Neopetrotaurines A-C, Isoquinoline Alkaloids with an Unprecedented Taurine Bridge from the Sponge Neopetrosia sp

Neopetrotaurines A-C (1-3), unusual alkaloids possessing two isoquinoline-derived moieties that are linked via a unique taurine bridge, were isolated from a Neopetrosia sp. marine sponge. These new compounds have proton-deficient structural scaffolds that are difficult to unambiguously assign using only conventional 2- and 3-bond 1H-13C and 1H-15N heteronuclear correlation data. Thus, the application of LR-HSQMBC and HMBC NMR experiments optimized to detect 4- and 5-bond long-range 1H-13C heteronuclear correlations facilitated the structure elucidation of these unusual taurine-bridged marine metabolites. Neopetrotaurines A-C (1-3) showed significant inhibition of transcription driven by the oncogenic fusion protein PAX3-FOXO1, which is associated with alveolar rhabdomyosarcoma, and cytotoxic activity against PAX3-FOXO1-positive cell lines.

Rare Caulamidine Hexacyclic Alkaloids from the Marine Ascidian Polyandrocarpa sp

Two new caulamidines C (2) and D (4) and three isocaulamidines B, C, and D (1, 3, and 5) along with the known compound caulamidine B (6) were isolated from the marine ascidian Polyandrocarpa sp. Their structures were elucidated by analysis of nuclear magnetic resonance (NMR) and electronic circular dichroism (ECD) data. Isocaulamidines have an altered pattern of N-methyl substitution (N-15 vs N-13 in the caulamidines) with a concomitant double-bond rearrangement to provide a new C-14/N-13 imine functionality. Caulamidine C (2) and isocaulamidine C (3) are the first members of this alkaloid family with two chlorine substituents in the core 6H-2,6-naphthyridine ring system.

Dentithecamides A-H, Diacylated Zoanthoxanthin Derivatives with PAX3-FOXO1 Inhibitory Activity from the Hydroid Dentitheca habereri

Chemical investigation of the marine hydroid Dentitheca habereri led to the identification of eight new diacylated zoanthoxanthin alkaloids, named dentithecamides A-H (1-8), along with three previously reported analogues, zoamides B-D (9-11). The structures of compounds 1-11 were elucidated by spectroscopic and spectrometric analyses, including IR, HRESIMS, and NMR experiments, and by comparison with literature data. Compounds 1-11 are the first zoanthoxanthin alkaloids to be reported from a hydroid. Dentithecamides A (1) and B (2) along with zoamides B-D (9-11), which all share a conformationally mobile cycloheptadiene core, inhibited PAX3-FOXO1 regulated transcriptional activity and thus provided a structural framework for the potential development of more potent PAX3-FOXO1 inhibitors.

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.

Sinularamides A-G, Terpenoid-Derived Spermidine and Spermine Conjugates with Casitas B-Lineage Lymphoma Proto-Oncogene B (Cbl-b) Inhibitory Activities from a Sinularia sp. Soft Coral

An extract of a Sinularia sp. soft coral showed inhibitory activity against the E3-ubiquitin ligase casitas B-lineage lymphoma proto-oncogene B (Cbl-b). Subsequent bioassay-guided separation of the extract provided a series of terpenoid-derived spermidine and spermine amides that were named sinularamides A-G (1-7). Compounds 1-7 represent new natural products; however, sinularamide A (1) was previously reported as a synthetic end product. The structures of sinularamides A-G (1-7) were elucidated by analysis of spectroscopic and spectrometric data from NMR, IR, and HRESIMS experiments and by comparison with literature data. All of the isolated compounds showed Cbl-b inhibitory activities with IC₅₀ values that ranged from approximately 6.5 to 33 μM.

Neopetrothiazide: An Intriguing Pentacyclic Thiazide Alkaloid from the Sponge Neopetrosia sp

Neopetrothiazide (1), a pentacyclic isoquinoline quinone, was isolated from a Neopetrosia sp. sponge. The structure elucidation was facilitated by utilizing long-range heteronuclear single quantum multiple bond correlation (LR-HSQMBC) and heteronuclear multiple bond correlation (HMBC) nuclear magnetic resonance (NMR) pulse sequences optimized to detect four- and five-bond ¹H-¹³C heteronuclear correlations. These NMR experiments can help assign proton-deficient structural motifs like neopetrothiazide (1), which has 14 contiguous nonprotonated centers (C, N, and S). Neopetrothiazide (1), with an unprecedented thiazide-fused structural scaffold, is the first natural product containing a thiazide moiety.

NMR characterization of rearranged staurosporine aglycone analogues from the marine sponge Damiria sp

The indolocarbazole family of bisindole alkaloids is best known for the natural product staurosporine, a protein kinase C inhibitor that belongs to the indolo[2,3-a]carbazole structural class. A large number of other indolo[2,3-a]carbazoles have subsequently been isolated and identified, but other isomeric forms of indolocarbazole natural products have rarely been reported. An extract of the marine sponge Damiria sp., which represents an understudied genus, provided two novel alkaloids named damirines A (1) and B (2). Their structures were assigned by comprehensive NMR spectroscopic analyses, and for compound 2, this included application of the LR-HSQMBC pulse sequence, a long-range heteronuclear correlation experiment that has particular utility for defining proton-deficient scaffolds. The damirines represent a new hexacyclic carbon-nitrogen framework comprised of an indolo[3,2-a]carbazole fused with either an aminoimidazole or a imidazolone ring. Compound 1 showed selective cytotoxic properties toward six different cell lines in the NCI-60 cancer screen.

Four New Pregnane-10,2-carbolactones from an Epipolasis sp. Marine Sponge

Four new pregnane steroids, 3β,4β,16β-trihydroxypregna-5,17-diene-10,2-carbolactone (1), 16β-acetoxy-3β,4β-dihydroxypregna-5,17-diene-10,2-carbolactone (2), 12β-acetoxy-3β,4β,16β-trihydroxypregna-5,17-diene-10,2-carbolactone (3), and 12β,16β-diacetoxy-3β,4β-dihydroxypregna-5,17-diene-10,2-carbolactone (4) were isolated from an extract of an Epipolasis sp. marine sponge. The structures of the new compounds were determined by extensive NMR spectroscopic analysis and comparison with data from previously reported compounds.

Swinhopeptolides A and B: Cyclic Depsipeptides from the Sponge Theonella swinhoei That Inhibit Ras/Raf Interaction

Two new cyclic depsipeptides named swinhopeptolides A (1) and B (2) have been isolated from the marine sponge Theonella swinhoei cf. verrucosa, collected from Papua New Guinea. They each contain 11 diverse amino acid residues and 13-carbon polyketide moieties attached at the N-terminus. Compounds 1 and 2 each exist as two conformers in DMSO-d₆ due to cis/trans isomerism of the proline residue, and their structures were successfully assigned by extensive NMR analyses complemented by chemical degradation and derivatization studies. Swinhopeptolide B (2) contains a previously undescribed 2,6,8-trimethyldeca-(2E,4E,6E)-trienoic acid moiety N-linked to a terminal serine residue. Swinhopeptolides A (1) and B (2) showed significant inhibition of the Ras/Raf signaling pathway with IC₅₀ values of 5.8 and 8.5 μM, respectively.

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.

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.

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.

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.

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.

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.

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.

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-1_RF with EC₅₀ values of 23 and 27 nM, respectively.

Characterization of pomiferin triacetate as a novel mTOR and translation inhibitor

Deregulation of the phosphatidylinositol 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR)-70kDa ribosomal protein S6 kinase 1 (p70(S6K)) pathway is commonly observed in many tumors. This pathway controls proliferation, survival, and translation, and its overactivation is associated with poor prognosis for tumor-associated survival. Current efforts focus on the development of novel inhibitors of this pathway. In a cell-based high-throughput screening assay of 15,272 pure natural compounds, we identified pomiferin triacetate as a potent stabilizer of the tumor suppressor programmed cell death 4 (Pdcd4). Mechanistically, pomiferin triacetate appeared as a general inhibitor of the PI3K-Akt-mTOR-p70(S6K) cascade. Interference with this pathway occurred downstream of Akt but upstream of p70(S6K). Specifically, mTOR kinase emerged as the molecular target of pomiferin triacetate, with similar activities against mTOR complexes 1 and 2. In an in vitro mTOR kinase assay pomiferin triacetate dose-dependently inhibited mTOR with an IC50 of 6.2 μM. Molecular docking studies supported the interaction of the inhibitor with the catalytic site of mTOR. Importantly, pomiferin triacetate appeared to be highly selective for mTOR compared to a panel of 17 lipid and 50 protein kinases tested. As a consequence of the mTOR inhibition, pomiferin triacetate efficiently attenuated translation. In summary, pomiferin triacetate emerged as a novel and highly specific mTOR inhibitor with strong translation inhibitory effects. Thus, it might be an interesting lead structure for the development of mTOR- and translation-targeted anti-tumor therapies.

Inhibition of hypoxia inducible factor-2 transcription: isolation of active modulators from marine sponges

Renal or kidney cancer accounts for about 3% of all cancer cases reported each year in the U.S. Molecular signatures that define the cancer, such as the loss of functional VHL, are found in both sporadic and familial cases of cancer. In clear cell renal cancer, the transcription factor HIF-2α has been shown to have a distinct role in tumorigenesis. Our laboratories developed a cell-based screen to identify modulators of HIF-2α. Screening of the NCI's Natural Product Extract Repository resulted in the identification of 10 sponge extracts, from which 12 compounds were isolated. The biological evaluation of these compounds will be discussed including evaluation of HIF-1α vs HIF-2α selectivity and the isolated compounds' effects on mRNA from several pathways regulated by HIF.

Flabelliferins A and B, sesterterpenoids from the South Pacific sponge Carteriospongia flabellifera

Two new sesterterpenoids named flabelliferins A (1) and B (2) were isolated from the lipophilic extract of the sponge Cateriospongia flabellifera, collected in the South Pacific near Vanuatu. The structure and absolute configuration of these two compounds were assigned by a combination of one- and two-dimensional NMR spectroscopy and by Mosher's ester analysis. Flabelliferin A (1) has a rare 25-homocheilanthane carbon skeleton, while flabelliferin B (2) is a 24-nor-25-homoscalarane sesterterpenoid.

Botryllamides: natural product inhibitors of ABCG2

ABCG2 is a membrane-localized, human transporter protein that has been demonstrated to reduce the intracellular accumulation of substrates through ATP-dependent efflux. Highly expressed in placental syncytiotrophoblasts, brain microvasculature, and the gastrointestinal tract, ABCG2 has been shown to mediate normal tissue protection as well as limit oral bioavailability of substrate compounds. Development of ABCG2 inhibitors for clinical use may allow increased penetration of therapeutic agents into sanctuary sites and increased gastrointestinal absorption. Previously identified inhibitors have lacked potency or specificity or were toxic at concentrations needed to inhibit ABCG2; none are in clinical development. A previously developed high-throughput assay measuring inhibition of ABCG2-mediated pheophorbide a transport was applied to natural product extract libraries. Among the active samples were extracts from the marine ascidian Botryllus tyreus. Bioassay-guided fractionation resulted in purification of a series of botryllamides. Ten botryllamides were obtained, two of which (designated I and J) were novel. Activity against ABCG2 was confirmed by assessing the ability of the compounds to inhibit ABCG2-mediated BODIPY-prazosin transport in ABCG2-transfected HEK293 cells, compete with [(125)I]-iodoarylazidoprazosin (IAAP) labeling of ABCG2, stimulate ABCG2-associated ATPase activity, and reverse ABCG2-mediated resistance.

A selective small-molecule nuclear factor-kappaB inhibitor from a high-throughput cell-based assay for "activator protein-1 hits"

NSC 676914 has been identified as a selective nuclear factor-kappaB (NF-kappaB) inhibitor that does not inhibit cell proliferation. This compound was originally identified in a high-throughput cell-based assay for activator protein-1 (AP-1) inhibitors using synthetic compound libraries and the National Cancer Institute natural product repository. NSC 676914 shows activity against NF-kappaB in luciferase reporter assays at concentrations much less than the IC50 for AP-1. A serum response element reporter used as a specificity control and indicator of cell proliferation was relatively insensitive to the compound. Pretreatment with NSC 676914 is here shown to repress 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced IkappaB-alpha phosphorylation and translocation of p65/50 to the nucleus but not the processing of p52 from p100, suggesting the inhibition of NF-kappaB regulator IKKbeta rather than IKKalpha. Inhibition of NF-kappaB activation occurred as a consequence of blocking phosphorylation of IKK. Induction of IkappaB-alpha phosphorylation by TPA was diminished by pretreatment of NSC 676914 even at 1.1 mumol/L. In contrast, kinases c-Jun-NH2-kinase and extracellular signal-regulated kinases 1 and 2, important for AP-1 activation, showed no significant repression by this compound. Furthermore, a Matrigel invasion assay with breast cancer cell lines and a transformation assay in mouse JB6 cells revealed that TPA-induced invasion and transformation responses were completely repressed by this compound. These results suggest that NSC 676914 could be a novel inhibitor having potential therapeutic activity to target NF-kappaB for cancer treatment or prevention.

HIV-1 ribonuclease H inhibitory phenolic glycosides from Eugenia hyemalis

Three new galloyl arbutins, hyemalosides A-C (1-3), along with nine known compounds were isolated from the evergreen tree Eugenia hyemalis. The structures of compounds 1-3 were determined by analysis of NMR and MS data. Compounds 1-3 inhibited HIV-1 RNase H in vitro with IC50 values of 1.46, >18, and 1.19 microM, respectively. However, in a XTT-based cell viability assay using the human T-cell line CEM-SS infected with HIV-1 RT, none of the compounds inhibited the cytopathic effect of HIV-1 infection at the highest dose tested (20 microg/mL).

New inhibitors of ABCG2 identified by high-throughput screening

In order to identify novel inhibitors of the ATP-binding cassette transporter, ABCG2, a high-throughput assay measuring the accumulation of the ABCG2 substrate pheophorbide a in ABCG2-overexpressing NCI-H460 MX20 cells was used to screen libraries of compounds. Out of a library of 7,325 natural products and synthetic compounds from the National Cancer Institute/Developmental Therapeutics Program collection, 18 were found to inhibit ABCG2 at 10 micromol/L. After eliminating flavonoids and compounds of limited availability from the 18 original compounds, 10 of the 11 remaining compounds reversed mitoxantrone resistance in NCI-H460/MX20 cells and prevented ABCG2-mediated BODIPY-prazosin transport in ABCG2-transfected HEK293 cells, confirming an interaction with ABCG2. Based on the activity profiles and the availability of materials, five inhibitors were examined for their ability to compete with [(125)I]iodoarylazidoprazosin labeling of ABCG2, increase binding of the anti-ABCG2 antibody 5D3, and prevent P-glycoprotein or multidrug resistance protein 1-mediated transport. At a concentration of 20 micromol/L, all of the compounds reduced iodoarylazidoprazosin labeling by 50% to 80% compared with controls. All five compounds also increased 5D3 labeling of ABCG2, indicating that these compounds are inhibitors but not substrates of ABCG2. None of the compounds affected P-glycoprotein-mediated rhodamine 123 transport, whereas three affected multidrug resistance protein-1-mediated calcein transport at 25 mumol/L, suggesting that the compounds are relatively specific for ABCG2. These five novel inhibitors of ABCG2 activity may provide a basis for further investigation of ABCG2 function and its relevance in multidrug resistance.

Atomic-resolution crystal structure of the antiviral lectin scytovirin

The crystal structures of the natural and recombinant antiviral lectin scytovirin (SVN) were solved by single-wavelength anomalous scattering and refined with data extending to 1.3 A and 1.0 A resolution, respectively. A molecule of SVN consists of a single chain 95 amino acids long, with an almost perfect sequence repeat that creates two very similar domains (RMS deviation 0.25 A for 40 pairs of Calpha atoms). The crystal structure differs significantly from a previously published NMR structure of the same protein, with the RMS deviations calculated separately for the N- and C-terminal domains of 5.3 A and 3.7 A, respectively, and a very different relationship between the two domains. In addition, the disulfide bonding pattern of the crystal structures differs from that described in the previously published mass spectrometry and NMR studies.

Natural products active in aberrant c-Kit signaling

Development of modulators of constitutively active, kinase domain mutants of c-Kit has proved to be very difficult. Therefore, a high-throughput differential cytotoxicity assay was developed to screen for compounds that preferentially kill cells expressing constitutively active c-Kit. The cells used in the assay, murine IC2 mast cells, express either the D814Y activating mutation (functionally equivalent to human D816Y) or wild type protein. This assay is robust and highly reproducible. When applied to libraries of natural product extracts (followed by assay-guided fractionation), two differentially active compounds were identified. To assess possible mechanisms of action, the active compounds were tested for inhibitory activity against a panel of signaling enzymes (including wild type and mutant c-Kit). Neither of the compounds significantly affected any of the 73 enzymes tested. The effects of commercially available modulators of known signaling components were also assessed using the screening assay. None of these inhibitors reproduced the differential activity seen with the natural products. Finally, both compounds were found to affect mitochondrial potential in cells expressing c-Kit(D814Y). These results suggest that the newly identified natural products may provide new avenues for intervention in aberrant c-Kit signaling pathways.

The novel fold of scytovirin reveals a new twist for antiviral entry inhibitors

The solution structure of the potent 95 residue anti-HIV protein scytovirin has been determined and two carbohydrate-binding sites have been identified. This unique protein, containing five structurally important disulfide bonds, demonstrates a novel fold with no elements of extended regular secondary structure. Scytovirin contains two 39 residue sequence repeats, differing in only three amino acid residues, and each repeat has primary sequence similarity to chitin binding proteins. Both sequence repeats form similarly structured domains, with the exception of one region. The result is two carbohydrate-binding sites with substantially different affinities. The unusual fold clusters aromatic residues in both sites, suggesting a binding mechanism similar to other known hevein-like carbohydrate-binding proteins but differing in carbohydrate specificity. Scytovirin, originally isolated from the cyanobacterium Scytonema varium, holds potential as an HIV entry inhibitor for both therapeutic and prophylactic anti-HIV applications. The high-resolution structural studies reported are an important initial step in unlocking the therapeutic potential of scytovirin.

A high-throughput cell-based assay for inhibitors of ABCG2 activity

ABCG2 is a member of the adenosine triphosphate (ATP)-binding cassette family of multidrug transporters associated with resistance of tumor cells to many cytotoxic agents. Evaluation of modulators of ABCG2 activity has relied on methods such as drug sensitization, biochemical characterization, and transport studies. To search for novel inhibitors of ABCG2, a fluorescent cell-based assay was developed for application in high-throughput screening. Accumulation of pheophorbide a (PhA), an ABCG2-specific substrate, forms the basis for the assay in NCI-H460/MX20 cells overexpressing wild-type ABCG2. Treatment of these cells with 10 microM fumitremorgin C (FTC), a specific ABCG2 inhibitor, increased cell accumulation of PhA to 5.6 times control (Z' 0.5). Validation included confirmation with known ABCG2 inhibitors: FTC, novobiocin, tariquidar, and quercetin. Verapamil, reported to inhibit P-glycoprotein but not ABCG2, had insignificant activity. Screening of a library of 3523 natural products identified 11 compounds with high activity (> or = 50% of FTC, confirmed by reassay), including 3 flavonoids, members of a family of compounds that include ABCG2 inhibitors. One of the inhibitors detected, eupatin, was moderately potent (IC50 of 2.2 microM) and, like FTC, restored sensitivity of resistant cells to mitoxantrone. Application of this assay to other libraries of synthetic compounds and natural products is expected to identify novel inhibitors of ABCG2 activity.

Anti-HIV Cyclotides

The cyclotides are a recently discovered, structurally unique family of bioactive plant peptides. Their discovery spawned a series of structural analyses, synthetic efforts, and studies to define the biosynthesis and biological properties of these novel peptide metabolites. Cyclotides have a head-to-tail cyclized amino acid backbone and a conserved cystine knot motif that provides an extremely stable structural framework. They all share a common global fold and are highly resistant to denaturation and to cleavage by proteolytic enzymes. However, these macrocyclic peptides are quite permissive to amino acid substitutions or additions in several peripheral loop regions, since changes in these loops do not alter the core cyclotide structure. These features make cyclotides attractive templates for incorporating desired amino acid sequences and then delivering these peptide sequences in a well defined, highly stable framework. Cyclotides likely function in a defensive role in the source plants since they exhibit a broad spectrum of antimicrobial activity and are detrimental to the growth and survival of herbivorous insects. Cyclotides are gene-encoded polypeptides that are cleaved from larger precursor proteins and then cyclized. This review summarizes research done on a subset of cyclotides that were discovered due to their HIV inhibitory properties. It details the isolation and characterization of these compounds and describes this work in the context of our current state of knowledge of the entire cyclotide family.

Oligosaccharide and Glycoprotein Microarrays as Tools in HIV Glycobiology

Defining HIV envelope glycoprotein interactions with host factors or binding partners advances our understanding of the infectious process and provides a basis for the design of vaccines and agents that interfere with HIV entry. Here we employ carbohydrate and glycoprotein microarrays to analyze glycan-dependent gp120-protein interactions. In concert with new linking chemistries and synthetic methods, the carbohydrate arrays combine the advantages of microarray technology with the flexibility and precision afforded by organic synthesis. With these microarrays, we individually and competitively determined the binding profiles of five gp120 binding proteins, established the carbohydrate structural requirements for these interactions, and identified a potential strategy for HIV vaccine development.

Isolation and characterization of anti-HIV peptides fromDorstenia contrajervaandTreculia obovoidea

Using a high throughput screen based on the interaction of the HIV-1 gp41 ectodomain with the virucidal protein cyanovirin-N (CV-N), we isolated two new peptides which inhibited the binding of CV-N to gp41 and which subsequently showed anti-HIV activity in a whole cell assay. A 5-kDa (contrajervin) and 10 kDa (treculavirin) peptide were isolated from Dorstenia contrajerva and Treculia obovoidea, respectively. Treculavirin was composed of two subunits, each containing 50 amino acid residues, which are covalently linked by at least one disulfide bond between the subunits. Both peptides were shown to bind to gp41 and gp120 and to inhibit the cytopathic effects of HIV-1(RF) infection in a human T-lymphoblastoid cell line (CEM-SS).

Solution Structure of the Cyclotide Palicourein

The cyclotides are a family of disulfide-rich proteins from plants. They have the characteristic structural features of a circular protein backbone and a knotted arrangement of disulfide bonds. Structural and biochemical studies of the cyclotides suggest that their unique physiological stability can be loaned to bioactive peptide fragments for pharmaceutical and agricultural development. In particular, the cyclotides incorporate a number of solvent-exposed loops that are potentially suitable for epitope grafting applications. Here, we determine the structure of the largest known cyclotide, palicourein, which has an atypical size and composition within one of the surface-exposed loops. The structural data show that an increase in size of a palicourein loop does not perturb the core fold, to which the thermodynamic and chemical stability has been attributed. The cyclotide core fold, thus, can in principle be used as a framework for the development of useful pharmaceutical and agricultural bioactivities.

A potent novel anti-HIV protein from the cultured cyanobacterium Scytonema varium

A new anti-HIV protein, scytovirin, was isolated from aqueous extracts of the cultured cyanobacterium Scytonema varium. The protein displayed potent anticytopathic activity against laboratory strains and primary isolates of HIV-1 with EC50 values ranging from 0.3 to 22 nM. Scytovirin binds to viral coat proteins gp120, gp160, and gp41 but not to cellular receptor CD4 or other tested proteins. This unique protein consists of a single 95-amino acid chain with significant internal sequence duplication and 10 cysteines forming five intrachain disulfide bonds.

A novel anti-HIV macrocyclic peptide from Palicourea condensata

A 37 amino acid cyclic polypeptide has been isolated from the organic extract of the tropical tree Palicourea condensata. Palicourein (1) is the largest of a growing family of plant peptides that contain a cyclized amino acid backbone cross-linked via three internal disulfide bridges. Palicourein inhibits the in vitro cytopathic effects of HIV-1RF infection of CEM-SS cells with an EC50 value of 0.1 microM and an IC50 value of 1.5 microM.

Laxifloranone, a new phloroglucinol derivative from Marila laxiflora

A new polyisoprenylated phloroglucinol derivative has been isolated from the twigs of Marila laxiflora and characterized on the basis of 1D and 2D NMR spectra. Laxifloranone (1) shows moderate inhibition of the cytopathic effects of in vitro HIV infection.

Alertenone, a dimer of suberosenone from Alertigorgia sp

Bioassay-guided fractionation of organic extracts of the gorgonian Alertigorgia sp. has yielded the previously known suberosenone (1), a cytotoxic tricyclic sesquiterpene of the quadrone class, and alertenone (2), a dimer of suberosenone. The structure of 2 was determined by spectral analysis; the 1D TOCSY experiment was particularly useful in the structure elucidation. Comparison of the in vitro cytotoxicity of alertenone and suberosenone revealed that the dimeric alertenone was devoid of cytotoxicity below 35 microg/mL. In a hollow-fiber assay model of in vivo activity, suberosenone exhibited some growth inhibition of two of six tumor cell lines tested.

Pyranocoumarins from tropical species of the genus Calophyllum: a chemotaxonomic study of extracts in the National Cancer Institute collection

(+)-Calanolide A, a novel dipyranocoumarin from the Malesian tree Calophyllum lanigerum var. austrocoriaceum, and a closely related compound, (-)-calanolide B, isolated from Calophyllum teysmannii var. inophylloide, are representatives of a distinct class of nonnucleoside HIV-1 specific reverse-transcriptase inhibitor under development as an AIDS chemotherapeutic. NCI repository specimens totalling 315 organic extracts from 31 taxa of Calophyllum were analyzed for related pyranocoumarins using a simple TLC system. A total of 127 extracts was initially classified as "positive"; eight out of the 31 taxa examined, representing perhaps 28 species already described (1/7-1/8 of all the species in this genus), contained prenylated coumarins, suggesting that these compounds, while sometimes abundantly present, are not widespread in the genus. Representative members of the TLC-positive extracts were partitioned between CH2C12 and 25% aqueous MeOH; the CH2C12-soluble materials were then analyzed by TLC and 1H NMR to confirm the presence of pyranocoumarins. The anti-HIV activity of the partitioned extracts are also presented. This study suggested that there are several distinctive coumarin chemotaxonomic markers distinguishing species of this genus.

Isolation and characterization of new anti-HIV and cytotoxic leads from plants, marine, and microbial organisms

New cytotoxic isomalabaricane triterpenes have been isolated from a sponge Stelletta sp. (1-7); anti-HIV pterocarpans (8 and 9) and isoflavanoids (12-16 and 18) were elucidated from two tropical plants in the genus Erythrina; and anti-HIV enniatins (20 and 22-23) were characterized from fungi in the genera Fusarium and Alternaria. The enniatins were evaluated for in vivo anti-HIV activity in the hollow fiber assay.

Preparative separation of naphthyltetrahydroisoquinoline alkaloids from Ancistrocladus korupensis by centrifugal partition chromatography

Crude extracts of Ancistrocladus korupensis contain a complex mixture of naphthyltetrahydroisoquinoline alkaloids, including the human immunodeficiency virus-inhibitory dimeric alkaloids michellamines A and B and the antimalarial monomeric korupensamines A-D. The efficient separation of michellamines A and B from these extracts has been accomplished by centrifugal partition chromatography. The chromatographic conditions used on a multi-channel cartridge unit (Sanki LLN) have been successfully scaled up with a newly developed, stacked-disk type centrifugal partition chromatography unit (Sanki NMF) for separating larger amounts of alkaloid mixtures with similar resolution. A refined, three-step process (solvent-solvent partitioning, centrifugal partition chromatography and HPLC) has been developed and applied to the scaled-up production of michellamine B for preclinical drug development.

A chemical screening strategy for the dereplication and prioritization of HIV-inhibitory aqueous natural products extracts

A relatively high percentage (ca. 15%) of aqueous extracts from terrestrial plants, cyanobacteria, and marine invertebrates and algae has exhibited activity in the National Cancer Institute's primary AIDS-antiviral screen. By removal of anionic polysaccharides in a first stage of dereplication, we have eliminated from further consideration a considerable number of these extracts. However, a still substantial proportion of the active extracts remained, from which we wished to select and prioritize a small percentage for our detailed bioassay-directed fractionation studies. Therefore, a chemical screening protocol, utilizing various solid-phase extraction cartridges, has been developed for a second-stage dereplication and to assist in prioritization of these extracts for our further investigations.