 encodes an enzyme involved in fatty
acid elongation
Resistance is observed followinglaboratory derived mutations in FKS1 or
GNS1
• Other mechanisms (?)
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Future Directions to AvoidDevelopment of Resistance
• Proper dosing strategies
• Restricted and well-defined indications forprophylaxis with azoles
Fungi will continue to develop NEWresistance mechanisms!..
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Final word
• Antifungal resistance is a complex, gradualand multifactorial issue
• Several uncertainties remain
• Molecular assays to detect resistance are notsimple
• The best way to improve the efficacy ofantifungal therapy is to improve the immunestatus of the host