Antineoplastic agents. 379. Synthesis of phenstatin phosphate

Development of QTMP: A promising anticancer agent through NP-Privileged Motif-Driven structural modulation

In this study of creating new molecules from clinical trial agents, an approach of Combretastatin structural modulation with the installation of NP-privileged motifs was considered, and a series of trimethoxyphenyl-2-aminoimidazole with functionalized quinolines and isoquinolines was investigated. An exciting method of quinoline C3-H iodination coupled with imidazopyridine-C3-H arylation and hydrazine-mediated fused-ring cleavage enabled synthesizing a class of compounds with two specific unsymmetric aryl substitutions. Interestingly, three compounds (6, 11, and 13) strongly inhibited HeLa cell proliferation with a half-maximal inhibitory concentration (10-46 nM). Among the compounds, compound 6 (QTMP) showed stronger antiproliferative ability than CA-4 (a clinical trial agent) in various cancer cell lines, including cervical, lung, breast, highly metastatic breast, and melanoma cells. QTMP inhibited the assembly of purified tubulin, depolymerized microtubules of A549 lung carcinoma cells, produced defective spindles, and arrested the cells in the G2/M phase. Further, QTMP binds to the colchicine site in tubulin with a dissociation constant of 5.0 ± 0.6 µM. QTMP displayed higher aqueous stability than CA-4 at 37 °C. Further, in silico analysis of QTMP indicated excellent drug-like properties, including good aqueous solubility, balanced hydrophilicity-lipophilicity, and high GI-absorption ability. The results together suggest that QTMP has anticancer potential.

Design, synthesis and biological evaluation of a novel bioactive indane scaffold 2-(diphenylmethylene)c-2,3-dihydro-1H-inden-1-one with potential anticancer activity

Over the past decades, designing of privileged structures has emerged as a useful approach to the discovery and optimisation of novel biologically active molecules, and many have been successfully exploited across and within different target families. Examples include indole, quinolone, isoquinoline, benzofuran and chromone, etc. In the current study, we focus on synthesising a novel hybrid scaffold constituting naturally occurring benzophenone (14) and indanone (22) ring systems, leading to a general structure of 2-(diphenylmethylene)-2,3-dihydro-1H-inden-1-one (23). It was hypothesised this new hybrid system would provide enhanced anti-cancer activity owing to the presence of the common features associated with the tubulin binding small molecule indanocine (10) and the estrogen receptor (ER) antagonist tamoxifen (24). Key hybrid molecules were successfully synthesised and characterised, and the in vitro cytotoxicity assays were performed against cancer cell lines: MCF7 (breast) and SKBR3 (breast), DU145 (prostate) and A549 (lung). The methyl-, chloro- and methoxy-, para-substituted benzophenone hybrids displayed the greatest degree of cytotoxicity and the E-configuration derivatives 45, 47 and 49 being significantly most potent. We further verified that the second benzyl moiety of this novel hybrid scaffold is fundamental to enhance the cytotoxicity, especially in the SKBR3 (HER2+) by the E-methyl lead molecule 47, MCF7 (ER+) by 45 and 49, and A549 (NSCLC) cell lines by 49. These hybrid molecules also showed a significant accumulation of SKBR3 cells at S-phase of the cell cycle after 72 hrs, which demonstrates besides of being cytotoxic in vitro against SKBR3 cells, 47 disturbs the replication and development of this type of cancer causing a dose-dependent cell cycle arrest at S-phase. Our results suggest that DNA damage might be involved in the induction of SKBR3 cell death caused by the hybrid molecules, and therefore, this novel system may be an effective suppressor of HER2+/Neu-driven cancer growth and progression. The present study points to potential structural optimisation of the series and encourages further focussed investigation of analogues of this scaffold series toward their applications in cancer chemoprevention or chemotherapy.

Design, synthesis and structure-activity relationship studies on erianin analogues as pyruvate carboxylase inhibitors in hepatocellular carcinoma cells

A series of novel erianin analogues were designed and synthesized based on the bioisosterism principle by altering the two aromatic rings of erianin, the substituents on the rings and the linker between them. The analogues were evaluated as pyruvate carboxylase (PC) inhibitors in hepatocellular carcinoma cells. It was found that compounds 35 and 36, where fluorine replaces a hydroxyl group, exhibited higher activity than erianin (IC50 value of 17.30 nM) in liver cancer cells with IC50 values of 15.15 nM and 10.05 nM, respectively. Additionally, at a concentration of 10 nM, compounds 35 and 36 inhibited PC with inhibitory rates of 39.10% and 40.15%, respectively, exhibiting nearly identical inhibitory activity to erianin (inhibitory rate of 40.07%). Additionally, a computer simulation docking study demonstrated the basis for better interactions between the receptors and ligands. The fluorine atom of 35 can not only form hydrogen bonds with Lys-1043 (NH⋯F, 2.04 Å), but also form fluorine bonds with the carbonyl groups of Lys-1043 (3.67 Å) and Glu-1046 (3.70 Å), due to the different orientations of the halogens on the B ring warhead. Conversely, the chlorine atom of 34 can only form alkyl hydrophobic interactions with the alkane chain in Lys-1043. Fluorinated compounds 35 and 36 also show better chemical stability and higher log P (clog P = 3.89 for 35 and 36) values than that of erianin (clog P = 3.07), and may be used as candidate compounds for further drug development.

Antiproliferative and Tubulin-Destabilising Effects of 3-(Prop-1-en-2-yl)azetidin-2-Ones and Related Compounds in MCF-7 and MDA-MB-231 Breast Cancer Cells

A series of novel 3-(prop-1-en-2-yl)azetidin-2-one, 3-allylazetidin-2-one and 3-(buta-1,3-dien-1-yl)azetidin-2-one analogues of combretastatin A-4 (CA-4) were designed and synthesised as colchicine-binding site inhibitors (CBSI) in which the ethylene bridge of CA-4 was replaced with a β-lactam (2-azetidinone) scaffold. These compounds, together with related prodrugs, were evaluated for their antiproliferative activity, cell cycle effects and ability to inhibit tubulin assembly. The compounds demonstrated significant in vitro antiproliferative activities in MCF-7 breast cancer cells, particularly for compounds 9h, 9q, 9r, 10p, 10r and 11h, with IC₅₀ values in the range 10-33 nM. These compounds were also potent in the triple-negative breast cancer (TBNC) cell line MDA-MB-231, with IC₅₀ values in the range 23-33 nM, and were comparable with the activity of CA-4. The compounds inhibited the polymerisation of tubulin in vitro, with significant reduction in tubulin polymerization, and were shown to interact at the colchicine-binding site on tubulin. Flow cytometry demonstrated that compound 9q arrested MCF-7 cells in the G₂/M phase and resulted in cellular apoptosis. The antimitotic properties of 9q in MCF-7 human breast cancer cells were also evaluated, and the effect on the organization of microtubules in the cells after treatment with compound 9q was observed using confocal microscopy. The immunofluorescence results confirm that β-lactam 9q is targeting tubulin and resulted in mitotic catastrophe in MCF-7 cells. In silico molecular docking supports the hypothesis that the compounds interact with the colchicine-binding domain of tubulin. Compound 9q is a novel potent microtubule-destabilising agent with potential as a promising lead compound for the development of new antitumour agents.

Palladium-Catalyzed Carbonylative Synthesis of Diaryl Ketones from Arenes and Arylboronic Acids through C(sp2)-H Thianthrenation

The development of mild methodology for converting inert C-H bonds to value-added molecules has been an attractive research topic during the last few decades as it offers efficient preparation. Meanwhile, diaryl ketones hold potent applications in antitumor drugs, the agrochemical industry, and synthetic chemistry. Herein, we report versatile palladium-catalyzed carbonylative cross-coupling reactions of aryl thianthrenium salts with arylboronic acids. Arenes were transformed site selectively via C(sp²)-H thianthrenation, and various desired diaryl ketones were produced in good to excellent yields.

Design, synthesis, and biological evaluation of diaryl heterocyclic derivatives targeting tubulin polymerization with potent anticancer activities

A series of diaryl heterocyclic analogues were designed and synthesized as tubulin polymerization inhibitors. Among them, compound 6y showed the highest antiproliferative activity against HCT-116 colon cancer cell line with an IC₅₀ values of 2.65 μM. Compound 6y also effectively inhibited tubulin polymerization in vitro (IC₅₀ of 10.9 μM), and induced HCT-116 cell cycle arrest in G2/M phase. In addition, compound 6y exhibited high metabolic stability on human liver microsomes (T_1/2 = 106.2 min). Finally, 6y was also effective in suppressing tumor growth in a HCT-116 mouse colon model without apparent toxicity. Collectively, these results suggest that 6y represents a new class of tubulin inhibitors deserving further investigation.

Discovery of N-benzylarylamide derivatives as novel tubulin polymerization inhibitors capable of activating the Hippo pathway

Novel N-benzylarylamide saderivatives were designed and synthesized, and their antiproliferative activities were explored. Some of 51 target compounds exhibited potent inhibitory activities against MGC-803, HCT-116 and KYSE450 cells with IC₅₀ values in two-digit nanomolar. Compound I-33 (MY-875) displayed the most potent antiproliferative activities against MGC-803, HCT-116 and KYSE450 cells (IC₅₀ = 0.027, 0.055 and 0.067 μM, respectively) and possessed IC₅₀ values ranging from 0.025 to 0.094 μM against other 11 cancer cell lines. Further mechanism studies indicated that compound I-33 (MY-875) inhibited tubulin polymerization (IC₅₀ = 0.92 μM) by targeting the colchicine bingding site of tubulin. Compound I-33 (MY-875) disrupted the construction of the microtubule networks and affected the mitosis in MGC-803 and SGC-7901 cells. In addition, although it acted as a colchicine binding site inhibitor, compound I-33 (MY-875) also activated the Hippo pathway to promote the phosphorylation status of MST and LATS, resulting in the YAP degradation in MGC-803 and SGC-7901 cells. Due to the degradation of YAP, the expression levels of TAZ and Axl decreased. Because of the dual actions on colchicine binding site and Hippo pathway, compound I-33 (MY-875) dose-dependently inhibited cell colony formatting ability, arrested cells at the G2/M phase and induced cells apoptosis in MGC-803 and SGC-7901 cells. Moreover, compound I-33 (MY-875) could regulate the levels of cell cycle and apoptosis regulatory proteins in MGC-803 and SGC-7901 cells. Furthermore, molecular docking analysis suggested that the hydrogen bond and hydrophobic interactions made compound I-33 (MY-875) well bind into the colchicine binding site of tubulin. Collectively, compound I-33 (MY-875) is a novel anti-gastric cancer agent and deserves to be further investigated for cancer therapy by targeting the colchicine binding site of tubulin and activating the Hippo pathway.

Design, Synthesis, In Vitro Biological Activity Evaluation and Stabilized Nanostructured Lipid Carrier Formulation of Newly Synthesized Schiff Bases-Based TMP Moieties

A series of novel Schiff bases-based TMP moieties have been designed and synthesized as potential anticancer agents. The target Schiff bases were screened for their cytotoxic activity against the MDA-MB-231 breast cancer cell line. Most of the tested molecules revealed good cytotoxic activity, especially compounds 4h, 4j and 5d. Being the most potent, compound 4h showed good tubulin polymerization inhibition activity as revealed by immunofluorescence analysis and ELISA assay. Additionally, compound 4h was screened for cell cycle disturbance and apoptosis induction. Pre-G1 apoptosis and cell growth halt at the G2/M phase were discovered to be caused by it. Moreover, compound 4h induced apoptosis via p53 and Bax activation, as well as reduced the level of Bcl-2. Additionally, the most potent compound 4h was lodged on nanostructured lipid carriers (NLCs). 23 full factorial design was involved to govern the influence of the fabrication variables on the in vitro characters of the casted NLCs. F3 was picked as the optimum formula exhibiting dominant desirability value 0.805, EE% 95.6 ± 2.4, PS 222.4 ±18.7, PDI 0.23 ± 0.05 and ZP −39.2 ± 3.9 Mv. Furthermore, F3 affirmed improved solubility and release over the drug suspension. In the comparative cytotoxic activity, F3 was capable of diminishing the IC50 by around 2.15 times for pure 4h, while nearly close to the IC50 of the reference drug. Thus, NLCs could be a potential platform for boosted antitumor activity.

A Review of the Recent Developments of Molecular Hybrids Targeting Tubulin Polymerization

Microtubules are cylindrical protein polymers formed from αβ-tubulin heterodimers in the cytoplasm of eukaryotic cells. Microtubule disturbance may cause cell cycle arrest in the G2/M phase, and anomalous mitotic spindles will form. Microtubules are an important target for cancer drug action because of their critical role in mitosis. Several microtubule-targeting agents with vast therapeutic advantages have been developed, but they often lead to multidrug resistance and adverse side effects. Thus, single-target therapy has drawbacks in the effective control of tubulin polymerization. Molecular hybridization, based on the amalgamation of two or more pharmacophores of bioactive conjugates to engender a single molecular structure with enhanced pharmacokinetics and biological activity, compared to their parent molecules, has recently become a promising approach in drug development. The practical application of combined active scaffolds targeting tubulin polymerization inhibitors has been corroborated in the past few years. Meanwhile, different designs and syntheses of novel anti-tubulin hybrids have been broadly studied, illustrated, and detailed in the literature. This review describes various molecular hybrids with their reported structural–activity relationships (SARs) where it is possible in an effort to generate efficacious tubulin polymerization inhibitors. The aim is to create a platform on which new active scaffolds can be modeled for improved tubulin polymerization inhibitory potency and hence, the development of new therapeutic agents against cancer.

Indole-Based Tubulin Inhibitors: Binding Modes and SARs Investigations

Tubulin inhibitors can interfere with normal cell mitosis and inhibit cell proliferation through interfering with the normal structure and function of microtubules, forming spindle filaments. Indole, as a privileged pharmacological skeleton, has been widely used in anti-cancer inhibitors. A variety of alkaloids containing an indole core obtained from natural sources have been proven to inhibit tubulin polymerization, and an ever-increasing number of synthetic indole-based tubulin inhibitors have been reported. Among these, several kinds of indole-based derivatives, such as TMP analogues, aroylindoles, arylthioindoles, fused indole, carbazoles, azacarbolines, alkaloid nortopsentin analogues and bis-indole derivatives, have shown good inhibition activities towards tubulin polymerization. The binding modes and SARs investigations of synthetic indole derivatives, along with a brief mechanism on their anti-tubulin activity, are presented in this review.

Synthesis and Antiproliferative Evaluation of 3-Chloroazetidin-2-ones with Antimitotic Activity: Heterocyclic Bridged Analogues of Combretastatin A-4

Antimitotic drugs that target tubulin are among the most widely used chemotherapeutic agents; however, the development of multidrug resistance has limited their clinical activity. We report the synthesis and biological properties of a series of novel 3-chloro-β-lactams and 3,3-dichloro-β-lactams (2-azetidinones) that are structurally related to the tubulin polymerisation inhibitor and vascular targeting agent, Combretastatin A-4. These compounds were evaluated as potential tubulin polymerisation inhibitors and for their antiproliferative effects in breast cancer cells. A number of the compounds showed potent activity in MCF-7 breast cancer cells, e.g., compound 10n (3-chloro-4-(3-hydroxy-4-methoxy-phenyl)-1-(3,4,5-trimethoxyphenyl)azetidin-2-one) and compound 11n (3,3-dichloro-4-(3-hydroxy-4-methoxyphenyl)-1-(3,4,5-trimethoxyphenyl)-azetidin-2-one), with IC₅₀ values of 17 and 31 nM, respectively, and displayed comparable cellular effects to those of Combretastatin A-4. Compound 10n demonstrated minimal cytotoxicity against non-tumorigenic HEK-293T cells and inhibited the in vitro polymerisation of tubulin with significant G₂/M phase cell cycle arrest. Immunofluorescence staining of MCF-7 cells confirmed that β-lactam 10n caused a mitotic catastrophe by targeting tubulin. In addition, compound 10n promoted apoptosis by regulating the expression of pro-apoptotic protein BAX and anti-apoptotic proteins Bcl-2 and Mcl-1. Molecular docking was used to explore the potential molecular interactions between novel 3-chloro-β-lactams and the amino acid residues of the colchicine binding active site cavity of β-tubulin. Collectively, these results suggest that 3-chloro-2-azetidinones, such as compound 10n, could be promising lead compounds for further clinical anti-cancer drug development.

Supported Palladium-Catalyzed Carbonylative Synthesis of Diaryl Ketones from Aryl Bromides and Arylboronic Acids

A palladium supported on graphitic carbon nitride (Pd/g-C₃ N₄ ) catalyzed carbonylative reaction of aryl bromides and arylboronic acids by has been developed for the construction of diaryl ketones. Using benzene-1,3,5-triyl triformate (TFBen) as the CO source, the reaction proceeded well to give various diaryl ketones in moderate to good yields.

3-Methyl-imidazo[2,1-b]thiazole derivatives as a new class of antifolates: Synthesis, in vitro/in vivo bio-evaluation and molecular modeling simulations

Inhibiting the Dihydrofolate reductase (DHFR) enzyme has been validated in multiple clinical manifestations related to bacterial infection, malaria, and multiple types of cancer. Herein, novel series of 3-methyl-imidazo[2,1-b] thiazole-based analogs were synthesized and biologically evaluated for their in vitro inhibitory profile towards DHFR. Compounds 22 and 23 exhibited potent inhibitory profile targeting DHFR (IC₅₀ 0.079 and 0.085 µM, respectively comparable to MTX IC₅₀ 0.087 µM). Compounds 22 and 23 showed promising cytotoxicity against MCF7 breast cancer cell lines inducing cell cycle arrest and apoptosis. Furthermore, Compound 23 showed its potential to reduce body weight and tumor volume significantly, using Ehrlich ascites carcinoma (EAC) solid tumor animal model of breast cancer, compared to control-treated groups. Further, molecular modeling simulations validated the potential of 22 and 23 to have high affinity binding towards Arg22 and Phe31 residues via π-π interaction and hydrogen bonding within DHFR binding pocket. Computer-assisted ADMET study suggested that the newly synthesized analogs could have high penetration to the blood brain barrier (BBB), better intestinal absorption, non-inhibitors of CYP2D6, adequate plasma protein binding and good passive oral absorption. The obtained model and pattern of substitution could be used for further development of DHFR inhibitors.

Cytotoxic substituted indolizines as new colchicine site tubulin polymerisation inhibitors

A potential microtubule destabilising series of new indolizine derivatives was synthesised and tested for their anticancer activity against a panel of 60 human cancer cell lines. Compounds 11a, 11b, 15a, and 15j showed a broad spectrum of growth inhibitory activity against cancer cell lines representing leukaemia, melanoma and cancer of lung, colon, central nervous system, ovary, kidney, breast, and prostate. Among them, compound 11a was distinguishable by its excellent cytostatic activity, showing GI₅₀ values in the range of 10–100 nM on 43 cell lines. The less potent compounds 15a and 15j in terms of GI₅₀ values showed a high cytotoxic effect against tested colon cancer, CNS cancer, renal cancer and melanoma cell lines and only on few cell lines from other types of cancer. In vitro assaying revealed tubulin polymerisation inhibition by all active compounds. Molecular docking showed good complementarity of active compounds with the colchicine binding site of tubulin.

Recent advances in research of colchicine binding site inhibitors and their interaction modes with tubulin

Microtubules have been a concerning target of cancer chemotherapeutics for decades, and several tubulin-targeted agents, such as paclitaxel, vincristine and vinorelbine, have been approved. The colchicine binding site is one of the primary targets on microtubules and possesses advantages compared with other tubulin-targeted agents, such as inhibitors of tumor vessels and overcoming P-glycoprotein overexpression-mediated multidrug resistance. This study reviews and summarizes colchicine binding site inhibitors reported in recent years with structural studies via the crystal structures of complexes or computer simulations to discover new lead compounds. We are attempting to resolve the challenge of colchicine site agent research.

Synthesis and Biological Evaluation of 1-(Diarylmethyl)-1H-1,2,4-triazoles and 1-(Diarylmethyl)-1H-imidazoles as a Novel Class of Anti-Mitotic Agent for Activity in Breast Cancer

We report the synthesis and biochemical evaluation of compounds that are designed as hybrids of the microtubule targeting benzophenone phenstatin and the aromatase inhibitor letrozole. A preliminary screening in estrogen receptor (ER)-positive MCF-7 breast cancer cells identified 5-((2H-1,2,3-triazol-1-yl)(3,4,5-trimethoxyphenyl)methyl)-2-methoxyphenol 24 as a potent antiproliferative compound with an IC₅₀ value of 52 nM in MCF-7 breast cancer cells (ER+/PR+) and 74 nM in triple-negative MDA-MB-231 breast cancer cells. The compounds demonstrated significant G₂/M phase cell cycle arrest and induction of apoptosis in the MCF-7 cell line, inhibited tubulin polymerisation, and were selective for cancer cells when evaluated in non-tumorigenic MCF-10A breast cells. The immunofluorescence staining of MCF-7 cells confirmed that the compounds targeted tubulin and induced multinucleation, which is a recognised sign of mitotic catastrophe. Computational docking studies of compounds 19e, 21l, and 24 in the colchicine binding site of tubulin indicated potential binding conformations for the compounds. Compounds 19e and 21l were also shown to selectively inhibit aromatase. These compounds are promising candidates for development as antiproliferative, aromatase inhibitory, and microtubule-disrupting agents for breast cancer.

Synthesis, biological evaluation, and molecular docking analysis of phenstatin based indole linked chalcones as anticancer agents and tubulin polymerization inhibitors

A library of new phenstatin based indole linked chalcone compounds (9a-z and 9aa-ad) were designed and synthesized. Of these, compound 9a with 1-methyl, 2- and 3-methoxy substituents in the aromatic ring was efficacious against the human oral cancer cell line SCC-29B, spheroids, and in a mouse xenograft model of oral cancer AW13516. Compound 9a exhibited anti-cancer activity through disrupting cellular integrity and affecting glucose metabolism-which is a hallmark of cancer. The cellular architecture was affected by inhibition of tubulin polymerization as observed by an immunofluorescence assay on 9a-treated SCC-29B cells. An in vitro tubulin polymerization kinetics assay provided evidence of direct interaction of 9a with tubulin. This physical interaction between tubulin and compound 9a was further confirmed by Surface Plasmon Resonance (SPR) analysis. Molecular docking experiments and validations revealed that compound 9a interacts and binds at the colchicine binding site of tubulin and at active sites of key enzymes in the glucose metabolism pathway. Based on in silico modeling, biophysical interactions, and pre-clinical observations, 9a consisting of phenstatin based indole-chalcone scaffolds, can be considered as an attractive tubulin polymerization inhibitor candidate for developing anti-cancer therapeutics.

Synthetic and medicinal chemistry of phthalazines: Recent developments, opportunities and challenges

Fused diaza-heterocycles constitute the core structure of numerous bioactive natural products and effective therapeutic drugs. Among them, phthalazines have been recognized as remarkable structural leads in medicinal chemistry due to their wide application in pharmaceutical and agrochemical industries. Accessing such challenging pharmaceutical agents/drug candidates with high chemical complexity through synthetically efficient approaches remains an attractive goal in the contemporary medicinal chemistry and drug discovery arena. In this review, we focus on the recent developments in the synthetic routes towards the generation of phthalazine-based active pharmaceutical ingredients and their biological potential against various targets. The general reaction scope of these innovative and easily accessible strategies was emphasized focusing on the functional group tolerance, substrate and coupling partner compatibility/limitation, the choice of catalyst, and product diversification. These processes were also accompanied by the mechanistic insights where deemed appropriate to demonstrate meaningful information. Moreover, the rapid examination of the structure-activity relationship analyses around the phthalazine core enabled by the pharmacophore replacement/integration revealed the generation of robust, efficient, and more selective compounds with pronounced biological effects. A large variety of in silico methods and ADME profiling tools were also employed to provide a global appraisal of the pharmacokinetics profile of diaza-heterocycles. Thus, the discovery of new structural leads offers the promise of improving treatments for various tropical diseases such as tuberculosis, leishmaniasis, malaria, Chagas disease, among many others including various cancers, atherosclerosis, HIV, inflammatory, and cardiovascular diseases. We hope this review would serve as an informative collection of structurally diverse molecules enabling the generation of mature, high-quality, and innovative routes to support the drug discovery endeavors.

Methylsulfanylpyridine based diheteroaryl isocombretastatin analogs as potent anti-proliferative agents

Isocombretastatins are the not isomerizable 1,1-diarylethene isomers of combretastatins. Both families of antimitotics are poorly soluble and new analogs with improved water solubility are needed. The ubiquitous 3,4,5-trimethoxyphenyl ring and most of its replacements contribute to the solubility problem. 39 new compounds belonging to two series of isocombretastatin analogs with 2-chloro-6-methylsulfanyl-4-pyridinyl or 2,6-bis(methylsulfanyl)-4-pyridinyl moieties replacing the 3,4,5-trimethoxyphenyl have been synthesized and their antimitotic activity and aqueous solubility have been studied. We show here that 2-chloro-6-methylsulfanylpyridines are more successful replacements than 2,6-bis(methylsulfanyl)pyridines, giving highly potent tubulin inhibitors and cytotoxic compounds with improved water solubilities. The optimal combination is with indole rings carrying polar substitutions at the three position. The resulting diheteroaryl isocombretastatin analogs showed potent cytotoxic activity against human cancer cell lines caused by tubulin inhibition, as shown by in vitro tubulin polymerization inhibitory assays, cell cycle analysis, and confocal microscopy studies. Cell cycle analysis also showed apoptotic responses following G₂/M arrest after treatment. Conformational analysis and docking studies were applied to propose binding modes of the compounds at the colchicine site of tubulin and were in good agreement with the observed SAR. 2-Chloro-6-methylsulfanylpyridines represent a new and successful trimethoxyphenyl ring substitution for the development of improved colchicine site ligands.

Non-hydrogen bond catalyst-mediated diastereoselective conjugate additions of 5H-oxazol-4-ones to o-hydroxyphenyl-substituted p-quinone methides

An efficient DBU-catalyzed conjugate addition of 5H-oxazol-4-ones to o-hydroxyphenyl-substituted p-quinone methides has been developed, affording the valuable diarylmethanes in high yields with excellent diastereoselectivity. This strategy demonstrates a robust access to a wide range of diarylmethane derivatives possessing biologically significant o-hydroxyphenol and p-hydroxyphenol moieties under mild reaction conditions.

Bioreductively Activatable Prodrug Conjugates of Combretastatin A-1 and Combretastatin A-4 as Anticancer Agents Targeted toward Tumor-Associated Hypoxia

The natural products combretastatin A-1 (CA1) and combretastatin A-4 (CA4) function as potent inhibitors of tubulin polymerization and as selective vascular disrupting agents (VDAs) in tumors. Bioreductively activatable prodrug conjugates (BAPCs) can enhance selectivity by serving as substrates for reductase enzymes specifically in hypoxic regions of tumors. A series of CA1-BAPCs incorporating nor-methyl, mono-methyl, and gem-dimethyl nitrothiophene triggers were synthesized together with corresponding CA4-BAPCs, previously reported by Davis (Mol. Cancer Ther. 2006, 5 (11), 2886), for comparison. The CA4-gem-dimethylnitrothiophene BAPC 45 proved exemplary in comparison to its nor-methyl 43 and mono-methyl 44 congeners. It was stable in phosphate buffer (pH 7.4, 24 h), was cleaved (25%, 90 min) by NADPH-cytochrome P450 oxidoreductase (POR), was inactive (desirable prodrug attribute) as an inhibitor of tubulin polymerization (IC₅₀ > 20 μM), and demonstrated hypoxia-selective activation in the A549 cell line [hypoxia cytotoxicity ratio (HCR) = 41.5]. The related CA1-gem-dimethylnitrothiophene BAPC 41 was also promising (HCR = 12.5) with complete cleavage (90 min) upon treatment with POR. In a preliminary in vivo dynamic bioluminescence imaging study, BAPC 45 (180 mg/kg, ip) induced a decrease (within 4 h) in light emission in a 4T1 syngeneic mouse breast tumor model, implying activation and vascular disruption.

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.

Potent colchicine-site ligands with improved intrinsic solubility by replacement of the 3,4,5-trimethoxyphenyl ring with a 2-methylsulfanyl-6-methoxypyridine ring

Colchicine site antimitotic agents typically suffer from low aqueous solubilities and are formulated as phosphate prodrugs of phenolic groups. These hydroxyl groups are the aim of metabolic transformations leading to resistance. There is an urgent need for more intrinsically soluble analogues lacking these hydroxyl groups. The 3,4,5-trimethoxyphenyl ring of combretastatin A-4 is a liability in terms of solubility but it is considered essential for high cytotoxic and tubulin polymerization inhibitory (TPI) activity. We have synthesized 36 new analogues of combretastatin A-4 replacing the trimethoxyphenyl moiety with more polar pyridine based moieties, measured their aqueous solubility, and studied their anti-proliferative effects against 3 human cancer cell lines. We show here that pyridine rings can be successful replacements for the trimethoxyphenyl ring, resulting in potent and more soluble analogues. The more straightforward replacement, a 2,6-dimethoxypyridine ring led to inactive analogues, but a 2-methoxy-6-methylsulfanylpyridine moiety led to active analogues when combined with different B rings. This replacement led to potent cytotoxic activity against sensitive human cancer cell lines due to tubulin inhibition, as shown by cell cycle analysis, confocal microscopy, and tubulin polymerization inhibitory activity studies. Cell cycle analysis also showed apoptotic responses following treatment. Docking studies suggested binding at the colchicine site of tubulin and provided a good agreement with the observed SAR. A 2-methoxy-6-methylsulfanylpyridine moiety is a good trimethoxyphenyl ring replacement for the development of new colchicine site ligands.

Application of triazoles as bioisosteres and linkers in the development of microtubule targeting agents

The triazole ring system has emerged as an exciting prospect in the optimization studies of promising lead molecules in the quest for new drugs for clinical usage. Several marketed drugs possess these versatile moieties that are used in a wide range of medical indications. This stems from the unique intrinsic properties of triazoles, which impart stability to the basic pharmacophoric unit with an added advantage of being a bioisostere of different chemical functionalities. In the last decade, the use of triazoles as bioisosteres and linkers in the development of microtubule targeting agents has been extensively investigated. The present review highlights the advances in this promising area of drug discovery and development.

Developments of isoCombretastatin A-4 derivatives as highly cytotoxic agents

Combretastatin A-4 (CA-4) is a natural anti-cancer agent isolated in 1989 from the African willow tree, Combretum caffrum. Due to its chemical simplicity, this (Z)-stilbene has been the subject of many structural modifications mainly to improve its chemical and metabolic stability. Beside a large number of synthetic analogues, isoCombretastatin A-4 (isoCA-4), has proved to be a solution of choice since this non-natural isomer of CA-4 is stable, easier to synthesize and has equivalent antitumor properties as CA-4. In this review, we will present the structure-activity relationships (SARs) around isoCA-4 since its discovery in 2007. In a first part, we will describe some alternatives to replace the phenol B-ring of isoCA-4, then we will focus on the variations made on the 1,1-ethylene double bond and then, we will evocate very recent exiting results concerning the possible replacements of the 3,4,5-trimethoxyphenyl A-ring of isoCA-4 by suitable heterocycles.

Design, Synthesis, Molecular Modelling and Anticancer Activities of New Fused Phenanthrolines

Three series of fused pyrrolophenanthroline derivatives were designed as analogues of phenstatin and synthesized in two steps starting with 1,7-phenanthroline, 4,7-phenanthroline and 1,10-phenanthroline, respectively. Two (Compounds 8a and 11c) of the four compounds tested against a panel of sixty human cancer cell lines of the National Cancer Institute (NCI) exhibited significant growth inhibition activity on several cell lines. Compound 11c showed a broad spectrum in terms of antiproliferative efficacy with GI₅₀ values in the range of 0.296 to 250 μM. Molecular docking studies indicated that Compounds 8a and 11c are accommodated in the colchicine binding site of tubulin in two different ways.

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.

Synthesis and evaluation of antiproliferative microtubule-destabilising combretastatin A-4 piperazine conjugates

Microtubules are a validated clinical target for the treatment of many cancers. We describe the design, synthesis, biochemical evaluation, and molecular modelling studies of a series of analogues of the microtubule-destabilising agent, combretastatin A-4 (CA-4). Our series of 33 novel compounds contain the CA-4 core structure with modifications to the stilbene linking group, and are predominantly piperazine derivatives. Synthesis was achieved in a two-step process by firstly obtaining the acrylic acid via a Perkin reaction using microwave enhanced synthesis, followed by coupling using either DCC or Mukaiyama's reagent. All target compounds were screened for antiproliferative activity in MCF-7 breast cancer cells. Hydroxyl derivative (E)-3-(4-hydroxy-3-methoxyphenyl)-1-(4-phenylpiperazin-1-yl)-2-(3,4,5-trimethoxyphenyl) propenone (4m) displayed potent antiproliferative activity (IC50 = 190 nM). Two amino-containing derivatives, (E)-3-(3-amino-4-methoxyphenyl)-1-(4-phenylpiperazin-1-yl)-2-(3,4,5-trimethoxyphenyl)prop-2-en-1-one (4q) and (E)-3-(3-amino-4-methoxyphenyl)-1-(4-(p-tolyl)piperazin-1-yl)-2-(3,4,5-trimethoxyphenyl)prop-2-en-1-one (4x), were the most potent with IC50 values of 130 nM and 83 nM respectively. Representative compounds were shown to depolymerise tubulin, induce G2/M arrest and apoptosis in MCF-7 cells but not peripheral blood mononuclear cells, and induce cleavage of the DNA repair enzyme poly ADP ribose polymerase (PARP) in MCF-7 cells. Modelling studies predict that the compounds bind to tubulin within the colchicine-binding site. These compounds are a valuable addition to the library of CA-4 analogues and 4m, 4q and 4x will be developed further as novel, water-soluble molecules targeting microtubules.

Colchicine-Binding Site Inhibitors from Chemistry to Clinic: A Review

It is over 50 years since the discovery of microtubules, and they have become one of the most important drug targets for anti-cancer therapies. Microtubules are predominantly composed of the protein tubulin, which contains a number of different binding sites for small-molecule drugs. There is continued interest in drug development for compounds targeting the colchicine-binding site of tubulin, termed colchicine-binding site inhibitors (CBSIs). This review highlights CBSIs discovered through diverse sources: from natural compounds, rational design, serendipitously and via high-throughput screening. We provide an update on CBSIs reported in the past three years and discuss the clinical status of CBSIs. It is likely that efforts will continue to develop CBSIs for a diverse set of cancers, and this review provides a timely update on recent developments.

Synthesis, molecular modelling and anticancer evaluation of new pyrrolo[1,2-b]pyridazine and pyrrolo[2,1-a]phthalazine derivatives

Two new series of heterocyclic derivatives with potential anticancer activity, in which a pyrrolo[1,2-b]pyridazine or a pyrrolo[2,1-a]phthalazine moiety was introduced in place of the 3'-hydroxy-4'-methoxyphenyl ring of phenstatin have been synthesised and their structure-activity relationship (SAR) was studied. Fourteen of the new compounds were evaluated for their in vitro cytotoxic activity by National Cancer Institute (NCI) against 60 human tumour cell lines panel. The best five compounds in terms of in vitro growth inhibition were screened in the second stage five dose-response studies, three of them showing a very good antiproliferative activity with GI50<100 nM on several cell lines including colon, ovarian, renal, prostate, brain and breast cancer, melanoma and leukemia. Docking experiments on the biologically active compounds showed a good compatibility with the colchicine binding site of tubulin.

The Masked Polar Group Incorporation (MPGI) Strategy in Drug Design: Effects of Nitrogen Substitutions on Combretastatin and Isocombretastatin Tubulin Inhibitors

Colchicine site ligands suffer from low aqueous solubility due to the highly hydrophobic nature of the binding site. A new strategy for increasing molecular polarity without exposing polar groups—termed masked polar group incorporation (MPGI)—was devised and applied to nitrogenated combretastatin analogues. Bulky ortho substituents to the pyridine nitrogen hinder it from the hydrophobic pocket while increasing molecular polarity. The resulting analogues show improved aqueous solubilities and highly potent antiproliferative activity against several cancer cell lines of different origin. The more potent compounds showed moderate tubulin polymerization inhibitory activity, arrested the cell cycle of treated cells at the G₂/M phase, and subsequently caused apoptotic cell death represented by the cells gathered at the subG₀/G₁ population after 48 h of treatment. Annexin V/Propidium Iodide (PI) double-positive cells observed after 72 h confirmed the induction of apoptosis. Docking studies suggest binding at the colchicine site of tubulin in a similar way as combretastatin A4, with the polar groups masked by the vicinal substituents. These results validate the proposed strategy for the design of colchicine site ligands and open a new road to increasing the aqueous solubility of ligands binding in apolar environments.

Activated carbon/Brønsted acid-promoted aerobic benzylic oxidation under "on-water" condition: Green and efficient synthesis of 3-benzoylquinoxalinones as potent tubulin inhibitors

Green chemistry is becoming the favored approach to preparing drug molecules in pharmaceutical industry. Herein, we developed a clean and efficient method to synthesize 3-benzoylquinoxalines via activated carbon promoted aerobic benzylic oxidation under "on-water" condition. Moreover, biological studies with this class of compounds reveal an antiproliferative profile. Further structure modifications are performed and the investigations exhibited that the most active 12a could inhibit the microtubule polymerization by binding to tubulin and thus induce multipolar mitosis, G2/M phase arrest, and apoptosis of cancer cells. In addition, molecular docking studies allow the rationalization of the pharmacodynamic properties observed. Our systematic studies provide not only guidance for applications of O₂/AC/H₂O system, but also a new scaffold targeting tubulin for antitumor agent discovery.

Structure Guided Design, Synthesis, and Biological Evaluation of Novel Benzosuberene Analogues as Inhibitors of Tubulin Polymerization

A promising design paradigm for small-molecule inhibitors of tubulin polymerization that bind to the colchicine site draws structural inspiration from the natural products colchicine and combretastatin A-4 (CA4). Our previous studies with benzocycloalkenyl and heteroaromatic ring systems yielded promising inhibitors with dihydronaphthalene and benzosuberene analogues featuring phenolic (KGP03 and KGP18) and aniline (KGP05 and KGP156) congeners emerging as lead agents. These molecules demonstrated dual mechanism of action, functioning both as potent vascular disrupting agents (VDAs) and as highly cytotoxic anticancer agents. A further series of analogues was designed to extend functional group diversity and investigate regioisomeric tolerance. Ten new molecules were effective inhibitors of tubulin polymerization (IC₅₀ < 5 μM) with seven of these exhibiting highly potent activity comparable to CA4, KGP18, and KGP03. For one of the most effective agents, dose-dependent vascular shutdown was demonstrated using dynamic bioluminescence imaging in a human prostate tumor xenograft growing in a rat.

3-Vinylazetidin-2-Ones: Synthesis, Antiproliferative and Tubulin Destabilizing Activity in MCF-7 and MDA-MB-231 Breast Cancer Cells

Microtubule-targeted drugs are essential chemotherapeutic agents for various types of cancer. A series of 3-vinyl-β-lactams (2-azetidinones) were designed, synthesized and evaluated as potential tubulin polymerization inhibitors, and for their antiproliferative effects in breast cancer cells. These compounds showed potent activity in MCF-7 breast cancer cells with an IC₅₀ value of 8 nM for compound 7s 4-[3-Hydroxy-4-methoxyphenyl]-1-(3,4,5-trimethoxyphenyl)-3-vinylazetidin-2-one) which was comparable to the activity of Combretastatin A-4. Compound 7s had minimal cytotoxicity against both non-tumorigenic HEK-293T cells and murine mammary epithelial cells. The compounds inhibited the polymerisation of tubulin in vitro with an 8.7-fold reduction in tubulin polymerization at 10 μM for compound 7s and were shown to interact at the colchicine-binding site on tubulin, resulting in significant G2/M phase cell cycle arrest. Immunofluorescence staining of MCF-7 cells confirmed that β-lactam 7s is targeting tubulin and resulted in mitotic catastrophe. A docking simulation indicated potential binding conformations for the 3-vinyl-β-lactam 7s in the colchicine domain of tubulin. These compounds are promising candidates for development as antiproiferative microtubule-disrupting agents.

Hybrid cis-stilbene Molecules: Novel Anticancer Agents

The growing interest in anticancer hybrids in the last few years has resulted in a great number of reports on hybrid design, synthesis and bioevaluation. Many novel multi-target-directed drug candidates were synthesized, and their biological activities were evaluated. For the design of anticancer hybrid compounds, the molecules of stilbenes, aromatic quinones, and heterocycles (benzimidazole, imidazole, pyrimidine, pyridine, pyrazole, quinoline, quinazoline) were applied. A distinct group of hybrids comprises the molecules built with natural compounds: Resveratrol, curcumin, coumarin, and oleanolic acid. In this review, we present the studies on bioactive hybrid molecules of a well-known tubulin polymerization inhibitor, combretastatin A-4 and its analogs with other pharmacologically active entities. The mechanism of anticancer activity of selected hybrids is discussed considering the structure-activity relationship.

Design, synthesis and biological evaluation of quinoline-indole derivatives as anti-tubulin agents targeting the colchicine binding site

A series of novel isocombretastatin A-4 (isoCA-4) analogs were designed and synthesized by replacing 3,4,5-trimethoylphenyl and isovanillin of isoCA-4 with quinoline and indole moieties, respectively. The structure activity relationships (SARs) of these synthesized quinoline-indole derivatives have been intensively investigated. Two compounds 27c and 34b exhibited the most potent activities against five cancer cell lines with IC₅₀ values ranging from 2 to 11 nM, which were comparable to those of Combretastatin A-4 (CA-4, 1). Further mechanism investigations revealed that 34b effectively inhibited the microtubule polymerization by binding to the colchicine site of tubulin. Further cellular mechanism studies elucidated that 34b disrupted cell microtubule networks, arrested the cell cycle at G2/M phase, induced apoptosis and depolarized mitochondria of K562 cells. Moreover, 34b displayed potent anti-vascular activity in both wound healing and tube formation assays. Importantly, 27c and 34b significantly inhibited tumor growth in H22 xenograft models without apparent toxicity, suggesting that 27c and 34b deserve further research as potent antitumor agents for cancer therapy.

Synthesis of dihydronaphthalene analogues inspired by combretastatin A-4 and their biological evaluation as anticancer agents

The natural products colchicine and combretastatin A-4 (CA4) have provided inspiration for the discovery and development of a wide array of derivatives and analogues that inhibit tubulin polymerization through a binding interaction at the colchicine site on β-tubulin. A water-soluble phosphate prodrug salt of CA4 (referred to as CA4P) has demonstrated the ability to selectively damage tumor-associated vasculature and ushered in a new class of developmental anticancer agents known as vascular disrupting agents (VDAs). Through a long-term program of structure activity relationship (SAR) driven inquiry, we discovered that the dihydronaphthalene molecular scaffold provided access to small-molecule inhibitors of tubulin polymerization. In particular, a dihydronaphthalene analogue bearing a pendant trimethoxy aryl ring (referred to as KGP03) and a similar aroyl ring (referred to as KGP413) were potent inhibitors of tubulin polymerization (IC50 = 1.0 and 1.2 μM, respectively) and displayed low nM cytotoxicity against human cancer cell lines. In order to enhance water-solubility for in vivo evaluation, the corresponding phosphate prodrug salts (KGP04 and KGP152, respectively) were synthesized. In a preliminary in vivo study in a SCID-BALB/c mouse model bearing the human breast tumor MDA-MB-231-luc, a 99% reduction in signal was observed with bioluminescence imaging (BLI) 4 h after IP administration of KGP152 (200 mg kg-1) indicating reduced tumor blood flow. In a separate study, disruption of tumor-associated blood flow in a Fischer rat bearing an A549-luc human lung tumor was observed by color Doppler ultrasound following administration of KGP04 (15 mg kg-1).

Combretastatin-Inspired Heterocycles as Antitubulin Anticancer Agents

Combretastatin (CA-4) and its analogues are undergoing several clinical trials for treating different types of tumors. In this work, the antiproliferative activity of a series of 2-aminoimidazole-carbonyl analogs of clinically relevant combretastatins A-4 (CA-4) and A-1 was evaluated using a cell-based assay. Among the compounds tested, C-13 and C-21 displayed strong antiproliferative activities against HeLa cells. C-13 inhibited the proliferation of lung carcinoma (A549) cells more potently than combretastatin A-4. C-13 also retarded the migration of A549 cells. Interestingly, C-13 displayed much stronger antiproliferative effects against breast carcinoma and skin melanoma cells compared to noncancerous breast epithelial and skin fibroblast cells. C-13 strongly disassembled cellular microtubules, perturbed the localization of EB1 protein, inhibited mitosis in cultured cells, and bound to tubulin at the colchicine site and inhibited the polymerization of reconstituted microtubules in vitro. C-13 treatment increased the level of reactive oxygen species and induced apoptosis via poly(ADP-ribose) polymerase-cleavage in HeLa cells. The results revealed the importance of the 2-aminoimidazole-carbonyl motif as a double bond replacement in combretastatin and indicated a pharmacodynamically interesting pattern of H-bond acceptors/donors and requisite syn-templated aryls.