Melissa M. Conte

Naseseazines A and B: A New Dimeric Diketopiperazine Framework from a Marine-Derived Actinomycete, Streptomyces sp.

Chemical analysis of a Streptomyces sp. (CMB-MQ030) isolated from a Fijian marine sediment yielded two new diketopiperazines, naseseazines A and B (1, 2), featuring a new dimeric framework. Structures were determined by detailed spectroscopic analysis and C(3) Marfeys analysis.

A search for BACE inhibitors reveals new biosynthetically related pyrrolidones, furanones and pyrroles from a southern Australian marine sponge, Ianthella sp.

Fractionation of a southern Australian marine sponge, Ianthella sp., yielded sixteen metabolites including a new class of pyrrolidone, ianthellidones A-F (1-6), a new class of furanone, ianthellidones G-H (7-8), new and known lamellarins, lamellarins O1 (9), O2 (10), O (11) and Q (12), plus the known 4-hydroxybenzaldehyde (13), 4-hydroxybenzoic acid (14), 4-methoxybenzoic acid (15) and ethyl 4-hydroxybenzoate (16). Structures for all Ianthella metabolites were determined by detailed spectroscopic analysis, supported by a plausible biosynthetic relationship. The ianthellidones were non-cytotoxic towards two human colon cancer cell lines (SW620 and SW620 Ad300), as well as Gram +ve and Gram -ve bacteria, and a fungus. Ianthellidone F (6) and lamellarins O2 (10) and O (11) displayed modest BACE inhibitory properties (IC(50) > 10 μM), while lamellarin O1 (9) was more potent (IC(50) < 10 μM). Lamellarin O (11) exhibited modest cytotoxicity towards SW620 and SW620 Ad300 cell lines (IC(50) > 22 μM), was an inhibitor of the multi-drug resistance efflux pump P-glycoprotein, and displayed selective growth inhibitory activity against the Gram +ve bacterium Bacillus subtilis (ATCC 6633) (IC(50) 2.5 μM).

Heronamides A–C, new polyketide macrolactams from an Australian marine-derived Streptomyces sp. A biosynthetic case for synchronized tandem electrocyclization

A Streptomyces sp. isolated from a shallow water sediment sample collected off Heron Island, Australia, afforded three new polyketide macrolactams, heronamides A-C (1-3). Structures were assigned to the heronamides on the basis of detailed spectroscopic analysis, chemical derivatization and biosynthetic considerations. A plausible biosynthetic pathway is proposed in which key carbocyclic ring transformations proceed via an unprecedented synchronized tandem electrocyclization. This biosynthesis provides a framework for the assignment of complete relative configurations across all heronamides, and inspires an attractive biomimetic strategy for future total syntheses. Heronamide C elicits a dramatic and reversible non-cytotoxic effect on mammalian cell morphology.

Nocardiopsins: new FKBP12-binding macrolide polyketides from an Australian marine-derived actinomycete, Nocardiopsis sp.

A marine-derived actinomycete, Nocardiopsis sp. (CMB-M0232), obtained from a sediment sample collected at a depth of 55 m off the coast of Brisbane, Australia, yielded two new macrolide polyketides. Structures for nocardiopsins A and B were assigned by detailed spectroscopic analysis, degradation and chemical derivatization. A Marfeys analysis revealed an unexpected acid-mediated partial racemization of the L-pipecolic acid incorporated within the nocardiopsins. The scope of this racemization was assessed against a selection of natural and synthetic N-acyl pipecolic acids. While the nocardiopsins are not antibacterial, antifungal or cytotoxic, they do exhibit low-micromolar binding to the immunophilin FKBP12, consistent with their structural and biosynthetic relationship to the immunosuppressive agents FK506 and rapamycin. The nocardiopsins represent a new point of entry into what has been a valuable, exclusive and reclusive region of bioactive chemical space--that surrounding the FK506/rapamycin pharmacophore.

New dictyodendrins as BACE inhibitors from a southern Australian marine sponge, Ianthella sp.

Chemical analysis of a southern Australian marine sponge, an Ianthella sp., yielded dictyodendrins F–J (1–5) as new examples of a rare class of marine alkaloid. Structures were assigned on the basis of detailed spectroscopic analysis, while biosynthetic considerations suggested a relationship between the dictyodendrins and co-metabolites belonging to the lamellarin and ianthellidone structure classes. The dictyodendrins 1 and 3–5 exhibited significant BACE inhibitory activity (IC50 1–2 μM), with the differential cytotoxicity displayed by 1–4 towards two human colon cancer cell lines (IC50 2–16 μM) marking them as both cytotoxins and probable substrates for the multi-drug resistance efflux pump P-glycoprotein. The dictyodendrins 1–5 did not inhibit growth of Gram −ve bacteria or fungi, but 1, 3, and 4 were selective Gram +ve antibacterials (IC50 1–3 μM). Dictyodendrin J (5), with its unique seco-carbon skeleton and unusual 1,2-diketone functionality, exhibited a promising non-cytotoxic biological activity profile, inclusive of significant BACE inhibitory activity (IC50 2 μM), supportive of further investigation.

Mirabilins revisited: polyketide alkaloids from a southern Australian marine sponge, Clathria sp.

Chemical investigation of a southern Australian marine sponge, Clathria sp., yielded the known mirabilins C, F and G, together with three new analogues, mirabilins H-J. For the first time mirabilins C and F are documented as the underivatized natural products, and a complete absolute stereochemistry is assigned to mirabilin F. Mirabilin I represents the first member of this structure class to incorporate a trans-fused ring junction. Structures for all mirabilins are assigned on the basis of detailed spectroscopic analysis. A plausible polyketide origin is proposed, linking all mirabilins and related sponge alkaloids. Mirabilin cytotoxicity against several human cancer cell lines is discussed.

Franklinolides A–C from an Australian Marine Sponge Complex: Phosphodiesters Strongly Enhance Polyketide Cytotoxicity

Rare discovery: Three novel polyketide phosphodiesters, franklinolidesa A-C, were identified as the cytotoxic components from an Australian marine sponge complex. Preliminary structure-activity relationship studies, using in vitro cytotoxicity and cell proliferation assays, demonstrated that the introduction of the phosphodiester moiety significantly improved the cytotoxicity by 30 to >300 fold.

Kinase Inhibitor Scaffolds against Neurodegenerative Diseases from a Southern Australian Ascidian, Didemnum sp.

Screening a library of Southern Australian and Antarctic marine invertebrates and algae for inhibitors of neurodegenerative disease kinase targets casein kinase 1 (CK1δ), cyclin‐dependent kinase 5 (CDK5) and glycogen synthase kinase 3β (GSK3β) identified a Western Australian Didemnum species (CMB‐02127) as a high‐priority specimen. Chemical fractionation returned the known aromatic alkaloids ningalins B–D as the major metabolites, together with six minor metabolites, the new ningalins E–G and the known hexacyclic pyrrole alkaloids lamellarins Z, G and A6. All structures were assigned by detailed spectroscopic analysis and literature comparisons, and the structural assignments were supported by biosynthetic considerations. The ningalins showed potent and broad inhibition across the three kinases, while the lamellarins were generally more selective for CDK5. Docking studies using published X‐ray crystal structures of CDK5 revealed both scaffolds target the ATP binding pocket.

Heterofibrins: inhibitors of lipid droplet formation from a deep-water southern Australian marine sponge, Spongia (Heterofibria) sp.

A bioassay-guided search for inhibitors of lipid droplet formation in a deep-water southern Australian marine sponge, Spongia (Heterofibria) sp., yielded six new compounds, fatty acids heterofibrins A1 (1) and B1 (4), along with related monolactyl and dilactyl esters, heterofibrins A2 (2), B2 (5), A3 (3) and B3 (6). Heterofibrin structures were assigned on the basis of detailed spectroscopic analysis, with comparison to chiral synthetic model compounds. All heterofibrins possess a diyne-ene moiety, while the monolactyl and dilactyl moiety featured in selected heterofibrins is unprecedented in the natural products literature. SAR by co-metabolite studies on the heterofibrins confirmed them to be non-cytotoxic, with the carboxylic acids 1 and 4 inhibiting lipid droplet formation in A431 fibroblast cell lines. Such inhibitors have potential application in the management of obesity, diabetes and atherosclerosis