MRP1 not MDR1 gene expression is the predominant mechanism of acquired multidrug resistance in two prostate carcinoma cell lines

Multidrug resistant prostate cancer cell lines DU 0.03 and PC 0.03 were established from the parental prostate cancer cell lines DU145 and PC-3 respectively by stepwise selection in doxorubicin (DOX) from 0.001 to 0.03 &mgr;g/ml. As cells adapted to each concentration of DOX. the drug concentration was increased by 0.001 &mgr;g/ml. The chemosensitivity of each line was determined by growth inhibition assay. The DU 0.03 and PC 0.03 lines exhibit a 5-10-fold and 1.3-2.8-fold increase in resistance to anthracyclines, vinblastine (VLB) and mitozantrone (Mito), respectively. Verapamil (5 &mgr;M) partially reversed the resistance to the anthracycline and completely reversed the resistance to VLB and Mito. Drug kinetic studies measured by intracellular accumulation of (3)H-daunorubicin demonstrated a 3 fold decrease in the level of intracellular (3)H-daunorubicin in the PC 0.03 and DU 0.03 resistant lines compared with their respective parental line. This effect was partially reversed by 5 &mgr;M verapamil. The expression of MDR1 and MRP genes was analysed by Northern blotting and RT-PCR. P-glycoprotein (Pgp) and MRP protein were tested by immunocytochemistry staining using the monoclonal antibodies J-SB1. C219 and MRK16 (Pgp) and MRPm6 and MRPr1 (MRP). Neither Northern blot analysis nor the more sensitive RT-PCR demonstrated detectable MDR1 transcripts in any of the prostate cancer cell lines and the three Pgp monoclonal antibodies failed to reveal expression of Pgp. A 2-4-fold increase in MRP1 mRNA levels in the drug resistant DU 0.03 and PC 0.03 lines were demonstrated by both Northern blotting and RT-PCR consistent with the findings observed after staining by the two specific monoclonal antibodies, MRPm6 and MRPr1. Southern blot analysis demonstrated a 2-fold increase in the MRP1 gene copy number in the PC 0.03 line but not in the DU 0.03 line, suggesting that the overexpression of the MRP gene was regulated at the level of transcription in the latter line. We conclude that MRP1 not MDR1 overexpression. contributes to acquired drug resistance in these two prostate cancer cell lines. Prostate Cancer and Prostatic Diseases (2000) 3, 66-75

Altered multidrug resistance phenotype caused by anthracycline analogues and cytosine arabinoside in myeloid leukemia

The expression of P-glycoprotein (Pgp) is often increased in acute myeloid leukemia (AML). However, little is known of the regulation of Pgp expression by cytotoxics in AML. We examined whether Pgp expression and function in leukemic blasts was altered after a short exposure to cytotoxics. Blasts were isolated from 19 patients with AML (15 patients) or chronic myeloid leukemia in blastic transformation (BT-CML, 4 patients). Pgp expression and function were analyzed by flow cytometric analysis of MRK 16 binding and Rhodamine 123 retention, respectively. At equitoxic concentrations, ex vivo exposure for 16 hours to the anthracyclines epirubicin (EPI), daunomycin (DAU), idarubicin (IDA), or MX2 or the nucleoside analogue cytosine arabinoside (AraC) differentially upregulated MDR1/Pgp expression in Pgp-negative and Pgp-positive blast cells. In Pgp-negative blasts, all four anthracyclines and AraC significantly increased Pgp expression (P =.01) and Pgp function (P =.03). In contrast, MX2, DAU, and AraC were the most potent in inducing Pgp expression and function in Pgp positive blasts (P <.05). A good correlation between increased Pgp expression and function was observed in Pgp-negative (r =.90, P =.0001) and Pgp-positive blasts (r =.77, P =.0002). This increase in Pgp expression and function was inhibited by the addition of 1 micromol/L PSC 833 to blast cells at the time of their exposure to these cytotoxics. In 1 patient with AML, an increase in Pgp levels was observed in vivo at 4 and 16 hours after the administration of standard chemotherapy with DAU/AraC. Upregulation of Pgp expression was also demonstrated ex vivo in blasts harvested from this patient before the commencement of treatment. In 3 other cases (1 patient with AML and 2 with BT-CML) in which blasts were Pgp negative at the time of initial clinical presentation, serial samples at 1 to 5 months after chemotherapy showed the presence of Pgp-positive blasts. All 3 patients had refractory disease. Interestingly, in all 3 cases, upregulation of Pgp by cytotoxics was demonstrated ex vivo in blasts harvested at the time of presentation. These data suggest that upregulation of the MDR1 gene may represent a normal response of leukemic cells to cytotoxic stress and may contribute to clinical drug resistance.

Induction of insulin-like growth factor binding protein expression by ICI 182,780 in a tamoxifen-resistant human breast cancer cell line

Earlier studies in our laboratory demonstrated that the steroidal antiestrogen ICI 182,780 is very effective in abolishing the tamoxifen-resistant proliferation of MCF 7/5-23 cells. In addition, preliminary binding studies showed that ICI 182,780 increased the binding of insulin-like growth factor (IGF)-I to the MCF 7/5-23 cells, although this finding was not the result of an increase in the expression of the insulin-like growth factor-I receptor (IGF-IR). Hence, we reasoned that the inhibition of tamoxifen-resistant cell growth by ICI 182,780 might have been due to increased expression of insulin-like growth factor binding proteins (IGFBPs). We observed the up-regulation of non-insulin-suppressible IGF-I binding in both the tamoxifen-sensitive MCF 7/5-21 cell line (1.5-fold) and the tamoxifen-resistant MCF 7/5-23 cell line (2.5-fold) after 5 days of treatment with ICI 182,780 (10(-7) M) in serum-free medium, suggesting a role for cell-associated IGFBPs. Affinity cross-linking experiments confirmed the presence of an IGF-I:IGFBP complex of approximately 38-kDa in tamoxifen or ICI 182,780-treated cells. Western ligand blots showed higher levels of a soluble 30-kDa IGFBP in media conditioned by either of the subclones that had been treated with ICI 182,780, an effect consistently opposed by estrogen (E2: 10(-9) M). RT-PCR showed higher levels of IGFBP-5 mRNA than any of the other known IGFBPs, suggesting that this was the major IGFBP subtype. The protein was subsequently identified by Western immunoblotting as IGFBP-5. In conclusion, we postulate that this may be a mechanism contributing to the greater potency of ICI 182,780 in the growth inhibition of the MCF 7/5-23, tamoxifen-resistant cell line.

Frequency of the fragile X syndrome in Chinese mentally retarded populations is similar to that in Caucasians

Fragile X syndrome is recognized as the most common inherited cause of mental retardation in western countries. The prevalence of the fragile X syndrome in Asian populations is uncertain. We report a multi-institutional collaborative study of molecular screening for the fragile X syndrome from 1,127 Chinese mentally retarded (MR) individuals. We found that 2.8% of the Chinese MR population screened by DNA analysis had the fragile X full mutation. Our screening indicated that the fragile X syndrome prevalence was very close to that of Caucasian subjects. In addition, we found that 62.5% of fragile X chromosomes had a single haplotype for DXS548-FRAXAC1 (21-18 repeats) which was present in only 9.7% of controls. This unique distribution of microsatellite markers flanking the FMR1 CGG repeats suggests that the fragile X syndrome in Chinese populations, as in the Caucasian, may also be derived from founder chromosomes.

Altered expression of the IGF-1 receptor in a tamoxifen-resistant human breast cancer cell line

The relationship between oestrogen (E2) and insulin-like growth factor-one (IGF-1) was examined in both tamoxifen-sensitive (MCF 7/5-21) and tamoxifen-resistant (MCF 7/5-23) subclones of the MCF 7 cell line. Both subclones were grown in defined, serum-free (SF) medium over a period of 7 days with the addition of E2 or IGF-1 or a combination of both agents. Growth of both MCF 7/5-21 and 7/5-23 cells was stimulated (245% and 350%, respectively) by E2. However, only the growth of MCF 7/5-23 cells was stimulated (266%) by IGF-1. A combination of E2 and IGF-1 significantly enhanced MCF 7/5-21 and 7/5-23 cell growth (581% and 695%, respectively). E2-induced IGF-1 receptor (IGF-1R) levels (as measured by 125I-IGF-1 binding and Northern analyses) in only MCF 7/5-23 cells. This effect was partially inhibited by tamoxifen. In medium containing serum, the growth of only the MCF 7/5-23 cells was significantly inhibited by the IGF-1R monoclonal antibody, alphaIR-3. The detection of E2-induced expression of IGF-2 using RT-PCR was demonstrated in the MCF 7/5-23 cells. These experiments indicate that E2 may sensitize tamoxifen-resistant MCF 7/5-23 cells to the growth stimulatory actions of IGF-2 via up-regulation of the IGF-1R and describes a cell-survival mechanism that may manifest itself as tamoxifen resistance.

Induction of MDR1 gene expression by anthracycline analogues in a human drug resistant leukaemia cell line

The effects of 4-demethoxydaunorubicin (idarubicin, IDA) and MX2, a new morpholino-anthracycline, on up-regulation of the MDR1 gene in the low-level multidrug resistant (MDR) cell line CEM/A7R were compared at similar concentrations (IC10, IC50 and IC90) over a short time exposure (4 and 24 h). The chemosensitivity of each drug was determined by a 3-day cell growth inhibition assay. Compared with epirubicin (EPI), IDA and MX2 were 17- and eightfold more effective in the CEM/A7R line respectively. No cross-resistance to 5-FU was seen in the CEM/A7R line. Verapamil (5 microM) and PSC 833 (1 microM), which dramatically reversed resistance to EPI in the CEM/A7R line, had no sensitizing effect on the resistance of this line to MX2, but slightly decreased resistance to IDA. The sensitivity to 5-FU was unchanged by these modulators. The induction of MDR1 mRNA expression by IDA, MX2 and 5-FU was analysed by Northern blotting and semiquantitatively assessed by scanning Northern blots on a phosphorimager. The relative level of MDR1 expression was expressed as a ratio of MDR1 mRNA to the internal RNA control glyceraldehyde-3-phosphate dehydrogenase (GAPDH). IDA, MX2 and 5-FU differentially up-regulated MDR1 mRNA in the CEM/A7R line in a dose-dependent manner. Both IDA and MX2 induced MDR1 expression within 4 h. 5-FU up-regulated MDR1 expression only when drug exposure was prolonged to 24 h. Based on MRK 16 binding, flow cytometric analysis of P-glycoprotein (Pgp) expression paralleled the increase in MDR1 mRNA levels. For the three anthracyclines, the increase in MDR1 expression was stable in cells grown in the absence of drug for more than 3 weeks after drug treatment. The induction of MDR1 expression by 5-FU was transient, associated with a rapid decrease in the increased Pgp levels which returned to baseline 72 h after the removal of 5-FU. This study demonstrates that MDR1 expression can be induced by analogues of anthracyclines not pumped by Pgp, and that this induction appears to be stable despite a 3-week drug-free period.

Altered methylation of the human MDR1 promoter is associated with acquired multidrug resistance

One of the most important forms of drug resistance in acute myeloid leukemia is the multidrug resistance (MDR) phenotype, which is characterized by the expression of the MDR1 gene product, P-glycoprotein. Although a number of factors affect MDR1 gene expression, the genetic events that "switch on" the human MDR1 gene in tumor cells that were previously P-glycoprotein negative have remained elusive. Here, we report evidence that the methylation status of the human MDR1 promoter may serve as a basis for this "switch." Based on Southern analysis using methylation-sensitive and methylation-insensitive restriction enzymes, a tight correlation was found between MDR phenotype and demethylation of the 5' region of the MDR1 gene in a human T cell leukemia cell line. Similar results were obtained from the analysis of P-glycoprotein-positive and P-glycoprotein-negative samples of chronic lymphocytic leukemia. Treatment of the cell lines with the demethylating agent 5'-azadeoxycytidine altered the methylation pattern of the MDR1 promoter in P-glycoprotein-negative cells to resemble that of P-glycoprotein-positive cells and activated the promoter such that MDR1 mRNA was now detectable. Treatment also resulted in an increased resistance to epirubicin and decreased daunomycin accumulation, both of which were reversible by verapamil, a characteristic of the classical MDR phenotype in cells expressing P-glycoprotein. These results suggest that the MDR phenotype may be acquired as a result of changes in methylation of the MDR1 promoter.

Cyclosporin A and PSC 833 prevent up-regulation of MDR1 expression by anthracyclines in a human multidrug-resistant cell line

We have previously demonstrated that within 24 h of exposure of the CEM/A7R cell line to epirubicin (EPI), MDR1 gene expression is induced. The aim of the current study was to investigate the role of cyclosporin A (CyA) and PSC 833, two biochemical modulators of the classical multidrug-resistant phenotype, in this model. CEM/A7R cells were exposed to EPI in the presence or absence of various concentrations of CyA or PSC 833. MDR1 expression was assessed using Northern blot analysis and quantitated using a phosphorimager. P-glycoprotein (P-gp) expression was analyzed by the determination of MRK16 binding using flow cytometry. P-gp function was measured in an assay of [3H]daunomycin accumulation. The coincubation of CyA or PSC 833 with EPI prevented the increase in MDR1 gene expression induced by EPI alone. This effect of the two modulators was dose dependent. Neither modulator alone had any significant effect on the expression of MDR1. In these experiments, changes in MDR1 expression correlated with changes in P-gp levels (based on MRK16 binding) and P-gp function. Thus, both PSC 833 and CyA appear to prevent the induction of MDR1 gene expression caused by the short-term exposure of CEM/A7R cells to EPI.

Inheritance of chromosome 7 is associated with a drug-resistant phenotype in somatic cell hybrids

A major form of drug resistance in tumour cells known as classical multidrug resistance (MDR) is associated with the overexpression of the mdr1 gene product, the membrane protein P-glycoprotein (P-gp), which acts as an energy-dependent drug efflux pump. In this study the inheritance of P-gp expression was examined using hybrids formed after somatic cell fusion between a drug-sensitive human T-cell leukaemia cell line, CEM/CCRF, and a drug-resistant derivative, CEM/A7, which is characterized by a clonal chromosomal duplication dup(7)(q11.23q31.2). Fourteen hybrids, chosen at random, were analysed by reverse transcriptase-polymerase chain reaction (RT-PCR) and by binding studies involving the monoclonal antibody MRK16, which recognises an external P-gp epitope. Only two hybrids were positive for both MRK16 antibody labelling and mdr1 mRNA. Partial karyotypic analysis of all hybrids revealed that only the MRK16-positive hybrids contained the duplication in chromosome 7 seen in the CEM/A7 parental MDR line. Therefore, P-gp overexpression in the MRK16-positive hybrids may be linked to the inheritance of chromosome 7 from CEM/A7 and possibly associated with the chromosome 7 abnormality.

The pure antiestrogen ICI 182,780 binds to a high-affinity site distinct from the estrogen receptor

Both estrogen receptor-positive (ER+), tamoxifen-sensitive (5-21) and tamoxifen-resistant (5-23) subclones of the parental MCF-7 breast cancer cell line were used in competitive ligand binding studies involving either [3H]ICI 182,780 or 4-OH-[3H]tamoxifen (4OHT) displacement by unlabelled estradiol (E2) or the antiestrogens (AE) 4OHT and ICI 182,780. Neither radioligand was displaced significantly by E2 over a range of concentrations; binding was predominantly inhibited by the corresponding radio-inert ligand. Scatchard analysis of the data revealed that the binding capacities of both cell lines for ICI 182,780 were approximately 7-fold greater than the previously determined number of ER sites per cell, with the affinity being an order of magnitude less than that of E2 for ER. No difference was found between the TAM-sensitive and -resistant cells in their binding of either AE. When cells were preincubated with either E2, TAM or 4OHT at a high, fixed concentration to block the ER or AE binding sites (AEBS), respectively, displaceable binding of [3H]ICI 182,780 was still observed, indicating binding at a site other than the classical ER or previously described AEBS. Our results suggest that there is a specific, saturable and relatively high-affinity binding site for ICI 182,780 in MCF 5-21 and MCF 5-23 breast cancer cells. However, the physiological relevance of this binding site requires further clarification because in cell growth assays, E2 (at 1/10 the dose of ICI 182,780) overcame the inhibitory effect of the antiestrogen in both of the cell lines.

Rapid up-regulation of mdr1 expression by anthracyclines in a classical multidrug-resistant cell line

Studies were carried out in a variant human multidrug-resistant (MDR) cell line CEM/A7R, which expresses very low levels of mdr1 mRNA and P-glycoprotein (P-gp). The induction of mdr1 RNA expression by three anthracyclines, (doxorubicin, daunorubicin, epirubicin), VP-16 and two vinca alkaloids (vincristine, vinblastine) was semiquantitatively assessed by scanning Northern blots on a phosphorimager. The relative level of mdr1 expression was expressed as ratio of mdr1 to the internal RNA (actin). A significant increase (P < 0.02) in expression of mdr1 was noted within 4 hrs of exposure to 1.5 micrograms ml-1 daunorubicin or epirubicin. Neither vinblastine nor vincristine had any effect on mdr1 levels after an 8 h exposure. With increasing concentrations of daunorubicin or epirubicin in a fixed 24 h time period, mdr1 expression increased, although a biphasic response was seen. Based on MRK 16 binding, an increase in P-gp levels was seen in the CEM/A7R line after a 24 h exposure to 1 microgram ml-1 daunorubicin or epirubicin. The rapid increase in mdr1 expression after a short period of exposure to doxorubicin, daunorubicin or epirubicin suggests that induction of mdr1 expression may have an important role in the development of drug-resistant tumours.

Clinical significance of multiple hypothalamic-pituitary functions assessment in patients with Turner's syndrome

Hypothalamic-pituitary functions in 26 cases of Turner syndrome were assessed with a combined stimulation test. The results showed that the peak GH levels of 12 cases were less than 10 micrograms/L; 3 patients were demonstrated as having an even TSH response, while another one with a delayed TSH peak, and other 4 had high basal values and consistent exaggerated TSH responses to TRH; all patients showed increased basal and peak LH and FSH levels but 5, whose LH and FSH secretion patterns were similar to normal. 12 cases have been treated with individualized protocols and followed up for 12 months or more, of them the growth velocity all increased, especially those with hypothyroidism or with a BA less than 13. It is suggested that multiple functions of hypothalamic-pituitary axis in Turner patients be evaluated as early as possible, in order that proper treatment could be adopted and their growth and development improved.

Cellular and karyotypic characterization of two doxorubicin resistant cell lines isolated from the same parental human leukemia cell line

Separate mechanisms underlying the multidrug resistant (MDR) phenotype were identified in 2 independent approaches to select tumour cells resistant to low concentrations of doxorubicin (Dox) from the sensitive T cell leukemia cell line CCRF-CEM. The CEM/A7 cell line was selected at an initial concentration of 0.005 microgram/ml of Dox and maintained at 0.07 microgram/ml. In contrast, the CEM/A5 line was selected using an initial concentration of 0.01 microgram/ml and maintained in Dox at a concentration of 0.05 microgram/ml. P-glycoprotein expression was demonstrated in the CEM/A7 line but not the CEM/A5 line. Amplification of the mdrI gene was not observed in the CEM/A7 cell line. Both cell lines showed cross-resistance to a number of structurally unrelated cytotoxic drugs including anthracyclines and etoposide (VP-16), although only the CEM/A7 line was cross resistant to Vinca alkaloids. Immunoblots of total cell lysates of the CEM/A5 line have revealed almost undetectable levels of topoisomerase II alpha and beta in this line. Cytogenetic analyses of both lines revealed numerous karyotypic abnormalities which were present in the parental cell line as well as both resistant cell lines. The CEM/A7 line also demonstrated a duplication of part of the long arm of chromosome 7 which included the region containing the mdrI gene, a finding not seen in the parental or CEM/A5 line. CEM/A5, however, demonstrated an abnormality of chromosome 7, outside the region of the mdrI gene, and it also contained a deletion of the short arm of chromosome 2. Abnormalities in this latter region of genome have been associated with non-P-glycoprotein-mediated MDR.

Circumvention of tamoxifen resistance by the pure anti-estrogen ICI 182,780

Both primary and acquired resistance to the growth-inhibitory effects of anti-estrogens (e.g., tamoxifen) limits the clinical usefulness of these drugs in the treatment of breast cancer. The new, steroidal anti-estrogen ICI 182,780 was tested for its ability to inhibit the proliferation of a tamoxifen-resistant variant of the parental MCF-7 human breast-cancer cell line. Two cell lines cloned from the MCF-7 line were used for these experiments: a tamoxifen-sensitive line, MCF 5-21, and a tamoxifen-resistant line, MCF 5-23. Compared with tamoxifen, ICI 182,780 appeared to be 150 and 1540 times more effective in inhibiting cell growth in the 5-21 and 5-23 sub-lines respectively. ICI 182,780 completely circumvented tamoxifen resistance at a concentration of (5 to 10) x 10(-9) M in this model. Based on IC50 concentrations, the 5-23 line was 22-fold more resistant to tamoxifen than the 5-21 line, but only 2-fold more resistant to ICI 182,780, reducing relative resistance by 10-fold in the resistant line. There were no differences in ER parameters between the 2 lines. ER numbers/cell were: 40500 and 34800 and the KD 0.48 and 0.15 x 10(-9) M in the 5-21 and 5-23 cells respectively. In the 5-23 cells, the concentrations of ICI 182,780 and tamoxifen resulting in a 50% inhibition of 3H-estradiol binding were 2.3 x 10(-8) M and 1 x 10(-6) M, respectively (cf. estradiol 0.89 x 10(-9) M). Thus, one potential mechanism for the increased effectiveness of ICI 182,780 may relate to the increased affinity of this drug for the estrogen receptor as compared with tamoxifen.

Differential effects of estrogen, tamoxifen and the pure antiestrogen ICI 182,780 in human drug-resistant leukemia cell lines

ICI 182,780, a potent, new steroidal antiestrogen without apparent agonist activity, appears to be a potent modulator of the classic multidrug resistance (MDR) phenotype in the CEM/A7, CEM/VLB100 and K562/VIN100 MDR cell lines. This reagent had no effect on the respective parental CCRF-CEM and K562 cell lines. The use of 1.25 microM ICI 182,780 resulted in a 6- to 7-fold decrease in doxorubicin resistance in the CEM/A7 and CEM/VLB100 cell lines. A dose-response effect was observed at ICI 182,780 concentrations of up to 5 microM. As compared with tamoxifen (TAM), ICI 182,780 was 2 and 4 times more effective in the K562/VIN100 and CEM/A7 cell lines, respectively. ICI 182,780 at 0.625 microM increased [3H]-daunomycin uptake (P < 0.0001) as effectively as 5 microM TAM in the resistant CEM/A7 line. Drug-efflux studies showed that 5 microM ICI 182,780 significantly decreased drug efflux as compared with 5 microM TAM (P < 0.0001). Estradiol (EST) at 10 microM increased doxorubicin resistance by 1.2-1.3 times in the CEM/A7 and CEM/VLB100 cell lines and significantly decreased drug accumulation (P = 0.002) and retention (P < 0.001) in the CEM/A7 cell line. However, the addition of 10 microM EST to 1-2 microM ICI 182,780 did not inhibit the ability of ICI 182,780 to modulate doxorubicin resistance in the two resistant cell lines. Using reverse-phase high-performance liquid chromatography (HPLC) to measure lipophilicity, we found no apparent association between the ability of ICI 182,780, TAM or EST to modulate resistance and their relative hydrophobicity.

Clinical application of a rapid, functional assay for multidrug resistance based on accumulation of the fluorescent dye, fluo-3

A rapid and simple functional assay for P-glycoprotein (Pgp) using flow cytometry to measure the accumulation of the flurophore fluo-3 has been applied to samples from patients with B-cell chronic lymphocytic leukaemia (B-CLL). Peripheral blood lymphocytes from 37 patients with B-CLL were studied for Pgp. Pgp expression, using MRK-16, a monoclonal antibody recognising an external surface epitope of Pgp, was detected in 92% of patients with B-CLL. The functional assays for Pgp expression were positive in 78 and 59% of patients using the fluo-3 and doxorubicin (dox) assays, respectively. When compared with the MRK-16 assay, the fluo-3 assay had a sensitivity of 82% compared to a sensitivity of 56% for the dox assay (P = 0.004). The specificity of the fluo-3 and dox assays could not be evaluated because of the low number of MRK-16 negative CLL cells.

Circumvention of doxorubicin resistance in multi-drug resistant human leukaemia and lung cancer cells by the pure antioestrogen ICI 164384

ICI 164384, a new steroidal antioestrogen, entirely devoid of oestrogenic activity, modulates doxorubicin resistance in vitro. At non-cytotoxic concentrations, ICI 164384 potentiated the cytotoxicity of doxorubicin in a dose-dependent manner in both the classical multi-drug resistant (MDR) human leukaemia cell lines CEM/VLB 100 and CEM/VLB 1000 and the human small cell lung cancer cell line H69 LX4. ICI 164384 had no effect on the two respective parental cell lines, CEM/CCRF and H69 P. None of these cell lines expressed the oestrogen receptor. In comparative studies at concentrations ranging from 1.25 to 10 mumols/l, ICI 164384 was significantly more effective (1.2-6-fold) than tamoxifen in reducing the IC50 of doxorubicin in the CEM/VLB 100 line. In resistant cells, ICI 164384 increased 3H-daunomycin accumulation in a dose-dependent manner and was significantly more effective than tamoxifen at concentrations ranging from 2.5 to 10 mumol/l. ICI 164384 reduced the efflux of daunomycin from resistant cells more effectively than tamoxifen. These studies suggest that ICI 164384 is an effective modulator of MDR.

Combined use of cyclosporin A and verapamil in modulating multidrug resistance in human leukemia cell lines

This study describes the synergistic interaction of two biochemical modulators, cyclosporin A (CyA) and verapamil (Vp), in multidrug-resistant cells, the highly resistant and moderately resistant variants (CEM/VLB 1000 and CEM/VLB 100) of the parental drug-sensitive T-cell leukemia cell line CEM/CCRF. In the absence of either modulator, the 50% inhibitory concentration for Adriamycin in these cell lines was 270 +/- 10.6 (SD) micrograms/ml, 96 +/- 8.5 micrograms/ml, and 1.5 +/- 0.1 micrograms/ml, respectively. CyA and Vp dramatically reduced multidrug resistance in CEM/VLB 100 and CEM/VLB 1000 in a dose-dependent manner but had no effect on the sensitivity of the parental line to Adriamycin. At a CyA concentration of 8.3 mumol (10 micrograms/ml), the 50% inhibitory concentration of Adriamycin of CEM/VLB 1000 and CEM/VLB 100 fell to 5.9 +/- 0.9 micrograms/ml and 3.3 +/- 0.6 micrograms/ml, respectively. Similarly at a Vp concentration of 10 mumol the 50% inhibitory concentration of Adriamycin of CEM/VLB 1000 and CEM/VLB 100 fell to 23.7 +/- 3.7 micrograms/ml and 5.7 +/- 0.2 micrograms/ml, respectively. More importantly, CyA and Vp showed significant synergism when tested in combination in the moderately resistant line at concentrations normally seen after the clinical administration of these modulators. Synergy was also present when both drugs were tested in the highly resistant variant. These data indicate the need for in vivo studies, given the potential clinical importance of these observations.

Effect of cyclosporin and verapamil on the cellular kinetics of daunorubicin

Both cyclosporin and verapamil modulate the multidrug-resistant (MDR) phenotype in the classical MDR cell lines, CEM/VLB100 and CEM/VLB1000. Initial studies demonstrated a significant reduction in daunorubicin accumulation in the two resistant lines compared with the drug-sensitive parent line CEM/CCRF. Both cyclosporin and verapamil increased drug accumulation in the resistant lines. This effect was dose-dependent although a plateau occurred in CEM/VLB100 cells at concentrations of cyclosporin exceeding 4.2 mumol/l. Cyclosporin 4.2 mumol/l and verapamil 10 mumol/l significantly increased daunorubicin uptake and reduced drug efflux in the CEM/VLB100 and CEM/VLB1000 lines. At low clinical concentrations of cyclosporin (0.8-1.6 mumol/l and verapamil (1-2 mumol/l), there was a synergistic increase in drug accumulation in the two resistant cell lines (P less than 0.007). These data suggest that cyclosporin modulates the classical MDR phenotype by altering the cellular kinetics of daunorubicin. The in vitro synergistic action of cyclosporin and verapamil could be interesting clinically.