Structural requirements for the interaction of combretastatins with tubulin: how important is the trimethoxy unit?

Design, synthesis, docking studies and biological screening of 2-pyrimidinyl-2, 3-dihydro-1H-naphtho [1, 2-e][1, 3] oxazines as potent tubulin polymerization inhibitors

A series of novel substituted 2-pyrimidinyl-2,3-dihydro-1H-naphtho[1,2-e][1, 3]oxazine analogs have been designed and synthesized based on structure-activity relationships from 2-naphthol, substituted pyrimidinyl amines and formalin through ring closure by one-pot three component reaction. These derivatives were evaluated for their in vitro cytotoxicity, cell cycle assay and their inhibitory effect on tubulin polymerization. From the MTT assay, it is clear that most of the synthesized compounds displayed potent cytotoxic activities on HeLa (cervical cancer) and B16F10 (melanoma) cancerous cell lines. The compounds 6b and 6k were found to be more effective against HeLa cell lines and exhibited significant cytotoxicity (with IC50 values 1.26 ± 0.12 µM and 1.16 ± 0.27 µM respectively), accumulation of HeLa cells in G2/M phase and exhibiting induced apoptosis. The immunohistochemistry and fluorescence assays showed that these compounds 6b and 6k inhibited the microtubule assembly in human cervical cancer cells (HeLa) at 2 µM concentration. Furthermore, molecular docking studies of these molecules revealed their better-fit potential as anticancer molecules and have a high affinity for colchicine binding site, indicating more inhibitory potential at the cellular level. Our studies suggest that the newly synthesized compounds may become promising leads for the development of new anti-cancer agents.Communicated by Ramaswamy H. Sarma.

CuI-mediated synthesis of 1-aryl-5,6,7-trimethoxybenzimidazoles as potent antitubulin agents

In situ CuI-mediated cyclization methodology helped yield benzimidazoles with different substitution manner, such as 1,2-diarylbenzimidazoles (4 and 5) and 1-arylbenzimidazoles (6-15). The result of structure-activity relationship (SAR) study confirmed the significance of the 5,6,7-trimethoxybenzimidazole moiety, and the representative derivatives (8-10) exhibited marked antiproliferative activity against A549, HCT-116, and PC-3 cells; in addition, they are able to inhibit the polymerization of tubulin. Among them, compound 10 inhibited the growth of A549, HCT-116, and PC-3 cells with a mean IC₅₀ value of 0.07 μM, and its IC₅₀ value of tubulin polymerization is 0.26 μM.

Synthesis and Biological Evaluation of Highly Active 7-Anilino Triazolopyrimidines as Potent Antimicrotubule Agents

Two different series of fifty-two compounds, based on 3′,4′,5′-trimethoxyaniline (7a–ad) and variably substituted anilines (8a–v) at the 7-position of the 2-substituted-[1,2,4]triazolo [1,5-a]pyrimidine nucleus, had moderate to potent antiproliferative activity against A549, MDA-MB-231, HeLa, HT-29 and Jurkat cancer cell lines. All derivatives with a common 3-phenylpropylamino moiety at the 2-position of the triazolopyrimidine scaffold and different halogen-substituted anilines at its 7-position, corresponding to 4′-fluoroaniline (8q), 4′-fluoro-3′-chloroaniline (8r), 4′-chloroaniline (8s) and 4′-bromoaniline (8u), displayed the greatest antiproliferative activity with mean IC50′s of 83, 101, 91 and 83 nM, respectively. These four compounds inhibited tubulin polymerization about 2-fold more potently than combretastatin A-4 (CA-4), and their activities as inhibitors of [3H]colchicine binding to tubulin were similar to that of CA-4. These data underlined that the 3′,4′,5′-trimethoxyanilino moiety at the 7-position of the [1,2,4]triazolo [1,5-a]pyrimidine system, which characterized compounds 7a–ad, was not essential for maintaining potent antiproliferative and antitubulin activities. Compounds 8q and 8r had high selectivity against cancer cells, and their interaction with tubulin led to the accumulation of HeLa cells in the G2/M phase of the cell cycle and to apoptotic cell death through the mitochondrial pathway. Finally, compound 8q significantly inhibited HeLa cell growth in zebrafish embryos.

A Review on Mechanistic Insight of Plant Derived Anticancer Bioactive Phytocompounds and Their Structure Activity Relationship

Cancer is a disorder that rigorously affects the human population worldwide. There is a steady demand for new remedies to both treat and prevent this life-threatening sickness due to toxicities, drug resistance and therapeutic failures in current conventional therapies. Researchers around the world are drawing their attention towards compounds of natural origin. For decades, human beings have been using the flora of the world as a source of cancer chemotherapeutic agents. Currently, clinically approved anticancer compounds are vincristine, vinblastine, taxanes, and podophyllotoxin, all of which come from natural sources. With the triumph of these compounds that have been developed into staple drug products for most cancer therapies, new technologies are now appearing to search for novel biomolecules with anticancer activities. Ellipticine, camptothecin, combretastatin, curcumin, homoharringtonine and others are plant derived bioactive phytocompounds with potential anticancer properties. Researchers have improved the field further through the use of advanced analytical chemistry and computational tools of analysis. The investigation of new strategies for administration such as nanotechnology may enable the development of the phytocompounds as drug products. These technologies have enhanced the anticancer potential of plant-derived drugs with the aim of site-directed drug delivery, enhanced bioavailability, and reduced toxicity. This review discusses mechanistic insights into anticancer compounds of natural origins and their structural activity relationships that make them targets for anticancer treatments.

Synthesis and characterization of novel combretastatin analogues of 1,1-diaryl vinyl sulfones, with antiproliferative potential via in-silico and in-vitro studies

Novel 1,1-diaryl vinyl-sulfones analogues of combretastatin CA-4 were synthesized via Suzuki-Miyaura coupling method and screened for in-vitro antiproliferative activity against four human cancer cell lines: MDA-MB 231(breast cancer), HeLa (cervical cancer), A549 (lung cancer), and IMR-32 (neuroblast cancer), along with a normal cell line HEK-293 (human embryonic kidney cell) by employing 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. The compounds synthesised had better cytotoxicity against the A549 and IMR-32 cell lines compared to HeLa and MDA-MB-231 cell lines. The synthesized compounds also showed significant activity on MDA-MB-231 cancer cell line with IC₅₀ of 9.85-23.94 µM, and on HeLa cancer cell line with IC₅₀ of 8.39-11.70 µM relative to doxorubicin having IC₅₀ values 0.89 and 1.68 µM respectively for MDA-MB-231 and HeLa cell lines. All the synthesized compounds were not toxic to the growth of normal cells, HEK-293. They appear to have a higher binding affinity for the target protein, tubulin, PDB ID = 5LYJ (beta chain), relative to the reference compounds, CA4 (- 7.1 kcal/mol) and doxorubicin (- 7.2 kcal/mol) except for 4E, 4M, 4N and 4O. The high binding affinity for beta-tubulin did not translate into enhanced cytotoxicity but the compounds (4G, 4I, 4J, 4M, 4N, and 4R, all having halogen substituents) that have a higher cell permeability (as predicted in-silico) demonstrated an optimum cytotoxicity against the tested cell lines in an almost uniform manner for all tested cell lines. The in-silico study provided insight into the role that cell permeability plays in enhancing the cytotoxicity of this class of compounds and as potential antiproliferative agents.

A facile synthesis of diaryl pyrroles led to the discovery of potent colchicine site antimitotic agents

Three different series of cis-restricted analogues of combretastatin A-4 (CA-4), corresponding to thirty-nine molecules that contained a pyrrole nucleus interposed between the two aryl rings, were prepared by a palladium-mediated coupling approach and evaluated for their antiproliferative activity against six human cancer cell lines. In the two series of 1,2-diaryl pyrrole derivatives, results suggested that the presence of the 3',4',5'-trimethoxyphenyl moiety at the N-1 position of the pyrrole ring was more favorable for antiproliferative activity. In the series of 3,4-diarylpyrrole analogues, three compounds (11i-k) exhibited maximal antiproliferative activity, showing excellent antiproliferative activity against the CA-4 resistant HT-29 cells. Inhibition of tubulin polymerization of selected 1,2 pyrrole derivatives (9a, 9c, 9o and 10a) was similar to that observed with CA-4, while the isomeric 3,4-pyrrole analogues 11i-k were generally from 1.5- to 2-fold more active than CA-4. Compounds 11j and 11k were the only compounds that showed activity as inhibitors of colchicine binding comparable to that CA-4. Compound 11j had biological properties consistent with its intracellular target being tubulin. This compound was able to block the cell cycle in metaphase and to induce significant apoptosis at a concentration of 25 nM, following the mitochondrial pathway, with low toxicity for normal cells. More importantly, compound 11j exerted activity in vivo superior to that of CA-4P, being able to significantly reduce tumor growth in a syngeneic murine tumor model even at the lower dose tested (5.0 mg/kg).

Computational modeling and target synthesis of monomethoxy-substituted o-diphenylisoxazoles with unexpectedly high antimitotic microtubule destabilizing activity

The ability of monomethoxy-substituted o-diphenylisoxazoles 2a-d to interact with the colchicine site of tubulin was predicted using computational modeling, docking studies, and calculation of binding affinity. The respective molecules were synthesized in high yields by three steps reaction using easily available benzaldehydes, acetophenones, and arylnitromethanes as starting material. The calculated antitubulin effect was confirmed in vivo in a sea urchin embryo model. Compounds 2a and 2c showed high antimitotic microtubule destabilizing activity compared to that of CA4. Isoxazole 2a also exhibited significant cytotoxicity against human cancer cells in NCI60 screen. For the first time, isoxazole-linked CA4 derivatives 2a and 2c with only one methoxy substituent were identified as potent antimitotic microtubule destabilizing agents. These molecules could be considered as promising structures for further optimization.

Design, synthesis and screening of benzimidazole containing compounds with methoxylated aryl radicals as cytotoxic molecules on (HCT-116) colon cancer cells

A novel series of benzimidazole derivatives with methoxylated aryl groups was designed and synthesized as molecules with potential cytotoxic activity. In vitro cytotoxic activity over HCT-116 cells showed that N-(benzimidazothiazolone)acetamides 11a, 11b and 11c were found to be the most cytotoxic compounds compared camptothecin (CPT). The tested compounds had a dual topoisomerase I-β (Topo I-β) and tubulin inhibiting activities when compared to CPT and Podophyllotoxin (Podo) where, compounds l0a, l0b, 11a and 11b exhibited a potent inhibitory activity on Topo I-β enzyme in nano-molar concentration, on the other hand, compounds 12b and 13b exhibited the best inhibitory activity β-tubulin polymerization. Results of the cell cycle analysis as well as the results of annexin-V on HCT-116 cells showed that benzimidazothiazoles 12b and 13b had a pro-apoptotic activity higher than CPT by 1.33- and 1.30-folds, respectively. Moreover, the concentration of p53, Bax/Bcl-2 ratio and caspase 3/7 increased in compounds l0b, 11b, l2b, 13b, especially, compounds 11b and 13b exhibited an increased level of these mediators than CPT. Finally, compound 11b regulated the radiosensitizing activity of the HCT-116 cells by modulating the chromosomal instability.

More Than Resveratrol: New Insights into Stilbene-Based Compounds

The concept of a scaffold concerns many aspects at different steps on the drug development path. In medicinal chemistry, the choice of relevant "drug-likeness" scaffold is a starting point for the design of the structure dedicated to specific molecular targets. For many years, the chemical uniqueness of the stilbene structure has inspired scientists from different fields such as chemistry, biology, pharmacy, and medicine. In this review, we present the outstanding potential of the stilbene-based derivatives. Naturally occurring stilbenes, together with powerful synthetic chemistry possibilities, may offer an excellent approach for discovering new structures and identifying their therapeutic targets. With the development of scientific tools, sophisticated equipment, and a better understanding of the disease pathogenesis at the molecular level, the stilbene scaffold has moved innovation in science. This paper mainly focuses on the stilbene-based compounds beyond resveratrol, which are particularly attractive due to their biological activity. Given the "fresh outlook" about different stilbene-based compounds starting from stilbenoids with particular regard to isorhapontigenin and methoxy- and hydroxyl- analogues, the update about the combretastatins, and the very often overlooked and underestimated benzanilide analogues, we present a new story about this remarkable structure.

More Than Resveratrol: New Insights into Stilbene-Based Compounds

The concept of a scaffold concerns many aspects at different steps on the drug development path. In medicinal chemistry, the choice of relevant "drug-likeness" scaffold is a starting point for the design of the structure dedicated to specific molecular targets. For many years, the chemical uniqueness of the stilbene structure has inspired scientists from different fields such as chemistry, biology, pharmacy, and medicine. In this review, we present the outstanding potential of the stilbene-based derivatives. Naturally occurring stilbenes, together with powerful synthetic chemistry possibilities, may offer an excellent approach for discovering new structures and identifying their therapeutic targets. With the development of scientific tools, sophisticated equipment, and a better understanding of the disease pathogenesis at the molecular level, the stilbene scaffold has moved innovation in science. This paper mainly focuses on the stilbene-based compounds beyond resveratrol, which are particularly attractive due to their biological activity. Given the "fresh outlook" about different stilbene-based compounds starting from stilbenoids with particular regard to isorhapontigenin and methoxy- and hydroxyl- analogues, the update about the combretastatins, and the very often overlooked and underestimated benzanilide analogues, we present a new story about this remarkable structure.

Synthesis and Cytotoxicity Studies of Stilbene Long-Chain Fatty Acid Conjugates

A series of 16 conjugates of the tubulin polymerization inhibitor combretastatin A4 (CA-4) and other functionally related stilbene with four 18-carbon fatty acids, namely, stearic, oleic, linoleic, and linolenic acids, have been synthesized in good yields. These new derivatives have been evaluated against the KB-3-1 (human epidermoid carcinoma), NCI-H460 (human lung cancer), HEK293 (human embryonic kidney), and MCF-7 (human breast adenocarcinoma) cell lines for antiproliferative activity, with the exhibited cytotoxic activities comparable with those of CA-4 and colchicine. Compounds 22 and 23, CA-4 conjugates of linoleic and linolenic acids, respectively, were determined to have exhibited the most active in vitro assays, with compound 23 exhibiting very similar activity to the parent compound against the NCI-H460 cell line. Our studies further delineated the structurally required Z-geometry of the stilbene moiety and that conjugation of the less active E-stilbenes with the most active fatty acid had minimal or no improvement in their respective activities.

Tubulin inhibitors: Discovery of a new scaffold targeting extra-binding residues within the colchicine site through anchoring substituents properly adapted to their pocket by a semi-flexible linker

Bis-hydrazides 13a-h were designed and synthesized as potential tubulin inhibitors selectively targeting the colchicine site between α- and β-tubulin subunits. The newly designed ring-B substituents were assisted at their ends by 'anchor groups' which are expected to exert binding interaction(s) with new additional amino acid residues in the colchicine site (beyond those amino acids previously reported to interact with reference inhibitors as CA-4 and colchicine). Conformational flexibility of bis-hydrazide linker assisted these 'extra-binding' properties through reliving ligands' strains in the final ligand-receptor complexes. Compound 13f displayed the most promising computational and biological study results in the series: MM/GBSA binding energy of -62.362 kcal/mol (extra-binding to Arg α:221, Thr β:353 & Lys β:254); 34% NCI-H522 cells' death (at 10 µM), IC₅₀ = 0.073 µM (MTT assay); significant cell cycle arrest at G2/M phase; 11.6% preG1 apoptosis induction and 83.1% in vitro tubulin inhibition (at concentration = IC₅₀). Future researchers in bis-hydrazide tubulin inhibitors are advised to consider the 2-chloro-N-(4-substituted-phenyl)acetamide derivatives as compound 13f due to extra-binding properties of their ring B.

β-Lactams with antiproliferative and antiapoptotic activity in breast and chemoresistant colon cancer cells

A series of novel 1,4-diaryl-2-azetidinone analogues of combretastatin A-4 (CA-4) have been designed, synthesised and evaluated in vitro for antiproliferative activity, antiapoptotic activity and inhibition of tubulin polymerisation. Glucuronidation of CA-4 by uridine 5-diphosphoglucuronosyl transferase enzymes (UGTs) has been identified as a mechanism of resistance in cancer cells. Potential sites of ring B glucuronate conjugation are removed by replacing the B ring meta-hydroxy substituent of selected series of β-lactams with alternative substituents e.g. F, Cl, Br, I, CH₃. The 3-phenyl-β-lactam 11 and 3-hydroxy-β-lactam 46 demonstrate improved activity over CA-4 in CA-4 resistant HT-29 colon cancer cells (IC₅₀ = 9 nM and 3 nM respectively compared with IC₅₀ = 4.16 μM for CA-4), while retaining potency in MCF-7 breast cancer cells (IC₅₀ = 17 nM and 22 nM respectively compared with IC₅₀ = for 4 nM for CA-4). Compound 46 binds at the colchicine site of tubulin, and strongly inhibits tubulin assembly at micromolar concentrations comparable to CA-4. In addition, compound 46 induced mitotic arrest at low concentration in both cell lines MCF-7 and HT-29 together with downregulation of expression of antiapoptotic proteins Mcl-1, Bcl-2 and survivin in MCF-7 cells. These novel antiproliferative and antiapoptotic β-lactams are potentially useful scaffolds in the development of tubulin-targeting agents for the treatment of breast cancers and chemoresistant colon cancers.

Design, Synthesis and Biological Activities of New Pyrazole Derivatives Possessing Both Coxib and Combretastatins Pharmacophores

In our efforts to discover novel multi-target agents having better antitumor activities than celecoxib, 21 new aryl-substituted pyrazole derivatives possessing cis-diphenylethylene scaffold were mostly synthesized by a one-pot approach to ethyl 1,4,5-triaryl-1H-pyrazole-3-carboxylates via an improved Claisen condensation - Knorr reaction sequence. The cytotoxic effects of these compounds against three human cancer cell lines HT-29, Hep-G2, MCF-7 as well as their inhibition of NO production were studied. Results showed that incorporation of the important pharmacophoric groups of two original molecules celecoxib and combretastatin A-4 in a single molecule plays an important role in determining a better biological activities of the new coxib-hybrided compounds.

Anti-tumor Properties of Stilbene-based Resveratrol Analogues: Recent Results

Recent literature about stilbene-based analogues of resveratrol (1) has been reviewed, and a total of 94 compounds are reported (see structures 4 – 97), selected either for their promising anti-tumor properties or as comparative terms in SAR studies. As a general outline, these recent literature data confirm the previously reported observation that minimal modification in the nature and position of the substituents on the stilbene nucleus may cause large variations in their biological activity and, more specifically, in their anti-tumor properties. Among the polyhydroxylated stilbenes, it has been established that those with either a catechol or pyrogallol moiety are far better radical scavengers than either 1 or other analogues lacking an ortho-dihydroxy group, and this property was shown to be related to pro-apoptotic activity. In the large majority of cases where couples of E- and Z-isomers were evaluated for either cytotoxic or pro-apoptotic activity, the Z-isomers were significantly more active than their E analogues; nevertheless, a general rule stating that stilbenoids with Z configuration of the double bond display a considerably higher antiproliferative activity than their E-isomers cannot be considered as established. A variety of methoxystilbenes has been reported recently: in many cases these analogues showed either potent antiproliferative and pro-apoptotic activity or strong inhibition of TNFα-induced activation of NF- kB. Globally considered, polymethoxystilbenes are a sub-group of great interest among the resveratrol analogues: these analogues appear worthy of a deeper evaluation also in connection with their potential anti-angiogenic properties. In addition, in vivo studies indicate that methoxystilbenes undergo different metabolic conversion and have a higher bioavailability than resveratrol. The potent activity of some amino- and halogenated stilbenes is undoubtedly worthy of attention, but the toxicity of these compounds to normal cells has rarely been evaluated. In conclusion, the synthesis and evaluation of stilbene-based resveratrol analogues proved to be a highly active field of research and has recently afforded compounds with either cytotoxic or pro-apoptotic activity in the nanomolar range. Nevertheless, the exact structural determinants to optimize the anti-tumor properties of these compounds and details of their mechanism of action remain to be clarified.

Sea Urchin Embryo Model As a Reliable in Vivo Phenotypic Screen to Characterize Selective Antimitotic Molecules. Comparative evaluation of Combretapyrazoles, -isoxazoles, -1,2,3-triazoles, and -pyrroles as Tubulin-Binding Agents

A series of both novel and reported combretastatin analogues, including diarylpyrazoles, -isoxazoles, -1,2,3-triazoles, and -pyrroles, were synthesized via improved protocols to evaluate their antimitotic antitubulin activity using in vivo sea urchin embryo assay and a panel of human cancer cells. A systematic comparative structure-activity relationship studies of these compounds were conducted. Pyrazoles 1i and 1p, isoxazole 3a, and triazole 7b were found to be the most potent antimitotics across all tested compounds causing cleavage alteration of the sea urchin embryo at 1, 0.25, 1, and 0.5 nM, respectively. These agents exhibited comparable cytotoxicity against human cancer cells. Structure-activity relationship studies revealed that compounds substituted with 3,4,5-trimethoxyphenyl ring A and 4-methoxyphenyl ring B displayed the highest activity. 3-Hydroxy group in the ring B was essential for the antiproliferative activity in the diarylisoxazole series, whereas it was not required for potency of diarylpyrazoles. Isoxazoles 3 with 3,4,5-trimethoxy-substituted ring A and 3-hydroxy-4-methoxy-substituted ring B were more active than the respective pyrazoles 1. Of the azoles substituted with the same set of other aryl pharmacophores, diarylpyrazoles 1, 4,5-diarylisoxazoles 3, and 4,5-diaryl-1,2,3-triazoles 7 displayed similar strongest antimitotic antitubulin effect followed by 3,4-diarylisoxazoles 5, 1,5-diaryl-1,2,3-triazoles 8, and pyrroles 10 that showed the lowest activity. Introduction of the amino group into the heterocyclic core decreased the antimitotic antitubulin effect of pyrazoles, triazoles, and to a lesser degree of 4,5-diarylisoxazoles, whereas potency of the respective 3,4-diarylisoxazoles was increased.

Influence of charge transfer on the isomerisation of stilbene derivatives for application in cancer therapy

The photoisomerisation of non-toxic trans-combretastatin CA4 to its cytotoxic cis isomer demonstrates the high potential of this and similar compounds for localised cancer therapy. The introduction of intramolecular charge-transfer character by altering the substituents of combretastatin systems opens up possibilities to tailor these stilbene derivatives to the special demands of anticancer drugs. In this TDDFT study we explore how absorption wavelengths for both the trans and cis isomers can be red shifted to enable deeper light penetration into tissue and how the trans → cis and cis → trans isomerisations are affected by charge transfer effects to different degrees.

Evaluation of Sydnone-Based Analogues of Combretastatin A-4 Phosphate (CA4P) as Vascular Disrupting Agents for Use in Cancer Therapy

The combretastatins have attracted significant interest as small-molecule therapies for cancer due to their ability to function as vascular disrupting agents. We have successfully prepared a range of combretastatin analogues that are based on a novel sydnone heterocycle core, and their potential as tubulin binders has been assessed in vitro and in vivo. The most potent candidate was found to disrupt microtubules and affect cellular morphology at sub-micromolar levels. Moreover, it was found to bind reversibly to tubulin and significantly increase endothelial cell monolayer permeability, in a similar manner to combretastatin A4. Surprisingly, the compound did not exhibit efficacy in vivo, possibly due to rapid metabolism.

Computational Design and Synthesis of Novel Fluoro-Analogs of Combretastatins A-4 and A-1

Combretastatin A-1 (CA-1) and combretastatin A-4 (CA-4) isolated from the African bush willow Combretum caffrum are highly potent tubulin polymerization inhibitors, possessing strong antitumor activities because of their vascular disrupting properties. Extensive SAR studies have been done for CA-4 analogs. Because of poor solubility, water-soluble prodrugs of CA-4 and CA-1 have been developed, which are currently in human clinical trials. Fluorine plays an important role in the current drug discovery and development due to its unique properties. Thus, several fluorine-containing analogs of CA-4/CA-1 have been studied. However, no analogs, which have a CF₃O-, CF₂HO- or CF₃- group instead of the 4'-methoxy group in the B ring, have been investigated. Therefore, we set out to design and synthesize those novel fluoro-analogs of CA-4/CA-1. For the design of the new analogs, we took a structure-based design approach based on the X-ray crystal structure of colchicine-tubulin complex (PDB: 4O2B) and computational docking analysis using the AutoDock Vina program. A library of novel fluoro-analogs of CA-4/CA-1 was generated and their docking energy scores obtained. It was found that those novel fluoro-analogs exhibited better docking energy scores than CA-4/CA-1. Also, docking poses of all of these fluoro-analogs were virtually superimposable and very good fit to the colchine binding site. Among 15 compounds designed and analyzed, we have synthesized 5 compounds and evaluated their cytotoxicity against drug-sensitive and multidrug-resistant cancer cell lines. All fluoro-analogs exhibited strong cytotoxicity even against multidrug-resistant cell line. However, the critical activity of this class of compounds is its vascular disrupting activity. Thus, further biological evaluations are warranted for those novel fluoro-analogs of CA-4/CA-1.

Synthesis and Antiproliferative Activity of Novel Heterocyclic Indole-Trimethoxyphenyl Conjugates

The synthesis and biological evaluation of a series of novel heterocyclic indole derivatives is described. The consolidation of the combretastatin and bisindolylmaleimide templates towards the inclusion of a novel heterocyclic ring proffered a versatile pharmacophore with which to pursue chemical diversification. Given literature precedent, maleimide was initially investigated in this role and the bioactivity assessed by measurement of NCI-60 cell panel growth. Subsequently, a range of 5-aminopyrazoles was designed and developed to explore the specific effect of heterocycle hydrogen bonding on cell growth. The unique electronic nature of the 5-aminopyrazole moiety allowed for regiospecific monosubstitution on different sites of the ring, such as thiourea substitution at the N(1) position for derivative 45 or trifluoroacetylation on the 5-amino position for 43. Further derivatisation led to the ultimate development of bicyclic pyrazolotriazinedione 41 and pyrimidine 42 systems. The antiproliferative activities of these 3,4-diaryl-5-aminopyrazoles were assessed using the NCI-60 cell screen, disclosing the discovery of distinct selectivity profiles towards a number of cell lines, such as SNB-75 CNS cancer, UO-31 and CAKI-1 renal cancer cells. A series of DNA topological assays discounted the interaction with topoisomerase II as a putative mechanism of action.

Development of combretastatins as potent tubulin polymerization inhibitors

The combretastatins are isolated from South African tree combretum caffrum kuntze. The lead compound combretastatin A-4 has displayed remarkable cytotoxic effect in a wide variety of preclinical tumor models and inhibits tubulin polymerization by interacting at colchicine binding site of microtubule. However, the structural simplicity of C A-4 is favorable for synthesis of various derivatives projected to induce rapid and selective vascular shutdown in tumors. Majority of the molecules have shown excellent antiproliferative activity and are able to inhibit tubulin polymerization as well as possible mechanisms of action have been investigated. In this review article, the synthesis and structure-activity relationships of C A-4 and immense number of its synthetic derivatives with various modifications on the A, B-rings, bridge carbons and their anti mitotic activities are discussed.

Tubulin-binding dibenz[c,e]oxepines: Part 2. Structural variation and biological evaluation as tumour vasculature disrupting agents

5,7-Dihydro-3,9,10,11-tetramethoxybenz[c,e]oxepin-4-ol 1, prepared from a dibenzyl ether precursor via Pd-catalysed intramolecular direct arylation, possesses broad-spectrum in vitro cytotoxicity towards various tumour cell lines, and induces vascular shutdown, necrosis and growth delay in tumour xenografts in mice at sub-toxic doses. The biological properties of 1 and related compounds can be attributed to their ability to inhibit microtubule assembly at the micromolar level, by binding reversibly to the same site of the tubulin αβ-heterodimer as colchicine 2 and the allocolchinol, N-acetylcolchinol 4.

Methylene versus carbonyl bridge in the structure of new tubulin polymerization inhibitors with tricyclic A-rings

The phenothiazine group has been identified as a suitable A ring in the structure of tubulin polymerization inhibitors. In our search to identify more potent inhibitors, a study of different isosteric tricyclic groups as new potential A rings was first realized and permitted to identify 1-azaphenothiazine and iminodibenzyl as favorable modulations providing compounds with improved activity against tubulin. An investigation of the methylene group as the connector between the A and B rings revealed that the "CH₂" bridge was tolerated, improving the biological potency when the A unit was of phenothiazine, 1-azaphenothiazine or iminodibenzyl type. Molecules 6-8 and 12 showed increased biological activity in comparison to parent phenstatin 2 on COLO 205 colon cancer cell line. The most antineoplastic agent in the current study was phenothiazine 5 displaying a GI₅₀ of 25nM against the melanoma MDA-MB-435 cell line.

Indole molecules as inhibitors of tubulin polymerization: potential new anticancer agents, an update (2013–2015)

Discovery of new indole-based tubulin polymerization inhibitors will continue to dominate the synthetic efforts of many medicinal chemists working in the field. The indole ring system is an essential part of several tubulin inhibitors identified in the recent years. The present review article will update the synthesis, anticancer and tubulin inhibition activities of several important new indole classes such as 2-phenylindoles (28, 29 & 30), oxindoles (35 & 38), indole-3-acrylamides (44), indolines (46), aroylindoles (49), carbozoles (75, 76 & 82), azacarbolines (87) and annulated indoles (100–105).

Novel iodoacetamido benzoheterocyclic derivatives with potent antileukemic activity are inhibitors of STAT5 phosphorylation

Signal Transducer and Activator of Transcription 5 (STAT5) protein, a component of the STAT family of signaling proteins, is considered to be an attractive therapeutic target because of its involvement in the progression of acute myeloid leukemia. In an effort to discover potent molecules able to inhibit the phosphorylation-activation of STAT5, twenty-two compounds were synthesized and evaluated on the basis of our knowledge of the activity of 2-(3',4',5'-trimethoxybenzoyl)-3-iodoacetamido-6-methoxy benzo[b]furan derivative 1 as a potent STAT5 inhibitor. Most of these molecules, structurally related to compound 1, were characterized by the presence of a common 3',4',5'-trimethoxybenzoyl moiety at the 2-position of different benzoheterocycles such as benzo[b]furan, benzo[b]thiophene, indole and N-methylindole. Effects on biological activity of the iodoacetamido group and of different moieties (methyl and methoxy) at the C-3 to C-7 positions were examined. In the series of benzo[b]furan derivatives, moving the iodoacetylamino group from the C-4 to the C-5 or C-6 positions did not significantly affect antiproliferative activity. Compounds 4, 15, 20 and 23 blocked STAT5 signals and induced apoptosis of K562 BCR-ABL positive cells. For compound 23, the trimethoxybenzoyl moiety at the 2-position of the benzo[b]furan core was not essential for potent inhibition of STAT5 activation.

Anticancer phototherapy using activation of E-combretastatins by two-photon-induced isomerization

The photoisomerization of relatively nontoxic E-combretastatins to clinically active Z-isomers is shown to occur in solution through both one- and two-photon excitations at 340 and 625 nm, respectively. The photoisomerization is also demonstrated to induce mammalian cell death by a two-photon absorption process at 625 nm. Unlike conventional photodynamic therapy (PDT), the mechanism of photoisomerization is oxygen-independent and active in hypoxic environments such as in tumors. The use of red or near-infrared (NIR) light for two-photon excitation allows greater tissue penetration than conventional UV one-photon excitation. The results provide a baseline for the development of a novel phototherapy that overcomes nondiscriminative systemic toxicity of Z-combretastatins and the limitations of PDT drugs that require the presence of oxygen to promote their activity, with the added benefits of two-photon red or NIR excitation for deeper tissue penetration.

Synthesis, anticancer activity and molecular docking studies on a series of heterocyclic trans-cyanocombretastatin analogues as antitubulin agents

A series of heterocyclic combretastatin analogues have been synthesized and evaluated for their anticancer activity against a panel of 60 human cancer cell lines. The most potent compounds were two 3,4,5-trimethoxy phenyl analogues containing either an (Z)-indol-2-yl (8) or (Z)-benzo[b]furan-2-yl (12) moiety; these compounds exhibited GI50 values of <10 nM against 74% and 70%, respectively, of the human cancer cell lines in the 60-cell panel. Compounds 8, and 12 and two previously reported compounds in the same structural class, i.e. 29 and 31, also showed potent anti-leukemic activity against leukemia MV4-11 cell lines with LD50 values = 44 nM, 47 nM, 18 nM, and 180 nM, respectively. From the NCI anti-cancer screening results and the data from the in vitro toxicity screening on cultured AML cells, seven compounds: 8, 12, 21, 23, 25, 29 and 31 were screened for their in vitro inhibitory activity on tubulin polymerization in MV4-11 AML cells; at 50 nM, 8 and 29 inhibited polymerization of tubulin by >50%. The binding modes of the three most active compounds (8, 12 and 29) to tubulin were also investigated utilizing molecular docking studies. All three molecules were observed to bind in the same hydrophobic pocket at the interface of α- and β-tubulin that is occupied by colchicine, and were stabilized by van der Waals' interactions with surrounding tubulin residues. The results from the tubulin polymerization and molecular docking studies indicate that compounds 8 and 29 are the most potent anti-leukemic compounds in this structural class, and are considered lead compounds for further development as anti-leukemic drugs.

Natural antitubulin agents: importance of 3,4,5-trimethoxyphenyl fragment

Microtubules are polar cytoskeletal filaments assembled from head-to-tail and comprised of lateral associations of α/β-tubulin heterodimers that play key role in various cellular processes. Because of their vital role in mitosis and various other cellular processes, microtubules have been attractive targets for several disease conditions and especially for cancer. Antitubulin is the most successful class of antimitotic agents in cancer chemotherapeutics. The target recognition of antimitotic agents as a ligand is not much explored so far. However, 3,4,5-trimethoxyphenyl fragment has been much highlighted and discussed in such type of interactions. In this review, some of the most important naturally occurring antimitotic agents and their interactions with microtubules are discussed with a special emphasis on the role of 3,4,5-trimethoxyphenyl unit. At last, some emerging naturally occurring antimitotic agents have also been tabulated.

Studies on indolizines. Evaluation of their biological properties as microtubule-interacting agents and as melanoma targeting compounds

With the aim of investigating new analogues of phenstatin with an indolizin-3-yl unit, in particular as the B-ring, three new series of compounds (6-8, 9-34 and 54) were synthesized and tested for interactions with tubulin polymerization and evaluated for cytotoxicity on an NCI-60 human cancer cell lines panel. The replacement of the 3'-hydroxy-4'-methoxyphenyl B-ring of phenstatin with substituted indolizine unit results in the conservation of both antitubulin and cytotoxic effect. Indolizines 9 and 17 were the most effective in the present study and showed the highest antiproliferative effect on melanoma cell lines MDA-MB-435 (GI50 = 30 nM) and could serve as new lead compounds for the development of anti-cancer therapeutics.

Cytotoxicity studies of novel combretastatin and pterostilbene derivatives

We synthesised seven 2-aminestilbenes with methoxy substitents in reactions of dinitrostilbenes with sodium azide. In order to study the positioning of the nitro groups, the optimum structure of obtained stilbenes using the DFT B3LYP/6-311++G(2d,p) method was calculated. Very interesting aspect of this regioselectivity reaction is the fact that in all substrates and synthetized compounds the nitro groups in position 2 were not coplanar whereas the para-nitro groups were coplanar with respect to the benzene ring. Due to unique features of stilbene derivatives, such as antitumor agents, we undertook the studies on the biological properties of new stilbene derivatives. Using five cancer cell lines, we investigated the effects of 2-aminestilbenes with methoxy substitents on cell growth.

Synthesis, antimitotic and antivascular activity of 1-(3',4',5'-trimethoxybenzoyl)-3-arylamino-5-amino-1,2,4-triazoles

A new class of compounds that incorporated the structural motif of the 1-(3',4',5'-trimethoxtbenzoyl)-3-arylamino-5-amino-1,2,4-triazole molecular skeleton was synthesized and evaluated for their antiproliferative activity in vitro, interactions with tubulin, and cell cycle effects. The most active agent, 3c, was evaluated for antitumor activity in vivo. Structure-activity relationships were elucidated with various substituents on the phenyl ring of the anilino moiety at the C-3 position of the 1,2,4-triazole ring. The best results for inhibition of cancer cell growth were obtained with the p-Me, m,p-diMe, and p-Et phenyl derivatives 3c, 3e, and 3f, respectively, and overall, these compounds were more or less as active as CA-4. Their vascular disrupting activity was evaluated in HUVEC cells, with compound 3c showing activity comparable with that of CA-4. Compound 3c almost eliminated the growth of syngeneic hepatocellular carcinoma in Balb/c mice, suggesting that 3c could be a new antimitotic agent with clinical potential.