Topsentin, bromotopsentin, and dihydrodeoxybromotopsentin: antiviral and antitumor bis(indolyl)imidazoles from Caribbean deep-sea sponges of the family Halichondriidae. Structural and synthetic studies

Marine-Derived Bisindoles for Potent Selective Cancer Drug Discovery and Development

Marine-derived bisindoles exhibit structural diversity and exert anti-cancer influence through multiple mechanisms. Comprehensive research has shown that the development success rate of drugs derived from marine natural products is four times higher than that of other natural derivatives. Currently, there are 20 marine-derived drugs used in clinical practice, with 11 of them demonstrating anti-tumor effects. This article provides a thorough review of recent advancements in anti-tumor exploration involving 167 natural marine bisindole products and their derivatives. Not only has enzastaurin entered clinical practice, but there is also a successfully marketed marine-derived bisindole compound called midostaurin that is used for the treatment of acute myeloid leukemia. In summary, investigations into the biological activity and clinical progress of marine-derived bisindoles have revealed their remarkable selectivity, minimal toxicity, and efficacy against various cancer cells. Consequently, they exhibit immense potential in the field of anti-tumor drug development, especially in the field of anti-tumor drug resistance. In the future, these compounds may serve as promising leads in the discovery and development of novel cancer therapeutics.

Discovery and Preliminary Structure-Activity Investigation of 3-Substituted-1H-imidazol-5-yl-1H-indoles with In Vitro Activity towards Methicillin-Resistant Staphylococcus aureus

Antibiotics have been the cornerstone of modern medicine saving lives by virtue of being able to cure infectious diseases and to prevent infections in those who are immune compromised. Their intense use has led to a surging increase in the incidence of antibiotic-resistant bacteria resulting in a desperate need for antibiotics with new mechanisms of action. As part of our search for new antimicrobials we have screened an in-house library of compounds and identified two 3-substituted-1H-imidazol-5-yl-1H-indoles as weak growth inhibitors (MIC 16 µg/mL) against methicillin-resistant Staphylococcus aureus (MRSA). An extensive library of analogues was prepared using the Van Leusen three-component reaction, biological evaluation of which led to the identification of two analogues (26 and 32) with favorable anti-MRSA activity (MIC ≤ 0.25 µg/mL) which also lacked cytotoxic or hemolytic properties. The screening campaign also identified two derivatives, a phenethyl-indole-imidazole 57 and a 5-phenyl-1H-imidazole 111 that were non-toxic selective antifungals towards Cryptococcus neoformans. These results have identified 3-substituted-1H-imidazol-5-yl-1H-indoles and 5-phenyl-1H-imidazoles as new structural scaffolds for further investigation as anti-MRSA and anti-C. neoformans agents, respectively.

Marine Demospongiae: A Challenging Treasure of Bioactive Compounds

In the last decades, it has been demonstrated that marine organisms are a substantial source of bioactive compounds with possible biotechnological applications. Marine sponges, in particular those belonging to the class of Demospongiae, have been considered among the most interesting invertebrates for their biotechnological potential. In this review, particular attention is devoted to natural compounds/extracts isolated from Demospongiae and their associated microorganisms with important biological activities for pharmacological applications such as antiviral, anticancer, antifouling, antimicrobial, antiplasmodial, antifungal and antioxidant. The data here presented show that this class of sponges is an exciting source of compounds, which are worth developing into new drugs, such as avarol, a hydroquinone isolated from the marine sponge Disidea avara, which is used as an antitumor, antimicrobial and antiviral drug.

Concise Syntheses of Marine (Bis)indole Alkaloids Meridianin C, D, F, and G and Scalaridine A via One-Pot Masuda Borylation-Suzuki Coupling Sequence

N-Protected 3-iodoindoles were reacted with (di)azine halides in a sequentially Pd-catalyzed one-pot fashion, i.e., by Masuda borylation–Suzuki coupling (MBSC) sequence. This methodology was successfully applied to the concise syntheses of marine indole alkaloids meridianin C, D, F, and G, as well as to the bisindole alkaloid scalaridine A, which were obtained in moderate to excellent yield.

Marine Natural Products as a Source of Drug Leads against Respiratory Viruses: Structural and Bioactive Diversity

Respiratory viruses, including influenza virus, respiratory syncytial virus, coronavirus, etc., have seriously threatened the human health. For example, the outbreak of severe acute respiratory syndrome coronavirus, SARS, affected a large number of countries around the world. Marine organisms, which could produce secondary metabolites with novel structures and abundant biological activities, are an important source for seeking effective drugs against respiratory viruses. This report reviews marine natural products with activities against respiratory viruses, the emphasis of which was put on structures and antiviral activities of these natural products. This review has described 167 marinederived secondary metabolites with activities against respiratory viruses published from 1981 to 2019. Altogether 102 references are cited in this review article.

1,2,4-Oxadiazole Topsentin Analogs with Antiproliferative Activity against Pancreatic Cancer Cells, Targeting GSK3β Kinase

A new series of topsentin analogs, in which the central imidazole ring of the natural lead was replaced by a 1,2,4-oxadiazole moiety, was efficiently synthesized. All derivatives were pre-screened for antiproliferative activity against the National Cancer Institute (NCI-60) cell lines panel. The five most potent compounds were further investigated in various pancreatic ductal adenocarcinoma (PDAC) cell lines, including SUIT-2, Capan-1, and Panc-1 cells, eliciting EC₅₀ values in the micromolar and sub-micromolar range, associated with significant reduction of cell migration. These remarkable results might be explained by the effects of these new topsentin analogues on epithelial-to-mesenchymal transition markers, including SNAIL-1/2 and metalloproteinase-9. Moreover, flow cytometric analysis after Annexin V-FITC and propidium iodide staining demonstrated that these derivatives enhanced apoptosis of PDAC cells. Keeping with these data, the PathScan intracellular signaling and ELISA array revealed cleavage of caspase-3 and PARP and a significant inhibition of GSK3β phosphorylation, suggesting this kinase as a potential downstream target of our novel compounds. This was further supported by a specific assay for the evaluation of GSK3β activity, showing IC₅₀ values for the most active compounds against this enzyme in the micromolar range.

Microbiome analysis of healthy and diseased sponges Lubomirskia baicalensis by using cell cultures of primmorphs

Endemic sponges (Demosponges, Lubomirskiidae) dominate the fauna of the littoral zone of Lake Baikal. These freshwater sponges live in symbiosis with diverse eukaryotes and prokaryotes, including chlorophyll-containing microalgae. Within the last 5 years, the incidence of sponge disease and mortality events in Lake Baikal has increased. The etiology and ecology of these events remain unknown, in part because of the lack of models to study sponge-microbe interactions. In this work, we tested the use of primmorph cell cultures of Lubomirskia baicalensis as a tool for investigating the microbiomes of sponges. We infected primmorphs, cultured in vitro, with samples from diseased sponges and observed, by microscopy, disease symptoms, including loss of green symbionts, associated with mass die-off events. Subsequent sequencing of 16S rRNA gene fragments revealed that the microbiome community of healthy sponge and primmorphs formed a group separate from the community of diseased sponges and infected primmorphs. This confirms the suitability of the primmorph cell culture as a model sponge system. We also discovered mass mortality of green symbionts (Chlorophyta) was associated with a shift in the microbial communities of sponges/primmorphs. Microbes in diseased sponges, and infected primmorphs, belonged mainly to the phyla Bacteroidetes and Proteobacteria and these families Flavobacteriaceae, Burkholderiaceae, and Moraxellaceae. Primmorphs cell culture may provide a model to study interactions between these bacteria and their host and elucidate the cause of mass mortality events.

Photoprotective Activity of Topsentin, A Bis(Indole) Alkaloid from the Marine Sponge Spongosorites genitrix, by Regulation of COX-2 and Mir-4485 Expression in UVB-Irradiated Human Keratinocyte Cells

Skin is an important barrier to protect the body from environmental stress. However, exposure to ultraviolet radiation (UV) and various environmental oxidative stresses can cause skin inflammation. Cyclooxygenase-2 (COX-2) is an inducible enzyme that mediates the formation of prostaglandin E2 (PGE2) against internal and external inflammatory stimulations. Therefore, the inhibition of COX-2 is an important approach to maintain skin health and prevent skin inflammation and carcinogenesis. Topsentin, a bis(indolyl)imidazole alkaloid isolated from the marine sponge Spongosorites genitrix, has been reported to exhibit anti-tumor and anti-microbial activities. However, the effect of topsentin on skin inflammation and its underlying molecular mechanism has not been elucidated. In the present study, we identified the photoprotective effects of topsentin on UVB irradiated human epidermal keratinocyte HaCaT cells. Topsentin suppresses COX-2 expression and its upstream signaling pathways, AP-1 and MAPK. Furthermore, topsentin inhibits miR-4485, a new biomarker selected from a microarray, and its target gene tumor necrosis factor alpha induced protein 2 (TNF-α IP2). The photoprotective effect of topsentin was also confirmed in a reconstructed human skin model. These findings suggest that topsentin may serve as a potential candidate for cosmetic formulations with skin inflammatory-mediated disorder.

Design and synthesis of newer 1,3,4-oxadiazole and 1,2,4-triazole based Topsentin analogues as anti-proliferative agent targeting tubulin

A set of two series of 1,3,4-oxadiazole (11a-n) and 1,2,4-Triazole (12a, c, e, g, h, j-n) based topsentin analogues were prepared by replacing imizadole moiety of topsentin through a multistep synthesis starting from indole. All the compounds synthesized were submitted for single dose (10 µM) screening against a NCI panel of 60-human cancer cell lines. Among all cancer cell lines, colon (HCC-2998) and Breast (MCF-7, T-47D) cancer cell lines were found to be more susceptible for this class of compounds. Among the compounds tested, compounds 11a, 11d, 11f, 12e and 12h, were exhibited good anti-proliferative activity against various cancer cell lines. Compounds 11d, 12e and 12h demonstrated better activity with IC₅₀ 2.42 µM, 3.06 µM, and 3.30 µM respectively against MCF-7 human cancer cell line than that of the standard drug doxorubicin IC₅₀ 6.31 µM. Furthermore, 11d induced cell cycle arrest at G0/G1 phase and also disrupted mitochondrial membrane potential with reducing cell migration potential of MCF-7 cells in dose dependent manner. In vitro microtubule polymerization assays found that compound 11d disrupt tubulin dynamics by inhibiting tubulin polymerization with IC₅₀ 3.89 μM compared with standard nocodazole (IC₅₀ 2.49 μM). In silico docking studies represented that 11d was binding at colchicine binding site of β-tubulin. Compound 11d emerged as lead molecule from the library of compounds tested and this may serve as a template for further drug discovery.

Bisindole Alkaloids from a New Zealand Deep-Sea Marine Sponge Lamellomorpha strongylata

Chemical investigation of the secondary metabolites of a rare New Zealand deep-sea sponge, Lamellomorpha strongylata, resulted in the isolation of twenty-one indole alkaloids, including two new bisindoles—(Z)-coscinamide D (1), (E)-coscinamide D (2)—and four compounds isolated for the first time as natural products—lamellomorphamides A (3), B (4), C (5) and D (6). In addition, fifteen previously reported natural products were isolated, seven of which are seco analogs of hamacanthin alkaloids. The one sponge produces enantiomerically pure but opposite configurations of compounds that only differ in the number of bromines, suggesting enantiodivergent biosynthesis. In addition, four compounds were isolated as partial racemates, suggesting these compounds are biosynthesized via two independent routes.

Brominated Bisindole Alkaloids from the Celtic Sea Sponge Spongosorites calcicola

As part of an ongoing program to identify new bioactive compounds from Irish marine bioresources, we selected the subtidal sponge Spongosorites calcicola for chemical study, as fractions of this species displayed interesting cytotoxic bioactivities and chemical profiles. The first chemical investigation of this marine species led to the discovery of two new bisindole alkaloids of the topsentin family, together with six other known indole alkaloids. Missing the usual central core featured by the representatives of these marine natural products, the new metabolites may represent key biosynthetic intermediates for other known bisindoles. These compounds were found to exhibit weak cytotoxic activity against HeLa tumour cells, suggesting a specificity towards previously screened carcinoma and leukaemia cells.

Design and Synthesis of Anti-Cancer Chimera Molecules Based on Marine Natural Products

In this paper, the chemical conjugation of marine natural products with other bioactive molecules for developing an advanced anti-cancer agent is described. Structural complexity and the extraordinary biological features of marine natural products have led to tremendous research in isolation, structural elucidation, synthesis, and pharmacological evaluation. In addition, this basic scientific achievement has made it possible to hybridize two or more biologically important skeletons into a single compound. The hybridization strategy has been used to identify further opportunities to overcome certain limitations, such as structural complexity, scarcity problems, poor solubility, severe toxicity, and weak potency of marine natural products for advanced development in drug discovery. Further, well-designed marine chimera molecules can function as a platform for target discovery or degradation. In this review, the design, synthesis, and biological evaluation of recent marine chimera molecules are presented.

Three-Component One-Pot Synthesis of Highly Functionalized Bis-Indole Derivatives

In this study, we detail the development of a concise and efficient three-component protocol for the regioselective synthesis of highly functionalized bis-indoles through a one-pot, two-step sequential process starting from enaminones 1, indoles 2, and acenaphthylene-1,2-dione 3 that is catalyzed by piperidine and p-methyl benzenesulfonic acid. This protocol has several advantages including simplicity of experimental operation, high efficiency of bond formation, ready availability and low cost of starting materials, environmentally benign conditions, and target molecular diversity.

On the Mechanism of Action of Dragmacidins I and J, Two New Representatives of a New Class of Protein Phosphatase 1 and 2A Inhibitors

Two new brominated bis(indole) alkaloids, dragmacidins I (1) and J (2), showing low micromolar cytostatic activity, along with three known congeners were isolated from the Tanzanian sponge Dragmacidon sp. and their structures determined by the analysis of their NMR and MS data. From the study of their mechanism of action, it can be concluded that the mitotic arrest at metaphase in treated tumor cells, mediated by inhibition of PP1 and/or PP2A phosphatases is involved in the observed antiproliferative activity. Differences in their bioactivities were rationalized, and a plausible binding mode is proposed on the basis of computational simulations.

Synthesis and biological activities of new bis-indole derivatives via microwave irradiation

Three new series of bis-indole derivatives were synthesized based on p-phenylenediamine (2–4, 5 and 6) and 4,4′-ethylenedianiline moieties (7–9) using facile and efficient condensation of three positional isomeric indole-carboxaldehyde derivatives (1a–c) with bifunctional amines upon microwave irradiation. The symmetric dimeric indole derivatives 2–4 as well as non-symmetric analogues 5 and 6 were obtained by in situ condensation of the respective positional 3-, 2- and 5-isomeric indole-carboxaldehydes with p-phenylenediamine, while compounds 7–9 resulted from respective condensation based on 4,4′-ethylenedianiline. Structures of the obtained compounds were deduced by advanced spectroscopic methods (1H NMR, 13C NMR and MS). In agar diffusion assay, derivative 6 showed moderate antibacterial activity against various Gram positive and negative bacteria, while derivative 7 displayed moderate activity against several Gram positive bacteria. However, in Resazurin assay employing the human cervix carcinoma cell line (KB-3-1), derivatives 2–9 turned out to be inactive.

Dragmacidin G, a Bioactive Bis-Indole Alkaloid from a Deep-Water Sponge of the Genus Spongosorites

A deep-water sponge of the genus Spongosorites has yielded a bis-indole alkaloid which we have named dragmacidin G. Dragmacidin G was first reported by us in the patent literature and has recently been reported by Hitora et al. from a sponge of the genus Lipastrotheya. Dragmacidin G is the first in this series of compounds to have a pyrazine ring linking the two indole rings. It also has a rare N-(2-mercaptoethyl)-guanidine side chain. Dragmacidin G shows a broad spectrum of biological activity including inhibition of methicillin-resistant Staphylococcus aureus, Mycobacterium tuberculosis, Plasmodium falciparum, and a panel of pancreatic cancer cell lines.

Synthesis of 3-acylindoles by oxidative rearrangement of 2-aminochalcones using a hypervalent iodine reagent and cyclization sequence

An efficient one-pot 3-acylindole synthesis by oxidative rearrangement of 2-aminochalcones and sequential cyclization has been developed.

Discovery of Topsentin Alkaloids and Their Derivatives as Novel Antiviral and Anti-phytopathogenic Fungus Agents

Topsentin alkaloids and their derivatives were designed, synthesized, and characterized on the basis of NMR and mass spectroscopy. The antiviral activities against tobacco mosaic virus (TMV) and anti-phytopathogenic fungus activities of these alkaloids were evaluated for the first time. Alkaloids 1c, 1e, 2b, and 2d displayed significantly higher antiviral activities against TMV than Ribavirin, emerging as new lead compounds for anti-TMV research. Further fungicidal activity tests against 14 kinds of phytopathogenic fungi revealed that these alkaloids displayed broad-spectrum fungicidal activities. Topsentin derivative 2d with 4-5 mg/kg EC₅₀ values against Sclerotinia sclerotiorum (Lib.), Rhizoctonia solani (Kuhn), and Botrytis cinerea (Pers.) emerged as a new lead compound for fungicidal research. Current studies provide support for the application of topsentin alkaloids as novel agrochemicals.

Dragmacidins G and H, Bisindole Alkaloids Tethered by a Guanidino Ethylthiopyrazine Moiety, from a Lipastrotethya sp. Marine Sponge

LCMS analysis of the extract and a cytotoxicity assay of the HPLC fractions generated from a small-scale extract of a Lipastrotethya sp. marine sponge demonstrated the presence of bisindole alkaloids that were associated with the cytotoxic activity. Two bisindole alkaloids tethered by a guanidino ethylthiopyrazine moiety, dragmacidins G (1) and H (2), were isolated, and their structures were assigned by analysis of the MS and NMR data. They showed moderate cytotoxic activity against HeLa cells.

A Metal-Free Protocol for Aminofunctionalization of Olefins Using TsNBr2

N,N-Dibromo-p-toluene sulfonamide (TsNBr2) has been found to be an effective reagent for various aminofunctionalization reactions. This reagent behaves both as an electrophilic bromine source as well as amine to react with olefins under different conditions to yield aminoether, imidazoline, diamine and amino bromine. The reaction proceeds rapidly under mild conditions with high regioselectivity. Olefins react with TsNBr2 in moist THF to form δ-amino ether at room temperature. Treatment of TsNBr2 with olefin in MeCN at room temperature produced imidazoline in high yield. Further modification of the reaction condition resulted in the development of a one-step procedure for the synthesis of N-acetyl,N'-tosyl diamine derivatives directly from olefin. When the olefin was treated with 2.4 mol equiv of TsNBr2 in the presence of K2CO3, N,N'-ditosyl diamine derivative was obtained in moderate yield. Instantaneous formation of aminobromine was observed when an olefin was treated with the reagent in dry CH2Cl2 at room temperature.

3-[4-(1H-indol-3-yl)-1,3-thiazol-2-yl]-1H-pyrrolo[2,3-b]pyridines, nortopsentin analogues with antiproliferative activity

A new series of nortopsentin analogues, in which the imidazole ring of the natural product was replaced by thiazole and the indole unit bound to position 2 of the thiazole ring was substituted by a 7-azaindole moiety, was efficiently synthesized. Two of the new nortopsentin analogues showed good antiproliferative effect against the totality of the NCI full panel of human tumor cell lines (~60) having GI₅₀ values ranging from low micromolar to nanomolar level. The mechanism of the antiproliferative effect of these derivatives, investigated on human hepatoma HepG2 cells, was pro-apoptotic, being associated with externalization of plasma membrane phosphatidylserine and mitochondrial dysfunction. Moreover, the compounds induced a concentration-dependent accumulation of cells in the subG0/G1phase, while confined viable cells in G2/M phase.

Synthesis and antiproliferative activity of thiazolyl-bis-pyrrolo[2,3-b]pyridines and indolyl-thiazolyl-pyrrolo[2,3-c]pyridines, nortopsentin analogues

Two new series of nortopsentin analogues, in which the imidazole ring of the natural product was replaced by thiazole and indole units were both substituted by 7-azaindole moieties or one indole unit was replaced by a 6-azaindole portion, were efficiently synthesized. Compounds belonging to both series inhibited the growth of HCT-116 colorectal cancer cells at low micromolar concentrations, whereas they did not affect the viability of normal-like intestinal cells. A compound of the former series induced apoptosis, evident as externalization of plasma membrane phosphatidylserine (PS), and changes of mitochondrial trans-membrane potential, while blocking the cell cycle in G2/M phase. In contrast, a derivative of the latter series elicited distinct responses in accordance with the dose. Thus, low concentrations (GI₃₀) induced morphological changes characteristic of autophagic death with massive formation of cytoplasmic acid vacuoles without apparent loss of nuclear material, and with arrest of cell cycle at the G1 phase, whereas higher concentrations (GI₇₀) induced apoptosis with arrest of cell cycle at the G1 phase.

Metal free access to quinolines via C–C bond cleavage of styrenes

A new metal free self-sorting tandem reaction between styrenes and anilines to access 2,4-disubstituted quinolines, involving simultaneous C–C and C–N bond formation along with a C–C bond cleavage, has been developed.

Alkaloids from marine invertebrates as important leads for anticancer drugs discovery and development

The present review describes research on novel natural antitumor alkaloids isolated from marine invertebrates. The structure, origin, and confirmed cytotoxic activity of more than 130 novel alkaloids belonging to several structural families (indoles, pyrroles, pyrazines, quinolines, and pyridoacridines), together with some of their synthetic analogs, are illustrated. Recent discoveries concerning the current state of the potential and/or development of some of them as new drugs, as well as the current knowledge regarding their modes of action, are also summarized. A special emphasis is given to the role of marine invertebrate alkaloids as an important source of leads for anticancer drug discovery.

Bioactive Secondary Metabolites from Acid Mine Waste Extremophiles

The extremophilic microbes of the Berkeley Pit Lake are a valuable source of new and interesting secondary metabolites. It is of particular interest that these acidophilic microbes produce small molecule inhibitors of pathways associated with low pH and high Eh. These same small molecules also inhibit molecular pathways induced by reactive oxygen species (ROS) and inflammation in mammalian cells. Low pH is a hallmark of inflammation and high Eh is one of ROS, so the suitability of this collection as a source of bioactive metabolites is actually quite biorational. Compound isolation was guided by inhibition of caspase-1 and matrix metalloproteinase-3, and active compounds were sent to the National Cancer Institute-Developmental Therapeutics Program and Memorial Sloan Kettering Cancer center for evaluation as either antiproliferative or cytotoxic agents.

Synthesis of tetra-substituted imidazoles and 2-imidazolines by Ni(0)-catalyzed dehydrogenation of benzylic-type imines

A one pot methodology using simple imines allowed the formation of N-substituted imidazoles and imidazolines.

Marine Bi-, Bis-, and Trisindole Alkaloids

This chapter, covering the chemistry literature up until June 2013 and comprising 142 references, records the chemical structures of 130 bi-, bis-, and trisindole alkaloids isolated from a plethora of marine phyla including bacteria, algae, bryozoans, sponges, mollusks, hard corals, and ascidians. While the vast majority of bisindoles have been isolated from marine sponges, biindoles are more commonly found in red algae species than sponges. Trisindoles are far less common than bisindoles in the marine environment and have been limited to two species of sponge and a single species of marine microbe. Antimicrobial activity and cytotoxicity dominate the bioactivities explored for selected members of this family of alkaloids. Synthetic approaches to 28 natural products are presented in 33 schemes, and in the absence of any in vivo biosynthetic studies, the putative biosyntheses of eight bisindole metabolites are presented.

Synthesis and antiproliferative activity of 2,5-bis(3'-indolyl)pyrroles, analogues of the marine alkaloid nortopsentin

2,5-bis(3′-Indolyl)pyrroles, analogues of the marine alkaloid nortopsentin, were conveniently prepared through a three step procedure in good overall yields. Derivatives 1a and 1b exhibited concentration-dependent antitumor activity towards a panel of 42 human tumor cell lines with mean IC₅₀ values of 1.54 μM and 0.67 μM, respectively. Investigating human tumor xenografts in an ex-vivo clonogenic assay revealed selective antitumor activity, whereas sensitive tumor models were scattered among various tumor histotypes.