SMAC Mimetic/IAP Inhibitor Birinapant Enhances Radiosensitivity of Glioblastoma Multiforme

Birinapant is a novel SMAC peptidomimetic molecule in clinical development. It suppresses the inhibitor of apoptosis proteins (IAPs) and promotes cytochrome-C/Apaf-1/caspase-9 activation to induce effective apoptosis. Because IAP inhibition has been shown to enhance the sensitivity of cancer cells to radiation, we investigated the role of birinapant in radiosensitization of glioblastoma cells in vitro and in vivo. Two glioblastoma cell lines, U-251 and U-87, were used to analyze radiosensitization in vitro with 7-AAD cell death/apoptosis and clonogenic assays. Subcutaneous flank (U-251 and U-87) and intracranial orthotopic (U-251) xenografts in nude mice were used to evaluate radiosensitization in vivo. TNF-α levels in media and serum were measured using electrochemiluminescence. Radiosensitization in vitro was more prominent for U-251 cells than for U-87 cells. In vivo, in both tumor models, significant tumor growth delay was observed with combination treatment compared to radiation alone. There was a survival benefit with combination treatment in the orthotopic U-251 model. TNF-α levels in media correlated directly with radiation dose in vitro. These findings show that birinapant can enhance the radiosensitivity of glioblastoma cell lines in cell-based assays and tumor models via radiation-induced TNF-α. Further study into the use of birinapant with radiation therapy is warranted.

Abstract 1648: ImmunoGraft® platform for the evaluation of Immuno-Oncology agents in PDX tumors models

Recent breakthroughs in Immunotherapy have given new hope to treating previously untreatable tumor types and provide a better tolerated alternative to standard agents. There is an unmet need for a pre-clinical platform to test potential immune-oncology therapeutics that would also provide a tool to examine the mechanisms of response to better predict clinical outcomes. We have previously presented the Champions ImmunoGraft®, an innovative pre-clinical model enabling immunotherapeutic agents to be evaluated for efficacy in solid tumors. This platform is more reflective of the human tumor microenvironment (both immune and non-immune cell-based) and may be one of the most translationally-relevant models to date for screening therapies targeting the immune system. However, optimization of humanization and PDX implantation protocols that allow a broader reconstitution of cell linage and higher engraftment rate are necessary to further improve the pre-clinical evaluation of immune-oncology therapeutics and enhance the value of this modality for patient’s benefit.

To this end, immune-compromised NOG (PrkdcscidIl2rgtm1Sug) mice were reconstituted (humanized) with human CD34+ cells using optimized procedure and blood was collected at different time points post engraftment to check for the major leukocyte linages. At ten weeks after humanization, mature human CD45+ cells comprised close to 50% of the leukocytes detected in the circulation and secondary lymphoid tissues of the humanized animals. As a result of improved methodology of the reconstitution protocol we have achieved an 85% success rate of humanization with a shortened engraftment period utilizing fewer CD34+ cells that maintain humanization reconfirmed at 22 weeks post reconstitution.

Champions TumorGraft® database contains more than one-thousand clinically relevant well-annotated PDX models. We have complied data from extensive genetic and protein expression analyses to design applicable ImmunoGraft® studies for testing of Immuno-Oncology agents. We have identified tumors expressing key markers that may predict response to immunotherapy agents including PDL-1, CD40, IDO-1 and IDO-2 as well as tumors that harbor a high mutational load. Here we present the efficacy and clinically relevant endpoints of two well-studied checkpoint inhibitors Ipilimumab and Pembrolizumab in this highly translational PDX platform.

Citation Format: David Cerna, Daniel Ciznadija, Bhavna Verma, David Sidransky, Evgeny Izumchenko, Angela Davies, Maria Mancini. ImmunoGraft® platform for the evaluation of Immuno-Oncology agents in PDX tumors models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1648. doi:10.1158/1538-7445.AM2017-1648

Abstract A40: The ImmunoGraftTM: A humanized mouse model for translational assessment of immunotherapy in solid tumors

Background: Therapeutics reactivating the immune system have demonstrated promise, with durable objective responses in patients with a variety of solid tumors. Despite these successes, current animal models do not reliably identify immunotherapeutic targets with the greatest clinical potential, due in part to differences between human and murine immune systems. Hence, development of robust preclinical tools to test such drugs against human tumors in the context of an allogeneic immune system remains an imperative. We have previously demonstrated the generation of its ImmunoGraft platform, whereby two technologies, the patient-derived xenograft (PDX) and humanized mice (immunodeficient mice reconstituted with a human immune system), are combined in a single platform. We now report on the utility of the ImmunoGraft for assessing the effect of immune-modulating agents in solid tumors.

Materials and Methods: Immune-compromised NOG (PrkdcscidIl2rgtm1Sug) mice were reconstituted with human CD34+ cells and monitored for the expansion of human immune cells (humanized). Humanized mice were engrafted with solid tumors that had been subjected to histocompatibility typing and characterized for a number of molecular markers, including PD-L1 expression. Tumor growth in the ImmunoGrafts was compared against non-humanized counterparts, as well as the level of immune reconstitution. Finally, ImmunoGrafts were treated with drugs blocking the immune checkpoints CTLA4 and PD1 and human immune activation and tumor growth inhibition evaluated.

Results: Mature human CD45+ cells comprised close to 50% of the leukocytes detected in the circulation and lymphoid organs of humanized mice. Solid tumors, including NSCLC, melanoma, and head and neck cancer, were successfully engrafted in the humanized mice. Moderate to high expression of PD-L1 was found in approximately 80% of these tumors. ImmunoGrafts treated with anti-CTLA4 or anti-PD1 antibodies exhibited systemic immune responses characterized by robust proliferation of splenic and circulating huCD3+ T cells, as well as activated huCD4+ Th1 cells. There was also an increase in tumor-infiltrating huCD8+ cytotoxic T lymphocytes and huCD68+ macrophages, along with elevated secretion of human-specific cytokines. Tumor growth inhibition, and in some instances tumor regression, was demonstrated in treated ImmunoGrafts. The magnitude of growth inhibition correlated with the level of immune activation.

Conclusion : The ImmunoGraft is an innovative pre-clinical model enabling immunotherapeutic agents to be evaluated for efficacy in solid tumors. This platform is more reflective of the human tumor microenvironment (both immune and non-immune cell-based) and may be one of the most translationally-relevant models to date for screening therapies targeting the immune system. To gauge the clinical potential of the ImmunoGraft, a retrospective analysis is currently ongoing using PDX models developed from patients treated with immuno-oncology drugs. The ImmunoGraft has the potential to revolutionize translational drug discovery and development for immunotherapeutic agents in oncology.

Citation Format: Gilson Baia, David Vasquez, David Cerna, Daniel Ciznadija, David Sidransky, Jennifer Jaskowiak, Lindsay Ryland, Angela Davies, Amanda Katz, Keren Paz. The ImmunoGraftTM: A humanized mouse model for translational assessment of immunotherapy in solid tumors. [abstract]. In: Proceedings of the AACR Special Conference: Patient-Derived Cancer Models: Present and Future Applications from Basic Science to the Clinic; Feb 11-14, 2016; New Orleans, LA. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(16_Suppl):Abstract nr A40.

Abstract A8: The ImmunoGraft: A humanized mouse model for translational assessment of immunotherapy in solid tumors

Background: Therapeutics reactivating the immune system have demonstrated promise, with durable objective responses in patients with a variety of solid tumors. Despite these successes, current animal models do not reliably identify immunotherapeutic targets with the greatest clinical potential, due in part to differences between human and murine immune systems. Hence, development of robust preclinical tools to test such drugs against human tumors in the context of an allogeneic immune system remains an imperative. We have previously demonstrated the generation of its ImmunoGraftTM platform, whereby two technologies, the patient-derived xenograft (PDX) and humanized mice (immunodeficient mice reconstituted with a human immune system), are combined in a single platform. We now report on the utility of the ImmunoGraftTM for assessing the effect of immune-modulating agents in solid tumors.

Materials and Methods: Immune-compromised NOG (PrkdcscidIl2rgtm1Sug) mice were reconstituted with human CD34+ cells and monitored for the expansion of human immune cells (humanized). Humanized mice were engrafted with solid tumors that had been subjected to histocompatibility typing and characterized for a number of molecular markers, including PD-L1 expression. Tumor growth in the ImmunoGraftsTM was compared against non-humanized counterparts, as well as the level of immune reconstitution. Finally, ImmunoGraftsTM were treated with drugs blocking the immune checkpoints CTLA4 and PD1 and human immune activation and tumor growth inhibition evaluated.

Results: Mature human CD45+ cells comprised close to 50% of the leukocytes detected in the circulation and lymphoid organs of humanized mice. Solid tumors, including NSCLC, melanoma, and head and neck cancer, were successfully engrafted in the humanized mice. Moderate to high expression of PD-L1 was found in approximately 80% of these tumors. ImmunoGraftsTM treated with anti-CTLA4 or anti-PD1 antibodies exhibited systemic immune responses characterized by robust proliferation of splenic and circulating huCD3+ T cells, as well as activated huCD4+ Th1 cells. There was also an increase in tumor-infiltrating huCD8+ cytotoxic T lymphocytes and huCD68+ macrophages, along with elevated secretion of human-specific cytokines. Tumor growth inhibition, and in some instances tumor regression, was demonstrated in treated ImmunoGraftsTM. The magnitude of growth inhibition correlated with the level of immune activation.

Conclusion: The ImmunoGraftTM is an innovative pre-clinical model enabling immunotherapeutic agents to be evaluated for efficacy in solid tumors. This platform is more reflective of the human tumor microenvironment (both immune and non-immune cell-based) and may be one of the most translationally-relevant models to date for screening therapies targeting the immune system. To gauge the clinical potential of the ImmunoGraftTM, a retrospective analysis is currently ongoing using PDX models developed from patients treated with immuno-oncology drugs. The ImmunoGraftTMhas the potential to revolutionize translational drug discovery and development for immunotherapeutic agents in oncology.

Citation Format: Gilson Baia, David Vasquez, David Cerna, Daniel Ciznadija, David Sidransky, Amanda Katz, Keren Paz. The ImmunoGraft: A humanized mouse model for translational assessment of immunotherapy in solid tumors. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A8.

Abstract 1599: Radiosensitization of GBM by a novel peptidomimetic of the Second Mitochondria-derived Activator of Caspases (SMAC)

TL32711 is a novel small molecule peptidomimetic of the SMAC (Second Mitochondria-derived Activator of Caspases) that promotes caspase activation in the cytochrome c/Apaf-1/caspase-9 pathway. Normally SMAC is located in the mitochondria; however it is released into the cytosol when cells undergo apoptosis. Release into the cytosol and cleavage of SMAC promotes caspase-9 activation by binding to inhibitors of apoptosis proteins, IAPs. Removing the inhibitory activity of IAPs increases cells’ sensitivity to apoptotic stimuli and abrogates NF-κB survival signaling. Thus, by targeting and eliminating IAPs, SMAC mimetics re-activate death signals in cancer cells, inhibit resistance mechanisms and induce tumor death by apoptosis.

This study was undertaken to determine radiation sensitization potential of TL32711 in GBM using in vitro and in vivo approaches. TL32711 alone did not cause any killing effects in GBM cells. However, in vitro apoptosis was increased when TL32711 was combined with radiation in GBM cell lines. In vivo data demonstrated that TL32711 alone delayed the growth of implanted xenograft tumors that were further delayed when combined with radiation. The survival of mice was greatly increased in TL32711 plus radiation treated GBM intracranial orthotopic model when compared to TL32711 or radiation treatment alone. Mechanistically, TL32711 confers radiation sensitization by abrogating NFκB activity that is elevated by radiation-induced TNF-α. Taken together, this is the first report that provides strong evidence that TL32711 in combination with ionizing radiation has potential to increase the effectiveness of radiotherapy.

Funded by NCI Contract No. HHSN261200800001E.

Citation Format: David Cerna, Donna Carter, Naoko Takebe, C. Norman Coleman, Mansoor M. Ahmed, Stephen Yoo. Radiosensitization of GBM by a novel peptidomimetic of the Second Mitochondria-derived Activator of Caspases (SMAC). [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1599. doi:10.1158/1538-7445.AM2013-1599

Radiation survivors: understanding and exploiting the phenotype following fractionated radiation therapy

Radiation oncology modalities such as intensity-modulated and image-guided radiation therapy can reduce the high dose to normal tissue and deliver a heterogeneous dose to tumors, focusing on areas deemed at highest risk for tumor persistence. Clinical radiation oncology produces daily doses ranging from 1 to 20 Gy, with tissues being exposed to 30 or more daily fractions. Hypothesizing the cells that survive fractionated radiation therapy have a substantially different phenotype than the untreated cells, which might be exploitable for targeting with molecular therapeutics or immunotherapy, three prostate cancer cell lines (PC3, DU145, and LNCaP) and normal endothelial cells were studied to understand the biology of differential effects of multifraction (MF) radiation of 0.5, 1, and/or 2 Gy fraction to 10 Gy total dose, and a single dose of 5 and 10 Gy. The resulting changes in mRNA, miRNA, and phosphoproteome were analyzed. Significant differences were observed in the MF radiation exposures including those from the 0.5 Gy MF that produces little cell killing. As expected, p53 function played a major role in response. Pathways modified by MF include immune response, DNA damage, cell-cycle arrest, TGF-β, survival, and apoptotic signal transduction. The radiation-induced stress response will set forth a unique platform for exploiting the effects of radiation therapy as "focused biology" for cancer treatment in conjunction with molecular targeted or immunologically directed therapy. Given that more normal tissue is treated, albeit to lower doses with these newer techniques, the response of the normal tissue may also influence long-term treatment outcome.

Fractionated radiation alters oncomir and tumor suppressor miRNAs in human prostate cancer cells

We have previously demonstrated that prostate carcinoma cells exposed to fractionated radiation differentially expressed more genes compared to single-dose radiation. To understand the role of miRNA in regulation of radiation-induced gene expression, we analyzed miRNA expression in LNCaP, PC3 and DU145 prostate cancer cells treated with single-dose radiation and fractionated radiation by microarray. Selected miRNAs were studied in RWPE-1 normal prostate epithelial cells by RT-PCR. Fractionated radiation significantly altered more miRNAs as compared to single-dose radiation. Downregulation of oncomiR-17-92 cluster was observed only in the p53 positive LNCaP and RWPE-1 cells treated with single-dose radiation and fractionated radiation. Comparison of miRNA and mRNA data by IPA target filter analysis revealed an inverse correlation between miR-17-92 cluster and several targets including TP53INP1 in p53 signaling pathway. The base level expressions of these miRNAs were significantly different among the cell lines and did not predict the radiation outcome. Tumor suppressor miR-34a and let-7 miRNAs were upregulated by fractionated radiation in radiosensitive LNCaP (p53 positive) and PC3 (p53-null) cells indicating that radiation-induced miRNA expression may not be regulated by p53 alone. Our data support the potential for using fractionated radiation to induce molecular targets and radiation-induced miRNAs may have a significant role in predicting radiosensitivity.

Inhibition of nicotinamide phosphoribosyltransferase (NAMPT) activity by small molecule GMX1778 regulates reactive oxygen species (ROS)-mediated cytotoxicity in a p53- and nicotinic acid phosphoribosyltransferase1 (NAPRT1)-dependent manner

Cancer cells undergo mitosis more frequently than normal cells and thus have increased metabolic needs, which in turn lead to higher than normal reactive oxygen species (ROS) production. Higher ROS production increases cancer cell dependence on ROS scavenging systems to balance the increased ROS. Selectively modulating intracellular ROS in cancers by exploiting cancer dependence on ROS scavenging systems provides a useful therapeutic approach. Essential to developing these therapeutic strategies is to maintain physiologically low ROS levels in normal tissues while inducing ROS in cancer cells. GMX1778 is a specific inhibitor of nicotinamide phosphoribosyltransferase, a rate-limiting enzyme required for the regeneration of NAD(+) from nicotinamide. We show that GMX1778 increases intracellular ROS in cancer cells by elevating the superoxide level while decreasing the intracellular NAD(+) level. Notably, GMX1778 treatment does not induce ROS in normal cells. GMX1778-induced ROS can be diminished by adding nicotinic acid (NA) in a NA phosphoribosyltransferase 1 (NAPRT1)-dependent manner, but NAPRT1 is lost in a high frequency of glioblastomas, neuroblastomas, and sarcomas. In NAPRT1-deficient cancer cells, ROS induced by GMX1778 was not susceptible to treatment with NA. GMX1778-mediated ROS induction is p53-dependent, suggesting that the status of both p53 and NAPRT1 might affect tumor apoptosis, as determined by annexin-V staining. However, as determined by colony formation, GMX1778 long term cytotoxicity in cancer cells was only prevented by the addition of NA to NAPRT1-expressing cells. Exposure to GMX1778 may be a novel way of inducing ROS selectively in NAPRT1-negative tumors without inducing cytotoxic ROS in normal tissue.

Abstract 1460: Comparison of radiosensitizing effects of several PARP inhibitors under clinical investigation

Targeting tumor DNA repair is an attractive strategy in selectively killing tumors while sparing normal tissues. Inhibiting Poly (ADP-ribose) polymerase (PARP) in cancers with BRCA1 and BRCA2 mutations has been shown to be a promising approach either as a single agent or in combination therapy with chemotherapeutics or ionizing radiation. To systematically compare the radiosensitizing efficacy of various PARP inhibitors we have evaluated the long-term cytotoxicity of ABT-888, AZD2281, MK4827, and BSI-201 in clonogenic survival assays in combination with ionizing radiation. For these studies we used the breast cancer cell lines MDA-MB 231 BR, a subline of MDA-MB-231 selected for its tendency to metastasize to brain (BRCA1+/−) and MX-1 (BRCA1 deletion and BRCA2 mutation). Furthermore, we have evaluated PARP inhibitors under normoxic and hypoxic conditions (21% O2 versus 0.1% O2). In MDA-MB 231 BR cells, at a clinically relevant and achievable concentration for ABT-888 (J Clin Oncol. 2009; 27(16)) and AZD2281(N Engl J Med. 2009; 361(2)), and at 1 uM MK4827 showed radiosensitizing effects under both normoxic and hypoxic conditions. BSI-201 at a clinically achievable concentration (ASCO 2010, Abstract 2012) did not exhibit radiosensitizing effects under the same conditions. When MX-1 cells were treated with BSI-201 under hypoxic conditions (0.1% O2) the cytotoxicity of BSI-201 was evident. Taken together, the results show that ABT-888, AZD2281 and MK4827 show promising radiosensitizing effects under both normoxic and hypoxic conditions whereas the cytotoxicity of BSI-201 was observed only under hypoxic conditions in BRCA1/2 mutant carrier MX-1cells.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1460. doi:1538-7445.AM2012-1460

Abstract C132: Radiosensitization of solid tumors by an MDM2 inhibitor (HLI373989)

The tumor suppressor p53 is a key molecule in the prevention of cancer. More than half of all human cancers have mutation or loss of the p53 gene. A significant percentage of the other half contain overexpression of Hdm2 which serves as a ubiquitin ligase (E3) to target p53 for degradation. While it has been established that an inhibitor of Hdm2 ubiquitin E3 ligase activity HLI373 (5-(3-dimethylaminopropylamino)-3,10-dimethyl-10H-pyrimido(4,5-b)quinoline-2,4-dione, NSC373989), selectively kills tumor cells harboring wild-type p53, we investigated the effect of the Hdm2 inhibitor HLI373, in the response to ionizing radiation in solid tumor lines expressing wild-type, mutant or null p53 in vitro and in vivo. We now report HLI373 enhances radiosensitivity in a panel of human tumor cell lines in p53 deficient and competent cell lines using clonogenic assay. HCT116 cells exposed to HLI373 (5–7.5 uM) for 16 hours before and maintained in the medium after irradiation had an increase in radiosensitivity. Cell cycle distribution of treated cells after 16 hours was measured using flow cytometry. In a comparison of wildtype and null cell lines, cell cycle arrest in G1 was only detected in cells with active p53 at 5uM of HLI373. In contrast, cells with a null or mutated p53 progressed to and arrested in G2. In addition, wild type p53 cells pretreated with 5 uM HLI373 and irradiated continued to be arrested in G1 and did not progress to a G2. However, cells without p53 treated with HLI373 and radiation increased in accumulation in G2. Treatment with higher dosages of HLI373 (>10uM) resulted in the accumulation of cells in G2 regardless of p53 status. These results indicate that HLI373 enhances tumor cell radiosensitivity in vitro and agents such as HLI373 may have potential for use in the clinics.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C132.

Abstract 2668: Radiosensitization of solid tumor cell lines by an MDM2 inhibitor

The tumor suppressor p53 is a key molecule in the prevention of cancer. More than half of all human cancers have mutation or loss of the p53 gene. A significant percentage of the other half contain overexpression of Hdm2 which serves as a ubiquitin ligase (E3) to target p53 for degradation. While it has been established that an inhibitor of Hdm2 ubiquitin E3 ligase activity HLI373 (5-(3-dimethylaminopropylamino)-3,10-dimethyl-10H-pyrimido(4,5-b)quinoline-2,4-dione, NSC373989), selectively kills tumor cells harboring wild-type p53, we investigated the effect of the Hdm2 inhibitor HLI373, in the response to ionizing radiation in solid tumor lines expressing wild-type, mutant or null p53 in vitro and in vivo. We now report HLI373 enhances radiosensitivity in a panel of human tumor cell lines in p53 deficient and competent cell lines using clonogenic assay. HCT116 cells exposed to HLI373 (5-7.5 uM) for 16 hours before and maintained in the medium after irradiation had an increase in radiosensitivity. Cell cycle distribution of treated cells after 16 hours was measured using flow cytometry. In a comparison of wildtype and null cell lines, cell cycle arrest in G1 was only detected in cells with active p53 at 5uM of HLI373. In contrast, cells with a null or mutated p53 progressed to and arrested in G2. In addition, wild type p53 cells pretreated with 5 uM HLI373 and irradiated continued to be arrested in G1 and did not progress to a G2. However, cells without p53 treated with HLI373 and radiation increased in accumulation in G2. Treatment with higher dosages of HLI373 (>10uM) resulted in the accumulation of cells in G2 regardless of p53 status. These results indicate that HLI373 enhances tumor cell radiosensitivity in vitro and agents such as HLI373 may have potential for use in the clinics.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2668. doi:10.1158/1538-7445.AM2011-2668

Fractionated radiation therapy can induce a molecular profile for therapeutic targeting

To examine the possibility of using fractionated radiation in a unique way with molecular targeted therapy, gene expression profiles of prostate carcinoma cells treated with 10 Gy radiation administered either as a single dose or as fractions of 2 Gy × 5 and 1 Gy × 10 were examined by microarray analysis. Compared to the single dose, the fractionated irradiation resulted in significant increases in differentially expressed genes in both cell lines, with more robust changes in PC3 cells than in DU145 cells. The differentially expressed genes (>twofold change; P < 0.05) were clustered and their ontological annotations evaluated. In PC3 cells genes regulating immune and stress response, cell cycle and apoptosis were significantly up-regulated by multifractionated radiation compared to single-dose radiation. Ingenuity Pathway Analysis (IPA) of the differentially expressed genes revealed that immune response and cardiovascular genes were in the top functional category in PC3 cells and cell-to-cell signaling in DU145 cells. RT-PCR analysis showed that a flexure point for gene expression occurred at the 6th-8th fraction and AKT inhibitor perifosine produced enhanced cell killing after 1 Gy × 8 fractionated radiation in PC3 and DU145 cells compared to single dose. This study suggests that fractionated radiation may be a uniquely exploitable, non-oncogene-addiction stress pathway for molecular therapeutic targeting.

CCND1 and ZNF217 gene amplification is equally frequent in BRCA1 and BRCA2 associated and non-BRCA breast cancer

Breast cancer associated with BRCA1 and BRCA2 gene mutations differs from non-BRCA tumors in several respects. We determined whether there was any difference in CCND1 (11q13) and ZNF217 (20q13) gene amplification with respect to BRCA status. Of 40 breast cancer samples examined, 15 and 9 were from BRCA1 and BRCA2 mutation carriers, respectively, and 16 from patients without mutation. Fluorescence in situ hybridization showed that eight tumors exhibited CCND1 amplification (20%; 3 BRCA1, 3 BRCA2, 2 non-BRCA). ZNF217 amplification was observed in three of 38 cases (8%; 2 BRCA1, 1 non-BRCA). There was no significant difference in CCND1 and ZNF217 amplification between BRCA1, BRCA2 and non-BRCA tumors. CCND1 amplification was associated with decreased disease-free (P = 0.045) and overall survival (P = 0.015). BRCA1 tumors with CCND1 amplification were estrogen receptor negative, in contrast to CCND1 amplified BRCA2 and non-BRCA tumors, suggesting that concurrent CCND1 amplification and estrogen and progesterone receptor negativity may predict germline BRCA1 gene mutation. All ZNF217 amplified tumors were of the medullary histological type (P = 0.002). There was no statistical correlation between CCND1 and ZNF217 amplification and estrogen receptor, progesterone receptor, and ERBB2 expression and TNM classification. CCND1 amplification did not correlate with EGFR expression.

Abstract 3046: Differential microRNA patterns in prostate carcinoma cells after single and fractionated radiation

Our previous mRNA microarray analysis showed that fractionated radiation induced more differentially expressed genes compared to single dose radiation in PC3 and DU145 cells. MicroRNAs (miRNAs) are an important class of non-coding small RNAs capable of regulating gene expression at the translational level. To understand the role of miRNAs in regulation of radiation-induced gene expression patterns, we studied the expression levels of miRNAs by microarray analysis in prostate cancer cells. Methods: PC3 and DU145 cells were exposed to 5 Gy and 10Gy either as a single dose radiation or multi-fractionated (0.5Gyx10 and 1Gyx10) radiation. RNA was extracted at 24h after the final dose of radiation and miRNA microarray analysis was done using Agilent human miRNA Microarray Kit (V2). The chip contains probes for 723 human and 76 human viral microRNAs from the Sanger database v.10.1. Data were analyzed using GeneSpring software (Agilent technologies). Results: Of the total 723 miRNAs represented in the array, 141 miRNAs were differentially expressed (&gt; 1.5 fold change) by the 4 radiation protocols. Significant difference in the miRNA expression pattern was noted between the two cell lines. The number of radiation - induced miRNAs was higher in PC3 cells than in DU145 cells. In PC3 cells fractionated radiation resulted in more down regulated miRNAs compared to single dose radiation. In our previous study immune response genes were significantly up regulated by fractionated radiation in PC3 cells but not in DU145 cells. In the present study Mir-146a was significantly up regulated in DU145 cells after fractionated radiation but not in PC3 cells suggesting an inverse correlation between the differential expression of mir-146a and immune response genes in both cell lines. This study also demonstrated significant differences in let-7 family of miRNAs in the two cell lines and also between single and fractionated radiation. In PC3 cells, 8 of the let-7 miRNAs were upregulated after fractionated radiation whereas in DU145 cells they were either downregulated or not changed. Conclusion: Fractionated radiation resulted in expression of more number of miRNAs compared to single dose radiation. We are currently in the process of evaluating radiation-induced differential gene expression changes by a combined approach of mRNA, miRNA and protein array analysis to identify radiation induced molecular targets for cancer therapy.

This work was supported by the Intramural Research Program of Center for Cancer Research, National Cancer Institute, National Institutes of Health.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3046.

Abstract 2506: Palomid 529, a PI3K/Akt/mTOR dual TORC1/2 inhibitor, is a radiosensitizer with effect in both subcutaneous and orthotopic U251 glioblastoma tumor xenograft models

Palomid 529 (P529) is a small molecule drug created through three generations of computational design. P529 is an anti-tumor agent able to target and inhibit the PI3K/Akt/mTOR signal transduction pathway, specifically as an allosteric dual TORC1/TORC2 inhibitor causing the dissociation of the TORC complexes. Furthermore, P529 has been shown to broadly inhibit cell lines of the NCI 60 cell screen, inhibit HIF-1α, and show tumor growth delay in murine tumor xenograft models. P529 is able to cross the blood-brain barrier and show little or no effect of two transporters (P-gp and Bcrp1) in brain uptake of P529. Here we show that P529 has a more than additive radiation sensitizing activity inhibiting tumor growth in U251 glioblastoma tumor xenograft models both subcutaneous and orthotopic. P529 was administered in a dose-dependent manner at 25, 50 and 95 mg/kg oral dosing every day for 5 days with 4 Gy radiation given on day three. Tumor xenografts in both subcutaneous and orthotopic models were evaluated daily for tumor size or survival respectively up to 40 days post tumor implantation. P529 treatment alone showed marked decrease in tumor growth (subcutaneous) and increase in survival (orthotopic). With addition of radiation, there was a synergistic increase in activity of P529 on the inhibition of U251 tumor growth and survival. All in all, work described here provides evidence that P529 has activity as a radiosensitizer in murine models of glioblastoma and the possibility of efficacious activity in treatment of human patients under a novel mechanism of inhibition of the PI3K/Akt/mTOR pathway.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2506.

Abstract B98: Induction of metabolic and oxidative stresses by the novel inhibitor of NAD+ biosynthesis, GMX 1778, for specific killing of human glioblastomas

Cancer therapy strategies that selectively target tumors while sparing normal tissues would provide enormous clinical benefit to cancer patients. Central to developing these strategies is identification of tumor specific vulnerabilities and molecular targeted agents targeting these vulnerabilities. GMX1777/1778, a novel inhibitor of NAD+ biosynthesis, is a potent and specific inhibitor of the nicotinamide adnine dinucleotide (NAD+) biosynthesis enzyme, phosphoribosyl transferase (NAMPRT). Since cancer cells have a high rate of NAD+ turnover, modulation of NAD+ biosynthesis would be an attractive target for GMX 1777/1778. In addition to the depletion of NAD+ level, we present evidence of inducing other metabolic and oxidative stresses in a tumor specific manner with minimal damage to normal tissues in vitro and in vivo. Taken together, the cancer therapeutic strategy presented here would provide a novel way of specific killing of tumors.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B98.

Cellular inhibition of checkpoint kinase 2 (Chk2) and potentiation of camptothecins and radiation by the novel Chk2 inhibitor PV1019 [7-nitro-1H-indole-2-carboxylic acid {4-[1-(guanidinohydrazone)-ethyl]-phenyl}-amide]

Chk2 is a checkpoint kinase involved in the ataxia telangiectasia mutated pathway, which is activated by genomic instability and DNA damage, leading to either cell death (apoptosis) or cell cycle arrest. Chk2 provides an unexplored therapeutic target against cancer cells. We recently reported 4,4'-diacetyldiphenylurea-bis(guanylhydrazone) (NSC 109555) as a novel chemotype Chk2 inhibitor. We have now synthesized a derivative of NSC 109555, PV1019 (NSC 744039) [7-nitro-1H-indole-2-carboxylic acid {4-[1-(guanidinohydrazone)-ethyl]-phenyl}-amide], which is a selective submicromolar inhibitor of Chk2 in vitro. The cocrystal structure of PV1019 bound in the ATP binding pocket of Chk2 confirmed enzymatic/biochemical observations that PV1019 acts as a competitive inhibitor of Chk2 with respect to ATP. PV1019 was found to inhibit Chk2 in cells. It inhibits Chk2 autophosphorylation (which represents the cellular kinase activation of Chk2), Cdc25C phosphorylation, and HDMX degradation in response to DNA damage. PV1019 also protects normal mouse thymocytes against ionizing radiation-induced apoptosis, and it shows synergistic antiproliferative activity with topotecan, camptothecin, and radiation in human tumor cell lines. We also show that PV1019 and Chk2 small interfering RNAs can exert antiproliferative activity themselves in the cancer cells with high Chk2 expression in the NCI-60 screen. These data indicate that PV1019 is a potent and selective inhibitor of Chk2 with chemotherapeutic and radiosensitization potential.

NS-398, ibuprofen, and cyclooxygenase-2 RNA interference produce significantly different gene expression profiles in prostate cancer cells

Cyclooxygenase-2 (COX-2) plays a significant role in tumor development and progression. Nonsteroidal anti-inflammatory drugs (NSAID) exhibit potent anticancer effects in vitro and in vivo by COX-2-dependent and COX-2-independent mechanisms. In this study, we used microarray analysis to identify the change of expression profile regulated by a COX-2-specific NSAID NS-398 (0.01 and 0.1 mmol/L), a nonspecific NSAID ibuprofen (0.1 and 1.5 mmol/L) and RNA interference (RNAi)-mediated COX-2 inhibition in PC3 prostate cancer cells. A total of 3,362 differentially expressed genes with 2-fold change and P<0.05 were identified. Low concentrations of NSAIDs and COX-2 RNAi altered very few genes (1-3%) compared with the higher concentration of NS-398 (17%) and ibuprofen (80%). Ingenuity Pathway Analysis was used for distributing the differentially expressed genes into biological networks and for evaluation of functional significance. The top 3 networks for both NSAIDs included functional categories of DNA replication, recombination and repair, and gastrointestinal disease. Immunoresponse function was specific to NS-398, and cell cycle and cellular movement were among the top functions for ibuprofen. Ingenuity Pathway Analysis also identified renal and urologic disease as a function specific for ibuprofen. This comprehensive study identified several COX-2-independent targets of NSAIDs, which may help explain the antitumor and radiosensitizing effects of NSAIDs. However, none of these categories were reflected in the identified networks in PC3 cells treated with clinically relevant low concentrations of NS-398 and ibuprofen or with COX-2 RNAi, suggesting the benefit to fingerprinting preclinical drug concentrations to improve their relevance to the clinical setting.

PX-478, an inhibitor of hypoxia-inducible factor-1alpha, enhances radiosensitivity of prostate carcinoma cells

Overexpression of hypoxia-inducible factor-1alpha (HIF-1alpha) in human tumors is associated with poor prognosis and poor outcome to radiation therapy. Inhibition of HIF-1alpha is considered as a promising approach in cancer therapy. The purpose of this study was to test the efficacy of a novel HIF-1alpha inhibitor PX-478 as a radiosensitizer under normoxic and hypoxic conditions in vitro. PC3 and DU 145 prostate carcinoma cells were treated with PX-478 for 20 hr, and HIF-1alpha protein level and clonogenic cell survival were determined under normoxia and hypoxia. Effects of PX-478 on cell cycle distribution and phosphorylation of H2AX histone were evaluated. PX-478 decreased HIF-1alpha protein in PC3 and DU 145 cells. PX-478 produced cytotoxicity in both cell lines with enhanced toxicity under hypoxia for DU-145. PX-478 (20 mumol/L) enhanced the radiosensitivity of PC3 cells irradiated under normoxic and hypoxic condition with enhancement factor (EF) 1.4 and 1.56, respectively. The drug was less effective in inhibiting HIF-1alpha and enhancing radiosensitivity of DU 145 cells compared to PC3 cells with EF 1.13 (normoxia) and 1.25 (hypoxia) at 50 mumol/L concentration. PX-478 induced S/G2M arrest in PC3 but not in DU 145 cells. Treatment of PC3 and DU 145 cells with the drug resulted in phosphorylation of H2AX histone and prolongation of gammaH2AX expression in the irradiated cells. PX-478 is now undergoing Phase I clinical trials as an oral agent. Although the precise mechanism of enhancement of radiosensitivity remains to be identified, this study suggests a potential role for PX-478 as a clinical radiation enhancer.

Postradiation sensitization of the histone deacetylase inhibitor valproic acid

PURPOSE

Preclinical studies evaluating histone deacetylase (HDAC) inhibitor-induced radiosensitization have largely focused on the preirradiation setting based on the assumption that enhanced radiosensitivity was mediated by changes in gene expression. Our previous investigations identified maximal radiosensitization when cells were exposed to HDAC inhibitors in both the preradiation and postradiation setting. We now expand on these studies to determine whether postirradiation exposure alone affects radiosensitivity.

EXPERIMENTAL DESIGN

The effects of the HDAC inhibitor valproic acid (VA) on postirradiation sensitivity in human glioma cell lines were evaluated using a clonogenic assay, exposing cells to VA up to 24 h after irradiation. DNA damage repair was evaluated using gammaH2AX and 53BP1 foci and cell cycle phase distribution was analyzed by flow cytometry. Western blot of acetylated gammaH2AX was done following histone extraction on AUT gels.

RESULTS

VA enhanced radiosensitivity when delivered up to 24 h after irradiation. Cells accumulated in G(2)-M following irradiation, although they returned to baseline at 24 h, mitigating the role of cell cycle redistribution in postirradiation sensitization by VA. At 12 h after irradiation, significant gammaH2AX and 53BP1 foci dispersal was shown in the control, although cells exposed to VA after irradiation maintained foci expression. VA alone had no effect on the acetylation or phosphorylation of H2AX, although it did acetylate radiation-induced gammaH2AX.

CONCLUSIONS

These results indicate that VA enhances radiosensitivity at times up to 24 h after irradiation, which has direct clinical application.

In vitro and in vivo radiosensitization induced by the ribonucleotide reductase inhibitor Triapine (3-aminopyridine-2-carboxaldehyde-thiosemicarbazone)

PURPOSE

Because ribonucleotide reductase (RR) plays a role in DNA repair, it may serve as a molecular target for radiosensitization. Unlike previously investigated RR inhibitors, Triapine potently inhibits both RR holoenzymes. Therefore, the effects of Triapine on tumor cell radiosensitivity were investigated.

EXPERIMENTAL DESIGN

The effects of Triapine on the in vitro radiosensitivity of three human tumor cell lines and one normal cell line were evaluated using a clonogenic assay. Growth delay was used to evaluate the effects of Triapine on in vivo tumor radiosensitivity. The levels of the RR subunits were determined using immunoblot analysis and DNA damage and repair were evaluated using gammaH2AX foci.

RESULTS

Exposure of the tumor cell lines to Triapine before or immediately after irradiation resulted in an increase in radiosensitivity. In contrast, Triapine enhanced the radiosensitivity of the normal fibroblast cell line only when the exposure was before irradiation. There were no consistent differences between cell lines with respect to the expression of the RR subunits. Whereas Triapine had no effect on radiation-induced gammaH2AX foci at 1 hour, the number of gammaH2AX foci per cell was significantly greater in the Triapine-treated cells at 24 hours after irradiation, suggesting the presence of unrepaired DNA damage. Triapine administration to mice bearing tumor xenografts immediately after irradiation resulted in a greater than additive increase in radiation-induced tumor growth delay.

CONCLUSIONS

These results indicate that Triapine can enhance tumor cell radiosensitivity in vitro and in vivo and suggest that this effect involves an inhibition of DNA repair.

Histone deacetylation as a target for radiosensitization

Due to an increase in the understanding of molecular radiobiology, strategies for enhancing tumor radiosensitivity have begun to focus on targeting the molecules and processes that regulate cellular radioresponse. Toward this end, histone acetylation has begun to receive considerable attention as a potential target for radiosensitization. Histone acetylation, which is determined by the competing actions of histone acetylases (HATs) and histone deacetylases (HDACs), plays a role in regulating chromatin structure and gene expression--two parameters that have long been considered determinants of radioresponse. As a means of modifying histone acetylation status, considerable effort has been put into the development of inhibitors of HDAC activity, which is often aberrant in tumor cells. This has led to the generation of a relatively large number of structurally diverse compounds that inhibit HDAC activity and result in histone hyperacetylation, and importantly, are applicable to patient treatment. Whereas a number of these HDAC inhibitors have antitumor activity in preclinical cancer models when delivered as single agents, recent studies have indicated that these compounds also significantly enhance tumor cell radiosensitivity. A structurally diverse set of HDAC inhibitors have been shown to enhance the in vitro radiosensitivity of human tumor cell lines generated from a spectrum of solid tumors. Moreover, HDAC inhibitors increased the radiosensitivity of human tumor xenografts. Although the mechanism responsible for this radiosensitization has not been definitely elucidated, data suggest that inhibiting the repair of radiation-induced DNA damage may be involved. Whereas HDAC inhibitors are currently in clinical trials as single modalities and in combination with chemotherapeutic agents, recent results suggest that these compounds may also enhance the antitumor effectiveness of radiotherapy.

ErbB3 expression predicts tumor cell radiosensitization induced by Hsp90 inhibition

The ability to identify tumors that are susceptible to a given molecularly targeted radiosensitizer would be of clinical benefit. Towards this end, we have investigated the effects of a representative Hsp90 inhibitor, 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17DMAG), on the radiosensitivity of a panel of human tumor cell lines. 17DMAG was previously shown to enhance the radiosensitivity of a number of human cell lines, which correlated with the loss of ErbB2. We now report on cell lines in which 17DMAG induced the degradation of ErbB2, yet had no effect on radiosensitivity. In a comparison of ErbB family members, ErbB3 protein was only detectable in cells resistant to 17DMAG-induced radiosensitization. To determine whether ErbB3 plays a casual role in this resistance, short interfering RNA (siRNA) was used to knockdown ErbB3 in the resistant cell line AsPC1. Whereas individual treatments with siRNA to ErbB3 or 17DMAG had no effect on radiosensitivity, the combination, which reduced both ErbB2 and ErbB3, resulted in a significant enhancement in AsPC1 radiosensitivity. In contrast to siRNA to ErbB3 or 17DMAG treatments only, AsPC1 cell exposure to the combination also resulted in a decrease in ErbB1 kinase activity. These results indicate that ErbB3 expression predicts for tumor cell susceptibility to and suggests that the loss of ErbB1 signaling activity is necessary for 17DMAG-induced radiosensitization. However, for cell lines sensitized by 17DMAG, treatment with siRNA to ErbB2, which reduced ErbB1 activity, had no effect on radiosensitivity. These results suggest that, whereas the loss of ErbB1 signaling may be necessary for 17DMAG-induced radiosensitization, it is not sufficient.

The structure of Sif2p, a WD repeat protein functioning in the SET3 corepressor complex

In Saccharomyces cerevisiae, the SIF2 gene product is an integral component of the Set3 complex (SET3C), an assembly of proteins with some homology to the human SMRT and N-CoR corepressor complexes. SET3C has histone deacetylase activity that is responsible for repressing a set of meiotic genes. We have determined the X-ray crystal structure of a 46 kDa C-terminal domain of a SET3C core protein, Sif2p to 1.55 A resolution and a crystallographic R-factor of 19.0%. This domain contains an unusual eight-bladed beta-propeller structure, which differs from other transcriptional corepressor structures such as yeast Tup1p and human groucho (Gro)/TLE1, which have only seven. We have demonstrated intact Sif2p is a tetramer and the N-terminal LisH (Lis-homology)-containing domain mediates tetramerization and interaction with another component of SET3C, Snt1p. Multiple sequence alignments indicate that a surface on the "top" of the protein is conserved among species, suggesting that it may play a common role in binding partner proteins. Since Sif2p appears to be the yeast homolog of human TBL1 and TBLR1, which function in the N-CoR/SMRT complexes, its structural and oligomeric properties are likely to be very similar.

Enhancement of in vitro and in vivo tumor cell radiosensitivity by the DNA methylation inhibitor zebularine

Aberrant DNA hypermethylation is a frequent finding in tumor cells, which has suggested that inhibition of DNA methylation may be an effective cancer treatment strategy. Because DNA methylation affects gene expression and chromatin structure, parameters considered to influence radioresponse, we investigated the effects of the DNA methylation inhibitor zebularine on the radiosensitivity of human tumor cells. Three human tumor cell lines were used in this study (MiaPaCa, DU145, and U251) and the methylation status of three genes frequently hypermethylated in tumor cells (RASSF1A, HIC-1, and 14-3-3sigma) was determined as a function of zebularine exposure. Zebularine resulted in DNA demethylation in a time-dependent manner, with the maximum loss of methylation detected by 48 hours. Treatment of cells with zebularine for 48 hours also resulted in an increase in radiosensitivity with dose enhancement factors of >1.5. As a measure of radiation-induced DNA damage, gammaH2AX expression was determined. Whereas zebularine had no effect on radiation-induced gammaH2AX foci at 1 hour, the number of gammaH2AX foci per cell was significantly greater in the zebularine-treated cells at 24 hours after irradiation, suggesting the presence of unrepaired DNA damage. Zebularine administration to mice reactivated gene expression in U251 xenografts; irradiation of U251 tumors in mice treated with zebularine resulted in an increase in radiation-induced tumor growth delay. These results indicate that zebularine can enhance tumor cell radiosensitivity in vitro and in vivo and suggest that this effect may involve an inhibition of DNA repair.

The 1.1-Å Structure of the Spindle Checkpoint Protein Bub3p Reveals Functional Regions

Bub3p is a protein that mediates the spindle checkpoint, a signaling pathway that ensures correct chromosome segregation in organisms ranging from yeast to mammals. It is known to function by co-localizing at least two other proteins, Mad3p and the protein kinase Bub1p, to the kinetochore of chromosomes that are not properly attached to mitotic spindles, ultimately resulting in cell cycle arrest. Prior sequence analysis suggested that Bub3p was composed of three or four WD repeats (also known as WD40 and beta-transducin repeats), short sequence motifs appearing in clusters of 4-16 found in many hundreds of eukaryotic proteins that fold into four-stranded blade-like sheets. We have determined the crystal structure of Bub3p from Saccharomyces cerevisiae at 1.1 angstrom and a crystallographic R-factor of 15.3%, revealing seven authentic repeats. In light of this, it appears that many of these repeats therefore remain hidden in sequences of other proteins. Analysis of random and site-directed mutants identifies the surface of Bub3p involved in checkpoint function through binding of Bub1p and Mad3p. Sequence alignments indicate that these surfaces are mostly conserved across Bub3 proteins from diverse species. A structural comparison with other proteins containing WD repeats suggests that these folds may bind partner proteins using similar surface areas on the top and sides of the propeller. The sequences composing these regions are the most divergent within the repeat across all WD repeat proteins and could potentially be modulated to provide specificity in partner protein binding without perturbation of the core structure.

Enhancement ofin vitro andin vivo tumor cell radiosensitivity by valproic acid

Valproic acid (VA) is a well-tolerated drug used to treat seizure disorders and has recently been shown to inhibit histone deacetylase (HDAC). Because HDAC modulates chromatin structure and gene expression, parameters considered to influence radioresponse, we investigated the effects of VA on the radiosensitivity of human brain tumor cells grown in vitro and in vivo. The human brain tumor cell lines SF539 and U251 were used in our study. Histone hyperacetylation served as an indicator of HDAC inhibition. The effects of VA on tumor cell radiosensitivity in vitro were assessed using a clonogenic survival assay and gammaH2AX expression was determined as a measure of radiation-induced DNA double strand breaks. The effect of VA on the in vivo radioresponse of brain tumor cells was evaluated according to tumor growth delay analysis carried out on U251 xenografts. Irradiation at the time of maximum VA-induced histone hyperacetylation resulted in significant increases in the radiosensitivity of both SF539 and U251 cells. The radiosensitization was accompanied by a prolonged expression of gammaH2AX. VA administration to mice resulted in a clearly detectable level of histone hyperacetylation in U251 xenografts. Irradiation of U251 tumors in mice treated with VA resulted in an increase in radiation-induced tumor growth delay. Valproic acid enhanced the radiosensitivity of both SF539 and U251 cell lines in vitro and U251 xenografts in vivo, which correlated with the induction of histone hyperacetylation. Moreover, the VA-mediated increase in radiation-induced cell killing seemed to involve the inhibition of DNA DSB repair.