Ernest Hamel

Isolation and structure of the strong cell growth and tubulin inhibitor combretastatin A-4

The African treeCombretum caffrum (Combretaceae) has been found to contain a powerful inhibitor of tubulin polymerization (IC502–3 μM), the growth of murine lymphocytic leukemia (L 1210 and P 388 with ED50≈0.003 μM and human colon cancer cell lines [(e.g. LoVo (ED50=0.005 μg/ml), HT29 (ED50 0.02 μg/ml, Colo 205 (ED50=0.07 μg/ml), DLD-1 (ED50=0.005 μg/ml) and HCT-15 (ED50=0.0009 μg/ml)] designated combretastatin A-4 (1c). The structure assigned by spectral techniques was confirmed by synthesis.

Activities of the Microtubule-stabilizing Agents Epothilones A and B with Purified Tubulin and in Cells Resistant to Paclitaxel (Taxol®)

Epothilones A and B, natural products with minimal structural analogy to taxoids, have effects similar to those of paclitaxel (Taxol®) in cultured cells and on microtubule protein, but differ from paclitaxel in retaining activity in multidrug-resistant cells. We examined interactions of the epothilones with purified tubulin and additional cell lines, including a paclitaxel-resistant ovarian carcinoma line with an altered β-tubulin. The epothilones, like paclitaxel, induced tubulin to form microtubules at low temperatures and without GTP and/or microtubule-associated proteins. The epothilones are competitive inhibitors of the binding of [3H]paclitaxel to tubulin polymers. The apparent Ki values for epothilones A and B were 1.4 and 0.7 μM by Hanes analysis and 0.6 and 0.4 μM by Dixon analysis. In the paclitaxel-sensitive human cell lines we examined, epothilone B had greater antiproliferative activity than epothilone A or paclitaxel, while epothilone A was usually less active than paclitaxel. A multidrug-resistant colon carcinoma line and the paclitaxel-resistant ovarian line retained sensitivity to the epothilones. With Potorous tridactylis kidney epithelial (PtK2) cells examined by indirect immunofluorescence, microtubule bundles appeared more rapidly following epothilone B treatment, and there were different proportions of various mitotic aberrations following treatment with different drugs.

Dolastatin 10, a powerful cytostatic peptide derived from a marine animal: Inhibition of tubulin polymerization mediated through the vinca alkaloid binding domain

Dolastatin 10, a cytostatic peptide containing several unique amino acid subunits, was isolated from the marine shell-less mollusk Dolabella auricularia (Pettit GR, Kamano Y, Herald CL, Tuinman AA, Boettner FE, Kizu H, Schmidt JM, Baczynskyj L, Tomer KB and Bontems RJ, J Am Chem Soc 109: 6883-6885, 1987). Since our preliminary studies demonstrated that dolastatin 10 inhibited tubulin polymerization and the binding of radiolabeled vinblastine to tubulin, an initial characterization of the properties of dolastatin 10 included a comparison to other antimitotic drugs interfering with vinca alkaloid binding to tubulin (vinblastine, maytansine, rhizoxin, and phomopsin A). Dolastatin 10 inhibited the growth of L1210 murine leukemia cells in culture, with a concordant rise in the mitotic index, and its IC50 value for cell growth was 0.5 nM. Comparable values for the other drugs were 0.5 nM for maytansine, 1 nM for rhizoxin, 20 nM for vinblastine, and 7 microM for phomopsin A. IC50 values were also obtained for the polymerization of purified tubulin in glutamate: 1.2 microM for dolastatin 10, 1.4 microM for phomopsin A, 1.5 microM for vinblastine, 3.5 microM for maytansine, and 6.8 microM for rhizoxin. Dolastatin 10 and vinblastine were comparable in their effects on microtubule assembly dependent on microtubule-associated proteins. Preliminary studies indicated that dolastatin 10, like vinblastine, causes formation of a cold-stable tubulin aggregate at higher drug concentrations. We confirmed that rhizoxin, phomopsin A, and maytansine also inhibit the binding of radiolabeled vinblastine and vincristine to tubulin. Dolastatin 10 and phomopsin A were the strongest inhibitors of these reactions, and rhizoxin the weakest. Dolastatin 10, phomopsin A, maytansine, vinblastine, and rhizoxin all inhibited tubulin-dependent GTP hydrolysis. The greatest inhibition of hydrolysis was observed with dolastatin 10 and phomopsin A, and the least inhibition with rhizoxin.

Evaluation of antimitotic agents by quantitative comparisons of their effects on the polymerization of purified tubulin

Most antimitotic compounds have highly specific interactions with tubulin, the major protein component of microtubules. It is, therefore, often desirable to characterize interactions of these agents with tubulin. In particular, quantitative comparisons between new and old (“standard”) agents, between different classes of agent, and between structural analogs (e.g., for a structure-activity relationship study) are important. Because antimitotic drugs have a variety of effects on tubulin and bind at multiple distinct sites on the protein, the tubulin assembly reaction is probably the only universally applicable reaction that can be analyzed. In my laboratory, we use the assembly of purified tubulin induced by higher concentrations of monosodium glutamate as our basic assay system. This report presents a detailed description of our current routine assay, including the effects of a variety of reaction components on the reaction. In addition, the variety of effects that reaction components can have on the quantitative results obtained with drugs, using the colchicine site drug combretastatin A-4 as a model compound, is described.

Natural products which interact with tubulin in the vinca domain: maytansine, rhizoxin, phomopsin A, dolastatins 10 and 15 and halichondrin B.

This paper summarizes published data on the interactions of tubulin with antimitotic compounds that inhibit the binding of vinca alkaloids to the protein. These are all relatively complex natural products isolated from higher plants, fungi and marine invertebrate animals. These agents are maytansine, rhizoxin, phomopsin A, dolastatins 10 and 15 and halichondrin B and their congeners. Effects on tubulin polymerization, ligand binding interactions and structure-activity relationships are emphasized.

Identification of cysteine 354 of beta-tubulin as part of the binding site for the A ring of colchicine.

The colchicine analog 3-chloroacetyl-3-demethylthiocolchicine (3CTC) is a competitive inhibitor of colchicine binding to tubulin, binds to tubulin at 37°C, but not at 0°C, and covalently reacts with β-tubulin at 37°C, but not at 0°C, in a reaction inhibited by colchicine site drugs. The approximate intramolecular distance between the oxygen at position C-3 in 3CTC and the chlorine atom of the 3-chloroacetyl group is 3 Å. Using decylagarose chromatography, we purified β-tubulin that had reacted with 3-(chloromethyl-[14C]carbonyl)-3-demethylthiocolchicine ([14C]3CTC). This β-tubulin was digested with formic acid, cyanogen bromide, endoproteinase Glu-C, or endoproteinase Lys-C, and the radiolabeled peptide(s) were isolated. The sequences of these peptides indicated that as much as 90% of the covalent reaction between the [14C]3CTC and β-tubulin occurred at cysteine 354. This finding indicates that the C-3 oxygen atom of colchicinoids is within 3 Å of the sulfur atom of the Cys-354 residue, suggests that the colchicine A ring lies between Cys-354 and Cys-239, based on the known 9 Å distance between these residues, and may indicate that the tropolone C ring lies between the peptide region containing Cys-239 and the amino-terminal β-tubulin sequence, based on the labeling pattern observed following direct photoactivation of tubulin-bound colchicine.

Glutamate-induced polymerization of tubulin: Characteristics of the reaction and application to the large-scale purification of tubulin

Abstract A three-stage method is presented for the large-scale purification of calf brain tubulin, exploiting the ability of high concentrations of glutamate to stabilize tubulin and, in the presence of GTP, to induce its polymerization. The purified tubulin contained 1.9 mol of guanine nucleotide per mole of protein and was essentially free of nucleoside diphosphate kinase and ATPase activities. The glutamate-induced polymerization reaction required GTP, while ATP, CTP, and UTP were completely ineffective. Polymerization was temperature dependent and cold reversible and was inhibited by colchicine, GDP, and Ca 2+ , but no absolute requirement for Mg 2+ could be demonstrated. The purified tubulin had only a minimal GTPase activity unless glutamate was present, and the hydrolytic reaction varied with the glutamate concentration. At higher glutamate levels the onset of hydrolysis was closely linked to the onset of polymerization with a rapid burst of GTP hydrolysis ending as polymerization approached its plateau, followed by a slower, linear rate of hydrolysis. Initially the molar ratio of GTP hydrolyzed to tubulin polymerized was about 1:1. At lower glutamate concentrations GTP hydrolysis did not appear to be linked to tubulin polymerization.

Antitumor 2,3-dihydro-2-(aryl)-4(1H)-quinazolinone derivatives: Interactions with tubulin

A series of derivatives of 2,3-dihydro-2-(aryl)-4(1H)-quinazolinone (DHQZ) with known antitumor activity was re-evaluated in the National Cancer Institute cancer cell line screen. Analysis by the COMPARE algorithm suggested that their cytotoxicity derived from interactions with tubulin. Significant inhibition of tubulin assembly and of the binding of radiolabeled colchicine to tubulin was demonstrated with several of the compounds, particularly NSC 145669, 175635, and 175636. The DHQZ derivatives are structurally analogous to a number of antimitotic agents, flavonols and derivatives of 2-styrylquinazolin-4(3H)-one and of 2-phenyl-4-quinolone. Structure-activity analogies between these agents, the combretastatins, and the colchicinoids were analyzed and summarized.

Asymmetric Synthesis of 2,3-Dihydro-2-arylquinazolin-4-ones: Methodology and Application to a Potent Fluorescent Tubulin Inhibitor with Anticancer Activity

For several decades the 2,3-dihydroquinazolinone (DHQZ) heterocycle has been known to possess a variety of important biological and medicinal properties. Despite the many interesting facets of these molecules, synthetic access to nonracemic DHQZ analogues has remained elusive. Herein, we disclose a synthetic route that allows access to either enantiomer of a variety of DHQZ derivatives. We illustrate the utility of this chemistry with the asymmetric preparation and biological evaluation of a new chiral fluorescent tubulin binding agent with extremely potent antiproliferative properties against human cancer cells. A computational rationale for the increased potency of the (S)-enantiomer over the (R)-enantiomer is given, based on the crystal structure of alpha,beta-tubulin complexed with colchicine. Taking advantage of the inherent fluorescence of these molecules, confocal images of GMC-5-193 (compound 7) in the cytoplasm of human melanoma cells (MDA-MB-435) cells are presented.

Discovery of 7-Hydroxy-6-methoxy-2-methyl-3-(3,4,5- trimethoxybenzoyl)benzo[b]furan (BNC105), a Tubulin Polymerization Inhibitor with Potent Antiproliferative and Tumor Vascular Disrupting Properties

A structure-activity relationship (SAR) guided design of novel tubulin polymerization inhibitors has resulted in a series of benzo[b]furans with exceptional potency toward cancer cells and activated endothelial cells. The potency of early lead compounds has been substantially improved through the synergistic effect of introducing a conformational bias and additional hydrogen bond donor to the pharmacophore. Screening of a focused library of potent tubulin polymerization inhibitors for selectivity against cancer cells and activated endothelial cells over quiescent endothelial cells has afforded 7-hydroxy-6-methoxy-2-methyl-3-(3,4,5-trimethoxybenzoyl)benzo[b]furan (BNC105, 8) as a potent and selective antiproliferative. Because of poor solubility, 8 is administered as its disodium phosphate ester prodrug 9 (BNC105P), which is rapidly cleaved in vivo to return the active 8. 9 exhibits both superior vascular disrupting and tumor growth inhibitory properties compared with the benchmark agent combretastatin A-4 disodium phosphate 5 (CA4P).