Elucidation of Spirodactylone, a Polycyclic Alkaloid from the Sponge Dactylia sp., and Nonenzymatic Generation from the Co-metabolite Denigrin B

Spirodactylone (1), a hexacyclic indolizidone alkaloid possessing a novel spiro ring system, was isolated from the marine sponge Dactylia sp. The structure was elucidated by extensive spectroscopic methods including application of the LR-HSQMBC NMR pulse sequence. Oxidative cyclization of denigrin B (2), an aryl-substituted 2-oxo-pyrroline derivative that was also isolated from the sponge extract, provided material identical to spirodactylone (1). This confirmed the assigned structure and provides insight into the probable biogenesis of 1.

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.

Biotransformed Metabolites of the Hop Prenylflavanone Isoxanthohumol

A metabolic conversion study on microbes is known as one of the most useful tools to predict the xenobiotic metabolism of organic compounds in mammalian systems. The microbial biotransformation of isoxanthohumol (1), a major hop prenylflavanone in beer, has resulted in the production of three diastereomeric pairs of oxygenated metabolites (2–7). The microbial metabolites of 1 were formed by epoxidation or hydroxylation of the prenyl group, and HPLC, NMR, and CD analyses revealed that all of the products were diastereomeric pairs composed of (2S)- and (2R)- isomers. The structures of these metabolic compounds were elucidated to be (2S,2″S)- and (2R,2″S)-4′-hydroxy-5-methoxy-7,8-(2,2-dimethyl-3-hydroxy-2,3-dihydro-4H-pyrano)-flavanones (2 and 3), (2S)- and (2R)-7,4′-dihydroxy-5-methoxy-8-(2,3-dihydroxy-3-methylbutyl)-flavanones (4 and 5) which were new oxygenated derivatives, along with (2R)- and (2S)-4′-hydroxy-5-methoxy-2″-(1-hydroxy-1-methylethyl)dihydrofuro[2,3-h]flavanones (6 and 7) on the basis of spectroscopic data. These results could contribute to understanding the metabolic fates of the major beer prenylflavanone isoxanthohumol that occur in mammalian system.

Harnessing Natural Product Diversity for Fluorophore Discovery: Naturally Occurring Fluorescent Hydroxyanthraquinones from the Marine Crinoid Pterometra venusta

Fluorescent small molecules are important tools in many aspects of modern biology. A two-stage evaluation process involving fluorescence screening and live-cell imaging was developed to facilitate the identification of new fluorescent probes from extracts housed within the NCI Natural Products Repository. To this end, over 2000 extracts and prefractionated samples were examined, including an extract from the marine crinoid Pterometra venusta. An optically guided evaluation involving stepwise fluorescence screening and live-cell imaging was developed to enable the isolation of fluorescent natural products. These efforts resulted in the isolation of six hydroxyanthraquinone compounds, three of which are new natural products. These purified metabolites were examined for their potential as cellular imaging probes, and they demonstrate that natural product libraries can be a good source of new fluorescent agents.

Synthesis and Biological Evaluation of New Madecassic Acid Derivatives Targeting ERK Cascade Signaling

In the present study, a series of novel madecassic acid derivatives was synthesized and screened against the National Cancer Institute's 60 human cancer cell line panel. Among them, compounds 5, 12, and 17 displayed potent and highly differential antiproliferative activity against 80% of the tumor cells harboring the B-Raf^V600E mutation within the nanomolar range. Structure-activity analysis revealed that a 5-membered A ring containing an α,β-unsaturated aldehyde substituted at C-23 with a 2-furoyl group seems to be crucial to produce this particular growth inhibition signature. In silico analysis of the cytotoxicity pattern of these compounds identified two highly correlated clinically approved drugs with known B-Raf^V600E inhibitory activity. Follow-up analysis revealed inhibition of the ERK signaling pathway through the reduction of cellular Raf protein levels is a key mechanism of action of these compounds. In particular, 17 was the most potent compound in suppressing tumor growth of B-Raf^V600E-mutant cell lines and displayed the highest reduction of Raf protein levels among the tested compounds. Taken together, this study revealed that modifications of madecassic acid structure can provide molecules with potent anticancer activity against cell lines harboring the clinically relevant B-Raf^V600E mutation, with compound 17 identified as a promising lead for the development of new anticancer drugs.

Macrophilones from the Marine Hydroid Macrorhynchia philippina Can Inhibit ERK Cascade Signaling

Six new macrophilone-type pyrroloiminoquines were isolated and identified from an extract of the marine hydroid Macrorhynchia philippina. The proton-deficient and heteroatom-rich structures of macrophilones B-G (2-7) were elucidated by spectroscopic analysis and comparison of their data with those of the previously reported metabolite macrophilone A (1). Compounds 1-7 are the first pyrroloiminoquines to be reported from a hydroid. The macrophilones were shown to inhibit the enzymatic conjugation of SUMO to peptide substrates, and macrophilones A (1) and C (3) exhibit potent and selective cytotoxic properties in the NCI-60 anticancer screen. Bioinformatic analysis revealed a close association of the cytotoxicity profiles of 1 and 3 with two known B-Raf kinase inhibitory drugs. While compounds 1 and 3 showed no kinase inhibitory activity, they resulted in a dramatic decrease in cellular protein levels of selected components of the ERK signal cascade. As such, the chemical scaffold of the macrophilones could provide small-molecule therapeutic leads that target the ERK signal transduction pathway.

Abstract 14: Preclinical evaluation of novel HIF-1α/P300 binding inhibitors

A key component of tumor progression and metastasis is the hypoxic response. The hypoxic response regulates angiogenesis, tumor invasion, and metabolism. The Hypoxia Inducible Factor (HIF) is the transcriptional system responsible for the hypoxic response. The inhibition of the hypoxic response via inhibition of the HIF-1 pathway by disrupting its association with the transcriptional coactivator p300 presents a potential therapeutic target for multiple cancers where HIF is upregulated. To that end, we describe the preclinical development of previously described novel marine pyrroloiminoquinone alkaloids found using a HIF-1α/p300 assay in a high throughput screen of extracts from the National Cancer Institute's Natural Products Repository. Pyrroloiminoquinone alkaloids are a novel class of HIF-1α inhibitors, which interrupt the protein−protein interaction between HIF-1α and p300 and consequently reduce HIF-related transcription. The 2 lead candidates, discorhabdin L and H were assessed in inhibition of angiogenesis in an in vitro HUVEC assay and ex vivo rat aortic ring assay. The compounds were also evaluated for in vivo efficacy in prostate cancer cell tumor xenografts. Results show that development of these compounds for clinical use is warranted and may prevent the progression of multiple tumor types.

Citation Format: Jonathan D. Strope, Emily M. Harris, Shaunna L. Beedie, Cindy H. Chau, Kristina M. Cook, Christopher J. Schofield, Kirk R. Gustafson, William D. Figg. Preclinical evaluation of novel HIF-1α/P300 binding inhibitors [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 14.

Cytotoxic Triterpenes from Salacia crassifolia and Metabolite Profiling of Celastraceae Species

The new pentacyclic triterpene 11β-hydroxypristimerin (1), along with the known metabolites pristimerin (2), 6-oxopristimerol (3) and vitideasin (4), were isolated from a Salacia crassifolia root wood extract, following a bioassay-guided fractionation approach. Both the extract and the purified triterpenes displayed pronounced cytotoxic activity against human cancer cell lines. The NCI-60 cell line screen revealed that compound 2 was the most active, with a mean GI₅₀ of 0.17 μM, while compound 1 had a mean GI₅₀ of 8.7 μM. A COMPARE analysis of the screening results showed that pristimerin is likely to be the main compound responsible for the cytotoxic activity of the extract (mean GI₅₀ of 0.3 μg·mL⁻¹). A targeted search for pristimerin and related derivatives using LC-MS/MS revealed the presence of pristimerin (2) and 6-oxopristimerol (3) in all Celastraceae species examined and in all plant parts tested, while vitideasin (4) was only detected in the genus Salacia.

Enhanced measurement of residual chemical shift anisotropy for small molecule structure elucidation

A method is introduced to measure residual chemical shift anisotropies conveniently and accurately in the mesophase of poly-γ-(benzyl-l-glutamate). The alignment amplitude is substantially enhanced over common methods which greatly benefits measurements particularly on sp3 carbons. The approach offers significant improvements in data accuracy and utility for small molecule structure determination.

Unequivocal determination of caulamidines A and B: application and validation of new tools in the structure elucidation tool box

Newly described NMR experimental approaches can provide valuable structural details and a complementary means of structure verification.

Ambiguities and errors in the structural assignment of organic molecules hinder both drug discovery and total synthesis efforts. Newly described NMR experimental approaches can provide valuable structural details and a complementary means of structure verification. The caulamidines are trihalogenated alkaloids from a marine bryozoan with an unprecedented structural scaffold. Their unique carbon and nitrogen framework was deduced by conventional NMR methods supplemented by new experiments that define 2-bond heteronuclear connectivities, reveal very long-range connectivity data, or visualize the ^35,37Cl isotopic effect on chlorinated carbons. Computer-assisted structural elucidation (CASE) analysis of the spectroscopic data for caulamidine A provided only one viable structural alternative. Anisotropic NMR parameters, specifically residual dipolar coupling and residual chemical shift anisotropy data, were measured for caulamidine A and compared to DFT-calculated values for the proposed structure, the CASE-derived alternative structure, and two energetically feasible stereoisomers. Anisotropy-based NMR experiments provide a global, orthogonal means to verify complex structures free from investigator bias. The anisotropic NMR data were fully consistent with the assigned structure and configuration of caulamidine A. Caulamidine B has the same heterocyclic scaffold as A but a different composition and pattern of halogen substitution. Caulamidines A and B inhibited both wild-type and drug-resistant strains of the malaria parasite Plasmodium falciparum at low micromolar concentrations, yet were nontoxic to human cells.

Structure Revision of Poecillastrin C and the Absolute Configuration of the β-Hydroxyaspartic Acid Residue

The planar structure of poecillastrin C (1) was revised through selective reduction of the ester carbon. The absolute configuration of the β-hydroxyaspartic acid (OHAsp) residue was determined to be d-threo by Marfey's analysis. The acid hydrolysate of the reduction product of 1 liberated (2R,3R)-2-amino-3,4-dihydroxybutanoic acid, demonstrating that the β-carboxyl group in poecillastrin C was esterified. The structures of poecillastrins B-D and 73-deoxychondropsin A were also revised.

Macrophilone A: Structure Elucidation, Total Synthesis, and Functional Evaluation of a Biologically Active Iminoquinone from the Marine Hydroid Macrorhynchia philippina

A previously uncharacterized pyrroloiminoquinone natural product, macrophilone A, was isolated from the stinging hydroid Macrorhynchia philippina. The structure was assigned utilizing long-range NMR couplings and DFT calculations and proved by a concise, five-step total synthesis. Macrophilone A and a synthetic analogue displayed potent biological activity, including increased intracellular reactive oxygen species levels and submicromolar cytotoxicity toward lung adenocarcinoma cells.

Characterization and Synthesis of Eudistidine C, a Bioactive Marine Alkaloid with an Intriguing Molecular Scaffold

An extract of Eudistoma sp. provided eudistidine C (1), a heterocyclic alkaloid with a novel molecular framework. Eudistidine C (1) is a racemic natural product composed of a tetracyclic core structure further elaborated with a p-methoxyphenyl group and a phenol-substituted aminoimidazole moiety. This compound presented significant structure elucidation challenges due to the large number of heteroatoms and fully substituted carbons. These issues were mitigated by application of a new NMR pulse sequence (LR-HSQMBC) optimized to detect four- and five-bond heteronuclear correlations and the use of computer-assisted structure elucidation software. Synthesis of eudistidine C (1) was accomplished in high yield by treating eudistidine A (2) with 4(2-amino-1H-imidazol-5-yl)phenol (4) in DMSO. Synthesis of eudistidine C (1) confirmed the proposed structure and provided material for further biological characterization. Treatment of 2 with various nitrogen heterocycles and electron-rich arenes provided a series of analogues (5-10) of eudistidine C. Chiral-phase HPLC resolution of epimeric eudistidine C provided (+)-(R)-eudistidine C (1a) and (-)-(S)-eudistidine C (1b). The absolute configuration of these enantiomers was assigned by ECD analysis. (-)-(S)-Eudistidine C (1b) modestly inhibited interaction between the protein binding domains of HIF-1α and p300. Compounds 1, 2, and 6-10 exhibited significant antimalarial activity against Plasmodium falciparum.

Cembranoids from a Chinese Collection of the Soft Coral Lobophytum crassum

Ten new cembrane-based diterpenes, locrassumins A–G (1–7), (–)-laevigatol B (8), (–)-isosarcophine (9), and (–)-7R,8S-dihydroxydeepoxysarcophytoxide (10), were isolated from a South China Sea collection of the soft coral Lobophytum crassum, together with eight known analogues (11–18). The structures of the new compounds were determined by extensive spectroscopic analysis and by comparison with previously reported data. Locrassumin C (3) possesses an unprecedented tetradecahydrobenzo[3,4]cyclobuta[1,2][8]annulene ring system. Compounds 1, 7, 12, 13, and 17 exhibited moderate inhibition against lipopolysaccharide (LPS)-induced nitric oxide (NO) production with IC₅₀ values of 8–24 μM.

Screening and Biological Effects of Marine Pyrroloiminoquinone Alkaloids: Potential Inhibitors of the HIF-1α/p300 Interaction

Inhibition of the hypoxia-inducible factor 1α (HIF-1α) pathway by disrupting its association with the transcriptional coactivator p300 inhibits angiogenesis and tumor development. Development of HIF-1α/p300 inhibitors has been hampered by preclinical toxicity; therefore, we aimed to identify novel HIF-1α/p300 inhibitors. Using a cell-free assay designed to test compounds that block HIF-1α/p300 binding, 170 298 crude natural product extracts and prefractionated samples were screened, identifying 25 active extracts. One of these extracts, originating from the marine sponge Latrunculia sp., afforded six pyrroloiminoquinone alkaloids that were identified as positive hits (IC50 values: 1-35 μM). Luciferase assays confirmed inhibition of HIF-1α transcriptional activity by discorhabdin B (1) and its dimer (2), 3-dihydrodiscorhabdin C (3), makaluvamine F (5), discorhabdin H (8), discorhabdin L (9), and discorhabdin W (11) in HCT 116 colon cancer cells (0.1-10 μM, p < 0.05). Except for 11, all of these compounds also reduced HIF-1α transcriptional activity in LNCaP prostate cancer cells (0.1-10 μM, p < 0.05). These effects occurred at noncytotoxic concentrations (<50% cell death) under hypoxic conditions. At the downstream HIF-1α target level, compound 8 (0.5 μM) significantly decreased VEGF secretion in LNCaP cells (p < 0.05). In COLO 205 colon cancer cells no activity was shown in the luciferase or cytotoxicity assays. Pyrroloiminoquinone alkaloids are a novel class of HIF-1α inhibitors, which interrupt the protein-protein interaction between HIF-1α and p300 and consequently reduce HIF-related transcription.

Structurally Diverse Metabolites from the Soft Coral Sinularia verruca Collected in the South China Sea

Nineteen metabolites with diverse structures, including the rare pyrroloindoline alkaloid verrupyrroloindoline (1), the unprecedented highly fused benzosesquiterpenoid verrubenzospirolactone (2), the new asteriscane-type sesquiterpenoid 10-deoxocapillosanane D (3), and the two new cyclopentenone derivatives (4S*,5S*)-4-hydroxy-5-(hydroxymethyl)-2,3-dimethyl-4-pentylcyclopent-2-en-1-one (4) and (S)-4-hydroxy-5-methylene-2,3-dimethyl-4-pentylcyclopent-2-en-1-one (5), were isolated from a South China Sea collection of the soft coral Sinularia verruca. Eleven previously described marine metabolites (7-15, 18, and 19) were also obtained as well as three new EtOH-adduct artifacts (6, 16, and 17). The structures of the new compounds were elucidated by extensive spectroscopic analysis and by comparison with previously reported data. Compounds 4, 5, and 16 showed protection against the cytopathic effects of HIV-1 infection with EC50 values of 5.8-34 μM, and 4, 6, and 16 exhibited inhibition against LPS-induced NO production with IC50 values of 24-28 μM.

Stelliosphaerols A and B, Sesquiterpene-Polyol Conjugates from an Ecuadorian Fungal Endophyte

Endophytic fungi are plant tissue-associated fungi that represent a rich resource of unexplored biological and chemical diversity. As part of an ongoing effort to characterize Amazon rainforest-derived endophytes, numerous fungi were isolated and cultured from plants collected in the Yasuní National Park in Ecuador. Of these samples, phylogenetic and morphological data revealed a previously undescribed fungus in the order Pleosporales that was cultured from the tropical tree Duroia hirsuta. Extracts from this fungal isolate displayed activity against Staphylococcus aureus and were thus subjected to detailed chemical studies. Two compounds with modest antibacterial activity were isolated, and their structures were elucidated using a combination of NMR spectroscopic analysis, LC-MS studies, and chemical degradation. These efforts led to the identification of stelliosphaerols A (1) and B (2), new sesquiterpene-polyol conjugates that are responsible, at least in part, for the S. aureus inhibitory activity of the fungal extract.

Deguelins, Natural Product Modulators of NF1-Defective Astrocytoma Cell Growth Identified by High-Throughput Screening of Partially Purified Natural Product Extracts

A high-throughput screening assay for modulators of Trp53/NF1 mutant astrocytoma cell growth was adapted for use with natural product extracts and applied to a novel collection of prefractionated/partially purified extracts. Screening 68 427 samples identified active fractions from 95 unique extracts, including the terrestrial plant Millettia ichthyotona. Only three of these extracts showed activity in the crude extract form, thus demonstrating the utility of a partial purification approach for natural product screening. The NF1 screening assay was used to guide purification of active compounds from the M. ichthyotona extract, which yielded the two rotenones deguelin (1) and dehydrodeguelin (2). The deguelins have been reported to affect growth of a number of cancer cell lines. They potently inhibited growth of only one of a panel of NF1/Trp53 mutant murine astrocytoma cell lines, possibly related to epigenetic factors, but had no effect on the growth of normal astrocytes. These results suggest the potential utility of deguelins as tools for further investigating NF1 astrocytoma cell growth. These bioprobes were identified only as a result of screening partially purified natural product extracts.

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.

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.

Abstract 2928: Withanolide E sensitizes renal carcinoma cells to TRAIL-induced apoptosis by increasing cFLIP degradation

Withanolide E (WE), a steroidal lactone from Physalis peruviana, was found to be highly active for sensitizing renal carcinoma cells and a number of other human cancer cells to TRAIL-mediated apoptosis. WE, the most potent and least toxic of 5 TRAIL sensitizing withanolides identified, enhanced death receptor-mediated apoptotic signaling via a rapid decline in the levels of both cFLIPL and cFLIPS proteins. Normal non-transformed human renal epithelial cells were completely resistant to the WE plus TRAIL combination. Other mechanisms by which TRAIL sensitizers have been reported to work: generation of reactive oxygen species (ROS), changes in various pro-and anti-apoptotic protein expression, death receptor upregulation, activation of intrinsic (mitochondrial) apoptotic pathways, ER stress, and proteasomal inhibition proved to be irrelevant to withanolide E activity. Loss of cFLIPL and cFLIPS was predominantly due to destabilization and/or aggregation of the proteins subsequently leading to their proteasomal degradation. Since cFLIPL and cFLIPS are reported to be HSP90 client proteins, and WE can inhibit HSP90 activity, the effect of WE on various other HSP90 client proteins was assessed. WE treatment of ACHN renal carcinoma cells altered the stability of a limited number of HSP90 client proteins when compared to the well-known HSP90 inhibitor geldanamycin. The destabilization of cFLIP proteins provides a potentially novel mechanism for sensitizing cancer cells to TRAIL mediated apoptosis. Sensitization of human renal carcinoma cells to TRAIL-induced apoptosis by WE and its lack of toxicity were confirmed in animal studies, in which a combination treatment of WE with an agonist antibody to TRAIL death receptor 5 (DR5) provided a significant therapeutic benefit compared to either agent alone in a renal cancer xenograft model. Due to its novel activity, WE is a promising reagent for analysis of mechanisms of TRAIL resistance, for understanding HSP90 function, and for further therapeutic development. Various structural analogs of WE are currently being tested for their TRAIL-sensitizing activity to provide more insight into Structure Activity Relationships (SAR) for these compounds. In marked contrast to bortezomib, among the best currently available TRAIL sensitizers, WE's more specific mechanism of action suggests minimal toxic side effects might accompany its combination with TRAIL death receptor agonists for cancer therapy.

Citation Format: Brooks D. Alan, Curtis J. Henrich, Karen L. Erickson, Cheryl L. Thomas, Heidi R. Bokesch, Poonam Tewary, Candace R. Thompson, Richard J. Pompei, Kirk R. Gustafson, James B. McMahon, Thomas J. Sayers. Withanolide E sensitizes renal carcinoma cells to TRAIL-induced apoptosis by increasing cFLIP degradation. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2928. doi:10.1158/1538-7445.AM2015-2928

Stellettapeptins A and B, HIV-inhibitory cyclic depsipeptides from the marine sponge Stelletta sp

Two new HIV-inhibitory depsipeptides, stellettapeptins A (1) and B (2), were isolated from an extract of the marine sponge Stelletta sp., collected from northwestern Australia. Structures of these cyclic nonribosomal peptides were elucidated on the basis of extensive NMR data analysis, and chemical degradation and derivatization studies. Stellettapeptins contain numerous nonproteinogenic amino acid residues and they are the first peptides reported to contain a 3-hydroxy-6,8-dimethylnon-4-(Z)-enoic acid moiety. Compounds 1 and 2 potently inhibit infection of human T-lymphoblastoid cells by HIV-1RF with EC50 values of 23 and 27 nM, respectively.

Structural Elucidation and Synthesis of Eudistidine A: An Unusual Polycyclic Marine Alkaloid that Blocks Interaction of the Protein Binding Domains of p300 and HIF-1α

Low oxygen environments are a hallmark of solid tumors, and transcription of many hypoxia-responsive genes needed for survival under these conditions is regulated by the transcription factor HIF-1 (hypoxia-inducible factor 1). Activation of HIF-1 requires binding of its α-subunit (HIF-1α) to the transcriptional coactivator protein p300. Inhibition of the p300/HIF-1α interaction can suppress HIF-1 activity. A screen for inhibitors of the protein binding domains of p300 (CH1) and HIF-1α (C-TAD) identified an extract of the marine ascidian Eudistoma sp. as active. Novel heterocyclic alkaloids eudistidines A (1) and B (2) were isolated from the extract, and their structures assigned by spectroscopic analyses. They contain an unprecedented tetracyclic core composed of two pyrimidine rings fused with an imidazole ring. Eudistidine A (1) was synthesized in a concise four-step sequence featuring a condensation/cyclization reaction cascade between 4-(2-aminophenyl)pyrimidin-2-amine (3) and 4-methoxy-phenylglyoxal (4), while eudistidine B (2) was synthesized in a similar fashion with glyoxylic acid (5) in place of 4. Naturally occurring eudistidine A (1) effectively inhibited CH1/C-TAD binding with an IC50 of 75 μM, and synthetic 1 had similar activity. The eudistidine A (1) scaffold, which can be synthesized in a concise, scalable manner, may provide potential therapeutic lead compounds or molecular probes to study p300/HIF-1α interactions and the role these proteins play in tumor response to low oxygen conditions. The unique structural scaffolds and functional group arrays often found in natural products make these secondary metabolites a rich source of new compounds that can disrupt critical protein-protein binding events.

Withanolide E sensitizes renal carcinoma cells to TRAIL-induced apoptosis by increasing cFLIP degradation

Withanolide E, a steroidal lactone from Physalis peruviana, was found to be highly active for sensitizing renal carcinoma cells and a number of other human cancer cells to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis. Withanolide E, the most potent and least toxic of five TRAIL-sensitizing withanolides identified, enhanced death receptor-mediated apoptotic signaling by a rapid decline in the levels of cFLIP proteins. Other mechanisms by which TRAIL sensitizers have been reported to work: generation of reactive oxygen species (ROS), changes in pro-and antiapoptotic protein expression, death receptor upregulation, activation of intrinsic (mitochondrial) apoptotic pathways, ER stress, and proteasomal inhibition proved to be irrelevant to withanolide E activity. Loss of cFLIP proteins was not due to changes in expression, but rather destabilization and/or aggregation, suggesting impairment of chaperone proteins leading to degradation. Indeed, withanolide E treatment altered the stability of a number of HSP90 client proteins, but with greater apparent specificity than the well-known HSP90 inhibitor geldanamycin. As cFLIP has been reported to be an HSP90 client, this provides a potentially novel mechanism for sensitizing cells to TRAIL. Sensitization of human renal carcinoma cells to TRAIL-induced apoptosis by withanolide E and its lack of toxicity were confirmed in animal studies. Owing to its novel activity, withanolide E is a promising reagent for the analysis of mechanisms of TRAIL resistance, for understanding HSP90 function, and for further therapeutic development. In marked contrast to bortezomib, among the best currently available TRAIL sensitizers, withanolide E's more specific mechanism of action suggests minimal toxic side effects.