Tubulysin analogs incorporating desmethyl and dimethyl tubuphenylalanine derivatives

A series of tubulysin analogs in which one of the stereogenic centers of tubuphenylalanine was eliminated were synthesized. All compounds were tested for antiproliferative activity towards ovarian cancer cells and for inhibition of tubulin polymerization. The dimethyl analogs were generally more active than the desmethyl analogs, and four analogs have tubulin polymerization IC₅₀ values similar to combretastatin A4 and the hemiasterlin analog HTI-286.     

Synthesis and evaluation of diaryl sulfides and diaryl selenide compounds for antitubulin and cytotoxic activity

We have devised a procedure for the synthesis of analogs of combretastatin A-4 (CA-4) containing sulfur and selenium atoms as spacer groups between the aromatic rings. CA-4 is well known for its potent activity as an inhibitor of tubulin polymerization, and its prodrugs combretastatin A-4 phosphate (CA-4P) and combretastatin A-1 phosphate (CA-1P) are being investigated as antitumor agents that cause tumor vascular collapse in addition to their activity as cytotoxic compounds. Here we report the preparation of two sulfur analogs and one selenium analog of CA-4. All synthesized compounds, as well as several synthetic intermediates, were evaluated for inhibition of tubulin polymerization and for cytotoxic activity in human cancer cells. Compounds 3 and 4 were active at nM concentration against MCF-7 breast cancer cells. As inhibitors of tubulin polymerization, both 3 and 4 were more active than CA-4 itself. In addition, 4 was the most active of these agents against 786, HT-29 and PC-3 cancer cells. Molecular modeling binding studies are also reported for compounds 1, 3, 4 and CA-4 to tubulin within the colchicine site.     

Design, synthesis, biological evaluation and molecular modeling of 1,3,4-oxadiazoline analogs of combretastatin-A4 as novel antitubulin agents

A total of 20 novel 1,3,4-oxadiazoline analogs (6a-6t) of combretastatin A-4 with naphthalene ring were designed, synthesized, and evaluated for biological activities as potential tubulin polymerization inhibitors. Among these compounds, 6n showed the most potent antiproliferative activities against multiple cancer cell lines and retained the microtubule disrupting effects. Docking simulation was performed to insert compound 6n into the crystal structure of tubulin to determine the probable binding model. These results indicated oxadiazoline compounds bearing the naphthyl moiety are promising tubulin inhibitors.     

Design, synthesis and biological evaluation of dihydronaphthalene and benzosuberene analogs of the combretastatins as inhibitors of tubulin polymerization in cancer chemotherapy

A novel series of dihydronaphthalene and benzosuberene analogs bearing structural similarity to the combretastatins in terms of 1,2-diarylethene, trimethoxyphenyl, and biaryl functionality has been synthesized. The compounds have been evaluated in regard to their ability to inhibit tubulin assembly and for their cytotoxicity against selected human cancer cell lines. From this series of compounds, benzosuberene analogs 2 and 4 inhibited tubulin assembly at concentrations comparable to that of combretastatin A-4 (CA4) and combretastatin A-1 (CA1). Furthermore, analog 4 demonstrated remarkable cytotoxicity against the three human cancer cell lines evaluated (for example GI(50)=0.0000032 microM against DU-145 prostate carcinoma).     

Lead identification of conformationally restricted benzoxepin type combretastatin analogs: synthesis,antiproliferative activity,and tubulin effects

We have synthesized a series of polymethoxylated rigid analogs of combretastatin A-4 which contain a benzoxepin ring in place of the usual ethylene bridge present in the natural combretastatin products. The compounds display antiproliferative activity when evaluated against the MCF-7 and MDA human breast carcinoma cell lines. 5-(3-Hydroxy-4-methoxyphenyl)-4-(3,4,5-trimethoxyphenyl)-2,3-dihydro-benzoxepine (11g) was found to be the most potent product when evaluated against the MCF-7 breast cancer cell line. A brief computational study of the structure–activity relationship for the synthesized compounds is presented. These 4,5-diarylbenzoxepins are identified as potentially useful scaffolds for the further development of antitumor agents which target tubulin.     

Synthesis and Biological Evaluation of 3-Alkyl-1,5-Diaryl-1H-Pyrazoles as Rigid Analogues of Combretastatin A-4 with Potent Antiproliferative Activity

A series of novel 3-alkyl-1,5-diaryl-1H-pyrazoles were synthesized as combretastatin A-4 (CA-4) analogues and evaluated for antiproliferative activity against three human cancer cell lines (SGC-7901, A549 and HT-1080). Most of the target compounds displayed moderate to potent antiproliferative activity, and 7k was found to be the most potent compound. Structure-activity relationships indicated that compounds with a trimethoxyphenyl A-ring at the N-1 position of the pyrazole skeleton were more potent than those with the A-ring at the C-5 position. Tubulin polymerization and immunostaining experiments revealed that 7k potently inhibited tubulin polymerization and disrupted tubulin microtubule dynamics in a manner similar to CA-4. Computational modelling demonstrated that the binding of 7k to the colchicine binding site on microtubules may involve a similar mode as CA-4.     

Synthesis,cytotoxic evaluation and molecular docking study of 2-alkylthio-4-(2,3,4-trimethoxyphenyl)-5-aryl-thiazoles as tubulin polymerization inhibitors

A series of cis-restricted 2-alkylthio-4-(2,3,4-trimethoxyphenyl)-5-aryl-thiazole analogues of combretastatin A-4 were synthesized and investigated for inhibition of cell proliferation against three cancer cell lines, HT-29, MCF-7, and AGS, and a normal mouse fibroblastic cell line, NIH-3T3, using an MTT assay. The biological study showed that 2-(methylthio) substituted compounds showed little cytotoxic activity against the four cell lines. In contrast, the presence of the 2-(benzylthio) group on the thiazole ring resulted in a significant improvement in cytotoxic activity relative to the 2-(methylthio) substituted derivatives. Furthermore, the inhibition of tubulin polymerization by some potent compounds was evaluated. All the compounds studied were moderate tubulin polymerization inhibitors. The flow cytometry analysis confirmed that the synthesized compounds led to cell cycle arrest at the G₂/M phase. Docking simulation was performed to insert these compounds into the crystal structure of tubulin at the colchicine binding site to determine a probable binding model.     

Novel sulfonate analogues of combretastatin A-4: potent antimitotic agents

Sulfonate analogues of combretastatin A-4 have been prepared. These compounds compete with colchicine and combretastatin A-4 for the colchicine binding site on tubulin and are potent inhibitors of tubulin polymerization and cell proliferation. Importantly, these compounds also inhibit the proliferation of P-glycoprotein positive (+) cancer cells, which are resistant to many other antitumor agents.     

Isocombretastatins A: 1,1-Diarylethenes as potent inhibitors of tubulin polymerization and cytotoxic compounds

Isocombretastatins A are 1,1-diarylethene isomers of combretastatins A. We have synthesized the isomers of combretastatin A-4, deoxycombretastatin A-4, 3-amino-deoxycombretastatin A-4 (AVE-8063), naphthylcombretastatin and the N-methyl- and N-ethyl-5-indolyl analogues of combretastatin A-4. Analogues with a 2,3,4-trimethoxyphenyl ring instead of the 3,4,5-trimethoxyphenyl ring have also been prepared. The isocombretastatins A strongly inhibit tubulin polymerization and are potent cytotoxic compounds, some of them with IC₅₀s in the nanomolar range. This new family of tubulin inhibitors shows higher or comparable potency when compared to phenstatin or combretastatin analogues. These results suggest that one carbon bridges with a geminal diaryl substitution can successfully replace the two carbon bridge of combretastatins and that the carbonyl group of phenstatins is not essential for high potency.     

Interaction of 4-arylcoumarin analogues of combretastatins with microtubule network of HBL100 cells and binding to tubulin

The synthesis of different 4-arylcoumarin analogues of combretastatin A-4 led to the identification of two new compounds (1 and 2) with potent cytotoxic activity on a CEM leukemia cell line and a third one completely inactive (compound 3). It was suggested that the cytotoxicity of compounds 1 and 2 may be related to their interaction with microtubules and tubulin, since these compounds inhibit microtubule formation from purified tubulin in vitro [Bailly et al. (2003) J. Med. Chem. 46 (25), 5437-5444]. In the present study, tubulin was identified as the main target of these molecules. We studied structure-activity relationships of these compounds using biological experiments specific for tubulin binding. The modification of cell cycle progression induced by compounds 1 and 2 was characterized by an apoptotic induction on human breast cells (HBL100). In addition, these two molecules disturbed cell survival by depolymerizing the microtubule network, leading to a mitotic block. We then determined the thermodynamic parameters of their interaction with purified tubulin by fluorescence spectroscopy and isothermal microcalorimetry. These results, together with a superimposition of the molecule on colchicine in the X-ray-determined three-dimensional structure model of tubulin-colchicine complex, allowed us to identify the pharmacophore of the combretastatin A-4 analogues responsible for their biological activity.     

A boronic acid chalcone analog of combretastatin A-4 as a potent anti-proliferation agent

Chalcones represent a class of natural products that inhibits tubulin assembly. In this study we designed and synthesized boronic acid analogs of chalcones in an effort to compare biological activities with combretastatin A-4, a potent inhibitor of tubulin polymerization. Systematic evaluation of the positional effects of the carbonyl moiety towards inhibition of tubulin polymerization, cancer cell proliferation and angiogenesis revealed that placement of the carbonyl adjacent to the trimethoxybenzene A-ring resulted in more active compounds than when the carbonyl group was placed adjacent to the C-ring. Our study identified a boronic acid chalcone with inhibition towards 16 human cancer cell lines in the 10–200 nM range, and another three cell lines with GI₅₀-values below 10 nM. Furthermore, this drug has significant anti-angiogenesis effects demonstrated by HUVEC tube formation and aortic ring assay.     

Isoxazole-type derivatives related to combretastatin A-4, synthesis and biological evaluation

Novel combretastatin analogues bearing various five-membered heterocycles with consecutive oxygen and nitrogen atoms, in place of the olefinic bridge of CA4, have been synthesized (isoxazole, isoxazoline, oxadiazole, etc). These compounds have been evaluated for cytotoxicity and their ability to inhibit the tubulin assembly. On the basis of the relative position of the aromatic A- and B-rings on the heterocyclic moiety, they could be split in two classes, the alpha,gamma- or alpha,beta-diaryl heterocyclic derivatives. In the first series, the 3,5-diaryloxadiazole 9a displayed comparable antitubulin activity to that of CA4, but was devoid of cytotoxic effects. Among the alpha,beta-diaryl heterocyclic derivatives, the 4,5-diarylisoxazole 35 exhibited greater antitubulin activity than that of CA4 (0.75 vs 1.2 microM), but modest antiproliferative activity. These data showed that minor alteration in the chemical structure of the heterocyclic ring and its relative orientation with regard to the two phenyl rings of CA4 could dramatically influence the tubulin binding properties.     

Chloroalkyl piperazine and nitrogen mustard porphyrins: synthesis and anticancer activity

Fifteen new chloroalkyl piperazine and nitrogen mustard porphyrins have been synthesized by the direct condensation of chloroalkyl piperazine, nitrogen mustard benzaldehyde, and pyrrole. Each porphyrin bears 1-4 chloroalkyl piperazine or nitrogen mustard moieties, which have been used as drugs. The Lindsey method was modified to synthesize chloroalkyl piperazine and nitrogen mustard porphyrins. To successfully synthesize chloroalkyl piperazine and nitrogen mustard porphyrins, catalyst acidity was proved to be the key factor, while the ratio of pyrrole to aldehyde had great influence on product yield. The synthetic chloroalkyl piperazine and nitrogen mustard porphyrins were characterized by elementary analysis, MS, (1)H NMR, IR, and UV-vis. Their anticancer activity to bel-7404 liver cancer cells was tested by the MTT assay. Most of the synthetic porphyrins had good anticancer activity toward bel-7404 liver cancer cells in the absence of light. These compounds might be potential anticancer medicines.     

Synthesis and biological evaluation of 1,2,3-triazole linked aminocombretastatin conjugates as mitochondrial mediated apoptosis inducers

A series of 1,2,3-triazole linked aminocombretastatin conjugates were synthesized and evaluated for cytotoxicity, inhibition of tubulin polymerization and apoptosis inducing ability. Most of the conjugates exhibited significant anticancer activity against some representative human cancer cell lines and two of the conjugates 6d and 7c displayed potent cytotoxicity with IC₅₀ values of 53 nM and 44 nM against A549 human lung cancer respectively, and were comparable to combretastatin A-4 (CA-4). SAR studies revealed that 1-benzyl substituted triazole moiety with an amide linkage at 3-position of B-ring of the combretastatin subunit are more active compared to 2-position. G2/M cell cycle arrest was induced by these conjugates 6d and 7c and the tubulin polymerization assay (IC₅₀ of 1.16 μM and 0.95 μM for 6d and 7c, respectively) as well as immunofluorescence analysis showed that these conjugates effectively inhibit microtubule assembly at both molecular and cellular levels in A549 cells. Colchicine competitive binding assay suggested that these conjugates bind at the colchicine binding site of tubulin as also observed from the docking studies. Further, mitochondrial membrane potential, ROS generation, caspase-3 activation assay, Hoechst staining and DNA fragmentation analysis revealed that these conjugates induce cell death by apoptosis.     

Design,synthesis and biological studies of novel tubulin inhibitors

A series of compounds originally derived from the vascular endothelial growth factor receptor tyrosine kinase inhibitor, SU5416, were synthesized and evaluated. The most potent compound in this series, compound 3, which structurally resembles the potent anti-microtubule agent combretastatin A-4, inhibited tubulin polymerization and showed potent growth inhibitory activities on both prostate and breast cancer lines with IC₅₀ values in the low nanomolar range.     

A 2-step synthesis of Combretastatin A-4 and derivatives as potent tubulin assembly inhibitors

A series of combretastatin derivatives were designed and synthesised by a two-step stereoselective synthesis by use of Wittig olefination followed by Suzuki cross-coupling. Interestingly, all new compounds (2a-2i) showed potent cell-based antiproliferative activities in nanomolar concentrations. Among the compounds, 2a, 2b and 2e were the most active across three cancer cell lines. In addition, these compounds inhibited the polymerisation of tubulin in vitro more efficiently than CA-4. They caused cell cycle arrest in G₂/M phase further confirming their ability to inhibit tubulin polymerisation.     

Synthesis and biological evaluation of pyrimidine bridged combretastatin derivatives as potential anticancer agents and mechanistic studies

A number of pyrimidine bridged combretastatin derivatives were designed, synthesized and evaluated for anticancer activities against breast cancer (MCF-7) and lung cancer (A549) cell lines using MTT assays. Most of the synthesized compounds displayed good anticancer activity with IC₅₀ values in low micro-molar range. Compounds 4a and 4p were found most potent in the series with IC₅₀ values of 4.67 µM & 3.38 µM and 4.63 µM & 3.71 µM against MCF7 and A549 cancer cell lines, respectively. Biological evaluation of these compounds showed that selective cancer cell toxicity (in vitro using human lung and breast cancer cell lines) might be due to the inhibition of antioxidant enzymes instigating elevated ROS levels which triggers intrinsic apoptotic pathways. These compounds were found nontoxic to the normal human primary cells. Compound 4a, was found to be competitive inhibitor of colchicine and in the tubulin binding assay it showed tubulin polymerization inhibition potential comparable to colchicine. The molecular modeling studies also showed that the synthesized compounds fit well in the colchicine-binding pocket.     

Synthesis and antitumor activity of benzils related to combretastatin A-4

A series of benzil derivatives related to combretastatin A-4 (CA-4) have been synthesized by oxidation of diarylalkynes promoted by PdI(2) in DMSO. Using this new protocol, 14 benzils were prepared in good to excellent yields and their biological activity has been delineated. Several benzils exhibited excellent antiproliferative activity: for example, 4j and 4k bearing the greatest resemblance to CA-4 and AVE-8062, respectively, were found to inhibit cell growth at the nanomolar level (20-50nM) on four human tumor cell lines. Flow cytometric analysis indicates that these compounds act as antimitotics and arrest the cell cycle in G(2)/M phase. A cell-based assay indicated that compounds 4j and 4k displayed a similar inhibition of tubulin assembly with an IC(50) value similar to CA-4. These results clearly demonstrated that the Z-double bond of CA-4 can be replaced by a 1,2-diketone unit without significant loss of cytotoxicity and inhibition of tubulin assembly potency.     

Kinetic Data on the Inhibition of High-affinity Choline Transport into Rat Forebrain Synaptosomes by Choline-like Compounds and Nitrogen Mustard Analogues

Abstract: A series of choline analogues and nitrogen mustard derivatives were evaluated as inhibitors of high-affinity transport of choline in rat forebrain synaptosomes. When synaptosomes were preincubated for 10 min with choline mustard aziridinium ion, monoethylcholine and monoethylcholine mustard aziridinium ion, the agents appeared to be equipotent as inhibitors of high-affinity uptake (K_i=2.63, 3.15 and 2.72 μm , respectively). Acetylcholine mustard aziridinium ion was less potent than these compounds (K_i= 27.8 μm ), but it was more potent than ethoxycholine and ethoxycholine mustard aziridinium ion (K_i= 500 and 403 μm ) as a blocker of choline transport. From study with these compounds it was concluded that the high-affinity choline transport mechanism shows specificity for hydroxylated compounds over those in which the same hydroxyl has been acetylated (10-fold) and that the carbonyl oxygen of the acetylated analogues is important, as its removal (to form the ethylether derivative) decreased affinity another 20-fold. The presence of an aziridinium ring on the quaternary nitrogen in place of two methyl groups did not affect the blocking of transport at 10 min of inhibitor preincubation and replacement of a methyl group on the nitrogen by an ethyl group did not alter affinity for the high-affinity carrier. The aziridinium ring on the nitrogen of the mustard analogues was important, however, in determining the extent of reversibility of the binding of these agents to the carrier protein. Choline transport was not restored by washing synaptosomes that were incubated with choline mustard aziridinium ion or monoethylcholine mustard aziridinium ion, but was readily obtained in washed synaptosomes preincubated with monoethylcholine, hemicholinium-3, or pyrrolcholine. The results indicate that the mustard analogues may be potent alkylators of the high-affinity choline carrier and thus, useful agents in monitoring acetylcholine turnover in systems where the carrier is blocked.     

Design, synthesis, and biological evaluation of combretastatin nitrogen-containing derivatives as inhibitors of tubulin assembly and vascular disrupting agents

A series of analogs with nitro or serinamide substituents at the C-2'-, C-5'-, or C-6'-position of the combretastatin A-4 (CA4) B-ring was synthesized and evaluated for cytotoxic effects against heart endothelioma cells, blood flow reduction to tumors in SCID mice, and as inhibitors of tubulin polymerization. The synthesis of these analogs typically featured a Wittig reaction between a suitably functionalized arylaldehyde and an arylphosphonium salt followed by separation of the resultant E- and Z-isomers. Several of these nitrogen-modified CA4 derivatives (both amino and nitro) demonstrate significant inhibition of tubulin assembly as well as cytotoxicity and in vivo blood flow reduction. 2'-Aminostilbenoid 7 and 2'-amino-3'-hydroxystilbenoid 29 proved to be the most active in this series. Both compounds, 7 and 29, have the potential for further pro-drug modification and development as vascular disrupting agents for treatment of solid tumor cancers and certain ophthalmological diseases.