Potent, orally active heterocycle-based combretastatin A-4 analogues: synthesis, structure-activity relationship, pharmacokinetics, and in vivo antitumor activity evaluation

Synthesis and evaluation of azetidinone analogues of combretastatin A-4 as tubulin targeting agents

The synthesis and antiproliferative activity of a new series of rigid analogues of combretastatin A-4 are described which contain the 1,4-diaryl-2-azetidinone (β-lactam) ring system in place of the usual ethylene bridge present in the natural combretastatin stilbene products. These novel compounds are also substituted at position 3 of the β-lactam ring with an aryl ring. A number of analogues showed potent nanomolar activity in human MCF-7 and MDA-MB-231 breast cancer cell lines, displayed in vitro inhibition of tubulin polymerization, and did not cause significant cytotoxicity in normal murine breast epithelial cells. 4-(4-Methoxyaryl)-substituted compound 32, 4-(3-hydroxy-4-methoxyaryl)-substituted compounds 35 and 41, and the 3-(4-aminoaryl)-substituted compounds 46 and 47 displayed the most potent antiproliferative activity of the series. β-Lactam 41 in particular showed subnanomolar activity in MCF-7 breast cancer cells (IC₅₀= 0.8 nM) together with significant in vitro inhibition of tubulin polymerization and has been selected for further biochemical assessment. These novel β-lactam compounds are identified as potentially useful scaffolds for the further development of antitumor agents that target tubulin.

Discovery of novel 2-aryl-4-benzoyl-imidazoles targeting the colchicines binding site in tubulin as potential anticancer agents

A series of 2-aryl-4-benzoyl-imidazoles (ABI) was synthesized as a result of structural modifications based on the previous set of 2-aryl-imidazole-4-carboxylic amide (AICA) derivatives and 4-substituted methoxylbenzoyl-aryl-thiazoles (SMART). The average IC(50) of the most active compound (5da) was 15.7 nM. ABI analogues have substantially improved aqueous solubility (48.9 μg/mL for 5ga vs 0.909 μg/mL for SMART-1, 0.137 μg/mL for paclitaxel, and 1.04 μg/mL for combretastatin A4). Mechanism of action studies indicate that the anticancer activity of ABI analogues is through inhibition of tubulin polymerization by interacting with the colchicine binding site. Unlike paclitaxel and colchicine, the ABI compounds were equally potent against multidrug resistant cancer cells and the sensitive parental melanoma cancer cells. In vivo results indicated that 5cb was more effective than DTIC in inhibiting melanoma xenograph tumor growth. Our results suggest that the novel ABI compounds may be developed to effectively treat drug-resistant tumors.

Lead identification of conformationally restricted β-lactam type combretastatin analogues: synthesis, antiproliferative activity and tubulin targeting effects

The synthesis and study of the structure-activity relationships of a series of rigid analogues of combretastatin A-4 are described which contain the 1,4-diaryl-2-azetidinone (β-lactam) ring system in place of the usual ethylene bridge present in the natural combretastatin stilbene products. The 1,4-diaryl-2-azetidinones are unsubstituted at C-3, or contain methyl substituent(s) at C-3. The most potent compounds 12d and 12e display antiproliferative activity at nanomolar concentrations when evaluated against the MCF-7 and MDA-MB-231 human breast carcinoma cell lines. 12d exerts antimitotic effects through an inhibition of tubulin polymerisation and subsequent G2/M arrest of the cell cycle in human MDA-MB-231 breast cancer cells, with similar activity to that of CA-4. These novel β-lactam compounds are identified as potentially useful scaffolds for the further development of antitumour agents which target tubulin.

C-H bonds as ubiquitous functionality: a general approach to complex arylated imidazoles via regioselective sequential arylation of all three C-H bonds and regioselective N-alkylation enabled by SEM-group transposition

Imidazoles are an important group of the azole family of heterocycles frequently found in pharmaceuticals, drug candidates, ligands for transition metal catalysts, and other molecular functional materials. Owing to their wide application in academia and industry, new methods and strategies for the generation of functionalized imidazole derivatives are in demand. We here describe a general and comprehensive approach for the synthesis of complex aryl imidazoles, where all three C-H bonds of the imidazole core can be arylated in a regioselective and sequential manner. We report new catalytic methods for selective C5- and C2-arylation of SEM-imidazoles and provide a mechanistic hypothesis for the observed positional selectivity based on electronic properties of C-H bonds and the heterocyclic ring. Importantly, aryl bromides and low-cost aryl chlorides can be used as arene donors under practical laboratory conditions. To circumvent the low reactivity of the C-4 position, we developed the SEM-switch that transfers the SEM-group from N-1 to N-3 nitrogen and thus enables preparation of 4-arylimidazoles and sequential C4-C5-arylation of the imidazole core. Furthermore, selective N3-alkylation followed by the SEM-group deprotection (trans-N-alkylation) allows for regioselective N-alkylation of complex imidazoles. The sequential C-arylation enabled by the SEM-switch allowed us to produce a variety of mono-, di-, and triarylimidazoles using diverse bromo- and chloroarenes. Using our approach, the synthesis of individual compounds or libraries of analogues can begin from either the parent imidazole or a substituted imidazole, providing rapid access to complex imidazole structures.

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

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

Synthesis and antitumor activity of 1,5-disubstituted 1,2,4-triazoles as cis-restricted combretastatin analogues

A series of 1-aryl-5-(3',4',5'-trimethoxyphenyl) derivatives and their related 1-(3',4',5'-trimethoxyphenyl)-5-aryl-1,2,4-triazoles, designed as cis-restricted combretastatin analogues, were synthesized and evaluated for antiproliferative activity, inhibitory effects on tubulin polymerization, cell cycle effects, and apoptosis induction. Their activity was greater than, or comparable with, that of the reference compound CA-4. Flow cytometry studies showed that HeLa and Jurkat cells treated with the most active compounds 4l and 4o were arrested in the G2/M phase of the cell cycle in a concentration dependent manner. This effect was accompanied by apoptosis of the cells, mitochondrial depolarization, generation of reactive oxygen species, activation of caspase-3, and PARP cleavage. Compound 4l was also shown to have potential antivascular activity, since it induced endothelial cell shape change in vitro and disrupted the sprouting of endothelial cells in the chick aortic ring assay.

Oxazole-bridged combretastatin A analogues with improved anticancer properties

Three new oxazole-bridged combretastatin A analogues with additional functional groups at the B-ring [-SMe, -OH, p-quinone] were tested for antiproliferative activity and specificity on human HL-60 leukemia, 518A2 melanoma, and colon carcinomas HCT-116 (wt)/(p53(-/-)) and HT-29 cells. While all oxazoles, except quinone 8, were efficacious against HCT-116 cells at submicromolar IC(50) values (48 h incubation), only thioanisole 5 achieved this potency in combretastatin-refractory HT-29 cells by significant upregulation of p21(cip1/waf1) associated with an S/G(2) cell-cycle arrest.

Discovery of a potent tubulin polymerization inhibitor: synthesis and evaluation of water-soluble prodrugs of benzophenone analog

Prodrugs have proven to be very useful in enhancing aqueous solubility of sparingly water-soluble drugs, thereby increasing in vivo efficacy without a need of special excipients. In vitro and in vivo evaluations of a number of amino acid prodrugs of 1, a previously identified potent tubulin polymerization inhibitor and cytotoxic against various cancer cell lines led to the discovery of 3·HCl (l-valine attached) which is highly efficacious in mouse xenografts bearing human cancer. Pharmacokinetic analysis in rats revealed that compound 1 was released immediately upon administration of 3·HCl intravenously, with rapid clearance of 3·HCl indicating the effective cleavage of prodrug. Compound 3·HCl (CKD-516) has now been progressed to phase 1 clinical trial.

L-Proline Triflate as an Efficient and Reusable Catalyst for the One-Pot Synthesis of 2,4,5-Trisubstituted Imidazoles and 1,2,4,5-Tetrasubstituted Imidazoles

L-Proline triflate has been used as an efficient catalyst for an improved and rapid one-pot synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles in good yields. The benefit of this approach is its operational simplicity and the catalyst can be easily recovered and reused without any loss of activity.

Synthesis and antitumor-evaluation of cyclopropyl-containing combretastatin analogs

Several derivatives of combretastatin have been prepared bearing a cyclopropyl unit instead of the natural occurring cis-double bond. Final products and synthetic intermediates were evaluated for their cytotoxic properties in two human cancer cell lines.

Synthesis and in-vitro antitumor activities of some mannich bases of 9-alkyl-1,2,3,4-tetrahydrocarbazole-1-ones

A novel series of 2-substituted aminomethyl-9-alkyl-1,2,3,4-tetrahydrocarbazole-1-ones 5a-q was synthesized via aminomethylation of 9-alkyl-1,2,3,4-tetrahydrocarbazole-1-ones 4a-e with hydrochlorides of the respective amines 6a-m. The structures of these newly synthesized compounds were characterized by (1)H-NMR, MS, and elemental analysis. All the compounds were tested for their cytotoxic activity in vitro against four human tumor cell lines including human non-small lung cancer cells (A549), human gastric adenocarcinoma (SGC), human colon cancer cell (HCT116), human myeoloid leukemia cells (K562), and one multi-drug resistant subline (KB-VCR). Most compounds showed moderate to potent cytotoxic activity against the tested cell lines. Preliminary mechanism research indicated that the most promising compound, 2-diethylaminomethyl-9-methyl-1,2,3,4-tetrahydrocarbazole-1-one 5c, exhibited a potential inhibitory effect against microtubule.

Discovery of 4-amino and 4-hydroxy-1-aroylindoles as potent tubulin polymerization inhibitors

1-Aroylindoline, 1-aroyl-1,2,3,4-tetrahydroquinoline, and 1-aroylindole derivatives were synthesized and evaluated for anticancer activity. The 4-amino and 4-hydroxy-1-aroylindoles 26 and 27 with IC 50 of 0.9 and 0.6 microM, respectively, exhibited antitubulin activity superior or comparable to that of colchicine and combretastatin A-4. They also showed antiproliferative activity with IC 50 of 0.3-5.4 nM in a set of human cancer cell lines.

Molecular modeling approaches to study the binding mode on tubulin of microtubule destabilizing and stabilizing agents

Tubulin targeting agents constitute an important class of anticancer drugs. By acting either as microtubule stabilizers or destabilizers, they disrupt microtubule dynamics, thus inducing mitotic arrest and, ultimately, cell death by apoptosis. Three different binding sites, whose exact location on tubulin has been experimentally detected, have been identified so far for antimitotic compound targeting microtubules, namely the taxoid, the colchicine and the vinka alkaloid binding site. A number of ligand- and structure-based molecular modeling studies in this field has been reported over the years, aimed at elucidating the binding modes of both stabilizing and destabilizing agent, as well as the molecular features responsible for their efficacious interaction with tubulin. Such studies are described in this review, focusing on information provided by different modeling approaches on the structural determinants of antitubulin agents and the interactions with the binding pockets on tubulin emerged as fundamental for antitumor activity.To describe molecular modeling approaches applied to date to molecules known to bind microtubules, this paper has been divided into two main parts: microtubule destabilizing (Part 1) and stabilizing (Part 2) agents. The first part includes structure-based and ligand-based approaches to study molecules targeting colchicine (1.1) and vinca alkaloid (1.2) binding sites, respectively. In the second part, the studies performed on microtubule-stabilizing antimitotic agents (MSAA) are described. Starting from the first representative compound of this class, paclitaxel, molecular modeling studies (quantitative structure-activity relationships - QSAR - and structure-based approaches), performed on natural compounds acting with the same mechanism of action and temptative common pharmacophoric hypotheses for all of these compounds, are reported.

Synthesis and biological evaluation of imidazol-2-one derivatives as potential antitumor agents

A new series of aryl substituted imidazol-2-one derivatives structurally related to combretastatin A-4 (CA-4) were synthesized and evaluated for their cytotoxic activities in vitro against various human cancer cell lines including MDR cell line. The cytotoxic effects of compounds 7b and 7i proved to be similar to or greater than that of docetaxel. The highly active compound 7b also exhibited excellent inhibitory activity on tumor growth in vivo.

Design, synthesis, and biological testing of pyrazoline derivatives of combretastatin-A4

Fourteen N-acetylated and non-acetylated 3,4,5-tri- or 2,5-dimethoxypyrazoline analogs of combretastatin-A4 (1) were synthesized. A non-acetylated derivative (5a) with the same substituents as CA-4 (1) was the most active compound in the series, with IC(50) values of 2.1 and 0.5 microM in B16 and L1210 cell lines, respectively. In contrast, a similar compound with an acetyl group at N1 of the pyrazoline ring (6g) showed poor activity in the cell lines studied. A cell-based assay indicated that compound 5a caused extensive microtubule depolymerization with an EC(50) value of 7.1 microM in A-10 cells while no activity was seen with the acetylated compound. Molecular modeling studies showed that these compounds possess a twisted conformation similar to CA-4 (1).

Antimitotic activities of 2-phenylindole-3-carbaldehydes in human breast cancer cells

Small molecules such as indoles are attractive as inhibitors of tubulin polymerization. Thus a number of 2-phenylindole-3-carbaldehydes with lipophilic substituents in both aromatic rings was synthesized and evaluated for antitumor activity in MDA-MB 231 and MCF-7 breast cancer cells. Some 5-alkylindole derivatives with a 4-methoxy group in the 2-phenyl ring strongly inhibit the growth of breast cancer cells with IC(50) values of 5-20nM. Their action can be rationalized by the cell cycle arrest in G(2)/M phase due to the inhibition of tubulin polymerization.