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。     

川芎嗪联合三氧化二砷逆转K562/ADM细胞多药耐药的实验研究

目的探讨川芎嗪与三氧化二砷联合逆转耐药人红白血病细胞株K562/ADM多药耐药的效果。方法采用WST-8法测定细胞的药敏性及抗药性逆转,应用流式细胞术检测细胞凋亡、细胞内ADM浓度、P-gp蛋白的表达,采用免疫细胞化学二步法检测细胞GST-π表达。结果非细胞毒性浓度的TMP（20μg/ml）及As2O3（0.5μmol/L）可降低ADM对K562/ADM细胞的IC50（P〈0.05）,2种药物联合应用对ADM的逆转倍数明显高于两者单独应用（P〈0.05）,而且也高于两者单独应用之和;两者以非细胞毒性浓度联合应用提高K562/ADM细胞内ADM浓度和细胞凋亡百分率,作用大于两药单独应用,并且明显下调细胞P-gp和GST-π表达（P〈0.05,P〈0.01）。结论非细胞毒性剂量的TMP和As2O3,均可部分逆转有多药耐药表型的细胞株K562/ADM对阿霉素的耐药性,两者联合应用效果优于单独应用,具有协同作用,其机制可能与下调P-gp和GST-π表达有关。     

人骨肉瘤耐药细胞系MG-63/PTX的建立及多药耐药机制研究

目的 建立人骨肉瘤细胞多药耐药亚系,探讨骨肉瘤细胞多药耐药的发生机制.方法 采用紫杉醇大剂量间断冲击培养法,诱导建立紫杉醇耐药骨肉瘤细胞系(MG-63/PTX),并运用MTT法检测6种药物敏感性变化及细胞生长规律变化,流式细胞仪分析亲本细胞系和耐药细胞系的细胞膜表面P-gp蛋白表达率、罗丹明123排出情况,RT-PCR检测细胞内耐药基因的变化,免疫细胞化学法检测细胞内P-gp蛋白的表达,Westernblotting法检测细胞内P-gp蛋白的表达.结果 历时12个月建成人骨肉瘤耐药细胞系MG-63/PTX,能在含PTX1μg/mL的培养基中稳定生长并传代,其对PTX的耐药指数为69.8,并与卡铂、顺铂、表柔比星、甲氨蝶呤、阿霉素等多种化疗药物存在不同程度的交叉耐药性;罗丹明123排出实验显示MG-63/PTX内罗丹明含量明显低于亲本细胞;RT-PCR结果显示在MG-63/PTX细胞系中存在MDR1、MRP1、LRP、ABCG2基因的表达,而在MG-63细胞系中无MDR1基因的表达.免疫细胞化学、流式细胞仪及Western blotting实验均显示在MG-63/PTX中存在P-gp的表达.结论 MDR1/ P-gp在多药耐药的特性上起着至关重要的作用,而且骨肉瘤多药耐药细胞亚系为进一步研究骨肉瘤耐药特征及逆转方法打下基础.     

ID3基因在逆转骨肉瘤顺铂耐药中的作用及其机制的研究

目的 探讨ID3基因在逆转骨肉瘤顺铂耐药中的表达及对骨肉瘤细胞耐药、凋亡及RhoE、P-gp、Cas-pase-3表达的影响.方法 MTT法检测顺铂处理对U-2 OS及U-2 OS/DDP细胞生长功能的影响,qRT-PCR及Western blot法检测U-2 OS及U-2 OS/DDP细胞中ID3的表达;用ID3过表达载体转染U-2 OS/DDP细胞,以空脂质体转染的细胞作为对照组,各组细胞培养24 h后,用Western blot法检测细胞中RhoE、P-gp、Caspase-3蛋白表达变化;MTT检测耐药细胞耐药性的逆转作用;流式细胞仪检测细胞凋亡.结果 顺铂耐药株U-2 OS/DDP细胞耐药能力明显高于U-2 OS细胞;ID3在顺铂耐药株U-2 OS/DDP细胞表达水平低于U-2 OS细胞;ID3基因过表达对骨肉瘤细胞耐药性具有逆转作用;ID3基因过表达可促进U-2 OS/DDP细胞凋亡;ID3过表达后,P-gp的表达下调,RhoE和Caspase-3的表达上调,差异有统计学意义(P﹤0.05).结论 ID3基因在顺铂耐药株U-2 OS/DDP细胞中表达水平较低,过表达ID3基因能逆转U-2 OS/DDP细胞耐药情况,并通过调节RhoE、P-gp、Caspase-3蛋白表达促进细胞凋亡,其可能的机制与耐药蛋白的调控有关.     

人肺腺癌紫杉醇耐药细胞株的建立及其生物学特性研究

目的建立耐紫杉醇的人肺腺癌细胞株模型SPC-A1/Taxol,并初步研究其生物学特性。方法应用浓度递增和短时作用法,从人肺腺癌细胞株SPC-A1中诱导分离出对紫杉醇耐药的细胞亚株(SPC-A1/Taxol)。采用MTT法检测耐药细胞的耐药指数及对抗癌药物的敏感性;光镜和透射电镜观察细胞形态;以流式细胞术检测该耐药细胞的细胞周期分布;以免疫细胞化学染色检测多药耐药基因(mdr1)的表达产物P-糖蛋白(P-gp)。结果经MTT法检测,SPC-A1/Taxol细胞的紫杉醇半数抑制浓度(IC50)是亲代SPC-A1细胞的759.46倍,对Taxotere、NVB、ADM呈高度耐药状态,对VP-16和HCPT有轻度的耐药,而对DDP、GEM以及中药榄香烯乳无交叉耐药现象。耐药细胞的群体倍增时间是亲代细胞的1.32倍;细胞形态未见明显改变;G0 G1细胞比例减少,S期细胞增多。SPC-A1细胞无P-gp表达,SPC-A1/Taxol则高表达P-gp。结论SPC-A1/Taxol肺腺癌细胞株是一个明确的多药耐药模型,具有耐药细胞的基本生物学特性,推测其多药耐药性与P-gp的高表达相关。     

虎杖苷逆转人结肠癌细胞株HT-29奥沙利铂耐药性及机制

目的:探讨虎杖苷对结肠癌耐药细胞株HT-29奥沙利铂(oxaliplatin,OXA)耐药性的逆转作用并探讨其可能的作用机制。方法:采用逐步增加药物浓度的方法建立奥沙利铂耐药结肠癌细胞株HT-29/OXA。MTT和CCK-8法测定虎杖苷对HT-29/OXA细胞的耐药性逆转作用,流式细胞术检测细胞凋亡、周期变化,qRT-PCR 检测各组细胞LRP(lung resistance protein)和P-gp(P-glycoprotein)mRNA表达水平,Western blot 检测各组细胞LRP和P-gp蛋白的表达水平。结果:虎杖苷使HT-29/OXA细胞对奥沙利铂的敏感性增加,耐药性得到部分逆转(P＜0.05)。联合应用对结肠癌耐药细胞株HT-29/OXA生长增殖具有明显抑制作用并且能够通过改变细胞周期引起凋亡(P＜0.05)。作用后LRP mRNA和P-gp mRNA表达水平降低,同时下调了LRP和P-gp 蛋白表达(P＜0.05)。结论:虎杖苷部分逆转HT-29/OXA细胞对奥沙利铂的耐药性,其机制与降低细胞内LRP基因从而导致P-gp 的表达降低有关。     

二藤散结方对SGC7901/VCR的多药耐药影响及与P-gp表达的关系

目的 观察二藤散结方对人胃癌多药耐药细胞SGC7901/VCR的影响及与P-糖蛋白(P-gp)表达的关系.方法 体外培养SGC7901/VCR,采用MTT法、流式细胞仪、免疫组化法检测二藤散结方对SGC7901/VCR的影响及与细胞膜P-gp表达的关系.结果 二藤散结方能逆转SGC7901/VCR的多药耐药性,增加SGC7901/VCR细胞内ADM浓度,下调SGC7901/VCR细胞膜P-gp的表达.结论 二藤散结方能逆转SGC7901/VCR的多药耐药性,逆转机制与细胞膜P-gp表达减弱有关.     

热疗联合 IL-2逆转人肺腺癌细胞 A549/CDDP 的作用及其可能机制

目的：观察热疗联合白介素-2（IL-2）对人肺腺癌细胞株 A549/ CDDP 的耐药逆转作用,并探讨其可能作用机制。方法采用细胞培养技术,分别培养人肺腺癌细胞株 A549及其耐药细胞株 A549/ CDDP。42℃热疗,联合或不联合200 u·mL -1的 IL-2,同时在2μg·mL -1的 CDDP 作用下,分别干预敏感细胞株和耐药细胞株2 h 后,采用 MTT 法检测2种不同细胞对 CDDP 的敏感性,流式细胞仪间接免疫荧光法检测热疗、IL-2等不同条件作用下细胞中 P -糖蛋白（P-gp）、多药耐药蛋白（MRP）、肺耐药蛋白（LRP）的表达差异,以及细胞内荧光药物 CDDP 的聚集量的变化。结果分别联用热疗、IL-2时,较单用 CDDP,A549/ CDDP细胞的抑制率得到提高,A549/ CDDP 细胞 P-gp、MRP 表达下降,细胞内荧光强度增强,差异均有统计学意义（P 均﹤0.05）。热疗联合 IL-2合用 CDDP 时,A549/ CDDP 细胞的抑制率进一步提高,A549/ CDDP 细胞P-gp、MRP 表达明显下降,细胞内荧光强度显著增强,差异均有统计学意义（P 均﹤0.05）,但 LRP 的表达差异无统计学意义（P ﹥0.05）。结论热疗、IL-2分别能部分逆转 A549/ CDDP 细胞对 CDDP 的耐药性;热疗联合 IL-2可进一步增强其耐药逆转效应。其逆转耐药机制,推测可能与抑制 P-gp、MRP 表达,增加细胞内药物 CDDP 的积聚有关。     

三苯氧胺对淋巴细胞mdr1表达阳性的非小细胞肺癌患者多药耐药性的逆转作用

目的:探讨经多次化疗的非小细胞肺癌(NSCLC)肿瘤细胞与外周血淋巴细胞(PBL)多药耐药基因(mdrl)表达蛋白P-gp含量及其相关性;临床观察三苯氧胺对已产生多药耐药的NSCLC患者耐药性的逆转作用.方法:应用流式细胞免疫学方法,对57例多次接受过化疗的NSCLC肿瘤组织细胞及PBL的P-gp含量进行定量研究,对PBL中P-gp表达阳性的48例患者均采用EMP方案化疗,其25例在化疗过程中同时予三苯氧胺口服,观察其临床治疗效果.结果:肿瘤细胞P-gp含量与其对应的PBL的P-gp含量呈正相关,PBL的P-gp含量可间接反应NSCLC细胞对化疗药物的耐受程度;三苯氧胺可部分逆转NSCLC细胞的耐药性,显著提高EMP方案对耐药的NSCLC患者的疗效.结论:通过检测NSCLC患者PBL P-gp的含量可间接反应肿瘤细胞的耐药程度,对已耐药的患者在化疗的同时合用三苯氧胺可明显提高化疗方案的有效率.     

LY294002与化疗药物联用对白血病K562细胞多药耐药的逆转作用

背景与目的: 传统逆转白血病多药耐药的药物由于不良反应大而限制了其在临床中的应用,进一步研究白血病多药耐药产生机制和有效逆转靶点成为攻克白血病多药耐药的关键.为此,本研究探讨LY294002[磷脂酰肌醇-3-激酶(P13-K/Akt)通路抑制剂]对人类白血病K562细胞多药耐药的逆转作用.方法: 锥虫蓝拒染法测定细胞生长增殖.Western印迹榆测K562/S和K562/D细胞中P-gp及p-Akt的表达.流式细胞术检测细胞内药物积聚.结果: K562/D细胞对柔红霉素(DNR)、多柔比星(ADR)、长春新碱(VCR)、依托泊苷(VP16)交叉耐药,相对其亲本细胞的耐药倍数分别为65、52、134和50.DNR诱导了K562/D细胞的P-gp、p-Akt过度表达.LY294002使K562/D细胞内药物积聚增加,部分逆转了K562/D细胞对DNR、ADR、VCR、VP16的耐药性(相对耐药倍数降至23、21、63和29),而对敏感细胞K562/S的耐药性无影响. 结论:LY294002部分逆转K562/D细胞的多药耐药,可能于DNR诱导K562/D细胞的P-gp、p-Akt过度表达而LY294002抑SUP13-K/Akt信号转导通路有关.     

蛋白激酶C激活剂及抑制剂对紫杉醇耐药的卵巢癌细胞中P-糖蛋白表达及功能的影响

目的:探讨PKC(protein kinase C)激活剂及抑制剂对紫杉醇诱导的卵巢癌耐药细胞系A2780/Taxol中P-gp(P-glyco-protein)的表达及功能的影响。方法:免疫细胞化学SP法检测P-gp和PKC-α在耐药细胞A2780/Taxol及其亲本细胞中的表达;Western Blot检测PKC激动剂佛波脂(PMA)和抑制剂十字孢碱(SP)作用后P-gp在2种细胞上的表达水平;以罗丹明123(Rohdamin123,R123)作为荧光探针用流式细胞仪检测PMA及SP对细胞膜P-gp功能的影响。结果:P-gp在耐药细胞A2780/Taxol中呈阳性表达,在亲本细胞中不表达;PMA处理细胞,细胞内R123的平均荧光强度明显减低,Pgp的功能增强而表达量无明显变化;SP处理细胞,细胞内R123的平均荧光强度明显增加,Pgp的功能减弱而表达也无明显减少。结论:PKC通过对P-gp功能的调节而参与卵巢癌紫杉醇耐药的形成。     

Overexpression of sorcin results in multidrug resistance in gastric cancer cells with up-regulation of P-gp

Sorcin, a calcium-binding protein was found up-regulated in the vincristine-induced multi-drug resistance (MDR) gastric cancer cell line SGC7901/VCR, over its parental SGC7901 cells in our previous proteomic studies. The present study explored the role and mechanism of sorcin in the development of MDR in gastric cancer. We constructed the recombinant plasmids FLAG-sorsin-pcDNA3.1 containing the full open reading frame of sorcin and a FLAG affinity tag. Overexpression of sorcin by gene transfection was able to confer drug resistance to vincristine, adriamycin, taxol and 5-fluorouracil in SGC7901 cells. Down-regulation of sorcin expression by sorcin antisense oligonucleutides, (ASO) increased sensitivity to vincristine. The intracellular concentration of vincristine in SGC7901 cells decreased in sorcin transfected cells and increased in sorcin ASO-transfected cells, indicating that sorcin had a direct or indirect function on pumping the drug out of cells. Overexpression of sorcin up-regulated the expression of P-gp and P-gp inhibitor verapamil partially reversed the sorcin-mediated MDR in SGC7901 cell, suggesting that regulation of P-gp might be one of the mechanisms of sorcin-mediated MDR. The further study of the interaction protein of sorcin may be helpful for understanding the mechanisms of MDR in gastric cancer and developing possible strategies to treat gastric cancer.     

The acridonecarboxamide GF120918 potently reverses P-glycoprotein-mediated resistance in human sarcoma MES-Dx5 cells

The doxorubicin-selected, P-glycoprotein (P-gp)-expressing human sarcoma cell line MES-Dx5 showed the following levels of resistance relative to the non-P-gp-expressing parental MES-SA cells in a 72 h exposure to cytotoxic drugs: etoposide twofold, doxorubicin ninefold, vinblastine tenfold, taxotere 19-fold and taxol 94-fold. GF120918 potently reversed resistance completely for all drugs. The EC50s of GF120918 to reverse resistance of MES-Dx5 cells were: etoposide 7+/-2 nM, vinblastine 19+/-3 nM, doxorubicin 21+/-6 nM, taxotere 57+/-14 nM and taxol 91+/-23 nM. MES-Dx5 cells exhibited an accumulation deficit relative to the parental MES-SA cells of 35% for [3H]-vinblastine, 20% for [3H]-taxol and [14C]-doxorubicin. The EC50 of GF120918, to reverse the accumulation deficit in MES-Dx5 cells, ranged from 37 to 64 nM for all three radiolabelled cytotoxics. [3H]-vinblastine bound saturably to membranes from MES-Dx5 cells with a KD of 7.8+/-1.4 nM and a Bmax of 5.2+/-1.6 pmol mg(-1) protein. Binding of [3H]-vinblastine to P-gp in MES-Dx5 membranes was inhibited by GF120918 (K = 5+/-1 nM), verapamil (Ki = 660+/-350 nM) and doxorubicin (Ki = 6940+/-2100 nM). Taxol, an allosteric inhibitor of [3H]-vinblastine binding to P-gp, could only displace 40% of [3H]-vinblastine (Ki = 400+/-140 nM). The novel acridonecarboxamide derivative GF120918 potently overcomes P-gp-mediated multidrug resistance in the human sarcoma cell line MES-Dx5. Detailed analysis revealed that five times higher GF120918 concentrations were needed to reverse drug resistance to taxol in the cytotoxicity assay compared to doxorubicin, vinblastine and etoposide. An explanation for this phenomenon had not been found.     

Multidrug Resistance Reversal Activity of Taxoids from Taxus cuspidate in KB-C2 and 2780AD Cells

Some non-taxol-type taxoids having neither an oxetane ring at C-4 and C-5 nor an N-acylphenylisoserine group at C-13, such as taxuspine C, 2'-desacetoxyaustrospicatine, and 2-desacetoxytaxinine J, which were isolated from the Japanese yew Taxus cuspidata , increased cellular accumulation of vincristine (VCR) in multidrug-resistant 2780AD cells as potently as verapamil, and efficiently inhibited [³H]azidopine photolabeling of P-glycoprotein (P-gp). Taxuspine C, 2'-desacetoxyaustrospicatine, and 2-desacetoxytaxinine J at 10μ M completely reversed the resistance to colchicine, VCR, and taxol in KB-C2 cells, which overexpress P-gp, while taxinine and taxinine M showed no effect. Taxuspine C, 2'-desacetoxyaustrospicatine, and 2-desacetoxytaxinine J may be candidate pharmaceuticals for reversing multidrug resistance (MDR) and also may be good modifiers of MDR in cancer chemotherapy.     

Multidrug resistance reversal activity of taxoids from Taxus cuspidata in KB-C2 and 2780AD cells.

Some non-taxol-type taxoids having neither an oxetane ring at C-4 and C-5 nor an N-acylphenyl-isoserine group at C-13, such as taxuspine C, 2'-desacetoxyaustrospicatine, and 2-desacetoxytaxinine J, which were isolated from the Japanese yew Taxus cuspidata, increased cellular accu-mulation of vincristine (VCR) in multidrug-resistant 2780AD cells as potently as verapamil, and efficiently inhibited [(3)H]azidopine photolabeling of P-glycoprotein (P-gp). Taxuspine C, 2'-desacetoxyaustrospicatine, and 2-desacetoxytaxinine J at 10 microM completely reversed the resistance to colchicine, VCR, and taxol in KB-C2 cells, which overexpress P-gp, while taxinine and taxinine M showed no effect. Taxuspine C, 2'-desacetoxyaustrospicatine, and 2-desacetoxytaxinine J may be candidate pharmaceuticals for reversing multidrug resistance (MDR) and also may be good modifiers of MDR in cancer chemotherapy.     

喉癌中COX-2表达与多药耐药的相关性研究

[目的]研究喉癌中COX-2与P-gp表达的关系,探讨其在喉癌耐药中的作用。[方法]采用免疫组化SP法检测喉癌中的COX-2的表达情况和COX-2阳性组、COX-2阴性组中P-gp的表达情况。[结果]COX-2在喉癌组织中阳性表达率为39.33%,在正常组织中仅5.00%,差异有显著性(P<0.05),COX-2表达与喉癌临床分期及淋巴结转移密切相关。COX-2阳性组中P-gp阳性率为74.29%;COX-2阴性组中为16.67%(P<0.05)。P-gp表达与COX-2表达呈正相关。[结论]COX-2可能参与了P-gp介导的多药耐药的产生,通过干预COX-2的表达可减少肿瘤耐药。     

The interaction of bortezomib with multidrug transporters: implications for therapeutic applications in advanced multiple myeloma and other neoplasias

Purpose

Bortezomib is an important agent in multiple myeloma treatment, but resistance in cell lines and patients has been described. The main mechanisms of resistance described in cancer fall into one of two categories, pharmacokinetic resistance (PK), e.g. over expression of drug efflux pumps and pharmacodynamic resistance, e.g. apoptosis resistance or altered survival pathways, where the agent reaches an appropriate concentration, but this fails to propagate an appropriate cell death response. Of the known pump mechanisms, P-glycoprotein (P-gp) is the best studied and considered to be the most important in contributing to general PK drug resistance. Resistance to bortezomib is multifactorial and there are conflicting indications that cellular overexpression of P-gp may contribute to resistance agent. Hence, better characterization of the interactions of this drug with classical resistance mechanisms should identify improved treatment applications.

Methods

Cell lines with different P-gp expression levels were used to determine the relationship between bortezomib and P-gp. Coculture system with stromal cells was used to determine the effect of the local microenvironment on the bortezomib–elacridar combination. To further assess P-gp function, intracellular accumulation of P-gp probe rhodamine-123 was utilised.

Results

In the present study, we show that bortezomib is a substrate for P-gp, but not for the other drug efflux transporters. Bortezomib activity is affected by P-gp expression and conversely, the expression of P-gp affect bortezomib’s ability to act as a P-gp substrate. The local microenvironment did not alter the cellular response to bortezomib. We also demonstrate that bortezomib directly affects the expression and function of P-gp.

Conclusions

Our findings strongly support a role for P-gp in bortezomib resistance and, therefore, suggest that combination of a P-gp inhibitor and bortezomib in P-gp positive myeloma would be a reasonable treatment combination to extend efficacy of this important drug.     

非霍奇金淋巴瘤多药耐药蛋白在淋巴结与骨髓液中表达的实验研究

 目的　探讨初发非霍奇金淋巴瘤（NHL）骨髓液和淋巴结活组织两种标本中多药耐药蛋白的表达及临床价值评估。方法　采用流式细胞术（FCM）活细胞免疫荧光法检测41例初治NHL淋巴结活组织中和37例骨髓液中瘤细胞多药耐药蛋白及反转录-聚合酶链反应（RT-PCR）半定量检测mdr1mRNA的表达,分析其与临床分期、恶性分级、乳酸脱氢酶（LDH）异常增高等的相关性。结果　两种标本中三种耐药基因单独及联合过度表达率无明显差异;淋巴结组织中,P-gp阳性表达在Ⅰ／Ⅱ期和Ⅲ／Ⅳ期之间有差别（P＝0.046）,与LDH异常增高相关（P＜0.05）,与恶性度分级无关（P＞0.1）;肺耐药蛋白（LRP）与三者都存在明显相关（P＜0.05）。而骨髓液中仅LRP与恶性分级和LDH异常增高明显相关（P＝0.01）,并未显示P-gp表达在不同分期的差异性;淋巴结中P-gp+的CR低于P-gp-（37.5 %,84.8 %;P＜0.01）;LRP+与LRP-的CR之间亦存在差别（53.3 %,88.5 %;P＜0.01）。结论　用淋巴结活组织进行耐药蛋白检测比骨髓能提供更准确的临床信息,且在肿瘤尚未发生骨髓转移时,取淋巴组织亦能进行耐药蛋白检测,及早指导临床治疗及预后。     

The influence of expression of P-glycoprotein on the penetration of anticancer drugs through multicellular layers

The success of chemotherapy in the treatment of solid tumours may be limited by cellular mechanisms leading to drug resistance and/or by the slow penetration of drugs through tissue, resulting in a steep concentration gradient from tumour blood vessels. One mechanism leading to the development of multidrug resistance is overexpression of the membrane-based export pump P-glycoprotein (P-gp). The relationship between expression of P-gp by constituent cells and the penetration of P-gp substrates through tissue was studied by comparing the penetration of P-gp substrates through multicellular layers derived from either wild-type or P-gp overexpressing cell lines. P-gp reversal agents were added to confirm the contribution of P-gp in influencing the penetration of its substrates. Our data indicate: 1) penetration of the P-gp substrates, 99mTc-sestaMIBI and 14C-doxorubicin, is greater through multicellular layers formed from P-gp overexpressing cell lines as compared with wild-type cells; 2) the addition of agents that inhibit the function of P-gp results in decreased penetration of these substrates through multicellular layers with P-gp expression. There was no effect of P-gp reversal agents on penetration of 14C-sucrose or of 3H-5-fluorouracil (non-substrate controls). Our data suggest that the administration of agents that inhibit the function of P-gp might have opposing effects on therapeutic index in solid tumours: increased sensitivity of perivascular tumour cells but decreased penetration of P-gp substrates to more distal cells. These effects may explain, in part, the limited therapeutic benefit for solid tumours that has accrued from use of agents that reverse the effects of P-gp.