- Email: [email protected]
Molecular mechanisms of resistance to taxanes and therapeutic implications
MINI-REVIEWS Molecular mechanisms of resistance to taxanes and therapeutic implications Franco Zunino, Giuliana Cassinelli, Donatella Polizzi, Paola Perego Istituto Nazionale per lo Studio e la Cura dei Tumori, 20133 Milan, Italy
Abstract The mechanism of resistance to taxanes has not been fully elucidated. Since Taxol is a substrate for P-glycoprotein, overexpression of this transport system is recognized as a relevant mechanism of resistance. Additional mechanisms include changes of microtubule structure and/or composition resulting in reduced drug binding to the target. Current efforts are directed at clarifying the role of cellular response to drug-induced damage to cytoskeleton and mitotic spindle. Downstream events, such as control of cell cycle progression and regulation of cell death pathways, are likely to play a relevant role in cellular sensitivity to antimicrotubule agents.The identification of resistance factors and critical determinants of antitumor efficacy of microtubule-stabilizing agents is essential to (i) improve their therapeutic efficacy; and (ii) to design non-cross-resistant compounds.The present review discusses the possible therapeutic implications of the recent progress in the field of resistance to taxanes. © 1999 Harcourt Publishers Ltd
INTRODUCTION
T
axanes have emerged as one of the most effective and promising class of antitumor agents developed over the last two decades.The interest of taxanes is related to their clinical efficacy against several solid tumors (in particular, breast and ovarian carcinoma) and to their peculiar mechanism of action.1 Unlike vinca alkaloids, taxanes preferentially bind to microtubules, rather than to tubulin dimers, at sites distinct from those of traditional antimitotic agents.2,3 In contrast to other tubulin binders (colchicines and vinca alkaloids), taxanes affect microtubule dynamics by promoting assembly of microtubules and inhibiting tubulin disassembly. Microtubules are dynamic polymers involved in a variety of critical cellular functions including mitosis, intracellular transport, and other cytoskeleton-related activities.3 Although the cytotoxic and antitumor effects of antimicrotubule agents are generally ascribed to their effects on the mitotic spindle and to mitotic arrest,4 the contribution of other microtubule-mediated cellular effects cannot be ruled out.5 A major limitation of the clinical use of taxanes is the development of cellular resistance, which is multifactorial in nature. The clinical relevance of the mechanisms of resistance to taxanes from in vitro studies remains to be defined. The clinical success of taxanes in the treatment of solid tumors has stimulated intensive efforts to identify novel analogs or novel antitubulin agents effective against Taxol-resistant tumors and to elucidate the cellular determinants of the sensitivity/resistance
status of tumor cells. Therefore, this review focuses on the available data concerning: (a) the mechanisms of tumor resistance to taxanes, with particular reference to the cellular response resulting in cell death; and (b) the development of non-cross-resistant analogs. CELLULAR BASIS OF RESISTANCE TO TAXANES Several lines of evidence support that the cellular sensitivity and tumor responsiveness to taxanes are the result of (a) expression of defence factors which reduce the intracellular content of active drug (pre-target events); (b) alterations affecting drug-target interaction; and (c) factors influencing cellular response to the cytoskeletal damage and to alteration of mitotic spindle function (Table 1). Defence factors Taxol and Taxotere belong to a large group of natural compounds which are known to be substrates for P-glycoprotein.6 Increased levels of P-glycoprotein are common in some tumor types.This protein, in addition to be overexpressed in multidrug-resistant (MDR) tumor cells, is also expressed in certain normal tissues and may have a role in determining drug absorption and tissue distribution of the drugs recognized by this transport system. The overexpression of P-glycoprotein in the intestinal mucosa strongly limits the oral bioavailability of Taxol.7 The marginal efficacy of Taxol against primary brain tumors is consistent with the drug inability to cross the intact blood-brain barrier, since P-glycoprotein is highly expressed in human brain capillary endothelial cells.8,9 Although the clinical relevance of the MDR phenotype in the development of resistance to Taxol in responsive tumor types remains to be defined, it is now evident that the recognition by the P-glycoprotein influences the clinical pharmacology of Taxol, as suggested by the effect of a P-glycoprotein blocker (cyclosporin A) on the pharmacokinetics of Taxol.10 In contrast to P-glycoprotein, the multidrug resistance-associated protein (MRP), a transport system associated with a P-glycoprotein-independent MDR phenotype,11 plays a marginal (if any) role in resistance to taxanes.12,13 Indeed, expression of MRP may confer only a low level of resistance to Taxol.11 The partial cross-resistance of MRP-overexpressing tumor cell lines to taxanes can, at least in part, be attributed to a concomitant expression of
Table 1 Mechanisms of resistance to taxanes Defence factors
Drug-target interaction
Cellular response
Intracellular drug accumulation Alteration in metabolism and/or subcellular distribution Alterations of tubulin (mutations) Altered β-tubulin isotype expression Changes in microtubule-associated proteins (MAPs, survivin) Signaling pathways (growth factor receptorand/or microtubule-mediated) Regulation of cell cycle (mitotic checkpoint) Control of apoptosis and cell death signals
1999 Harcourt Publishers Ltd Drug Resistance Updates (1999) 2, 351–357 DOI: 10.1054/drup.1999.0108, available online at http://www.idealibrary.com on
351
Zunino et al. other resistance mechanisms, which is a general feature of the MDR phenotype characterized by high level of resistance to the selecting agents.14 The possible involvement of other ‘pre-target’ events (intracellular drug metabolism, subcellular distribution) remains to be established.15 The antagonistic effects of glutathione depletion on Taxol cytotoxicity suggest that the intracellular metabolism affects microtubule dynamics rather than cellular pharmacokinetics.16 Drug-target interaction Resistance to Taxol is associated with multiple alterations of its intracellular target, including modification of tubulin levels, altered electrophoretic mobility of α- or β-tubulin isoforms and acetylation of α-tubulin.3,17,18 The contribution of these alterations to the Taxol-resistant phenotype has not yet been elucidated. Indeed, both reduced and increased tubulin levels have been found in Taxol-resistant cell lines.3,17 Taxol-resistant cell lines have shown a number of alterations involving total tubulin content or tubulin isotype content or expression of microtubule-associated proteins.All these alterations may represent a relevant aspect of variable sensitivity of tumor cells to antimicrotubule agents. Alterations involving β-tubulin might be more critical than others due to the fact that Taxol binds to the β-tubulin subunit of the microtubule. Such alterations fall into two categories: (a) altered composition of β-tubulin isotypes; and (b) β-tubulin mutations. Six different β-tubulin isotypes are known, and several pieces of evidence suggest that their composition may affect cellular response to Taxol. In vitro, microtubule dynamics, measured as sensitivity to bound Taxol, have been shown to be influenced by isotype composition.19 In cell systems, increased expression of specific β-tubulin isotypes has been found to be associated with Taxol resistance of cell lines of different tumor types including small cell lung and ovarian carcinoma.20 A specific relationship between Taxol resistance and this alteration is supported by the possibility of decreasing Taxol resistance by using antisense oligonucleotides (class III β tubulin isotype).20 Mutations of tubulin isotype genes have been documented in Taxol-resistant cell lines.21 The clinical relevance of this alteration appear to be supported by a recent observation indicating that mutations of β-tubulin gene in NSCLC is a predictive parameter of response to Taxol.22 The microtubular network is composed of tubulin dimers and a number of microtubule associated proteins (MAPs), which are likely to play a regulatory role in microtubule dynamic stability.23,24 Therefore, changes in both tubuline isotype content and MAP proteins could influence the sensitivity of drugs whose mechanism of action affects the stability or function of these critical cellular structures.25 Role of growth factor receptors and signal transduction pathways Alterations in the expression or activation of genes involved in the regulation of cell proliferation or apoptosis may affect the sensitivity to cytotoxic agents including taxanes. The effects on cell sensitivity resulting from abnormal expression of regulatory genes may be complex and sometimes contradictory because of opposing influences in different experimental systems.26 The role of altered expression of 352
Drug Resistance Updates (1999) 2, 351–357
1999 Harcourt Publishers Ltd
HER-2/neu and EGF receptors as a determinant of sensitivity/resistance status of tumor cells is still matter of debate.A correlation between Taxol resistance and HER-2/neu expression level has been described in cells expressing different levels of HER-2/neu.27 Consistent with this observation, downregulation of HER-2/neu expression by adenovirus type 5 E1A sensitizes the cells to Taxol.28 A similar effect was found in SKOV-3 ovarian carcinoma cells, which exhibit overexpression of HER-2/neu.29 The inhibition of tyrosine kinase activity in HER-2/neu-overexpressing breast cancer cells produces sensitization to Taxol.30 The co-expression of HER-2/neu and P-glycoprotein results in a high degree of resistance to Taxol.31 Sensitization to Taxol of breast cancer cells overexpressing both HER2/neu and P-glycoprotein may be achieved by either downregulation of cell surface HER-2/neu protein with a specific monoclonal antibody or by using blockers of P-glycoprotein (verapamil).These results support the interaction and cooperation of multiple alterations in determining resistance to taxanes and provide a rational basis for the therapeutic approach of combining anti-HER-2/neu antibody (Herceptin) with Taxol.32 The effects of EGF receptors appear to be different from those mediated by HER-2/neu receptors.26 Expression and activation of EGF receptors seems to enhance the sensitivity to cytotoxic agents including taxanes. Human tumor xenografts overexpressing EGF receptors were found to be hypersensitive to taxanes.33 Recently it has been shown that taxanes activate signaling cascade which involves the protein kinase A (PKA), Ras and MAP kinase as intermediates of drug action.34 A recent study performed on human MCF-10A mammary epithelial cells expressing EGF receptors and their transformed sublines overexpressing c-Ha-ras or HER-2/neu or both genes shows that overexpression of Ha-ras increased sensitivity to taxanes.35 By contrast, cells transformed by HER-2/neu or both neu and ras genes exhibited relative resistance to taxanes. Downregulation of type I PKA with an antisense oligonucleotide, which inhibits HER-2/neu expression, restored sensitivity to taxane. PKA is also involved in the mitogenic signaling following EGF receptor activation and therefore in the MAP kinase pathway.34 MAP kinase has been implicated in the phosphorylation of MAPs, which play a role in microtubular assembly.23 Activation of PKA induced phosphorylation of Bcl-2, thus inhibiting its anti-apoptotic activity.36 The available data suggest a complex interplay between the signaling pathway activated by drug-target interaction and the signal transduction mediated by activation of growth factor receptors.This interpretation is consistent with the antiangiogenetic effects of taxanes. The possible involvement of the signaling cascade in determining the sensitivity to taxanes is also supported by upregulation of caveolin-1 in Taxol resistant cells.37 Caveolin-1 is involved in the regulatory mechanism of PKA.38 Apoptosis Effective antitumor agents cause cell killing by inducing apoptosis.39,40 It is conceivable that alterations in any of the genes that participate in the apoptotic pathway can influence drug sensitivity. DNA damaging agents may be potent initiators of p53-dependent apoptosis.39 Mutations of p53
Resistance mechanisms to taxanes gene can presumably cause a relative resistance to genotoxic agents,41 although the influence of the p53 gene status depends upon the tumor type, the biological context and nature of the cytotoxic lesion. Apoptosis induced by other types of cellular damage (e.g. alterations of mitotic spindle function) is not mediated by p53.42 However, p53 plays a role in control of genome integrity at various cell cycle checkpoints including the G2/M boundary.43,44 Loss or alteration of spindle checkpoint control may have a relevant influence in the cellular outcome following microtubular damage. Several studies have shown an increased sensitivity of cells with mutant p53 to taxanes.45,46 A p53 mutant ovarian carcinoma cell line, IGROV-1/Pt1, selected for resistance to cisplatin, exhibits collateral sensitivity to Taxol.47 The mechanism by which mutation of p53 changes the sensitivity to taxanes is unknown. Changes in the expression in p53 gene targets might affect drug sensitivity. Relevant to this point is the observation that p53 negatively regulates the expression of MAP4, a microtubule-stabilizing protein.24 Thus, overexpression of MAP4 provides a plausible explanation of the increased sensitivity to taxanes of mutant p53 cells, since polymerized microtubules favor drug binding. P53 itself likely plays a direct or indirect role in the mitotic checkpoint.43,44 Molecular regulatory defects at this checkpoint could drive different pathways of cell death.44 In support of this interpretation is the finding that p53 missense mutations of a common class II confers a dominant, gain-of-function phenotype that disrupts the mitotic spindle checkpoint.48 A recent study described a p53-inducible gene coding for a microtubule-binding protein with G2 phase specific expression.49 This protein is induced by wild-type p53 under the same conditions in which the microtubule-associated MAP4 is downregulated, suggesting a functional interplay between these protein at G2/M boundary.Accumulating evidence supports an involvement of p53 status in determining sensitivity to taxanes. Considering the frequency of p53 mutations in human tumors, the ability of taxanes to activate p53-independent apoptosis represents a relevant pharmacological feature as suggested in a clinical study performed in ovarian carcinoma.50 The p53 status can be regarded as one of the important functions influencing cellular sensitivity to a large number of agents; however, its functional status will not generate high degree of resistance. While the microtubule stabilization is recognized as the primary effect of taxanes, the downstream biochemical events that lead to cell death are not well defined.Although the signaling cascade activated by agents that disrupt mitotic apparatus or damage cytoskeleton may be distinct from the cellular response to DNA damage, downstream regulation of the apoptotic pathway appears to be related to that involved in response to genotoxic stress.The bcl-2 gene family plays a central role in the regulation of apoptosis subsequent to microtubule stabilization.35,51,52 Overexpression of the antiapoptotic proteins, Bcl-2 and Bcl-xL, induced resistance to Taxol in human HL-60 leukemia cells.12 The apoptotic pathway activated by taxanes is associated with phosphorylation of Bcl-2, which is an important regulatory event.51,52 Indeed, hyperphosphorylated Bcl-2 does not bind the pro-apoptotic Bax protein, resulting in increased level of free Bax.53
Conversely, overexpression of the pro-apoptotic proteins Bax and Bad sensitizes human ovarian carcinoma cells to Taxol.54 A number of protein kinases, including Raf-1,51 PKA35 and other kinases involved in the regulation of cellular proliferation and cell cycle progression,55 may be directly or indirectly responsible for Bcl-2 phosphorylation.Again, these observations provide possible explanations for the complex relationships between cellular sensitivity to taxanes and tumor-specific alterations involving signal transduction pathways. In addition to Bcl-2, also Bcl-XL is phosphorylated by antimicrotubule agents.56 However, the precise role of levels of Bcl-2 protein and its phosphorylation as determinant of resistance/sensitivity status is still controversal. For example, a prostate carcinoma cell line lacking Bcl-2 expression was resistant to taxane-induced apoptosis57 and a number of human tumor xenografts overexpressing Bcl-2 exhibit hypersensitivity to taxanes.33 Since Bcl-2 phosphorylation may also be induced by effective DNA damaging agents,58 the possibility exists that Bcl-2 phosphorylation is a common event associated with different cell death pathways that are probably related to the onset of apoptosis. Recently, a new protein associated with microtubules of the mitotic spindle has been reported.59 This protein, survivin, has been implicated in the control of mitotic spindle checkpoint and apoptosis.The survivin gene is expressed in non-small cell lung carcinoma;60 however, its role in determining resistance of this tumor type remain unknown. Although cytotoxic effects of taxanes likely reflect induction of apoptosis, the difficulty to establish a precise correlation between apoptosis-related factors and sensitivity/resistance status presumably reflects not only the complexity of regulation of apoptosis but also the activation of a cellular response involving different cell death pathways. For example, in an human osteosarcoma cell line lacking p53 function apoptosis is induced through the CD95/CD95L system.61 Again, this pathway involves Bcl-2 phosphorylation. STRUCTURE-ACTIVITY RELATIONSHIPS AND DEVELOPMENT OF NOVEL TAXANES Identification of novel taxanes endowed with improved efficacy is expected on the basis of structure-activity relationship studies performed to define molecular features relevant for drug-target interaction and for ability to overcome P-glycoprotein-mediated multidrug resistance.These studies have documented that the interaction of taxanes with tubulin is affected by conformation of the side-chain at the 13-position of the taxane ring (Fig. 1) and by the presence of proper hydrophobic and hydrophilic substituents in both side chain and taxane ring.62–66 In particular, the optimal drug activity is related to the presence of hydroxyl group at C-2′ and to the 2′R and 3′S configuration. However, the presence of the 3′phenyl group and the benzoic acid at 3′-amino group are not essential for drug activity and they can be successfully replaced by alkyl substituents.63,64 Replacement of the aromatic substituents at the 3′- and 3′-N-positions with alkyl groups or alkyl esters, respectively, resulted in superior activity compared to that of Taxol. More importantly, in the 3′-substituted derivatives, protection of the hydroxyl groups at C1, 1999 Harcourt Publishers Ltd Drug Resistance Updates (1999) 2, 351–357
353
Zunino et al.
Fig. 1 Structure of Taxol (paclitaxel).
C10 and C14 (as in the case derivatives of 14β-hydroxy-10deacetylbaccatin III) with appropriate substituents enables the drug to overcome typical multidrug resistance.64,65 These observations provide indirect evidence that different and specific interactions must be involved in determining drug affinity for the intracellular target and in drug recognition by the P-glycoprotein. This interpretation is also supported by the finding that epothilones, a novel class of cytotoxic macrolides with a Taxol-like mechanism (i.e. inhibitors of microtubule depolymerization), exhibit efficacy against MDR tumors.67 Other novel structurally unrelated compounds with Taxol-like activity but having the ability to overcome MDR-mediated resistance (elentherobin, discodermolide) have been described.68,69 Based on these observations, a common pharmacophore for compounds that stabilize microtubule has been identified.70 Conversely, natural taxoids (taxuspines) exhibit reduced cytotoxic activity but ability to modulate MDR resistance. 71 Taken together, all these observations support the possibility to discriminate essential requirements for effective drug-target interaction in future drug design. This possibility is supported by evidence that a novel taxane analog is able to completely overcome P-glycoprotein-mediated resistance and is more effective than Taxol.33,64 CONCLUSIONS AND FUTURE DIRECTIONS The cellular resistance to taxanes is a very complex phenomenon involving (a) defence factors, such as intracellular drug accumulation or distribution, (pre-target events) or (b) alterations of the microtubule structure and/or composition, resulting in changes of microtubule dynamics or drug-target interactions (Table 1).The clinical relevance of the resistance 354
Drug Resistance Updates (1999) 2, 351–357
1999 Harcourt Publishers Ltd
factors identified in cell systems remains to be defined. Several lines of evidence support the hypothesis that the antitumor efficacy of taxanes may be due not only to the absence of resistance mechanisms, but also to the expression of sensitivity determinants, such as cell ability to activate an efficient apoptotic pathway.72 Co-expression and/or cooperation between multiple factors may determine cell sensitivity status.73 Understanding the signal transduction pathway and possible cross-talk among different pathways (growth factor receptor-mediated or microtubule-mediated) may provide a rational basis for exploiting the therapeutic potential of taxanes. Relevant to this point is the observation that Taxol may induce CD95-mediated apoptosis.61,74 Since this pathway is potentiated by cytokines, such as TNFα and IFN-γ,75 studies of combination of cytokines with Taxol could be designed in tumors expressing the CD95 receptor. A phase I study of such combination has been performed in prostate cancer patients.76 The definition of the molecular/biological context that favors sensitivity to taxanes is expected to improve therapeutic strategies.The p53-regulated changes of expression of MAP4 provide an explanation for opposite influence of p53 gene status on sensitivity to taxanes and vinca alkaloids and may have implications in the rational design of novel combinations.25,77 The recognition of the antiangiogenetic properties of Taxol and other antimicrotubule agents may provide a basis for additional clinical applications.78 A promising approach in the improvement of microtubule-targeted therapies is the development of non-crossresistant analogs33,79 or novel antitubulin agents.3,67,68,80 Many of these new compounds exhibit activity against tumor cells expressing the MDR phenotype.An important finding for the future design of more effective antimicrotubule agents is the
Resistance mechanisms to taxanes identification of a common pharmacophore for specific drug-target interaction.70 A major drawback of the therapeutic use of taxanes is not only the development of drug resistance, but drug toxicity. Thus, identification of novel agents with a better tolerability and improved therapeutic index is essential to exploit therapeutic potential of antimicrotubule agents. A novel taxane with an improved pharmacological profile in terms of both toxicity and activity against Taxolresistant tumors has been selected as a promising candidate for clinical evaluation.33 In spite of an increased cytotoxic potency and efficacy against tumors with the MDR phenotype, the analog is characterized by a better tolerability than Taxol at effective doses, thus suggesting an increased antitumor selectivity. Acknowledgements
This work was partially supported by the Associazione Italiana per la Ricerca sul Cancro, Milan, and by the Ministero della Sanita’, Rome, Italy
Received 17 November 1999; Accepted 8 December 1999 Correspondence to: Franco Zunino, Istituto Nazionale Tumori,Via Venezian 1, 20133 Milan, Italy.Tel: +39 02 2390267; Fax: +39 02 2390692; E.mail [email protected]
REFERENCES 1. Rowinski EK, Donehower RC. Paclitaxel (Taxol). In:Wood AJJ, ed., Drug therapy. N Engl J Med 1995: 1004–1012. 2. Horwitz SB, Cohen D, Rao S, Ringel I, Shen H-J,Yang C-PH.Taxol: mechanisms of action and resistance. Monogr Natl Cancer Inst 1993; 15: 55–61. 3. Dumontet C, Sikic BI. Mechanisms of action of and resistance to antitubulin agents: microtubule dynamics, drug transport, and cell death. J Clin Oncol 1999; 17: 1061–1070. 4. Jordan MA,Wendell K, Gardiner S, Derry WB, Copp H,Wilson L. Mitotic block induced in HeLa cells by low concentrations of paclitaxel (Taxol) results in abnormal mitotic exit and apoptotic cell death. Cancer Res 1996; 56: 816–825. 5. Fan W. Possible mechanisms of paclitaxel-induced apoptosis. Biochem Pharmacol 1999; 57: 1215–1221. 6. Pastan I, Gottesman MM. Multidrug resistance.Annu Rev Med 1991; 42: 277–286. 7. Sparreboom A,Van Asperen J, Mayer U et al. Limited oral bioavailability and active epithelial excretion of paclitaxel (Taxol) caused by P-glycoprotein in the intestine. Proc Natl Acad Sci USA 1997; 94: 2031–2035. 8. Balmaceda C.Advances in brain tumor chemosensitivity. Curr Opin Oncol 1998; 10: 194–200. 9. Hosli P, Sappino AP, de Tribolet N, Dietrich PY. Malignant glioma: should chemotherapy be overthrown by experimental treatments? Ann Oncol 1998; 9: 589–600. 10. Van Asperen J,Van Tellingen O,Van der Valk MA, Rozenhart M, Beijnen JH. Enhanced oral absorption and decreased elimination of paclitaxel in mice cotreated with cyclosporin A. Clin Cancer Res 1998; 4: 2293–2297.
11. Cole SPC, Sparks KE, Fraser K et al. Pharmacological characterization of multidrug resistant MRP-transfected human tumor cells. Cancer Res 1994; 54: 5902–5910. 12. Huang Y, Ibrado AM, Reed JC et al. Co-expression of several molecular mechanisms of multidrug resistance and their significance for paclitaxel cytotoxicity in human AML HL-60 cells. Leukemia 1997; 11: 253–257. 13. Binaschi M, Supino R, Gambetta RA et al. MRP gene overexpression in a human doxorubicin-resistant SCLC cell line: alterations in cellular pharmacokinetics and in pattern of crossresistance. Int J Cancer 1995; 62: 84–89. 14. Vanhoefer U, Cao S, Harstrick A, Seeber S, Rustum YM. Comparative antitumor efficacy of docetaxel and paclitaxel in nude mice bearing human tumor xenografts that overexpress the multidrug resistance protein (MRP).Ann Oncol 1997; 8: 1221–1228. 15. Royer I, Monsarrat B, Sonnier M,Wright M, Cresteil T. Metabolism of docetaxel by human cytochromes P450: interactions with paclitaxel and other antineoplastic drugs. Cancer Res 1996; 56: 58–65. 16. Liebmann JE, Hahn SM, Cook JA, Lipschultz C, Mitchell JB, Kaufman DC. Glutathione depletion by L-buthionine sulfoximine antagonizes Taxol cytotoxicity. Cancer Res 1993; 53: 2066–2070. 17. Nishio K, Saijo N. Cytoskeletons and antimitotic agents developed in Japan.Anti-Cancer Drug Design 1999; 14: 133–141. 18. Ohta S, Nishio K, Kubota N et al. Characterization of a Taxolresistant human small-cell lung cancer cell line. Jpn J Cancer Res 1994; 85: 290–297. 19. Derry WB,Wilson L, Khan IA, Luduena RF, Jordan MA.Taxol differentially modulates the dynamics of microtubules assembled from unfractionated and purified beta-tybulin isotypes. Biochemistry 1997; 36: 3554–3562. 20. Kavallaris M, Burkhart CA, Horwitz SB.Antisense oligonucleotides to class III beta-tubulin sensitize drug-resistant cells to Taxol. Br J Cancer 1999; 80: 1020–1025. 21. Giannakakou P, Sackett DL, Kang YK et al. Paclitaxel-resistant human ovarian cancer cells have mutant beta-tubulins that exhibit impaired paclitaxel-driven polymerization. J Biol Chem 1997; 272: 17118–17125. 22. Monzó M, Rosell R, Sanchez JJ et al. Paclitaxel resistance in nonsmall-cell lung cancer associated with beta-tubulin gene mutations. J Clin Oncol 1999; 17: 1786–1793. 23. Nishio K,Arioka H, Ishida T et al. Enhanced interaction between tubulin and microtubule-associated protein 2 via inhibition of MAP kinase and CDC2 kinase by paclitaxel. Int J Cancer 1995; 63: 688–693. 24. Zhang CC,Yang J-M,White E, Murphy M, Levine A, Hait WN.The role of MAP4 expression in the sensitivity to paclitaxel and resistance to vinca alkaloids in p53 mutant cells. Oncogene 1998; 16: 1617–1624. 25. Jordan MA,Wilson L. Microtubules and actin filaments: dynamic targets for cancer chemotherapy. Curr Opin Cell Biol 1998; 10: 123–130. 26. Mendelsohn J, Fan Z. Epidermal growth factor receptor family and chemosensitization. J Natl Cancer Inst 1997; 89: 341–343. 27. Yu D, Liu B,Tan M, Li J,Wang S-S, Hung M-C. Overexpression of c-erbB-2/neu in breast cancer cells confers increased resistance to Taxol via mdr-1-independent mechanisms. Oncogene 1996; 13: 1359–1365.
1999 Harcourt Publishers Ltd Drug Resistance Updates (1999) 2, 351–357
355
Zunino et al. 28. Ueno NT,Yu D, Hung MC. Chemosensitization of HER-2/neuoverexpressing human breast cancer cells to paclitaxel (Taxol) by adenovirus type 5 E1A. Oncogene 1997; 15: 953–960. 29. Brader KR,Wolf JK, Hung MC et al.Adenovirus E1A expression enhances the sensitivity of an ovarian cancer cell line to multiple cytotoxic agents through an apoptotic mechanism. Clin Cancer Res 1997; 3: 2017–2024. 30. Zhang L, Lau YK, Xia W, Hortobagyi GN, Hung MC.Tyrosine kinase inhibitor emodin suppresses growth of HER-2/neu-overexpressing breast cancer cells in athymic mice and sensitizes these cells to the inhibitory effect of paclitaxel. Clin Cancer Res 1999; 5: 343–353. 31. Yu D, Liu B, Jing T et al. Overexpression of both p185c-erbB2 and p170mdr-1 renders breast cancer cells highly resistant to Taxol. Oncogene 1998; 16: 2087–2094. 32. Baselga J, Norton L,Albanell J et al. Recombinant humanized anti HER2 antibody (Herceptin) enhances the antitumor activity of paclitaxel and doxorubicin against HER2/neu overexpressing human breast cancer xenografts. Cancer Res 1998; 58: 2825–2831. 33. Polizzi D, Pratesi G,Tortoreto M, Supino R, Riva A, Bombardelli E, Zunino F.A novel taxane with improved tolerability and therapeutic activity in a panel of human tumor xenografts. Cancer Res 1999; 59: 1036–1040. 34. Ciardiello F,Tortora G. Interactions between the epidermal growth factor receptor and type I protein kinase A: biological significance and therapeutic implications. Clin Cancer Res 1998; 4: 821–828. 35. Ciardiello F, Caputo R, Pomatico G et al. Resistance to taxanes is induced by c-erbB-2 overexpression in human MCF-10A mammary epithelial cells and is blocked by combined treatment with an antisense oligonucleotide targeting type I protein kinase A. Int J Cancer 1999; in press. 36. Srivastava RK, Srivastava AR, Korsmeyer SJ, Nesterova M, ChoChung YS, Longo DL. Inolvement of microtubules in the regulation of Bcl2 phosphorylation and apoptosis through cyclic AMPdependent protein kinase. Mol Cell Biol 1998; 18: 3509–3517. 37. Yang C-PH, Galbiati F,Volonté D, Horwitz SB, Lisanti MP. Upregulation of caveolin-1 and caveolae organelles in Taxolresistant A549 cells. FEBS Letters 1998; 439: 368–372. 38. Razani B, Rubin CS, Lisanti MP. Regulation of cAMP-mediated signal transduction via interaction of caveolins with the catalytic subunit of protein kinase A. J Biol Chem 1999; 274: 26353–26360. 39. Zunino F, Perego P, Pilotti S, Pratesi G, Supino R,Arcamone F. Role of apoptotic response in cellular resistance to cytotoxic agents. Pharmacol Ther 1997; 76: 177–185. 40. Wu GS, El-Deiry WS.Apoptotic death of tumor cells correlates with chemosensitivity, independent of p53 or Bcl-2. Clin Cancer Res 1996; 2: 623–633. 41. Perego P, Giarola M, Righetti SC et al.Association between cisplatin resistance and mutation of p53 gene and reduced bax expression in ovarian carcinoma cell systems. Cancer Res 1996; 56: 556–562. 42. Vasey PA, Jones NA, Jenkins S, Dive C, Brown R. Cisplatin, camptothecin, and Taxol sensitivities of cells with p53-associated multidrug resistance. Mol Pharmacol 1996; 50: 1536–1540. 43. Rudner AD, Murray AW.The spindle assembly checkpoint. Curr Opin Cell Biol 1996; 8: 773–780. 44. Sorger PK, Dobles M,Tournebize R, Hyman AA. Coupling cell division and cell death to microtubule dynamics. Curr Opin Cell Biol 1997; 9: 807–814. 45. Wahl AF, Dinaldson KL, Fairchild C et al. Loss of normal p53 function confers sensitization to Taxol by increasing G2/M arrest and apoptosis. Nature Med 1996; 2: 72–79.
356
Drug Resistance Updates (1999) 2, 351–357
1999 Harcourt Publishers Ltd
46. Vikhanskaya F,Vignati S, Beccaglia P et al. Inactivation of p53 in a human ovarian cancer cell line increases the sensitivity to paclitaxel by inducing G2/M arrest and apoptosis. Exp Cell Res 1998; 241: 96–101. 47. Perego P, Romanelli S, Carenini N et al. Ovarian cancer cisplatinresistant cell lines: multiple changes including collateral sensitivity to Taxol.Ann Oncol 1998; 9: 423–430. 48. Gualberto A,Aldape K, Kozakiewicz K,Tlsty TD.An oncogenic form of p53 confers a dominant, gain-of-function phenotype that disrupts spindle checkpoint control. Proc Natl Acad Sci USA 1998; 95: 5166–5171. 49. Utrera R, Collavin L, Lazarevic D, Delia D, Schneider C.A novel p53-inducible gene coding for a microtubule-localized protein with G2-phase-specific expression. EMBO J 1998; 17: 5015–5025. 50. Lavarino C, Delia D, Di Palma S, Zunino F, Pilotti S. P53 in drug resistance in ovarian cancer. Lancet 1997; 349: 1556. 51. Blagosklonny MV, Schulte T, Nguyen P,Trepel J, Neckers LM.Taxolinduced apoptosis and phosphorylation of Bcl-2 protein involves c-Raf-1 and represents a novel c-Raf-1 signal transduction pathway. Cancer Res 1996; 56: 1851–1854. 52. Wang LG, Liu XM, Kreis W, Budman DR.The effect of antimicrotubule agents on signal transduction pathways of apoptosis: a review. Cancer Chemother Pharmacol 1999; 44: 355–361. 53. Korsmeyer SJ. BCL-2 gene family and the regulation of programmed cell death. Cancer Res 1999; 59: 1693–1700. 54. Strobel T,Tai Y-T, Korsmeyer S, Cannistra SA. BAD partly reverses paclitaxel resistance in human ovarian cancer cells. Oncogene 1998; 17: 2419–2427. 55. Srivastava KK, Mi Q-S, Hardwick JM, Longo DL. Deletion of the loop region of Bcl-2 completely blocks paclitaxel-induced apoptosis. Proc. Natl.Acad. Sci USA 1999; 96: 3775–3780. 56. Poruchynsky MS,Wang EE, Rudin CM, Blagosklonny MV, Fojo T. Bcl-xL is phosphorylated in malignant cells following microtubule disruption. Cancer Res. 1998; 58: 3331–3338. 57. Haldar S, Chintapalli J, Croce CM.Taxol induces Bcl-2 phosphorylation and death of prostate cancer cells. Cancer Res 1996; 56: 1253–1255. 58. De Cesare M, Pratesi G, Giusti A, Polizzi D, Zunino F. Stimulation of the apoptotic response as a basis for the therapeutic synergism of lonidamine and cisplatin in combination in human tumour xenografts. Br J Cancer 1998; 77: 434–439. 59. Li F,Ambrosini G, Chu EY et al. Control of apoptosis and mitotic spindle checkpoint by survivin. Nature 1998; 396: 580–584. 60. Monzó M, Rosell R, Felip E et al.A novel anti-apoptosis gene: reexpression of survivin messenger RNA as a prognosis marker in non-small-cell lung cancers. J Clin Oncol 1999; 17: 2100–2104. 61. Pucci B, Bellincampi L,Tafani M, Masciullo V, Melino G, Giordano A. Paclitaxel induces apoptosis in saos-2 cells with CD95L upregulation and Bcl-2 phosphorylation. Exp Cell Res 1999; 252: 134–143. 62. Guenard D, Gueritte-Voegelein F, Dubois J, Potier P. Structureactivity relationships of Taxol and taxotere analogues. Monogr Natl Cancer Inst 1993; 15: 79–82. 63. Ojima I, Slater JC, Michaud E et al. Syntheses and structure-activity relationships of the second-generation antitumor taxoids: exceptional activity against drug-resistant cancer cells. J Med Chem 1996; 39: 3889–3896. 64. Distefano M, Scambia G, Ferlini C et al.Anti-proliferative activity of a new class of taxanes (14β-hydroxy-10-deacetylbaccatin III
Resistance mechanisms to taxanes
65.
66.
67.
68.
69.
70.
71.
72.
derivatives) on multidrug-resistance-positive human cancer cells. Int J Cancer 1997; 72: 844–850. Ojima I, Slater JC, Kuduk SD et al. Syntheses and structure-activity relationships of taxoids derived from 14β-hydroxy-10deacetylbaccatin III. J. Med. Chem. 1997; 40: 267–278. Ojima I, Kuduk SD, Pera P,Veith JM, Bernacki RJ. Synthesis and structure-activity relationships of nonaromatic taxoids: effects of alkyl and alkenyl ester groups on cytotoxicity. J Med Chem 1997; 40: 279–285. Chou T-C, Zhang X-G, Harris CR et al. Desoxyepothilone B is curative against human tumor xenografts that are refractory to paclitaxel. Proc Natl Acad Sci USA 1998; 95: 15798–15802. Kowalski RJ, Giannakakou P, Gunasekera SP, Longley RE, Day BW, Hamel E.The microtubule-stabilizing agent discodermolide competitively inhibits the binding of paclitaxel (Taxol) to tubulin polymers, enhances tubulin nucleation reactions more potently than paclitaxel, and inhibits the growth of paclitaxel-resistant cells. Mol Pharmacol 1997; 52: 613–622. McDaid H, Bhattacharya SK, Chen K-T et al. Structure-activity profiles of eleutherobin analogs and their cross-resistance in Taxolresistant cell lines. Cancer Chemother Pharmacol 1999; 44: 131–137. Ojima I, Chakravarty S, Inoue T et al.A common pharmacophore for cytotoxic natural products that stabilize microtubules. Proc Natl Acad Sci USA 1999; 96: 4256–4261. Kobayashi J, Hosoyama H,Wang X-x, Shigemori H, Sudo Y,Tsuruo T. Modulation of multidrug resistance by taxuspine C and other taxoids from japanese yew. Bioorg Med Chem Lett 1998; 8: 1555–1558. Milross CG, Mason KA, Hunter NR, Chung WK, Peters LJ, Milas L. Relationship of mitotic arrest and apoptosis to antitumor effect of paclitaxel. J Natl Cancer Inst 1996; 88: 1308–1314.
73. Strobel T, Kraeft S-K, Chen LB, Cannistra SA. BAX expression is associated with enhanced intracellular accumulation of paclitaxel: a novel role for BAX during chemotherapy-induced cell death. Cancer Res 1998; 58: 4776–4781. 74. Lanni JS, Lowe SW, Licitra EJ, Liu JO, Jacks T. p53-independent apoptosis induced by paclitaxel through an indirect mechanism. Proc Natl Acad Sci USA 1997; 94: 9679–9683. 75. Weller M, Frei K, Groscurth P, Krammer PH,Yonekawa Y, Fontana A.Anti-Fas/APO-1 antibody-mediated apoptosis of cultures human glioma cells. Induction and modulation of sensitivity by cytokines. J Clin Invest 1994; 94: 954–964. 76. DiPaola RS, Rafi MM,Vyas V et al. Phase I clinical and pharmacologic study of 13-cis-retinoic acid, interferon alfa, and paclitaxel in patients with prostate cancer and other advanced malignancies. J Clin Oncol 1999; 17: 2213–2218. 77. Zhang CC,Yang JM, Bash-Babula J et al. DNA damage increases sensitivity to vinca alkaloids and decreases sensitivity to taxanes through p53-dependent repression of microtubule-associated protein 4. Cancer Res 1999; 59: 3663–3670. 78. Klauber N, Parangi S, Flynn E, Hamel E, D’Amato RJ. Inhibition of angiogenesis and breast cancer in mice by the microtubule inhibitors 2-methoxyestradiol and Taxol. Cancer Res 1997; 57: 81–86. 79. De Vincenzo R, Scambia G, Ferlini C et al.Antiproliferative activity of colchicine analogues on MDR-positive and MDR-negative human cancer cell lines.Anti-Cancer Drug Design 1998; 13: 19–33. 80. Shan B, Medina JC, Santha E et al. Selective, covalent modification of β-tubulin residue Cys-239 by T138067, an antitumor agent with in vivo efficacy against multidrug-resistant tumors. Proc Natl Acad Sci USA 1999; 96: 5686–5691.
1999 Harcourt Publishers Ltd Drug Resistance Updates (1999) 2, 351–357
357
Abstract The mechanism of resistance to taxanes has not been fully elucidated. Since Taxol is a substrate for P-glycoprotein, overexpression of this transport system is recognized as a relevant mechanism of resistance. Additional mechanisms include changes of microtubule structure and/or composition resulting in reduced drug binding to the target. Current efforts are directed at clarifying the role of cellular response to drug-induced damage to cytoskeleton and mitotic spindle. Downstream events, such as control of cell cycle progression and regulation of cell death pathways, are likely to play a relevant role in cellular sensitivity to antimicrotubule agents.The identification of resistance factors and critical determinants of antitumor efficacy of microtubule-stabilizing agents is essential to (i) improve their therapeutic efficacy; and (ii) to design non-cross-resistant compounds.The present review discusses the possible therapeutic implications of the recent progress in the field of resistance to taxanes. © 1999 Harcourt Publishers Ltd
INTRODUCTION
T
axanes have emerged as one of the most effective and promising class of antitumor agents developed over the last two decades.The interest of taxanes is related to their clinical efficacy against several solid tumors (in particular, breast and ovarian carcinoma) and to their peculiar mechanism of action.1 Unlike vinca alkaloids, taxanes preferentially bind to microtubules, rather than to tubulin dimers, at sites distinct from those of traditional antimitotic agents.2,3 In contrast to other tubulin binders (colchicines and vinca alkaloids), taxanes affect microtubule dynamics by promoting assembly of microtubules and inhibiting tubulin disassembly. Microtubules are dynamic polymers involved in a variety of critical cellular functions including mitosis, intracellular transport, and other cytoskeleton-related activities.3 Although the cytotoxic and antitumor effects of antimicrotubule agents are generally ascribed to their effects on the mitotic spindle and to mitotic arrest,4 the contribution of other microtubule-mediated cellular effects cannot be ruled out.5 A major limitation of the clinical use of taxanes is the development of cellular resistance, which is multifactorial in nature. The clinical relevance of the mechanisms of resistance to taxanes from in vitro studies remains to be defined. The clinical success of taxanes in the treatment of solid tumors has stimulated intensive efforts to identify novel analogs or novel antitubulin agents effective against Taxol-resistant tumors and to elucidate the cellular determinants of the sensitivity/resistance
status of tumor cells. Therefore, this review focuses on the available data concerning: (a) the mechanisms of tumor resistance to taxanes, with particular reference to the cellular response resulting in cell death; and (b) the development of non-cross-resistant analogs. CELLULAR BASIS OF RESISTANCE TO TAXANES Several lines of evidence support that the cellular sensitivity and tumor responsiveness to taxanes are the result of (a) expression of defence factors which reduce the intracellular content of active drug (pre-target events); (b) alterations affecting drug-target interaction; and (c) factors influencing cellular response to the cytoskeletal damage and to alteration of mitotic spindle function (Table 1). Defence factors Taxol and Taxotere belong to a large group of natural compounds which are known to be substrates for P-glycoprotein.6 Increased levels of P-glycoprotein are common in some tumor types.This protein, in addition to be overexpressed in multidrug-resistant (MDR) tumor cells, is also expressed in certain normal tissues and may have a role in determining drug absorption and tissue distribution of the drugs recognized by this transport system. The overexpression of P-glycoprotein in the intestinal mucosa strongly limits the oral bioavailability of Taxol.7 The marginal efficacy of Taxol against primary brain tumors is consistent with the drug inability to cross the intact blood-brain barrier, since P-glycoprotein is highly expressed in human brain capillary endothelial cells.8,9 Although the clinical relevance of the MDR phenotype in the development of resistance to Taxol in responsive tumor types remains to be defined, it is now evident that the recognition by the P-glycoprotein influences the clinical pharmacology of Taxol, as suggested by the effect of a P-glycoprotein blocker (cyclosporin A) on the pharmacokinetics of Taxol.10 In contrast to P-glycoprotein, the multidrug resistance-associated protein (MRP), a transport system associated with a P-glycoprotein-independent MDR phenotype,11 plays a marginal (if any) role in resistance to taxanes.12,13 Indeed, expression of MRP may confer only a low level of resistance to Taxol.11 The partial cross-resistance of MRP-overexpressing tumor cell lines to taxanes can, at least in part, be attributed to a concomitant expression of
Table 1 Mechanisms of resistance to taxanes Defence factors
Drug-target interaction
Cellular response
Intracellular drug accumulation Alteration in metabolism and/or subcellular distribution Alterations of tubulin (mutations) Altered β-tubulin isotype expression Changes in microtubule-associated proteins (MAPs, survivin) Signaling pathways (growth factor receptorand/or microtubule-mediated) Regulation of cell cycle (mitotic checkpoint) Control of apoptosis and cell death signals
1999 Harcourt Publishers Ltd Drug Resistance Updates (1999) 2, 351–357 DOI: 10.1054/drup.1999.0108, available online at http://www.idealibrary.com on
351
Zunino et al. other resistance mechanisms, which is a general feature of the MDR phenotype characterized by high level of resistance to the selecting agents.14 The possible involvement of other ‘pre-target’ events (intracellular drug metabolism, subcellular distribution) remains to be established.15 The antagonistic effects of glutathione depletion on Taxol cytotoxicity suggest that the intracellular metabolism affects microtubule dynamics rather than cellular pharmacokinetics.16 Drug-target interaction Resistance to Taxol is associated with multiple alterations of its intracellular target, including modification of tubulin levels, altered electrophoretic mobility of α- or β-tubulin isoforms and acetylation of α-tubulin.3,17,18 The contribution of these alterations to the Taxol-resistant phenotype has not yet been elucidated. Indeed, both reduced and increased tubulin levels have been found in Taxol-resistant cell lines.3,17 Taxol-resistant cell lines have shown a number of alterations involving total tubulin content or tubulin isotype content or expression of microtubule-associated proteins.All these alterations may represent a relevant aspect of variable sensitivity of tumor cells to antimicrotubule agents. Alterations involving β-tubulin might be more critical than others due to the fact that Taxol binds to the β-tubulin subunit of the microtubule. Such alterations fall into two categories: (a) altered composition of β-tubulin isotypes; and (b) β-tubulin mutations. Six different β-tubulin isotypes are known, and several pieces of evidence suggest that their composition may affect cellular response to Taxol. In vitro, microtubule dynamics, measured as sensitivity to bound Taxol, have been shown to be influenced by isotype composition.19 In cell systems, increased expression of specific β-tubulin isotypes has been found to be associated with Taxol resistance of cell lines of different tumor types including small cell lung and ovarian carcinoma.20 A specific relationship between Taxol resistance and this alteration is supported by the possibility of decreasing Taxol resistance by using antisense oligonucleotides (class III β tubulin isotype).20 Mutations of tubulin isotype genes have been documented in Taxol-resistant cell lines.21 The clinical relevance of this alteration appear to be supported by a recent observation indicating that mutations of β-tubulin gene in NSCLC is a predictive parameter of response to Taxol.22 The microtubular network is composed of tubulin dimers and a number of microtubule associated proteins (MAPs), which are likely to play a regulatory role in microtubule dynamic stability.23,24 Therefore, changes in both tubuline isotype content and MAP proteins could influence the sensitivity of drugs whose mechanism of action affects the stability or function of these critical cellular structures.25 Role of growth factor receptors and signal transduction pathways Alterations in the expression or activation of genes involved in the regulation of cell proliferation or apoptosis may affect the sensitivity to cytotoxic agents including taxanes. The effects on cell sensitivity resulting from abnormal expression of regulatory genes may be complex and sometimes contradictory because of opposing influences in different experimental systems.26 The role of altered expression of 352
Drug Resistance Updates (1999) 2, 351–357
1999 Harcourt Publishers Ltd
HER-2/neu and EGF receptors as a determinant of sensitivity/resistance status of tumor cells is still matter of debate.A correlation between Taxol resistance and HER-2/neu expression level has been described in cells expressing different levels of HER-2/neu.27 Consistent with this observation, downregulation of HER-2/neu expression by adenovirus type 5 E1A sensitizes the cells to Taxol.28 A similar effect was found in SKOV-3 ovarian carcinoma cells, which exhibit overexpression of HER-2/neu.29 The inhibition of tyrosine kinase activity in HER-2/neu-overexpressing breast cancer cells produces sensitization to Taxol.30 The co-expression of HER-2/neu and P-glycoprotein results in a high degree of resistance to Taxol.31 Sensitization to Taxol of breast cancer cells overexpressing both HER2/neu and P-glycoprotein may be achieved by either downregulation of cell surface HER-2/neu protein with a specific monoclonal antibody or by using blockers of P-glycoprotein (verapamil).These results support the interaction and cooperation of multiple alterations in determining resistance to taxanes and provide a rational basis for the therapeutic approach of combining anti-HER-2/neu antibody (Herceptin) with Taxol.32 The effects of EGF receptors appear to be different from those mediated by HER-2/neu receptors.26 Expression and activation of EGF receptors seems to enhance the sensitivity to cytotoxic agents including taxanes. Human tumor xenografts overexpressing EGF receptors were found to be hypersensitive to taxanes.33 Recently it has been shown that taxanes activate signaling cascade which involves the protein kinase A (PKA), Ras and MAP kinase as intermediates of drug action.34 A recent study performed on human MCF-10A mammary epithelial cells expressing EGF receptors and their transformed sublines overexpressing c-Ha-ras or HER-2/neu or both genes shows that overexpression of Ha-ras increased sensitivity to taxanes.35 By contrast, cells transformed by HER-2/neu or both neu and ras genes exhibited relative resistance to taxanes. Downregulation of type I PKA with an antisense oligonucleotide, which inhibits HER-2/neu expression, restored sensitivity to taxane. PKA is also involved in the mitogenic signaling following EGF receptor activation and therefore in the MAP kinase pathway.34 MAP kinase has been implicated in the phosphorylation of MAPs, which play a role in microtubular assembly.23 Activation of PKA induced phosphorylation of Bcl-2, thus inhibiting its anti-apoptotic activity.36 The available data suggest a complex interplay between the signaling pathway activated by drug-target interaction and the signal transduction mediated by activation of growth factor receptors.This interpretation is consistent with the antiangiogenetic effects of taxanes. The possible involvement of the signaling cascade in determining the sensitivity to taxanes is also supported by upregulation of caveolin-1 in Taxol resistant cells.37 Caveolin-1 is involved in the regulatory mechanism of PKA.38 Apoptosis Effective antitumor agents cause cell killing by inducing apoptosis.39,40 It is conceivable that alterations in any of the genes that participate in the apoptotic pathway can influence drug sensitivity. DNA damaging agents may be potent initiators of p53-dependent apoptosis.39 Mutations of p53
Resistance mechanisms to taxanes gene can presumably cause a relative resistance to genotoxic agents,41 although the influence of the p53 gene status depends upon the tumor type, the biological context and nature of the cytotoxic lesion. Apoptosis induced by other types of cellular damage (e.g. alterations of mitotic spindle function) is not mediated by p53.42 However, p53 plays a role in control of genome integrity at various cell cycle checkpoints including the G2/M boundary.43,44 Loss or alteration of spindle checkpoint control may have a relevant influence in the cellular outcome following microtubular damage. Several studies have shown an increased sensitivity of cells with mutant p53 to taxanes.45,46 A p53 mutant ovarian carcinoma cell line, IGROV-1/Pt1, selected for resistance to cisplatin, exhibits collateral sensitivity to Taxol.47 The mechanism by which mutation of p53 changes the sensitivity to taxanes is unknown. Changes in the expression in p53 gene targets might affect drug sensitivity. Relevant to this point is the observation that p53 negatively regulates the expression of MAP4, a microtubule-stabilizing protein.24 Thus, overexpression of MAP4 provides a plausible explanation of the increased sensitivity to taxanes of mutant p53 cells, since polymerized microtubules favor drug binding. P53 itself likely plays a direct or indirect role in the mitotic checkpoint.43,44 Molecular regulatory defects at this checkpoint could drive different pathways of cell death.44 In support of this interpretation is the finding that p53 missense mutations of a common class II confers a dominant, gain-of-function phenotype that disrupts the mitotic spindle checkpoint.48 A recent study described a p53-inducible gene coding for a microtubule-binding protein with G2 phase specific expression.49 This protein is induced by wild-type p53 under the same conditions in which the microtubule-associated MAP4 is downregulated, suggesting a functional interplay between these protein at G2/M boundary.Accumulating evidence supports an involvement of p53 status in determining sensitivity to taxanes. Considering the frequency of p53 mutations in human tumors, the ability of taxanes to activate p53-independent apoptosis represents a relevant pharmacological feature as suggested in a clinical study performed in ovarian carcinoma.50 The p53 status can be regarded as one of the important functions influencing cellular sensitivity to a large number of agents; however, its functional status will not generate high degree of resistance. While the microtubule stabilization is recognized as the primary effect of taxanes, the downstream biochemical events that lead to cell death are not well defined.Although the signaling cascade activated by agents that disrupt mitotic apparatus or damage cytoskeleton may be distinct from the cellular response to DNA damage, downstream regulation of the apoptotic pathway appears to be related to that involved in response to genotoxic stress.The bcl-2 gene family plays a central role in the regulation of apoptosis subsequent to microtubule stabilization.35,51,52 Overexpression of the antiapoptotic proteins, Bcl-2 and Bcl-xL, induced resistance to Taxol in human HL-60 leukemia cells.12 The apoptotic pathway activated by taxanes is associated with phosphorylation of Bcl-2, which is an important regulatory event.51,52 Indeed, hyperphosphorylated Bcl-2 does not bind the pro-apoptotic Bax protein, resulting in increased level of free Bax.53
Conversely, overexpression of the pro-apoptotic proteins Bax and Bad sensitizes human ovarian carcinoma cells to Taxol.54 A number of protein kinases, including Raf-1,51 PKA35 and other kinases involved in the regulation of cellular proliferation and cell cycle progression,55 may be directly or indirectly responsible for Bcl-2 phosphorylation.Again, these observations provide possible explanations for the complex relationships between cellular sensitivity to taxanes and tumor-specific alterations involving signal transduction pathways. In addition to Bcl-2, also Bcl-XL is phosphorylated by antimicrotubule agents.56 However, the precise role of levels of Bcl-2 protein and its phosphorylation as determinant of resistance/sensitivity status is still controversal. For example, a prostate carcinoma cell line lacking Bcl-2 expression was resistant to taxane-induced apoptosis57 and a number of human tumor xenografts overexpressing Bcl-2 exhibit hypersensitivity to taxanes.33 Since Bcl-2 phosphorylation may also be induced by effective DNA damaging agents,58 the possibility exists that Bcl-2 phosphorylation is a common event associated with different cell death pathways that are probably related to the onset of apoptosis. Recently, a new protein associated with microtubules of the mitotic spindle has been reported.59 This protein, survivin, has been implicated in the control of mitotic spindle checkpoint and apoptosis.The survivin gene is expressed in non-small cell lung carcinoma;60 however, its role in determining resistance of this tumor type remain unknown. Although cytotoxic effects of taxanes likely reflect induction of apoptosis, the difficulty to establish a precise correlation between apoptosis-related factors and sensitivity/resistance status presumably reflects not only the complexity of regulation of apoptosis but also the activation of a cellular response involving different cell death pathways. For example, in an human osteosarcoma cell line lacking p53 function apoptosis is induced through the CD95/CD95L system.61 Again, this pathway involves Bcl-2 phosphorylation. STRUCTURE-ACTIVITY RELATIONSHIPS AND DEVELOPMENT OF NOVEL TAXANES Identification of novel taxanes endowed with improved efficacy is expected on the basis of structure-activity relationship studies performed to define molecular features relevant for drug-target interaction and for ability to overcome P-glycoprotein-mediated multidrug resistance.These studies have documented that the interaction of taxanes with tubulin is affected by conformation of the side-chain at the 13-position of the taxane ring (Fig. 1) and by the presence of proper hydrophobic and hydrophilic substituents in both side chain and taxane ring.62–66 In particular, the optimal drug activity is related to the presence of hydroxyl group at C-2′ and to the 2′R and 3′S configuration. However, the presence of the 3′phenyl group and the benzoic acid at 3′-amino group are not essential for drug activity and they can be successfully replaced by alkyl substituents.63,64 Replacement of the aromatic substituents at the 3′- and 3′-N-positions with alkyl groups or alkyl esters, respectively, resulted in superior activity compared to that of Taxol. More importantly, in the 3′-substituted derivatives, protection of the hydroxyl groups at C1, 1999 Harcourt Publishers Ltd Drug Resistance Updates (1999) 2, 351–357
353
Zunino et al.
Fig. 1 Structure of Taxol (paclitaxel).
C10 and C14 (as in the case derivatives of 14β-hydroxy-10deacetylbaccatin III) with appropriate substituents enables the drug to overcome typical multidrug resistance.64,65 These observations provide indirect evidence that different and specific interactions must be involved in determining drug affinity for the intracellular target and in drug recognition by the P-glycoprotein. This interpretation is also supported by the finding that epothilones, a novel class of cytotoxic macrolides with a Taxol-like mechanism (i.e. inhibitors of microtubule depolymerization), exhibit efficacy against MDR tumors.67 Other novel structurally unrelated compounds with Taxol-like activity but having the ability to overcome MDR-mediated resistance (elentherobin, discodermolide) have been described.68,69 Based on these observations, a common pharmacophore for compounds that stabilize microtubule has been identified.70 Conversely, natural taxoids (taxuspines) exhibit reduced cytotoxic activity but ability to modulate MDR resistance. 71 Taken together, all these observations support the possibility to discriminate essential requirements for effective drug-target interaction in future drug design. This possibility is supported by evidence that a novel taxane analog is able to completely overcome P-glycoprotein-mediated resistance and is more effective than Taxol.33,64 CONCLUSIONS AND FUTURE DIRECTIONS The cellular resistance to taxanes is a very complex phenomenon involving (a) defence factors, such as intracellular drug accumulation or distribution, (pre-target events) or (b) alterations of the microtubule structure and/or composition, resulting in changes of microtubule dynamics or drug-target interactions (Table 1).The clinical relevance of the resistance 354
Drug Resistance Updates (1999) 2, 351–357
1999 Harcourt Publishers Ltd
factors identified in cell systems remains to be defined. Several lines of evidence support the hypothesis that the antitumor efficacy of taxanes may be due not only to the absence of resistance mechanisms, but also to the expression of sensitivity determinants, such as cell ability to activate an efficient apoptotic pathway.72 Co-expression and/or cooperation between multiple factors may determine cell sensitivity status.73 Understanding the signal transduction pathway and possible cross-talk among different pathways (growth factor receptor-mediated or microtubule-mediated) may provide a rational basis for exploiting the therapeutic potential of taxanes. Relevant to this point is the observation that Taxol may induce CD95-mediated apoptosis.61,74 Since this pathway is potentiated by cytokines, such as TNFα and IFN-γ,75 studies of combination of cytokines with Taxol could be designed in tumors expressing the CD95 receptor. A phase I study of such combination has been performed in prostate cancer patients.76 The definition of the molecular/biological context that favors sensitivity to taxanes is expected to improve therapeutic strategies.The p53-regulated changes of expression of MAP4 provide an explanation for opposite influence of p53 gene status on sensitivity to taxanes and vinca alkaloids and may have implications in the rational design of novel combinations.25,77 The recognition of the antiangiogenetic properties of Taxol and other antimicrotubule agents may provide a basis for additional clinical applications.78 A promising approach in the improvement of microtubule-targeted therapies is the development of non-crossresistant analogs33,79 or novel antitubulin agents.3,67,68,80 Many of these new compounds exhibit activity against tumor cells expressing the MDR phenotype.An important finding for the future design of more effective antimicrotubule agents is the
Resistance mechanisms to taxanes identification of a common pharmacophore for specific drug-target interaction.70 A major drawback of the therapeutic use of taxanes is not only the development of drug resistance, but drug toxicity. Thus, identification of novel agents with a better tolerability and improved therapeutic index is essential to exploit therapeutic potential of antimicrotubule agents. A novel taxane with an improved pharmacological profile in terms of both toxicity and activity against Taxolresistant tumors has been selected as a promising candidate for clinical evaluation.33 In spite of an increased cytotoxic potency and efficacy against tumors with the MDR phenotype, the analog is characterized by a better tolerability than Taxol at effective doses, thus suggesting an increased antitumor selectivity. Acknowledgements
This work was partially supported by the Associazione Italiana per la Ricerca sul Cancro, Milan, and by the Ministero della Sanita’, Rome, Italy
Received 17 November 1999; Accepted 8 December 1999 Correspondence to: Franco Zunino, Istituto Nazionale Tumori,Via Venezian 1, 20133 Milan, Italy.Tel: +39 02 2390267; Fax: +39 02 2390692; E.mail [email protected]
REFERENCES 1. Rowinski EK, Donehower RC. Paclitaxel (Taxol). In:Wood AJJ, ed., Drug therapy. N Engl J Med 1995: 1004–1012. 2. Horwitz SB, Cohen D, Rao S, Ringel I, Shen H-J,Yang C-PH.Taxol: mechanisms of action and resistance. Monogr Natl Cancer Inst 1993; 15: 55–61. 3. Dumontet C, Sikic BI. Mechanisms of action of and resistance to antitubulin agents: microtubule dynamics, drug transport, and cell death. J Clin Oncol 1999; 17: 1061–1070. 4. Jordan MA,Wendell K, Gardiner S, Derry WB, Copp H,Wilson L. Mitotic block induced in HeLa cells by low concentrations of paclitaxel (Taxol) results in abnormal mitotic exit and apoptotic cell death. Cancer Res 1996; 56: 816–825. 5. Fan W. Possible mechanisms of paclitaxel-induced apoptosis. Biochem Pharmacol 1999; 57: 1215–1221. 6. Pastan I, Gottesman MM. Multidrug resistance.Annu Rev Med 1991; 42: 277–286. 7. Sparreboom A,Van Asperen J, Mayer U et al. Limited oral bioavailability and active epithelial excretion of paclitaxel (Taxol) caused by P-glycoprotein in the intestine. Proc Natl Acad Sci USA 1997; 94: 2031–2035. 8. Balmaceda C.Advances in brain tumor chemosensitivity. Curr Opin Oncol 1998; 10: 194–200. 9. Hosli P, Sappino AP, de Tribolet N, Dietrich PY. Malignant glioma: should chemotherapy be overthrown by experimental treatments? Ann Oncol 1998; 9: 589–600. 10. Van Asperen J,Van Tellingen O,Van der Valk MA, Rozenhart M, Beijnen JH. Enhanced oral absorption and decreased elimination of paclitaxel in mice cotreated with cyclosporin A. Clin Cancer Res 1998; 4: 2293–2297.
11. Cole SPC, Sparks KE, Fraser K et al. Pharmacological characterization of multidrug resistant MRP-transfected human tumor cells. Cancer Res 1994; 54: 5902–5910. 12. Huang Y, Ibrado AM, Reed JC et al. Co-expression of several molecular mechanisms of multidrug resistance and their significance for paclitaxel cytotoxicity in human AML HL-60 cells. Leukemia 1997; 11: 253–257. 13. Binaschi M, Supino R, Gambetta RA et al. MRP gene overexpression in a human doxorubicin-resistant SCLC cell line: alterations in cellular pharmacokinetics and in pattern of crossresistance. Int J Cancer 1995; 62: 84–89. 14. Vanhoefer U, Cao S, Harstrick A, Seeber S, Rustum YM. Comparative antitumor efficacy of docetaxel and paclitaxel in nude mice bearing human tumor xenografts that overexpress the multidrug resistance protein (MRP).Ann Oncol 1997; 8: 1221–1228. 15. Royer I, Monsarrat B, Sonnier M,Wright M, Cresteil T. Metabolism of docetaxel by human cytochromes P450: interactions with paclitaxel and other antineoplastic drugs. Cancer Res 1996; 56: 58–65. 16. Liebmann JE, Hahn SM, Cook JA, Lipschultz C, Mitchell JB, Kaufman DC. Glutathione depletion by L-buthionine sulfoximine antagonizes Taxol cytotoxicity. Cancer Res 1993; 53: 2066–2070. 17. Nishio K, Saijo N. Cytoskeletons and antimitotic agents developed in Japan.Anti-Cancer Drug Design 1999; 14: 133–141. 18. Ohta S, Nishio K, Kubota N et al. Characterization of a Taxolresistant human small-cell lung cancer cell line. Jpn J Cancer Res 1994; 85: 290–297. 19. Derry WB,Wilson L, Khan IA, Luduena RF, Jordan MA.Taxol differentially modulates the dynamics of microtubules assembled from unfractionated and purified beta-tybulin isotypes. Biochemistry 1997; 36: 3554–3562. 20. Kavallaris M, Burkhart CA, Horwitz SB.Antisense oligonucleotides to class III beta-tubulin sensitize drug-resistant cells to Taxol. Br J Cancer 1999; 80: 1020–1025. 21. Giannakakou P, Sackett DL, Kang YK et al. Paclitaxel-resistant human ovarian cancer cells have mutant beta-tubulins that exhibit impaired paclitaxel-driven polymerization. J Biol Chem 1997; 272: 17118–17125. 22. Monzó M, Rosell R, Sanchez JJ et al. Paclitaxel resistance in nonsmall-cell lung cancer associated with beta-tubulin gene mutations. J Clin Oncol 1999; 17: 1786–1793. 23. Nishio K,Arioka H, Ishida T et al. Enhanced interaction between tubulin and microtubule-associated protein 2 via inhibition of MAP kinase and CDC2 kinase by paclitaxel. Int J Cancer 1995; 63: 688–693. 24. Zhang CC,Yang J-M,White E, Murphy M, Levine A, Hait WN.The role of MAP4 expression in the sensitivity to paclitaxel and resistance to vinca alkaloids in p53 mutant cells. Oncogene 1998; 16: 1617–1624. 25. Jordan MA,Wilson L. Microtubules and actin filaments: dynamic targets for cancer chemotherapy. Curr Opin Cell Biol 1998; 10: 123–130. 26. Mendelsohn J, Fan Z. Epidermal growth factor receptor family and chemosensitization. J Natl Cancer Inst 1997; 89: 341–343. 27. Yu D, Liu B,Tan M, Li J,Wang S-S, Hung M-C. Overexpression of c-erbB-2/neu in breast cancer cells confers increased resistance to Taxol via mdr-1-independent mechanisms. Oncogene 1996; 13: 1359–1365.
1999 Harcourt Publishers Ltd Drug Resistance Updates (1999) 2, 351–357
355
Zunino et al. 28. Ueno NT,Yu D, Hung MC. Chemosensitization of HER-2/neuoverexpressing human breast cancer cells to paclitaxel (Taxol) by adenovirus type 5 E1A. Oncogene 1997; 15: 953–960. 29. Brader KR,Wolf JK, Hung MC et al.Adenovirus E1A expression enhances the sensitivity of an ovarian cancer cell line to multiple cytotoxic agents through an apoptotic mechanism. Clin Cancer Res 1997; 3: 2017–2024. 30. Zhang L, Lau YK, Xia W, Hortobagyi GN, Hung MC.Tyrosine kinase inhibitor emodin suppresses growth of HER-2/neu-overexpressing breast cancer cells in athymic mice and sensitizes these cells to the inhibitory effect of paclitaxel. Clin Cancer Res 1999; 5: 343–353. 31. Yu D, Liu B, Jing T et al. Overexpression of both p185c-erbB2 and p170mdr-1 renders breast cancer cells highly resistant to Taxol. Oncogene 1998; 16: 2087–2094. 32. Baselga J, Norton L,Albanell J et al. Recombinant humanized anti HER2 antibody (Herceptin) enhances the antitumor activity of paclitaxel and doxorubicin against HER2/neu overexpressing human breast cancer xenografts. Cancer Res 1998; 58: 2825–2831. 33. Polizzi D, Pratesi G,Tortoreto M, Supino R, Riva A, Bombardelli E, Zunino F.A novel taxane with improved tolerability and therapeutic activity in a panel of human tumor xenografts. Cancer Res 1999; 59: 1036–1040. 34. Ciardiello F,Tortora G. Interactions between the epidermal growth factor receptor and type I protein kinase A: biological significance and therapeutic implications. Clin Cancer Res 1998; 4: 821–828. 35. Ciardiello F, Caputo R, Pomatico G et al. Resistance to taxanes is induced by c-erbB-2 overexpression in human MCF-10A mammary epithelial cells and is blocked by combined treatment with an antisense oligonucleotide targeting type I protein kinase A. Int J Cancer 1999; in press. 36. Srivastava RK, Srivastava AR, Korsmeyer SJ, Nesterova M, ChoChung YS, Longo DL. Inolvement of microtubules in the regulation of Bcl2 phosphorylation and apoptosis through cyclic AMPdependent protein kinase. Mol Cell Biol 1998; 18: 3509–3517. 37. Yang C-PH, Galbiati F,Volonté D, Horwitz SB, Lisanti MP. Upregulation of caveolin-1 and caveolae organelles in Taxolresistant A549 cells. FEBS Letters 1998; 439: 368–372. 38. Razani B, Rubin CS, Lisanti MP. Regulation of cAMP-mediated signal transduction via interaction of caveolins with the catalytic subunit of protein kinase A. J Biol Chem 1999; 274: 26353–26360. 39. Zunino F, Perego P, Pilotti S, Pratesi G, Supino R,Arcamone F. Role of apoptotic response in cellular resistance to cytotoxic agents. Pharmacol Ther 1997; 76: 177–185. 40. Wu GS, El-Deiry WS.Apoptotic death of tumor cells correlates with chemosensitivity, independent of p53 or Bcl-2. Clin Cancer Res 1996; 2: 623–633. 41. Perego P, Giarola M, Righetti SC et al.Association between cisplatin resistance and mutation of p53 gene and reduced bax expression in ovarian carcinoma cell systems. Cancer Res 1996; 56: 556–562. 42. Vasey PA, Jones NA, Jenkins S, Dive C, Brown R. Cisplatin, camptothecin, and Taxol sensitivities of cells with p53-associated multidrug resistance. Mol Pharmacol 1996; 50: 1536–1540. 43. Rudner AD, Murray AW.The spindle assembly checkpoint. Curr Opin Cell Biol 1996; 8: 773–780. 44. Sorger PK, Dobles M,Tournebize R, Hyman AA. Coupling cell division and cell death to microtubule dynamics. Curr Opin Cell Biol 1997; 9: 807–814. 45. Wahl AF, Dinaldson KL, Fairchild C et al. Loss of normal p53 function confers sensitization to Taxol by increasing G2/M arrest and apoptosis. Nature Med 1996; 2: 72–79.
356
Drug Resistance Updates (1999) 2, 351–357
1999 Harcourt Publishers Ltd
46. Vikhanskaya F,Vignati S, Beccaglia P et al. Inactivation of p53 in a human ovarian cancer cell line increases the sensitivity to paclitaxel by inducing G2/M arrest and apoptosis. Exp Cell Res 1998; 241: 96–101. 47. Perego P, Romanelli S, Carenini N et al. Ovarian cancer cisplatinresistant cell lines: multiple changes including collateral sensitivity to Taxol.Ann Oncol 1998; 9: 423–430. 48. Gualberto A,Aldape K, Kozakiewicz K,Tlsty TD.An oncogenic form of p53 confers a dominant, gain-of-function phenotype that disrupts spindle checkpoint control. Proc Natl Acad Sci USA 1998; 95: 5166–5171. 49. Utrera R, Collavin L, Lazarevic D, Delia D, Schneider C.A novel p53-inducible gene coding for a microtubule-localized protein with G2-phase-specific expression. EMBO J 1998; 17: 5015–5025. 50. Lavarino C, Delia D, Di Palma S, Zunino F, Pilotti S. P53 in drug resistance in ovarian cancer. Lancet 1997; 349: 1556. 51. Blagosklonny MV, Schulte T, Nguyen P,Trepel J, Neckers LM.Taxolinduced apoptosis and phosphorylation of Bcl-2 protein involves c-Raf-1 and represents a novel c-Raf-1 signal transduction pathway. Cancer Res 1996; 56: 1851–1854. 52. Wang LG, Liu XM, Kreis W, Budman DR.The effect of antimicrotubule agents on signal transduction pathways of apoptosis: a review. Cancer Chemother Pharmacol 1999; 44: 355–361. 53. Korsmeyer SJ. BCL-2 gene family and the regulation of programmed cell death. Cancer Res 1999; 59: 1693–1700. 54. Strobel T,Tai Y-T, Korsmeyer S, Cannistra SA. BAD partly reverses paclitaxel resistance in human ovarian cancer cells. Oncogene 1998; 17: 2419–2427. 55. Srivastava KK, Mi Q-S, Hardwick JM, Longo DL. Deletion of the loop region of Bcl-2 completely blocks paclitaxel-induced apoptosis. Proc. Natl.Acad. Sci USA 1999; 96: 3775–3780. 56. Poruchynsky MS,Wang EE, Rudin CM, Blagosklonny MV, Fojo T. Bcl-xL is phosphorylated in malignant cells following microtubule disruption. Cancer Res. 1998; 58: 3331–3338. 57. Haldar S, Chintapalli J, Croce CM.Taxol induces Bcl-2 phosphorylation and death of prostate cancer cells. Cancer Res 1996; 56: 1253–1255. 58. De Cesare M, Pratesi G, Giusti A, Polizzi D, Zunino F. Stimulation of the apoptotic response as a basis for the therapeutic synergism of lonidamine and cisplatin in combination in human tumour xenografts. Br J Cancer 1998; 77: 434–439. 59. Li F,Ambrosini G, Chu EY et al. Control of apoptosis and mitotic spindle checkpoint by survivin. Nature 1998; 396: 580–584. 60. Monzó M, Rosell R, Felip E et al.A novel anti-apoptosis gene: reexpression of survivin messenger RNA as a prognosis marker in non-small-cell lung cancers. J Clin Oncol 1999; 17: 2100–2104. 61. Pucci B, Bellincampi L,Tafani M, Masciullo V, Melino G, Giordano A. Paclitaxel induces apoptosis in saos-2 cells with CD95L upregulation and Bcl-2 phosphorylation. Exp Cell Res 1999; 252: 134–143. 62. Guenard D, Gueritte-Voegelein F, Dubois J, Potier P. Structureactivity relationships of Taxol and taxotere analogues. Monogr Natl Cancer Inst 1993; 15: 79–82. 63. Ojima I, Slater JC, Michaud E et al. Syntheses and structure-activity relationships of the second-generation antitumor taxoids: exceptional activity against drug-resistant cancer cells. J Med Chem 1996; 39: 3889–3896. 64. Distefano M, Scambia G, Ferlini C et al.Anti-proliferative activity of a new class of taxanes (14β-hydroxy-10-deacetylbaccatin III
Resistance mechanisms to taxanes
65.
66.
67.
68.
69.
70.
71.
72.
derivatives) on multidrug-resistance-positive human cancer cells. Int J Cancer 1997; 72: 844–850. Ojima I, Slater JC, Kuduk SD et al. Syntheses and structure-activity relationships of taxoids derived from 14β-hydroxy-10deacetylbaccatin III. J. Med. Chem. 1997; 40: 267–278. Ojima I, Kuduk SD, Pera P,Veith JM, Bernacki RJ. Synthesis and structure-activity relationships of nonaromatic taxoids: effects of alkyl and alkenyl ester groups on cytotoxicity. J Med Chem 1997; 40: 279–285. Chou T-C, Zhang X-G, Harris CR et al. Desoxyepothilone B is curative against human tumor xenografts that are refractory to paclitaxel. Proc Natl Acad Sci USA 1998; 95: 15798–15802. Kowalski RJ, Giannakakou P, Gunasekera SP, Longley RE, Day BW, Hamel E.The microtubule-stabilizing agent discodermolide competitively inhibits the binding of paclitaxel (Taxol) to tubulin polymers, enhances tubulin nucleation reactions more potently than paclitaxel, and inhibits the growth of paclitaxel-resistant cells. Mol Pharmacol 1997; 52: 613–622. McDaid H, Bhattacharya SK, Chen K-T et al. Structure-activity profiles of eleutherobin analogs and their cross-resistance in Taxolresistant cell lines. Cancer Chemother Pharmacol 1999; 44: 131–137. Ojima I, Chakravarty S, Inoue T et al.A common pharmacophore for cytotoxic natural products that stabilize microtubules. Proc Natl Acad Sci USA 1999; 96: 4256–4261. Kobayashi J, Hosoyama H,Wang X-x, Shigemori H, Sudo Y,Tsuruo T. Modulation of multidrug resistance by taxuspine C and other taxoids from japanese yew. Bioorg Med Chem Lett 1998; 8: 1555–1558. Milross CG, Mason KA, Hunter NR, Chung WK, Peters LJ, Milas L. Relationship of mitotic arrest and apoptosis to antitumor effect of paclitaxel. J Natl Cancer Inst 1996; 88: 1308–1314.
73. Strobel T, Kraeft S-K, Chen LB, Cannistra SA. BAX expression is associated with enhanced intracellular accumulation of paclitaxel: a novel role for BAX during chemotherapy-induced cell death. Cancer Res 1998; 58: 4776–4781. 74. Lanni JS, Lowe SW, Licitra EJ, Liu JO, Jacks T. p53-independent apoptosis induced by paclitaxel through an indirect mechanism. Proc Natl Acad Sci USA 1997; 94: 9679–9683. 75. Weller M, Frei K, Groscurth P, Krammer PH,Yonekawa Y, Fontana A.Anti-Fas/APO-1 antibody-mediated apoptosis of cultures human glioma cells. Induction and modulation of sensitivity by cytokines. J Clin Invest 1994; 94: 954–964. 76. DiPaola RS, Rafi MM,Vyas V et al. Phase I clinical and pharmacologic study of 13-cis-retinoic acid, interferon alfa, and paclitaxel in patients with prostate cancer and other advanced malignancies. J Clin Oncol 1999; 17: 2213–2218. 77. Zhang CC,Yang JM, Bash-Babula J et al. DNA damage increases sensitivity to vinca alkaloids and decreases sensitivity to taxanes through p53-dependent repression of microtubule-associated protein 4. Cancer Res 1999; 59: 3663–3670. 78. Klauber N, Parangi S, Flynn E, Hamel E, D’Amato RJ. Inhibition of angiogenesis and breast cancer in mice by the microtubule inhibitors 2-methoxyestradiol and Taxol. Cancer Res 1997; 57: 81–86. 79. De Vincenzo R, Scambia G, Ferlini C et al.Antiproliferative activity of colchicine analogues on MDR-positive and MDR-negative human cancer cell lines.Anti-Cancer Drug Design 1998; 13: 19–33. 80. Shan B, Medina JC, Santha E et al. Selective, covalent modification of β-tubulin residue Cys-239 by T138067, an antitumor agent with in vivo efficacy against multidrug-resistant tumors. Proc Natl Acad Sci USA 1999; 96: 5686–5691.
1999 Harcourt Publishers Ltd Drug Resistance Updates (1999) 2, 351–357
357