Identification of fungal natural products with potent inhibition in Toxoplasma gondii

In an effort to identify novel compounds with potent inhibition against Toxoplasma gondii, a phenotypic screen was performed utilizing a library of 683 pure compounds derived primarily from terrestrial and marine fungi. An initial screen with a fixed concentration of 5 µM yielded 91 hits with inhibition comparable to an equal concentration of artemisinin. These compounds were then triaged based on known biological and chemical concerns and liabilities. From these, 49 prioritized compounds were tested in a dose response format with T. gondii and human foreskin fibroblasts (HFFs) for cytotoxicity. Ten compounds were identified with an IC50 less than 150 nM and a selectivity index (SI) greater than 100. An additional eight compounds demonstrated submicromolar IC50 and SI values equal to or greater than 35. While the majority of these scaffolds have been previously implicated against apicomplexan parasites, their activities in T. gondii were largely unknown. Herein, we report the T. gondii activity of these compounds with chemotypes including xanthoquinodins, peptaibols, heptelidic acid analogs, and fumagillin analogs, with multiple compounds demonstrating exceptional potency in T. gondii and limited toxicity to HFFs at the highest concentrations tested.

IMPORTANCE

Current therapeutics for treating toxoplasmosis remain insufficient, demonstrating high cytotoxicity, poor bioavailability, limited efficacy, and drug resistance. Additional research is needed to develop novel compounds with high efficacy and low cytotoxicity. The success of artemisinin and other natural products in treating malaria highlights the potential of natural products as anti-protozoan therapeutics. However, the exploration of natural products in T. gondii drug discovery has been less comprehensive, leaving untapped potential. By leveraging the resources available for the malaria drug discovery campaign, we conducted a phenotypic screen utilizing a set of natural products previously screened against Plasmodium falciparum. Our study revealed 18 compounds with high potency and low cytotoxicity in T. gondii, including four novel scaffolds with no previously reported activity in T. gondii. These new scaffolds may serve as starting points for the development of toxoplasmosis therapeutics but could also serve as tool compounds for target identification studies using chemogenomic approach.

Genomic and metabolomic diversity within a familial population of Aspergillus flavus

Aspergillus flavus is an agriculturally significant micro-fungus having potential to contaminate food and feed crops with toxic secondary metabolites such as aflatoxin (AF) and cyclopiazonic acid (CPA). Research has shown A. flavus strains can overcome heterokaryon incompatibility and undergo meiotic recombination as teleomorphs. Although evidence of recombination in the AF gene cluster has been reported, the impacts of recombination on genotype and metabolomic phenotype in a single generation are lacking. In previous studies, we paired an aflatoxigenic MAT1-1 A. flavus strain with a non-aflatoxigenic MAT1-2 A. flavus strain that had been tagged with green fluorescent protein and then 10 F1 progenies (a mix of fluorescent and non-fluorescent) were randomly selected from single-ascospore colonies and broadly examined for evidence of recombination. In this study, we determined four of those 10 F1 progenies were recombinants because they were not vegetatively compatible with either parent or their siblings, and they exhibited other distinctive traits that could only result from meiotic recombination. The other six progenies examined shared genomic identity with the non-aflatoxigenic, fluorescent, and MAT1-2 parent, but were metabolically distinct. This study highlights phenotypic and genomic changes that may occur in a single generation from the outcrossing of sexually compatible strains of A. flavus.

Antiplasmodial peptaibols act through membrane directed mechanisms

Our previous study identified 52 antiplasmodial peptaibols isolated from fungi. To understand their antiplasmodial mechanism of action, we conducted phenotypic assays, assessed the in vitro evolution of resistance, and performed a transcriptome analysis of the most potent peptaibol, HZ NPDG-I. HZ NPDG-I and 2 additional peptaibols were compared for their killing action and stage dependency, each showing a loss of digestive vacuole (DV) content via ultrastructural analysis. HZ NPDG-I demonstrated a stepwise increase in DV pH, impaired DV membrane permeability, and the ability to form ion channels upon reconstitution in planar membranes. This compound showed no signs of cross resistance to targets of current clinical candidates, and 3 independent lines evolved to resist HZ NPDG-I acquired nonsynonymous changes in the P. falciparum multidrug resistance transporter, pfmdr1. Conditional knockdown of PfMDR1 showed varying effects to other peptaibol analogs, suggesting differing sensitivity.

microbeMASST: a taxonomically informed mass spectrometry search tool for microbial metabolomics data

microbeMASST, a taxonomically informed mass spectrometry (MS) search tool, tackles limited microbial metabolite annotation in untargeted metabolomics experiments. Leveraging a curated database of >60,000 microbial monocultures, users can search known and unknown MS/MS spectra and link them to their respective microbial producers via MS/MS fragmentation patterns. Identification of microbe-derived metabolites and relative producers without a priori knowledge will vastly enhance the understanding of microorganisms' role in ecology and human health.

Persephacin Is a Broad-Spectrum Antifungal Aureobasidin Metabolite That Overcomes Intrinsic Resistance in Aspergillus fumigatus

Fungi pose a persistent threat to humankind with worrying indications that emerging and re-emerging pathogens (e.g., Candida auris, Coccidioides spp., drug-resistant Aspergilli, and more) exhibit resistance to the limited number of approved antifungals. To address this problem, our team is exploring endophytic fungi as a resource for the discovery of new antifungal natural products. The rationale behind this decision is based on evidence that endophytes engage with plants in mutualistic relationships wherein some fungi actively participate by producing chemical defense measures that suppress pathogenic microorganisms. To improve the odds of bioactive metabolite discovery, we developed a new hands-free laser-cutting system capable of generating >50 plant samples per minute that, in turn, enabled our team to prepare and screen large numbers of endophytic fungi. One of the fungal isolates obtained in this way was identified as an Elsinoë sp. that produced a unique aureobasidin analogue, persephacin (1). Some distinctive features of 1 are the absence of both phenylalanine residues combined with the incorporation of a novel amino acid residue, persephanine (9). Compound 1 exhibits potent antifungal effects against a large number of pathogenic yeast (including several clinical C. auris strains), as well as phylogenetically diverse filamentous fungi (e.g., Aspergillus fumigatus). In an ex vivo eye infection model, compound 1 outperformed standard-of-care treatments demonstrating the ability to suppress fluconazole-resistant Candida albicans and A. fumigatus at a concentration (0.1% solution) well below the clinically recommended levels used for fluconazole and natamycin (2% and 5% solutions, respectively). In 3D tissue models for acute dermal and ocular safety, 1 was found to be nontoxic and nonirritating at concentrations required to elicit antifungal activity. Natural product 1 appears to be a promising candidate for further investigation as a broad-spectrum antifungal capable of controlling a range of pathogens that negatively impact human, animal, and plant health.

A Taxonomically-informed Mass Spectrometry Search Tool for Microbial Metabolomics Data

MicrobeMASST, a taxonomically-informed mass spectrometry (MS) search tool, tackles limited microbial metabolite annotation in untargeted metabolomics experiments. Leveraging a curated database of >60,000 microbial monocultures, users can search known and unknown MS/MS spectra and link them to their respective microbial producers via MS/MS fragmentation patterns. Identification of microbial-derived metabolites and relative producers, without a priori knowledge, will vastly enhance the understanding of microorganisms' role in ecology and human health.

Resolving a Natural Product Cold Case: Elucidation of Fusapyrone Structure and Absolute Configuration and Demonstration of Their Fungal Biofilm Disrupting Properties

Fusapyrones are fungal metabolites, which have been reported to have broad-spectrum antibacterial and antifungal properties. Despite the first members of this chemical class being described three decades prior, many aspects of their structures have remained unresolved, thereby constraining efforts to fully understand structure-activity relationships within this metabolite family and impeding the design of streamlined syntheses. Among the main challenges posed by fusapyrones is the incorporation of several single and groups of stereocenters separated by atoms with freely rotating bonds, which have proven unyielding to spectroscopic analyses. In this study, we obtained a series of new (2-5 and 7-9) and previously reported fusapyrones (1 and 6), which were subjected to a combination of spectroscopic, chemical, and computational techniques enabling us to offer proposals for their full structures, as well as provide a pathway to reinterpreting the absolute configurations of other published fusapyrone metabolites. Biological testing of the fusapyrones revealed their abilities to inhibit and disrupt biofilms made by the human fungal pathogen, Candida albicans. These results show that fusapyrones reduce hyphae formation in C. albicans, as well as decrease the surface adherence capabilities of planktonic cells and cells transitioning into early-stage biofilm formation.

Appraisal of Fungus-Derived Xanthoquinodins as Broad-Spectrum Anti-Infectives Targeting Phylogenetically Diverse Human Pathogens

Xanthoquinodins make up a distinctive class of xanthone-anthraquinone heterodimers reported as secondary metabolites from several fungal species. Through a collaborative multi-institutional screening program, a fungal extract prepared from a Trichocladium sp. was identified that exhibited strong inhibitory effects against several human pathogens (Mycoplasma genitalium, Plasmodium falciparum, Cryptosporidium parvum, and Trichomonas vaginalis). This report focuses on one of the unique samples that exhibited a desirable combination of biological effects: namely, it inhibited all four test pathogens and demonstrated low levels of toxicity toward HepG2 (human liver) cells. Fractionation and purification of the bioactive components and their congeners led to the identification of six new compounds [xanthoquinodins NPDG A1-A5 (1-5) and B1 (6)] as well as several previously reported natural products (7-14). The chemical structures of 1-14 were determined based on interpretation of their 1D and 2D NMR, HRESIMS, and electronic circular dichroism (ECD) data. Biological testing of the purified metabolites revealed that they possessed widely varying levels of inhibitory activity against a panel of human pathogens. Xanthoquinodins A1 (7) and A2 (8) exhibited the most promising broad-spectrum inhibitory effects against M. genitalium (EC50 values: 0.13 and 0.12 μM, respectively), C. parvum (EC50 values: 5.2 and 3.5 μM, respectively), T. vaginalis (EC50 values: 3.9 and 6.8 μM, respectively), and P. falciparum (EC50 values: 0.29 and 0.50 μM, respectively) with no cytotoxicity detected at the highest concentration tested (HepG2 EC50 > 25 μM).

Identification of Fungus-Derived Natural Products as New Antigiardial Scaffolds

There is an unmet need for effective therapies for treating diseases associated with the intestinal parasite Giardia lamblia. In this study, a library of chemically validated purified natural products and fungal extracts was screened for chemical scaffolds that can inhibit the growth of G. lamblia. The phenotypic screen led to the identification of several previously unreported classes of natural product inhibitors that block the growth of G. lamblia. Hits from phenotypic screens of these naturally derived compounds are likely to possess a variety of mechanisms of action not associated with clinically used nitroimidazole and thiazolide compounds. They may therefore be effective against current drug-resistant parasite strains. IMPORTANCE There is a direct link between widespread prevalence of clinical giardiasis and poverty. This may be one of the reasons why giardiasis is a significant contributor to diarrheal morbidity, stunting, and death of children in resource-limited communities around the world. FDA-approved treatments for giardiasis include metronidazole, related nitroimidazole drugs, and albendazole. However, a substantial number of clinical infections are resistant to these treatments. The depth of the challenge is partly exacerbated by a lack of investment in the discovery and development of novel agents for treatment of giardiasis. Applicable interventions must include new drug development strategies that will result in the identification of effective therapeutics, particularly those that are inexpensive and can be quickly advanced to clinical uses, such as products from nature. This study identified novel chemical scaffolds from fungi that can form the basis of future medicinal chemistry optimization of novel antigiardial agents.

In Vitro Susceptibility and Resistance of Mycoplasma genitalium to Nitroimidazoles

Mycoplasma genitalium is a sexually transmitted reproductive tract pathogen of men and women. M. genitalium infections are increasingly difficult to treat due to poor efficacy of doxycycline and acquired resistance to azithromycin and moxifloxacin. A recent clinical trial suggested that metronidazole may improve cure rates for women with pelvic inflammatory disease and reduced the detection of M. genitalium when included with standard doxycycline plus ceftriaxone treatment. As data regarding susceptibility of mycoplasmas to nitroimidazoles are lacking in the scientific literature, we determined the in vitro susceptibility of 10 M. genitalium strains to metronidazole, secnidazole, and tinidazole. MICs ranged from 1.6 to 12.5 μg/mL for metronidazole, 3.1 to 12.5 μg/mL for secnidazole, and 0.8 to 6.3 μg/mL for tinidazole. None of these agents was synergistic with doxycycline in checkerboard broth microdilution assays. Tinidazole was superior to metronidazole and secnidazole in terms of MIC and time-kill kinetics and was bactericidal (>99.9% killing) at concentrations below reported serum concentrations. Mutations associated with nitroimidazole resistance were identified by whole-genome sequencing of spontaneous resistant mutants, suggesting a mechanism for reductive activation of the nitroimidazole prodrug by a predicted NAD(P)H-dependent flavin mononucleotide (FMN) oxidoreductase. The presence of oxygen did not affect MICs of wild-type M. genitalium, but a nitroimidazole-resistant mutant was defective for growth under anaerobic conditions, suggesting that resistant mutants may have a fitness disadvantage in anaerobic genital sites. Clinical studies are needed to determine if nitroimidazoles, especially tinidazole, are effective for eradicating M. genitalium infections in men and women.

Visualization of Electron Density Changes Along Chemical Reaction Pathways

We propose a simple procedure for visualizing the electron density changes (EDC) during a chemical reaction, which is based on a mapping of rectangular grid points for a stationary structure into (distorted) positions around atoms of another stationary structure. Specifically, during a small step along the minimum energy pathway (MEP), the displacement of each grid point is obtained as a linear combination of the motion of all atoms, with the contribution from each atom scaled by the corresponding Hirshfeld weight. For several reactions (identity SN2, Claisen rearrangement, Diels-Alder reaction, [3+2] cycloaddition, and phenylethyl mercaptan attack on pericosine A), our EDC plots showed an expected reduction of electron densities around severed bonds (or those with the bond-order lowered), with the opposite observed for newly-formed or enhanced chemical bonds. The EDC plots were also shown for copper triflate catalyzed N2O fragmentation, where the N-O bond weakening initially occurred on a singlet surface, but continued on a triplet surface after reaching the minimum-energy crossing point (MECP) between the two potential energy surfaces.

Tolypocladamides A-G: Cytotoxic Peptaibols from Tolypocladium inflatum

Seven new peptaibols named tolypocladamides A-G have been isolated from an extract of the fungus Tolypocladium inflatum, which inhibits the interaction between Raf and oncogenic Ras in a cell-based high-throughput screening assay. Each peptaibol contains 11 amino acid residues, an octanoyl or decanoyl fatty acid chain at the N-terminus, and a leucinol moiety at the C-terminus. The peptaibol sequences were elucidated on the basis of 2D NMR and mass spectral fragmentation analyses. Amino acid configurations were determined by advanced Marfey's analyses. Tolypocladamides A-G caused significant inhibition of Ras/Raf interactions with IC50 values ranging from 0.5 to 5.0 μM in a nanobioluminescence resonance energy transfer (NanoBRET) assay; however, no interactions were observed in a surface plasmon resonance assay for binding of the compounds to wild type or G12D mutant Ras constructs or to the Ras binding domain of Raf. NCI 60 cell line testing was also conducted, and little panel selectivity was observed.

Assessing Microbial Metabolic and Biological Diversity to Inform Natural Product Library Assembly

The pressing need for novel chemical matter to support bioactive compound discovery has led natural product researchers to explore a wide range of source organisms and environments. One of the implicit guiding principles behind those efforts is the notion that sampling different environments is critical to accessing unique natural products. This idea was tested by comparing fungi from disparate biomes: aquatic sediments from Lake Michigan (USA) and terrestrial samples taken from the surrounding soils. Matched sets of Penicillium brevicompactum, Penicillium expansum, and Penicillium oxalicum from the two source environments were compared, revealing modest differences in physiological performance and chemical output. Analysis of LC-MS/MS-derived molecular feature data showed no source-dependent differences in chemical richness. High levels of scaffold homogeneity were also observed with 78-83% of scaffolds shared among the terrestrial and aquatic Penicillium spp. isolates. A comparison of the culturable fungi from the two biomes indicated that certain genera were more strongly associated with aquatic sediments (e.g., Trichoderma, Pseudeurotium, Cladosporium, and Preussia) versus the surrounding terrestrial environment (e.g., Fusarium, Pseudogymnoascus, Humicola, and Acremonium). Taken together, these results suggest that focusing efforts on sampling the microbial resources that are unique to an environment may have a more pronounced effect on enhancing the sought-after natural product diversity needed for chemical discovery and screening collections.

Dried Plum's Polyphenolic Compounds and Carbohydrates Contribute to Its Osteoprotective Effects and Exhibit Prebiotic Activity in Estrogen Deficient C57BL/6 Mice

Evidence of dried plum’s benefits on bone continues to emerge. This study investigated the contribution of the fruit’s polyphenol (PP) and carbohydrate (CHO) components on a bone model of postmenopausal osteoporosis to explore their prebiotic activity. Osteopenic ovariectomized mice were fed diets supplemented with dried plum, a crude extract of dried plum’s polyphenolic compounds, or the PP or CHO fraction of the crude extract. The effects of treatments on the bone phenotype were assessed at 5 and 10 weeks as well as the prebiotic activity of the different components of dried plum. Both the CHO and PP fractions of the extract contributed to the effects on bone with the CHO suppressing bone formation and resorption, and the PP temporally down-regulating formation. The PP and CHO components also altered the gut microbiota and cecal short chain fatty acids. These findings demonstrate that the CHO as well as the PP components of dried plum have potential prebiotic activity, but they have differential roles in mediating the alterations in bone formation and resorption that protect bone in estrogen deficiency.

Identification of Leucinostatins from Ophiocordyceps sp. as Antiparasitic Agents against Trypanosoma cruzi

Safe and effective treatments for Chagas disease, a potentially fatal parasitic infection associated with cardiac and gastrointestinal pathology and caused by the kinetoplastid parasite Trypanosoma cruzi, have yet to be developed. Benznidazole and nifurtimox, which are currently the only available drugs against T. cruzi, are associated with severe adverse effects and questionable efficacy in the late stage of the disease. Natural products have proven to be a rich source of new chemotypes for other infectious agents. We utilized a microscopy-based high-throughput phenotypic screen to identify inhibitors of T. cruzi from a library of natural product samples obtained from fungi procured through a Citizen Science Soil Collection Program (https://whatsinyourbackyard.org/) and the Great Lakes (USA) benthic environment. We identified five leucinostatins (A, B, F, NPDG C, and NPDG D) as potent inhibitors of the intracellular amastigote form of T. cruzi. Leucinostatin B also showed in vivo suppression of T. cruzi in a mouse model of Chagas disease. Given prior reports that leucinostatins A and B have antiparasitic activity against the related kinetoplastid Trypanosoma brucei, our findings suggest a potential cross-trypanocidal compound class and provide a platform for the further chemical derivatization of a potent chemical scaffold against T. cruzi.

Altertoxin II, a Highly Effective and Specific Compound against Ewing Sarcoma

A screening program designed to identify natural products with selective cytotoxic effects against cell lines representing different types of pediatric solid tumors led to the identification of altertoxin II as a highly potent and selective cytotoxin against Ewing sarcoma cell lines. Altertoxin II, but not the related compounds altertoxin I and alteichin, was highly effective against every Ewing sarcoma cell line tested, with an average 25-fold selectivity for these cells as compared to cells representing other pediatric and adult cancers. Mechanism of action studies revealed that altertoxin II causes DNA double-strand breaks, a rapid DNA damage response, and cell cycle accumulation in the S phase. Our studies also demonstrate that the potent effects of altertoxin II are partially dependent on the progression through the cell cycle, because the G₁ arrest initiated by a CDK4/6 inhibitor decreased antiproliferative potency more than 10 times. Importantly, the cell-type-selective DNA-damaging effects of altertoxin II in Ewing sarcoma cells occur independently of its ability to bind directly to DNA. Ultimately, we found that altertoxin II has a dose-dependent in vivo antitumor efficacy against a Ewing sarcoma xenograft, suggesting that it has potential as a therapeutic drug lead and will be useful to identify novel targets for Ewing-sarcoma-specific therapies.

Building Natural Product Libraries Using Quantitative Clade-Based and Chemical Clustering Strategies

The success of natural product-based drug discovery is predicated on having chemical collections that offer broad coverage of metabolite diversity. We propose a simple set of tools combining genetic barcoding and metabolomics to help investigators build natural product libraries aimed at achieving predetermined levels of chemical coverage. It was found that such tools aided in identifying overlooked pockets of chemical diversity within taxa, which could be useful for refocusing collection strategies. We have used fungal isolates identified as Alternaria from a citizen-science-based soil collection to demonstrate the application of these tools for assessing and carrying out predictive measurements of chemical diversity in a natural product collection. Within Alternaria, different subclades were found to contain nonequivalent levels of chemical diversity. It was also determined that a surprisingly modest number of isolates (195 isolates) was sufficient to afford nearly 99% of Alternaria chemical features in the data set. However, this result must be considered in the context that 17.9% of chemical features appeared in single isolates, suggesting that fungi like Alternaria might be engaged in an ongoing process of actively exploring nature's metabolic landscape. Our results demonstrate that combining modest investments in securing internal transcribed spacer (ITS)-based sequence information (i.e., establishing gene-based clades) with data from liquid chromatography-mass spectrometry (i.e., generating feature accumulation curves) offers a useful route to obtaining actionable insights into chemical diversity coverage trends in a natural product library. It is anticipated that these outcomes could be used to improve opportunities for accessing bioactive molecules that serve as the cornerstone of natural product-based drug discovery. IMPORTANCE Natural product drug discovery efforts rely on libraries of organisms to provide access to diverse pools of compounds. Actionable strategies to rationally maximize chemical diversity, rather than relying on serendipity, can add value to such efforts. Readily implementable biological (i.e., ITS sequence analysis) and chemical (i.e., mass spectrometry-based feature and scaffold measurements) diversity assessment tools can be employed to monitor and adjust library development tactics in real time. In summary, metabolomics-driven technologies and simple gene-based specimen barcoding approaches have broad applicability to building chemically diverse natural product libraries.

Cyclic Tetrapeptide HDAC Inhibitors with Improved Plasmodium falciparum Selectivity and Killing Profile

Cyclic tetrapeptide histone deacetylase inhibitors represent a promising class of antiplasmodial agents that epigenetically disrupt a wide range of cellular processes in Plasmodium falciparum. Unfortunately, certain limitations, including reversible killing effects and host cell toxicity, prevented these inhibitors from further development and clinical use as antimalarials. In this study, we present a series of cyclic tetrapeptide analogues derived primarily from the fungus Wardomyces dimerus that inhibit P. falciparum with low nanomolar potency and high selectivity. This cyclic tetrapeptide scaffold was diversified further via semisynthesis, leading to the identification of several key structural changes that positively impacted the selectivity, potency, and in vitro killing profiles of these compounds. We confirmed their effectiveness as HDAC inhibitors through the inhibition of PfHDAC1 catalytic activity, in silico modeling, and the hyperacetylation of histone H4. Additional analysis revealed the in vitro inhibition of the most active epoxide-containing analogue was plasmodistatic, exhibiting reversible inhibitory effects upon compound withdrawal after 24 or 48 h. In contrast, one of the new diacetyloxy semisynthetic analogues, CTP-NPDG 19, displayed a rapid and irreversible action against the parasite following compound exposure for 24 h.

Identification of natural product modulators of Merkel cell carcinoma cell growth and survival

Merkel cell carcinoma (MCC) is a rare, but aggressive skin cancer the incidence of which has increased significantly in recent years. The majority of MCCs have incorporated Merkel cell polyomavirus (VP-MCC) while the remainder are virus-negative (VN-MCC). Although a variety of therapeutic options have shown promise in treating MCC, there remains a need for additional therapeutics as well as probes for better understanding MCC. A high-throughput screening campaign was used to assess the ability of > 25,000 synthetic and natural product compounds as well as > 20,000 natural product extracts to affect growth and survival of VN-MCC and VP-MCC cell lines. Sixteen active compounds were identified that have mechanisms of action reported in the literature along with a number of compounds with unknown mechanisms. Screening results with pure compounds suggest a range of potential targets for MCC including DNA damage, inhibition of DNA or protein synthesis, reactive oxygen species, and proteasome inhibition as well as NFκB inhibition while also suggesting the importance of zinc and/or copper binding. Many of the active compounds, particularly some of the natural products, have multiple reported targets suggesting that this strategy might be a particularly fruitful approach. Processing of several active natural product extracts resulted in the identification of additional MCC-active compounds. Based on these results, further investigations focused on natural products sources, particularly of fungal origin, are expected to yield further potentially useful modulators of MCC.

Roseabol A, a New Peptaibol from the Fungus Clonostachys rosea

A new 11 amino acid linear peptide named roseabol A (1) and the known compound 13-oxo-trans-9,10-epoxy-11(E)-octadecenoic acid (2) were isolated from the fungus Clonostachys rosea. Combined NMR and MS analysis revealed that roseabol A (1) contained amino acid residues characteristic of the peptaibol family of peptides such as isovaline, α-aminoisobutyric acid, hydroxyproline, leucinol, and an N-terminal isovaleric acid moiety. The amino acid sequence was established by a combination of NMR studies and tandem MS fragmentation analyses, and the absolute configurations of the constituent amino acids of 1 were determined by the advanced Marfey’s method. Compound 2 showed inhibitory activity against Merkel cell carcinoma, a rare and difficult-to-treat type of skin cancer, with an IC₅₀ value of 16.5 μM.

Yuanhuacine Is a Potent and Selective Inhibitor of the Basal-Like 2 Subtype of Triple Negative Breast Cancer with Immunogenic Potential

The heterogeneity of triple negative breast cancer (TNBC) has led to efforts to further subtype this disease with the hope of identifying new molecular liabilities and drug targets. Furthermore, the finding that TNBC is the most inherently immunogenic type of breast cancer provides the potential for effective treatment with immune checkpoint inhibitors and immune adjuvants. Thus, we devised a dual screen to identify compounds from natural product extracts with TNBC subtype selectivity that also promote the expression of cytokines associated with antitumor immunity. These efforts led to the identification of yuanhuacine (1) as a potent and highly selective inhibitor of the basal-like 2 (BL2) subtype of TNBC that also promoted an antitumor associated cytokine signature in immune cells. The mechanism of action of yuanhuacine for both phenotypes depends on activation of protein kinase C (PKC), defining a novel target for the treatment of this clinical TNBC subtype. Yuanhuacine showed potent antitumor efficacy in animals bearing BL2 tumors further demonstrating that PKC could function as a potential pharmacological target for the treatment of the BL2 subtype of TNBC.

An Integrated Strategy for the Detection, Dereplication, and Identification of DNA-Binding Biomolecules from Complex Natural Product Mixtures

A fundamental factor in natural product drug discovery programs is the necessity to identify the active component(s) from complex chemical mixtures. Whereas this has traditionally been accomplished using bioassay-guided fractionation, we questioned whether alternative techniques could supplement and, in some cases, even supplant this approach. We speculated that a combination of ligand-fishing methods and modern analytical tools (e.g., LC-MS and online natural product databases) offered a route to enhance natural product drug discovery. Herein, a candidate solution referred to as the lickety-split ligand-affinity-based molecular angling system (LLAMAS) is described. This approach utilizes an ultrafiltration-based LC-PDA-MS/MS-guided DNA-binding assay in combination with the (i) Global Natural Products Social Molecular Networking, (ii) Dictionary of Natural Products, and (iii) SciFinder platforms to identify DNA binders in complex chemical mixtures. LLAMAS was initially vetted in tests using known small-molecule DNA binders and then optimized to a 96-well plate-based format. A set of 332 plant samples used in traditional Chinese medicine was screened for DNA-binding activity with LLAMAS, resulting in the identification of seven DNA-binding molecules, including berberine (12), palmatine (13), coptisine (14), fangchinoline (15), tetrandrine (16), daurisoline (17), and dauricine (18). These results demonstrate that LLAMAS is an effective natural product discovery platform for the efficient identification and dereplication of DNA-binding molecules from complex mixtures.

Leveraging Peptaibol Biosynthetic Promiscuity for Next-Generation Antiplasmodial Therapeutics

Malaria remains a worldwide threat, afflicting over 200 million people each year. The emergence of drug resistance against existing therapeutics threatens to destabilize global efforts aimed at controlling Plasmodium spp. parasites, which is expected to leave vast portions of humanity unprotected against the disease. To address this need, systematic testing of a fungal natural product extract library assembled through the University of Oklahoma Citizen Science Soil Collection Program has generated an initial set of bioactive extracts that exhibit potent antiplasmodial activity (EC₅₀ < 0.30 μg/mL) and low levels of toxicity against human cells (less than 50% reduction in HepG2 growth at 25 μg/mL). Analysis of the two top-performing extracts from Trichoderma sp. and Hypocrea sp. isolates revealed both contained chemically diverse assemblages of putative peptaibol-like compounds that were responsible for their antiplasmodial actions. Purification and structure determination efforts yielded 30 new peptaibols and lipopeptaibols (1-14 and 28-43), along with 22 known metabolites (15-27 and 44-52). While several compounds displayed promising activity profiles, one of the new metabolites, harzianin NPDG I (14), stood out from the others due to its noteworthy potency (EC₅₀ = 0.10 μM against multi-drug-resistant P. falciparum line Dd2) and absence of gross toxicity toward HepG2 at the highest concentrations tested (HepG2 EC₅₀ > 25 μM, selectivity index > 250). The unique chemodiversity afforded by these fungal isolates serves to unlock new opportunities for translating peptaibols into a bioactive scaffold worthy of further development.

CRISPR-Cas9 Genome-Wide Knockout Screen Identifies Mechanism of Selective Activity of Dehydrofalcarinol in Mesenchymal Stem-like Triple-Negative Breast Cancer Cells

There are no targeted therapies available for triple-negative breast cancers (TNBCs) in part because they represent a heterogeneous group of tumors with diverse oncogenic drivers. Our goal is to identify targeted therapies for subtypes of these cancers using a mechanism-blind screen of natural product extract libraries. An extract from Desmanthodium guatemalense was 4-fold more potent for cytotoxicity against MDA-MB-231 cells, which represent the mesenchymal stem-like (MSL) subtype, as compared to cells of other TNBC subtypes. Bioassay-guided fractionation led to the isolation of six polyacetylenes, and subsequent investigations of plant sources known to produce polyacetylenes yielded six additional structurally related compounds. A subset of these compounds retained selective cytotoxic effects in MSL subtype cells. Studies suggest that these selective effects do not appear to be due to PPARγ agonist activities that have previously been reported for polyacetylenes. A CRISPR-Cas9-mediated gene knockout screen was employed to identify the mechanism of selective cytotoxic activity of the most potent and selective compound, dehydrofalcarinol (1a). This genomic screen identified HSD17B11, the gene encoding the enzyme 17β-hydroxysteroid dehydrogenase type 11, as a mediator of the selective cytotoxic effects of 1a in MDA-MB-231 cells that express high levels of this protein. The Project Achilles cancer dependency database further identified a subset of Ewing sarcoma cell lines as highly dependent on HSD17B11 expression, and it was found these were also highly sensitive to 1a. This report demonstrates the value of CRISPR-Cas9 genome-wide screens to identify the mechanisms underlying the selective activities of natural products.

Abstract 1786: High expression of NCX-1 in triple negative breast cancer cell lines identifies a potential biomarker for sensitivity to cardiac glycosides

Triple negative breast cancers (TNBCs) represent 15-20% of all breast cancers and are defined by lack of the estrogen and progesterone receptors and HER2 gene amplification. Therefore, TNBC patients do not benefit from available therapies targeting these receptors. The significant heterogeneity of TNBC precludes the expectation that a single molecular target will be identified for this disease. Our goal is to identify compounds with selective cytotoxic effects in subsets of TNBC cells to identify new therapeutic targets for TNBC subgroups. Our studies show that the TNBC cell lines BT-549 and Hs578T are distinct from other TNBC cells in that they are highly sensitive to 9 cardiac glycosides/cardenolides isolated from Calotropis gigantea as well as digoxin. BT-549 cells are 15-times more sensitive to the isolated cardiac glycoside calotropin and 9-times more sensitive to digoxin as compared to MDA-MB-231 TNBC cells. Cardiac glycosides bind to the Na+/K+ ATPase and inhibit its activity, leading to increased intracellular Na+, which results in higher intracellular Ca2+ through reversal of the Na+/Ca2+ exchanger (NCX). Notably, among the 10 cardiac glycosides/cardenolides, there was a significant correlation between potency for inhibition of purified Na+/K+ ATPase and cytotoxic potency and selectivity for BT-549 cells. These data suggest that inhibition of Na+/K+ ATPase is critical for the selective cytotoxic effects of these compounds. BT-549 and Hs578T cells express high levels of non-specific TRPC1/4 cation channels1 and low levels of SERCA2 Ca2+ pumps2 compared to other TNBC cells, suggesting that they have impaired ability to handle intracellular Ca2+. Consistent with this hypothesis, within 3 h, concentrations of calotropin and digoxin that are selectivity cytotoxic each initiated a significant increase in intracellular Ca2+ in BT-549 cells. To test whether sensitivity to cardiac glycosides was due to higher Na+/K+ ATPase expression, message and protein levels were evaluated in 10 TNBC cell lines. The results show that there is no correlation between levels of Na+/K+ ATPase and sensitivity to cardiac glycosides. In contrast, NCX-1 expression was found to be 120 and 60-fold higher in the sensitive BT-549 and Hs578T cells, respectively, as compared to the resistant TNBC cell lines. Ongoing studies are evaluating whether genetic manipulation of NCX-1 is sufficient to modulate sensitivity to cardiac glycosides. These findings demonstrate that NCX-1 expression is associated with the selective sensitivity of a subgroup of TNBC cell lines to the cytotoxic effects of cardiac glycosides. Our results suggest that NCX-1 could be a biomarker to identify TNBC patients that could ultimately benefit from use of a cardiac glycoside for anticancer indications.

1. Grant CV, et al. Breast Cancer Res Treat. 2019;177(2):345.

2. Varga K, et al. BMC Cancer. 2018;18(1):1029.

Citation Format: Petra E. Pederson, Shengxin Cai, Chase M. Carver, Tanja Grkovic, Barry R. O'Keefe, April L. Risinger, Robert H. Cichewicz, Susan L. Mooberry. High expression of NCX-1 in triple negative breast cancer cell lines identifies a potential biomarker for sensitivity to cardiac glycosides [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1786.

Triple-Negative Breast Cancer Cells Exhibit Differential Sensitivity to Cardenolides from Calotropis gigantea

Triple-negative breast cancers (TNBC) are aggressive and heterogeneous cancers that lack targeted therapies. We implemented a screening program to identify new leads for subgroups of TNBC using diverse cell lines with different molecular drivers. Through this program, we identified an extract from Calotropis gigantea that caused selective cytotoxicity in BT-549 cells as compared to four other TNBC cell lines. Bioassay-guided fractionation of the BT-549 selective extract yielded nine cardenolides responsible for the selective activity. These included eight known cardenolides and a new cardenolide glycoside. Structure-activity relationships among the cardenolides demonstrated a correlation between their relative potencies toward BT-549 cells and Na+/K+ ATPase inhibition. Calotropin, the compound with the highest degree of selectivity for BT-549 cells, increased intracellular Ca2+ in sensitive cells to a greater extent than in the resistant MDA-MB-231 cells. Further studies identified a second TNBC cell line, Hs578T, that is also highly sensitive to the cardenolides, and mechanistic studies were conducted to identify commonalities among the sensitive cell lines. Experiments showed that both cardenolide-sensitive cell lines expressed higher mRNA levels of the Na+/Ca2+ exchanger NCX1 than resistant TNBC cells. This suggests that NCX1 could be a biomarker to identify TNBC patients that might benefit from the clinical administration of a cardiac glycoside for anticancer indications.

Leucinostatins from Ophiocordyceps spp. and Purpureocillium spp. Demonstrate Selective Antiproliferative Effects in Cells Representing the Luminal Androgen Receptor Subtype of Triple Negative Breast Cancer

The structures of four leucinostatin analogues (1-4) from Ophiocordyceps spp. and Purpureocillium spp. were determined together with six known leucinostatins [leucinostatins B (5), A (6), B2 (7), A2 (8), F (9), and D (10)]. The structures of the metabolites were established using a combination of analytical methods including HRESIMS and MS/MS experiments, 1D and 2D NMR spectroscopy, chiral HPLC, and advanced Marfey's analysis of the acid hydrolysate, as well as additional empirical and chemical methods. Compounds 1-10 were evaluated for their biological effects on triple negative breast cancer (TNBC) cells. Leucinostatins 1-10 showed selective cytostatic activities in MDA-MB-453 and SUM185PE cells representing the luminal androgen receptor subtype of TNBC. This selective activity motivated further investigation into the mechanism of action of leucinostatin B (5). The results demonstrate that this peptidic fungal metabolite rapidly inhibits mTORC1 signaling in leucinostatin-sensitive TNBC cell lines, but not in leucinostatin-resistant cells. Leucinostatins have been shown to repress mitochondrial respiration through inhibition of the ATP synthase, and we demonstrated that both the mTORC1 signaling and LAR-selective activities of 5 were recapitulated by oligomycin. Thus, inhibition of the ATP synthase with either leucinostatin B or oligomycin is sufficient to selectively impede mTORC1 signaling and inhibit the growth of LAR-subtype cells.

Structure elucidation and absolute configuration of metabolites from the soil-derived fungus Dictyosporium digitatum using spectroscopic and computational methods

Following the discovery of a new class of compounds that inhibit the mucosa-associated lymphoid tissue lymphoma translocation 1 (MALT1) protease in a prior study, further chemical investigation of the Dictyosporium digitatum fungus resulted in the identification of 16 additional metabolites, including 12 undescribed compounds (1-12). The constitution and relative configuration of these new molecules were established by comprehensive NMR and HRMS analyses. Their absolute configurations were determined by employing Mosher's ester analysis and TDDFT ECD calculations. Two sesquiterpenes, dictyosporins A (1) and B (2), possess an undescribed eudesmen-type of structural scaffold. The ability of the isolated compounds to inhibit MALT1 proteolytic activity was evaluated, but none of them exhibited significant inhibition.

Using the Cancer Dependency Map to Identify the Mechanism of Action of a Cytotoxic Alkenyl Derivative from the Fruit of Choerospondias axillaris

An extract prepared from the fruit of Choerospondias axillaris exhibited differential cytotoxic effects when tested in a panel of pediatric cancer cell lines [Ewing sarcoma (A-673), rhabdomyosarcoma (SJCRH30), medulloblastoma (D283), and hepatoblastoma (Hep293TT)]. Bioassay-guided fractionation led to the purification of five new hydroquinone-based metabolites, choerosponols A-E (1-5), bearing unsaturated hydrocarbon chains. The structures of the natural products were determined using a combination of 1D and 2D NMR, HRESIMS, ECD spectroscopy, and Mosher ester analyses. The purified compounds were evaluated for their antiproliferative and cytotoxic activities, revealing that 1, which contains a benzofuran moiety, exhibited over 50-fold selective antiproliferative activity against Ewing sarcoma and medulloblastoma cells with growth inhibitory (GI50) values of 0.19 and 0.07 μM, respectively. The effects of 1 were evaluated in a larger panel of cancer cell lines, and these data were used in turn to interrogate the Project Achilles cancer dependency database, leading to the identification of the MCT1 transporter as a functional target of 1. These data highlight the utility of publicly available cancer dependency databases such as Project Achilles to facilitate the identification of the mechanisms of action of compounds with selective activities among cancer cell lines, which can be a major challenge in natural products drug discovery.

Local Phenomena Shape Backyard Soil Metabolite Composition

Soil covers most of Earth's continental surface and is fundamental to life-sustaining processes such as agriculture. Given its rich biodiversity, soil is also a major source for natural product drug discovery from soil microorganisms. However, the study of the soil small molecule profile has been challenging due to the complexity and heterogeneity of this matrix. In this study, we implemented high-resolution liquid chromatography-tandem mass spectrometry and large-scale data analysis tools such as molecular networking to characterize the relative contributions of city, state and regional processes on backyard soil metabolite composition, in 188 soil samples collected from 14 USA States, representing five USA climate regions. We observed that region, state and city of collection all influence the overall soil metabolite profile. However, many metabolites were only detected in unique sites, indicating that uniquely local phenomena also influence the backyard soil environment, with both human-derived and naturally-produced (plant-derived, microbially-derived) metabolites identified. Overall, these findings are helping to define the processes that shape the backyard soil metabolite composition, while also highlighting the need for expanded metabolomic studies of this complex environment.

Cholinesterase Inhibitory Arisugacins L-Q from a Penicillium sp. Isolate Obtained through a Citizen Science Initiative and Their Activities in a Phenotype-Based Zebrafish Assay

Phenotype-based screening of a fungal extract library yielded an active sample from a Penicillium sp. isolate that impaired zebrafish motility. Bioassay-guided purification led to the identification of 14 meroterpenoids including six new metabolites, arisugacins L-Q (4, 5, 8, and 12-14), seven known arisugacins (1-3, 6, 7, 9, and 10), and one known terreulactone (11). Their structures were determined using a combination of NMR and HRESIMS data, evidence secured from theoretical and experimental ECD spectra, and the modified Mosher's method. The purified compounds were tested in zebrafish embryos, as well as in vitro for cholinesterase inhibition activities. Compound 12 produced defects in myotome structure (metameric muscle, which is critical for locomotion) in vivo and showed the most potent and selective acetylcholinesterase inhibitory activity with an IC₅₀ of 191 nM in vitro. The phenotype assay was also used to reveal bioactivities for several previously reported arisugacins, which had failed to show activity in prior cell-based and in vitro testing. This study demonstrates that utilization of the zebrafish phenotype assay is an effective approach for the identification of bioactive extracts, is compatible with the bioassay-guided compound purification strategies, and offers a valuable tool for probing complex natural product sources to detect bioactive small molecules with potential therapeutic or other commercial applications.

Design and Application of a High-Throughput, High-Content Screening System for Natural Product Inhibitors of the Human Parasite Trichomonas vaginalis

It is estimated that Trichomonas vaginalis affects an astonishing 3.9% of the world's population, and while many of those infected are asymptomatic, progression of the disease can lead to serious health problems. Currently, the nitroimidazoles constitute the only drug class approved to treat trichomoniasis in the United States, which makes the spread of drug resistance a realistic concern. We developed a new image-based, high-throughput, and high-content assay for testing natural products (purified compounds and extracts) for antitrichomonal activity. Applying this assay system to a library of fungal natural product extracts led to the identification of three general classes of natural product inhibitors that exhibited moderate to strong activities against T. vaginalis: anthraquinones, xanthone-anthraquinone heterodimers, and decalin-linked tetramic-acid-containing metabolites. The tetramate natural products emerged as the most promising candidate molecules with pyrrolocin A (51) exhibiting potent activity against the parasite (EC50 = 60 nM), yet this metabolite showed limited toxicity to mammalian cell lines (selectivity index values of 100 and 167 versus 3T3 fibroblast and Ect1 normal cervical cells, respectively). The imaging-based assay system is a powerful tool for the bioassay-guided purification of single-component antitrichomonal biomolecules from complex natural product mixtures.

An Electrophilic Natural Product Provides a Safe and Robust Odor Neutralization Approach To Counteract Malodorous Organosulfur Metabolites Encountered in Skunk Spray

The anal secretions of skunks comprise several types of malodorous organosulfur compounds. The pungent metabolites are used defensively by skunks to repel threats posed by predators, and in many parts of the world, those perceived threats include humans and their pets. The extremely low thresholds for detection of the organosulfur metabolites make efforts to "de-skunk" people, animals, and clothing a process fraught with many challenges. The fungal-derived metabolite pericosine A (4) is a promiscuous yet stabile electrophilic compound that we propose is used by some fungi as a novel form of chemical defense. Our investigations have indicated that pericosine A readily reacts with skunk-spray secretions to transform them into odorless products. Mechanistic and computational studies suggested that pericosine A and its synthetic analogues react via S_N2'-type mechanisms with thiols and thioacetates under aqueous conditions to generate stable thioethers. Testing revealed that pericosine A did not cause skin or eye irritation and was highly effective at deodorizing skunk anal gland secretions when formulated to include adjunctive cosmetic ingredients.

Abstract 2141: Mechanistic studies of altertoxin II: A fungal metabolite with selective activity against Ewing sarcoma

Ewing sarcomas (ES) are bone and soft tissue tumors that affect children and adolescents. These cancers are driven by chromosomal translocations that most frequently result in expression of the abnormal EWSR1-FLI1 fusion protein, a transcriptional activator and modulator of RNA splicing, or similar fusion proteins. Despite significant advances in therapies for pediatric hematological malignancies over the past 30 years, there has been less progress in the development of curative therapies for advanced or metastatic pediatric solid tumors. Approximately 35% of ES are resistant to standard-of-care chemotherapeutic agents, and nearly 70% of ES patients with metastatic disease at the time of diagnosis die within 5 years. Therefore, there is a need to develop new, targeted therapies for ES that improve patient survival and decrease toxicity. We performed high-content phenotypic screens of natural product libraries to identify compounds with selective actions against pediatric solid tumor types and found that the fungal metabolite altertoxin II (ATXII) has highly selective cytotoxic activity against 6 ES cells lines compared to a panel of other pediatric and adult cancer cell lines. On average, the IC50 of ATXII is 94-fold higher (range 16- to 400-fold) in rhabdomyosarcoma (RMS) cells than in ES cells, indicating high selectivity for ES. Mechanistic studies were conducted to determine this compound’s mechanisms of actions in ES cells, with the goal of identifying new molecular targets for the treatment of ES. We found that 100 nM ATXII, a concentration that has no effect on RMS cell viability, induces cell cycle accumulation in ES cells in the S and G2/M phases, and rapid phosphorylation of histone H2A.X (gamma-H2A.X) and checkpoint kinase 1, suggesting ATXII selectively induces cell cycle arrest and DNA damage in ES cells. To determine if ES sensitivity to ATXII is a dominant or recessive phenotype, we hybridized sensitive EW8 cells (ES) with resistant Rh30 cells (RMS) and determined the sensitivity of these hybrids to ATXII. Five single-cell subclones derived from these hybrids were resistant to ATXII, suggesting that sensitivity to ATXII is a recessive phenotype in ES cells. Ongoing studies are aimed at determining the cellular uptake and binding kinetics of radiolabeled ATXII and identifying genes that mediate sensitivity and resistance to this compound by performing a genome-wide CRISPR/Cas9 knockout screen.

Citation Format: Andrew J. Robles, Wentao Dai, Robert H. Cichewicz, Susan L. Mooberry, Peter J. Houghton. Mechanistic studies of altertoxin II: A fungal metabolite with selective activity against Ewing sarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2141.

Natural-Product-Inspired Compounds as Countermeasures against the Liver Carcinogen Aflatoxin B1

Aflatoxin B₁ (AfB₁) ranks among the most potent liver carcinogens known, and the accidental or intentional exposure of humans and livestock to this toxin remains a serious global threat. One protective measure that had been proposed is employing small-molecule therapeutics capable of mitigating the toxicity of AfB₁; however, to date, these efforts have had little clinical success. To identify molecular scaffolds that reduce the toxicity of AfB₁, we developed a cell-based high-throughput high-content imaging assay that enabled our team to test natural products (pure compounds, fractions, and extracts) for protection of monolayers and spheroids composed of HepG2 liver cells against AfB₁. The spheroid assay showed notable potential for further development, as it afforded greater sensitivity of HepG2 cells to AfB1, which is believed to better mimic the in vivo response of hepatocytes to the toxin. One of the most bioactive compounds to arise from this investigation was alternariol-9-methyl ether (1, purified from an Alternaria sp. isolate), which inspired the synthesis and testing of several structurally related molecules. Based on these findings, it is proposed that several types of natural and synthetic polyarene molecules that have undergone oxidative functionalization (e.g., compounds containing 3-methoxyphenol moieties) are promising starting points for the development of new agents that protect against AfB₁ toxicity.

Anacolosins A-F and Corymbulosins X and Y, Clerodane Diterpenes from Anacolosa clarkii Exhibiting Cytotoxicity toward Pediatric Cancer Cell Lines

An extract of the plant Anacolosa clarkii was obtained from the NCI Natural Products Repository, and it showed cytotoxic activity toward several types of pediatric solid tumor cell lines. Bioassay-guided fractionation led to the purification of eight new clerodane diterpenes [anacolosins A-F (1-6) and corymbulosins X and Y (7 and 8)] and two known compounds (9 and 10) that contained an isozuelanin skeleton. The structures of the new natural products were determined using 1D and 2D NMR and HRESIMS data, while the relative and absolute configurations of the compounds were assessed using a combination of ¹H NMR coupling constant data, ROESY experiments, ECD (electronic circular dichroism) and VCD (vibrational circular dichroism) spectroscopy, chemical methods (including Mosher and 2-naphthacyl esterification), and chiral HPLC analyses. The purified natural products exhibited a range of cytotoxic activities against cell lines representing four pediatric cancer types (i.e., rhabdomyosarcoma, Ewing sarcoma, medulloblastoma, and hepatoblastoma) with total growth inhibitory (TGI) values in the range 0.2-4.1 μM. The rhabdomyosarcoma and medulloblastoma cell lines showed higher sensitivity to compounds 1-4, which are the first compounds reported to contain an isozuelanin skeleton and feature keto carbonyl groups at the C-6 positions. In contrast, the hepatoblastoma cell line was modestly more sensitive to 7-10, which contained a C-6 hydroxy group moiety.

In Situ Ring Contraction and Transformation of the Rhizoxin Macrocycle through an Abiotic Pathway

A Rhizopus sp. culture containing an endosymbiont partner ( Burkholderia sp.) was obtained through a citizen-science-based soil-collection program. An extract prepared from the pair of organisms exhibited strong inhibition of Ewing sarcoma cells and was selected for bioassay-guided fractionation. This led to the purification of rhizoxin (1), a potent antimitotic agent that inhibited microtubule polymerization, along with several new (2-5) and known (6) analogues of 1. The structures of 2-6 were established using a combination of NMR data analysis, while the configurations of the new stereocenters were determined using ROESY spectroscopy and comparison of GIAO-derived and experimental data for NMR chemical shift and ³ J_HH coupling values. Whereas compound 1 showed modest selectivity for Ewing sarcoma cell lines carrying the EWSR1/ FLI1 fusion gene, the other compounds were determined to be inactive. Chemically, compound 2 stands out from other rhizoxin analogues because it is the first member of this class that is reported to contain a one-carbon-smaller 15-membered macrolactone system. Through a combination of experimental and computational tests, we determined that 2 is likely formed via an acid-catalyzed Meinwald rearrangement from 1 because of the mild acidic culture environment created by the Rhizopus sp. isolate and its symbiont.

Identification of C-6 as a New Site for Linker Conjugation to the Taccalonolide Microtubule Stabilizers

The taccalonolides are a class of microtubule stabilizers that circumvent clinically relevant forms of drug resistance due to their unique mechanism of microtubule stabilization imparted by the covalent binding of the C-22-C-23 epoxide moiety to tubulin. A taccalonolide (8) with a fluorescein group attached with a linker at C-6 was generated, and biochemical and cell-based assays showed that it bound directly to tubulin and stabilized microtubules. This pharmacological probe has allowed, for the first time, a direct visualization of a taccalonolide binding to microtubules, verifying their cellular binding site. This C-6-modified taccalonolide showed potency comparable to the untagged compound in biochemical experiments; however, its potency was lower in cellular assays, presumably due to decreased cellular permeability. These studies provide a valuable tool to facilitate the further understanding of taccalonolide pharmacology and demonstrate that C-6 is a promising site for a linker to be added to this novel class of microtubule stabilizers for targeted drug delivery.

Secondary Metabolites from the Fungus Dictyosporium sp. and Their MALT1 Inhibitory Activities

Bioassay-guided separation of an extract from a Dictyosporium sp. isolate led to the identification of six new compounds, 1-6, together with five known compounds, 7-11. The structures of the new compounds were primarily established by extensive 1D and 2D NMR experiments. The absolute configurations of compounds 3-6 were determined by comparison of their experimental electronic circular dichroism (ECD) spectra with DFT quantum mechanical calculated ECD spectra. Compounds 3-5 possess novel structural scaffolds, and biochemical studies revealed that oxepinochromenones 1 and 7 inhibited the activity of MALT1 protease.

Abstract 2804: Identifying an optimal site for linker conjugation of the taccalonolides, novel microtubule stabilizing drugs

The taccalonolides are a structurally distinct class of microtubule stabilizers purified from the roots and rhizomes of plants of the genus Tacca. The covalent binding of taccalonolides bearing a C-22,23 epoxide to β-tubulin generates a distinct profile of microtubule stabilization that results in a high level of cellular persistence and the ability to circumvent multiple mechanisms of clinically relevant drug resistance, including expression of P-glycoprotein. The biological effects of 121 structurally distinct taccalonolides, including 23 isolated natural products and 98 semi-synthetic derivatives have been evaluated. These compounds range in potency from 0.6 nM to over 50 µM, allowing for detailed structure activity relationship studies.

Some of the most potent taccalonolide derivatives include semi-synthetic modifications at C-6, C-7, or C-15 in combination with C-22,23 epoxidation. While these compounds did not have an acceptable therapeutic window for in vivo antitumor activity by systemic administration, they demonstrated highly effective and exquisitely persistent antitumor activity in a drug-resistant tumor model when delivered directly into the tumor. Additional studies were undertaken to see whether large chemical moieties could be added at these sites to facilitate targeted delivery of these compounds. In contrast to C-7 and C-15 derivatives, which are cleaved, C-6 modifications are stable and retain microtubule stabilizing and cytotoxic activities. In particular, fluorescent C-6 taccalonolide conjugates allowed, for the first time, direct visualization of these compounds in live cells, which co-localized with bundled microtubules. Further probing of the functionality at taccalonolide C-6 will allow for more detailed mechanistic studies of drug transport and distribution as well as optimization of a linker that could be used for antibody-based drug delivery.

Citation Format: April L. Risinger, Lin Du, Antonius Ola, Robert H. Cichewicz, Susan L. Mooberry. Identifying an optimal site for linker conjugation of the taccalonolides, novel microtubule stabilizing drugs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2804.

Select polyphenolic fractions from dried plum enhance osteoblast activity through BMP-2 signaling

Dried plum supplementation has been shown to enhance bone formation while suppressing bone resorption. Evidence from previous studies has demonstrated that these responses can be attributed in part to the fruit's polyphenolic compounds. The purpose of this study was to identify the most bioactive polyphenolic fractions of dried plum with a focus on their osteogenic activity and to investigate their mechanisms of action under normal and inflammatory conditions. Utilizing chromatographic techniques, six fractions of polyphenolic compounds were prepared from a crude extract of dried plum. Initial screening assays revealed that two fractions (DP-FrA and DP-FrB) had the greatest osteogenic potential. Subsequent experiments using primary bone-marrow-derived osteoblast cultures demonstrated these two fractions enhanced extracellular alkaline phosphatase (ALP), an indicator of osteoblast activity, and mineralized nodule formation under normal conditions. Both fractions enhanced bone morphogenetic protein (BMP) signaling, as indicated by increased Bmp2 and Runx2 gene expression and protein levels of phosphorylated Smad1/5. DP-FrB was most effective at up-regulating Tak1 and Smad1, as well as protein levels of phospho-p38. Under inflammatory conditions, TNF-α suppressed ALP and tended to decrease nodule formation (P=.0674). This response coincided with suppressed gene expression of Bmp2 and the up-regulation of Smad6, an inhibitor of BMP signaling. DP-FrA and DP-FrB partially normalized these responses. Our results show that certain fractions of polyphenolic compounds in dried plum up-regulate osteoblast activity by enhancing BMP signaling, and when this pathway is inhibited by TNF-α, the osteogenic response is attenuated.

What lies beneath? Fungal diversity at the bottom of Lake Michigan and Lake Superior

Fungi are phylogenetically diverse organisms found in nearly every environment as key contributors to the processes of nutrient cycling and decomposition. To date, most fungal diversity has been documented from terrestrial habitats leaving aquatic habitats underexplored. In particular, comparatively little is known about fungi inhabiting freshwater lakes, particularly the benthic zone, which may serve as an untapped resource for fungal biodiversity. Advances in technology allowing for direct sequencing of DNA from environmental samples provide a new opportunity to investigate freshwater benthic fungi. In this study, we employed both culture-dependent and culture-independent methods to evaluate the diversity of fungi in one of the largest freshwater systems on Earth, the North American Laurentian Great Lakes. This study presents the first comprehensive survey of fungi from sediment from Lake Michigan and Lake Superior, resulting in 465 fungal taxa with only 7% of sequence overlap between these two methods. Additionally, culture-independent analyses of the ITS1 and ITS2 regions revealed 49% and 72%, respectively, of the OTUs did not match a described fungal taxonomic group below kingdom Fungi. The low level of sequence overlap between methods and high percentage of fungal taxa that can only be classified at the kingdom level suggests an immense amount of fungal diversity remains to be studied in these aquatic fungal communities.

Taccalonolide Microtubule Stabilizers Generated Using Semisynthesis Define the Effects of Mono Acyloxy Moieties at C-7 or C-15 and Disubstitutions at C-7 and C-25

The taccalonolides are a unique class of microtubule stabilizers isolated from Tacca spp. that have efficacy against drug-resistant tumors. Our previous studies have demonstrated that a C-15 acetoxy taccalonolide, AF, has superior in vivo antitumor efficacy compared to AJ, which bears a C-15 hydroxy group. With the goal of further improving the in vivo efficacy of this class of compounds, we semisynthesized and tested the biological activities of 28 new taccalonolides with monosubstitutions at C-7 or C-15 or disubstitutions at C-7 and C-25, covering a comprehensive range of substituents from formic acid to anthraquinone-2-carbonyl chloride. The resulting taccalonolide analogues with diverse C-7/C-15/C-25 modifications exhibited IC₅₀ values from 2.4 nM to >20 μM, allowing for extensive in vitro structure-activity evaluations. This semisynthetic strategy was unable to provide a taccalonolide with improved therapeutic window due to hydrolysis of substituents at C-7 or C-15 regardless of size or steric bulk. However, two of the most potent new taccalonolides, bearing isovalerate modifications at C-7 or C-15, demonstrated potent and highly persistent antitumor activity in a drug-resistant xenograft model when administered intratumorally. This study demonstrates that targeted delivery of the taccalonolides to the tumor could be an effective, long-lasting approach to treat drug-resistant tumors.

Structure-Activity Relationships of New Natural Product-Based Diaryloxazoles with Selective Activity against Androgen Receptor-Positive Breast Cancer Cells

Targeted therapies for ER+/PR+ and HER2-amplified breast cancers have improved patient survival, but there are no therapies for triple negative breast cancers (TNBC) that lack expression of estrogen and progesterone receptors (ER/PR), or amplification or overexpression of HER2. Gene expression profiling of TNBC has identified molecular subtypes and representative cell lines. An extract of the Texas native plant Amyris texana was found to have selective activity against MDA-MB-453 cells, a model of the luminal androgen receptor (LAR) subtype of TNBC. Bioassay-guided fractionation identified two oxazole natural products with selective activity against this cell line. Conducted analog synthesis and structure-activity relationship studies provided analogs with more potent and selective activity against two LAR subtype cell line models, culminating in the discovery of compound 30 (CIDD-0067106). Lead compounds discovered have potent and selective antiproliferative activities, and mechanisms of action studies show they inhibit the activity of the mTORC1 pathway.

Unique amalgamation of primary and secondary structural elements transform peptaibols into potent bioactive cell-penetrating peptides

Mass-spectrometry-based metabolomics and molecular phylogeny data were used to identify a metabolically prolific strain of Tolypocladium that was obtained from a deep-water Great Lakes sediment sample. An investigation of the isolate's secondary metabolome resulted in the purification of a 22-mer peptaibol, gichigamin A (1). This peptidic natural product exhibited an amino acid sequence including several β-alanines that occurred in a repeating ααβ motif, causing the compound to adopt a unique right-handed 311 helical structure. The unusual secondary structure of 1 was confirmed by spectroscopic approaches including solution NMR, electronic circular dichroism (ECD), and single-crystal X-ray diffraction analyses. Artificial and cell-based membrane permeability assays provided evidence that the unusual combination of structural features in gichigamins conferred on them an ability to penetrate the outer membranes of mammalian cells. Compound 1 exhibited potent in vitro cytotoxicity (GI50 0.55 ± 0.04 µM) and in vivo antitumor effects in a MIA PaCa-2 xenograft mouse model. While the primary mechanism of cytotoxicity for 1 was consistent with ion leakage, we found that it was also able to directly depolarize mitochondria. Semisynthetic modification of 1 provided several analogs, including a C-terminus-linked coumarin derivative (22) that exhibited appreciably increased potency (GI50 5.4 ± 0.1 nM), but lacked ion leakage capabilities associated with a majority of naturally occurring peptaibols such as alamethicin. Compound 22 was found to enter intact cells and induced cell death in a process that was preceded by mitochondrial depolarization.

Osteoclast Differentiation is Downregulated by Select Polyphenolic Fractions from Dried Plum via Suppression of MAPKs and Nfatc1 in Mouse C57BL/6 Primary Bone Marrow Cells

Background: Clinical and preclinical studies have shown that dietary supplementation with dried plum improves bone health. These osteoprotective effects are a result, in part, of the antiresorptive properties of the fruit, which appear to be mediated by its polyphenolic compounds. Objective: This study was designed to determine if certain fractions of the polyphenolic compounds in dried plums are responsible for the antiresorptive effects and whether they alter mitogen-activated protein kinase (MAPK) and calcium signaling, which are essential to osteoclast differentiation and activity, under normal and inflammatory conditions. Methods: Six polyphenolic fractions were derived from the total polyphenolic extract of dried plum based on solubility. Initial screening, with the use of the Raw 264.7 monocyte and macrophage cell line, showed that 3 fractions had the most marked capacity to downregulate osteoclast differentiation. This response was confirmed in 2 of the fractions by using primary bone marrow-derived cultures and in all subsequent experiments to determine how osteoclast differentiation and function were altered with a focus on these 2 fractions in primary cultures. Data were analyzed by using ANOVA followed by post hoc analyses. Results: Both of the polyphenol fractions decreased osteoclast differentiation and activity coincident with downregulating nuclear factor of activated T cells, cytoplasmic, calcineurin-dependent 1 (Nfatc1), which is required for osteoclast differentiation. Calcium signaling, essential for the auto-amplification of Nfatc1, was suppressed by the polyphenolic fractions under normal conditions as indicated by suppressed mRNA expression of costimulatory receptors osteoclast-associated receptor (Oscar), signaling regulatory protein β1 (Sirpb1), and triggering receptor expressed on myeloid cells 2 (Trem2). In contrast, in the presence of tumor necrosis factor α (TNF-α), only Sirpb1 was downregulated. In addition to calcium signaling, phosphorylation of extracellular signal-regulated kinase (Erk) and p38 MAPK, involved in the expression and activation of Nfatc1, was also suppressed by the polyphenolic fractions. Conclusion: These results show that certain types of polyphenolic compounds from dried plum downregulate calcium and MAPK signaling, resulting in suppression of Nfatc1 expression, which ultimately decreases osteoclast formation and activity.

Growth Inhibition of Colon Cancer and Melanoma Cells by Versiol Derivatives from a Paraconiothyrium Species

Bioassay-guided fractionation of a colon cancer growth inhibitory extract of the fungus Paraconiothyrium sp. led to the isolation of eight new versiol derivatives (1, 3-8, 10) along with two known compounds. The structures were elucidated by interpretation of combined MS and 2D NMR spectroscopic data. Compounds 8, 9, and 10 showed cell growth inhibition against COLO205 and KM12 cells, and both 8 and 9 displayed selectivity in their inhibition of melanoma cell lines in the NCI 60 one-dose test. In addition, compound 8 and the crude Paraconiothyrium sp. extract showed potent dose-dependent inhibitory effects in the five-dose NCI 60 cell line assay.