Radiosensitization by antisense anti-MDM2 mixed-backbone oligonucleotide in in vitro and in vivo human cancer models

MDM2 Inhibitors for Cancer Therapy: The Past, Present, and Future

Since its discovery over 35 years ago, MDM2 has emerged as an attractive target for the development of cancer therapy. MDM2's activities extend from carcinogenesis to immunity to the response to various cancer therapies. Since the report of the first MDM2 inhibitor more than 30 years ago, various approaches to inhibit MDM2 have been attempted, with hundreds of small-molecule inhibitors evaluated in preclinical studies and numerous molecules tested in clinical trials. Although many MDM2 inhibitors and degraders have been evaluated in clinical trials, there is currently no Food and Drug Administration (FDA)-approved MDM2 inhibitor on the market. Nevertheless, there are several current clinical trials of promising agents that may overcome the past failures, including agents granted FDA orphan drug or fast-track status. We herein summarize the research efforts to discover and develop MDM2 inhibitors, focusing on those that induce MDM2 degradation and exert anticancer activity, regardless of the p53 status of the cancer. We also describe how preclinical and clinical investigations have moved toward combining MDM2 inhibitors with other agents, including immune checkpoint inhibitors. Finally, we discuss the current challenges and future directions to accelerate the clinical application of MDM2 inhibitors. In conclusion, targeting MDM2 remains a promising treatment approach, and targeting MDM2 for protein degradation represents a novel strategy to downregulate MDM2 without the side effects of the existing agents blocking p53-MDM2 binding. Additional preclinical and clinical investigations are needed to finally realize the full potential of MDM2 inhibition in treating cancer and other chronic diseases where MDM2 has been implicated. SIGNIFICANCE STATEMENT: Overexpression/amplification of the MDM2 oncogene has been detected in various human cancers and is associated with disease progression, treatment resistance, and poor patient outcomes. This article reviews the previous, current, and emerging MDM2-targeted therapies and summarizes the preclinical and clinical studies combining MDM2 inhibitors with chemotherapy and immunotherapy regimens. The findings of these contemporary studies may lead to safer and more effective treatments for patients with cancers overexpressing MDM2.

The Role of Deferoxamine in Irradiated Breast Reconstruction: A Study of Oncologic Safety

BACKGROUND

Radiotherapy plays an essential role in the oncologic management of breast cancer. However, patients who undergo radiotherapy experience significantly more wound complications during the reconstructive process. Deferoxamine has immense potential to up-regulate angiogenesis and improve reconstructive outcomes. The purpose of this study was to determine the impact of deferoxamine on breast cancer cell proliferation in vitro, to delineate oncologic safety concerns regarding the use of deferoxamine as a regenerative therapeutic.

METHODS

The dose-dependent effect of radiation and deferoxamine on two triple-negative breast cancer cell lines (MDA-MB-231 and MDA-MB-468) was determined by means of MTS (percentage cell viability) and tumorsphere (sphere number) analysis. Radiation therapy and deferoxamine were delivered both individually and in combination, and all experiments were completed in triplicate. Intracellular iron, nuclear factor-κB localization, and apoptosis/necrosis assays were performed to delineate mechanism. Analysis of variance statistical analysis was performed using SPSS (p < 0.05).

RESULTS

For both cell lines, percentage viability and sphere number significantly decreased following exposure to 10 Gy of radiation. Surprisingly, the administration of 25 µM deferoxamine also significantly decreased each metric. The administration of deferoxamine (100 µM) in combination with radiation (10 Gy) resulted in significantly reduced percentage viability and sphere number compared with the administration of radiation alone. Deferoxamine treatment decreased intracellular iron, suppressed nuclear factor-κB activation, and induced apoptosis.

CONCLUSION

Radiation and deferoxamine significantly decrease breast cancer proliferation when delivered independently and in combination, suggesting deferoxamine may be safely used to facilitate improved reconstructive outcomes among triple-negative breast cancer survivors.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, V.

A novel small molecule inhibitor of MDM2-p53 (APG-115) enhances radiosensitivity of gastric adenocarcinoma

BACKGROUND

Gastric cancer is the leading cause of cancer related death worldwide. Radiation alone or combined with chemotherapy plays important role in locally advanced and metastatic gastric adenocarcinoma. MDM2-p53 interaction and downstream signaling affect cellular response to DNA damage which leads to cell cycle arrest and apoptosis. Therefore, restoring p53 function by inhibiting its interaction with MDM2 is a promising therapeutic strategy for cancer. APG-115 is a novel small molecule inhibitor which blocks the interaction of MDM2 and p53. In this study, we investigated that the radiosensitivity of APG-115 in gastric adenocarcinoma in vitro and in vivo.

METHODS

The role of APG-115 in six gastric cancer cells viability in vitro was determined by CCK-8 assay. The expression level of MDM2, p21, PUMA and BAX in AGS and MKN45 cell lines was measured via real-time PCR (RT-PCR). The function of treatment groups on cell cycle and cell apoptosis were detected through Flow Cytometry assay. Clonogenic assays were used to measure the radiosensitivity of APG-115 in p53 wild type gastric cancer cell lines. Western blot was conducted to detect the protein expressions of mdm2-p53 signal pathway. Xenograft models in nude mice were established to explore the radiosensitivity role of APG-115 in gastric cancer cells in vivo.

RESULTS

We found that radiosensitization by APG-115 occurred in p53 wild-type gastric cancer cells. Increasing apoptosis and cell cycle arrest was observed after administration of APG-115 and radiation. Radiosensitivity of APG-115 was mainly dependent on MDM2-p53 signal pathway. In vivo, APG-115 combined with radiation decreased xenograft tumor growth much more significantly than either single treatment. Moreover, the number of proliferating cells (Ki-67) significantly decreased in combination group compared with single treatment group.

CONCLUSIONS

In summary, we found that combination of MDM2-p53 inhibitor (APG-115) and radiotherapy can enhance antitumor effect both in vitro and in vivo. This is the first report on radiosensitivity of APG-115 which shed light on clinical trial of the combination therapy of radiation with APG-115 in gastric adenocarcinoma.

Simvastatin enhances the radiosensitivity of p53‑deficient cells via inhibition of mouse double minute 2 homolog

Simvastatin exhibits anticancer activities, but its molecular mechanisms and radiosensitizing effects relative to p53 status remain unclear. In this study, we investigated whether the combination of simvastatin and ionizing radiation (IR) would enhance the antitumor effects of IR alone in HCT116 p53+/+ and p53‑/- colon cancer cells. Using colony formation assays and a xenograft mouse model, we found that simvastatin potently stimulated radiosensitization of HCT116 p53‑/- cells and xenograft tumors. The combination of simvastatin with IR decreased G2/M arrest and delayed the repair of IR-induced DNA damage; however, no differences between the HCT116 p53+/+ and p53‑/- cells were evident. A further analysis revealed that simvastatin exhibited a novel function, namely, MDM2 suppression, regardless of p53 status. Interestingly, simvastatin induced radiosensitization by enhancing MDM2 suppression and elevating IR-induced p‑ATM foci formation compared with IR alone in HCT116 p53‑/- cells. Furthermore, simvastatin caused accumulations of the FOXO3a, E-cadherin, and p21 tumor suppressor proteins, which are downstream factors of MDM2, in HCT116 p53‑/- cells. In conclusion, simvastatin enhanced radiosensitivity by inducing MDM2 inhibition and increasing tumor suppressor protein levels in radioresistant HCT116 p53‑/- cells and xenografts. Overall, our novel findings suggest a scientific rationale for the clinical use of simvastatin as an MDM2 inhibitor and radiosensitizer for p53‑deficient colorectal tumor treatments.

Dual inhibition of nuclear factor kappa-B and Mdm2 enhance the antitumor effect of radiation therapy for pancreatic cancer

INTRODUCTION

Radiation therapy, alone or in combination with chemotherapy, is effective for patients with locally advanced and recurrent pancreatic cancer. Ionizing radiation induces cell cycle arrest and cell apoptosis through enhancement several signals such as p53, p21(Waf1/Cip1), and caspase. However, the therapeutic efficacy is attenuated by radiation-induced activation of NF-κB. Nafamostat mesilate, a synthetic serine protease inhibitor, inhibits NF-κB activation in pancreatic cancer. Therefore, we hypothesized that nafamostat mesilate inhibited radiation-induced activation of NF-κB and improves therapeutic outcome.

RESULTS

In combination group, NF-κB activation was significantly inhibited in comparison with that of radiation group. Nafamostat mesilate obviously down-regulated the expression levels of Mdm2 compared with control cells or irradiated cells. Consequently, p53 expression was stabilized inversely in correlation with Mdm2 protein expression level. The expression levels of p53, p21(Waf1/Cip1), cleaved caspase-3 and -8 were the highest in the combination group. Nafamostat mesilate enhanced ionizing radiation-induced cell apoptosis and G2/M cell cycle arrest. In combination group, cell proliferation and tumor growth were significantly slower than those in other groups.

CONCLUSION

Combination therapy of radiation with nafamostat mesilate exerts enhanced anti-tumor effect against human pancreatic cancer.

Inhibition of MDM2 by RG7388 confers hypersensitivity to X-radiation in xenograft models of childhood sarcoma

BACKGROUND

Curative therapy for childhood sarcoma presents challenges when complete resection is not possible. Ionizing radiation (XRT) is used as a standard modality at diagnosis or recurrence for childhood sarcoma; however, local recurrence is still problematic. Most childhood sarcomas are TP53 wild type at diagnosis, although approximately 5-10% have MDM2 amplification or overexpression.

PROCEDURES

The MDM2 inhibitor, RG7388, was examined alone or in combination with XRT (20Gy given in 2 Gy daily fractions) to immune-deficient mice bearing Rh18 (embryonal) or a total of 30 Gy in 2 Gy fractions to mice bearing Rh30 (alveolar) rhabdomyosarcoma xenografts. RG7388 was administered by oral gavage using two schedules (daily ×5; schedule 1 or once weekly; schedule 2). TP53-responsive gene products (p21, PUMA, DDB2, and MIC1) as well as markers of apoptosis were analyzed.

RESULTS

RG7388 showed no significant single agent antitumor activity. Twenty Grays XRT induced complete regressions (CR) of Rh18 with 100 percent tumor regrowth by week 7, but no tumor regrowth at 20 weeks when combined with RG7388. RG7388 enhanced time to recurrence combined with XRT in Rh30 xenografts compared to 30 Gy XRT alone. RG7388 did not enhance XRT-induced local skin toxicity. Combination treatments induced TP53 responsive genes more rapidly and to a greater magnitude than single agent treatments.

CONCLUSIONS

RG7388 enhanced the activity of XRT in both rhabdomyosarcoma models without increasing local XRT-induced skin toxicity. Changes in TP53-responsive genes were consistent with the synergistic activity of RG7388 and XRT in the Rh18 model.

MDM2 molecular imaging for the prediction of chemotherapeutic sensitivity in human breast cancer xenograft

The aim of the present study was to investigate the possible use of mouse double-minute 2 (MDM2) molecular imaging to predict chemotherapeutic sensitivity in breast cancer xenografts (BCXs). MCF-7 cells were transfected with MDM2 antisense oligonucleotides (ASONs), and MDM2 expression levels were determined by Western blotting. Cell viability was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay in MCF-7 cells transfected with ASONs and treated with paclitaxel. BCXs were established in nude mice by injection of ASONs, and tumor volumes were measured after paclitaxel treatment. MDM2 ASONs were labeled with 99mTc to generate an MDM2 molecular probe, and MDM2 expression levels were evaluated by imaging and Western blotting. MDM2 ASONs downregulated MDM2 expression in a dose-dependent manner and increased the rate of paclitaxel-induced cell growth inhibition. Imaging of tumors revealed significant differences in the tumor to skeletal muscle (T/M) ratio between groups. Tumor MDM2 protein expression was correlated with T/M ratios at 4 hours (R  =  .880) and 10 hours (R  =  .886). The effect of paclitaxel varied among nude mice bearing BCXs with different concentrations of ASONs, as shown by differences in tumor growth. MDM2 molecular imaging could be a promising method for predicting the sensitivity of BCXs to chemotherapy.

Development and validation of a rapid HPLC method for quantitation of SP-141, a novel pyrido[b]indole anticancer agent, and an initial pharmacokinetic study in mice

There is an increasing interest in targeting the MDM2 oncogene for cancer therapy. SP-141, a novel designed small molecule MDM2 inhibitor, exerts excellent in vitro and in vivo anticancer activity. To facilitate the preclinical development of this candidate anticancer agent, we have developed an HPLC method for the quantitative analysis of SP-141. The method was validated to be precise, accurate, and specific, with a linear range of 16.2-32,400 ng/mL in plasma, 16.2-6480 ng/mL in homogenates of brain, heart, liver, kidneys, lungs, muscle and tumor, and 32.4-6480 ng/mL in spleen homogenates. The lower limit of quantification was 16.2 ng/mL in plasma and all the tissue homogenates, except for spleen homogenates, where it was 32.4 ng/mL. The intra- and inter-assay precisions (coefficient of variation) were between 0.86 and 13.39%, and accuracies (relative errors) ranged from -8.50 to 13.92%. The relative recoveries were 85.6-113.38%. SP-141 was stable in mouse plasma, modestly plasma bound and metabolized by S9 microsomal enzymes. We performed an initial pharmacokinetic study in tumor-bearing nude mice, demonstrating that SP-141 has a short half-life in plasma and wide tissue distribution. In summary, this HPLC method can be used in future preclinical and clinical investigations of SP-141.

Effects of MDM2 inhibitors on vascular endothelial growth factor-mediated tumor angiogenesis in human breast cancer

BACKGROUND

Mouse double minute 2 (MDM2) is overexpressed in many malignant tumors, and MDM2 levels are associated with poor prognosis of several human cancers, including breast cancer. In the present study, we investigated the function of MDM2 in vascular endothelial growth factor (VEGF)-mediated tumor angiogenesis of breast cancer and the potential value of MDM2 as an anti-angiogenic therapy target for cancer therapy by inhibiting MDM2 with antisense oligonucleotides (ASO) or other antagonist nutlin-3.

METHODS

Anti-MDM2 ASO and nutlin-3 were evaluated for their in vitro and in vivo anti-angiogenesis activities in different human breast cancer models with a different p53 status: MCF-7 cell line containing wild-type p53 and MDA-MB-468 cell line containing mutant p53. MCF-7 and MDA-MB-468 cells were incubated with different concentrations of ASO or nutlin-3 for various periods of time. VEGF gene and protein expression in tumor cells was measured by qPCR and Western blot. The level of VEGF protein secreted in the culture supernatant of treated cells was quantified by enzyme-linked immunosorbent assay (ELISA). Nude mouse xenograft models were further established to determine their effects on tumor growth and angiogenesis. Serum levels of VEGF were measured by ELISA. VEGF expression and microvessel density in tumor tissues were studied by immunohistochemistry. Both angiogenesis and tumor growth were digitally quantified.

RESULTS

In both MCF-7 and MDA-MB-468 cells, VEGF expression and secretion were reduced, resulting from specific inhibition of MDM2 expression by ASO. In vivo assay, after administration of ASO, VEGF production reduced and anti-angiogenesis activity occurred in nude mice bearing MCF-7 or MDA-MB-468 xenograft. However, in both models treated with nutlin-3, VEGF production was not changed and anti-angiogenesis activity was not observed.

CONCLUSION

In summary, the ASO construct targeting MDM2 specifically suppresses VEGF expression in vitro and VEGF-mediated tumor angiogenesis in vivo in breast cancer. Furthermore, the suppression of VEGF expression subsequent to inhibition of MDM2 in p53 mutant cells suggests that MDM2 has a regulatory role on VEGF expression through a p53-independent mechanism.

The MDM2-p53 pathway revisited

The p53 tumor suppressor is a key transcription factor regulating cellular pathways such as DNA repair, cell cycle, apoptosis, angiogenesis, and senescence. It acts as an important defense mechanism against cancer onset and progression, and is negatively regulated by interaction with the oncoprotein MDM2. In human cancers, the TP53 gene is frequently mutated or deleted, or the wild-type p53 function is inhibited by high levels of MDM2, leading to downregulation of tumor suppressive p53 pathways. Thus, the inhibition of MDM2-p53 interaction presents an appealing therapeutic strategy for the treatment of cancer. However, recent studies have revealed the MDM2-p53 interaction to be more complex involving multiple levels of regulation by numerous cellular proteins and epigenetic mechanisms, making it imperative to reexamine this intricate interplay from a holistic viewpoint. This review aims to highlight the multifaceted network of molecules regulating the MDM2-p53 axis to better understand the pathway and exploit it for anticancer therapy.

Celecoxib enhances radiosensitivity of hypoxic glioblastoma cells through endoplasmic reticulum stress

BACKGROUND

Refractoriness of glioblastoma multiforme (GBM) largely depends on its radioresistance. We investigated the radiosensitizing effects of celecoxib on GBM cell lines under both normoxic and hypoxic conditions.

METHODS

Two human GBM cell lines, U87MG and U251MG, and a mouse GBM cell line, GL261, were treated with celecoxib or γ-irradiation either alone or in combination under normoxic and hypoxic conditions. Radiosensitizing effects were analyzed by clonogenic survival assays and cell growth assays and by assessing apoptosis and autophagy. Expression of apoptosis-, autophagy-, and endoplasmic reticulum (ER) stress-related genes was analyzed by immunoblotting.

RESULTS

Celecoxib significantly enhanced the radiosensitivity of GBM cells under both normoxic and hypoxic conditions. In addition, combined treatment with celecoxib and γ-irradiation induced marked autophagy, particularly in hypoxic cells. The mechanism underlying the radiosensitizing effect of celecoxib was determined to be ER stress loading on GBM cells.

CONCLUSION

Celecoxib enhances the radiosensitivity of GBM cells by a mechanism that is different from cyclooxygenase-2 inhibition. Our results indicate that celecoxib may be a promising radiosensitizing drug for clinical use in patients with GBM.

Radiosensitization in prostate cancer: mechanisms and targets

Prostate cancer is the second most commonly diagnosed cancer in American men over the age of 45 years and is the third most common cause of cancer related deaths in American men. In 2012 it is estimated that 241,740 men will be diagnosed with prostate cancer and 28,170 men will succumb to prostate cancer. Currently, radiation therapy is one of the most common definitive treatment options for localized prostate cancer. However, significant number of patients undergoing radiation therapy will develop locally persistent/recurrent tumours. The varying response rates to radiation may be due to 1) tumor microenvironment, 2) tumor stage/grade, 3) modality used to deliver radiation, and 4) dose of radiation. Higher doses of radiation has not always proved to be effective and have been associated with increased morbidity. Compounds designed to enhance the killing effects of radiation, radiosensitizers, have been extensively investigated over the past decade. The development of radiosensitizing agents could improve survival, improve quality of life and reduce costs, thus benefiting both patients and healthcare systems. Herin, we shall review the role and mechanisms of various agents that can sensitize tumours, specifically prostate cancer.

Combination of radiotherapy and adenovirus-mediated p53 gene therapy for MDM2-overexpressing hepatocellular carcinoma

The p53 gene plays a determinant role in radiation-induced cell death and its protein product is negatively regulated by MDM2. We investigated whether adenovirus-mediated modified p53 gene transfer, which blocks p53-MDM2 binding, is effective for radiation-induced cell death in hepatocellular carcinoma (HCC) at different MDM2 cellular levels. Human hepatocellular carcinoma cell lines expressing MDM2 at low levels (Huh7) and high levels (SK-Hep1) were used. Ad-p53 and Ad-p53vp are replication-deficient adenoviral vectors containing human wild-type or modified p53, respectively. The anti-tumor effect was highest for Ad-p53 + radiotherapy (RT) in the low-level MDM2 cells, whereas this effect was highest for Ad-p53vp + RT in the MDM2-overexpressing cells. In Huh-7 cells, Ad-p53 + RT decreased cell viability (32%) in vitro and inhibited tumor growth (enhancement factor, 1.86) in vivo. Additionally, p21 expression and apoptosis were increased. In contrast, in SK-Hep1 cells, Ad-p53vp + RT showed decreased cell viability (51%) in vitro and inhibition of tumor growth (enhancement factor, 3.07) in vivo. Caspase-3 expression and apoptosis were also increased. Adenovirus-expressing modified p53, which blocks p53-MDM2 binding, was effective in killing tumor cells overexpressing MDM2. Furthermore, the combination strategy for disruption of the p53-MDM2 interaction with RT demonstrated enhanced anti-tumor effects both in vitro and in vivo.

Time-course analysis of DNA damage response-related genes after in vitro radiation in H460 and H1229 lung cancer cell lines

Radiation is the most useful treatment modality for cancer patients. It initiates a series of signal cascades such as DNA damage response (DDR) signaling for repairing damaged DNA, arresting the cell cycle, and inducing cell death. Until now, few genes have been found to be regulated by radiation, which explains the molecular mechanisms of cellular responses to radiation. Although the transcriptional changes caused by radiation have been widely investigated, little is known about the direct evidence for the transcriptional control of DDR-related genes. Here, we examined the radiosensitivity of two non-small cell lung cancer cell lines (H460 and H1299), which have different p53 status. We monitored the time-dependent changes of 24 DDR-related gene expressions via microarray analysis. Based on the basal expression levels and temporal patterns, we further classified 24 DDR-related genes into four subgroups. Then, we also addressed the protein levels of several DDR-related genes such as TopBP1, Chk1 and Chk2, confirming the results of microarray analysis. Together, these results indicate that the expression patterns of DDR-related genes are associated with radiosensitivity and with the p53 statuses of H460 and H1299, which adds to the understanding of the complex biological responses to radiation.

Antisense MDM2 enhances E2F1-induced apoptosis and the combination sensitizes androgen-sensitive [corrected] and androgen-insensitive [corrected] prostate cancer cells to radiation

We have previously shown in separate studies that MDM2 knockdown via antisense MDM2 (AS-MDM2) and E2F1 overexpression via adenoviral-mediated E2F1 (Ad-E2F1) sensitized prostate cancer cells to radiation. Because E2F1 and MDM2 affect apoptosis through both common and independent pathways, we hypothesized that coupling these two treatments would result in increased killing of prostate cancer cells. In this study, the effect of Ad-E2F1 and AS-MDM2 in combination with radiation was investigated in three prostate cancer cell lines: LNCaP cells, LNCaP-Res cells [androgen insensitive with functional p53 and androgen receptor (AR)], and PC3 cells (androgen insensitive, p53(null), and AR(null)). A supra-additive radiosensitizing effect was observed in terms of clonogenic inhibition and induction of apoptosis (caspase-3 + caspase-7 activity) in response to Ad-E2F1 plus AS-MDM2 treatments in all three cell lines. In LNCaP and LNCaP-Res, these combination treatments elevated the levels of phospho-Ser(15) p53 with significant induction of p21(waf1/cip1), phospho-gammaH2AX, PUMA, and Bax levels and reduction of AR and bcl-2 expression. Similarly, AR(null) and p53(null) PC-3 cells showed elevated levels of Bax and phospho-gammaH2AX expression. These findings show that the combination of Ad-E2F1 and AS-MDM2 significantly increases cell death in prostate cancer cells exposed to radiation and that this effect occurs in the presence or absence of AR and p53.

Multiple cyclin kinase inhibitors promote bile acid-induced apoptosis and autophagy in primary hepatocytes via p53-CD95-dependent signaling

Previously, using primary hepatocytes residing in early G1 phase, we demonstrated that expression of the cyclin-dependent kinase (CDK) inhibitor protein p21Cip-1/WAF1/mda6 (p21) enhanced the toxicity of deoxycholic acid (DCA) + MEK1/2 inhibitor. This study examined the mechanisms regulating this apoptotic process. Overexpression of p21 or p27(Kip-1) (p27) enhanced DCA + MEK1/2 inhibitor toxicity in primary hepatocytes that was dependent on expression of acidic sphingomyelinase and CD95. Overexpression of p21 suppressed MDM2, elevated p53 levels, and enhanced CD95, BAX, NOXA, and PUMA expression; knockdown of BAX/NOXA/PUMA reduced CDK inhibitor-stimulated cell killing. Parallel to cell death processes, overexpression of p21 or p27 profoundly enhanced DCA + MEK1/2 inhibitor-induced expression of ATG5 and GRP78/BiP and phosphorylation of PKR-like endoplasmic reticulum kinase (PERK) and eIF2alpha, and it increased the numbers of vesicles containing a transfected LC3-GFP construct. Incubation of cells with 3-methyladenine or knockdown of ATG5 suppressed DCA + MEK1/2 inhibitor-induced LC3-GFP vesicularization and enhanced DCA + MEK1/2 inhibitor-induced toxicity. Expression of dominant negative PERK blocked DCA + MEK1/2 inhibitor-induced expression of ATG5, GRP78/BiP, and eIF2alpha phosphorylation and prevented LC3-GFP vesicularization. Knock-out or knockdown of p53 or CD95 abolished DCA + MEK1/2 inhibitor-induced PERK phosphorylation and prevented LC3-GFP vesicularization. Thus, CDK inhibitors suppress MDM2 levels and enhance p53 expression that facilitates bile acid-induced, ceramide-dependent CD95 activation to induce both apoptosis and autophagy in primary hepatocytes.

Nutlin-3 radiosensitizes hypoxic prostate cancer cells independent of p53

Nutlin-3 is a small-molecule inhibitor that acts to inhibit MDM2 binding to p53 and subsequent p53-dependent DNA damage signaling. Whether Nutlin-3 alters cell toxicity following DNA damage under oxic versus hypoxic conditions has not been studied. The potential radiosensitization (0-10 Gy) properties of Nutlin-3 (dose range, 2-10 micromol/L for up to 24 h) were investigated in vitro using three prostate cancer cell lines, 22RV1 [wild-type p53 (WTp53)], DU145 (mutated p53), and PC-3 (p53-null) under oxic (21% O(2)), hypoxic (0.2% O(2)), and anoxic (0% O(2)) conditions. As a single agent, Nutlin-3 (2-10 micromol/L) stabilized p53 and p21(WAF) levels and was toxic to WTp53-22RV1 cells (IC(50), 4.3 micromol/L) but had minimal toxicity toward p53-deficient cells (IC(50), >10 micromol/L). When combined with radiation under oxic conditions, Nutlin-3 decreased clonogenic survival in all three cell lines: 22RV1 [sensitizing enhancement ratio (SER), 1.24], DU145 (SER, 1.27), and PC-3 (SER, 1.12). Anoxia induced p53 protein expression in 22RV1 cells and this was augmented by Nutlin-3 treatment. Furthermore, Nutlin-3 was more effective as a radiosensitizer under hypoxic conditions particularly in WTp53-expressing cells: 22RV1 (SER, 1.78), DU145 (SER, 1.31), and PC-3 (SER, 1.28). The decrease in clonogenic survival with Nutlin-3 was not correlated to altered levels of radiation-induced apoptosis within the three cell lines. Our results indicate that Nutlin-3 can act as a radiosensitizer via p53-independent mechanisms under low O(2) levels. Nutlin-3 may be a useful adjunct to improve the therapeutic ratio using precision radiotherapy targeted to hypoxic cells and warrants further study in vivo.

Disruption of estrogen receptor alpha-p53 interaction in breast tumors: a novel mechanism underlying the anti-tumor effect of radiation therapy

Inactivation of tumor suppressor p53 is one of the most frequent events in cancer. Unlike many other cancers, however, p53 gene mutations are infrequent in breast cancers, as about 80% of breast tumors contain wild type p53. The mechanisms underlying functional inactivation of wild type p53 in breast cancer have remained elusive. Besides, how p53 gets activated in breast tumors subjected to radiation therapy remains unknown. We recently reported that in MCF-7 breast cancer cells, estrogen receptor alpha (ERalpha) directly binds to p53 and represses its function. Furthermore, the ERalpha-p53 interaction was disrupted by ionizing radiation. These observations have important translational implications especially as there are no reliable cellular or molecular criteria for rational radiotherapy for breast cancer. Here we report our studies towards addressing this important issue, using an MCF-7 breast cancer xenograft model in mice. Radiation effectively inhibits growth of these tumors and stabilizes p53, but has no observable effect on ERalpha protein level. Importantly, chromatin immunoprecipitation (ChIP) assays demonstrated that ERalpha interacts with p53 bound to endogenous target gene promoters in tumors in vivo, and this interaction is considerably reduced in response to radiotherapy although p53 level is increased. Concomitant with its effect on ERalpha-p53 interaction, radiation increases p53-mediated transcriptional activation of several target genes and increases p53-mediated transcriptional repression of survivin. Our studies show that disruption of ERalpha-p53 interaction in vivo resulting in restoration of functional p53 is a cellular response to radiation. Radiation could be affecting ERalpha and/or p53 directly or it could be influencing other proteins associated with the ERalpha-p53 complex. To the best of our knowledge, this is the first report on analysis of DNA-protein-protein interaction occurring on endogenous gene promoters in vivo in breast tumor tissues. These findings suggest that alleviating the inhibitory effect of ERalpha on p53 could be one of the molecular mechanisms underlying activation of p53 by radiation in breast tumors, and therefore, could be exploited to develop more effective ways of combining radiation therapy with systemic therapies such as hormonal therapy and chemotherapy.

Antisense, RNAi, and gene silencing strategies for therapy: mission possible or impossible?

Antisense oligonucleotides can regulate gene expression in living cells. As such, they regulate cell function and division, and can modulate cellular responses to internal and external stresses and stimuli. Although encouraging results from preclinical and clinical studies have been obtained and significant progress has been made in developing these agents as drugs, they are not yet recognized as effective therapeutics. Several major hurdles remain to be overcome, including problems with efficacy, off-target effects, delivery and side effects. The lessons learned from antisense drug development can help in the development of other oligonucleotide-based therapeutics such as CpG oligonucleotides, RNAi and miRNA.

Antisense-MDM2 sensitizes LNCaP prostate cancer cells to androgen deprivation, radiation, and the combination in vivo

PURPOSE

To test the effects of antisense (AS)-MDM2 alone and with androgen deprivation (AD), radiotherapy (RT), and AD + RT on wild-type LNCaP cells in an orthotopic in vivo model.

METHODS

Androgen-sensitive LNCaP cells were grown in the prostates of nude mice. Magnetic resonance imaging-based tumor volume and serum prostate-specific antigen (PSA) measurements were used to assess effects on tumor response. Tumor response was measured by biochemical and tumor volume failure definitions and doubling time estimates from fitted PSA and tumor volume growth curves. Expression of MDM2, p53, p21, and Ki-67 was quantified using immunohistochemical staining and image analysis of formalin-fixed tissue, analogous to methods used clinically.

RESULTS

Antisense-MDM2 significantly inhibited the growth of LNCaP tumors over the mismatch controls. The most significant increase in tumor growth delay and tumor doubling time was from AS-MDM2 + AD + RT, although the effect of AS-MDM2 + AD was substantial. Expression of MDM2 was significantly reduced by AS-MDM2 in the setting of RT.

CONCLUSIONS

This is the first in vivo investigation of the effects of AS-MDM2 in an orthotopic model and the first to demonstrate incremental sensitization when added to AD and AD + RT. The results with AD underscore the potential to affect micrometastatic disease, which is probably responsible for treatment failure in 30-40% of men with high-risk disease.

Curcumin, a Dietary Component, Has Anticancer, Chemosensitization, and Radiosensitization Effects by Down-regulating the MDM2 Oncogene through the PI3K/mTOR/ETS2 Pathway

The oncoprotein MDM2, a major ubiquitin E3 ligase of tumor suppressor p53, has been suggested as a novel target for human cancer therapy based on its p53-dependent and p53-independent activities. We have identified curcumin, which has previously been shown to have anticancer activity, as an inhibitor of MDM2 expression. Curcumin down-regulates MDM2, independent of p53. In a human prostate cancer cell lines PC3 (p53(null)), curcumin reduced MDM2 protein and mRNA in a dose- and time-dependent manner, and enhanced the expression of the tumor suppressor p21(Waf1/CIP1). The inhibitory effects occur at the transcriptional level and seem to involve the phosphatidylinositol 3-kinase/mammalian target of rapamycin/erythroblastosis virus transcription factor 2 pathway. Curcumin induced apoptosis and inhibited proliferation of PC3 cells in culture, but both MDM2 overexpression and knockdown reduced these effects. Curcumin also inhibited the growth of these cells and enhanced the cytotoxic effects of gemcitabine. When it was administered to tumor-bearing nude mice, curcumin inhibited growth of PC3 xenografts and enhanced the antitumor effects of gemcitabine and radiation. In these tumors, curcumin reduced the expression of MDM2. Down-regulation of the MDM2 oncogene by curcumin is a novel mechanism of action that may be essential for its chemopreventive and chemotherapeutic effects. Our observations help to elucidate the process by which mitogens up-regulate MDM2, independent of p53, and identify a mechanism by which curcumin functions as an anticancer agent.

Cutting-Edge Apoptosis-Based Therapeutics

Defects in programmed cell death or apoptosis are major hallmarks of cancer contributing to tumorigenesis, tumor progression, and therapy resistance. In the past decade, many of the pathways leading to apoptosis, as well as the molecular mechanisms blocking the death of tumor cells, have been elucidated. This detailed knowledge of the core apoptosis machinery is now being exploited for translation into novel cancer therapies in order to restore apoptosis induction in tumor cells. Strategies include activation of proapoptotic mediators such as death receptors, tumor protein p53, and second mitochondria-derived activator of caspases (SMAC)/DIABLO as well as inhibition of endogenous apoptosis inhibitors such as IAPs (inhibitor of apoptosis proteins) and BCL-2 (B-cell chronic lymphoid leukemia/lymphoma) proteins. Several approaches employing gene therapy and antisense strategies, recombinant biologics, or classic organic and combinatorial chemistry, have advanced into clinical trials or are already approved. This review looks at recent developments in apoptosis-based cancer therapies and highlights some very promising advances in drug design.

Mdm2 is critically and continuously required to suppress lethal p53 activity in vivo

There is currently much interest in the idea of restoring p53 activity in tumor cells by inhibiting Hdm2/Mdm2. However, it has remained unclear whether this would also activate p53 in normal cells. Using a switchable endogenous p53 mouse model, which allows rapid and reversible toggling of p53 status between wild-type and null states, we show that p53 is spontaneously active in all tested tissues of mdm2-deficient mice, triggering fatal pathologies that include ablation of classically radiosensitive tissues. In apoptosis-resistant tissues, spontaneous unbuffered p53 activity triggers profound inhibition of cell proliferation. Such acute spontaneous p53 activity occurs in the absence of any detectable p53 posttranslational modification, DNA damage, or p19ARF signaling and triggers rapid p53 degradation.

Antisense-based cancer therapeutics: are we there yet?

Despite significant advances that have been made in recent years, there is still an urgent need for novel, more effective and less toxic therapeutics for human cancer. Among many new molecular therapeutics being explored for cancer therapy, antisense oligonucleotides are a promising nucleic acid-based approach, with numerous antisense agents being evaluated in preclinical studies and several anticancer antisense drugs in clinical trials. Although there are still a few problems facing the development of antisense strategies for cancer therapy, with progress made in chemical modifications, target selection and drug delivery systems, antisense oligonucleotides are emerging as a novel approach to cancer therapy used alone or in combination with conventional treatments such as chemotherapy and radiation therapy.

Radiosensitization of lung cancer by nutlin, an inhibitor of murine double minute 2

p53 plays a critical role in cell cycle arrest and induction of apoptosis. Certain malignancies carry wild-type p53, which is frequently down-regulated by murine double minute 2 (MDM2) overexpression. Availability of a small-molecule inhibitor against MDM2, nutlin, has made it feasible to evaluate the anti-MDM2-based therapeutic strategies. The rationale for the current study is that functional p53 has been linked with improved responses to radiation treatment. Hence, this study evaluates the use of nutlin, a small-molecule inhibitor that blocks the interaction of p53 and MDM2, in sensitizing cancer cells to radiation. Expression of MDM2, p53, and p21 in both p53 wild-type and p53-defective lung cancer cell lines was examined. Clonogenic and 7-amino-actinomycin D studies were used to determine possible mechanisms of cell death. The combined effect of MDM2 inhibition and radiation on cell cycle was also studied. We found that radiosensitization by nutlin occurs in lung cancer cells with wild-type p53. There were increased apoptosis and cell cycle arrest following administration of nutlin and radiation. Furthermore, the combination of nutlin and radiation decreased the ability of endothelial cells to form vasculature, as shown by Matrigel assays. Our data suggest that nutlin is an effective radiosensitizer of p53 wild-type cells. The radiosensitizing effect seems to be at least partially due to induction of apoptosis and cell cycle arrest. In addition, nutlin may be an effective radiosensitizer of tumor vasculature.

Benzodiazepinedione inhibitors of the Hdm2:p53 complex suppress human tumor cell proliferation in vitro and sensitize tumors to doxorubicin in vivo

The activity and stability of the p53 tumor suppressor are regulated by the human homologue of the mouse double minute 2 (Hdm2) oncoprotein. It has been hypothesized that small molecules disrupting the Hdm2:p53 complex would allow for the activation of p53 and result in growth suppression. We have identified small-molecule inhibitors of the Hdm2:p53 interaction using our proprietary ThermoFluor microcalorimetry technology. Medicinal chemistry and structure-based drug design led to the development of an optimized series of benzodiazepinediones, including TDP521252 and TDP665759. Activities were dependent on the expression of wild-type (wt) p53 and Hdm2 as determined by lack of potency in mutant or null p53-expressing cell lines or cells engineered to no longer express Hdm2 and wt p53. TDP521252 and TDP665759 inhibited the proliferation of wt p53-expressing cell lines with average IC(50)s of 14 and 0.7 micromol/L, respectively. These results correlated with the direct cellular dissociation of Hdm2 from wt p53 observed within 15 minutes in JAR choriocarcinoma cells. Additional activities of these inhibitors in vitro include stabilization of p53 protein levels, up-regulation of p53 target genes in a DNA damage-independent manner, and induction of apoptosis in HepG2 cells. Administration of TDP665759 to mice led to an increase in p21(waf1/cip1) levels in liver samples. Finally, TDP665759 synergizes with doxorubicin both in culture and in an A375 xenograft model to decrease tumor growth. Taken together, these data support the potential utility of small-molecule inhibitors of the Hdm2:p53 interaction for the treatment of wt p53-expressing tumors.

Stabilization of E2F1 protein by MDM2 through the E2F1 ubiquitination pathway

Although previous studies suggested that the tumorigenicity of mouse double minute 2 (MDM2) was due to its negative regulation of p53, the p53-independent interactions may be equally as important. During recent studies utilizing MDM2 inhibitors, we noted that E2F transcription factor 1 (E2F1) was down regulated upon inhibition of MDM2, regardless of the p53 status of the cancer. The present study investigated the mechanisms responsible for the MDM2-mediated increase in E2F1 expression. MDM2 prolongs the half-life of the E2F1 protein by inhibiting its ubiquitination. MDM2 displaces SCF(SKP2), the E2F1 E3 ligase. Direct binding between MDM2 and E2F1 is necessary for the negative effects of MDM2 on E2F1 ubiquitination, and deletion of the MDM2 nuclear localization signal does not result in loss of the ability to increase the E2F1 protein level. The downregulation of E2F1 upon MDM2 inhibition was not due to either pRB or p14(Arf). In addition, E2F1 was responsible for at least part of the inhibition of cell proliferation induced by MDM2 knockdown. In conclusion, the present study provides evidence that stabilization of the E2F1 protein is likely another p53-independent component of MDM2-mediated tumorigenesis. More knowledge about the MDM2-E2F1 interaction may be helpful in developing novel anticancer therapies.

Antisense Bcl-2 sensitizes prostate cancer cells to radiation

BACKGROUND

Bcl-2 is anti-apoptotic and overexpression is associated with prostate tumor aggressiveness. We hypothesized that Bcl-2 has a role in prostate cancer radiation (RT) response. The relationship of Bcl-2 expression in four prostate cancer cell lines, and the effect of modulating expression with a Bcl-2 antisense oligonucleotide (G3139, Genasense, oblimersen sodium, Genta Incorporated), to RT was examined.

METHODS

The four cell lines studied were LNCaP (wild type-p53), PC3 (p53 null), Bcl-2 stably transfected LNCaP (LNCaP-BST), and Bcl-2 stably transfected PC3 (PC3-BST) cells. Cells were treated with antisense (AS) Bcl-2 alone or with RT (2-6 Gy). Following RT, cells were processed at 3-6 hr for Western blots, 18 hr for Annexin V staining and flow cytometric analysis, 24 hr for caspases 3+7 quantification by fluorometric assay, and immediately for clonogenic survival.

RESULTS

AS caused a significant reduction in Bcl-2 expression in all cell lines. P53 expression was elevated following RT treatment in LNCaP and LNCaP-BST cells. P21 was increased by RT treatment in all cell lines. AS caused a significant increase in caspase 3+7 activity over the mismatch (MM) controls in all cell lines. When AS was combined with RT, caspase 3+7 activity was further increased significantly over all other groups in all cell lines. Moreover, AS+RT resulted in significantly reduced clonogenic survival over MM+RT, which was dampened in the Bcl-2 overexpressing lines.

CONCLUSIONS

To our knowledge, these data demonstrate for the first time that a Bcl-2 specific AS oligonucleotide sensitizes prostate cancer cells to RT. p53 is not required for this effect.

Murine double minute 2 as a therapeutic target for radiation sensitization of lung cancer

Murine double minute 2 (MDM2) inhibits p53-mediated functions, which are essential for therapies using DNA-damaging agents. The purpose of this study was to determine whether MDM2 inhibition enhances the radiosensitivity of a lung cancer model. The effects of MDM2 inhibition on tumor vasculature were also studied. Transient transfection of H460 lung cancer cells and human umbilical vascular endothelial cells (HUVEC) with antisense oligonucleotides (ASODN) against MDM2 resulted in a reduced level of MDM2 and increased levels of p21 and p53. Clonogenic assays showed that inhibition of MDM2 greatly decreased cell survival following irradiation. Quantification of apoptotic cells by 7-aminoactinomycin D staining and of senescent cells by X-gal staining showed that both processes were significantly increased in H460 cells treated with MDM2-specific ASODN and radiation. H460 xenografts that were treated with MDM2 ASODN plus radiotherapy also showed significant growth delay (P < 0.001) and increased apoptosis by terminal deoxynucleotidyl transferase-mediated nick end labeling staining. HUVECs transfected with MDM2-specific ASODN showed impaired viability and migration with decreased tube formation. Doppler studies showed that tumor blood flow was compromised when H460 xenografts were treated with MDM2-specific ASODN and radiation. A combination of radiotherapy and inhibition of MDM2 through the antisense approach results in improved tumor control in the H460 lung cancer model. This implies that a similar strategy should be investigated among patients with locally advanced lung cancer, receiving thoracic radiotherapy.

MDM2 as MYCN transcriptional target: Implications for neuroblastoma pathogenesis

MYCN amplification is associated with an exceptionally poor prognosis in neuroblastoma. Furthermore, the crucial effectors of MYCN responsible for this aggressive subset of neuroblastoma await characterization. A critical negative regulator of the p53 tumor suppressor, MDM2, has been recently characterized in neuroblastoma cell lines as a transcriptional target of MYCN. Targeted inhibition of MYCN results in reduced MDM2 expression levels, with concomitant stabilization of p53 and stimulation of apoptosis in MYCN amplified neuroblastoma cell lines. These data suggest the possibility that MYCN-driven expression of MDM2 might play a role in counterbalancing the p53-dependent apoptotic pathways concurrently stimulated by over expression of MYC proteins. Mouse models of lymphoma have demonstrated that MDM2 expression, with decreased p53 activity, is critical for complete MYCC driven tumorigenesis. Our data suggest that a similar situation may apply for MYCN in neuroblastoma. Strategies for pharmacologic and genetic inhibition of MDM2 may prove to be an important new therapeutic approach in neuroblastoma.

Mdm2 in growth signaling and cancer

Genetic and biochemical evidence have demonstrated a direct link between Mdm2 and cancer development. Elevated expression of Mdm2 is observed in a significant proportion of different types of cancer. The major contribution of Mdm2 to the development of cancer is through a tight inhibition of the activities and stability of the tumor suppressor p53. However, extensive studies over the past few years have identified p53-independent functions of Mdm2, in the regulation of several important cellular processes and multiple signaling pathways. The promotion of cell cycle progression by Mdm2 is mediated via p53 inhibition, and by regulating the pRb/E2F complex. Mdm2 is an important mediator of growth and survival signaling in the PI3K/Akt pathway, an activator of certain steroid hormone receptors, and an inhibitor of the TGF-beta growth restrictive pathway. Thus, the impact on these pathways by deregulated Mdm2, as often observed in cancer, can be oncogenic in a permissible environment. This renders Mdm2 as an important target for the development of anti-cancer drugs.

Gene therapy strategies to improve the effectiveness of cancer radiotherapy

Having the ability to alter the genetic makeup of a cancer cell by gene transfer is a potentially powerful strategy for treating human cancer. However, a low efficiency of gene delivery in vivo and poor tumour specificity has prevented the widespread implementation of this technology in the clinic. Despite these formidable obstacles, the first successful application of gene therapy in the treatment of cancer may occur when it is combined with local modalities such as radiation therapy. A small number of gene therapy strategies have been evaluated in clinical trials in combination with external beam radiation therapy. The combined therapy has been well-tolerated and has not exacerbated the side effects of radiation therapy. Gene transfer and tumour cell destruction has been demonstrated in vivo. Although the results await confirmation in larger, prospective Phase III trials, there is suggestive evidence that the combined therapies may be demonstrating better than expected antitumour activity. Our vast knowledge of the molecular defects that drive the cancer process, coupled with our expanding understanding of the genes responsible for tumour cell radioresistance, have spawned the development of rational, targeted gene therapies designed to increase tumour cell radiosensitivity. Here, the results of the clinical trials conducted so far will be reviewed, followed by a description of new approaches under development at present.

Combined targeting of epidermal growth factor receptor and MDM2 by gefitinib and antisense MDM2 cooperatively inhibit hormone-independent prostate cancer

PURPOSE

The epidermal growth factor receptor (EGFR) may play a relevant role in the progression, hormone therapy resistance, and prognosis of prostate cancer patients. Also MDM2, a negative p53 regulator that interacts with retinoblastoma (Rb), E2F, p19(arf) and the ras-mitogen-activated protein kinase(MAPK) cascade plays an important role in prostate cancer progression and prognosis. On the basis of the EGFR and MDM2 role in integrating signaling pathways critical for prostate cancer progression, we investigated whether their selective combined blockade may have a cooperative antitumor effect in prostate cancer. For this purpose, we have used the EGFR tyrosine kinase inhibitor gefitinib (ZD1839, Iressa) and a second generation hybrid oligonucleotide antisense MDM2 (AS-MDM2), respectively.

EXPERIMENTAL DESIGN

Gefitinib and AS-MDM2 were administered to hormone-refractory and hormone-dependent human prostate cancer cells in vitro and to mice bearing tumor xenografts, evaluating the effects on growth, apoptosis, and protein expression, in vitro and in vivo.

RESULTS

We demonstrated that the combination of gefitinib and AS-MDM2 synergistically inhibits the growth of hormone-independent prostate cancer cells in vitro. This effect is accompanied by the inhibition of MDM2, phosphorylated Akt (pAkt), phosphorylated MAPK (pMAPK), and vascular endothelial growth factor (VEGF) expression and by Rb hypophosphorylation. The combination of the two agents in nude mice bearing the same hormone-independent tumors caused a potent cooperative antitumor effect. Tumor samples analysis confirmed the inhibition of MDM2, pAkt, pMAPK, VEGF, and basic fibroblast growth factor expression.

CONCLUSIONS

This study shows that EGFR and MDM2 play a critical role in the growth of prostate cancer, especially hormone-dependent, and that their combined blockade by gefitinib and AS-MDM2 causes a cooperative antitumor effect, supporting the clinical development of this therapeutic strategy.

Oncogenes as Novel Targets for Cancer Therapy (Part IV)

This is the final part of a four-part serial review on oncogenes and their potential use as targets for cancer therapy. Previous sections discussed various categories of oncogenes (growth factors, tyrosine kinases, intermediate signaling molecules, and transcription factors) and the advances made in various strategies being used to alter their actions. This part describes four oncogenes, MDM2, BCL2, XIAP, and Survivin, that are involved in regulation of the cell cycle and apoptosis.

Enhancement of Radiation Effect by Aphanamixis polystachya in Mice Transplanted with Ehrlich Ascites Carcinoma

The effect of radiation on tumor tissue can be optimized by adding radiosensitizing agents, in order to achieve a greater degree of tumor damage than expected from the use of either treatment alone. The ethanolic extract of Aphanamixis polystachya (APE) was tested in Swiss albino mice transplanted with Ehrlich ascites carcinoma (EAC) and exposed to various doses of gamma-radiation. EAC mice received 0, 10, 25, 50, 75, 100, 150 or 200 mg/kg body wt APE before exposure to 6 Gy gamma-radiation followed by once daily administration for another 8 consecutive days post-irradiation. The optimum radiosensitizing dose was found to be 50 mg/kg APE that was further tested in EAC mice exposed to 0, 1, 2, 4, 6 or 8 Gy hemi body gamma-radiation. The best effect of APE and radiation was observed for 6 Gy gamma-radiation. The splitting of 50 mg into two equal fractions of 25 mg and administering the split dose with a gap of 8 h on 1, 3, 5, 7 or 9 d of tumor inoculation resulted in an increased survival even when the drug was administered at late stages (day 5) of tumor development. The APE treatment before irradiation elevated lipid peroxidation followed by a reduction in the glutathione contents. Treatment of tumor bearing mice with APE before irradiation further reduced the activities of various antioxidant enzymes like glutathione peroxidase, glutathione-s-transferase, superoxide dismutase and catalase at different post last drug administration (PLDA) times.