Specific substitutions in the echinocandin target Fks1p account for reduced susceptibility of rare laboratory and clinical Candida sp. isolates

[In vitro activity of anidulafungin. Comparison with the activity of other echinocandins]

Anidulafungin is a new echinocandin that acts by inhibiting (1,3)-beta-D-glucan synthesis in the fungal cell wall. This agent is a semisynthetic lipopeptide synthesized from a fermentation product of Aspergillus nidulans. The spectrum of activity of anidulafungin includes Candida and Aspergillus, the two main etiological agents causing invasive fungal infections. This drug is also active against strains of these genera resistant to azoles or amphotericin B. However, anidulafungin is not active against Cryptococcus spp., Trichosporon spp., Fusarium spp. or Mucorales spp. Data on the activity of this drug against other species are limited and do not allow conclusions to be drawn or recommendations to be made. Echinocandin resistance is uncommon and has little clinical relevance.

[The mechanisms of resistance to echinocandin class of antifungal drugs]

The echinocandin (candin) class of antifungal drugs inhibit beta-1,3-glucan synthase and block synthesis of beta-1,3-glucan , an important polysaccharide in fungal cell walls. Candins are used widely for treatment of systemic infections caused by Candida and Aspergillus because of their high potency and low toxicity to humans. The incidence of candin resistance has been rare compared to that of azole resistance, although candin-resistant clinical isolates of C. albicans, C. glabrata, C. krusei and C. tropicalis have been reported in the USA and Europe in recent years. These isolates possess hundred-fold higher MIC values for candins than sensitive strains, as well as candin-resistant beta-1,3-glucan synthase activities. Their candin resistance is associated with amino acid substitutions in the echinocandin resistant region (Ech) of the FKS gene that encodes a catalytic subunit of the beta-1,3-glucan synthase. However, the effect of these amino acid substitutions on the drug-protein interaction and the molecular basis for the resistance is unknown. The exposure of fungi to candin drugs induces stress responses that activate networks involving transcriptional regulators and components controlling signal transduction of the pathways responsible for maintenance of fungal cell wall integrity. The fungal cell wall is still an attractive drug target and further investigation into the mechanisms of candin resistance and structural analysis of the beta-1,3-glucan synthase protein complex will facilitate the development of broad spectrum inhibitors of fungal cell wall synthesis.

Antifungal drug resistance mechanisms

Antifungal resistance is a prominent feature in the management of invasive mycoses, with important implications for morbidity and mortality. Microbiological resistance, the most common cause of refractory infection, is associated with a fungal pathogen for which an antifungal MIC is higher than average or within the range designated as the resistant breakpoint. Four major mechanisms of resistance to azoles have been described in Candida spp.: decreased intracellular drug concentration by activation of efflux systems or reduction of drug penetration, modification of the target site, upregulation of the target enzyme and development of bypass pathways. Conversely, echinocandins are a poor substrate for multidrug efflux transporters, and their mechanisms of resistance are associated with point mutations and/or overexpression of FKS1 and FKS2 genes. Acquired resistance to flucytosine results from defects in its metabolism through enzymatic mutations, whereas resistance to amphotericin B may be mediated by increased catalase activity or defects in ergosterol biosynthesis.

Overview of the changing epidemiology of candidemia

Candida spp. are currently the fourth most common cause of bloodstream infections in US hospitals, and the third most common cause of bloodstream infections in the intensive care unit. Over the last 2 decades there has been a shift towards a greater involvement of non-Candida albicans spp. as the cause of candidemia. Several of these non-albicans spp. (e.g., C. glabrata and C. krusei ) exhibit resistance to traditional triazole antifungals like fluconazole, and cross-resistance with newer triazoles, focusing attention on the first-line use of antifungals such as the echinocandins, which possess improved activity against fluconazole-resistant strains. Recent treatment guidelines from the Infectious Diseases Society of America (IDSA) recommend an echinocandin as primary therapy for nonneutropenic or neutropenic patients with moderately severe to severe candidiasis and for patients at risk for infection with a triazole-resistant strain. However, further improvement in candidemia-associated mortality will only be attainable with the development and validation of new diagnostic tools that will allow earlier detection, discrimination, and treatment of invasive candidiasis. Clinicians should remain vigilant to wider emergence of Candida spp. with echinocandin resistance.

Anidulafungin and its role in candida infections

Candida infections continue to play a significant role not only in critically ill and immunocompromised patients but also in non-compromised patients. The incidence of systemic fungal infections in the United States has been on the rise for the past 30 years. Anidulafungin and all echinocandins inhibit glucan synthase thus inhibiting the formation of 1,3-β-D-glucan which is an essential component of the fungal cell wall. The decrease in 1,3-β-D-glucan results in the osmotic lysis of the cell, resulting in fungicidal activity against candida. Anidulafungin is active against most species of candida and resistance to it is very rare. Two potential mechanisms conferring reduced susceptibility to the echinocandins are efflux and target alteration. The efflux pump associated with fluconazole resistance in Candida albicans can confer higher minimum inhibitory concentrations to caspofungin. The second mechanism of resistance is via mutations in the genes which code for 1,3 β-D-glucan synthase, specifically FKS1. Because of its spectrum of activity, fungicidal nature, and tolerability it is an attractive first-line therapeutic choice for treating candidemia in both non-neutropenic and neutropenic patients. Because it is available only parenterally its role in treating mucocutaneous candidiasis is primarily in patients unable to take oral therapy.

Effect of Candida glabrata FKS1 and FKS2 mutations on echinocandin sensitivity and kinetics of 1,3-beta-D-glucan synthase: implication for the existing susceptibility breakpoint

Thirteen Candida glabrata strains harboring a range of mutations in hot spot regions of FKS1 and FKS2 were studied. The mutations were linked to an echinocandin reduced susceptibility phenotype. Sequence alignments showed that 11 out of the 13 mutants harbored a mutation in FKS1 or FKS2 not previously implicated in echinocandin reduced susceptibility in C. glabrata. A detailed kinetic characterization demonstrated that amino acid substitutions in Fks1p and Fks2p reduced drug sensitivity in mutant 1,3-beta-D-glucan synthase by 2 to 3 log orders relative to that in wild-type enzyme. These mutations were also found to reduce the catalytic efficiency of the enzyme (Vmax) and to influence the relative expression of FKS genes. In view of the association of FKS mutations and reduced susceptibility of 1,3-beta-D-glucan synthase, an evaluation of the new CLSI echinocandin susceptibility breakpoint was conducted. Only 3 of 13 resistant fks mutants (23%) were considered anidulafungin or micafungin nonsusceptible (MIC > 2 microg/ml) by this criterion. In contrast, most fks mutants (92%) exceeded a MIC of >2 microg/ml with caspofungin. However, when MIC determinations were performed in the presence of 50% serum, all C. glabrata fks mutants showed MICs of > or = 2 microg/ml for the three echinocandin drugs. As has been observed with Candida albicans, the kinetic inhibition parameter 50% inhibitory concentration may be a better predictor of FKS-mediated resistance. Finally, the close association between FKS1/FKS2 hot spot mutations provides a basis for understanding echinocandin resistance in C. glabrata.

Anidulafungin is fungicidal and exerts a variety of postantifungal effects against Candida albicans, C. glabrata, C. parapsilosis, and C. krusei isolates

Anidulafungin targets the cell walls of Candida species by inhibiting beta-1,3-glucan synthase, thereby killing isolates and exerting prolonged postantifungal effects (PAFEs). We performed time-kill and PAFE experiments on Candida albicans (n = 4), C. glabrata (n = 3), C. parapsilosis (n = 3), and C. krusei (n = 2) isolates and characterized the PAFEs in greater detail. MICs were 0.008 to 0.125 microg/ml against C. albicans, C. glabrata, and C. krusei and 1.0 to 2.0 microg/ml against C. parapsilosis. During time-kill experiments, anidulafungin caused significant kills at 16x MIC (range, log 2.68 to 3.89) and 4x MIC (log 1.87 to 3.19), achieving fungicidal levels (>or=log 3) against nine isolates. A 1-hour drug exposure during PAFE experiments resulted in kills ranging from log 1.55 to 3.47 and log 1.18 to 2.89 (16x and 4x MIC, respectively), achieving fungicidal levels against four isolates. Regrowth of all 12 isolates was inhibited for >or=12 h after drug washout. Isolates of each species collected 8 h after a 1-hour exposure to anidulafungin (16x and 4x MIC) were hypersusceptible to sodium dodecyl sulfate (0.01 to 0.04%) and calcofluor white (40 microg/ml). Moreover, PAFEs were associated with major cell wall disturbances, as evident in electron micrographs of viable cells, and significant reductions in adherence to buccal epithelial cells (P <or= 0.01). Finally, three of four PAFE isolates tested were hypersusceptible to killing by J774 macrophages (P <or= 0.007). Our data suggest that the efficacy of anidulafungin in the treatment of candidiasis might stem from both direct fungicidal activity and indirect PAFEs that lessen the ability of Candida cells to establish invasive disease and to persist within infected hosts.

[Micafungin for therapy of invasive candidiasis in solid organ transplant recipients]

BACKGROUND

Micafungin is an echinocandin approved for the prevention of Candida spp. infection in hematopoietic stem cell transplantation and therapy of oesophageal candidiasis, disseminated candidiasis and candidemia in adults, children and neonates.

AIMS

To evaluate the role of micafungin for candidiasis therapy in solid organ transplant recipients.

METHODS

A medical literature review according to micafungin role for candidiasis therapy in transplant patients is performed.

RESULTS

Micafungin has shown fungicide activity against Candida species, including strains resistant or poorly susceptible to fluconazole. No dose adjustment is required when micafungin is administered in combination with other drugs used in transplant patients, excluding sirolimus, nifedipine and itraconazol. With these drugs, a minimal dose reduction is recommended. The results observed in transplant patients included in clinical trials are favourable and similar to results obtained in other kind of patients.

CONCLUSIONS

The clinical results, its safety profile and the low grade of medical interactions permit micafungin to be considered for therapy in specific groups of transplant patients.

Role for Fks1 in the intrinsic echinocandin resistance of Fusarium solani as evidenced by hybrid expression in Saccharomyces cerevisiae

The opportunistic mold Fusarium solani is intrinsically resistant to cell wall synthesis-inhibiting echinocandins (ECs), including caspofungin and micafungin. Mutations that confer acquired EC resistance in Saccharomyces cerevisiae and other normally susceptible yeast species have been mapped to the Fks1 gene; among these is the mutation of residue 639 from Phe to Tyr (F639Y) within a region designated hot spot 1. Fks1 sequence analysis identified the equivalent of Y639 in F. solani as well as in Scedosporium prolificans, another intrinsically EC-resistant mold. To test its role in intrinsic EC resistance, we constructed Fks1 hybrids in S. cerevisiae that incorporate F. solani hot spot 1 and flanking residues. Hybrid construction was accomplished by a PCR-based method that was validated by studies with Fks1 sequences from EC-susceptible Aspergillus fumigatus and paired EC-susceptible and -resistant Candida glabrata isolates. In support of our hypothesis, hybrid Fks1 incorporating F. solani hot spot 1 conferred significantly reduced EC susceptibility, 4- to 8-fold less than that of wild-type S. cerevisiae and 8- to 32-fold less than that of the same hybrid with an F639 mutation. We propose that Fks1 sequences represent determinants of intrinsic EC resistance in Fusarium and Scedosporium species and, potentially, other fungi.

Anidulafungin: a novel echinocandin for candida infections

A third echinocandin, anidulafungin, has recently been approved for Candida infections in the non-neutropenic patient. In the EU it is indicated for invasive candidiasis; in 2006 it was approved in the USA for candida esophagitis, candidemia, and two types of invasive infections, peritonitis and intra-abdominal abscesses. It is fungicidal against Candida species and fungistatic against Aspergillus species. In addition to its favorable tolerability in studies to date, it does not need adjustment for renal or hepatic insufficiency and has no known drug interactions. A steady state concentration can be achieved on day 2 following a loading dose of twice the maintenance concentration on day 1, and the drug is administered intravenously once daily. Cross resistance with other classes of antifungals is not a concern as it possesses a unique mechanism of action.

Differential in vitro activity of anidulafungin, caspofungin and micafungin against Candida parapsilosis isolates recovered from a burn unit

Recent studies suggest that differences in antifungal activity among echinocandins may exist. In this study, the activities of three echinocandins (anidulafungin, caspofungin, and micafungin) against Candida parapsilosis isolates from burn unit patients, healthcare workers and the hospital environment were determined. Additionally, the effect of these echinocandins on the cell morphology of caspofungin-susceptible and caspofungin-non-susceptible isolates was assessed using scanning electron microscopy (SEM). The C. parapsilosis isolates obtained from patients were susceptible to anidulafungin, but were less so to caspofungin and micafungin. Isolates obtained from healthcare workers or environmental sources were susceptible to all antifungals. SEM data demonstrated that although anidulafungin and caspofungin were equally active against a caspofungin-susceptible C. parapsilosis strain, they differed in their ability to damage a caspofungin-non-susceptible strain, for which lower concentrations of anidulafungin (1 mg/L) than of caspofungin (16 mg/L) were needed to induce cellular damage and distortion of the cellular morphology. To determine whether the difference in the antifungal susceptibility of C. parapsilosis isolates to anidulafungin as compared to the other two echinocandins could be due to different mutations in the FKS1 gene, the sequences of the 493-bp region of this gene associated with echinocandin resistance were compared. No differences in the corresponding amino acid sequences were observed, indicating that differences in activity between anidulafungin and the other echinocandins are not related to mutations in this region. The results of this study provide evidence that differences exist between the activities of anidulafungin and the other echinocandins.

Micafungin in the treatment of invasive candidiasis and invasive aspergillosis

Micafungin is an echinocandin antifungal agent available for clinical use in Japan, Europe, and the United States. Through inhibition of β-1,3-glucan production, an essential component of the fungal cell wall, micafungin exhibits potent antifungal activity against key pathogenic fungi, including Candida and Aspergillus species, while contributing minimal toxicity to mammalian cells. This activity is maintained against polyene and azole-resistant isolates. Pharmacokinetic and pharmacodynamic studies have demonstrated linear kinetics both in adults and children with concentration-dependent activity observed both in vitro and in vivo. Dosage escalation studies have also demonstrated that doses much higher than those currently recommended may be administered without serious adverse effects. Clinically, micafungin has been shown to be efficacious for the treatment of invasive candidiasis and invasive aspergillosis. Furthermore, the clinical effectiveness of micafungin against these infections occurs without the drug interactions that occur with the azoles and the nephrotoxicity observed with amphotericin B formulations. This review will focus on the pharmacology, clinical microbiology, mechanisms of resistance, safety, and clinical efficacy of micafungin in the treatment of invasive candidiasis and invasive aspergillosis.

Breakthrough Aspergillus fumigatus and Candida albicans double infection during caspofungin treatment: laboratory characteristics and implication for susceptibility testing

Caspofungin is used for the treatment of acute invasive candidiasis and as salvage treatment for invasive aspergillosis. We report characteristics of isolates of Candida albicans and Aspergillus fumigatus detected in a patient with breakthrough infection complicating severe gastrointestinal surgery and evaluate the capability of susceptibility methods to identify candin resistance. The susceptibility of C. albicans to caspofungin and anidulafungin was investigated by Etest, microdilution (European Committee on Antibiotic Susceptibility Testing [EUCAST] and CLSI), disk diffusion, agar dilution, and FKS1 sequencing and in a mouse model. Tissue was examined by immunohistochemistry, PCR, and sequencing for the presence of A. fumigatus and resistance mutations. The MICs for the C. albicans isolate were as follows: >32 microg/ml caspofungin and 0.5 microg/ml anidulafungin by Etest, 2 microg/ml caspofungin and 0.125 microg/ml anidulafungin by EUCAST methods, and 1 microg/ml caspofungin and 0.5 microg/ml anidulafungin by CLSI methods. Sequencing of the FKS1 gene revealed a mutation leading to an S645P substitution. Caspofungin and anidulafungin failed to reduce kidney CFU counts in animals inoculated with this isolate (P > 0.05 compared to untreated control animals), while both candins completely sterilized the kidneys in animals infected with a control isolate. Disk diffusion and agar dilution methods clearly separated the two isolates. Immunohistochemistry and sequencing confirmed the presence of A. fumigatus without FSK1 resistance mutations in liver and lung tissues. Breakthrough disseminated aspergillosis and candidiasis developed despite an absence of characteristic FKS1 resistance mutations in the Aspergillus isolates. EUCAST and CLSI methodology did not separate the candin-resistant clinical isolate from the sensitive control isolate as well as did the Etest and agar methods.

Correlating echinocandin MIC and kinetic inhibition of fks1 mutant glucan synthases for Candida albicans: implications for interpretive breakpoints

A detailed kinetic characterization of echinocandin inhibition was performed for mutant 1,3-beta-d-glucan synthase enzymes from clinical isolates of Candida albicans with nine different FKS1 mutations resulting in high MICs. Among 14 mutant Fks1p enzymes studied, the kinetic parameters 50% inhibitory concentration and K(i) increased 50-fold to several thousandfold relative to those for the wild type. Enzymes with mutations at Ser645 (S645P, S645Y, and S645F) within hot spot 1 showed the most prominent decrease in sensitivity, while those with mutations at the N- and C-terminal ends of hot spot 1 generally retained greater sensitivity to all three drugs. Kinetic inhibitions by caspofungin, micafungin, and anidulafungin were comparable among the fks1 mutant enzymes, although absolute values did vary with specific mutations. Amino acid substitutions in Fks1p did not alter K(m) values, although some mutations decreased the V(max). Given the association of FKS1 mutations with clinical resistance, an evaluation of the kinetic parameters for the inhibition of mutant 1,3-beta-D-glucan synthase as a function of the MIC enabled an independent evaluation of the recently adopted susceptibility breakpoint for echinocandin drugs. Overall, a breakpoint MIC of >or=2 microg/ml for caspofungin captured nearly 100% of fks1 C. albicans strains when a kinetic inhibition rise threshold of <or=50-fold for the K(i) was used as a measure of susceptibility. A similar MIC breakpoint for micafungin and anidulafungin was less inclusive, and a projected MIC of >or=0.5 microg/ml was required for >95% coverage of clinical isolates. However, when MIC determinations were performed in the presence of 50% serum, all fks1 mutants showed MIC values of >or=2 microg/ml for the three echinocandin drugs. The 1,3-beta-D-glucan synthase kinetic inhibition data support the proposed susceptibility breakpoint for caspofungin in C. albicans, but a lower susceptibility breakpoint (<or=0.5 microg/ml) may be more appropriate for anidulafungin and micafungin. Overall, the data indicate that MIC testing with caspofungin may serve as a surrogate marker for resistance among the class of echinocandin drugs.

[The fungal cell wall and the mechanism of action of anidulafungin]

The fungal cell wall is a structure with a high plasticity that protects the cell from different types of environmental stresses including changes in osmotic pressure. In addition to that, the cell wall allows the fungal cell to interact with its environment, since some of its proteins are adhesins and receptors. Some of its components are highly immunogenic. The structure of the fungal cell wall is unique to the fungi, and it is composed of glucan, chitin and glycoproteins. Since humans lack the components present in the cell walls of fungi, this structure is an excellent target for the development of antifungal drugs. Anidulafungin, like the rest of echinocandins acts on beta-1,3-D-glucan synthase inhibiting the formation of beta-1,3-D-glucan and causing, depending on the type of fungus, a fungicidal or either a fungistatic effect.

Mechanisms of resistance to antifungal agents: yeasts and filamentous fungi

Failure to respond to antifungal therapy could be due to in vitro resistance (intrinsic or developed during therapy) or clinical resistance; the latter is associated with numerous factors related to the host, the antifungal agent, or the infecting isolate. Recently, a susceptible MIC breakpoint ( < or =2 microg/ml) was designed for Candida spp. to all three available echinocandins, anidulafungin (Pfizer), caspofungin (Merck) and micafungin (Astellas) and treatment failures have been associated with MICs > 2 microg/ml. In some of these cases, clinical failure was associated with the genetic mutations described below. Azole and flucytosine breakpoints, and the echinocandin susceptible breakpoint, are useful when isolates are tested by CLSI standardized methods; breakpoints are also available by the EUCAST method. More recently, in vitro resistant MIC breakpoints have been assigned for filamentous fungi (moulds) vs. five antifungal agents, but these categories are not based on correlations of in vitro with in vivo response to therapy. However, itraconazole (Janssen), amphotericin B (Bristol-Myers) and voriconazole (Pfizer) clinical failures in aspergillosis have been correlated with MICs > 2 microg/ml. This article provides a review of reported resistance molecular mechanisms to antifungal agents since 2005; previous related reviews are also listed.

Functional analysis of Candida albicans GPI-anchored proteins: roles in cell wall integrity and caspofungin sensitivity

The outer layer of the Candida albicans cell wall is enriched in highly glycosylated proteins. The major class, the GlycosylPhosphatidylInositol (GPI)-anchored proteins are tethered to the wall by GPI-anchor remnants and include adhesins, glycosyltransferases, yapsins and superoxide dismutases. In silico analysis suggested that C. albicans possesses 115 putative GPI anchored proteins (GpiPs), almost twice the number reported for Saccharomyces cerevisiae. A global approach to characterise in silico predicted GpiPs has been initiated by generating a library of 45 mutants. This library was subjected to a screen for cell wall modifications by testing the cell wall integrity (SDS and Calcofluor White sensitivity) and response to caspofungin. We showed that, when caspofungin sensitivity was modified, in more than half of the cases the susceptibility can be correlated to the level of chitin and cell wall thickness: sensitive strains have low level of chitin and a thin cell wall. We also identified, for the first time, genes that when deleted lead to decreased caspofungin sensitivity: DFG5, PHR1, PGA4 and PGA62. The role of two unknown GpiPs, Pga31 and Pga62 in the cell wall structure and composition was clearly demonstrated during this study.

Development of caspofungin resistance following prolonged therapy for invasive candidiasis secondary to Candida glabrata infection

We report a case of Candida glabrata invasive candidiasis that developed reduced susceptibility to caspofungin during prolonged therapy. Pre- and posttreatment isolates were confirmed to be isogenic, and sequencing of hot spots known to confer echinocandin resistance revealed an F659V substitution within the FKS2 region of the glucan synthase complex.

Pyrosequencing to detect mutations in FKS1 that confer reduced echinocandin susceptibility in Candida albicans

Pyrosequencing was compared to Sanger dideoxy sequencing to detect mutations in FKS1 responsible for reduced echinocandin susceptibility in Candida albicans. These methods were in complete agreement for 10 of 12 clinical isolates with elevated echinocandin MICs, supporting the potential feasibility of pyrosequencing to detect mutations within diploid fungi.

Caspofungin-resistant Candida tropicalis strains causing breakthrough fungemia in patients at high risk for hematologic malignancies

We identified three cases of C. tropicalis strains causing breakthrough fungemia in allogeneic stem cell recipients receiving caspofungin prophylaxis and treatment. Three genetically unrelated isolates with high echinocandin MICs were identified. Each strain carried a characteristic mutation conferring an amino acid substitution within Fks1p hot spot 1.

Micafungin activity against Candida albicans with diverse azole resistance phenotypes

OBJECTIVES

The purpose of this study was to investigate whether mechanisms of azole resistance in Candida albicans contribute to reduced micafungin activity in vitro.

METHODS

MICs were determined for a collection of strains with well-characterized mechanisms of azole resistance obtained from systemic, oral and vaginal infections. This collection of strains includes those with resistance-associated phenotypes. All known molecular mechanisms of azole resistance are included in this set of isolates (alone or in combination). Micafungin activity was further investigated for a subset of isolates by agar dilution.

RESULTS

There was no correlation between any of the azole resistance mechanisms or resistance phenotypes and micafungin activity as determined by MIC, even in isolates with cross-resistance to multiple azole drugs. Overexpression of the ABC transporter CDR2 has been suggested to contribute to reduced echinocandin activity in agar dilution studies. By broth microdilution, there was no difference in MIC between the pump overexpressors and the collection as a whole. However, azole-resistant isolates from matched strains exhibited a small increase in their micafungin MICs relative to their susceptible controls. By agar dilution analysis, multiple CDR2-overexpressing strains exhibited reduced growth in the presence of micafungin relative to the laboratory strain SC5314.

CONCLUSIONS

Azole resistance mechanisms do not contribute to increased micafungin MIC as determined by broth microdilution. However, within sets of matched isolates, strains overexpressing CDR2 had a slight increase in micafungin MIC. Changes in micafungin susceptibility are associated with CDR2 overexpression in agar dilution tests.

Establishing in vitro-in vivo correlations for Aspergillus fumigatus: the challenge of azoles versus echinocandins

Two clinical isolates of Aspergillus fumigatus, designated AT and DK, were recently obtained from patients failing caspofungin and itraconazole therapy, respectively. The isolates were tested by microdilution for susceptibility to itraconazole, voriconazole, posaconazole, ravuconazole, and caspofungin and by Etest for susceptibility to amphotericin B and caspofungin. Susceptibility testing documented that the DK isolate was azole resistant (itraconazole and posaconazole MICs, >4 microg/ml; voriconazole MIC, 2 microg/ml; ravuconazole MIC, 4 microg/ml), and the resistance was confirmed in a hematogenous mouse model, with mortality and the galactomannan index as the primary and secondary end points. Sequencing of the cyp51A gene revealed the M220K mutation, conferring multiazole resistance. The Etest, but not microdilution, suggested that the AT isolate was resistant to caspofungin (MIC, >32 microg/ml). In the animal model, this isolate showed reduced susceptibility to caspofungin. Sequencing of the FKS1 gene revealed no mutations; the enzyme retained full sensitivity in vitro; and investigation of the polysaccharide composition showed that the beta-(1,3)-glucan proportion was unchanged. However, gene expression profiling by Northern blotting and real-time PCR demonstrated that the FKS gene was expressed at a higher level in the AT isolate than in the susceptible control isolate. To our knowledge, this is the first report to document the presence of multiazole-resistant clinical isolates in Denmark and to demonstrate reduced susceptibility to caspofungin in a clinical A. fumigatus isolate with increased expression of the FKS gene. Further research to determine the prevalence of resistance in A. fumigatus worldwide, and to develop easier and reliable tools for the identification of such isolates in routine laboratories, is warranted.

Mutations in the fks1 gene in Candida albicans, C. tropicalis, and C. krusei correlate with elevated caspofungin MICs uncovered in AM3 medium using the method of the European Committee on Antibiotic Susceptibility Testing

Mutations in two specific regions of the Fks1 subunit of 1,3-beta-D-glucan synthase are known to confer decreased caspofungin susceptibility on Candida spp. Clinical isolates of Candida spp. (404 Candida albicans, 62 C. tropicalis, and 21 C. krusei isolates) sent to the French National Reference Center were prospectively screened for susceptibility to caspofungin in vitro by the broth microdilution reference method of the Antifungal Susceptibility Testing Subcommittee of the European Committee on Antibiotic Susceptibility Testing (AFST-EUCAST). Twenty-eight isolates (25 C. albicans, 2 C. tropicalis, and 1 C. krusei isolate) for which the caspofungin MIC was above the MIC that inhibited 90% of the isolates of the corresponding species (MIC(90)) were subjected to molecular analysis in order to identify mutations in the fks1 gene. Substitutions in the deduced protein sequence of Fks1 were found for 8 isolates, and 20 isolates had the wild-type sequence. Among the six C. albicans isolates harboring mutations, six patterns were observed involving amino acid changes at positions 641, 645, 649, and 1358. For C. tropicalis, one isolate showed an L644W mutation, and for one C. krusei isolate, two mutations, L658W and L701M, were found. Two media, RPMI medium and AM3, were tested for their abilities to distinguish between isolates with wild-type Fks1 and those with mutant Fks1. In RPMI medium, caspofungin MICs ranged from 0.25 to 2 microg/ml for wild-type isolates and from 1 to 8 micro for mutant isolates. A sharper difference was observed in AM3: all wild-type isolates were inhibited by 0.25 micro of caspofungin, while caspofungin MICs for all mutant isolates were >or=0.5 microg/ml. These data demonstrate that clinical isolates of C. albicans, C. tropicalis, and C. krusei with decreased susceptibility to caspofungin in vitro have diverse mutations in the fks1 gene and that AM3 is potentially a better medium than RPMI for distinguishing between mutant and wild-type isolates using the AFST-EUCAST method.

Correlation of MIC with outcome for Candida species tested against caspofungin, anidulafungin, and micafungin: analysis and proposal for interpretive MIC breakpoints

The CLSI Antifungal Subcommittee followed the M23-A2 "blueprint" to develop interpretive MIC breakpoints for anidulafungin, caspofungin, and micafungin against Candida species. MICs of < or = 2 microg/ml for all three echinocandins encompass 98.8 to 100% of all clinical isolates of Candida spp. without bisecting any species group and represent a concentration that is easily maintained throughout the dosing period. Data from phase III clinical trials demonstrate that the standard dosing regimens for each of these agents may be used to treat infections due to Candida spp. for which MICs are as high as 2 microg/ml. An MIC predictive of resistance to these agents cannot be defined based on the data from clinical trials due to the paucity of isolates for which MICs exceed 2 microg/ml. The clinical data set included only three isolates from patients treated with an echinocandin (caspofungin) for which the MICs were > 2 microg/ml (two C. parapsilosis isolates at 4 microg/ml and one C. rugosa isolate at 8 microg/ml). Based on these data, the CLSI subcommittee has decided to recommend a "susceptible only" breakpoint MIC of < or = 2 microg/ml due to the lack of echinocandin resistance in the population of Candida isolates thus far. Isolates for which MICs exceed 2 microg/ml should be designated "nonsusceptible" (NS). For strains yielding results suggestive of an NS category, the organism identification and antimicrobial-susceptibility test results should be confirmed. Subsequently, the isolates should be submitted to a reference laboratory that will confirm the results by using a CLSI reference dilution method.

Micafungin for the prophylaxis and treatment of Candida infections

Invasive fungal infection is a significant cause of morbidity and mortality worldwide. The incidence of these infections is steadily increasing. In addition, strains resistant to many commonly used antifungal agents are becoming more prevalent. Many new antifungals have become commercially available in recent years, which have vastly improved the ability to treat these infections effectively. Micafungin is one of three commercially available echinocandins available for use in the USA. This class of agents possess a unique mechanism of action that helps to reduce toxicity while maintaining potent antifungal activity. Micafungin is currently approved for the treatment of esophageal candidiasis in adults and is the only in its class approved for the prophylaxis of Candida infection in patients who have undergone hematopoietic stem cell transplantation. It was also recently approved in the USA for the treatment of candidemia and other forms of invasive candiaisis (acute disseminated candiaisis, Candida peritonitis and abscess). In general, micafungin is well tolerated and has favorable safety and drug-interaction profiles.

[The role of anidulafungin therapy in solid organ transplant recipients]

Anidulafungin is a new echinocandin recently approved for the treatment of esophageal candidiasis, candidemia and other forms of invasive candidiasis, such as peritonitis and intra-abdominal abscesses in non-neutropenic patients. It is fungicidal against Candida spp. and fungistatic against Aspergillus spp. It is active against Pneumocystis jirovecii. In contrast, anidulafungin does not have activity against Cryptococcus neoformans, Zygomycetes or molds, other than Aspergillus spp. The drug is well tolerated, even in patients with renal or hepatic impairment. In contrast to other echinocandins, it does not significantly interfere with the cytochrome P450 pathway and has a low drug-drug interaction profile, including calcineurinic agents and other drugs used in transplant recipients. So far, anidulafungin appears to have an excellent safety profile with few adverse events and it promises a special consideration in the management of fungal infections happening in transplant recipients.

Advances in antifungal therapy

The prevalence of invasive fungal infections (IFIs) has increased over the past three decades owing to the increasing numbers of immunocompromised hosts. These infections are associated with significant morbidity and mortality. Recent significant advances in antifungal therapy include the broad-spectrum triazoles (voriconazole and posaconazole) and a new class of antifungals, the echinocandins (caspofungin, micafungin, and anidulafungin). New treatment strategies, such as combination therapy and pre-emptive therapy, are being investigated. There have also been significant improvements in diagnostics; the galactomannan enzyme immunoassay and the beta-glucan test are now part of the EORTC/MSG criteria for diagnosis of IFI. Despite these advances, there remain a number of unanswered questions regarding optimal management of serious fungal infections, and research continues to discover and develop new therapies and evaluate new management strategies.

Clinical evaluation of the Sensititre YeastOne colorimetric antifungal panel for antifungal susceptibility testing of the echinocandins anidulafungin, caspofungin, and micafungin

A commercially prepared, dried colorimetric microdilution panel (Sensititre YeastOne Trek Diagnostic Systems, Cleveland, OH) was compared in three different laboratories with the Clinical and Laboratory Standards Institute (CLSI) reference microdilution method by testing 2 quality control strains, 25 reproducibility strains, and 404 isolates of Candida spp. against anidulafungin, caspofungin, and micafungin. Reference MIC endpoints and YeastOne colorimetric endpoints were read after 24 h of incubation. YeastOne endpoints were determined to be the lowest concentration at which the color in the well changed from red (positive, indicating growth) to blue (negative, indicating no growth). Excellent essential agreement (within 2 dilutions) between the reference and colorimetric MICs was observed. Overall agreement was 100% for all three agents. Categorical agreement ranged from 99.3% (anidulafungin) to 100% (caspofungin, micafungin) and interlaboratory reproducibility was 99%. The YeastOne colorimetric method appears to be comparable to the CLSI reference method for testing the susceptibility of Candida spp. to the echinocandins anidulafungin, caspofungin, and micafungin.

Characterization of caspofungin susceptibilities by broth and agar in Candida albicans clinical isolates with characterized mechanisms of azole resistance

Caspofungin (CSP) susceptibilities of Candida albicans, as determined by broth microdilution methods, have not been found to be related to azole susceptibilities or resistance. In contrast, it has been observed that azole-resistant clinical isolates that overexpress the efflux pump gene CDR2 are less susceptible to CSP when tested using an agar dilution method commonly employed with Saccharomyces cerevisiae. The goal of this study was to further understand the effects of azole resistance mechanisms on CSP susceptibility testing. A collection of 69 isolates exhibiting known mechanisms of azole resistance and resistance-associated phenotypes were analyzed by broth microdilution methods to determine standard minimum inhibitory concentrations (MICs) for CSP. The same isolates were then analyzed as to their MIC to CSP by Etest strips, an agar-based method that has been shown generally to be comparable to broth methods. The MICs found with both methods were not significantly different. However, a collection of strains overexpressing the efflux pump CDR2 did exhibit a spectrum of CSP susceptibilities when examined by agar dilution susceptibility tests, ranging from standard to reduced susceptibilities. This work demonstrated that a change in CSP susceptibility with CDR2 overexpressing cells in agar dilution studies is a variable phenotype and it is not the result of growth conditions (i.e., broth versus agar).

Detection of caspofungin resistance in Candida spp. by Etest

The caspofungin susceptibilities of 28 Candida sp. clinical isolates, including 8 caspofungin-resistant isolates characterized by mutations in the Fks1 protein, were determined by the Etest in RPMI and AM3 media. Good discrimination between wild-type and mutant isolates was obtained. These results suggest that the Etest is valuable for the detection of caspofungin resistance in Candida spp.

A naturally occurring proline-to-alanine amino acid change in Fks1p in Candida parapsilosis, Candida orthopsilosis, and Candida metapsilosis accounts for reduced echinocandin susceptibility

Candida parapsilosis has emerged as a common cause of invasive fungal infection, especially in Latin America and in the neonatal setting. C. parapsilosis is part of a closely related group of organisms that includes the species Candida orthopsilosis and Candida metapsilosis. All three species show elevated MICs for the new echinocandin class drugs caspofungin, micafungin, and anidulafungin relative to other Candida species. Despite potential impacts on therapy, the mechanism behind this reduced echinocandin susceptibility has not been determined. In this report, we investigated the role of a naturally occurring Pro-to-Ala substitution at amino acid position 660 (P660A), immediately distal to the highly conserved hot spot 1 region of Fks1p, in the reduced-echinocandin-susceptibility phenotype. Kinetic inhibition studies demonstrated that glucan synthase from the C. parapsilosis group was 1 to 2 logs less sensitive to echinocandin drugs than the reference enzyme from C. albicans. Furthermore, clinical isolates of C. albicans and C. glabrata which harbor mutations at this equivalent position also showed comparable 2-log decreases in target enzyme sensitivity, which correlated with increased MICs. These mutations also resulted in 2.4- to 18.8-fold-reduced V(max) values relative to those for the wild-type enzyme, consistent with kinetic parameters obtained for C. parapsilosis group enzymes. Finally, the importance of the P660A substitution for intrinsic resistance was confirmed by engineering an equivalent P647A mutation into Fks1p of Saccharomyces cerevisiae. The mutant glucan synthase displayed characteristic 2-log decreases in sensitivity to the echinocandin drugs. Overall, these data firmly indicate that a naturally occurring P660A substitution in Fks1p from the C. parapsilosis group accounts for the reduced susceptibility phenotype.

Stimulation of chitin synthesis rescues Candida albicans from echinocandins

Echinocandins are a new generation of novel antifungal agent that inhibit cell wall beta(1,3)-glucan synthesis and are normally cidal for the human pathogen Candida albicans. Treatment of C. albicans with low levels of echinocandins stimulated chitin synthase (CHS) gene expression, increased Chs activity, elevated chitin content and reduced efficacy of these drugs. Elevation of chitin synthesis was mediated via the PKC, HOG, and Ca(2+)-calcineurin signalling pathways. Stimulation of Chs2p and Chs8p by activators of these pathways enabled cells to survive otherwise lethal concentrations of echinocandins, even in the absence of Chs3p and the normally essential Chs1p, which synthesize the chitinous septal ring and primary septum of the fungus. Under such conditions, a novel proximally offset septum was synthesized that restored the capacity for cell division, sustained the viability of the cell, and abrogated morphological and growth defects associated with echinocandin treatment and the chs mutations. These findings anticipate potential resistance mechanisms to echinocandins. However, echinocandins and chitin synthase inhibitors synergized strongly, highlighting the potential for combination therapies with greatly enhanced cidal activity.

Reduced Candida glabrata susceptibility secondary to an FKS1 mutation developed during candidemia treatment

We describe a case of recurring Candida glabrata infection in a 68-year-old African-American female on caspofungin therapy. The initial isolate was susceptible, but isolates recovered during following relapses were not. All isolates were clonal, and high-MIC strains contained a mutation in the highly conserved hot spot 1 region of Fks1p.

Current status of antifungal susceptibility testing methods

Antifungal susceptibility testing is a very dynamic field of medical mycology. Standardization of in vitro susceptibility tests by the Clinical and Laboratory Standards Institute (CLSI) and the European Committee for Antimicrobial Susceptibility Testing (EUCAST), and current availability of reference methods constituted the major remarkable steps in the field. Based on the established minimum inhibitory concentration (MIC) breakpoints, it is now possible to determine the susceptibilities of Candida strains to fluconazole, itraconazole, voriconazole, and flucytosine. Moreover, utility of fluconazole antifungal susceptibility tests as an adjunct in optimizing treatment of candidiasis has now been validated. While the MIC breakpoints and clinical significance of susceptibility testing for the remaining fungi and antifungal drugs remain yet unclear, modifications of the available methods as well as other methodologies are being intensively studied to overcome the present drawbacks and limitations. Among the other methods under investigation are Etest, colorimetric microdilution, agar dilution, determination of fungicidal activity, flow cytometry, and ergosterol quantitation. Etest offers the advantage of practical application and favorable agreement rates with the reference methods that are frequently above acceptable limits. However, MIC breakpoints for Etest remain to be evaluated and established. Development of commercially available, standardized colorimetric panels that are based on CLSI method parameters has added more to the antifungal susceptibility testing armamentarium. Flow cytometry, on the other hand, appears to offer rapid susceptibility testing but requires specified equipment and further evaluation for reproducibility and standardization. Ergosterol quantitation is another novel approach, which appears potentially beneficial particularly in discrimination of azole-resistant isolates from heavy trailers. The method is yet investigational and requires to be further studied. Developments in methodology and applications of antifungal susceptibility testing will hopefully provide enhanced utility in clinical guidance of antifungal therapy. However, and particularly in immunosuppressed host, in vitro susceptibility is and will remain only one of several factors that influence clinical outcome.

Attenuation of echinocandin activity at elevated concentrations: a review of the paradoxical effect

PURPOSE OF REVIEW

The echinocandins have been a welcome addition for the treatment of invasive fungal infections caused by Candida and Aspergillus species. Despite their excellent safety profile and clinical efficacy, concerns exist regarding an attenuation of activity at higher concentrations, known as the paradoxical effect. This article will review the literature describing this effect, the potential mechanisms responsible for it, and the clinical implications of this phenomenon.

RECENT FINDINGS

In-vitro studies have reported a paradoxical effect at higher concentrations against both Candida and Aspergillus species. Recent data have demonstrated this effect in Candida to be species related and echinocandin specific. Although not completely understood, studies have pointed towards involvement of the protein kinase C cell wall integrity and calcineurin pathways as well as increases in cell wall chitin content as potential mechanisms behind the effect. Although some in-vivo studies with echinocandins have reported a paradoxical increase in markers of invasive disease, clinical data are scarce.

SUMMARY

The clinical implications of the paradoxical attenuation of echinocandin activity observed in vitro and in vivo remain unknown. A complete understanding of this effect may further our knowledge of fungal responses to echinocandin cell wall damage and potentially improve treatment strategies.

In search of the holy grail of antifungal therapy

The ideal antifungal agent remains an elusive goal for treatment of life-threatening systemic fungal infections. Such an agent would have broad antifungal activity, low rates of resistance, flexible routes of administration, few associated adverse events, and limited drug-drug interactions. Only three of the seven classes of antifungal agents currently available are suitable for treatment of systemic infection: the polyenes, the azoles, and the echinocandins. None match all the characteristics of an ideal agent, the Holy Grail of antifungal therapy. Academia and industry need to collaborate in the search for new lead antifungal compounds using traditional screening methods as well as the new pharmacogenomics methods. Enhancing efficacy and reducing toxicity of the currently available therapeutic agents is also another important avenue of study. As an example, the Mycosis Research Center at the University of Mississippi Medical Center has identified pyogenic polyenes in commercial preparations of amphotericin B deoxycholate which correlate with infusion related toxicities. A highly purified formulation of amphotericin B appears promising, with a better therapeutic index compared to its parent compound as evidenced by results of in vitro and in vivo studies reviewed in this presentation.

In vitro susceptibility of invasive isolates of Candida spp. to anidulafungin, caspofungin, and micafungin: six years of global surveillance

The echinocandins are being used increasingly as therapy for invasive candidiasis. Prospective sentinel surveillance for the emergence of in vitro resistance to the echinocandins among invasive Candida sp. isolates is indicated. We determined the in vitro activities of anidulafungin, caspofungin, and micafungin against 5,346 invasive (bloodstream or sterile-site) isolates of Candida spp. collected from over 90 medical centers worldwide from 1 January 2001 to 31 December 2006. We performed susceptibility testing according to the CLSI M27-A2 method and used RPMI 1640 broth, 24-h incubation, and a prominent inhibition endpoint for determination of the MICs. Of 5,346 invasive Candida sp. isolates, species distribution was 54% C. albicans, 14% C. parapsilosis, 14% C. glabrata, 12% C. tropicalis, 3% C. krusei, 1% C. guilliermondii, and 2% other Candida spp. Overall, all three echinocandins were very active against Candida: anidulafungin (MIC50, 0.06 microg/ml; MIC90, 2 microg/ml), caspofungin (MIC50, 0.03 microg/ml; MIC90, 0.25 microg/ml), micafungin (MIC50, 0.015 microg/ml; MIC90, 1 microg/ml). More than 99% of isolates were inhibited by < or = 2 microg/ml of all three agents. Results by species (expressed as the percentages of isolates inhibited by < or = 2 microg/ml of anidulafungin, caspofungin, and micafungin, respectively) were as follows: for C. albicans, 99.6%, 100%, and 100%; for C. parapsilosis, 92.5%, 99.9%, and 100%; for C. glabrata, 99.9%, 99.9%, and 100%; for C. tropicalis, 100%, 99.8%, and 100%; for C. krusei, 100%, 100%, and 100%; and for C. guilliermondii, 90.2%, 95.1%, and 100%. There was no significant change in the activities of the three echinocandins over the 6-year study period and no difference in activity by geographic region. All three echinocandins have excellent in vitro activities against invasive strains of Candida isolated from centers worldwide. Our prospective sentinel surveillance reveals no evidence of emerging echinocandin resistance among invasive clinical isolates of Candida spp.

Recent advances and challenges in the treatment of invasive fungal infections

The frequency of invasive fungal infections (IFIs) has increased over the last decade with the rise in at-risk populations of patients. The morbidity and mortality of IFIs are high and management of these conditions is a great challenge. With the widespread adoption of antifungal prophylaxis, the epidemiology of invasive fungal pathogens has changed. Non-albicans Candida, non-fumigatus Aspergillus and moulds other than Aspergillus have become increasingly recognised causes of invasive diseases. These emerging fungi are characterised by resistance or lower susceptibility to standard antifungal agents. Invasive infections due to these previously rare fungi are therefore more difficult to treat. Recently developed antifungal agents provide the potential to improve management options and therapeutic outcomes of these infections. The availability of more potent and less toxic antifungal agents, such as second-generation triazoles and echinocandins, has led to considerable improvement in the treatment of IFIs. This article reviews the changing spectrum of invasive mycosis, the properties of recently developed antifungal agents and their role in the management of these infections.

A Ser678Pro substitution in Fks1p confers resistance to echinocandin drugs in Aspergillus fumigatus

An S678P substitution in Fks1p, the major subunit of glucan synthase, was sufficient to confer echinocandin resistance in Aspergillus fumigatus. The equivalent mutation in Candida spp. has been implicated in echinocandin resistance. This work demonstrates that modification of Fks1p is a conserved mechanism for echinocandin resistance in pathogenic fungi.

Recent advances in antifungal chemotherapy

For over 50 years, amphotericin B deoxycholate (AmBD) has been the 'gold standard' in antifungal chemotherapy, despite its frequent toxicities. However, improved treatment options for invasive fungal infections (IFIs) have been developed during the last 15 years. Newer antifungal agents, including less toxic lipid preparations of AmBD, triazoles and the echinocandins, have been added to our armamentarium against IFIs. Some of these newer drugs can now replace AmBD as primary therapy (e.g. caspofungin for candidiasis, voriconazole for aspergillosis), whilst others offer new therapeutic options for difficult-to-treat IFIs (e.g. posaconazole for zygomycosis, fusariosis and chromoblastomycosis). It is interesting that extended use of newer antifungals such as fluconazole, despite decreasing the mortality attributed to candidiasis, resulted in selection of species resistant to several antifungals (Candida krusei, Candida glabrata); whilst several publications suggest that prolonged use of voriconazole may expose severely immunocompromised patients to the risk of zygomycosis (breakthrough). On the other hand, the differences in the mode of action of newer antifungals such as echinocandins raise the question whether combination antifungal therapy is more effective than monotherapy. Finally, the availability of an oral formulation with excellent biosafety of several newer antifungals (e.g. posaconazole) makes them candidates for prophylactic or prolonged maintenance therapy.

The pharmacology and clinical use of caspofungin

Caspofungin was the first echinocandin to be licensed for the treatment of invasive fungal infections. Caspofungin has in vitro and in vivo activity against Candida spp. and Aspergillus spp., which constitute the majority of medically important opportunistic fungal pathogens. Caspofungin inhibits the synthesis of the 1,3-beta-glucan, with resultant osmotic instability and lysis. The pharmacology of caspofungin is relatively complex. Trafficking of drug into tissues is an important determinant of the shape of the concentration-time relationship. Caspofungin has demonstrated efficacy in experimental models of invasive candidiasis and aspergillosis, which reflect its activity in the treatment of oropharyngeal, esophageal and disseminated candidiasis, as well as salvage therapy for patients with invasive aspergillosis.

A comparative evaluation of properties and clinical efficacy of the echinocandins

With the increase in prevalence of fungal infections, newer antifungal agents are needed to effectively treat invasive disease, and at the same time minimize adverse effects from therapy. The echinocandins comprise a novel class of antifungals; their mechanism of action involves inhibiting 1,3-beta-D-glucan synthase, which is essential in cell wall synthesis for certain fungi. All three echinocandins are US FDA-approved for the treatment of esophageal candidiasis. Caspofungin and anidulafungin are licensed for the treatment of candidemia, and other select forms of invasive candidiasis. Micafungin is at present the only echinocandin approved for prophylaxis of fungal infections in hematopoietic stem cell transplants; whereas caspofungin is approved for empiric therapy of febrile neutropenia. Although all three echinocandins are active against Aspergillus, only caspofungin is presently approved for salvage therapy in invasive aspergillosis. Combination therapy with echinocandins plus other licensed antifungal therapy shows promise in treating invasive aspergillosis. This article will explore the similarities and differences among the echinocandins.