EFFECT OF DRUG-RESISTANCE REVERSORS ON EXPRESSIONS OF ONCOGENES OR TUMOR SUPPRESSOR ONCOGENES OF HUMAN TUMOR CELL LINES

FailureofchemotherapyresultsmainIyfrompresenceofanticancer-drugresistanceoftumor.Inthepastmorethantwentyyears,tenskindsofreversorsormodulatorsagainstdrugresistanceoftumorcell,havebeendiscovered.Thesereversorsarebelievedto'overcomeQrugresistanceoftumorceIlsthroughcompetitivecombinationwithanticancerdrugsforp-glycpproteinandg1utamine-Stransferase(suchasbuthioninesulfoximine,BSO)ormodulationofDNArepair(Suchasstreptozocin).Oncogenesortumorsuppresseroncogeneswerebeenrecentlyreportedtoplayagreat…     

Establishment of K562/ADM/VER cell subline resistant to verapamil and its resistant mechanism

Failure of chemotherapy for cancer results mainly from resistance to chemotherapy. Tens kind of reversors have been found to reverse resistance of tumor cell to anticancer drugs in vitro. Interestingly, tumor cells were observed to develop resistance to the resistance reversors recently. Multidrug resistant cells, EMT16 have been reported to produce resistance to verapamil (VER), a resistance reversor, from Reeve JG, et al.[1]. However, this fact has not been fully paid attention with no r…     

干扰素和环孢霉素A 协同逆转K562/ ADM细胞对阿霉素的耐药性

  目的 观察干扰素(α-Interleron，α-IFN)和环孢霉素A(Cyclosporine A,CsA)对白血病K562/ADM细胞耐药性的协同逆转效应。方法 以多药耐药基因／P-糖蛋白(Muhidrug resistance gene／P-glycoprotein，mdrl／P-gp)超表达的K562／ADM细胞为靶细胞，MTT比色法检测药物的细胞毒效应；流式细胞仪检测细胞P-糖蛋白(P-glycoprotein，P-gp)表达水平；激光共聚焦显微镜观察细胞内阿霉素含量变化。结果 K562／ADM细胞对阿霉素呈高度耐药性，并与柔红霉素和鬼臼乙叉甙交叉耐药，但与CsA无交叉耐药。CsA和α-IFN单独或联合应用均对K562／ADM细胞的耐药性有较强的抑制效应。流式细胞仪和激光共聚焦显微镜分析发现α-IFN和CsA单独或联合均不能下调细胞mdrl/P-gp的表达，反而应激性地刺激耐药细胞P-gp的合成增加，但可抑制P-gp的功能、增加K562／ADM细胞内阿霉素的积聚。结论 α-IFN和CsA联合可协同逆转耐药白血病细胞的耐药性，其作用机制为抑制P-gp的功能而非下调mdrl／P-gp的表达水平。     

Effect of ascorbic acid on DNA synthesis, intracellular accumulation of ADM and ADM resistance of tumor cell lines

Objective: To determine the effect of ascorbic acid (AA) on DNA synthesis, intracellular accumulation of ADM and ADM resistance of tumor cell lines. Methods: K562, K562/ADM and KB cell lines were used to study the effect of ascorbic acid on DNA synthesis, intracellular accumulation of ADM and ADM resistance by fluid scintillometry, MTT method, spectrofluorophotometry and immunocytochemistry. Results: Results showed that AA was capable of inhibiting DNA synthesis of K562 and K562/ADM in a dose-dependence fashion, but not KB cell line, and significantly reducing ADM sensitivity in K562 and KB cell lines, as well as potentiating obviously ADM resistance in K562/ADM cell line. Conclusion: These effects of AA may be closely correlated with significant elevation of intracellular accumulation of ADM in KB cell line, and significant reduction of that in K562 and K562/ADM cell lines but possibly not correlated with the expression of P-glycoprotein.     

Effects of permeability transition inhibition and decrease in cytochrome c content on doxorubicin toxicity in K562 cells

As mitochondria play a key role in the commitment to cell death, we have investigated the mitochondrial consequences of resistance to doxorubicin (DOX) in K562 cells. We found that the permeability transition pore (PTP) inhibitor cyclosporine A (CsA) failed to inhibit PTP opening in the resistant clone. Moreover, the Ca2+ loading capacity in the resistant clone was identical to that observed in the parent cells in the presence of CsA, suggesting that the PTP was already inhibited in a CsA-like manner in the resistant cells. In agreement with this proposal, the mitochondrial target of CsA cyclophilin D (CyD) decreased by half in the resistant cells. The levels of adenine nucleotide translocator, voltage anion-dependent channel, Bax, Bcl-2, Bcl-xL, AIF and Smac/Diablo, were similar in both cell lines, whereas cytochrome c content was divided by three in the resistant cells. Since P-glycoprotein inhibition did not restore DOX toxicity in the resistant cells, while DOX-induced cell death in the parent cells was prevented by either PTP inhibition or siRNA-induced decrease in cytochrome c content, we conclude that the inhibition of PTP opening and the decrease in cytochrome c content participate in the mechanism that makes K562 cells resistant to DOX.     

K562多药耐药细胞系中肿瘤干细胞样细胞对伊马替尼耐药机制的初步研究

Objective： To characterize a novel chronic myeloid leukemia （CML） cell line and to further elucidate the mechanisms of resistance to STI571. Methods： A novel K562 cell line （K562NP16） was achieved after exposure of the K562 cells to VP16. A small subpopulation （K562NP16 SP） that was capable of excluding Hoechst 33342 in the K562NP16 cell line was isolated by fiow cytometry sorting. The rest of the K562NP16 cells were classified as non-SP K562NP16. The mechanisms involved in K562NP16 SP cells which became resistant to STI571 were studied. Results： The levels of Bcr-Abl and Abl proteins were similar in the K562 cell line and in non-SP K562NP16 and K562NP16 SP cells. The multidrug-resistant gene 1 （MDR1） expression of the 170 kDa P-glycoprotein （P-gp） was detected in K562NP16 non-SP and K562NP16 SP cells but not in K562 cells. The expression levels of P-gp in the two K562NP16 cell lines were similar. Compared with non-SP K562/ VP16, the K562NP16 SP cells were more resistant to STI571. This resistance could hardly be reversed by many multidrug resistance inhibitors. In addition, in vivo study showed that the K562NP16 SP cells induced tumorigenesis in mice, while the K562NP16 non-SP cells failed to do so. Conclusion： A novel K562 cell line, K562NP16, was generated. A small side population K562NP16 SP cells, had high resistance to STI571 treatment and more tumorigenic than the K562 cells. It may represent the cancer stem cells of the K562NP16 cell line.     

HPLCASSAY FOR INTRACELLULAR ACCUMULATION OF VERAPAMIL IN VER-RESISTANT HUMAN LEUKEMIC CELL SUSLINES AND THEIR PARENTAL CELL LINES

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Re-Expression of Bone Marrow Proteoglycan-2 by 5-Azacytidine is associated with STAT3 Inactivation and Sensitivity Response to Imatinib in Resistant CML Cells

Background: Epigenetic silencing of tumor suppressor genes (TSG) is involved in development andprogression of cancers. Re-expression of TSG is inversely proportionate with STAT3 signaling pathways.Demethylation of DNA by 5-Azacytidine (5-Aza) results in re-expression of silenced TSG. Forced expression ofPRG2 by 5-Aza induced apoptosis in cancer cells. Imatinib is a tyrosine kinase inhibitor that potently inhibits BCR/ABL tyrosine kinase resulting in hematological remission in CML patients. However, majority of CML patients treatedwith imatinib would develop resistance under prolonged therapy. Methods: CML cells resistant to imatinib weretreated with 5-Aza and cytotoxicity of imatinib and apoptosis were determined by MTS and annexin-V, respectively.Gene expression analysis was detected by real time-PCR, STATs activity examined using Western blot and methylationstatus of PRG2 was determined by pyrosequencing analysis. Result: Expression of PRG2 was significantly higher inK562-R+5-Aza cells compared to K562 and K562-R (p=0.001). Methylation of PRG2 gene was significantly decreasedin K562-R+5-Aza cells compared to other cells (p=0.021). STAT3 was inactivated in K562-R+5-Aza cells which showedhigher sensitivity to imatinib. Conclusion: PRG2 gene is a TSG and its overexpression might induce sensitivity toimatinib. However, further studies are required to evaluate the negative regulations of PRG2 on STAT3 signaling.     

K562多药耐药细胞系中肿瘤干细胞样细胞对伊马替尼耐药机制的初步研究

 目的 进一步阐明一些高表达P-糖蛋白（P-gp）的慢性粒细胞白血病细胞对伊马替尼耐药的机制。方法 经过对K562细胞系长期的足叶乙苷（VP16）诱导和克隆筛选,建立一株耐药细胞系K562／VP16;利用干细胞高效能将Hoechst 33342 荧光染料泵出细胞的特性,采用流式细胞术,从K562／VP16细胞系中分选出一小群细胞,即边缘细胞（SP）,称为K562／VP16 SP细胞,并初步探讨其抗伊马替尼的机制。结果 bcr／abl和abl 蛋白在K562细胞、K562／VP16 SP细胞及非K562／VP16 SP细胞（non-SP K562／VP16）中的表达水平差异无统计学意义;P-gp在K562细胞中不表达,在K562／VP16 SP及non-SP K562／VP16细胞中均高表达且表达水平一致;与non-SP K562／VP16细胞比较,K562／VP16 SP细胞对伊马替尼的耐药性更强,并且这种抗性几乎不能被多种多药耐药逆转剂逆转;另外,体内外实验显示,K562／VP16细胞的致瘤性几乎全部来源于K562／VP16 SP细胞。结论 bcr／abl基因的扩增、过度表达和多药耐药基因及其蛋白表达产物P-gp的高表达,可能不是白血病细胞产生对伊马替尼临床耐药的重要机制;白血病细胞对伊马替尼具有一定的抗性,可能与数量极少的白血病干细胞有直接的关系。因此,这类数量极少的干细胞样的肿瘤细胞应当成为有效治疗肿瘤的靶细胞。     

血液肿瘤细胞对氧化砷的敏感性与其抗氧化能力的关系

目的　探讨血液肿瘤细胞对三氧化二砷 (As2 O3 )的敏感性和细胞抗氧化能力的关系。方法　应用 9个血液肿瘤细胞系 ,通过细胞活力、形态学和流式细胞仪检测细胞凋亡 ,并测定细胞系的谷胱甘肽 (GSH)含量和 4种抗氧化酶的活性。结果　除了HL 6 0、U937、K5 6 2和Jurkat细胞外 ,其他5个细胞对As2 O3 诱导的凋亡敏感。与敏感细胞系比较 ,As2 O3 耐受细胞系存在较高的GSH含量和(或 )过氧化氢酶活性。谷胱甘肽过氧化物酶、谷胱甘肽S转移酶和超氧化物歧化酶活性与细胞对As2 O3 诱导凋亡效应敏感性无明显相关。结论　细胞内GSH水平和过氧化氢酶的活性是决定血液肿瘤细胞对As2 O3 敏感性的重要因素。     

Reversal of the resistance to STI571 in human chronic myelogenous leukemia K562 cells

STI571, an Abl-specific tyrosine kinase inhibitor, selectively kills Bcr-Abl-containing cells in vitro and in vivo . However, some chronic myelogenous leukemia (CML) cell lines are resistant to STI571. We evaluated whether STI571 interacts with P-glycopro-tein (P-gp) and multidrug resistance protein 1 (MRP1), and examined the effect of agents that reverse multidrug resistance (MDR) on the resistance to SI571 in MDR cells. STI571 inhibited the [¹²⁵l]azidoagosterol A-photolabeling of P-gp, but not that of MRP1. K562/MDR cells that overexpress P-gp were 3.67 times more resistant to STI571 than the parental Philadelphia-chromosome-positive (Ph+) CML K562 cells, and this resistance was most effectively reversed by cepharanthine among the tested reversing agents. The concentration of STI571 required to completely inhibit tyrosine phosphorylation in K562/MDR cells was about 3 times higher than that in K562 cells, and cepharanthine abolished the difference. In KB-G2 cells that overexpress P-gp, but not Bcr-Abl, 2.5 μM STI571 partly reversed the resistance to vincristine (VCR), paclitaxel, etoposide (VP-16) and actinomycin D (ACD) but not to Adriamycin (ADM) or colchicine. STI571 increased the accumulation of VCR, but not that of ADM in KB-G2 cells. STI571 did not reverse resistance to any agent in KB/MRP cells that overexpress MRP1. These findings suggest that STI571 is a substrate for P-gp, but is less efficiently transported by P-gp than VCR, and STI571 is not a substrate for MRP1. Among the tested reversing agents that interact with P-gp, cepharanthine was the most effective agent for the reversal of the resistance to STI571 in K562/ MDR cells. Furthermore, STI571 itself was a potent reversing agent for MDR in P-gp-expressing KB-G2 cells.     

DNA-mediated transfer and cloning of a human multidrug-resistant gene of adriamycin-resistant myelogenous leukemia K562

We have successfully transferred and cloned a fragment of a human multidrug-resistant gene by using DNA-mediated gene transfer. Macromolecular DNA of human multidrug-resistant K562 cells was transfected to drug-sensitive mouse Ltk- cells to obtain a drug-resistant transfectant with a human resistant gene. Both primary and secondary transfectants showed similar patterns of cross-resistance to Adriamycin and vincristine. The mechanism of drug resistance of the transfectants was attributed to decreased retention of the drug. Three secondary transfectants obtained independently contained common Alu-containing EcoRI fragments 15, 6.5, 3.7, 2.6, and 1.9 kilobases long. The 2.6-kilobase EcoRI fragment was cloned from a lambda phage genomic library made from DNA of a secondary transfectant. The 2.6-kilobase fragment was detected in the primary and secondary transfectants but not in the parental Ltk-, Adriamycin-resistant Ltk-, and Adriamycin-resistant P388 cells. This sequence was found to be amplified in several multidrug-resistant cell lines such as Adriamycin-resistant ovarian carcinoma A2780 and colchicine-resistant KB carcinoma cells. The 2.6-kilobase fragment hybridized with a 4.5-kilobase mRNA which is overexpressed in the Adriamycin-resistant K562 cells and the Adriamycin-resistant A2780 cells but not detected in the parental K562 cells. The gene transferred and cloned in this study seems to be related to the P-glycoprotein gene as judged from the size of mRNA and its overexpression in some of the multidrug-resistant cell lines where P-glycoprotein was found to be highly expressed.     

The inhibition of ERK/MAPK not the activation of JNK/SAPK is primarily required to induce apoptosis in chronic myelogenous leukemic K562 cells

In this study, the downstream signaling of Bcr-Abl tyrosine kinase responsible for apoptosis resistance was investigated. DNA fragmentation, a hallmark of apoptosis, was observed after 2 days of herbimycin A treatment with a peak on 3 day. During the apoptosis induced by the treatment of herbimycin A, stress-activated protein kinase (SAPK) and p38 kinase were activated time- and dose-dependently, while extracellular signal-regulated kinase (ERK) was inhibited. However, apoptosis was induced by the treatment of PD98059, a specific inhibitor of MEK (MAPK or ERK kinase), not by the treatment of sorbitol, a strong activator of SAPK and p38 kinase. Although K562 cells were very resistant to sorbitol-induced apoptosis, DNA fragmentation was induced rapidly in Jurkat, HL-60 and U937 cells after exposure to sorbitol, despite that these apoptosis-sensitive cells have similar or lower activities of JNK/SAPK and p38 kinase compared with K562 cells after treatment of sorbitol. K562 cells had a much higher basal activity of ERK/MAPK than other apoptosis-sensitive cell lines, which were very susceptible to apoptosis induced by low dose of PD98059 compared with K562 cells. In HL-60 cells, sorbitol-induced apoptosis was prevented by the treatment of phorbol myristate 13-acetate (PMA), which activates the ERK/MAPK pathway, and this was blocked by PD98059. From these results, it could be suggested that the inhibition of ERK/MAPK not the activation of JNK/SAPK is primarily required to induce apoptosis in K562 cells.     

Impairment of methotrexate (MTX)-polyglutamate formation of MTX-resistant K562 cell lines

We examined the mechanism of methotrexate (MTX) resistance in five K562 cell subclones resistant to MTX. Based on a clonogenic assay, the IC50S of these MTX-resistant clones were 10 to 40 microM MTX, indicating 2,000 to 5,000-fold resistance as compared to that of the parent cell line. The doubling times of these MTX-resistant K562 cell lines are longer (27-60 hr) than that of the parent K562 cell line (24 hr). One-hour MTX accumulation in the resistant cells cells was 70-80% of that in parent cells. To investigate the formation of MTX-polyglutamates (MTX-PGs), resistant cells were incubated with 3H-MTX (1 or 10 microM) for 24 hr in the presence of thymidine and deoxyinosine to prevent cytotoxicity. MTX (-Glu1) and the polyglutamate metabolites (MTX-Glu2, -Glu3, -Glu4 and -Glu5) were analyzed by a high-pressure liquid chromatography (HPLC) technique. After a 24-hr incubation with 10 microM MTX, the total concentration of intracellular MTX reached 39 to 89 nmol/g protein, only 20 to 40% of the MTX level of the parent K562 cells. The HPLC analysis revealed that less than 2% of intracellular MTX was in the form of high-molecular MTX-PGs (MTX-Glu3, -Glu4 and -Glu5) in the five MTX-resistant K562 cell lines, while in the parent cells MTX-Glu3-5 comprised 46% of the total intracellular MTX. These data indicate the possibility that impairment of MTX-PG formation, with transport alteration, may be a special mechanism for the high level of resistance to this agent in human leukemic cells.     

Altered expression of DNA-topoisomerase IIalpha is associated with increased rate of spontaneous polyploidization in etoposide resistant K562 cells

Molecular mechanisms of drug resistance and alterations in chromosome number were studied in two independently etoposide selected clonal cell lines, K562-VP16-1 and K562-W16-2, and in bulk cell line K562-VPI6. In all cell lines was observed down-regulation of topo IIalpha gene expression, while topo IIbeta mRNA content was unchanged compared to parental cell line. Antiapoptotic bcl-2 mRNA content was decreased in all drug resistant cell variants, the level of bcl-X(L) mRNA was greatly diminished only in one of cell lines, K562-VP16-1. Proapoptotic bax mRNA was down regulated to an undetectable level in all resistant cell lines analysed. These data indicate that abrogation of bax-mediated apoptosis can be implicated in development of etoposide resisance. Cytogenetic analysis revealed increased rate of spontaneous polyploidization in K562-VPI6 bulk and K562-VP-16-2 cells, while in K562-VP-16-1 cells was observed only moderate accumulation of polyploid cells. The degree of changes in topo IIa but not bcl-2 family members expression correlated positively with dynamics of accumulation of polyploid cells. Our findings suggest that down regulation of topo IIalpha in association with p53 deficiency can confer chromosomal instability in etoposide-resistant K562 cells.     

Comparison of glutathione S-transferase activity between drug-resistant and-sensitive human tumor cells: Is glutathione S-transferase associated with multidrug resistance?

Summary We have studied the levels of glutathione S-transferase in drug-resistant and-sensitive human tumor cell lines to examine a possible involvement of glutathione S-transferase (GST) in multidrug resistance mechanisms. No increase in the activity of glutathione S-transferase was detected in myelogenous leukemia K562 resistant to adriamycin (K562/ADM), ovarian carcinoma cell line A2780 resistant to adriamycin (2780AD), or acute lymphoblastic leukemia cell line CCRF-CEM resistant to vinblastine (CEM-VLB100), compared with the drug-sensitive parent tumor cells. The human breast cancer cell lines Hattori and MCF-7 had a 12- to 63-fold lower level of glutathione S-transferase activity than K562, A2780, CCRF-CEM, and their drug-resistant sublines. Induction of ADM resistance in Hattori did not increase the activity of glutathione S-transferase. However, induction of colchicine resistance in MCF-7 resulted in a 70-fold increase in the activity of glutathione S-transferase. A revertant of the colchicine-resistant MCF-7 contained a level of glutathione S-transferase activity similar to that of the resistant subline. The increase of glutathione S-transferase activity did not alter the sensitivity of the cell to cytotoxic drugs. The increased activity was due to the appearance of glutathione S-transferase , as shown by enzyme inhibition using anti-glutathione S-transferase antibody. Our findings indicate that increased cellular glutathione S-transferase activity is not associated with the development of multidrug resistance.Abbreviations GST glutathione S-transferase - ADM adriamycin - VCR vincristine - VLB vinblastine     

Altered expression of topoisomerase IIalpha contributes to cross-resistant to etoposide K562/MX2 cell line by aberrant methylation

KRN 8602 (MX2) is a novel morpholino anthracycline derivative having the chemical structure 3'-deamino-3'-morpholino-13-deoxo-10-hydroxycarminomycin hydrochloride. To investigate the mechanisms of resistance to MX2, we established an MX2-resistant phenotype (K562/MX2) of the human myelogeneous leukaemia cell line (K562/P), by continuously exposing a suspension culture to increasing concentrations of MX2. K562/MX2 cells were more resistant to MX2 than the parent cells, and also showed cross-resistance to etoposide and doxorubicin. Topoisomerase (Topo) IIalpha protein levels in K562/MX2 cells were lower of those in K562/P cells on immunoblot analysis and decreased expression of Topo IIalpha mRNA was seen in K562/MX2 cells. Topoisomerase II catalytic activity was also reduced in the nuclear extracts from K562/MX2 cells when compared with K562/P cells. Aberrant methylated CpG of Topo IIalpha gene was observed in K562/MX2 cells when compared with the parent line on methylation-specific restriction enzyme analysis. To overcome the drug resistance to MX2 and etoposide, we investigated treatment with 5-Aza-2'-deoxycytidine (5AZ), which is a demethylating agent, in K562/MX2 cells. 5-Aza-2'-deoxycytidine treatment increased Topo IIalpha mRNA expression in K562/MX2 cells, but not in K562/P cells, and increased the cytotoxicity of MX2 and etoposide. Methylated CpG was decreased in K562/MX2 cells after 5AZ treatment. We concluded that the mechanism of drug resistance to MX2 and etoposide in K562/MX2 cells might be the combination of decreased expression of Topo IIalpha gene and increased methylation, and that 5AZ could prove to be a novel treatment for etoposide-resistant cell lines, such as K562/MX2.     

Synergistic effect of cyclosporin A and verapamil in overcoming vincristine resistance of multidrug-resistant cultured human leukemia cells

Reversal of vincristine (VCR) resistance by cyclosporin A (CyA) or the combination of CyA and verapamil (VER) was investigated by using four P-glycoprotein (P-gp)-associated human multidrug-resistant (MDR) cell lines (K562/ADM, KYO-1, HEL and CMK). Drug sensitivity was expressed as 50% inhibitory concentration (IC50). The degree of reversal of resistance was expressed as x-fold decrease by dividing the IC50 value without modifier(s) by that with modifier(s). CyA overcame P-gp-associated MDR significantly in all four MDR cell lines. Reversal of VCR resistance by CyA appeared to be dose-dependent. In the case of low-grade MDR cell lines (KYO-1, HEL and CMK), CyA at the low concentration of 0.5 microgram/ml was still effective. The degree of reversal of VCR resistance in this condition was greater (6.3- to 16-fold decrease) in the low-grade MDR cell lines than in a high-grade MDR cell line (K562/ADM) (2.9-fold decrease). At a high concentration (5 micrograms/ml) of CyA, however, it was greater (240-fold decrease) in the high-grade MDR cell lines than in the low-grade MDR cell line (20- to 100-fold decrease). This indicates that concentration of CyA required for overcoming drug resistance in MDR cells was dependent on the degree of drug resistance. CyA overcame VCR resistance more efficiently than VER. The combination of CyA and VER enhanced reversal of VCR resistance in a supra-additive or at least an additive manner and overcame VCR resistance at low concentrations of both modifiers that are clinically achievable with safety.     

Antiproliferating activity of the mitotic inhibitor pironetin against vindesine- and paclitaxel-resistant human small cell lung cancer H69 cells

Pironetin, isolated from Streptomyces sp., is a potent inhibitor of microtubule assembly and the first compound identified that covalently binds to alpha-tubulin at Lys352. We examined whether pironetin is an effective agent against human small cell lung cancer H69 cells, including two cell lines resistant to the microtubule-targeted drugs vindesine (H69/VDS) and paclitaxel (H69/Txl) that interact with beta-tubulin. Pironetin was found to be effective against these resistant cells as well as their parental cells. In addition, pironetin inhibited the growth of human leukemic K562 multidrug-resistant cells (K562/ADM), which have mdr1 gene expression, as well as the parental K562 cells. In these cell lines, including the parental and resistant cells, pironetin caused complete mitotic arrest; in addition, apoptosis inductions by 30 and 100 nM pironetin were observed. In this study, the new mitotic inhibitor, pironetin, was found to be effective not only against human tumor cell lines resistant to microtubule-targeted drugs, but also multidrug-resistant cells with mdr1 gene expression. These results suggest that pironetin is a useful agent for overcoming drug resistance in cancer chemotherapy.