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

Chitin Biosynthesis in Aspergillus Species

The fungal cell wall (FCW) is a dynamic structure responsible for the maintenance of cellular homeostasis, and is essential for modulating the interaction of the fungus with its environment. It is composed of proteins, lipids, pigments and polysaccharides, including chitin. Chitin synthesis is catalyzed by chitin synthases (CS), and up to eight CS-encoding genes can be found in Aspergillus species. This review discusses in detail the chitin synthesis and regulation in Aspergillus species, and how manipulation of chitin synthesis pathways can modulate fungal growth, enzyme production, virulence and susceptibility to antifungal agents. More specifically, the metabolic steps involved in chitin biosynthesis are described with an emphasis on how the initiation of chitin biosynthesis remains unknown. A description of the classification, localization and transport of CS was also made. Chitin biosynthesis is shown to underlie a complex regulatory network, with extensive cross-talks existing between the different signaling pathways. Furthermore, pathways and recently identified regulators of chitin biosynthesis during the caspofungin paradoxical effect (CPE) are described. The effect of a chitin on the mammalian immune system is also discussed. Lastly, interference with chitin biosynthesis may also be beneficial for biotechnological applications. Even after more than 30 years of research, chitin biosynthesis remains a topic of current interest in mycology.

The role of Candida albicans stress response pathways in antifungal tolerance and resistance

Human fungal pathogens are the causative agents of devastating diseases across the globe, and the increasing prevalence of drug resistance threatens to undermine the already limited treatment options. One prominent pathogen is the opportunistic fungus Candida albicans, which can cause both superficial and serious systemic infections in immunocompromised individuals. C. albicans antifungal drug resistance and antifungal tolerance are supported by diverse and expansive cellular stress response pathways. Some of the major players are the Ca²⁺-calmodulin-activated phosphatase calcineurin, the protein kinase C cell wall integrity pathway, and the molecular chaperone heat shock protein 90. Beyond these core signal transducers, several other enzymes and transcription factors have been implicated in both tolerance and resistance. Here, we highlight some of the major stress response pathways, key advances in identifying chemical matter to inhibit these pathways, and implications for C. albicans persistence in the host.

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Candida albicans can cause superficial and serious systemic infections in humans

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Stress response pathways regulate C. albicans antifungal resistance and tolerance

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Stress response regulators include calcineurin, Pkc1, Hsp90, and many others

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Stress response inhibitors could reduce the likelihood of fungi persisting in humans

Antifungal Susceptibility Testing: A Primer for Clinicians

Clinicians treating patients with fungal infections may turn to susceptibility testing to obtain information regarding the activity of different antifungals against a specific fungus that has been cultured. These results may then be used to make decisions regarding a patient's therapy. However, for many fungal species that are capable of causing invasive infections, clinical breakpoints have not been established. Thus, interpretations of susceptible or resistant cannot be provided by clinical laboratories, and this is especially true for many molds capable of causing severe mycoses. The purpose of this review is to provide an overview of susceptibility testing for clinicians, including the methods used to perform these assays, their limitations, how clinical breakpoints are established, and how the results may be put into context in the absence of interpretive criteria. Examples of when susceptibility testing is not warranted are also provided.

The Protein Kinase A-Dependent Phosphoproteome of the Human Pathogen Aspergillus fumigatus Reveals Diverse Virulence-Associated Kinase Targets

PKA is essential for the virulence of eukaryotic human pathogens. Understanding PKA signaling mechanisms is therefore fundamental to deciphering pathogenesis and developing novel therapies.

Protein kinase A (PKA) signaling plays a critical role in the growth and development of all eukaryotic microbes. However, few direct targets have been characterized in any organism. The fungus Aspergillus fumigatus is a leading infectious cause of death in immunocompromised patients, but the specific molecular mechanisms responsible for its pathogenesis are poorly understood. We used this important pathogen as a platform for a comprehensive and multifaceted interrogation of both the PKA-dependent whole proteome and phosphoproteome in order to elucidate the mechanisms through which PKA signaling regulates invasive microbial disease. Employing advanced quantitative whole-proteomic and phosphoproteomic approaches with two complementary phosphopeptide enrichment strategies, coupled to an independent PKA interactome analysis, we defined distinct PKA-regulated pathways and identified novel direct PKA targets contributing to pathogenesis. We discovered three previously uncharacterized virulence-associated PKA effectors, including an autophagy-related protein, Atg24; a CCAAT-binding transcriptional regulator, HapB; and a CCR4-NOT complex-associated ubiquitin ligase, Not4. Targeted mutagenesis, combined with in vitro kinase assays, multiple murine infection models, structural modeling, and molecular dynamics simulations, was employed to characterize the roles of these new PKA targets in growth, environmental and antimicrobial stress responses, and pathogenesis in a mammalian system. We also elucidated the molecular mechanisms of PKA regulation for these effectors by defining the functionality of phosphorylation at specific PKA target sites. We have comprehensively characterized the PKA-dependent phosphoproteome and validated PKA targets as direct regulators of infectious disease for the first time in any pathogen, providing new insights into PKA signaling and control over microbial pathogenesis.

Minimal Inhibitory Concentration (MIC)-Phenomena in Candida albicans and Their Impact on the Diagnosis of Antifungal Resistance

Antifungal susceptibility testing (AFST) of clinical isolates is a tool in routine diagnostics to facilitate decision making on optimal antifungal therapy. The minimal inhibitory concentration (MIC)-phenomena (trailing and paradoxical effects (PXE)) observed in AFST complicate the unambiguous and reproducible determination of MICs and the impact of these phenomena on in vivo outcome are not fully understood. We aimed to link the MIC-phenomena with in vivo treatment response using the alternative infection model Galleria mellonella. We found that Candida albicans strains exhibiting PXE for caspofungin (CAS) had variable treatment outcomes in the Galleria model. In contrast, C. albicans strains showing trailing for voriconazole failed to respond in vivo. Caspofungin- and voriconazole-susceptible C. albicans strains responded to the respective antifungal therapy in vivo. In conclusion, MIC data and subsequent susceptibility interpretation of strains exhibiting PXE and/or trailing should be carried out with caution, as both effects are linked to drug adaptation and treatment response is uncertain to predict.

The fungal cell wall as a target for the development of new antifungal therapies

In the past three decades invasive mycoses have globally emerged as a persistent source of healthcare-associated infections. The cell wall surrounding the fungal cell opposes the turgor pressure that otherwise could produce cell lysis. Thus, the cell wall is essential for maintaining fungal cell shape and integrity. Given that this structure is absent in host mammalian cells, it stands as an important target when developing selective compounds for the treatment of fungal infections. Consequently, treatment with echinocandins, a family of antifungal agents that specifically inhibits the biosynthesis of cell wall (1-3)β-D-glucan, has been established as an alternative and effective antifungal therapy. However, the existence of many pathogenic fungi resistant to single or multiple antifungal families, together with the limited arsenal of available antifungal compounds, critically affects the effectiveness of treatments against these life-threatening infections. Thus, new antifungal therapies are required. Here we review the fungal cell wall and its relevance in biotechnology as a target for the development of new antifungal compounds, disclosing the most promising cell wall inhibitors that are currently in experimental or clinical development for the treatment of some invasive mycoses.

Oral glucan synthase inhibitor SCY-078 is effective in an experimental murine model of invasive candidiasis caused by WT and echinocandin-resistant Candida glabrata

Background

Echinocandins are recommended as first-line therapy against Candida glabrata infections, although increased resistance to this class has been reported worldwide and they are currently only available for parenteral administration. SCY-078 is an investigational glucan synthase inhibitor that is orally available.

Objectives

To evaluate the in vivo efficacy of SCY-078 in an experimental model of invasive candidiasis due to WT and echinocandin-resistant C. glabrata isolates.

Methods

Neutropenic ICR mice were inoculated intravenously with a WT isolate (SCY-078 and caspofungin MICs 0.25 and 0.125 mg/L, respectively) or an echinocandin-resistant isolate (SCY-078 and caspofungin MICs 1 and 0.5 mg/L, respectively). Treatment with placebo, SCY-078 (8, 30 or 40 mg/kg orally every 12 h) or caspofungin (1 mg/kg by intraperitoneal injection once daily) began 24 h later. Kidney fungal burden was measured on day 8 post-inoculation.

Results

Significant reductions in kidney fungal burden were observed with 30 mg/kg SCY-078 against both isolates and with the 40 mg/kg dose against the echinocandin-resistant isolate. These results were supported by SCY-078 plasma concentration data at the higher doses, where levels above the MICs for both isolates were observed 12 h after the last oral dose. Reductions in fungal burden were also observed with caspofungin against the WT isolate, but not against the resistant isolate.

Conclusions

SCY-078 demonstrated in vivo efficacy against infections caused by both WT and echinocandin-resistant C. glabrata isolates in this experimental model. This orally available glucan synthase inhibitor has potential as a therapy against echinocandin-resistant C. glabrata infections.

Molecular Evolution of Antifungal Drug Resistance

The fungal pathogens Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus have transitioned from a rare curiosity to a leading cause of human mortality. The management of infections caused by these organisms is intimately dependent on the efficacy of antifungal agents; however, fungi that are resistant to these treatments are regularly isolated in the clinic, impeding our ability to control infections. Given the significant impact fungal pathogens have on human health, it is imperative to understand the molecular mechanisms that govern antifungal drug resistance. This review describes our current knowledge of the mechanisms by which antifungal drug resistance evolves in experimental populations and clinical settings. We explore current antifungal treatment options and discuss promising strategies to impede the evolution of drug resistance. By tackling antifungal drug resistance as an evolutionary problem, there is potential to improve the utility of current treatments and accelerate the development of novel therapeutic strategies.

The Aspergillus fumigatus Sialidase (Kdnase) Contributes to Cell Wall Integrity and Virulence in Amphotericin B-Treated Mice

Aspergillus fumigatus is a filamentous fungus that can cause a life-threatening invasive pulmonary aspergillosis (IPA) in immunocompromised individuals. We previously characterized an exo-sialidase from A. fumigatus that prefers the sialic acid substrate, 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (Kdn); hence it is a Kdnase. Sialidases are known virulence factors in other pathogens; therefore, the goal of our study was to evaluate the importance of Kdnase in A. fumigatus. A kdnase knockout strain (Δkdnase) was unable to grow on medium containing Kdn and displayed reduced growth and abnormal morphology. Δkdnase was more sensitive than wild type to hyperosmotic conditions and the antifungal agent, amphotericin B. In contrast, Δkdnase had increased resistance to nikkomycin, Congo Red and Calcofluor White indicating activation of compensatory cell wall chitin deposition. Increased cell wall thickness and chitin content in Δkdnase were confirmed by electron and immunofluorescence microscopy. In a neutropenic mouse model of invasive aspergillosis, the Δkdnase strain had attenuated virulence and a significantly lower lung fungal burden but only in animals that received liposomal amphotericin B after spore exposure. Macrophage numbers were almost twofold higher in lung sections from mice that received the Δkdnase strain, possibly related to higher survival of macrophages that internalized the Δkdnase conidia. Thus, A. fumigatus Kdnase is important for fungal cell wall integrity and virulence, and because Kdnase is not present in the host, it may represent a potential target for the development of novel antifungal agents.

Phosphorylation of Aspergillus fumigatus PkaR impacts growth and cell wall integrity through novel mechanisms

Protein kinase A (PKA) signaling is essential for growth and virulence of the fungal pathogen Aspergillus fumigatus. Little is known concerning the regulation of this pathway in filamentous fungi. Employing liquid chromatography-tandem mass spectroscopy, we identified novel phosphorylation sites on the regulatory subunit PkaR, distinct from those previously identified in mammals and yeasts, and demonstrated the importance of two phosphorylation clusters for hyphal growth and cell wall-stress response. We also identified key differences in the regulation of PKA subcellular localization in A. fumigatus compared with other species. This is the first analysis of the phosphoregulation of a PKA regulatory subunit in a filamentous fungus and uncovers critical mechanistic differences between PKA regulation in filamentous fungi compared with mammals and yeast species, suggesting divergent targeting opportunities.

Pharmacokinetics of antifungal drugs: practical implications for optimized treatment of patients

Introduction

Because of the high mortality of invasive fungal infections (IFIs), appropriate exposure to antifungals appears to be crucial for therapeutic efficacy and safety.

Materials and methods

This review summarises published pharmacokinetic data on systemically administered antifungals focusing on co-morbidities, target-site penetration, and combination antifungal therapy.

Conclusions and discussion

Amphotericin B is eliminated unchanged via urine and faeces. Flucytosine and fluconazole display low protein binding and are eliminated by the kidney. Itraconazole, voriconazole, posaconazole and isavuconazole are metabolised in the liver. Azoles are substrates and inhibitors of cytochrome P450 (CYP) isoenzymes and are therefore involved in numerous drug–drug interactions. Anidulafungin is spontaneously degraded in the plasma. Caspofungin and micafungin undergo enzymatic metabolism in the liver, which is independent of CYP. Although several drug–drug interactions occur during caspofungin and micafungin treatment, echinocandins display a lower potential for drug–drug interactions. Flucytosine and azoles penetrate into most of relevant tissues. Amphotericin B accumulates in the liver and in the spleen. Its concentrations in lung and kidney are intermediate and relatively low myocardium and brain. Tissue distribution of echinocandins is similar to that of amphotericin. Combination antifungal therapy is established for cryptococcosis but controversial in other IFIs such as invasive aspergillosis and mucormycosis.

A Novel Phosphoregulatory Switch Controls the Activity and Function of the Major Catalytic Subunit of Protein Kinase A in Aspergillus fumigatus

Invasive aspergillosis (IA), caused by the filamentous fungal pathogen Aspergillus fumigatus, is a major cause of death among immunocompromised patients. The cyclic AMP/protein kinase A (PKA) signaling pathway is essential for hyphal growth and virulence of A. fumigatus, but the mechanism of regulation of PKA remains largely unknown. Here, we discovered a novel mechanism for the regulation of PKA activity in A. fumigatus via phosphorylation of key residues within the major catalytic subunit, PkaC1. Phosphopeptide enrichment and tandem mass spectrometry revealed the phosphorylation of PkaC1 at four sites (S175, T331, T333, and T337) with implications for important and diverse roles in the regulation of A. fumigatus PKA. While the phosphorylation at one of the residues (T333) is conserved in other species, the identification of three other residues represents previously unknown PKA phosphoregulation in A. fumigatus Site-directed mutagenesis of the phosphorylated residues to mimic or prevent phosphorylation revealed dramatic effects on kinase activity, growth, conidiation, cell wall stress response, and virulence in both invertebrate and murine infection models. Three-dimensional structural modeling of A. fumigatus PkaC1 substantiated the positive or negative regulatory roles for specific residues. Suppression of PKA activity also led to downregulation of PkaC1 protein levels in an apparent novel negative-feedback mechanism. Taken together, we propose a model in which PkaC1 phosphorylation both positively and negatively modulates its activity. These findings pave the way for future discovery of fungus-specific aspects of this key signaling network.

IMPORTANCE

Our understanding of signal transduction networks in pathogenic fungi is limited, despite the increase in invasive fungal infections and rising mortality rates in the immunosuppressed patient population. Because PKA is known to be essential for hyphal growth and virulence of A. fumigatus, we sought to identify fungus-specific regulatory mechanisms governing PKA activity. In this study, we identify, for the first time, a novel mechanism for the regulation of PKA signaling in which differential phosphorylation of the PkaC1 catalytic subunit can lead to either positive or negative regulation of activity. Furthermore, we show that inactivation of PKA signaling leads to downregulation of catalytic subunit protein levels in a negative-feedback mechanism distinct from expression patterns previously reported in the yeasts. Our findings represent a divergence in the regulation of PKA signaling in A. fumigatus, which could potentially be exploited as a target and also open the avenue for discovery of fungus-specific downstream effectors of PKA.

The Paradoxical Effect of Echinocandins in Aspergillus fumigatus Relies on Recovery of the β-1,3-Glucan Synthase Fks1

Echinocandins target the fungal cell wall by inhibiting biosynthesis of the cell wall carbohydrate β-1,3-glucan. This antifungal drug class exhibits a paradoxical effect that is characterized by the resumption of growth of otherwise susceptible strains at higher drug concentrations (approximately 4 to 32 μg/ml). The nature of this phenomenon is still unknown. In this study, we analyzed the paradoxical effect of the echinocandin caspofungin on the pathogenic mold Aspergillus fumigatus Using a conditional fks1 mutant, we show that very high caspofungin concentrations exert an additional antifungal activity besides inhibition of the β-1,3-glucan synthase. This activity could explain the suppression of paradoxical growth at very high caspofungin concentrations. Additionally, we found that exposure to inhibitory caspofungin concentrations always causes initial growth deprivation independently of the capability of the drug concentration to induce the paradoxical effect. Paradoxically growing hyphae emerge from microcolonies essentially devoid of β-1,3-glucan. However, these hyphae expose β-1,3-glucan again, suggesting that β-1,3-glucan synthesis is restored. In agreement with this hypothesis, we found that expression of the β-1,3-glucan synthase Fks1 is an essential requirement for the paradoxical effect. Surprisingly, overexpression of fks1 renders A. fumigatus more susceptible, whereas reduced expression leads to hyphae that are more resistant to the growth-inhibitory and limited fungicidal activity of caspofungin. Upregulation of chitin synthesis appears to be of minor importance for the paradoxical effect, since paradoxically growing hyphae exhibit significantly less chitin than the growth-deprived parental microcolonies. Our results argue for a model where the paradoxical effect primarily relies on recovery of β-1,3-glucan synthase activity.

Characterization of the FKBP12-Encoding Genes in Aspergillus fumigatus

Invasive aspergillosis, largely caused by Aspergillus fumigatus, is responsible for a growing number of deaths among immunosuppressed patients. Immunosuppressants such as FK506 (tacrolimus) that target calcineurin have shown promise for antifungal drug development. FK506-binding proteins (FKBPs) form a complex with calcineurin in the presence of FK506 (FKBP12-FK506) and inhibit calcineurin activity. Research on FKBPs in fungi is limited, and none of the FKBPs have been previously characterized in A. fumigatus. We identified four orthologous genes of FKBP12, the human FK506 binding partner, in A. fumigatus and designated them fkbp12-1, fkbp12-2, fkbp12-3, and fkbp12-4. Deletional analysis of the four genes revealed that the Δfkbp12-1 strain was resistant to FK506, indicating FKBP12-1 as the key mediator of FK506-binding to calcineurin. The endogenously expressed FKBP12-1-EGFP fusion protein localized to the cytoplasm and nuclei under normal growth conditions but also to the hyphal septa following FK506 treatment, revealing its interaction with calcineurin. The FKBP12-1-EGFP fusion protein didn’t localize at the septa in the presence of FK506 in the cnaA deletion background, confirming its interaction with calcineurin. Testing of all deletion strains in the Galleria mellonella model of aspergillosis suggested that these proteins don’t play an important role in virulence. While the Δfkbp12-2 and Δfkbp12-3 strains didn’t show any discernable phenotype, the Δfkbp12-4 strain displayed slight growth defect under normal growth conditions and inhibition of the caspofungin-mediated “paradoxical growth effect” at higher concentrations of the antifungal caspofungin. Together, these results indicate that while only FKBP12-1 is the bona fide binding partner of FK506, leading to the inhibition of calcineurin in A. fumigatus, FKBP12-4 may play a role in basal growth and the caspofungin-mediated paradoxical growth response. Exploitation of differences between A. fumigatus FKBP12-1 and human FKBP12 will be critical for the generation of fungal-specific FK506 analogs to inhibit fungal calcineurin and treat invasive fungal disease.

Hitting the caspofungin salvage pathway of human-pathogenic fungi with the novel lasso peptide humidimycin (MDN-0010)

Fungal infections have increased dramatically in the last 2 decades, and fighting infectious diseases requires innovative approaches such as the combination of two drugs acting on different targets or even targeting a salvage pathway of one of the drugs. The fungal cell wall biosynthesis is inhibited by the clinically used antifungal drug caspofungin. This antifungal activity has been found to be potentiated by humidimycin, a new natural product identified from the screening of a collection of 20,000 microbial extracts, which has no major effect when used alone. An analysis of transcriptomes and selected Aspergillus fumigatus mutants indicated that humidimycin affects the high osmolarity glycerol response pathway. By combining humidimycin and caspofungin, a strong increase in caspofungin efficacy was achieved, demonstrating that targeting different signaling pathways provides an excellent basis to develop novel anti-infective strategies.

Hsp70 and the Cochaperone StiA (Hop) Orchestrate Hsp90-Mediated Caspofungin Tolerance in Aspergillus fumigatus

Aspergillus fumigatus is the primary etiologic agent of invasive aspergillosis (IA), a major cause of death among immunosuppressed patients. Echinocandins (e.g., caspofungin) are increasingly used as second-line therapy for IA, but their activity is only fungistatic. Heat shock protein 90 (Hsp90) was previously shown to trigger tolerance to caspofungin and the paradoxical effect (i.e., decreased efficacy of caspofungin at higher concentrations). Here, we demonstrate the key role of another molecular chaperone, Hsp70, in governing the stress response to caspofungin via Hsp90 and their cochaperone Hop/Sti1 (StiA in A. fumigatus). Mutation of the StiA-interacting domain of Hsp70 (C-terminal EELD motif) impaired thermal adaptation and caspofungin tolerance with loss of the caspofungin paradoxical effect. Impaired Hsp90 function and increased susceptibility to caspofungin were also observed following pharmacologic inhibition of the C-terminal domain of Hsp70 by pifithrin-μ or after stiA deletion, further supporting the links among Hsp70, StiA, and Hsp90 in governing caspofungin tolerance. StiA was not required for the physical interaction between Hsp70 and Hsp90 but had distinct roles in the regulation of their function in caspofungin and heat stress responses. In conclusion, this study deciphering the physical and functional interactions of the Hsp70-StiA-Hsp90 complex provided new insights into the mechanisms of tolerance to caspofungin in A. fumigatus and revealed a key C-terminal motif of Hsp70, which can be targeted by specific inhibitors, such as pifithrin-μ, to enhance the antifungal activity of caspofungin against A. fumigatus.

The Aspergillus fumigatus cell wall integrity signaling pathway: drug target, compensatory pathways, and virulence

Aspergillus fumigatus is the most important airborne fungal pathogen, causing severe infections with invasive growth in immunocompromised patients. The fungal cell wall (CW) prevents the cell from lysing and protects the fungus against environmental stress conditions. Because it is absent in humans and because of its essentiality, the fungal CW is a promising target for antifungal drugs. Nowadays, compounds acting on the CW, i.e., echinocandin derivatives, are used to treat A. fumigatus infections. However, studies demonstrating the clinical effectiveness of echinocandins in comparison with antifungals currently recommended for first-line treatment of invasive aspergillosis are still lacking. Therefore, it is important to elucidate CW biosynthesis pathways and their signal transduction cascades, which potentially compensate the inhibition caused by CW- perturbing compounds. Like in other fungi, the central core of the cell wall integrity (CWI) signaling pathway in A. fumigatus is composed of three mitogen activated protein kinases. Deletion of these genes resulted in severely enhanced sensitivity of the mutants against CW-disturbing compounds and in drastic alterations of the fungal morphology. Additionally, several cross-talk interactions between the CWI pathways and other signaling pathways are emerging, raising the question about their role in the CW compensatory mechanisms. In this review we focused on recent advances in understanding the CWI signaling pathway in A. fumigatus and its role during drug stress response and virulence.

Efficacy of the investigational echinocandin ASP9726 in a guinea pig model of invasive pulmonary aspergillosis

ASP9726 is an investigational echinocandin with in vitro activity against Aspergillus species. We evaluated the pharmacokinetics and efficacy of this agent in an established guinea pig model of invasive pulmonary aspergillosis. ASP9726 plasma concentrations were measured in guinea pigs administered either a single dose or multiple doses of this agent at 2.5, 5, and 10 mg/kg of body weight/day by subcutaneous injection. Immunosuppressed guinea pigs were inoculated with A. fumigatus AF293, and ASP9726 (2.5, 5, and 10 mg/kg/day), voriconazole (10 mg/kg by oral gavage twice daily), or caspofungin (3 mg/kg/day by intraperitoneal injection) was administered for 8 days. Changes in fungal burden were measured by enumerating CFU and by quantitative PCR of specimens from within the lungs, as well as by analysis of serum (1 → 3)-β-D-glucan and galactomannan. Lung histopathology was also evaluated. ASP9726 plasma concentrations increased in a dose-proportional manner, and the drug was well tolerated at each dose. Each dose of ASP9726, voriconazole, and caspofungin significantly reduced pulmonary fungal burden as measured by quantitative PCR and by determining (1 → 3)-β-D-glucan and galactomannan levels, but only voriconazole significantly reduced numbers of CFU. ASP9726 at 5 mg/kg also significantly improved survival. Histopathology demonstrated morphological changes in hyphae in animals exposed to ASP9726 and caspofungin, consistent with the activities of the echinocandins. These results suggest that ASP9726 may be efficacious for the treatment of invasive pulmonary aspergillosis.

Histone deacetylase inhibition as an alternative strategy against invasive aspergillosis

Invasive aspergillosis (IA) is a life-threatening infection due to Aspergillus fumigatus and other Aspergillus spp. Drugs targeting the fungal cell membrane (triazoles, amphotericin B) or cell wall (echinocandins) are currently the sole therapeutic options against IA. Their limited efficacy and the emergence of resistance warrant the identification of new antifungal targets. Histone deacetylases (HDACs) are enzymes responsible of the deacetylation of lysine residues of core histones, thus controlling chromatin remodeling and transcriptional activation. HDACs also control the acetylation and activation status of multiple non-histone proteins, including the heat shock protein 90 (Hsp90), an essential molecular chaperone for fungal virulence and antifungal resistance. This review provides an overview of the different HDACs in Aspergillus spp. as well as their respective contribution to total HDAC activity, fungal growth, stress responses, and virulence. The potential of HDAC inhibitors, currently under development for cancer therapy, as novel alternative antifungal agents against IA is discussed.

Heat shock protein 90 (Hsp90): A novel antifungal target against Aspergillus fumigatus

Invasive aspergillosis is a life-threatening and difficult to treat infection in immunosuppressed patients. The efficacy of current anti-Aspergillus therapies, targeting the cell wall or membrane, is limited by toxicity (polyenes), fungistatic activity and some level of basal resistance (echinocandins), or the emergence of acquired resistance (triazoles). The heat shock protein 90 (Hsp90) is a conserved molecular chaperone involved in the rapid development of antifungal resistance in the yeast Candida albicans. Few studies have addressed its role in filamentous fungi such as Aspergillus fumigatus, in which mechanisms of resistance may differ substantially. Hsp90 is at the center of a complex network involving calcineurin, lysine deacetylases (KDAC) and other client proteins, which orchestrate compensatory repair mechanisms of the cell wall in response to the stress induced by antifungals. In A. fumigatus, Hsp90 is a trigger for resistance to high concentrations of caspofungin, known as the paradoxical effect. Disrupting Hsp90 circuitry by different means (Hsp90 inhibitors, KDAC inhibitors and anti-calcineurin drugs) potentiates the antifungal activity of caspofungin, thus representing a promising novel antifungal approach. This review will discuss the specific features of A. fumigatus Hsp90 and the potential for antifungal strategies of invasive aspergillosis targeting this essential chaperone.

Trailing or paradoxical growth of Aspergillus flavus exposed to caspofungin is independent of genotype

There are limited data on in vitro susceptibility testing of echinocandins against Aspergillus species. The objective of this study was to describe the phenotypes of Aspergillus flavus observed on exposure to caspofungin in vitro and to test whether these phenotypes were associated with A. flavus genotypes. The caspofungin MICs of 37 A. flavus clinical isolates collected from 14 patients with invasive aspergillosis were determined using Etest assays. Caspofungin MICs ranged from 0.012 to 0.064 mg l(-1); the modal MIC was 0.023 mg l(-1) and the MIC₅₀ and MIC₉₀ were 0.032 and 0.064 mg l(-1), respectively. A clear end point was noted in 24 (65 %) isolates, whereas seven (19 %) displayed a trailing effect and six (16 %) showed paradoxical growth when exposed to caspofungin. In these A. flavus isolates, the absence of a significant population structure or genetic differentiation indicated that trailing or paradoxical growth phenotypes were independent of microsatellite genotype.

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.

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.

Transcriptional activation of heat shock protein 90 mediated via a proximal promoter region as trigger of caspofungin resistance in Aspergillus fumigatus

Invasive aspergillosis is a deadly infection for which new antifungal therapies are needed. Heat shock protein 90 (Hsp90) is an essential chaperone in Aspergillus fumigatus representing an attractive antifungal target. Using a thiamine-repressible promoter (pthiA), we showed that genetic repression of Hsp90 significantly reduced virulence in a murine model of invasive aspergillosis. Moreover, substituting the A. fumigatus hsp90 promoter with 2 artificial promoters (potef, pthiA) and the Candida albicans hsp90 promoter resulted in hypersensitivity to caspofungin and abolition of the paradoxical effect (resistance at high caspofungin concentrations). By inducing truncations in the hsp90 promoter, we identified a 100-base pair proximal sequence that triggers a significant increase of hsp90 expression (≥1.5-fold) and is essential for the paradoxical effect. Preventing this increase of hsp90 expression was sufficient to abolish the paradoxical effect and therefore optimize the antifungal activity of caspofungin.

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.

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.

Effect of nikkomycin Z and 50% human serum on the killing activity of high-concentration caspofungin against Candida species using time-kill methodology

Caspofungin and nikkomycin Z (NIK) efficacy alone and in combination were tested against seven Candida species showing or not showing paradoxical growth (PG) against caspofungin in time-kill test in RPMI-1640. Selected isolates against caspofungin and NIK were also tested in 50% serum. PG was always eliminated by NIK as well as by serum. In the serum, 1 and 16 μg/ml caspofungin yielded 0.14-4.0 and 0.34-4.0 log CFU decreases from the starting inocula for C. albicans, C. glabrata, C. tropicalis, and C. dubliniensis, respectively. CFU decrease (0.10-2.08 log) at 16 μg/ml, but not at lower caspofungin concentration was noted against C. parapsilosis, C. orthopsilosis, and C. metapsilosis. One C. parapsilosis isolate was not inhibited even by 16 μg/ml caspofungin. Caspofungin against C. albicans, C. glabrata, C. tropicalis, and C. dubliniensis maintained its activity in serum at even 1 μg/ml concentration. PG seems to an in vitro phenomenon, without clinical relevance.

The high-osmolarity glycerol- and cell wall integrity-MAP kinase pathways of Saccharomyces cerevisiae are involved in adaptation to the action of killer toxin HM-1

Fps1p is an aquaglyceroporin important for turgor regulation of Saccharomyces cerevisiae. Previously we reported the involvement of Fps1p in the yeast-killing action of killer toxin HM-1. The fps1 cells showed a high HM-1-resistant phenotype in hypotonic medium and an HM-1-susceptible phenotype in hypertonic medium. This osmotic dependency in HM-1 susceptibility was similar to those observed in Congo red, but different from those observed in other cell wall-disturbing agents. These results indicate that HM-1 exerts fungicidal activity mainly by binding and inserting into the yeast cell wall structure, rather than by inhibiting 1,3-β-glucan synthase. We next determined HM-1-susceptibility and diphospho-MAP kinase inductions in S. cerevisiae. In the wild-type cell, expressions of diphospho-Hog1p and -Slt2p, and mRNA transcription of CWP1 and HOR2, were induced within 1 h after an addition of HM-1. ssk1 and pbs2 cells, but not sho1 and hkr1 cells, showed HM-1-sensitive phenotypes and lacked inductions of phospho-Hog1p in response to HM-1. mid2, rom2 and bck1 cells showed HM-1-sensitive phenotypes and decreased inductions of phospho-Slt2p in response to HM-1. From these results, we postulated that the Sln1-Ypd1-Ssk1 branch of the high-osmolality glycerol (HOG) pathway and plasma membrane sensors of the cell wall integrity (CWI) pathway detect cell wall stresses caused by HM-1. We further suggested that activations of both HOG and CWI pathways have an important role in the adaptive response to HM-1 toxicity.

Echinocandins: are they all the same?

The discovery of echinocandins, and their development and approval, was hailed as a significant addition to our antifungal armamentarium, previously predominated by polyenes and azoles. To date, three echinocandins (anidulafungin, caspofungin, and micafungin) have been approved by the U.S. Food and Drug Administration for the treatment of fungal infections. Since all three echinocandins target the fungal cell wall and share a similar structural chemical backbone, they are perceived to be identical. However, a scientific literature review shows distinct differences among the echinocandins in terms of in vitro activity, fungicidal activity, post-antifungal effect, paradoxical effect, and activity on biofilms. More investigation is warranted to determine if the observed differences among the echinocandins can translate to clinical advantages.

Heat shock protein 90 is required for conidiation and cell wall integrity in Aspergillus fumigatus

Heat shock protein 90 (Hsp90) is a eukaryotic molecular chaperone. Its involvement in the resistance of Candida albicans to azole and echinocandin antifungals is well established. However, little is known about Hsp90's function in the filamentous fungal pathogen Aspergillus fumigatus. We investigated the role of Hsp90 in A. fumigatus by genetic repression and examined its cellular localization under various stress conditions. Failure to generate a deletion strain of hsp90 suggested that it is essential. Genetic repression of Hsp90 was achieved by an inducible nitrogen-dependent promoter (pniiA-Hsp90) and led to decreased spore viability, decreased hyphal growth, and severe defects in germination and conidiation concomitant with the downregulation of the conidiation-specific transcription factors brlA, wetA, and abaA. Hsp90 repression potentiated the effect of cell wall inhibitors affecting the β-glucan structure of the cell wall (caspofungin, Congo red) and of the calcineurin inhibitor FK506, supporting a role in regulating cell wall integrity pathways. Moreover, compromising Hsp90 abolished the paradoxical effect of caspofungin. Pharmacological inhibition of Hsp90 by geldanamycin and its derivatives (17-AAG and 17-DMAG) resulted in similar effects. C-terminal green fluorescent protein (GFP) tagging of Hsp90 revealed mainly cytosolic distribution under standard growth conditions. However, treatment with caspofungin resulted in Hsp90 accumulation at the cell wall and at sites of septum formation, further highlighting its role in cell wall stress compensatory mechanisms. Targeting Hsp90 with fungal-specific inhibitors to cripple stress response compensatory pathways represents an attractive new antifungal strategy.

Role of innate immune receptors in paradoxical caspofungin activity in vivo in preclinical aspergillosis

This study investigated the possible mechanisms underlying the paradoxical caspofungin activity in vivo in preclinical aspergillosis. We evaluated the activity of escalating doses of caspofungin in vivo in different preclinical models of invasive aspergillosis, including mice deficient for selected innate immune receptors. The therapeutic efficacy of caspofungin in experimental invasive aspergillosis was strictly dose dependent, being observed at doses of 0.1 and 1 mg/kg of body weight depending on the experimental models. Paradoxical increase in pulmonary fungal burden as well as inflammatory pathology was observed at the highest dose of caspofungin (5 mg/kg), occurred independently of the so-called Eagle effect and susceptibility to caspofungin in vitro, and was contingent upon the presence of TLR2, Dectin-1, and TLR9. Increased expression of Dectin-1 and TLR9 were observed upon exposure to caspofungin in vitro and in vivo. Together, these findings suggest that the net activity of caspofungin in vivo is orchestrated by the activation, directly or indirectly, of multiple innate immune receptors.

Overcoming fluconazole resistance in Candida albicans clinical isolates with tetracyclic indoles

Continuing efforts to discover novel means of combating fluconazole resistance in Candida albicans have identified an indole derivative that sensitizes strains demonstrating resistance to fluconazole. This tetracycle (3, ML229) does not appear to act through established Hsp90 or calcineurin pathways to chemosensitize C. albicans, as determined in Saccharomyces cerevisiae models, and may be a useful probe to uncover alternative resistance pathways.

Addition of DNase improves the in vitro activity of antifungal drugs against Candida albicans biofilms

Cells within Candida albicans biofilms display decreased susceptibility to most clinically used antifungal agents. We recently demonstrated that extracellular DNA (eDNA) plays an important role in biofilm integrity, as a component of the biofilm matrix. This study aimed at gaining insights into the contributions of eDNA to C. albicans biofilms antifungal susceptibility by the investigation of the impact of the combined use of deoxyribonuclease I (DNase) and antifungals to treat biofilms. Candida albicans biofilms were formed using a simple and reproducible 96-well plate-based method. The activity of the combined use of 0.13 mg l(-1) DNase and antifungals was estimated using the 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium hydroxide (XTT) reduction assay and total viable counts. Herein, we report the improved efficacy of amphotericin B when in combination with DNase against C. albicans biofilms, as assessed using XTT readings and viable counts. Furthermore, although DNase increased the efficacy of caspofungin in the reduction of mitochondrial activity, no changes were observed in terms of culturable cells. Deoxyribonuclease I did not affect biofilm cells susceptibility to fluconazole. This work suggests that agents that target processes affecting the biofilm structural integrity may have potential use as adjuvants of a catheter-lock therapy.

Candida species extracellular alcohols: production and effect in sessile cells

Cell-cell signaling alcohol molecules were recently identified in Candida albicans and Candida dubliniensis supernatants. To date, it is not known whether these molecules are produced by other Candida species and their role in biofilm formation is not fully clarified. Herein, Candida parapsilosis and Candida tropicalis extracellular alcohols production by planktonic cultures was analyzed by headspace-solid-phase microextraction and gas chromatography-mass spectrometry. Both these Candida species extracellular media contained E,E-farnesol, 1-dodecanol, 2-phenylethanol, and isoamyl alcohol but not E-nerolidol, as produced by C. albicans and C. dubliniensis. Moreover, the ability of these compounds to regulate C. albicans, C. dubliniensis, C. parapsilosis, and C. tropicalis sessile cells was assessed by adding the alcohols after 3 h of adhesion and 48 h of biofilm formation. After 24 h, biofilms were analyzed in terms of cellular mitochondrial activity and total biomass. E,E-Farnesol affected C. albicans and C. dubliniensis. E-Nerolidol and 1-dodecanol elicited C. parapsilosis and C. tropicalis changes in further biofilm development. C. tropicalis was affected by 2-phenylethanol and isoamyl alcohol triggered changes in C. albicans, C. dubliniensis, and C. tropicalis sessile cells. The results demonstrated that almost all of these alcohols are produced by these Candida species and also evidence the complexity of biofilm formation.

Comparative in vivo dose-dependent activity of caspofungin and anidulafungin against echinocandin-susceptible and -resistant Aspergillus fumigatus

BACKGROUND

Echinocandin resistance in Aspergillus species is rare. We examined if mutations in FKS1 would result in a complete loss of echinocandin activity in vivo in an experimental model of aspergillosis.

METHODS

Neutropenic mice were infected with either an echinocandin-susceptible Aspergillus fumigatus (AF 293) or an echinocandin-resistant A. fumigatus laboratory strain harbouring 'hot-spot' substitution in Fks1p (AF Ser678Pro). Mice then received daily treatment with either anidulafungin or caspofungin at varying dosages (0.25-16 mg/kg/day) for 5 days and Aspergillus lung fungal burden was assessed by quantitative real-time PCR.

RESULTS

Both strains produced histological evidence of progressive invasive pulmonary aspergillosis, but AF Ser678Pro was less virulent than AF 293, as evidenced by lower lung fungal burden and longer median survival time. At > 0.5 mg/kg/day, both anidulafungin and caspofungin reduced the lung fungal burden in neutropenic animals infected with AF 293, but had mixed efficacy against the resistant AF Ser678Pro strain. For caspofungin, the fungal burden was reduced only at doses <1 mg/kg/day. Anidulafungin also modestly reduced AF Ser678Pro lung fungal burden, but only at > 4 mg/kg/day.

CONCLUSIONS

Despite a lack of appreciable antifungal activity in vitro, both anidulafungin and caspofungin were still modestly effective in vivo against a laboratory-generated A. fumigatus mutant harbouring the Ser678Pro mutation in Fks1p. This persistent activity, combined with impaired fitness of the isolate in vivo, could partially explain why microbiologically documented echinocandin-resistance in Aspergillus species remains a rare clinical occurrence.

Differential activities of three families of specific beta(1,3)glucan synthase inhibitors in wild-type and resistant strains of fission yeast

Three specific β(1,3)glucan synthase (GS) inhibitor families, papulacandins, acidic terpenoids, and echinocandins, have been analyzed in Schizosaccharomyces pombe wild-type and papulacandin-resistant cells and GS activities. Papulacandin and enfumafungin produced similar in vivo effects, different from that of echinocandins. Also, papulacandin was the strongest in vitro GS inhibitor (IC(50) 10(3)-10(4)-fold lower than with enfumafungin or pneumocandin), but caspofungin was by far the most efficient antifungal because of the following. 1) It was the only drug that affected resistant cells (minimal inhibitory concentration close to that of the wild type). 2) It was a strong inhibitor of wild-type GS (IC(50) close to that of papulacandin). 3) It was the best inhibitor of mutant GS. Moreover, caspofungin showed a special effect for two GS inhibition activities, of high and low affinity, separated by 2 log orders, with no increase in inhibition. pbr1-8 and pbr1-6 resistances are due to single substitutions in the essential Bgs4 GS, located close to the resistance hot spot 1 region described in Saccharomyces and Candida Fks mutants. Bgs4(pbr)(1-8) contains the E700V change, four residues N-terminal from hot spot 1 defining a larger resistance hot spot 1-1 of 13 amino acids. Bgs4(pbr)(1-6) contains the W760S substitution, defining a new resistance hot spot 1-2. We observed spontaneous revertants of the spherical pbr1-6 phenotype and found that an additional A914V change is involved in the recovery of the wild-type cell shape, but it maintains the resistance phenotype. A better understanding of the mechanism of action of the antifungals available should help to improve their activity and to identify new antifungal targets.

Changes in cell wall synthesis and ultrastructure during paradoxical growth effect of caspofungin on four different Candida species

Paradoxical growth (PG) has been described for echinocandins and is characterized by cell growth at drug concentrations above the MIC. In this study, two isolates each of Candida albicans, C. tropicalis, C. orthopsilosis, and C. parapsilosis, all of which displaying PG in response to caspofungin, were subjected to MIC, minimal fungicidal concentration (MFC), and time-kill curve assays to evaluate the levels of PG. Cell wall components and ultrastructural modifications of the PG cells were also investigated. The results showed that when cell growth and survival were evaluated by MFC or time-kill curve assays, high concentrations of caspofungin did not show fungicidal activity against PG cells. Furthermore, for C. parapsilosis and C. orthopsilosis, time-kill curves were more discriminatory than MFCs in detecting the PG effect. The four different Candida species studied demonstrated similar alterations in cell wall components and ultrastructure associated with PG. In PG cells, β-1,3-glucan content decreased from 2.7- to 7.8-fold, whereas chitin content increased from 4.0- to 6.6-fold. An electron microscopy study of the PG cells revealed morphological alterations, clumping of cells, enlarged cells, the absence of filamentation, abnormal septa, and accumulation of chitin in the cell wall. Also, PG cells basically exhibited a single dark high-density layer in the cell wall, indicating the loss of the β-1,3-glucan layer. Our results present novel details about the ultrastructural alterations that occur in C. albicans, C. parapsilosis, C. orthopsilosis, and C. tropicalis during PG and show that chitin is the major component of the cell walls of PG cells. Stimulation of chitin synthesis may represent a rescue mechanism against caspofungin activity.

Differential Aspergillus lentulus echinocandin susceptibilities are Fksp independent

The recently described species Aspergillus lentulus exhibits differential and reduced susceptibilities to echinocandins and other antifungal drugs in vitro. A. lentulus isolates overall are less susceptible to caspofungin, although they maintain susceptibility to anidulafungin and micafungin. Mutations or polymorphisms in fks, the gene encoding the catalytic subunit of β-1,3-glucan synthase, are known to confer decreased susceptibility to echinocandins in Candida spp. and Aspergillus fumigatus. The analysis of the A. lentulus fks sequence did not reveal a polymorphism at any of the known hot-spot regions of the gene. Caspofungin and micafungin kinetic inhibition profiles of the A. lentulus glucan synthase were comparable to those from susceptible A. fumigatus enzymes. Although the basal cell wall chitin levels in A. lentulus averaged 60% of those in A. fumigatus, echinocandin treatment promoted the increase of cell wall chitin in both organisms, indicating that A. lentulus displays a compensatory chitin response similar to that of A. fumigatus. The data suggest that differential echinocandin susceptibilities in A. lentulus are independent of the echinocandin target, Fksp, and they emphasize the potential that the drugs' capacity to inhibit the target enzyme is unequal at the cellular level.

Susceptibility of Candida albicans biofilms to azithromycin, tigecycline and vancomycin and the interaction between tigecycline and antifungals

Despite growing data on antimicrobial lock therapy (ALT) in treating bacterial catheter-related bloodstream infections (CR-BSIs), ALT has not been established as a treatment option for CR-BSI caused by Candida albicans. Based on our finding that high-dose doxycycline exhibited antifungal activity against mature C. albicans biofilms, we evaluated additional antibacterial agents with Gram-positive activity [azithromycin, tigecycline (TIG) and vancomycin]. After screening these antibiotics, it was found that TIG had substantial antifungal activity against mature C. albicans biofilms. Therefore, TIG was assayed alone and in combination with fluconazole (FLC), amphotericin B (AmB) or caspofungin (CAS). TIG at 2048 μg/mL resulted in a >50% reduction in the growth of planktonic C. albicans cells. TIG inhibited the formation of biofilms from 128 μg/mL. Against mature biofilms, 2048 μg/mL TIG reduced metabolic activity by 84.2%. Furthermore, addition of 512 μg/mL TIG to FLC at all concentrations tested provided additional reduction in the metabolic activity of mature biofilms. However, this was not superior to 512 μg/mL TIG alone. TIG at 512 μg/mL increased the antifungal effect of lower concentrations of AmB (0.03125-0.25 μg/mL), but at 0.03125 μg/mL and 0.0625 μg/mL this effect was not superior to 512 μg/mL TIG alone. TIG inhibited the antifungal effect of higher concentrations of AmB (≥ 2 μg/mL). TIG at 512 μg/mL inhibited the antifungal activity of CAS at lower concentrations (0.25-8 μg/mL). These data indicate that high-dose TIG is highly active in vitro against planktonic cells, forming biofilms and mature biofilms of C. albicans.

Anti-Candida-biofilm activity of micafungin is attenuated by voriconazole but restored by pharmacological inhibition of Hsp90-related stress responses

We have conducted an in vitro evaluation of the efficacy of a voriconazole-micafungin combination against Candida albicans. When used alone, both micafungin and voriconazole decreased the metabolic activity of planktonic cells, but only micafungin displayed potent anti-biofilm activity. Their combination appeared to have an additive effect against planktonic cells. However, voriconazole significantly antagonized the fungicidal effect of micafungin against Candida biofilms. Time-lag experiments showed that pre-treatment with voriconazole induced resistance to micafungin in Candida biofilms. The micafungin-antagonizing effect of voriconazole persisted even when the biofilm was no longer exposed to voriconazole. In contrast, voriconazole addition after 24 h of micafungin treatment did not alter micafungin sensitivity. To investigate the mechanism of antagonism, we used inhibitors of Hsp90 and its effectors because Hsp90 seems to be implicated in the resistance to micafungin. These molecules reversed the voriconazole-induced resistance to micafungin which suggests that Hsp90-related stress responses are involved in the antagonism. Our results may provide clues as to the mechanism of increased drug resistance in Candida biofilms and raises concerns about the use of the voriconazole-micafungin combination in clinical settings.

Echinocandin pharmacodynamics: review and clinical implications

Echinocandins have made a significant impact in the treatment of select invasive fungal infections, most notably invasive candidiasis and aspergillosis. However, treatment outcomes for such infections are still less than optimal, prompting an examination of dosing and administration techniques in an attempt to exploit known pharmacodynamic properties and improve outcomes. Echinocandins generally exhibit concentration-dependent, fungicidal activity against Candida spp. and fungistatic activity against Aspergillus spp. However, increasing drug concentrations of echinocandins above the organism's MIC may result in a paradoxical increase in fungal growth as demonstrated in some in vitro and in vivo models (known most commonly as the 'Eagle effect'). Therefore, the potential impact of dose escalations on improving the clinical efficacy of echinocandins based on in vitro and animal models are uncertain and are still being evaluated. In addition, such strategies have to consider the potential for increased treatment-related toxicities and costs. To date, published clinical studies (both superiority and non-inferiority) demonstrating the potential for dose-related improvements in treatment outcomes have been limited to mucocutaneous and oesophageal candidiasis. Further research is needed to determine if a role exists for optimizing echinocandin pharmacodynamics in various clinical settings.

Transcriptional regulation of chitin synthases by calcineurin controls paradoxical growth of Aspergillus fumigatus in response to caspofungin

Attenuated activity of echinocandin antifungals at high concentrations, known as the "paradoxical effect," is a well-established phenomenon in Candida albicans and Aspergillus fumigatus. In the yeast C. albicans, upregulation of chitin biosynthesis via the protein kinase C (PKC), high-osmolarity glycerol response (HOG), and Ca(2+)/calcineurin signaling pathways is an important cell wall stress response that permits growth in the presence of high concentrations of echinocandins. However, nothing is known of the molecular mechanisms regulating the mold A. fumigatus and its paradoxical response to echinocandins. Here, we show that the laboratory strain of A. fumigatus and five of seven clinical A. fumigatus isolates tested display various magnitudes of paradoxical growth in response to caspofungin. Interestingly, none of the eight strains showed paradoxical growth in the presence of micafungin or anidulafungin. Treatment of the DeltacnaA and DeltacrzA strains, harboring gene deletions of the calcineurin A subunit and the calcineurin-dependent transcription factor, respectively, with high concentrations of caspofungin revealed that the A. fumigatus paradoxical effect is calcineurin pathway dependent. Exploring a molecular role for CnaA in the compensatory chitin biosynthetic response, we found that caspofungin treatment resulted in increased chitin synthase gene expression, leading to a calcineurin-dependent increase in chitin synthase activity. Taken together, our data suggest a mechanistic role for A. fumigatus calcineurin signaling in the chitin biosynthetic response observed during paradoxical growth in the presence of high-dose caspofungin treatment.

[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.