麻黄碱逆转K562/A02细胞多药耐药性的研究

目的：观察非细胞毒性质量浓度的麻黄碱对耐药的白血病细胞株（KS62／A02）多药耐药性的逆转作用，并探讨其逆转机制。方法：用MTT法检测麻黄碱的细胞毒作用；用流式细胞仪检测非细胞毒性浓度的麻黄碱处理后K562／A02细胞膜表面糖蛋白P170表达及功能的变化。结果：麻黄碱对K562／A02有一定的细胞毒作用，其非细胞质量浓度（IC10）为75mg·L^-1，非细胞毒性质量浓度的麻黄碱对K562／A02细胞对阿霉素的耐药性有部分逆转作用（5．67倍），作用于K562／A02细胞后，细胞膜糖蛋白P170的表达从（85．3±5．5）％下调至（34．8±1．2）％，DNR外渗试验显示，细胞内化疗药物的质量浓度明显增加。结论：麻黄碱通过下调K562／A02细胞膜糖蛋白P170的表达，抑制其将化疗药物“泵”出细胞外的功能，提高化疗药物在K562／A02细胞内的有效质量浓度，能部分逆转K562／A02细胞的多药耐药性。     

阿霉素纳米粒对MRP介导的膀胱肿瘤多药耐药细胞株EJ/MRP多药耐药性的逆转作用

目的 研究阿霉素纳米粒对多药耐药相关蛋白(MRP)介导的膀胱肿瘤多药耐药的逆转作用.方法 采用四甲基偶氮唑盐(MTT)法测定药物的体外杀伤作用,应用流式细胞术测定细胞内药物浓度.结果 阿霉素纳米粒对EJ细胞的细胞毒作用与阿霉素相似,EJ/MRP 细胞对阿霉素纳米粒较阿霉素敏感4.00倍.结论 阿霉素纳米粒通过增加耐药细胞内阿霉素浓度而有效逆转多药耐药.     

红霉素逆转入肝癌细胞BEL—7402多药耐药性的研究

目的 研究红霉素（ERY）对BEL－7402细胞（人肝癌细胞）多耐药性的逆转作用。方法 将BEL－7402细胞连续培养在含阿霉素（ADM）的培养液中诱导耐药细胞株BEL－7402／ADM,用cell-ELASA法检测细胞膜表面P－gp的表达,细胞毒试验采用MTT法,用荧光分光光度法测定细胞内ADM浓度。结果 BEL－7402／ADM细胞表面P－gp高度表达,除对ADM耐药外,对长春新碱（VCR）和丝裂霉素（MMC）也有不同程度的交叉耐药;ERY可增强ADM、VCR、MMC对BEL－7402。／ADM细胞的增殖抑制作用,可增加BEL－7402／ADM细胞内ADM的浓度而对细胞表面P-gp的表达没有影响。结论 ERY通过竞争性地饱和BEL－7402／ADM细胞表面P-gp通道,使细胞内药物外排减少、浓度增加,从而发挥对BEL－7402／ADM细胞多药耐药性的逆转作用。     

黄芪皂苷Ⅱ对人肝癌多药耐药细胞BEL-7402/FU 的逆转耐药作用

摘要目的：探讨黄芪皂苷II（AstragalosideII,ASⅡ）对肝癌多药耐药性的逆转作用。方法：MTT法检测人肝癌多药耐药细胞BEL-7402／FU对化疗药物的敏感性和逆转耐药倍数,RT-PCR法检测多药耐药蛋白-1（MDRl）基因表达,免疫细胞化学法检测P一糖蛋白（P-gP）蛋白表达水平,流式细胞仪检测AS1I对细胞内Rhodaminel23的蓄积影响。结果：BEL-7402／FU细胞对5-氟尿嘧啶（5-FU）、阿霉素、丝裂霉素的耐药倍数分别为19．64、1．98、1．92。0．04、0．08mg／mL的ASⅡ能增强5-FU对BEL-7402／FU的细胞毒作用,逆转耐药倍数分别为1．49,1．81。0．08、0．16mg／mL的ASⅡ作用BEL-7402／FU细胞24h后,能下调MDRlmRNA和P-gP蛋白表达水平,提高细胞内Rhodaminel23的荧光表达率。结论：ASⅡ能部分逆转肝癌多药耐药性,其机制可能与下调MDRl1TIRNA表达及抑制P-gP的功能与表达有关。     

阿霉素纳米粒对白血病多药耐药细胞株K562/ADR耐药性的逆转作用

目的:研究阿霉素纳米粒对人白血病多药耐药细胞株K562/ADR耐药性的逆转作用.方法:采用MTT法测定药物的体外杀伤作用,应用流式细胞术测定细胞内药物浓度.结果:阿霉素纳米粒和阿霉素对K562细胞细胞毒作用相似,K562/ADR对阿霉素纳米粒较阿霉素敏感2.63倍,细胞内阿霉素浓度显著增加可能是关键因素.结论:阿霉素纳米粒通过增加耐药细胞内阿霉素浓度有效逆转多药耐药.     

黄芪甲苷逆转耐药肝癌细胞株HepG2/GCS多药耐药的研究

目的探讨黄芪甲苷对耐药肝癌细胞株HepG2/GCS多药耐药的逆转作用及可能机制。方法以噻唑蓝(MTT)法测定黄芪甲苷的细胞毒作用和处理前后肝癌细胞对阿霉素的敏感性变化,通过Westernblot法检测细胞GCS蛋白的表达。结果黄芪甲苷在实验浓度范围内对HepG2、HepG2/GCS细胞均无明显毒性;浓度为40μg/ml的黄芪甲苷可逆转HepG2/GCS细胞对阿霉素的耐药性,逆转倍数为1.50,同时细胞内GCS蛋白的表达下降。结论黄芪甲苷具有逆转HepG2/GCS细胞多药耐药的作用,可能与下调细胞内GCS基因表达有关。     

板蓝根高级不饱和脂肪酸对耐药肝癌细胞株BEL-7404/ADM逆转作用实验

目的：板蓝根高级不饱和脂肪酸对肝癌耐经细胞株BEL－7404／ADM耐药逆转作用主机制研究。方法：MTT法观察药物对细胞的生长抑制作用；免疫组化法检测细胞P－糖蛋白（P－gp)和多药耐药相关蛋白（MRP）的表达；采用高效液相色谱法（HPLC）测定细胞内ADM的浓度。结果：耐药肝癌细胞株BEL－7404／ADM对多种化疗药物产生多药耐药性，耐药细胞P－gp和MRP的阳性率显著高于亲本细胞（P＜0．01）；板蓝根高级不饱和脂肪酸在非细胞毒剂量时与ADM合用后能逆转BEL－7404／ADM对ADM的耐药性；ADM与板蓝根高级不饱和脂肪酸合用时，其细胞内ADM的含量较单独使用明显增高。结论：肝癌细胞株BEL－7404／ADM对多种化疗药物具有多药耐药性，其多药耐药性与P－gp和MRP过量表达有关；板蓝根高级不饱和脂肪酸在非细胞毒剂量时能逆转BEL－7404／ADM对ADM的耐药性，其逆转作用可能与降低P－gp药物外排功能、增加细胞内药物浓度有关。     

重组人p53腺病毒注射液逆转人耐药胃癌MGC-803细胞耐药性的体外试验研究

目的:研究重组人p53腺病毒注射液(rAd-p53)对体外人耐药胃癌MGC-803细胞的逆转作用及其可能的机制。方法:通过紫杉醇由低到高剂量诱导人胃癌MGC-803细胞内多耐药基因表达;不同剂量rAd-p53作用于人耐药胃癌MGC-803细胞不同时间后,MTT法检测细胞增殖抑制率,并计算半数有效抑制浓度(IC50),流式细胞仪检测细胞周期及凋亡情况,免疫组织化学法和蛋白质印迹法测定多药耐药基因mdr1表达蛋白P糖蛋白(MDR1-Pgp)的表达。结果:rAd-p53可明显抑制人耐药胃癌MGC-803细胞增殖,呈时间-剂量依赖关系,作用24、48、72h的IC50分别为1889.85、998.44、354.91MOI;rAd-p53可阻滞人耐药胃癌MGC-803细胞周期于G2/M期并诱导其凋亡,可下调MDR1-Pgp蛋白表达。结论:人耐药胃癌MGC-803细胞的耐药性可能与MDR1-Pgp的高表达有关;rAd-p53可显著抑制人耐药胃癌MGC-803细胞的增殖并诱导耐药细胞凋亡,且呈剂量和时间依赖关系。     

槐耳颗粒逆转人乳腺癌细胞MCF-7耐药的初步机制

目的研究槐耳颗粒逆转乳腺癌细胞株MCF-7耐药的初步机制。方法使用四甲基偶氮唑蓝（MTT）比色法测定敏感／耐药乳腺癌细胞MCF-7-S／A对单药阿霉素（ADM）和槐耳颗粒的药物毒性,耐药倍数和槐耳颗粒对MCF-7／A的耐药逆转倍数,分别采用荧光定量逆转录-聚合酶链反应和免疫组化SP法测定多药耐药基因mdr1、多药耐药相关蛋白基因MRP的mRNA和其相应的表达产物P-gp、MRP蛋白,在MCF-7／S及非细胞毒性剂量的槐耳颗粒处理前后MCF-7／A上的表达。结果非细胞毒性剂量（0．01mg／ml）的槐耳颗粒能显著降低ADM对MCF-7／A的IC50（5．06μm）,与逆转前MCF-7／A的IC50（25．8μm）,相比差异有统计学意义（P〈0．01）,其逆转倍数为5．1倍;0．01mg／ml的槐耳颗粒使MCF-7／A细胞的耐药基因mdr1、MDR-1的mRNA以及相应的P-gp、MRP蛋白的表达水平均下调,与未加槐耳颗粒处理的对照组MCF-7／A相比有显著性差异（P〈0．01）。结论非细胞毒性剂量槐耳颗粒具有逆转MCF-7／A细胞耐药性的作用,逆转机制和其耐药基因mdr1、MDR-1的mRNA以及相应的P-gp、MRP蛋白的表达水平下调相关,暗示槐耳颗粒是一种有潜力的耐药逆转剂。     

苦瓜蛋白逆转K562／AO2细胞多药耐药性的研究

目的：研究非细胞毒性质量浓度苦瓜蛋白对耐阿霉素的人红白血病细胞株K562／AO2多药耐药性的逆转作用和促凋亡作用。方法：采用CCK-8法测定苦瓜蛋白的细胞毒性及其对K562／AO2细胞敏感性的影响,用流式细胞仪检测K562／AO2细胞经不同药物处理后细胞的凋亡情况。结果：苦瓜蛋白对K562／AO2细胞有一定的细胞毒作用,其非细胞毒性质量浓度为5μg／mL,非细胞毒性质量浓度苦瓜蛋白对K562／AO2细胞对阿霉素、长春新碱（VCR）和柔红霉素的耐药性都有部分逆转作用（分别为5．4、6．5和4．0倍）;5μg／mL苦瓜蛋白联合VCR诱导K562／AO2细胞凋亡,凋亡率为（19．38±1．06）％,而对照组为（1．64±0．27）％,单一苦瓜蛋白组为（3．79±0．82）％,单一VCR组为（9．83±0．98）％。结论：苦瓜蛋白能部分逆转人红白血病K562／AO2细胞对阿霉素、VCR和柔红霉素的耐药,一定剂量的苦瓜蛋白与VCR联合应用可增加肿瘤细胞凋亡率。     

Effect of curcumin on multidrug resistance in resistant human gastric carcinoma cell line SGC7901/VCR

AIM: To investigate the reversal effects of curcumin on multidrug resistance (MDR) in a resistant human gastric carcinoma cell line. METHODS: The cytotoxic effect of vincristine (VCR) was evaluated by MTT assay. The cell apoptosis induced by VCR was determined by propidium iodide (PI)-stained flow cytometry (FCM) and a morphological assay using acridine orange (AO)/ethidium bromide (EB) dual staining. P-glycoprotein (P-gp) function was demonstrated by the accumulation and efflux of rhodamine123 (Rh123) using FCM. The expression of P-gp and the activation of caspase-3 were measured by FCM using fluorescein isothiocyanate (FITC)-conjugated anti-P-gp and anti-cleaved caspase-3 antibodies, respectively. RESULTS: Curcumin, at concentrations of 5 micromol/L, 10 micromol/L, or 20 micromol/L, had no cytotoxic effect on a parent human gastric carcinoma cell line (SGC7901) or its VCR-resistant variant cell line (SGC7901/VCR). The VCR-IC50 value of the SGC7901/VCR cells was 45 times more than that of the SGC7901cells and the SGC7901/VCR cells showed apoptotic resistance to VCR. SGC7901/VCR cells treated with 5 micromol/L, 10 micromol/L, or 20 micromol/L curcumin decreased the IC50 value of VCR and promoted VCR-mediated apoptosis in a dose-dependent manner. Curcumin (10 micromol/L) increased Rh123 accumulation and inhibited the efflux of Rh123 in SGC7901/VCR cells, but did not change the accumulation and efflux of Rh123 in SGC7901 cells. P-gp was overexpressed in SGC7901/VCR cells, whereas it was downregulated after a 24-h treatment with curcumin (10 micromol/L). Resistant cells treated with 1 mumol/L VCR alone showed 77% lower levels of caspase-3 activation relative to SGC7901 cells, but the activation of caspase-3 in the resistant cell line increased by 44% when cells were treated with VCR in combination with curcumin. CONCLUSION: Curcumin can reverse the MDR of the human gastric carcinoma SGC7901/VCR cell line. This might be associated with decreased P-gp function and expression, and the promotion of caspase-3 activation in MDR cells.     

Oleanolic acid derivative methyl 3,11-dioxoolean-12-en-28-olate targets multidrug resistance related to ABCB1

Multidrug resistance (MDR) in leukemia patients is a great incentive to the development of new drugs. In a search for potential multidrug resistance modulators we tested a group of oleanolic acid (OA) analogues modified at C-3, C-11, C-12 and C-28 using an experimental model consisting of three human acute lymphoblastic leukemia cell lines (CCRF-CEM and the multidrug resistant sublines CCRF-VCR1000 and CCRF-ADR5000). The most effective compound, methyl 3,11-dioxoolean-12-en-28-olate (DIOXOL) was more potent in cell viability inhibition than its precursor - OA, and showed similar or even higher activity in the drug resistant than in the wild-type cells. Resistance factor (RF) values obtained for CCRF-VCR1000 and CCRF-ADR-5000 cells using MTT assay were 0.7 and 0.8 (24 h of treatment) and after 72 h of treatment 0.9 and 1.1, respectively. Moreover, 5 μM DIOXOL significantly reduced the expression of the ABCB1 gene in MDR cells by around 30%, and also decreased the level of P-gp protein. Compared to untreated control cells, DIOXOL treatment resulted in a significant P-gp decrease (30% in CCRF-ADR5000 and 50% in CCRF-VCR1000), that was detected by western blot and confirmed by flow cytometry analysis. Moreover, DIOXOL (at 10 μM) significantly inhibited P-gp transport function by more than twofold comparing to control, untreated cells that was demonstrated using rhodamine 123-based functional test. The compound exhibited synergistic activity with ABCB1 substrate - adriamycin in CCRF-VCR1000 cells, indicating partial but significant MDR reversing ability.     

Benzo[a]phenoxazines: a new group of potent P-glycoprotein inhibitors

The ability of fifteen novel phenoxazine derivatives (four phenoxazines and eleven benzo[a]phenoxazines) to modulate multidrug resistance (MDR) in a P-gp-overexpressing mouse T lymphoma cell line (L5178 MDR) was studied. A flow cytometric functional test, based on the differential accumulation of rhodamine 123 by sensitive and multidrug-resistant cells, was employed. Seven benzo[a]phenoxazines were observed to increase the amount of rhodamine 123 accumulated by resistant cells, i.e. to be new effective MDR modulators. The results allowed us to draw preliminary conclusions about the structural features of benzo[a]phenoxazines which are important for MDR modulation.     

N-糖基取代的邻苯二甲酰亚胺新衍生物的合成与肿瘤多药耐药逆转作用研究

沙利度胺（α-N-phthalimido-glutarimide，TLD）是一种具有抗血管生成和抗炎作用的药物，对多种实体瘤有效。本文研究了N-糖基取代的沙利度胺新衍生物（STA-35）对阿霉素（doxorubicin，ADR）引起的多药耐药（multidurg resistance，MDR）的调节作用。采用SRB法检测化合物对癌细胞的增殖抑制作用，应用流式细胞术测定P-糖蛋白（P-glycoprotein，P—gp）的功能，以免疫印迹方法考察P—gP的蛋白表达。实验结果表明，STA-35能够抑制人乳腺癌细胞MCF-7及其ADR耐药细胞MCF-7／ADR生长，耐药指数仅为1．19；并能增强MCF-7／ADR细胞对ADR的敏感性。此外，STA-35可以增加MCF-7／ADR细胞内罗丹明123（rhodamine 123，RH123）的聚积，减弱P—gP的功能，抑制P-gp的蛋白表达。该化合物具有多药耐药逆转作用，其分子机制可能与抑制P—gp的功能和蛋白表达相关。     

SB203580, a specific inhibitor of p38-MAPK pathway, is a new reversal agent of P-glycoprotein-mediated multidrug resistance.

P-glycoprotein (P-gp) is the plasma membrane transport pump responsible for efflux of chemotherapeutic agents from cells and is one of the systems that secures multidrug resistance (MDR) of neoplastic cells. In the present study, drug sensitive L1210 and multidrug resistant L1210/VCR (characterized by overexpression of P-gp) mouse leukemic cell lines were used as an experimental model. We have found that SB203580, a specific inhibitor of p38-MAPK pathway, significantly reduced the degree of the vincristine resistance in L1210/VCR cells. This phenomenon was accompanied by a decrease in the LC(50) value of vincristine from 3.203+/-0.521 to 0.557+/-0.082 microM. The LC(50) value of sensitive cells for vincristine was about 0.011 microM. The effect of SB203580 on L1210/VCR cells was associated with significantly increased intracellular accumulation of [3H]-vincristine in the concentration dependent manner. Prolonged exposure of resistant cells to 30 microM SB203580 did neither significantly influence the gene expression of P-gp, nor change the protein levels of p38-MAPK. Western blot analysis revealed that the MDR phenotype in L1210/VCR cells was associated with increased level and activity of cytosolic p38-MAPK. In resistant cells, the enhanced phosphorylation of both, p38-MAPK and ATF-2 (endogenous substrate for p38-MAPK) was found as well. In conclusion we could remark that SB203580, an inhibitor of p38 kinase pathway, reversed the MDR resistance of L1210/VCR cells. MDR phenotype of these cells is connected with increased levels and activities of p38-MAPK. These findings point to the possible involvement of the p38-MAPK pathway in the modulation of P-gp mediated multidrug resistance in the L1210/VCR mouse leukemic cell line. However, the mechanisms of SB203580 action should be further investigated.     

ZNF300基因在肝癌耐药细胞HepG2/VCR中的表达及功能初探

目的建立人肝癌HepG2/VCR耐药细胞株,检测ZNF300基因在HepG2/VCR中的表达并初步分析其在肝癌多药耐药(MDR)中发挥的功能。方法采用体外低浓度梯度递增的诱导方法建立长春新碱(VCR)获得性HepG2/VCR耐药细胞株。MTT法检测确定HepG2/VCR耐药细胞株对VCR的耐药性,用Western blot方法检测人锌指蛋白ZNF300基因编码的ZNF300及多药耐药基因编码的P糖蛋白(P-gp)在HepG2和HepG2/VCR细胞中的表达差异;在HepG2/VCR细胞中转染ZNF300基因正向或反向cDNA质粒后,MTT法检测VCR对耐药细胞IC50值的变化,Westernblot方法检测细胞内P-gp表达的影响。结果 MTT检测确认HepG2/VCR耐药细胞构建成功,Western blot检测发现耐药细胞中ZNF300及P-gp的表达相对于HepG2细胞明显增高。在HepG2/VCR细胞中转染正向ZNF300 cDNA质粒后,MTT和Western blot检测发现ZNF300过表达可使VCR对耐药细胞的IC50值增高,并使细胞内P-gp表达上调;在转染反向cDNA质粒Knockdown ZNF300基因表达后得到相反的结果。结论 ZNF300基因在HepG2/VCR耐药细胞中表达明显增高,并能通过上调耐药蛋白P-gp的表达促进肝癌细胞耐药性,可以作为逆转肝癌多药耐药的分子作用靶点。     

姜黄素衍生物C15对白血病K562/A02细胞多药耐药的逆转作用

目的: 探讨姜黄素衍生物C15对人白血病K562/A02细胞多药耐药(multidrug resistance,MOR)的逆转作用及其作用机制。方法: 四甲基偶氮唑蓝(MTT)法检测细胞增殖;流式细胞术检测P-糖蛋白(P-gp)外排泵功能和细胞周期;免疫印迹法检测蛋白表达;P-gp-Glo^TM Assay System试剂盒检测P-gp ATP水解酶(ATPase)活性。结果: C15对K562/A02细胞半数抑制浓度(IC₅₀)大于50 μmol·L^-1。对K562/A02细胞无明显细胞毒的浓度为2.5,5.0,10.0 μmol·L^-1的C15逆转对K562/A02细胞对阿霉素(ADR)耐药的倍数分别为2.60,5.39,11.39,对长春新碱(vincristine, VCR)耐药的倍数分别为4.50,18.07,124.35,但是对非P-gp底物的化疗药物顺铂(cisplatin, CIS)和敏感细胞K562基本无逆转效果。2.5,5.0,10.0 μmol·L^-1 C15可以增加耐药细胞K562/A02胞内罗丹明123(Rh-123)的蓄积量分别为1.93,2.30,2.47倍。C15增加阿霉素(adriamycin, ADR)在K562/A02细胞中的蓄积水平,降低P-gp介导的Rh-123外排速率。2.5,10.0 μmol·L^-1 C15与300 nmol·L^-1VCR联合作用后,可使K562/A02细胞的G2/M期比例从9.36%增加到67.57%和69.38%。C15对P-gp蛋白和ATPase的活性没有抑制作用。结论: C15可能是P-gp的非衣物型抑制剂,且具有逆转K562/A02细胞MDR的作用,该作用与其抑制细胞P-gp的外排泵功能有关。     

Reversal of P-gp mediated multidrug resistance in-vitro and in-vivo by FG020318

Overexpression of P-glycoprotein (P-gp) by tumours results in multidrug resistance (MDR) to structurally and functionally unrelated chemotherapeutic drugs. Combined therapy with MDR-related cytotoxins and MDR modulators is a promising strategy to overcome clinical MDR. This study was performed to explore the MDR reversal activity of a novel compound 2-[4-(2-pyridin-2-yl-vinyl) phenyl]-4,5-bis-(4-N,N-diethylaminophenyl)-1(H)-imidazole (FG020318) in-vitro and in-vivo. Tetrazolium (MTT) assay was used to evaluate the ability of FG020318 to reverse drug resistance in two P-gp-expressing tumour cell lines, KBv200 and MCF-7/adr. Intracellular doxorubicin accumulation was determined by fluorescence spectrophotometry in MCF-7/adr cell line. The effect of FG020318 on P-gp function was demonstrated by rhodamine 123 (Rh123) accumulation in KBv200 cells. KBv200 cell xenograft models were established to study the in-vivo effect of FG020318 on reversing MDR. FG020318 was not cytotoxic by itself against P-gp expressing KBv200 cells and MCF-7/adr cells and their parental drug-sensitive KB cells and MCF-7 cells. FG020318 could significantly increase the sensitivity of MDR cells to antitumour drugs including doxorubicin and vincristine in MCF-7/adr cells and KBv200 cells, respectively. It was much stronger than the positive control verapamil in reversal of MDR. FG020318 also increased the intracellular accumulation of doxorubicin in a concentration-dependent manner in MCF-7/adr cells, but did not affect the accumulation of doxorubicin in drug-sensitive MCF-7 cells. The Rh123 accumulation in resistant KBv200 cells was also increased by the addition of FG020318, but Rh123 accumulation was not affected by FG020318 in drug-sensitive KB cells. FG020318 potentiated the antitumour activity of vincristine to KBv200 xenografts and was an efficacious modulator in-vivo. Our results suggested that FG020318 was a highly potent, efficacious MDR modulator not only in-vitro but also in-vivo. The reversal of drug resistance by FG020318 was probably related to the increased anticancer drug accumulation and its inhibition of P-gp function of MDR tumour cells.     

Differential cytotoxic effects of docetaxel in a range of mammalian tumor cell lines and certain drug resistant sublinesin vitro

Thein vitro cytotoxic effects of docetaxel (Taxotere^®; RP56976, NSC688503) proved both time and concentration dependent. Amongst thirteen human cell lines from various tumor types, exposure to increasing concentrations of docetaxel over 24 hrs resulted in a plateau-shaped dose response curve, suggesting that increased cell kill becomes more dependent on increased exposure duration than on concentration. IC₅₀ concentrations (reducing survival by 50%) ranged from 0.13–3.3 ng/ml, with three neuroblastoma lines proving most sensitive and three breast and two colon carcinoma lines showing least sensitivity. There was significant cross-resistance to docetaxel in the classic multidrug resistant (MDR) Chinese hamster ovarian (CHO) CHRC5 line and the human lymphoblastoid CCRF-CEMVLB1000 line, as well as in two vincristine(VCR)-selected MDR MCF-7 sublines. All four of these MDR sublines overexpress P-glycoprotein (Pgp), as did a 6fold docetaxel-selected resistant CHO subline. As an apparent corollary, in two human teratoma lines selected for etoposide resistance and showing some cross-resistance to VCR and in two CHO sublines expressing low levels of VCR resistance, yet all proving Pgp positive, no docetaxel crossresistance was identified. Verapamil modulated docetaxel resistance only in sublines expressing resistance to the drug and overexpressing Pgp. Four other human tumor sublines selected for resistance to 5-fluorouracil, cisplatin or teniposide, showed a lack of cross-resistance to docetaxel. Furthermore, cross-resistance to docetaxel was not apparant in four epipodophyllotoxin-selected resistant sublines with alterations in topoisomerase II, indicating its effectiveness against tumor cells expressing the topoisomerase II-related MDR phenotype. Our observation that docetaxel cross-resistance was not automatically expressed by classic MDR tumour cells appears of interest and of potential clinical relevance.