Echinocandin drugs resistance.

G. Garcia-Effron and D. Perlin

Author address: 

New Jersey Medical School-UMDNJ, Newark, USA.

Abstract: 

Background. The treatment choices for invasive fungal infections are limited by the toxicity of the available drugs and by the limited number of fungal targets. The echinocandins drugs are the newest class of antifungal drugs and the first group targeting the fungal cell wall. The three echinocandin drugs, caspofungin (CSF), micafungin (MCF) and anidulafungin (ANF) share spectrum, mechanism of action, mechanism of resistance and in vivo potency. Spectrum and in vitro susceptibility breakpoints. Echinocandins are fungicidal against most Candida spp. and fungistatic against Aspergillus spp. and other moulds. Nevertheless, certain Candida species such as C. parapsilosis, C. orthopsilosis, C. metapsilosis and C. guilliermondii show intrinsic echinocandin reduced susceptibility phenotypes. It was determined for a wide range of Candida species that greater than 99% of isolates were inhibited by 2 μg/ml of either CSF, MCF, or ANF. Therefore, the CLSI Antifungal Subcommittee recently established an MIC of 2 μg/ml as an interpretive MIC breakpoint for susceptibility of Candida spp. to the three echinocandin drugs. Mechanism of action. Echinocandins inhibit β -1,3-D-glucan synthase, disrupting the structure of the growing cell wall causing osmotic instability and death of susceptible cells. β - 1,3-D-glucan synthase is a protein complex formed by catalytic subunits (Fksp) and a ubiquitous regulatory element (Rho1p). Echinocandin drugs target Fksp, which is encoded by 3 putative FKS genes in Saccharomyces spp., Candida spp. and Aspergillus spp. (FKS1, FKS2 and FKS3). Echinocandin mechanism of resistance. Global surveillance studies showed no changes in echinocandin mean MIC or MIC distributions since the approval of CSF in 2001. However, echinocandin clinical failures due to resistance are increasingly being reported. In Candida spp., intrinsic and secondary echinocandin resistance was exclusively linked to mutations in two conserved regions of the Fksp subunits termed hot spot (HS) 1 and 2. These mutations include non-conservative amino acid substitutions within HS1 (C. albicans Fks1p aa.: Phe641, Ser645, Asp648 and Phe649) and HS2 (C. albicans Fks1p: Trp1358, Arg1360 and Arg1361). The mutations at the C-terminal end of the HS1 and in the HS2 show the weakest phenotypes (CSF MIC=2 μg/ml). Amino acid substitutions at Phe641 and Ser645 show the highest frequency of substitutions (85% of the echinocandin resistant isolates) and the strongest phenotype (CSF MIC ≥ 4 μg/ml). The mutations are dominant, conferring crossresistance to all three echinocandin drugs as homo or heterozygosis genotypes. Murine infection models confirmed that fks mutant isolates are virulent and resistant to echinocandin treatment. The universal role of Fksp modifications as the mechanism of echinocandin resistance was confirmed for A. fumigatus laboratory mutants, Fusarium solani and Scedosporium spp. However, only a single clinical A. fumigatus isolate described so far showed reduced CSF susceptibility due to an increased expression of the FKS1 gene with no FKS mutations. β -1,3-D-glucan synthase complexes biochemistry. A detailed kinetic characterization of β -1,3-D-glucan synthase complexes demonstrated that amino acid substitutions in Fksp reduce drug sensitivity by 2-3 log orders relative to wild type enzymes. These mutations were also found to influence the relative expression of FKS genes and to reduce the catalytic efficiency of the enzyme (Vmax). These kinetic alterations translate into a decreased fitness in C. albicans fks mutants in a murine mixed infection model using wild type and mutant clonal strains. CLSI echinocandin breakpoints revision. In view of the association of FKS mutations and reduced susceptibility of β -1,3-D-glucan synthase, an evaluation of the new CLSI echinocandin susceptibility breakpoint was conducted in Candida spp. Overall, the β -1,3-Dglucan synthase kinetic inhibition data supported the proposed susceptibility breakpoint for CSF in C. albicans, but a lower susceptibility breakpoint (≥ 0.25 μg/ml) may be more appropriate for ANF and MCF. Also, the available data indicate that MIC testing with CSF may serve as a surrogate marker for resistance among the class of echinocandin drugs.
2009

abstract No: 

W11.2

Full conference title: 

4th Trends in Medical Mycology
    • TIMM 4th (2012)