Identification of Antimycobacterial Natural Products from a Library of Marine Invertebrate Extracts.

Tuberculosis (TB) remains a public health crisis, requiring the urgent identification of new anti-mycobacterial drugs. We screened several organic and aqueous marine invertebrate extracts for their in vitro inhibitory activity against the causative organism, Mycobacterium tuberculosis. Here, we report the results obtained for 54 marine invertebrate extracts. The chemical components of two of the extracts were dereplicated, using 1H NMR and HR-LCMS with GNPS molecular networking, and these extracts were further subjected to an activity-guided isolation process to purify the bioactive components. Hyrtios reticulatus yielded heteronemin 1 and Jaspis splendens was found to produce the bengamide class of compounds, of which bengamides P 2 and Q 3 were isolated, while a new derivative, bengamide S 5, was putatively identified and its structure predicted, based on the similarity of its MS/MS fragmentation pattern to those of other bengamides. The isolated bioactive metabolites and semi-pure fractions exhibited M. tuberculosis growth inhibitory activity, in the range <0.24 to 62.50 µg/mL. This study establishes the bengamides as potent antitubercular compounds, with the first report of whole-cell antitubercular activity of bengamides P 2 and Q 3.

New dihydroxycucurbitacin D's from the Namib desert endemic plant Acanthosicyos horridus (!nara).

Acanthosicyos horridus Welw. ex Hook.f. (!nara) is a leafless, thorny, melon-producing plant endemic to the hyper-arid Namib Desert. The methanol crude extract prepared from the ripe fruits of !nara afforded the known dihydroxycucurbitacin 7ß-hydroxy-23,24-dihydrocucurbitacin D (1), along with four new congeners 7ß,15ß-dihydroxy-23,24-dihydrocucurbitacin D (2), 25-O-ß-glucopyranosyl-7ß-hydroxy-23,24-dihydrocucurbitacin D (3), 25-O-ß-glucopyranosyl-7ß-hydroxy-23,24-dihydroisocucurbitacin D (4) and 25-O-ß-glucopyranosyl-7ß-hydroxy-23,24-dihydro-3-epi-isocucurbitacin D (5). These compounds were isolated through a combination of preparative normal phase thin-layer chromatography (TLC) and semi-preparative reversed phase high performance liquid chromatography (HPLC). Their structures were established by comprehensive analysis of HR-ESI-MS data, 1D and 2D NMR spectroscopic data and by comparison with literature values of similar cucurbitacins. The five isolated compounds exhibited poor cytotoxic activity against the MDA-MB-231 breast cancer cell line. To the best of our knowledge, this is the first report of glycosylated cucurbitacins in Acanthosicyos horridus.

Recifin A, Initial Example of the Tyr-Lock Peptide Structural Family, Is a Selective Allosteric Inhibitor of Tyrosyl-DNA Phosphodiesterase I.

Tyrosyl-DNA phosphodiesterase 1 (TDP1) is a molecular target for the sensitization of cancer cells to the FDA-approved topoisomerase inhibitors topotecan and irinotecan. High-throughput screening of natural product extract and fraction libraries for inhibitors of TDP1 activity resulted in the discovery of a new class of knotted cyclic peptides from the marine sponge Axinella sp. Bioassay-guided fractionation of the source extract resulted in the isolation of the active component which was determined to be an unprecedented 42-residue cysteine-rich peptide named recifin A. The native NMR structure revealed a novel fold comprising a four strand antiparallel ß-sheet and two helical turns stabilized by a complex disulfide bond network that creates an embedded ring around one of the strands. The resulting structure, which we have termed the Tyr-lock peptide family, is stabilized by a tyrosine residue locked into three-dimensional space. Recifin A inhibited the cleavage of phosphodiester bonds by TDP1 in a FRET assay with an IC50 of 190 nM. Enzyme kinetics studies revealed that recifin A can specifically modulate the enzymatic activity of full-length TDP1 while not affecting the activity of a truncated catalytic domain of TDP1 lacking the N-terminal regulatory domain (Δ1-147), suggesting an allosteric binding site for recifin A on the regulatory domain of TDP1. Recifin A represents both the first of a unique structural class of knotted disulfide-rich peptides and defines a previously unseen mechanism of TDP1 inhibition that could be productively exploited for potential anticancer applications.

Novel South African Rare Actinomycete Kribbella speibonae Strain SK5: A Prolific Producer of Hydroxamate Siderophores Including New Dehydroxylated Congeners.

In this paper, we report on the chemistry of the rare South African Actinomycete Kribbella speibonae strain SK5, a prolific producer of hydroxamate siderophores and their congeners. Two new analogues, dehydroxylated desferrioxamines, speibonoxamine 1 and desoxy-desferrioxamine D1 2, have been isolated, together with four known hydroxamates, desferrioxamine D1 3, desferrioxamine B 4, desoxy-nocardamine 5 and nocardamine 6, and a diketopiperazine (DKP) 7. The structures of 1-7 were characterized by the analysis of HRESIMS and 1D and 2D NMR data, as well as by comparison with the relevant literature. Three new dehydroxy desferrioxamine derivatives 8-10 were tentatively identified in the molecular network of K. speibonae strain SK5 extracts, and structures were proposed based on their MS/MS fragmentation patterns. A plausible spb biosynthetic pathway was proposed. To the best of our knowledge, this is the first report of the isolation of desferrioxamines from the actinobacterial genus Kribbella.

Isolation, Characterization and Antiproliferative Activity of New Metabolites from the South African Endemic Red Algal Species Laurencia alfredensis.

The marine red algae of the genus Laurencia have been widely studied for their structurally diverse and biologically active secondary metabolites. We report here the natural product investigation of the organic extract of a newly identified South African endemic species, Laurencia alfredensis. A sequence of column chromatography, preparative TLC and normal phase HPLC resulted in the isolation of eleven compounds comprising three labdane-type diterpenes (1-3), four polyether triterpenes (4-7), three cholestane-type ecdysteroids (8-10) and a glycolipid (11). Compounds 1-3, 5-8 and 10 have not previously been reported, while compound 9 is reported here for the first time from a natural source and the known compound 11 isolated for the first time from the genus Laurencia. The structural elucidation and the relative configuration assignments of the compounds were accomplished by extensive use of 1D- and 2D-NMR, HR-ESI-MS, UV and IR spectroscopic techniques, while the absolute configuration of compound 1 was determined by single-crystal X-ray diffraction analysis. All compounds were evaluated against the MDA-MB-231 breast and HeLa cervical cancer cell lines. Compound 2 exhibited low micromolar antiproliferative activity (IC50 = 9.3 µM) against the triple negative breast carcinoma and compound 7 was similarly active (IC50 = 8.8 µM) against the cervical cancer cell line.

Steroidal alkaloids from the marine sponge Corticium niger that inhibit growth of human colon carcinoma cells.

Bioinformatic analysis of data from the NCI-60 cell cytotoxicity screen revealed a subset of extracts that showed selective cytotoxic activity toward human colon carcinoma cell lines. Bioassay-guided fractionation of a colon cancer selective extract from a Philippines collection of the marine sponge Corticium niger provided two new steroidal alkaloids, plakinamines N (1) and O (2), along with two known compounds of the plakinamine class (3, 4). The structures of these compounds were elucidated by interpretation of combined MS and NMR spectroscopic data. Plakinamines N (1), O (2), and J (4) were tested for antiproliferative activity in the NCI-60 screen, and they showed enhanced inhibitory effects against all of the colon cell lines with mean GI50 values of 11.5, 2.4, and 1.4 µM, respectively.

Cytotoxicity of lapachol, ß-lapachone and related synthetic 1,4-naphthoquinones against oesophageal cancer cells.

Naphthoquinones have been found to have a wide range of biological activities, including cytotoxicity to cancer cells. The secondary metabolites lapachol, α- and ß-lapachone and a series of 25 related synthetic 1,4-naphthoquinones were screened against the oesophageal cancer cell line (WHCO1). Most of the compounds exhibited enhanced cytotoxicity (IC50 1.6-11.7 µM) compared to the current drug of choice cisplatin (IC50 = 16.5 µM). This study also established that the two new synthetic halogenated compounds 12a and 16a (IC50 = 3.0 and 7.3 µM) and the previously reported compound 11a (IC50 = 3.9 µM), were non-toxic to NIH3T3 normal fibroblast cells. Cell death of oesophageal cancer cells by processes involving PARP cleavage caused by 11a was shown to be associated with elevated c-Jun levels, suggesting a role for this pathway in the mechanism of action of this cohort of naphthoquinone compounds.

Cytotoxic and antioxidant marine prenylated quinones and hydroquinones.

Covering: 1972 to 2011. This review covers the literature of prenylated quinone, hydroquinone and naphthoquinone marine natural products with reported cytotoxic and/or antioxidant properties. The structures, biological activity and, where applicable, the syntheses of 159 cytotoxic/antioxidant compounds, isolated from various marine organisms, are presented, while trends in the distribution of these cytotoxic metabolites, across the different marine phyla, are highlighted. Marine prenylated quinones, hydroquinones and naphthoquinones are of mixed polyketide and terpenoid biogenesis and recent biosynthetic studies of selected compounds are discussed.