Resistance to microtubule-stabilizing drugs involves two events: beta-tubulin mutation in one allele followed by loss of the second allele

Development of 1,3-diynyl derivatives of noscapine as potent tubulin binding anticancer agents for the management of breast cancer

We developed 1,3-diynyl derivatives of noscapine (an opium alkaloid) through in silico combinatorial approach and screened out a panel of promising derivatives that bind tubulin and display anticancer activity. The selected derivatives such as 9-4-^tBu-Ph-Diyne (20p), 9-3,4-Di-Cl-Diyne (20k) and 9-3,4-Di-F-Diyne (22s) noscapinoids revealed improved predicted binding energy of -6.676 kcal/mol for 20p, -7.294 kcal/mol for 20k and -7.750 kcal/mol for 20s respectively in comparison to noscapine (-5.246 kcal/mol). These 1,3-diynyl derivatives (20p, 29k and 20s) were strategically synthesized in high yields by regioselective modification of noscapine scaffold and HPLC purified (purity is >96%). The decrease in intrinsic fluorescence of purified tubulin to 8.39%, 17.39% and 25.47% by 20p, 20k and 20s respectively, compared to control suggests their binding capability to tubulin. Their cytotoxicity activity was validated based on cellular studies using two human breast adenocarcinoma (MCF-7 and MDA-MB-231), a panel of primary breast tumor cells and one normal human embryonic kidney cell (293 T). The 1,3-diynyl noscapinoids, 20p, 20k and 20s inhibited cellular proliferation in all the cancer cells that ranged between 6.2 and 38.9 µM, without affecting the normal healthy cells (cytotoxicity is <5% at 100 µM). Further, these novel derivatives arrest cell cycle in the G2/M-phase, followed by induction of apoptosis to cancer cells. Thus, we conclude that 1,3-diynyl-noscapinoids have great potential to be a novel therapeutic agent for breast cancers.Communicated by Ramaswamy H. Sarma.

CLIP-170S is a microtubule +TIP variant that confers resistance to taxanes by impairing drug-target engagement

Taxanes are widely used cancer chemotherapeutics. However, intrinsic resistance limits their efficacy without any actionable resistance mechanism. We have discovered a microtubule (MT) plus-end-binding CLIP-170 protein variant, hereafter CLIP-170S, which we found enriched in taxane-resistant cell lines and patient samples. CLIP-170S lacks the first Cap-Gly motif, forms longer comets, and impairs taxane access to its MT luminal binding site. CLIP-170S knockdown reversed taxane resistance in cells and xenografts, whereas its re-expression led to resistance, suggesting causation. Using a computational approach in conjunction with the connectivity map, we unexpectedly discovered that Imatinib was predicted to reverse CLIP-170S-mediated taxane resistance. Indeed, Imatinib treatment selectively depleted CLIP-170S, thus completely reversing taxane resistance. Other RTK inhibitors also depleted CLIP-170S, suggesting a class effect. Herein, we identify CLIP-170S as a clinically prevalent variant that confers taxane resistance, whereas the discovery of Imatinib as a CLIP-170S inhibitor provides novel therapeutic opportunities for future trials.

Biotin conjugated organic molecules and proteins for cancer therapy: A review

The main transporter for biotin is sodium dependent multivitamin transporter (SMVT), which is overexpressed in various aggressive cancer cell lines such as ovarian (OV 2008, ID8), leukemia (L1210FR), mastocytoma (P815), colon (Colo-26), breast (4T1, JC, MMT06056), renal (RENCA, RD0995), and lung (M109) cancer cell lines. Furthermore, its overexpression was found higher to that of folate receptor. Therefore, biotin demand in the rapidly growing tumors is higher than normal tissues. Several biotin conjugated organic molecules has been reported here for selective delivery of the drug in cancer cell. Biotin conjugated molecules are showing higher fold of cytotoxicity in biotin positive cancer cell lines than the normal cell. Nanoparticles and polymer surface modified drugs and biotin mediated cancer theranostic strategy was highlighted in this review. The cytotoxicity and selectivity of the drug in cancer cells has enhanced after biotin conjugation.

Mitotic cell death induction by targeting the mitotic spindle with tubulin-inhibitory indole derivative molecules

Tubulin-targeting molecules are widely used cancer therapeutic agents. They inhibit microtubule-based structures, including the mitotic spindle, ultimately preventing cell division. The final fates of microtubule-inhibited cells are however often heterogeneous and difficult to predict. While recent work has provided insight into the cell response to inhibitors of microtubule dynamics (taxanes), the cell response to tubulin polymerization inhibitors remains less well characterized. Arylthioindoles (ATIs) are recently developed tubulin inhibitors. We previously identified ATI members that effectively inhibit tubulin polymerization in vitro and cancer cell growth in bulk cell viability assays. Here we characterise in depth the response of cancer cell lines to five selected ATIs. We find that all ATIs arrest mitotic progression, yet subsequently yield distinct cell fate profiles in time-lapse recording assays, indicating that molecules endowed with similar tubulin polymerization inhibitory activity in vitro can in fact display differential efficacy in living cells. Individual ATIs induce cytological phenotypes of increasing severity in terms of damage to the mitotic apparatus. That differentially triggers MCL-1 down-regulation and caspase-3 activation, and underlies the terminal fate of treated cells. Collectively, these results contribute to define the cell response to tubulin inhibitors and pinpoint potentially valuable molecules that can increase the molecular diversity of tubulin-targeting agents.

Creating chemotherapeutic-resistant breast cancer cell lines: advances and future perspectives

The development of resistance remains the most significant impediment to generating effective treatments for cancer. In the modern age of personalized medicine, it is of critical importance to understand the principles of both innate and acquired resistance to achieve the most effective therapeutic outcomes. Significant differences exist between cancer cells that exhibit innate resistance verses those that acquire resistance over time. Studying the acquisition of resistance is essential to obtaining a complete understanding of how treatments contribute to disease recurrence and progression. This review will evaluate the current understanding of chemotherapeutic resistance and its role in personalized medicine. This review will also explore how generating resistant cells in culture is essential to the development of improved cancer therapeutics.

Investigation of Paclitaxel Resistant R306C Mutation in β-Tubulin—A Computational Approach

Paclitaxel is the most effective chemotherapeutic agent used for the treatment of a broad spectrum of solid tumors. However, observed paclitaxel resistance in clinical trials presents one of the major obstacles for cancer chemotherapy. Most importantly, resistance due to β-tubulin mutations (R306C) has been intensely debated in recent years. Despite all efforts, mechanism of resistance is still not well understood. In this study, computational techniques were employed to uncover the effect of R306C mutation in the β-tubulin structure and its function. The tools such as I-Mutant, CUPSAT and Fold-X were employed to address the consequence of R306C mutation in the structural stability of β-tubulin. Further, molecular docking and molecular dynamics study was employed to understand the functional impact of β-tubulin mutation. Our results suggest that the R306C mutation causes a significant reduction in the binding affinity between β-tubulin and paclitaxel. Further, docked complex analysis indicates that destruction of conservative hydrogen bond maintained by the residues Arg282 and Gly360 should be responsible for the large conformation changes of the binding pocket in R306C mutant. Finally, molecular dynamics simulations study confirms the stable binding of paclitaxel with native type β-tubulin structure rather than mutant (R306C) type. We certainly believe that this study will provide useful guidance for the development of novel inhibitors that are less susceptible to drug resistance.

An antimitotic and antivascular agent BPR0L075 overcomes multidrug resistance and induces mitotic catastrophe in paclitaxel-resistant ovarian cancer cells

Paclitaxel plays a major role in the treatment of ovarian cancer; however, resistance to paclitaxel is frequently observed. Thus, new therapy that can overcome paclitaxel resistance will be of significant clinical importance. We evaluated antiproliferative effects of an antimitotic and antivascular agent BPR0L075 in paclitaxel-resistant ovarian cancer cells. BPR0L075 displays potent and broad-spectrum cytotoxicity at low nanomolar concentrations (IC₅₀ = 2–7 nM) against both parental ovarian cancer cells (OVCAR-3, SKOV-3, and A2780-1A9) and paclitaxel-resistant sublines (OVCAR-3-TR, SKOV-3-TR, 1A9-PTX10), regardless of the expression levels of the multidrug resistance transporter P-gp and class III β-tubulin or mutation of β-tubulin. BPR0L075 blocks cell cycle at the G2/M phase in paclitaxel-resistant cells while equal concentration of paclitaxel treatment was ineffective. BPR0L075 induces cell death by a dual mechanism in parental and paclitaxel-resistant ovarian cancer cells. In the parental cells (OVCAR-3 and SKOV-3), BPR0L075 induced apoptosis, evidenced by poly(ADP-ribose) polymerase (PARP) cleavage and DNA ladder formation. BPR0L075 induced cell death in paclitaxel-resistant ovarian cancer cells (OVCAR-3-TR and SKOV-3-TR) is primarily due to mitotic catastrophe, evidenced by formation of giant, multinucleated cells and absence of PARP cleavage. Immunoblotting analysis shows that BPR0L075 treatment induced up-regulation of cyclin B1, BubR1, MPM-2, and survivin protein levels and Bcl-XL phosphorylation in parental cells; however, in resistant cells, the endogenous expressions of BubR1 and survivin were depleted, BPR0L075 treatment failed to induce MPM-2 expression and phosphorylation of Bcl-XL. BPR0L075 induced cell death in both parental and paclitaxel-resistant ovarian cancer cells proceed through caspase-3 independent mechanisms. In conclusion, BPR0L075 displays potent cytotoxic effects in ovarian cancer cells with a potential to overcome paclitaxel resistance by bypassing efflux transporters and inducing mitotic catastrophe. BPR0L075 represents a novel microtubule therapeutic to overcome multidrug resistance and trigger alternative cell death by mitotic catastrophe in ovarian cancer cells that are apoptosis-resistant.

Understanding the basis of drug resistance of the mutants of αβ-tubulin dimer via molecular dynamics simulations

The vital role of tubulin dimer in cell division makes it an attractive drug target. Drugs that target tubulin showed significant clinical success in treating various cancers. However, the efficacy of these drugs is attenuated by the emergence of tubulin mutants that are unsusceptible to several classes of tubulin binding drugs. The molecular basis of drug resistance of the tubulin mutants is yet to be unraveled. Here, we employ molecular dynamics simulations, protein-ligand docking, and MMPB(GB)SA analyses to examine the binding of anticancer drugs, taxol and epothilone to the reported point mutants of tubulin--T274I, R282Q, and Q292E. Results suggest that the mutations significantly alter the tubulin structure and dynamics, thereby weaken the interactions and binding of the drugs, primarily by modifying the M loop conformation and enlarging the pocket volume. Interestingly, these mutations also affect the tubulin distal sites that are associated with microtubule building processes.

Overexpression of centrosomal protein Nlp confers breast carcinoma resistance to paclitaxel

Nlp (ninein-like protein), an important molecule involved in centrosome maturation and spindle formation, plays an important role in tumorigenesis and its abnormal expression was recently observed in human breast and lung cancers. In this study, the correlation between overexpression of Nlp and paclitaxel chemosensitivity was investigated to explore the mechanisms of resistance to paclitaxel and to understand the effect of Nlp upon apoptosis induced by chemotherapeutic agents. Nlp expression vector was stably transfected into breast cancer MCF-7 cells. With Nlp overexpression, the survival rates, cell cycle distributions and apoptosis were analyzed in transfected MCF-7 cells by MTT test and FCM approach. The immunofluorescent assay was employed to detect the changes of microtubule after paclitaxel treatment. Immunoblotting analysis was used to examine expression of centrosomal proteins and apoptosis associated proteins. Subsequently, Nlp expression was retrospectively examined with 55 breast cancer samples derived from paclitaxel treated patients. Interestingly, the survival rates of MCF-7 cells with Nlp overexpressing were higher than that of control after paclitaxel treatment. Nlp overexpression promoted G2-M arrest and attenuated apoptosis induced by paclitaxel, which was coupled with elevated Bcl-2 protein. Nlp expression significantly lessened the microtubule polymerization and bundling elicited by paclitaxel attributing to alteration on the structure or dynamics of β-tubulin but not on its expression. The breast cancer patients with high expression of Nlp were likely resistant to the treatment of paclitaxel, as the response rate in Nlp negative patients was 62.5%, whereas was 58.3 and 15.8% in Nlp (+) and Nlp (++) patients respectively (p = 0.015). Nlp expression was positive correlated with those of Plk1 and PCNA. These findings provide insights into more rational chemotherapeutic regimens in clinical practice, and more effective approaches might be developed through targeting Nlp to increase chemotherapeutic sensitivity.

Mutations in the β-tubulin binding site for peloruside A confer resistance by targeting a cleft significant in side chain binding

Peloruside A is a microtubule-stabilizing macrolide that binds to beta tubulin at a site distinct from the taxol site. The site was previously identified by H-D exchange mapping and molecular docking as a region close to the outer surface of the microtubule and confined in a cavity surrounded by a continuous loop of protein folded so as to center on Y340. We have isolated a series of peloruside A-resistant lines of the human ovarian carcinoma cell line A2780(1A9) to better characterize this binding site and the consequences of altering residues in it. Four resistant lines (Pel A-D) are described with single-base mutations in class I β-tubulin that result in the following substitutions: R306H, Y340S, N337D, and A296S in various combinations. The mutations are localized to peptides previously identified by Hydrogen-Deuterium exchange mapping, and center on a cleft in which the drug side chain appears to dock. The Pel lines are 10-15-fold resistant to peloruside A and show cross resistance to laulimalide but not to any other microtubule stabilizers. They show no cross-sensitivity to any microtubule destabilizers, nor to two drugs with targets unrelated to microtubules. Peloruside A induces G2/M arrest in the Pel cell lines at concentrations 10-15 times that required in the parental line. The cells show notable changes in morphology compared to the parental line.

New insights into mechanisms of resistance to microtubule inhibitors

Mechanisms to explain tumor cell resistance to drugs that target the microtubule cytoskeleton have relied on the assumption that the drugs act either to suppress microtubule dynamics or to perturb the balance between assembled and nonassembled tubulin. Recently, however, it was found that these drugs also alter the stability of microtubule attachment to centrosomes, and do so at the same concentrations that are needed to inhibit cell division. Based on this new information, a new model is presented that explains resistance resulting from a variety of molecular changes that have been reported in the literature. The improved understanding of drug action and resistance has important implications for chemotherapy with these agents.

Effectiveness of liposomal paclitaxel against MCF-7 breast cancer cells

Paclitaxel is an effective chemotherapeutic agent that is widely used for the treatment of several cancers, including breast, ovarian, and non-small-cell lung cancer. Due to its high lipophilicity, paclitaxel is difficult to administer and requires solubilization with Cremophor EL (polyethoxylated castor oil) and ethanol, which often lead to adverse side effects, including life-threatening anaphylaxis. Incorporation of paclitaxel in dimyristoylphosphatidylcholine:dimyristoylphosphatidylglycerol (DPPC:DMPG) liposomes can facilitate its delivery to cancer cells and eliminate the adverse reactions associated with the Cremophor EL vehicle. Accordingly, the effectiveness of liposomal paclitaxel on MCF-7 breast cancer cells was examined. The results from this study showed that (i) the lipid components of the liposomal formulation were nontoxic, (ii) the cytotoxic effects of liposomal paclitaxel were improved when compared with those seen with conventional paclitaxel, and (iii) the intracellular paclitaxel levels were higher in MCF-7 cells treated with the liposomal paclitaxel formulation. The results of these studies showed that delivery of paclitaxel as a liposomal formulation could be a promising strategy for enhancing its chemotherapeutic effects.

Cancer cells acquire mitotic drug resistance properties through beta I-tubulin mutations and alterations in the expression of beta-tubulin isotypes

BACKGROUND

Anti-mitotic compounds (microtubule de-stabilizers) such as vincristine and vinblastine have been shown clinically successful in treating various cancers. However, development of drug-resistance cells limits their efficacies in clinical situations. Therefore, experiments were performed to determine possible drug resistance mechanisms related to the application of anti-mitotic cancer therapy.

PRINCIPAL FINDINGS

A KB-derived microtubule de-stabilizer-resistant KB-L30 cancer cell line was generated for this study. KB-L30 cells showed cross-resistance to various microtubule de-stabilizers including BPR0L075, vincristine and colchicine through multiple-drug resistant (MDR)-independent mechanisms. Surprisingly, KB-L30 cells showed hyper-sensitivity to the microtubule-stabilizer, paclitaxel. Results of the RT-PCR analysis revealed that expression of both class II and III β-tubulin was down-regulated in KB-L30 cells as compared to its parental KB cancer cells. In addition, DNA sequencing analysis revealed six novel mutation sites present in exon four of the βI-tubulin gene. Computational modeling indicated that a direct relationship exists between βI-tubulin mutations and alteration in the microtubule assembly and dynamic instability in KB-L30 cells and this predicted model was supported by an increased microtubule assembly and reduced microtubule dynamic instability in KB-L30 cells, as shown by Western blot analysis.

CONCLUSIONS AND SIGNIFICANCE

Our study demonstrated that these novel mutations in exon four of the βI-tubulin induced resistance to microtubule de-stabilizers and hyper-sensitivity to microtubule stabilizer through an alteration in the microtubule assembly and dynamics in cancer cells. Importantly, the current study reveals that cancer cells may acquire drug resistance ability to anti-mitotic compounds through multiple changes in the microtubule networks. This study further provided molecular information in drug selection for patients with specific tubulin mutations.

Expressed sequence tags (ESTs) and single nucleotide polymorphisms (SNPs): What large-scale sequencing projects can tell us about ADME

To date over 800 complete genomes have been sequenced, with many more partially complete. Coupled with the large amount of mRNA transcript sequence data being produced from expression studies, there is now a daunting amount of information available to the research scientist. This review examines how this information may be best used, focusing on examples from sequences encoding absorption, distribution, metabolism and excretion (ADME)-related proteins in particular. Through the use of phylogenetic, splice variant and single nucleotide polymorphism (SNP) analysis, the review examines not only how insights into species-specific responses to drug exposure may be gained, but also how best to utilize this information to predict both individual human responses and the impact of population variance in response.

Biomarker discovery in ovarian cancer

Ovarian cancer is a leading cause of gynecologic cancer death among women. Tumors diagnosed early (in stage I) have a cure rate approaching 90%. However, because specific symptoms and screening tools are lacking, most ovarian cancers are very advanced when finally diagnosed. CA125 expression and pelvic ultrasonography are of limited efficacy in screening, and the search for new, complementary ovarian cancer biomarkers continues. New technology and research techniques have allowed the identification of over 100 possible tumor markers, many of which are still being evaluated for clinical relevance and several of which have entered clinical trials. Here, we review the methods of biomarker discovery, address the significance and functions of newly identified ovarian cancer tumor markers, and provide further insight into the future of ovarian cancer biomarkers.

The optimal therapeutic use of ixabepilone in patients with locally advanced or metastatic breast cancer

Background. The epothilones are a new class of microtubule-stabilizing drugs that exert potent antitumor activity against taxane-resistant and multidrug resistant cell lines. The most clinically advanced member of this class is the semisynthetic epothilone B derivative ixabepilone.

Purpose. This article reviews the preclinical and clinical data on ixabepilone in patients with locally advanced and metastatic breast cancer (MBC) and provides guidance for pharmacists on its optimal use. Data sources. PubMed and conference proceedings through August 2008.

Results. In preclinical studies, ixabepilone has demonstrated potent antitumor activity and low susceptibility to mechanisms that confer tumor resistance. Clinically meaningful benefits have been achieved with ixabepilone monotherapy in phase 2 trials of patients with MBC who have failed previous chemotherapies (anthracyclines, taxanes, or capecitabine). In a randomized, phase 3 trial, the combination of ixabepilone and capecitabine proved more effective than capecitabine alone after the failure of taxane and anthracycline regimens. At the recommended dose and schedule, the therapeutic ratio of ixabepilone is generally favorable, and its adverse effects (notably neutropenia and peripheral neuropathy) are generally manageable and reversible.

Conclusion. Ixabepilone represents an advance in the treatment of anthracycline - and taxane-pretreated MBC. Future studies will define its efficacy in combination with other drugs used in the treatment of MBC, as well as in other types of cancer. J Oncol Pharm Practice (2009) 15: 95—106.

Gene expression modulation is associated with gene amplification, supernumerary chromosomes and chromosome loss in antimony-resistant Leishmania infantum

Antimonials remain the first line drug against the protozoan parasite Leishmania but their efficacy is threatened by resistance. We carried out a RNA expression profiling analysis comparing an antimony-sensitive and -resistant (Sb2000.1) strain of Leishmania infantum using whole-genome 70-mer oligonucleotide microarrays. Several genes were differentially expressed between the two strains, several of which were found to be physically linked in the genome. MRPA, an ATP-binding cassette (ABC) gene known to be involved in antimony resistance, was overexpressed in the antimony-resistant mutant along with three other tandemly linked genes on chromosome 23. This four gene locus was flanked by 1.4 kb repeated sequences from which an extrachromosomal circular amplicon was generated in the resistant cells. Interestingly, gene expression modulation of entire chromosomes occurred in the antimony-resistant mutant. Southern blots analyses and comparative genomic hybridizations revealed that this was either due to the presence of supernumerary chromosomes or to the loss of one chromosome. Leishmania parasites with haploid chromosomes were viable. Changes in copy number for some of these chromosomes were confirmed in another antimony-resistant strain. Selection of a partial revertant line correlated antimomy resistance levels and the copy number of aneuploid chromosomes, suggesting a putative link between aneuploidy and drug resistance in Leishmania.

Pharmacogenetics influence treatment efficacy in childhood acute lymphoblastic leukemia

Pharmacogenetics covers the genetic variation affecting pharmacokinetics and pharmacodynamics, and their influence on drug-response phenotypes. The genetic variation includes an estimated 15 million single nucleotide polymorphisms (SNPs) and is a key determinator for the interindividual differences in treatment resistance and toxic side effects. As most childhood acute lymphoblastic leukemia treatment protocols include up to 13 different chemotherapeutic agents, the impact of individual SNPs has been difficult to evaluate. So far focus has mainly been on the widely used glucocorticosteroids, methotrexate, and thiopurines, or on metabolic pathways and transport mechanisms that are common to several drugs, such as the glutathione S-transferases. However, beyond the thiopurine methyltransferase polymorphisms, the candidate-gene approach has not established clear associations between polymorphisms and treatment response. In the future, high-throughput, low-cost, genetic platforms will allow screening of hundreds or thousands of targeted SNPs to give a combined gene-dosage effect (=individual SNP risk profile), which may allow pharmacogenetic-based individualization of treatment.

Lx2-32c, a novel taxane and its antitumor activities in vitro and in vivo

Lx2-32c, a novel taxane derivative, is a semisynthetic analogue from cephalomannine. Its antitumor activity in vivo and in vitro was investigated in this study. Lx2-32c was cytotoxic (IC50=1.7+/-1.6nM) to various human tumor cell lines after 72h incubation. In vitro it enhanced the rate of tubulin polymerization in a dose-dependent manner and induced the bundling of microtubule in BGC-823 cells with the mode similar to that of paclitaxel. As determined by flow cytometry, after either 12 or 24h exposure, Lx2-32c caused BGC-823 cells G2/M phase arrest in a time- and dose-dependent manner. Moreover, we demonstrated that Lx2-32c had significant antitumor activity on BGC-823 (human gastric carcinoma) and A549 (human non-small cell lung carcinoma) xenograft in nude mice. These data suggest that Lx2-32c is a microtubule-stabilizing agent, which has significant antitumor activity in vitro and in vivo.

Epothilones and new analogues of the microtubule modulators in taxane-resistant disease

BACKGROUND

Microtubule-stabilising agents typified by the epothilone class of drug have demonstrated promising activity in Phase II and III clinical trials.

OBJECTIVE

Data supporting the efficacy of these agents are reviewed and their potential use in taxane-refractory disease assessed.

METHODS

Preclinical evidence assessing the role of the spindle assembly checkpoint in determining the cellular response to microtubule stabilization are presented together with clinical data documenting the efficacy of non-taxane microtubule modulators.

RESULTS/CONCLUSIONS

Evidence suggests that microtubule-stabilising agents prolong activation of the spindle assembly checkpoint which may promote cancer cell death in mitosis or following mitotic exit. A weakened spindle assembly checkpoint is associated with altered sensitivity to agents targeting the microtubule and therefore pathways of drug resistance may be shared by these cytotoxic therapies. Preliminary clinical trial data do suggest modest activity of epothilones in truly taxane-resistant patient cohorts, indicating the potential niche for these agents in a molecularly undefined patient group, potentially implicating the role of P-glycoprotein in the acquisition of taxane-resistant disease. Trial data of these antimitotic agents will be presented together with their potential role in taxane-resistant disease and the implications for future clinical trial design.

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.

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.

Treatment of drug-eluting stent restenosis with the same versus different drug-eluting stent

OBJECTIVE

The authors aimed to compare the clinical outcomes with repeat drug-eluting stent (DES) implantation utilizing the same type versus an alternate DES type for in-stent restenosis (ISR) of DES.

BACKGROUND

: DES are proven as an effective treatment for bare metal ISR.

METHODS

A cohort of 116 patients previously treated with a sirolimus-eluting stent (SES) or a paclitaxel-eluting stent (PES) who presented with angiographic ISR were treated with repeat DES. Of these, 62 (53.4%) were treated with different DES and 54 (46.6%) were treated with the same DES. This cohort was followed for clinical events at 30 days, 6 months, and 1 year.

RESULTS

Baseline characteristics were similar except for more diabetes among patients receiving the different type of DES. Of the 116, overall 16.4% of the DES were implanted for previous ISR and 2.6% had previously received brachytherapy. At 6 months, the overall target vessel revascularization (TVR) rate was 12.2% for the entire cohort. The TVR-major adverse cardiac event (MACE) rate for the patients treated with different DES was 14.5% and 16.7% for the same DES (P = 0.750). Overall TVR rate at 1 year was 28.8%. The TVR-MACE was 32.6% for different DES and 35.0% for the same DES (P = 0.814).

CONCLUSIONS

Reimplantation of DES for the treatment of DES ISR (same or different) is safe but associated with a high rate of recurrences at 1 year regardless of the initial DES type. Other treatment modalities for ISR of DES should be considered to further improve the overall TVR-MACE.