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

Mechanisms of Antifungal Drug Resistance

Antifungal therapy is a central component of patient management for acute and chronic mycoses. Yet, treatment choices are restricted because of the sparse number of antifungal drug classes. Clinical management of fungal diseases is further compromised by the emergence of antifungal drug resistance, which eliminates available drug classes as treatment options. Once considered a rare occurrence, antifungal drug resistance is on the rise in many high-risk medical centers. Most concerning is the evolution of multidrug- resistant organisms refractory to several different classes of antifungal agents, especially among common Candida species. The mechanisms responsible are mostly shared by both resistant strains displaying inherently reduced susceptibility and those acquiring resistance during therapy. The molecular mechanisms include altered drug affinity and target abundance, reduced intracellular drug levels caused by efflux pumps, and formation of biofilms. New insights into genetic factors regulating these mechanisms, as well as cellular factors important for stress adaptation, provide a foundation to better understand the emergence of antifungal drug resistance.

An automated high-throughput cell-based multiplexed flow cytometry assay to identify novel compounds to target Candida albicans virulence-related proteins

Although three major classes of systemic antifungal agents are clinically available, each is characterized by important limitations. Thus, there has been considerable ongoing effort to develop novel and repurposed agents for the therapy of invasive fungal infections. In an effort to address these needs, we developed a novel high-throughput, multiplexed screening method that utilizes small molecules to probe candidate drug targets in the opportunistic fungal pathogen Candida albicans. This method is amenable to high-throughput automated screening and is based upon detection of changes in GFP levels of individually tagged target proteins. We first selected four GFP-tagged membrane-bound proteins associated with virulence or antifungal drug resistance in C. albicans. We demonstrated proof-of-principle that modulation of fluorescence intensity can be used to assay the expression of specific GFP-tagged target proteins to inhibitors (and inducers), and this change is measurable within the HyperCyt automated flow cytometry sampling system. Next, we generated a multiplex of differentially color-coded C. albicans strains bearing C-terminal GFP-tags of each gene encoding candidate drug targets incubated in the presence of small molecules from the Prestwick Chemical Library in 384-well microtiter plate format. Following incubation, cells were sampled through the HyperCyt system and modulation of protein levels, as indicated by changes in GFP-levels of each strain, was used to identify compounds of interest. The hit rate for both inducers and inhibitors identified in the primary screen did not exceed 1% of the total number of compounds in the small-molecule library that was probed, as would be expected from a robust target-specific, high-throughput screening campaign. Secondary assays for virulence characteristics based on null mutant strains were then used to further validate specificity. In all, this study presents a method for the identification and verification of new antifungal drugs targeted to fungal virulence proteins using C. albicans as a model fungal pathogen.

Epidemiology of echinocandin resistance in Candida

Echinocandins are the newest antifungal agents approved for use in treating Candida infections in the US. They act by interfering with 1,3-β-D-glucan synthase and therefore disrupt cell wall production and lead to Candida cell death. There is no intrinsic resistance to echinocandins among Candida species, and isolates from historic collections archived before the release of the echinocandins show no resistance. Resistance to the echinocandins remains low among most Candida species and ranges overall from 0-1%. Among isolates of Candida glabrata, the proportion of resistant isolates is higher and has been reported to be as high as 13.5% in at least one hospital. Antifungal resistance is due to specific amino acid mutations in the Fksp subunit(s) of the 1,3-β-D-glucan synthase protein which are localized to one of two hotspots. These mutations are being recognized in isolates from patients who have failed echinocandin therapy, and often lead to a poor outcome. While the future looks bright for the echinocandins against most Candida species, C. glabrata remains a species of concern and resistance rates of C. glabrata to the echinocandins should be monitored closely.

Positions and numbers of FKS mutations in Candida albicans selectively influence in vitro and in vivo susceptibilities to echinocandin treatment

Candidemia is the fourth most common kind of microbial bloodstream infection, with Candida albicans being the most common causative species. Echinocandins are employed as the first-line treatment for invasive candidiasis until the fungal species is determined and confirmed by clinical diagnosis. Echinocandins block the FKS glucan synthases responsible for embedding β-(1,3)-d-glucan in the cell wall. The increasing use of these drugs has led to the emergence of antifungal resistance, and elevated MICs have been associated with single-residue substitutions in specific hot spot regions of FKS1 and FKS2. Here, we show for the first time the caspofungin-mediated in vivo selection of a double mutation within one allele of the FKS1 hot spot 1 in a clinical isolate. We created a set of isogenic mutants and used a hematogenous murine model to evaluate the in vivo outcomes of echinocandin treatment. Heterozygous and homozygous double mutations significantly enhance the in vivo resistance of C. albicans compared with the resistance seen with heterozygous single mutations. The various FKS1 hot spot mutations differ in the degree of their MIC increase, substance-dependent in vivo response, and impact on virulence. Our results demonstrate that echinocandin EUCAST breakpoint definitions correlate with the in vivo response when a standard dosing regimen is used but cannot predict the in vivo response after a dose escalation. Moreover, patients colonized by a C. albicans strain with multiple mutations in FKS1 have a higher risk for therapeutic failure.

Target enzyme mutations confer differential echinocandin susceptibilities in Candida kefyr

Candida kefyr is an increasingly reported pathogen in patients with hematologic malignancies. We studied a series of bloodstream isolates that exhibited reduced echinocandin susceptibilities (RES). Clinical and surveillance isolates were tested for susceptibilities to all three echinocandins, and those isolates displaying RES to one or more echinocandins were selected for molecular and biochemical studies. The isolates were analyzed for genetic similarities, and a subset was analyzed for mutations in the echinocandin target gene FKS1 and glucan synthase echinocandin sensitivities using biochemical methods. The molecular typing did not indicate strong genetic relatedness among the isolates except for a series of strains recovered from a single patient. Two unrelated isolates with RES had previously uncharacterized FKS1 mutations: R647G and deletion of amino acid 641 (F641Δ). Biochemical analysis of the semipurified R647G glucan synthase generated differential echinocandin sensitivity (resistance to micafungin only), while the deletion of F641 resulted in a glucan synthase highly insensitive to all three echinocandins. The consecutive isolates from a single patient with RES all harbored the common S645P mutation, which conferred resistance to all three echinocandins. The MIC values paralleled the glucan synthase inhibition kinetic data, although the S645P isolates displayed relatively higher susceptibility to caspofungin (2 μg/ml) than the other two echinocandins (>8 μg/ml). These findings highlight novel and common FKS1 mutations in C. kefyr isolates. The observation of differential susceptibilities to echinocandins may provide important mechanistic insights for echinocandin antifungals.

Role of FKS Mutations in Candida glabrata: MIC values, echinocandin resistance, and multidrug resistance

Candida glabrata is the second leading cause of candidemia in U.S. hospitals. Current guidelines suggest that an echinocandin be used as the primary therapy for the treatment of C. glabrata disease due to the high rate of resistance to fluconazole. Recent case reports indicate that C. glabrata resistance to echinocandins may be increasing. We performed susceptibility testing on 1,380 isolates of C. glabrata collected between 2008 and 2013 from four U.S. cities, Atlanta, Baltimore, Knoxville, and Portland. Our analysis showed that 3.1%, 3.3%, and 3.6% of the isolates were resistant to anidulafungin, caspofungin, and micafungin, respectively. We screened 1,032 of these isolates, including all 77 that had either a resistant or intermediate MIC value with respect to at least one echinocandin, for mutations in the hot spot regions of FKS1 and FKS2, the major mechanism of echinocandin resistance. Fifty-one isolates were identified with hot spot mutations, 16 in FKS1 and 35 in FKS2. All of the isolates with an FKS mutation except one were resistant to at least one echinocandin by susceptibility testing. Of the isolates resistant to at least one echinocandin, 36% were also resistant to fluconazole. Echinocandin resistance among U.S. C. glabrata isolates is a concern, especially in light of the fact that one-third of those isolates may be multidrug resistant. Further monitoring of U.S. C. glabrata isolates for echinocandin resistance is warranted.

Current status of antifungal resistance and its impact on clinical practice

Mortality linked to invasive fungal diseases remains very high despite the availability of novel antifungals and new therapeutic strategies. Candida albicans and Aspergillus fumigatus account for most invasive mycosis produced by yeast or moulds, respectively. Other Candida non-albicans are increasingly being reported and newly emerging, as well as cryptic, filamentous fungi often cause disseminated infections in immunocompromised hosts. Management of invasive fungal infections is becoming a challenge as emerging fungal pathogens generally show poor response to many antifungals. The ability of reference antifungal susceptibility testing methods to detect emerging resistance patterns, together with the molecular characterization of antifungal resistance mechanisms, are providing useful information to optimize the effectiveness of antifungal therapy. The current status of antifungal resistance epidemiology with special emphasis on the molecular resistant mechanisms that have been described in the main pathogenic fungal species are reviewed.

Mechanisms of Candida biofilm drug resistance

Candida commonly adheres to implanted medical devices, growing as a resilient biofilm capable of withstanding extraordinarily high antifungal concentrations. As currently available antifungals have minimal activity against biofilms, new drugs to treat these recalcitrant infections are urgently needed. Recent investigations have begun to shed light on the mechanisms behind the profound resistance associated with the biofilm mode of growth. This resistance appears to be multifactorial, involving both mechanisms similar to conventional, planktonic antifungal resistance, such as increased efflux pump activity, as well as mechanisms specific to the biofilm lifestyle. A unique biofilm property is the production of an extracellular matrix. Two components of this material, β-glucan and extracellular DNA, promote biofilm resistance to multiple antifungals. Biofilm formation also engages several stress response pathways that impair the activity of azole drugs. Resistance within a biofilm is often heterogeneous, with the development of a subpopulation of resistant persister cells. In this article we review the molecular mechanisms underlying Candida biofilm antifungal resistance and their relative contributions during various growth phases.

Echinocandin failure case due to a previously unreported FKS1 mutation in Candida krusei

Echinocandins are the preferred therapy for invasive infections due to Candida krusei. We present here a case of clinical failure involving C. krusei with a characteristic FKS1 hot spot mutation not previously reported in C. krusei that was isolated after 14 days of treatment. Anidulafungin MICs were elevated by ≥ 5 dilution steps above the clinical breakpoint but by only 1 step for a Candida albicans isolate harboring the corresponding mutation, suggesting a notable species-specific difference in the MIC increase conferred by this mutation.

Modulation of Alternaria infectoria cell wall chitin and glucan synthesis by cell wall synthase inhibitors

The present work reports the effects of caspofungin, a β-1,3-glucan synthase inhibitor, and nikkomycin Z, an inhibitor of chitin synthases, on two strains of Alternaria infectoria, a melanized fungus involved in opportunistic human infections and respiratory allergies. One of the strains tested, IMF006, bore phenotypic traits that conferred advantages in resisting antifungal treatment. First, the resting cell wall chitin content was higher and in response to caspofungin, the chitin level remained constant. In the other strain, IMF001, the chitin content increased upon caspofungin treatment to values similar to basal IMF006 levels. Moreover, upon caspofungin treatment, the FKS1 gene was upregulated in IMF006 and downregulated in IMF001. In addition, the resting β-glucan content was also different in both strains, with higher levels in IMF001 than in IMF006. However, this did not provide any advantage with respect to echinocandin resistance. We identified eight different chitin synthase genes and studied relative gene expression when the fungus was exposed to the antifungals under study. In both strains, exposure to caspofungin and nikkomycin Z led to modulation of the expression of class V and VII chitin synthase genes, suggesting its importance in the robustness of A. infectoria. The pattern of A. infectoria phagocytosis and activation of murine macrophages by spores was not affected by caspofungin. Monotherapy with nikkomycin Z and caspofungin provided only fungistatic inhibition, while a combination of both led to fungal cell lysis, revealing a strong synergistic action between the chitin synthase inhibitor and the β-glucan synthase inhibitor against this fungus.

Micafungin: an evidence-based review of its place in therapy

Invasive fungal infections have increased throughout the world. Many of these infections occur in patients with multiple comorbidities who are receiving medications with the potential for interactions with antifungal therapy that could lead to renal and hepatic dysfunction. The second marketed echinocandin, micafungin, was approved in 2005 for the treatment of esophageal candidiasis and prophylaxis of invasive Candida infections in patients undergoing hematopoietic stem cell transplantation. The indication for use was later expanded to include candidemia, acute disseminated candidiasis, Candida abscesses, and peritonitis. Like other echinocandins it is fungicidal against Candida species, including those that are polyene- and azole-resistant and fungistatic against Aspergillus species. Its formulation is by the intravenous route only and it is dosed once daily without a loading dose as 85% of the steady state concentration is achieved after three daily doses. It has a favorable tolerability profile with no significant drug interactions and does not need adjustment for renal or hepatic insufficiency.

Breakthrough candidemia due to multidrug-resistant Candida glabrata during prophylaxis with a low dose of micafungin

We identified a case of breakthrough candidemia in a 25-year-old patient receiving micafungin prophylaxis (50 mg/day). Five Candida glabrata isolates were obtained from blood cultures and were classified as multidrug-resistant isolates, since all of them exhibited high MICs for echinocandin and azole drugs. A mutation (S663F) in hot spot 1 of the FKS2 gene was found in all five isolates. This mutation yielded a 1,3-β-D-glucan synthase enzyme with highly reduced sensitivities to echinocandin drugs.

Echinocandin antifungals: review and update

The echinocandins are a new and unique class of antifungal agents that act on the fungal cell wall by way of noncompetitive inhibition of the synthesis of 1,3-beta-glucans. All agents of this class are of parenteral formulation, with no oral preparations available. Caspofungin (Cancidas) was the first approved echinocandin, followed recently by micafungin (Mycamine) and anidulafungin (Eraxis). The precise role of the echinocandins in the antifungal armamentarium is still unfolding. Caspofungin is approved for the treatment of candidal esophagitis and candidemia, salvage therapy of Aspergillus infections and for empirical therapy of febrile neutropenia. Micafungin is likewise approved for candidal esophagitis, in addition to antifungal prophylaxis for hematopoietic stem cell transplant recipients. Anidulafungin is also approved for treatment of candidal esophagitis, as well as therapy of candidemia. There has been anecdotal use of these agents to treat less common fungal pathogens, as well as limited use as a component of combination antifungal therapy. The echinocandins are an important addition to the antifungal armamentarium in the treatment of fungal infections in both immunocompromised patients and those with normal immunity.

Paradoxical antifungal activity and structural observations in biofilms formed by echinocandin-resistant Candida albicans clinical isolates

Echinocandin-resistant clinical isolates of Candida albicans have been reported, and key-hot spot mutations in the FKS1 gene, which encodes a major glucan synthase subunit, have been identified in these (caspofungin-resistant [CAS-R]) strains. Although these mutations result in phenotypic resistance to echinocandins in planktonic cells, there is little data on antifungal susceptibilities of CAS-R C. albicans strains within biofilms. Thus, we analyzed biofilms formed by 12 C. albicans CAS-R clinical strains in which we previously identified FKS1 hot-spot mutations and compared the sessile antifungal and paradoxical activity of anidulafungin (ANID), caspofungin (CAS), and micafungin (MICA). Biofilms were formed in a 96-well static microplate model and assayed using both tetrazolium-salt reduction and crystal violet assays, as well as examination by scanning electron microscopy. We first sought to assess biofilm formation and structure in these fks1 mutants and found that the biofilm mass and metabolic activities were reduced in most of the fks1 mutants as compared with reference strain SC5314. Structural analyses revealed that the fks1 mutant biofilms were generally less dense and had a clear predominance of yeast and pseudohyphae, with unusual "pit"-like cell surface structures. We also noted that sessile minimum inhibitory concentrations (MICs) to ANID, CAS, and MICA were higher than planktonic MICs of all but one strain. The majority of strains demonstrated a paradoxical effect (PE) to particular echinocandins, in either planktonic or sessile forms. Overall, biofilms formed by echinocandin-resistant clinical isolates demonstrated varied PEs to echinocandins and were structurally characterized by a preponderance of yeast, pseudohyphae, and pit-like structures.

Development of a Luminex-based multiplex assay for detection of mutations conferring resistance to Echinocandins in Candida glabrata

Echinocandins are the recommended treatment for invasive candidiasis due to Candida glabrata. Resistance to echinocandins is known to be caused by nonsynonymous mutations in the hot spot-1 (HS1) regions of the FKS1 and FKS2 genes, which encode a subunit of the β-1,3-glucan synthase, the target of echinocandins. Here, we describe the development of a microsphere-based assay using Luminex MagPix technology to identify mutations in the FKS1 HS1 and FKS2 HS1 domains, which confer in vitro echinocandin resistance in C. glabrata isolates. The assay is rapid and can be performed with high throughput. The assay was validated using 102 isolates that had FKS1 HS1 and FKS2 HS1 domains previously characterized by DNA sequencing. The assay was 100% concordant with DNA sequencing results. The assay was then used for high-throughput screening of 1,032 C. glabrata surveillance isolates. Sixteen new isolates with mutations, including a mutation that was new to our collection (del659F), were identified. This assay provides a rapid and cost-effective way to screen C. glabrata isolates for echinocandin resistance.

Candida infections, causes, targets, and resistance mechanisms: traditional and alternative antifungal agents

The genus Candida includes about 200 different species, but only a few species are human opportunistic pathogens and cause infections when the host becomes debilitated or immunocompromised. Candida infections can be superficial or invasive. Superficial infections often affect the skin or mucous membranes and can be treated successfully with topical antifungal drugs. However, invasive fungal infections are often life-threatening, probably due to inefficient diagnostic methods and inappropriate initial antifungal therapies. Here, we briefly review our current knowledge of pathogenic species of the genus Candida and yeast infection causes and then focus on current antifungal drugs and resistance mechanisms. An overview of new therapeutic alternatives for the treatment of Candida infections is also provided.

Nanoscale effects of caspofungin against two yeast species, Saccharomyces cerevisiae and Candida albicans

Saccharomyces cerevisiae and Candida albicans are model yeasts for biotechnology and human health, respectively. We used atomic force microscopy (AFM) to explore the effects of caspofungin, an antifungal drug used in hospitals, on these two species. Our nanoscale investigation revealed similar, but also different, behaviors of the two yeasts in response to treatment with the drug. While administration of caspofungin induced deep cell wall remodeling in both yeast species, as evidenced by a dramatic increase in chitin and decrease in β-glucan content, changes in cell wall composition were more pronounced with C. albicans cells. Notably, the increase of chitin was proportional to the increase in the caspofungin dose. In addition, the Young modulus of the cell was three times lower for C. albicans cells than for S. cerevisiae cells and increased proportionally with the increase of chitin, suggesting differences in the molecular organization of the cell wall between the two yeast species. Also, at a low dose of caspofungin (i.e., 0.5× MIC), the cell surface of C. albicans exhibited a morphology that was reminiscent of cells expressing adhesion proteins. Interestingly, this morphology was lost at high doses of the drug (i.e., 4× MIC). However, the treatment of S. cerevisiae cells with high doses of caspofungin resulted in impairment of cytokinesis. Altogether, the use of AFM for investigating the effects of antifungal drugs is relevant in nanomedicine, as it should help in understanding their mechanisms of action on fungal cells, as well as unraveling unexpected effects on cell division and fungal adhesion.

Echinocandins: production and applications

The first echinocandin-type antimycotic (echinocandin B) was discovered in the 1970s. It was followed by the isolation of more than 20 natural echinocandins. These cyclic lipo-hexapeptides are biosynthesized on non-ribosomal peptide synthase complexes by different ascomycota fungi. They have a unique mechanism of action; as non-competitive inhibitors of β-1,3-glucan synthase complex they target the fungal cell wall. Results of the structure-activity relationship experiments let us develop semisynthetic derivatives with improved properties. Three cyclic lipohiexapeptides (caspofungin, micafungin and anidulafungin) are currently approved for use in clinics. As they show good fungicidal (Candida spp.) or fungistatic (Aspergillus spp.) activity against the most important human pathogenic fungi including azole-resistant strains, they are an important addition to the antifungal armamentarium. Some evidence of acquired resistance against echinocandins has been detected among Candida glabrata strains in recent years, which enhanced the importance of data collected on the mechanism of acquired resistance developing against the echinocandins. In this review, we show the structural diversity of natural echinocandins, and we summarize the emerging data on their mode of action, biosynthesis and industrial production. Their clinical significance as well as the mechanism of natural and acquired resistance is also discussed.

Stepwise development of a homozygous S80P substitution in Fks1p, conferring echinocandin resistance in Candida tropicalis

Three Candida tropicalis isolates were obtained from a patient with acute lymphoblastic leukemia. The first isolate was susceptible to all drug classes, while isolates 2 and 3, obtained after 8 and 8.5 weeks of caspofungin treatment, respectively, were resistant to the three echinocandins. Multilocus sequence genotyping suggested a clonal relation among all isolates. FKS1 sequencing revealed a stepwise development of a heterozygous and finally a homozygous mutation, leading to S80S/P and S80P amino acid substitutions.

Chitin and glucan, the yin and yang of the fungal cell wall, implications for antifungal drug discovery and therapy

The structural carbohydrate polymers glucan and chitin compliment and reinforce each other in a dynamic process to maintain the integrity and physical strength of the fungal cell wall. The assembly of chitin and glucan in the cell wall of the budding yeast Saccharomyces cerevisiae and the polymorphic human pathogen Candida albicans are essential processes that involve a range of fungal-specific enzymes and regulatory networks. The fungal cell wall is, therefore, an attractive target for novel therapies as host cells lack many cell wall-related proteins. The most recent class of antifungal drug approved for clinical use, the echinocandins, targets the synthesis of cell wall β(1-3)glucan. The echinocandins are effective at treating invasive and bloodstream Candida infections and are now widely used in the clinic. However, there have been sporadic reports of breakthrough infections in patients undergoing echinocandin therapy. The acquisition of point mutations in the FKS genes that encode the catalytic β(1-3)glucan synthase subunits, the target of the echinocandins, has emerged as a dominant resistance mechanism. Cells with elevated chitin levels are also less susceptible to echinocandins and in addition, treatment with sub-MIC echinocandin activates cell wall salvage pathways that increase chitin synthesis to compensate for reduced glucan production. The development of drugs targeting the cell wall has already proven to be beneficial in providing an alternative class of drug for use in the clinic. Other cell wall targets such as chitin synthesis still hold great potential for drug development but careful consideration should be given to the capacity of fungi to manipulate their walls in a dynamic response to cell wall perturbations.

Evaluation of caspofungin susceptibility testing by the new Vitek 2 AST-YS06 yeast card using a unique collection of FKS wild-type and hot spot mutant isolates, including the five most common candida species

FKS mutant isolates associated with breakthrough or failure cases are emerging in clinical settings. Discrimination of these from wild-type (wt) isolates in a routine laboratory setting is complicated. We evaluated the ability of caspofungin MIC determination using the new Vitek 2 AST-Y06 yeast susceptibility card to correctly identify the fks mutants from wt isolates and compared the performance to those of the CLSI and EUCAST reference methods. A collection of 98 Candida isolates, including 31 fks hot spot mutants, were included. Performance was evaluated using the FKS genotype as the "gold standard" and compared to those of the CLSI and EUCAST methodologies. The categorical agreement for Vitek 2 was 93.9%, compared to 88.4% for the CLSI method and 98.7% for the EUCAST method. Vitek 2 misclassified 19.4% (6/31) of the fks mutant isolates as susceptible, in contrast to <4% for each of the reference methods. The overall essential agreement between the CLSI method and Vitek 2 MICs was 92.6% (88/95) but was substantially lower for fks mutant isolates (78.6% [22/28]). Correct discrimination between susceptible and intermediate Candida glabrata isolates was not possible, as the revised species-specific susceptibility breakpoint was not included in the Vitek 2 detection range (MIC of ≤0.250 to ≥4 mg/liter). In conclusion, the Vitek 2 allowed correct categorization of all wt isolates as susceptible. However, despite an acceptable categorical agreement, it failed to reliably classify isolates harboring fks hot spot mutations as intermediate or resistant, which was in part due to the fact that the detection range did not span the susceptibility breakpoint for C. glabrata.

Elevated chitin content reduces the susceptibility of Candida species to caspofungin

The echinocandin antifungal drugs inhibit synthesis of the major fungal cell wall polysaccharide β(1,3)-glucan. Echinocandins have good efficacy against Candida albicans but reduced activity against other Candida species, in particular Candida parapsilosis and Candida guilliermondii. Treatment of Candida albicans with a sub-MIC level of caspofungin has been reported to cause a compensatory increase in chitin content and to select for sporadic echinocandin-resistant FKS1 point mutants that also have elevated cell wall chitin. Here we show that elevated chitin in response to caspofungin is a common response in various Candida species. Activation of chitin synthesis was observed in isolates of C. albicans, Candida tropicalis, C. parapsilosis, and C. guilliermondii and in some isolates of Candida krusei in response to caspofungin treatment. However, Candida glabrata isolates demonstrated no exposure-induced change in chitin content. Furthermore, isolates of C. albicans, C. krusei, C. parapsilosis, and C. guilliermondii which were stimulated to have higher chitin levels via activation of the calcineurin and protein kinase C (PKC) signaling pathways had reduced susceptibility to caspofungin. Isolates containing point mutations in the FKS1 gene generally had higher chitin levels and did not demonstrate a further compensatory increase in chitin content in response to caspofungin treatment. These results highlight the potential of increased chitin synthesis as a potential mechanism of tolerance to caspofungin for the major pathogenic Candida species.

Mutants in the Candida glabrata glycerol channels are sensitized to cell wall stress

Many fungal species use glycerol as a compatible solute with which to maintain osmotic homeostasis in response to changes in external osmolarity. In Saccharomyces cerevisiae, intracellular glycerol concentrations are regulated largely by the high osmolarity glycerol (HOG) response pathway, both through induction of glycerol biosynthesis and control of its flux through the plasma membrane Fps1 glycerol channel. The channel activity of Fps1 is also controlled by a pair of positive regulators, Rgc1 and Rgc2. In this study, we demonstrate that Candida glabrata, a fungal pathogen that possesses two Fps1 orthologs and two Rgc1/-2 orthologs, accumulates glycerol in response to hyperosmotic stress. We present an initial characterization of mutants with deletions in the C. glabrata FPS1 (CAGL0C03267 [www.candidagenome.org]) and FPS2 (CAGL0E03894) genes and find that a double mutant accumulates glycerol, experiences constitutive cell wall stress, and is hypersensitive to treatment by caspofungin, an antifungal agent that targets the cell wall. This mutant is cleared more efficiently in mouse infections than is wild-type C. glabrata by caspofungin treatment. Finally, we demonstrate that one of the C. glabrata RGC orthologs complements an S. cerevisiae rgc1 rgc2 null mutant, supporting the conclusion that this regulatory assembly is conserved between these species.

Fks1 and Fks2 are functionally redundant but differentially regulated in Candida glabrata: implications for echinocandin resistance

The echinocandins caspofungin, micafungin, and anidulafungin, inhibitors of cell wall β-1,3-glucan synthesis, were recently elevated to first-line agents for treating infections due to the azole-refractory yeast Candida glabrata. In Candida albicans, echinocandin resistance is strictly associated with mutations in Fks1, a large integral membrane protein and putative β-1,3-glucan synthase, while mutations in both Fks1 and its paralog Fks2 (but not Fks3) have been associated with resistance in C. glabrata. To further explore their function, regulation, and role in resistance, C. glabrata fks genes were disrupted and subjected to mutational analysis, and their differential regulation was explored. An fks1Δ fks2Δ double disruptant was not able to be generated; otherwise, all three single and remaining two double disruptants displayed normal growth and echinocandin susceptibility, indicating Fks1-Fks2 redundancy. Selection on echinocandin-containing medium for resistant mutants was dependent on strain background: only fks1Δ and fks1Δ fks3Δ strains consistently yielded mutants exhibiting high-level resistance, all with Fks2 hot spot 1 mutations. Thus, Fks1-Fks2 redundancy attenuates the rate of resistance; further analysis showed that it also attenuates the impact of resistance-conferring mutations. Growth of the fks1Δ and, especially, fks1Δ fks3Δ strains was specifically susceptible to the calcineurin inhibitor FK506. Relatedly, FK506 addition or calcineurin gene CMP2 disruption specifically reversed Fks2-mediated resistance of laboratory mutants and clinical isolates. RNA analysis suggests that transcriptional control is not the sole mechanism by which calcineurin modulates Fks2 activity.

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.

RNA sequencing revealed novel actors of the acquisition of drug resistance in Candida albicans

BACKGROUND

Drug susceptible clinical isolates of Candida albicans frequently become highly tolerant to drugs during chemotherapy, with dreadful consequences to patient health. We used RNA sequencing (RNA-seq) to analyze the transcriptomes of a CDR (Candida Drug Resistance) strain and its isogenic drug sensitive counterpart.

RESULTS

RNA-seq unveiled differential expression of 228 genes including a) genes previously identified as involved in CDR, b) genes not previously associated to the CDR phenotype, and c) novel transcripts whose function as a gene is uncharacterized. In particular, we show for the first time that CDR acquisition is correlated with an overexpression of the transcription factor encoding gene CZF1. CZF1 null mutants were susceptible to many drugs, independently of known multidrug resistance mechanisms. We show that CZF1 acts as a repressor of β-glucan synthesis, thus negatively regulating cell wall integrity. Finally, our RNA-seq data allowed us to identify a new transcribed region, upstream of the TAC1 gene, which encodes the major CDR transcriptional regulator.

CONCLUSION

Our results open new perspectives of the role of Czf1 and of our understanding of the transcriptional and post-transcriptional mechanisms that lead to the acquisition of drug resistance in C. albicans, with potential for future improvements of therapeutic strategies.

Echinocandin Resistance in Candida Species: Mechanisms of Reduced Susceptibility and Therapeutic Approaches

OBJECTIVE:

To summarize published data regarding mechanisms of reduced echinocandin susceptibility in Candida spp., the impact of echinocandin resistance on the fitness and virulence of Candida isolates, and current and future treatment approaches.

DATA SOURCES:

A search of MEDLINE databases (1966-September 2011) was conducted.

STUDY SELECTION AND DATA EXTRACTION:

Databases were searched using the terms echinocandin, resistance, and Candida. Citations from publications were reviewed for additional references.

DATA SYNTHESIS:

Echinocandins have in vitro activity against most Candida spp. and are first-line agents in the treatment of candidemia. However, case reports describing echinocandin treatment failure due to resistant isolates have been published. Reduced echinocandin susceptibility has been shown to occur via 3 main mechanisms: (1) adaptive stress responses, which result in elevated cell wall chitin content and paradoxical growth in vitro at supra minimum inhibitory concentrations (MICs); (2) acquired FKS mutations, which confer reduced glucan synthase sensitivity, elevated MICs, and are associated with clinical failure; and (3) intrinsic FKS mutations, which are naturally occurring mutations in C. parapsilosis and C. guilliermondii, which confer elevated MIC levels but a lower level of reduced glucan synthase sensitivity compared with acquired FKS mutations. Some FKS mutants have been shown to have significantly reduced fitness and virulence versus wild type isolates and may contribute to the low incidence of echinocandin resistance reported in large surveillance studies. Treatment strategies evaluated for FKS mutants include echinocandin dose escalation and combination with agents such as calcineurin inhibitors, HSP90 inhibitors, and chitin synthase inhibitors.

CONCLUSIONS:

While the incidence of echinocandin resistance in Candida spp. is low, it can present a significant therapeutic challenge, especially in multidrug-resistant Candida isolates. Dose escalation is unlikely to be effective in treating FKS mutant isolates, and significant adverse effects limit the clinical use of agents evaluated as combination therapy. Patients with infections failing to respond to echinocandin therapy should undergo susceptibility testing and be treated with an alternative antifungal agent if possible.

Caspofungin for the treatment of immunocompromised and severely ill children and neonates with invasive fungal infections

Caspofungin is the first member of the echinocandin class of antifungals to receive an indication for the use in infants, children, and adolescents from the United States Food and Drug Administration. Daily doses of 50 mg/m(2) result in pharmacokinetic parameters that are similar to those observed in adults. Although fewer data are available, the response rates in pediatric patients who received caspofungin either as treatment or empiric therapy in clinical trials are similar to those reported in adults. In addition, caspofungin appears to be generally safe and well tolerated in this population. This represents a significant step forward in the treatment of invasive fungal infections within this population, as caspofungin is associated with few clinically significant drug-interactions and toxicities compared to other antifungals, such as the azoles and amphotericin B.

Echinocandin resistance due to simultaneous FKS mutation and increased cell wall chitin in a Candida albicans bloodstream isolate following brief exposure to caspofungin

Echinocandins are first-line agents for treating severe invasive candidiasis. Glucan synthase gene (FKS1) mutations lead to echinocandin resistance but the role of enhanced chitin expression is not well recognized in clinical isolates. We report a case of bloodstream Candida albicans infection with both Fks1 hotspot mutation and elevated cell wall chitin.

Population dynamics and the evolution of antifungal drug resistance in Candida albicans

Candida albicans is an important human fungal pathogen. Resistance to all major antifungal agents has been observed in clinical isolates of Candida spp. and is a major clinical challenge. The rise and expansion of drug-resistant mutants during exposure to antifungal agents occurs through a process of adaptive evolution, with potentially complex population dynamics. Understanding the population dynamics during the emergence of drug resistance is important for determining the fundamental principles of how fungal pathogens evolve for resistance. While few detailed reports that focus on the population dynamics of C. albicans currently exist, several important features on the population structure and adaptive landscape can be elucidated from existing evolutionary studies in in vivo and in vitro systems.

The echinocandin B producer fungus Aspergillus nidulans var. roseus ATCC 58397 does not possess innate resistance against its lipopeptide antimycotic

Aspergillus nidulans var. roseus ATCC 58397 is an echinocandin B (ECB) producer ascomycete with great industrial importance. As demonstrated by ECB/caspofungin sensitivity assays, A. nidulans var. roseus does not possess any inherent resistance to echinocandins, and its tolerance to these lipopeptide antimycotics are even lower than those of the non-producer A. nidulans FGSC A4 strain. Under ECB producing conditions or ECB exposures, A. nidulans var. roseus induced its ECB tolerance via up-regulating elements of the chitin biosynthetic machinery and, hence, through changing dynamically the composition of its own cell wall. Importantly, although the specific β-1,3-glucan synthase activity was elevated, these changes reduced the β-glucan content of hyphae considerably, but the expression of fksA, encoding the catalytic subunit of β-1,3-glucan synthase, the putative target of echinocandins in the aspergilli, was not affected. These data suggest that compensatory chitin biosynthesis is the centerpiece of the induced ECB tolerance of A. nidulans var. roseus. It is important to note that the induced tolerance to ECB (although resulted in paradoxical growth at higher ECB concentrations) was accompanied with reduced growth rate and, under certain conditions, even sensitized the fungus to other stress-generating agents like SDS. We hypothesize that although ECB-resistant mutants may arise in vivo in A. nidulans var. roseus cultures, their widespread propagation is severely restricted by the disadvantageous physiological effects of such mutations.

β(1,3)-glucan synthase complex from Alternaria infectoria, a rare dematiaceous human pathogen

The fungal cell wall polymer β-(1,3)-D-glucan is synthesized by the enzyme β-(1,3)- D-glucan synthase that is a complex composed of at least two proteins, Rho1p and Fks1p. Here, we report the nucleotide sequence of a single FKS gene and of the regulatory unit, RHO1 from the dematiaceous pathogenic fungus Alternaria infectoria. The predicted AiFks and AiRho share, respectively, 93% and 100% identity with that of Drechslera tritici-repentis. We also report that the sensitivity to caspofungin of eight different A. infectoria clinical strains is similar, with a MIC > 32 µg/ml and a MEC of 1 µg/ml, except for one strain which had a MEC of 1.4 µg/ml. This same strain exhibited one substitution at the hot spot 2, S1405A, compatible with less susceptible phenotypes, with the other seven strains having no mutations in either hot spot 1 or 2. The relative quantification of the expression of AiFKS and of AiRHO demonstrated a decrease in response to an exposure to caspofungin at 0.5 µg/ml.

Differential in vivo activities of anidulafungin, caspofungin, and micafungin against Candida glabrata isolates with and without FKS resistance mutations

We recently observed that the micafungin MICs for some Candida glabrata fks hot spot mutant isolates are less elevated than those for the other echinocandins, suggesting that the efficacy of micafungin may be differentially dependent on such mutations. Three clinical C. glabrata isolates with or without (S3) fks hot spot mutations R83 (Fks2p-S663F) and RR24 (Fks1p-S629P) and low, medium, and high echinocandin MICs, respectively, were evaluated to assess the in vivo efficacy in an immunocompetent mouse model using three doses of each echinocandin. Drug concentrations were determined in plasma and kidneys by high-performance liquid chromatography (HPLC). A pharmacokinetic-pharmacodynamic mathematical model was used to define the area under the concentration-time curve (AUC) that produced half- and near-maximal activity. Micafungin was equally efficacious against the S3 and R83 isolates. The estimates for the AUCs of each echinocandin that induced half-maximal effect (E(50)s) were 194.2 and 53.99 mg · h/liter, respectively. In contrast, the maximum effect (E(max)) for caspofungin was higher against S3 than R83, but the estimates for E(50) were similar (187.1 and 203.5 mg · h/liter, respectively). Anidulafungin failed to induce a ≥1-log reduction for any of the isolates (AUC range, 139 to 557 mg · h/liter). None of the echinocandins were efficacious in mice challenged with the RR24 isolate despite lower virulence (reduced maximal growth, prolonged lag phase, and lower kidney burden). The AUC associated with half-maximal effect was higher than the average human exposure for all drug-dose-bug combinations except micafungin and the R83 isolate. In conclusion, differences in micafungin MICs are associated with differential antifungal activities in the animal model. This study may have implications for clinical practice and echinocandin breakpoint determination, and further studies are warranted.

Antifungal resistance and new strategies to control fungal infections

Despite improvement of antifungal therapies over the last 30 years, the phenomenon of antifungal resistance is still of major concern in clinical practice. In the last 10 years the molecular mechanisms underlying this phenomenon were extensively unraveled. In this paper, after a brief overview of currently available antifungals, molecular mechanisms of antifungal resistance will be detailed. It appears that major mechanisms of resistance are essential due to the deregulation of antifungal resistance effector genes. This deregulation is a consequence of point mutations occurring in transcriptional regulators of these effector genes. Resistance can also follow the emergence of point mutations directly in the genes coding antifungal targets. In addition we further describe new strategies currently undertaken to discover alternative therapy targets and antifungals. Identification of new antifungals is essentially achieved by the screening of natural or synthetic chemical compound collections. Discovery of new putative antifungal targets is performed through genome-wide approaches for a better understanding of the human pathogenic fungi biology.

Elevated cell wall chitin in Candida albicans confers echinocandin resistance in vivo

Candida albicans cells with increased cell wall chitin have reduced echinocandin susceptibility in vitro. The aim of this study was to investigate whether C. albicans cells with elevated chitin levels have reduced echinocandin susceptibility in vivo. BALB/c mice were infected with C. albicans cells with normal chitin levels and compared to mice infected with high-chitin cells. Caspofungin therapy was initiated at 24 h postinfection. Mice infected with chitin-normal cells were successfully treated with caspofungin, as indicated by reduced kidney fungal burdens, reduced weight loss, and decreased C. albicans density in kidney lesions. In contrast, mice infected with high-chitin C. albicans cells were less susceptible to caspofungin, as they had higher kidney fungal burdens and greater weight loss during early infection. Cells recovered from mouse kidneys at 24 h postinfection with high-chitin cells had 1.6-fold higher chitin levels than cells from mice infected with chitin-normal cells and maintained a significantly reduced susceptibility to caspofungin when tested in vitro. At 48 h postinfection, caspofungin treatment induced a further increase in chitin content of C. albicans cells harvested from kidneys compared to saline treatment. Some of the recovered clones had acquired, at a low frequency, a point mutation in FKS1 resulting in a S645Y amino acid substitution, a mutation known to confer echinocandin resistance. This occurred even in cells that had not been exposed to caspofungin. Our results suggest that the efficacy of caspofungin against C. albicans was reduced in vivo due to either elevation of chitin levels in the cell wall or acquisition of FKS1 point mutations.

Current perspectives on echinocandin class drugs

It has been nearly a decade since caspofungin was approved for clinical use as the first echinocandin class antifungal agent, followed by micafungin and anidulafungin. The echinocandin drugs target the fungal cell wall by inhibiting the synthesis of β-1,3-D-glucan, a critical cell wall component of many pathogenic fungi. They are fungicidal for Candida spp. and fungistatic for moulds, such as Aspergillus fumigatus, where they induce abnormal morphology and growth properties. The echinocandins have a limited antifungal spectrum but are highly active against most Candida spp., including azole-resistant strains and biofilms. As they target glucan synthase, an enzyme absent in mammalian cells, the echinocandins have a favorable safety profile. They show potent MIC and epidemiological cutoff values against susceptible Candida and Aspergillus isolates, and the frequency of resistance is low. When clinical breakthrough occurs, it is associated with high MIC values and mutations in Fks subunits of glucan synthase, which can reduce the sensitivity of the enzyme to the drug by several thousand-fold. Such strains were not adequately captured by an early clinical breakpoint for susceptibility prompting a revised lower value, which addresses the FKS resistance mechanism and new pharmacokinetic/pharmacodynamic studies. Elevated MIC values unlinked to therapeutic failure can occur and result from adaptive cell behavior, which is FKS-independent and involves the molecular chaperone Hsp90 and the calcineurin pathway. Mutations in FKS1 and/or FKS2 alter the kinetic properties of glucan synthase, which reduces the relative fitness of mutant strains causing them to be less pathogenic. The echinocandin drugs also modify the cell wall architecture exposing buried glucans, which in turn induce a variety of important host immune responses. Finally, the future for glucan synthase inhibitors looks bright with the development of new orally active compounds.

Fitness and virulence costs of Candida albicans FKS1 hot spot mutations associated with echinocandin resistance

The identification of FKS1 mutations in Candida albicans associated with echinocandin resistance has raised concerns over the spread of drug-resistant strains. We studied the impact of fks1 mutations on C. albicans virulence and fitness. Compared with wild-type strains for FKS1, echinocandin-resistant C. albicans strains with homozygous fks1 hot-spot mutations had reduced maximum catalytic capacity of their glucan synthase complexes and thicker cell walls attributable to increased cell wall chitin content. The fks1 mutants with the highest chitin contents had reduced growth rates and impaired filamentation capacities. Fks1 mutants were hypovirulent in fly and mouse models of candidiasis, and this phenotype correlated with the cell wall chitin content. In addition, we observed reduced fitness of echinocandin-resistant C. albicans in competitive mixed infection models. We conclude that fks1 mutations that confer echinocandin resistance come at fitness and virulence costs, which may limit their epidemiological and clinical impact.

Hsp90 governs dispersion and drug resistance of fungal biofilms

Fungal biofilms are a major cause of human mortality and are recalcitrant to most treatments due to intrinsic drug resistance. These complex communities of multiple cell types form on indwelling medical devices and their eradication often requires surgical removal of infected devices. Here we implicate the molecular chaperone Hsp90 as a key regulator of biofilm dispersion and drug resistance. We previously established that in the leading human fungal pathogen, Candida albicans, Hsp90 enables the emergence and maintenance of drug resistance in planktonic conditions by stabilizing the protein phosphatase calcineurin and MAPK Mkc1. Hsp90 also regulates temperature-dependent C. albicans morphogenesis through repression of cAMP-PKA signalling. Here we demonstrate that genetic depletion of Hsp90 reduced C. albicans biofilm growth and maturation in vitro and impaired dispersal of biofilm cells. Further, compromising Hsp90 function in vitro abrogated resistance of C. albicans biofilms to the most widely deployed class of antifungal drugs, the azoles. Depletion of Hsp90 led to reduction of calcineurin and Mkc1 in planktonic but not biofilm conditions, suggesting that Hsp90 regulates drug resistance through different mechanisms in these distinct cellular states. Reduction of Hsp90 levels led to a marked decrease in matrix glucan levels, providing a compelling mechanism through which Hsp90 might regulate biofilm azole resistance. Impairment of Hsp90 function genetically or pharmacologically transformed fluconazole from ineffectual to highly effective in eradicating biofilms in a rat venous catheter infection model. Finally, inhibition of Hsp90 reduced resistance of biofilms of the most lethal mould, Aspergillus fumigatus, to the newest class of antifungals to reach the clinic, the echinocandins. Thus, we establish a novel mechanism regulating biofilm drug resistance and dispersion and that targeting Hsp90 provides a much-needed strategy for improving clinical outcome in the treatment of biofilm infections.

Candida albicans and Aspergillus fumigatus are the most common causative agents of fungal infections worldwide. Both species can form biofilms on host tissues and indwelling medical devices that are highly resistant to antifungal treatment. Here we implicate the molecular chaperone Hsp90 as a key regulator of biofilm dispersion and drug resistance. Compromising Hsp90 function reduced biofilm formation of C. albicans in vitro and impaired dispersal of biofilm cells, potentially blocking their capacity to serve as reservoirs for infection. Further, compromise of Hsp90 function abrogated resistance of C. albicans biofilms to the most widely deployed class of antifungal, the azoles, both in vitro and in a mammalian model of catheter-associated candidiasis. Key drug resistance regulators were depleted upon reduction of Hsp90 levels in planktonic but not biofilm conditions, suggesting that Hsp90 regulates drug resistance through different mechanisms in these distinct cellular states. Reduction of Hsp90 markedly reduced levels of matrix glucan, a carbohydrate important for C. albicans biofilm drug resistance. Inhibition of Hsp90 also reduced resistance of A. fumigatus biofilms to the newest class of antifungal, the echinocandins. Thus, targeting Hsp90 provides a promising strategy for the treatment of biofilm infections caused by diverse fungal species.

Prior caspofungin exposure in patients with hematological malignancies is a risk factor for subsequent fungemia due to decreased susceptibility in Candida spp.: a case-control study in Paris, France

Infections due to caspofungin-resistant Candida isolates in patients exposed to caspofungin therapy are increasing. We report here a nested case-control study which aimed at identifying factors associated with bloodstream infections caused by Candida spp. having reduced susceptibility to caspofungin (CRSC) in adults suffering from hematological malignancies. In univariate and multivariate analyses, infections with CRSC were associated with caspofungin exposure in the previous 30 days (odds ratio [OR] = 5.25; 95% confidence interval [95% CI], 1.68-16.35) and with an age of ≤ 65 years (OR = 3.27; 95% CI, 1.26-8.50).

Discovery of a novel class of orally active antifungal beta-1,3-D-glucan synthase inhibitors

The echinocandins are a class of semisynthetic natural products that target β-1,3-glucan synthase (GS). Their proven clinical efficacy combined with minimal safety issues has made the echinocandins an important asset in the management of fungal infection in a variety of patient populations. However, the echinocandins are delivered only parenterally. A screen for antifungal bioactivities combined with mechanism-of-action studies identified a class of piperazinyl-pyridazinones that target GS. The compounds exhibited in vitro activity comparable, and in some cases superior, to that of the echinocandins. The compounds inhibit GS in vitro, and there was a strong correlation between enzyme inhibition and in vitro antifungal activity. In addition, like the echinocandins, the compounds caused a leakage of cytoplasmic contents from yeast and produced a morphological response in molds characteristic of GS inhibitors. Spontaneous mutants of Saccharomyces cerevisiae with reduced susceptibility to the piperazinyl-pyridazinones had substitutions in FKS1. The sites of these substitutions were distinct from those conferring resistance to echinocandins; likewise, echinocandin-resistant isolates remained susceptible to the test compounds. Finally, we present efficacy and pharmacokinetic data on an example of the piperazinyl-pyridazinone compounds that demonstrated efficacy in a murine model of Candida glabrata infection.

Pharmacodynamics of echinocandins against Candida glabrata: requirement for dosage escalation to achieve maximal antifungal activity in neutropenic hosts

Candida glabrata is a leading cause of disseminated candidiasis. The echinocandins are increasingly used as first-line agents for the treatment of patients with this syndrome, although the optimal regimen for the treatment of invasive Candida glabrata infections in neutropenic patients is not known. We studied the pharmacokinetics (PK) and pharmacodynamics (PD) of micafungin, anidulafungin, and caspofungin in a neutropenic murine model of disseminated Candida glabrata infection to gain further insight into optimal therapeutic options for patients with this syndrome. A mathematical model was fitted to the data and used to bridge the experimental results to humans. The intravenous inoculation of Candida glabrata in mice was followed by logarithmic growth throughout the experimental period (101 h). A dose-dependent decline in fungal burden was observed following the administration of 0.1 to 20 mg/kg of body weight every 24 h for all three agents. The exposure-response relationships for each drug partitioned into distinct fungistatic and fungicidal components of activity. Surprisingly, the average human drug exposures following currently licensed regimens were predicted to result in a fungistatic antifungal effect. Higher human dosages of all three echinocandins are required to induce fungicidal effects in neutropenic hosts.

Epidemiology and antifungal resistance in invasive candidiasis

The epidemiology of Candida infections has changed over the last two decades: The number of patients suffering from such infections has increased dramatically and the Candida species involved have become more numerous as Candida albicans is replaced as an infecting agent by various non-C. albicans species (NAC). At the same time, additional antifungal agents have become available. The different Candida species may vary in their susceptibility for these various antifungals. This draws more attention to in vitro susceptibility testing. Unfortunately, several different test methods exist that may deliver different results. Moreover, clinical breakpoints (CBP) that classify test results into susceptible, intermediate and resistant are controver- sial between CLSI and EUCAST. Therefore, clinicians should be aware that interpretations may vary with the test system being followed by the microbiological laboratory. Thus, knowledge of actual MIC values and pharmacokinetic properties of individual antifungal agents is important in delivering appropriate therapy to patients

Caspofungin therapy in immunocompromised children and neonates

The prevalence of invasive fungal infections is increasing and the infections are becoming a major problem in immunocompromised children and neonates. Fortunately, there has been a recent surge in the development of new antifungal agents. Caspofungin, the first licensed echinocandin, is a novel class of antifungal and is approved for use in children 3 months of age or older for the treatment of invasive candidiasis, salvage therapy for invasive aspergillosis and as empirical therapy for febrile neutropenia. This article reviews the published data on the use of caspofungin in immunocompromised children and neonates with invasive fungal infections.