On the total synthesis and preliminary biological evaluations of 15(R) and 15(S) aza-dEpoB: a Mitsunobu inversion at C15 in pre-epothilone fragments

Meayamycin inhibits pre-messenger RNA splicing and exhibits picomolar activity against multidrug-resistant cells

FR901464 is a potent antitumor natural product that binds to the splicing factor 3b complex and inhibits pre-mRNA splicing. Its analogue, meayamycin, is two orders of magnitude more potent as an antiproliferative agent against human breast cancer MCF-7 cells. Here, we report the picomolar antiproliferative activity of meayamycin against various cancer cell lines and multidrug-resistant cells. Time-dependence studies implied that meayamycin may form a covalent bond with its target protein(s). Meayamycin inhibited pre-mRNA splicing in HEK-293 cells but not alternative splicing in a neuronal system. Meayamycin exhibited specificity toward human lung cancer cells compared with nontumorigenic human lung fibroblasts and retained picomolar growth-inhibitory activity against multidrug-resistant cells. These data suggest that meayamycin is a useful chemical probe to study pre-mRNA splicing in live cells and is a promising lead as an anticancer agent.

Theoretical basis, experimental design, and computerized simulation of synergism and antagonism in drug combination studies

The median-effect equation derived from the mass-action law principle at equilibrium-steady state via mathematical induction and deduction for different reaction sequences and mechanisms and different types of inhibition has been shown to be the unified theory for the Michaelis-Menten equation, Hill equation, Henderson-Hasselbalch equation, and Scatchard equation. It is shown that dose and effect are interchangeable via defined parameters. This general equation for the single drug effect has been extended to the multiple drug effect equation for n drugs. These equations provide the theoretical basis for the combination index (CI)-isobologram equation that allows quantitative determination of drug interactions, where CI < 1, = 1, and > 1 indicate synergism, additive effect, and antagonism, respectively. Based on these algorithms, computer software has been developed to allow automated simulation of synergism and antagonism at all dose or effect levels. It displays the dose-effect curve, median-effect plot, combination index plot, isobologram, dose-reduction index plot, and polygonogram for in vitro or in vivo studies. This theoretical development, experimental design, and computerized data analysis have facilitated dose-effect analysis for single drug evaluation or carcinogen and radiation risk assessment, as well as for drug or other entity combinations in a vast field of disciplines of biomedical sciences. In this review, selected examples of applications are given, and step-by-step examples of experimental designs and real data analysis are also illustrated. The merging of the mass-action law principle with mathematical induction-deduction has been proven to be a unique and effective scientific method for general theory development. The median-effect principle and its mass-action law based computer software are gaining increased applications in biomedical sciences, from how to effectively evaluate a single compound or entity to how to beneficially use multiple drugs or modalities in combination therapies.

Synthesis and characterization of the 7-(4-aminomethyl-1H-1,2,3-triazol-1-yl) analogue of kabiramide C

The 7-(4-aminomethyl-1H-1,2,3-triazol-1-yl) analogue of kabiramide C (5) was synthesized by using the Mitsunobu reaction and 1,3-dipolar cycloaddition. This compound and the intermediate compounds 2 and 4 were shown to bind tightly to G-actin in a 1:1 complex and exhibited the same degree of cytotoxicity as 1. Compound 5 serves as a key intermediate for the synthesis of actin-directed optical probes and drugs.

Anticancer therapy with novel tubulin-interacting drugs

Antimitotic agents that target tubulin, including the taxanes and vinca alkaloids, are important components of current anticancer therapy. Whilst these antimitotic drugs are highly effective in the treatment of a number of cancers, both acquired and intrinsic resistance to these agents is a major clinical problem. Furthermore, the systemic toxicity, and in some cases lack of oral availability, make these agents less than ideal. Recently much effort has been directed on the isolation and synthesis of new antimitotic drugs that target the tubulin/microtubule system and display efficacy against drug-refractory carcinomas. Newly described compounds include structurally diverse natural products, such as dolastatin, epothilones and discodermolide, derivatives and structural analogues of traditional antimitotics, and novel synthetic molecules. Additionally, new developments in drug targeting are improving efficacy and therapeutic indices of traditional agents. A number of promising 'new generation' antimitotics are now undergoing clinical testing. These new agents are reviewed here in terms of their mechanism(s) of action on microtubules, effectiveness against drug-resistant tumour cells and clinical potential.

Subcellular distribution of epothilones in human tumor cells

Epothilones are a new class of natural and potent antineoplastic agents that stabilize microtubules. Although 12,13-epoxide derivatives are potent antiproliferative agents, the activities of the corresponding 12,13-olefin analogs are significantly decreased. These data were confirmed for two new analogs, 6-propyl-EpoB (pEB) and 6-propyl-EpoD (pED), in comparison with the natural compounds EpoB/EpoD, by using human A431, MCF7, and MDR1-overexpressing NCI/Adr cells. By using tritiated pEB/pED, compound uptake, release, and nuclear accumulation were investigated in A431 and NCI/Adr cells. In these cells, epothilones can principally be recognized and exported by Verapamil-sensitive efflux pumps, which are not identical to MDR1. The degree of export depends on the structure, olefin vs. epoxide-analog, and also on the intracellular drug concentration. The accumulation of pED used at 3.5 or 70 nM, respectively, was increased in the presence of 10 microM Verapamil in both cell lines 2- to 8-fold. In contrast, the intracellular levels of pEB were affected by Verapamil only at 3.5 nM pEB in NCI/Adr (2-fold) and not in A431 cells. In addition, strong nuclear accumulation was observed for pEB (40-50%) but not paclitaxel or pED (5-15%) in both cell lines. Our study suggests that differences in growth inhibitory efficacy between epoxide and olefin analogs may be based on different mechanisms of drug accumulation and subcellular distribution.

The synthesis, discovery, and development of a highly promising class of microtubule stabilization agents: curative effects of desoxyepothilones B and F against human tumor xenografts in nude mice

We have evaluated two synthetic epothilone analogues lacking the 12,13-epoxide functionality, 12,13-desoxyepothilone B (dEpoB), and 12,13-desoxyepothilone F (dEpoF). The concentrations required for 50% growth inhibition (IC(50)) for a variety of anticancer agents were measured in CCRF-CEM/VBL1000 cells (2,048-fold resistance to vinblastine). By using dEpoB, dEpoF, aza-EpoB, and paclitaxel, the IC(50) values were 0.029, 0.092, 2.99, and 5.17 microM, respectively. These values represent 4-, 33.5-, 1,423- and 3,133-fold resistance, respectively, when compared with the corresponding IC(50) in the parent [nonmultiple drug-resistant (MDR)] CCRF-CEM cells. We then produced MDR human lung carcinoma A549 cells by continuous exposure of the tumor cells to sublethal concentrations of dEpoB (1.8 yr), vinblastine (1.2 yr), and paclitaxel (1.8 yr). This continued exposure led to the development of 2.1-, 4,848-, and 2,553-fold resistance to each drug, respectively. The therapeutic effect of dEpoB and paclitaxel was also compared in vivo in a mouse model by using various tumor xenografts. dEpoB is much more effective in reducing tumor sizes in all MDR tumors tested. Analysis of dEpoF, an analog possessing greater aqueous solubility than dEpoB, showed curative effects similar to dEpoB against K562, CCRF-CEM, and MX-1 xenografts. These results indicate that dEpoB and dEpoF are efficacious antitumor agents with both a broad chemotherapeutic spectrum and wide safety margins.

On the interactivity of complex synthesis and tumor pharmacology in the drug discovery process: total synthesis and comparative in vivo evaluations of the 15-aza epothilones

The total syntheses of 12,13,15-desoxy-15(S)-aza-epothilone B (aza-dEpoB; dEpoB-lactam) and 12,13,15-desoxy-15(R)-aza-epothilone B (15-epi-aza-dEpoB; 15-epi-dEpoB-lactam) have been accomplished via a highly convergent strategy. We have also successfully oxidized 12,13,15-desoxy-15(S)-aza-epothilone B to aza-epothilone B (aza-EpoB; EpoB-lactam). Aza-epothilone B has been advanced to phase I clinical trials by the Bristol-Myers Squibb group. Our synthesis is efficient and was amenable to the production of significant quantities of these lactams. Using our fully synthetically derived lactams, in vitro and in vivo studies were conducted in comparison with advanced clinical candidates, 12,13-desoxyepothilone B and 12,13-desoxyepothilone F, also derived by total synthesis.

The 12,13-diol cyclization approach for a truly stereocontrolled total synthesis of epothilone B and the synthesis of a conformationally restrained analogue

A highly convergent and stereocontrolled synthesis of epothilone B (1) has been developed. The epoxide moiety in 1 was generated by regioselective mesylation and base treatment of the 12,13-diol 30 which was formed by a chelate Cram controlled Grignard addition of 14 and methyl ketone 13. Both fragments were synthesized from the chiral carbon pool precursors (S)-citronellol and (S)-lactic acid, respectively. A highly diastereoselective aldol addition of epoxy-aldehyde 7 and the known Southern hemisphere ketone 8 delivered the full carbon skeleton, containing all the stereogenic centers of 1. Functional group manipulation, macrolactonization and removal of two protecting groups then yielded 1. The spatial closeness of the C4-beta-methyl and C6-methyl group in the crystal structure of 1 inspired us to connect them through a methylene bridge to give a cyclohexanone derivative. Thus, the Northern hemisphere aldehyde 7 was added to the enolate of the cyclohexanone 47. Further manipulations and macrolactonization delivered the conformationally restrained epothilone derivative 42.

Catalytic Antibody Route to the Naturally Occurring Epothilones: Total Synthesis of Epothilones A-F

Naturally occurring epothilones have been synthesized starting from enantiomerically pure aldol compounds 9-11, which were obtained by antibody catalysis. Aldolase antibody 38C2 catalyzed the resolution of (+/-)-9 by enantioselective retro-aldol reaction to afford 9 in 90% ee at 50 % conversion. Compounds 10 and 11 were obtained in more than 99% ee at 50% conversion by resolution of their racemic mixtures using newly developed aldolase antibodies 84G3, 85H6 or 93F3. Compounds 9, 10 and 11 were resolved in multigram quantities and then converted to the epothilones by metathesis processes, which were catalyzed by Grubbs' catalysts.