Recent Advances in Combretastatin A-4 Inspired Inhibitors of Tubulin Polymerization-An Update

Cancer is one of the leading causes of fatality and mortality worldwide. Investigations on developing therapeutic strategies for cancer are supported throughout the world. The massive achievements in molecular sciences involving biochemistry, molecular chemistry, medicine, and pharmacy, and high throughput techniques such as genomics and proteomics have helped to create new potential drug targets for cancer treatment. Microtubules are very attractive targets for cancer therapy because of the crucial roles they play in cell division. In recent years, lots of efforts have been put into the identification of new microtubule-targeting agents (MTAs) in anticancer therapy. Combretastatin A-4 (CA-4) is a natural compound that binds to microtubules’ colchicine binding site and inhibits microtubule polymerization. Due to CA-4’s structural simplicity, many analogs have been synthesized. This article summarizes the new molecule development efforts to reach CA-4 analogs by modifications on its pharmacophore groups, published since 2015.

Identification and Validation of Novel Microtubule Suppressors with an Imidazopyridine Scaffold through Structure Based Virtual Screening and Docking.

Targeting the colchicine binding site of alpha/beta tubulin microtubules can lead to suppression of microtubule dynamics, cell cycle arrest and apoptosis. Therefore, development of microtubule (MT) inhibitors is considered a promising route to anticancer agents. Our approach to identify novel scaffolds as MT inhibitors depends on a 3D-structure based pharmacophore approach and docking using three programmes MOE, Autodock and BUDE (Bristol University Docking Engine) to screen a library of virtual compounds. From this work we identified the compound 7-(3-Hydroxy-4-methoxy-phenyl)-3-(3-trifluoromethyl-phenyl)-6,7-dihydro-3H-imidazo[4,5-b] pyridin-5-ol (6) as a novel inhibitor scaffold. This compound inhibited several types of cancer cell proliferation at low micromolar concentrations with low toxicity. Compound 6 caused cell cycle arrest in the G2/M phase and blocked tubulin polymerization at low micromolar concentration (IC50 = 6 micromolar, inducing apoptosis via activation of caspase 9, increasing the level of the pro-apoptotic protein Bax and decreasing the level of the anti-apoptotic protein Bcl2. In summary, our approach identified a lead compound with potential antimitotic and antiproliferative activity.

Combrestatin derivatives as microtubular assembly inhibitors at colchicine binding site

: Colchicine binding site in microtubules is among the most flourishing target for anticancer remedy. Microtubule inhibitor drugs, including combrestatin phosphate, paclitaxel and Vinca alkaloids, were formerly considered to exert their activity primarily by increasing or decreasing the cellular microtubule mass. This review describes the microtubular assembly along with the combrestatin derivatives as microtubules inhibitors, the structures of compounds known to interact with colchicines binding site, and their possible mechanism of action. Additionally, we have also discussed the utility of other heterocyclic rings and their combrestatin derivatives in treating cancer. Colchicines binding site represents a stimulating new molecular target in the design of combrestatin drugs.

New Multi-Targeted Antiproliferative Agents: Design and Synthesis of IC261-Based Oxindoles as Potential Tubulin, CK1 and EGFR Inhibitors

A series of 3-benzylideneindolin-2-one compounds was designed and synthesized based on combretastatin A-4 and compound IC261, a dual casein kinase (CK1)/tubulin polymerization inhibitor, taking into consideration the pharmacophore required for EGFR-tyrosine kinase inhibition. The new molecular entities provoked significant growth inhibition against PC-3, MCF-7 and COLO-205 at a 10 μM dose. Compounds 6-chloro-3-(2,4,6-trimethoxybenzylidene) indolin-2-one, 4b, and 5-methoxy-3-(2,4,6-trimethoxybenzylidene)indolin-2-one, 4e, showed potent activity against the colon cancer COLO-205 cell line with an IC50 value of 0.2 and 0.3 μM. A mechanistic study demonstrated 4b’s efficacy in inhibiting microtubule assembly (IC50 = 1.66 ± 0.08 μM) with potential binding to the colchicine binding site (docking study). With an IC50 of 1.92 ± 0.09 μg/mL, 4b inhibited CK1 almost as well as IC261. Additionally, 4b and 4e were effective inhibitors of EGFR-TK with IC50s of 0.19 μg/mL and 0.40 μg/mL compared to Gifitinib (IC50 = 0.05 μg/mL). Apoptosis was induced in COLO-205 cells treated with 4b, with apoptotic markers dysregulated. Caspase 3 levels were elevated to more than three-fold, while Cytochrome C levels were doubled. The cell cycle was arrested in the pre-G1 phase with extensive cellular accumulation in the pre-G1 phase, confirming apoptosis induction. Levels of cell cycle regulating proteins BAX and Bcl-2 were also defective. The binding interaction patterns of these compounds at the colchicine binding site of tubulin and the Gifitinib binding site of EGFR were verified by molecular docking, which adequately matched the reported experimental result. Hence, 4b and 4e are considered promising potent multitarget agents against colon cancer that require optimization.

The Spicy Story of Cannabimimetic Indoles

The Sterling Research Group identified pravadoline as an aminoalkylindole (AAI) non-steroidal anti-inflammatory pain reliever. As drug design progressed, the ability of AAI analogs to block prostaglandin synthesis diminished, and antinociceptive activity was found to result from action at the CB1 cannabinoid receptor, a G-protein-coupled receptor (GPCR) abundant in the brain. Several laboratories applied computational chemistry methods to ultimately conclude that AAI and cannabinoid ligands could overlap within a common binding pocket but that WIN55212-2 primarily utilized steric interactions via aromatic stacking, whereas cannabinoid ligands required some electrostatic interactions, particularly involving the CB1 helix-3 lysine. The Huffman laboratory identified strategies to establish CB2 receptor selectivity among cannabimimetic indoles to avoid their CB1-related adverse effects, thereby stimulating preclinical studies to explore their use as anti-hyperalgesic and anti-allodynic pharmacotherapies. Some AAI analogs activate novel GPCRs referred to as “Alkyl Indole” receptors, and some AAI analogs act at the colchicine-binding site on microtubules. The AAI compounds having the greatest potency to interact with the CB1 receptor have found their way into the market as “Spice” or “K2”. The sale of these alleged “herbal products” evades FDA consumer protections for proper labeling and safety as a medicine, as well as DEA scheduling as compounds having no currently accepted medical use and a high potential for abuse. The distribution to the public of potent alkyl indole synthetic cannabimimetic chemicals without regard for consumer safety contrasts with the adherence to regulatory requirements for demonstration of safety that are routinely observed by ethical pharmaceutical companies that market medicines.

Computational repurposing of benzimidazole anthelmintic drugs as potential colchicine binding site inhibitors

Background: Although some benzimidazole-based anthelmintic drugs are found to possess anticancer activity, their modes of binding interactions have not been reported. Methodology: In this study, we aimed to investigate the binding interactions and electronic configurations of nine benzimidazole-based anthelmintics against one of the well-known cancer targets (tubulin protein). Results: Binding affinities of docked benzimidazole drugs into colchicine-binding site were calculated where flubendazole > oxfendazole > nocodazole > mebendazole. Flubendazole was found to bind more efficiently with tubulin protein than other drugs. Quantum mechanics studies revealed that the electron density of HOMO of flubendazole and mebendazole together with their molecular electrostatic potential map are closely similar to that of nocodazole. Conclusion: Our study has ramifications for considering repurposing of flubendazole as a promising anticancer candidate.