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ANTIFUNGAL DRUGS Modes of Action Mechanisms of Resistance

ANTIFUNGAL DRUGS Modes of Action Mechanisms of Resistance. Sevtap Arikan, MD Hacettepe University Medical School Ankara Turkey. MOST COMMON FUNGAL PATHOGENS. Dermatophytes Candida Aspergillus Cryptococcus Rhizopus . POLYENES Amphotericin B, nystatin AZOLES

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ANTIFUNGAL DRUGS Modes of Action Mechanisms of Resistance

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  1. ANTIFUNGAL DRUGSModes of ActionMechanisms of Resistance Sevtap Arikan, MD Hacettepe University Medical School Ankara Turkey

  2. MOST COMMON FUNGAL PATHOGENS • Dermatophytes • Candida • Aspergillus • Cryptococcus • Rhizopus • ...

  3. POLYENES Amphotericin B, nystatin AZOLES Imidazoles: Ketoconazole.. Triazoles: Fluconazole, itraconazole, voriconazole, posaconazole, ravuconazole ALLYLAMINES Terbinafine, butenafine MORPHOLINE Amorolfine FLUORINATED PYRIMIDINE Flucytosine ECHINOCANDINS Caspofungin, anidulafungin, micafungin PEPTIDE-NUCLEOSIDE Nikkomycin Z TETRAHYDROFURAN DERIVATIVES Sordarins, azasordarins OTHER Griseofulvin ANTIFUNGAL DRUGS--by structure

  4. MODES of ACTION

  5. Membrane disrupting agents Amphotericin B, nystatin Ergosterol synthesis inhibitors Azoles, allylamines, morpholine Nucleic acid inhibitor Flucytosine Anti-mitotic (spindle disruption) Griseofulvin Glucan synthesis inhibitors Echinocandins Chitin synthesis inhibitor Nikkomycin Protein synthesis inhibitors Sordarins, azasordarins ANTIFUNGAL DRUGS--by mode of action

  6. 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

  7. AMPHOTERICIN B generates pores in the membrane Clin Microbiol Rev 1999; 12: 501

  8. 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

  9. Ergosterol synthesis

  10. Azoles, allylamines & morpholines inhibit specific ENZYMES Clin Microbiol Rev 1998; 11: 382

  11. TARGETS for antifungal activity • Ergosterol (Cell membrane) Drug-ergosterol interaction Inhibition 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

  12. FLUCYTOSINE (5-fluorocytosine) Cytosine permease 5-FC cytosine deaminase 5-FU 5-FU 5-FU uracil phosphoribosyl 5-fluorouridilic acid (FUMP) transferase (UPRTase) FUMPphosphorylation 5-fluorodeoxyuridine monophosphate thymidylate synthase inhibitor inhibits DNA synthesis 5-fluoro-UTP incorporated into RNA disrupts protein synthesis

  13. TARGETS for antifungal activity • Ergosterol (Cell membrane) Drug-ergosterol interaction Inhibition 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

  14. SORDARINS, AZASORDARINS • EF3: A target in protein synthesis machinery unique to FUNGI • GM 237354... (sordarins) GW 471558... (azasordarins) • Yet investigational

  15. TARGETS for antifungal activity • Ergosterol (Cell membrane) Drug-ergosterol interaction Inhibition 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

  16. ECHINOCANDINSCaspofungin is licensed • Inhibition of β-(1-3) glucan synthesis (of glucan synthase ??) • Secondary reduction in ergosterol & lanosterol • Increase in chitin • Kills hyphae at their growth tips and branching points • Buds fail to seperate from the mother cell • Yields osmotically sensitive fungal cells

  17. TARGETS for antifungal activity • Ergosterol (Cell membrane) Drug-ergosterol interaction Inhibition 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

  18. NIKKOMYCIN • Competitive inhibition of chitin synthase • Yet investigational

  19. MECHANISMS OF RESISTANCE

  20. RESISTANCE is.. CLINICAL IN VITRO MOLECULAR

  21. 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)

  22. Clinical Resistance is a Multifactorial Issue • HOST Immune status Site of infection Severity of infection Foreign devices Noncompliance with drug regimen • FUNGUS Initial MIC Cell type: Yeast/hyphae.. Genomic stability Biofilm production Population bottlenecks • DRUG Fungistatic nature Dosing Pharmacokinetics Drug-drug interactions

  23. Resistance to Amphotericin B • Technical difficulties in detection of resistance in vitro • In vivo resistance is rare C. lusitaniae, C. krusei C. neoformans Trichosporon spp. A. terreus S. apiospermum Fusarium spp. ...

  24. Mechanisms of Amphotericin B Resistance • Reduced ergosterol content (defective ERG2 or 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 • (?)

  25. Resistance to Azoles • Well-known particularly for fluconazole • Data available also for other azoles • A significant clinical problem RESISTANCE TO FLUCONAZOLE PRIMARYC. krusei Aspergillus C. glabrata C. norvegensis... SECONDARYC. albicans C. dubliniensis...

  26. Mechanisms of Resistance to Azoles • 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) superfamily proteins (MDR, CDR of Candida) • Changes in sterol and/or phospholipid composition of fungal cell membrane (decreased permeability)

  27. Azole ResistanceMolecular Aspects • Single point mutation of ERG11 gene Altered lanosterol demethylase • Overexpression of ERG11 gene Increased production of lanosterol demethylase • Alterations in ERG3 or ERG5 genes Production of low affinity sterols • Increase in mRNA levels of CDR1 or MDR1 genes Decreased accumulation of the azole in fungal cell

  28. If your fungus is susceptible to azoles.. Clin Microbiol Rev 1998; 11: 382

  29. If it is azole-resistant.. Clin Microbiol Rev 1998; 11: 382

  30. Secondary Resistance in C. albicans to Fluconazole CID 1997; 25: 908-910

  31. Resistance to Terbinafine • Very rare • Primary resistance to terbinafine in a T.rubrum strain (ICAAC 2001, abst. no. J-104) • Mechanism: (?) CDR1-mediated efflux (possible)

  32. Resistance to Flucytosine • PRIMARY non-albicansCandidaC. neoformans Aspergillus (highest) • SECONDARYC. albicans C. neoformans Secondary resistance develops following flucytosine MONOtherapy.

  33. Mechanisms of Resistance to Flucytosine • Loss of permease activity • Loss of cytosine deaminase activity • Decrease in the activity of UPRTase

  34. Flucytosine ResistanceMolecular 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 detectable UPRTase activity

  35. Resistance to Echinocandins PRIMARYC. neoformans Fusarium spp. SECONDARY (?) The only licensed member is caspofungin (Jan 2001, USA). Resistant mutants due to therapy are not available.

  36. Echinocandin ResistanceMolecular Aspects • FKS1 encodes glucan synthase • GNS1 encodes an enzyme involved in fatty acid elongation Resistance is observed following laboratory derived mutations in FKS1 or GNS1 • Other mechanisms (?)

  37. Future Directions to Avoid Development of Resistance • Proper dosing strategies • Restricted and well-defined indications for prophylaxis with azoles  Fungi will continue to develop NEW resistance mechanisms!..

  38. Final word • Antifungal resistance is a complex, gradual and multifactorial issue • Several uncertainties remain • Molecular assays to detect resistance are not simple • The best way to improve the efficacy of antifungal therapy is to improve the immune status of the host

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