The Seco-Taxane IDN5390 Is Able to Target Class III β-Tubulin and to Overcome Paclitaxel Resistance

Quantitative analysis of the effect of tubulin isotype expression on sensitivity of cancer cell lines to a set of novel colchicine derivatives

BACKGROUND

A maximum entropy approach is proposed to predict the cytotoxic effects of a panel of colchicine derivatives in several human cancer cell lines. Data was obtained from cytotoxicity assays performed with 21 drug molecules from the same family of colchicine compounds and correlate these results with independent tubulin isoform expression measurements for several cancer cell lines. The maximum entropy method is then used in conjunction with computed relative binding energy values for each of the drug molecules against tubulin isotypes to which these compounds bind with different affinities.

RESULTS

We have found by using our analysis that alphabetaI and alphabetaIII tubulin isoforms are the most important isoforms in establishing predictive response of cancer cell sensitivity to colchicine derivatives. However, since alphabetaI tubulin is widely distributed in the human body, targeting it would lead to severe adverse side effects. Consequently, we have identified tubulin isotype alphabetaIII as the most important molecular target for inhibition of microtubule polymerization and hence cancer cell cytotoxicity. Tubulin isotypes alphabetaI and alphabetaII are concluded to be secondary targets.

CONCLUSIONS

The benefit of being able to correlate expression levels of specific tubulin isotypes and the resultant cell death effect is that it will enable us to better understand the origin of drug resistance and hence design optimal structures for the elimination of cancer cells. The conclusion of the study described herein identifies tubulin isotype alphabetaIII as a target for optimized chemotherapy drug design.

Computational design and biological testing of highly cytotoxic colchicine ring A modifications

Microtubules are the primary target for many anti-cancer drugs, the majority of which bind specifically to beta-tubulin. The existence of several beta-tubulin isotypes, coupled with their varied expression in normal and cancerous cells provides a platform upon which to construct selective chemotherapeutic agents. We have examined five prevalent human beta-tubulin isotypes and identified the colchicine-binding site as the most promising for drug design based on specificity. Using this binding site as a template, we have designed several colchicine derivatives and computationally probed them for affinity to the beta-tubulin isotypes. These compounds were synthesized and subjected to cytotoxicity assays to determine their effectiveness against several cancerous cell lines. We observed a correlation between computational-binding predictions and experimentally determined IC(50) values, demonstrating the utility of computational screening in the design of more effective colchicine derivatives. The most promising derivative exhibited an IC(50) approximately threefold lower than values previously reported for either colchicine or paclitaxel, demonstrating the utility of computational design and assessment of binding to tubulin.

Class III beta-tubulin expression and in vitro resistance to microtubule targeting agents

BACKGROUND

Class III beta-tubulin overexpression is a marker of resistance to microtubule disruptors in vitro, in vivo and in the clinic for many cancers, including breast cancer. The aims of this study were to develop a new model of class III beta-tubulin expression, avoiding the toxicity associated with chronic overexpression of class III beta-tubulin, and study the efficacy of a panel of clinical and pre-clinical drugs in this model.

METHODS

MCF-7 (ER+ve) and MDA-MB-231 (ER-ve) were either transfected with pALTER-TUBB3 or siRNA-tubb3 and 24 h later exposed to test compounds for a further 96 h for proliferation studies. RT-PCR and immunoblotting were used to monitor the changes in class III beta-tubulin mRNA and protein expression.

RESULTS

The model allowed for subtle changes in class III beta-tubulin expression to be achieved, which had no direct effect on the viability of the cells. Class III beta-tubulin overexpression conferred resistance to paclitaxel and vinorelbine, whereas downregulation of class III beta-tubulin rendered cells more sensitive to these two drugs. The efficacy of the colchicine-site binding agents, 2-MeOE2, colchicine, STX140, ENMD1198 and STX243 was unaffected by the changes in class III beta-tubulin expression.

CONCLUSION

These data indicate that the effect of class III beta-tubulin overexpression may depend on where the drug's binding site is located on the tubulin. Therefore, this study highlights for the first time the potential key role of targeting the colchicine-binding site, to develop new treatment modalities for taxane-refractory breast cancer.

Tubulin targets in the pathobiology and therapy of glioblastoma multiforme. I. Class III beta-tubulin

Glioblastoma multiforme (GBM) is the most common and deadliest form of primary brain cancer in adults. Despite advances in molecular biology and genetics of gliomas currently there is no effective treatment or promising molecularly targeted experimental therapeutic strategies for these tumors. In previous studies we have shown aberrant overexpression of the class III beta-tubulin isotype (betaIII-tubulin) in GBM and have proposed that this change may reflect perturbations in microtubule dynamics associated with glioma tumorigenesis, tumor progression and malignant transformation into GBM. This minireview focuses on microtubules and tubulin as emerging targets in potential therapy of GBM using a new class of betaIII-tubulin-targeted drugs in the light of recent developments concerning the function and potential role of this isotype in clinically aggressive tumor behavior, cancer stem cells, tumor hypoxia and chemoresistance to tubulin binding agents, principally taxanes.

Paclitaxel directly binds to Bcl-2 and functionally mimics activity of Nur77

We reported previously that Bcl-2 is paradoxically down-regulated in paclitaxel-resistant cancer cells. We reveal here that paclitaxel directly targets Bcl-2 in the loop domain, thereby facilitating the initiation of apoptosis. Molecular modeling revealed an extraordinary similarity between the paclitaxel binding sites in Bcl-2 and beta-tubulin, leading us to speculate that paclitaxel could be mimetic of an endogenous peptide ligand, which binds both proteins. We tested the hypothesis that paclitaxel mimics Nur77, which, like paclitaxel, changes the function of Bcl-2. This premise was confirmed by Nur77 interacting with both paclitaxel targets (Bcl-2 and beta-tubulin) and a peptide sequence mimicking the Nur77 structural region, thus reproducing the paclitaxel-like effects of tubulin polymerization and opening the permeability transition pore channel in mitochondria. This discovery could help in the development of novel anticancer agents with nontaxane skeleton as well as in identifying the clinical subsets responsive to paclitaxel-based therapy.

Novel C-seco-taxoids possessing high potency against paclitaxel-resistant cancer cell lines overexpressing class III beta-tubulin

Novel C-seco-taxoids were synthesized from 10-deacetylbaccatin III and their potencies evaluated against drug-sensitive and drug-resistant cancer cell lines. The drug-resistant cell lines include ovarian cancer cell lines resistant to cisplatin, topotecan, adriamycin and paclitaxel overexpressing class III beta-tubulin, A2780TC1 and A2780TC3. The last two cell lines were selected through chronic exposure of A2780wt to paclitaxel and Pgp blocker cyclosporine. All novel C-seco-taxoids exhibited remarkable potency against A2780TC1 and A2780TC3 cell lines, and no cross resistance to cisplatin- and topotecan-resistant cell lines, A2780CIS and A2780TOP. Four of those C-seco-taxoids exhibit much higher activities than IDN5390 against paclitaxel-resistant cell lines, A2780ADR, A2780TC1 and A2780TC3. SB-CST-10202 possesses the best all-round high potencies across different drug-resistant cell lines. Molecular modeling studies, including molecular dynamics simulations, on the drug-protein complexes of class I and III beta-tubulins were performed to identify possible cause of the remarkable potency of these C-seco-taxoids against paclitaxel-resistant cell lines overexpressing class III beta-tubulin.

Recent advances in the chemistry and biology of new generation taxoids

Among the numerous chemotherapeutic drugs, paclitaxel and docetaxel are among the most widely used against various types of cancer. However, these drugs cause undesirable side effects as well as drug resistance. Therefore, it is essential to develop "taxane" anticancer agents with better pharmacological properties and improved activity especially against drug-resistant cancers. Several laboratories have performed extensive SAR studies on paclitaxel. Our SAR studies have led to the development of numerous highly potent novel second- and third-generation taxoids with systematic modifications at the C-2, C-10, and C-3' positions. The third-generation taxoids showed virtually no difference in potency against drug-resistant and drug-sensitive cell lines. Some of the new generation taxoids also exhibited excellent cytotoxicity against pancreatic cell lines expressing multidrug-resistant genes. We have also designed taxoids with strategic fluorine incorporation to investigate their effects on the cytotoxicity and the blockage of known metabolic pathways. Furthermore, we have successfully employed computational biology analysis to design novel macrocyclic taxoids to mimic the bioactive conformation of paclitaxel. This account describes our work on the design, synthesis, and biological evaluation of these novel taxoids, which has led to the discovery of very promising candidates for further preclinical studies.

Molecular mechanisms of patupilone resistance

Patupilone is an epothilone in advanced clinical development that has shown promising efficacy in heavily pretreated patients. This study aimed at characterizing the mechanisms of patupilone activity in resistant patients. To this end, we generated patupilone-resistant cells using two cellular models, the first characterized by high chemosensitivity and low class III beta-tubulin (TUBB3) expression (A2780), and the second by low chemosensitivity and high TUBB3 expression (OVCAR-3). The obtained cell lines were named EPO3 and OVCAR-EPO, respectively. The same selection procedure was done in A2780 cells to generate a paclitaxel-resistant cell line (TAX50). Factors of resistance are expected to increase in the drug-resistant cell lines, whereas factors of drug sensitivity will be down-regulated. Using this approach, we found up-regulation of TUBB3 in TAX50, but not EPO3, cells, showing that TUBB3 mediates the resistance to paclitaxel but not to patupilone. Moreover, TUBB3 was a factor of patupilone sensitivity because OVCAR-EPO cells exhibited a dramatic reduction of TUBB3 and a concomitant sensitization to hypoxia and cisplatin-based chemotherapy. To identify the mechanisms underlying patupilone resistance, tubulin genes were sequenced, thereby revealing that a prominent mechanism of drug resistance is represented by point mutations in class I beta-tubulin. Overall, these results suggest that paclitaxel and patupilone have nonoverlapping mechanisms of resistance, thus allowing the use of patupilone for those patients relapsing after paclitaxel-based chemotherapy. Furthermore, patupilone represents a promising first-line option for the treatment of high-risk ovarian cancer patients, who exhibit high TUBB3 levels and poor response to standard paclitaxel-platin chemotherapy.

TP53, BCL-2 and BAX analysis in 199 ovarian cancer patients treated with taxane-platinum regimens

OBJECTIVE

In cell line studies, BCL-2 and BAX proteins interfere with cancer response to taxanes. This issue has not received much attention with regard to taxane-platinum (TP)-treated ovarian cancer patients.

METHODS

We evaluated prognostic/predictive significance of BCL-2 and BAX with regard to TP53 status. Immunohistochemical analysis was performed on 199 ovarian carcinomas FIGO stage IIB-IV treated with TP; the results were analyzed by the Cox and logistic regression models.

RESULTS

Clinicopathological parameters (residual tumor size, FIGO stage and/or tumor grade, but not patient's age) were the only or the strongest predictors of patient's outcome. Platinum highly sensitive response showed a positive association with TP53 accumulation (p=0.045). As in our previously published analysis on platinum-cyclophosphamide-treated group, complete remission showed a borderline negative (paradoxic) association with high BAX expression in the whole group (p=0.058) and with BCL-2 expression in the TP53(-) group (p=0.058).

CONCLUSION

Our results suggest that TP53, BCL-2 and BAX proteins carry some predictive potential in taxane-platinum-treated ovarian cancer patients, auxiliary to clinicopathological factors. We have confirmed on another patient group that clinical importance of BCL-2 may depend on TP53 status.

Role of microRNAs in drug-resistant ovarian cancer cells

OBJECTIVES

Chemotherapy is the preferred therapeutic approach for the therapy of advanced ovarian cancer, but a successful long-term treatment is prevented by the development of drug resistance. Recent works have underlined the involvement of non-coding RNAs, microRNAs (miRNAs) in cancer development, with several conjectures regarding their possible involvement in the evolution of drug resistance. This work was aimed to identify selected microRNAs involved in the development of chemoresistance in ovarian cancer.

METHODS

High-throughput analysis of the miRNA profile in a panel of paclitaxel- (A2780TAX, A2780TC1 and A2780TC3) and cisplatin-resistant (A2780CIS) cells was assessed using a microarray platform and subsequent validation with qPCR and Northern blots. Downstream target validation was performed for miR-130a and the target M-CSF.]

RESULTS

Six miRNAs (let-7e, miR-30c, miR-125b, miR-130a and miR-335) were always diversely expressed in all the resistant cell lines. Let-7e was upregulated in A2780TAX cells, while it was downregulated in the other resistant cell lines. The opposite phenomenon was obtained for miR-125b, which was downregulated in A2780TAX and upregulated in the other cell lines. The miR-30c, miR-130a and miR-335 were downregulated in all the resistant cell lines, thereby suggesting a direct involvement in the development of chemoresistance. Finally downstream target validation was proven for the miR-130a, whose downregulation was linked to the translational activation of the M-CSF gene, a known resistance factor for ovarian cancer.

CONCLUSIONS

Our results indicate that ovarian cancer drug resistance is associated with a distinct miRNA fingerprint, and miRNA microarrays could represent a prognostic tool to monitor the chemotherapy outcome.

Class III beta-tubulin is a component of the mitotic spindle in multiple cell types

The findings of this study show that Class III beta-tubulin is a component of the mitotic spindle in multiple cell types. Class III beta-tubulin has been widely used as a neuron-specific marker, but it has been detected also in association with breast and pancreatic cancers. In this study, we describe a novel finding of Class III beta-tubulin in a subpopulation of cells in malignant peripheral nerve sheath tumor. The findings of this study also show that Class III beta-tubulin is expressed by normal mesenchymal and epithelial cells (fibroblasts and keratinocytes), two transitional cell carcinoma cell lines, and neurofibroma Schwann cells, as shown by immunolabeling and Western transfer analysis using two different Tuj-1 antibodies that are specific for Class III beta-tubulin. The corresponding mRNA was detected using RT-PCR and whole human genome microarrays. Both antibodies localized Class III beta-tubulin to the mitotic spindle and showed a colocalization with alpha-tubulin. The immunoreaction became visible in early prophase, and the most intense immunoreaction was detected during metaphase and anaphase when microtubules were connected to the kinetochores on chromosomes. Class III beta-tubulin-specific immunoreaction lasted to the point when the midbody of cytokinesis became detectable.

Paclitaxel and docetaxel resistance: molecular mechanisms and development of new generation taxanes

Taxanes represent one of the most promising classes of anticancer agents. Unfortunately, their clinical success has been limited by the insurgence of cellular resistance, mainly mediated by the expression of the MDR phenotype or by microtubule alterations. However, the remarkable relevance of paclitaxel and docetaxel in clinical oncology stimulated intensive efforts in the last decade to identify new derivatives endowed with improved activities towards resistant tumor cells, resulting in a huge number of novel natural and synthetic taxanes. Among them, several structurally different derivatives were found to exhibit a promising behavior against the MDR phenotype in terms of either MDR inhibiting properties, or enhanced cytotoxicity compared to parental drugs, or both. On the other hand, only in more recent years have the first taxanes retaining activity against resistant cancer cells bearing alterations of the tubulin/microtubule system emerged. This review describes the main molecular mechanisms of resistance to paclitaxel and docetaxel identified so far, focusing on the advances achieved in the development of new taxanes potentially useful for the treatment of resistant tumors.

Cellular mechanisms of resistance to anthracyclines and taxanes in cancer: intrinsic and acquired

Taxanes and anthracyclines are two of the most potent and broadly effective classes of chemotherapeutic agents. However, resistance to these agents is common and significantly limits their potential. As such, there is a great need to understand the mechanisms underlying de novo and acquired resistance to these agents. Beyond the resistance barrier lies even greater potential to significantly alter the natural course of human cancer. This review discusses what we currently understand about the mechanisms of resistance to taxanes and anthracyclines. Preclinical models suggest a role for ATP-binding cassette transporters, tubulin isoforms, microtubule-associated proteins, tubulin gene mutations, and mitotic checkpoint signaling proteins in resistance to taxanes. Preclinical models also suggest that drug transport proteins, antioxidant defenses, apoptotic signaling, and topoisomerase modulation may mediate anthracycline resistance. Many of these hypotheses remain untested in appropriately designed clinical studies, but limited clinical evidence will be reviewed. Epothilones represent a novel class of non-taxane microtubule stabilizing agents with distinct drug-resistance profiles. Potential mechanisms behind these differences and their potential role in the treatment of both taxane- and anthracycline-refractory patients are discussed.

Is class III beta-tubulin a predictive factor in patients receiving tubulin-binding agents?

On the basis of preclinical studies that show overexpression of class III beta-tubulin is associated with resistance to tubulin-binding agents, several investigators have addressed the relation between class III beta-tubulin and outcome in patients treated with such agents. High expression of class III beta-tubulin has been found to be correlated either with low response rates in patients treated with regimens containing taxanes or vinorelbine or with reduced survival in patients with non-small-cell lung cancer, in breast, ovarian, and gastric cancers, and in cancers of unknown primary site. Two studies have shown patients with advanced non-small-cell lung cancer receiving paclitaxel whose tumours expressed high levels of class III beta-tubulin had a lower response to paclitaxel and shorter survival, whereas this variable was not found to be predictive in patients receiving regimens without tubulin-binding agents. Conversely, analysis of samples from patients in the JBR-10 trial, which compared adjuvant chemotherapy to no further therapy in operable non-small-cell lung cancer, showed that chemotherapy seemed to overcome the negative prognostic effect of high levels of expression of class III beta-tubulin and the greatest benefit from cisplatin/vinorelbine was seen in patients with high levels of expression of class III beta-tubulin. Further analyses in operable and advanced non-small-cell lung cancer showed a relation between high expression of class III beta-tubulin and baseline factors such as age under 60 years, adenocarcinoma and large-cell carcinoma histologies, and advanced stage of disease. These results suggest that class III beta-tubulin could be both a prognostic and a predictive factor. Large randomised studies are warranted to determine the prognostic or predictive value of class III beta-tubulin in different settings and tumours.

TP53 status and taxane-platinum versus platinum-based therapy in ovarian cancer patients: a non-randomized retrospective study

Background

Taxane-platinum therapy (TP) has replaced platinum-based therapy (PC or PAC, DNA damaging chemotherapy) in the postoperative treatment of ovarian cancer patients; however, it is not always effective. TP53 protein plays a differential role in response to DNA-damaging agents and taxanes. We sought to define profiles of patients who benefit the most from TP and also of those who can be treated with PC.

Methods

We compared the effectiveness of PC/PAC (n = 253) and TP (n = 199) with respect to tumor TP53 accumulation in ovarian cancer patients with FIGO stage IIB-IV disease; this was a non-randomized retrospective study. Immunohistochemical analysis was performed on 452 archival tumors; univariate and multivariate analysis by the Cox's and logistic regression models was performed in all patients and in subgroups with [TP53(+)] and without TP53 accumulation [TP53(-)].

Results

The advantage of taxane-platinum therapy over platinum-based therapy was seen in the TP53(+), and not in the TP53(-) group. In the TP53(+) group taxane-platinum therapy enhanced the probability of complete remission (p = .018), platinum sensitivity (p = .014), platinum highly sensitive response (p = .038) and longer survival (OS, p = .008). Poor tumor differentiation diminished the advantage from taxane-platinum therapy in the TP53(+) group. In the TP53(-) group PC/PAC was at least equally efficient as taxane-platinum therapy and it enhanced the chance of platinum highly sensitive response (p = .010). However, in the TP53(-) group taxane-platinum therapy possibly diminished the risk of death in patients over 53 yrs (p = .077). Among factors that positively interacted with taxane-platinum therapy in some analyses were endometrioid and clear cell type, FIGO III stage, bulky residual tumor, more advanced age of patient and moderate tumor differentiation.

Conclusion

Our results suggest that taxane-platinum therapy is particularly justified in patients with TP53(+) tumors or older than 53 years. In the group of patients ≤53 yrs and with TP53(-) tumors platinum-based therapy is possibly equally efficient. We provide hints for planning randomized trials to verify these observations.

Strategies for the development of novel Taxol-like agents

Taxol, the first microtubule stabilizer identified, is one of the most important new anticancer drugs to be brought to the clinic in the past 20 yr. The clinical success of TaxolTM led to the development of a second-generation taxane, docetaxel (Taxotere), and multiple third-generation taxane derivatives are under development. Non-taxane microtubule-stabilizers of diverse chemical structures, including the epothilones and discodermolide, show promising preclinical activities and several epothilones are progressing through clinical trials. One important advantage of the new stabilizers is their ability to circumvent drug resistance mechanisms. The clinical development of these new classes of agents suggests that microtubule stabilizers will continue to be important drugs for the treatment of cancer. This chapter provides a brief history of Taxol and the discovery and development status of other classes of microtubule stabilizers. Although all microtubule-stabilizers share similar mechanisms of action, interesting subtle differences among the stabilizers are being detected. This chapter also provides some strategies for identifying the differences among microtubule stabilizers that may help prioritize them for development and clinical use.

Hypoxia induces class III beta-tubulin gene expression by HIF-1alpha binding to its 3' flanking region

Class III beta-tubulin (TUBB3) overexpression represents a major mechanism of drug resistance to microtubule interacting agents such as taxanes and Vinca alkaloids. Here, we tested hypoxia as a possible inducer of TUBB3. The effects of hypoxia on TUBB3 expression were monitored at mRNA and protein level in A2780, in its paclitaxel-resistant counterpart (TC1) and in HeLa cells. Hypoxia was a strong inducer of TUBB3 in A2780, but not in TC1 and HeLa cells. In A2780 HIF-1alpha was knocked down using RNA interference and TUBB3 expression was assessed in normoxia and hypoxia. The silencing abolished the hypoxia-dependent increase of TUBB3, thereby demonstrating that HIF-1alpha mediates TUBB3 induction in hypoxia. To investigate this phenomenon, the 5' flanking region of human TUBB3 was cloned upstream GFP as a reporter. This region contained the promoter gene, but activity of the reporter was unaffected by hypoxia. Thus, we looked at the 3' flanking region and, at +168 nucleotides from the stop codon, an HIF-1alpha binding site was proven to be active in hypoxia, using a construct in which the site was cloned downstream GFP as reporter gene. Deletion of the site in the construct abolished GFP enhancement upon hypoxia. Chromatin immunoprecipitation revealed the engagement by HIF-1alpha of this site in hypoxia. Methylation analysis of this 3' enhancer showed that it was free of methylation in 70% of cells in A2780, while in less than 16% in both TC1 and HeLa cells, thereby suggesting that TUBB3 increase upon hypoxia is abolished through methylation of the 3' enhancer.

Taxane's substituents at C3' affect its regioselective metabolism: different in vitro metabolism of cephalomannine and paclitaxel

To investigate how taxane's substituents at C3' affect its metabolism, we compared the metabolism of cephalomannine and paclitaxel, a pair of analogs that differ slightly at the C3' position. After cephalomannine was incubated with human liver microsomes in an NADPH-generating system, two monohydroxylated metabolites (M1 and M2) were detected by liquid chromatography/tandem mass spectrometry. C4'' (M1) and C6alpha (M2) were proposed as the possible hydroxylation sites, and the structure of M1 was confirmed by (1)H NMR. Chemical inhibition studies and assays with recombinant human cytochromes P450 (P450s) indicated that 4''-hydroxycephalomannine was generated predominantly by CYP3A4 and 6alpha-hydroxycephalomannine by CYP2C8. The overall biotransformation rate between paclitaxel and cephalomannine differed slightly (184 vs. 145 pmol/min/mg), but the average ratio of metabolites hydroxylated at the C13 side chain to C6alpha for paclitaxel and cephalomannine varied significantly (15:85 vs. 64:36) in five human liver samples. Compared with paclitaxel, the major hydroxylation site transferred from C6alpha to C4'', and the main metabolizing P450 changed from CYP2C8 to CYP3A4 for cephalomannine. In the incubation system with rat or minipig liver microsomes, only 4''-hydroxycephalomannine was detected, and its formation was inhibited by CYP3A inhibitors. Molecular docking by AutoDock suggested that cephalomannine adopted an orientation in favor of 4''-hydroxylation, whereas paclitaxel adopted an orientation favoring 3'-p-hydroxylation. Kinetic studies showed that CYP3A4 catalyzed cephalomannine more efficiently than paclitaxel due to an increased V(m). Our results demonstrate that relatively minor modification of taxane at C3' has major consequence on the metabolism.

Taxanes, microtubules and chemoresistant breast cancer

The taxanes, paclitaxel and docetaxel are microtubule-stabilizing agents that function primarily by interfering with spindle microtubule dynamics causing cell cycle arrest and apoptosis. However, the mechanisms underlying their action have yet to be fully elucidated. These agents have become widely recognized as active chemotherapeutic agents in the treatment of metastatic breast cancer and early-stage breast cancer with benefits gained in terms of overall survival (OS) and disease-free survival (DFS). However, even with response to taxane treatment the time to progression (TTP) is relatively short, prolonging life for a matter of months, with studies showing that patients treated with taxanes eventually relapse. This review focuses on chemoresistance to taxane treatment particularly in relation to the spindle assembly checkpoint (SAC) and dysfunctional regulation of apoptotic signaling. Since spindle microtubules are the primary drug targets for taxanes, important SAC proteins such as MAD2, BUBR1, Synuclein-gamma and Aurora A have emerged as potentially important predictive markers of taxane resistance, as have specific checkpoint proteins such as BRCA1. Moreover, overexpression of the drug efflux pump MDR-1/P-gp, altered expression of microtubule-associated proteins (MAPs) including tau, stathmin and MAP4 may help to identify those patients who are most at risk of recurrence and those patients most likely to benefit from taxane treatment.

Anticancer drugs from nature--natural products as a unique source of new microtubule-stabilizing agents

This review article provides an overview on the current state of research in the area of microtubule-stabilizing agents from natural sources, with a primary focus on the biochemistry, biology, and pharmacology associated with these compounds. A variety of natural products have been discovered over the last decade to inhibit human cancer cell proliferation through a taxol-like mechanism. These compounds represent a whole new range of structurally diverse lead structures for anticancer drug discovery.

Investigational agents against platinum-resistant ovarian cancer

Ovarian cancer is still the fourth cause of death by cancer among women and is the most fatal among gynaecological tumours. The goal of treatment for patients with recurrent, platinum-resistant (platinum-free interval < 6 months) ovarian cancer is the palliation of symptoms because no evidence indicates that present therapies may prolong survival in this setting of patients. Successful management of these patients depends on the identification of agents that are not cross-resistant with platinum compounds. The development of molecular biology is providing us with new information on the molecular basis of cancer, its mechanism of initiation and progression, and supply the need of a more patient-tailored therapy where specific tumours are treated with specific drugs. This paper reports and discusses new developments in the treatment of platinum-resistant ovarian cancer patients. The authors present proteomic advances, including the HER kinases, the 26S proteasome and the angiogenesis pathway. The opportunities to change the treatment of ovarian cancer will require creative clinical trial design but the next 10 years promise to be filled with therapeutic advances for patients with ovarian cancer.

Activity-dependent neuroprotective protein: from gene to drug candidate

Activity-dependent neuroprotective protein (ADNP) is essential for brain formation. The gene encoding ADNP is highly conserved and abundantly expressed in the brain. ADNP contains a homeobox profile and a peptide motif providing neuroprotection against a variety of cytotoxic insults. ADNP mRNA and protein expression responds to brain injury and oscillates as a function of the estrus cycle. The plastic nature of ADNP expression is correlated with brain protection and an association between neuroendocrine regulation and neuroprotection is put forth with ADNP as a focal point. Further understanding of neuroprotective molecules should pave the path to better diagnostics and therapies. In this respect, structure-activity studies have identified a short 8 amino acid peptide in ADNP/NAPVSIPQ (NAP) that provides potent neuroprotection. NAP is currently in clinical development for neuroprotection.

Pharmacokinetics and Metabolism in Mice of IDN 5390 (13-(N-Boc-3-i-butylisoserinoyl)-C-7,8-seco-10-deacetylbaccatin III), a New Oral C-seco-Taxane Derivative with Antiangiogenic Property Effective on Paclitaxel-Resistant Tumors

IDN 5390 (13-(N-Boc-3-i-butylisoserinoyl)-C-7,8-seco-10-deacetylbaccatin III) is a new taxane, derived from 7,8-C-seco-10-deacetylbaccatin, selected for its ability to inhibit angiogenesis, mainly by acting on endothelial cell motility, and for its selective activity on class III beta-tubulin. In vivo, IDN 5390 shows activity against paclitaxel-sensitive and -resistant tumors when administered on a prolonged, continuous dosage schedule. We studied the pharmacokinetics and bioavailabilty of the drug in mice after single and repeated oral treatment. IDN 5390 was rapidly absorbed after oral administration, with good bioavailability (43%). After intravenous injection, it was extensively distributed in tissue, mainly the liver, kidney, and heart, with low but persistent levels in brain. The kinetics appear dose-dependent with a clearance of 2.6, 1.4, and 0.9 l/kg at, respectively, 60, 90, and 120 mg/kg, and a half-life 24, 36, and 54 min. After prolonged daily oral doses given for 2 weeks, we found that there was a decrease in drug availability; i.e., the area under the concentration-time curve value after p.o. daily administration on day 14 was 2-fold lower than that on day 1. Metabolism plays a major role in elimination of the drug, and at least 12 metabolites were identified in feces and urine. The percentage excreted as metabolites after an oral dose (42%) was higher than that after the i.v. dose (33%), suggesting a first-pass effect. Four metabolites were found in plasma at detectable levels; one of them, with restored taxane scaffold, is a species 3 times more potent than IDN 5390, possibly contributing to the observed anti-tumor activity.

Peptide neuroprotection through specific interaction with brain tubulin

This study aimed to identify the neuronal target for the potent neuroprotective peptide NAP. When added to pheochromocytoma cells (neuronal model), NAP was found in the intracellular milieu and was co-localized with microtubules. NAP induced neurite outgrowth and protected primary neurons against microtubule-associated ZnCl2 toxicity. Rapid microtubule reorganization into distinct microtubules ensued after NAP addition to both pheochromocytoma cells and primary cerebral cortical neurons, but not to fibrobalsts. While binding neuronal tubulin and protecting pheochromocytoma cells against oxidative stress, NAP did not bind tubulin extracted from fibroblasts, nor did it protect those cells against oxidative stress. Affinity chromatography identified the brain-specific betaIII-tubulin as a major NAP binding protein. Paclitaxel (a microtubule aggregating agent that interacts with beta-tubulin) reduced NAP tubulin binding. Thus, the underlying mechanism for the neuroprotection offered by NAP is targeting neuronal microtubules that are essential for neuronal survival and function.

Binding studies of taxanes to human serum albumin by bioaffinity chromatography and circular dichroism

The binding to human serum albumin (HSA) of the antitumoural drug paclitaxel and of several structural analogues has been characterized by bioaffinity chromatography and circular dichroism. A ranking of the taxanes was obtained for their affinity to the protein by measuring their retention times on an albumin chromatographic column. This also allowed the calculation of the drug bound percentage. Affinity resulted significantly affected by the nature of the isoserinic side chain, the presence of the 1,14-carbonate moiety and the substituent at C-7, showing that the hydrophobicity of the drug is fundamental in the binding process. The analysis demonstrated that the organic solvent highly alters the interaction mechanism of taxanes to the protein and so the affinity results. Circular dichroism experiments supported this hypothesis. Furthermore, taxanes binding to the serum carrier was characterized by displacement chromatography, by adding into the mobile phase selected competitors, (S)-ibuprofen and valproic acid, that are known to bind to specific binding sites on HSA. These experiments established a non-cooperative binding mechanism.

Class III beta-tubulin overexpression is a marker of poor clinical outcome in advanced ovarian cancer patients

PURPOSE

Overexpression of beta III tubulin has been involved in paclitaxel resistance in several experimental models. We investigated the role of beta III tubulin as predictor of clinical outcome in ovarian cancer patients given platinum/paclitaxel treatment. We also investigated whether beta III tubulin expression could be modified after the selective pressure represented by chemotherapy in vivo.

EXPERIMENTAL DESIGN

The study was designed to include a series of consecutive ovarian cancer patients with unresectable disease at time of first surgery, who underwent interval debulking surgery with pathologic assessment of response to treatment with platinum/paclitaxel chemotherapy. Immunostaining was done on formalin-fixed, paraffin-embedded tissue sections from pretreatment and posttreatment tissue biopsies by using the polyclonal rabbit anti-class III beta-tubulin antibody.

RESULTS

beta III Tubulin immunoreaction was observed in 51 of 62 (82.2%) cases. beta III Tubulin positivity was neither associated with clinicopathologic variables nor with pathologic response to chemotherapy. Significantly lower percentages of beta III tubulin positivity were observed in posttreatment (range, 5-80%; median, 20%) versus pretreatment (range 10-100%; median, 40%) tissue biopsies (P = 0.0011). Cases with high beta III tubulin expression showed a worse overall survival with respect to cases with low beta III tubulin expression (median overall survival, 25 versus 46 months; P = 0.002). Multivariate analysis showed that high content of beta III tubulin remains independently associated with a worse prognosis.

CONCLUSIONS

Assessment of beta III tubulin could be useful to identify poor prognosis ovarian cancer patients candidates to more aggressive and/or targeted therapy.

Stimulation of erythrocyte phosphatidylserine exposure by paclitaxel

Side effects of cytostatic treatment include development of anemia resulting from either decreased generation or accelerated clearance of circulating erythrocytes. Recent experiments revealed a novel kind of stress-induced erythrocyte death, i.e. eryptosis, which is characterized by enhanced cytosolic Ca(2+) levels, increased ceramide formation and exposure of phosphatidylserine at the cell surface. The present study explored whether cytostatic treatment with paclitaxel (Taxol) triggers eryptosis. Blood was drawn from cancer patients before and after infusion of 175 mg/m2 Taxol. The treatment significantly decreased the hematocrit and significantly increased the percentage of annexin-V-binding erythrocytes in vivo (by 37%). In vitro incubation of human erythrocytes with 10 microM paclitaxel again significantly increased annexin-V-binding (by 129%) and augmented the increase of annexin-V-binding following cellular stress. The enhanced phosphatidylserine exposure was not dependent on caspase-activity but paralleled by erythrocyte shrinkage, increase of cytosolic Ca(2+) activity, ceramide formation and activation of calpain. Phosphatidylserine exposure was similarly induced by docetaxel but not by carboplatin or doxorubicin. Moreover, eryptosis was triggered by the Ca(2+) ionophore ionomycin (10 microM). In mice, ionomycin-treated eryptotic erythrocytes were rapidly cleared from circulating blood and sequestrated into the spleen. In conclusion, our data strongly suggest that paclitaxel-induced anemia is at least partially due to induction of eryptosis.

Conjugated indole-imidazole derivatives displaying cytotoxicity against multidrug resistant cancer cell lines

We report herein the SAR studies of a series of indole-imidazole compounds. that demonstrate substantial in vitro anti-proliferative activities against cancer cell lines, including multidrug resistance (MDR) phenotypes. The in vitro cytotoxic effects have been demonstrated across a wide array of tumor types, including hematologic and solid tumor cell lines of various origins (e.g., leukemia, breast, colon, and uterine).

The betaI/betaIII-tubulin isoforms and their complexes with antimitotic agents

Both microtubule destabilizer and stabilizer agents are important molecules in anticancer therapy. In particular, paclitaxel has been demonstrated to be effective for the treatment of ovarian, breast, and nonsmall cell lung carcinomas. It has been shown that emergence of resistance against this agent correlates with an increase in the relative abundance of tubulin isoform betaIII and that the more recently discovered IDN5390 can be effectively used once resistance has emerged. In this paper, we analyze the binding modes of these antimitotic agents to type I and III isoforms of beta-tubulin by computational methods. Our results are able to provide a molecular explanation of the experimental data. Using the same protocol, we could also show that no preference for any of the two isoforms can be detected for epothilone A, a potentially very interesting drug for which no data about the emergence of resistance is currently available. Our analysis provides structural insights about the recognition mode and the stabilization mechanism of these antimitotic agents and provides useful suggestions for the design of more potent and selective antimitotic agents.

Potentiation of taxol efficacy and by discodermolide in ovarian carcinoma xenograft-bearing mice

PURPOSE

To evaluate the drug combination of discodermolide and Taxol in human ovarian cancer cells and in an in vivo model of ovarian carcinoma.

EXPERIMENTAL DESIGN

The combination index method was used to evaluate the interaction of Taxol and discodermolide in human ovarian SKOV-3 carcinoma cells. Data were correlated with alterations in cell cycle distribution and caspase activation. In addition, SKOV-3 xenograft-bearing mice were treated with either Taxol, discodermolide, or a combination of both drugs given concurrently to evaluate the antitumor efficacy and toxicity of this combination. The Matrigel plug assay and CD31 immunohistochemistry were done to assess antiangiogenic effects.

RESULTS

Taxol and discodermolide interact synergistically over a range of concentrations and molar ratios that cause drug-induced aneuploidy in ovarian carcinoma cells. In SKOV-3 xenograft-bearing mice, the combination is significantly superior to either single agent, and induces tumor regressions without notable toxicities. Immunohistochemical analysis of CD31 and Matrigel plug analysis show decreased vessel formation in mice treated with the combination relative to either drug alone.

CONCLUSIONS

The synergistic activity of Taxol and discodermolide in cells is most potent at drug concentrations that result in drug-induced aneuploidy rather than mitotic arrest. Moreover, in an animal model of ovarian carcinoma, this is a well-tolerated combination that induces tumor regressions and suppresses angiogenesis. These data confirm the potency of this combination and support the use of concurrent low doses of Taxol and discodermolide for potential use in cancer therapeutics.

Synthesis and biological evaluation of methoxylated analogs of the newer generation taxoids IDN5109 and IDN5390

Starting from 10-deacetylbaccatin III (7), the 2-debenzoyl-2-m-methoxybenzoyl analogs of the newer generation taxoids IDN5109 (3) and IDN5390 (4) were synthesized. The biological evaluation of these compounds (5 and 6, respectively) showed a general increase of cytotoxicity, as observed in first-generation anticancer taxanes.