Tumor vasculature is an important target in cancer treatment. Two distinct vasculartargeting therapeutic strategies are applied to attack cancer cells indirectly. The antiangiogenic approach intervenes in the neovascularization processes and blocks the formation of new blood vessels, while th e antivascular approach targets the established tumor blood vessels, making vascular shutdown and resulting in rapid haemorrhagic necrosis and tumor cell death. A number of compounds with diverse structural scaffolds have been designed to target tumor vasculature and they are called vascular disrupting agents (VDAs). The biological or ligand-directed VDAs utilize antibodies, peptides or growth factors to deliver toxins or pro-coagulants or proapoptotic affectors to tumor-related blood vessels, while the small-molecule VDAs selectively target tumor blood vessels and have little effects on the normal endothelium. Among the small-molecule VDAs, the tubulin colchicine binding site inhibitors have been extensively studied and many of them have entered the clinical trials, including CA-4P, CA-1P, AVE8062, OXi4503, CKD-516, BNC105P, ABT-751, CYT- 997, ZD6126, NPI-2358, MN-029 and EPC2407. This review makes a summary of the small-molecule VDAs in clinical developments and highlights some potential VDA leads or candidates for the treatment of tumors.

By targeting a new binding region at the interface between αβ-tubulin heterodimers at the colchicine binding site, we designed a series of 7-substituted 1-methyl-1,4-dihydroindeno[1,2-c]pyrazoles as potential tubulin polymerization inhibitors. Among the compounds synthesized, 2-(6-ethoxy-3-(3-ethoxyphenylamino)-1-methyl-1,4-dihydroindeno[1,2-c]pyrazol-7-yloxy)acetamide 6a and 2-(6-ethoxy-3-(3-ethoxyphenylamino)-1-methyl-1,4-dihydroindeno[1,2-c]pyrazol-7-yloxy)-N-hydroxyacetamide 6n showed noteworthy low nanomolar potency against HepG2, Hela, PC3, and MCF-7 cancer cell lines. In mechanism studies, 6a inhibited tubulin polymerization and disorganized microtubule in A549 cells by binding to tubulin colchicine binding site. 6a arrested A549 cells in G2/M phase that was related to the alterations in the expression of cyclin B1 and p-cdc2. 6a induced A549 cells apoptosis through the activation of caspase-3 and PARP. In addition, 6a inhibited capillary tube formation in a concentration-dependent manner. In nonsmall cell lung cancer xenografts mouse model, 6a suppressed tumor growth by 59.1% at a dose of 50 mg/kg (ip) without obvious toxicity, indicating its in vivo potential as anticancer agent.

Taking the previously discovered 1-methyl-1,4-dihydroindeno­[1,2c]­pyrazol derivative LL01 as a lead, systematic structural modifications were made at the phenolic 6- and 7-positions and the aniline at the 3-position of the indenopyrazole core to investigate the SARs and to improve water solubility. Among the designed indenopyrazoles ID01–ID33, a series of potent MTAs were identified. As the hydrochloride salt(s), ID09 and ID33 showed excellent aqueous solubility and favorable Log P value and displayed noteworthily low nanomolar potency against a variety of tumor cells, including those taxol-resistant ones. They inhibited tubulin polymerization, disrupted cellular microtubule networks by targeting the colchicine site, and promoted HepG2 cell cycle arrest and cell apoptosis. In the HepG2 xenograft mouse model, ID09 and ID33 effectively inhibited tumor growth at an oral dose of 25 mg/kg. At an intravenous (iv) injection dose of 10 mg/kg every other day, ID09 suppressed tumor growth by 68% without obvious toxicity.