Differential modulation of doxorubicin toxicity to multidrug and intrinsically drug resistant cell lines by anti-oestrogens and their major metabolites

Tetrandrine Interaction with ABCB1 Reverses Multidrug Resistance in Cancer Cells Through Competition with Anti-Cancer Drugs Followed by Downregulation of ABCB1 Expression

The overexpression of ABC transporters induced by anticancer drugs has been found to be the main cause of multidrug resistance. It is actually also a strategy by which cancer cells escape being killed. Tetrandrine is a natural product extracted from the stem of Tinospora crispa. In this study, tetrandrine showed synergistic cytotoxic activity in combinational use with chemotherapeutic drugs, such as Doxorubicin, Vincristine, and Paclitaxel, in both drug-induced and MDR1 gene-transfected cancer cells that over-expressed ABCB1/P-glycoprotein. Tetrandrine stimulated P-glycoprotein ATPase activity, decreased the efflux of [³H]-Paclitaxel and increased the intracellular accumulation of [³H]-Paclitaxel in KB-C2 cells. Furthermore, SW620/Ad300 and KB-C2 cells pretreated with 1 μM tetrandrine for 72 h decreased P-glycoprotein expression without changing its cellular localization. This was demonstrated through Western blotting and immunofluorescence analysis. Interestingly, down-regulation of P-glycoprotein expression was not correlated with gene transcription, as the MDR1 mRNA level exhibited a slight fluctuation in SW620/Ad300 and KB-C2 cells at 0, 24, 48, and 72 h treatment time points. In addition, molecular docking analysis predicted that tetrandrine had inhibitory potential with the ABCB1 transporter. Our results suggested that tetrandrine can antagonize MDR in both drug-selected and MDR1 gene-transfected cancer cells by down regulating the expression of the ABCB1 transporter, followed by increasing the intracellular concentration of chemotherapeutic agents. The combinational therapy using tetrandrine and other anticancer drugs could promote the treatment efficiency of drugs that are substrates of ABCB1.

Efficacy of tamoxifen in combination with docetaxel in patients with advanced non-small-cell lung cancer pretreated with platinum-based chemotherapy

The aim of this study was to evaluate the efficacy and safety of the combination of docetaxel (TXT) plus tamoxifen (TAM) in advanced non-small-cell lung cancer (NSCLC) patients who had received platinum-based first-line chemotherapy. A total of 120 advanced NSCLC patients pretreated with platinum-based chemotherapy were randomized into two treatment groups (the TXT and TXT+TAM groups) in a 1 : 1 ratio. Reversal of P-glycoprotein (P-gp) expression, tumor response, progression-free survival, overall survival, and safety were evaluated on an intention-to-treat basis. The median number of cycles of allocated chemotherapy was four in each treatment group (range: 2-6 cycles). The overall response rate and disease control rate in the TXT+TAM group were significantly higher than those in the TXT group (36.7 vs. 15.0% for overall response rate, P=0.007; 85.0 vs. 68.3% for disease control rate, P=0.031). The combination of TXT and TAM could effectively reverse P-gp expression in tumor tissues and provide a significant survival benefit for advanced NSCLC patients compared with TXT alone (11.6 vs. 9.1 months, P=0.030). In addition, in the TXT+TAM group, patients achieving P-gp reversal had a significantly greater median progression-free survival and overall survival than nonreversal patients. Furthermore, the combined therapy showed a safety profile comparable to that of TXT. The combination of TXT and TAM may be an effective and safe treatment option for advanced NSCLC patients who have already developed P-gp-mediated multidrug resistance.

Inhibitory effects of tamoxifen and doxorubicin, alone and in combination, on the proliferation of the MG63 human osteosarcoma cell line

The present study aimed to compare the combined effect of tamoxifen (TAM) and doxorubicin (ADM) with the individual effects of TAM and ADM alone on the MG63 human osteosarcoma cell line. Estrogen receptor (ER) expression was detected in the MG63 cells using reverse transcription PCR. The morphological changes during the inhibition of cell growth were observed using an inverted microscope and a 3-(4, 5-dimethy1-2-thiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) colorimetric assay following the individual or combined addition of TAM and ADM. ERα and ERβ expression was detected in the MG63 cells. The typical apoptotic cell morphology was observed in all groups, with the exception of the control group. The MTT colorimetric analysis demonstrated that the rate of inhibition of cell proliferation in the combination group was significantly increased compared with that in the other groups (P<0.05). ERα and ERβ expression was detected in the MG63 human osteosarcoma cells. TAM and ADM alone were able to inhibit cell proliferation. The combination of TAM and ADM significantly enhanced the inhibitory effect, partly through the enhanced sensitivity of the cells to ADM by TAM, which caused the inhibition of cell proliferation and apoptosis.

A mathematical model for thermosensitive liposomal delivery of Doxorubicin to solid tumour

The effectiveness of anticancer treatments is often hampered by the serious side effects owing to toxicity of anticancer drugs and their undesirable uptake by healthy cells in vivo. Thermosensitive liposome-mediated drug delivery has been developed as part of research efforts aimed at improving therapeutic efficacy while reducing the associated side effect. Since multiple steps are involved in the transport of drug-loaded liposomes, drug release, and its uptake, mathematical models become an indispensible tool to analyse the transport processes and predict the outcome of anticancer treatment. In this study, a computational model is developed which incorporates the key physical and biochemical processes involved in drug delivery and cellular uptake. The model has been applied to idealized tumour geometry, and comparisons are made between continuous infusion of doxorubicin and thermosensitive liposome-mediated delivery. Results show that thermosensitive liposome-mediated delivery performs better in reducing drug concentration in normal tissues, which may help lower the risk of associated side effects. Compared with direct infusion over a 2-hour period, thermosensitive liposome delivery leads to a much higher peak intracellular concentration of doxorubicin, which may increase cell killing in tumour thereby enhancing the therapeutic effect of the drug.

Recurrent Male Breast Cancer Accompanied by Carcinomatous Pleuritis That Responded to Combination Therapy with High-Dose Toremifene and Docetaxel

BACKGROUND: Male breast cancer (MBC) is a rare disease with no standard treatment compared to female breast cancer. There are very few reports that go beyond second-line chemotherapy and endocrine therapy for advanced and recurrent MBC. CASE REPORT: This report presents a case of recurrent MBC accompanied by carcinomatous pleuritis that responded to combination therapy with high-dose toremifene (TOR) and docetaxel (DOC). A 63-year-old male patient had previously undergone a modified radical mastectomy for left breast cancer and received several series of systemic chemotherapy and endocrine therapy. He complained of severe dyspnea, and was admitted to our hospital due to a massive left pleural effusion caused by carcinomatous pleuritis. He received combination therapy with high-dose TOR and biweekly DOC. The pleural effusion disappeared without any severe side effects after 4 cycles of the therapy. Thereafter, his disease stabilized for 1 year without re-increase of tumor markers under continuous treatment with the combination therapy. CONCLUSIONS: High-dose TOR and DOC might be effective even beyond second-line chemotherapy and endocrine therapy for MBC to overcome potential multiple drug resistance and diminish inevitable side effects.

Differential effects of toremifene on doxorubicin, vinblastine and Tc-99m-sestamibi in P-glycoprotein-expressing breast and head and neck cancer cell lines

The effect of toremifene on P-glycoprotein-mediated multidrug resistance (MDR) in breast and head and neck cancer cell lines was measured in vitro and in vivo. Pgp expression was low and high, respectively, in drug-sensitive (MCF7-S, KB) and drug-resistant (MCF7-R, MCF7-R1, KBV1) cell lines. Toremifene (7.5 microM) significantly enhanced cytoplasmic and nuclear accumulation of doxorubicin in drug-resistant cells. Toremifene (10 microM) increased the in vitro cytotoxicity of doxorubicin in drug-resistant breast cancer cells (13-fold and 21-fold for MCF7-R and MCF7-R1, respectively) without affecting the sensitivity of MCF7-S cells. Similarly, toremifene (10 microM) caused a 12-fold increase in the sensitivity of KBV1 cells to vinblastine. In contrast, toremifene (5 microM) reduced the net uptake of the radiolabelled Pgp substrate, Tc-99m-sestamibi, in the Pgp-overexpressing cell lines by factors of 0.32 and 0.42 for MCF7-R1 and KBV1 cells, respectively (p < 0.01), and, to a lesser extent, by corresponding factors of 0.89 and 0.86 in the drug-sensitive cell lines (p < 0.05 and p > 0.05, respectively). In nude mice bearing both KB and KBV1 xenograft tumours, significantly higher tumour levels of Tc-99m-sestamibi were recorded in KB tumours compared with KBV1 tumours. After 3 days of treatment with intraperitoneal toremifene (25 mg/kg), tumour levels of Tc-99m-sestamibi were reduced in KB and KBV1 tumours but only statistically significantly for KB tumours. Toremifene is a potent MDR modulating agent with respect to chemotherapeutic agents but has the opposite effect with respect to Tc-99m-sestamibi. This finding is of importance in view of the widespread use of Tc-99m-sestamibi as an imaging surrogate for a chemotherapeutic agent.

Multifactorial activities of nonsteroidal antiestrogens against leukemia

The antileukemic activity of nonsteroidal antiestrogens was investigated. Tamoxifen, clomiphene and nafoxidine caused a decrease in viability of the estrogen receptor-negative T-lymphoblastic leukemia cell line CCRF/CEM, nafoxidine being the most active. A combination of clomiphene and genistein resulted in a synergistic cytotoxic effect when applied to Molt-3, another T-lymphblastic leukemic cell line. The antiestrogens arrested the cells at G(0)/G(1) phase and induced apoptosis. Using the CCRF/VCR(1000) cell line, which is resistant to vincristine, it was observed that the effect of nafoxidine on modulating drug resistance was manifested at a lower concentration than that causing a direct cytotoxic effect. Nafoxidine inhibited the Pgp pump activity as measured by rhodamine 123 efflux. Combination with verapamil was found to be more effective in abrogating the pump activity. This study points to the multifactorial activities of nonsteroidal antiestrogens against lymphoblastic leukemia and implies their potential use in clinical treatment as antileukemic drugs.

Paclitaxel and tamoxifen: An active regimen for patients with metastatic melanoma

BACKGROUND

In early trials of paclitaxel administered as a 24-hour infusion, an overall response rate of 16% was reported for patients with metastatic melanoma. Paclitaxel is a natural product-based agent and is thus subject to the problem of multidrug resistance (MDR). Tamoxifen is an agent that can abrogate MDR and potentially enhance the effect of paclitaxel. A Phase II trial of the combination was undertaken with previously treated patients.

METHODS

Patients with metastatic cutaneous or mucosal melanoma who were previously treated with the Dartmouth chemotherapy regimen (dacarbazine, carmustine, cisplatin, and tamoxifen) were evaluated. Paclitaxel was administered at a dose of 225 mg/m(2) intravenously over 3 hours every 3 weeks. All patients also took tamoxifen 40 mg orally daily. Treatment continued until disease progression.

RESULTS

Twenty-one patients completed at least two cycles of paclitaxel and were evaluable for response. Five responses were observed, 1 complete response, and 4 partial responses, for an overall response rate of 24%. The combination was well tolerated. The most common nonhematologic side effects were myalgia and paresthesia. Hematologic toxicity was mild. No patients developed neutropenic fever.

CONCLUSIONS

This is the first report of a Phase II trial evaluating paclitaxel as a 3-hour infusion in melanoma patients. The 3-hour infusion is well tolerated and results in little myelosuppression and minimal neurotoxicity. The contribution of tamoxifen is difficult to evaluate because plasma levels were not measured. It is possible that a higher response rate might be observed with larger doses of tamoxifen. Further investigation of paclitaxel in the treatment of patients with metastatic melanoma is warranted.

A new PCR MIMIC strategy to quantify low mdr1 mRNA levels in drug resistant cell lines and AML blast samples

Determination of the MDR-phenotype in patients suffering from AML is an important hallmark of treatment outcome but is often complicated by technical problems in P-gp assessment. A PCR-MIMIC strategy was employed to construct PCR-fragments for a competitive and quantitative mdr1 reverse transcription-PCR-assay. Using K562 cells, which had been selected for drug resistance to the epipodophyllotoxin VP16, a stepwise increase of mdr1 levels depending on the concentration of VP16 was shown with the MIMIC technique. Comparison of mdr1 levels in drug selected K562 cells with the corresponding levels for P-gp and functional data indicated a mRNA threshold that has to be exceeded for the full expression of the MDR-phenotype. Subsequently mdr1 levels of 34 samples of de novo acute myeloid leukemia were determined with the PCR-MIMIC strategy. Ten patient samples could be identified with elevated mdr1 levels which were substantially lower than the levels observed in the MDR-cell line K 562 0.7 microM VP16. Outcome analysis revealed that eight of the ten patients had an unfavourable prognosis and did not achieve CR after induction chemotherapy. Coexpression of mdr1 and CD 34 was not associated with CR in all examined cases. Moreover all these patients had unfavourable cytogenetic aberrations. These data indicate a sensitive technique with applicability in patient material.

First-line treatment with mitoxantrone, methotrexate, vincristine, and carboplatine (MIMOC) plus cyclical hormonotherapy with tamoxifen and megestrol acetate in advanced breast cancer

Fifty patients with stage IIIB and IV breast cancer entered a prospective study receiving combination chemotherapy consisting of mitoxantrone (8 mg/m2) on day 1, methotrexate (30 mg/m2) on day 1, vincristine (1 mg/m2) on day 2, and carboplatine (250 mg/m2) on day 2 (MIMOC), plus cyclical hormonotherapy with tamoxifen (20 mg daily, days 1-10) and megestrol acetate (160 mg daily, days 11-21). The regimen was repeated every 3 weeks. None had received chemotherapy for advanced disease, although 17 patients had previously received adjuvant chemotherapy and 21 had received adjuvant hormonotherapy with tamoxifen. Twenty-seven patients had positive estrogen receptor (ER+) status, and 23 negative estrogen receptor (ER-) status. Response was observed in 31 (62%) of the 50 analyzed patients (95% CI: 48.5-75.4%), with 5 complete responses (10%). A significantly better response rate was observed in ER+ patients (p = 0.03). The median duration of response was 16 months, and the median time to disease progression was 18 months. The median overall survival was 19 months (27 for responders and 7 for nonresponders). ER+ patients had a higher probability of survival (p = 0.02). Toxicity was moderate. Nausea/vomiting and myelotoxicity were the main side effects. In conclusion, MIMOC plus cyclical hormonotherapy represents a well-tolerated and effective first-line treatment for advanced breast cancer. The observed difference in response and survival in favor of ER+ patients warrants further investigation.

Tamoxifen induces oxidative stress and apoptosis in oestrogen receptor-negative human cancer cell lines

Recent data have demonstrated that the anti-oestrogen tamoxifen (TAM) is able to facilitate apoptosis in cancer cells not expressing oestrogen receptor (ER). In an attempt to identify the biochemical pathway for this phenomenon, we investigated the role of TAM as an oxidative stress agent. In two ER-negative human cancer cell lines, namely T-leukaemic Jurkat and ovarian A2780 cancer cells, we have demonstrated that TAM is able to generate oxidative stress, thereby causing thiol depletion and activation of the transcriptional factor NF-kappaB. As described for other oxidative agents, TAM was able to induce either cell proliferation or apoptosis depending on the dose. When used at the lowest dose tested (0.1 microM), a slight proliferative effect of TAM was noticed in terms of cell counts and DNA synthesis rate, whereas at higher doses (10 microM) a consistent occurrence of apoptosis was detected. Importantly, the induction of apoptosis by TAM is not linked to down-regulation or functional inactivation by phosphorylation of the antiapoptotic bcl-2 protein.

Inhibition of the membrane translocation and activation of protein kinase C, and potentiation of doxorubicin-induced apoptosis of hepatocellular carcinoma cells by tamoxifen

Hepatocellular carcinoma (HCC) is characterized by high drug resistance to currently available chemotherapeutic agents. In a prospective clinical study, we have demonstrated that high-dose tamoxifen significantly enhanced the therapeutic efficacy of doxorubicin in patients with far-advanced HCC. In a search for a possible mechanism, we found that tamoxifen at a clinically achievable concentration (2.5 microM) significantly enhanced doxorubicin-induced cytotoxicity and apoptosis of Hep-3B cells, a multidrug resistance (MDR)-1 expressing HCC cell line. This synergistic cytotoxic effect of tamoxifen, at this concentration, however, was not mediated by MDR inhibition. Instead, as evidenced by both western blot and immunofluorescence studies, tamoxifen inhibited the cytoplasmic-membrane translocation of protein kinase C (PKC)-alpha. 12-O-Tetradecanoylphorbol-13-acetate (TPA) restored the membrane translocation of PKC-alpha and abrogated the synergistic cytotoxicity of tamoxifen. We also showed that tamoxifen, at this concentration, did not directly affect the enzyme activity of PKC. Further, membrane translocation of other membrane-bound proteins, such as Ras protein, was similarly inhibited by tamoxifen, but could not be restored by the addition of TPA. Together, these data suggested that tamoxifen may act on the cytoplasmic membrane, and thereby inhibit PKC-alpha translocation to the membrane where it is activated. We hypothesize that high-dose tamoxifen may be an effective modulator of doxorubicin in the treatment of HCC, and suggest that biochemical modulation of PKC as a measure to improve systemic chemotherapy for HCC deserves further investigation.

Interaction with type II estrogen binding sites and antiproliferative activity of tamoxifen and quercetin in human non-small-cell lung cancer

The antiestrogen tamoxifen is thought to antagonize the effects of estrogens by competing with them for estrogen receptor (ER) binding. However, tarnoxifen can also reverse multidrug resistance, synergize with cisplatin cytotoxicity, and inhibit growth in ER-negative lung cancer cells. In addition to ERs, rat and human target tissues contain a second binding macromolecule termed the type II estrogen binding site (type II EBS). It has been shown that tamoxifen and flavonoids, a widely distributed class of natural substances with a variety of biologic actions, bind to type II EBS and inhibit the growth of several tumor cell types. At present, conflicting data about ERs and an absence of data about type II EBSs exist for lung tumors. We have tested non-small-cell lung carcinoma cell lines and primary tumor cells for the presence of ERs and type II EBSs and have evaluated the effects of tamoxifen and quercetin (pentahydroxyflavone) on the growth of these cells. Using a whole-cell assay and nuclear and cytosolic radiobinding experiments with [3H]estradiol as tracer, we have found that SK-LU1, SW900, ChaGo-K-1, H441, H661, and A549 cells, as well as primary tumors, bind estrogen specifically. This binding results mainly from the presence of a large number of type II EBSs, whereas ERs are absent or present at low concentrations. Type II EBSs bound tamoxifen and quercetin with similar affinity. Cell counts and a thymidine incorporation assay showed that both compounds inhibit cell growth in a concentration-dependent manner at concentrations ranging from 10 nM to 1 microM. Neither ipriflavone, an isoflavone, nor rutin, the 3-rhamnosylglucoside of quercetin, bound type II EBSs or inhibited cell growth. These findings suggest that tamoxifen and quercetin could regulate lung cancer cell growth through a binding interaction with type II EBSs. This mechanism could also be active in vivo, in that we have observed that nuclear and cytosolic type II EBSs were present in all primary lung cancers tested (n = 12), and that tamoxifen and quercetin were effective in inhibiting in vitro bromodeoxyuridine (BrdU) incorporation and proliferation-cell nuclear antigen expression by neoplastic cells in these cancers.

Combined effects of epirubicin and tamoxifen on the cell-cycle phases in estrogen-receptor-negative Ehrlich ascites tumor cells

Laboratory and clinical data suggest some interactions between cytotoxic agents and tamoxifen. The mechanisms of these interactions differ in estrogen-receptor-negative cell lines. The ability of tamoxifen to modify the effects of epirubicin on the cell-cycle phases of estrogen-receptor-negative Ehrlich's carcinoma ascitic cells (EATC) was studied in mice. The results showed that combination of tamoxifen with epirubicin decreased the thymidine labelling index more effectively than did either drug alone. Adding tamoxifen to epirubicin treatment induced both an early S-phase and G2-M-phase arrest and a later G0-G1-phase arrest in EATC. An increase of S0 cells in the quiescent fraction could play a role in these changes, and some of these quiescent cells may not be viable, causing them to die later. In conclusion, the data suggest that continuous exposure to tamoxifen might modify the effects of epirubicin via cell-cycle perturbations.

Modulatory effect of tamoxifen and ICI 182,780 on adriamycin resistance in MCF-7 human breast-cancer cells

In this study the ability of the new pure anti-estrogen ICI 182,780 to modulate the cytotoxic action of adriamycin (ADR) on parental and ADR-resistant MCF-7 (MCF-7 ADRr) human breast-cancer cells was investigated and compared with that of tamoxifen (TAM). TAM enhanced ADR cytotoxicity in MCF-7 ADRr cells in a dose-related manner, but this effect was slight or absent in MCF-7 WT. In contrast, ICI 182,780 was able to enhance ADR toxicity both in MCF-7 ADRr and in the parental cell line. ICI 182,780 was up to 2.5-fold more effective than TAM in reducing the IC50 of ADR in MCF-7 ADRr cells. Analysis of the data by the isobole method showed that the combination ADR/TAM and ADR/ICI 182,780 produced synergistic anti-proliferative activity in MCF-7 ADRr cells. Because ADR resistance in these cells is associated with the expression of high levels of P-glycoprotein (Pgp), we evaluated the effect of anti-estrogens on Pgp expression and activity. Both ICI 182,780 and TAM failed to modulate Pgp expression as assessed by flow cytometry and Western-blot analysis, performed using the monoclonal antibodies MM4.17 and C 219, which are specific for an external or an internal determinant respectively. Pgp activity was investigated by flow cytometry measuring the extrusion of ADR and the cationic dye Rhodamine 123 (Rh 123). ICI 182,780, but not TAM, reduced the activity of Pgp in MCF-7 ADRr cells. Flow cytometry was also used to investigate cell-cycle modifications induced by ADR in MCF-7 ADRr cells, both in the presence and in the absence of anti-estrogens. After 72 hr, higher doses induced an arrest of cells at the G2/M phase. The same effect was visible when lower doses of ADR were combined with ICI 182,780 or TAM. In terms of cell-cycle-blocking activity ICI 182,780 was largely more effective than TAM.

Synergistic effects of tamoxifen and cepharanthine for circumventing the multidrug resistance

We examined the synergistic effects of tamoxifen (TAM) and cepharanthine (CEP) for doxorubicin (DOX) sensitivity using MTT assay. The augmentation of DOX sensitivity by TAM and CEP was significantly correlated with the P-glycoprotein expression. The cytotoxic effect of DOX with TAM and CEP was significantly higher than that of DOX alone, or DOX with TAM, and this synergistic effect was dominant in cell lines with high expression of P-glycoprotein. It was also examined that the intracellular concentration of DOX was increased in combined exposure of TAM and CEP, compared with the exposure of TAM, because TAM and CEP promoted the influx and inhibited the efflux of DOX. Thus, TAM and CEP might be able to circumvent DOX-resistance for treatment in cancer patients.

The effects of cyclosporin A, tamoxifen, and medroxyprogesterone acetate on the enhancement of adriamycin cytotoxicity in primary cultures of human breast epithelial cells

Adriamycin (Adr), the single most active agent used in the treatment of breast cancer, may become ineffective as treatment progresses due to the development of multidrug resistant (MDR) tumors. A major mechanism associated with MDR is increased P-glycoprotein (Pgp) expression. This study examined the abilities of the anti-estrogen tamoxifen (TAM) and the progestin medroxyprogesterone acetate (MPA) as well as cyclosporin A (CsA), a known resistance modifier, to enhance the cytotoxic effects of Adr on human breast epithelial cells (HBEC) in primary culture. Pgp and estrogen receptor (ER) expression were determined in each of the cultures by immunocytochemical assays using the monoclonal antibodies C219 and H222 Sp gamma, respectively. The Adr-sensitive, Pgp-, ER+ MCF-7 cell line and the Adr-resistant, Pgp+, ER- MCF7-AdrR cell line were used as controls. Primary cultures were categorized as HBEC from tissues with or without previous chemotherapy. Pgp was detected in 1 of the 15 cell cultures from tissues without previous chemotherapy and in 5 of the 6 cell cultures from tissues previously exposed to chemotherapy. Incubation with either CsA or MPA plus Adr enhanced Adr toxicity in Pgp+ but not Pgp- cell cultures, whereas TAM had no effect on the sensitivity of any of the cultures. Of the 21 primary cultures of HBEC, 3 were ER+. There was no correlation between the enhancement of Adr cytotoxicity and ER status. The data suggest that MPA as well as CsA may be useful as modifying agents in overcoming Pgp-associated multidrug resistance.

P-glycoprotein-mediated multidrug resistance: experimental and clinical strategies for its reversal

The study of the cellular, biochemical, and molecular biology and pharmacology of MDR has provided one of the most active and exciting areas within cancer research and one that holds great promise for translation into clinical benefit. While convincing evidence for the functional role of P-gp in mediating clinical drug resistance in humans remains elusive, studies of the clinical expression of P-gp and trials of chemosensitizers with cancer chemotherapy suggest "resistance modification" strategies may be effective in some tumors with intrinsic or acquired drug resistance. However, even if P-gp-associated MDR proves to be a relevant and reversible cause of clinical drug resistance, numerous problems remain to be solved before effective clinical chemosensitization may be achieved. Such factors as absorption, distribution, and metabolism; the effect of chemosensitizers on chemotherapeutic drug clearance; toxicity to normal tissues expressing P-gp; and the most efficacious modulator regimens all remain to be defined in vivo. Clearly, the identification of more specific, potent, and less clinically toxic chemosensitizers for clinical use remains critical to the possible success of this approach. Nonetheless, the finding that a number of pharmacological agents can antagonize a well-characterized form of experimental drug resistance provides promise for potential clinical applications. Further study of chemosensitizers in humans and the rational design of novel chemosensitizers with improved activity should define the importance of MDR in clinically resistant cancer.

Mechanisms of action of endocrine treatment in breast cancer

Endocrine treatment plays an important role in the therapy of breast cancer. While the basic mechanisms are understood, additional mechanisms may be of importance to their action and they may also contribute to the mechanism(s) of acquired resistance. Currently, several novel drugs are entering into clinical trials. Observations of the absence or presence of cross resistance to novel 'pure' steroidal antiestrogens and the non-steroidal tamoxifen may add important information to our understanding of the mechanisms of action of both classes of drugs. Similarly, exploration of different aromatase inhibitors in sequence or concert, as well as the combining of different endocrine treatment options may be warranted. Additionally, alterations in different biochemical parameters such as growth factors should not only be carefully explored in relation to treatment options but should also be followed during the course of treatment to asess alterations over time and in relation to the development of drug resistance.

Collateral sensitivity of multidrug resistant cells to narcotic analgesics is due to effects on the plasma membrane

It has previously been demonstrated that opiates interact directly with P-glycoprotein in drug resistant Chinese hamster ovary (CHO) cells (Callaghan, R. and Riordan, J.R. (1993) J. Biol. Chem. 268, 16059-16064). In this study we have examined the effects of several opiates on the growth of drug sensitive and resistant CHO and human MCF7 cell lines. The growth of P-glycoprotein expressing cells was inhibited by the opiates pentazocine, pethidine and naloxone to a greater extent than in drug sensitive cells. Since P-glycoprotein is localised at the plasma membrane the effects of opiates on membrane biophysical properties were investigated. The opiates caused a fluidizing effect in membranes from P-glycoprotein expressing cells and decreased the basal level of P-glycoprotein phosphorylation. In addition, they were able to increase the leakage of the membrane impermeant compound 6-carboxyfluorescein entrapped in model membrane vesicles. The ability to alter membrane biophysical properties correlated with the inhibitory effects on growth of drug resistant cells. These results suggest that the collateral sensitivity of P-glycoprotein expressing cell lines to opiates is mediated by the drugs' effects on the plasma membrane.

Effects of tamoxifen on growth and apoptosis of estrogen-dependent and -independent human breast cancer cells

BACKGROUND

Apoptosis ("programmed cell death") is an active process characterized by prominent nuclear changes and DNA cleavage, which distinguishes it from cellular necrosis. In this study we investigated whether tamoxifen (TAM) treatment of estrogen receptor ER(+) MCF-7 and ER(-) MDA-231 human breast cancer cells resulted in cytotoxicity and cellular changes typical of apoptosis.

METHODS

Cytotoxicity was measured using a tetrazolium assay. Cellular morphologic changes were observed using transmission electron microscopy. DNA cleavage was assessed using 1.6% agarose gel electrophoresis and was also quantitated biochemically.

RESULTS

Exposure of cells to TAM for 24 h resulted in dose-dependent cytotoxicity, and MCF-7 cells were somewhat more sensitive to TAM. TAM induced chromatin condensation around the nuclear periphery in both cell lines, changes typical of apoptosis. TAM-induced cytotoxicity correlated with dose-dependent DNA cleavage, which showed the characteristic "internucleosomal ladder." DNA cleavage occurred at a slightly lower TAM dose and occurred somewhat sooner in MCF-7 cells. TAM-induced DNA cleavage in MCF-7 cells was inhibited by the protein synthesis inhibitor cycloheximide, the RNA synthesis inhibitor actinomycin D, and by 17 beta-estradiol. However, in MDA-231 cells, DNA cleavage was inhibited by cycloheximide, partially but not significantly inhibited by actinomycin D, and not inhibited by 17 beta-estradiol.

CONCLUSIONS

TAM induces typical apoptosis in ER(+) or ER(-) human breast cancer cells. TAM induction of apoptosis in MCF-7 cells involves the estrogen receptor, and requires the synthesis of new protein and mRNA. TAM induction of apoptosis in MDA-231 cells depends primarily on protein synthesis. TAM-induced cytotoxicity and DNA damage appear to be explained in part by the induction of apoptosis.

Interaction of tamoxifen with the multidrug resistance P-glycoprotein

Tamoxifen is an anti-oestrogen which is currently being assessed as a prophylactic for women at high risk of breast cancer. Taxoxifen has also been shown to reverse multidrug resistance in P-glycoprotein (P-gp)-expressing cells, although the mechanism of action is unknown. In this study we demonstrate that tamoxifen interacts directly with P-gp. Plasma membranes from P-gp-expressing cells bound [3H]tamoxifen in a specific and saturable fashion. A 180 kDa membrane protein in these membranes, labelled by the affinity analogue tamoxifen aziridine and azidopine, was shown to be P-gp. Tamoxifen reduced the binding of vinblastine and azidopine to P-gp, and tamoxifen increased [3H]vinblastine accumulation in P-gp-expressing cells to levels approaching those in non-P-gp-expressing cells. However, the cellular accumulation of [3H]tamoxifen itself was not influenced by the presence of P-gp. Thus, tamoxifen appears to reverse multidrug resistance by binding to P-gp and inhibiting the transport of cytotoxic drugs, but does not itself appear to be transported by the protein.

P-glycoprotein immunostaining correlates with ER and with high Ki67 expression but fails to predict anthracycline resistance in patients with advanced breast cancer

In an attempt to further define the clinical utility of p-glycoprotein immunostaining in breast cancer, we examined 101 specimens from patients with advanced breast cancer. There was a significant correlation between estrogen receptor status and p-glycoprotein expression but only for low levels of p-glycoprotein. Premenopausal status appeared to correlate with increased p-glycoprotein expression, but this probably reflects patient selection as premenopausal patients had higher prior exposure to anthracyclines and were more likely to have received chemotherapy as initial treatment. P-glycoprotein expression was highly significantly correlated with expression of the proliferation related antigen Ki67, suggesting that p-glycoprotein expression may well be cell cycle dependent, with overexpression occurring in rapidly cycling cells. These findings may explain reported findings of modulation of p-glycoprotein expression by agents such as anti-oestrogens. P-glycoprotein positive staining did not, however, predict chemotherapy treatment failure or survival duration.

Effect of tamoxifen on mitoxantrone cytotoxicity in drug-sensitive and multidrug-resistant MCF-7 cells

The influence of the antiestrogen tamoxifen (TAM) on the activity of mitoxantrone (MXN), was evaluated against wild-type MCF-7/WT and their multidrug-resistant variant MCF-7/ADR cells. Multidrug resistance (MDR) in this cell line which was selected for resistance to Adriamycin (ADR), is associated with increased expression of P-glycoprotein (P-gp). In a clonogenic assay it was observed that TAM (1-10 microM) significantly enhanced the activity of MXN in the MCF-7/ADR but not in the drug-sensitive cell line. Isobologram analysis indicated that the effect of the combination was additive in the parental MCF-7/WT cells and strongly synergistic in the MDR MCF-7/ADR cells. Also, TAM (10 microM) caused a three-fold increase in the steady-state levels (Css) of MXN in MCF-7/ADR cells but did not modulate MXN levels in MCF-7/WT cells. The observed synergism in MCF-7/ADR cells was perhaps due to the increase in Css of MXN that may involve interaction of TAM with P-gp. The combination of MXN and TAM may be useful in the treatment of drug-sensitive and drug-resistant breast cancer.

Tamoxifen blocks chloride channels. A possible mechanism for cataract formation

Tamoxifen is an antiestrogen frequently used in the treatment of breast cancer and is currently being assessed as a prophylactic for those at high risk of developing tumors. We have found that tamoxifen and its derivatives are high-affinity blockers of specific chloride channels. This blockade appears to be independent of the interaction of tamoxifen with the estrogen receptor and therefore reflects an alternative cellular target. One of the clinical side effects of tamoxifen is impaired vision and cataract. Chloride channels in the lens of the eye were shown to be essential for maintaining normal lens hydration and transmittance. These channels were blocked by tamoxifen and, in organ culture, tamoxifen led to lens opacity associated with cataracts at clinically relevant concentrations. These data suggest a molecular mechanism by which tamoxifen can cause cataract formation and have implications for the clinical use of tamoxifen and related antiestrogens.

Reversal of P-glycoprotein-mediated multidrug resistance by pure anti-oestrogens and novel tamoxifen derivatives

In this study the ability of five novel anti-oestrogens [4-iodotamoxifen, pyrrolidino-4-iodotamoxifen, ethyl bromide tamoxifen (EBTx), ICI 164,384 (ICI 164) and ICI 182,780] to alter drug toxicity to multidrug resistant cell lines have been compared. The effect of these compounds on ATP-dependent vinblastine (VBL) transport was also tested using inside-out vesicles (IOV) prepared from highly P-glycoprotein (Pgp)-expressing CCRF-CEM/VBL1000 cells. The pure anti-oestrogen ICI 164 was most effective, enhancing doxorubicin and VBL toxicity to MCF-7Adr cells 25- and 35-fold, respectively, and was also the best inhibitor of ATP-dependent [3H]VBL accumulation by IOV. Pure anti-oestrogens, tamoxifen and iodotamoxifens completely reversed VBL resistance in the mdr1 transfected lung cancer cell line, S1/1.1, where resistance relative to wild-type cells was mediated solely by Pgp. The membrane impermeant tamoxifen derivative EBTx did not modify drug resistance, yet was as effective an inhibitor of VBL accumulation by inside-out Pgp-positive vesicles as tamoxifen. This indicates an intracellular role for tamoxifen and its derivatives in the modulation of Pgp-mediated drug resistance.

Modulation of doxorubicin-toxicity by tamoxifen in multidrug-resistant tumor cells in vitro and in vivo

Modulation of the resistance of tumors offers new strategies to improve the therapeutical treatment of cancer. In this report, the anti-oestrogen tamoxifen was investigated in multidrug-resistant tumor cells in vitro and in vivo. The doxorubicin-resistance of L 1210/DOX-tumor cells, which express the multidrug-resistance phenotype, could be completely circumvented by addition of 1 microgram/ml tamoxifen. In contrast, no increased effect could be observed in the parental L 1210 tumor cells or in cytosine arabinoside-resistant L 1210 cells not expressing the multidrug-resistance phenotype. Thus, the enhancing effect of tamoxifen was restricted only to the multidrug-resistant L 1210/DOX tumor cells. Similar to the in vitro experiments, a significant reduction in the growth in solid tumors of mice by the combined treatment of doxorubicin and tamoxifen was again observed only in the multidrug-resistant L 1210/DOX tumors.

Selective toxicity of TGF-alpha-PE40 to EGFR-positive cell lines: selective protection of low EGFR-expressing cell lines by EGF

The sensitivity of human breast and lung cancer cell lines to TGF-alpha-PE40, a novel chimeric recombinant cytotoxin composed of two independent domains, (i) TGF-alpha and (ii) a 40 kDa segment of the Pseudomonas exotoxin protein, PE-40, was investigated. Toxicity varied widely, correlated with epidermal growth factor receptor (EGFR) levels (P = 0.01) and was greatly reduced by EGF, indicating that binding of TGF-alpha-PE40 to EGFR is important in mediating toxicity. Cell lines expressing low EGFR levels were most highly protected by EGF, indicating that normal (low EGFR-expressing) tissue may be selectively protected by EGF in vivo. P-glycoprotein did not confer resistance to TGF-alpha-PE40, and toxicity was unaffected by multidrug resistance-modulating agents (cyclosporin A, tamoxifen, verapamil), indicating a role for TGF-alpha-PE40 in the clinical management of drug-resistant tumours.

The role of DT-diaphorase in determining the sensitivity of human tumor cells to tirapazamine (SR 4233)

PURPOSE

To determine the dependency of the aerobic and hypoxic toxicity of tirapazamine on the intracellular activity of DT-diaphorase.

METHODS AND MATERIALS

A panel of 18 human cell lines comprising predominantly small cell and nonsmall cell lung cancer and breast cancer lines were used. The activity of DT-diaphorase was determined in cytosolic preparations from cell lysates. The toxicity of tirapazamine was determined using the MTT assay after either 96 or 3 h aerobic exposure or 3 h treatment in hypoxia.

RESULTS

The cell lines exhibited a 5000-fold range in DT-diaphorase activity. In toxicity experiments, values of IC50 range from 10.2-120 microM and from 155-1230 for 96 and 3 h aerobic exposures, respectively. In N2, IC50s ranged from 8-55 microM. None of the toxicity values correlate with activity of DT-diaphorase, neither did the ratio of aerobic:hypoxic toxicity (differential toxicity).

CONCLUSION

The expression of DT-diaphorase in human tumor cells does not affect the toxicity of tirapazamine.

Anthracycline antibiotics in cancer therapy. Focus on drug resistance

30 years ago an anthracycline antibiotic was shown to have antineoplastic activity. This led to the development of well over 1000 analogues with a vast spectrum of biochemical characteristics. Many biological actions have been described. The original anthracyclines are active against many types of cancer and are an integral part of several curative combinations. They are ineffective against other tumours. Although some analogues show an altered spectrum of activity or an improved therapeutic index relative to the older agents, it is not clear that cardiotoxicity can be totally avoided with these agents. Primary and secondary resistance to anthracyclines remain major clinical problems. Pharmacokinetic studies have been of limited help in explaining this. Overexpression of a surface-membrane permeability glycoprotein (Pgp) was identified in ovarian cancer of patients who had clinical multidrug resistance in 1985. This led the way for the discovery of a number of resistance mechanisms in vitro. Some of these have been found in more than 1 type of cell line, and more than 1 mechanism may exist in a single cell. Additional resistance proteins have been identified, qualitative and quantitative alterations of topoisomerase II have been described, and some mechanisms in other systems have not yet been identified. Some of these may prove to be important in clinical drug resistance. Drugs such as calcium antagonists and cyclosporin, studied initially for their ability to block the Pgp pump, appear to be heterogeneous in this capacity and may have additional sites of action. It will be critical for clinical studies to define the precise resistance mechanism(s) that must be reversed. To date this has been difficult, even in trials ostensibly dealing with the original Pgp. Liposomes can potentially alter toxicity and target drug delivery to specific sites. In addition, they may permit the use of lipophilic drugs that would otherwise be difficult to administer systemically. Resistant tumours may be sensitive to anthracyclines delivered by liposomes. To reduce cardiac toxicity, administering doxorubicin (adriamycin) by slow infusion through a central-venous line should be considered whenever feasible. Monitoring of cardiac ejection fraction and the use of endomyocardial biopsy will permit patients to be treated safely after they reach the dose threshold at which heart failure begins to be a potential risk. A number of structurally modified anthracyclines with the potential advantages of decreased cardiotoxicity and avoidance of multidrug resistance mechanisms are entering clinical trials. Meanwhile, the vast weight of clinical experience leaves doxorubicin as a well tolerated and effective choice for most potentially anthracycline-sensitive tumours.