Isolation and characterization of novel human immunodeficiency virus integrase inhibitors from fungal metabolites

Biosynthesis, biological activities, and structure-activity relationships of decalin-containing tetramic acid derivatives isolated from fungi

Covering: up to December 2023Decalin-containing tetramic acid derivatives, especially 3-decalinoyltetramic acids (3-DTAs), are commonly found as fungal secondary metabolites. Numerous biological activities of this class of compounds, such as antibiotic, antiviral, antifungal, antiplasmodial, and antiprotozoal properties, have been the subject of ongoing research. For this reason, these molecules have attracted a lot of interest from the scientific community and various efforts including semi-synthesis, co-culturing with bacteria and biosynthetic gene sequencing have been made to obtain more derivatives. In this review, 3-DTAs are classified into four major groups based on the absolute configuration of the bicyclic decalin ring. Their biosynthetic pathways, various biological activities, and structure-activity relationship are then introduced.

Cell Culture Evaluation Hints Widely Available HIV Drugs Are Primed for Success if Repurposed for HTLV-1 Prevention

With an estimated 10 million people infected, the deltaretrovirus human T-cell lymphotropic virus type 1 (HTLV-1) is the second most prevalent pathogenic retrovirus in humans after HIV-1. Like HIV-1, HTLV-1 overwhelmingly persists in a host via a reservoir of latently infected CD4+ T cells. Although most patients are asymptomatic, HTLV-1-associated pathologies are often debilitating and include adult T-cell leukaemia/lymphoma (ATLL), which presents in mature adulthood and is associated with poor prognosis with short overall survival despite treatment. Curiously, the strongest indicator for the development of ATLL is the acquisition of HTLV-1 through breastfeeding. There are no therapeutic or preventative regimens for HTLV-1. However, antiretrovirals (ARVs), which target the essential retrovirus enzymes, have been developed for and transformed HIV therapy. As the architectures of retroviral enzyme active sites are highly conserved, some HIV-specific compounds are active against HTLV-1. Here, we expand on our work, which showed that integrase strand transfer inhibitors (INSTIs) and some nucleoside reverse transcriptase inhibitors (NRTIs) block HTLV-1 transmission in cell culture. Specifically, we find that dolutegravir, the INSTI currently recommended as the basis of all new combination antiretroviral therapy prescriptions, and the latest prodrug formula of the NRTI tenofovir, tenofovir alafenamide, also potently inhibit HTLV-1 infection. Our results, if replicated in a clinical setting, could see transmission rates of HTLV-1 and future caseloads of HTLV-1-associated pathologies like ATLL dramatically cut via the simple repurposing of already widely available HIV pills in HTLV-1 endemic areas. Considering our findings with the old medical saying "it is better to prevent than cure", we highly recommend the inclusion of INSTIs and tenofovir prodrugs in upcoming HTLV-1 clinical trials as potential prophylactics.

Investigation of in vitro antimicrobial, antioxidant and antiproliferative activities of Nostoc calcicola biosynthesized gold nanoparticles

The cyanobacteria are the promising candidate for synthesizing gold nanoparticles (AuNPs), due to their ability to accumulate heavy metals from the cellular environment and additionally contain varied bioactive compounds as reducing and stabilizing agents. This study describes the N2-fixing cyanobacterium Nostoc calcicola-mediated bioreduction of AuNPs and the inherent antimicrobial, antioxidant, and antiproliferative activities in vitro. Biosynthesized Nc-AuNPs were characterized by spectral characterization techniques. The formation of AuNPs was physically confirmed by the colour change from pale green to dark violet. The UV-Vis analysis, further, proved the reduction in Nc-AuNPs with the cyanobacterium and showed a spectral peak at 527 nm. FESEM-EDX images suggested the surface morphology of the NPs as spherical, cuboidal, and size between 20 and 140 nm. The antimicrobial studies of Nc-AuNPs were carried out by agar-well diffusion method and MIC values against five pathogenic bacterial and two fungal strains were noted. The AuNPs exhibited potential antimicrobial activity against h-pathogenic bacteria with inhibitory zones ranging at 11-18 mm; against fungi ranging at 13-17 mm. Significant antioxidant potentialities were explored by a DPPH assay with an IC50 value of 55.97 μg/ mL. Furthermore, in the anticancer efficacy assay, the Nc-AuNPs inhibited cellular proliferation in human breast adenocarcinoma and cervical cancer cell lines at IC50 concentration, 37.3 μg/ml, and 44.5 μg/ml, respectively. Conclusively, N. calcicola would be an excellent source for synthesizing stable colloidal AuNPs that had significant credibility as phycological (algal) nanomedicines as novel prodrugs with multiple bioactivities.

A comprehensive overview on the role of phytocompounds in human immunodeficiency virus treatment

Acquired immune deficiency syndrome (AIDS) is a worldwide epidemic caused by human immunodeficiency virus (HIV) infection. Newer medicines for eliminating the viral reservoir and eradicating the virus are urgently needed. Attempts to locate relatively safe and non-toxic medications from natural resources are ongoing now. Natural-product-based antiviral candidates have been exploited to a limited extent. However, antiviral research is inadequate to counteract for the resistant patterns. Plant-derived bioactive compounds hold promise as powerful pharmacophore scaffolds, which have shown anti-HIV potential. This review focuses on a consideration of the virus, various possible HIV-controlling methods and the recent progress in alternative natural compounds with anti-HIV activity, with a particular emphasis on recent results from natural sources of anti-HIV agents. Please cite this article as: Mandhata CP, Sahoo CR, Padhy RN. A comprehensive overview on the role of phytocompounds in human immunodeficiency virus treatment. J Integr Med. 2023; Epub ahead of print.

Isolation, Screening, and Active Metabolites Identification of Anti-Vibrio Fungal Strains Derived From the Beibu Gulf Coral

The Beibu Gulf harbors abundant underexplored marine microbial resources, which are rich in diversified secondary metabolites. The genera Vibrio is a well-known pathogenic bacterium of aquatic animals. In this study, 22 fungal strains were isolated and identified from the Beibu Gulf coral via the serial dilution method and internal transcribed spacer (ITS) sequence analysis, which were further divided into three branches by phylogenetic tree analysis. The crude extracts of them via small-scale fermentation were selected for the screening of inhibitory activity against Vibrio alginalyticus, Vibrio coralliilyticus, Vibrio harveyi, Vibrio parahaemolyticus, Vibrio owensii, and Vibrio shilonii. The results showed that eight fungal extracts displayed anti-Vibrio activity via the filter paper disk assay. Several of them showed strong inhibitory effects. Then, two tetramic acid alkaloids, equisetin (1) and 5'-epiequisetin (2), were identified from Fusarium equiseti BBG10 by bioassay-guided isolation, both of which inhibited the growth of Vibrio spp. with the MIC values of 86-132 μg/ml. The scanning electron microscope results showed that cell membranes of Vibrio became corrugated, distorted or ruptured after treatment with 1 and 2. Taken together, this study provided eight fungal isolates with anti-Vibrio potentials, and two alkaloid-type antibiotics were found with anti-Vibrio effects from the bioactive strain F. equiseti BBG10. Our findings highlight the importance of exploring promising microbes from the Beibu Gulf for the identification of anti-Vibrio for future antibiotic development.

Exploring the Synthetic Potential of γ-Lactam Derivatives Obtained from a Multicomponent Reaction. Applications as Antiproliferative Agents

A study on the reactivity of 3-amino α,β-unsaturated γ-lactam derivatives obtained from a multicomponent reaction is presented. Key features of the substrates are the presence of an endocyclic α,β-unsaturated amide moiety and an enamine functionality. Following different synthetic protocols, the functionalization at three different positions of the lactam core is achieved. In the presence of a soft base, under thermodynamic conditions, the functionalization at C-4 takes place where the substrates behave as enamines, while the use of a strong base, under kinetic conditions, leads to the formation of C-5-functionalized γ-lactams, in the presence of ethyl glyoxalate, through a highly diastereoselective vinylogous aldol reaction. Moreover, the nucleophilic addition of organometallic species allows the functionalization at C-3, through the imine tautomer, affording γ-lactams bearing tetrasubstituted stereocenters, where the substrates act as imine electrophiles. Taking into account the advantage of the presence of a chiral stereocenter in C-5 substituted γ-lactams, further diastereoselective transformations are also explored, leading to novel bicyclic substrates holding a fused γ and δ-lactam skeleton. Remarkably, an example of a highly stereoselective formal [3+3] cycloaddition reaction of chiral γ-lactam substrates is reported for the synthesis of 1,4-dihidropyridines, where a non-covalent attractive interaction of a carbonyl group with an electron-deficient arene seems to drive the stereoselectivity of the reaction to the exclusive formation of the cis isomer. In order to unambiguously determine the substitution pattern resulting from the diverse reactions, an extensive characterization of the substrates is detailed through 2D NMR and/or X-ray experiments. Likewise, applications of the substrates as antiproliferative agents against lung and ovarian cancer cells are also described.

Equisetin is an anti-obesity candidate through targeting 11β-HSD1

Obesity is increasingly prevalent globally, searching for therapeutic agents acting on adipose tissue is of great importance. Equisetin (EQST), a meroterpenoid isolated from a marine sponge-derived fungus, has been reported to display antibacterial and antiviral activities. Here, we revealed that EQST displayed anti-obesity effects acting on adipose tissue through inhibiting adipogenesis in vitro and attenuating HFD-induced obesity in mice, doing so without affecting food intake, blood pressure or heart rate. We demonstrated that EQST inhibited the enzyme activity of 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), a therapeutic target of obesity in adipose tissue. Anti-obesity properties of EQST were all offset by applying excessive 11β-HSD1's substrates and 11β-HSD1 inhibition through knockdown in vitro or 11β-HSD1 knockout in vivo. In the 11β-HSD1 bypass model constructed by adding excess 11β-HSD1 products, EQST's anti-obesity effects disappeared. Furthermore, EQST directly bond to 11β-HSD1 protein and presented remarkable better intensity on 11β-HSD1 inhibition and better efficacy on anti-obesity than known 11β-HSD1 inhibitor. Therefore, EQST can be developed into anti-obesity candidate compound, and this study may provide more clues for developing higher effective 11β-HSD1 inhibitors.

Recent advances in the chemo-biological characterization of decalin natural products and unraveling of the workings of Diels-Alderases

The Diels–Alder (DA) reaction refers to a [4 + 2] cycloaddition reaction that falls under the category of pericyclic reactions. It is a reaction that allows regio- and stereo-selective construction of two carbon–carbon bonds simultaneously in a concerted manner to generate a six-membered ring structure through a six-electron cyclic transition state. The DA reaction is one of the most widely applied reactions in organic synthesis, yet its role in biological systems has been debated intensely over the last four decades. A survey of secondary metabolites produced by microorganisms suggests strongly that many of the compounds possess features that are likely formed through DA reactions, and most of them are considered to be catalyzed by enzymes that are commonly referred to as Diels–Alderases (DAases). In recent years, especially over the past 10 years or so, we have seen an accumulation of a substantial body of work that substantiates the argument that DAases indeed exist and play a critical role in the biosynthesis of complex metabolites. This review will cover the DAases involved in the biosynthesis of decalin moieties, which are found in many of the medicinally important natural products, especially those produced by fungi. In particular, we will focus on a subset of secondary metabolites referred to as pyrrolidine-2-one-bearing decalin compounds and discuss the decalin ring stereochemistry and the biological activities of those compounds. We will also look into the genes and enzymes that drive the biosynthetic construction of those complex natural products, and highlight the recent progress made on the structural and mechanistic understanding of DAases, especially regarding how those enzymes exert stereochemical control over the [4 + 2] cycloaddition reactions they catalyze.

Multicomponent Synthesis of Unsaturated γ-Lactam Derivatives. Applications as Antiproliferative Agents through the Bioisosterism Approach: Carbonyl vs. Phosphoryl Group

We report efficient synthetic methodologies for the preparation of 3-amino and 3-hydroxy 3-pyrrolin-2-ones (unsaturated γ-lactams) through a multicomponent reaction of amines, aldehydes and acetylene or pyruvate derivatives. The densely substituted γ-lactam substrates show in vitro cytotoxicity, inhibiting the growth of the carcinoma human tumor cell lines RKO (human colon epithelial carcinoma), SKOV3 (human ovarian carcinoma) and A549 (carcinomic human alveolar basal epithelial cell). In view of the possibilities for the diversity of the substituents that offer a multicomponent, synthetic methodology, an extensive structure–activity profile is presented. In addition, the bioisosteric replacement of the flat ester group by a tetrahedral phosphonate or phosphine oxide moiety in γ-lactam substrates leads to increased growth inhibition activity. Cell morphology analysis and flow cytometry assays indicate that the main pathway by which our compounds induce cytotoxicity is based on the activation of the intracellular apoptotic mechanism.

Naturally Occurring Terpenes: A Promising Class of Organic Molecules to Address Influenza Pandemics

Since the olden times, infectious diseases have largely affected human existence. The newly emerged infections are excessively caused by viruses that are largely associated with mammal reservoirs. The casualties of these emergencies are significantly influenced by the way human beings interact with the reservoirs, especially the animal ones. In our review we will consider the evolutionary and the ecological scales of such infections and their consequences on the public health, with a focus on the pathogenic influenza A virus. The nutraceutical properties of fungal and plant terpene-like molecules will be linked to their ability to lessen the symptoms of viral infections and shed light on their potential use in the development of new drugs. New challenging methods in antiviral discovery will also be discussed in this review. The authors believe that pharmacognosy is the "wave of future pharmaceuticals", as it can be continually produced and scaled up under eco-friendly requirements. Further diagnostic methods and strategies however are required to standardise those naturally occurring resources.

Drug Design Strategies to Avoid Resistance in Direct-Acting Antivirals and Beyond

Drug resistance is prevalent across many diseases, rendering therapies ineffective with severe financial and health consequences. Rather than accepting resistance after the fact, proactive strategies need to be incorporated into the drug design and development process to minimize the impact of drug resistance. These strategies can be derived from our experience with viral disease targets where multiple generations of drugs had to be developed to combat resistance and avoid antiviral failure. Significant efforts including experimental and computational structural biology, medicinal chemistry, and machine learning have focused on understanding the mechanisms and structural basis of resistance against direct-acting antiviral (DAA) drugs. Integrated methods show promise for being predictive of resistance and potency. In this review, we give an overview of this research for human immunodeficiency virus type 1, hepatitis C virus, and influenza virus and the lessons learned from resistance mechanisms of DAAs. These lessons translate into rational strategies to avoid resistance in drug design, which can be generalized and applied beyond viral targets. While resistance may not be completely avoidable, rational drug design can and should incorporate strategies at the outset of drug development to decrease the prevalence of drug resistance.

Fungal natural products galaxy: Biochemistry and molecular genetics toward blockbuster drugs discovery

Secondary metabolites synthesized by fungi have become a precious source of inspiration for the design of novel drugs. Indeed, fungi are prolific producers of fascinating, diverse, structurally complex, and low-molecular-mass natural products with high therapeutic leads, such as novel antimicrobial compounds, anticancer compounds, immunosuppressive agents, among others. Given that these microorganisms possess the extraordinary capacity to secrete diverse chemical scaffolds, they have been highly exploited by the giant pharma companies to generate small molecules. This has been made possible because the isolation of metabolites from fungal natural sources is feasible and surpasses the organic synthesis of compounds, which otherwise remains a significant bottleneck in the drug discovery process. Here in this comprehensive review, we have discussed recent studies on different fungi (pathogenic, non-pathogenic, commensal, and endophytic/symbiotic) from different habitats (terrestrial and marines), the specialized metabolites they biosynthesize, and the drugs derived from these specialized metabolites. Moreover, we have unveiled the logic behind the biosynthesis of vital chemical scaffolds, such as NRPS, PKS, PKS-NRPS hybrid, RiPPS, terpenoids, indole alkaloids, and their genetic mechanisms. Besides, we have provided a glimpse of the concept behind mycotoxins, virulence factor, and host immune response based on fungal infections.

Starting or Switching to an Integrase Inhibitor-Based Regimen Affects PTSD Symptoms in Women with HIV

As the use of Integrase inhibitor (INSTI)-class antiretroviral medications becomes more common to maintain long-term viral suppression, early reports suggest the potential for CNS side-effects when starting or switching to an INSTI-based regimen. In a population already at higher risk for developing mood and anxiety disorders, these drugs may have significant effects on PTSD scale symptom scores, particularly in women with HIV (WWH). A total of 551 participants were included after completing ≥ 1 WIHS study visits before and after starting/switching to an INSTI-based ART regimen. Of these, 14% were ART naïve, the remainder switched from primarily a protease inhibitor (PI) or non-nucleoside reverse transcriptase inhibitor (NNRTI)-based regimen. Using multivariable linear mixed effects models, we compared PTSD Civilian Checklist subscale scores before and after a "start/switch" to dolutegravir (DTG), raltegravir (RAL), or elvitegravir (EVG). Start/switch to EVG improved re-experiencing subscale symptoms (P's < 0.05). Switching to EVG improved symptoms of avoidance (P = 0.01). Starting RAL improved arousal subscale symptoms (P = 0.03); however, switching to RAL worsened re-experiencing subscale symptoms (P < 0.005). Starting DTG worsened avoidance subscale symptoms (P = 0.03), whereas switching to DTG did not change subscale or overall PTSD symptoms (P's > 0.08). In WWH, an EVG-based ART regimen is associated with improved PTSD symptoms, in both treatment naïve patients and those switching from other ART. While a RAL-based regimen was associated with better PTSD symptoms than in treatment naïve patients, switching onto a RAL-based regimen was associated with worse PTSD symptoms. DTG-based regimens either did not affect, or worsened symptoms, in both naïve and switch patients. Further studies are needed to determine mechanisms underlying differential effects of EVG, RAL and DTG on stress symptoms in WWH.

Pyrrolocin C and equisetin inhibit bacterial acetyl-CoA carboxylase

Antimicrobial resistance is a growing global health and economic concern. Current antimicrobial agents are becoming less effective against common bacterial infections. We previously identified pyrrolocins A and C, which showed activity against a variety of Gram-positive bacteria. Structurally similar compounds, known as pyrrolidinediones (e.g., TA-289, equisetin), also display antibacterial activity. However, the mechanism of action of these compounds against bacteria was undetermined. Here, we show that pyrrolocin C and equisetin inhibit bacterial acetyl-CoA carboxylase (ACC), the first step in fatty acid synthesis. We used transcriptomic data, metabolomic analysis, fatty acid rescue and acetate incorporation experiments to show that a major mechanism of action of the pyrrolidinediones is inhibition of fatty acid biosynthesis, identifying ACC as the probable molecular target. This hypothesis was further supported using purified proteins, demonstrating that biotin carboxylase is the inhibited component of ACC. There are few known antibiotics that target this pathway and, therefore, we believe that these compounds may provide the basis for alternatives to current antimicrobial therapy.

New Tetramic Acids Comprising of Decalin and Pyridones From Chaetomium olivaceum SD-80A With Antimicrobial Activity

Cycloaddition reactions such as intramolecular Diels–Alder (IMDA) are extremely important in constructing multicyclic scaffolds with diverse bioactivities. Using MycB as a biomarker, three new polyketides – Chaetolivacines A (1), B (3), and C (4) – with one known compound Myceliothermophin E (2) comprising of decalin and 4-hydroxy-2-pyridones were obtained from the culture of Chaetomium olivaceum SD-80A under the guidance of gene mining. The structures of these compounds were established using detailed 1D, 2D NMR, and high-resolution electron spray ionization mass spectroscopy (HRESIMS) analysis. The relative and absolute configurations of the compounds 1, 3, and 4 were elucidated by NOESY and ECD. The biosynthesis pathways of these compounds were proposed, which involves in three key genes ChaA [polyketide synthase-non-ribosomal peptide synthetases (PKS-NRPS)], ChaB, and ChaC. Compounds 1–4 were tested for their antimicrobial activities, and compounds 2 and 3 showed moderate bioactivity against Staphylococcus aureus (SA) and methicillin-resistant S. aureus (MRSA) with MIC values of 15.8 and 27.1 μM. The results showed that configuration of C-21 in 3 and 4 is important for anti-SA and anti-MRSA activities. This study reveals the significant potential of the genus Chaetomium in producing new PKS-NRPS, therefore increasing the speed in the mining for new sources of antimicrobial agents.

Hyalodendrins A and B, New Decalin-Type Tetramic Acid Larvicides from the Endophytic Fungus Hyalodendriella sp. Ponipodef12

Two new decalin/tetramic acid hybrid metabolites, hyalodendrins A (1) and B (2) were isolated from plant endophytic fungus Hyalodendriella sp. Ponipodef12. The structures of the new compounds were elucidated by analysis of the spectroscopic data, including NMR, HRMS and ECD, and by chemical conversion. Compounds 1 and 2 were phomasetin analogues, and both showed potent larvicidal activity against the fourth-instar larvae of Aedes aegypti with the median lethal dose (LC₅₀) values of 10.31 and 5.93 μg/mL, respectively.

Antiviral Agents From Fungi: Diversity, Mechanisms and Potential Applications

Viral infections are amongst the most common diseases affecting people worldwide. New viruses emerge all the time and presently we have limited number of vaccines and only few antivirals to combat viral diseases. Fungi represent a vast source of bioactive molecules, which could potentially be used as antivirals in the future. Here, we have summarized the current knowledge of fungi as producers of antiviral compounds and discuss their potential applications. In particular, we have investigated how the antiviral action has been assessed and what is known about the molecular mechanisms and actual targets. Furthermore, we highlight the importance of accurate fungal species identification on antiviral and other natural products studies.

Antiviral Agents From Fungi: Diversity, Mechanisms and Potential Applications

Viral infections are amongst the most common diseases affecting people worldwide. New viruses emerge all the time and presently we have limited number of vaccines and only few antivirals to combat viral diseases. Fungi represent a vast source of bioactive molecules, which could potentially be used as antivirals in the future. Here, we have summarized the current knowledge of fungi as producers of antiviral compounds and discuss their potential applications. In particular, we have investigated how the antiviral action has been assessed and what is known about the molecular mechanisms and actual targets. Furthermore, we highlight the importance of accurate fungal species identification on antiviral and other natural products studies.

Fungal Lactamases: Their Occurrence and Function

Fungi are absorptive feeders and thus must colonize and ramify through their substrate to survive. In so doing they are in competition, particularly in the soil, with myriad microbes. These microbes use xenobiotic compounds as offensive weapons to compete for nutrition, and fungi must be sufficiently resistant to these xenobiotics. One prominent mechanism of xenobiotic resistance is through production of corresponding degrading enzymes. As typical examples, bacterial β-lactamases are well known for their ability to degrade and consequently confer resistance to β-lactam antibiotics, a serious emerging problem in health care. We have identified many fungal genes that putatively encode proteins exhibiting a high degree of similarity to β-lactamases. However, fungal cell walls are structurally different from the bacterial peptidoglycan target of β-lactams. This raises the question, why do fungi have lactamases and what are their functions? Previously, we identified and characterized one Fusarium verticillioides lactamase encoding gene (FVEG_08291) that confers resistance to the benzoxazinoid phytoanticipins produced by maize, wheat, and rye. Since benzoxazinoids are γ-lactams with five-membered rings rather than the four-membered β-lactams, we refer to the predicted enzymes simply as lactamases, rather than β-lactamases. An overview of fungal genomes suggests a strong positive correlation between environmental niche complexity and the number of fungal lactamase encoding genes, with soil-borne fungi showing dramatic amplification of lactamase encoding genes compared to those fungi found in less biologically complex environments. Remarkably, Fusarium species frequently possess large (>40) numbers of these genes. We hypothesize that many fungal hydrolytic lactamases are responsible for the degradation of plant or microbial xenobiotic lactam compounds. Alignment of protein sequences revealed two conserved patterns resembling bacterial β-lactamases, specifically those possessing PFAM domains PF00753 or PF00144. Structural predictions of F. verticillioides lactamases also suggested similar catalytic mechanisms to those of their bacterial counterparts. Overall, we present the first in-depth analysis of lactamases in fungi, and discuss their potential relevance to fitness and resistance to antimicrobials in the environment.

Potential of small-molecule fungal metabolites in antiviral chemotherapy

Various viral diseases, such as acquired immunodeficiency syndrome, influenza, and hepatitis, have emerged as leading causes of human death worldwide. Scientific endeavor since invention of DNA-dependent RNA polymerase of pox virus in 1967 resulted in better understanding of virus replication and development of various novel therapeutic strategies. Despite considerable advancement in every facet of drug discovery process, development of commercially viable, safe, and effective drugs for these viruses still remains a big challenge. Decades of intense research yielded a handful of natural and synthetic therapeutic options. But emergence of new viruses and drug-resistant viral strains had made new drug development process a never-ending battle. Small-molecule fungal metabolites due to their vast diversity, stereochemical complexity, and preapproved biocompatibility always remain an attractive source for new drug discovery. Though, exploration of therapeutic importance of fungal metabolites has started early with discovery of penicillin, recent prediction asserted that only a small percentage (5-10%) of fungal species have been identified and much less have been scientifically investigated. Therefore, exploration of new fungal metabolites, their bioassay, and subsequent mechanistic study bears huge importance in new drug discovery endeavors. Though no fungal metabolites so far approved for antiviral treatment, many of these exhibited high potential against various viral diseases. This review comprehensively discussed about antiviral activities of fungal metabolites of diverse origin against some important viral diseases. This also highlighted the mechanistic details of inhibition of viral replication along with structure-activity relationship of some common and important classes of fungal metabolites.

Establishment of the Inducible Tet-On System for the Activation of the Silent Trichosetin Gene Cluster in Fusarium fujikuroi

The PKS-NRPS-derived tetramic acid equisetin and its N-desmethyl derivative trichosetin exhibit remarkable biological activities against a variety of organisms, including plants and bacteria, e.g., Staphylococcus aureus. The equisetin biosynthetic gene cluster was first described in Fusarium heterosporum, a species distantly related to the notorious rice pathogen Fusarium fujikuroi. Here we present the activation and characterization of a homologous, but silent, gene cluster in F. fujikuroi. Bioinformatic analysis revealed that this cluster does not contain the equisetin N-methyltransferase gene eqxD and consequently, trichosetin was isolated as final product. The adaption of the inducible, tetracycline-dependent Tet-on promoter system from Aspergillus niger achieved a controlled overproduction of this toxic metabolite and a functional characterization of each cluster gene in F. fujikuroi. Overexpression of one of the two cluster-specific transcription factor (TF) genes, TF22, led to an activation of the three biosynthetic cluster genes, including the PKS-NRPS key gene. In contrast, overexpression of TF23, encoding a second Zn(II)₂Cys₆ TF, did not activate adjacent cluster genes. Instead, TF23 was induced by the final product trichosetin and was required for expression of the transporter-encoding gene MFS-T. TF23 and MFS-T likely act in consort and contribute to detoxification of trichosetin and therefore, self-protection of the producing fungus.

Pyrrolocin A, a 3-Decalinoyltetramic Acid with Selective Biological Activity, Isolated from Amazonian Cultures of the Novel Endophyte Diaporthales sp. E6927E

Natural products remain an important source of new therapeutics for emerging drug-resistant pathogens like Candida albicans, which particularly affects immunocompromised patients. A bioactive 3-decalinoyltetramic acid, pyrrolocin A, was isolated from extracts of a novel Amazonian fungal endophyte, E6927E, of the Diaporthales family. The structure of the natural product was solved using NMR and CD spectroscopy and it is structurally related to the fungal setins, equisetin and phomasetin, which are well-characterized tetramic acid antibiotics specific for Gram-positive organisms. We show that the compound inhibits growth of Staphylococcus aureus and Enterococcus faecalis. It shows selective and potent bioactivity against fungal strains, with an MIC of 4 μg/mL for C. albicans, 100 μg/mL for Aspergillus sp. and greater than 100 μg/mL for Saccharomyces cerevisiae. Further, the compound is less toxic to mammalian cells (IC50 = 150 μg/mL), with an inhibitory concentration greater than forty times that for C. albicans. Pyrrolocin A retained potent activity against eight out of seventeen strains of clinical Candida sp. isolates tested.

Potential Antiviral Agents from Marine Fungi: An Overview

Biodiversity of the marine world is only partially subjected to detailed scientific scrutiny in comparison to terrestrial life. Life in the marine world depends heavily on marine fungi scavenging the oceans of lifeless plants and animals and entering them into the nutrient cycle by. Approximately 150 to 200 new compounds, including alkaloids, sesquiterpenes, polyketides, and aromatic compounds, are identified from marine fungi annually. In recent years, numerous investigations demonstrated the tremendous potential of marine fungi as a promising source to develop new antivirals against different important viruses, including herpes simplex viruses, the human immunodeficiency virus, and the influenza virus. Various genera of marine fungi such as Aspergillus, Penicillium, Cladosporium, and Fusarium were subjected to compound isolation and antiviral studies, which led to an illustration of the strong antiviral activity of a variety of marine fungi-derived compounds. The present review strives to summarize all available knowledge on active compounds isolated from marine fungi with antiviral activity.

Unified synthesis of tirandamycins and streptolydigins

The comprehensive approach to tirandamycin and streptolydigin antibiotics has been developed utilizing a cinchona alkaloid-catalyzed asymmetric Morita–Baylis–Hillman reaction.

The past, present and future of secondary metabolite research in the Dothideomycetes

The Dothideomycetes represents a large and diverse array of fungi in which prominent plant pathogens are over-represented. Species within the Cochliobolus, Alternaria, Pyrenophora and Mycosphaerella (amongst others) all cause diseases that threaten food security in many parts of the world. Significant progress has been made over the past decade in understanding how some of these pathogens cause disease at a molecular level. It is reasonable to suggest that much of this progress can be attributed to the increased availability of genome sequences. However, together with revealing mechanisms of pathogenicity, these genome sequences have also highlighted the capacity of the Dothideomycetes to produce an extensive array of secondary metabolites, far greater than originally thought. Indeed, it is now clear that we appear to have only scratched the surface to date in terms of the identification of secondary metabolites produced by these fungi. In the first half of this review, we examine the current status of secondary metabolite research in the Dothideomycetes and highlight the diversity of the molecules discovered thus far, in terms of both structure and biological activity. In the second part of this review, we survey the emerging techniques and technologies that will be required to shed light on the vast array of secondary metabolite potential that is encoded within these genomes. Experimental design, analytical chemistry and synthetic biology are all discussed in the context of how they will contribute to this field.

Fungal metabolites diversity in maize and associated human dietary exposures relate to micro-climatic patterns in Malawi

This study investigated the diversity of fungal metabolites in maize across four agro-ecological zones of Malawi. A total of 90 maize samples (for human consumption), collected from farmsteads, were analysed for 235 fungal metabolites using liquid chromatography-tandem mass spectrometry. A total of 65 metabolites were found in the samples. 75% of samples from the hottest agro-ecological zone contained either aflatoxins, fumonisins, deoxynivalenol, zearalenone; or a combination thereof in levels exceeding European Union (EU) maximum levels, whereas the related fraction was only 17% in the cool temperature zone. Aflatoxins, citrinin, 3-nitropropionic acid, monocerin and equisetin were most prevalent and in higher levels in samples from hot agro-ecological zones, whereas deoxynivalenol, nivalenol, zearalenone and aurofusarin were most prevalent in cool agro-ecologies. On the basis of per-capita maize consumption, estimated daily intakes for all samples from hot ecologies were well above the JECFA's provisional maximum tolerable daily intake (PMTDI) of 2.0 μg/kg body weight (bw)/day for fumonisins, whereas the PMTDI of 1.0 μg/kg bw/day for deoxynivalenol was exceeded in relatively more (90%) samples from the cool highlands than the other zones. These results demonstrate the influence of micro-climatic conditions on mycotoxin prevalence patterns and underscores the need for development of agro-ecological specific mycotoxin dietary exposure management strategies.

Tetramic acid analogues produced by coculture of Saccharopolyspora erythraea with Fusarium pallidoroseum

Coculture of the fungus Fusarium pallidoroseum with the bacterium Saccharopolyspora erythraea was found to produce three new decalin-type tetramic acid analogues related to equisetin. The structures were determined by spectroscopic methods. The absolute configurations were established by circular dichroism spectroscopy and comparing the data with those of equisetin.

Two related pyrrolidinedione synthetase loci in Fusarium heterosporum ATCC 74349 produce divergent metabolites

Equisetin synthetase (EqiS), from the filamentous fungus Fusarium heterosporum ATCC 74349, was initially assigned on the basis of genetic knockout and expression analysis. Increasing inconsistencies in experimental results led us to question this assignment. Here, we sequenced the F. heterosporum genome, revealing two hybrid polyketide-peptide proteins that were candidates for the equisetin synthetase. The surrounding genes in both clusters had the needed auxiliary genes that might be responsible for producing equisetin. Genetic mutation, biochemical analysis, and recombinant expression in the fungus enabled us to show that the initially assigned EqiS does not produce equisetin but instead produces a related 2,4-pyrrolidinedione, fusaridione A, that was previously unknown. Fusaridione A is methylated in the 3-position of the pyrrolidinedione, which has not otherwise been found in natural products, leading to spontaneous reverse-Dieckmann reactions. A newly described gene cluster, eqx, is responsible for producing equisetin.

Epi-trichosetin, a new undecaprenyl pyrophosphate synthase inhibitor, produced by Fusarium oxysporum FKI-4553

A new compound, designated epi-trichosetin (1), was isolated along with the known compound trichosetin (2) from the culture broth of Fusarium oxysporum FKI-4553 by solvent extraction, silica gel column chromatography and reversed-phase HPLC. The structure of 1 was elucidated by comparing various spectral data with those of 2, revealing that 1 was a stereoisomer of 2. Compounds 1 and 2 inhibited the undecaprenyl pyrophosphate synthase activity of Staphylococcus aureus with IC50 values of 83 and 30 μM, respectively, and showed antimicrobial activity, particularly against Gram-positive bacteria, including methicillin-sensitive and -resistant S. aureus.

Library-based discovery and characterization of daphnane diterpenes as potent and selective HIV inhibitors in Daphne gnidium

Despite the existence of an extended armamentarium of effective synthetic drugs to treat HIV, there is a continuing need for new potent and affordable drugs. Given the successful history of natural product based drug discovery, a library of close to one thousand plant and fungal extracts was screened for antiretroviral activity. A dichloromethane extract of the aerial parts of Daphne gnidium exhibited strong antiretroviral activity and absence of cytotoxicity. With the aid of HPLC-based activity profiling, the antiviral activity could be tracked to four daphnane derivatives, namely, daphnetoxin (1), gnidicin (2), gniditrin (3), and excoecariatoxin (4). Detailed anti-HIV profiling revealed that the pure compounds were active against multidrug-resistant viruses irrespective of their cellular tropism. Mode of action studies that narrowed the site of activity to viral entry events suggested a direct interference with the expression of the two main HIV co-receptors, CCR5 and CXCR4, at the cell surface by daphnetoxin (1).

Raltegravir: first in class HIV integrase inhibitor

On October 16, 2007, the US Food and Drug Administration (FDA) approved raltegravir for treatment of human immunodeficiency virus (HIV)-1 infection in combination with other antiretroviral agents in treatment-experienced adult patients who have evidence of viral replication and HIV-1 strains resistant to multiple antiretroviral agents. Raltegravir is first in a novel class of antiretroviral drugs known as integrase inhibitors. It has demonstrated potent anti HIV activity in both antiretroviral treatment-naïve and experienced patients. The most common adverse events reported with raltegravir during phase 2 and 3 clinical trials were diarrhea, nausea, and headache. Laboratory abnormalities include mild elevations in liver transaminases and creatine phosphokinase.

Extended use of raltegravir in the treatment of HIV-1 infection: optimizing therapy

Raltegravir is the first licensed compound in 2007 of the new integrase inhibitor drug class. At the dose of 400 mg twice daily, raltegravir showed a potent antiviral action in antiretroviral-naïve patients when associated with tenofovir and emtricitabine. Raltegravir was also found to be highly active in antiretroviral-experienced patients with virological failure and displaying multiresistant virus, as shown with the BENCHMRK and ANRS 139 TRIO trials. Finally, the use of raltegravir was assessed in the context of a switch strategy in antiretroviral-experienced patients with virological success [human immunodeficiency virus type 1 (HIV-1) RNA below detection limit], highlighting the following mandatory criteria in this strategy: the nucleoside reverse transcriptase inhibitors associated with raltegravir have to be fully active. In the different studies, raltegravir had a favorable safety and tolerability profile. In the clinical situation a switch in virologically suppressed patients receiving a protease inhibitor, an improvement of the lipid profile was observed. Overall, when analyzing the Phase II and III trials together, only a few patients on raltegravir discontinued for adverse events. The development of resistance to raltegravir mainly involved three resistance mutations in integrase gene: Q148H/K/R, N155H, and Y143C/H/R. In conclusion, raltegravir improved the clinical management of HIV-1 infection both in antiretroviral-naïve and in antiretroviral-experienced patients.

Clinical use of raltegravir: a review

In the last 5 years there have been important achievements in the field of HIV therapy with the availability of new classes of antiretrovirals together with a new generation of old classes. Among them, the development of raltegravir, the first commercially available component of integrase inhibitors, brought an extraordinary improvement in salvage therapy for HIV-infected patients, allowing for virologic suppression even in multi-experienced subjects, who until recently had limited treatment options. It was subsequently approved for initial treatment combined with a nucleoside/nucleotide backbone, and also became an interesting option for naive HIV-infected populations. Its unique characteristics, the limited potential for significant pharmacokinetic interactions with other drugs and excellent safety profile have increased interest in its use in different settings.

Identification of the tirandamycin biosynthetic gene cluster from Streptomyces sp. 307-9

The structurally intriguing bicyclic ketal moiety of tirandamycin is common to several acyl-tetramic acid antibiotics, and is a key determinant of biological activity. We have identified the tirandamycin biosynthetic gene cluster from the environmental marine isolate Streptomyces sp. 307-9, thus providing the first genetic insight into the biosynthesis of this natural product scaffold. Sequence analysis revealed a hybrid polyketide synthase-nonribosomal peptide synthetase gene cluster with a colinear domain organization, which is entirely consistent with the core structure of the tirandamycins. We also identified genes within the cluster that encode candidate tailoring enzymes for elaboration and modification of the bicyclic ketal system. Disruption of tamI, which encodes a presumed cytochrome P450, led to a mutant strain deficient in production of late stage tirandamycins that instead accumulated tirandamycin C, an intermediate devoid of any post assembly-line oxidative modifications.

Phoma Saccardo: distribution, secondary metabolite production and biotechnological applications

Phoma Sacc. is an ubiquitous fungus, which has been reported from plants, soil, human beings, animals, and air. Some species of Phoma like P. sorghina, P. herbarum, P. exigua var. exigua, P. macrostoma, P. glomerata, Phoma macdonaldii, Phoma tracheiphila, Phoma proboscis, P. multirostrata, and Phoma foveata secrete phytotoxin and anthraquinone pigments as secondary metabolites, which have great potential for the biological control of weeds, and can be exploited for the production of mycopesticides, agrophytochemicals, and dyes. Some other species produce pharmaceutically active metabolites, viz., Sirodesmins, Phomenoic acid, Phomenolactone, Phomadecalins, Phomactin A, Phomasetin, Squalestatin-1 (S1), and Squalestatin-2 (S2). The secondary metabolites secreted by some species of Phoma are antitumor, antimicrobial, and anti-HIV. Equisetin and Phomasetin obtained from species of Phoma are useful against AIDS. The main goal of the present review is to discuss secondary metabolite production by species of Phoma and their utilization as antibiotics and as biocontrol agents.

Strand transfer inhibitors of HIV-1 integrase: bringing IN a new era of antiretroviral therapy

HIV-1 integrase (IN) is one of three essential enzymes (along with reverse transcriptase and protease) encoded by the viral pol gene. IN mediates two critical reactions during viral replication; firstly 3'-end processing (3'EP) of the double-stranded viral DNA ends and then strand transfer (STF) which joins the viral DNA to the host chromosomal DNA forming a functional integrated proviral DNA. IN is a 288 amino acid protein containing three functional domains, the N-terminal domain (NTD), catalytic core domain (CCD) and the C-terminal domain (CTD). The CCD contains three conserved catalytic residues, Asp64, Asp116 and Glu152, which coordinate divalent metal ions essential for the STF reaction. Intensive research over the last two decades has led to the discovery and development of small molecule inhibitors of the IN STF reaction (INSTIs). INSTIs are catalytic inhibitors of IN, and act to chelate the divalent metal ions in the CCD. One INSTI, raltegravir (RAL, Merck Inc.) was approved in late 2007 for the treatment of HIV-1 infection in patients with prior antiretroviral (ARV) treatment experience and was recently approved also for first line therapy. A second INSTI, elvitegravir (EVG, Gilead Sciences, Inc.) is currently undergoing phase 3 studies in ARV treatment-experienced patients and phase 2 studies in ARV naïve patients as part of a novel fixed dose combination. Several additional INSTIs are in early stage clinical development. This review will discuss the discovery and development of this novel class of antiretrovirals. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

Isolation and characterization of tirandamycins from a marine-derived Streptomyces sp

The novel dienoyl tetramic acids tirandamycin C (1) and tirandamycin D (2) with activity against vancomycin-resistant Enterococcus faecalis were isolated from the marine environmental isolate Streptomyces sp. 307-9, which also produces the previously identified compounds tirandamycins A (3) and B (4). Spectroscopic analysis of 1 and 2 indicated structural similarity to 3 and 4, with differences only in the pattern of pendant oxygenation on the bicyclic ketal system. The isolation of these putative biosynthetic intermediates was enabled by their sequestration on an adsorbent resin during early stationary-phase fermentation.

Raltegravir in the management of HIV-infected patients

Raltegravir has recently been licensed for the treatment of HIV-1 infection. Currently its use is limited to treatment-experienced patients and subjects with resistant virus. In addition to its activity in the setting of resistance and treatment failure, it appears to have great potential for first-line therapy and as a switch option for subjects with intolerance to other agents, as well. Overall tolerability in clinical trials was excellent, and the toxicity profile is non-overlapping with other agents, with no clear neuropsychiatric, gastrointestinal, or metabolic toxicity. Its metabolization occurs mainly via UGT1A1 rather than by the CYP450 system, resulting in a relatively unproblematic drug interaction profile. The independence of the compound from “boosting” of drug levels with ritonavir is an attractive feature for many patients suffering from ritonavir-associated side effects. However, it has to be dosed twice daily.

The unique effect of raltegravir on the establishment of viral latency makes it a logical component of treatment attempts aiming at reducing and controlling this viral sanctuary.

This review summarizes the clinical view on the role of this novel compound in HIV therapy.

HIV-1 integrase inhibitors: 2005-2006 update

HIV-1 integrase (IN) catalyzes the integration of proviral DNA into the host genome, an essential step for viral replication. Inhibition of IN catalytic activity provides an attractive strategy for antiretroviral drug design. Currently two IN inhibitors, MK-0518 and GS-9137, are in advanced stages of human clinical trials. The IN inhibitors in clinical evaluation demonstrate excellent antiretroviral efficacy alone or in combination regimens as compared to previously used clinical antiretroviral agents in naive and treatment-experienced HIV-1 infected patients. However, the emergence of viral strains resistant to clinically studied IN inhibitors and the dynamic nature of the HIV-1 genome demand a continued effort toward the discovery of novel inhibitors to keep a therapeutic advantage over the virus. Continued efforts in the field have resulted in the discovery of compounds from diverse chemical classes. In this review, we provide a comprehensive report of all IN inhibitors discovered in the years 2005 and 2006.

Development of integrase inhibitors for treatment of AIDS: An overview

HIV-1 integrase (IN) is an essential enzyme for retroviral replication. It is involved in the integration of HIV DNA into host chromosomal DNA. The unique properties of IN makes it an ideal target for drug design. First, there appears to have no functional equivalent in human cells and the reactions catalyzed by IN are unique. Second, IN is absolutely required for viral replication and mutations in a number of key residues block the viral replication. Third, IN has been validated as a legitimate target and the results from the molecules like S-1,360, JKT-303 which are under phase II/III clinical trials suggest synergistic effect with reverse transcriptase (RT) and protease (PR) inhibitors. During the past 10 years a plethora of inhibitors have been identified and some were shown to be selective against IN and block viral replication. The classes under which inhibitors of integrase can be classified are catechol-containing hydroxylated aromatics, diketoacid-containing aromatics, quninolines and others (non-catechol containing). In the present article we review all the recent small molecules reported to inhibit recombinant HIV-1 IN under these heads. It seems likely that the efficient use of HIV IN as target for rational design can give potent anti-HIV agents, which can be used alone or in combination regimens with other classes of anti-HIV drugs.

In-Silico docking of HIV-1 integrase inhibitors reveals a novel drug type acting on an enzyme/DNA reaction intermediate

Background

HIV-1 integrase (IN) is an emerging drug target, as IN strand transfer inhibitors (INSTIs) are proving potent antiretroviral agents in clinical trials. One credible theory sees INSTIs as docking at the cellular (acceptor) DNA-binding site after IN forms a transitional complex with viral (donor) DNA. However, mapping of the DNA and INSTI binding sites within the IN catalytic core domain (CCD) has been uncertain.

Methods

Structural superimpositions were conducted using the SWISS PDB and Cn3D free software. Docking simulations of INSTIs were run by a widely validated genetic algorithm (GOLD).

Results

Structural superimpositions suggested that a two-metal model for HIV-1 IN CCD in complex with small molecule, 1-(5-chloroindol-3-yl)-3-(tetrazoyl)-1,3-propandione-ene (5CITEP) could be used as a surrogate for an IN/viral DNA complex, because it allowed replication of contacts documented biochemically in viral DNA/IN complexes or displayed by a crystal structure of the IN-related enzyme Tn5 transposase in complex with transposable DNA. Docking simulations showed that the fitness of different compounds for the catalytic cavity of the IN/5CITEP complex significantly (P < 0.01) correlated with their 50% inhibitory concentrations (IC₅₀s) in strand transfer assays in vitro. The amino acids involved in inhibitor binding matched those involved in drug resistance. Both metal binding and occupation of the putative viral DNA binding site by 5CITEP appeared to be important for optimal drug/ligand interactions. The docking site of INSTIs appeared to overlap with a putative acceptor DNA binding region adjacent to but distinct from the putative donor DNA binding site, and homologous to the nucleic acid binding site of RNAse H. Of note, some INSTIs such as 4,5-dihydroxypyrimidine carboxamides/N-Alkyl-5-hydroxypyrimidinone carboxamides, a highly promising drug class including raltegravir/MK-0518 (now in clinical trials), displayed interactions with IN reminiscent of those displayed by fungal molecules from Fusarium sp., shown in the 1990s to inhibit HIV-1 integration.

Conclusion

The 3D model presented here supports the idea that INSTIs dock at the putative acceptor DNA-binding site in a IN/viral DNA complex. This mechanism of enzyme inhibition, likely to be exploited by some natural products, might disclose future strategies for inhibition of nucleic acid-manipulating enzymes.