Interrogating the bioactive pharmacophore of the latrunculin chemotype by investigating the metabolites of two taxonomically unrelated sponges

Asymmetric synthesis of a stereopentade fragment toward latrunculins

Latrunculins are marine toxins used in cell biology to block actin polymerization. The development of new synthetic strategies and methods for their synthesis is thus important in order to improve, modulate or control this biological value. The total syntheses found in the literature all target similar disconnections, especially an aldol strategy involving a recurrent 4-acetyl-1,3-thiazolidin-2-one ketone partner. Herein, we describe an alternative disconnection and subsequent stereoselective transformations to construct a stereopentade amenable to latrunculin and analogue synthesis, starting from (+)-β-citronellene. Key stereoselective transformations involve an asymmetric Krische allylation, an aldol reaction under 1,5-anti stereocontrol, and a Tishchenko-Evans reduction accompanied by a peculiar ester transposition, allowing to install key stereogenic centers of the natural products.

Spectrin: an alternate target for cytoskeletal drugs

Cytoskeletal drugs having enormous therapeutic potential act on the cytoskeletal components like actin, tubulin either by promoting polymerization or destabilizing the same. Here we present the interaction of the popular cytoskeletal drugs such as taxol, latrunculin and cytochalasin with spectrin, a huge protein with multi domains that forms the cytoskeletal network. Particularly, the actin binding domain of spectrin regulates the dynamics of the actin cytoskeleton. We followed the binding of these drugs to its actin binding domain and intact spectrin as well. These drugs bind with moderate affinity (K_b ∼ 10⁴ M^-1) and the interaction with actin binding domain is entropy driven and hydrophobic in nature as determined by Van't Hoff plot. The docking studies and molecular dynamics simulations further corroborate the experimental findings. Particularly the higher binding constants in the case of latrunculin and cytochalasin to the actin binding domain of spectrin suggest the binding sites are presumably located in its actin binding domain.Communicated by Ramaswamy H. Sarma.

Magnificines A and B, Antimicrobial Marine Alkaloids Featuring a Tetrahydrooxazolo[3,2-a]azepine-2,5(3H,6H)-dione Backbone from the Red Sea Sponge Negombata magnifica

Investigation of the Red Sea sponge Negombata magnifica gave two novel alkaloids, magnificines A and B (1 and 2) and a new β-ionone derivative, (±)-negombaionone (3), together with the known latrunculin B (4) and 16-epi-latrunculin B (5). The analysis of the NMR and HRESIMS spectra supported the planar structures and the relative configurations of the compounds. The absolute configurations of magnificines A and B were determined by the analysis of the predicted and experimental ECD spectra. Magnificines A and B possess a previously unreported tetrahydrooxazolo[3,2-a]azepine-2,5(3H,6H)-dione backbone and represent the first natural compounds in this class. (±)-Negombaionone is the first β-ionone of a sponge origin. Compounds 1-3 displayed selective activity against Escherichia coli in a disk diffusion assay with inhibition zones up to 22 mm at a concentration of 50 µg/disc and with MIC values down to 8.0 µM. Latrunculin B and 16-epi-latrunculin B inhibited the growth of HeLa cells with IC₅₀ values down to 1.4 µM.

Marine-Derived Macrolides 1990-2020: An Overview of Chemical and Biological Diversity

Macrolides are a significant family of natural products with diverse structures and bioactivities. Considerable effort has been made in recent decades to isolate additional macrolides and characterize their chemical and bioactive properties. The majority of macrolides are obtained from marine organisms, including sponges, marine microorganisms and zooplankton, cnidarians, mollusks, red algae, bryozoans, and tunicates. Sponges, fungi and dinoflagellates are the main producers of macrolides. Marine macrolides possess a wide range of bioactive properties including cytotoxic, antibacterial, antifungal, antimitotic, antiviral, and other activities. Cytotoxicity is their most significant property, highlighting that marine macrolides still encompass many potential antitumor drug leads. This extensive review details the chemical and biological diversity of 505 macrolides derived from marine organisms which have been reported from 1990 to 2020.

Construction of sulfur-containing moieties in the total synthesis of natural products

This review surveys the total syntheses of sulfur-containing natural products where sulfur atoms are introduced with different sulfurization agents to construct related sulfur-containing moieties.

New marine natural products from sponges (Porifera) of the order Dictyoceratida (2001 to 2012); a promising source for drug discovery, exploration and future prospects

The discovery of new drugs can no longer rely primarily on terrestrial resources, as they have been heavily exploited for over a century. During the last few decades marine sources, particularly sponges, have proven to be a most promising source of new natural products for drug discovery. This review considers the order Dictyoceratida in the Phylum Porifera from which the largest number of new marine natural products have been reported over the period 2001-2012. This paper examines all the sponges from the order Dictyoceratida that were reported as new compounds during the time period in a comprehensive manner. The distinctive physical characteristics and the geographical distribution of the different families are presented. The wide structural diversity of the compounds produced and the variety of biological activities they exhibited is highlighted. As a representative of sponges, insights into this order and avenues for future effective natural product discovery are presented. The research institutions associated with the various studies are also highlighted with the aim of facilitating collaborative relationships, as well as to acknowledge the major international contributors to the discovery of novel sponge metabolites. The order Dictyoceratida is a valuable source of novel chemical structures which will continue to contribute to a new era of drug discovery.

Marine natural products

This review covers the literature published in 2013 for marine natural products (MNPs), with 982 citations (644 for the period January to December 2013) 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 (1163 for 2013), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Macrocycles as potential therapeutic agents in neglected diseases

Macrocycles possess desirable properties that make them promising candidates for the discovery of novel drugs. They present structural features to favor bioactive conformations, selectivity to the receptors, cell permeability and metabolic stability. More effective and nontoxic drugs to cure human African trypanosomiasis, Leishmaniasis and Chagas disease are needed, especially because resistance has been detected. Therefore, major efforts should be made for investigation in new bioactive compounds exhibiting different mechanisms of action. Macrocycles might fulfill the expectations for the development of new drugs to treat those diseases. In the current review, we focus on macrocycles exhibiting biological activities as antitrypanosomal and/or antileishmanial. The isolation, synthetic and biological studies of this class of compounds published from 2005 to 2014 are summarized.

Total synthesis of (+)-18-epi-latrunculol A: development of a synthetic route

The evolution of an enantioselective total synthesis of (+)-18-epi-latrunculol A, a congener of the marine-sponge-derived latrunculins A and B, is reported. Key steps include a late-stage Mitsunobu macrolactonization to construct the 16-membered macrolactone, a mild Carreira alkynylation to unite the northern and southern hemispheres, a diastereoselective, acid-mediated δ-hydroxy enone cyclization/equilibration sequence, and a functional-group-tolerant cross-metathesis to access the enone cyclization precursor.

Scrutinizing the scaffolds of marine biosynthetics from different source organisms: Gram-negative cultured bacterial products enter center stage

Compounds from macro marine organisms are presumed to owe their biosynthetic origins to associated microbial symbionts, although few definitive examples exist. An upsurge in the recent literature from 2012 to 2013 has shown that four compounds previously reported from macro marine organisms are in fact biosynthesized by non-photosynthetic Gram-negative bacteria (NPGNB). Structural parallels between compounds isolated from macro marine organisms and NPGNB producers form the basis of this review. Although less attention has been given to investigating the chemistry of NPGNB sources, there exists a significant list of structural parallels between NPGNB and macro marine organism-derived compounds. Alternatively, of the thousands of compounds isolated from Gram-positive actinomycetes, few structural parallels with macro marine organisms are known. A summary of small molecules isolated from marine NPGNB sources is presented, including compounds isolated from marine myxobacteria. From this assemblage of structural parallels and diverse chemical structures, it is hypothesized that the potential for the discovery of inspirational molecules from NPGNB sources is vast and that the recent spike in the literature of macro marine compounds owing their biosynthetic origin to NPGNB producers represents a turning point in the field.

Differential binding of latrunculins to G-actin: a molecular dynamics study

Latrunculins are unique macrolides containing a thiazolidinone moiety. Latrunculin A (1), latrunculin B (2), 16-epi-latrunculin B (3), and latrunculin T (4) were isolated from the Red Sea sponge Negombata magnifica. In the present study, after testing compounds 2-4 for cytotoxic activity, they were docked into the crystal structure of G-actin and subjected to binding energy calculation and a 20 ns MD simulation. The modeling study shows that latrunculins binding depends on both hydrophobic interaction of the macrocycle as well as H bonding of the thiazolidinone ring with Asp157 and Thr186. It was noticed that epimerization at C16 of latrunculin B was well tolerated as it could form an alternative H bonding network. However, opening of the macrocyclic ring deteriorates the actin binding due to reduced hydrophobicity. MD simulation showed that latrunculin B (2) possesses a more significant stabilizing effect on G-actin than latrunculin T (4) and could efficiently hinder the flattening transition of G-actin into F-actin. These findings could explain, at the molecular level, the impact of epimerization and macrolide ring-opening on latrunculins activity, an issue that has not been addressed before. Also, the study gives insights into the mechanism of cytotoxicity of diverse latrunculins and provides direction for future lead optimization studies.

Total synthesis of (+)-18-epi-latrunculol A

An enantioselective total synthesis of the cytotoxic latrunculin congener (+)-18-epi-latrunculol A has been achieved. Key steps in the synthetic route include an acid-mediated enone cyclization/equilibration sequence, a Carreira alkynylation, and a late-stage Mitsunobu macrolactonization to construct the macrolide skeleton.

Structures and cytotoxic evaluation of new and known acyclic Ene-Ynes from an American Samoa Petrosia sp. Sponge

Four new compounds, (-)-petrosynoic acids A-D (1-4), and five known congeners, pellynols A (5), C (6), D (7), F (8), and I (9), were isolated from a Petrosia sp. marine sponge collected in American Samoa. Isolation work was guided by cytotoxicity against human lung cancer cells (H460). The structures of the C31-C33 polyacetylenes (1-9) were determined on the basis of 1D- and 2D-NMR analysis, mass spectrometry, and comparison of specific rotation values. Compounds 1-9 were found to be broadly cytotoxic with limited selectivity for cancer cells, as they were all moderately active against the A2058 (melanoma), H522-T1 (lung), and H460 (lung) human cancer cell lines as well as IMR-90 quiescent human fibroblast cells.

Inhibition of interdomain motion in g-actin by the natural product latrunculin: a molecular dynamics study

As part of the cytoskeleton, actin is essential for the morphology, motility, and division of eukaryotic cells. Recent X-ray fiber diffraction studies have shown that the conformation of monomeric actin is flattened upon incorporation into the filament by a relative rotation of its two major domains. The antiproliferative activity of latrunculin, a macrolide toxin produced by sponges, seems to be related to its binding to monomeric actin and inhibition of polymerization. Yet, the mechanism of inhibition is not known in detail. Here, multiple explicit water molecular dynamics simulations show that latrunculin binding hinders the conformational transition related to actin polymerization. In particular, the presence of latrunculin at the interface of the two major domains of monomeric actin reduces the correlated displacement of Domain 2 with respect to Domain 1. Moreover, higher rotational flexibility between the two major domains is observed in the absence of ATP as compared to ATP-bound actin, offering a possible explanation as to why actin polymerizes more favorably in the absence of nucleotides.

Natural product libraries to accelerate the high-throughput discovery of therapeutic leads

A high-throughput (HT) paradigm generating LC-MS-UV-ELSD-based natural product libraries to discover compounds with new bioactivities and or molecular structures is presented. To validate this methodology, an extract of the Indo-Pacific marine sponge Cacospongia mycofijiensis was evaluated using assays involving cytoskeletal profiling, tumor cell lines, and parasites. Twelve known compounds were identified including latrunculins (1-4, 10), fijianolides (5, 8, 9), mycothiazole (11), aignopsanes (6, 7), and sacrotride A (13). Compounds 1-5 and 8-11 exhibited bioactivity not previously reported against the parasite T. brucei, while 11 showed selectivity for lymphoma (U937) tumor cell lines. Four new compounds were also discovered including aignopsanoic acid B (13), apo-latrunculin T (14), 20-methoxy-fijianolide A (15), and aignopsane ketal (16). Compounds 13 and 16 represent important derivatives of the aignopsane class, 14 exhibited inhibition of T. brucei without disrupting microfilament assembly, and 15 demonstrated modest microtubule-stabilizing effects. The use of removable well plate libraries to avoid false positives from extracts enriched with only one or two major metabolites is also discussed. Overall, these results highlight the advantages of applying modern methods in natural products-based research to accelerate the HT discovery of therapeutic leads and/or new molecular structures using LC-MS-UV-ELSD-based libraries.

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.

Linking chemical and microbial diversity in marine sponges: possible role for poribacteria as producers of methyl-branched fatty acids

Many marine sponges contain massive numbers of largely uncultivated, phylogenetically diverse bacteria that seem to be important contributors to the chemistry of these animals. Insights into the diversity, origin, distribution, and function of their metabolic gene communities are crucial to dissect the chemical ecology and biotechnological potential of sponge symbionts. This study reveals a sharp dichotomy between high and low microbial abundance sponges with respect to polyketide synthase (PKS) gene content, the presence of methyl-branched fatty acids, and the presence of members of the symbiotic candidate phylum "Poribacteria". For the symbiont-rich sponge Cacospongia mycofijiensis, a source of the tubulin-inhibiting fijianolides (=laulimalides), near-exhaustive large-scale sequencing of PKS gene-derived PCR amplicons was conducted. Although these amplicons exhibit high diversity at the sequence level, almost all of them belong to a single, architecturally unique group of PKSs present in "Poribacteria" and are proposed to synthesize methyl-branched fatty acids. Three components of this PKS were studied in vitro, providing initial insight into its biochemistry.

Biostructural features of additional jasplakinolide (jaspamide) analogues

The cyclodepsipeptide jasplakinolide (1) (aka jaspamide), isolated previously from the marine sponge Jaspis splendens, is a unique cytotoxin and molecular probe that operates through stabilization of filamentous actin (F-actin). We have recently disclosed that two analogues of 1, jasplakinolides B (3) and E, were referred to the National Cancer Institute's (NCI) Biological Evaluation Committee, and the objective of this study was to reinvestigate a Fijian collection of J. splendens in an effort to find jasplakinolide congeners with similar biological properties. The current efforts have afforded six known jasplakinolide analogues (4-7, 9, 10), two structures requiring revision (8 and 14), and four new congeners of 1 (11-13, 15) including open-chain derivatives and structures with modified β-tyrosine residues. Compounds were evaluated for biological activity in the NCI's 60 cell line screen and in a microfilament disruption assay in both HCT-116 and HeLa cells. These two phenotypic screens provide evidence that each cytotoxic analogue, including jasplakinolide B (3), operates by modification of microfilaments. The new structure jasplakinolide V (13) has also been selected for study by the NCI's Biological Evaluation Committee. In addition, the results of a clonogenic dose-response study on jasplakinolide are presented.

Isolation, structural elucidation, and absolute stereochemistry of enigmazole A, a cytotoxic phosphomacrolide from the Papua New Guinea marine sponge Cinachyrella enigmatica

Enigmazole A (1), a novel phosphate-containing macrolide, was isolated from a Papua New Guinea collection of the marine sponge Cinachyrella enigmatica. The structure of 1, including the absolute stereochemistry at all eight chiral centers, was determined by a combination of spectroscopic analyses and a series of microscale chemical derivatization studies. Compound 1 is comprised of an 18-membered phosphomacrolide that contains an embedded exomethylene-substituted tetrahydropyran ring and an acyclic portion that spans an embedded oxazole moiety. Two additional analogues, 15-O-methylenigmazole A and 13-hydroxy-15-O-methylenigmazole A, were also isolated and assigned. The enigmazoles are the first phosphomacrolides from a marine source and 1 exhibited significant cytotoxicity in the NCI 60-cell line antitumor screen, with a mean GI(50) of 1.7 microM.

Semisynthetic latrunculin derivatives as inhibitors of metastatic breast cancer: biological evaluations, preliminary structure-activity relationship and molecular modeling studies

The microfilament cytoskeleton protein actin plays an important role in cell biology and affects cytokinesis, morphogenesis, and cell migration. These functions usually fail and become abnormal in cancer cells. The marine-derived macrolides latrunculins A and B, from the Red Sea sponge Negombata magnifica, are known to reversibly bind actin monomers, forming 1:1 stoichiometric complexes with G-actin, disrupting its polymerization. To identify novel therapeutic agents for effective treatment of metastatic breast cancer, several semisynthetic derivatives of latrunculin A with diverse steric, electrostatic, and hydrogen bond donor and acceptor properties were rationally prepared. Analogues were designed to modulate the binding affinity toward G-actin. Examples of these reactions are esterification, acetylation, and N-alkylation. Semisynthetic latrunculins were then tested for their ability to inhibit pyrene-conjugated actin polymerization, and subsequently assayed for their antiproliferative and anti-invasive properties against MCF7 and MDA-MB-231 cells using MTT and invasion assays, respectively.

Assessing pressurized liquid extraction for the high-throughput extraction of marine-sponge-derived natural products

In order to compare the utility of standard solvent partitioning (SSP) versus accelerated solvent extraction (ASE), a series of experiments were performed and evaluated. Overall yields, solvent consumption, processing time, and chemical stability of the fractions obtained by both methods were compared. Five marine sponges were selected for processing and analysis containing 12 structurally distinct, bioactive natural products. Extracts generated using SSP and ASE were assessed for chemical degradation using comparative LC MS-ELSD. The extraction efficiency (EE) of the ASE apparatus was 3 times greater than the SSP method on average, while the total extraction yields (TEY) were roughly equivalent. Furthermore, the ASE methodology required only 2 h to process each sample versus 80 h for SSP, and the LC MS-ELSD from extracts of both methods appeared comparable. These results demonstrate that ASE can serve as an effective high-throughput methodology for extracting marine organisms to streamline the discovery of novel and bioactive natural products.

New structures and bioactivity properties of jasplakinolide (jaspamide) analogues from marine sponges

The goal of this study was to isolate and study additional jasplakinolide analogues from two taxonomically distinct marine sponges including two Auletta spp. and one Jaspis splendens. This led to the isolation of jasplakinolide (1) and eleven jasplakinolide analogues (3-13) including seven new analogues (6-10, 12, and 13). Structure elucidation of the new compounds was based on a combination of 1D and 2D NMR analysis, optical rotation, circular dichroism, and preparation of Mosher's esters. Five of the new compounds are oxidized tryptophan derivatives of 1, including a unique quinazoline derivative (9). Compounds 1, 3, 5-8, and 11 were evaluated in the NCI 60 cell line screen, and all compounds were tested in a microfilament disruption assay. Jasplakinolide B (11) exhibited potent cytotoxicity (GI(50) < 1 nM vs human colorectal adenocarcinoma (HCT-116) cells) but did not exhibit microfilament-disrupting activity at 80 nM.

Development and validation of a rapid method for the detection of latrunculol A in plasma

Latrunculol A is a recently discovered 6,7-dihydroxy analog of the potent actin inhibitor latrunculin A. Latrunculol A has exhibited greater cytotoxicity than latrunculin A against both murine and human colon tumor cell lines in vitro. Currently, there are no reports regarding the bioavailability of latrunculol A in vivo. This study was undertaken as a prelude to pharmacokinetic assessments and it is the first work where bioavailability of latrunculol A was studied. In the present work, a simple plasma preparation and a rapid HPLC method have been developed. Mouse plasma containing latrunculol A was first treated by acetonitrile and then centrifuged at 14,000 rpm at 4 °C for 25 min. The supernatant was injected in an HPLC system comprising a Waters Symmetry NH₂ column, a mobile phase of acetonitrile/water (95/5, v/v), a flow rate of 1.0 mL/min, at 220 nm. The method was validated by parameters including a good linear correlation, a limit of quantification of 9 ng/mL, and a good precision with a coefficient variation of 1.65, 1.86, and 1.26% for 20, 400, and 800 ng/mL, respectively. With this simple method, excellent separation and sensitivity of latrunculol A are achieved, thus allowing a rapid analysis of the plasma samples for absorption, distribution, and metabolism studies.

The aignopsanes, a new class of sesquiterpenes from selected chemotypes of the sponge Cacospongia mycofijiensis

A survey of individual specimens of northern Papua New Guinea derived Cacospongia mycofijiensis has yielded novel sesquiterpenes, aignopsanoic acid A (1), methyl aignopsanoate A (2), and isoaignopsanoic acid A (3). The structures and absolute configurations of 1-3 were established using NMR data, X-ray crystallography results, and an analysis of CD properties. Two of these metabolites, 1 and 2, were moderately active against Trypanosoma brucei, the parasite responsible for sleeping sickness.

A selective account of effective paradigms and significant outcomes in the discovery of inspirational marine natural products

Marine natural products continue to be a source of significant molecular structures that serve as a stimulus to seed further significant research. This account reviews some of the major advances in the study of marine biomolecules made at UC Santa Cruz over more than three decades. The continuing challenge of discovery and characterization of what we term "inspirational molecular structures" will be presented in a comprehensive fashion. Examples of privileged molecular structures and their impact on biomedicinal research will be an important theme. The three major groups of organisms explored include seaweeds, sponges, and marine-derived fungi, and the study of their active principles has greatly benefited from synergistic collaborations with both academic and biopharmaceutical groups. The concluding sections of this chronicle will touch on prospects for future outcomes involving new sources and strategies.