New cytotoxic N-methylated beta-carboline alkaloids from the marine ascidian Eudistoma gilboverde

Photocatalytic enantioselective Minisci reaction of β-carbolines and application to natural product synthesis

A highly efficient enantioselective direct C-H functionalization of β-carbolines via a Minisci-type radical process under a photo-redox and chiral phosphoric acid cooperative catalytic system has been disclosed. Through this protocol, a wide range of C1 aminoalkylated β-carbolines were constructed directly with high levels of enantioselectivities from readily available β-carbolines and alanine-derived redox-active esters. This transformation allows straightforward access to highly valuable enantioenriched β-carbolines, which are an intriguing structural motif in valuable natural products and synthetic bio-active compounds. This protocol has been utilized as a highly efficient synthetic strategy for the concise asymmetric total synthesis of marine alkaloids eudistomin X, (+)-eudistomidin B and (+)-eudistomidin I.

Simple naturally occurring β-carboline alkaloids – role in sustainable theranostics

This review is a brief treatise on some simple β-carboline alkaloids that are abundantly available in plants, animals and foodstuff. These alkaloids are well known for their pharmacological action as well as their allelopathic behaviour. The focus of this review is on sustainable use of naturally occurring compounds in safeguarding human health and protecting our environment at large i.e. the prospective applications of these molecules for Sustainable Theranostics. The review commences with an initial introduction to the β-carboline alkaloids, followed by an outlay of their geographical distribution and natural abundance, then the basic structure and building units of the simplest β-carboline alkaloids have been mentioned. This is followed by a discussion on the important methods of extraction from natural sources both plants and animals. Then the foundation for the use of these alkaloids in Sustainable Theranostics has been built by discussing their interesting photophysics, interactions with important biological molecules and an extensive survey of their therapeutic potential and allelopathic behaviour. Finally the review ends with a silver lining mentioning the future prospective applications of these alkaloids with special relevance to sustainability issues.

Marine biome-derived secondary metabolites, a class of promising antineoplastic agents: A systematic review on their classification, mechanism of action and future perspectives

Cancer is one of the most deadly diseases on the planet. Over the past decades, numerous antineoplastic compounds have been discovered from natural resources such as medicinal plants and marine species as part of multiple drug discovery initiatives. Notably, several marine flora (e.g. Ascophyllum nodosum, Sargassum thunbergii) have been identified as a rich source for novel cytotoxic compounds of different chemical forms. Despite the availability of enormous chemically enhanced new resources, the anticancer potential of marine flora and fauna has received little attention. Interestingly, numerous marine-derived secondary metabolites (e.g., Cytarabine, Trabectedin) have exhibited anticancer effects in preclinical cancer models. Most of the anticancer drugs obtained from marine sources stimulated apoptotic signal transduction pathways in cancer cells, such as the intrinsic and extrinsic pathways. This review highlights the sources of different cytotoxic secondary metabolites obtained from marine bacteria, algae, fungi, invertebrates, and vertebrates. Furthermore, this review provides a comprehensive overview of the utilisation of numerous marine-derived cytotoxic compounds as anticancer drugs, as well as their modes of action (e.g., molecular target). Finally, it also discusses the future prospects of marine-derived drug developments and their constraints.

A comprehensive overview of β-carbolines and its derivatives as anticancer agents

β-Carboline alkaloids are a family of natural and synthetic products with structural diversity and outstanding antitumor activities. This review summarizes research developments of β-carboline and its derivatives as anticancer agents, which focused on both natural and synthetic monomers as well as dimers. In addition, the structure-activity relationship (SAR) analysis of β-carboline monomers and dimers are summarized and mechanism of action of β-carboline and its derivatives are also presented. A few possible research directions, suggestions and clues for future work on the development of novel β-carboline-based anticancer agents with improved expected activities and lesser toxicity are also provided.

Marine Natural Products from Tunicates and Their Associated Microbes

Marine tunicates are identified as a potential source of marine natural products (MNPs), demonstrating a wide range of biological properties, like antimicrobial and anticancer activities. The symbiotic relationship between tunicates and specific microbial groups has revealed the acquisition of microbial compounds by tunicates for defensive purpose. For instance, yellow pigmented compounds, "tambjamines", produced by the tunicate, Sigillina signifera (Sluiter, 1909), primarily originated from their bacterial symbionts, which are involved in their chemical defense function, indicating the ecological role of symbiotic microbial association with tunicates. This review has garnered comprehensive literature on MNPs produced by tunicates and their symbiotic microbionts. Various sections covered in this review include tunicates' ecological functions, biological activities, such as antimicrobial, antitumor, and anticancer activities, metabolic origins, utilization of invasive tunicates, and research gaps. Apart from the literature content, 20 different chemical databases were explored to identify tunicates-derived MNPs. In addition, the management and exploitation of tunicate resources in the global oceans are detailed for their ecological and biotechnological implications.

Spectroscopic and quantum chemical characterization of the ground and lowest electronically excited singlet and triplet states of halo- and nitro-harmines in aqueous media

Halogenated and nitro β-carboline (βCs) alkaloids have garnered increasing interest for their role in a broad range of biological, pharmacological and biotechnological processes. Addressing their spectroscopic and photophysical properties provide tools to further explore the presence of these alkaloids in complex biological matrices. In addition, these studies help to elucidate processes where these alkaloids are involved. The UV-visible and steady-state room temperature fluorescence of bromo- and nitro-harmines in an aqueous environment at different pHs, low-temperature phosphorescence (at 77 K) and quantum yields of singlet oxygen production are reported herein. Singlet (S0 and S1) and triplet (T1) electronic states are further analyzed using density functional theory (DFT) and the results compared with experimental data. Data are discussed in the framework of potential biotechnological applications of these βC alkaloids.

β-Carbolines as potential anticancer agents

β-Carbolines are indole alkaloids having a tricyclic pyrido[3,4-b]indole ring in their structure. Since the isolation of first β-carboline from Peganum harmala in 1841, the isolation and synthesis of various β-carboline derivatives surged in the following centuries. β-Carboline derivatives due to their widespread availability from natural sources, structural flexibility, quick reactivity and interaction with varied anticancer targets such as DNA (intercalation, groove binding, etc.), enzymes (GPX4, topoisomerases, kinases, etc.) and proteins (tubulin, ABCG2/BRCP1, etc.) have established themselves as promising lead compounds for the synthesis of various anticancer active agents. The current review covers the synthesis and isolation, anticancer activity, mechanism of action and SAR of various β-carboline containing molecules, its derivatives and congeners.

Molecular docking studies of gyrase inhibitors: weighing earlier screening bedrock

For any antimicrobial assay, a standard drug is used to compare the bactericidal efficiency of the bioactive compound under screening. The standard drugs have different targets that may be intracellular or membrane located. The location of the target is believed to be determining the bioactivity of the drug depending on the drug's access to its target. Therefore, different drugs must have a different magnitude in exhibiting the biological effect. However, in most of the published literature about the screening of bioactive compounds on antimicrobial activity, generally, the standard drug is randomly chosen while comparing against the bioactive compound of interest. Further, the antimicrobial activity is inferred by comparing the randomly chosen standard drugs without knowing the physicochemical parameters of the standard drug and the test molecule. It is just like an unfair comparison of the impact of a bullet with the impact of an explosive in a combat scene. Computer-based strategies for structure-based drug discovery presents a valuable alternative to the costly and time-consuming process of random screening. The docking studies provide better insights into the binding mechanism of substrate and inhibitor at the molecular level. The evaluation of such a comparison of bioactive compounds against randomly selected standard drugs through a customized virtual screening pipeline showed 57% false positives, 18% true positive, 17% true negative, 8% false-negative results. This study directs for mandatory cheminformatics-based assessment of the bioactive compounds before choosing the standard drug to compare with.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40203-020-00064-9.

Bimetallic Catalyzed N-arylation Used in Synthesis of Novel β-carbolines Derivatives

Background:

Natural occurring β-Carbolines alkaloids are abundant in the plant kingdom or other organisms, and they were found to possess good antitumor activity through multiple mechanisms. Based on previous summarized SARs of β-carboline derivatives, the modification on pyridine ring would have a great impact on their antitumor activities. Therefore, we plan to synthesized arylated β-carboline-3-amides to find more valuable β-Carbolines derivatives.

Methods:

A novel bimetallic Pd(OAc)2/AgOAc catalyst system was developed for the amidation of aryl iodides under acid condition. A series of N-arylated β-carbolines derivatives were synthesized using this method. The structures of these compounds were confirmed by 1H NMR, 13C NMR and HRMS, and their in vitro antiproliferative activity was investigated against HepG2 and Hela tumor cell lines by MTT assay.

Results:

Eleven N-arylated β-carboline-3-amides were synthesized using this bimetallic catalyzed method in 58-98% yields. These synthesized N-arylated compounds showed no antiproliferative activity at 20 μM.

Conclusion:

We have discovered an efficient and bimetallic catalytic system allowing the Narylation of secondary acyclic amides. This is the first report that N-arylation of aliphatic secondary acyclic amides under acid condition.

N-Methyl-β-carbolinium Salts and an N-Methylated 8-Oxoisoguanine as Acetylcholinesterase Inhibitors from a Solitary Ascidian, Cnemidocarpa irene

New brominated β-carbolines irenecarbolines A (1) and B (4) along with known β-carbolines 2 and 3 and a new 8-oxoisoguanine derivative, 5, were isolated from a solitary ascidian, Cnemidocarpa irene. The structures of these compounds were determined on the basis of their spectral data. All, except for 3, inhibited the action of acetylcholinesterase (AchE). The activities of 1 and 5 were comparable to those of galantamine, a clinically used AchE inhibitor. Compounds 1 and 2 were found to be present in high concentrations in blood, and fluorescence was observed in certain types of cells found in the blood of the tunicate.

Natural Products Diversity of Marine Ascidians (Tunicates; Ascidiacea) and Successful Drugs in Clinical Development

This present study reviewed the chemical diversity of marine ascidians and their pharmacological applications, challenges and recent developments in marine drug discovery reported during 1994-2014, highlighting the structural activity of compounds produced by these specimens. Till date only 5% of living ascidian species were studied from <3000 species, this study represented from family didemnidae (32%), polyclinidae (22%), styelidae and polycitoridae (11-12%) exhibiting the highest number of promising MNPs. Close to 580 compound structures are here discussed in terms of their occurrence, structural type and reported biological activity. Anti-cancer drugs are the main area of interest in the screening of MNPs from ascidians (64%), followed by anti-malarial (6%) and remaining others. FDA approved ascidian compounds mechanism of action along with other compounds status of clinical trials (phase 1 to phase 3) are discussed here in. This review highlights recent developments in the area of natural products chemistry and biotechnological approaches are emphasized.

Ascidian bioresources: common and variant chemical compositions and exploitation strategy - examples of Halocynthia roretzi, Styela plicata, Ascidia sp. and Ciona intestinalis

To explore abundant marine ascidian bioresources, four species from two orders have been compared in their chemical compositions. After a universal separation of the animal body into two fractions, all tunics have been found rich in carbohydrate contents, while all inner body tissues are richer in proteins. Cellulose is present almost exclusively in the tunics and more in the order Stolidobranchia, while more sulfated polysaccharides are present in Phlebobranchia species. Almost all proteins are collagens with a high essential amino acid index and high delicious amino acid (DAA) content. All fractions also have high contents of good-quality fatty acids and trace minerals but low toxic element contents, with different sterols and glycosaminoglycans. There are species-specific characteristics observed for vanadium accumulation and sterol structures which are also meaningful for ascidian chemotaxonomy and resource exploitation. It is suggested that in addition to the present utilizations of tunics for cellulose production and of some species' inner body tissues as human food, one should explore all species' inner body tissues as human foods and all tunics as food or animal feed with the contained cellulose as dietary fiber. Collagens, sulfated polysaccharides, glycosaminoglycans, sterols and trace elements could be explored as byproducts for, e.g. pharmaceutical and chemical industries.

Chemical and photochemical properties of chloroharmine derivatives in aqueous solutions

In water, chloroharmines follow very distinctive thermal and photochemical pH- and O₂-dependent-reaction pathways.

PhI(OAc)2-mediated one-pot oxidative decarboxylation and aromatization of tetrahydro-β-carbolines: synthesis of norharmane, harmane, eudistomin U and eudistomin I

Iodobenzene diacetate was employed as a mild and efficient reagent for one-pot oxidative decarboxylation of tetrahydro-β-carboline acids and dehydrogenation of tetrahydro-β-carbolines to access the corresponding aromatic β-carbolines. To the best of our knowledge this is the first synthesis of β-carbolines via a one-pot oxidative decarboxylation at ambient temperature. The utility of this protocol has been demonstrated in the synthesis of β-carboline alkaloids norharmane (2o), harmane (2p), eudistomin U (9) and eudistomin I (12).

Structure elucidation and anticancer activity of 7-oxostaurosporine derivatives from the Brazilian endemic tunicate Eudistoma vannamei

The present study reports the identification of two new staurosporine derivatives, 2-hydroxy-7-oxostaurosporine (1) and 3-hydroxy-7-oxostaurosporine (2), obtained from mid-polar fractions of an aqueous methanol extract of the tunicate Eudistoma vannamei, endemic to the northeast coast of Brazil. The mixture of 1 and 2 displayed IC₅₀ values in the nM range and was up to 14 times more cytotoxic than staurosporine across a panel of tumor cell lines, as evaluated using the MTT assay.

Diving for drugs: tunicate anticancer compounds

The marine biosphere boasts tremendous biodiversity replete with structurally unique, active and selective secondary metabolites. Bioprospecting for antitumor compounds has been rewarding, and tunicates have been especially successful in yielding prospective cancer therapies. These compounds are now subjected to clinical trials in Europe and the USA. With the ongoing search for potent and specific anticancer drugs, in this article we discuss the unique perspectives, compounds and opportunities afforded by this rich source of potential pharmaceuticals. We discuss marine-derived antitumor drugs, their structures, and their various types and levels of antitumor activities in bench and bedside efforts.

Occurrence of halogenated alkaloids

Once considered to be isolation artifacts or chemical "mistakes" of nature, the number of naturally occurring organohalogen compounds has grown from a dozen in 1954 to >5000 today. Of these, at least 25% are halogenated alkaloids. This is not surprising since nitrogen-containing pyrroles, indoles, carbolines, tryptamines, tyrosines, and tyramines are excellent platforms for biohalogenation, particularly in the marine environment where both chloride and bromide are plentiful for biooxidation and subsequent incorporation into these electron-rich substrates. This review presents the occurrence of all halogenated alkaloids, with the exception of marine bromotyrosines where coverage begins where it left off in volume 61 of The Alkaloids. Whereas the biological activity of these extraordinary compounds is briefly cited for some examples, a future volume of The Alkaloids will present full coverage of this topic and will also include selected syntheses of halogenated alkaloids. Natural organohalogens of all types, especially marine and terrestrial halogenated alkaloids, comprise a rapidly expanding class of natural products, in many cases expressing powerful biological activity. This enormous proliferation has several origins: (1) a revitalization of natural product research in a search for new drugs, (2) improved compound characterization methods (multidimensional NMR, high-resolution mass spectrometry), (3) specific enzyme-based and other biological assays, (4) sophisticated collection methods (SCUBA and remote submersibles for deep ocean marine collections), (5) new separation and purification techniques (HPLC and countercurrent separation), (6) a greater appreciation of traditional folk medicine and ethobotany, and (7) marine bacteria and fungi as novel sources of natural products. Halogenated alkaloids are truly omnipresent in the environment. Indeed, one compound, Q1 (234), is ubiquitous in the marine food web and is found in the Inuit from their diet of whale blubber. Given the fact that of the 500,000 estimated marine organisms--which are the source of most halogenated alkaloids--only a small percentage have been investigated for their chemical content, it is certain that myriad new halogenated alkaloids are awaiting discovery. For example, it is estimated that nearly 4000 species of bryozoans have not been examined for their chemical content. The few species that have been studied contain some extraordinary halogenated alkaloids, such as hinckdentine A (610) and the chartellines (611-613). Of the estimated 1.5 million species of fungi, secondary metabolites have been characterized from only 5000 species. The future seems bright for the collector of halogenated alkaloids!

A journey under the sea: the quest for marine anti-cancer alkaloids

The alarming increase in the global cancer death toll has fueled the quest for new effective anti-tumor drugs thorough biological screening of both terrestrial and marine organisms. Several plant-derived alkaloids are leading drugs in the treatment of different types of cancer and many are now being tested in various phases of clinical trials. Recently, marine-derived alkaloids, isolated from aquatic fungi, cyanobacteria, sponges, algae, and tunicates, have been found to also exhibit various anti-cancer activities including anti-angiogenic, anti-proliferative, inhibition of topoisomerase activities and tubulin polymerization, and induction of apoptosis and cytotoxicity. Two tunicate-derived alkaloids, aplidin and trabectedin, offer promising drug profiles, and are currently in phase II clinical trials against several solid and hematologic tumors. This review sheds light on the rich array of anti-cancer alkaloids in the marine ecosystem and introduces the most investigated compounds and their mechanisms of action.

A survey of marine natural compounds and their derivatives with anti-cancer activity reported in 2010

Although considerable progress in oncology therapeutics has been achieved in the last century, cancer remains one of major death causes in the World and for this reason, the development of novel cancer drugs remains a pressing need. Natural marine compounds represent an interesting source of novel leads with potent chemotherapeutic or chemo-preventive activities. In the last decades, structure-activity-relationship studies have led to the development of naturally-derived or semi-synthetic analogues with improved bioactivity, a simplified synthetic target or less toxicity. We aim here to review a selection of natural compounds with reported anticancer activity isolated of marine sources and their associated analogues published in 2010.

Gold from the sea: marine compounds as inhibitors of the hallmarks of cancer

Cancer is one of the most deadly diseases in the world. Although advances in the field of chemo-preventive and therapeutic medicine have been made regularly over the last ten years, the search for novel anticancer treatments continues. In this field, the marine environment, with its rich variety of organisms, is a largely untapped source of novel compounds with potent antitumor activity. Although many reviews of marine anticancer compounds have been published, we focus here on selected marine compounds that act on the six hallmarks of cancer presented namely self-sufficiency in growth signals, insensitivity to anti-growth signals, evasion of apoptosis, limitless replication, sustained angiogenesis and tissue invasion and metastasis.

Room-temperature aromatization of tetrahydro-β-carbolines by 2-iodoxybenzoic acid: utility in a total synthesis of eudistomin U

2-Iodoxybenzoic acid is a convenient reagent for the dehydrogenation of tetrahydro-β-carbolines to their aromatic forms under mild conditions. The utility of the method was demonstrated in a total synthesis of the marine indole alkaloid eudistomin U.

Structure of pseudocerosine, an indolic azafulvene alkaloid from the flatworm Pseudoceros indicus

The rim of the tunic of the flatworm Pseudoceros indicus is characterized by blue dots on a white background. The isolation and structure elucidation of the blue pigment is reported. It is shown by extensive analysis of spectroscopic data to be an indolic azafulvene, which has been named pseudocerosine.

Isolation, structure elucidation, and synthesis of eudistomides A and B, lipopeptides from a Fijian ascidian Eudistoma sp

Eudistomides A (1) and B (2), two new cyclic peptides, were isolated from a Fijian ascidian Eudistoma sp. These five-residue cystine-linked cyclic peptides are flanked by a C-terminal methyl ester and a 12-oxo- or 12-hydroxy-tetradecanoyl moiety. The complete structures of the eudistomides were determined using a combination of spectroscopic and chemical methods. Chiral HPLC analysis revealed that all five amino acid residues in 1 and 2 had the L-configuration. Total synthesis of eudistomides A (1) and B (2) confirmed the proposed structures. Enantioselective lipase-catalyzed hydrolysis of a mixture of C-35 acetoxy epimers indicated a 35R absolute configuration for 2.

Cytotoxic activity of a dichloromethane extract and fractions obtained from Eudistoma vannamei (Tunicata: Ascidiacea)

This study consists of the bioassay-guided fractionation of the dichloromethane extract from Eudistoma vannamei and the pharmacological characterization of the active fractions. The dried hydromethanolic extract dissolved in aqueous methanol was partitioned with dichloromethane and chromatographed on a silica gel flash column. The anti-proliferative effect was monitored by the MTT assay. Four of the latest fractions, numbered 14 to 17, which held many chemical similarities amongst each other, were found to be the most active. The selected fractions were tested for viability, proliferation and death induction on cultures of HL-60 promyeloblastic leukemia cells. The results suggested that the observed cytotoxicity is related to apoptosis induction.

Marine pharmacology in 2001-2: antitumour and cytotoxic compounds

During 2001 and 2002, marine antitumour pharmacology research aimed at the discovery of novel antitumour agents was published in 175 peer-reviewed articles. The purpose of this paper is to present a structured Review of the antitumour and cytotoxic properties of 97 marine natural products, many of them novel compounds that belong to diverse structural classes, including polyketides, terpenes, steroids, and peptides. The organisms yielding these bioactive compounds comprise a taxonomically diverse group of marine invertebrate animals, algae, fungi and bacteria. Antitumour pharmacological studies were conducted with 30 structurally characterised natural marine products in a number of experimental and clinical models which further defined their mechanisms of action. Particularly potent in vitro cytotoxicity data generated with murine and human tumour cell lines was reported for 67 novel marine chemicals with as yet undetermined mechanisms of action. Noteworthy, is the fact that marine anticancer research was sustained by a collaborative effort, involving researchers from Australia, Brazil, Canada, Denmark, Egypt, France, Germany, Italy, Japan, Netherlands, New Zealand, The Philippines, Russia, Singapore, South Korea, Thailand, Taiwan, Turkey, Spain, Switzerland, Taiwan, Thailand, Turkey, and the United States. Finally, this 2001-2 overview of the marine pharmacology literature highlights the fact that the discovery of novel marine antitumour agents has continued at the same pace as during 1998, 1999 and 2000.

Bioactive Marine Alkaloids

The chapter deals with bioactive marine alkaloids. The chemistry and biological activities of pyridoacridines, pyrroloacridines, indoles, β-carbolines, pyrroles, isoquinolines, and tyrosine derived alkaloids have been discussed and reviewed.

Alkaloids and a pimarane diterpenoid from Strychnos vanprukii

From the stem of Strychnos vanprukii, a gluco-indole alkaloid, 3,4-dehydropalicoside, and a pimarane diterpenoid, 7 beta-hydroxypimara-8,15-dien-14-one, were isolated together with four known alkaloids: palicoside, 3,4,5,6-tetradehydropalicoside, akagerine and 17-O-methylakagerine. The structures of these compounds were elucidated based on spectroscopic evidence.

Identification of brain proteins that interact with 2-methylnorharman. An analog of the parkinsonian-inducing toxin, MPP+

N-Methylated beta-carbolines, including 2-methylnorharman, are structural and functional analogs of the parkinsonian-inducing toxin, MPP+. We are investigating N-methylated beta-carbolines, including 2-methylnorharman, as possible etiologic factors in the pathogenesis of Parkinson's disease. The cellular targets of N-methylated beta-carboline-mediated cytotoxicity are unknown; therefore, we used the T7Select Phage Display System in a novel approach to identify brain proteins that bind to 2-methylnorharman. We incubated (biopanned) immobilized 2-methylnorharman with a phage display cDNA library that expressed a library of human brain proteins on the surface of bacteriophage T7. We washed off unbound phage, amplified the phage that were bound to 2-methylnorharman, and enriched for toxin-interacting phage by repeating the biopanning and amplification steps. The cDNA sequences from the toxin-interacting phage were used to derive the amino acid sequences of the phage-displayed proteins. Five of the six 2-methylnorharman-interacting proteins may have relevance to Parkinson's disease: alpha-tubulin, paraoxonase, dorfin, fatty acid binding protein, and platelet-activating factor acetylhydrolase. Dorfin has sequence homology with parkin, which is interesting because mutations in the parkin gene associate with early-onset Parkinson's disease. Our findings are the basis for future studies aimed at determining whether 2-methylnorharman affects the function of these specific proteins in vitro and in vivo.