Resistant Human Tumor Cell Lines Research Articles

Ameliorative Potential of Tamoxifen/Thymoquinone Combination in Patients with Breast Cancer: A Biochemical and Immunohistochemical Study

Background: Breast cancer is one of the most common types of malignancies in females worldwide. Targeting the estrogen receptors alone with tamoxifen, reduces the incidence of estrogen receptor positive tumors but the possibility of development of tamoxifen resistance remains the most important adverse effect. Thymoquinone is an important constituent of the Nigella sativa plant (Black seed). It was found to be cytotoxic in several types of multidrug resistant human tumor cell lines. Objective: To detect the effects of the combination between tamoxifen and thymoquinone in patients with breast cancer. Methods: Eighty female patients with breast cancer were divided into four equal groups: control untreated group, tamoxifen treated group, thymoquinone treated group and tamoxifen + thymoquinone treated group. Tissue malondialdehyde (MDA), catalase (CAT), superoxide dismutase (SOD), tumor necrosis factor-alpha (TNF-α), interleukin 6 (IL-6) and transforming growth factor beta1 (TGF-β1) were determined in the tumor tissues. Parts of the tumor were subjected to immunohistochemical examination. The survival rate and the relapse rate were measured every year for five years. Results: Tamoxifen or thymoquinone alone or in combination induced significant increase in the 5-years survival rate, tumor CAT and SOD with significant decrease in the relapse rate, tumor tissue MDA, TNF-α, IL-6 and TGF-β1 and alleviated the immunohistochemical changes with significant increase in tissue caspase-3 expression and significant decrease in tissue bcl2 compared to the control untreated group. Conclusion: Tamoxifen/ thymoquinone combination had a better effect than each of these drugs alone on patients with breast cancer. So, the addition of thymoquinone to tamoxifen may represent a new therapeutic modality for management of breast cancer.

The eEF2 Kinase Confers Resistance to Nutrient Deprivation by Blocking Translation Elongation

Metabolic adaptation is essential for cell survival during nutrient deprivation. We report that eukaryotic elongation factor 2 kinase (eEF2K), which is activated by AMP-kinase (AMPK), confers cell survival under acute nutrient depletion by blocking translation elongation. Tumor cells exploit this pathway to adapt to nutrient deprivation by reactivating the AMPK-eEF2K axis. Adaptation of transformed cells to nutrient withdrawal is severely compromised in cells lacking eEF2K. Moreover, eEF2K knockdown restored sensitivity to acute nutrient deprivation in highly resistant human tumor cell lines. Invivo, overexpression of eEF2K rendered murine tumors remarkably resistant to caloric restriction. Expression of eEF2K strongly correlated with overall survival in human medulloblastoma and glioblastoma multiforme. Finally, C.elegans strains deficient in efk-1, the eEF2K ortholog, were severely compromised in their response to nutrient depletion. Our data highlight a conserved role for eEF2K in protecting cells from nutrient deprivation and in conferring tumor cell adaptation to metabolic stress. PAPERCLIP:

Long-circulating monensin nanoparticles for the potentiation of immunotoxin and anticancer drugs.

The carboxylic ionophore monensin was formulated into long-circulating nanoparticles with the help of polyethylene glycol/poly (DL-lactide-co-glycolide) diblock copolymers, in an attempt to enhance the cytotoxicity of a ricin-based immunotoxin, anti-My9, and anticancer drugs like adriamycin and tamoxifen. This study looked into various aspects involving the preparation (using a homogenizer and an EmulsiFlex homogenizer-extrusion device) and lyophilization of long-circulating monensin nanoparticles (LMNP) of particle size < 200 nm in diameter. The particle size of LMNP was reduced from 194 nm to 160 nm by passing the nanoparticles through an EmulsiFlex, before freeze-drying. There was a 4.8-83.7% increase in the particle size of LMNP after freeze-drying, which was dependent upon the manufacturing conditions such as use of the EmulsiFlex for size reduction before freeze-drying, the freezing method (rapid/slow) and the concentration of lyoprotectant (mannitol or trehalose) employed for freeze-drying. LMNP freeze-dried with 2.4% of trehalose showed minimal size change (< 9%) after freeze-drying. Further, the freezing method was found to have negligible effect on the particle size of LMNP freeze-dried with trehalose in comparison with mannitol. The entrapment efficiency of monensin in LMNP was found to be 14.2 +/- 0.3%. The LMNP were found to be spherical in shape and smooth in surface texture as observed by atomic force microscopy. In-vitro release of monensin from LMNP in phosphate buffered saline (PBS) pH 7.4 or PBS supplemented with 10% human serum indicated that there was an initial rapid release of about 40% in the first 8 h followed by a fairly slow release (about 20%) in the next 88 h. In-vivo studies conducted with Sprague-Dawley rats showed that 20% of monensin remained in circulation 4-8 h after the intravenous administration of LMNP. An in-vitro dye-based cytotoxicity assay (MTS/PMS method) showed that there was 500 times and 5 times potentiation of the cytotoxicity of anti-My9 immunotoxin by LMNP (5 x 10(-8) M of monensin) in HL-60 sensitive and resistant human tumour cell lines, respectively. Further, LMNP (5 x 10(-8) M of monensin) potentiated the cytotoxicity of adriamycin in MCF 7 and SW 620 cell lines by 100 fold and 10 fold, respectively, and that of tamoxifen by 44 fold in MCF 7 cell line as assessed by crystal violet dye uptake assay. Our results suggest that it is possible to prepare LMNP possessing appropriate particle size (< 200 nm), monensin content and in-vitro and in-vivo release characteristics with the help of a homogenizer and an EmulsiFlex homogenizer-extrusion device. LMNP can be freeze-dried with minimal increase in particle size by using a suitable concentration of a lyoprotectant like trehalose. Furthermore, LMNP could potentiate the cytotoxicity of immunotoxin, adriamycin and tamoxifen by 5-500 fold in-vitro, which will be further investigated in-vivo in a suitable animal model.

Preclinical antitumor activity of the oral platinum analog satraplatin

Satraplatin is an orally available platinum analog. The purpose of this study was to better characterize satraplatin's preclinical antitumor efficacy in a variety of sensitive and resistant human tumor cell lines and in a prostate cancer xenograft model and to evaluate the effect of satraplatin on PSA expression and/or secretion in a prostate cancer cell line. Satraplatin and its primary metabolite JM-118 were preclinically tested for their cytotoxic activity in a range of cancer cells including: human prostate, those forming the NCI drug screening panel, and those resistant to anti-cancer drugs. Also, the antiproliferative efficacy of satraplatin was tested in vivo in a human prostate cancer model. The effect of satraplatin and JM-118 on PSA transcription was measured by quantitative real time PCR. Satraplatin and JM-118 inhibited in vitro and in vivo the growth of prostate cancer cells in a dose-dependent fashion. The IC50 cytotoxicity values for satraplatin ranged from 1 to 3 microM for androgen-insensitive cells and was 11 microM for the androgen-sensitive cell line. Interestingly, JM-118 was up to 16-fold more potent than satraplatin. Oral administration of satraplatin to nude mouse PC-3 xenograft models inhibited the growth of these human tumors. Satraplatin had no direct effect on PSA transcription and the observed decrease in secreted PSA correlated with a decrease in cell number. When evaluated in the NCI drug-screening panel, satraplatin was most active in leukemia and small cell lung cancer cell lines. Both satraplatin and JM-118 were tested on cells resistant to chemotherapeutic agents. Satraplatin and JM-118 were equally active in the cisplatin-resistant A129cp80 ovarian carcinoma cell line, with activity comparable to that observed in the parent line. Neither expression of MDR1, BCRP, MRP1, nor altered tubulin or topoisomerase I were found to mediate resistance to satraplatin or JM-118. Although these resistance mechanisms contribute to drug resistance for a number of chemotherapeutics, they do not appear to play a role in satraplatin resistance. These results demonstrate that satraplatin and JM-118 have preclinical antitumor activity in human prostate cancer and other tumor types as well, including several cell lines displaying drug resistance to cisplatin, docetaxel and mitoxantrone. In addition, the results suggest that PSA should be further evaluated as a relevant marker of clinical response in patients with prostate cancer treated with satraplatin.

The ability to overcome multidrug resistance of tumor cell lines by novel acridine cytostatics with condensed heterocyclic rings.

Two recently synthesized groups of acridine cytostatics containing fused heterocyclic ring(s): pyrazoloacridines (PAC) and pyrazolopyrimidoacridines (PPAC) were tested in regard to their in vitro cytotoxic activity towards a panel of sensitive and resistant human tumor cell lines. The obtained results corroborate our earlier hypothesis on the essential role of heterocyclic ring fused to the acridine moiety in the ability of acridine cytostatics to overcome multidrug resistance of tumor cells. The presence, location and kind of substituents considerably influenced both the cytotoxic activity of the derivatives and their ability to overcome multidrug resistance. The same factors also affected the cytostatics ability to differentiate between tumor cell lines with various types of drug exporting pumps.

Examination of the effects of oxidation and ring closure on the cytotoxicities of the platinum complexes of N-(2-hydroxyethyl)ethane-1,2-diamine and ethane-1,2-diamine-N,N′-diacetic acid

The crystal structures, electrochemical properties and cytotoxicities of platinum(II) and platinum(IV) complexes of the multidentate ligands N-(2-hydroxyethyl)ethane-1,2-diamine (NNOH) and ethane-1,2-diamine-N,N′-diacetic acid (H2enda) are reported. In the platinum(II) state the NNOH and H2enda ligands act as bidentate ligands, coordinating through the two amine groups with the hydroxyethyl and carboxylate groups remaining uncoordinated. Oxidation with hydrogen peroxide followed by refluxing yields the ring closed Pt(IV) complexes in which the NNOH and H2enda ligands are deprotonated and coordinate via the two amine groups and either the deprotonated alcohol group in the case of NNO or both carboxylato groups in the case of enda. The platinum(IV) complex of NNO is 2- to 5-fold more active against a panel of cisplatin sensitive and resistant human tumour cell lines than is the platinum(II) complex, whereas in the case of enda, the reverse is true. Ring closure to occupy both axial sites apparently leads to deactivation of platinum(IV) complexes, but a single closure does not necessarily do so.

TUMOR NECROSIS FACTOR-α REPRESSES ANDROGEN SENSITIVITY IN THE LNCaP PROSTATE CANCER CELL LINE

TUMOR NECROSIS FACTOR-α REPRESSES ANDROGEN SENSITIVITY IN THE LNCaP PROSTATE CANCER CELL LINE

Double high-dose chemotherapy with stem cell rescue (HD-SCR) in patients with breast cancer - effect of sequence.

A preliminary analysis of our double high-dose chemotherapy with stem cell rescue (HD-SCR) clinical trial for breast cancer, and preclinical cross-resistant studies, suggested that melphalan (M) adversely affected response to subsequent chemotherapy, i.e., that the sequence of alkylating agents (AAs) might affect response. We, therefore, constructed and examined preclinical models to determine whether prior exposure to M, in fact, adversely affected response to other therapy. The purpose of the study was to determine whether the sequence of AAs, specifically the prior use of M, adversely affected response to subsequent treatment. The methods employed were the following: (1) Human tumor cell lines rendered resistant by in vitro sequential exposure to five different AAs were developed. The resistant cell lines were examined for cross-resistance to alkylating and other agents. (2) In vivo studies in the p388 mouse leukemia for resistance and cross-resistance among the AAs. (3) In vivo studies of the effect of sequence of AAs on response in mice bearing EMT6 breast cancer. (4) The double transplant model was developed in the mouse and the sequence of high-dose AAs was studied. (5) Biochemical and reverse transcriptase-polymerase chain reaction (RT-PCR) studies of the various resistant tumor cell lines. (1) The in vitro human tumor cells resistant to M were cross-resistant in 57% of tests to other AAs. In contrast, resistance for other AAs crossed to other agents in only 10 to 20% of tests. (2) The in vivo studies of p388 indicated that resistance to M commonly crossed to other AAs and many non-AAs. (3) The results for the mouse breast cancer (EMT6) studies of the sequence of AAs again indicated that M employed first markedly reduced responsiveness to subsequent treatment, particularly with AAs. (4) The double transplant model: again, M first markedly reduced response to other agents. (5) The in vitro resistant human tumor cell lines, particularly the breast cancer cell line MCF7, were found to contain high concentrations of glutathione S1 transferase gamma, which is consistent with that mechanism being responsible for resistance. The sequence of alkylating agent treatment may substantially influence response. Melphalan, particularly, produces resistance that commonly crosses to the other AAs. Mechanistic studies indicate significant changes in glutathione S1 transferase, a known mechanism for broadly based resistance to AAs.

Folic acid-enhanced synergy for the combination of trimetrexate plus the glycinamide ribonucleotide formyltransferase inhibitor 4-[2-(2-Amino-4-oxo-4,6,7,8-tetrahydro-3 H-pyrimidino[5,4,6][1,4]thiazin-6-yl)-( S)-ethyl]-2,5-thienoylamino- l-glutamic acid (AG2034) : Comparison across sensitive and resistant human tumor cell lines

Folic acid (PteGlu)-enhanced intense synergy has been observed between nonpolyglutamylatable dihydrofolate reductase (DHFR) inhibitors and polyglutamylatable inhibitors of other folate-requiring enzymes, such as glycinamide ribonucleotide formyltransferase (GARFT) and thymidylate synthase. Since this phenomenon is potentially therapeutically useful, we explored its universality by examining the combined action of a DHFR inhibitor, trimetrexate (TMQ), with a GARFT inhibitor, 4-[2-(2-amino-4-oxo-4,6,7,8-tetrahydro-3 H-pyrimidino[5,4,6][1,4]thiazin-6-yl)-( S)-ethyl]-2,5-thienoylamino- l-glutamic acid (AG2034), in eight human cultured cell lines. Using a 96-well plate cell growth inhibition assay, four ileocecal adenocarcinoma cell lines [HCT-8, HCT-8/DW2 (Tomudex-resistant), HCT-8/DF2 (Tomudex-/FdUrd-resistant), and HCT-8/50 (adapted to 50 nM PteGlu)], three head and neck carcinoma cell lines [A253, FaDu, and Hep-2/500 (FdUrd-resistant)], and a non-small cell lung carcinoma cell line [H460] were treated for 96 hr with TMQ + AG2034 in the presence of 2.3 or 40 μM PteGlu. Cell growth was measured with the sulforhodamine B assay at the end of this period. Drug interactions were assessed by fitting a 7-parameter model including a synergism parameter, α, to data with weighted nonlinear regression. Isobologram analysis was also applied. At 2.3 μM PteGlu, cells exhibited similar intensities of Loewe synergy for the combination of TMQ + AG2034. Loewe synergy was abolished in HCT-8/50 cells cultured and studied in 50 nM PteGlu. At 40 μM PteGlu, the intensity of the combined action in all cell lines was increased. However, the most intense Loewe synergy was seen with HCT-8, HCT-8/DF2, H460, FaDu, A253, and Hep-2/500 cells, whereas the HCT-8/50 subculture showed less of the phenomenon, and PteGlu enhancement was the least with HCT-8/DW2, a subline deficient in folylpolyglutamate synthetase (FPGS). The universality of the PteGlu-enhanced intense synergy phenomenon is suggested. Impaired FPGS activity and low-folate adaptation prior to treatment significantly lessen the degree of PteGlu enhancement.

Effect of cisplatin on the clinically relevant radiosensitivity of human cervical carcinoma cell lines

To evaluate the effect of clinically relevant levels of cisplatin on the radiosensitivity of human cervical tumor cells, and to estimate what changes in local control rates might be expected to accrue from the concomitant use of cisplatin during fractionated radiotherapy. The effects of concomitant cisplatin (1 microgram/ml, a typical intratumor concentration) on the clinically relevant radiosensitivity, i.e., surviving fraction after 2 G (SF2) values, was determined in 19 cloned human cervical tumor cell lines. These early passage cell lines had SF2 values ranging from 0.26 to 0.87. The concomitant administration of cisplatin reduced the clinically relevant radiosensitivity in the majority (11 out of 19) of the human tumor cell lines investigated. In only 4 out of 19 was any radiosensitization observed, and in 4 out of 19 cell lines there was no significant change in radiosensitivity. However, the sum of the independent cell killing by radiation and cisplatin, was approximately twofold higher than after radiation alone. There was no apparent dependence of the cisplatin-induced changes in SF2 values upon the level of cell killing by cisplatin. However, there is a suggestion that concomitant cisplatin administration may have a differential effect in inherently radiosensitive and resistant human tumor cell lines. Our data suggest that concomitant cisplatin/radiotherapy regimens may result in a higher level of local tumor control, but primarily through additive toxicity and not through radiosensitization. Future improvements in local tumor control may, thus, be derived by increasing the total dose of cisplatin.

Synthesis and in vitro and in vivo antitumor activity of a series of trans platinum antitumor complexes.

The synthesis of a series of platinum complexes of trans coordination geometry [centered around the general formula, trans-ammine(amine)dichlorodihydroxoplatinum(IV) plus corresponding tetrachloroplatinum(IV) or Pt(II) counterparts] is described as part of a drug discovery program to identify more effective platinum-based anticancer drugs, particularly targeted toward the circumvention of resistance to cisplatin. Complexes have been evaluated for antitumor activity using in vitro and in vivo tumor models. In vitro against a panel of cisplatin-sensitive and -resistant human tumor cell lines (predominantly ovarian), many of the trans platinum complexes studied (e.g., 1, R = cyclohexyl) exhibited comparable potency to cisplatin and also overcame acquired cisplatin resistance, where resistance was due mainly to either reduced drug uptake or enhanced platinum-DNA adduct removal. Moreover, 14 trans complexes showed significant in vivo antitumor activity against the subcutaneous murine ADJ/PC6 plasmacytoma model; all were platinum(IV) complexes, 13/14 possessing axial hydroxo ligands the other possessing axial ethylcarbamato ligands. Where tested, all of their respective platinum(II) or tetrachloroplatinum(IV) counterparts were inactive. Notably, three dihydroxoPt(IV) complexes (18, 29, 34) (R = c-hexyl, c-heptyl, and 1-adamantyl) retained some efficacy against a cisplatin-resistant variant of the ADJ/PC6. Compounds 18 (trans-[PtCl2(OH)2NH3-(RNH2)]) R = c-C6H11, 22, R = Me3C, 27, R = n-C6H13, 28, R = PhCH2, and 36 (trans-[PtBr2(OH)2NH3(c-C6H11NH2)]) also produced evidence of antitumor activity (> 5 days growth delay) against subcutaneously grown advanced stage human ovarian carcinoma xenografts. These data demonstrate that a series of trans-ammine(amine)dichlorodihydroxoplatinum(IV) complexes are active in vivo against both murine and human subcutaneous tumor models and represent potential leads to a new generation of platinum-based anticancer drug.

Cooperative P-glycoprotein mediated daunorubicin transport into DNA-loaded plasma membrane vesicles

Most of the multidrug resistant human tumor cell lines overexpress the MDR1 gene product P-glycoprotein (P-gp) which is believed to function as an energy-dependent drug efflux pump. Here we describe a novel method that allows the kinetic characterization of P-gp-mediated active drug transport. This method is based on the fluorescence quenching of anthracyclines transported into DNA-loaded plasma membrane vesicles. The uptake of daunorubicin (DNR) into the plasma membrane vesicles was saturable in terms of the extravesicular DNR concentration with a K m of 1.5 ± 0.1 μM. This transport occured by a cooperative process with a Hill coefficient close to 2 for DNR. A model is discussed in which P-gp pumps two molecules of drug per turnover.

Evaluation of S9788 as a potential modulator of drug resistance against human tumour sublines expressing differing resistance mechanisms In Vitro

Significant activity has been identified using S9788, a triazineaminopiperidine derivative, as a new modulator of multi-drug resistance against a series of drug-resistant human tumour-cell lines in vitro. Maximal non-cytotoxic concentrations (i.e., those resulting in < or = 10% cytotoxicity) of S9788 or verapamil were tested in combination with vinblastine, Adriamycin or vincristine and cytotoxicity was evaluated using a clonogenic assay, or the metabolic dye reduction MTT assay, or by monitoring growth inhibition. Under these conditions, the extent of resistance modulation by verapamil and by S9788 was comparable in the various tumour cell lines tested, although a definite concentration-dependent modulation was noted with both compounds. The highest dose-modification factors were noted in the highly vinblastine-resistant classic multi-drug-resistant subline CEM/VLB100, although resistance reversal was only partial. Resistance modulation by both verapamil and S9788 was noted in 4 drug-selected resistant sublines and 4 "intrinsically" resistant human tumour cell lines, which all exhibited significant P-glycoprotein expression. In contrast, in 2 drug-resistant human tumour sublines (GLC4/ADR and CEM/VM-1) characterized by altered topoisomerase-II activity and proving to be P-glycoprotein-negative, no resistance modulation relative to parental cells was observed. These data are consistent with the proposal that resistance modulation is mediated by interaction between S9788 and P-glycoprotein and support its clinical evaluation in patients with P-glycoprotein-positive tumours.

RICIN-MEDIATED CELL-LYSIS AND APOPTOSIS OF DRUG SENSITIVE AND RESISTANT TUMOR-CELLS

Ricin is a potent plant toxin that inhibits protein synthesis of eukaryotic cells resulting eventually in cell death. The mechanism of ricin-mediated cytotoxicity on human tumor cell lines was examined in view of the recent findings that bacterial toxins like diphtheria toxin (DTX) and Pseudomonas aeruginosa exotoxin A (PEA) cause programmed cell death or apoptosis in addition to their activity of inhibition of protein synthesis. Ricin is shown to be cytolytic in a short term assay (less-than-or-equal-to 24 h) and cytostatic in a long term assay (greater-than-or-equal-to 72 h) as determined by the MTT assay. The cytostatic activity of ricin paralleled its protein synthesis inhibitory activity whereas ricin-mediated cytolytic activity did not correlate with protein synthesis inhibition. Ricin is shown to mediate programmed cell death or apoptosis against several drug sensitive and drug resistant cell lines. DNA fragmentation is initially detected following 4 h of treatment. Although cytotoxicity by ricin is energy dependent, it is not reduced by the addition of a lysosomotropic agent NH4Cl. We have recently demonstrated that the bacterial toxins DTX and PEA synergize with tumor necrosis factor-alpha (TNF-alpha) (J Immunol 147: 2609-2616, 1991 and J Immunol 149: 2089-2094, 1992). However, unlike DTX or PEA, ricin did not synergize with TNF-alpha, suggesting that ricin and DTX/PEA may mediate their cytolytic activity by different mechanisms. These findings demonstrate that ricin can mediate both protein synthesis inhibition and programmed cell death or apoptosis against a variety of drug sensitive and resistant human tumor cell lines. The significance of these findings in cancer therapy is discussed.

Retinoic acid receptor and retinoid X receptor expression in retinoic acid—resistant human tumor cell lines

Retinoic acid (RA) has profound effects on cell proliferation and differentiation both in vitro and in vivo. Many human cell lines are known to be sensitive to the growth-inhibitory action of RA. We analyzed established human solid tumor-derived cell lines for their RA sensitivity. Growth inhibition by RA in monolayer was examined by [3H]thymidine incorporation and cell proliferation. Here we report that 11 widely used human cell lines were RA resistant. The majority are carcinoma derived (A-431, BT-20, C-41, ACHN, HCT116, 293, A549, and PA-1); two are sarcoma derived (Saos-2 and A673); and one is a melanoma cell line (A-375). Since nuclear retinoid receptors are implicated in the biological effects of RA, we examined the expression of retinoic acid receptors (RARs) RAR alpha, RAR beta, RAR gamma, and the retinoid X receptors (RXRs) RXR alpha, RXR beta, and RXR gamma in the RA-resistant cell lines by northern blotting and by RNase protection analysis for RAR beta. RAR alpha transcripts were constitutively expressed in all cell lines. By contrast, RAR beta was expressed in only seven RA-resistant cell lines (Saos-2, ACHN, 293, A549, A-375, A673, and PA-1), and its level was enhanced by RA in some cases. In most cell lines, RAR gamma expression was low and was not affected by RA. The RXR genes showed a very distinct expression pattern in the group of selected cell lines. In general, RXR alpha was the most abundantly expressed subtype, RXR beta was expressed at low levels, and RXR gamma could not be detected. In none of the RA-resistant cell lines was RXR expression modulated by RA. The results presented here indicate that the resistance of these human tumor cell lines to RA cannot be simply correlated with expression of RAR or RXR or both.

A monoclonal antibody detecting cell surface epitope on some drug resistant human tumour cell lines

In the present study, we report the derivation of a monoclonal antibody against a MDR human ovarian carcinoma cell line, A2780.AD (AD) (Rogan et al., 1984), which recognises a cell surface antigen whose association with drug resistance appears independent of P-glycoprotein

The activity of verapamil as a resistance modifier in vitro in drug resistant human tumour cell lines is not stereospecific

The l-isomer of verapamil is a more potent calcium antagonist than the d-isomer. We have examined the two stereoisomers of verapamil for their ability to increase the chemosensitivity in vitro of three drug resistant cell lines (2780AD, MCF7/Adr r and H6 9LX10). Neither racemic verapamil nor its individual isomers had any effect on the drug sensitivity of the parent cell lines (A2780, MCF7 and NCI-H69). Verapamil (6.6 μM) increased the sensitivity of all three resistant cell lines to Adriamycin® by 10–12-fold. This activity was concentration dependent and was maximal at 6–7 μM. The increase in sensitivity was only 2–3-fold at 2,μM, the maximum plasma concentration achieved in patients. Both the d- and l-isomers of verapamil alone at 6.6 μM were as effective as racemic verapamil and the d-isomer demonstrated the same concentration dependent activity as racemic verapamil. The total cellular Adriamycin® concentration of both 2780AD and MCF7/Adr r was increased by two-fold in the presence of verapamil (6.6 μM). Both d- and l-verapamil alone increased the amount of drug accumulated to the same extent as racemic verapamil. These results indicate that the resistance modification activity of verapamil is not stereospecific. Use of d-verapamil alone in patients could increase the maximum tolerated plasma concentrations of verapamil and thus d-verapamil may be a more effective resistance modifier in vivo than racemic verapamil.