antifung drg 02

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ANTIFUNGAL DRUGSModes of Action

Mechanisms of ResistanceSevtap Arikan, MD

Hacettepe University Medical SchoolAnkara Turkey



MOST COMMON FUNGAL

PATHOGENS• Dermatophytes

• Candida

• Aspergillus

• Cryptococcus

• Rhizopus

• ...



MODES of ACTION



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



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



TERB

Ergosterol synthesis



SORDARINS,

AZASORDARINS• EF3: A target in protein synthesis machineryunique to FUNGI

• GM 237354... (sordarins)

GW 471558... (azasordarins)

• Yet investigational



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



NIKKOMYCIN

• Competitiveinhibition of chitin

synthase

• Yet investigational



RESISTANCE is..

IN VITRO

CLINICAL

MOLECULAR



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)



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• (?)



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)



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



If it is azole-resistant..

Clin Microbiol Rev 1998; 11: 382



Secondary Resistance inC. albicans

to Fluconazole

CID 1997; 25: 908-910



Resistance to Flucytosine

• PRIMARY non-albicans Candida C. neoformans

Aspergillus (highest)

• SECONDARY C. albicans

C. neoformans

Secondary resistance develops followingflucytosine MONOtherapy.



Mechanisms of Resistance to

Flucytosine• Loss of permease activity

• Loss of cytosine deaminase activity

• Decrease in the activity of UPRTase



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



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.



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 (?)



Future Directions to AvoidDevelopment of Resistance

• Proper dosing strategies

• Restricted and well-defined indications forprophylaxis with azoles

Fungi will continue to develop NEWresistance mechanisms!..



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

antifung drg 02

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