Marine natural products offer an abundant source of pharmacologically active agents with great diversity and complexity, and the potential to produce valuable therapeutic entities. Indole alkaloids is one of the important class of marine-derived secondary metabolites, with wide occurrence amongst variety of marine sources such as sponges, tunicates, algae, worms and microorganisms and have been extensively studied for their biological activities. Among this chemical family, a sponge-derived bis-indole alkaloid fascaplysin (1) exhibited broad range of bioactivities including antibacterial, antifungal, antiviral, anti-HIV-1-RTase, p56 tyrosine kinase inhibition, antimalarial, anti-angiogenic, antiproliferative activity against numerous cancer cell lines, specific inhibition of cyclin-dependent kinase-4 (IC(50) 350 nM) and action as a DNA intercalator. In the present review, the chemical diversity of natural as well as synthetic analogues of fascaplysin has been reviewed with a detailed account on synthetic reports and pharmacological studies. Our analysis of the structure-activity relationships of this family of compounds highlights the existence of various potential leads for the development of novel anticancer agents.

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In this study, we for the first time explored the cellular and molecular mechanism of anticancer properties of fascaplysin, a marine sponge-derived alkaloid. Our study demonstrated that fascaplysin induced a cooperative interaction between apoptotic and autophagic pathways to induce cytotoxicity in HL-60 cells. Fascaplysin treatment not only activated pro-apoptotic events like PARP-1 cleavage and caspase activation but also triggered autophagy signaling as shown by the increased expression of LC3-II, ATG7and beclin. Interestingly, it was found that use of pan-caspase inhibitor completely reversed the fascaplysin mediated cell death as analyzed by MTT and cell cycle assays. It was observed that cell death as well as the expression of pro-death proteins was partially reversed, when key autophagy mediators ATG7 was silenced by siRNA in fascaplysin treated cells. Cooperative involvement of autophagy and apoptotic signaling in cytotoxicity was confirmed when combined silencing of pro-apototic (PARP-1) and autophagic (ATG-7) signaling by respective siRNA's lead to substantial rescue of cell death induced by fascaplysin. Although, apoptosis and autophagy are two independent cell death pathways, our findings provide detailed insight by which both the pathways acted cooperatively to elicit fascaplysin induced cell death in HL-60 cells. Our findings provide molecular insight into the anti-cancer potential of fascaplysin by showing that both autophagic and apoptotic signaling can work together in the induction of cell death.

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The marine natural product fascaplysin (1) is a potent Cdk4 (cyclin-dependent kinase 4)-specific inhibitor, but is toxic to all cell types possibly because of its DNA-intercalating properties. Through the design and synthesis of numerous fascaplysin analogues, we intended to identify inhibitors of cancer cell growth with good therapeutic window with respect to normal cells. Among various non-planar tryptoline analogues prepared, N-(biphenyl-2-yl) tryptoline (BPT, 6) was identified as a potent inhibitor of cancer cell growth and free from DNA-binding properties owing to its non-planar structure. This compound was tested in over 60 protein kinase assays. It displayed inhibition of Cdk4-cyclin D1 enzyme in vitro far more potently than many other kinases including Cdk family members. Although it blocks growth of cancer cells deficient in the mitotic-spindle checkpoint at the G0/G1 phase of the cell cycle, the block occurs primarily at the G2/M phase. BPT inhibits tubulin polymerization in vitro and acts as an enhancer of tubulin depolymerization of paclitaxel-stabilized tubulin in live cells. Western blot analyses indicated that, in p53-positive cells, BPT upregulates the expression of p53, p21 and p27 proteins, whereas it downregulates the expression of cyclin B1 and Cdk1. BPT selectively kills SV40-transformed mouse embryonic hepatic cells and human fibroblasts rather than untransformed cells. BPT inhibited the growth of several human cancer cells with an IC50 <1 μM. The pharmacokinetic study in BALB/c mice indicated good plasma exposure after intravenous administration. It was found to be efficacious at 1/10th the maximum-tolerated dose (1000 mg/kg) against human tumours derived from HCT-116 (colon) and NCI-H460 (lung) cells in SCID (severe-combined immunodeficient) mice models. BPT is a relatively better anticancer agent than fascaplysin with an unusual ability to block two overlapping yet crucial phases of the cell cycle, mitosis and G0/G1. Its ability to effectively halt tumour growth in human tumour-bearing mice would suggest that BPT has the potential to be a candidate for further clinical development.

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Biphenyl-4-carboxylic acid-[2-(1H-indol-3-yl)-ethyl]-methylamide 1 (CA224) is a nonplanar analogue of fascaplysin (2) that specifically inhibits Cdk4-cyclin D1 in vitro. Compound 1 blocks the growth of cancer cells at G0/G1 phase of the cell cycle. It also blocks the cell cycle at G2/M phase, which is explained by the fact that it inhibits tubulin polymerization. Additionally, it acts as an enhancer of depolymerization for taxol-stabilized tubulin. Western blot analyses of p53-positive cancer cells treated with compound 1 indicated upregulation of p53, p21, and p27 proteins together with downregulation of cyclin B1 and Cdk1. Compound 1 selectively induces apoptosis of SV40 large T-antigen transformed cells and significantly reduces colony formation efficiency, in a dose-dependent manner, of lung cancer cells. It is efficacious at 1/10th of the MTD against human tumors derived from HCT-116 and NCI-H460 cells in SCID mouse models. The promising efficacy of compound 1 in human xenograft models as well as its excellent therapeutic window indicates its potential for clinical development.