New inhibitors of tubulin polymerisation

B-ring-modified isocombretastatin A-4 analogues endowed with interesting anticancer activities

A novel class of isocombretastatin A-4 (isoCA-4) analogues with modifications at the 3'-position of the B-ring by replacement with C-linked substituents was studied. Exploration of the structure-activity relationships of theses analogues led to the identification of several compounds that exhibit excellent antiproliferative activities in the nanomolar concentration range against H1299, MDA-MB231, HCT116, and K562 cancer cell lines; they also inhibit tubulin polymerization with potency similar to that of isoCA-4. 1,1-Diarylethylenes 8 and 17, respectively with (E)-propen-3-ol and propyn-3-ol substituents at the 3'-position of the B-ring, proved to be the most active in this series. Both compounds led to the arrest of various cancer cell lines at the G(2) /M phase of the cell cycle and strongly induced apoptosis. Docking of compounds 8 and 17 in the colchicine binding site indicated that their C3' substituents guide the positioning of the B-ring in a manner different from that observed for isoCA-4.

Synthesis and biological evaluation of phenstatin metabolites

Previous investigations on the incubation of phenstatin with rat and human microsomal fractions revealed the formation of nine main metabolites. The structures of eight of these metabolites have been now confirmed by synthesis and their biological properties have been reported. Eaton's reagent was utilized as a convenient condensing agent, allowing, among others, a simple multigram scale preparation of phenstatin. Synthesized metabolites and related compounds were evaluated for their antiproliferative activity in the NCI-60 cancer cell line panel, and for their effect on microtubule assembly. Metabolite 23 (2'-methoxyphenstatin) exhibited the most potent in vitro cytotoxic activity: inhibition of the growth of K-562, NCI-H322M, NCI-H522, KM12, M14, MDA-MB-435, NCI/ADR-RES, and HS 578T cell lines with GI(50) values <10nM. It also showed more significant tubulin polymerization inhibitory activity than parent phenstatin (3) (IC(50)=3.2 μM vs 15.0 μM) and induced G2/M arrest in murine leukemia DA1-3b cells. The identification of this active metabolite led to the design and synthesis of analogs with potent in vitro cytotoxicity and inhibition of microtubule assembly.

In vitrometabolism of a triclyclic alkaloid (M445526) in human liver microsomes and hepatocytes

The in vitro metabolism of M445,526 (ZD6,126 phenol) was investigated by incubating [(14)C]-M445,526 at a concentration of 10 microg ml(-1) with human hepatic microsomes (4 mg ml(-1)) or human hepatocytes (2 x 10(6) cells ml(-1)) for up to 180 min. Following incubation with microsomes and hepatocytes, up to 78% and 40% of [(14)C]-M445,526 was metabolized after 180 and 120 min, respectively. High-performance liquid chromatography (HPLC) with radiochemical detection confirmed extensive metabolism of [(14)C]-M445,526 by microsomes and hepatocytes. Mass spectrometry and (1)H-NMR spectroscopy enabled structural identification of up to eight metabolites. Human liver microsomes formed one major (O-desmethyl) and three minor (a further O-desmethyl and two different hydroxylated) phase I metabolites. Human hepatocytes produced one major metabolite, a sulphate conjugate of the major O-desmethyl metabolite formed by microsomes. Four minor metabolites were also formed, primarily by O-demethylation with subsequent glucuronidation. Taken collectively, [(14)C]-M445,526 underwent extensive in vitro metabolism by human liver fractions. These data were confirmed by subsequent human in vivo studies.

[(2-Phenylindol-3-yl)methylene]propanedinitriles inhibit the growth of breast cancer cells by cell cycle arrest in G2/M phase and apoptosis

Cell cycle arrest of malignant cells is an important option for cancer treatment. In this study, we modified the structure of antimitotic 2-phenylindole-3-carbaldehydes by condensation with malononitrile. The resulting methylene propanedinitriles inhibited the growth of MDA-MB 231 and MCF-7 breast cancer cells with IC(50) values below 100 nM. Though they exhibited similar structure-activity relationships as the aldehydes, they did not inhibit tubulin polymerization but were capable of blocking the cell cycle in G(2)/M phase. The cell cycle arrest was accompanied by apoptosis as demonstrated by the activation of caspases 3 and 9. Since the new 2-phenylindole derivatives also inhibited the growth of transplanted MXT mouse mammary tumors, they are interesting candidates for further development.

Colchicine glycorandomization influences cytotoxicity and mechanism of action

The reaction of 70 unprotected, diversely functionalized free reducing sugars with methoxyamine-appended colchicine led to the production of a 58-member glycorandomized library. High-throughput cytotoxicity assays revealed glycosylation to modulate specificity and potency. Library members were also identified which, unlike the parent natural product (a destabilizer), stabilized in vitro tubulin polymerization in a manner similar to taxol. This study highlights a simple extension of neoglycorandomization toward amine-bearing scaffolds and the potential benefit of glycosylating nonglycosylated natural products.

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.

9-Benzylidene-naphtho[2,3-b]thiophen-4-ones and benzylidene-9(10H)-anthracenones as novel tubulin interacting agents with high apoptosis-inducing activity

Tubulin-binding 9-benzylidene-naphtho[2,3-b]thiophen-4-ones 1a and 1b and benzylidene-9(10H)-anthracenone 2 were evaluated for their ability to induce cell death. We examined the effect of the molecules on cell cycle progression, organization of microtubule networks, and apoptosis induction. As determined by flow cytometry, cancer cells were predominantly arrested in metaphase with 4N DNA before cell death occurred. By using indirect immunofluorescence techniques we visualized microtubule depolymerization recognizable by short microtubule fragments scattered around the nucleus. The incubation with 1a and 2 resulted in chromatin condensation, nuclear fragmentation, and cell shrinkage, which are, among others, typical features of apoptotic cell death. Furthermore, time- and dose-dependent induction of apoptosis in SH-SY5Y cells was detected via cleavage of Ac-DEVD-AMC, a fluorigenic substrate for caspase-3. We observed a lower apoptotic activity in neuroblastoma cells overexpressing Bcl-xL, suggesting activation of the mitochondrial apoptosis pathway. Western blot analysis demonstrated that caspase-3, an apoptosis mediator, was activated in a time-dependent manner after exposure of SH-SY5Y cells to drugs 1a and 2. Taken together, the agents investigated in the present study display strong apoptosis-inducing activity and therefore show promise for the development of novel chemotherapeutics.

Synthesis and biological activity of analogues of the antimicrotubule agent N,beta,beta-trimethyl-L-phenylalanyl-N(1)-[(1S,2E)-3-carboxy-1-isopropylbut-2-enyl]- N(1),3-dimethyl-L-valinamide (HTI-286)

Hemiasterlin, a tripeptide isolated from marine sponges, induces microtubule depolymerization and mitotic arrest in cells. HTI-286, an analogue from an initial study of the hemiasterlins, is presently in clinical trials. In addition to its potent antitumor effects, 2 has the advantage of circumventing the P-glycoprotein-mediated resistance that hampers the efficacy of other antimicrotubule agents such as paclitaxel and vincristine in animal models. This paper describes an in-depth study of the structure--activity relationships of analogues of 2, their effects on microtubule polymerization, and their in vitro and in vivo anticancer activity. Regions of the molecule necessary for potent activity are identified. Groups tolerant of modification, leading to novel analogues, are reported. Potent analogues identified through in vivo studies in tumor xenograft models include one superior analogue, HTI-042.

Novel benzylidene-9(10H)-anthracenones as highly active antimicrotubule agents. Synthesis, antiproliferative activity, and inhibition of tubulin polymerization

A novel series of 10-benzylidene-9(10H)-anthracenones and 10-(phenylmethyl)-9(10H)-anthracenones were synthesized and evaluated for antiproliferative activity in an assay based on K562 leukemia cells. The 3-hydroxy-4-methoxybenzylidene analogue 9h was found to be the most active compound (IC(50) K562: 20 nM). Structure-activity relationships are also considered. The highly active compound 9h and the 2,4-dimethoxy-3-hydroxybenzylidene analogue 9l were tested against five tumor cell lines using the XTT assay, including multidrug resistant phenotypes. Induction of cell death in a variety of tumor cell lines was determined in a monolayer assay using propidium iodide. Noteworthy, all compounds within the series induced elongations in K562 cells similar to vinblastine-treated cells. The effect of the lead compound 9h on K562 cell growth was associated with cell cycle arrest in G2/M. Concentrations for 50% KB/HeLa cells arrested in G2/M after treatment with 9h and 9l were determined and found to be in the range of 0.2 microM. Additionally, we monitored the dose dependent caspase-3-like protease activity in K562 cells and MCF-7/Casp-3 cells treated with 9h, indicating induction of apoptosis. Western blotting analysis demonstrated that 9h caused a shift in tubulin concentration from the polymerized state found in the cell pellet to the unpolymerized state found in the cell supernatant. Seven compounds strongly inhibited tubulin polymerization with activities higher or comparable to those of the reference compounds such as colchicine, podophyllotoxin, and nocodazole. In general, the antiproliferative activity correlated with inhibition of tubulin polymerization. The most active compounds strongly displaced [(3)H]colchicine from its binding site in the tubulin, yielding IC(50) values 3- to 4-fold lower than that of colchicine. The novel benzylidene-9(10H)-anthracenones described in the present study constitute an interesting group of highly active and easily accessible antimitotic agents that inhibit tubulin polymerization.

A convergent approach to cryptophycin 52 analogues: synthesis and biological evaluation of a novel series of fragment a epoxides and chlorohydrins

Cryptophycin 52 is a synthetic derivative of Cryptophycin 1, a potent antimicrotubule agent isolated from cyanobacteria. In an effort to increase the potency and water solubility of the molecule, a structure-activity relationship study (SAR) was initiated around the phenyl ring of fragment A. These Cryptophycin 52 analogues were accessed using a Wittig olefination reaction between various triphenylphosphonium salts and a key intermediate aldehyde prepared from Cryptophycin 53. Substitution on the phenyl ring of fragment A was well tolerated, and several of these analogues were equally or more potent than Cryptophycin 52 when evaluated in vitro in the CCRF-CEM leukemia cell line and in vivo against a murine pancreatic adenocarcinoma.

A fluorescence-based high-throughput assay for antimicrotubule drugs

With the advent of combinatorial chemistry and the extensive libraries of potential drugs produced from it, there is a growing need for rapid sensitive, high-throughput screening for drug potency. Microtubules are important targets for anticancer agents, and new antimicrotubule compounds are of continued interest in drug development. The in vitro potency of antimicrotubule drugs may be evaluated by measuring the extent of tubulin assembly. The extent of polymerization is proportional to the turbidity of the solution, which usually has been measured as apparent absorption. The turbidity method has inherent problems that hinder its adaptation to a high-throughput format, such as a requirement for high protein concentrations and a high coefficient of variation. We present here a high-throughput assay for antimicrotubule activity in which fluorescence is used to monitor microtubule assembly. Both assembly-inhibiting and assembly-promoting compounds can be evaluated. The assay is rapid and easy to perform, and the data are reliable, with good accuracy and reproducibility.

Small molecule antagonists of proteins

The identification of small molecule antagonists of protein function is at the core of the pharmaceutical industry. Successful approaches to this problem, including screening and rational design, have been developed over the years to identify antagonists of enzymes and cellular receptors. These methods have been extended to the search for inhibitors of protein-protein interactions. While the very possibility of designing a small molecule inhibitor for such interactions was once doubted, there are examples of such inhibitors that are currently marketed products and many more inhibitors in various stages of research and development. Here we review the progress in identifying and designing small molecule protein inhibitors, with particular attention to those that block protein-protein interactions. We also discuss the physical character of protein-protein interfaces, and the resulting implications for small molecule lead discovery and design.

Recent advances in the field of tubulin polymerization inhibitors

In recent years, enormous progress has been made in the field of tubulin targeting agents. Several companies and academic laboratories have entered this field and competition has become very strong. Nevertheless, clinically promising compounds often face substantial limitations, such as high systemic toxicity, poor water solubility and bioavailability, as well as complex synthesis and isolation procedures. As a drawback of established drugs, like paclitaxel or the vinca alkaloids, the outcome of cancer chemotherapy is often affected by the emergence of the multidrug resistance phenotype. Among the recently disclosed tubulin polymerization inhibitors, there are several interesting low molecular weight compounds with improved oral bioavailability and demonstrated activity against multi-drug resistance positive phenotypes. As documented by the imidazole-based combretastatin analogs, to name just one example, chemical optimization of a lead structure resulted in compounds with potent in vitro and in vivo activity along with appropriate pharmacodynamic and pharmacokinetic requirements for a potential therapeutic candidate. Currently, several compounds are undergoing Phase I or Phase II clinical trials, among them orally bioavailable sulfonamides or dolastatin 10. Several other compounds are close to entering Phase I trials. The purpose of this review is to focus on the most recent advances in tubulin polymerization inhibitors from a survey of the published patent literature and related publications between late 1999 and April 2002. However, biological data, especially for the inhibition of tubulin polymerization, obtained from different laboratories cannot easily be compared.

Synthesis and biological evaluation of 2-indolyloxazolines as a new class of tubulin polymerization inhibitors. Discovery of A-289099 as an orally active antitumor agent

A series of indole containing oxazolines has been discovered as a result of structural modifications of the lead compound A-105972. The compounds exert their anticancer activity through inhibition of tubulin polymerization by binding at the colchicine site. A-289099 was identified as an orally active antimitotic agent active against various cancer cell lines including those that express the MDR phenotype. The anticancer activity, pharmacokinetics, and an efficient and enantioselective synthesis of A-289099 are described.

Novel sulfonate derivatives: potent antimitotic agents

The synthesis and biological evaluation of novel sulfonate analogues of E-7010 are reported. Several of the compounds are potent inhibitors of cell proliferation and tubulin polymerization. Importantly, these compounds are also active against P-glycoprotein positive (+) cancer cells, which are resistant to many other antitumor agents.