Design, synthesis, and biological evaluation of novel C14-C3'BzN-linked macrocyclic taxoids

Novel macrocyclic paclitaxel congeners were designed to mimic the bioactive conformation of paclitaxel. Computational analysis of the "REDOR-Taxol" structure revealed that this structure could be rigidified by connecting the C14 position of the baccatin moiety and the ortho position of C3'N-benzoyl group (C3'BzN), which are ca. 7.5 A apart, with a short linker (4-6 atoms). 7-TES-14beta-allyloxybaccatin III and (3R,4S)-1-(2-alkenylbenzoyl)-beta-lactams were selected as key components, and the Ojima-Holton coupling afforded the corresponding paclitaxel-dienes. The Ru-catalyzed ring-closing metathesis (RCM) of paclitaxel-dienes gave the designed 15- and 16-membered macrocyclic taxoids. However, the RCM reaction to form the designed 14-membered macrocyclic taxoid did not proceed as planned. Instead, the attempted RCM reaction led to the occurrence of an unprecedented novel Ru-catalyzed diene-coupling process, giving the corresponding 15-membered macrocyclic taxoid (SB-T-2054). The biological activities of the novel macrocyclic taxoids were evaluated by tumor cell growth inhibition (i.e., cytotoxicity) and tubulin-polymerization assays. Those assays revealed high sensitivity of cytotoxicity to subtle conformational changes. Among the novel macrocyclic taxoids evaluated, SB-T-2054 is the most active compound, which possesses virtually the same potency as that of paclitaxel. The result may also indicate that SB-T-2054 structure is an excellent mimic of the bioactive conformation of paclitaxel. Computational analysis for the observed structure-activity relationships is also performed and discussed.     

Effects of novel taxanes SB-T-1213 and IDN5109 on tubulin polymerization and mitosis

SB-T-1213 and IDN5109 are semisynthetic, orally available taxanes that are up to 400-fold more active than paclitaxel against drug-resistant cells. IDN5109 is in clinical trials. We investigated the primary target for SB-T-1213 and IDN5109 and whether the compounds interact with microtubules differently than paclitaxel. Unlike paclitaxel, at 1-10 microM both novel taxanes initiate microtubule polymerization in vitro with no lag. They enhance polymerization equally or more potently than paclitaxel. SB-T-1213 induces unusual microtubules with attached extra protofilaments or open sheets, and IDN5109 induces large protofilamentous sheets. Both inhibit HeLa cell proliferation, block mitosis at the metaphase/anaphase transition, bundle microtubules at high drug concentrations, and induce abnormal metaphase spindles and apoptosis. They target microtubules but alter their polymerization and structure differently than paclitaxel. These differences may play a role in their enhanced cytotoxicity and efficacy.     

Rotational-echo double-resonance NMR distance measurements for the tubulin-bound Paclitaxel conformation

The important anticancer drug Taxol (paclitaxel, PTX) owes its unique activity to its ability to bind to tubulin in a stoichiometric ratio and promote its assembly into microtubules. The conformation of the microtubule-bound drug has been the focus of numerous research efforts, since the inability of polymerized tubulin to form crystals precludes structure proof by X-ray crystallography. Likewise, although the alpha,beta-tubulin dimer structure has been solved by electron crystallography, the 3.7 A resolution is too low to permit direct determination of either ligand conformation or binding pose. In this article, we present experimental results from 2H{19F} REDOR NMR that provide direct confirmation that paclitaxel adopts a T-shaped conformation when it is bound to tubulin.     

新型紫杉烷类衍生物的制备及生物活性评价

多药耐药性问题是导致第一代紫杉烷药物在临床化疗失败的主要原因。本文对紫杉醇C7、C10、C14、C3′多个位点的取代基进行改造,针对合成的6个新型的紫杉烷化合物,在体外考察其对多药耐药肿瘤细胞株以及人结肠癌HCT-116干细胞的增殖抑制活性,实验结果表明6个化合物的抗多药耐药活性均优于紫杉醇。采用P-gp高表达的犬肾细胞MDCK-MDR1进一步研究高活性候选化合物JT-3与P-gp的相互作用。以此研发抗多药耐药型的新一代紫杉烷类药物,对开发扩大抗癌新适应症的新一代紫杉烷类抗癌药意义重大。     

Chemistry and chemical biology of taxane anticancer agents

Taxol (paclitaxel) and Taxotere (docetaxel) are currently considered to be among the most important anticancer drugs in cancer chemotherapy. The anticancer activity of these drugs is ascribed to their unique mechanism of action, i.e., causing mitotic arrest in cancer cells, leading to apoptosis through inhibition of the depolymerization of microtubules. Although both paclitaxel and docetaxel possess potent antitumor activity, treatment with these drugs often results in a number of undesirable side effects, as well as multidrug resistance (MDR). Therefore, it has become essential to develop new anticancer agents with superior pharmacological properties, improved activity against various classes of tumors, and fewer side effects. This paper describes an account of our research on the chemistry of paclitaxel and taxoid anticancer agents at the biomedical interface, including: 1. The structure-activity relationship (SAR) study of taxoids leading to the development of the "second-generation" taxoids, which possess exceptional activity against drug-resistant cancer cells expressing the MDR phenotype. 2. Development of fluorinated taxoids to study the bioactive conformation of paclitaxel and photoaffinity labeling taxoids for mapping of the drug-binding domain on both microtubules and P-glycoprotein. 3. The synthesis of novel macrocyclic taxoids for the investigation into the common pharmacophore for microtubule stabilizing anticancer agents.     

T-Taxol and the electron crystallographic density in beta-tubulin

[chemical structure: see text]. T-Taxol has been proposed as the bioactive conformation on beta-tubulin and subsequently utilized in the design of a series of highly active bridged taxane analogues. A modified T-form with a reversed C-13 side chain orientation has recently been proposed as an equally plausible bioactive shape. A comparison of the two spatial alternatives within the tubulin binding site electron crystallographic density suggests strongly that T-Taxol is the bioactive conformation.     

Studies on taxol biosynthesis. Preparation of taxa-4(20),11(12)-dien-5 alpha-acetoxy-10 beta-ol by deoxygenation of a taxadiene tetraacetate obtained from Japanese yew

Taxa-4(20),11(12)-dien-5 alpha-acetoxy-10 beta-ol 6 has been identified as an early stage intermediate involved in the biosynthesis of taxol (Paclitaxel). This compound has been efficiently prepared by Barton deoxygenation of the C-2- and C-14-hydroxyl groups on a derivative semisynthetically prepared from taxa-4(20),11(12)-dien-2 alpha,5 alpha,10 beta-triacetoxy-14 beta-(2-methyl)butyrate (7), a major taxoid metabolite isolated from Japanese yew heart wood. The synthetic methodology is amenable for the preparation of isotopically labeled congeners that will be useful to probe further intermediate steps in the biosynthesis of taxol.     

Analysis of MS/MS fragmentation of taxoids

The fragmentation pathways of seven types of taxoids were investigated by using a LC-MS/MS method, namely: (1) neutral taxoids with a C-4(20) double bond; (2) taxoids with a C-4(20) double bond and oxygenation at C-14; (3) 5-cinnamoyl taxoids with a C-4(20) double bond; (4) a basic taxoid with a C-4(20) double bond; (5) a taxoid with a C-4(20) epoxide; (6) taxoids with an oxetane ring; and (7) taxoids with an oxetane ring and a phenylisoserine C-13 side chain. Depending on the class of core structure and the substitution pattern, each taxoid gave either the molecular adduct ion [M+NH4]+ or [M+H]+. In the MS/MS, the molecular adduct ion gave characteristic product ions corresponding to the loss of water, acetic acid, benzoic acid, and cinnamic acid or the phenylisoserine group. These could reflect the difference of the substitutions and structural modifications and should be utilized for the structure elucidation oftaxoids by LC-MS.     

Competitive mass spectrometry binding assay for characterization of three binding sites of tubulin

Tubulin is an attractive and established target for anticancer therapy. To date, the only method to determine the binding of inhibitor to tubulin has been competitive radioligand binding assays. We developed a non‐radioactive mass spectrometry (MS) binding assay to study the tubulin binding of colchicine, vinblastine and paclitaxel and to identify which of these three binding sites that a novel inhibitor binds. The method involves a very simple step of separating the unbound ligand from macromolecules using ultrafiltration. The unbound ligand in the filtrate can be accurately determined using highly sensitive and specific liquid chromatography tandem mass spectrometry (LC‐MS/MS) method using multiple reaction monitoring (MRM) mode. The assay was validated using podophyllotoxin, vincristine and docetaxel, drugs that compete to the colchicine‐, vinblastine‐ and paclitaxel‐binding sites in tubulin, respectively. This competitive binding assay allowed the reliable detection of interactions of these drugs with three binding sites on tubulin. This method was subsequently applied to determine the tubulin‐binding site of 4‐substituted methoxylbenzoyl‐aryl‐thiazoles (SMART‐H), a potent antitubulin agent developed in our laboratory. The results indicated that SMART‐H specifically and reversibly bound only to the colchicine‐binding site, but not to vinblastine‐ or paclitaxel sites. This new non‐radioligand binding method to determine the binding site on tubulin will function as a useful tool to study the binding sites of tubulin inhibitors. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrochemical Studies of Paclitaxel Interaction with Tubulin

Paclitaxel can stabilize microtubules and induce microtubule assembly, so people areinterested in the interaction of paclitaxel with tubulin. This interaction has been studiedby fluorometry'. but the electrochemical method has not been adopted to study it so far.Pig-brain tubulin was purified by temperature-depen4ent assemly(disassemblyprocedure=, protein concentrations were estimated by Bradford method and its purity wasdetermined by SDS-polyacrylamide gel electrophoresis (295 % purity). A …     

Novel d-seco paclitaxel analogues: synthesis, biological evaluation, and model testing

Four new D-secopaclitaxel analogues were synthesized from paclitaxel. The key step of the synthesis involved the opening of the D-ring by Jones oxidation. Two of the compounds had been predicted to be nearly as active as paclitaxel in a minireceptor model of the binding site on tubulin, but all were biologically inactive in an in vitro cytotoxic assay and a tubulin assembly assay. The biological results identify a weakness in our predictive minireceptor model and suggest a corrective remedy in which additional amino acids are needed to accommodate ligand-protein steric effects around the oxetane ring. These changes to the model lead to correct predictions of the bioactivity. Conformational analysis and dynamics simulations of the compounds showed that the 4-acetyl substituent is as important as the oxetane in determining the A ring conformation.     

Synthesis and biological evaluation of novel macrocyclic paclitaxel analogues

[reaction: see text] This work describes the synthesis of two novel macrocyclic taxoid constructs by ring-closing olefin metathesis (RCM) and their biological evaluation. Computational studies examine conformational profiles of 1 and 2 for their fit to the beta-tubulin binding site determined by electron crystallography. The results support the hypothesis that paclitaxel binds to microtubules in a "T" conformation.     

Synthesis of novel taxoid analogue containing sulfur group on C-13 side-chain: 2′-deoxy-2′-epi-mercaptopaclitaxel

Paclitaxel analogues with a thiol group in place of the hydroxyl group on the C-13 side-chain constitute an interesting avenue of research for the study of new taxoid compounds. A synthetic route for the preparation of 2′-deoxy-2′-epi-mercaptopaclitaxel with high ee has been developed. The key step is the regio-controlled ring-opening reaction of the trans-oxazoline derivative with thiolacetic acid, which allows the introduction of the sulfur-containing group onto the side-chain.     

A new paclitaxel prodrug for use in ADEPT strategy

A new paclitaxel prodrug intended for use in Antibody-Directed Prodrug Therapy (ADEPT) or Prodrug Monotherapy (PMT) has been prepared. This prodrug was originally designed to be activated into the drug by human beta-glucuronidase. In order to enhance the liberation rate of paclitaxel, an elongated spacer system including a nitro-aromatic derivative and a N,N'-methylethylenediamine was incorporated between the sugar moiety and the drug. Indeed, this new prodrug proved to be activated significantly faster than a former paclitaxel prodrug containing a conventional spacer.     

A systematic SAR study of C10 modified paclitaxel analogues using a combinatorial approach

A library with 63 paclitaxel analogues modified at the C10 position of paclitaxel has been prepared using parallel solution phase synthesis. Most of the C10 analogues were slightly less active than paclitaxel in the tubulin assembly assay and had reduced potency in the B16 melanoma and MCF-7 cell line cytotoxicity assays. These modifications at C10, however, did not lead to the total loss of activity, indicating that the C10 moiety of paclitaxel may not be directly involved in the drug-microtubule interactions, but could influence its binding affinity to P-glycoprotein. Approximately 50% of the analogues demonstrated better activity against the drug resistant cell line MCF7-ADR. However, the increase in activity was 10-fold at most. This result demonstrates that the cytotoxicity against this drug resistant cancer cell line is sensitive to structural changes at the C10 position of paclitaxel. It was also found that the presence of a nitrogen atom in the C10 substituent might play a role in the interaction of analogues with microtubules.     

SAR and pH stability of cyano-substituted epothilones

[formula: see text] 3-Cyano epothilones 15-18 are the only examples of non-hydroxy C-3-substituted analogues. Their tubulin binding affinity and cytotoxicity provide meaningful structure-activity relationship information on the dependence of C-1/C-3 conformation upon activity. 12-Cyano epothilone 24 has improved pH stability over epothilone B, and its activity further supports the hypothesis that C-12 stereochemistry is not critical for tubulin affinity.     

Diastereoselective synthesis of 1-α-phenylethyl-3,3-dimethyldiaziridine. Conformation and configuration of three-membered ring nitrogen heterocycles containing an asymmetric substituent on the nitrogen atom

The synthesis of 1-(S)--phenylethyl-3,3-dimethyldiaziridine from N-(S)--phenylacetonimine and hydroxylamine-O-sulfonic acid has been carried out with high diastereoselectivity. The absolute configuration of the diastereomers was established by comparison with oxaaziridine analogs based on their NMR and CD spectra, and atom-atomic potential calculations. A test has been proposed to establish the absolute configuration of N--phenylethyl-substituted three-membered ring heterocycles using NMR, and the mechanism of epimination of imines is discussed, which is similar to their oxidation mechanism.Communication 60 in the series Asymmetric Nitrogen; for Communication 59, see [1].Translated from Geterotsiklicheskikh Soedinenii, No. 11, pp. 1461–1465, November, 1988.     

Investigation on the interaction between anthracyclines and DNA in the presence of paclitaxel by resonance light scattering technique

We have developed a method to investigate the interaction between DNA-targeted anthracyclines and DNA in the presence of the drug paclitaxel. It is based on resonance light scattering (RLS) and on the finding that anthracyclines when bound to DNA undergo a dramatic enhancement in their RLS intensities, while paclitaxel does not display such an effect. However, the RLS intensities of the anthracyclines-DNA associates are remarkably enhanced again on addition of paclitaxel. UV-visible spectra reveal interactions between paclitaxel and anthracyclines, but no reaction between paclitaxel and DNA. Consequently, paclitaxel, though not DNA-targeted, can improve the DNA-binding capabilities of anthracyclines. Binding constants between anthracyclines and DNA, and improved efficiency of paclitaxel on the DNA-binding capabilities of anthracyclines were calculated. The DNA binding constants of doxorubicin, epirubicin, and mitoxantrone, respectively, are 4.53?×?10⁵?L?mol^?1, 6.05?×?10⁵?L?mol^?1, and 9.47?×?10⁵?L?mol^?1. The improved values in presence of paclitaxel are 78%, 47% and 19%. We also have investigated the effects of drug concentrations and the order of adding the drugs. Displacement studies (using methylene blue as a competitive agent) provided additional information on the mechanisms of the interaction between paclitaxel and anthracyclines.

Studies on taxol biosynthesis. Preparation of 5α-acetoxytaxa-4(20),11-dien-2α,10β-diol derivatives by deoxygenation of a taxadiene tetra-acetate obtained from Japanese yew

The putative metabolite, 5α-acetoxytaxa-4(20),11-dien-2α,10β-diol (7), which is a promising candidate as a biosynthetic pathway triol in taxol biosynthesis, has been prepared by Barton deoxygenation of the C-14-hydroxyl group of a differentially protected derivative of natural 2α,5α,10β-triacetoxy-14β-(2-methyl)-butyryloxytaxa-4(20),11-diene (8), a major taxoid metabolite isolated from Japanese Yew heart wood. The synthetic protocol devised, is amenable for the preparation of isotopically labeled congeners that will be useful to probe further intermediate steps in the biosynthesis of taxol.