Recent developments in tubulin polymerization inhibitors: An overview

Evaluation of the pharmacokinetics, tissue distribution and excretion studies of YMR-65, a tubulin polymerization inhibitor with potential anticancer activity, in rats using UPLC-MS/MS

1. YMR-65, 5-(5-bromo-1-methyl-1H-indol-3-yl)-3-(3-methoxyphenyl)-4, 5-dihydro-1H-pyrazole-1-carboxamide, is a new tubulin polymerization inhibitor with encouraging anticancer activity. 2. The validated ultra-performance liquid chromatography-tandem mass spectrometer (UPLC-MS/MS) method was successfully applied to the pharmacokinetics, tissue distribution and excretion study of YMR-65 after oral and intravenous administration. The area under concentration-time curve (AUC_0-∞) for YMR-65 were 151.67 ± 54.48 and 459.45 ± 49.23 ng/ml*h for oral and intravenous administration at the dosage of 1.5 mg/kg, respectively and the oral bioavailability was about 33.01%. Moreover, YMR-65 was extensively distributed in heart, liver, spleen, lung, kidney, stomach, intestine and testis and the highest were detected in heart, followed by stomach, intestine and liver. The majority of YMR-65 was excreted via feces and its accumulative excretion ratio during the period of 96 h was 19.83 ± 3.01%, but only 1.54 ± 0.37 and 0.215 ± 0.026% for urine within 96 h and bile within 10 h after intravenous administration, respectively, though the fecal and urine excretion were incomplete within 96 h. 3. In summary, this study defined the pharmacokinetic characteristics of YMR-65 in vivo and the important data can be a useful resource for further research and development.

Bisindole-oxadiazole hybrids, T3P® -mediated synthesis, and appraisal of their apoptotic, antimetastatic, and computational Bcl-2 binding potential

In the pursuit of novel anticancer leads, new bisindole-oxadiazoles were synthesized using propyl phosphonic anhydride as a mild and efficient reagent. The molecule, 3-[5-(1H-indol-3-ylmethyl)-1,3,4-oxadiazol-2-yl]-1H-indole (3a) exhibited selective cytotoxicity to MCF-7 cells with a cell cycle arrest in the G1 phase. The mechanism of cytotoxicity of 3a involved caspase-2-dependent apoptotic pathway with characteristic apoptotic morphological alterations as observed in acridine orange/ethidium bromide and Hoechst staining. The wound healing migratory assay exhibited an intense impairment in the motility of MCF-7 cells on incubation with 3a. Docking simulations with anti-apoptotic protein Bcl-2, which is also involved in cancer metastasis displayed good affinity and high binding energy of 3a into the well characterized BH3 binding site. The positive correlation between the Bcl-2 binding studies and the results of in vitro investigations exemplifies compound 3a as a lead molecule exhibiting MCF-7 differential cytotoxicity via apoptotic mode of cell death in addition to its anti-metastatic activity.

Design, synthesis and structure-activity relationship of 3,6-diaryl-7H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazines as novel tubulin inhibitors

A novel series of 3,6-diaryl-7H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazines were designed, synthesized and biologically evaluated as vinylogous CA-4 analogues, which involved a rigid [1,2,4]triazolo[3,4-b][1,3,4]thiadiazine scaffold to fix the configuration of (Z,E)-butadiene linker of A-ring and B-ring. Among these rigidly vinylogous CA-4 analogues, compounds 4d, 5b, 5i, 6c, 6e, 6g, 6i and 6k showed excellent antiproliferative activities against SGC-7901, A549 and HT-1080 cell lines with IC₅₀ values at the nanomolar level. Compound 6i showed the most highly active antiproliferative activity against the three human cancer cell lines with an IC₅₀ values of 0.011-0.015 µM, which are comparable to those of CA-4 (IC₅₀ = 0.009-0.013 µM). Interestingly, SAR studies revealed that 3,4-methylenedioxyphenyl, 3,4-dimethoxyphenyl, 3-methoxyphenyl and 4-methoxyphenyl could replace the classic 3,4,5-trimethoxyphenyl in CA-4 structure and keep antiproliferative activity in this series of designed compounds. Tubulin polymerization experiments showed that 6i could effectively inhibit tubulin polymerization, which was corresponded with CA-4, and immunostaining experiments suggested that 6i significantly disrupted microtubule/tubulin dynamics. Furthermore, 6i potently induced cell cycle arrest at G₂/M phase in SGC-7901 cells. Competitive binding assays and docking studies suggested that compound 6i binds to the tubulin perfectly at the colchicine binding site. Taken together, these results revealed that 6i may become a promising lead compound for new anticancer drugs discovery.

Recent advances in combretastatin based derivatives and prodrugs as antimitotic agents

The dynamic and crucial role of tubulin in different cellular functions rendered it a promising target in anticancer drug development. Combretastatin A-4 (CA-4), an inhibitor of tubulin polymerization isolated from natural sources, is a lead molecule with significant cytotoxicity against tumour cells. Owing to its non polar nature it exhibits low solubility in natural biological fluids, thereby prompting the development of new CA-4 based derivatives. The modification of this lead molecule was mostly carried out by keeping the crucial cis-orientation of the double bond intact, along with a trimethoxyphenyl aromatic ring, by employing different approaches. The issue of solubility was also addressed by the development of water soluble prodrugs of CA-4. The present review highlights the investigations into the parallel development of both new CA-4 based derivatives and prodrugs in the past few years.

Antileishmanial activity and tubulin polymerization inhibition of podophyllotoxin derivatives on Leishmania infantum

Leishmania microtubules play an important role not only in cell division, but also in keeping the shape of the parasite and motility of its free-living stages. Microtubules result from the self-assembly of alpha and beta tubulins, two phylogenetically conserved and very abundant eukaryotic proteins in kinetoplastids. The colchicine binding domain has inspired the discovery and development of several drugs currently in clinical use against parasites. However, this domain is less conserved in kinetoplastids and may be selectively targeted by new compounds. This report shows the antileishmanial effect of several series of compounds (53), derived from podophyllotoxin (a natural cyclolignan isolated from rhizomes of Podophyllum spp.) and podophyllic aldehyde, on a transgenic, fluorescence-emitting strain of Leishmania infantum. These compounds were tested on both promastigotes and amastigote-infected mouse splenocytes, and in mammalian – mouse non-infected splenocytes and liver HepG2 cells – in order to determine selective indexes of the drugs. Results obtained with podophyllotoxin derivatives showed that the hydroxyl group at position C-7α was a structural requisite to kill the parasites. On regards podophyllic aldehyde, derivatives with C9-aldehyde group integrated into a bicyclic heterostructure displayed more potent antileishmanial effects and were relatively safe for host cells. Docking studies of podophyllotoxin and podophyllic aldehyde derivatives showed that these compounds share a similar pattern of interaction at the colchicine site of Leishmania tubulin, thus pointing to a common mechanism of action. However, the results obtained suggested that despite tubulin is a remarkable target against leishmaniasis, there is a poor correlation between inhibition of tubulin polymerization and antileishmanial effect of many of the compounds tested, fact that points to alternative pathways to kill the parasites.

In Vivo and Mechanistic Studies on Antitumor Lead 7-Methoxy-4-(2-methylquinazolin-4-yl)-3,4-dihydroquinoxalin-2(1H)-one and Its Modification as a Novel Class of Tubulin-Binding Tumor-Vascular Disrupting Agents

7-Methoxy-4-(2-methylquinazolin-4-yl)-3,4-dihydroquinoxalin-2(1H)-one (2), a promising anticancer lead previously identified by us, inhibited tumor growth by 62% in mice at 1.0 mg/kg without obvious signs of toxicity. Moreover, compound 2 exhibited extremely high antiproliferative activity in the NIH-NCI 60 human tumor cell line panel, with low to sub-nanomolar GI₅₀ values (10^-10 M level). It also showed a suitable balance between aqueous solubility and lipophilicity, as well as moderate metabolic stability in vivo. Mechanistic studies using Mayer's hematoxylin and eosin and immunohistochemistry protocols on xenograft tumor tissues showed that 2 inhibited tumor cell proliferation, induced apoptosis, and disrupted tumor vasculature. Moreover, evaluation of new synthetic analogues (6a-6t) of 2 indicated that appropriate 2-substitution on the quinazoline ring could enhance antitumor activity and improve druglike properties. Compound 2 and its analogues with a 4-(2-methylquinazolin-4-yl)-3,4-dihydroquinoxalin-2(1H)-one scaffold thus represent a novel class of tubulin-binding tumor-vascular disrupting agents (tumor-VDAs) that target established blood vessels in tumors.

An Updated Review on Marine Anticancer Compounds: The Use of Virtual Screening for the Discovery of Small-Molecule Cancer Drugs

Marine secondary metabolites are a promising source of unexploited drugs that have a wide structural diversity and have shown a variety of biological activities. These compounds are produced in response to the harsh and competitive conditions that occur in the marine environment. Invertebrates are considered to be among the groups with the richest biodiversity. To date, a significant number of marine natural products (MNPs) have been established as antineoplastic drugs. This review gives an overview of MNPs, both in research or clinical stages, from diverse organisms that were reported as being active or potentially active in cancer treatment in the past seventeen years (from January 2000 until April 2017) and describes their putative mechanisms of action. The structural diversity of MNPs is also highlighted and compared with the small-molecule anticancer drugs in clinical use. In addition, this review examines the use of virtual screening for MNP-based drug discovery and reveals that classical approaches for the selection of drug candidates based on ADMET (absorption, distribution, metabolism, excretion, and toxicity) filtering may miss potential anticancer lead compounds. Finally, we introduce a novel and publically accessible chemical library of MNPs for virtual screening purposes.

Synthesis and biological evaluation of novel indole-pyrimidine hybrids bearing morpholine and thiomorpholine moieties

Based on our previous screening hit compound 1, a series of novel indole-pyrimidine hybrids possessing morpholine or thiomorpholine moiety were synthesized via an efficient one-pot multistep synthetic method. The antiproliferative activities of the synthesized compounds were evaluated in vitro against four cancer cell lines including HeLa, MDA-MB-231, MCF-7, and HCT116. The results revealed that most compounds possessed moderate to excellent potency. The IC₅₀ values of the most promising compound 15 are 0.29, 4.04, and 9.48 μM against MCF-7, HeLa, and HCT116 cell lines, respectively, which are 48.0, 4.9, and 1.8 folds more active than the lead compound 1. Moreover, fluorescence-activated cell sorting analysis revealed that compound 14 showing the highest activity against HeLa (IC₅₀ = 2.51 μM) displayed a significant effect on G₂/M cell-cycle arrest in a concentration-dependent manner in HeLa cell line. In addition, representative nine active hybrids were evaluated for tubulin polymerization inhibitory activities, and compound 15 exhibited the most potent anti-tubulin activity showing 42% inhibition at 10 μM. These preliminary results encourage a further investigation on indole-pyrimidine hybrids for the development of potent anticancer agents that inhibit tubulin polymerization.

Anti-mitotic agents: Are they emerging molecules for cancer treatment?

Mutations in cancer cells frequently result in cell cycle alterations that lead to unrestricted growth compared to normal cells. Considering this phenomenon, many drugs have been developed to inhibit different cell-cycle phases. Mitotic phase targeting disturbs mitosis in tumor cells, triggers the spindle assembly checkpoint and frequently results in cell death. The first anti-mitotics to enter clinical trials aimed to target tubulin. Although these drugs improved the treatment of certain cancers, and many anti-microtubule compounds are already approved for clinical use, severe adverse events such as neuropathies were observed. Since then, efforts have been focused on the development of drugs that also target kinases, motor proteins and multi-protein complexes involved in mitosis. In this review, we summarize the major proteins involved in the mitotic phase that can also be targeted for cancer treatment. Finally, we address the activity of anti-mitotic drugs tested in clinical trials in recent years.

N-Heterocyclic (4-Phenylpiperazin-1-yl)methanones Derived from Phenoxazine and Phenothiazine as Highly Potent Inhibitors of Tubulin Polymerization

We report here a series of 27 10-(4-phenylpiperazin-1-yl)methanones derived from tricyclic heterocycles which were screened for effects on tumor cell growth, inhibition of tubulin polymerization, and induction of cell cycle arrest. Several analogues, among them the 10-(4-(3-methoxyphenyl)piperazine-1-carbonyl)-10H-phenoxazine-3-carbonitrile (16o), showed excellent antiproliferative properties, with low nanomolar GI₅₀ values (16o, mean GI₅₀ of 3.3 nM) against a large number (93) of cancer cell lines. Fifteen compounds potently inhibited tubulin polymerization. Analysis of cell cycle by flow cytometry revealed that inhibition of tumor cell growth was related to an induction of G2/M phase cell cycle blockade. Western blotting and molecular docking studies suggested that these compounds bind efficiently to β-tubulin at the colchicine binding site. Our studies demonstrate the suitability of the phenoxazine and phenothiazine core and also of the phenylpiperazine moiety for the development of novel and potent tubulin polymerization inhibitors.

Mechanism of antiproliferative action of a new d-secoestrone-triazole derivative in cervical cancer cells and its effect on cancer cell motility

Cervical cancer is the fourth most frequently diagnosed tumor and the fourth leading cause of cancer death in females worldwide. Cervical cancer is predominantly related with human papilloma virus (HPV) infection, with the most oncogenic types being HPV-18 and -16. Our previous studies demonstrated that some d-secoestrone derivatives exert pronounced antiproliferative activity. The aim of the current investigation was to characterize the mechanism of action of d-secoestrone-triazole (D-SET) on three cervical cancer cell lines with different pathological backgrounds. The growth-inhibitory effects of D-SET were determined by a standard MTT assay. We have found that D-SET exerts a pronounced growth-inhibitory effect on HPV 18-positive HeLa and HPV-negative C-33 A cells, but it has no substantial inhibitory activity on HPV 16-positive SiHa or on intact fibroblast MRC-5 cell lines. After 24h incubation, cells showed the morphological and biochemical signs of apoptosis determined by fluorescent double staining, flow cytometry and caspase-3 activity assay. Besides the elevation of the ratio of cells in the subG1 phase, flow cytometric analysis revealed a cell cycle arrest at G2/M in both HeLa and C-33 A cell lines. To distinguish the G2/M cell population immunocytochemical flow cytometric analysis was performed on HeLa cells. The results show that D-SET significantly increases the ratio of phosphorylated histone H3, indicating cell accumulation in the M phase. Additionally, D-SET significantly increased the maximum rate of microtube formation measured by an in vitro tubulin polymerization assay. Besides its direct antiproliferative activity, the antimigratory property of D-SET has been investigated. Our results demonstrate that D-SET significantly inhibits the migration and invasion of HeLa cells after 24h incubation. These results suggests that D-SET is a potent antiproliferative agent against HPV 16+ and HPV-negative cervical cancer cell lines, with an efficacious motility-inhibiting activity against HPV 16+ cells. Accordingly D-SET can be regarded as a potential drug candidate with a promising new mechanism of action among the antiproliferative steroids, potentially allowing for the design of novel anticancer agents.

Synthesis, in vitro and in vivo evaluation of new hybrids of millepachine and phenstatin as potent tubulin polymerization inhibitors

In this paper, a series of millepachine derivatives were synthesized and evaluated as tubulin polymerization inhibitors.

Synthesis and evaluation of selenium-containing indole chalcone and diarylketone derivatives as tubulin polymerization inhibition agents

Sixteen new selenium-containing indole chalcone and diarylketone derivatives were synthesized and evaluated as tubulin polymerization inhibitors. Compound 25b exhibited the most potent antiproliferative activities and effectively inhibited tubulin polymerization (IC₅₀ = 2.1 ± 0.27 μM).

Synthesis and Bioevaluation of 3,6-Diaryl-[1,2,4]triazolo[4,3-b] Pyridazines as Antitubulin Agents

A series of 3,6-diaryl-[1,2,4]triazolo[4,3-b]pyridazines were designed as a class of vinylogous CA-4 analogues. The easily isomerized (Z,E)-butadiene linker of vinylogous CA-4 was replaced by a rigid [1,2,4]triazolo[4,3-b]pyridazine scaffold. Twenty-one target compounds were synthesized and exhibited moderate to potent antiproliferative activity. The compound 4q with a 3-amino-4-methoxyphenyl moiety as the B-ring, comparable to CA-4 (IC₅₀ = 0.009-0.012 μM), displayed the highly active antiproliferative activity against SGC-7901, A549, and HT-1080 cell lines with IC₅₀ values of 0.014, 0.008, and 0.012 μM, respectively. Tubulin polymerization experiments indicated that 4q effectively inhibited tubulin polymerization, and immunostaining assay revealed that 4q significantly disrupted tubulin microtubule dynamics. Moreover, cell cycle studies revealed that compound 4q dramatically arrested cell cycle progression at G2/M phase in A549 cells. Molecular modeling studies showed that 4q could bind to the colchicine binding site on microtubules.

Deacetylisovaltratum disrupts microtubule dynamics and causes G2/M-phase arrest in human gastric cancer cells in vitro

Aim:

Deacetylisovaltratum (DI) is isolated from the traditional Chinese herbal medicine Patrinia heterophylla Bunge, which exhibits anti-cancer activity. Here, we investigated the effects of DI on human gastric carcinoma cell lines in vitro and elucidated its anti-cancer mechanisms.

Methods:

Human gastric carcinoma AGS and HGC-27 cell lines were treated with DI, and cell viability was detected with MTT assay. Cell cycle stages, apoptosis and mitochondrial membrane potential were measured using flow cytometry. Protein levels were analyzed by Western blotting. Tubulin polymerization assays and immunofluorescence were used to characterize the tubulin polymerization process.

Results:

DI inhibited the cell viability of AGS and HGC-27 cells in a dose- and time-dependent manner with IC₅₀ values of 12.0 and 28.8 μmol/L, respectively, at 24 h of treatment. Treatment with DI (10–100 μmol/L) dose-dependently promoted tubulin polymerization, and induced significant G₂/M cell cycle arrest in AGS and HGC-27 cells. Moreover, DI treatment disrupted mitochondrial membrane potential and induced caspase-dependent apoptosis in AGS and HGC-27 cells.

Conclusion:

DI induces G₂/M-phase arrest by disrupting tubulin polymerization in human gastric cancer cells, which highlights its potent anti-cancer activity and potential application in gastric cancer therapy.

Interactions of long-chain homologues of colchicine with tubulin

Several colchicine analogues in which the N-acetyl residue has been replaced by aliphatic, straight-chain acyl moieties, have been synthesized. These compounds show high cytotoxic activity at the nanomolar level against the tumoral cell lines HT-29, MCF-7 and A549. Some of them exhibit activities in the picomolar range against the HT-29 line and are thus two to three orders of magnitude more cytotoxic than colchicine. In this specific cell line, the activities were found to be closely related to the length of the acyl carbon chain, an increase in the latter giving rise to an increase in the cytotoxicity with a maximum in the range of 10-12 carbon atoms, followed by a decrease in activity with still longer chains. Some of the compounds inhibit microtubule assembly and induce the formation of abnormal polymers and present in most cases better apparent affinity constants than colchicine. In addition, at IC₅₀ concentrations the analogues block the cell cycle of A549 cells in the G2/M phase. Molecular docking studies suggest that, while interactions of the colchicine analogues with the colchicine binding site at β-tubulin are still present, the increase in the acyl chain length leads to the progressive development of new interactions, not present in colchicine itself, with the neighboring α-tubulin subunit. Indeed, sufficiently long acyl chains span the intradimer interface and contact with a hydrophobic groove in α-tubulin. It is worth noting that some of the compounds show cytotoxicity at concentrations three orders of magnitude lower than colchicine. Their pharmacological use in cancer therapy could possibly be performed with lower dosages and be thus endowed with less acute toxicity problems than in the case of colchicine.

Synthesis, anti-cancer evaluation of benzenesulfonamide derivatives as potent tubulin-targeting agents

A series of benzenesulfonamide derivatives were synthesized and evaluated for their anti-proliferative activity and interaction with tubulin. These new derivatives showed significant activities against cellular proliferative and tubulin polymerization. Compound BA-3b proved to be the most potent compound with IC50 value ranging from 0.007 to 0.036 μM against seven cancer cell lines, and three drug-resistant cancer cell lines, which indicated a promising anti-cancer agent. The target tubulin was also verified by dynamic tubulin polymerization assay and tubulin intensity assay.

Structural and Biochemical Characterization of the Interaction of Tubulin with Potent Natural Analogues of Podophyllotoxin

Four natural analogues of podophyllotoxin obtained from the Mexican medicinal plant Bursera fagaroides, namely, acetyl podophyllotoxin (2), 5'-desmethoxy-β-peltatin A methyl ether (3), 7',8'-dehydro acetyl podophyllotoxin (4), and burseranin (5), have been characterized, and their interactions with tubulin have been investigated. Cytotoxic activity measurements, followed by immunofluorescence microscopy and flow cytometry studies, demonstrated that these compounds disrupt microtubule networks in cells and cause cell cycle arrest in the G2/M phase in the A549 cell line. A tubulin binding assay showed that compounds 1-4 were potent assembly inhibitors, displaying binding to the colchicine site with Kb values ranging from 11.75 to 185.0 × 10(5) M(-1). In contrast, burseranin (5) was not able to inhibit tubulin assembly. From the structural perspective, the ligand-binding epitopes of compounds 1-3 have been mapped using STD-NMR, showing that B and E rings are the major points for interaction with the protein. The obtained results indicate that the inhibition of tubulin assembly of this family of compounds is more effective when there are at least two methoxyl groups at the E ring, along with a trans configuration of the lactone ring in the aryltetralin lignan core.

Reactive Oxygen Species Mediates the Synergistic Activity of Fenretinide Combined with the Microtubule Inhibitor ABT-751 against Multidrug-Resistant Recurrent Neuroblastoma Xenografts

ABT-751 is a colchicine-binding site microtubule inhibitor. Fenretinide (4-HPR) is a synthetic retinoid. Both agents have shown activity against neuroblastoma in laboratory models and clinical trials. We investigated the antitumor activity of 4-HPR + the microtubule-targeting agents ABT-751, vincristine, paclitaxel, vinorelbine, or colchicine in laboratory models of recurrent neuroblastoma. Drug cytotoxicity was assessed in vitro by a fluorescence-based assay (DIMSCAN) and in subcutaneous xenografts in nu/nu mice. Reactive oxygen species levels (ROS), apoptosis, and mitochondrial depolarization were measured by flow cytometry; cytochrome c release and proapoptotic proteins were measured by immunoblotting. 4-HPR + ABT-751 showed modest additive or synergistic cytotoxicity, mitochondrial membrane depolarization, cytochrome c release, and caspase activation compared with single agents in vitro; synergism was inhibited by antioxidants (ascorbic acid, α-tocopherol). 4-HPR + ABT-751 was highly active against four xenograft models, achieving multiple maintained complete responses. The median event-free survival (days) for xenografts from 4 patients combined was control = 28, 4-HPR = 49, ABT-751 = 77, and 4-HPR + ABT-751 > 150 (P < 0.001). Apoptosis (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, TUNEL) was significantly higher in 4-HPR + ABT-751-treated tumors than with single agents (P < 0.01) and was inhibited by ascorbic acid and α-tocopherol (P < 0.01), indicating that ROS from 4-HPR enhanced the activity of ABT-751. 4-HPR also enhanced the activity against neuroblastoma xenografts of vincristine or paclitaxel, but the latter combinations were less active than 4-HPR + ABT-751. Our data support clinical evaluation of 4-HPR combined with ABT-751 in recurrent and refractory neuroblastoma. Mol Cancer Ther; 15(11); 2653-64. ©2016 AACR.

The Interference of Selected Cytotoxic Alkaloids with the Cytoskeleton: An Insight into Their Modes of Action

Alkaloids, the largest group among the nitrogen-containing secondary metabolites of plants, usually interact with several molecular targets. In this study, we provide evidence that six cytotoxic alkaloids (sanguinarine, chelerythrine, chelidonine, noscapine, protopine, homoharringtonine), which are known to affect neuroreceptors, protein biosynthesis and nucleic acids, also interact with the cellular cytoskeleton, such as microtubules and actin filaments, as well. Sanguinarine, chelerythrine and chelidonine depolymerized the microtubule network in living cancer cells (Hela cells and human osteosarcoma U2OS cells) and inhibited tubulin polymerization in vitro with IC50 values of 48.41 ± 3.73, 206.39 ± 4.20 and 34.51 ± 9.47 μM, respectively. However, sanguinarine and chelerythrine did not arrest the cell cycle while 2.5 μM chelidonine arrested the cell cycle in the G₂/M phase with 88.27% ± 0.99% of the cells in this phase. Noscapine and protopine apparently affected microtubule structures in living cells without affecting tubulin polymerization in vitro, which led to cell cycle arrest in the G2/M phase, promoting this cell population to 73.42% ± 8.31% and 54.35% ± 11.26% at a concentration of 80 μM and 250.9 μM, respectively. Homoharringtonine did not show any effects on microtubules and cell cycle, while the known microtubule-stabilizing agent paclitaxel was found to inhibit tubulin polymerization in the presence of MAPs in vitro with an IC50 value of 38.19 ± 3.33 μM. Concerning actin filaments, sanguinarine, chelerythrine and chelidonine exhibited a certain effect on the cellular actin filament network by reducing the mass of actin filaments. The interactions of these cytotoxic alkaloids with microtubules and actin filaments present new insights into their molecular modes of action.

Recent Advances in chemistry and pharmacology of 2-methoxyestradiol: An anticancer investigational drug

2-Methoxyestradiol (2ME2), an estrogen hormone metabolite is a potential cancer chemotherapeutic agent. Presently, it is an investigational drug under various phases of clinical trials alone or in combination therapy. Its anticancer activity has been attributed to its antitubulin, antiangiogenic, pro-apoptotic and ROS induction properties. This anticancer drug candidate has been explored extensively in last twenty years for its detailed chemistry and pharmacology. Present review is an update of its chemistry and biological activity. It also extends an assessment of potential of 2ME2 and its analogues as possible anticancer drug in future.

New imidazoquinoxaline derivatives: Synthesis, biological evaluation on melanoma, effect on tubulin polymerization and structure-activity relationships

Microtubules are considered as important targets of anticancer therapy. EAPB0503 and its structural imidazo[1,2-a]quinoxaline derivatives are major microtubule-interfering agents with potent anticancer activity. In this study, the synthesis of several new derivatives of EAPB0503 is described, and the anticancer efficacy of 13 novel derivatives on A375 human melanoma cell line is reported. All new compounds show significant antiproliferative activity with IC50 in the range of 0.077-122μM against human melanoma cell line (A375). Direct inhibition of tubulin polymerization assay in vitro is also assessed. Results show that compounds 6b, 6e, 6g, and EAPB0503 highly inhibit tubulin polymerization with percentages of inhibition of 99%, 98%, 90%, and 84% respectively. Structure-activity relationship studies within the series are also discussed in line with molecular docking studies into the colchicine-binding site of tubulin.

Synthesis, Evaluation, and Mechanism Study of Novel Indole-Chalcone Derivatives Exerting Effective Antitumor Activity Through Microtubule Destabilization in Vitro and in Vivo

Twenty-nine novel indole-chalcone derivatives were synthesized and evaluated for antiproliferative activity. Among them, 14k exhibited most potent activity, with IC50 values of 3-9 nM against six cancer cells, which displayed a 3.8-8.7-fold increase in activity when compare with compound 2. Further investigation revealed 14k was a novel tubulin polymerization inhibitor binding to the colchicine site. Its low cytotoxicity toward normal human cells and nearly equally potent activity against drug-resistant cells revealed the possibility for cancer therapy. Cellular mechanism studies elucidated 14k arrests cell cycle at G2/M phase and induces apoptosis along with the decrease of mitochondrial membrane potential. Furthermore, good metabolic stability of 14k was observed in mouse liver microsomes. Importantly, 14k and its phosphate salt 14k-P inhibited tumor growth in xenograft models in vivo without apparent toxicity, which was better than the reference compound CA-4P and 2. In summary, 14k deserves consideration for cancer therapy.

Combretastatin A-4 analogues with benzoxazolone scaffold: Synthesis, structure and biological activity

In order to design and synthesize a new class of heterocyclic analogues of natural combretastatin A-4 and its synthetic derivative AVE8062, the benzoxazolone ring was selected as a scaffold for a bioisosteric replacement of the ring B of both molecules. A library of 28 cis- and trans-styrylbenzoxazolones was obtained by a modified Wittig reaction under Boden's conditions. Structures of the newly synthesized compounds bearing the 3,4,5-trimethoxy-, 3,4-dimethoxy-, 3,5-dimethoxy-, and 4-methoxystyryl fragment at position 4, 5, 6 or 7 of benzoxazolone core were determined on the basis of spectral and X ray data. The in vitro cytotoxicity of styrylbenzoxazolones against different cell lines was examined. Stilbene derivative 16Z, (Z)-3-methyl-6-(3,4,5-trimethoxystyryl)-2(3H)-benzoxazolone, showed highest antiproliferative potential of the series, with IC50 of 0.25 μM against combretastatin resistant cell line HT-29, 0.19 μM against HepG2, 0.28 μM against EA.hy926 and 0.73 μM against K562 cells. Furthermore, the results of flow cytometric analysis confirmed that 16Z induced cell cycle arrest in G2/M phase in the cell lines like combretastatin A-4. This arrest is followed by an abnormal exit of cells from mitosis without cytokinesis into a pseudo G1-like multinucleate state leading to late apoptosis and cell death. Accordingly, synthetic analogue 16Z was identified as the most promising potential anticancer agent in present study, and was selected as lead compound for further detailed investigations.

C1, a highly potent novel curcumin derivative, binds to tubulin, disrupts microtubule network and induces apoptosis

C1 is one of the most potent curcumin analogues identified till date which inhibits proliferation of various cancer cell lines. C1 binds to tubulin and depolymerized microtubules of MCF-7 cells. C1 altered the expression of apoptotic proteins and induces p53-dependent apoptosis.

We have synthesized a curcumin derivative, 4-{5-(4-hydroxy-3-methoxy-phenyl)-2-[3-(4-hydroxy-3-methoxy-phenyl)-acryloyl]-3-oxo-penta-1,4-dienyl}-piperidine-1-carboxylic acid tert-butyl ester (C1) that displays much stronger antiproliferative activity against various types of cancer cells including multidrug resistance cells than curcumin. C1 depolymerized both interphase and mitotic microtubules in MCF-7 cells and also inhibited the reassembly of microtubules in these cells. C1 inhibited the polymerization of purified tubulin, disrupted the lattice structure of microtubules and suppressed their GTPase activity in vitro. The compound bound to tubulin with a dissociation constant of 2.8±1 μM and perturbed the secondary structures of tubulin. Further, C1 treatment reduced the expression of Bcl2, increased the expression of Bax and down regulated the level of a key regulator of p53, murine double minute 2 (Mdm2) (S166), in MCF-7 cells. C1 appeared to induce p53 mediated apoptosis in MCF-7 cells. Interestingly, C1 showed more stability in aqueous buffer than curcumin. The results together showed that C1 perturbed microtubule network and inhibited cancer cells proliferation more efficiently than curcumin. The strong antiproliferative activity and improved stability of C1 indicated that the compound may have a potential as an anticancer agent.

Dual targeting of microtubule and topoisomerase II by α-carboline derivative YCH337 for tumor proliferation and growth inhibition

Both microtubule and topoisomerase II (Top2) are important anticancer targets and their respective inhibitors are widely used in combination for cancer therapy. However, some combinations could be mutually antagonistic and drug resistance further limits their therapeutic efficacy. Here we report YCH337, a novel α-carboline derivative that targets both microtubule and Top2, eliciting tumor proliferation and growth inhibition and overcoming drug resistance. YCH337 inhibited microtubule polymerization by binding to the colchicine site and subsequently led to mitotic arrest. It also suppressed Top2 and caused DNA double-strand breaks. It disrupted microtubule more potently than Top2. YCH337 induced reversible mitotic arrest at low concentrations but persistent DNA damage. YCH337 caused intrinsic and extrinsic apoptosis and decreased MCL-1, cIAP1 and XIAP proteins. In this aspect, YCH337 behaved differently from the combination of vincristine and etoposide. YCH337 inhibited proliferation of tumor cells with an averaged IC₅₀ of 0.3 μM. It significantly suppressed the growth of HT-29 xenografts in nude mice too. Importantly, YCH337 nearly equally killed different-mechanism-mediated resistant tumor cells and corresponding parent cells. Together with the novelty of its chemical structure, YCH337 could serve as a promising lead for drug development and a prototype for a dual microtubule/Top2 targeting strategy for cancer therapy.

The synthesis and evaluation of new benzophenone derivatives as tubulin polymerization inhibitors

Inspired by the potent inhibition activity of phenstatin and millepachine against cancer cell growth, a series of new benzophenone derivatives were synthesized and evaluated as tubulin polymerization inhibitors.

Identification of gallic acid based glycoconjugates as a novel tubulin polymerization inhibitors

A novel class of gallic acid based glycoconjugates were designed and synthesized as potential anticancer agents. Among all the compounds screened, compound 2a showed potent anticancer activity against breast cancer cells. The latter resulted in tubulin polymerization inhibition and induced G2/M cell cycle arrest, generation of reactive oxygen species, mitochondrial depolarization and subsequent apoptosis in breast cancer cells. In addition, ultraviolet-visible spectroscopy and fluorescence quenching studies of the compound with tubulin confirmed direct interaction of compounds with tubulin. Molecular modeling studies revealed that it binds at the colchicine binding site in tubulin. Further, 2a also exhibited potent in vivo anticancer activity in LA-7 syngeneic rat mammary tumor model. Current data projects its strong candidature to be developed as anticancer agent.

Combretastatins: more than just vascular targeting agents?

Several prodrugs of the naturally occurring combretastatins have undergone extensive clinical evaluation as vascular targeting agents (VTAs). Their increased selectivity toward endothelial cells together with their innate ability to rapidly induce vascular shutdown and inhibit tumor growth at doses up to 10-fold less than the maximum tolerated dose led to the clinical evaluation of combretastatins as VTAs. Tubulin is well established as the molecular target of the combretastatins and the vast majority of its synthetic derivatives. Furthermore, tubulin is a highly validated molecular target of many direct anticancer agents routinely used as front-line chemotherapeutics. The unique vascular targeting properties of the combretastatins have somewhat overshadowed their development as direct anticancer agents and the delineation of the various cell death pathways and anticancer properties associated with such chemotherapeutics. Moreover, the ongoing clinical trial of OXi4503 (combretastatin-A1 diphosphate) together with preliminary preclinical evaluation for the treatment of refractory acute myelogenous leukemia has successfully highlighted both the indirect and direct anticancer properties of combretastatins. In this review, we discuss the development of the combretastatins from nature to the clinic. The various mechanisms underlying combretastatin-induced cell cycle arrest, mitotic catastrophe, cell death, and survival are also reviewed in an attempt to further enhance the clinical prospects of this unique class of VTAs.

Triazole linked mono carbonyl curcumin-isatin bifunctional hybrids as novel anti tubulin agents: Design, synthesis, biological evaluation and molecular modeling studies

Keeping in view the limitations associated with currently available anticancer drugs, molecular hybrids of mono carbonyl curcumin and isatin tethered by triazole ring have been synthesized and evaluated for in vitro cytotoxicity against THP-1, COLO-205, HCT-116, A549, HeLa, CAKI-I, PC-3, MiaPaca-2 human cancer cell lines. The results revealed that the compounds SA-1 to SA-9, SB-2, SB-3, SB-4, SB-7 and SC-2 showed a good range of IC50 values against THP-1, COLO-205, HCT-116 and PC-3 cell lines, while the other four cell lines among these were found to be almost resistant. Structure activity relationship revealed that the nature of Ring X and substitution at position R influences the activity. Methoxy substituted phenyl ring as Ring X and H as R were found to be the ideal structural features. The most potent compounds (SA-2, SA-3, SA-4, SA-7) were further tested for tubulin inhibition. Compound SA-2 was found to significantly inhibit the tubulin polymerization (IC50=1.2 μM against HCT-116). Compound SA-2, moreover, lead to the disruption of microtubules as confirmed by immunofluorescence technique. The significant cytotoxicity and tubulin inhibition by SA-2 was streamlined by molecular modeling studies where it was docked at the curcumin binding site of tubulin.

Stabilizing versus destabilizing the microtubules: a double-edge sword for an effective cancer treatment option?

Microtubules are dynamic and structural cellular components involved in several cell functions, including cell shape, motility, and intracellular trafficking. In proliferating cells, they are essential components in the division process through the formation of the mitotic spindle. As a result of these functions, tubulin and microtubules are targets for anticancer agents. Microtubule-targeting agents can be divided into two groups: microtubule-stabilizing, and microtubule-destabilizing agents. The former bind to the tubulin polymer and stabilize microtubules, while the latter bind to the tubulin dimers and destabilize microtubules. Alteration of tubulin-microtubule equilibrium determines the disruption of the mitotic spindle, halting the cell cycle at the metaphase-anaphase transition and, eventually, resulting in cell death. Clinical application of earlier microtubule inhibitors, however, unfortunately showed several limits, such as neurological and bone marrow toxicity and the emergence of drug-resistant tumor cells. Here we review several natural and synthetic microtubule-targeting agents, which showed antitumor activity and increased efficacy in comparison to traditional drugs in various preclinical and clinical studies. Cryptophycins, combretastatins, ombrabulin, soblidotin, D-24851, epothilones and discodermolide were used in clinical trials. Some of them showed antiangiogenic and antivascular activity and others showed the ability to overcome multidrug resistance, supporting their possible use in chemotherapy.