Subcellular distribution of epothilones in human tumor cells

Anticancer properties of indole derivatives as IsoCombretastatin A-4 analogues

In this study, a variety of original ligands related to Combretastatin A-4 and isoCombretastatin A-4, able to inhibit the tubulin polymerization into microtubules, was designed, synthesized, and evaluated. Our lead compound 15d having a quinazoline as A-ring and a 2-substituted indole as B-ring separated by a N-methyl linker displayed a remarkable sub-nanomolar level of cytotoxicity (IC₅₀ < 1 nM) against 9 human cancer cell lines.

Nuclear DNA Damage and Repair in Normal Ovarian Cells Caused by Epothilone B

This study was designed to assess, whether a new chemotherapeutic microtubule inhibitor, Epothilone B (EpoB, Patupilone), can induce DNA damage in normal ovarian cells (MM14.Ov), and to evaluate if such damage could be repaired. The changes were compared with the effect of paclitaxel (PTX) commonly employed in the clinic. The alkaline comet assay technique and TUNEL assay were used. The kinetics of DNA damage formation and the level of apoptotic cells were determined after treatment with IC50 concentrations of EpoB and PTX. It was observed that PTX generated significantly higher apoptotic and genotoxic changes than EpoB. The peak was observed after 48 h of treatment when the DNA damage had a maximal level. The DNA damage induced by both tested drugs was almost completely repaired. As EpoB in normal cells causes less damage to DNA it might be a promising anticancer drug with potential for the treatment of ovarian tumors.

Investigation of epothilone B-induced cell death mechanisms in human epithelial cancer cells -in consideration of combined treatment with ionizing radiation

Epothilone B was shown to have promising chemo- and radiosensitizing effects on cells, but the mechanisms underlying cell death remain ambiguous. The aim of the study was to examine selected cell death pathways on the basis of FaDu and A549 cells. Western blot analyses were used for investigation of specific apoptotic markers. Immunofluorescence imaging and flow cytometry were utilized for examination of cell death mechanisms. DNA-staining was used for studying influence of epothilone B on micronucleus rate. We showed that epothilone B can initiate cell death via apoptosis and mitotic catastrophe, but induction of cell death was cell type specific.

Dendritic polyglycerol sulfate as a novel platform for paclitaxel delivery: pitfalls of ester linkage

In this study, dendritic polyglycerol sulfate (dPGS) is evaluated as a delivery platform for the anticancer, tubulin-binding drug paclitaxel (PTX). The conjugation of PTX to dPGS is conducted via a labile ester linkage. A non-sulfated dendritic polyglycerol (dPG) is used as a control, and the labeling with an indocarbocyanine dye (ICC) renders multifunctional conjugates that can be monitored by fluorescence microscopy. The conjugates are characterized by (1)H NMR, UV-vis measurements, and RP-HPLC. In vitro cytotoxicity of PTX and dendritic conjugates is evaluated using A549 and A431 cell lines, showing a reduced cytotoxic efficacy of the conjugates compared to PTX. The study of uptake kinetics reveals a linear, non saturable uptake in tumor cells for dPGS-PTX-ICC, while dPG-PTX-ICC is hardly taken up. Despite the marginal uptake of dPG-PTX-ICC, it prompts tubulin polymerization to a comparable extent as PTX. These observations suggest a fast ester hydrolysis and premature drug release, as confirmed by HPLC measurements in the presence of plasma enzymes.

Epothilones

Epothilones A and B are naturally occurring microtubule stabilizers with nanomolar or even sub-nanomolar activity against human cancer cells in vitro and potent in vivo antitumor activity against multidrug-resistant tumors. Over the last decade, ten epothilonetype agents have entered clinical trials in humans; of these, the epothilone B lactam ixabepilone (BMS-247550; Ixempra®) was approved by the FDA for breast cancer treatment in 2007. Numerous synthetic and semisynthetic analogs of epothilones have been prepared and their in vitro and (in selected cases) in vivo biological activity has been determined, producing a wealth of SAR information on this compound family. This chapter will provide a brief summary of the in vitro and in vivo biological properties of epothilone B (Epo B). The major part of the discussion will then be organized around those epothilone analogs that have entered clinical development. For each analog the underlying synthetic chemistry and the most important preclinical features will be reviewed, together with the properties of some important related structures.

Synthesis, biological evaluation, and structure-activity relationships of tri- and tetrasubstituted olefins related to isocombretastatin A-4 as new tubulin inhibitors

The synthesis and structure-activity relationships associated with a series of 1,1-diarylethylene tubulin polymerization inhibitors 3 and 4 are described. The key step for their preparation involves a palladium-catalyzed coupling of N-arylsulfonylhydrazones with aryl halides, thus providing flexible and convergent access to tri- and tetrasubstituted 1,1-diarylolefins 3 and 4 related to isocombretastatin A-4 (isoCA-4). These compounds have been evaluated for tubulin polymerization inhibitory activity as well as for cytotoxic activity. The most potent compounds are 1,1-diaryl-2-methoxyethylenes 4b, 4d and 4e having a trisubstituted double bond. They exhibited good antiproliferative activity against various human cancer cell lines (GI(50) = 8-80 nM). Compounds 4b and 4e strongly inhibited tubulin polymerization with IC(50) values of 2 and 3 μM, respectively, and induced cell cycle arrest in the G(2)/M phase in the K562 cell line. Docking studies in the colchicine binding site of tubulin allowed identification of residues most likely to interact with these inhibitors and explain their potent anti-tubulin activity.

Effect of epothilone B on cell cycle, metabolic activity, and apoptosis induction on human epithelial cancer cells-under special attention of combined treatment with ionizing radiation

In recent studies, epothilone B was shown to have a cytotoxic and radiosensitizing effect on cells. The aim of our investigation was to explain this impact by examining the mode of action of epothilone B on FaDu and A549 tumor cells. Flow cytometry was used for cell cycle distribution and for the evaluation of apoptosis. Metabolic activity was studied by proliferation assay. Influence on nuclei morphology was investigated by DNA-staining. We showed that epothilone B-induced G2/M accumulation is the main rationale for drug-induced radiosensitivity. The cytotoxic effect resulted in apoptotic cell death, decreased metabolic activity, and formation of multinucleated cells.

Radiosensitizing effect of epothilone B on human epithelial cancer cells

BACKGROUND

A combined modality treatment employing radiation and chemotherapy plays a central role in the management of solid tumors. In our study, we examined the cytotoxic and radiosensitive effect of the microtubule stabilizer epothilone B on two human epithelial tumor cell lines in vitro and its influence on the microtubule assembly.

METHODS

Cancer cells were treated with epothilone B in proliferation assays and in combination with radiation in colony-forming assays. For the analysis of ionizing radiation-induced DNA damage and the influence of the drug on its repair a γH2AX foci assay was used. To determine the effect of epothilone B on the microtubule assembly in cells and on purified tubulin, immunofluorescence staining and tubulin polymerization assay, respectively, were conducted.

RESULTS

Epothilone B induced a concentration- and application-dependent antiproliferative effect on the cells, with IC(50) values in the low nanomolar range. Colony forming assays showed a synergistic radiosensitive effect on both cell lines which was dependent on incubation time and applied concentration of epothilone B. The γH2AX assays demonstrated that ionizing radiation combined with the drug resulted in a concentration-dependent increase in the number of double-strand breaks and suggested a reduction in DNA repair capacity. Epothilone B produced enhanced microtubule bundling and abnormal spindle formation as revealed by immunofluorescence microscopy and caused microtubule formation from purified tubulin.

CONCLUSION

The results of this study showed that epothilone B displays cytotoxic antitumor activity at low nanomolar concentrations and also enhances the radiation response in the tumor cells tested; this may be induced by a reduced DNA repair capacity triggered by epothilone B. It was also demonstrated that epothilone B in fact targets microtubules in a more effective manner than paclitaxel.

Evaluation of activity and combination strategies with the microtubule-targeting drug sagopilone in breast cancer cell lines

Sagopilone, a fully synthetic epothilone, is a microtubule-stabilizing agent optimized for high in vitro and in vivo activity against a broad range of tumor models, including those resistant to paclitaxel and other systemic treatments. Sagopilone development is accompanied by translational research studies to evaluate the molecular mode of action, to recognize mechanisms leading to resistance, to identify predictive response biomarkers, and to establish a rationale for combination with different therapies. Here, we profiled sagopilone activity in breast cancer cell lines. To analyze the mechanisms of mitotic arrest and apoptosis and to identify additional targets and biomarkers, an siRNA-based RNAi drug modifier screen interrogating 300 genes was performed in four cancer cell lines. Defects of the spindle assembly checkpoint (SAC) were identified to cause resistance against sagopilone-induced mitotic arrest and apoptosis. Potential biomarkers for resistance could therefore be functional defects like polymorphisms or mutations in the SAC, particularly in the central SAC kinase BUB1B. Moreover, chromosomal heterogeneity and polyploidy are also potential biomarkers of sagopilone resistance since they imply an increased tolerance for aberrant mitosis. RNAi screening further demonstrated that the sagopilone-induced mitotic arrest can be enhanced by concomitant inhibition of mitotic kinesins, thus suggesting a potential combination therapy of sagopilone with a KIF2C (MCAK) kinesin inhibitor. However, the combination of sagopilone and inhibition of the prophase kinesin KIF11 (EG5) is antagonistic, indicating that the kinesin inhibitor has to be highly specific to bring about the required therapeutic benefit.

The subcellular distribution of small molecules: a meta-analysis

To explore the extent to which current knowledge about the organelle-targeting features of small molecules may be applicable toward controlling the accumulation and distribution of exogenous chemical agents inside cells, molecules with known subcellular localization properties (as reported in the scientific literature) were compiled into a single data set. This data set was compared to a reference data set of approved drug molecules derived from the DrugBank database, and to a reference data set of random organic molecules derived from the PubChem database. Cheminformatic analysis revealed that molecules with reported subcellular localizations were comparably diverse. However, the calculated physicochemical properties of molecules reported to accumulate in different organelles were markedly overlapping. In relation to the reference sets of DrugBank and PubChem molecules, molecules with reported subcellular localizations were biased toward larger, more complex chemical structures possessing multiple ionizable functional groups and higher lipophilicity. Stratifying molecules based on molecular weight revealed that many physicochemical properties' trends associated with specific organelles were reversed in smaller vs larger molecules. Most likely, these reversed trends are due to the different transport mechanisms determining the subcellular localization of molecules of different sizes. Molecular weight can be dramatically altered by tagging molecules with fluorophores or by incorporating organelle targeting motifs. Generally, in order to better exploit structure-localization relationships, subcellular targeting strategies would benefit from analysis of the biodistribution effects resulting from variations in the size of the molecules.

Molecular mode of action and role of TP53 in the sensitivity to the novel epothilone sagopilone (ZK-EPO) in A549 non-small cell lung cancer cells

Sagopilone, an optimized fully synthetic epothilone, is a microtubule-stabilizing compound that has shown high in vitro and in vivo activity against a broad range of human tumor models. We analyzed the differential mechanism of action of sagopilone in non-small cell lung cancer cell lines in vitro. Sagopilone inhibited proliferation of non-small cell lung cancer cell lines at lower nanomolar concentration. The treatment with sagopilone caused strong disturbances of cellular cytoskeletal organization. Two concentration-dependent phenotypes were observed. At 2.5 nM sagopilone or 4 nM paclitaxel an aneuploid phenotype occur whereas a mitotic arrest phenotype was induced by 40 nM sagopilone or paclitaxel. Interestingly, treatment with 2.5 nM of sagopilone effectively inhibited cell proliferation, but - compared to high concentrations (40 nM) - only marginally induced apoptosis. Treatment with a high versus a low concentration of sagopilone or paclitaxel regulates a non-overlapping set of genes, indicating that both phenotypes substantially differ from each other. Genes involved in G2/M phase transition and the spindle assembly checkpoint, like Cyclin B1 and BUBR1 were upregulated by treatment with 40 nM sagopilone. Unexpectedly, also genes involved in DNA damage response were upregulated under that treatment. In contrast, treatment of A549 cells with a low concentration of sagopilone revealed an upregulation of direct transcriptional target genes of TP53, like CDKN1A, MDM2, GADD45A, FAS. Knockdown of TP53, which inhibited the transcriptional induction of TP53 target genes, led to a significant increase in apoptosis induction in A549 cells when treated with a low concentration of sagopilone. The results indicate that activation of TP53 and its downstream effectors like CDKN1A by low concentrations of sagopilone is responsible for the relative apoptosis resistance of A549 cells and might represent a mechanism of resistance to sagopilone.

Diversity through semisynthesis: the chemistry and biological activity of semisynthetic epothilone derivatives

Epothilones are myxobacterial natural products that inhibit human cancer cell growth through the stabilization of cellular microtubules (i.e., a "taxol-like" mechanism of action). They have proven to be highly productive lead structures for anticancer drug discovery, with at least seven epothilone-type agents having entered clinical trials in humans over the last several years. SAR studies on epothilones have included a large number of fully synthetic analogs and semisynthetic derivatives. Previous reviews on the chemistry and biology of epothilones have mostly focused on analogs that were obtained by de novo chemical synthesis. In contrast, the current review provides a comprehensive overview on the chemical transformations that have been investigated for the major epothilones A and B as starting materials, and it discusses the biological activity of the resulting products. Many semisynthetic epothilone derivatives have been found to exhibit potent effects on human cancer cell growth and several of these have been advanced to the stage of clinical development. This includes the epothilone B lactam ixabepilone (Ixempra(®), which has been approved by the FDA for the treatment of advanced and metastatic breast cancer.

Making epothilones fluoresce: design, synthesis, and biological characterization of a fluorescent N12-aza-epothilone (azathilone)

A green fluorescent 12-aza-epothilone (azathilone) derivative has been prepared through the attachment of the 4-nitro-2,1,3-benzoxadiazole (NBD) fluorophore to the 12-nitrogen atom of the azamacrolide core structure. While less potent than natural epothilones or different N12-acylated azathilone derivatives, NBD-azathilone (3) promotes tubulin assembly, inhibits cancer cell proliferation in vitro and arrests the cell cycle at the G2/M transition. Most significantly, the binding of 3 to cellular microtubules (MTs) could be directly visualized by confocal fluorescence microscopy. Based on competition binding experiments with laulimalide-stabilized MTs in vitro, the N12-Boc substituted azathilone 1, Epo A, and NBD-azathilone (3) all interact with the same tubulin-binding site. Computational studies provided a structural model of the complexes between beta-tubulin and 1 or 3, respectively, in which the NBD moiety of 3 or the BOC moiety of 1 directly and specifically contribute to MT binding. Collectively, these data demonstrate that the cellular effects of 3 and, by inference, also of other azathilones are the result of their interactions with the cellular MT network.

Sagopilone (ZK-EPO): from a natural product to a fully synthetic clinical development candidate

BACKGROUND

Tubulin is among the most established and clinically validated targets in oncology. The taxanes, paclitaxel and docetaxel, stabilize microtubules and have shown significant clinical activity, but factors such as the development of resistance can limit their clinical use. The epothilones are a novel class of natural microtubule-stabilizing products with potential activity in an expanded spectrum of tumour indications.

OBJECTIVE

In an extensive lead optimization programme, we selected sagopilone from 350 compounds produced by total synthesis because of its combination of potent activity and good tolerability in tumour models. It is the first fully synthetic epothilone in clinical development.

METHODS

Here we review the directed optimization of the natural product epothilone B to produce sagopilone, along with its mechanism of action, preclinical data and emerging clinical results.

RESULTS/CONCLUSIONS

We show how this optimization process translated into superior preclinical activity, coupled with a favourable tolerability profile. Activity has been determined in a number of animal models, including those from tumours resistant to other systemic treatments. The approach used to develop sagopilone may become more common as structure-driven research is increasingly employed to exploit the enormous potential of natural products, in parallel with other targeted approaches, heralding a new era of anticancer therapy.

Epothilones: a novel class of microtubule-stabilizing drugs for the treatment of cancer

Microtubule-targeted anticancer drugs are effective in treating various cancers but are limited in use due to development of resistance and unacceptable toxicities. The epothilones are a novel class of microtubule-stabilizing anticancer drugs and may have a role in treating taxane-resistant cancers. Revised and updated data from several clinical studies for ixabepilone were recently published and subsequently resulted in ixabepilone becoming the first epothilone approved as monotherapy or in combination for treatment of locally advanced or metastatic breast cancer. BMS-310705, patupilone, KOS-862, KOS-1584 and ZK-EPO are epothilones that have been developed. Although peripheral sensory neuropathy and neutropenia are the dose-limiting toxicities for ixabepilone, these dose-limiting toxicities are ixabepilone specific. This review will discuss the current preclinical, clinical pharmacokinetic and pharmacodynamic, efficacy and toxicity data of the epothilones.

Improved cellular pharmacokinetics and pharmacodynamics underlie the wide anticancer activity of sagopilone

Sagopilone (ZK-EPO) is the first fully synthetic epothilone undergoing clinical trials for the treatment of human tumors. Here, we investigate the cellular pathways by which sagopilone blocks tumor cell proliferation and compare the intracellular pharmacokinetics and the in vivo pharmacodynamics of sagopilone with other microtubule-stabilizing (or tubulin-polymerizing) agents. Cellular uptake and fractionation/localization studies revealed that sagopilone enters cells more efficiently, associates more tightly with the cytoskeleton, and polymerizes tubulin more potently than paclitaxel. Moreover, in contrast to paclitaxel and other epothilones [such as the natural product epothilone B (patupilone) or its partially synthetic analogue ixabepilone], sagopilone is not a substrate of the P-glycoprotein efflux pumps. Microtubule stabilization by sagopilone caused mitotic arrest, followed by transient multinucleation and activation of the mitochondrial apoptotic pathway. Profiling of the proapoptotic signal transduction pathway induced by sagopilone with a panel of small interfering RNAs revealed that sagopilone acts similarly to paclitaxel. In HCT 116 colon carcinoma cells, sagopilone-induced apoptosis was partly antagonized by the knockdown of proapoptotic members of the Bcl-2 family, including Bax, Bak, and Puma, whereas knockdown of Bcl-2, Bcl-X(L), or Chk1 sensitized cells to sagopilone-induced cell death. Related to its improved subcellular pharmacokinetics, however, sagopilone is more cytotoxic than other epothilones in a large panel of human cancer cell lines in vitro and in vivo. In particular, sagopilone is highly effective in reducing the growth of paclitaxel-resistant cancer cells. These results underline the processes behind the therapeutic efficacy of sagopilone, which is now evaluated in a broad phase II program.

Epothilones as lead structures for new anticancer drugs--pharmacology, fermentation, and structure-activity-relationships

Epothilones (Epo's) A and B are naturally occurring microtubule-stabilizers, which inhibit the growth of human cancer cells in vitro at low nM or sub-nM concentrations. In contrast to taxol (paclitaxel, Taxol) epothilones are also active against different types of multidrug-resistant cancer cell lines in vitro and against multidrug-resistant tumors in vivo. Their attractive preclinical profile has made epothilones important lead structures in the search for improved cytotoxic anticancer drugs and Epo B (EPO906, patupilone) is currently undergoing Phase III clinical trials. Numerous synthetic and semisynthetic analogs have been prepared since the absolute stereochemistry of epothilones was first disclosed in mid-1996 and their in vitro biological activity has been determined. Apart from generating a wealth of SAR information, these efforts have led to the identification of at least six compounds (in addition to Epo B), which are currently at various stages of clinical evaluation in humans. The most advanced of these compounds, Epo B lactam BMS-247550 (ixabepilone), has recently obtained FDA approval for the treatment of metastatic and advanced breast cancer. This chapter will first provide a summary of the basic features of the biological profile of Epo B in vitro and in vivo. This will be followed by a review of the processes that have been developed for the fermentative production of Epo B. The main part of the chapter will focus on the most relevant aspects of the epothilone SAR with regard to effects on tubulin polymerization, in vitro antiproliferative activity, and in vivo antitumor activity. Particular emphasis will be placed on work conducted in the authors' own laboratories, but data from other groups will also be included. In a final section, the current status of those epothilone analogs undergoing clinical development will be briefly discussed.

A novel class of tubulin inhibitors that exhibit potent antiproliferation and in vitro vessel-disrupting activity

PURPOSE

Since anticancer agents that interfere with microtubule function are in widespread use and have a broad spectrum of activity against both hematological malignancies and solid tumors, there is an urgent need to develop novel tubulin inhibitors with broader activities and avoiding drug resistance.

METHODS AND RESULTS

In this study, we describe the characterization of select lead compounds from a novel class of indazole-based tubulin inhibitors. Three lead compounds, TH-337, TH-482 and TH-494, exhibit potent antiproliferative activity against cell lines derived from human pancreatic carcinoma, human breast adenocarcinoma and human colorectal adenocarcinoma cells. The three compounds were also tested for cytotoxicity against a panel of clinically relevant drug resistant cancer cell lines that either overexpress the drug resistance pumps MDR-1, MRP-1 and BCRP-1 or have altered Topoisomerase II activity. TH-482 and -494 retained cytotoxic activities against all of the resistant cell lines tested; however, TH-337 exhibited decreased cytotoxicity in the cell line overexpressing BCRP-1, indicating that TH-337 is a substrate of that pump. We show that TH-482's antiproliferative activity is due to cell cycle arrest at the G(2)/M phase. We demonstrate that TH-482 binds specifically to the colchicine site of tubulin and that it inhibits tubulin polymerization in vitro in a concentration-dependent manner. The in vitro anti-vascular activities of TH-482 were assessed using the HUVEC-C cell line. TH-482 inhibits in vitro neovessel formation and disrupts pre-established vessels using HUVEC-C cells. TH-482 also increases permeability of vascular endothelial cells in a concentration- and time-dependent manner.

CONCLUSIONS

TH-482 demonstrates potent in vitro efficacy as a novel tubulin-targeted anti-proliferative and anti-vascular agent and notably is more potent in antiproliferative assays than the benchmark compound combretastatin A-4. These results identify TH-482 as a potent tubulin inhibitor, and support the investigation of its in vivo efficacy and pharmacokinetic properties as the prototype of a new class of anti-tubulin agents.

Peripheral blood mononuclear and tumor cell pharmacodynamics of the novel epothilone B analogue, ixabepilone

BACKGROUND

We previously demonstrated that peak microtubule bundle formation (MBF) in peripheral blood mononuclear cells (PBMCs) occurs at the end of drug infusion and correlates with drug pharmacokinetics (PK). In the current study, a new expanded evaluation of drug target effect was undertaken.

PATIENTS AND METHODS

Patients with advanced solid malignancies were treated with ixabepilone 40 mg/m2 administered as a 1-h i.v. infusion every 3 weeks. Blood, plasma, and tumor tissue sampling was carried out to characterize pharmacodynamics and PK.

RESULTS

Forty-seven patients were treated with 141 cycles of ixabepilone. In both PBMCs (n=27) and tumor cells (n=9), peak MBF occurred at the end of infusion; however, at 24-72 h after drug infusion, the number of cells with MBF was significantly greater in tumor cells, relative to PBMCs. A Hill model (EC50=109.65 ng/ml; r2=0.94) was fitted, which demonstrated a relationship between percentage of PBMCs with MBF and plasma ixabepilone concentration. The percentage of PBMCs with MBF at the end of infusion also correlated with severity of neutropenia (P=0.050).

CONCLUSIONS

Plasma ixabepilone concentration and severity of neutropenia correlate with the level of MBF in PBMCs. Therefore, this technically straightforward assay should be considered as a complement to the clinical development of novel microtubule-binding agents.

Patupilone (epothilone B) inhibits growth and survival of multiple myeloma cells in vitro and in vivo

In this study, we investigated the in vitro and in vivo efficacy of patupilone (epothilone B, EPO906), a novel nontaxane microtubule stabilizing agent, in treatment of multiple myeloma (MM). Patupilone directly inhibited growth and survival of MM cells, including those resistant to conventional chemotherapies, such as the taxane paclitaxel. Patupilone induced G2M arrest of MM cells, with subsequent apoptosis. Interleukin-6 (IL-6) and insulin-like growth factor-1 (IGF-1), 2 known growth and survival factors for MM, did not protect MM.1S cells against patupilone-induced cell death. Proliferation of MM cells induced by adherence to bone marrow stromal cells (BMSCs) was also inhibited by patupilone and was paralleled by down-regulation of vascular endothelial growth factor (VEGF) secretion. Importantly, stimulation of cells from patients with MM, either with IL-6 or by adherence to BMSCs, enhanced the anti-proliferative and proapoptotic effects of patupilone. Moreover, patupilone was effective against MM cell lines that overexpress the MDR1/P-glycoprotein multidrug efflux pump. In addition, patupilone was effective in slowing tumor growth and prolonging median survival of mice that received orthotopical transplants with MM tumor cells. Taken together, these preclinical findings suggest that patupilone may be a safe and effective drug in the treatment of MM, providing the framework for clinical studies to improve patient outcome in MM. (Blood. 2005;105:350-357)

Disorazol A1, a highly effective antimitotic agent acting on tubulin polymerization and inducing apoptosis in mammalian cells

Disorazol A1, a macrocyclic polyketide compound that is produced by the myxobacterium Sorangium cellulosum showed a remarkably high cytostatic activity. It inhibited the proliferation of different cancer cell lines including a multidrug-resistant KB line at low picomolar levels. In presence of disorazol A1, the nuclei of the cells increased in size and the cells often became multinucleate. Low concentrations of disorazol (<100 pM) induced an apoptotic process, characterized by enhanced capase-3 activity and DNA laddering, and abnormal, multipolar mitotic spindles. Low concentrations also induced an accumulation of p53 protein in the nucleus. At higher concentrations, we observed an accumulation of the cells in the G2/M-phase of the cell cycle, and a depletion of microtubules. In vitro, disorazol A1 inhibited the polymerization of tubulin in a concentration-dependent manner and independently of microtubule-associated proteins. Correspondingly it induced a complete depolymerization of microtubules prepared in vitro. Formation of defined degradation structures was not observed. Disorazol is a novel, highly effective antimitotic agent. Efforts are going on to develop it as an anticancer drug.