The ascidian-derived metabolites with antimicrobial properties

Among the sub-phylum of Tunicate, ascidians represent the most abundant class of marine invertebrates, with 3000 species by heterogeneous habitat, that is, from shallow water to deep sea, already reported. The chemistry of these sessile filter-feeding organisms is an attractive reservoir of varied and peculiar bioactive compounds. Most secondary metabolites isolated from ascidians stand out for their potential as putative therapeutic agents in the treatment of several illnesses like microbial infections. In this review, we present and discuss the antibacterial activity shown by the main groups of ascidian-derived products, such as sulfur-containing compounds, meroterpenes, alkaloids, peptides, furanones, and their derivatives. Moreover, the direct evidence of a symbiotic association between marine ascidians and microorganisms shed light on the real producers of many extremely potent marine natural compounds. Hence, we also report the antibacterial potential, joined to antifungal and antiviral activity, of metabolites isolated from ascidian-associate microorganisms by culture-dependent methods

Antiplasmodial activity of p-substituted benzyl thiazinoquinone derivatives and their potential against parasitic infections

Malaria is a life-threatening disease and, what is more, the resistance to available antimalarial drugs is a recurring problem. The resistance of Plasmodium falciparum malaria parasites to previous generations of medicines has undermined malaria control efforts and reversed gains in child survival. This paper describes a continuation of our ongoing efforts to investigate the effects against Plasmodium falciparum strains and human microvascular endothelial cells (HMEC-1) of a series of methoxy p-benzyl-substituted thiazinoquinones designed starting from a pointed antimalarial lead candidate. The data obtained from the newly tested compounds expanded the structure-activity relationships (SARs) of the thiazinoquinone scaffold, indicating that antiplasmodial activity is not affected by the inductive effect but rather by the resonance effect of the introduced group at the para position of the benzyl substituent. Indeed, the current survey was based on the evaluation of antiparasitic usefulness as well as the selectivity on mammalian cells of the tested p-benzyl-substituted thiazinoquinones, upgrading the knowledge about the active thiazinoquinone scaffold

Insights into Cytotoxic Behavior of Lepadins and Structure Elucidation of the New Alkaloid Lepadin L from the Mediterranean Ascidian Clavelina lepadiformis

The chemical investigation of the Mediterranean ascidian Clavelina lepadiformis has led to the isolation of a new lepadin, named lepadin L, and two known metabolites belonging to the same family, lepadins A and B. The planar structure and relative configuration of the decahydroquinoline ring of lepadin L were established both by means of HR-ESIMS and by a detailed as extensive analysis of 1D and 2D NMR spectra. Moreover, microscale derivatization of the new alkaloid lepadin L was performed to assess the relative configuration of the functionalized alkyl side chain. Lepadins A, B, and L were tested for their cytotoxic activity on a panel of cancer cell lines (human melanoma [A375], human breast [MDA-MB-468], human colon adenocarcinoma [HT29], human colorectal carcinoma [HCT116], and mouse myoblast [C2C12]). Interestingly, a deeper investigation into the mechanism of action of the most cytotoxic metabolite, lepadin A, on the A375 cells has highlighted its ability to induce a strongly inhibition of cell migration, G2/M phase cell cycle arrest and a dose-dependent decrease of cell clonogenity, suggesting that it is able to impair self-renewing capacity of A375 cells

Assignment of the Absolute Configuration of Phosphoeleganin via Synthesis of Model Compounds

The full absolute configuration assignment of phosphoeleganin (1), a recently discovered marine-derived phosphorylated polyketide with protein tyrosine phosphatase 1B inhibitory activity, was achieved. It was based on the synthesis of model diasteroisomeric compounds of the C-8-C-12 segment portion of phosphoeleganin, chiral derivatization methods, and application of the universal NMR database concept

Investigating the antiparasitic potential of the marine sesquiterpene avarone, its reduced form avarol, and the novel semisynthetic thiazinoquinone analogue thiazoavarone

The chemical analysis of the sponge Dysidea avara afforded the known sesquiterpene quinone avarone, along with its reduced form avarol. To further explore the role of the thiazinoquinone scaffold as an antiplasmodial, antileishmanial and antischistosomal agent, we converted the quinone avarone into the thiazinoquinone derivative thiazoavarone. The semisynthetic compound, as well as the natural metabolites avarone and avarol, were pharmacologically investigated in order to assess their antiparasitic properties against sexual and asexual stages of Plasmodium falciparum, larval and adult developmental stages of Schistosomamansoni (eggs included), and also against promastigotes and amastigotes of Leishmania infantum and Leishmania tropica. Furthermore, in depth computational studies including density functional theory (DFT) calculations were performed. A toxic semiquinone radical species which can be produced starting both from quinone- and hydroquinone-based compounds could mediate the anti-parasitic effects of the tested compounds

Microwave-Assisted Synthesis of 2-Methyl-1H-indole-3-carboxylate Derivatives via Pd-Catalyzed Heterocyclization

Indole moiety is well-known as a superlative framework in many natural products and synthetic pharmaceuticals. Herein, we report an efficient procedure to synthesize a series of functionalized 2-methyl-1H-indole-3-carboxylate derivatives from commercially available anilines properly functionalized by different electron-withdrawing and -donating groups through a palladiumcatalyzed intramolecular oxidative coupling. The conversion of a variety of enamines into the relevant indole was optimized by exposing the neat mixture of reactants to microwave irradiation, obtaining the desired products in excellent yields and high regioselectivity. The synthesized compounds were confirmed by 1H and 13C spectroscopic means as well as by high-resolution mass spectrometry

Spectroscopic properties of two 5′-(4-dimethylamino)azobenzene conjugated G-quadruplex forming oligonucleotides

The synthesis of two 5'-end (4-dimethylamino)azobenzene conjugated G-quadruplex forming aptamers, the thrombin binding aptamer (TBA) and the HIV-1 integrase aptamer (T30695), was performed. Their structural behavior was investigated by means of UV, CD, fluorescence spectroscopy, and gel electrophoresis techniques in K+-containing buffers and water-ethanol blends. Particularly, we observed that the presence of the 5'-(4-dimethylamino)azobenzene moiety leads TBA to form multimers instead of the typical monomolecular chair-like G-quadruplex and almost hampers T30695 G-quadruplex monomers to dimerize. Fluorescence studies evidenced that both the conjugated G-quadruplexes possess unique fluorescence features when excited at wavelengths corresponding to the UV absorption of the conjugated moiety. Furthermore, a preliminary investigation of the trans-cis conversion of the dye incorporated at the 5'-end of TBA and T30695 showed that, unlike the free dye, in K+-containing water-ethanol-triethylamine blend the trans-to-cis conversion was almost undetectable by means of a standard UV spectrophotometer

MS Dereplication for Rapid Discovery of Structurally New or Novel Natural Products

In order to accelerate the isolation and characterisation of structurally new or novel natural products, it is crucial to develop efficient strategies that prioritise samples with greatest promise early in the workflow so that resources can be utilised in a more efficient and cost-effective manner. Two complementary approaches have been developed: One is based on targeted identification of known compounds held in a database based on high resolution MS and predicted LC retention time data [1]. The second is an MS metrics-based approach where the software algorithm calculates metrics for sample novelty, complexity, and diversity after interrogating databases of known compounds, and contaminants. These metrics are then used to prioritise samples for isolation and structure elucidation work [2]. Both dereplication approaches have been validated using natural product extracts resulting in the isolation and characterization of new or novel natural products

Exploitation of natural products in the search of drug-like molecules and leads: a fascinating resource

Natural compounds are characterized by a considerable chemical and structural diversity alongside to be considered as the most valuable and prolific source of new drugs up to now. Not surprisingly, nature remains a key inspirational factor in the search for new therapeutic agents. In this frame, marine environment holds a central role furnishing several secondary metabolites which possess peculiar functionality groups on their backbone and a wide ranging of biological activities. Despite this promising potential, the main disadvantage of marine-derived compounds is represented by their limited availability. Therefore, always more frequently marine metabolites are useful not only as effective drugs but also as inspiration for countless synthetic drugs. During the PhD activity, the whole of my work was managed in two different topics: a) Structural and pharmacological characterization of novel secondary metabolites isolated from marine organisms. This task implied the extensive application of most spectroscopic (mainly NMR) and spectrometric techniques, often assisted by computational studies, as well as the development of efficient method for stereochemical assignment, including chemical derivatization and synthesis. The whole of these studies led to the isolation of two novel alkaloids, polyaurine A and B, from the Caribbean ascidian Polycarpa aurata featuring unusual structural moieties. Polyaurine A with its peculiar N-methylguanidine group resulted in the impairment of the development of eggs-laid by Schistosoma mansoni. The chemical investigation of the sesquiterpenoid quinones/hydroquinones-rich sponge Dysidea avara afforded the isolation of avarone and avarol. According to the substantiated redox properties of different quinones against Plasmodium and the similarities in physio-pathological processes of that protozoon and the blood fluke Schistosoma, these derivatives were evaluated against malaria and some neglected tropical diseases (i.e. schistosomiasis and leishmaniasis) since there is an urgent need of new therapeutic alternatives to keep up with the widespread parasite resistance. Moreover, my PhD activity was also focused in completing the configurational assignment of phosphoeleganin, an acyclic phosphorylated polyketide with inhibitory activity against protein tyrosine phosphatase 1B. The univocal stereochemical elucidation required the synthesis of the 8,9-anti stereoisomers of the tetradecan-5,8,9-triol as diastereoisomeric model compounds directed to the application of UDB concept. Accordingly, the widening of NMR database as useful support for stereostructural elucidation has encouraged the synthesis of several simplified analogues of the marine metabolite. b) Design and synthesis of quinone molecules inspired by bioactive marine natural products for antiparasitic and cytotoxicity screenings. Taking into account the antiplasmodial effects of some marine compounds endowed with the thiazinoquinone bicyclic moiety and also the antischistosomal properties of several effective antimalarials, a rationalized chemical library of thiazinoquinones has been synthesised inspired by the marine cytotoxic aplidinone A. The adopted synthetic scheme reflected the compelling priority of an efficient and inexpensive procedure for neglected diseases. Some requirements appeared crucial for the antiparasitic activity: a specific regiochemistry of the thiazinoquinone system strongly affected the antiparasitic potency whereas the nature of the side-chain substituents on the quinone moiety could address the selective toxicity against mammalian cells or specific developmental stages of parasites. Computational studies provided useful insights on putative mechanism of action exerted by thiazinoquinones supporting the one-electron reduction bioactivation step with subsequent formation of a toxic semiquinone radical species. Overall these data clearly point out the thiazinoquinone scaffold as potential new lead structure for neglected disease drugs discovery. To further exploit the versatility of the synthetic protocol, a small series of prenylated quinones and related thiazinoquinones has been prepared in order to broaden the knowledge on the antiproliferative effects of these compounds. Among these, a geranylquinone derivative exerted a cytostatic activity through G0/G1 cell-cycle arrest in BxPC-3 cells

The Ascidian-Derived Metabolites with Antimicrobial Properties

Among the sub-phylum of Tunicate, ascidians represent the most abundant class of marine invertebrates, with 3000 species by heterogeneous habitat, that is, from shallow water to deep sea, already reported. The chemistry of these sessile filter-feeding organisms is an attractive reservoir of varied and peculiar bioactive compounds. Most secondary metabolites isolated from ascidians stand out for their potential as putative therapeutic agents in the treatment of several illnesses like microbial infections. In this review, we present and discuss the antibacterial activity shown by the main groups of ascidian-derived products, such as sulfur-containing compounds, meroterpenes, alkaloids, peptides, furanones, and their derivatives. Moreover, the direct evidence of a symbiotic association between marine ascidians and microorganisms shed light on the real producers of many extremely potent marine natural compounds. Hence, we also report the antibacterial potential, joined to antifungal and antiviral activity, of metabolites isolated from ascidian-associate microorganisms by culture-dependent methods