Induction of autophagy-dependent and caspase- and microtubule-acetylation-independent cell death by phytochemical-stabilized gold nanopolygons in colorectal adenocarcinoma cells.

Collective functionalization of the phytochemicals of medicinal herbs on nanoparticles is emerging as a potential cancer therapeutic strategy. This study presents the facile synthesis of surface-functionalized gold nanoparticles using Bacopa monnieri (Brahmi; Bm) phytochemicals and their therapeutically relevant mechanism of action in the colorectal cancer cell line, HT29. The nanoparticles were characterized using UV-visible spectroscopy, TEM-EDAX, zeta potential analysis, TGA, FTIR and 1H NMR spectroscopy, and HR-LC-MS. The particles (Bm-GNPs) were of polygonal shape and were stable against aggregation. They entered the target cells and inhibited the viability and clonogenicity of the cells with eight times more antiproliferative efficacy (25 ± 1.5 µg mL-1) than Bm extract (Bm-EX). In vitro studies revealed that Bm-GNPs bind tubulin (a protein crucial in cell division and a target of anticancer drugs) and disrupt its helical structure without grossly altering its tertiary conformation. Like other antitubulin agents, Bm-GNPs induced G2/M arrest and ultimately killed the cells, as confirmed using flow cytometry analyses. ZVAD-FMK-mediated global pan-caspase inhibition and the apparent absence of cleaved caspase-3 in treated cells indicated that the death did not involve the classic apoptosis pathway. Cellular ultrastructure analyses, western immunoblots, and in situ immunofluorescence visualization of cellular microtubules revealed microtubule-acetylation-independent induction of autophagy as the facilitator of cell death. Together, the data indicate strong antiproliferative efficacy and a possible mechanism of action for these designer nanoparticles. Bm-GNPs, therefore, merit further investigations, including preclinical evaluations, for their therapeutic potential as inducers of non-apoptotic cell death.

Anthraquinones from the roots of Morinda scabrida Craib exhibit antiproliferative activity against A549 lung cancer cells and antitubulin polymerization.

Six anthraquinones were isolated from Morinda scabrida Craib, an unexplored species of Morinda found in the tropical forest of Thailand. All six anthraquinones showed cytotoxicity against A549 lung cancer cells, with the most active compound, nordamnacanthal (MS01), exhibiting the IC50 value of 16.3 ± 2.5 µM. The cytotoxic effect was dose-dependent and led to cell morphological changes characteristic of apoptosis. In addition, flow cytometric analysis showed dose-dependent apoptosis induction and the G2/M phase cell cycle arrest, which was in agreement with the tubulin polymerization inhibitory activity of MS01. Molecular docking analysis illustrated the binding between MS01 and the α/ß-tubulin heterodimer at the colchicine binding site, and UV-visible absorption spectroscopy revealed the DNA binding capacity of MS01.

Identification of dual binding mode of Orthodiffenes towards human topoisomerase-I and α-tubulin: exploring the potential role in anti-cancer activity via in silico study.

The drugs prescribed for targeting the tumour growth comprise of chemotherapy regimen involving combinations to cell-cycle phase specific target receptors. The combination therapy with Topoisomerase-I (Topo-I) & anti-tubulin agents are in the clinical trial stages and have scope for identifying new chemical entities with dual binding and inhibiting potential. The checkpoint proteins present at the interface of cell-cycle phases are considered the link between these two that establish the connectivity across the two phases of cell-cycle. In the present study, this potential cross-link or dual targeting is explored via in silico analysis on the natural molecules, Orthodiffene (OD) A-F which are reported from the medicinal plant, Orthosiphon diffusus. These molecules have been reported to possess significant cytotoxicity against Jurkat and HL-60 cancer cells lines in vitro. A detailed in silico analysis on OD-series molecules to evaluate their plausible anticancer mechanism & potential, as well as their in situ ADMET profile study is reported here. The DFT analysis, molecular modelling and molecular dynamics (MD) collectively establishes Topoisomerase-I & α-Tubulin proteins to be the putative target responsible for the cytotoxic activities of OD-B. Orthodiffene series molecules found to be abiding by Lipinksi's rule of 5 for orally bioavailable drug molecule. The present data & study are useful for further exploration of developing new chemical entities based on the structures of OD-series molecules as dual-target inhibitors of Topo-I & tubulin proteins with better efficacies.Communicated by Ramaswamy H. Sarma.

Bis-nor-diterpene from Cnidoscolus quercifolius (Euphorbiaceae) induces tubulin depolymerization-mediated apoptosis in BRAF-mutated melanoma cells.

A phytochemical investigation of cytotoxic extract and fractions of Cnidoscolus quercifolius Pohl led to isolation of five terpenoids, including three lupane-type triterpenes (1-3) and two bis-nor-diterpenes (4-5). Compounds 4 (phyllacanthone) and 5 (favelanone) are commonly found in this species and have unique chemical structure. Although their cytotoxic activity against cancer cells has been previously reported, the anticancer potential of these molecules remains poorly explored. In this paper, the antimelanoma potential of phyllacanthone (PHY) was described for the first time. Cell viability assay showed a promising cytotoxic activity (IC50 = 40.9 µM) against chemoresistant human melanoma cells expressing the BRAF oncogenic mutation (A2058 cell line). After 72 h of treatment, PHY inhibited cell migration and induced apoptosis and cell cycle arrest (p < 0.05). Immunofluorescence assay showed that the pro-apoptotic effect of PHY is probably associated with tubulin depolymerization, resulting in cytoskeleton disruption of melanoma cells. Molecular docking investigation confirmed this hypothesis given that satisfactory interaction between PHY and tubulin was observed, particularly at the colchicine binding site. These results suggest PHY from C. quercifolius could be potential leader for the design of new antimelanoma drugs.

In Silico Exploration of Novel Tubulin Inhibitors: A Combination of Docking and Molecular Dynamics Simulations, Pharmacophore Modeling, and Virtual Screening.

Microtubules play a critical role in mitosis and cell division and are regarded as an excellent target for anticancer therapy. Although microtubule-targeting agents have been widely used in the clinical treatment of different human cancers, their clinical application in cancer therapy is limited by both intrinsic and acquired drug resistance and adverse toxicities. In a previous work, we synthesized compound 9IV-c, ((E)-2-(3,4-dimethoxystyryl)-6,7,8-trimethoxy-N-(3,4,5-trimethoxyphenyl)quinoline-4-amine) that showed potent activity against multiple human tumor cell lines, by targeting spindle formation and/or the microtubule network. Accordingly, in this study, to identify potent tubulin inhibitors, at first, molecular docking and molecular dynamics studies of compound 9IV-c were performed into the colchicine binding site of tubulin; then, a pharmacophore model of the 9IV-c-tubulin complex was generated. The pharmacophore model was then validated by Güner-Henry (GH) scoring methods and receiver operating characteristic (ROC) analysis. The IBScreen database was searched by using this pharmacophore model as a screening query. Finally, five retrieved compounds were selected for molecular docking studies. These efforts identified two compounds (b and c) as potent tubulin inhibitors. Investigation of pharmacokinetic properties of these compounds (b and c) and compound 9IV-c displayed that ligand b has better drug characteristics compared to the other two ligands.

Two New Cytotoxic Maytansinoids Targeting Tubulin from Trewia nudiflora.

Two new maytansinoids, N-methyltreflorine (1: ) and methyltrewiasine (2: ), were isolated from the dried fruits of Trewia nudiflora, together with three known congeners (3:  - 5: ). Their structures were elucidated by spectroscopic methods, and the absolute configuration of 1: and 2: was determined by X-ray crystallographic analysis. Compounds 1:  - 5: exhibited strong cytotoxicity against human tumor cell lines, including HeLa, MV-4 - 11, and MCF-7, with IC50 values ranging from 0.12 to 11 nM. Compounds 1: and 4: also showed inhibitory activity against the MCF-7/ADR cell line with IC50 values of 13 and 28 nM, respectively. Compounds 1: and 2: significantly inhibited tubulin polymerization in vitro with IC50 values of 3.6 and 3.2 µM, respectively.

Blinking Fluorescent Probes for Tubulin Nanoscopy in Living and Fixed Cells.

Here we report a small molecule tubulin probe for single-molecule localization microscopy (SMLM), stimulated emission depletion (STED) microscopy and MINFLUX nanoscopy, which can be used in living and fixed cells. We explored a series of taxane derivatives containing spontaneously blinking far-red dye hydroxymethyl silicon-rhodamine (HMSiR) and found that the linker length profoundly affects the probe permeability and off-targeting in living cells. The best performing probe, HMSiR-tubulin, is composed of cabazitaxel and the 6'-regioisomer of HMSiR bridged by a C6 linker. Microtubule diameter of ≤50 nm was routinely measured in SMLM experiments on living and fixed cells. HMSiR-tubulin allows a complementary use of different nanoscopy techniques for investigating microtubule functions and developing imaging methods. For the first time, we resolved the inner microtubule diameter of 16 ± 5 nm by optical nanoscopy and thereby demonstrated the utility of a self-blinking dye for MINFLUX imaging.

3D-QSAR-Based Pharmacophore Modeling, Virtual Screening, and Molecular Docking Studies for Identification of Tubulin Inhibitors with Potential Anticancer Activity.

In this study, we aimed to develop a pharmacophore-based three-dimensional quantitative structure activity relationship (3D-QSAR) for a set including sixty-two cytotoxic quinolines (1-62) as anticancer agents with tubulin inhibitory activity. A total of 279 pharmacophore hypotheses were generated based on the survival score to build QSAR models. A six-point pharmacophore model (AAARRR.1061) was identified as the best model which consisted of three hydrogen bond acceptors (A) and three aromatic ring (R) features. The model showed a high correlation coefficient (R 2 = 0.865), cross-validation coefficient (Q 2 = 0.718), and F value (72.3). The best pharmacophore model was then validated by the Y-Randomization test and ROC-AUC analysis. The generated 3D contour maps were used to reveal the structure activity relationship of the compounds. The IBScreen database was screened against AAARRR.1061, and after calculating ADMET properties, 10 compounds were selected for further docking study. Molecular docking analysis showed that compound STOCK2S-23597 with the highest docking score (-10.948 kcal/mol) had hydrophobic interactions and can form four hydrogen bonds with active site residues.

Antioxidant and Antiproliferative Activity of The Ethanolic Extract of Equisetum Myriochaetum and Molecular Docking of Its Main Metabolites (Apigenin, Kaempferol, and Quercetin) on ß-Tubulin.

Equisetum myriochaetum is a semi-aquatic plant found on riverbanks that is commonly used in traditional medicine as a diuretic agent. Additionally, the genus Equisetum stands out for its content of the flavonoid kaempferol, a well-known antiproliferative agent. Therefore, in this study, E. myriochaetum ethanolic extract was tested in vitro against a cervical cancer cell line (SiHa). Additionally, the antioxidative activity was evaluated through a 2,2-diphenyl-1-picrilhidrazil (DPPH) assay. Finally, a molecular docking analysis of apigenin, kaempferol, and quercetin on the active site of ß-tubulin was performed to investigate their potential mechanism of action. All fractions of E. myriochaetum ethanolic extract showed antioxidative activity. Fraction 14 displayed an antiproliferative capacity with a half maximal inhibitory concentration (IC50) value of 6.78 µg/mL against SiHa cells.

4'-O-Methylbroussochalcone B as a novel tubulin polymerization inhibitor suppressed the proliferation and migration of acute myeloid leukaemia cells.

BACKGROUND: Recent years, survival rates of human with high-risk acute myeloid leukaemia (AML) have not raised substantially. This research aimed to investigate the role of 4'-O-Methylbroussochalcone B, for the treatment of human AML. METHODS: Firstly, we evaluated the effects of six chalcones on AML cells activity by MTT assay. Immunofluorescence staining, tubulin polymerization assay and N,N'-ethylenebis (iodoacetamide) (EBI) competition assay were performed on ML-2 cells. Transwell and apoptosis assay were also utilized in ML-2 cells and OCI-AML5 cells. The expressions of migration-related proteins, apoptosis-related proteins and Wnt/ß-catenin pathway were detected by Western Blot. RESULTS: The results found six chalcones exhibited the anti-proliferative activity against different AML cell lines. Based on the results of immunofluorescence staining, tubulin polymerization assay and EBI competition assay, 4'-O-Methylbroussochalcone B was discovered to be a novel colchicine site tubulin polymerization inhibitor. 4'-O-Methylbroussochalcone B could induce apoptosis, inhibit proliferation and migration of ML-2 cells and OCI-AML5 cells. The cells were arrested in the G2-M phase by the treatment of 4'-O-Methylbroussochalcone B. In addition, 4'-O-Methylbroussochalcone B regulated MAPK and Wnt/ß-catenin pathways in AML cells. CONCLUSION: 4'-O-Methylbroussochalcone B might inhibit proliferation and migration of the AML cells by MAPK and Wnt/ß-catenin pathways as a tubulin polymerization inhibitor. It is promising for 4'-O-Methylbroussochalcone B to become a new drug to treat AML.

A Comprehension into Target Binding and Spatial Fingerprints of Noscapinoid Analogues as Inhibitors of Tubulin.

BACKGROUND: Owing to its potential to interfere in microtubule dynamics in the mitotic phase of cell cycle and selectively induce apoptosis in cancer cells without affecting normal cells, noscapine and its synthetic analogues have been investigated by other research groups in different cell lines for their capability to be used as anti-cancer agents. OBJECTIVE: The present study is focused on the investigation of the mode of binding of noscapinoids with tubulin, prediction of target binding affinities and mapping of their spatial fingerprints (shape and electrostatic). METHODS: Molecular docking assisted alignment based 3D-QSAR was used on a dataset (43 molecules) having an inhibitory activity (IC50 = 1.2-250 µM) against human lymphoblast (CEM) cell line. RESULTS AND CONCLUSION: Key amino acid residues of target tubulin were mapped for the binding of most potent noscapine analogue (Compound 11) and were compared with noscapine. Spatial fingerprints of noscapinoids for favorable tubulin inhibitory activity were generated and are proposed herewith for further pharmacophoric amendments of noscapine analogues to design and develop novel potent noscapine based anti-cancer agents that may enter into drug development pipeline.

1,3,4-oxadiazole and its derivatives: A review on recent progress in anticancer activities.

The 1,3,4-oxadiazole nucleus is a biologically imperative scaffold possesses numerous biological activities. The broad and potent activity of 1,3,4-oxadiazole and their derivatives has established them as important pharmacological scaffolds especially in the treatment of cancer disease. Several di-, tri-, aromatic, and heterocyclic substituted 1,3,4-oxadiazole derivatives have been reported to possess potent anticancer activity. These substituted 1,3,4-oxadiazoles had shown different mechanism of action and participated in anticancer drug discovery and development. This review is complementary to earlier reviews and aims to review the work reported on anticancer activities of 1,3,4-oxadiazole derivatives from year 2000 to the beginning of 2020.

High-resolution X-ray structure of three microtubule-stabilizing agents in complex with tubulin provide a rationale for drug design.

Microtubule is a key component of cytoskeleton and has been considered as an important target for the treatment of cancer. In particular, the tubulin taxane-site inhibitors such as taxol analogs and epothilones have achieved great success in clinical trials. However, the structural basis of many taxane-site inhibitors is still lacking in exploring their mechanism of action. We here reported crystal complex structures for three taxane-site inhibitors, Ixabepilone, Epothilone B, and Epothilone D, which were determined to 2.4 Å, 2.4 Å, and 2.85 Å, respectively. The crystal structures revealed that these taxane-site inhibitors possess similar binding modes to that of Epothilone A at the taxane site, e.g. making critical hydrogen-bonding interactions with multiple residues on the M-loop, which facilitating the tubulin polymerization. Furthermore, we summarized the binding modes of almost all taxane-site inhibitors and identified novel taxane-site ligands with simpler chemical structures through virtual screening. On this basis, new derivatives with higher binding affinity to tubulin were designed and developed, which can form additional hydrogen bond interactions with tubulin. Overall, this work determined the mechanism of action of epothilones and provided a structural basis to design reasonably novel taxane-site inhibitors with simpler structure and improved pharmacokinetic properties.

Design, synthesis, biological evaluation and molecular docking studies of new chalcone derivatives containing diaryl ether moiety as potential anticancer agents and tubulin polymerization inhibitors.

A novel series of chalcone derivatives containing diaryl ether moiety (5a-5p) were designed, synthesized and evaluated their anti-tubulin polymerization activities and anticancer activities. Among them, compound 5b with 4-methoxy substitution on right aromatic ring was found to be most active on MCF-7, HepG2 and HCT116 cancer cell lines, with IC50 values of 3.44 ± 0.19, 4.64 ± 0.23, and 6.31 ± 0.27 µM, respectively. In vitro tubulin polymerization assay showed that 5b could effectively inhibit tubulin polymerization. Further mechanism studies revealed that 5b could induce G2/M phase arrest and cell apoptosis. Molecular docking studies revealed that 5b interact and bind at the colchicine binding site of the tubulin.

Targeting cellular microtubule by phytochemical apocynin exhibits autophagy-mediated apoptosis to inhibit lung carcinoma progression and tumorigenesis.

BACKGROUND: Lung cancer is the leading cause of cancer-related deaths worldwide. Several targets have been identified for lung cancer therapy, amongst which 'Microtubule' and its dynamics are the most widely studied and used in therapy. Tubulin-microtubule polymer dynamics are highly sought after targets in the field of anti-cancer drug designing. Natural compounds are important sources for developing anticancer therapeutics owing to their efficacy and lower cytotoxicity. Evidence suggested that therapeutic targeting of microtubule by natural compounds is amongst the most widely used interventions in numerous cancer therapies including lung cancer. PURPOSE: To determine the efficacy of apocynin (a natural compound) in suppressing the progression of lung carcinoma both in vitro and in vivo, along with the identification of targets and the underlying mechanism for developing a novel therapeutic approach. METHODS: We have demonstrated themicrotubule depolymerizing role of apocynin by established protocols in cellular and cell-free system. The efficacy of apocynin to inhibit lung carcinoma progression was studied on A549 cells.The tumoricidal ability of apocynin was studied in BALB/c mice model as well.Mice were classified into 4 groups namely-group II mice as tumor control; group III-IV mice asalso tumor-induced but treated with differential apocynin doses whereas group I mice were kept as normal. RESULTS: Apocynin, showed selective cytotoxicity towards lung cancer cells rather than normal lung fibroblast cells. Apocynin inhibited oncogenic properties including growth, proliferation (p < 0.05), colony formation (p < 0.05), invasion (p < 0.05) and spheroid formation (p < 0.05) in lung cancer cells. Apart from other established properties, apocynin was found to be a novel and potent component to bind with tubulin and depolymerize cellular microtubule network. Apocynin mediated cellular microtubule depolymerization was the driving mechanism to trigger autophagy-mediated apoptotic cell death (p < 0.05) which in turn retarded lung cancer progression. Furthermore, apocynin showed tumoricidal characteristics to inhibit lung tumorigenesis in mice as well. CONCLUSION: Targeting tubulin-microtubule equilibrium with apocynin could be the key regulator to catastrophe cellular catabolic processes to mitigate lung carcinoma. Thus, apocynin could be a potential therapeutic agent for lung cancer treatment.

Virtual screening identification of novel chemical inhibitors for aberrant interactions between pathogenic mutant SOD1 and tubulin.

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disease caused by selective motor neuron death. Mutations in the gene encoding copper/zinc superoxide dismutase (SOD1) belong to one of the four major mutation clusters responsible for pathogenesis of ALS. Toxic gain-of-function (not loss-of-function) of SOD1 mutants causes motor neuron degeneration. Aberrant protein-protein interactions (PPI) between mutant SOD1 and other proteins are involved in this toxic gain-of-function. Therefore, PPI inhibitors of mutant SOD1 not only increase understanding of ALS pathogenesis, but can also be used as novel therapeutics for ALS. Although it is challenging to identify PPI inhibitors, prior knowledge of the binding site can increase success probability. We have previously reported that tubulin interacts with N-terminal residues 1-23 of mutant SOD1. In the present study, we performed virtual screening by docking simulation of 32,791 compounds using this N-terminal binding site as prior knowledge. An established assay system for interaction inhibition between mutant SOD1-tubulin was used as an in-house model system to identify mutant SOD1 PPI inhibitors, with the goal of developing novel therapeutics for ALS. Consequently, five of six assay-executable compounds among top-ranked compounds during docking simulation inhibited the mutant SOD1-tubulin interaction in vitro. Binding mode analysis predicted that some inhibitors might bind the tubulin binding site of G85R SOD1 by pi electron interaction with the aromatic ring of the Trp32 residue of G85R SOD1. Our screening methods may contribute to the identification of lead compounds for treating ALS.

Structure Elucidation, Conformation, and Configuration of Cytotoxic 6-Heptyl-5,6-dihydro-2 H-pyran-2-ones from Hyptis Species and Their Molecular Docking to α-Tubulin.

Cytotoxic 6-heptyl-5,6-dihydro-2 H-pyran-2-ones are chemical markers of Hyptis (Lamiaceae) and are responsible for some of the therapeutic properties of species with relevance to traditional medicine. The present investigation describes the isolation of known pectinolides A-C (1-3), in addition to the new pectinolides I-M (4-8), from two Mexican collections of H. pectinata by HPLC. The novel biosynthetically related monticolides A (9) and B (10) were also isolated by high-speed countercurrent chromatography from H. monticola, an endemic species of the Brazilian southeastern high-altitude regions. A combination of chemical correlations, chiroptical measurements, and Mosher ester NMR analysis was used to confirm their absolute configuration. The utility of DFT-NMR chemical shifts and JH-H calculations was assessed for epimer differentiation. Molecular docking studies indicated that 6-heptyl-5,6-dihydro-2 H-pyran-2-ones have a high affinity for the pironetin-binding site of α-tubulin, which may be a possible mechanism contributing to the cytotoxic potential of these small and flexible molecules.

Evaluation of Lignans from Piper cubeba against Schistosoma mansoni Adult Worms: A Combined Experimental and Theoretical Study.

Six dibenzylbutyrolactonic lignans ((-)-hinokinin (1), (-)-cubebin (2), (-)-yatein (3), (-)-5-methoxyyatein (4), dihydrocubebin (5) and dihydroclusin (6)) were isolated from Piper cubeba seed extract and evaluated against Schistosoma mansoni. All lignans, except 5, were able to separate the adult worm pairs and reduce the egg numbers during 24 h of incubation. Lignans 1, 3 and 4 (containing a lactone ring) were the most efficient concerning antiparasitary activity. Comparing structures 3 and 4, the presence of the methoxy group at position 5 appears to be important for this activity. Considering 1 and 3, it is possible to see that the substitution pattern change (methylenedioxy or methoxy groups) in positions 3' and 4' alter the biological response, with 1 being the second most active compound. Computational calculations suggest that the activity of compound 4 can be correlated with the largest lipophilicity value.

The design, synthesis and evaluation of selenium-containing 4-anilinoquinazoline hybrids as anticancer agents and a study of their mechanism.

Inhibition of tubulin polymerization is one of the significant strategies in the treatment of cancer. Inspired by the excellent antitumor activity of EP128495 and the beneficial biological activities of selenium compounds, a series of new selenium-containing 4-anilinoquinazoline hybrids were synthesized and evaluated as tubulin polymerization inhibitors. An anti-proliferative activity assay showed that most of the compounds inhibited human sensitive cancer cells at low nanomolar concentrations. A mechanism study revealed that the optimal compound 5a disrupted microtubule dynamics, decreased the mitochondrial membrane potential and arrested HeLa cells in the G2/M phase, finally resulting in cellular apoptosis.

Indirubin, a bis-indole alkaloid binds to tubulin and exhibits antimitotic activity against HeLa cells in synergism with vinblastine.

Indirubin, a bis-indole alkaloid used in traditional Chinese medicine has shown remarkable anticancer activity against chronic myelocytic leukemia. The present work was aimed to decipher the underlying molecular mechanisms responsible for its anticancer attributes. Our findings suggest that indirubin inhibited the proliferation of HeLa cells with an IC50 of 40 µM and induced a mitotic block. At concentrations higher than its IC50, indirubin exerted a moderate depolymerizing effect on the interphase microtubular network and spindle microtubules in HeLa cells. Studies with goat brain tubulin indicated that indirubin bound to tubulin at a single site with a dissociation constant of 26 ±â€¯3 µM and inhibited the in vitro polymerization of tubulin into microtubules in the presence of glutamate as well as microtubule-associated proteins. Molecular docking analysis and molecular dynamics simulation studies indicate that indirubin stably binds to tubulin at the interface of the α-ß tubulin heterodimer. Further, indirubin stabilized the binding of colchicine on tubulin and promoted the cysteine residue modification by 5,5'-dithiobis-2-nitrobenzoic acid, indicating towards alteration of tubulin conformation upon binding. In addition, we found that indirubin synergistically enhanced the anti-mitotic and anti-proliferative activity of vinblastine, a known microtubule-targeted agent. Collectively our studies indicate that perturbation of microtubule polymerization dynamics could be one of the possible mechanisms behind the anti-cancer activities of indirubin.