Lik Tong Tan

Filamentous tropical marine cyanobacteria: a rich source of natural products for anticancer drug discovery

A plethora of structurally novel bioactive secondary metabolites have been reported from the prokaryotic filamentous marine cyanobacteria in the past few decades. In addition to the production of harmful toxins, these marine blue-green algae are emerging as an important source of anticancer drugs. The majority of these potent biomolecules, including the dolastatins, curacin A, hectochlorin, the apratoxins, and the lyngbyabellins, belongs to the mixed polyketide–polypeptide structural class. Furthermore, a high proportion of these natural products target eukaryotic cytoskeleton, such as tubulin and actin microfilaments, making them an attractive source of potential anticancer drugs. In recent years, a number of potent marine cyanobacteria have also been reported to modulate cell death and apoptosis in cancer cells as well as target enzymes such as histone deacetylase. A number of marine cyanobacterial compounds have also served as structural templates for the generation of new drug leads, further attesting to the importance of these marine microbes as an important source of new pharmaceuticals. This review serves to highlight the chemistry and biology of selected anticancer marine cyanobacterial natural products exhibiting significant biological activities in the nanomolar or submicromolar range, and their discussion will be based on the different modes of action.

Lagunamides A and B: Cytotoxic and Antimalarial Cyclodepsipeptides from the Marine Cyanobacterium Lyngbya majuscula

Lagunamides A (1) and B (2) are new cyclic depsipeptides isolated from the marine cyanobacterium Lyngbya majuscula obtained from Pulau Hantu Besar, Singapore. The planar structural characterization of these molecules was achieved by extensive spectroscopic analysis, including 2D NMR experiments. In addition to Marfeys method and (3)J(H-H) coupling constant values, a modified method based on Moshers reagents and analysis using LC-MS was deployed for the determination of the absolute configuration. Lagunamides A and B displayed significant antimalarial properties, with IC(50) values of 0.19 and 0.91 μM, respectively, when tested against Plasmodium falciparum. Lagunamides A and B also possessed potent cytotoxic activity against P388 murine leukemia cell lines, with IC(50) values of 6.4 and 20.5 nM, respectively. Furthermore, these cyanobacterial compounds exhibited moderate antiswarming activities when tested against Pseudomonas aeruginosa PA01.

Hantupeptin A, a cytotoxic cyclic depsipeptide from a Singapore collection of Lyngbya majuscula.

Chemical investigation of the marine cyanobacterium Lyngbya majuscula from Pulau Hantu Besar, Singapore, has led to the isolation of a cyclodepsipeptide, hantupeptin A (1). The planar structure of 1 was assigned on the basis of extensive 1D and 2D NMR spectroscopic experiments. The absolute configuration of the amino and hydroxyl acid residues in the molecule was determined by application of the advanced Marfey method, chiral HPLC analysis, and Moshers method. Hantupeptin A showed cytotoxicity to MOLT-4 leukemia cells and MCF-7 breast cancer cells with IC(50) values of 32 and 4.0 microM, respectively.

Biochemical Studies of the Lagunamides, Potent Cytotoxic Cyclic Depsipeptides from the Marine Cyanobacterium Lyngbya majuscula

Lagunamides A (1) and B (2) are potent cytotoxic cyclic depsipeptides isolated from the filamentous marine cyanobacterium, Lyngbya majuscula, from Pulau Hantu, Singapore. These compounds are structurally related to the aurilide-class of molecules, which have been reported to possess exquisite antiproliferative activities against cancer cells. The present study presents preliminary findings on the selectivity of lagunamides against various cancer cell lines as well as their mechanism of action by studying their effects on programmed cell death or apoptosis. Lagunamide A exhibited a selective growth inhibitory activity against a panel of cancer cell lines, including P388, A549, PC3, HCT8, and SK-OV3 cells, with IC50 values ranging from 1.6 nM to 6.4 nM. Morphological studies showed blebbing at the surface of cancer cells as well as cell shrinkage accompanied by loss of contact with the substratum and neighboring cells. Biochemical studies using HCT8 and MCF7 cancer cells suggested that the cytotoxic effect of 1 and 2 might act via induction of mitochondrial mediated apoptosis. Data presented in this study warrants further investigation on the mode of action and underscores the importance of the lagunamides as potential anticancer agents.

Bouillonamide: A Mixed Polyketide–Peptide Cytotoxin from the Marine Cyanobacterium Moorea bouillonii

The tropical marine cyanobacterium, Moorea bouillonii, has gained recent attention as a rich source of bioactive natural products. Continued chemical investigation of this cyanobacterium, collected from New Britain, Papua New Guinea, yielded a novel cytotoxic cyclic depsipeptide, bouillonamide (1), along with previously reported molecules, ulongamide A and apratoxin A. Planar structure of bouillonamide was established by extensive 1D and 2D NMR experiments, including multi-edited HSQC, TOCSY, HBMC, and ROESY experiments. In addition to the presence of α-amino acid residues, compound 1 contained two unique polyketide-derived moieties, namely a 2-methyl-6-methylamino-hex-5-enoic acid (Mmaha) residue and a unit of 3-methyl-5-hydroxy-heptanoic acid (Mhha). Absolute stereochemistry of the α-amino acid units in bouillonamide was determined mainly by Marfey’s analysis. Compound 1 exhibited mild toxicity with IC50’s of 6.0 µM against the neuron 2a mouse neuroblastoma cells.

Chapter 4 – Marine Cyanobacteria: A Treasure Trove of Bioactive Secondary Metabolites for Drug Discovery

The prokaryotic marine cyanobacteria are a prolific source of novel bioactive secondary metabolites. To date, more than 400 metabolites have been isolated from various marine cyanobacterial strains, particularly from the tropical filamentous Lyngbya and Symploca genera. These molecules are nitrogen containing and belong mostly to the polyketide–polypeptide structural class. This chapter highlights a total of 112 new nitrogen-containing marine cyanobacterial compounds published between January 2007 and August 2011. A high proportion of these metabolites, including bisebromoamide, hoiamides, and largazole, are structurally novel and they possess high potency as cytotoxins, neurotoxins, and antiprotozoal agents. In addition, new analogs of previously reported cytotoxic molecules, such as apratoxin, lyngbyabellin, and lyngbyastatin/dolastatin 13 class of compounds, were also isolated. Further, a number of marine cyanobacterial metabolites interfere with specific cellular targets, such as tubulin, actin, and histone deacetylase, making them an attractive source of natural products for drug discovery.

DEVELOPMENT OF DIRECT SERIAL COUPLING OF HYDROPHILIC INTERACTION LIQUID CHROMATOGRAPHY AND REVERSED-PHASE COLUMNS FOR THE SIMULTANEOUS ANALYSIS OF ANTI-TUBERCULOSIS DRUGS

The use of HPLC-MS and -UV systems are preferred methods for the detection of pharmaceutical drugs. In this study, direct serial coupling of HILIC to a reversed-phase column via a zero-volume union was applied to resolve a mixture of first-line anti-tuberculosis drugs, including rifampicin (RIF), isoniazid (INH), pyrazinamide (PZA), ethambutol (EMB), and streptomycin (STP). These analytes, detected by either UV or MS, were chosen for their wide range of polarities and their analysis was achieved in a single HPLC injection with a 16 min runtime per sample and a flow rate of 0.6 mL/min. In addition, optimization of the chromatographic conditions required to separate the antibiotics in the direct serial coupling setup are also discussed.

Benderamide A, a Cyclic Depsipeptide from a Singapore Collection of Marine Cyanobacterium cf. Lyngbya sp.

Benderamide A (1), a (S)-2,2-dimethyl-3-hydroxy-7-octynoic acid (S-Dhoya)-containing cyclic depsipeptide that belongs to the kulolide superfamily, was isolated from a Singapore collection of cf. Lyngbya sp. marine cyanobacterium using a bioassay-guided approach. While the planar structure of 1 was elucidated using a combination of 1D and 2D NMR experiments and MS analysis, the absolute configuration was subsequently achieved using the results obtained from Marfey’s analysis, comparative analysis of nuclear overhauser effect spectroscopy (NOESY) with the known compound 3, and one dimensional-nuclear overhauser effect (1D-NOE). Although 1 did not display antiproliferative activity against MCF7 breast cancer cells, the presence of an Ala instead of Gly suggests a possible mechanistic pathway to explain the consequential decrease in cytotoxicity compared to the closely related 2. In addition, results obtained from an LC–MS/MS-based molecular networking algorithm revealed two other closely related compounds encouraging further identification and isolation from the same marine cyanobacterium extract.

Molecular Targets of Anticancer Agents from Filamentous Marine Cyanobacteria

The prokaryotic marine cyanobacteria, especially the filamentous forms, are known to produce a plethora of structurally unique natural products. A majority of these molecules are nitrogen-containing, belonging to the hybrid polyketide-polypeptide structural class. Various activities of clinical significance have been attributed to these molecules, ranging from anticancer, neuromodulating to antiprotozoal properties. Particularly in the area of cancer therapy, a number of potent marine cyanobacterial compounds, including dolastatins 10, 15, and largazole, have been identified as anticancer drug leads and are being further developed synthetically for clinical usage. The high potencies of these compounds are due to their exquisite interactions or interference with cellular pathways, specific macromolecules, or enzymes, such as the JAK-STAT signaling pathway, histone deacetylase, proteasome, protein kinase C, actin and microtubule filaments. This mini review covers more than 90 references and features more than 40 anticancer compounds, consisting of marine cyanobacterial natural products and their synthetic analogues. Biological data of these compounds are discussed based on their molecular targets.