Arundifungin, a novel antifungal compound produced by fungi: biological activity and taxonomy of the producing organisms

The limitless endophytes: their role as antifungal agents against top priority pathogens

Multi resistant fungi are on the rise, and our arsenal compounds are limited to few choices in the market such as polyenes, pyrimidine analogs, azoles, allylamines, and echinocandins. Although each of these drugs featured a unique mechanism, antifungal resistant strains did emerge and continued to arise against them worldwide. Moreover, the genetic variation between fungi and their host humans is small, which leads to significant challenges in new antifungal drug discovery. Endophytes are still an underexplored source of bioactive secondary metabolites. Many studies were conducted to isolate and screen endophytic pure compounds with efficacy against resistant yeasts and fungi; especially, Candida albicans, C. auris, Cryptococcus neoformans and Aspergillus fumigatus, which encouraged writing this review to critically analyze the chemical nature, potency, and fungal source of the isolated endophytic compounds as well as their novelty features and SAR when possible. Herein, we report a comprehensive list of around 320 assayed antifungal compounds against Candida albicans, C. auris, Cryptococcus neoformans and Aspergillus fumigatus in the period 1980-2024, the majority of which were isolated from fungi of orders Eurotiales and Hypocreales associated with terrestrial plants, probably due to the ease of laboratory cultivation of these strains. 46% of the reviewed compounds were active against C. albicans, 23% against C. neoformans, 29% against A. fumigatus and only 2% against C. auris. Coculturing was proved to be an effective technique to induce cryptic metabolites absent in other axenic cultures or host extract cultures, with Irperide as the most promising compounds MIC value 1 μg/mL. C. auris was susceptible to only persephacin and rubiginosin C. The latter showed potent inhibition against this recalcitrant strain in a non-fungicide way, which unveils the potential of fungal biofilm inhibition. Further development of culturing techniques and activation of silent metabolic pathways would be favorable to inspire the search for novel bioactive antifungals.

Natural products targeting the synthesis of β(1,3)-D-glucan and chitin of the fungal cell wall. Existing drugs and recent findings

BACKGROUND

During the last three decades systemic fungal infections associated to immunosuppressive therapies have become a serious healthcare problem. Clinical development of new antifungals is an urgent requirement. Since fungal but not mammalian cells are encased in a carbohydrate-containing cell wall, which is required for the growth and viability of fungi, the inhibition of cell wall synthesizing machinery, such as β(1,3)-D-glucan synthases (GS) and chitin synthases (CS) that catalyze the synthesis of β(1-3)-D-glucan and chitin, respectively, represent an ideal mode of action of antifungal agents. Although the echinocandins anidulafungin, caspofungin and micafungin are clinically well-established GS inhibitors for the treatment of invasive fungal infections, much effort must still be made to identify inhibitors of other enzymes and processes involved in the synthesis of the fungal cell wall.

PURPOSE

Since natural products (NPs) have been the source of several antifungals in clinical use and also have provided important scaffolds for the development of semisynthetic analogues, this review was devoted to investigate the advances made to date in the discovery of NPs from plants that showed capacity of inhibiting cell wall synthesis targets. The chemical characterization, specific target, discovery process, along with the stage of development are provided here.

METHODS

An extensive systematic search for NPs against the cell wall was performed considering all the articles published until the end of 2020 through the following scientific databases: NCBI PubMed, Scopus and Google Scholar and using the combination of the terms "natural antifungals" and "plant extracts" with "fungal cell wall".

RESULTS

The first part of this review introduces the state of the art of the structure and biosynthesis of the fungal cell wall and considers exclusively those naturally produced GS antifungals that have given rise to both existing semisynthetic approved drugs and those derivatives currently in clinical trials. According to their chemical structure, natural GS inhibitors can be classified as 1) cyclic lipopeptides, 2) glycolipids and 3) acidic terpenoids. We also included nikkomycins and polyoxins, NPs that inhibit the CS, which have traditionally been considered good candidates for antifungal drug development but have finally been discarded after enduring unsuccessful clinical trials. Finally, the review focuses in the most recent findings about the growing field of plant-derived molecules and extracts that exhibit activity against the fungal cell wall. Thus, this search yielded sixteen articles, nine of which deal with pure compounds and seven with plant extracts or fractions with proven activity against the fungal cell wall. Regarding the mechanism of action, seven (44%) produced GS inhibition while five (31%) inhibited CS. Some of them (56%) interfered with other components of the cell wall. Most of the analyzed articles refer to tests carried out in vitro and therefore are in early stages of development.

CONCLUSION

This report delivers an overview about both existing natural antifungals targeting GS and CS activities and their mechanisms of action. It also presents recent discoveries on natural products that may be used as starting points for the development of potential selective and non-toxic antifungal drugs.

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.

Endophytic Fungi of Olive Tree

In addition to the general interest connected with investigations on biodiversity in natural contexts, more recently the scientific community has started considering occurrence of endophytic fungi in crops in the awareness of the fundamental role played by these microorganisms on plant growth and protection. Crops such as olive tree, whose management is more and more frequently based on the paradigm of sustainable agriculture, are particularly interested in the perspective of a possible applicative employment, considering that the multi-year crop cycle implies a likely higher impact of these symbiotic interactions. Aspects concerning occurrence and effects of endophytic fungi associated with olive tree (Olea europaea) are revised in the present paper.

New Chromones from a Marine-Derived Fungus, Arthrinium sp., and Their Biological Activity

Five new chromone derivatives, arthones A⁻E (1⁻5), together with eight known biogenetically related cometabolites (6⁻13), were isolated from a deep-sea-derived fungus Arthrinium sp. UJNMF0008. Their structures were assigned by detailed analyses of spectroscopic data, while the absolute configurations of 1 and 5 were established by electronic circular dichroism (ECD) calculations and that of 2 was determined by modified Mosher ester method. Compounds 3 and 8 exhibited potent antioxidant property with DPPH and ABTS radical scavenging activities, with IC50 values ranging from 16.9 to 18.7 μM. Meanwhile, no compounds indicated obvious bioactivity in our antimicrobial and anti-inflammatory assays at 50.0 μM.

Anti-Candida albicans natural products, sources of new antifungal drugs: A review

INTRODUCTION

Candida albicans is the most prevalent fungal pathogen in humans. Due to the development of drug resistance, there is today a need for new antifungal agents for the efficient management of C. albicans infections. Therefore, we reviewed antifungal activity, mechanisms of action, possible synergism with antifungal drugs of all natural substances experimented to be efficient against C. albicans for future.

METHODS

An extensive and systematic review of the literature was undertaken and all relevant abstracts and full-text articles analyzed and included in the review.

REVIEW

A total of 111 documents were published and highlighted 142 anti-C. albicans natural products. These products are mostly are reported in Asia (44.37%) and America (28.17%). According to in vitro model criteria, from the 142 natural substances, antifungal activity can be considered as important for 40 (28.20%) and moderate for 24 (16.90%). Sixteen products have their antifungal activity confirmed by in vivo gold standard experimentation. Microbial natural products, source of antifungals, have their antifungal mechanism well described in the literature: interaction with ergosterol (polyenes), inhibition 1,3-β-d-glucan synthase (Echinocandins), inhibition of the synthesis of cell wall components (chitin and mannoproteins), inhibition of sphingolipid synthesis (serine palmitoyltransferase, ceramide synthase, inositol phosphoceramide synthase) and inhibition of protein synthesis (sordarins). Natural products from plants mostly exert their antifungal effects by membrane-active mechanism. Some substances from arthropods are also explored to act on the fungal membrane. Interestingly, synergistic effects were found between different classes of natural products as well as between natural products and azoles.

CONCLUSION

Search for anti-C. albicans new drugs is promising since the list of natural substances, which disclose activity against this yeast is today long. Investigations must be pursued not only to found more new anti-Candida compounds from plants and organisms but also to carried out details on molecules from already known anti-Candida compounds and to more elucidate mechanisms of action.

A phylogenetic re-evaluation of Arthrinium

Although the genus Arthrinium (sexual morph Apiospora) is commonly isolated as an endophyte from a range of substrates, and is extremely interesting for the pharmaceutical industry, its molecular phylogeny has never been resolved. Based on morphology and DNA sequence data of the large subunit nuclear ribosomal RNA gene (LSU, 28S) and the internal transcribed spacers (ITS) and 5.8S rRNA gene of the nrDNA operon, the genus Arthrinium is shown to belong to Apiosporaceae in Xylariales. Arthrinium is morphologically and phylogenetically circumscribed, and the sexual genus Apiospora treated as synonym on the basis that Arthinium is older, more commonly encountered, and more frequently used in literature. An epitype is designated for Arthrinium pterospermum, and several well-known species are redefined based on their morphology and sequence data of the translation elongation factor 1-alpha (TEF), beta-tubulin (TUB) and internal transcribed spacer (ITS1, 5.8S, ITS2) gene regions. Newly described are A. hydei on Bambusa tuldoides from Hong Kong, A. kogelbergense on dead culms of Restionaceae from South Africa, A. malaysianum on Macaranga hullettii from Malaysia, A. ovatum on Arundinaria hindsii from Hong Kong, A. phragmites on Phragmites australis from Italy, A. pseudospegazzinii on Macaranga hullettii from Malaysia, A. pseudosinense on bamboo from The Netherlands, and A. xenocordella from soil in Zimbabwe. Furthermore, the genera Pteroconium and Cordella are also reduced to synonymy, rejecting spore shape and the presence of setae as characters of generic significance separating them from Arthrinium.

Chemical Investigations of a Deep Water Marine-Derived Fungus: Simple Amino Acid Derivatives from an Arthrinium sp

The saltwater culture of an Arthrinium sp. derived from a marine sediment collected at −550 meters was a source of tyrosol (1) and a new compound, tyrosol carbamate (2). This is only the third report of novel secondary metabolites discovered from the saltwater culture of a deep-water marine-derived fungus.

Assessing resistance to the echinocandin antifungal drug caspofungin in Candida albicans by profiling mutations in FKS1

Resistance of clinical isolates of Candida albicans to the echinocandin drug caspofungin is slowly emerging and is linked to mutations in short conserved regions in the FKS1 gene. The most prominent changes occurred at the serine 645 position in Fks1p with substitutions of proline, tyrosine, and phenylalanine. An allele-specific real-time PCR molecular-beacon assay was developed for rapid identification of drug resistance by targeting FKS1 mutations. Mutations altering serine 645 were reliably identified in both heterozygous and homozygous states. The molecular-beacon assay was used to evaluate two large collections of spontaneous mutants from separate strains of C. albicans with resistance (MICs, >16 microg/ml) to caspofungin with the goal of understanding the relationship between FKS1 mutations and echinocandin resistance. Of 85 resistant isolates recovered, all were identified with mutations in FKS1; 93% showed changes at Ser645, with 62% displaying a characteristic S645P substitution expressed as either a homozygous or a heterozygous mutation in FKS1. Two other prominent amino acid substitutions, S645Y and S645F, were found at frequencies of 22% and 8%, respectively. Three new mutations were also identified: T1922C, G1932T, and C1934G, encoding F641S, L644F, and S645C substitutions, respectively. One strain had the double amino acid substitution L644F and S645C. Allele-specific probes were combined in a multiplex assay for reliable screening of known FKS1 mutations. These data support the importance of FKS1p substitutions in echinocandin resistance and demonstrate the feasibility of applying molecular screening for routine resistance assessment.

In vitro conidial germination in Arthrinium aureum and Arthrinium phaeospermum

This paper describes the microscopic details of conidial germination and the influence of pH, sodium chloride concentration, 3% glucose, 3% saccharose and ultraviolet irradiation on the conidial germination in Arthrinium species. Under laboratory conditions, germination started after an incubation period of 90 minutes in 2% malt extract broth at 25 degrees C. In vitro, the conidia of Arthrinium species have a very low percentage of germination (A. phaeospermum: 7.9%; A. aureum: 15.8%). Conidia of this genus have a characteristic equatorial slit. Conidia may break spontaneously at this slit, releasing their cytoplasmic contents. Arthrinium phaeospermum attains its optimum germination percentage when its conidia are suspended in a sterile saline solution (pH 3.5) in a water bath at 20 degrees C for 15 minutes before being inoculated on 2% malt extract agar. Conidial suspensions of A. aureum may be held in the same conditions, but for 30 minutes.

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.

Testing the limits of biological tolerance to arsenic in a fungus isolated from the River Tinto

The Tinto river in Spain, with its high acidity and heavy metal concentrations (As, Cu, Cr, Zn), is an example of an environment hostile to life. Yet despite these extreme conditions, the site possesses a great diversity of eukaryotic life forms. We report the isolation of a filamentous fungus able to grow at 200 mM arsenic ( approximately 15 000 p.p.m.), i.e. a concentration 20-fold above that withstood by the reference microorganisms Escherichia coli, Saccharomyces cerevisiae and Aspergillus nidulans, and 200 times greater than that tolerated by Aspergillus niger. Based on morphological, physiological and genotypic criteria, the strain belongs to the genus Aspergillus. High concentrations of the metalloid induced vacuolation, suggesting that this organelle is someway connected to arsenic tolerance. Concentrations that are lethal to other organisms do not stress Aspergillus sp. P37. The fungus was capable of removing arsenic from culture media. In addition to arsenic hyper-resistance, it also displayed a polyresistant phenotype to copper and chromium.

Microbial natural products as a source of antifungals

The vast number and variety of chemotherapeutic agents isolated from microbial natural products and used to treat bacterial infections have greatly contributed to the improvement of human health during the past century. However, only a limited number of antifungal agents (polyenes and azoles, plus the recently introduced caspofungin acetate) are currently available for the treatment of life-threatening fungal infections. Furthermore, the prevalence of systemic fungal infections has increased significantly during the past decade. For this reason, the development of new antifungal agents, preferably with novel mechanisms of action, is an urgent medical need. A selection of antifungal agents in early stages of development, produced by micro-organisms, is summarized in this review. The compounds are classified according to their mechanisms of action, covering inhibitors of the synthesis of cell wall components (glucan, chitin and mannoproteins), of sphingolipid synthesis (serine palmitoyltransferase, ceramide synthase, inositol phosphoceramide synthase and fatty acid elongation) and of protein synthesis (sordarins). In addition, some considerations related to the chemotaxonomy of the producing organisms and some issues relevant to antifungal drug discovery are also discussed.

Antimicrobial activity of ergokonin A from Trichoderma longibrachiatum

AIMS

Natural fungal products were screened for antifungal compounds. The mode of action of one of the hits found and the taxonomy of the producing organism were analysed.

METHODS AND RESULTS

An extract from a Trichoderma species showed a more potent activity in an agar-based assay against the null mutant fks1::HIS strain than against the wild-type strain, suggesting that it could contain a glucan synthesis inhibitor. The active component was identified as the known compound ergokonin A. The compound exhibited activity against Candida and Aspergillus species, but was inactive against Cryptococcus species. It induced alterations in the hyphal morphology of Aspergillus fumigatus. The identification of the producing isolate was confirmed by sequencing of the rDNA internal transcribed spacers and comparison with the sequences of other Trichoderma species. The analysis showed that the producing fungus had a high homology with other strains classified as Trichoderma longibrachiatum and its teleomorph Hypocrea schweinitzii.

CONCLUSIONS

The antifungal activity spectrum of ergokonin A and the morphology alterations induced on A. fumigatus are consistent with glucan synthesis as the target for ergokonin A. The production of ergokonin A is not uncommon, but is probably restricted to Trichoderma species.

SIGNIFICANCE AND IMPACT OF THE STUDY

The discovery that ergokonin A could be an inhibitor of glucan synthesis, having a structure very different to other inhibitors, increases the likelihood that orally active agents with this fungal-specific mode of action may be developed.