Echinocandin-Resistant Candida: Molecular Methods and Phenotypes

Yeast species-specific, differential inhibition of β-1,3-glucan synthesis by poacic acid and caspofungin

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Poacic acid antifungal activity is both strains and species dependent for a range of Candida species.

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The calcineurin pathway regulates poacic acid sensitivity in C. albicans.

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Point mutations in β-1,3-glucan synthase Fks1 differentially affect poacic acid and echinocandin sensitivity.

The rise of widespread antifungal resistance fuels the need to explore new classes of inhibitory molecules as potential novel inhibitors. Recently a plant natural product poacic acid (PA) was shown to inhibit β-1,3-glucan synthesis, and to have antifungal activity against a range of plant pathogens and against Saccharomyces cerevisiae. As with the echinocandins, such as caspofungin, PA targets the synthesis of cell wall β-1,3-glucan and has potential utility in the treatment of medically important fungi. However, the antifungal activity of PA against human pathogenic Candida species has not been explored and the precise mode of action of this compound is not understood. Here, we show that PA sensitivity is regulated by the calcineurin pathway and that susceptibility to PA varied significantly between Candida species, but did not correlate with in vitro β-glucan synthase activity, cell wall β-glucan content or the sensitivity of the species to caspofungin. Strains with point mutations (S645Y or S645P) in the hotspot1 region of the β-1,3-glucan synthase subunit Fks1, had decreased sensitivity to caspofungin but increased sensitivity to PA. C. guilliermondii, C. orthopsilosis, and C. parapsilosis were more sensitive to PA than C. albicans, C. dubliniensis, C. tropicalis, and C. glabrata. These observations suggest that there are significant differences in the mode of action of PA and caspofungin and that PA or PA analogues are not likely to have broad spectrum activity in the treatment of Candida infections.

Acquired Multidrug Antifungal Resistance in Candida lusitaniae during Therapy

Candida lusitaniae is usually susceptible to echinocandins. Beta-1,3-glucan synthase encoded by FKS genes is the target of echinocandins. A few missense mutations in the C. lusitaniae FKS1 hot spot 1 (HS1) have been reported. We report here the rapid emergence of antifungal resistance in C. lusitaniae isolated during therapy with amphotericin B (AMB), caspofungin (CAS), and azoles for treatment of persistent candidemia in an immunocompromised child with severe enterocolitis and visceral adenoviral disease. As documented from restriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA (RAPD) analysis, the five C. lusitaniae isolates examined were related to each other. From antifungal susceptibility and molecular analyses, 5 different profiles (P) were obtained. These profiles included the following: profile 1 (P1) (CAS MIC [μg/ml], 0.5; fluconazole [FLC] MIC, 0.25), determined while the patient was being treated with liposomal AMB for 3 months; P2 (FLC MIC [μg/ml], 0.25; CAS MIC, 4), while the patient was being treated with CAS for 2 weeks; P3 (CAS MIC [μg/ml], 0.5; FLC MIC, 32), while the patient was being treated with azoles and CAS initially followed by azoles alone for a week; P4 (CAS MIC [μg/ml], 8; FLC MIC, 8), while the patient was being treated with both drugs for 3 weeks; and P5 (AMB MIC [μg/ml], 0.125; CAS MIC, 8), while the patient was being treated with AMB and FLC for 2 weeks. CAS resistance was associated with resistance not only to micafungin and anidulafungin but also to AMB. Analysis of CAS resistance revealed 3 novel FKS1 mutations in CAS-resistant isolates (S638Y in P2; S631Y in P4; S638P in P5). While S638Y and -P are within HS1, S631Y is in close proximity to this domain but was confirmed to confer candin resistance using a site-directed mutagenesis approach. FLC resistance could be linked with overexpression of major facilitator gene 7 (MFS7) in C. lusitaniae P2 and P4 and was associated with resistance to 5-flurocytosine. This clinical report describes resistance of C. lusitaniae to all common antifungals. While candins or azole resistance followed monotherapy, multidrug antifungal resistance emerged during combined therapy.

Mechanisms of echinocandin antifungal drug resistance

Fungal infections due to Candida and Aspergillus species cause extensive morbidity and mortality, especially among immunosuppressed patients, and antifungal therapy is critical to patient management. Yet only a few drug classes are available to treat invasive fungal diseases, and this problem is compounded by the emergence of antifungal resistance. Echinocandin drugs are the preferred choice to treat candidiasis. They are the first cell wall-active agents and target the fungal-specific enzyme glucan synthase, which catalyzes the biosynthesis of β-1,3-glucan, a key cell wall polymer. Therapeutic failures occur rarely among common Candida species, with the exception of Candida glabrata, which is frequently multidrug resistant. Echinocandin resistance in susceptible species is always acquired during therapy. The mechanism of resistance involves amino acid changes in hot-spot regions of Fks subunits of glucan synthase, which decrease the sensitivity of the enzyme to drug. Cellular stress response pathways lead to drug adaptation, which promotes the formation of resistant fks strains. Clinical factors promoting echinocandin resistance include empiric therapy, prophylaxis, gastrointestinal reservoirs, and intra-abdominal infections. A better understanding of the echinocandin-resistance mechanism, along with cellular and clinical factors promoting resistance, will facilitate more effective strategies to overcome and prevent echinocandin resistance.

Echinocandin resistance, susceptibility testing and prophylaxis: implications for patient management

This article addresses the emergence of echinocandin resistance among Candida species, mechanisms of resistance, factors that promote resistance and confounding issues surrounding standard susceptibility testing. Fungal infections remain a significant cause of global morbidity and mortality, especially among patients with underlying immunosupression. Antifungal therapy is a critical component of patient management for acute and chronic diseases. Yet, therapeutic choices are limited due to only a few drug classes available to treat systemic disease. Moreover, the problem is exacerbated by the emergence of antifungal resistance, which has resulted in difficult to manage multidrug resistant strains. Echinocandin drugs are now the preferred choice to treat a range of candidiasis. These drugs target and inhibit the fungal-specific enzyme glucan synthase, which is responsible for the biosynthesis of a key cell wall polymer. Therapeutic failures involving acquisition of resistance among susceptible organisms like Candida albicans is largely a rare event. However, in recent years, there is an alarming trend of increased resistance among strains of Candida glabrata, which in many cases are also resistant to azole drugs. Echinocandin resistance is always acquired during therapy and the mechanism of resistance is well established to involve amino acid changes in "hot-spot" regions of the Fks subunits carrying the catalytic portion of glucan synthase. These changes significantly decrease the sensitivity of the enzyme to drug resulting in higher MIC values. A range of drug responses, from complete to partial refractory response, is observed depending on the nature of the amino acid substitution, and clinical responses are recapitulated in pharmacodynamic models of infection. The cellular processes promoting the formation of resistant Fks strains involve complex stress response pathways, which yield a variety of adaptive compensatory genetic responses. Stress-adapted cells become drug tolerant and can form stable drug resistant FKS mutations with continued drug exposure. A major concern for resistance detection is that classical broth microdilution techniques show significant variability among clinical microbiology laboratories for certain echinocandin drugs and Candida species. The consequence is that susceptible strains are misclassified according to established clinical breakpoints, and this has led to confusion in the field. Clinical factors that appear to promote echinocandin resistance include the expanding use of antifungal agents for empiric therapy and prophylaxis. Furthermore, host reservoirs such as biofilms in the gastrointestinal tract or intra-abdominal infections can seed development of resistant organisms during therapy. A fundamental understanding of the primary molecular resistance mechanism, along with cellular and clinical factors that promote resistance emergence, is critical to develop better diagnostic tools and therapeutic strategies to overcome and prevent echinocandin resistance.

Use of anidulafungin as a surrogate marker to predict susceptibility and resistance to caspofungin among 4,290 clinical isolates of Candida by using CLSI methods and interpretive criteria

This study addressed the application of anidulafungin as a surrogate marker to predict the susceptibility of Candida to caspofungin due to unacceptably high interlaboratory variation of caspofungin MIC values. CLSI reference broth microdilution methods and species-specific interpretive criteria were used to test 4,290 strains of Candida (eight species), including 71 strains with documented fks mutations. Caspofungin MIC values were compared with those of anidulafungin to determine the percentage of categorical agreement (CA) and very major (VME), major (ME), and minor error rates, as well as the ability to detect fks mutants. For all 4,290 isolates the CA was 97.1% (0.2% VME and ME, 2.5% minor errors) using anidulafungin as the surrogate. Among the 62 isolates of Candida albicans (4 isolates), C. tropicalis (5 isolates), C. krusei (4 isolates), C. kefyr (2 isolates), and C. glabrata (47 isolates) that were nonsusceptible (NS; either intermediate [I] or resistant [R]) to both caspofungin and anidulafungin, 52 (83.8%) contained a mutation in fks1 or fks2. Eight mutants of C. glabrata, two of C. albicans, and one each of C. tropicalis and C. krusei were classified as susceptible (S) to both antifungal agents. The remaining 7 mutants (2 C. albicans and 5 C. glabrata) were susceptible to one of the agents and either intermediate or resistant to the other. Using the epidemiological cutoff value (ECV) of 0.12 μg/ml for both caspofungin and anidulafungin to differentiate wild-type (WT) from non-WT strains of C. glabrata, 42 of the 55 (76.4%) C. glabrata mutants were non-WT and 8 of the 55 (14.5%) were WT for both agents (90.9% concordance). Anidulafungin can accurately serve as a surrogate marker to predict S and R of Candida to caspofungin.

Use of micafungin as a surrogate marker to predict susceptibility and resistance to caspofungin among 3,764 clinical isolates of Candida by use of CLSI methods and interpretive criteria

Due to unacceptably high interlaboratory variation in caspofungin MIC values, we evaluated the use of micafungin as a surrogate marker to predict the susceptibility of Candida spp. to caspofungin using reference methods and species-specific interpretive criteria. The MIC results for 3,764 strains of Candida (eight species), including 73 strains with fks mutations, were used. Caspofungin MIC values and species-specific interpretive criteria were compared with those of micafungin to determine the percent categorical agreement (%CA) and very major error (VME), major error (ME), and minor error rates as well as their ability to detect fks mutant strains of Candida albicans (11 mutants), Candida tropicalis (4 mutants), Candida krusei (3 mutants), and Candida glabrata (55 mutants). Overall, the %CA was 98.8% (0.2% VMEs and MEs, 0.8% minor errors) using micafungin as the surrogate marker. Among the 60 isolates of C. albicans (9 isolates), C. tropicalis (5 isolates), C. krusei (2 isolates), and C. glabrata (44 isolates) that were nonsusceptible (either intermediate or resistant) to both caspofungin and micafungin, 54 (90.0%) contained a mutation in fks1 or fks2. An additional 10 C. glabrata mutants, two C. albicans mutants, and one mutant each of C. tropicalis and C. krusei were classified as susceptible to both antifungal agents. Using the epidemiological cutoff values (ECVs) of 0.12 μg/ml for caspofungin and 0.03 μg/ml for micafungin to differentiate wild-type (WT) from non-WT strains of C. glabrata, 80% of the C. glabrata mutants were non-WT for both agents (96% concordance). Micafungin may serve as an acceptable surrogate marker for the prediction of susceptibility and resistance of Candida to caspofungin.

Candida guilliermondii and other species of candida misidentified as Candida famata: assessment by vitek 2, DNA sequencing analysis, and matrix-assisted laser desorption ionization-time of flight mass spectrometry in two global antifungal surveillance programs

Candida famata (teleomorph Debaryomyces hansenii) has been described as a medically relevant yeast, and this species has been included in many commercial identification systems that are currently used in clinical laboratories. Among 53 strains collected during the SENTRY and ARTEMIS surveillance programs and previously identified as C. famata (includes all submitted strains with this identification) by a variety of commercial methods (Vitek, MicroScan, API, and AuxaColor), DNA sequencing methods demonstrated that 19 strains were C. guilliermondii, 14 were C. parapsilosis, 5 were C. lusitaniae, 4 were C. albicans, and 3 were C. tropicalis, and five isolates belonged to other Candida species (two C. fermentati and one each C. intermedia, C. pelliculosa, and Pichia fabianni). Additionally, three misidentified C. famata strains were correctly identified as Kodomaea ohmeri, Debaryomyces nepalensis, and Debaryomyces fabryi using intergenic transcribed spacer (ITS) and/or intergenic spacer (IGS) sequencing. The Vitek 2 system identified three isolates with high confidence to be C. famata and another 15 with low confidence between C. famata and C. guilliermondii or C. parapsilosis, displaying only 56.6% agreement with DNA sequencing results. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) results displayed 81.1% agreement with DNA sequencing. One strain each of C. metapsilosis, C. fermentati, and C. intermedia demonstrated a low score for identification (<2.0) in the MALDI Biotyper. K. ohmeri, D. nepalensis, and D. fabryi identified by DNA sequencing in this study were not in the current database for the MALDI Biotyper. These results suggest that the occurrence of C. famata in fungal infections is much lower than previously appreciated and that commercial systems do not produce accurate identifications except for the newly introduced MALDI-TOF instruments.

Optimizing Echinocandin dosing and susceptibility breakpoint determination via in vivo pharmacodynamic evaluation against Candida glabrata with and without fks mutations

Echinocandins are a preferred therapy for invasive candidiasis due to their potency and broad spectrum. Resistance, especially in Candida glabrata, is an emerging threat to their use. Pharmacodynamic (PD) studies examining reduced susceptibility secondary to fks mutations in C. glabrata are lacking. The current study explored PD targets for anidulafungin, caspofungin, and micafungin in an in vivo invasive candidiasis model against 11 C. glabrata isolates with known or putative fks mutations. The PD targets were compared to those of 8 wild-type (WT) isolates. The MIC ranges in the WT group were 0.03 to 0.25 mg/liter for anidulafungin, 0.03 to 0.25 mg/liter for caspofungin, and 0.01 to 0.06 mg/liter for micafungin. The MIC ranges for mutants were 0.06 to 4, 0.25 to 16, and 0.13 to 8 mg/liter for the same compounds, respectively. The mean free drug 24-h area under the concentration-time curve (AUCf)/MIC ratio associated with a stasis endpoint for the WT group was 13.2 for anidulafungin, 2.04 for caspofungin, and 6.78 for micafungin. Comparative values for mutants were 3.43, 2.67, and 0.90, respectively. Pharmacokinetic data from patients suggest that the C. glabrata PD targets needed for success in this model could be achieved based on MIC values of 0.25 mg/liter for anidulafungin, 2 mg/liter for caspofungin, and 0.5 mg/liter for micafungin. These values are higher than recently identified epidemiology cutoff values (ECVs). The results suggest that drug-specific MIC breakpoints could be increased for caspofungin and micafungin against C. glabrata and could include organisms with mutations in fks-1 and fks-2. While identification of genetic mutants is epidemiologically important, the phenotype (MIC) provides a better predictor of therapeutic efficacy.

Progress in antifungal susceptibility testing of Candida spp. by use of Clinical and Laboratory Standards Institute broth microdilution methods, 2010 to 2012

Antifungal susceptibility testing of Candida has been standardized and refined and now may play a useful role in managing Candida infections. Important new developments include validation of 24-h reading times for all antifungal agents and the establishment of species-specific epidemiological cutoff values (ECVs) for the systemically active antifungal agents and both common and uncommon species of Candida. The clinical breakpoints (CBPs) for fluconazole, voriconazole, and the echinocandins have been revised to provide species-specific interpretive criteria for the six most common species. The revised CBPs not only are predictive of clinical outcome but also provide a more sensitive means of identifying those strains with acquired or mutational resistance mechanisms. This brief review serves as an update on the new developments in the antifungal susceptibility testing of Candida spp. using Clinical and Laboratory Standards Institute (CLSI) broth microdilution (BMD) methods.

Evaluation of fungal-specific fluorescent labeled echinocandin probes as diagnostic adjuncts

The diagnosis of invasive fungal infections from radiographic imaging is non-specific and problematic. As a first step toward increasing specificity, we describe the development of a broad-spectrum fungal-specific targeting molecule, which when modified with a fluorescent label fully retains its targeting properties, and provides a basis for future imaging applications.

Frequency of decreased susceptibility and resistance to echinocandins among fluconazole-resistant bloodstream isolates of Candida glabrata

The echinocandin class of antifungal agents is considered to be the first-line treatment of bloodstream infections (BSI) due to Candida glabrata. Recent reports of BSI due to strains of C. glabrata resistant to both fluconazole and the echinocandins are of concern and prompted us to review the experience of two large surveillance programs, the SENTRY Antimicrobial Surveillance Program for the years 2006 through 2010 and the Centers for Disease Control and Prevention population-based surveillance conducted in 2008 to 2010. The in vitro susceptibilities of 1,669 BSI isolates of C. glabrata to fluconazole, voriconazole, anidulafungin, caspofungin, and micafungin were determined by CLSI broth microdilution methods. Fluconazole MICs of ≥64 μg/ml were considered resistant. Strains for which anidulafungin and caspofungin MICs were ≥0.5 μg/ml and for which micafungin MICs were ≥0.25 μg/ml were considered resistant. A total of 162 isolates (9.7%) were resistant to fluconazole, of which 98.8% were nonsusceptible to voriconazole (MIC > 0.5 μg/ml) and 9.3%, 9.3%, and 8.0% were resistant to anidulafungin, caspofungin, and micafungin, respectively. There were 18 fluconazole-resistant isolates that were resistant to one or more of the echinocandins (11.1% of all fluconazole-resistant isolates), all of which contained an acquired mutation in fks1 or fks2. By comparison, there were no echinocandin-resistant strains detected among 110 fluconazole-resistant isolates of C. glabrata tested in 2001 to 2004. These data document the broad emergence of coresistance over time to both azoles and echinocandins in clinical isolates of C. glabrata.