Glucan synthase inhibitors as antifungal agents

Antifungal effects of echinocandins diminish when exposed to intestinal lumen contents: a finding with potentially significant clinical implications

Echinocandins, a prominent class of antifungals, are known for their broad-spectrum activity and favorable safety profiles. However, their bioavailability and efficacy via oral route are suboptimal. In this study, caspofungin and micafungin, the two most commonly used echinocandins, were evaluated in various in vitro environments simulating intestinal lumen. The results revealed that while both antifungals are effective in standard medium, their efficacy significantly diminishes in the presence of human small bowel aspirates and bovine bile. The study suggests that bowel contents and specifically bile acids may be a suppressive component, hindering the antifungal effects of echinocandins. This novel exploration sheds light on the poor oral bioavailability of echinocandins. The findings imply that echinocandins alone, regardless of administration route, may not be optimal for gastrointestinal (GI) fungal infections or invasive fungal infections originating from intestinal translocation. Further clinical investigations are warranted to validate and expand upon these observations.

An evaluation of Rezafungin: the latest treatment option for adults with candidemia and invasive candidiasis

INTRODUCTION

Invasive fungal infections, especially candidemia and invasive candidiasis, continue to cause substantial morbidity and mortality. In addition, the emergence of drug-resistant Candida species, notably C. glabrata and C. auris, along with limitations in available treatments, highlights the urgent need for novel, effective antifungal agents.

AREAS COVERED

This review discusses the results of in vitro studies evaluating the spectrum and highlights the pharmacokinetic/pharmacodynamic properties. It also includes discussions on two key clinical studies that assess safety, tolerability, and efficacy.

EXPERT OPINION

Rezafungin has demonstrated comparable efficacy to other echinocandins in two clinical studies and exhibits in vitro activity against a broad range of Candida species and Aspergillus spp. It has a favorable safety profile with minimal side effects, and no drug interactions or effects on QT intervals. In contrast to other echinocandins, it demonstrates dose-dependent killing, a prolonged half-life, and low clearance make it suitable for once-weekly dosing, which is supported by clinical trials confirming its efficacy. Rezafungin offers a promising option for the outpatient management of difficult to treat fungal infections. It has become a valuable addition to the antifungal arsenal, with the potential to reduce hospital length of stay and hospitalization costs and combat drug-resistant Candida species.

Candidiasis: From cutaneous to systemic, new perspectives of potential targets and therapeutic strategies

Candidiasis is an infection caused by fungi from a Candida species, most commonly Candida albicans. C. albicans is an opportunistic fungal pathogen typically residing on human skin and mucous membranes of the mouth, intestines or vagina. It can cause a wide variety of mucocutaneous barrier and systemic infections; and becomes a severe health problem in HIV/AIDS patients and in individuals who are immunocompromised following chemotherapy, treatment with immunosuppressive agents or after antibiotic-induced dysbiosis. However, the immune mechanism of host resistance to C. albicans infection is not fully understood, there are a limited number of therapeutic antifungal drugs for candidiasis, and these have disadvantages that limit their clinical application. Therefore, it is urgent to uncover the immune mechanisms of the host protecting against candidiasis and to develop new antifungal strategies. This review synthesizes current knowledge of host immune defense mechanisms from cutaneous candidiasis to invasive C. albicans infection and documents promising insights for treating candidiasis through inhibitors of potential antifungal target proteins.

Ibrexafungerp: A First-in-Class Oral Triterpenoid Glucan Synthase Inhibitor

Ibrexafungerp (formerly SCY-078 or MK-3118) is a first-in-class triterpenoid antifungal or “fungerp” that inhibits biosynthesis of β-(1,3)-D-glucan in the fungal cell wall, a mechanism of action similar to that of echinocandins. Distinguishing characteristics of ibrexafungerp include oral bioavailability, a favourable safety profile, few drug–drug interactions, good tissue penetration, increased activity at low pH and activity against multi-drug resistant isolates including C. auris and C. glabrata. In vitro data has demonstrated broad and potent activity against Candida and Aspergillus species. Importantly, ibrexafungerp also has potent activity against azole-resistant isolates, including biofilm-forming Candida spp., and echinocandin-resistant isolates. It also has activity against the asci form of Pneumocystis spp., and other pathogenic fungi including some non-Candida yeasts and non-Aspergillus moulds. In vivo data have shown IBX to be effective for treatment of candidiasis and aspergillosis. Ibrexafungerp is effective for the treatment of acute vulvovaginal candidiasis in completed phase 3 clinical trials.

Echinocandins: structural diversity, biosynthesis, and development of antimycotics

Abstract

Echinocandins are a clinically important class of non-ribosomal antifungal lipopeptides produced by filamentous fungi. Due to their complex structure, which is characterized by numerous hydroxylated non-proteinogenic amino acids, echinocandin antifungal agents are manufactured semisynthetically. The development of optimized echinocandin structures is therefore closely connected to their biosynthesis. Enormous efforts in industrial research and development including fermentation, classical mutagenesis, isotope labeling, and chemical synthesis eventually led to the development of the active ingredients caspofungin, micafungin, and anidulafungin, which are now used as first-line treatments against invasive mycosis. In the last years, echinocandin biosynthetic gene clusters have been identified, which allowed for the elucidation but also engineering of echinocandin biosynthesis on the molecular level. After a short description of the history of echinocandin research, this review provides an overview of the current knowledge of echinocandin biosynthesis with a special focus of the diverse structural elements, their biosynthetic background, and structure−activity relationships.

Key points

• Complex and highly oxidized lipopeptides produced by fungi.

• Crucial in the design of drugs: side chain, solubility, and hydrolytic stability.

• Genetic methods for engineering biosynthesis have recently become available.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-020-11022-y.

Evolutionary Overview of Molecular Interactions and Enzymatic Activities in the Yeast Cell Walls

Fungal cell walls are composed of a polysaccharide network that serves as a scaffold in which different glycoproteins are embedded. Investigation of fungal cell walls, besides simple identification and characterization of the main cell wall building blocks, covers the pathways and regulations of synthesis of each individual component of the wall and biochemical reactions by which they are cross-linked and remodeled in response to different growth phase and environmental signals. In this review, a survey of composition and organization of so far identified and characterized cell wall components of different yeast genera including Saccharomyces, Candida, Kluyveromyces, Yarrowia, and Schizosaccharomyces are presented with the focus on their cell wall proteomes.

Rezafungin-Mechanisms of Action, Susceptibility and Resistance: Similarities and Differences with the Other Echinocandins

Rezafungin (formerly CD101) is a new β-glucan synthase inhibitor that is chemically related with anidulafungin. It is considered the first molecule of the new generation of long-acting echinocandins. It has several advantages over the already approved by the Food and Drug Administration (FDA) echinocandins as it has better tissue penetration, better pharmacokinetic/phamacodynamic (PK/PD) pharmacometrics, and a good safety profile. It is much more stable in solution than the older echinocandins, making it more flexible in terms of dosing, storage, and manufacturing. These properties would allow rezafungin to be administered once-weekly (intravenous) and to be potentially administered topically and subcutaneously. In addition, higher dose regimens were tested with no evidence of toxic effect. This will eventually prevent (or reduce) the selection of resistant strains. Rezafungin also has several similarities with older echinocandins as they share the same in vitro behavior (very similar Minimum Inhibitory Concentration required to inhibit the growth of 50% of the isolates (MIC50) and half enzyme maximal inhibitory concentration 50% (IC50)) and spectrum, the same target, and the same mechanisms of resistance. The selection of FKS mutants occurred at similar frequency for rezafungin than for anidulafungin and caspofungin. In this review, rezafungin mechanism of action, target, mechanism of resistance, and in vitro data are described in a comparative manner with the already approved echinocandins.

Moniliophthora perniciosa development: key genes involved in stress-mediated cell wall organization and autophagy

Moniliophthora perniciosa is a basidiomycete responsible for the witches' broom disease in cacao (Theobroma cacao L.). Chitin synthase (CHS), chitinase (CHIT) and autophagy (ATG) genes have been associated to stress response preceding the formation of basidiocarp. An analysis of literature mining, interactomics and gene expression was developed to identify the main proteins related to development, cell wall organization and autophagy in M. perniciosa. TORC2 complex elements were identified and were involved in the response to the nutrient starvation during the fungus development stages preceding the basidiocarp formation. This complex interacted with target proteins related to cell wall synthesis and to polarization and cell division (FKS1, CHS, CDC42, ROM2). Autolysis and autophagy processes were associated to CHIT2, ATG8 and to the TORC1 complex (TOR1 and KOG1), which is central in the upstream signalization of the stress response due to nutrient starvation and growth regulation. Other important elements that participate to steps preceding basidiocarp formation were also identified (KOG1, SSZ1, GDI1, FKS1, CCD10, CKS1, CDC42, RHO1, AVO1, BAG7). Similar gene expression patterns during fungus reproductive structure formation and when treated by rapamycin (a nutritional related-autophagy stress agent) were observed: cell division related-genes were repressed while those related to autolysis/autophagy were overexpressed.

Echinocandins: The Expanding Antifungal Armamentarium

The echinocandins are large lipopeptide molecules that, since their discovery approximately 41 years ago, have emerged as important additions to the expanding armamentarium against invasive fungal diseases. Echinocandins exert in vitro and in vivo fungicidal action against most Candida species and fungistatic action against Aspergillus species. However, the population of patients at risk for developing invasive fungal infections continues to increase. New therapeutic strategies using echinocandins are needed to improve clinical outcomes in patients with invasive fungal disease.

In silico screening for identification of novel β-1,3-glucan synthase inhibitors using pharmacophore and 3D-QSAR methodologies

The enzyme β-1,3-glucan synthase, which catalyzes the synthesis of β-1,3-glucan, an essential and unique structural component of the fungal cell wall, has been considered as a promising target for the development of less toxic anti-fungal agents. In this study, a robust pharmacophore model was developed and structure activity relationship analysis of 42 pyridazinone derivatives as β-1,3-glucan synthase inhibitors were carried out. A five-point pharmacophore model, consisting of two aromatic rings (R) and three hydrogen bond acceptors (A) was generated. Pharmacophore based 3D-QSAR model was developed for the same reported data sets. The generated 3D-QSAR model yielded a significant correlation coefficient value (R² = 0.954) along with good predictive power confirmed by the high value of cross-validated correlation coefficient (Q² = 0.827). Further, the pharmacophore model was employed as a 3D search query to screen small molecules database retrieved from ZINC to select new scaffolds. Finally, ADME studies revealed the pharmacokinetic efficiency of these compounds.

Proteins involved in building, maintaining and remodeling of yeast cell walls

The cell wall defines the shape and provides osmotic stability to the yeast cell. It also serves to anchor proteins required for communication of the yeast cell with surrounding molecules and other cells. It is synthesized as a complex structure with β-1,3-glucan chains forming the basic network to which β-1,6-glucan, chitin and a number of mannoproteins are attached. Synthesis, maintaining and remodeling of this complex structure require a set of different synthases, hydrolases and transglycosidases whose concerted activities provide necessary firmness but at the same time flexibility of the wall moiety. The present state of comprehension of the interplay of these proteins in the yeast cell wall is the subject of this article.

Discovery and development of first in class antifungal caspofungin (CANCIDAS®)--a case study

Covering: 1985 to 2001.This paper describes a fifteen year journey from concept to clinical discovery and development of the first in class caspofungin acetate (CANCIDAS®) a parenteral antifungal agent. Caspofungin is a semisynthetic derivative of pneumocandin B0, a naturally occurring, lipophilic cyclic peptide isolated from the fungus, Glarea lozoyensis. While the echinocandins had been previously studied for antifungal activity by several organizations, the class was dropped for a variety of reasons. Merck subsequently initiated a research program leading to the discovery and development of caspofungin. The multitude of challenges that ensued during the discovery and development process and which were successfully resolved by multi-disciplinary teams constitute the content of this article. The article consists of five sections that describe the discovery and development of caspofungin in chronological order: (i) discovery of the natural product pneumocandin B0 from fungal fermentations, (ii) fermentation development to improve the titer of pneumocandin B0 to make it commercially viable, (iii) semisynthetic modification by medicinal chemistry to successfully improve the properties of pneumocandin B0 leading to the discovery of caspofungin, (iv) development of commercial semisynthesis and purification and formulation development to improve stability and (v) clinical development and approval of CANCIDAS® as an antifungal drug which subsequently saved thousands of lives.

Design, Synthesis, and Molecular Docking of 1-(1-(4-Chlorophenyl)-2-(phenylsulfonyl)ethylidene)-2-phenylhydrazine as Potent Nonazole Anticandidal Agent

1-(1-(4-Chlorophenyl)-2-(phenylsulfonyl)ethylidene)-2-phenylhydrazine (13) was designed and synthesized as potential nonazole anticandidal agent and precisely characterized by IR,1H NMR,13C NMR, and ESI-MS. The anti-Candidaactivity of13was evaluated against fourCandidaspecies (C. albicans, C. krusei, C. parapsilosis, andC. glabrata). Compound13displayed good anticandidal activities (MIC=0.39, 0.195, 0.39, and 1.56 μmol/mL, resp.) in comparison with that of the standard drug fluconazole (MIC=0.195, inactive, 1.56, and 1.56 μmol/mL, resp.) againstC. albicans, C. krusei, C. parapsilosis, andC. glabrata, respectively. A molecular modeling of the newly synthesized compound13was built in order to investigate its mode of action towards the prospective target cytochrome P450-dependent enzyme lanosterol 14α-demethylase (PDB-code: 1EA1). The docking results showed a similar binding interaction of13and fluconazole at the active site of CYT P450 14α-sterol demethylase. Furthermore, compound13showed no cytotoxicity against normal human breast cell line MCF10A.

Natural products: a continuing source of novel drug leads

BACKGROUND

Nature has been a source of medicinal products for millennia, with many useful drugs developed from plant sources. Following discovery of the penicillins, drug discovery from microbial sources occurred and diving techniques in the 1970s opened the seas. Combinatorial chemistry (late 1980s), shifted the focus of drug discovery efforts from Nature to the laboratory bench.

SCOPE OF REVIEW

This review traces natural products drug discovery, outlining important drugs from natural sources that revolutionized treatment of serious diseases. It is clear Nature will continue to be a major source of new structural leads, and effective drug development depends on multidisciplinary collaborations.

MAJOR CONCLUSIONS

The explosion of genetic information led not only to novel screens, but the genetic techniques permitted the implementation of combinatorial biosynthetic technology and genome mining. The knowledge gained has allowed unknown molecules to be identified. These novel bioactive structures can be optimized by using combinatorial chemistry generating new drug candidates for many diseases.

GENERAL SIGNIFICANCE

The advent of genetic techniques that permitted the isolation / expression of biosynthetic cassettes from microbes may well be the new frontier for natural products lead discovery. It is now apparent that biodiversity may be much greater in those organisms. The numbers of potential species involved in the microbial world are many orders of magnitude greater than those of plants and multi-celled animals. Coupling these numbers to the number of currently unexpressed biosynthetic clusters now identified (>10 per species) the potential of microbial diversity remains essentially untapped.

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.

New Fks hot spot for acquired echinocandin resistance in Saccharomyces cerevisiae and its contribution to intrinsic resistance of Scedosporium species

Echinocandins represent a new antifungal group with potent activity against Candida species. These lipopeptides inhibit the synthesis of β-1,3-glucan, the major cell wall polysaccharide. Acquired resistance or reduced echinocandin susceptibility (RES) is rare and associated with mutations in two "hot spot" regions of Fks1 or Fks2, the probable β-1,3-glucan synthases. In contrast, many fungi demonstrate intrinsic RES for reasons that remain unclear. We are using Saccharomyces cerevisiae to understand the basis for RES by modeling echinocandin-Fks interaction. Previously characterized mutations confer cross-RES; we screened for mutations conferring differential RES, implying direct interaction of that Fks residue with a variable echinocandin side chain. One mutant (in an fks1Δ background) exhibited ≥16-fold micafungin and anidulafungin versus caspofungin RES. Sequencing identified a novel Fks2 mutation, W714L/Y715N. Equivalent W695L/Y696N and related W695L/F/C mutations in Fks1 generated by site-directed mutagenesis and the isolation of a W695L-equivalent mutation in Candida glabrata confirmed the role of the new "hot spot 3" in RES. Further mutagenesis expanded hot spot 3 to Fks1 residues 690 to 700, yielding phenotypes ranging from cross-RES to differential hypersusceptibility. Fks1 sequences from intrinsically RES Scedosporium species revealed W695F-equivalent substitutions; Fks1 hybrids expressing Scedosporium prolificans hot spot 3 confirmed that this substitution imparts RES.

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

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

The echinocandins: comparison of their pharmacokinetics, pharmacodynamics and clinical applications

Caspofungin, micafungin and anidulafungin are three drugs of the echinocandin class of antifungals available for intravenous treatment of invasive candidiasis and aspergillosis. They exhibit high in vitro and in vivo activities against Candida spp. and Aspergillus spp. In various clinical studies investigating candidemia and invasive candidiasis, Candida esophagitis, and fever in neutropenia, the clinical efficacy of the echinocandin tested was similar to that of established antifungals. Antifungal activity against strains no longer susceptible to conventional antifungal agents, such as fluconazole and amphotericin B suggests that echinocandins can be used as salvage therapy in life-threatening fungal infections. There is no cross-resistance to other antifungals. Excellent safety and tolerability of treatment with caspofungin has been documented over a total of 4.3 million patient days. Echinocandins are poor substrates of the cytochrome P450 enzyme family and can be safely co-administered with most drugs without the need for dosage adaptation. No dose reduction is required in renal impairment. A reduction in the daily maintenance dose has been recommended for caspofungin, but not for micafungin and anidulafungin in patients presenting with mild to moderate hepatic failure.

Candida albicans and Candida glabrata clinical isolates exhibiting reduced echinocandin susceptibility

A recognized hotspot for mutations conferring reduced echinocandin susceptibility (RES) is residue S645 of Candida albicans Gsc1(Fks1). We report that the mutation F641Y is associated with RES in a C. albicans isolate. The analogous Fks2 residue is mutated F to V in a Candida glabrata RES isolate; the introduction of this mutation into susceptible C. glabrata confirmed its role in RES. Y641-equivalent Fks residues were identified in intrinsically RES Fusarium species and Candida guilliermondii.

Caspofungin: a review of its use in the treatment of fungal infections

Caspofungin (Cancidas) is the first of a new class of antifungal agents, the echinocandins, that inhibit the synthesis of the fungal cell wall component beta-(1,3)-D-glucan. Caspofungin is administered once daily by slow intravenous infusion and is used to treat infections caused by Candida spp. and Aspergillus spp. Caspofungin is a valuable new antifungal agent with a novel mechanism of action. In comparative clinical trials, caspofungin was no less effective than liposomal amphotericin B in the empirical treatment of neutropenic patients with persistent fever, amphotericin B deoxycholate in the treatment of invasive candidiasis or fluconazole in the treatment of oesophageal candidiasis. Caspofungin also displayed broadly similar efficacy to amphotericin B deoxycholate in oesophageal or oropharyngeal candidiasis and was effective as salvage therapy in patients with invasive aspergillosis who were refractory to or intolerant of standard therapy. The tolerability profile of caspofungin was similar to that of fluconazole and superior to that of amphotericin B deoxycholate and liposomal amphotericin B. Therefore, in the appropriate indications, caspofungin is a viable alternative to amphotericin B deoxycholate, liposomal amphotericin B or fluconazole.

The Smk1p MAP kinase negatively regulates Gsc2p, a 1,3-beta-glucan synthase, during spore wall morphogenesis in Saccharomyces cerevisiae

Spore formation in Saccharomyces cerevisiae involves the sequential deposition of multiple spore wall layers between the prospore membranes that surround each meiotic product. The Smk1p mitogen-activated protein (MAP) kinase plays a critical role in spore formation, but the proteins that interact with Smk1p to regulate spore morphogenesis have not been described. Using mass spectrometry, we identify Gsc2p as a Smk1p-associated protein. Gsc2p is a 1,3-beta-glucan synthase subunit involved in synthesizing an inner spore wall layer. We find that 1,3-beta-glucan synthase activity is elevated in smk1 mutants, suggesting that SMK1 negatively regulates GSC2. Although deposition of the two inner spore wall layers is normal in smk1 mutants, deposition of the outer layers is aberrant. However, eliminating GSC2 activity restores normal deposition of the third spore wall layer in smk1 mutants, indicating that negative regulation of GSC2 by SMK1 is important for spore wall deposition. Our findings suggest a model for the coordination of spore wall layer deposition in which Smk1p facilitates the transition between early and late phases of spore wall deposition by inhibiting a spore wall-synthesizing enzyme important for early phases of spore wall deposition.

Utility of mould susceptibility testing

PURPOSE OF REVIEW

As new antifungal agents were introduced for the treatment of fungal infections, reliable methods were developed or adapted for the in-vitro susceptibility testing of yeasts and moulds (filamentous fungi). This paper reviews the available methods for antifungal susceptibility testing of moulds as well as the scant data that have been published since 2002 regarding the clinical implications of in-vitro testing.

RECENT FINDINGS

Caspofungin and voriconazole have been recently licensed for treatment of certain mould infections that are usually refractory to treatment with established agents. The introduction of new agents and the frequent reports of mould resistance have underscored the role of the laboratory in patient management and initiated the study of mechanisms of resistance in moulds. In 2002, the US National Committee for Clinical Laboratory Standards M38-P document moved to the approved level of development (M38-A document). Although combination therapy and synergistic studies with the new agents have been documented, very little information is available on in-vitro/in-vivo correlations for moulds.

SUMMARY

The ultimate goal of in-vitro testing is the prediction of the clinical outcome of therapy. The use of National Committee for Clinical Laboratory Standards procedures has led to increased interlaboratory agreement of minimum inhibitory concentrations. Reproducibility of minimum inhibitory concentrations for yeasts has facilitated the establishment of interpretive breakpoints for fluconazole and itraconazole versus Candida spp., but breakpoints are not available for any antifungal against mould species. Although some insights have been documented regarding the potential utility of in-vitro testing for moulds, further documentation of in-vitro versus in-vivo data is needed.

Cell wall alpha-1,3-glucan is required to anchor the Cryptococcus neoformans capsule

Cryptococcus neoformans is an opportunistic pathogen responsible for serious disease in humans. Critical for virulence of this fungus is an elaborate polysaccharide capsule, which impedes the host immune response. We found that association of the capsule with the cell requires a specific component of the cell wall, alpha-1,3-glucan. Post-transcriptional inhibition of alpha-1,3-glucan synthase expression, using double-stranded RNA interference, yields cells that are unable to assemble a capsule although they generate its polysaccharide components. The resulting cryptococci are slow-growing and acapsular. This finding demonstrates a novel mode of polysaccharide attachment and an important application of RNA interference in fungi. The elimination of the capsule by reducing the expression of a single gene suggests a potential avenue for antifungal chemotherapy.

Antifungal susceptibility survey of 2,000 bloodstream Candida isolates in the United States

Candida bloodstream isolates (n = 2,000) from two multicenter clinical trials carried out by the National Institute of Allergy and Infectious Diseases Mycoses Study Group between 1995 and 1999 were tested against amphotericin B (AMB), flucytosine (5FC), fluconazole (FLU), itraconazole (ITR), voriconazole (VOR), posaconazole (POS), caspofungin (CFG), micafungin (MFG), and anidulafungin (AFG) using the NCCLS M27-A2 microdilution method. All drugs were tested in the NCCLS-specified RPMI 1640 medium except for AMB, which was tested in antibiotic medium 3. A sample of isolates was also tested in RPMI 1640 supplemented to 2% glucose and by using the diluent polyethylene glycol (PEG) in lieu of dimethyl sulfoxide for those drugs insoluble in water. Glucose supplementation tended to elevate the MIC, whereas using PEG tended to decrease the MIC. Trailing growth occurred frequently with azoles. Isolates were generally susceptible to AMB, 5FC, and FLU. Rates of resistance to ITR approached 20%. Although no established interpretative breakpoints are available for the candins (CFG, MFG, and AFG) and the new azoles (VOR and POS), they all exhibited excellent antifungal activity, even for those strains resistant to the other aforementioned agents.

Echinocandin antifungal drugs

The echinocandins are large lipopeptide molecules that are inhibitors of beta-(1,3)-glucan synthesis, an action that damages fungal cell walls. In vitro and in vivo, the echinocandins are rapidly fungicidal against most Candida spp and fungistatic against Aspergillus spp. They are not active at clinically relevant concentrations against Zygomycetes, Cryptococcus neoformans, or Fusarium spp. No drug target is present in mammalian cells. The first of the class to be licensed was caspofungin, for refractory invasive aspergillosis (about 40% response rate) and the second was micafungin. Adverse events are generally mild, including (for caspofungin) local phlebitis, fever, abnormal liver function tests, and mild haemolysis. Poor absorption after oral administration limits use to the intravenous route. Dosing is once daily and drug interactions are few. The echinocandins are widely distributed in the body, and are metabolised by the liver. Results of studies of caspofungin in candidaemia and invasive candidiasis suggest equivalent efficacy to amphotericin B, with substantially fewer toxic effects. Absence of antagonism in combination with other antifungal drugs suggests that combination antifungal therapy could become a general feature of the echinocandins, particularly for invasive aspergillosis.

Invasive oesophageal candidiasis: current and developing treatment options

Oesophageal candidiasis is frequently one of the first signs of HIV infection, and a marker of HIV disease. Approximately 10% of patients with AIDS or other immunodeficiency, whether due to an underlying disease, chemotherapy or radiation therapy, will experience oesophageal candidiasis during their lifetime. In addition, unless the underlying immunodeficiency is corrected, approximately 60% of patients will experience a relapse within 6 months of the initial infection. The systemic azoles have gradually replaced the use of amphotericin B for oesophageal candidiasis, and are generally safely used and effective agents for this infection. A concern in some of these patients is the appearance of antifungal-refractory oesophageal candidiasis, which frequently leads to a vicious cycle of poor oral intake, weight loss, malnutrition and wasting syndrome, with occasional mortality due to malnutrition. Newer antifungals such as voriconazole and caspofungin, which are more potent in vitro and have a broader spectrum of activity, including activity against fluconazole-resistant Candida species are a welcome addition to the antifungal armamentarium that may be used in the management of refractory mucosal candidiasis.

Caspofungin

Caspofungin, the first inhibitor of fungal beta-1,3 glucan synthesis to receive approval by the United States Food and Drug Administration, is effective for the treatment of mucosal and invasive candidiasis and invasive aspergillosis. It is also active in vitro and in animal models against a number of other filamentous and dimorphic endemic fungi and in animal models of Pneumocystis carinii infection. In vitro studies and some animal studies almost always indicate an absence of antagonism when caspofungin is combined with azole or polyene antifungal agents. Caspofungin has an excellent safety profile. Caspofungin may prove to be useful in empirical therapy for suspected invasive fungal infections. Additional clinical trial data that expand our knowledge of the usefulness of caspofungin for these and other mycoses, including its administration in combination with other antifungal agents, is anticipated. Caspofungin is an important addition to the antifungal pharmacopoeia.

Caspofungin

Echinocandins are a new class of antifungal agents with a novel mechanism of action (interference with fungal cell wall synthesis). Caspofungin (Cancidas), Caspofungin MSD) is the first echinocandin to be approved and is administered intravenously. Caspofungin 50 mg/day had similar efficacy to intravenous fluconazole 200 mg/day and was at least as effective as intravenous amphotericin B 0.5 mg/kg/day in patients with oesophageal candidiasis in two randomised, double-blind studies. A favourable combined clinical and endoscopic response occurred in 81% of caspofungin recipients versus 85% of fluconazole recipients and in 74% of caspofungin recipients versus 63% of amphotericin B recipients. A favourable combined response rate of approximate, equals 90% and approximate, equals 60% occurred in the stratum of patients with oesophageal candidiasis who received caspofungin or amphotericin B in a third randomised, double-blind study. Caspofungin (70 mg loading dose followed by 50 mg/day) had similar efficacy to intravenous amphotericin B (0.7-1.0 mg/kg/day in patients with neutropenia and 0.6-0.7 mg/kg/day in patients without neutropenia) in patients with invasive candidiasis in a double-blind, randomised trial. A favourable overall response occurred in 73.4% of caspofungin recipients and in 61.7% of amphotericin B recipients. In a noncomparative study, salvage therapy with caspofungin (70 mg loading dose followed by 50 mg/day) was effective in patients with invasive aspergillosis who were refractory to or did not tolerate standard antifungal therapy. A favourable response (complete plus partial response) occurred in 37 of 83 patients (45%). Caspofungin was generally well tolerated in clinical trials; it had similar tolerability to intravenous fluconazole and was better tolerated than intravenous amphotericin B. Significantly fewer caspofungin than amphotericin B recipients reported chills, fever, nausea or infusion-related adverse events. In conclusion, caspofungin is a valuable new antifungal agent with a novel mechanism of action. In comparative trials, caspofungin had similar efficacy to fluconazole and was at least as effective as amphotericin B in oesophageal candidiasis and had similar efficacy to amphotericin B in invasive candidiasis. In addition, caspofungin had similar tolerability to fluconazole and was better tolerated than amphotericin B in these indications. Caspofungin was also effective in patients with invasive aspergillosis who were refractory to or intolerant of standard antifungal agents. Thus, caspofungin provides an alternative to triazoles or amphotericin B in oesophageal candidiasis and an alternative to amphotericin B in invasive candidiasis, as well as being an effective salvage therapy in invasive aspergillosis.