The treatment of intracranial malignant gliomas using orally administered tamoxifen therapy: preliminary results in a series of "failed" patients

Tamoxifen Induces Cytotoxic Autophagy in Glioblastoma

Glioblastomas (GBMs) are the most common and aggressive primary human malignant brain tumors. 4-Hydroxy tamoxifen (OHT) is an active metabolite of the tamoxifen (TMX) prodrug and a well-established estrogen receptor (ER) and estrogen-related receptor antagonist. A recent study from our laboratory demonstrated that OHT induced ER-independent malignant peripheral nerve sheath tumor (MPNST) cell death by autophagic degradation of the prosurvival protein Kirsten rat sarcoma viral oncogene homolog. Because both MPNST and GBM are glial in cell origin, we hypothesized that OHT could mediate similar effects in GBM. OHT induced a concentration-dependent reduction in cell viability that was largely independent of caspase activation in a human GBM cell line and 2 patient-derived xenolines. Further, OHT induced both cytotoxic autophagy and a concentration-dependent decrease in epidermal growth factor receptor (EGFR) protein levels. A GBM cell line expressing EGFR variant III (EGFRvIII) was relatively resistant to OHT-induced death and EGFRvIII was refractory to OHT-induced degradation. Thus, OHT induces GBM cell death through a caspase-independent, autophagy-related mechanism and should be considered as a potential therapeutic agent in patients with GBM whose tumors express wild-type EGFR.

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.

Prospective serial proton MR spectroscopic assessment of response to tamoxifen for recurrent malignant glioma

OBJECTIVE

Early prediction of imminent failure during chemotherapy for malignant glioma has the potential to guide proactive alterations in treatment before frank tumor progression. We prospectively followed patients with recurrent malignant glioma receiving tamoxifen chemotherapy using proton magnetic resonance spectroscopic imaging ((1)H-MRSI) to identify intratumoral metabolic changes preceding clinical and radiological failure.

METHODS

We performed serial (1)H-MRSI examinations to assess intratumoral metabolite intensities in 16 patients receiving high-dose oral tamoxifen monotherapy for recurrent malignant glioma (WHO grade III or IV) as part of a phase II clinical trial. Patients were followed until treatment failure, death, or trial termination.

RESULTS

Patients were officially classified as responders (7 patients) or non-responders (9 patients) 8 weeks into treatment. At 8 weeks, responders and non-responders had different intratumoral intensities across all measured metabolites except choline. Beyond 8 weeks, metabolite intensities remained stable in all responders, but changed again with approaching disease progression. Choline, lipid, choline/NAA, and lactate/NAA were significantly elevated (P < 0.02), while creatine (P < 0.04) was significantly reduced, compared to stabilized levels on average 4 weeks prior to failure. Lactate was significantly elevated (P = 0.036) fully 8 weeks prior to failure. In one patient who was still responding to tamoxifen at the conclusion of the trial, metabolite intensities never deviated from 8-week levels for the duration of follow-up.

CONCLUSIONS

Characteristic global intratumoral metabolic changes, detectable on serial (1)H-MRSI studies, occur in response to chemotherapy for malignant glioma and may predict imminent treatment failure before actual clinical and radiological disease progression.

Targeted therapy for malignant gliomas

The identification of markers that are associated with tumour but not normal tissue has allowed the development of highly-specific targeted therapies. Monoclonal antibodies, either alone or linked to radioisotopes or toxins, have provided a powerful tool for research, as well as the basis for promising therapeutic agents with less side effects than standard radiotherapy or chemotherapy. A new class of drugs, the tyrosine kinase inhibitors, which interfere with the function of key molecules in cancer-promoting pathways, have had a dramatic effect in haematological malignancy and are being trialled in solid tumours, including glioma. Although the problem of achieving specific, high-level delivery of these various agents to tumours in the brain remains a major issue, encouraging early results with some targeted agents support the attractive theoretical principles of this new paradigm. Further work to identify new molecular targets and to develop agents exploiting them, is needed, as well as confirmation of their safety and efficacy by clinical trials.

A phase II study of carboplatin and chronic high-dose tamoxifen in patients with recurrent malignant glioma

PURPOSE

To determine the response rate, time to disease progression, survival, and toxicity of intravenous carboplatin and chronic oral high-dose tamoxifen in patients with recurrent malignant gliomas.

PATIENTS AND METHODS

Patients with histological confirmation of recurrent malignant gliomas were eligible for this multicenter phase II trial. Treatment consisted of 400 mg/m2 carboplatin intravenously every 4 weeks and oral high dose chronic tamoxifen (80 mg bid in women and 100 mg bid in men).

RESULTS

Twenty seven patients met the eligibility criteria and were evaluable for response. The histological subtypes were: 16 (59%) glioblastoma multiforme (GBM), malignant astrocytoma (5 patients), malignant mixed glioma (5 patients), and glioblastoma/gliosarcoma (1 patient). Twenty-two patients (82%) had an ECOG performance status of 0 or 1. No complete responses were observed, 4 patients (15%) achieved a partial response, and 14 patients (52%) had stable disease. Median time to progression was 3.65 months (95%CI 2.56, 4.83). Median overall survival was 14.09 months (95%CI 7.06, 19.91). One patient with a recurrent GBM had a sustained partial response and is progression free 81 months since starting treatment. Another patient with mixed malignant oligoastrocytoma also had a prolonged partial response (lasting 63 months) and is alive 84 months after treatment for recurrence. The most frequently reported grade 3 or 4 toxicities were fatigue (19%), nausea (11%) and anorexia (11%).

CONCLUSIONS

Carboplatin and high dose tamoxifen has similar response rates to other regimens for recurrent malignant gliomas and are probably equivalent to those found using tamoxifen as monotherapy. Long-lasting periods of disease free survival in some patients (particularly those with malignant mixed oligo astrocytomas) were found.

Agents with Selective Estrogen Receptor (ER) Modulator Activity Induce ApoptosisIn vitroandIn vivoin ER-Negative Glioma Cells

Tamoxifen, a member of the selective estrogen receptor modulator (SERM) family, is widely used in the treatment of estrogen receptor (ER)-expressing breast cancer. It has previously been shown that high-dose tamoxifen has cytotoxic activity against glioma cells, but whether this effect is drug specific or represents a general property of SERMs is unknown. In this study, we demonstrate that tamoxifen and CC-8490, a novel benzopyranone with SERM activity, induce glioma cell apoptosis in a dose- and time-dependent manner. Moreover, administration of tamoxifen and CC-8490 suppresses tumor growth in vivo and extends animal survival in glioma xenograft models. None of the eight glioma cell lines examined express either ER-alpha or -beta, suggesting the mechanism for tamoxifen- and CC-8490-induced glioma cell apoptosis is independent of the ER signaling pathway. Complementary DNA microarray expression profiling allowed us to identify a subset of genes specifically regulated by tamoxifen and CC-8490, and not by other apoptotic stimuli, including nuclear factor (NF)-kappaB with its target genes IEX-3, SOD2, IL6, and IL8. We demonstrate that suppression of NF-kappaB activation markedly enhances SERM-induced apoptosis, suggesting a role for NF-kappaB in protecting glioma cells from SERM-induced cytotoxicity. These findings demonstrate for the first time that a SERM other than tamoxifen can induce glioma cell apoptosis in vitro and in vivo and that the clinical efficacy of SERMs for the treatment of malignant gliomas could potentially be enhanced by simultaneous inhibition of the NF-kappaB pathway.

Tamoxifen increases photodynamic therapeutic response of U87 and U25ln human glioma cells

We tested the hypothesis that Tamoxifen (TMX), an inhibitor of protein kinase C (PKC), augments the cytotoxicity of photodynamic therapy (PDT) treatment of human (U87) and (U25ln) glioma cells. U87 and U25ln glioma cells were plated and treated with PDT using Photofrin as the sensitizer. Cells were treated with Photofrin at various doses and with various optical (632 nm) irradiation intensities 24 h later. Cells were also treated with Photofrin at a fixed dose alone and with various doses of Tamoxifen and subjected to laser treatment 24 h later. Tumor response was tested using the (3-94,5-dimethyl-2-yl)-2,5-diphenyl-tetrazolium (MTT) method. Total toxicity of U87 cells was achieved with PDT at all doses of Photofrin (1, 2.5, 5, 10 microg/ml) with irradiation densities equal to or greater than 200 mJ/cm2. Using an irradiation intensity of 100 mJ/cm2, U87 and U25ln cells were killed in a Photofrin dose-dependent manner. Significant cytotoxicity was detected with Photofrin doses of 5 microg/ml (p < 0.05) and 10 microg/ml (p < 0.001). Tamoxifen at a dose of 500 microg/ml and higher, significantly increased the toxicity of the PDT response with 5 microg/ml Photofrin and 100 mJ/cm2 (p < 0.05). In summary, our data demonstrate that Tamoxifen significantly enhances the Photofrin PDT activity of U87 and U25ln human glioma cells.

Proteases and the biology of glioma invasion

Despite optimal clinical treatment, the prognosis for malignant gliomas remains poor. One of the primary reasons for treatment failure is not diffuse dissemination, but local invasion. Recently, there has been an increase in information regarding specific molecules that determine the aggressiveness and invasion potential of high-grade astrocytic tumors. In particular, expression of matrix metalloproteases in high-grade gliomas appears to correlate with tissue invasiveness. It is the purpose of the present review to describe the connection between alterations in growth-related genes, protease activity, and tumor biology, and how these connections may suggest potential novel therapeutic targets.

Biological mechanisms of glioma invasion and potential therapeutic targets

The current understanding of glioma biology reveals targets for anti-invasive therapy which include manipulations of extracellular matrix and receptors, growth factors and cytokines, proteases, cytoskeletal components, oncogenes and tumor suppressor genes. A better understanding of the complex regulation and the signalling molecules involved in glioma invasion is still needed in order to design new and effective treatment modalities towards invasive tumor cells. Representative and valid in vitro experimental systems and animal models of gliomas are necessary for the characterization of the invasive phenotype and further development of anti-invasive therapy. In the future, it will probably be important to move from comparative genomic modelling through protein characterization based on advanced proteomic techniques to analyse tissue samples, where the aim for gliomas should be to compare invaded and non-invaded tissue. This will hopefully render promising new therapeutic targets for gliomas.

Surgery, tamoxifen, carboplatin, and radiotherapy in the treatment of newly diagnosed glioblastoma patients

A historically controlled phase II study was undertaken to investigate the efficacy and toxicity of a postoperative treatment consisting of high-dose continuous tamoxifen, carboplatin and radiotherapy in patients with newly diagnosed glioblastoma. Between 1995 and 1998, 50 patients with newly diagnosed glioblastomas underwent surgery and were subsequently treated with 200 mg day(-1) tamoxifen continuously, 3 cycles of carboplatin (300 mg m(-2)), and radiotherapy. Survival data for a historical control group were calculated from respective prognostic indices and were obtained from studies with comparable patient populations treated with operation and radiotherapy only. In our study, the median time to tumor progression was 30 weeks and the median survival time (MST) 55 weeks (95% confidence interval: 46-63 weeks). The MST of the control group (48 weeks) showed to be within this interval. In addition to already known prognostic factors in malignant gliomas (age, Karnofsky performance score, extent of tumor resection), the gender (females lived longer than males, p = 0.0025) showed to influence survival. Serious side effects (thrombosis, pulmonary embolism) occurred in 6 patients. A high incidence of multifocal tumor recurrences (33%), which might be related to study-treatment, was observed. In conclusion, the combined therapy failed to demonstrate a higher efficacy than standard treatment for glioblastoma patients.

Treatment of supratentorial glioblastoma multiforme with radiotherapy and a combination of BCNU and tamoxifen: a phase II study

From May 1990 to November 1994, 70 consecutive patients suffering from glioblastoma multiforme were treated following surgery with conventional radiotherapy and adjuvant IV BCNU administered alone or in combination with tamoxifen. Twenty-five patients received BCNU alone (control group A) while 24 patients also received 40 mg of tamoxifen (TMX) PO daily (group B) and 21 received 100 mg of TMX PO daily (group C). There were no significant differences between the 3 groups concerning age, type of resection and median post-operative Karnofsky performance status (KPS). Blood toxicity over grade II occurred in 33.5% of patients receiving TMX versus 12% of patients treated with BCNU alone (p < 0.05). Deep venous thrombosis complications were observed in 4 patients of each TMX group, whereas they were not observed in the control group (p < 0.04). Median time to tumor progression (MTTP) was 35 weeks in the control group and 27 weeks in both TMX groups B and C. Median survival time (MST) was 56, 66 and 51 weeks, respectively. These results suggest that the addition of TMX to standard treatment of glioblastomas does not affect the time to tumor progression and overall survival but may increase the risk of deep venous thrombosis or nitrosourea-induced blood toxicity.

Use of tamoxifen in hepatocellular carcinoma: a review and paradigm shift

Hepatocellular carcinoma is often diagnosed at a late, inoperable stage for which there are no uniformly efficacious treatment available presently. The oral anti-oestrogen drug, tamoxifen, has been used in such patients, based on the belief that the growth of hepatocellular carcinoma is promoted by endogenous oestrogen via a receptor-mediated process. In this review, we examine the trials reported in the literature using tamoxifen in hepatocellular carcinoma. Randomized controlled trials with tamoxifen have so far revealed mixed results. We propose that this may be due to the fact that the mechanism of action of tamoxifen in hepatocellular carcinoma is via an oestrogen-receptor independent pathway that requires much higher doses of tamoxifen for activation than those used in the trials so far. Thus there must be a paradigm shift to dissociate the action of tamoxifen from oestrogen receptors in hepatocellular carcinoma. This means that future trials with tamoxifen in hepatocellular carcinoma should use higher doses of tamoxifen, at least four to eight-fold that of the dose that is efficacious in an oestrogen-receptor dependent mechanism.

Radiation therapy and high-dose tamoxifen in the treatment of patients with diffuse brainstem gliomas: results of a Brazilian cooperative study. Brainstem Glioma Cooperative Group

PURPOSE

The efficacy of radiation therapy (RT) combined with tamoxifen (TX) was tested in patients diagnosed with diffuse brainstem gliomas in a multicenter trial.

PATIENTS AND METHODS

TX was administered orally (maintenance dose: 200 mg/m(2) per day) along with conventional local RT and then continued for 52 additional weeks. Survival, tumoral radiologic response, and toxicity were evaluated. Compliance was assessed using pharmacokinetic measurements.

RESULTS

Of 29 patients, 27 completed RT (median dose, 54 Gy). Of 22 assessable patients, 11 (50%) had an objective radiologic response. The mean TX steady-state serum level was 2.44 micromol/L +/- 1.02 micromol/L. Only three patients completed the entire course of treatment without tumoral progression or significant toxicity. Common side effects included nausea and vomiting. Hepatotoxicity (five patients), neurotoxicity (two patients), venous thrombosis (one patient), bilateral ovarian cysts (two patients), and transient neutropenia (one patient) were also observed. Median survival was 10.3 months. Only four patients remain alive without tumoral progression. The 1-year survival rate (mean +/- SD) was 37.0% +/- 9.5%.

CONCLUSION

This treatment combination produced no significant change in the overall poor prognosis of these patients. Most tumors responded initially to treatment but recurred as the study progressed. A minority of patients seemed to benefit from the extended use of TX. Generally, treatment was well tolerated, with good patient compliance, but we recommend continuous close monitoring for side effects. Based on our poor results, we recommend that alternative treatments be tested in patients with this type of tumor.

Tamoxifen inhibits particulate-associated protein kinase C activity, and sensitises cultured human glioblastoma cells not to etoposide but to gamma-radiation and BCNU

We investigated the potential mechanisms of tamoxifen cytotoxicity in the U-373, U-138, and U-87 human glioblastoma cell lines, namely interference with protein kinase C (PKC) activity, the oestrogen receptor, and/or the production of transforming growth factor beta 1 (TGF-beta 1). We further examined the effects of tamoxifen on the cytotoxicity exerted by gamma-radiation, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), and etoposide in this cell line panel. Thus, the cells were treated for 4 days with tamoxifen, gamma-radiation, purified recombinant human TGF-beta 1 (rhTGF-beta 1), BCNU, or etoposide, either alone or at certain combinations. Cellular responses were evaluated with the sulphorhodamine B assay, as well as by multiple drug effect analysis, and related to PKC activities in particulate and cellular fractions; cellular oestrogen receptor contents; and the influence of rhTGF-beta 1 on cell growth. Tamoxifen inhibited cell proliferation as well as the phosphorylation capacity of the particulate, but not of the cytosolic fractions dose-dependently, at comparable kinetics, and at IC50 values of approximately 15 microM. At these concentrations, tamoxifen acted synergistically with gamma-radiation (4- to 6-fold) and additively with BCNU (approximately 2-fold), but did not affect etoposide cytotoxicity. The cells were negative to immunostaining for the oestrogen receptor, and rhRGF-beta 1 did not influence their growth up to 100 nm. Our data suggest that tamoxifen can sensitise cultured glioblastoma cells not to etoposide but to gamma-radiation and BCNU, possibly through interference with membrane PKC, supporting its evaluation in experimental protocols for primary malignant gliomas.

Clinical and radiographic response in three children with recurrent malignant cerebral tumors with high-dose tamoxifen

The purpose of this study was to deliver tamoxifen as antiangiogenic therapy to children with recurrent progressive malignant brain tumors. Tamoxifen was administered orally in very high dosage to one child as monotherapy and to two children in combination with oral etoposide and dexamethasone. One boy was diagnosed with high-grade astrocytoma in the brain stem, one girl with anaplastic ependymoma of the fourth ventricule, and one girl with high-grade astrocytoma in the midbrain. Conventional treatment with multiple surgeries, first- and second-line chemotherapy, and external beam therapy had failed. Tumor reduction was seen in radiographic images together with clinical improvement in 2 children, and clinical and radiographic halting of tumor progression was demonstrated in the patient with anaplastic ependymoma. None of the patients developed complications from the treatment. Follow up of the patients ranged from 15 to 30 months with a mean of 17 months. These encouraging preliminary results suggest a potential for this type of therapy. More studies are needed to start clinical trials and prove that angiostatic activity may contribute to the therapeutic effect of antiestrogens in estrogen receptor-negative tumors.

Irinotecan therapy in adults with recurrent or progressive malignant glioma

PURPOSE

To determine the activity, toxicity, and pharmacokinetics of irinotecan (CPT-11, Camptosar; Pharmacia & Upjohn, Kalamazoo, MI) in the treatment of adults with progressive, persistent, or recurrent malignant glioma.

PATIENTS AND METHODS

Patients with progressive or recurrent malignant gliomas were enrolled onto this study between October 1996 and August 1997. CPT-11 was given as a 90-minute intravenous (i.v.) infusion at a dose of 125 mg/m2 once weekly for 4 weeks followed by a 2-week rest, which comprised one course. Plasma concentrations of CPT-11 and its metabolites, SN-38 and SN-38 glucuronide (SN-38G), were determined in a subset of patients.

RESULTS

All 60 patients who enrolled (36 males and 24 females) were treated with CPT-11 and all were assessable for toxicity, response, and survival. Pharmacokinetic data were available in 32 patients. Nine patients (15%; 95% confidence interval, 6% to 24%) had a confirmed partial response, and 33 patients (55%) achieved stable disease lasting more than two courses (12 weeks). Toxicity observed during the study was limited to infrequent neutropenia, nausea, vomiting, and diarrhea. CPT-11, SN-38, and SN-38G area under the plasma concentration-time curves through infinite time values in these patients were approximately 40%, 25%, and 25%, respectively, of those determined previously in patients with metastatic colorectal cancer not receiving antiepileptics or chronic dexamethasone treatment.

CONCLUSION

Response results document that CPT-11, given with a standard starting dose and treatment schedule, has activity in patients with recurrent malignant glioma. However, the low incidence of severe toxicity and low plasma concentrations of CPT-11 and SN-38 achieved in this patient population suggest that concurrent treatment with anticonvulsants and dexamethasone enhances drug clearance.

Tamoxifen radiosensitization in human glioblastoma cell lines

OBJECT

A combined tamoxifen and radiation therapy is being used in clinical trials to treat glioblastoma multiforme (GBM). The rationale behind this therapy is that tamoxifen is a radiosensitizer. However, the evidence for this is weak. The authors, therefore, examined the effect of combined radiation-tamoxifen therapy in three GBM cell lines of human origin.

METHODS

The GBM cell lines were exposed to different concentrations (0.3-5 microg/ml) of tamoxifen and subsequently irradiated at varying doses (0.8-5 Gy). Tumor growth inhibition was measured using a proliferation assay. The interaction of tamoxifen and radiation therapies was quantified using the combination index method, which distinguishes whether a combined antitumor effect is synergistic, additive, or antagonistic. At high doses of tamoxifen or radiation there was significant inhibition of tumor cell proliferation. At low doses of either therapeutic agent, there was little effect. In one cell line, synergism occurred at high doses of tamoxifen and radiation. In the other two cell lines, an additive effect was observed. In only one of the three cell lines was there synergy between tamoxifen and radiation at doses that significantly inhibited tumor proliferation.

CONCLUSIONS

Because synergy could not be demonstrated in all three cell lines at active dosages, the clinical combination of tamoxifen and radiation therapies may not be of benefit to all patients.

Procarbazine and high-dose tamoxifen as a second-line regimen in recurrent high-grade gliomas: a phase II study

PURPOSE

A phase II study was conducted in patients with high-grade gliomas that recurred after surgery plus radiotherapy and a first-line nitrosourea-based regimen. Our aim was to investigate the efficacy of procarbazine (PCB) combined with high-dose tamoxifen in relation to tumor control, toxicity, and time to progression (TTP).

PATIENTS AND METHODS

Fifty-three patients were treated with procarbazine in repeated 30-day courses at 100 mg/m2/d plus tamoxifen 100 mg/d, with a 30-day interval between courses. Thirty-four patients had been pretreated with a first-line nitrosourea-based chemotherapy regimen (group A), and 19 patients had also been pretreated with a second-line chemotherapy regimen consisting of carboplatin and teniposide (group B). Twenty-one of the patients had also been procarbazine pretreated, whereas the remaining 32 patients were not procarbazine pretreated.

RESULTS

The response was assessed in 51 patients, 28 of whom had glioblastoma multiforme (GBM) and 23 of whom had anaplastic astrocytoma (AA). There were two complete responses (CR) (4%) and 13 partial responses (PR) (25.5%). The overall response rate (CR + PR) was 29.5% (SE, 6.4; 95% confidence interval [CI], 23 to 35.8). Seventeen patients (32%) had stable disease (SE, 6.2; 95% CI, 21 to 33.6). The median TTP was 13 weeks for patients with GBM and 33 weeks for patients with AA (P = .006). The median survival time (MST) was 27 weeks for patients with GBM and 57 weeks for those with AA (P = .006).

CONCLUSION

Combined PCB and tamoxifen as a second-line regimen gave a reasonably high response rate in patients with heavily pretreated high-grade gliomas. However, although it resulted in an improvement in the patients' quality of life and/or performance status, it was not followed by an increased TTP or MST.

Protein kinase C inhibition by UCN-01 induces apoptosis in human glioma cells in a time-dependent fashion

Recent studies in our laboratory have shown that UCN-01 (7-hydroxystaurosporine), which is a derivative of the non-selective protein kinase inhibitor staurosporine that exhibits relative selectivity for protein kinase C (PKC), is a potent inhibitor of glioma growth in in vitro and in vivo models. This agent exhibits both cytotoxic and cytostatic effects, depending on the time period of drug exposure. In the present study, we examined whether UCN-01-induced cytotoxicity correlated with the induction of apoptosis, and characterized further the time course of this process as a prelude to application of UCN-01 in clinical trials. We first demonstrated that the cytotoxic effects of UCN-01 were associated with the induction of morphological features of apoptosis. Secondly, we identified electrophoretic features of apoptosis semiquantitatively at a series of time points using field inversion gel electrophoresis. These studies showed a peak in the induction of high-molecular-weight DNA fragmentation after 3-6 days of drug treatment. Thirdly, we measured the percentage of cells undergoing apoptosis at various time points using a terminal transferase-catalyzed in situ end-labeling technique, which confirmed a time- and concentration-dependent increase in apoptotic cell numbers. This correlated with a progressive decrease in the percentage of cells that were viable as assessed by trypan blue exclusion. Cell killing peaked within 2-4 days after beginning UCN-01 treatment, but continued at a lower level in the ensuing days. Taken together, these studies demonstrated that extended periods of exposure to UCN-01 are needed for optimal manifestation of cytotoxic effects against glioma cells, a factor that must be taken into consideration in the design of future clinical trials with this agent for malignant gliomas.

Tamoxifen modulation of carboplatin cytotoxicity in a human U-138 glioma cell line

Glioma cells express high protein kinase C (PKC) activity, which may represent an important therapeutic target. Tamoxifen (TAM) has moderate PKC-inhibiting activity, blocking DNA synthesis and cellular proliferation in human glioma cells at concentrations that can be achieved therapeutically. Carboplatin (CBDCA), a second-generation platinum derivative, induces intra- and interstrand DNA-protein crosslinks producing inhibition of tumor-cell growth. In the present study, the effect of TAM, CBDCA, and the combination of both was evaluated against the human established U-138 glioma cell line during the exponential growth phase (48-72 h) by means of both the Biorad protein assay (BPA) method and Trypan blue exclusion study (TBES). Both TAM and CBDCA reduced the cellular growth rate, with a median 50%-inhibiting concentration (IC50) of 12.5 microM for TAM and 350 microM for CBDCA. The U-138 glioma cell line showed a moderate response to 100 microM of CBDCA, with < or = 10% reduction of the growth rate. The association of both chemotherapeutic agents induced a 98% reduction of the IC50 dose of TAM (0.1 microM), and a 71% reduction of the IC50 dose of CBDCA (100 microM). During the combinational TAM CBDCA exposure we observed a cytotoxic effect of TAM at concentrations lower than 0.1 microM, not recognized using it as a single drug. The differences observed among the IC50 doses (TAM, CBDCA, TAM-CBDCA) and among treated and untreated matched control cells were statistically significant (P < 0.01). Our results confirm previous observations about the efficacy in vitro of TAM against human glioma cell lines and show a marked enhancement of this activity by CBDCA.

Tamoxifen and carboplatin combinational treatment of high-grade gliomas. Results of a clinical trial on newly diagnosed patients

Between April, 1992 and December, 1995, forty consecutive patients with a cerebral malignant glioma (WHO Grade III and IV) were enrolled in a trial consisting in surgery and post-operative administration of radiotherapy (4500-6000 cGy), carboplatin (CBDCA; dose of 450-600 mg/m2), and oral tamoxifen (TAM; at doses of 40, 80 or 120 mg/day). Two patients of the TAM group died in the postoperative period from a pulmonary embolism and myocardial infarction, respectively. The patients (all dosages combined) had a median survival time of 13 months from the time of diagnosis. The 12-month and 24-month survival rates were 52% and 32%, respectively. The median relapse-free survival time was 7 months. Patients treated with higher doses of TAM (80-120 mg/day) demonstrated a longer median survival rate (13 months both) and a longer 12-month survival result (58% and 76%, respectively). Patients who assumed TAM for a period longer than 3 months (group +3) have a higher median survival rate (16 months) and better 12-month and 24-month results (62% and 40%, respectively). Moreover, the median relapse-free survival time was 10 months (versus 6 months in group -3; p = 0.0038). However, it is not possible to exclude that patients of group +3 had a slower growing or a stable tumor and were well enough to assume TAM for a longer period. The results observed in the TAM-group have been compared with those of 40 matched controls treated with surgery, radiotherapy and CBDCA. These patients had a median survival time of 9 months (p = 0.04) and the 12-month and 24-month survival rates were 30% and 0%, respectively. The median relapse-free survival time was 4 months (p = 0.0014). These data suggest a potential role for combinational TAM-CBDCA therapy in the post-operative treatment of cerebral malignant gliomas; further clinical phase III trials, especially those with higher dosages of TAM are warranted.

High dose oral tamoxifen and subcutaneous interferon alpha-2a for recurrent glioma

Chemotherapeutic regimens in present use for recurrent glioma have substantial toxicity. Activity against recurrent gliomas has been reported for both tamoxifen and interferon alpha, agents that have more acceptable toxicity profiles and that can be administered in an outpatient setting. We tested the efficacy and toxicity of the combination of high-dose tamoxifen and interferon alpha in adults with recurrent glioma in a phase II trial. Eligible patients had radiographically measurable recurrent gliomas of any grade after initial radiation therapy. Interferon-alpha [6 x 10(6) U subcutaneously three times per week] and tamoxifen (240 mg/m2/day orally) were administered continuously. Treatment response was assessed at 6 week intervals using clinical and radiographic criteria. Eighteen patients (11 males and 7 females) were enrolled. Median age was 41 years (range 23-61 years). All patients had gliomas that progressed after radiation therapy and nitrosourea chemotherapy. The histologic diagnosis of the original tumor was glioblastoma multiforme in 8 patients, anaplastic astrocytoma in 5 patients, astrocytoma in 4 patients and mixed malignant glioma in 1 patient. Reversible moderate to severe neurological toxicity manifested by dizziness and unsteady gait was seen at tamoxifen doses of 240 mg/m2/day. Although the initial tamoxifen dose was reduced to 120 mg/m2/day, moderate neurotoxicity was noted at this dose as well and the trial was closed early. The combination of oral tamoxifen (120 to 240 mg/m2/day) and subcutaneous interferon-alpha [6 x 10(6) U three times per week] was associated with significant neurotoxicity in this group of recurrent glioma patients, resulting in early study closure. Of 16 evaluable patients, 12 had progressive disease after one cycle of treatment, 3 had stable disease, and there was one minor response. Gradual dose escalation may be required if similar patients are to be treated with high dose tamoxifen in conjunction with interferon.

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.

Signal transduction pathways and their relevance in human astrocytomas

Aberrations in a number of signal transduction pathways have been identified as playing a key role in the molecular pathogenesis of astrocytomas and their progression to high grade glioblastoma multiforme (GBM). GBMs are characterized by overexpression of the Platelet Derived Growth Factor Receptors (PDGFR) and their ligands (PDGF), as well as the Epidermal Growth Factor Receptor (EGF-R). These receptors activate the Ras pathway, a key cellular signal transduction pathway, culminating in the activation of a wide range of Ras-dependent cellular events. GBMs have also been found to either overexpression or lose expression of various Protein Kinase C (PKC) isoforms. Major strides are being made in developing pharmacological agents which specifically inhibit these growth factor receptors and intracellular signal transduction pathways. Elucidating the role of these pathways in GBMs is thus of major clinical importance, as these novel molecularly-targeted agents may prove of use in the clinical management of GBMs in the future.

The effect of calphostin C, a potent photodependent protein kinase C inhibitor, on the proliferation of glioma cells in vitro

Recent studies have suggested that the proliferation of malignant gliomas may result from activation of protein kinase C (PKC)-mediated pathways; conversely, inhibition of PKC may provide a strategy for blocking tumor growth. In the current studies, we examined the effect of a novel PKC inhibitor, calphostin C, which is a selective, highly potent, photo-activatable inhibitor of the PKC regulatory domain, on the proliferation and viability of three established and three low-passage malignant glioma cell lines, four low-passage low-grade glioma cell lines, and in adult human and neonatal rat non-neoplastic astrocyte cell lines in vitro. Under light-treated conditions, calphostin C consistently inhibited cell proliferation in each of the tumor cell lines and in the neonatal rat astrocyte cell line with a 50% effective concentration of 30 to 50 ng/ml (40 to 60 nm), which was comparable to the previously reported median inhibitory concentration (IC50) for PKC inhibition by calphostin C. Complete elimination of proliferation was achieved at concentrations of 50 to 100 ng/ml (60 to 125 nM). Cell viability decreased sharply with calphostin C concentrations of 100 to 300 ng/ml (125 to 380 nM). In contrast, under light-shielded conditions, calphostin C had a comparatively modest effect on cell proliferation and viability, with a median effective concentration of approximately 300 ng/ml. No significant inhibition of proliferation was noted in the non-neoplastic adult astrocyte cell line under either light-treated or light-shielded conditions. These findings provide further evidence that PKC may play an essential role in mediating the proliferation of both benign and malignant glioma cells in vitro and may also contribute to the proliferation of non-neoplastic immature astrocytes. Light-sensitive inhibition of proliferation and viability by agents such as calphostin C may provide a novel strategy for applying photodynamic therapy to the treatment of neoplastic glial cells.

RU 58668: further in vitro and in vivo pharmacological data related to its antitumoral activity

Previous studies with the pure antiestrogen RU 58668 showed that this compound proved to be highly antiproliferative in vitro, and to be the only antiestrogenic compound so far known to induce long-term regression of MCF-7 tumours implanted into nude mice. In order to obtain more insight into the therapeutic potential of this molecule, we performed a new set of experiments in vitro and in vivo in comparison with tamoxifen and/or ICI 182,780. In vitro, 1 nM RU 58668 induced an accumulation of MCF-7 cells in G0/G1 phases of the cell cycle within 48 h and, in contrast to trans-4-hydroxy-tamoxifen, blocked the invasiveness of ras-transfected MCF-7 cells into the chick embryo heart during the three weeks of co-culture. An in vivo dose-effect relationship study showed that RU 58668 induced a regression of MCF-7 tumour with as low a dose as 10 mg/kg/week, and that such an effect can not be obtained either with a sublethal dose of adriamycin or with ICI 182,780, (2-250 mg/kg/week). This reduction in the tumour volumes accords with histological modifications of the tumours, which showed a decrease in the ratio of epithelial cells over the tumoral mass, and with a concomitant decrease in their regrowth potential when reimplanted into naive nude mice. Taken together, these results suggest a promising usefulness for RU 58668 in the treatment of metastatic breast cancer in women.

Blocking of glioma proliferation in vitro and in vivo and potentiating the effects of BCNU and cisplatin: UCN-01, a selective protein kinase C inhibitor

Seven-hydroxystaurosporine (UCN-01) is a derivative of the nonselective protein kinase inhibitor staurosporine that exhibits significant selectivity for protein kinase C (PKC) in comparison to a variety of other intracellular kinases and appears to be well tolerated in vivo at concentrations sufficient to achieve effective inhibition of PKC. Because recent studies have indicated that the proliferation of malignant gliomas may result from activation of PKC-mediated pathways and, conversely, may be inhibited by blocking PKC, the authors examined the efficacy of this agent as an inhibitor of proliferation in three established and three low-passage malignant glioma cell lines in vitro. A striking inhibition of proliferation was produced by UCN-01 in each of the cell lines, with a median effective concentration of 20 to 100 nM, which correlated with the median in vitro PKC inhibitory concentration of 20 to 60 nM for this agent in the U-87 and SG-388 glioma cell lines. Inhibition-recovery studies of clonogenic activity indicated that UCN-01 had both cytostatic and cytotoxic effects on the treated cells. Proliferation resumed after short-term (6- and 24-hour) exposures to this agent; in contrast, with longer exposures, recovery of proliferative activity was severely compromised. In addition, UCN-01 enhanced the inhibition of glioma cell proliferation achieved with conventional chemotherapeutic agents, exhibiting synergistic effects with cisplatin and additive effects with 1,3-bis(2-chloroethyl)-1-nitrosourea. In vivo studies in which UCN-01 was administered by continuous intraperitoneal infusion in subcutaneous and intracranial intraparenchymal nude rat models demonstrated significant activity against U-87 glioma xenografts at dose levels that were well tolerated. It is concluded that UCN-01 is an effective agent for the inhibition of glioma proliferation in vitro and in vivo and has potential for clinical applicability in the treatment of human gliomas.

Signal transduction for proliferation of glioma cells in vitro occurs predominantly through a protein kinase C-mediated pathway

Previous work has demonstrated that glioma cells have very high protein kinase C (PKC) enzyme activity when compared to non-malignant glia, and that their PKC activity correlates with their proliferation rate. The purpose of this study was to determine whether the elevated PKC activity in glioma is secondary to an autonomously active PKC isoform implying oncogenic transformation, or whether this activity is driven by upstream ligand-receptor tyrosine kinase interactions. We treated established human glioma cell lines A172, U563 or U251 with either the highly selective PKC inhibitor CGP 41 251, or with genistein, a tyrosine kinase inhibitor. The proliferation rate and PKC activity of all the glioma lines was reduced by CGP 41 251; the IC50 values for inhibiting cell proliferation corresponded to the IC50v values for inhibition of PKC activity. Genistein also inhibited cell proliferation, with IC50 proliferation values approximating those for inhibition of tyrosine kinase activity in cell free protein extracts. Importantly, in genistein-treated cells, downstream PKC enzyme activity was dose dependently reduced such that the correlation coefficient for effects of genistein on proliferation rate and PKC activity was 0.92. These findings suggest that upstream tyrosine kinase linked events, rather than an autonomously functioning PKC, result in the high PKC activity observed in glioma. Finally, fetal calf serum (FCS) evoked a strong mitogenic effect on glioma cell lines. This mitogenic activity was completely blocked by CGP 41 251, suggesting that although the many mitogens in FCS for glioma cells signal initially through genistein-inhibitable tyrosine kinases, they ultimately channel through a PKC-dependent pathway. We conclude that proliferative signal transduction in glioma cells occurs through a predominantly PKC-dependent pathway and that selectively targeting this enzyme provides an approach to glioma therapy.

Apoptosis of human glioma cells in response to calphostin C, a specific protein kinase C inhibitor

Calphostin C acts at the regulatory domain as a highly selective inhibitor of protein kinase C (PKC), and staurosporine acts at the catalytic domain as a nonspecific PKC inhibitor. The authors investigated the capacity of calphostin C and staurosporine to promote apoptotic fragmentation of DNA in four human glioma cell lines. The exposure of glioma cell lines to 100 nM calphostin C for 2 to 8 hours induced a decrease in particulate PKC activities and exposure for 16 to 24 hours produced a concentration-dependent increase in internucleosomal DNA cleavage on agarose gel electrophoresis. In addition, the human glioma cells showed the classic morphological features of apoptosis: cell shrinkage, nuclear condensation, and the formation of apoptotic bodies. A 24-hour exposure to staurosporine failed to induce internucleosomal DNA fragmentation at concentrations generally used to achieve maximum inhibition of enzyme activity (50 nM) but promoted fragmentation at considerably higher concentration (more than 200 nM). Deoxyribonucleic acid fragments obtained from cells exposed to 100 nM calphostin C for 16 to 24 hours possessed predominantly 5'-phosphate termini, consistent with the action of a Ca++/Mg(++)-dependent endonuclease. Northern and Western blot analyses revealed that the exposure to 100 nM calphostin C for 4 hours failed to alter bcl-2 transcript and protein, but exposure for more than 8 hours decreased the amount of bcl-2 transcript and protein. Together, these observations suggest that calphostin C is capable of inducing apoptotic DNA fragmentation and cell death in a highly concentration dependent manner in human glioma cells and that the apoptosis is closely associated with the decrease in transcription and translation of bcl-2.

Inhibitory effects of tamoxifen and tumor necrosis factor alpha on human glioblastoma cells

We reported previously that tumor necrosis factor alpha (TNF alpha) inhibited proliferation and invasiveness of human malignant glial cells. Because tamoxifen, an estrogen antagonist, has also been shown to inhibit growth of such cells, we hypothesized that a combination of tamoxifen and TNF alpha might be more effective than either reagent alone. TNF alpha (1-100 ng/ml) or tamoxifen (80 ng/ml-2 micrograms/ml) alone inhibited proliferation of a human glioblastoma cell line (WITG3) in a dose-dependent fashion; in combination, tamoxifen and TNF alpha yielded additive growth inhibition. Apoptotic cells characterized by nuclear fragmentation were detectable after 48 h of TNF alpha or tamoxifen exposure and were significantly increased by combination treatment. In non-neoplastic human astroglia and fibroblasts, proliferation was unaffected by tamoxifen, and enhanced by TNF alpha as previously reported. Staurosporine (2-50 nM), which has been reported to augment the effects of TNF alpha, was less effective than tamoxifen against WITG3 and, in addition, was markedly inhibitory to non-neoplastic glial cells. Binding studies yielded no evidence of WITG3 estrogen or progesterone receptors, nor of tamoxifen effects on TNF alpha receptors. Data suggest that TNF alpha and tamoxifen in combination display growth-regulatory properties, which (a) are more inhibitory to human glioblastoma cells than either agent alone, (b) do not affect non-neoplastic glia, (c) do not require either estrogen/progesterone receptors or alteration of external TNF alpha receptors, and (d) may involve apoptosis.

Paradoxical elevation of Ki-67 labeling with protein kinase inhibition in malignant gliomas

The monoclonal antibody Ki-67 recognizes a nuclear antigen expressed in the G1, S, G2, and M phase of the cell cycle and has been used extensively as an indicator of cellular proliferation in malignant gliomas, both in the laboratory and clinically. Recently, protein kinase C (PKC) inhibitors have been demonstrated to inhibit malignant glioma growth both in in vitro and in vivo. This study was undertaken to determine whether Ki-67 could function as an indicator of cellular proliferation rate after PKC inhibition in gliomas and to explore cell cycle specificity of such inhibition. Both established and low-passage malignant glioma cell lines have previously been shown to be sensitive to growth inhibition by the PKC inhibitors staurosporine and tamoxifen in vitro (IC50 in the nanomolar and micromolar ranges, respectively), as measured by cell numbers, [3H]thymidine uptake, and flow-cytometric DNA analysis. However, in the same cells that are inhibited by staurosporine and tamoxifen on these assays, and on the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl tetrazolium bromide (MTT) assay in the present study, the Ki-67 labeling index paradoxically increased in a dose-related manner with the same treatments, as measured by immunohistochemistry and confirmed by flow cytometry. For example, in established line U-87, a 20.5% decrease in thymidine uptake and a 28.5% decrease in absorbance on the MTT assay produced by tamoxifen at 1 microM was associated with an increase in Ki-67 labeling from 42% to 62%; staurosporine, which produces a 78.8% decrease in thymidine uptake in cell line A-172 at 10 nM, produced an increase in Ki-67 labeling from 19% to 32%. In this regard, Ki-67 labeling of glioblastoma tissue from a patient treated with high-dose tamoxifen yielded results within the range of 10% to 15% (consistent with values seen in untreated glioblastoma), despite tumor regression with treatment. The authors' interpretation of these results is that these PKC inhibitors are halting the cell cycle in the G1 phase or the G1-S transition (beyond G0 but before S-phase), resulting in a paradoxical increase in labeling while arresting growth. Two important implications from these observations are that Ki-67 is not a reliable indicator of cellular proliferation after treatment with PKC inhibitors and that these inhibitors used at the doses given above halt cell growth in a phase-specific manner.

Tamoxifen and related compounds protect against lipid peroxidation in isolated nuclei: relevance to the potential anticarcinogenic benefits of breast cancer prevention and therapy with tamoxifen?

Tamoxifen, 4-hydroxytamoxifen, nafoxidine, 17 beta-oestradiol and ICI 164,384 were all found to protect rat liver nuclei against Fe(III)-ascorbate dependent lipid peroxidation. The order of effectiveness of these compounds was 4-hydroxytamoxifen > 17 beta-oestradiol > nafoxidine > tamoxifen > ICI 164,384. This protection by tamoxifen against the formation of the genotoxic reactive-intermediates and products of lipid peroxidation in the nuclear membrane could be important in the prevention of nuclear DNA damage and thus carcinogenesis. This possible anticarcinogenic benefit of tamoxifen treatment could be important in long-term therapy with tamoxifen (and future derivatives) and in its proposed use in the prevention of breast cancer.

Examination of the natural protein substrates affected by staurosporine in the developing cerebral cortex

The protein substrates affected by staurosporine (SP), the most potent inhibitor of protein kinases yet described, are unknown. In order to approach this problem we incubated cerebral cortex tissue with 0, 20, 50 and 100 nM of SP using [32P]orthophosphate as radioactive precursor. The analysis of the phosphoproteins were made with a modified high resolution two dimensional gel electrophoresis, followed by autoradiography. We detected several proteins affected by SP. Specially noticeable was an approximately 55 kDa protein which strikingly diminished the intensity of phosphorylation. However, the reverse phenomenon was also observed. To the best of our knowledge this is the first examination of protein substrates affected by SP in intact tissue.

The antioxidant action of synthetic oestrogens involves decreased membrane fluidity: relevance to their potential use as anticancer and cardioprotective agents compared to tamoxifen?

The synthetic oestrogens diethylstilboestrol, hexoestrol and 17 alpha-ethynyloestradiol are known to be good antioxidants, and we now report that they decrease membrane fluidity, in ox-brain phospholipid liposomes. The order of effectiveness was diethylstilboestrol > hexoestrol > 17 alpha-ethynyloestradiol and a good positive correlation was demonstrated between decreased membrane fluidity and antioxidant ability (measured as inhibition of liposomal lipid peroxidation: correlation coefficient, r = 0.99). This ability of diethylstilboestrol, hexoestrol and 17 alpha-ethynyloestradiol to decrease membrane fluidity is suggested, therefore, to be the mechanism of their antioxidant action. The membrane-modulating antioxidant action of these synthetic oestrogens is compared to that of tamoxifen and their potential use as anticancer and cardioprotective agents is discussed.

A comparison of the relative chemosensitivity of human gliomas to tamoxifen and n-desmethyltamoxifen in vitro

Tamoxifen has been shown to inhibit the proliferation of human gliomas in vitro. This inhibition is independent of tamoxifen's known anti-estrogenic properties. Tamoxifen is an inhibitor of protein kinase C (PKC), a calcium- and phospholipid-dependent serine kinase which plays a critical role in the proliferation of certain cell lines. Gliomas overexpress PCK, and their growth rate is coupled to the level of this key enzyme. As such, the effect of tamoxifen may be mediated by its inhibitory effect on PKC. To further investigate this possibility, we compared the chemosensitivity of cultured glioma lines to both tamoxifen and N-desmethyltamoxifen (DMT). DMT is the major metabolite of tamoxifen in humans and is a ten-fold more potent inhibitor of PKC. Seven lines were tested using the standard MTT assay, which quantitates metabolically active cells colorimetrically using a tetrazolium dye. Four of the seven lines were also tested using a tritiated thymidine uptake assay. In the MTT assay, all seven lines showed significantly greater sensitivity to DMT, while three of the four lines tested in the thymidine uptake assay were more sensitive to DMT. Correlation between the two assays was good. The dose of tamoxifen required to produce a 50% inhibition of optical absorbance or thymidine uptake (ID50) was typically five- to ten-fold greater than the ID50 for DMT, approximating the relative strength of the two compounds as PKC inhibitors. In addition to providing some support for the ypothesis that triphenylethylenes inhibit gliomas via PKC inhibition, these findings have clinical significance.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective actions of tamoxifen and 4-hydroxytamoxifen against oxidative damage to human low-density lipoproteins: a mechanism accounting for the cardioprotective action of tamoxifen?

Tamoxifen and 4-hydroxytamoxifen protect isolated human low-density lipoproteins (LDLs) against copper-ion-dependent lipid peroxidation: 4-hydroxytamoxifen is more protective than tamoxifen or 17 beta-oestradiol. 4-Hydroxytamoxifen and 17 beta-oestradiol also prevent the increase in the electrophoretic mobility of LDL caused by exposure to copper ions, presumably by protection of the apoprotein B of LDL against oxidative modification. Our observations may help to account for the cardioprotective benefits reported to be associated with tamoxifen therapy and prophylaxis in breast cancer.

Tamoxifen inhibits lipid peroxidation in cardiac microsomes. Comparison with liver microsomes and potential relevance to the cardiovascular benefits associated with cancer prevention and treatment by tamoxifen

Tamoxifen and 4-hydroxytamoxifen were both good inhibitors of iron-dependent lipid peroxidation in rat cardiac microsomes. Tamoxifen was also a good inhibitor of lipid peroxidation in liposomes prepared from the phospholipid obtained from rat liver microsomes. In a modified rat liver microsomal system containing a sufficiently low amount of peroxidizable phospholipid to make it comparable with the rat cardiac microsomal system, tamoxifen and 4-hydroxytamoxifen were of similar effectiveness as in the cardiac system. Tamoxifen is known to lower serum cholesterol levels, and the findings reported here indicate that the drug might also protect heart cell membranes against peroxidative damage. Potential cardioprotective and antiatherosclerotic benefits of tamoxifen are discussed in relation to the drug's use in cancer prevention and treatment.