Antineoplastic agents. 443. Synthesis of the cancer cell growth inhibitor hydroxyphenstatin and its sodium diphosphate prodrug

Lewis Acid-Catalyzed Benzannulation of Vinyloxiranes with 3-Formylchromones or 1,4-Quinones for Diversely Functionalized 2-Hydroxybenzophenones, 1,4-Naphthoquinones, and Anthraquinones

A facile and convenient protocol for the regioselective construction of functionalized 2-hydroxybenzophenones is described. This protocol involves the Sc(OTf)3/BF3·OEt2-catalyzed benzannulation of 2-vinyloxirans with 3-formylchromone, which involves cascade in situ diene formation, [4 + 2] cycloaddition, elimination, and ring-opening strategies. Moreover, it provides an expedited synthetic pathway to access biologically intriguing 1,4-naphthoquinones and anthraquinones including vitamin K3 and tectoquinone. The synthesized compounds also hold potential for use as UV filters and show promise as chemosensors for Cu2+ and Mg2+ ions.

Synthesis and biological evaluation of structurally diverse 6-aryl-3-aroyl-indole analogues as inhibitors of tubulin polymerization

The synthesis and evaluation of small-molecule inhibitors of tubulin polymerization remains a promising approach for the development of new therapeutic agents for cancer treatment. The natural products colchicine and combretastatin A-4 (CA4) inspired significant drug discovery campaigns targeting the colchicine site located on the beta-subunit of the tubulin heterodimer, but so far these efforts have not yielded an approved drug for cancer treatment in human patients. Interest in the colchicine site was enhanced by the discovery that a subset of colchicine site agents demonstrated dual functionality as both potent antiproliferative agents and effective vascular disrupting agents (VDAs). Our previous studies led to the discovery and development of a 2-aryl-3-aroyl-indole analogue (OXi8006) that inhibited tubulin polymerization and demonstrated low nM IC50 values against a variety of human cancer cell lines. A water-soluble phosphate prodrug salt (OXi8007), synthesized from OXi8006, displayed promising vascular disrupting activity in mouse models of cancer. To further extend structure-activity relationship correlations, a series of 6-aryl-3-aroyl-indole analogues was synthesized and evaluated for their inhibition of tubulin polymerization and cytotoxicity against human cancer cell lines. Several structurally diverse molecules in this small library were strong inhibitors of tubulin polymerization and of MCF-7 and MDA-MB-231 human breast cancer cells. One of the most promising analogues (KGP591) caused significant G2/M arrest of MDA-MB-231 cells, disrupted microtubule structure and cell morphology in MDA-MB-231 cells, and demonstrated significant inhibition of MDA-MB-231 cell migration in a wound healing (scratch) assay. A phosphate prodrug salt, KGP618, synthesized from its parent phenolic precursor, KGP591, demonstrated significant reduction in bioluminescence signal when evaluated in vivo against an orthotopic model of kidney cancer (RENCA-luc) in BALB/c mice, indicative of VDA efficacy. The most active compounds from this series offer promise as anticancer therapeutic agents.

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 Biological Evaluation of Prodrugs of 666-15 as Inhibitors of CREB-Mediated Gene Transcription

cAMP-response element binding protein (CREB) is a transcription factor involved in multiple cancers. Chemical inhibitors of CREB represent potential anticancer agents. We previously identified 666-15 as a potent CREB inhibitor. While 666-15 showed efficacious anticancer activity in vivo through intraperitoneal (IP) injection, its oral bioavailability is limited. To increase its oral bioavailability, we describe synthesis and evaluation of prodrugs based on 666-15. The amino acid esters were attempted, but they were not stable for detailed characterization. The corresponding sulfate and phosphates were prepared. The sulfate of 666-15 was too stable to release 666-15 while the phosphates were converted into 666-15 with half-lives of ∼2 h. Phosphate 3 was also a potent CREB inhibitor with anti-breast cancer activity. Furthermore, compound 3 showed much improved oral bioavailability at 38%. These studies support that 3 can be used as an oral CREB inhibitor while IP administration of 666-15 is preferred for in vivo applications.

Green Tandem [5C + 1C] Cycloaromatization of α-Alkenoyl Ketene Dithioacetals and Nitroethane in Water: Eco-Friendly Synthesis of Ortho-Acylphenols

For the first time, an eco-friendly and sustainable tandem [5C + 1C] cycloaromatization of α-alkenoyl ketene dithioacetals and nitroethane in water for the efficient synthesis of ortho-acylphenols was reported. In refluxing water, a range of α-alkenoyl ketene dithioacetals and nitroethane smoothly underwent tandem Michael addition/cyclization/aromatization reactions in the presence of 2.0 equivalents of DBU to provide various ortho-acylphenols in excellent yields. The green approach to ortho-acylphenols not only avoided the use of harmful organic solvents, which could result in serious environmental and safety issues, but also exhibited fascinating features such as good substrate scope, excellent yields, simple purification for desired products, ease of scale-up, and reusable aqueous medium.

Water-Tolerant ortho-Acylation of Phenols

A metal-free, water-tolerant, and one-pot process for ortho-acylation of phenols promoted by the iodine source/hydrogen peroxide system has been developed. This transformation undergoes ether formation, iodocyclization, C-C bond cleavage, and oxidative hydrolysis in a one-step manner, which is supported by control experiments.

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.

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.

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.

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.

Transition-metal-free cascade benzannulations for synthesizing 2-hydroxybenzophenones

A set of cascade benzannulations of readily accessible chromone-3-carboxaldehydes and γ-nitroaldehydes for synthesizing biologically relevant 2-hydroxybenzophenones has been developed.

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.

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.

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.

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).

Integrating Metal-Catalyzed C-H and C-O Functionalization To Achieve Sterically Controlled Regioselectivity in Arene Acylation

One major goal of organometallic chemists is the direct functionalization of the bonds most recurrent in organic molecules: C-H, C-C, C-O, and C-N. An even grander challenge is C-C bond formation when both precursors are of this category. Parallel to this is the synthetic goal of achieving reaction selectivity that contrasts with conventional methods. Electrophilic aromatic substitution (EAS) via Friedel-Crafts acylation is the most renowned method for the synthesis of aryl ketones, a common structural motif of many pharmaceuticals, agrochemicals, fragrances, dyes, and other commodity chemicals. However, an EAS synthetic strategy is only effective if the desired site for acylation is in accordance with the electronic-controlled regioselectivity of the reaction. Herein we report steric-controlled regioselective arene acylation with salicylate esters via iridium catalysis to access distinctly substituted benzophenones. Experimental and computational data indicate a unique reaction mechanism that integrates C-O activation and C-H activation with a single iridium catalyst without an exogenous oxidant or base. We disclose an extensive exploration of the synthetic scope of both the arene and the ester components, culminating in the concise synthesis of the potent anticancer agent hydroxyphenstatin.

Metal- and O2-Free Oxidative C-C Bond Cleavage of Aromatic Aldehydes

An oxidative C-C cleavage of aldehydes requiring neither metals nor O₂ was discovered. Homobenzylic aldehydes and α-substituted homobenzylic aldehydes were cleaved to benzylic aldehydes and ketones, respectively, using nitrosobenzene as an oxidant. This reaction is chemoselective for aromatic aldehydes, as an aliphatic aldehyde was unreactive under these conditions, and other reactive functionality such as ketones and free alcohols are tolerated. A mechanism accounting for the fate of the lost carbon is proposed.

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.

Bioreductively activatable prodrug conjugates of phenstatin designed to target tumor hypoxia

A variety of solid tumor cancers contain significant regions of hypoxia, which provide unique challenges for targeting by potent anticancer agents. Bioreductively activatable prodrug conjugates (BAPCs) represent a promising strategy for therapeutic intervention. BAPCs are designed to be biologically inert until they come into contact with low oxygen tension, at which point reductase enzyme mediated cleavage releases the parent anticancer agent in a tumor-specific manner. Phenstatin is a potent inhibitor of tubulin polymerization, mimicking the chemical structure and biological activity of the natural product combretastatin A-4. Synthetic approaches have been established for nitrobenzyl, nitroimidazole, nitrofuranyl, and nitrothienyl prodrugs of phenstatin incorporating nor-methyl, mono-methyl, and gem-dimethyl variants of the attached nitro compounds. A series of BAPCs based on phenstatin have been prepared by chemical synthesis and evaluated against the tubulin-microtubule protein system. In a preliminary study using anaerobic conditions, the gem-dimethyl nitrothiophene and gem-dimethyl nitrofuran analogues were shown to undergo efficient enzymatic cleavage in the presence of NADPH cytochrome P450 oxidoreductase. Each of the eleven BAPCs evaluated in this study demonstrated significantly reduced inhibitory activity against tubulin in comparison to the parent anti-cancer agent phenstatin (IC50=1.0μM). In fact, the majority of the BAPCs (seven of the eleven analogues) were not inhibitors of tubulin polymerization (IC50>20μM), which represents an anticipated (and desirable) attribute for these prodrugs, since they are intended to be biologically inactive prior to enzyme-mediated cleavage to release phenstatin.

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.

Synthesis of novel morpholine conjugated benzophenone analogues and evaluation of antagonistic role against neoplastic development

A series of novel 4-benzyl-morpholine-2-carboxylic acid N'-[2-(4-benzoyl-phenoxy)-acetyl]-hydrazide derivatives 8a-j has been synthesized from (4-hydroxy-aryl)-aryl methanones through a multi-step reaction sequence and then evaluated for anti-proliferative activity in vitro against various types of neoplastic cells of mouse and human such as DLA, EAC, MCF-7 and A549 cells. From the cytotoxic studies and structural activity relationship of compounds 8a-j, it is clear that methyl group on the B ring of benzophenone is essential for antiproliferative activity and bromo at ortho position (compound 8b) and methyl at para position (compound 8f) on A ring of benzophenone are significant for extensive anti-mitogenic activity. Investigation on clonogenesis and Fluorescence-activated cell sorting suggests that compounds 8b and 8f have the potency to exhibit the prolonged activity with cell cycle arrest on G2/M phase against cancer progression. Further, the compounds 8b and 8f inhibit murine ascites lymphoma through caspase activated DNase mediated apoptosis.

Synthesis and Biological Evaluation of Benzocyclooctene-based and Indene-based Anticancer Agents that Function as Inhibitors of Tubulin Polymerization

The natural products colchicine and combretastatin A-4 (CA4) have been inspirational for the design and synthesis of structurally related analogues and spin-off compounds as inhibitors of tubulin polymerization. The discovery that a water-soluble phosphate prodrug salt of CA4 (referred to as CA4P) is capable of imparting profound and selective damage to tumor-associated blood vessels paved the way for the development of a new therapeutic approach for cancer treatment utilizing small-molecule inhibitors of tubulin polymerization that also act as vascular disrupting agents (VDAs). Combination of salient structural features associated with colchicine and CA4 led to the design and synthesis of a variety of fused aryl-cycloalkyl and aryl-heterocyclic compounds that function as inhibitors of tubulin polymerization. Prominent among these compounds is a benzosuberene analogue (referred to as KGP18), which demonstrates sub-nM cytotoxicity against human cancer cell lines and functions (when administered as a water-soluble prodrug salt) as a VDA in mouse models. Structure activity relationship considerations led to the evaluation of benzocyclooctyl [6,8 fused] and indene [6,5 fused] ring systems. Four benzocyclooctene and four indene analogues were prepared and evaluated biologically. Three of the benzocyclooctene analogues were active as inhibitors of tubulin polymerization (IC50 < 5 μM), and benzocyclooctene phenol 23 was comparable to KGP18 in terms of potency. The analogous indene-based compound 31 also functioned as an inhibitor of tubulin polymerization (IC50 = 11 μM) with reduced potency. The most potent inhibitor of tubulin polymerization from this group was benzocyclooctene analogue 23, and it was converted to its water-soluble prodrug salt 24 to assess its potential as a VDA. Preliminary in vivo studies, which utilized the MCF7-luc-GFP-mCherry breast tumor in a SCID mouse model, demonstrated that treatment with 24 (120 mg/kg) resulted in significant vascular shutdown, as evidenced by bioluminescence imaging at 4 h post administration, and that the effect continued at both 24 and 48 h. Contemporaneous studies with CA4P, a clinically relevant VDA, were carried out as a positive control.

2-Anilino-3-Aroylquinolines as Potent Tubulin Polymerization Inhibitors

Several 2-anilino-3-aroylquinolines were designed, synthesized, and screened for their cytotoxic activity against five human cancer cell lines: HeLa, DU-145, A549, MDA-MB-231, and MCF-7. Their IC50 values ranged from 0.77 to 23.6 μm. Among the series, compounds 7 f [(4-fluorophenyl)(2-((4-fluorophenyl)amino)quinolin-3-yl)methanone] and 7 g [(4-chlorophenyl)(2-((4-fluorophenyl)amino)quinolin-3-yl)methanone] showed remarkable antiproliferative activity against human lung cancer and prostate cancer cell lines. The IC50 values for inhibiting tubulin polymerization were 2.24 and 2.10 μm for compounds 7 f and 7 g, respectively, and were much lower than that of the reference compound E7010 [N-(2-(4-hydroxyphenylamino)pyridin-3-yl)-4-methoxybenzenesulfonamide]. Furthermore, flow cytometric analysis revealed that these compounds arrest the cell cycle at the G2 /M phase, leading to apoptosis. Apoptosis was also confirmed by mitochondrial membrane potential, Annexin V-FITC assay, and intracellular ROS generation. Immunohistochemistry, western blot, and tubulin polymerization assays showed that these compounds disrupt tubulin polymerization. Molecular docking studies revealed that these compounds bind efficiently to β-tubulin at the colchicine binding site.

Design, synthesis, and biological evaluation of water-soluble amino acid prodrug conjugates derived from combretastatin, dihydronaphthalene, and benzosuberene-based parent vascular disrupting agents

Targeting tumor vasculature represents an intriguing therapeutic strategy in the treatment of cancer. In an effort to discover new vascular disrupting agents with improved water solubility and potentially greater bioavailability, various amino acid prodrug conjugates (AAPCs) of potent amino combretastatin, amino dihydronaphthalene, and amino benzosuberene analogs were synthesized along with their corresponding water-soluble hydrochloride salts. These compounds were evaluated for their ability to inhibit tubulin polymerization and for their cytotoxicity against selected human cancer cell lines. The amino-based parent anticancer agents 7, 8, 32 (also referred to as KGP05) and 33 (also referred to as KGP156) demonstrated potent cytotoxicity (GI50=0.11-40nM) across all evaluated cell lines, and they were strong inhibitors of tubulin polymerization (IC50=0.62-1.5μM). The various prodrug conjugates and their corresponding salts were investigated for cleavage by the enzyme leucine aminopeptidase (LAP). Four of the glycine water-soluble AAPCs (16, 18, 44 and 45) showed quantitative cleavage by LAP, resulting in the release of the highly cytotoxic parent drug, whereas partial cleavage (<10-90%) was observed for other prodrugs (15, 17, 24, 38 and 39). Eight of the nineteen AAPCs (13-16, 42-45) showed significant cytotoxicity against selected human cancer cell lines. The previously reported CA1-diamine analog and its corresponding hydrochloride salt (8 and 10, respectively) caused extensive disruption (at a concentration of 1.0μM) of human umbilical vein endothelial cells growing in a two-dimensional tubular network on matrigel. In addition, compound 10 exhibited pronounced reduction in bioluminescence (greater than 95% compared to saline control) in a tumor bearing (MDA-MB-231-luc) SCID mouse model 2h post treatment (80mg/kg), with similar results observed upon treatment (15mg/kg) with the glycine amino-dihydronaphthalene AAPC (compound 44). Collectively, these results support the further pre-clinical development of the most active members of this structurally diverse collection of water-soluble prodrugs as promising anticancer agents functioning through a mechanism involving vascular disruption.

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

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

Structural interrogation of benzosuberene-based inhibitors of tubulin polymerization

The discovery of 3-methoxy-9-(30,40,50-trimethoxyphenyl)-6,7-dihydro-5H-benzo[7]annulen-4-ol (a benzosuberene-based analogue referred to as KGP18) was originally inspired by the natural products colchicine and combretastatin A-4 (CA4). The relative structural simplicity and ease of synthesis of KGP18, coupled with its potent biological activity as an inhibitor of tubulin polymerization and its cytotoxicity (in vitro) against human cancer cell lines, has resulted in studies focused on new analogue design and synthesis. Our goal was to probe the relationship of structure to function in this class of anticancer agents. A series of twenty-two new benzosuberene-based analogues of KGP18 was designed and synthesized. These compounds vary in their methoxylation pattern and separately incorporate trifluoromethyl groups around the pendant aryl ring for the evaluation of the effect of functional group modifications on the fused six-membered aromatic ring. In addition, the 8,9-saturated congener of KGP18 has been synthesized to assess the necessity of unsaturation at the carbon atom bearing the pendant aryl ring. Six of the molecules from this benzosuberene-series of compounds were active (IC50 < 5 lM) as inhibitors of tubulin polymerization while four analogues were comparable (IC50 approximately 1 lM) in their tubulin inhibitory activity to CA4 and KGP18. The potency of a bis-trifluoromethyl analogue 74 and the unsaturated KGP18 derivative 73 as inhibitors of tubulin assembly along with their moderate cytotoxicity suggested the potential utility of these compounds as vascular disrupting agents (VDAs) to selectively target microvessels feeding tumors. Accordingly, water-soluble and DMSO-soluble phosphate prodrug salts of each were synthesized for preliminary in vivo studies to assess their potential efficacy as VDAs.

Synthesis of phenstatin/isocombretastatin-chalcone conjugates as potent tubulin polymerization inhibitors and mitochondrial apoptotic inducers

A series of phenstatin/isocombretastatin–chalcones were synthesized and screened for their cytotoxic activity against various human cancer cell lines. Some representative compounds exhibited significant antiproliferative activity against a panel of sixty human cancer cell lines of the NCI, with GI50 values in the range of 0.11 to 19.0 μM. Three compounds (3b, 3c and 3e) showed a broad spectrum of antiproliferative efficacy on most of the cell lines in the sub-micromolar range. In addition, all the synthesized compounds (3a–l and 4a–l) displayed moderate to excellent cytotoxicity against breast cancer cells such as MCF-7 and MDA-MB-231 with IC50 values in the range of 0.5 to 19.9 μM. Moreover, the tubulin polymerization assay and immunofluorescence analysis results suggest that some of these compounds like 3c and 3e exhibited significant inhibitory effect on the tubulin assembly with an IC50 value of 0.8 μM and 0.6 μM respectively. A competitive binding assay suggested that these compounds bind at the colchicine-binding site of tubulin. A cell cycle assay revealed that these compounds arrest at the G2/M phase and lead to apoptotic cell death. Furthermore, this was confirmed by Hoechst 33258 staining, activation of caspase 9, DNA fragmentation, Annexin V-FITC and mitochondrial membrane depolarization. Molecular docking studies indicated that compounds like 3e occupy the colchicine binding site of tubulin.

Discovery of indolizines containing triazine moiety as new leads for the development of antitumoral agents targeting mitotic events

A new family of 3-aroylindolizines bearing a dimethoxytriazine unit in their position 1 was designed, synthesized and evaluated for their ability to inhibit tubulin polymerization and cellular growth in vitro. Compound 39 was the best candidate in the current study with a GI50 value of 870 nM on SNB-75 CNS cancer cells and of 920 nM on MDA-MB-231/ATCC breast cancer cells. The standard NCI Compare results indicated that indolizine 39 may target PLK1 (polo-like kinase 1).

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.

Synthesis, cytotoxicity, docking study, and tubulin polymerization inhibitory activity of novel 1-(3,4-dimethoxyphenyl)-5-(3,4,5-trimethoxyphenyl)-1H-1,2,4-triazole-3-carboxanilides

A series of novel 1-(3,4-methoxyphenyl)-5-(3,4,5-trimethoxyphenyl)-1H-1,2,4-triazole-3-carboxylic acid derivatives (4a-n) were synthesized and evaluated for their in vitro cytotoxic activity against the growth of four different human cell lines (hepatocarcinoma HepG2, breast adenocarcinoma MCF-7, colon carcinoma DLD-1, and leukemia HL-60). The anilides of m-anisidine 4e, o-anisidine 4f, and 3,5-difluoroaniline 4l demonstrated best results on MCF-7 cells and mean IC50 values of 7.79, 10.79, and 13.20 µM, respectively. The compounds produced a significant reduction in cellular microtubules at a concentration of 25 µg/mL, for microtubule loss. Molecular modeling studies involving compounds 4d, 4e, 4f, and 4l with the colchicine binding site of α,β-tubulin revealed hydrogen bonding and hydrophobic interactions with several amino acids in the colchicine binding site of β-tubulin.

Rational approaches, design strategies, structure activity relationship and mechanistic insights for anticancer hybrids

A Hybrid drug which comprises the incorporation of two drug pharmacophores in one single molecule are basically designed to interact with multiple targets or to amplify its effect through action on another bio target as one single molecule or to counterbalance the known side effects associated with the other hybrid part(.) The present review article offers a detailed account of the design strategies employed for the synthesis of anticancer agents via molecular hybridization techniques. Over the years, the researchers have employed this technique to discover some promising chemical architectures displaying significant anticancer profiles. Molecular hybridization as a tool has been particularly utilized for targeting tubulin protein as exemplified through the number of research papers. The microtubule inhibitors such as taxol, colchicine, chalcones, combretasatin, phenstatins and vinca alkaloids have been utilized as one of the functionality of the hybrids and promising results have been obtained in most of the cases with some of the tubulin based hybrids exhibiting anticancer activity at nanomolar level. Linkage with steroids as biological carrier vector for anticancer drugs and the inclusion of pyrrolo [2,1-c] [1,4]benzodiazepines (PBDs), a family of DNA interactive antitumor antibiotics derived from Streptomyces species in hybrid structure based drug design has also emerged as a potential strategy. Various heteroaryl based hybrids in particular isatin and coumarins have also been designed and reported to posses' remarkable inhibitory potential. Apart from presenting the design strategies, the article also highlights the structure activity relationship along with mechanistic insights revealed during the biological evaluation of the hybrids.

Synthesis of 1-amino-2-aroyl/acetylnaphthalenes through a base mediated one pot inter and intramolecular C–C bond formation strategy

Base mediated synthesis of highly functionalized aroyl/acetylnaphthalenes by the reaction of 2-(1-cyano-2,2-bis(methylthio)vinyl)benzonitrile and aryl methyl ketone or acetone has been reported.

Synthesis of structurally diverse benzosuberene analogues and their biological evaluation as anti-cancer agents

Diversely functionalized, fused aryl-alkyl ring systems hold a prominent position as well-established molecular frameworks for a variety of anti-cancer agents. The benzosuberene (6,7 fused, also referred to as dihydro-5H-benzo[7]annulene and benzocycloheptene) ring system has emerged as a valuable molecular core component for the development of inhibitors of tubulin assembly, which function as antiproliferative anti-cancer agents and, in certain cases, as vascular disrupting agents (VDAs). Both a phenolic-based analogue (known as KGP18, compound 39) and its corresponding amine-based congener (referred to as KGP156, compound 45), which demonstrate strong inhibition of tubulin assembly (low micromolar range) and potent cytotoxicity (picomolar range for KGP18 and nanomolar range for KGP156) are noteworthy examples of such benzosuberene-based compounds. In order to extend the structure-activity relationship (SAR) knowledge base related to benzosuberene anti-cancer agents, a series of eleven analogues (including KGP18) were prepared in which the methoxylation pattern on the pendant aryl ring as well as functional group incorporation on the fused aryl ring were varied. The synthetic approach to these compounds featured a sequential Wittig olefination, reduction, Eaton's reagent-mediated cyclization strategy to achieve the core benzosuberone intermediate, and represented a higher-yielding synthesis of KGP18 (which we prepared previously through a ring-expansion strategy). Incorporation of a fluorine or chlorine atom at the 1-position of the fused aryl ring or replacement of one of the methoxy groups with hydrogen (on the pendant aryl ring of KGP18) led to benzosuberene analogues that were both strongly inhibitory against tubulin assembly (IC50 approximately 1.0 μM) and strongly cytotoxic against selected human cancer cell lines (for example, GI50=5.47 nM against NCI-H460 cells with fluoro-benzosuberene analogue 37). A water-soluble phosphate prodrug salt of KGP18 (referred to as KGP265, compound 44) and a water-soluble serinamide salt (compound 48) of KGP156 were also synthesized and evaluated in this study.

Synthesis of a 2-aryl-3-aroyl indole salt (OXi8007) resembling combretastatin A-4 with application as a vascular disrupting agent

The natural products colchicine and combretastatin A-4 are potent inhibitors of tubulin assembly, and they have inspired the design and synthesis of a large number of small-molecule, potential anticancer agents. The indole-based molecular scaffold is prominent among these SAR modifications, leading to a rapidly increasing number of agents. The water-soluble phosphate prodrug 33 (OXi8007) of 2-aryl-3-aroylindole-based phenol 8 (OXi8006) was prepared by chemical synthesis and found to be strongly cytotoxic against selected human cancer cell lines (GI₅₀ = 36 nM against DU-145 cells, for example). The free phenol, 8 (OXi8006), was a strong inhibitor (IC₅₀ = 1.1 μM) of tubulin assembly. The corresponding phosphate prodrug 33 (OXi8007) also demonstrated pronounced interference with tumor vasculature in a preliminary in vivo study utilizing a SCID mouse model bearing an orthotopic PC-3 (prostate) tumor as imaged by color Doppler ultrasound. The combination of these results provides evidence that the indole-based phosphate prodrug 33 (OXi8007) functions as a vascular disrupting agent that may prove useful for the treatment of cancer.

Synthesis of novel chromene derivatives of expected antitumor activity

Inhibition of tubulin polymerization is one of the important tactics in cancer therapy. Since 4-aryl-4H-chromene derivatives are found to be microtubule-binding agents via interfering with tubulin polymerization so we decide to concentrate our exploration efforts on the combination of this nucleus with 5-, 6-, and/or 7-memebered heterocyclic moieties in a novel series of compounds to explore the effect that might result from this combination. Ten novel compounds were selected for anticancer screening assay against MCF-7 breast cancer cell line in comparison to colchicine as positive control and most of them showed excellent activity.