Antitumour potential of BPT: a dual inhibitor of cdk4 and tubulin polymerization.

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.

Chemistry and biology of fascaplysin, a potent marine-derived CDK-4 inhibitor.

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.

Meridianins: marine-derived potent kinase inhibitors.

Marine invertebrates are a rich source of novel, bioactive secondary metabolites and have attracted a great deal of attention from scientists in the fields of chemistry, pharmacology, ecology, and molecular biology. This profilic natural source has produced several antitumor secondary metabolites and amongst these, indole alkaloids are of wide occurrence. Meridianins A-G (1-7) are indole alkaloids isolated from tunicate Aplidium meridianum and are known to inhibit variety of protein kinases associated with cancer and neurodegenerative diseases. These compounds also exhibited promising antiproliferative activity in several cancer cell lines. Amongst natural meridianins, meridianin E (5) showed potent and selective inhibition of CDK-1 and CDK-5. Several synthetic meridianin analogs exhibited potent and selective inhibition of glycogen synthase-3 (GSK-3) and dual-specificity tyrosine-phosphorylation regulated kinase 1A (Dyrk-1A) which are known to be implicated in progression of Alzheimer's disease. The present review provides the critical account of isolation, medicinal chemistry and pharmacology of meridianins. 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 and anti-Alzheimer's agents.

Cyclin-dependent kinase inhibition by flavoalkaloids.

Chromone alkaloids and flavoalkaloids are an important group of natural products possessing promising medicinal properties. A chromone alkaloid rohitukine is a major bioactive chemical constituent of plant Dysoxylum binectariferum (Meliaceae) Hook. which is phylogenetically related to the Ayurvedic plant, D. malabaricum Bedd. used for treatment of rheumatoid arthritis. This chromone alkaloid led to discovery of two synthetic flavoalkaloids: flavopiridol (Sanofi) and P-276-00 (Piramal) which have reached to advanced stages of clinical development for cancer treatment. Flavopiridol (Alvocidib; L868275; HMR-1275; NSC 649890 of Sanofi-Aventis + NCI) is approved as an orphan drug for treatment of chronic lymphocytic leukemia and is currently undergoing phase II studies as monotherapy and also as in combination regimes with traditional chemotherapy agents. P-276-00 (12) is currently in phase II clinical studies for advanced refractory neoplasms and multiple myeloma. Extensive amount of medicinal chemistry efforts have been reported on these flavoalkaloids. Flavopiridol demonstrated potent and specific in vitro inhibition of variety of cyclindependent kinases with clear block in cell cycle progression at the G1/S and G2/M phases. Preclinical studies demonstrated the capacity of flavopiridol to induce programmed cell death, promote differentiation, inhibit angiogenic processes and modulate transcriptional events. The co-crystallised structure of deschloro-flavopiridol with CDK-2 is available and key interactions in the ATP binding site have been reported. Flavopiridol has also been studied for the treatment of arthritis and atherosclerotic plaque formation. The present review comprises discovery, medicinal chemistry, pharmacology and preclinical/clinical development of flavoalkaloids as CDK inhibitors.

Modulation of k-Ras signaling by natural products.

Ras proteins regulate diverse cellular pathways that are important in the growth and spread of malignancies, including cell proliferation, cell cycle regulation, cell survival, angiogenesis and cell migration. These proteins lack the conventional transmembrane or hydrophobic domain typical of membrane associated proteins. Being small and hydrophilic in nature, these proteins undergo four-stage post-translational lipid modifications viz. prenylation, AAX proteolysis, carboxymethylation and palmitoylation for membrane localization which is important for their function. Therefore, enzymes involved in these modifications viz. farnesyl transferase (FTase), geranylgeranyl transferase-I (GGTase-I), geranylgeranyl transferase-II (GGTase-II), Ras converting enzyme-1 (Rce-1) and isoprenyl cysteine methyl transferase (ICMT) are emerging as potential therapeutic targets for the discovery of newer anticancer therapeutics. Several natural products have shown modulation of these post-translational enzymes. In the present review, natural products isolated from terrestrial as well as marine sources showing ability to modulate these k-Ras post-translational targets and their promise as potential anticancer agents have been discussed. A total of 157 natural products with 141 corresponding references have been covered.