Phase II Study of the Liposomal Formulation of Eribulin (E7389-LF) in Combination with Nivolumab: Results from the Gastric Cancer Cohort

PURPOSE

E7389-LF is a liposomal formulation of the microtubule dynamics inhibitor eribulin and has shown preliminary efficacy in the treatment of gastric cancer. Study 120, a phase Ib/II open-label study, assessed efficacy and safety of E7389-LF in combination with nivolumab, a programmed cell death (PD)-1 inhibitor. This report focuses on the gastric cancer cohort within the expansion phase.

PATIENTS AND METHODS

Eligible patients had unresectable, measurable gastric cancer, progression following a platinum drug plus fluoropyrimidine (1L), and a taxane-containing regimen (2L). The primary objective of the expansion phase was objective response rate, secondary objectives included safety and PFS, and exploratory objectives included overall survival and biomarker evaluation. Patients received E7389-LF 2.1 mg/m2 in combination with nivolumab 360 mg every 3 weeks, both as intravenous infusions. Tumor responses were assessed every 6 weeks by the investigators per RECIST v1.1. Plasma and tumor biomarkers were assessed.

RESULTS

In the 31 patients who received E7389-LF in combination with nivolumab, the objective response rate was 25.8% [confidence interval (CI), 11.9-44.6]. The median progression-free survival was 2.69 months (95% CI, 1.91-2.99) and median overall survival was 7.85 months (95% CI, 4.47-not estimable). The most common treatment-related TEAE of any grade were neutropenia (77.4%), leukopenia (74.2%), and decreased appetite (51.6%). E7389-LF in combination with nivolumab significantly increased CD8-positive cells at C2D1 (P = 0.039), and six of seven vascular markers and four IFNγ-related markers showed increases from C1D1.

CONCLUSIONS

Promising antitumor activity was observed with E7389-LF in combination with nivolumab in patients with gastric cancer, and no new safety signals were observed, compared with either monotherapy.

Design, synthesis and biological evaluation of 1,2,3-triazole benzothiazole derivatives as tubulin polymerization inhibitors with potent anti-esophageal cancer activities

In this work, we utilized the molecular hybridization strategy to design and synthesize novel 1,2,3-triazole benzothiazole derivatives K1-26. The antiproliferative activities against MGC-803, Kyse30 and HCT-116 cells were explored, and their structure-activity relationship were preliminarily conducted and summarized. Among them, compound K18, exhibited the strongest proliferation inhibitory activity, with esophageal cancer cells Kyse30 and EC-109 being the most sensitive to its effects (IC50 values were 0.042 and 0.038 μM, respectively). Compound K18 effectively inhibited tubulin polymerization (IC50 = 0.446 μM), thereby hindering tubulin polymerize into filamentous microtubules in Kyse30 and EC-109 cells. Additionally, compound K18 induced the degradation of oncogenic protein YAP via the UPS pathway. Based on these dual molecular-level effects, compound K18 could induce G2/M phase arrest and cell apoptosis in Kyse30 and EC-109 cells, as well as regulate the expression levels of cell cycle and apoptosis-related proteins. In summary, our findings highlight a novel 1,2,3-triazole benzothiazole derivative K18, which possesses significant potential for treating esophageal cancers.

Discovery of novel arylamide derivatives containing piperazine moiety as inhibitors of tubulin polymerisation with potent liver cancer inhibitory activity

In this work, a series of novel arylamide derivatives containing piperazine moiety were designed and synthesised as tubulin polymerisation inhibitors. Among 25 target compounds, compound 16f (MY-1121) exhibited low nanomolar IC50 values ranging from 0.089 to 0.238 μM against nine human cancer cells. Its inhibitory effects on liver cancer cells were particularly evident with IC50 values of 89.42 and 91.62 nM for SMMC-7721 and HuH-7 cells, respectively. Further mechanism studies demonstrated that compound 16f (MY-1121) could bind to the colchicine binding site of β-tubulin and directly act on β-tubulin, thus inhibiting tubulin polymerisation. Additionally, compound 16f (MY-1121) could inhibit colony forming ability, cause morphological changes, block cell cycle arrest at the G2 phase, induce cell apoptosis, and regulate the expression of cell cycle and cell apoptosis related proteins in liver cancer cells. Overall, the promising bioactivities of compound 16f (MY-1121) make the novel arylamide derivatives have the value for further development as tubulin polymerisation inhibitors with potent anticancer activities.

New Combretastatin Analogs as Anticancer Agents: Design, Synthesis, Microtubules Polymerization Inhibition, and Molecular Docking Studies

A new series of 4-(4-methoxyphenyl)-5-(3,4,5-trimethoxyphenyl)-4H-1,2,4-triazole-3-thiol derivatives were synthesized as analogs for the anticancer drug combretastatin A-4 (CA-4) and characterized using FT-IR, 1 H-NMR, 13 CNMR, and HR-MS techniques. The new CA-4 analogs were designed to meet the structural requirements of the highest expected anticancer activity of CA-4 analogs by maintaining ring A 3,4,5-trimethoxyphenyl moiety, and at the same time varying the substituents effect of the triazole moiety (ring B). In silico analysis indicated that compound 3 has higher total energy and dipole moment than colchicine and the other analogs, and it has excellent distribution of electron density and is more stable, resulting in an increased binding affinity during tubulin inhibition. Additionally, compound 3 was found to interact with three apoptotic markers, namely p53, Bcl-2, and caspase 3. Compound 3 showed strong similarity to colchicine, and it has excellent pharmacokinetics properties and a good dynamic profile. The in vitro anti-proliferation studies showed that compound 3 is the most cytotoxic CA-4 analog against cancer cells (IC50 of 6.35 μM against Hep G2 hepatocarcinoma cells), and based on its selectivity index (4.7), compound 3 is a cancer cytotoxic-selective agent. As expected and similar to colchicine, compound 3-treated Hep G2 hepatocarcinoma cells were arrested at the G2/M phase resulting in induction of apoptosis. Compound 3 tubulin polymerization IC50 (9.50 μM) and effect on Vmax of tubulin polymerization was comparable to that of colchicine (5.49 μM). Taken together, the findings of the current study suggest that compound 3, through its binding to the colchicine-binding site at β-tubulin, is a promising microtubule-disrupting agent with excellent potential to be used as cancer therapeutic agent.

[3 + 2]-Annulation of pyridinium ylides with 1-chloro-2-nitrostyrenes unveils a tubulin polymerization inhibitor

Indolizines and pyrazolo[1,5-a]pyridines were prepared via [3 + 2]-cycloaddition of pyridinium ylides to 1-chloro-2-nitrostyrenes. The synthesized molecules were evaluated for antiproliferative activities against a BE(2)-C neuroblastoma cell line with several compounds decreasing the viability of cancer cells. Indolizine 9db showed higher potency than that of all-trans-retinoic acid, an approved cancer drug. Mechanistically, it was found to inhibit tubulin polymerization and it is thus proposed that the discovered chemistry can be exploited for the development of novel microtubule-targeting anticancer agents.

Novel Beta-Tubulin-Immobilized Nanoparticles Affinity Material for Screening β-Tubulin Inhibitors from a Complex Mixture

In order to efficiently screen and isolate β-tubulin inhibitors, β-tubulin was immobilized on core-shell PMMA/CS (poly(methyl methacrylate)/Chitosan) nanoparticles to produce a new type of immobilized affinity material named β-tubulin-immobilized nanoparticles (β-TIN). The selectivity and adsorption performance of β-TIN were characterized using various control drugs. The β-TIN, the paclitaxel molecularly imprinted ploymers (MIP), and the C18 adsorbing material were compared for selectivity and enrichment ratio. Microtubule-targeting antitumor compounds were screened and isolated from a typical Chinese medicine, Chloranthus multistachys, by β-TIN. Three active compounds (curcolnol, zedoarofuran, and codonolactone) in Chloranthus multistachys extract were captured successfully. Microscale thermophoresis demonstrated that these three compounds strongly bind to β-tubulin, and the dissociation constants (K_d) between the three active compounds and β-tubulin were 1820 ± 0.68 nM, 1640 ± 0.52 nM, and 284 ± 1.00 nM, respectively. Moreover, the binding affinity between codonolactone and β-tubulin was greater than that between paclitaxel and β-tubulin. The antitumor activities of the three compounds were confirmed by the microtubule inhibition model, and the results showed a similar antitumor mechanism as paclitaxel. Molecular dynamics simulations were performed to preliminarily investigate the potential binding sites and the structure-activity relationship between the three active molecules and β-tubulin. Our study is the first to report the use of this novel material which is highly efficient in capturing low-content β-tubulin inhibitors from a complex mixture. The three screened compounds exhibited potential antineoplastic activity, and these lead compounds utilize a new mechanism of action with promising development prospects. Because β-TIN is easily prepared, displays excellent adsorption and selectivity for targets, and can effectively maintain the steric conformation and activities of target proteins, it will be very useful in the screening of lead compounds for different drug target proteins.

Proteomics investigation reveals cell death-associated proteins of basidiomycete fungus Trametes versicolor treated with Ferruginol

Ferruginol has antifungal activity against wood-rot fungi (basidiomycetes). However, specific research on the antifungal mechanisms of ferruginol is scarce. Two-dimensional gel electrophoresis and fluorescent image analysis were employed to evaluate the differential protein expression of wood-rot fungus Trametes versicolor treated with or without ferruginol. Results from protein identification of tryptic peptides via liquid chromatography–electrospray ionization tandem mass spectrometry (LC–ESI-MS/MS) analyses revealed 17 protein assignments with differential expression. Downregulation of cytoskeleton β-tubulin 3 indicates that ferruginol has potential to be used as a microtubule-disrupting agent. Downregulation of major facilitator superfamily (MFS)–multiple drug resistance (MDR) transporter and peroxiredoxin TSA1 were observed, suggesting reduction in self-defensive capabilities of T. versicolor. In addition, the proteins involved in polypeptide sorting and DNA repair were also downregulated, while heat shock proteins and autophagy-related protein 7 were upregulated. These observations reveal that such cellular dysfunction and damage caused by ferruginol lead to growth inhibition and autophagic cell death of fungi.

Two anti-microtubular drugs for two differential responses: a rice cell line resistant to EPC remains susceptible to oryzalin

Sensitivity to the two anti-microtubular drugs oryzalin and EPC (ethyl-N-phenylcarbamate) is shown to be uncoupled in the rice EPC-resistant ER31d cell line, derived from the corresponding ER31 mutant. The ER31d cell line grows in the presence of EPC but it remains susceptible to oryzalin. In the presence of concentrations of EPC up to 0.4 mM, ER31d cells remain viable maintaining cell anisotropy and detectable cortical microtubule array. The amount of α- and β-tubulin is also maintained high through a regulatory mechanism that operates at post-transcriptional level. In contrast, all these cellular and molecular parameters are heavily affected by the addition of 1 μM oryzalin. Also, the pattern of post-translationally modified α-tubulins changes in the ER31d cells compared to that of their Nihon-Masari wild type line of reference. The different response elicited by the two herbicides is discussed in relation to a possible differential sensitivity of the cortical MT array, that may in turn relate to their different tubulin-binding specificities and chemical structure.

Immunodetection of retinoblastoma-related protein and its phosphorylated form in interphase and mitotic alfalfa cells

Plant retinoblastoma-related (RBR) proteins are primarily considered as key regulators of G(1)/S phase transition, with functional roles in a variety of cellular events during plant growth and organ development. Polyclonal antibody against the C-terminal region of the Arabidopsis RBR1 protein also specifically recognizes the alfalfa 115 kDa MsRBR protein, as shown by the antigen competition assay. The MsRBR protein was detected in all cell cycle phases, with a moderate increase in samples representing G(2)/M cells. Antibody against the human phospho-pRb peptide (Ser807/811) cross-reacted with the same 115 kDa MsRBR protein and with the in vitro phosphorylated MsRBR protein C-terminal fragment. Phospho-MsRBR protein was low in G(1) cells. Its amount increased upon entry into the S phase and remained high during the G(2)/M phases. Roscovitine treatment abolished the activity of alfalfa MsCDKA1;1 and MsCDKB2;1, and the phospho-MsRBR protein level was significantly decreased in the treated cells. Colchicine block increased the detected levels of both forms of MsRBR protein. Reduced levels of the MsRBR protein in cells at stationary phase or grown in hormone-free medium can be a sign of the division-dependent presence of plant RBR proteins. Immunolocalization of the phospho-MsRBR protein indicated spots of variable number and size in the labelled interphase nuclei and high signal intensity of nuclear granules in prophase. Structures similar to phospho-MsRBR proteins cannot be recognized in later mitotic phases. Based on the presented western blot and immunolocalization data, the possible involvement of RBR proteins in G(2)/M phase regulation in plant cells is discussed.

Polyketide decarboxylative chain termination preceded by o-sulfonation in curacin a biosynthesis

Biosynthetic innovation in natural product systems is driven by the recruitment of new genes and enzymes into these complex pathways. Here, an unprecedented decarboxylative chain termination mechanism is described for the polyketide synthase of curacin A, an anticancer lead compound isolated from the marine cyanobacterium Lyngbya majuscula. The unusual chain termination module containing adjacent sulfotransferase (ST) and thioesterase (TE) catalytic domains embedded in CurM was biochemically characterized. The TE was proved to catalyze a hydrolytic chain release of the polyketide chain elongation intermediate. Moreover, a selective ST-mediated sulfonation of the (R)-beta-hydroxyl group was found to precede TE-mediated hydrolysis, triggering a successive decarboxylative elimination and resulting in the formation of a rare terminal olefin in the final metabolite.

Synthesis and antimitotic activity of novel 2-methoxyestradiol analogs--Part II

The syntheses and antimitotic activity of several novel 18a-homo-analogs of 2-methoxyestradiol are described. Structural modifications of the parent 2-methoxy-18a-homoestradiol include introduction of unsaturation in the D-ring and methylation of the 17-OH. Of seven analogs synthesized, one has demonstrated superior biological activities compared to 2-methoxyestradiol. The relationship between biological activity and the conformational preference of the 13-ethyl group as determined by computational analysis is discussed.

Identification and mitotic partitioning strategies of vacuoles in the unicellular red alga Cyanidioschyzon merolae

Cyanidioschyzon merolae is considered as a suitable model system for studies of organelle differentiation, proliferation and partitioning. Here, we have identified and characterized vacuoles in this organism and examined the partitioning of vacuoles using fluorescence and electron microscopy. Vacuoles were stained with the fluorescent aminopeptidase substrate 7-amino-4-chloromethylcoumarin L: -arginine amide, acidotrophic dyes quinacrine and LysoTracker, and 4',6-diamidino-2-phenyl indole, which, at a high concentration, stains polyphosphate. Vacuoles have been shown to be approximately 500 nm in diameter with a mean of around five per interphase cell. The vacuolar H(+)-ATPase inhibitor concanamycin A blocked the accumulation of quinacrine in the vacuoles, suggesting the presence of the enzyme on these membranes. Electron microscopy revealed that the vacuoles were single membrane-bound organelles with an electron-dense substance, often containing a thick layer surrounding the membrane. Immunoelectron microscopy using an anti-vacuolar-H(+)-pyrophosphatase antibody revealed the presence of the enzyme on these membranes. In interphase cells, vacuoles were distributed in the cytoplasm, while in mitotic cells they were localized adjacent to the mitochondria. Filamentous structures were observed between vacuoles and mitochondria. Vacuoles were distributed almost evenly to daughter cells and redistributed in the cytoplasm after cytokinesis. The change in localization of vacuoles also happened in microtubule-disrupted cells. Since no actin protein or filaments have been detected in C. merolae, this result suggests an intrinsic mechanism for the movement of vacuoles that differs from commonly known mechanisms mediated by microtubules and actin filaments.

Indole, a core nucleus for potent inhibitors of tubulin polymerization

Microtubules are the basic components of cell structure, which take part in a wide number of pivotal cellular functions. Drugs that are able to modulate the microtubule assembly either by inhibition of tubulin polymerization or by blocking microtubule disassembly are of great interest in anti-cancer therapy. Several tubulin polymerization inhibitors characterized by the presence of an indole nucleus have been obtained from natural sources or have been prepared by semi-synthesis. In the last decade an ever increasing number of synthetic indoles have been reported. We have reviewed anti-tubulin agents obtained by synthesis having an indole as core nucleus. The synthesis, the biological activity, and the structure - activity relationship aspects of 3-formyl-2-phenylindoles, heterocombretastatins, diarylindoles, 2-aroylindoles, D-24851, 2-aryl-3-aroylindoles, 3-aroyl- and 1-aroylindoles, and arylthioindoles are discussed.

Salt stress affects cortical microtubule organization and helical growth in Arabidopsis

Cortical microtubule arrays are critical in determining the growth axis of diffusely growing plant cells, and various environmental and physiological factors are known to affect the array organization. Microtubule organization is partly disrupted in the spiral1 mutant of Arabidopsis thaliana, which displays a right-handed helical growth phenotype in rapidly elongating epidermal cells. We show here that mutations in the plasma membrane Na(+)/H(+) antiporter SOS1 and its regulatory kinase SOS2 efficiently suppressed both microtubule disruption and helical growth phenotypes of spiral1, and that sos1 and sos2 roots in the absence of salt stress exhibited altered helical growth response to microtubule-interacting drugs at low doses. Salt stress also altered root growth response to the drugs in wild-type roots. Suppression of helical growth appeared to be specific to spiral1 since other helical growth mutants were not rescued. The effects of sos1 in suppressing spiral1 defects and in causing abnormal drug responses were nullified in the presence of the hkt1 Na(+) influx carrier mutation in roots but not in hypocotyls. These results suggest that cytoplasmic salt imbalance caused by insufficient SOS1 activity compromises cortical microtubule functions in which microtubule-localized SPIRAL1 is specifically involved.

Solidification/stabilization of arsenic containing solid wastes using portland cement, fly ash and polymeric materials

Stabilization/solidification (S/S) is used as a pre-landfill waste treatment technology that aims to make hazardous industrial wastes safe for disposal. Cement-based solidification/stabilization technology is widely used because it offer assurance of chemical stabilization of many contaminants and produce a stable form of waste. The leaching behavior of arsenic from a solidified/stabilized waste was studied to obtain information about their potential environmental risk. Activated alumina (AA) contaminated with arsenic was used as a waste, which was stabilized/solidified (S/S) using ordinary portland cement (C), fly ash (FA), calcium hydroxide (CH) and various polymeric materials such as polystyrene and polymethyl methacrylate (PMMA). Toxicity characteristics leaching procedure (TCLP) and semi-dynamic leach tests were conducted to evaluate the leaching behavior of arsenic. Formations of calcite along with precipitate formation of calcium arsenite were found to be responsible for low leaching of arsenic from the stabilized/solidified samples. Effective diffusivity of arsenic ion from the matrix and leachablity index was also estimated. Minimum leaching of the contaminant was observed in matrix having AA+C+FA+CH due to the formation of calcite.

Microtubule interactions with chemically diverse stabilizing agents: thermodynamics of binding to the paclitaxel site predicts cytotoxicity

The interactions of microtubules with most compounds described as stabilizing agents have been studied. Several of them (lonafarnib, dicumarol, lutein, and jatrophane polyesters) did not show any stabilizing effect on microtubules. Taccalonolides A and E show paclitaxel-like effects in cells, but they were not able to modulate in vitro tubulin assembly or to bind microtubules, which suggests that other factors are involved in their cellular effects. The binding constants of epothilones, eleutherobin, discodermolide, sarcodictyins, 3,17beta-diacetoxy-2-ethoxy-6-oxo-B-homo-estra-1,3,5(10)-triene, and dictyostatin to the paclitaxel site; the critical concentrations of ligand-induced assembly; and their cytotoxicity in carcinoma cells have been measured, and correlations between these parameters have been determined. The inhibition of cell proliferation correlates better with the binding enthalpy change than with the binding constants, suggesting that large, favorable enthalpic contribution to the binding is desired to design paclitaxel site drugs with higher cytotoxicity.

New cytotoxic agents: a review of the literature

The goal of treatment for patients with advanced cancer is to prolong survival, control symptoms, and reduce disease-related complications. Despite the introduction of many cytotoxic agents during the past decade, only modest improvement in survival has been achieved. In order to urgently improve these situations, new cytotoxic agents as well as molecular-targeted agents are now under investigation. In this article, we reviewed the latest information regarding antitumor activity, toxicity, pharmacokinetics, and clinical application of the new cytotoxic agents.

A common pharmacophore for a diverse set of colchicine site inhibitors using a structure-based approach

Modulating the structure and function of tubulin and microtubules is an important route to anticancer therapeutics, and therefore, small molecules that bind to tubulin and cause mitotic arrest are of immense interest. A large number of synthetic and natural compounds with diverse structures have been shown to bind at the colchicine site, one of the major binding sites on tubulin, and inhibit tubulin assembly. Using the recently determined X-ray structure of the tubulin:colchicinoid complex as the template, we employed docking studies to determine the binding modes of a set of structurally diverse colchicine site inhibitors. These binding models were subsequently used to construct a comprehensive, structure-based pharmacophore that in combination with molecular dynamics simulations confirms and extends our understanding of binding interactions at the colchicine site.

Design and synthesis of non-hydroxamate histone deacetylase inhibitors: identification of a selective histone acetylating agent

A series of suberoylanilide hydroxamic acid (SAHA)-based non-hydroxamates was designed, synthesized, and evaluated for their histone deacetylase (HDAC) inhibitory activity. Among these, methyl sulfoxide 15 inhibited HDACs in enzyme assays and caused hyperacetylation of histone H4 while not inducing the accumulation of acetylated alpha-tubulin in HCT116 cells.

Synthesis and biological activity of mustard derivatives of combretastatins

A series of chimeric compounds bearing the combretastatin and the nitrogen mustard cores were synthesized. All the compounds were cytotoxic and inhibited tubulin polymerization. When combretastatin was joined to chlorambucil via an ester linkage, the resultant compound proved to be significantly more potent than the two compounds put together. When combretastatin was joined to nitrogen mustard via an ether linkage or when a true hybrid was synthesized, loss of potency was observed. Nonetheless, these latter compounds appeared to be more efficacious and surprisingly were able to inhibit tubulin depolymerization at high concentrations.

Two photoaffinity analogues of the tripeptide, hemiasterlin, exclusively label alpha-tubulin

A synthetic analogue of the tripeptide hemiasterlin, designated HTI-286, depolymerizes microtubules, is a poor substrate for P-glycoprotein, and inhibits the growth of paclitaxel-resistant tumors in xenograft models. Two radiolabeled photoaffinity analogues of HTI-286, designated 4-benzoyl-N,beta,beta-trimethyl-l-phenylalanyl-N(1)-[(1S,2E)-3-carboxy-1-isopropylbut-2-enyl]-N(1),3-dimethyl-l-valinamide (probe 1) and N,beta,beta-trimethyl-l-phenylalanyl-4-benzoyl-N-[(1S,2E)-3-carboxy-1-isopropyl-2-butenyl]-N,beta,beta-trimethyl-l-phenylalaninamide (probe 2), were made to help identify HTI-286 binding sites in tubulin. HTI-286, probe 1, and probe 2 had similar affinities for purified tubulin [apparent K(D(app)) = 0.2-1.1 microM], inhibited polymerization of purified tubulin approximately 80%, and were potent inhibitors of cell growth (IC(50) = 1.0-22 nM). Both radiolabeled probes labeled exclusively alpha-tubulin. Labeling by [(3)H]probe 1 was inhibited by probe 1, HTI-286, vinblastine, or dolastatin 10 (another peptide antimitotic agent that depolymerizes microtubules) but was either unaffected or enhanced (at certain temperatures) by colchicine or paclitaxel. [(3)H]Probe 1 also labeled exclusively tubulin in cytosolic extracts of whole cells. The major, if not exclusive, contact site for probe 1 was mapped to residues 314-339 of alpha-tubulin and corresponds to the sheet 8 and helix 10 region. This region is known to (1) have longitudinal interactions with beta-tubulin across the interdimer interface, (2) have lateral interactions with adjacent protofilaments, and (3) contact the N-terminal region of stathmin, a protein that induces depolymerization of tubulin. Binding of probe 1 to this region may alter the conformation of tubulin outside the labeling domain, since enzymatic removal of the C-terminus of only alpha-tubulin by subtilisin after, but not before, photolabeling is blocked by probe 1. These results suggest that hemiasterlin is in close contact with alpha-tubulin and may span the interdimer interface so that it contacts the vinblastine- and dolastatin 10-binding sites believed to be in beta-tubulin. In addition, we speculate that antimitotic peptides mimic the interaction of stathmin with tubulin.

Review: tubulin function, action of antitubulin drugs, and new drug development

Anticancer agents that interfere with microtubulin function are in widespread use in man and have a broad spectrum of activity against both hematological malignancies and solid tumors. The mechanisms of actions of these agents have been better defined during the past decade, indicating that there are distinct binding sites for these agents and that they interfere with microtubulin dynamics (growth and shortening of tubules) at low concentrations and only evoke microtubulin aggregation or dissociation at high concentrations. Tubulin has been recently described in the nucleus of cells and in mitochondria. Downstream events from tubulin binding are believed to be critical events for the generation of apoptosis in the malignant cell. The effects of vinca alkaloids and taxanes are distinct, suggesting that the interference with the tubulin cap by high-affinity binding of effective agents is not the only mechanism of cytotoxic effect, and the low-affinity binding of drug, which distorts microtubulin function, may also be important. The epothilones share some of the binding characteristics of the taxanes and are in clinical trials because of cytoxic activity in taxane resistant cells. Tubulin has additional target sites for anticancer drugs including interference with the binding and function of microtubule associated proteins and interference with motor proteins which are essential for the transport of substances within the cell. Because many of these microtubule associated proteins have an ATP binding site, both computer-aided design and combinatorial chemistry techniques can be used to make agents to interfere with their function analogous to imatinib mesylate (Gleevec). Agents that interfere with the motor protein kinesin are entering clinical trials.

Antimitotic activity and reversal of breast cancer resistance protein-mediated drug resistance by stilbenoids from Bletilla striata

Eight stilbenoids, 1-(p-hydroxybenzyl)-4,8-dimethoxyphenanthrene-2,7-diol (1), 2,7-dihydroxy-1,3-bis(p-hydroxybenzyl)-4-methoxy-9,10-dihydrophenanthrene (2), 4,7-dihydroxy-1-(p-hydroxybenzyl)-2-methoxy-9,10-dihydrophenanthrene (3), 3,3'-dihydroxy-2',6'-bis(p-hydroxybenzyl)-5-methoxybibenzyl (4), 3',5-dihydroxy-2-(p-hydroxybenzyl)-3-methoxybibenzyl (5), blestriarenes B (6) and C (7), and blestrianol A (8) have been isolated by the guidance of inhibitory effect of tubulin polymerization from the tubers of Bletilla striata (Orchidaceae). Among them, both of bisbenzyls 4 and 5 inhibited the polymerization of tubulin at IC(50) 10muM, respectively. Furthermore bisbenzyl 4 potentiated the cytotoxicity of SN-38 in BCRP-transduced K562 (K562/BCRP) cells.

New antitubulin derivatives in the combretastatin A4 series: synthesis and biological evaluation

Two series of combretastatin A4 derivatives (acrylamide=carboxamide and carbamate) were synthesized in order to improve the water solubility and stabilize the cis-configuration of the double bond. Their cytotoxic effects were evaluated against MCF-7, KB-3-1 and IGROV human cancer cell lines, as well as their inhibitory activity on tubulin polymerization. Results were compared to those of carboxamide 1, chosen as reference. Potent inhibitions were observed on both tests in the carboxamide series, particularly for compound 4d bearing a fluorine group in replacement of the 3-hydroxyl of CA4. In contrast, most of the carbamates were either inactive or displayed only moderate cytotoxicities. Interestingly, a submicromolar IC(50) was measured on MCF-7 cells for 6g, although this compound was totally devoid of antitubulin activity.

Synthesis and evaluation of double bond substituted combretastatins

A series of combretastatins substituted with epoxides, amides and small alkyl groups has been synthesised and evaluated for cytotoxicity and their ability to inhibit the assembly of tubulin. The methyl and ethyl substituted phenols 36, 44 have shown potent antimitotic effects whilst exhibiting reduced cytotoxicity.

Synthesis and activity of novel analogs of hemiasterlin as inhibitors of tubulin polymerization: modification of the A segment

Analogs of hemiasterlin (1) and HTI-286 (2), which contain various aromatic rings in the A segment, were synthesized as potential inhibitors of tubulin polymerization. The structure-activity relationships related to stereo- and regio-chemical effects of substituents on the aromatic ring in the A segment were studied. Analogs, which carry a meta-substituted phenyl ring in the A segment show comparable activity for inhibition of tubulin polymerization to 2, as well as in the cell proliferation assay using KB cells containing P-glycoprotein, compared to those of 1 and 2.

Genotoxicity of inorganic lead salts and disturbance of microtubule function

Lead compounds are known genotoxicants, principally affecting the integrity of chromosomes. Lead chloride and lead acetate induced concentration-dependent increases in micronucleus frequency in V79 cells, starting at 1.1 microM lead chloride and 0.05 microM lead acetate. The difference between the lead salts, which was expected based on their relative abilities to form complex acetato-cations, was confirmed in an independent experiment. CREST analyses of the micronuclei verified that lead chloride and acetate were predominantly aneugenic (CREST-positive response), which was consistent with the morphology of the micronuclei (larger micronuclei, compared with micronuclei induced by a clastogenic mechanism). The effects of high concentrations of lead salts on the microtubule network of V79 cells were also examined using immunofluorescence staining. The dose effects of these responses were consistent with the cytotoxicity of lead(II), as visualized in the neutral-red uptake assay. In a cell-free system, 20-60 microM lead salts inhibited tubulin assembly dose-dependently. The no-observed-effect concentration of lead(II) in this assay was 10 microM. This inhibitory effect was interpreted as a shift of the assembly/disassembly steady-state toward disassembly, e.g., by reducing the concentration of assembly-competent tubulin dimers. The effects of lead salts on microtubule-associated motor-protein functions were studied using a kinesin-gliding assay that mimics intracellular transport processes in vitro by quantifying the movement of paclitaxel-stabilized microtubules across a kinesin-coated glass surface. There was a dose-dependent effect of lead nitrate on microtubule motility. Lead nitrate affected the gliding velocities of microtubules starting at concentrations above 10 microM and reached half-maximal inhibition of motility at about 50 microM. The processes reported here point to relevant interactions of lead with tubulin and kinesin at low dose levels.

Novel diarylsulfonylurea derivatives as potent antimitotic agents

A novel series of diarylsulfonylurea derivatives were synthesized and evaluated for interaction with tubulin and for cytotoxicity against human cancer cell lines. These derivatives demonstrated good inhibitory activity against tubulin polymerization, which was well correlated with promising antiproliferative activity as well as G2/M phase cell cycle arrest. Furthermore, several compounds were also efficacious against multidrug-resistant cancer cells, which are resistant to many other known microtubule inhibitors.

D-piece modifications of the hemiasterlin analog HTI-286 produce potent tubulin inhibitors

Modifications of the D-piece carboxylic acid group of the hemiasterlin analog HTI-286 gave tubulin inhibitors which were potent cytotoxic agents in taxol resistant cell lines expressing P-glycoprotein. Amides derived from proline had potency comparable to HTI-286. Reduction of the carboxylic acid to ketones and alcohols or its conversion to acidic heterocycles also gave potent analogs. Synthetic modifications of the carboxylic acid could be carried out selectively using a wide range of synthetic reagents. Proline analog 3 was found to be effective in a human xenograft model in athymic mice.

Tubulin inhibitors. Synthesis and biological activity of HTI-286 analogs with B-segment heterosubstituents

Modifications of the B-segment of HTI-286 (2) produced a class of analogs incorporating heteroatom-substituents. The structure-activity relationship was studied. Analogs bearing methylsulfide and fluoride groups exhibited potency comparable to that of the parent compound HTI-286 and to paclitaxel in cytotoxicity assays against KB-3-1 cell lines. These analogs were more potent than paclitaxel against P-glycoprotein expressing KB-8-5 and KB-V1 cell lines. Several analogs showed strong inhibition of tubulin polymerization.

-NH-Dansyl Isocolchicine Exhibits a Significantly Improved Tubulin-Binding Affinity and Microtubule Inhibition in Comparison to Isocolchicine by Binding Tubulin through Its A and B Rings

Structure-activity relationship studies have established that the A and C rings of colchicine comprise the minimum structural feature necessary for high affinity drug-tubulin binding. Thus, colchicine acts as a bifunctional ligand by making two points of attachment to the protein. Furthermore, analogues belonging to the iso series of colchicine are virtually inactive in binding to tubulin and inhibiting microtubule assembly. In the present study, we found that the substitution of a hydrophobic dansyl group on the B-ring side chain (C7 position) of isocolchicine reverses the structural alterations at the C ring and the newly synthesized -NH-dansyl isocolchicine restores the lost biological activity of the compound. It inhibits microtubule assembly efficiently with an IC(50) value of 10 microM and competes with [(3)H]colchicine for binding to tubulin. Moreover, although -NH-dansyl colchicine binding to tubulin involves two steps, the -NH-dansyl isocolchicine-tubulin interaction has been found to occur via a one-step process. Also, the affinity constant of the -NH-dansyl isocolchicine-tubulin interaction is roughly only 3 times lower than that of the -NH-dansyl colchicine-tubulin interaction. These results suggest that the enhanced microtubule inhibitory ability of -NH-dansyl isocolchicine is therefore related to the affinity of the drug-tubulin interaction and not to any conformational changes upon binding tubulin. We also observed that the competition of -NH-dansyl isocolchicine with [(3)H]colchicine for binding to tubulin was dependent on the tubulin concentration. In conclusion, this paper for the first time indicates that a biologically active bifuntional colchicine analogue can be designed where the drug binds tubulin through its A and B rings, while the C ring remains inactive.

Glutamylated and glycylated tubulin isoforms in the aberrant sperm axoneme of the gall-midge fly, Asphondylia ruebsaameni

The axonemal organization expressed in the sperm flagella of the cecidomyiid dipteran Asphondylia ruebsaameni is unconventional, being characterized by the presence of an exceedingly high number of microtubular doublets and by the absence of both the inner dynein arms and the central pair/radial spoke complex. Consequently, its motility, both in vivo and in vitro, is also peculiar. Using monoclonal antibodies directed against posttranslational modifications, we have analyzed the presence and distribution of glutamylated and glycylated tubulin isoforms in this aberrant axonemal structure, and compared them with those of a reference insect species (Apis mellifera), endowed with a conventional axoneme. Our results have shown that the unorthodox structure and motility of the Asphondylia axoneme are concomitant with: (1). a very low glutamylation extent in the alpha-tubulin subunit, (2). a high level of glutamylation in the beta-subunit, (3). an extremely low total extent of glycylation, with regard to both monoglycylated and polyglycylated sites, either in alpha- or in beta-tubulin, (4). the presence of a strong labeling of glutamylated tubulin isoforms at the proximal end of the axoneme, and (5). a uniform distribution of glutamylated as well as glycylated isoforms along the rest of the axoneme. Thus, our data indicate that tubulin molecular heterogeneity is much lower in the Asphondylia axoneme than in the conventional 9+2 axoneme with regard to both isoform content and isoform distribution along the axoneme.

Synthesis and anti-tubulin activity of a 3'-(4-azidophenyl)-3'-dephenylpaclitaxel photoaffinity probe

The synthesis and biological evaluation of a novel paclitaxel photoaffinity probe is described. The synthesis involved the preparation of an azide-containing C13 side chain through a Staudinger cycloaddition followed by a lipase-mediated kinetic resolution to obtain the azetidinone in 99% ee. Coupling of the enantiopure side chain precursor to 7-TES-baccatin III and subsequent silyl ether deprotection afforded 3'-(4-azidophenyl)-3'-dephenylpaclitaxel, which was shown to be as active as paclitaxel in tubulin assembly and cytotoxicity assays.

Arylthioindoles, potent inhibitors of tubulin polymerization

Several arylthioindoles had excellent activity as inhibitors both of tubulin polymerization and of the growth of MCF-7 human breast carcinoma cells. Methyl 3-[(3,4,5-trimethoxyphenyl)thio]-5-methoxy-1H-indole-2-carboxylate (21), the most potent derivative, showed IC(50) = 2.0 microM, 1.6 times more active than colchicine and about as active as combretastatin A-4 (CSA4). Compound 21 inhibited the growth of the MCF-7 cells at IC(50) = 13 nM. Colchicine and CSA4 had 13 nM and 17 nM IC(50) values, respectively, with these cells.

Quantitative Structure−Activity Relationship (5D-QSAR) Study of Combretastatin-like Analogues as Inhibitors of Tubulin Assembly

A molecular modeling study was carried out to develop a predictive model for combretastatin-like analogues populating the colchicine-binding site of beta-tubulin. A series of compounds built around a framework including two aromatic groups linked by various moieties such as alkenes (stilbenes), enones (chalcones), or ethers was selected for the study. The 5D-QSAR model was developed stepwise. First a model was generated for the chalcone series (19 compounds, 71 conformations), then for the stilbene series (18 compounds, 59 conformations), and finally for the combined dataset (47 ligands, 160 conformers). Although the models for the chalcone and stilbene series appeared slightly different when represented by QSAR colored surfaces, the combined model seems to reconcile the differences without compromise and represents a highly predictive model for compounds that bind to the colchicine-binding site of tubulin.

Tubulin-polymerization inhibitors derived from thalidomide

2-(2,6-Diisopropylphenyl)-5-hydroxy-1H-isoindole-1,3-dione (5HPP-33), which was obtained during our previous structural development studies on thalidomide, was revealed to possess potent tubulin-polymerization-inhibiting activity, comparable to that of the known tubulin-polymerization inhibitors, rhizoxin and colchicine. A major metabolite of thalidomide, 5-hydroxythalidomide, which possesses a hydroxyl group at the position corresponding to that of 5HPP-33, also showed moderate inhibitory activity.

Syntheses and bioactivities of macrocyclic paclitaxel bis-lactones

Five macrocyclic paclitaxel bis-lactones and their corresponding open chain taxoids were synthesized as models of the tubulin-binding conformation of paclitaxel. Macrocyclic lactones with a 19-21-membered ring underwent isomerization to form smaller rings. The lactones were evaluated for cytotoxicity and tubulin-polymerization ability. All five macrocyclic paclitaxel lactones were active, but less so than paclitaxel, while the rearranged macrocyclic lactones and the corresponding open-chain taxoids were much less active or inactive.

Probing the interaction of HTI-286 with tubulin using a stilbene analogue

HTI-286 is a synthetic analogue of the natural product hemiasterlin. HTI-286 is a potent antitumor agent that induces tubulin oligomerization. To investigate the binding stoichiometry and the binding site during this ligand-induced tubulin association, we synthesized an analogue of HTI-286 containing the chromophore stilbene. Using the distinct absorbance of the stilbene analogue, we determined the amounts of inhibitors bound to different tubulin oligomers by analytical ultracentrifugation. Herein we describe our findings based on these experiments. At the ratio of inhibitor to protein equal to or greater than 1, the stilbene analogue induces oligomerization of tubulin to a ring structure. The binding stoichiometry in the ring is one inhibitor per tubulin monomer (defined as an alpha/beta-heterodimer). At the ratio of inhibitor to protein less than 1, tubulin forms multiple intermediates, with the binding stoichiometry less than one inhibitor per tubulin monomer for all intermediates. The stable complex between the inhibitor and tubulin monomer was not detected under our experimental conditions. The binding site of the stilbene analogue does not overlap with the classic tubulin-binding agent, colchicine.

Integrity of actin fibers and microtubules influences metastatic tumor cell adhesion

Tumor cell adhesion within host organ microvasculature, its stabilization and invasion into host organ parenchyma appear to be important steps during formation of distant metastasis. These interactions of circulating tumor cells with the host organs occur in the presence of fluid shear forces and soluble and cellular environmental conditions of the blood that can modulate their cellular responses and possibly their metastatic efficiency. Cytoskeletal components, such as actin filaments and microtubules, can regulate biophysical characteristics and cellular signaling of the circulating cells. Therefore, we investigated the role of these cytoskeletal structures for early steps during metastasis formation in vivo and in vitro. Using an intravital observation technique, tumor cell adhesion of colon carcinoma cells within the hepatic microcirculation of rats and their invasion into liver parenchyma was observed. Disruption of actin filaments increased cell adhesion, whereas tubulin disruption inhibited adhesive interactions in vivo. The impairment of the cytoskeleton modulated adhesion-mediated cell signaling via focal adhesion kinase (FAK) and paxillin under flow conditions in vitro. In the presence of fluid flow, focal adhesions were enlarged and hyperphosphorylated, whereas stress fibers were reduced compared to static cell adhesion. Disruption of microtubules, however, partially inhibited these effects. Combining the in vivo and in vitro results, our study suggested that changes in cell rigidity and avidity of cell adhesion molecules after disruption of cytoskeletal components appear to be more important for initial adhesive interactions in vivo than their interference with adhesion-mediated cellular signal transduction.

Oxaline, a fungal alkaloid, arrests the cell cycle in M phase by inhibition of tubulin polymerization

Oxaline and neoxaline, fungal alkaloids, were found to inhibit cell proliferation and to induce cell cycle arrest at the G(2)/M phase in Jurkat cells. CBP501 (a peptide corresponding to amino acids 211-221 of Cdc25C phosphatase), which inhibits the G(2) checkpoint, did not affect the G(2)/M arrest caused by oxaline, suggesting that oxaline causes M phase arrest but not G(2) phase arrest. The Cdc2 phosphorylation level of oxaline-treated cell lysate was lower than that of the control cells, indicating that oxaline arrests the M phase. Oxaline disrupted cytoplasmic microtubule assembly in 3T3 cells. Furthermore, oxaline inhibited polymerization of microtubule protein and purified tubulin dose-dependently in vitro. In a binding competition assay, oxaline inhibited the binding of [(3)H]colchicine to tubulin, but not that of [(3)H]vinblastine. These results indicate that oxaline inhibits tubulin polymerization, resulting in cell cycle arrest at the M phase.

Antimicrotubular drugs binding to vinca domain of tubulin

Studies on vinca domain binding drugs were done in great details by a number of workers as it is recognized as a potential target for anticancer drug development. Their structures, properties, mode of action, success and failures as potential anticancer drug have been discussed in short details in this review. Among these drugs rhizoxin and maytansine are competitive inhibitors, and bind at the vinblastine binding site of tubulin where as others are non-competitive inhibitors. Besides binding, these drugs also differ in the extent of GTP hydrolysis, GTP exchange and in the stabilization of colchicine binding site. The toxicity level of these drugs towards the host cells and the extent of efflux of drugs by the P-glycoprotein mediated pump are also discussed.

Synthesis, modeling, and anti-tubulin activity of a D-seco paclitaxel analogue

[reaction: see text] We have previously described a model of paclitaxel-microtubule binding that led to the prediction that analogues of paclitaxel lacking any D ring could stabilize microtubules as well as paclitaxel if the substituent present at C4 did not have unfavorable steric interactions with the binding pocket. We report the synthesis of a 4-methyl paclitaxel analogue, compound 1, which bears this prediction out. Compound 1 is as potent as paclitaxel at microtubule stabilization in vitro; however, it has only about one-four-hundredth the cytotoxicity of paclitaxel.

Complexes of gamma-tubulin with nonreceptor protein tyrosine kinases Src and Fyn in differentiating P19 embryonal carcinoma cells

Nonreceptor protein tyrosine kinases of the Src family have been shown to play an important role in signal transduction as well as in regulation of microtubule protein interactions. Here we show that gamma-tubulin (gamma-Tb) in P19 embryonal carcinoma cells undergoing neuronal differentiation is phosphorylated and forms complexes with protein tyrosine kinases of the Src family, Src and Fyn. Elevated expression of both kinases during differentiation corresponded with increased level of proteins phosphorylated on tyrosine. Immunoprecipitation experiments with antibodies against Src, Fyn, gamma-tubulin, and with anti-phosphotyrosine antibody revealed that gamma-tubulin appeared in complexes with these kinases. In vitro kinase assays showed tyrosine phosphorylation of proteins in gamma-tubulin complexes isolated from differentiated cells. Pretreatment of cells with Src family selective tyrosine kinase inhibitor PP2 reduced the amount of phosphorylated gamma-tubulin in the complexes. Binding experiments with recombinant SH2 and SH3 domains of Src and Fyn kinases revealed that protein complexes containing gamma-tubulin bound to SH2 domains and that these interactions were of SH2-phosphotyrosine type. The combined data suggest that Src family kinases might have an important role in the regulation of gamma-tubulin interaction with tubulin dimers or other proteins during neurogenesis.

Celogenamide A, a new cyclic peptide from the seeds of Celosia argentea

A new cyclic nonapeptide, celogenamide A, has been isolated from the seeds of Celosia argentea, and the structure including absolute stereochemistry was determined by using extensive 2D NMR and MS-MS methods and chemical means.

Biosynthetic pathway and gene cluster analysis of curacin A, an antitubulin natural product from the tropical marine cyanobacterium Lyngbya majuscula

Curacin A (1) is a potent cancer cell toxin obtained from strains of the tropical marine cyanobacterium Lyngbya majuscula found in Curaçao. Its structure is unique in that it contains the sequential positioning of a thiazoline and cyclopropyl ring, and it exerts its potent cell toxicity through interaction with the colchicine drug binding site on microtubules. A series of stable isotope-labeled precursors were fed to cultures of curacin A-producing strains and, following NMR analysis, allowed determination of the metabolic origin of all atoms in the natural product (one cysteine, 10 acetate units, two S-adenosyl methionine-derived methyl groups) as well as several unique mechanistic insights. Moreover, these incorporation experiments facilitated an effective gene cloning strategy that allowed identification and sequencing of the approximately 64 kb putative curacin A gene cluster. The metabolic system is comprised of a nonribosomal peptide synthetase (NRPS) and multiple polyketide synthases (PKSs) and shows a very high level of collinearity between genes in the cluster and the predicted biochemical steps required for curacin biosynthesis. Unique features of the cluster include (1) all but one of the PKSs are monomodular multifunctional proteins, (2) a unique gene cassette that contains an HMG-CoA synthase likely responsible for formation of the cyclopropyl ring, and (3) a terminating motif that is predicted to function in both product release and terminal dehydrative decarboxylation.