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

New Discorhabdin D Analogues from Latrunculia spp. Sponges

Chemical profiles of Latrunculia kaakaariki and Latrunculia brevis were investigated, resulting in the isolation of five new discorhabdin D derivatives 1 and 3-6. Their planar structures were solved by combination of NMR and HR-MS, while J-based configurational analysis, computational techniques, and semisynthetic methods were used for the establishment of their absolute configurations. New natural products were tested for their growth inhibitory activity against the NCI-60 human tumor cell line panel, and two compounds 5 and 6 showed low micromolar potency.

Marine-Derived Leads as Anticancer Candidates by Disrupting Hypoxic Signaling through Hypoxia-Inducible Factors Inhibition

The inadequate vascularization seen in fast-growing solid tumors gives rise to hypoxic areas, fostering specific changes in gene expression that bolster tumor cell survival and metastasis, ultimately leading to unfavorable clinical prognoses across different cancer types. Hypoxia-inducible factors (HIF-1 and HIF-2) emerge as druggable pivotal players orchestrating tumor metastasis and angiogenesis, thus positioning them as prime targets for cancer treatment. A range of HIF inhibitors, notably natural compounds originating from marine organisms, exhibit encouraging anticancer properties, underscoring their significance as promising therapeutic options. Bioprospection of the marine environment is now a well-settled approach to the discovery and development of anticancer agents that might have their medicinal chemistry developed into clinical candidates. However, despite the massive increase in the number of marine natural products classified as 'anticancer leads,' most of which correspond to general cytotoxic agents, and only a few have been characterized regarding their molecular targets and mechanisms of action. The current review presents a critical analysis of inhibitors of HIF-1 and HIF-2 and hypoxia-selective compounds that have been sourced from marine organisms and that might act as new chemotherapeutic candidates or serve as templates for the development of structurally similar derivatives with improved anticancer efficacy.

Total Synthesis of (+)-Discorhabdin V

The discorhabdin natural products are a large subset of pyrroloiminoquinone alkaloids with a myriad of biological activities. Despite garnering much synthetic attention, few members have thus far been completed, particularly those featuring a bridging carbon-nitrogen bond that is found in numerous discorhabdins, including discorhabdin V. Herein we report the first total synthesis and full stereochemical assignment of (+)-discorhabdin V. To access the pyrroloiminoquinone we developed a convergent N-alkylation/oxidative aminocyclization/bromination cascade that joins two key components, which are both made on multigram scale. An intramolecular Heck reaction then forms the quaternary carbon center in an intermediate containing the carbon-nitrogen bridge, and a reductive N,O-acetal cyclization sequence introduces the final piperidine ring. Furthermore, we have established the relative configuration of (+)-discorhabdin V through experimental NOESY data and DP4 NMR probability calculations. The absolute configuration of the natural product has also been determined by circular dichroism and the use of an amino acid derived chiral starting material. Our work represents one of only two reports of a total synthesis of a nitrogen-bridged discorhabdin and paves the way for future biological evaluation of such compounds.

An Investigation of Structure-Activity Relationships and Cell Death Mechanisms of the Marine Alkaloids Discorhabdins in Merkel Cell Carcinoma Cells

A library of naturally occurring and semi-synthetic discorhabdins was assessed for their effects on Merkel cell carcinoma (MCC) cell viability. The set included five new natural products and semi-synthetic compounds whose structures were elucidated with NMR, HRMS, and ECD techniques. Several discorhabdins averaged sub-micromolar potency against the MCC cell lines tested and most of the active compounds showed selectivity towards virus-positive MCC cell lines. An investigation of structure-activity relationships resulted in an expanded understanding of the crucial structural features of the discorhabdin scaffold. Mechanistic cell death assays suggested that discorhabdins, unlike many other MCC-active small molecules, do not induce apoptosis, as shown by the lack of caspase activation, annexin V staining, and response to caspase inhibition. Similarly, discorhabdin treatment failed to increase MCC intracellular calcium and ROS levels. In contrast, the rapid loss of cellular reducing potential and mitochondrial membrane potential suggested that discorhabdins induce mitochondrial dysfunction leading to non-apoptotic cell death.

Marine Natural Compound (Neviotin A) Displays Anticancer Efficacy by Triggering Transcriptomic Alterations and Cell Death in MCF-7 Cells

We investigated the anticancer mechanism of a chloroform extract of marine sponge (Haliclona fascigera) (sample C) in human breast adenocarcinoma (MCF-7) cells. Viability analysis using MTT and neutral red uptake (NRU) assays showed that sample C exposure decreased the proliferation of cells. Flow cytometric data exhibited reactive oxygen species (ROS), nitric oxide (NO), dysfunction of mitochondrial potential, and apoptosis in sample C-treated MCF-7 cells. A qPCR array of sample C-treated MCF-7 cells showed crosstalk between different pathways of apoptosis, especially BIRC5, BCL2L2, and TNFRSF1A genes. Immunofluorescence analysis affirmed the localization of p53, bax, bcl2, MAPKPK2, PARP-1, and caspase-3 proteins in exposed cells. Bioassay-guided fractionation of sample C revealed Neviotin A as the most active compound triggering maximum cell death in MCF-7, indicating its pharmacological potency for the development of a drug for the treatment of human breast cancer.

Unified Divergent Total Synthesis of Discorhabdin B, H, K, and Aleutianamine via the Late-Stage Oxidative N,S-Acetal Formation

This study achieved the total syntheses of (+)-discorhabdin B, (-)-discorhabdin H, (+)-discorhabdin K, and (-)-aleutianamine. A phenethylamine fragment bearing a o-pivaloylthio group, corresponding to the D/E/G ring moiety, was prepared from benzothiophen-2-carboxylic acid methyl ester and condensed with a known pyrroloiminoquinone derivative. The adduct was subjected to [bis(trifluoroacetoxy)iodo]benzene (PIFA)-promoted oxidative spirocyclization to furnish the A/B/C/D/E spirocyclohexadienone fused with pyrroloiminoquinone. The total synthesis of (±)-discorhabdin B was completed via the key construction of the highly strained G ring with the N,S-acetal moiety featuring a newly developed CuBr2-mediated oxidative cascade cyclization. The stereocontrolled total synthesis of (+)-discorhabdin B was accomplished by a diastereoselective PIFA-promoted oxidative spirocyclization using a chiral thioester. (-)-Disocrhabdin H and (+)-discorhabdin K were synthesized by the site- and face-selective thia-Michael addition of l-ovothiol A to (+)-N-Ts-discorhabdin B with the concomitant formation of the F ring by forming the C2-N18 bond. The total synthesis of (-)-aleutianamine was achieved via a skeletal rearrangement initiated by the Luche reduction of the dienone moiety of (+)-N-Ts-discorhabdin B.

Heterocyclic Iminoquinones and Quinones from the National Cancer Institute (NCI, USA) COMPARE Analysis

This review uses the National Cancer Institute (NCI) COMPARE program to establish an extensive list of heterocyclic iminoquinones and quinones with similarities in differential growth inhibition patterns across the 60-cell line panel of the NCI Developmental Therapeutics Program (DTP). Many natural products and synthetic analogues are revealed as potential NAD(P)H:quinone oxidoreductase 1 (NQO1) substrates, through correlations to dipyridoimidazo[5,4-f]benzimidazoleiminoquinone (DPIQ), and as potential thioredoxin reductase (TrxR) inhibitors, through correlations to benzo[1,2,4]triazin-7-ones and pleurotin. The strong correlation to NQO1 infers the enzyme has a major influence on the amount of the active compound with benzo[e]perimidines, phenoxazinones, benz[f]pyrido[1,2-a]indole-6,11-quinones, seriniquinones, kalasinamide, indolequinones, and furano[2,3-b]naphthoquinones, hypothesised as prodrugs. Compounds with very strong correlations to known TrxR inhibitors had inverse correlations to the expression of both reductase enzymes, NQO1 and TrxR, including naphtho[2,3-b][1,4]oxazepane-6,11-diones, benzo[a]carbazole-1,4-diones, pyranonaphthoquinones (including kalafungin, nanaomycin A, and analogues of griseusin A), and discorhabdin C. Quinoline-5,8-dione scaffolds based on streptonigrin and lavendamycin can correlate to either reductase. Inhibitors of TrxR are not necessarily (imino)quinones, e.g., parthenolides, while oxidising moieties are essential for correlations to NQO1, as with the mitosenes. Herein, an overview of synthetic methods and biological activity of each family of heterocyclic imino(quinone) is provided.

Current Perspectives on Pyrroloiminoquinones: Distribution, Biosynthesis and Drug Discovery Potential

Pyrroloiminoquinones are a group of cytotoxic alkaloids most commonly isolated from marine sponges. Structurally, they are based on a tricyclic pyrrolo[4,3,2-de]quinoline core and encompass marine natural products such as makaluvamines, tsitsikammamines and discorhabdins. These diverse compounds are known to exhibit a broad spectrum of biological activities including anticancer, antiplasmodial, antimicrobial, antifungal and antiviral activities as well as the inhibition of several key cellular enzymes. The resurgence of interest in pyrroloiminoquinones and the convoluted understanding regarding their biological activities have prompted this review. Herein, we provided a concise summary of key findings and recent developments pertaining to their structural diversity, distribution, biogenesis, and their potential as chemical probes for drug development, including a discussion of promising synthetic analogs.

Anticancer Activities of Marine-Derived Phenolic Compounds and Their Derivatives

Since the middle of the last century, marine organisms have been identified as producers of chemically and biologically diverse secondary metabolites which have exerted various biological activities including anticancer, anti-inflammatory, antioxidant, antimicrobial, antifouling and others. This review primarily focuses on the marine phenolic compounds and their derivatives with potent anticancer activity, isolated and/or modified in the last decade. Reports on the elucidation of their structures as well as biosynthetic studies and total synthesis are also covered. Presented phenolic compounds inhibited cancer cells proliferation or migration, at sub-micromolar or nanomolar concentrations (lamellarins D (37), M (38), K (39), aspergiolide B (41), fradimycin B (62), makulavamine J (66), mayamycin (69), N-acetyl-N-demethylmayamycin (70) or norhierridin B (75)). In addition, they exhibited anticancer properties by a diverse biological mechanism including induction of apoptosis or inhibition of cell migration and invasive potential. Finally, phlorotannins 1–7 and bromophenols 12–29 represent the most researched phenolic compounds, of which the former are recognized as protective agents against UVB or gamma radiation-induced skin damages. Finally, phenolic metabolites were assorted into six main classes: phlorotannins, bromophenols, flavonoids, coumarins, terpenophenolics, quinones and hydroquinones. The derivatives that could not be attributed to any of the above-mentioned classes were grouped in a separate class named miscellaneous compounds.

A Soft Spot for Chemistry-Current Taxonomic and Evolutionary Implications of Sponge Secondary Metabolite Distribution

Marine sponges are the most prolific marine sources for discovery of novel bioactive compounds. Sponge secondary metabolites are sought-after for their potential in pharmaceutical applications, and in the past, they were also used as taxonomic markers alongside the difficult and homoplasy-prone sponge morphology for species delineation (chemotaxonomy). The understanding of phylogenetic distribution and distinctiveness of metabolites to sponge lineages is pivotal to reveal pathways and evolution of compound production in sponges. This benefits the discovery rate and yield of bioprospecting for novel marine natural products by identifying lineages with high potential of being new sources of valuable sponge compounds. In this review, we summarize the current biochemical data on sponges and compare the metabolite distribution against a sponge phylogeny. We assess compound specificity to lineages, potential convergences, and suitability as diagnostic phylogenetic markers. Our study finds compound distribution corroborating current (molecular) phylogenetic hypotheses, which include yet unaccepted polyphyly of several demosponge orders and families. Likewise, several compounds and compound groups display a high degree of lineage specificity, which suggests homologous biosynthetic pathways among their taxa, which identifies yet unstudied species of this lineage as promising bioprospecting targets.

Natural products as potent inhibitors of hypoxia-inducible factor-1α in cancer therapy

Hypoxia is a prominent feature of tumors. Hypoxia-inducible factor-1α (HIF-1α), a major subunit of HIF-1, is overexpressed in hypoxic tumor tissues and activates the transcription of many oncogenes. Accumulating evidence has demonstrated that HIF-1α promotes tumor angiogenesis, metastasis, metabolism, and immune evasion. Natural products are an important source of antitumor drugs and numerous studies have highlighted the crucial role of these agents in modulating HIF-1α. The present review describes the role of HIF-1α in tumor progression, summarizes natural products used as HIF-1α inhibitors, and discusses the potential of developing natural products as HIF-1α inhibitors for the treatment of cancer.

Effects of Sponge-Derived Alkaloids on Activities of the Bacterial α-D-Galactosidase and Human Cancer Cell α-N-Acetylgalactosaminidase

During a search for glycosidase inhibitors among marine natural products, we applied an integrated in vitro and in silico approach to evaluate the potency of some aaptamines and makaluvamines isolated from marine sponges on the hydrolyzing activity of α-N-acetylgalactosaminidase (α-NaGalase) from human cancer cells and the recombinant α-D-galactosidase (α-PsGal) from a marine bacterium Pseudoalteromonas sp. KMM 701. These alkaloids showed no direct inhibitory effect on the cancer α-NaGalase; but isoaaptamine (2), 9-demethylaaptamine (3), damirone B (6), and makaluvamine H (7) reduced the expression of the enzyme in the human colorectal adenocarcinoma cell line DLD-1 at 5 μM. Isoaaptamine (2), 9-demethylaaptamine (3), makaluvamine G (6), and zyzzyanone A (7) are slow-binding irreversible inhibitors of the bacterial α-PsGal with the inactivation rate constants (k_inact) 0.12 min⁻¹, 0.092 min⁻¹, 0.079 min⁻¹, and 0.037 min⁻¹, as well as equilibrium inhibition constants (K_i) 2.70 µM, 300 µM, 411 µM, and 105 µM, respectively. Docking analysis revealed that these alkaloids bind in a pocket close to the catalytic amino acid residues Asp451 and Asp516 and form complexes, due to π-π interactions with the Trp308 residue and hydrogen bonds with the Lys449 residue. None of the studied alkaloids formed complexes with the active site of the human α-NaGalase.

Metabolites from Marine Sponges and Their Potential to Treat Malarial Protozoan Parasites Infection: A Systematic Review

Malaria is an infectious disease caused by protozoan parasites of the Plasmodium genus through the bite of female Anopheles mosquitoes, affecting 228 million people and causing 415 thousand deaths in 2018. Artemisinin-based combination therapies (ACTs) are the most recommended treatment for malaria; however, the emergence of multidrug resistance has unfortunately limited their effects and challenged the field. In this context, the ocean and its rich biodiversity have emerged as a very promising resource of bioactive compounds and secondary metabolites from different marine organisms. This systematic review of the literature focuses on the advances achieved in the search for new antimalarials from marine sponges, which are ancient organisms that developed defense mechanisms in a hostile environment. The principal inclusion criterion for analysis was articles with compounds with IC50 below 10 µM or 10 µg/mL against P. falciparum culture. The secondary metabolites identified include alkaloids, terpenoids, polyketides endoperoxides and glycosphingolipids. The structural features of active compounds selected in this review may be an interesting scaffold to inspire synthetic development of new antimalarials for selectively targeting parasite cell metabolism.

Unlocking the Diversity of Pyrroloiminoquinones Produced by Latrunculid Sponge Species

Sponges of the Latrunculiidae family produce bioactive pyrroloiminoquinone alkaloids including makaluvamines, discorhabdins, and tsitsikammamines. The aim of this study was to use LC-ESI-MS/MS-driven molecular networking to characterize the pyrroloiminoquinone secondary metabolites produced by six latrunculid species. These are Tsitsikamma favus, Tsitsikamma pedunculata, Cyclacanthia bellae, and Latrunculia apicalis as well as the recently discovered species, Tsitsikamma nguni and Tsitsikamma michaeli. Organic extracts of 43 sponges were analyzed, revealing distinct species-specific chemical profiles. More than 200 known and unknown putative pyrroloiminoquinones and related compounds were detected, including unprecedented makaluvamine-discorhabdin adducts and hydroxylated discorhabdin I derivatives. The chemical profiles of the new species T. nguni closely resembled those of the known T. favus (chemotype I), but with a higher abundance of tsitsikammamines vs. discorhabdins. T. michaeli sponges displayed two distinct chemical profiles, either producing mostly the same discorhabdins as T. favus (chemotype I) or non- or monobrominated, hydroxylated discorhabdins. C. bellae and L. apicalis produced similar pyrroloiminoquinone chemistry to one another, characterized by sulfur-containing discorhabdins and related adducts and oligomers. This study highlights the variability of pyrroloiminoquinone production by latrunculid species, identifies novel isolation targets, and offers fundamental insights into the collision-induced dissociation of pyrroloiminoquinones.

Marine Pharmacology in 2016-2017: Marine Compounds with Antibacterial, Antidiabetic, Antifungal, Anti-Inflammatory, Antiprotozoal, Antituberculosis and Antiviral Activities; Affecting the Immune and Nervous Systems, and Other Miscellaneous Mechanisms of Action

The review of the 2016-2017 marine pharmacology literature was prepared in a manner similar as the 10 prior reviews of this series. Preclinical marine pharmacology research during 2016-2017 assessed 313 marine compounds with novel pharmacology reported by a growing number of investigators from 54 countries. The peer-reviewed literature reported antibacterial, antifungal, antiprotozoal, antituberculosis, and antiviral activities for 123 marine natural products, 111 marine compounds with antidiabetic and anti-inflammatory activities as well as affecting the immune and nervous system, while in contrast 79 marine compounds displayed miscellaneous mechanisms of action which upon further investigation may contribute to several pharmacological classes. Therefore, in 2016-2017, the preclinical marine natural product pharmacology pipeline generated both novel pharmacology as well as potentially new lead compounds for the growing clinical marine pharmaceutical pipeline, and thus sustained with its contributions the global research for novel and effective therapeutic strategies for multiple disease categories.

Chemistry, Chemotaxonomy and Biological Activity of the Latrunculid Sponges (Order Poecilosclerida, Family Latrunculiidae)

Marine sponges are exceptionally prolific sources of natural products for the discovery and development of new drugs. Until now, sponges have contributed around 30% of all natural metabolites isolated from the marine environment. Family Latrunculiidae Topsent, 1922 (class Demospongiae Sollas, 1885, order Poecilosclerida Topsent, 1928) is a small sponge family comprising seven genera. Latrunculid sponges are recognized as the major reservoirs of diverse types of pyrroloiminoquinone-type alkaloids, with a myriad of biological activities, in particular, cytotoxicity, fuelling their exploration for anticancer drug discovery. Almost 100 pyrroloiminoquinone alkaloids and their structurally related compounds have been reported from the family Latrunculiidae. The systematics of latrunculid sponges has had a complex history, however it is now well understood. The pyrroloiminoquinone alkaloids have provided important chemotaxonomic characters for this sponge family. Latrunculid sponges have been reported to contain other types of metabolites, such as peptides (callipeltins), norditerpenes and norsesterpenes (trunculins) and macrolides (latrunculins), however, the sponges containing latrunculins and trunculins have been transferred to other sponge families. This review highlights a comprehensive literature survey spanning from the first chemical investigation of a New Zealand Latrunculia sp. in 1986 until August 2020, focusing on the chemical diversity and biological activities of secondary metabolites reported from the family Latrunculiidae. The biosynthetic (microbial) origin and the taxonomic significance of pyrroloiminoquinone related alkaloids are also discussed.

Synthetic Makaluvamine Analogs Decrease c-Kit Expression and Are Cytotoxic to Neuroendocrine Tumor Cells

In an effort to discover viable systemic chemotherapeutic agents for neuroendocrine tumors (NETs), we screened a small library of 18 drug-like compounds obtained from the Velu lab against pulmonary (H727) and thyroid (MZ-CRC-1 and TT) neuroendocrine tumor-derived cell lines. Two potent lead compounds (DHN-II-84 and DHN-III-14) identified from this screening were found to be analogs of the natural product makaluvamine. We further characterized the antitumor activities of these two compounds using pulmonary (H727), thyroid (MZ-CRC-1) and pancreatic (BON) neuroendocrine tumor cell lines. Flow cytometry showed a dose-dependent increase in apoptosis in all cell lines. Induction of apoptosis with these compounds was also supported by the decrease in myeloid cell leukemia-1 (MCL-1) and X-chromosome linked inhibitor of apoptosis (XIAP) detected by Western blot. Compound treatment decreased NET markers chromogranin A (CgA) and achaete-scute homolog 1 (ASCL1) in a dose-dependent manner. Moreover, the gene expression analysis showed that the compound treatment reduced c-Kit proto-oncogene expression in the NET cell lines. Induction of apoptosis could also have been caused by the inhibition of c-Kit expression, in addition to the known mechanisms such as damage of DNA by topoisomerase II inhibition for this class of compounds. In summary, makaluvamine analogs DHN-II-84 and DHN-III-14 induced apoptosis, decreased neuroendocrine tumor markers, and showed promising antitumor activity in pulmonary, thyroid, and pancreatic NET cell lines, and hold potential to be developed as an effective treatment to combat neuroendocrine tumors.

Exploration of the Electrophilic Reactivity of the Cytotoxic Marine Alkaloid Discorhabdin C and Subsequent Discovery of a New Dimeric C-1/N-13-Linked Discorhabdin Natural Product

The cytotoxic marine natural product discorhabdin C contains a 2,6-dibromo-cyclohexa-2,5-diene moiety, previously proposed to be a critical feature required for biological activity. We have determined that the dienone-ring of discorhabdin C is indeed electrophilic, reacting with thiol and amine nucleophiles, affording debrominated adducts. In the case of reaction with 1-aminopentane the product contains an unusual C-2/N-18 ring closed, double-hydrate moiety. This electrophilic reactivity also extends to proteins, with lysozyme-discorhabdin C adducts being detected by ESI mass spectrometry. These results prompted further examination of an extract of discorhabdin C-producing sponge, Latrunculia (Latrunculia) trivetricillata, leading to the isolation and characterisation of a new example of a C-1/N-13 linked discorhabdin dimer that shared structural similarities with the 1-aminopentane-discorhabdin C adduct. To definitively assess the influence of the dienone moiety of discorhabdin C on cytotoxicity, a semi-synthetic hydrogenation derivative was prepared, affording a didebrominated ring-closed carbinolamine that was essentially devoid of tumour cell line cytotoxicity. Antiparasitic activity was assessed for a set of 14 discorhabdin alkaloids composed of natural products and semi-synthetic derivatives. Three compounds, (-)-discorhabdin L, a dimer of discorhabdin B and the discorhabdin C hydrogenation carbinolamine, exhibited pronounced activity towards Plasmodium falciparum K1 (IC₅₀ 30–90 nM) with acceptable to excellent selectivity (selectivity index 19–510) versus a non-malignant cell line.

Creating and screening natural product libraries

Covering: up to 2020The National Cancer Institute of the United States (NCI) has initiated a Cancer Moonshot program entitled the NCI Program for Natural Product Discovery. As part of this effort, the NCI is producing a library of 1 000 000 partially purified natural product fractions which are being plated into 384-well plates and provided to the research community free of charge. As the first 326 000 of these fractions have now been made available, this review seeks to describe the general methods used to collect organisms, extract those organisms, and create a prefractionated library. Importantly, this review also details both cell-based and cell-free bioassay methods and the adaptations necessary to those methods to productively screen natural product libraries. Finally, this review briefly describes post-screen dereplication and compound purification and scale up procedures which can efficiently identify active compounds and produce sufficient quantities of natural products for further pre-clinical development.

Marine Natural Products with High Anticancer Activities

This review covers recent literature from 2012-2019 concerning 170 marine natural products and their semisynthetic analogues with strong anticancer biological activities. Reports that shed light on cellular and molecular mechanisms and biological functions of these compounds, thus advancing the understanding in cancer biology are also included. Biosynthetic studies and total syntheses, which have provided access to derivatives and have contributed to the proper structure or stereochemistry elucidation or revision are mentioned. The natural compounds isolated from marine organisms are divided into nine groups, namely: alkaloids, sterols and steroids, glycosides, terpenes and terpenoids, macrolides, polypeptides, quinones, phenols and polyphenols, and miscellaneous products. An emphasis is placed on several drugs originating from marine natural products that have already been marketed or are currently in clinical trials.

Tridiscorhabdin and Didiscorhabdin, the First Discorhabdin Oligomers Linked with a Direct C-N Bridge from the Sponge Latrunculia biformis Collected from the Deep Sea in Antarctica

Guided by LC-MS/MS molecular networking-based metabolomics and cytotoxic activity, two new discorhabdin-type alkaloids, tridiscorhabdin (1) and didiscorhabdin (2), were isolated from the sponge Latrunculia biformis, collected from the Weddell Sea (Antarctica) at -291 m depth. Their structures were established by HRESIMS, NMR, [α]_D, and ECD data coupled with DFT calculations. Both compounds bear a novel C-N bridge (C-1/N-13) between discorhabdin monomers, and 1 represents the first trimeric discorhabdin molecule isolated from Nature. Tridiscorhabdin (1) exhibited strong cytotoxic activity against the human colon cancer cell line HCT-116 (IC₅₀ value 0.31 μM).

New Discorhabdin B Dimers with Anticancer Activity from the Antarctic Deep-Sea Sponge Latrunculia biformis

Latrunculia sponges represent a rich source of discorhabdin-type pyrroloiminoquinone alkaloids, a few of which comprise a dimeric structure. The anticancer-activity-guided isolation of the n-hexane subextract of the Antarctic deep-sea sponge Latrunculia biformis yielded the known compound (-)-(1R,2R,6R,8S,6'S)-discorhabdin B dimer (1) and two new derivatives, (-)-(1S,2R,6R,8S,6'S)-discorhabdin B dimer (2) and (-)-(1R,2R,6R,8S,6'S)-16',17'-dehydrodiscorhabdin B dimer (3). The chemical structures of compounds 1-3 were elucidated by means of HR-ESIMS, NMR, [], ECD spectroscopy, and a comparison with the previously reported discorhabdin analogs. Compounds 1 and 2 showed significant in vitro anticancer activity against the human colon cancer cell line (HCT-116), with IC₅₀ values of 0.16 and 2.01 µM, respectively. Compared to monomeric discorhabdins, dimeric discorhabdins are very rare in Nature. This study adds two new discorhabdin dimers (2 and 3) to this small pyrroloiminoquinone subfamily. This is also the first report of compound 1 as a natural product and the first assessment of its in vitro anticancer activity.

The Cancer Clock Is (Not) Ticking: Links between Circadian Rhythms and Cancer

Circadian rhythms regulate many physiological and behavioral processes, including sleep, metabolism and cell division, which have a 24-h oscillation pattern. Rhythmicity is generated by a transcriptional–translational feedback loop in individual cells, which are synchronized by the central pacemaker in the brain and external cues. Epidemiological and clinical studies indicate that disruption of these rhythms can increase both tumorigenesis and cancer progression. Environmental changes (shift work, jet lag, exposure to light at night), mutations in circadian regulating genes, and changes to clock gene expression are recognized forms of disruption and are associated with cancer risk and/or cancer progression. Experimental data in animals and cell cultures further supports the role of the cellular circadian clock in coordinating cell division and DNA repair, and disrupted cellular clocks accelerate cancer cell growth. This review will summarize studies linking circadian disruption to cancer biology and explore how such disruptions may be further altered by common characteristics of tumors including hypoxia and acidosis. We will highlight how circadian rhythms might be exploited for cancer drug development, including how delivery of current chemotherapies may be enhanced using chronotherapy. Understanding the role of circadian rhythms in carcinogenesis and tumor progression will enable us to better understand causes of cancer and how to treat them.

New Discorhabdin Alkaloids from the Antarctic Deep-Sea Sponge Latrunculia biformis

The sponge genus Latrunculia is a prolific source of discorhabdin type pyrroloiminoquinone alkaloids. In the continuation of our research interest into this genus, we studied the Antarctic deep-sea sponge Latrunculia biformis that showed potent in vitro anticancer activity. A targeted isolation process guided by bioactivity and molecular networking-based metabolomics yielded three known discorhabdins, (-)-discorhabdin L (1), (+)-discorhabdin A (2), (+)-discorhabdin Q (3), and three new discorhabdin analogs (-)-2-bromo-discorhabdin D (4), (-)-1-acetyl-discorhabdin L (5), and (+)-1-octacosatrienoyl-discorhabdin L (6) from the MeOH-soluble portion of the organic extract. The chemical structures of 1-6 were elucidated by extensive NMR, HR-ESIMS, FT-IR, [α]_D, and ECD (Electronic Circular Dichroism) spectroscopy analyses. Compounds 1, 5, and 6 showed promising anticancer activity with IC₅₀ values of 0.94, 2.71, and 34.0 µM, respectively. Compounds 1-6 and the enantiomer of 1 ((+)-discorhabdin L, 1e) were docked to the active sites of two anticancer targets, topoisomerase I-II and indoleamine 2,3-dioxygenase (IDO1), to reveal, for the first time, the binding potential of discorhabdins to these proteins. Compounds 5 and 6 are the first discorhabdin analogs with an ester function at C-1 and 6 is the first discorhabdin bearing a long-chain fatty acid at this position. This study confirms Latrunculia sponges to be excellent sources of chemically diverse discorhabdin alkaloids.

Target Identification of the Marine Natural Products Dictyoceratin-A and -C as Selective Growth Inhibitors in Cancer Cells Adapted to Hypoxic Environments

Hypoxia-adapted cancer cells in tumors contribute to the pathological progression of cancer. The marine spongean sesquiterpene phenols dictyoceratin-A (1) and -C (2) have been shown to induce hypoxia-selective growth inhibition in cultured cancer cells and exhibit in vivo antitumor effects. These compounds inhibit the accumulation of hypoxia-inducible factor-1α (HIF-1α), which is a drug target in hypoxia-adapted cancer cells, under hypoxic conditions. However, the target molecules of compounds 1 and 2, which are responsible for decreasing HIF-1α expression under hypoxic conditions, remain unclear. In this study, we synthesized probe molecules for compounds 1 and 2 to identify their target molecules and found that both compounds bind to RNA polymerase II-associated protein 3 (RPAP3), which is a component of the R2TP/Prefoldin-like (PEDL) complex. In addition, RPAP3-knockdown cells showed a phenotype similar to that of compound-treated cells.

Advances in inhibition of protein-protein interactions targeting hypoxia-inducible factor-1 for cancer therapy

Hypoxia is a common characteristic of many types of solid tumors and is associated with tumor propagation, malignant progression, and resistance to anti-cancer therapy. HIF-1 pathway is one of the survival pathways activated in tumor in response to hypoxia. In hypoxic condition, hypoxia-inducible factor-1α (HIF-1α) is stabilized and translocated into nucleus where it forms heterodimer with HIF-1β and regulates the expression of a plethora of genes involved in different processes, such as cell proliferation, differentiation, apoptosis, vascularization/angiogenesis, tumor invasion and metastasis. Recruitment of co-activator p300 or CBP to HIF-1α is critical to the transactivation activity of HIF-1 dimer, therefore, small molecules which can block the dimerization of HIF-1α and HIF-1β or inhibit the interaction between HIF-1α and p300 can function as inhibitors of HIF-1 and have the potential to be developed as novel therapies for the treatment of human cancers. In this review, recent progress of small molecular inhibitors of protein-protein interactions targeting HIF-1 is summarized, the mechanism of functions of these compounds and their potential usage as anti-cancer agents have also been discussed.

Obstructive Sleep Apnea Activates HIF-1 in a Hypoxia Dose-Dependent Manner in HCT116 Colorectal Carcinoma Cells

Obstructive sleep apnea (OSA) affects a significant proportion of the population and is linked to increased rates of cancer development and a worse cancer outcome. OSA is characterized by nocturnal intermittent hypoxia and animal models of OSA-like intermittent hypoxia show increased tumor growth and metastasis. Advanced tumors typically have regions of chronic hypoxia, activating the transcription factor, HIF-1, which controls the expression of genes involved in cancer progression. Rapid intermittent hypoxia from OSA has been proposed to increase HIF-1 activity and this may occur in tumors. The effect of exposing a developing tumor to OSA-like intermittent hypoxia is largely unknown. We have built a cell-based model of physiological OSA tissue oxygenation in order to study the effects of intermittent hypoxia in HCT116 colorectal cancer cells. We found that HIF-1α increases following intermittent hypoxia and that the expression of HIF-target genes increases, including those involved in glycolysis, the hypoxic pathway and extracellular matrix remodeling. Expression of these genes acts as a 'hypoxic' signature which is associated with a worse prognosis. The total dose of hypoxia determined the magnitude of change in the hypoxic signature rather than the frequency or duration of hypoxia-reoxygenation cycles per se. Finally, transcription of HIF1A mRNA differs in response to chronic and intermittent hypoxia suggesting that HIF-1α may be regulated at the transcriptional level in intermittent hypoxia and not just by the post-translational oxygen-dependent degradation pathway seen in chronic hypoxia.

Molecular Networking Reveals Two Distinct Chemotypes in Pyrroloiminoquinone-Producing Tsitsikamma favus Sponges

The temperate marine sponge, Tsitsikamma favus, produces pyrroloiminoquinone alkaloids with potential as anticancer drug leads. We profiled the secondary metabolite reservoir of T. favus sponges using HR-ESI-LC-MS/MS-based molecular networking analysis followed by preparative purification efforts to map the diversity of new and known pyrroloiminoquinones and related compounds in extracts of seven specimens. Molecular taxonomic identification confirmed all sponges as T. favus and five specimens (chemotype I) were found to produce mainly discorhabdins and tsitsikammamines. Remarkably, however, two specimens (chemotype II) exhibited distinct morphological and chemical characteristics: the absence of discorhabdins, only trace levels of tsitsikammamines and, instead, an abundance of unbranched and halogenated makaluvamines. Targeted chromatographic isolation provided the new makaluvamine Q, the known makaluvamines A and I, tsitsikammamine B, 14-bromo-7,8-dehydro-3-dihydro-discorhabdin C, and the related pyrrolo-ortho-quinones makaluvamine O and makaluvone. Purified compounds displayed different activity profiles in assays for topoisomerase I inhibition, DNA intercalation and antimetabolic activity against human cell lines. This is the first report of makaluvamines from a Tsitsikamma sponge species, and the first description of distinct chemotypes within a species of the Latrunculiidae family. This study sheds new light on the putative pyrroloiminoquinone biosynthetic pathway of latrunculid sponges.

Targeted Isolation of Tsitsikammamines from the Antarctic Deep-Sea Sponge Latrunculia biformis by Molecular Networking and Anticancer Activity

The Antarctic deep-sea sponge Latrunculia (Latrunculia) biformis Kirkpatrick, 1908 (Class Demospongiae Sollas, Order Poecilosclerida Topsent, Latrunculiidae Topsent) was selected for chemical analyses due to its potent anticancer activity. Metabolomic analysis of its crude extract by HRMS/MS-based molecular networking showed the presence of several clusters of pyrroloiminoquinone alkaloids, i.e., discorhabdin and epinardin-type brominated pyridopyrroloquinolines and tsitsikammamines, the non-brominated bis-pyrroloiminoquinones. Molecular networking approach combined with a bioactivity-guided isolation led to the targeted isolation of the known pyrroloiminoquinone tsitsikammamine A (1) and its new analog 16,17-dehydrotsitsikammamine A (2). The chemical structures of the compounds 1 and 2 were elucidated by spectroscopic analysis (one-dimensional (1D) and two-dimensional (2D) NMR, HR-ESIMS). Due to minute amounts, molecular modeling and docking was used to assess potential affinities to potential targets of the isolated compounds, including DNA intercalation, topoisomerase I-II, and indoleamine 2,3-dioxygenase enzymes. Tsitsikammamines represent a small class of pyrroloiminoquinone alkaloids that have only previously been reported from the South African sponge genus Tsitsikamma Samaai & Kelly and an Australian species of the sponge genus Zyzzya de Laubenfels. This is the first report of tsitsikammamines from the genus Latrunculia du Bocage and the successful application of molecular networking in the identification of comprehensive chemical inventory of L.biformis followed by targeted isolation of new molecules. This study highlights the high productivity of secondary metabolites of Latrunculia sponges and may shed new light on their biosynthetic origin and chemotaxonomy.

Preclinical Evaluation of Discorhabdins in Antiangiogenic and Antitumor Models

Elements of the hypoxia inducible factor (HIF) transcriptional system, a key regulator of the cellular hypoxic response, are up-regulated in a range of cancer cells. HIF is fundamentally involved in tumor angiogenesis, invasion, and energy metabolism. Inhibition of the transcriptional activity of HIF may be of therapeutic benefit to cancer patients. We recently described the identification of two marine pyrroloiminoquinone alkaloids with potent activity in inhibiting the interaction between the oncogenic transcription factor HIF-1α and the coactivator protein p300. Herein, we present further characterization data for these two screening hits: discorhabdin H (1) and discorhabdin L (2), with a specific focus on their anti-angiogenic and anti-tumor effects. We demonstrated that only discorhabdin L (2) possesses excellent anti-angiogenic activity in inhibiting endothelial cell proliferation and tube formation, as well as decreasing microvessel outgrowth in the ex vivo rat aortic ring assay. We further showed that discorhabdin L (2) significantly inhibits in vivo prostate tumor growth in a LNCaP xenograft model. In conclusion, our findings suggest that discorhabdin L (2) represents a promising HIF-1α inhibitor worthy of further drug development.

Molecular Targets of Active Anticancer Compounds Derived from Marine Sources

Over the past decades, a number of novel compounds, which are produced in the marine environment, have been found to exhibit the anticancer effects. This review focuses on molecular targets of marine-derived anticancer candidates in clinical and preclinical studies. They are kinases, transcription factors, histone deacetylase, the ubiquitin-proteasome system, and so on. Specific emphasis of this review paper is to provide information on the optimization of new target compounds for future research and development of anticancer drugs, based on the identification of structures of these target molecules and parallel compounds.

Downregulation of decidual SP1 and P300 is associated with severe preeclampsia

Preeclampsia (PE) is a pregnancy-induced disorder characterized by hypertension and proteinuria after 20 weeks of gestation, affecting 5-7% of pregnancies worldwide. So far, the etiology of PE remains poorly understood. Abnormal decidualization is thought to contribute to the development of PE. SP1 belongs to the Sp/KLF superfamily and can recruit P300 to regulate the transcription of several genes. SP1 is also very important for decidualization as it enhances the expression of tissue factor. In this study, we investigated the expression of SP1 and P300 in deciduae and their relationship with PE. A total of 42 decidua samples were collected, of which 21 were from normal pregnant (NP) and 21 from severe PE. SP1 and P300 expression in deciduae and the levels of SP1 and P300 in cultured human endometrial stromal cells (hESCs) and primary hESCs during decidualization were determined. To further investigate the role of SP1 and P300 in human decidualization, RNA interference was used to silence SP1 and P300 in hESCs and primary hESCs. The following results were obtained. We found that the expressions of SP1 and P300 were reduced in decidual tissues with PE compared to those from NP. In the in vitro model of induction of decidualization, we found an increase in both SP1 and P300 levels. Silencing of SP1 and P300 resulted in abnormal decidualization and a significant reduction of decidualization markers such as insulin-like growth factor-binding protein1 and prolactin. Furthermore, the expression of vascular endothelial growth factor was also decreased upon SP1 and P300 silencing. Similar results were observed in primary hESCs. Our results suggest that SP1 and P300 play an important role during decidualization. Dysfunction of SP1 and P300 leads to impaired decidualization and might contribute to PE.

Marine natural products

Covering: 2016. Previous review: Nat. Prod. Rep., 2017, 34, 235-294This review covers the literature published in 2016 for marine natural products (MNPs), with 757 citations (643 for the period January to December 2016) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1277 in 432 papers for 2016), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included.

Recent Advances in the Synthesis of Spiroheterocycles via N-Heterocyclic Carbene Organocatalysis

Spiroheterocycles are regarded as a privileged framework because of their wide distribution in various natural products and synthetic molecules and promising bioactivities. This review focuses on the recent advances in the synthesis of spiroheterocycles by using the strategy of N-heterocyclic carbene (NHC) organocatalysis, and is organized based on the stereoselectivity and the reactive intermediates. According to the stereochemistry, this review was divided into two main parts, covering racemic and enantioselective versions. In each part, we firstly describe the synthetic transformations using nucleophilic Breslow intermediates, and then discuss the reactions that employ electrophilic acylazolium or radical cation intermediates. With those distinct catalytic activation modes of NHC organocatlysis, we expect this synthetic protocol will possibly produce new molecules with structural novelty and complexity, which may warrant further research in the field of drug discovery.

Another Look at Pyrroloiminoquinone Alkaloids-Perspectives on Their Therapeutic Potential from Known Structures and Semisynthetic Analogues

This study began with the goal of identifying constituents from Zyzzya fuliginosa extracts that showed selectivity in our primary cytotoxicity screen against the PANC-1 tumor cell line. During the course of this project, which focused on six Z. fuliginosa samples collected from various regions of the Indo-Pacific, known compounds were obtained consisting of nine makaluvamine and three damirone analogues. Four new acetylated derivatives were also prepared. High-accuracy electrospray ionization mass spectrometry (HAESI-MS) m/z ions produced through MS² runs were obtained and interpreted to provide a rapid way for dereplicating isomers containing a pyrrolo[4,3,2-de]quinoline core. In vitro human pancreas/duct epithelioid carcinoma (PANC-1) cell line IC₅₀ data was obtained for 16 compounds and two therapeutic standards. These results along with data gleaned from the literature provided useful structure activity relationship conclusions. Three structural motifs proved to be important in maximizing potency against PANC-1: (i) conjugation within the core of the ABC-ring; (ii) the presence of a positive charge in the C-ring; and (iii) inclusion of a 4-ethyl phenol or 4-ethyl phenol acetate substituent off the B-ring. Two compounds, makaluvamine J (9) and 15-O-acetyl makaluvamine J (15), contained all three of these frameworks and exhibited the best potency with IC₅₀ values of 54 nM and 81 nM, respectively. These two most potent analogs were then tested against the OVCAR-5 cell line and the presence of the acetyl group increased the potency 14-fold from that of 9 whose IC₅₀ = 120 nM vs. that of 15 having IC₅₀ = 8.6 nM.

Endophytic fungi from mangrove inhibit lung cancer cell growth and angiogenesis in vitro

The secondary metabolites of mangrove-derived endophytic fungi contain multiple substances with novel structures and biological activities. In the present study, three types of mangrove plants, namely Kandelia candel, Rhizophora stylosa and Rhizophoraceae from Zhanjiang region including the leaves, roots and stems were collected, and endophytic fungi were isolated, purified and identified from these mangrove plants. MTT assay was used to observe the effects of the isolated endophytic fungi on the growth of A549 and NCI-H460 lung cancer cells. The effect of the endophytic fungi on lung cancer angiogenesis in vitro induced by the HPV-16 E7 oncoprotein was observed. Our results showed that 28 strains of endophytic fungi were isolated, purified and identified from the three types of mangrove plants. Ten strains of endophytic fungi significantly suppressed the growth of A549 and NCI-H460 cells. The average inhibitory rates in the A549 cells were 64.4, 59.5, 81.9, 43.9, 58.3, 56.2, 48.3, 42.4, 93.0 and 49.7%, respectively. The average inhibitory rates in the NCI-H460 cells were 41.2, 49.3, 82.7, 40.7, 53.9, 52.6, 56.8, 64.3, 91.0 and 45.6%, respectively. Particularly, three strains of endophytic fungi markedly inhibited HPV-16 E7 oncoprotein‑induced lung cancer angiogenesis in vitro. These findings contribute to the further screening of potential chemotherapeutic agents from mangrove-derived endophytic fungi.

Hypoxia inducible factor down-regulation, cancer and cancer stem cells (CSCs): ongoing success stories

In cancers, hypoxia inducible factor 1 (HIF-1) is an over-expressed transcription factor, which regulates a large set of genes involved in tumour vascularization, metastases, and cancer stem cells (CSCs) formation and self-renewal. This protein has been identified as a relevant target in oncology and several HIF-1 modulators are now marketed or in advanced clinical trials. The purpose of this review is to summarize the advances in the understanding of its regulation and its inhibition, from the medicinal chemist point of view. To this end, we selected in the recent literature relevant examples of "hit" compounds, including small-sized organic molecules, pseudopeptides and nano-drugs, exhibiting in vitro and/or in vivo both anti-HIF-1 and anti-tumour activities. Whenever possible, a particular emphasis has been dedicated to compounds that selectively target CSCs.