The antifungal pipeline: a reality check

Harnessing calcineurin-FK506-FKBP12 crystal structures from invasive fungal pathogens to develop antifungal agents

Calcineurin is important for fungal virulence and a potential antifungal target, but compounds targeting calcineurin, such as FK506, are immunosuppressive. Here we report the crystal structures of calcineurin catalytic (CnA) and regulatory (CnB) subunits complexed with FK506 and the FK506-binding protein (FKBP12) from human fungal pathogens (Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans and Coccidioides immitis). Fungal calcineurin complexes are similar to the mammalian complex, but comparison of fungal and human FKBP12 (hFKBP12) reveals conformational differences in the 40s and 80s loops. NMR analysis, molecular dynamic simulations, and mutations of the A. fumigatus CnA/CnB-FK506-FKBP12-complex identify a Phe88 residue, not conserved in hFKBP12, as critical for binding and inhibition of fungal calcineurin. These differences enable us to develop a less immunosuppressive FK506 analog, APX879, with an acetohydrazine substitution of the C22-carbonyl of FK506. APX879 exhibits reduced immunosuppressive activity and retains broad-spectrum antifungal activity and efficacy in a murine model of invasive fungal infection.

Global Transcriptomic Analysis of the Candida albicans Response to Treatment with a Novel Inhibitor of Filamentation

These results from whole-genome transcriptional profiling provide further insights into the biological activity and mode of action of a small-molecule inhibitor of C. albicans filamentation. This information will assist in the development of novel antivirulence strategies against C. albicans infections.

The opportunistic pathogenic fungus Candida albicans can cause devastating infections in immunocompromised patients. Its ability to undergo a morphogenetic transition from yeast to filamentous forms allows it to penetrate tissues and damage tissues, and the expression of genes associated with a number of pathogenetic mechanisms is also coordinately regulated with the yeast-to-hypha conversion. Therefore, it is widely considered that filamentation represents one of the main virulence factors of C. albicans. We have previously identified N-[3-(allyloxy)-phenyl]-4-methoxybenzamide (compound 9029936) as the lead compound in a series of small-molecule inhibitors of C. albicans filamentation and characterized its activity both in vitro and in vivo. This compound appears to be a promising candidate for the development of alternative antivirulence strategies for the treatment of C. albicans infections. In this study, we performed RNA sequencing analysis of samples obtained from C. albicans cells grown under filament-inducing conditions in the presence or absence of this compound. Overall, treatment with compound 9029936 resulted in 618 upregulated and 702 downregulated genes. Not surprisingly, some of the most downregulated genes included well-characterized genes associated with filamentation and virulence such as SAP5, ECE1 (candidalysin), and ALS3, as well as genes that impact metal chelation and utilization. Gene ontology analysis revealed an overrepresentation of cell adhesion, iron transport, filamentation, biofilm formation, and pathogenesis processes among the genes downregulated during treatment with this leading compound. Interestingly, the top upregulated genes suggested an enhancement of vesicular transport pathways, particularly those involving SNARE interactions.

IMPORTANCE These results from whole-genome transcriptional profiling provide further insights into the biological activity and mode of action of a small-molecule inhibitor of C. albicans filamentation. This information will assist in the development of novel antivirulence strategies against C. albicans infections.

Exploring the resistance mechanisms in Trichosporon asahii: Triazoles as the last defense for invasive trichosporonosis

Trichosporon asahii has recently been recognized as an emergent fungal pathogen able to cause invasive infections in neutropenic cancer patients as well as in critically ill patients submitted to invasive medical procedures and broad-spectrum antibiotic therapy. T. asahii is the main pathogen associated with invasive trichosporonosis worldwide. Treatment of patients with invasive trichosporonosis remains a controversial issue, but triazoles are mentioned by most authors as the best first-line antifungal therapy. There is mounting evidence supporting the claim that fluconazole (FLC) resistance in T. asahii is emerging worldwide. Since 2000, 15 publications involving large collections of T. asahii isolates described non-wild type isolates for FLC and/or voriconazole. However, very few papers have addressed the epidemiology and molecular mechanism of antifungal resistance in Trichosporon spp. Data available suggest that continuous exposure to azoles can induce mutations in the ERG11 gene, resulting in resistance to this class of antifungal drugs. A recent report characterizing T. asahii azole-resistant strains found several genes differentially expressed and highly mutated, including genes related to the Target of Rapamycin (TOR) pathway, indicating that evolutionary modifications on this pathway induced by FLC stress may be involved in developing azole resistance. Finally, we provided new data suggesting that hyperactive efflux pumps may play a role as drug transporters in FLC resistant T. asahii strains.

Pulmonary cryptococcosis: A review of pathobiology and clinical aspects

Pulmonary cryptococcosis is an important opportunistic invasive mycosis in immunocompromised patients, but it is also increasingly seen in immunocompetent patients. The main human pathogens are Cryptococcus neoformans and C. gattii, which have a worldwide distribution. In contrast to cryptococcal meningitis, pulmonary cryptococcosis is still underdiagnosed because of limitations in diagnostic tools. It can mimic lung cancer, pulmonary tuberculosis, bacterial pneumonia, and other pulmonary mycoses both clinically and radiologically. Pulmonary nodules are the most common radiological feature, but these are not specific to pulmonary cryptococcosis. The sensitivity of culture of respiratory samples for Cryptococcus is poor and a positive result may also reflect colonisation. Cryptococcal antigen (CrAg) with lateral flow device is a fast and sensitive test and widely used on serum and cerebrospinal fluid, but sera from patients with pulmonary cryptococcosis are rarely positive in the absence of disseminated disease. Detection of CrAg from respiratory specimens might assist the diagnosis of pulmonary cryptococcosis but there are very few data. Molecular detection techniques such as multiplex reverse transcription polymerase chain reaction (RT-PCR) could also provide better sensitivity but these still require validation for respiratory specimens. The first line of treatment for pulmonary cryptococcosis is fluconazole, or amphotericin B and flucytosine for those with central nervous system involvement. Pulmonary cryptococcosis worsens the prognosis of cryptococcal meningitis. In this review, we summarize the biological aspects of Cryptococcus and provide an update on the diagnosis and management of pulmonary cryptococcosis.

Role of lipid transporters in fungal physiology and pathogenicity

The fungal cell wall and membrane are the most common targets of antifungal agents, but the potential of membrane lipid organization in regulating drug-target interactions has yet to be investigated. Energy-dependent lipid transporters have been recently associated with virulence and drug resistance in many pathogenic fungi. To illustrate this view, we discuss (i) the structural and biological aspects of ATP-driven lipid transporters, comprising P-type ATPases and ATP-binding cassette transporters, (ii) the role of these transporters in fungal physiology and virulence, and (iii) the potential of lipid transporters as targets for the development of novel antifungals. These recent observations indicate that the lipid-trafficking machinery in fungi is a promising target for studies on physiology, pathogenesis and drug development.

Biomimetic Nanopillared Surfaces Inhibit Drug Resistant Filamentous Fungal Growth

Filamentous fungi are invasive and multidrug resistant pathogens that commonly contaminate biomedical devices and implants. Once spherical fungal spores attach to a surface, they exhibit germ tube development, hyphal growth, and robust biofilm formation. Nanotopography found on plants, reptiles, and insect wings possess bactericidal properties during prokaryotic cell adhesion. Here, we demonstrate the application of biomimetic nanopillars that inhibit eukaryotic filamentous fungal growth and possess fungicidal properties. Furthermore, many spores on the nanopillars appeared deflated, while those on the flat surfaces remained spherical and intact. These antifungal phenomena provide promising applications in antifouling biointerfaces for biomedical devices and implants.

A new acridone with antifungal properties against Candida spp. and dermatophytes, and antibiofilm activity against C. albicans

AIM

The increase in the number of fungal infections worldwide, coupled with the limitations of current antifungal chemotherapy, demand the development of safe and effective new antifungals. Here, we presented the synthesis of a novel acridone (M14) and its antifungal properties against Candida and dermatophytes species.

METHODS AND RESULTS

A series of 17 acridones was designed, synthesized and tested for its antifungal activity. The minimum inhibitory concentration (MIC) was determined by the broth microdilution method. Only the acridone M14 showed growth-inhibitory activity against reference strains and clinical isolates of Candida and dermatophytes, with MIC range of 7·81-31·25 µg ml^-1 . Moreover, M14 exhibited fungicidal activity and prevented biofilm formation by C. albicans as well as reduced the viability of preformed biofilms, even at sub-MICs. The confocal laser scanning microscopy analysis revealed that C. albicans hyphal growth was completely inhibited in the presence of M14. Similarly, there was a severe inhibition on hyphal growth of Trichophyton rubrum. We also found that M14 has relatively low toxicity to human fibroblasts.

CONCLUSIONS

The new acridone M14 has antifungal properties against Candida spp. and dermatophytes, and antibiofilm activity against C. albicans. In addition, M14 is relatively selective to fungal cells compared to human normal cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

Because of its in vitro antifungal activity, anti-Candida biofilm effect and moderate cytotoxicity towards normal human cell, M14 may serve as a valuable lead compound to develop a new antifungal agent.

Discovery of Simplified Sampangine Derivatives with Potent Antifungal Activities against Cryptococcal Meningitis

Cryptococcal meningitis (CM) is associated with high morbidity and mortality. Current antifungal drug therapy for CM has the following challenges: limited efficacy, significant side effects, emerging drug resistance, and unavailability in highly needed countries. There is an urgent need to develop novel CM therapeutic agents with a new mode of action. On the basis of the antifungal natural product sampangine, herein, novel simplified isoxazole derivatives were identified to possess excellent inhibitory activity against Cryptococcus neoformans (C. neoformans). Particularly, compound 9a was highly active (the minimum inhibitory concentration of 80% inhibition, MIC₈₀ = 0.031 μg/mL) and significantly inhibited biofilm formation, melanin, and urease production of C. neoformans. 9a had good blood-brain barrier (BBB) permeability and effectively reduced the brain fungal burden in a murine model of cryptococcosis. The antifungal mechanism of compound 9a was preliminarily investigated by transmission electron microscopy and flow cytometry. It was able to cause necrocytosis of C. neoformans cells and cell cycle arrest in the G1/S phase. Isoxazole compound 9a represents a promising lead compound for the development of novel CM therapeutic agents.

Resisting resistance: gearing up for war

Where do we stand in our fight against antimicrobial resistance (AMR)? Many antimicrobials may lose their clinical efficacy, particularly due to the rise of multidrug-resistant (MDR) and extended drug-resistant (XDR) pathogens, including bacteria, fungi, and parasites. We need weapons against them all. Society must come together against these pathogens, just like we did against HIV, cancer, and heart disease. This opinion piece is, first and foremost, a call to arms for all partners involved in the war against AMR. Even more so, it is an attempt to highlight the positives in a seemingly long line of failures, and to identify the current set of challenges we must work on. So, how do we win the war against AMR? We must learn from the past, so we can act in the present, to save the future.

Combined Antifungal Resistance and Biofilm Tolerance: the Global Threat of Candida auris

The enigmatic yeast Candida auris has emerged over the last decade and rapidly penetrated our consciousness. The global threat from this multidrug-resistant yeast has generated a call to arms from within the medical mycology community. Over the past decade, our understanding of how this yeast has spread globally, its clinical importance, and how it tolerates and resists antifungal agents has expanded. This review highlights the clinical importance of antifungal resistance in C. auris and explores our current understanding of the mechanisms associated with azole, polyene, and echinocandin resistance. We also discuss the impact of phenotypic tolerance, with particular emphasis on biofilm-mediated resistance, and present new pipelines of antifungal drugs that promise new hope in the management of C. auris infection.

Facilitators of adaptation and antifungal resistance mechanisms in clinically relevant fungi

Opportunistic fungal pathogens can cause a diverse range of diseases in humans. The increasing rate of fungal infections caused by strains that are resistant to commonly used antifungals results in difficulty to treat diseases, with accompanying high mortality rates. Existing and newly emerging molecular resistance mechanisms rapidly spread in fungal populations and need to be monitored. Fungi exhibit a diversity of mechanisms to maintain physiological resilience and create genetic variation; processes which eventually lead to the selection and spread of resistant fungal pathogens. To prevent and anticipate this dispersion, the role of evolutionary factors that drive fungal adaptation should be investigated. In this review, we provide an overview of resistance mechanisms against commonly used antifungal compounds in the clinic and for which fungal resistance has been reported. Furthermore, we aim to summarize and elucidate potent generators of genetic variability across the fungal kingdom that aid adaptation to stressful environments. This knowledge can lead to recognizing potential niches that facilitate fast resistance development and can provide leads for new management strategies to battle the emerging resistant populations in the clinic and the environment.

Broad-spectrum antifungal activity of spirooxindolo-pyrrolidine tethered indole/imidazole hybrid heterocycles against fungal pathogens

Invasive fungal infections are one of the leading causes of nosocomial bloodstream infections with a limited treatment option. A series of derivatized spirooxindolo-pyrrolidine tethered indole and imidazole heterocyclic hybrids have been synthesized, and their antifungal activity against fungal strains were determined. Here we characterize the antifungal activity of a specific spirooxindolo-pyrrolidine hybrid, dubbed compound 9c, a spirooxindolo-pyrrolidine tethered imidazole synthesized with a 2-chloro and trifluoromethoxy substituent. The compound 9c exhibited no cytotoxicity against mammalian cell line at concentrations that inhibited fungal strains. Compound 9c also significantly inhibited the fungal hyphae and biofilm formation. Our results indicate that spirooxindolo-pyrrolidine heterocyclic hybrids potentially represent a broad class of chemical agents with promising antifungal potential.

Insights into the development of chemical probes for RNA

Over the past decade, the RNA revolution has revealed thousands of non-coding RNAs that are essential for cellular regulation and are misregulated in disease. While the development of methods and tools to study these RNAs has been challenging, the power and promise of small molecule chemical probes is increasingly recognized. To harness existing knowledge, we compiled a list of 116 ligands with reported activity against RNA targets in biological systems (R-BIND). In this survey, we examine the RNA targets, design and discovery strategies, and chemical probe characterization techniques of these ligands. We discuss the applicability of current tools to identify and evaluate RNA-targeted chemical probes, suggest criteria to assess the quality of RNA chemical probes and targets, and propose areas where new tools are particularly needed. We anticipate that this knowledge will expedite the discovery of RNA-targeted ligands and the next phase of the RNA revolution.

Mass Spectrometry-Based Proteomics of Fungal Pathogenesis, Host-Fungal Interactions, and Antifungal Development

The prevalence of fungal diseases is increasing on a global scale, ranging from acute to systemic infections caused by commensal or pathogenic microorganisms, often associated with the immune status of the host. Morbidity and mortality rates remain high and our ability to treat fungal infections is challenged by a limited arsenal of antifungal agents and the emergence of drug resistant pathogens. There is a high demand for new approaches to elucidate the fungal mechanisms of pathogenesis and the interplay between host and pathogen to discover novel treatment options. Moreover, the need for improved drug efficacy and reduced host toxicity requires the identification and characterization of antifungal biological targets and molecular mechanisms of action. Mass spectrometry (MS)-based proteomics is a rapidly advancing field capable of addressing these priorities by providing comprehensive information on the dynamics of cellular processes, modifications, and interactions. In this Review, we focus on applications of MS-based proteomics in a diverse array of fungal pathogens and host systems to define and distinguish the molecular details of fungal pathogenesis and host–fungal interactions. We also explore the emerging role of MS-based proteomics in the discovery and development of novel antifungal therapies and provide insight into the future of MS-based proteomics in fungal biology.

Oriented Assembly of Cell-Mimicking Nanoparticles via a Molecular Affinity Strategy for Targeted Drug Delivery

Cell membrane cloaking is an emerging field in drug delivery in which specific functions of parent cells are conferred to newly formed biomimetic vehicles. A growing variety of delivery systems with diverse surface properties have been utilized for this strategy, but it is unclear whether the affinity of membrane-core pairs could guarantee effective and proper camouflaging. In this study, we propose a concise and effective "molecular affinity" strategy using the intracellular domain of transmembrane receptors as "grippers" during membrane coating. Red blood cell (RBC) membranes and cationic liposomes were adopted for fabrication, and a peptide ligand derived from the cytoplasmic protein P4.2 was prepared to specifically recognize the cytoplasmic domain of band 3, a key transmembrane receptor of erythrocytes. Once anchored onto the liposome surface, the P4.2-derived peptide would interact with the isolated RBC membrane, forming a "hidden peptide button", which ensures the right-side-out orientation. The membrane-coated liposomes exhibited an appropriate size distribution around 100 nm and high stability, with superior circulation durations compared with those of conventional PEGylated liposomes. Importantly, they possessed the ability to target Candida albicans by the interaction between the pathogenic fungus and host erythrocytes and to neutralize hemotoxin secreted by the pathogenic fungi. The curative effect of the model drug was thus substantially improved. In summary, the "molecular affinity" strategy may provide a powerful and universal approach for the construction of cell membrane-coated biomaterials and nanomedicines at both laboratory and industrial scales.

The antifungal and Cryptococcus neoformans virulence attenuating activity of Pelargonium sidoides extracts

ETHNOPHARMACOLOGICAL RELEVANCE

Limitations of clinical antifungal treatments and drug-resistance are drivers of the search for novel antifungal strategies. Extracts prepared from the tubers of the medicinal plant, Pelargonium sidoides, are known for their antiviral and antibacterial activities and are used in ethnomedicine for the treatment of acute respiratory infections. Their impact on fungi has not been well characterised. Here, we provide a first report on the antifungal activity of a P. sidoides aerial tissue extract against Cryptococcus neoformans as well as the effects of both tuber and aerial tissue extracts on selected virulence factors.

AIM OF THE STUDY

Novel antimicrobial strategies that target multiple cellular pathways or make use of anti-pathogenic compounds that inhibit virulence factors have been proposed. This work aimed to evaluate P. sidoides plant parts for their anticryptococcal activity and antipathogenic properties on selected virulence factors.

MATERIALS AND METHODS

The antifungal activity of crude P. sidoides tuber and aerial tissue extracts (15% m/m ethanol) were compared using a modified colourimetric antifungal susceptibility test. Fungicidal activity of the extracts was confirmed by plate counts. To test yeast resistance to the extracts, it was conditioned by multiple passages in sub-lethal doses followed by antifungal susceptibility testing. Cytotoxicity of the extracts was tested with a blood agar haemolysis assay. Extracts were evaluated for the presence of multiple bioactive compounds by solid-phase fractionation and visualisation by thin-layer chromatography in combination with bioassays. The influence of extracts on the production of the polysaccharide capsule, ergosterol content as well as laccase and urease activities were also evaluated. Cell surface variations after extract exposure were visualised by scanning electron microscopy (SEM).

RESULTS

Both tuber and aerial tissue extracts were fungicidal and contained multiple bioactive compounds which constrained the development of antifungal resistance. No haemolytic activity was observed, and the extracts did not appear to target ergosterol biosynthesis. However, the extracts displayed anti-pathogenic potential by significantly inhibiting laccase and urease activity while also significantly reducing capsule size. SEM revealed notable cell surface variations and provided support for the observed reduction in capsule size.

CONCLUSIONS

Our results provide support to the exploration of medicinal plants as sources of alternative antifungal therapies and the potential use of multicomponent inhibition and or virulence attenuation for next-generation treatment strategies. Our data also provide relevant information that may support the further use of P. sidoides in traditional medicines as well as in commercialised phytopharmaceuticals.

Ionic Liquids Chemical Stress Triggers Sphingoid Base Accumulation in Aspergillus nidulans

Understanding stress responses and signaling pathways in fungi became a fundamental need for the discovery of new specific antifungal targets for fighting emerging life-threatening pathogens and drug resistance. Ionic liquids constitute a unique class of chemicals, which structural diversity and tunable physical and chemical properties can provide a great diversity of stimuli. In this study, we propose the use of ionic liquids as tools to unravel signaling of stress responses in the filamentous fungus Aspergillus nidulans. We assessed how three ionic liquids with distinct effects over the cell wall and plasma membrane affect the biosynthesis of sphingolipids and accumulation of free sphingoid bases in this fungus. The stress imposed by each ionic liquid triggered the sphingolipid biosynthetic pathway and led to distinct profiles of sphingoid bases accumulation. Dodecyltributylphosphonium chloride and 1-decyl-3-methylimidazolium chloride induced the accumulation of sphingosine and of a yet unknown sphingoid base, respectively, while cholinium decanoate did not seem to accumulate any of these intermediates. This study brings further light to the roles of sphingoid bases in A. nidulans. In particular, sphingosine as a possible response mediator to cell wall damage induced by dodecyltributylphosphonium chloride, and involvement of an unknown sphingoid base in the response to plasma membrane permeabilization caused by 1-decyl-3-methylimidazolium chloride. In addition, we completed the genetic assignment of the glucosylceramide pathway in A. nidulans through the identification of the sphingolipid Δ4-desaturase gene (AN4405). The knowledge established reinforces the idea of targeting sphingolipids biosynthesis in the search of improved antifungal compounds.

How We Approach Combination Antifungal Therapy for Invasive Aspergillosis and Mucormycosis in Transplant Recipients

Invasive aspergillosis and mucormycosis are life-threatening infections in solid organ and hematopoietic cell transplant recipients. Despite medical advancements in the care of these patients and the availability of new mold-active drugs, the outcomes remain suboptimal. Therefore, there has been increased interest in the use of combination antifungal therapy, in hopes that leveraging the possible in vitro synergy of these agents will improve the prognosis of invasive mold disease. However, there has been a large disconnect between the results of experimental and clinical investigations, as clinical studies have not unequivocally demonstrated the superiority of combination therapy over monotherapy. This is particularly true for mucormycosis, where the rarity of the condition has made it nearly impossible to prospectively study novel therapeutic strategies. We review the current standard of antifungal therapy and the preclinical and clinical data addressing the merit of combination therapy, and we provide guidance to optimize the management of these mycoses.

Emerging Anticancer Activity of Candidal Glucoseamine-6-Phosphate Synthase Inhibitors upon Nanoparticle-Mediated Delivery

Numerous glutamine analogues have been reported as irreversible inhibitors of the glucosamine-6-phosphate (GlcN-6-P) synthase in pathogenic Candida albicans in the last 3.5 decades. Among the reported inhibitors, the most effective N³-(4-methoxyfumaroyl)-l-2,3-diaminopropanoic acid (FMDP) has been extensively studied in order to develop its more active analogues. Several peptide-FMDP conjugates were tested to deliver FMDP to its subcellularly located GlcN-6-P synthase target. However, the rapid development of fungal resistance to FMDP-peptides required development of different therapeutic approaches to tackle antifungal resistance. In the current state of the global antifungal resistance, subcellular delivery of FMDP via free diffusion or endocytosis has become crucial. In this study, we report on in vitro nanomedical applications of FMDP and one of its ketoacid analogues, N³- trans-4-oxo-4-phenyl-2-butenoyl-l-2,3-diaminopropanoic acid (BADP). FMDP and BADP covalently attached to polyethylene glycol-coated iron oxide/silica core-shell nanoparticles are tested against intrinsically multidrug-resistant C. albicans. Three different human cancer cell lines potentially overexpressing the GlcN-6-P synthase enzyme are tested to demonstrate the immediate inhibitory effects of nanoparticle conjugates against mammalian cells. It is shown that nanoparticle-mediated delivery transforms FMDP and BADP into strong anticancer agents by inhibiting the growth of the tested cancer cells, whereas their anti-Candidal activity is decreased. This study discusses the emerging inhibitory effect of the FMDP/BADP-nanoparticle conjugates based on their cellular internalization efficiency and biocompatibility.

Synthesis of Simplified Azasordarin Analogs as Potential Antifungal Agents

A new series of simplified azasordarin analogs was synthesized using as key steps a Diels-Alder reaction to generate a highly substituted bicyclo[2.2.1]heptane core, followed by a subsequent nitrile alkylation. Several additional strategies were investigated for the generation of the key tertiary nitrile or aldehyde thought to be required for inhibition at the fungal protein eukaryotic elongation factor 2. This new series also features a morpholino glycone previously reported in semisynthetic sordarin derivatives with broad spectrum antifungal activity. Despite a lack of activity against Candida albicans for these early de novo analogs, the synthetic route reported here permits more comprehensive modifications of the bicyclic core and structure-activity relationship studies that were not heretofore possible.

Understanding the riboflavin biosynthesis pathway for the development of antimicrobial agents

Life on earth depends on the biosynthesis of riboflavin, which plays a vital role in biological electron transport processes. Higher mammals obtain riboflavin from dietary sources; however, various microorganisms, including Gram-negative pathogenic bacteria and yeast, lack an efficient riboflavin-uptake system and are dependent on endogenous riboflavin biosynthesis. Consequently, the inhibition of enzymes in the riboflavin biosynthesis pathway would allow selective toxicity to a pathogen and not the host. Thus, the riboflavin biosynthesis pathway is an attractive target for designing novel antimicrobial drugs, which are urgently needed to address the issue of multidrug resistance seen in various pathogens. The enzymes involved in riboflavin biosynthesis are lumazine synthase (LS) and riboflavin synthase (RS). Understanding the details of the mechanisms of the enzyme-catalyzed reactions and the structural changes that occur in the enzyme active sites during catalysis can facilitate the design and synthesis of suitable analogs that can specifically inhibit the relevant enzymes and stop the generation of riboflavin in pathogenic bacteria. The present review is the first compilation of the work that has been carried out over the last 25 years focusing on the design of inhibitors of the biosynthesis of riboflavin based on an understanding of the mechanisms of LS and RS. This review aimed to address the fundamental advances in our understanding of riboflavin biosynthesis as applied to the rational design of a novel class of inhibitors. These advances have been aided by X-ray structures of ligand-enzyme complexes, rotational-echo, double-resonance nuclear magnetic resonance spectroscopy, high-throughput screening, virtual screenings, and various mechanistic probes.

Broad spectrum triazoles for invasive mould infections in adults: Which drug and when?

Invasive mould infections are an increasing cause of morbidity and mortality globally, mainly due to increasing numbers of immunocompromised individuals at risk for fungal infections. The introduction of broad spectrum triazoles, which are much better tolerated compared to conventional amphotericin B formulations, has increased survival, particularly in invasive mould infection. However, early initiation of appropriate antifungal treatment remains a major predictor of outcome in invasive mould infection, but despite significant advances in diagnosis of these diseases, early diagnosis remains a challenge. As a result, prophylaxis with mould-active triazoles is widely used for those patients at highest risk for invasive mould infection, including patients with prolonged neutropenia after induction chemotherapy for acute myeloid leukemia and patients with graft-versus-host-disease. Posaconazole is the recommended drug of choice for antimould prophylaxis in these high-risk patients. Voriconazole has its primary role in treatment of invasive aspergillosis but not in prophylaxis. Recently, isavuconazole has been introduced as an excellent alternative to voriconazole for primary treatment of invasive aspergillosis in patients with hematological malignancies. Compared to voriconazole, isavuconazole and posaconazole have broader activity against moulds and are therefore also an option for treatment of mucormycosis in the presence of intolerance or contraindications against liposomal amphotericin B.

Management of febrile neutropenia in the perspective of antimicrobial de-escalation and discontinuation

Introduction: Infections are among the most frequent complications in patients with hematological and oncological diseases. They might be classified as fever of unknown origin and microbiologically or clinically documented infections. Optimal duration of antimicrobial treatment is still unclear in these patients.Areas covered: We provide an overview on the management of febrile neutropenia in the perspective of antimicrobial de-escalation and discontinuation.Expert opinion: Patients with febrile high-risk neutropenia should be treated empirically with an anti-pseudomonal agent such as piperacillin/tazobactam. Several clinical studies support the assumption that the primary antibiotic regimen might be safely discontinued prior to neutrophil reconstitution if the patient is afebrile for several days and all infection-related symptoms have been resolved. Primary empirical treatment with carbapenems or antibiotic combinations should commonly only be considered in selected patient subgroups, such as patients with severe neutropenic sepsis or colonization with multidrug-resistant bacteria. Preemptive antifungal treatment guided by lung imaging and other parameters (e.g. serial Aspergillus galactomannan antigen screening) might reduce the consumption of antifungals compared to the classical empirical approach.Multidrug-resistant pathogens are emerging, and novel anti-infective agents under development are scarce. Therefore, a rational use of antimicrobials based on the principles of antibiotic stewardship is crucial.

Candida albicans: An Emerging Yeast Model to Study Eukaryotic Genome Plasticity

Saccharomyces cerevisiae and Schizosaccharomyces pombe have served as uncontested unicellular model organisms, as major discoveries made in the field of genome biology using yeast genetics have proved to be relevant from yeast to humans. The yeast Candida albicans has attracted much attention because of its ability to switch between a harmless commensal and a dreaded human pathogen. C. albicans bears unique features regarding its life cycle, genome structure, and dynamics, and their links to cell biology and adaptation to environmental challenges. Examples include a unique reproduction cycle with haploid, diploid, and tetraploid forms; a distinctive organisation of chromosome hallmarks; a highly dynamic genome, with extensive karyotypic variations, including aneuploidies, isochromosome formation, and loss-of-heterozygosity; and distinctive links between the response to DNA alterations and cell morphology. These features have made C. albicans emerge as a new and attractive unicellular model to study genome biology and dynamics in eukaryotes.

Discovery of Carboline Derivatives as Potent Antifungal Agents for the Treatment of Cryptococcal Meningitis

Clinical treatment of cryptococcal meningitis (CM) remains a significant challenge because of the lack of effective and safe drug therapies. Developing novel CM therapeutic agents with novel chemical scaffolds and new modes of action is of great importance. Herein, new β-hexahydrocarboline derivatives are shown to possess potent anticryptococcal activities. In particular, compound A4 showed potent in vitro and in vivo anticryptococcal activity with good metabolic stability and blood-brain barrier permeability. Compound A4 was orally active and could significantly reduce brain fungal burdens in a murine model of CM. Moreover, compound A4 could inhibit several virulence factors of Cryptococcus neoformans and might act by a new mode of action. Preliminary mechanistic studies revealed that compound A4 induced DNA double-stranded breaks and cell cycle arrest at the G2 phase by acting on the Cdc25c/CDK1/cyclin B pathway. Taken together, β-hexahydrocarboline A4 represents a promising lead compound for the development of next-generation CM therapeutic agents.

Linking Cellular Morphogenesis with Antifungal Treatment and Susceptibility in Candida Pathogens

Fungal infections are a growing public health concern, and an increasingly important cause of human mortality, with Candida species being amongst the most frequently encountered of these opportunistic fungal pathogens. Several Candida species are polymorphic, and able to transition between distinct morphological states, including yeast, hyphal, and pseudohyphal forms. While not all Candida pathogens are polymorphic, the ability to undergo morphogenesis is linked with the virulence of many of these pathogens. There are also many connections between Candida morphogenesis and antifungal drug treatment and susceptibility. Here, we review how Candida morphogenesis-a key virulence trait-is linked with antifungal drugs and antifungal drug resistance. We highlight how antifungal therapeutics are able to modulate morphogenesis in both sensitive and drug-resistant Candida strains, the shared signaling pathways that mediate both morphogenesis and the cellular response to antifungal drugs and drug resistance, and the connection between Candida morphology, drug resistance, and biofilm growth. We further review the development of anti-virulence drugs, and targeting Candida morphogenesis as a novel therapeutic strategy to target fungal pathogens. Together, this review highlights important connections between fungal morphogenesis, virulence, and susceptibility to antifungals.

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.

The Yin and Yang of Current Antifungal Therapeutic Strategies: How Can We Harness Our Natural Defenses?

Fungal infections have aroused much interest over the last years because of their involvement in several human diseases. Immunocompromission due to transplant-related therapies and malignant cancer treatments are risk factors for invasive fungal infections, but also aggressive surgery, broad-spectrum antibiotics and prosthetic devices are frequently associated with infectious diseases. Current therapy is based on the administration of antifungal drugs, but the occurrence of resistant strains to the most common molecules has become a serious health-care problem. New antifungal agents are urgently needed and it is essential to identify fungal molecular targets that could offer alternatives for development of treatments. The fungal cell wall and plasma membrane are the most important structures that offer putative new targets which can be modulated in order to fight microbial infections. The development of monoclonal antibodies against new targets is a valid therapeutic strategy, both to solve resistance problems and to support the immune response, especially in immunocompromised hosts. In this review, we summarize currently used antifungal agents and propose novel therapeutic approaches, including new fungal molecular targets to be considered for drug development.

The expanding use of matrix-assisted laser desorption/ionization-time of flight mass spectroscopy in the diagnosis of patients with mycotic diseases

INTRODUCTION

Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) has emerged as a powerful new tool to identify human fungal pathogens and has radically altered the diagnostic mycology workflow at many medical centers around the world. Areas covered: While most experience is with the identification of yeasts, including species of Candida and Cryptococcus, there is ongoing work investigating the role of MALDI-TOF MS to detect molds, including species of Aspergillus, Fusarium, Scedosporium, and Mucormyctes as well as thermally dimorphic fungi. Expert commentary: In this paper, we review the current knowledge about this important new platform and examine how its expanding use may impact molecular diagnostics and patient care in the years ahead.

Sugar Sensing and Signaling in Candida albicans and Candida glabrata

Candida species, such as Candida albicans and Candida glabrata, cause infections at different host sites because they adapt their metabolism depending on the available nutrients. They are able to proliferate under both nutrient-rich and nutrient-poor conditions. This adaptation is what makes these fungi successful pathogens. For both species, sugars are very important nutrients and as the sugar level differs depending on the host niche, different sugar sensing systems must be present. Saccharomyces cerevisiae has been used as a model for the identification of these sugar sensing systems. One of the main carbon sources for yeast is glucose, for which three different pathways have been described. First, two transporter-like proteins, ScSnf3 and ScRgt2, sense glucose levels resulting in the induction of different hexose transporter genes. This situation is comparable in C. albicans and C. glabrata, where sensing of glucose by CaHgt4 and CgSnf3, respectively, also results in hexose transporter gene induction. The second glucose sensing mechanism in S. cerevisiae is via the G-protein coupled receptor ScGpr1, which causes the activation of the cAMP/PKA pathway, resulting in rapid adaptation to the presence of glucose. The main components of this glucose sensing system are also conserved in C. albicans and C. glabrata. However, it seems that the ligand(s) for CaGpr1 are not sugars but lactate and methionine. In C. glabrata, this pathway has not yet been investigated. Finally, the glucose repression pathway ensures repression of respiration and repression of the use of alternative carbon sources. This pathway is not well characterized in Candida species. It is important to note that, apart from glucose, other sugars and sugar-analogs, such as N-acetylglucosamine in the case of C. albicans, are also important carbon sources. In these fungal pathogens, sensing sugars is important for a number of virulence attributes, including adhesion, oxidative stress resistance, biofilm formation, morphogenesis, invasion, and antifungal drug tolerance. In this review, the sugar sensing and signaling mechanisms in these Candida species are compared to S. cerevisiae.

Screening and Antifungal Activity of a β-Carboline Derivative against Cryptococcus neoformans and C. gattii

Background

Cryptococcosis is a fungal disease of bad prognosis due to its pathogenicity and the toxicity of the drugs used for its treatment. The aim of this study was to investigate the medicinal potential of carbazole and β-carboline alkaloids and derivatives against Cryptococcus neoformans and C. gattii.

Methods

MICs were established in accordance with the recommendations of the Clinical and Laboratory Standards Institute for alkaloids and derivatives against C. neoformans and C. gattii genotypes VNI and VGI, respectively. A single active compound was further evaluated against C. neoformans genotypes VNII, VNIII, and VNIV, C. gattii genotypes VGI, VGIII, and VGIV, Candida albicans ATCC 36232, for cytotoxicity against the MRC-5 lineage of human fibroblasts and for effects on fungal cells (cell wall, ergosterol, and leakage of nucleic acids).

Results

Screening of 11 compounds revealed 8-nitroharmane as a significant inhibitor (MIC 40 μg/mL) of several C. neoformans and C. gattii genotypes. It was not toxic to fibroblasts (IC₅₀ > 50 µg/mL) nor did it alter fungal cell walls or the concentration of ergosterol in C. albicans or C. neoformans. It increased leakage of substances that absorb at 260 nm.

Conclusions

The synthetic β-carboline 8-nitroharmane significantly inhibits pathogenic Cryptococcus species and is interesting as a lead compound towards new therapy for Cryptococcus infections.

Targeting intrinsic cell death pathways to control fungal pathogens

Fungal pathogens pose an increasing threat to public health. Limited clinical drug regimens and emerging drug-resistant isolates challenge infection control. The global burden of human fungal pathogens is estimated at 1 billion infections and 1.5 million deaths annually. In addition, plant fungal pathogens increasingly threaten global food resources. Novel strategies are needed to combat emerging fungal diseases and pan-resistant fungi. An untapped mechanistically novel approach is to pharmacologically activate the intrinsic cell death pathways encoded by pathogenic fungi. This strategy is analogous to new anti-cancer therapeutics now entering the clinic. Here we summarize the best understood examples of cell death mechanisms encoded by pathogenic fungi, contrast these to mammalian cell death pathways, and highlight the gaps in knowledge towards identifying potential death effectors as druggable targets.

A Structural View on Medicinal Chemistry Strategies against Drug Resistance

The natural phenomenon of drug resistance is a widespread issue that hampers the performance of drugs in many major clinical indications. Antibacterial and antifungal drugs are affected, as well as compounds for the treatment of cancer, viral infections, or parasitic diseases. Despite the very diverse set of biological targets and organisms involved in the development of drug resistance, the underlying molecular mechanisms have been identified to understand the emergence of resistance and to overcome this detrimental process. Detailed structural information on the root causes for drug resistance is nowadays frequently available, so next-generation drugs can be designed that are anticipated to suffer less from resistance. This knowledge-based approach is essential for fighting the inevitable occurrence of drug resistance.

Highly N2-selective allylation of NH-1,2,3-triazoles with allenamides mediated by N-iodosuccinimide

We report a new protocol to synthesize N²-allyl-substituted 1,2,3-triazoles via NIS mediated allylation of allenamides with NH-1,2,3-triazoles.

Developing collaborative works for faster progress on fungal respiratory infections in cystic fibrosis

Cystic fibrosis (CF) is the major genetic inherited disease in Caucasian populations. The respiratory tract of CF patients displays a sticky viscous mucus, which allows for the entrapment of airborne bacteria and fungal spores and provides a suitable environment for growth of microorganisms, including numerous yeast and filamentous fungal species. As a consequence, respiratory infections are the major cause of morbidity and mortality in this clinical context. Although bacteria remain the most common agents of these infections, fungal respiratory infections have emerged as an important cause of disease. Therefore, the International Society for Human and Animal Mycology (ISHAM) has launched a working group on Fungal respiratory infections in Cystic Fibrosis (Fri-CF) in October 2006, which was subsequently approved by the European Confederation of Medical Mycology (ECMM). Meetings of this working group, comprising both clinicians and mycologists involved in the follow-up of CF patients, as well as basic scientists interested in the fungal species involved, provided the opportunity to initiate collaborative works aimed to improve our knowledge on these infections to assist clinicians in patient management. The current review highlights the outcomes of some of these collaborative works in clinical surveillance, pathogenesis and treatment, giving special emphasis to standardization of culture procedures, improvement of species identification methods including the development of nonculture-based diagnostic methods, microbiome studies and identification of new biological markers, and the description of genotyping studies aiming to differentiate transient carriage and chronic colonization of the airways. The review also reports on the breakthrough in sequencing the genomes of the main Scedosporium species as basis for a better understanding of the pathogenic mechanisms of these fungi, and discusses treatment options of infections caused by multidrug resistant microorganisms, such as Scedosporium and Lomentospora species and members of the Rasamsonia argillacea species complex.

Scedosporium and Lomentospora: an updated overview of underrated opportunists

Species of Scedosporium and Lomentospora are considered as emerging opportunists, affecting immunosuppressed and otherwise debilitated patients, although classically they are known from causing trauma-associated infections in healthy individuals. Clinical manifestations range from local infection to pulmonary colonization and severe invasive disease, in which mortality rates may be over 80%. These unacceptably high rates are due to the clinical status of patients, diagnostic difficulties, and to intrinsic antifungal resistance of these fungi. In consequence, several consortia have been founded to increase research efforts on these orphan fungi. The current review presents recent findings and summarizes the most relevant points, including the Scedosporium/Lomentospora taxonomy, environmental distribution, epidemiology, pathology, virulence factors, immunology, diagnostic methods, and therapeutic strategies.

Cationic Amphiphiles Induce Macromolecule Denaturation and Organelle Decomposition in Pathogenic Yeast

Cationic amphiphiles are a large and diverse class of antimicrobial agents. Although their mode of action is not fully resolved, it is generally accepted that these antimicrobials perturb the structural integrity of the plasma membrane leading to the microbial cell disruption. Here we report on the development of inherently fluorescent antifungal cationic amphiphiles and on the study of their effects on cells of Candida, one of the most common fungal pathogens in humans. Fluorescent images of Candida yeast cells that express a fluorescent reporter protein revealed that the cationic amphiphiles rapidly accumulated in the cytosol and led to structural changes in proteins and DNA. Using fluorescent organelle-specific dyes, we showed that these antifungal agents also caused organelle disassembly in Candida cells. The results of this study indicate that, in designing antifungal cationic amphiphiles for clinical use, the intracellular activities of these molecules must be addressed to avoid undesired side effects to mammalian cells.

Twice-Weekly Tacrolimus Can Overcome Pharmacologic Interaction and Help in the Successful Treatment of Pulmonary Aspergillosis in a Liver Transplant Recipient

Drug-drug interactions between azoles and calcineurin inhibitors can cause issues for organ transplant specialists. Clinical practice guidelines for the treatment of solid-organ transplant recipients with invasive aspergillosis infection are lacking. Here, we present a patient who developed pulmonary aspergillosis after liver transplant. The patient had prolonged treatment with echinocandin that was not effective. A drug-drug interaction between azoles and tacrolimus caused issues for the clinical physician. We adjusted the doses, and the patient was successfully treated. A reduction in the tacrolimus dose, intensive monitoring of associated parameters, and elimination of risk exposures are important for a favorable outcome.

Cationic surfactants as antifungal agents

Fungi-in being responsible for causing diseases in animals and humans as well as environmental contaminations in health and storage facilities-represent a serious concern to health security. Surfactants are a group of chemical compounds used in a broad spectrum of applications. The recently considered potential employment of cationic surfactants as antifungal or fungistatic agents has become a prominent issue in the development of antifungal strategies, especially if such surface-active agents can be synthesized in an eco-friendly manner. In this review, we describe the antifungal effect and the reported mechanisms of action of several types of cationic surfactants and also include a discussion of the contribution of these surfactants to the inhibition of yeast-based-biofilm formation. Furthermore, the putative mechanism of arginine-based tensioactive compounds as antifungal agents and their applications are also analyzed.

Functional Characterization of Cryptococcal Genes: Then and Now

Site-directed mutagenesis enables researchers to switch a gene of interest off for functional characterization of the gene. In the pathogenic yeasts, Cryptococcus neoformans and sister species C. deneoformans, this is almost exclusively achieved by introducing DNA into cells through either biolistic transformation or electroporation. The targeted gene is then disrupted by homologous recombination (HR) between the gene and the transforming DNA. Both techniques have downsides; biolistic transformation equipment is very expensive, limiting the use thereof to well-resourced laboratories, and HR occurs at extremely low frequencies in electroporated cryptococcal cells, making this method unappealing for gene targeting when not making use of additional modifications or methods to enhance HR in these cells. One approach to increase the frequency of HR in electroporated cryptococcal cells have recently been described. In this approach, CRISPR-Cas9 technology is utilized to form a double strand break in the targeted gene where after the occurrence of HR seems to be higher. The less expensive electroporation technique can therefore be used to deliver the CRISPR-Cas9 components into cells to disrupt a gene of interest, but only if the CRISPR components can be maintained for long enough in cells to enable their expression. Maintenance of episomal DNA occurs readily in C. deneoformans, but only under certain conditions in C. neoformans. In addition, CRISPR-Cas9 allows for gene complementation in order to fulfill Falkow’s molecular Koch’s postulates and adds other novel methods for studying genes as well, such as the addition of a fluorophore to an inactive Cas9 enzyme to highlight the location of a gene in a chromosome. These developments add less expensive alternatives to current methods, which could lead to more research on this yeast in developing countries where cryptococcal infections are more prevalent and researchers have access to more clinical isolates.

Fungal Diseases in the 21st Century: The Near and Far Horizons

Fungal diseases became a major medical problem in the second half of the 20th century when advances in modern medicine together with the HIV epidemic resulted in large numbers of individuals with impaired immunity. Fungal diseases are difficult to manage because they tend to be chronic, hard to diagnose, and difficult to eradicate with antifungal drugs. This essay considers the future of medical mycology in the 21st century, extrapolating from current trends. In the near horizon, the prevalence of fungal diseases is likely to increase, as there will be more hosts with impaired immunity and drug resistance will inevitably increase after selection by antifungal drug use. We can expect progress in the development of new drugs, diagnostics, vaccines, and immunotherapies. In the far horizon, humanity may face new fungal diseases in association with climate change. Some current associations between chronic diseases and fungal infections could lead to the establishment of fungi as causative agents, which will greatly enhance their medical importance. All trends suggest that the importance of fungal diseases will increase in the 21st century, and enhanced human preparedness for this scourge will require more research investment in this group of infectious diseases.

Evolutionary Emergence of Drug Resistance in Candida Opportunistic Pathogens

Fungal infections, such as candidiasis caused by Candida, pose a problem of growing medical concern. In developed countries, the incidence of Candida infections is increasing due to the higher survival of susceptible populations, such as immunocompromised patients or the elderly. Existing treatment options are limited to few antifungal drug families with efficacies that vary depending on the infecting species. In this context, the emergence and spread of resistant Candida isolates are being increasingly reported. Understanding how resistance can evolve within naturally susceptible species is key to developing novel, more effective treatment strategies. However, in contrast to the situation of antibiotic resistance in bacteria, few studies have focused on the evolutionary mechanisms leading to drug resistance in fungal species. In this review, we will survey and discuss current knowledge on the genetic bases of resistance to antifungal drugs in Candida opportunistic pathogens. We will do so from an evolutionary genomics perspective, focusing on the possible evolutionary paths that may lead to the emergence and selection of the resistant phenotype. Finally, we will discuss the potential of future studies enabled by current developments in sequencing technologies, in vitro evolution approaches, and the analysis of serial clinical isolates.

Nystatin-like Pseudonocardia polyene B1, a novel disaccharide-containing antifungal heptaene antibiotic

Polyene macrolides such as nystatin A1 and amphotericin B belong to a large family of very valuable antifungal polyketide compounds typically produced by soil actinomycetes. Recently, nystatin-like Pseudonocardia polyene (NPP) A1 has been identified as a unique disaccharide-containing tetraene antifungal macrolide produced by Pseudonocardia autotrophica. Despite its significantly increased water solubility and decreased hemolytic activity, its antifungal activity remains limited compared with that of nystatin A1. In this study, we developed NPP B1, a novel NPP A1 derivative harboring a heptaene core structure, by introducing two amino acid substitutions in the putative NADPH-binding motif of the enoyl reductase domain in module 5 of the NPP A1 polyketide synthase NppC. The low level NPP B1 production yield was successfully improved by eliminating the native plasmid encoding a polyketide biosynthetic gene cluster present in P. autotrophica. In vitro and in vivo antifungal activity and toxicity studies indicated that NPP B1 exhibited comparable antifungal activity against Candida albicans and was less toxic than the most potent heptaene antifungal, amphotericin B. Moreover, NPP B1 showed improved pharmacokinetic parameters compared to those of amphotericin B, suggesting that NPP B1 could be a promising candidate for development into a pharmacokinetically improved and less-toxic polyene antifungal antibiotic.