Investigation of Indolglyoxamide and Indolacetamide Analogues of Polyamines as Antimalarial and Antitrypanosomal Agents

Pure compound screening has previously identified the indolglyoxylamidospermidine ascidian metabolites didemnidine A and B (2 and 3) to be weak growth inhibitors of Trypanosoma brucei rhodesiense (IC₅₀ 59 and 44 μM, respectively) and Plasmodium falciparum (K1 dual drug resistant strain) (IC₅₀ 41 and 15 μM, respectively), but lacking in selectivity (L6 rat myoblast, IC₅₀ 24 μM and 25 μM, respectively). To expand the structure–activity relationship of this compound class towards both parasites, we have prepared and biologically tested a library of analogues that includes indoleglyoxyl and indoleacetic “capping acids”, and polyamines including spermine (PA3-4-3) and extended analogues PA3-8-3 and PA3-12-3. 7-Methoxy substituted indoleglyoxylamides were typically found to exhibit the most potent antimalarial activity (IC₅₀ 10–92 nM) but with varying degrees of selectivity versus the L6 rat myoblast cell line. A 6-methoxyindolglyoxylamide analogue was the most potent growth inhibitor of T. brucei (IC₅₀ 0.18 μM) identified in the study: it, however, also exhibited poor selectivity (L6 IC₅₀ 6.0 μM). There was no apparent correlation between antimalarial and anti-T. brucei activity in the series. In vivo evaluation of one analogue against Plasmodium berghei was undertaken, demonstrating a modest 20.9% reduction in parasitaemia.     

New Antimicrobial Bromotyrosine Analogues from the Sponge Pseudoceratina purpurea and Its Predator Tylodina corticalis

Bioassay-guided fractionation of extracts from temperate Australian collections of the marine sponge Pseudoceratina purpurea resulted in the isolation and characterisation of two new and six known bromotyrosine-derived alkaloids with antibiotic activity. Surprisingly, a single specimen of the mollusc Tylodina corticalis, which was collected while feeding on P. purpurea, contained only a few of the compounds found in the sponge suggesting selective accumulation and chemical modification of sponge metabolites.     

Eleganolone,a Diterpene from the French Marine Alga Bifurcaria bifurcata Inhibits Growth of the Human Pathogens Trypanosoma brucei and Plasmodium falciparum

Organic extracts of 20 species of French seaweed have been screened against Trypanosoma brucei rhodesiense trypomastigotes, the parasite responsible for sleeping sickness. These extracts have previously shown potent antiprotozoal activities in vitro against Plasmodium falciparum and Leishmania donovani. The selectivity of the extracts was also evaluated by testing cytotoxicity on a mammalian L6 cell line. The ethyl acetate extract of the brown seaweed, Bifurcaria bifurcata, showed strong trypanocidal activity with a mild selectivity index (IC₅₀ = 0.53 µg/mL; selectivity index (SI) = 11.6). Bio-guided fractionation led to the isolation of eleganolone, the main diterpenoid isolated from this species. Eleganolone contributes only mildly to the trypanocidal activity of the ethyl acetate extract (IC₅₀ = 45.0 µM, SI = 4.0). However, a selective activity against P. falciparum erythrocytic stages in vitro has been highlighted (IC₅₀ = 7.9 µM, SI = 21.6).     

Smenamides A and B,Chlorinated Peptide/Polyketide Hybrids Containing a Dolapyrrolidinone Unit from the Caribbean Sponge Smenospongia aurea. Evaluation of Their Role as Leads in Antitumor Drug Research

An in-depth study of the secondary metabolites contained in the Caribbean sponge Smenospongia aurea led to the isolation of smenamide A (1) and B (2), hybrid peptide/polyketide compounds containing a dolapyrrolidinone unit. Their structures were elucidated using high-resolution ESI-MS/MS and homo- and heteronuclear 2D NMR experiments. Structures of smenamides suggested that they are products of the cyanobacterial metabolism, and 16S rRNA metagenomic analysis detected Synechococcus spongiarum as the only cyanobacterium present in S. aurea. Smenamides showed potent cytotoxic activity at nanomolar levels on lung cancer Calu-1 cells, which for compound 1 is exerted through a clear pro-apoptotic mechanism. This makes smenamides promising leads for antitumor drug design.     

New Oxidized Zoanthamines from a Canary Islands Zoanthus sp.

Three new norzoanthamine-type alkaloids, named 2-hydroxy-11-ketonorzoanthamide B (1), norzoanthamide B (2) and 15-hydroxynorzoanthamine (3), were isolated from Zoanthus sp. specimens collected at the Canary Islands. Their structures were determined by interpretation of NMR and HR-ESIMS data. Relative configurations of their chiral centers were proposed on the basis of ROESY spectra and by comparison of their spectroscopic data with those of the well-known compound, norzoanthamine.     

The Brown Alga Stypopodium zonale (Dictyotaceae): A Potential Source of Anti-Leishmania Drugs

This study evaluated the anti-Leishmania amazonensis activity of a lipophilic extract from the brown alga Stypopodium zonale and atomaric acid, its major compound. Our initial results revealed high inhibitory activity for intracellular amastigotes in a dose-dependent manner and an IC₅₀ of 0.27 μg/mL. Due to its high anti-Leishmania activity and low toxicity toward host cells, we fractionated the lipophilic extract. A major meroditerpene in this extract, atomaric acid, and its methyl ester derivative, which was obtained by a methylation procedure, were identified by nuclear magnetic resonance (NMR) spectroscopy. Both compounds inhibited intracellular amastigotes, with IC₅₀ values of 20.2 μM (9 μg/mL) and 22.9 μM (10 μg/mL), and selectivity indexes of 8.4 μM and 11.5 μM. The leishmanicidal activity of both meroditerpenes was independent of nitric oxide (NO) production, but the generation of reactive oxygen species (ROS) may be at least partially responsible for the amastigote killing. Our results suggest that the lipophilic extract of S. zonale may represent an important source of compounds for the development of anti-Leishmania drugs.     

Stereochemical Determination of Five-Membered Cyclic Ether Acetogenins Using a Spin-Spin Coupling Constant Approach and DFT Calculations

Five-membered rings are of particular interest, due to their presence in some of the most common molecules in chemistry and biology. Despite their apparent simplicity, the structural resolution of these rings is complex, due to their inherent conformational flexibility. Here, we describe an application of a recently reported simple and efficient NMR protocol based on the measurement of spin-spin coupling constants to achieve the challenging relative configurations of five new halogenated C₁₅ tetrahydrofuranyl-acetogenins, marilzafurollenes A–D (1–4) and 12-acetoxy-marilzafurenyne (5), isolated from the red alga, Laurencia marilzae. Although DFT chemical shift calculations were used to connect remote stereocenters, the NMR-based approach seems advantageous over computational techniques in this context, as the presence of halogens may interfere with reliable calculations.     

Natural Products from Marine Fungi—Still an Underrepresented Resource

Marine fungi represent a huge potential for new natural products and an increased number of new metabolites have become known over the past years, while much of the hidden potential still needs to be uncovered. Representative examples of biodiversity studies of marine fungi and of natural products from a diverse selection of marine fungi from the author’s lab are highlighting important aspects of this research. If one considers the huge phylogenetic diversity of marine fungi and their almost ubiquitous distribution, and realizes that most of the published work on secondary metabolites of marine fungi has focused on just a few genera, strictly speaking Penicillium, Aspergillus and maybe also Fusarium and Cladosporium, the diversity of marine fungi is not adequately represented in investigations on their secondary metabolites and the less studied species deserve special attention. In addition to results on recently discovered new secondary metabolites of Penicillium species, the diversity of fungi in selected marine habitats is highlighted and examples of groups of secondary metabolites produced by representatives of a variety of different genera and their bioactivities are presented. Special focus is given to the production of groups of derivatives of metabolites by the fungi and to significant differences in biological activities due to small structural changes.     

Cephalopods as Vectors of Harmful Algal Bloom Toxins in Marine Food Webs

Here we summarize the current knowledge on the transfer and accumulation of harmful algal bloom (HAB)-related toxins in cephalopods (octopods, cuttlefishes and squids). These mollusks have been reported to accumulate several HAB-toxins, namely domoic acid (DA, and its isomers), saxitoxin (and its derivatives) and palytoxin (and palytoxin-like compounds) and, therefore, act as HAB-toxin vectors in marine food webs. Coastal octopods and cuttlefishes store considerably high levels of DA (amnesic shellfish toxin) in several tissues, but mainly in the digestive gland (DG)—the primary site of digestive absorption and intracellular digestion. Studies on the sub-cellular partitioning of DA in the soluble and insoluble fractions showed that nearly all DA (92.6%) is found in the cytosol. This favors the trophic transfer of the toxins since cytosolic substances can be absorbed by predators with greater efficiency. The available information on the accumulation and tissue distribution of DA in squids (e.g., in stranded Humboldt squids, Dosidicus gigas) is scarcer than in other cephalopod groups. Regarding paralytic shellfish toxins (PSTs), these organisms accumulate them at the greatest extent in DG >> kidneys > stomach > branchial hearts > posterior salivary glands > gills. Palytoxins are among the most toxic molecules identified and stranded octopods revealed high contamination levels, with ovatoxin (a palytoxin analogue) reaching 971 μg kg⁻¹ and palytoxin reaching 115 μg kg⁻¹ (the regulatory limit for PlTXs is 30 μg kg⁻¹ in shellfish). Although the impacts of HAB-toxins in cephalopod physiology are not as well understood as in fish species, similar effects are expected since they possess a complex nervous system and highly developed brain comparable to that of the vertebrates. Compared to bivalves, cephalopods represent a lower risk of shellfish poisoning in humans, since they are usually consumed eviscerated, with exception of traditional dishes from the Mediterranean area.     

Marine Organisms as Alkaloid Biosynthesizers of Potential Anti-Alzheimer Agents

The incidence of neurodegenerative diseases, such as Alzheimer’s disease (AD), increases continuously demanding the urgent development of anti-Alzheimer’s agents. Marine organisms (MO) have to create their own defenses due to the adverse environment where they live and so synthesize several classes of compounds, such as akaloids, to defend themselves. Therefore, the identification of marine natural products with neuroprotective effects is a necessity. Being that AD is not only a genetic but also an environmental complex disease, a treatment for AD remains to discover. As the major clinical indications (CI) of AD are extracellular plaques formed by β-amyloid (Aβ) protein, intracellular neurofibrillary tangles (NFTs) formed by hyper phosphorylated τ-protein, uncommon inflammatory response and neuron apoptosis and death caused by oxidative stress, alkaloids that may decrease CI, might be used against AD. Most of the alkalolids with those properties are derivatives of the amino acid tryptophan mainly with a planar indole scaffold. Certainly, alkaloids targeting more than one CI, multitarget-directed ligands (MTDL), have the potential to become a lead in AD treatment. Alkaloids to have a maximum of activity against CI, should be planar and contain halogens and amine quaternization.     

Helicusin E,Isochromophilone X and Isochromophilone XI: New Chloroazaphilones Produced by the Fungus Bartalinia robillardoides Strain LF550

Microbial studies of the Mediterranean sponge Tethya aurantium led to the isolation of the fungus Bartalinia robillardoides strain LF550. The strain produced a number of secondary metabolites belonging to the chloroazaphilones. This is the first report on the isolation of chloroazaphilones of a fungal strain belonging to the genus Bartalinia. Besides some known compounds (helicusin A (1) and deacetylsclerotiorin (2)), three new chloroazaphilones (helicusin E (3); isochromophilone X (4) and isochromophilone XI (5)) and one new pentaketide (bartanolide (6)) were isolated. The structure elucidations were based on spectroscopic analyses. All isolated compounds revealed different biological activity spectra against a test panel of four bacteria: three fungi; two tumor cell lines and two enzymes.     

Sponge-Derived Kocuria and Micrococcus spp. as Sources of the New Thiazolyl Peptide Antibiotic Kocurin

Forty four marine actinomycetes of the family Microccocaceae isolated from sponges collected primarily in Florida Keys (USA) were selected from our strain collection to be studied as new sources for the production of bioactive natural products. A 16S rRNA gene based phylogenetic analysis showed that the strains are members of the genera Kocuria and Micrococcus. To assess their biosynthetic potential, the strains were PCR screened for the presence of secondary metabolite genes encoding nonribosomal synthetase (NRPS) and polyketide synthases (PKS). A small extract collection of 528 crude extracts generated from nutritional microfermentation arrays was tested for the production of bioactive secondary metabolites against clinically relevant strains (Bacillus subtilis, methicillin-resistant Staphylococcus aureus (MRSA), Acinetobacter baumannii and Candida albicans). Three independent isolates were shown to produce a new anti-MRSA bioactive compound that was identified as kocurin, a new member of the thiazolyl peptide family of antibiotics emphasizing the role of this family as a prolific resource for novel drugs.     

A new hydroxylated nonaprenylhydroquinone from the Mediterranean marine sponge Sarcotragus spinosulus

Chemical investigation of the Mediterranean sponge Sarcotragus spinosulus led to the isolation of a new hydroxylated nonaprenylhydroquinone, along with two known metabolites, hepta- and octaprenylhydroquinones. The structure of the new metabolite was assigned by extensive 1D and 2D NMR analyses and MS studies. The antileukemic effect of the three compounds towards the chronic myelogenous leukemia (CML) cells line K562 was also evaluated.     

Synthesis and Evaluation of Some New Aza-B-homocholestane Derivatives as Anticancer Agents

Using analogues of some marine steroidal oximes as precursors, a series of aza-B-homocholestane derivatives possessing different substituted groups at the 3-position of the steroidal nucleus were synthesized. Their biological activity against cancer cell proliferation was determined with multiple cancer cell lines. Aza-B-homocholestane derivatives possessing 3-hydroxyl, 3-hydroximino and 3-thiosemicarbazone groups displayed remarkable cytotoxicity to cancer cells via apoptosis inducing mechanism. Compounds 5, 10, 12, 15 and 18 exhibited better potency to inhibit cancer cell proliferation. In addition, compound 15 was further evaluated with three dimensional (3D) multicellular spheroids assay to determine its potency against spheroid growth. The structure-activity relationship (SAR) generated in the studies is valuable for the design of novel chemotherapeutic agents.     

Oleic acid produced by a marine Vibrio spp. acts as an anti-Vibrio parahaemolyticus agent

It is known that some strains of Vibrio parahaemolyticus are responsible for gastroenteric diseases caused by the ingestion of marine organisms contaminated with these bacterial strains. Organic products that show inhibitory activity on the growth of the pathogenic V. parahaemolyticus were extracted from a Vibrio native in the north of Chile. The inhibitory organic products were isolated by reverse phase chromatography and permeation by Sephadex LH20, and were characterized by spectroscopic and spectrometric techniques. The results showed that the prevailing active product is oleic acid, which was compared with standards by gas chromatography and high-performance liquid chromatography (HPLC). These active products might be useful for controlling the proliferation of pathogenic clones of V. parahaemolyticus.     

Toward the synthesis and biological screening of a cyclotetrapeptide from marine bacteria

The first synthesis of a naturally occurring tetrapeptide cyclo-(isoleucyl-prolyl-leucyl-alanyl) has been achieved using a solution-phase technique via coupling of dipeptide segments Boc-L-Pro-L-Leu-OH and L-Ala-L-Ile-OMe. Deprotection of the linear tetrapeptide unit and its subsequent cyclization gave a cyclopeptide, identical in all aspects to the naturally occurring compound. Bioactivity results indicated the antifungal and antihelmintic potential of the synthesized peptide against pathogenic dermatophytes and earthworms.     

Potential Chemopreventive Activity of a New Macrolide Antibiotic from a Marine-Derived Micromonospora sp

Agents capable of inducing phase II enzymes such as quinone reductase 1 (QR1) are known to have the potential of mediating cancer chemopreventive activity. As part of a program to discover novel phase II enzyme-inducing molecules, we identified a marine-derived actinomycete strain (CNJ-878) that exhibited activity with cultured Hepa 1c1c7 cells. Based on this activity, a new macrolide, juvenimicin C (1), as well as 5-O-α-l-rhamnosyltylactone (2), were isolated from the culture broth of a Micromonospora sp. Compound 1 enhanced QR1 enzyme activity and glutathione levels by two-fold with CD values of 10.1 and 27.7 μM, respectively. In addition, glutathione reductase and glutathione peroxidase activities were elevated. This is the first reported member of the macrolide class of antibiotics found to mediate these responses.     

Pseudonocardians A-C, new diazaanthraquinone derivatives from a deap-sea actinomycete Pseudonocardia sp. SCSIO 01299

Pseudonocardians A–C (2–4), three new diazaanthraquinone derivatives, along with a previously synthesized compound deoxynyboquinone (1), were produced by the strain SCSIO 01299, a marine actinomycete member of the genus Pseudonocardia, isolated from deep-sea sediment of the South China Sea. The structures of compounds 1–4 were determined by mass spectrometry and NMR experiments (¹H, ¹³C, HSQC, and HMBC). The structure of compound 1, which was obtained for the first time from a natural source, was confirmed by X-ray analysis. Compounds 1–3 exhibited potent cytotoxic activities against three tumor cell lines of SF-268, MCF-7 and NCI-H460 with IC₅₀ values between 0.01 and 0.21 μm, and also showed antibacterial activities on Staphylococcus aureus ATCC 29213, Enterococcus faecalis ATCC 29212 and Bacillus thuringensis SCSIO BT01, with MIC values of 1–4 μg mL⁻¹.     

Cyanobacterial Metabolite Calothrixins: Recent Advances in Synthesis and Biological Evaluation

The marine environment is host to unparalleled biological and chemical diversity, making it an attractive resource for the discovery of new therapeutics for a plethora of diseases. Compounds that are extracted from cyanobacteria are of special interest due to their unique structural scaffolds and capacity to produce potent pharmaceutical and biotechnological traits. Calothrixins A and B are two cyanobacterial metabolites with a structural assembly of quinoline, quinone, and indole pharmacophores. This review surveys recent advances in the synthesis and evaluation of the biological activities of calothrixins. Due to the low isolation yields from the marine source and the promise this scaffold holds for anticancer and antimicrobial drugs, organic and medicinal chemists around the world have embarked on developing efficient synthetic routes to produce calothrixins. Since the first review appeared in 2009, 11 novel syntheses of calothrixins have been published in the efforts to develop methods that contain fewer steps and higher-yielding reactions. Calothrixins have shown their potential as topoisomerase I poisons for their cytotoxicity in cancer. They have also been observed to target various aspects of RNA synthesis in bacteria. Further investigation into the exact mechanism for their bioactivity is still required for many of its analogs.