Limitations of radiosensitization by direct telomerase inhibition to treat high-risk medulloblastoma

Medulloblastoma (MB) is the most common malignant pediatric brain tumor. Previous studies have elucidated the genomic landscape of MB leading to the recognition of four core molecular subgroups (WNT, SHH, group 3 and group 4) with distinct clinical outcomes. Group 3 has the worst prognosis of all MB. Radiotherapy (RT) remains a major component in the treatment of poor prognosis MB but is rarely curative alone and is associated with acute and long-term toxicities. A hallmark of cancer cells is their unlimited proliferative potential which correlates closely with telomere length. The vast majority of malignant tumors activate telomerase to maintain telomere length, whereas this activity is barely detectable in most normal human somatic tissues, making telomerase inhibition a rational therapeutic target in the setting of cancer recurrence and therapy resistance. We and others have previously shown that short telomeres confer sensitivity to ionizing radiation (IR) suggesting that telomerase inhibition mediated telomere shortening will improve the efficacy of RT while minimizing its side effects. Here, we investigated the efficacy of the combination of IR with IMT, a potent telomerase inhibitor, in an in vivo model of group 3 MB. Our results indicate that although IMT inhibited MB telomerase activity resulting in telomere shortening and delayed tumor growth, the combination with IR did not prevent tumor recurrence and did not improve survival compared to the treatment with IR alone. Together, these findings suggest that the radiosensitization by direct telomerase inhibition is not an effective approach to treat high-risk pediatric brain tumors.

Sensitization to Ionizing Radiation by MEK Inhibition Is Dependent on SNAI2 in Fusion-Negative Rhabdomyosarcoma

In fusion-negative rhabdomyosarcoma (FN-RMS), a pediatric malignancy with skeletal muscle characteristics, >90% of high-risk patients have mutations that activate the RAS/MEK signaling pathway. We recently discovered that SNAI2, in addition to blocking myogenic differentiation downstream of MEK signaling in FN-RMS, represses proapoptotic BIM expression to protect RMS tumors from ionizing radiation (IR). As clinically relevant concentrations of the MEK inhibitor trametinib elicit poor responses in preclinical xenograft models, we investigated the utility of low-dose trametinib in combination with IR for the treatment of RAS-mutant FN-RMS. We hypothesized that trametinib would sensitize FN-RMS to IR through its downregulation of SNAI2 expression. While we observed little to no difference in myogenic differentiation or cell survival with trametinib treatment alone, robust differentiation and reduced survival were observed after IR. In addition, IR-induced apoptosis was significantly increased in FN-RMS cells treated concurrently with trametinib, as was increased BIM expression. SNAI2's role in these processes was established using overexpression rescue experiments, where overexpression of SNAI2 prevented IR-induced myogenic differentiation and apoptosis. Moreover, combining MEK inhibitor with IR resulted in complete tumor regression and a 2- to 4-week delay in event-free survival (EFS) in preclinical xenograft and patient-derived xenograft models. Our findings demonstrate that the combination of MEK inhibition and IR results in robust differentiation and apoptosis, due to the reduction of SNAI2, which leads to extended EFS in FN-RMS. SNAI2 thus is a potential biomarker of IR insensitivity and target for future therapies to sensitize aggressive sarcomas to IR.

SNAI2-Mediated Repression of BIM Protects Rhabdomyosarcoma from Ionizing Radiation

Ionizing radiation (IR) and chemotherapy are mainstays of treatment for patients with rhabdomyosarcoma, yet the molecular mechanisms that underlie the success or failure of radiotherapy remain unclear. The transcriptional repressor SNAI2 was previously identified as a key regulator of IR sensitivity in normal and malignant stem cells through its repression of the proapoptotic BH3-only gene PUMA/BBC3. Here, we demonstrate a clear correlation between SNAI2 expression levels and radiosensitivity across multiple rhabdomyosarcoma cell lines. Modulating SNAI2 levels in rhabdomyosarcoma cells through its overexpression or knockdown altered radiosensitivity in vitro and in vivo. SNAI2 expression reliably promoted overall cell growth and inhibited mitochondrial apoptosis following exposure to IR, with either variable or minimal effects on differentiation and senescence, respectively. Importantly, SNAI2 knockdown increased expression of the proapoptotic BH3-only gene BIM, and chromatin immunoprecipitation sequencing experiments established that SNAI2 is a direct repressor of BIM/BCL2L11. Because the p53 pathway is nonfunctional in the rhabdomyosarcoma cells used in this study, we have identified a new, p53-independent SNAI2/BIM signaling axis that could potentially predict clinical responses to IR treatment and be exploited to improve rhabdomyosarcoma therapy. SIGNIFICANCE: SNAI2 is identified as a major regulator of radiation-induced apoptosis in rhabdomyosarcoma through previously unknown mechanisms independent of p53.

Biodegradable nanoparticle delivery of inactivated swine influenza virus vaccine provides heterologous cell-mediated immune response in pigs

Swine influenza virus (SwIV) causes considerable economic loss to pig industry, and some SwIV are zoonotic. This study was conducted to evaluate the cross-protective efficacy of PLGA (poly lactic-co-glycolic acid) nanoparticles (NPs) encapsulated SwIV vaccine in pigs. Killed SwIV H1N2 (δ lineage) antigens (KAg) were encapsulated in PLGA NPs of 200–300 nm (PLGA-KAg NPs), and influenza antibody-free pigs were prime-boost vaccinated intranasally as mist and challenged using a heterologous, virulent and zoonotic SwIV H1N1 (γ lineage). PLGA-KAg NPs induced maturation of pig macrophages and dendritic cells in vitro. In vaccinated pigs, PLGA-KAg NPs induced antigen specific lymphocyte proliferation and enhanced the frequency of T-helper/memory cells and cytotoxic T cells in peripheral blood mononuclear cells (PBMCs). In virus challenged pigs, the PLGA-KAg NPs vaccine rescued virus induced clinical fever, reduced the gross lung pathology, reduced the virus load in the lung sections with complete clearance of the virus from the lungs of most of the pigs; but the nasal virus shedding was not reduced. Immunologically, at post-challenge day 6 in a recall response in PBMCs of PLGA KAg NPs vaccinated pigs, a significant increase in IFN-γ secreting T cells against both vaccine and challenge viruses were detected. However, humoral immune response in those pigs was not augmented. In conclusion, intranasal delivery of PLGA NPs based SwIV induced cross-protective response through specific cell-mediated response. Future studies are aimed at boosting the mucosal antibody response.

Biodegradable nanoparticle delivery of inactivated swine influenza virus vaccine provides heterologous cell-mediated immune response in pigs

Swine influenza virus (SwIV) is one of the important zoonotic pathogens. Current flu vaccines have failed to provide cross-protection against evolving viruses in the field. Poly(lactic-co-glycolic acid) (PLGA) is a biodegradable FDA approved polymer and widely used in drug and vaccine delivery. In this study, inactivated SwIV H1N2 antigens (KAg) encapsulated in PLGA nanoparticles (PLGA-KAg) were prepared, which were spherical in shape with 200 to 300nm diameter, and induced maturation of antigen presenting cells in vitro. Pigs vaccinated twice with PLGA-KAg via intranasal route showed increased antigen specific lymphocyte proliferation and enhanced the frequency of T-helper/memory and cytotoxic T cells (CTLs) in peripheral blood mononuclear cells (PBMCs). In PLGA-KAg vaccinated and heterologous SwIV H1N1 challenged pigs, clinical flu symptoms were absent, while the control pigs had fever for four days. Grossly and microscopically, reduced lung pathology and viral antigenic mass in the lung sections with clearance of infectious challenge virus in most of the PLGA-KAg vaccinated pig lung airways were observed. Immunologically, PLGA-KAg vaccine irrespective of not significantly boosting the mucosal antibody response, it augmented the frequency of IFN-γ secreting total T cells, T-helper and CTLs against both H1N2 and H1N1 SwIV. In summary, inactivated influenza virus delivered through PLGA-NPs reduced the clinical disease and induced cross-protective cell-mediated immune response in a pig model. Our data confirmed the utility of a pig model for intranasal particulate flu vaccine delivery platform to control flu in humans.

Inhibition of MEK confers hypersensitivity to X-radiation in the context of BRAF mutation in a model of childhood astrocytoma

PURPOSE

Curative therapy for childhood glioma presents challenges when complete resection is not possible. Patients with recurrent low-grade tumors or anaplastic astrocytoma may receive radiation treatment; however, the long-term sequellae from radiation treatment can be severe. As many childhood gliomas are associated with activation of BRAF, we have explored the combination of ionizing radiation with MEK inhibition in a model of BRAF-mutant anaplastic astrocytoma.

EXPERIMENTAL DESIGN

The regulation of TORC1 signaling by BRAF was examined in BT-40 (BRAF mutant) and BT-35 (BRAF wild type) xenografts, in a cell line derived from the BT-40 xenograft and two adult BRAF mutant glioblastoma cell lines. The effect of MEK inhibition (selumetinib), XRT (total dose 10 Gy as 2 Gy daily fractions), or the combination of selumetinib and XRT was evaluated in subcutaneous BT-40 xenografts.

RESULTS

Inhibition of MEK signaling by selumetinib suppressed TORC1 signaling only in the context of the BRAF-mutant both in vitro and in vivo. Inhibition of MEK signaling in BT-40 cells or in xenografts lead to a complete suppression of FANCD2 and conferred hypersensitivity to XRT in BT-40 xenografts without increasing local skin toxicity.

CONCLUSIONS

Selumetinib suppressed TORC1 signaling in the context of BRAF mutation. Selumetinib caused a rapid downregulation of FANCD2 and markedly potentiated the effect of XRT. These data suggest the possibility of potentiating the effect of XRT selectively in tumor cells by MEK inhibition in the context of mutant BRAF or maintaining tumor control at lower doses of XRT that would decrease long-term sequelae.

Radiation therapy may increase metastatic potential in alveolar rhabdomyosarcoma

BACKGROUND

We previously determined that radiation could be safely administered using a mouse-flank in vivo model to both alveolar (Rh30) and embryonal (Rh18) rhabdomyosarcoma xenografts. Mice from both tumor lines in this experiment developed metastases, an event not previously described with these models. We sought to determine if radiation-induced changes in gene expression underlie an increase in the metastatic behavior of these tumor models.

PROCEDURE

Parental Rh18 and Rh30 xenografts, as well as tumor that recurred locally after radiotherapy (Rh18RT and Rh30RT), were grown subcutaneously in the flanks of SCID mice and then subjected to either fractionated radiotherapy or survival surgery alone. Metastasis formation was monitored and recorded. Gene expression profiling was also performed on RNA extracted from parental, recurrent, and metastatic tissue of both tumor lines.

RESULTS

Rh30 and Rh30RT xenografts demonstrated metastases only if they were exposed to fractionated radiotherapy, whereas Rh18 and Rh18RT xenografts experienced significantly fewer metastatic events when treated with fractionated radiotherapy compared to survival surgery alone. Mean time to metastasis formation was 40 days in the recurrent tumors and 73 days in the parental xenografts. Gene expression profiling noted clustering of Rh30 recurrent and metastatic tissue that was independent of the parental Rh30 tissue. Rh18RT xenografts lost radiosensitivity compared to parental Rh18.

CONCLUSION

Radiation therapy can significantly decrease the formation of metastases in radio-sensitive tumors (Rh18) and may induce a more pro-metastatic phenotype in radio-resistant lines (Rh30).

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.

Abstract 1614: Inhibition of MDM2 by RG7388 confers hypersensitivity to X-radiation in xenograft models of childhood sarcoma

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, and approximately 5-10 per cent have MDM2 amplification or overexpression. Thus, reconstitution of a functional TP53 pathway, through inhibition of MDM2-mediated TP53 degradation, is an attractive anticancer strategy to enhance the activity of cytotoxic therapies that induce apoptosis through TP53-dependent pathways. RG7388, is a second generation MDM2 inhibitor. However, combining MDM2 inhibitors with myelosuppressive therapies may be problematic as the p53-MDM2 auto-regulatory loop in normal megakaryocytopoiesis suggests that thrombocytopenia may be an on-target toxicity. Here we have evaluated the antitumor activity of RG7388 alone or in combination with XRT in two rhabdomyosarcoma xenografts (Rh18 [embryonal], and Rh30 [alveolar]).

RG7388 was administered at 80 mg/kg daily x 5 (schedule 1) or 100 mg/kg BID q7 days x 2 (schedule 2). XRT (2 Gy daily fractions) was given to a cumulative dose of 20 Gy (Rh18) or 30 Gy (Rh30). XRT (20 Gy) induced complete regressions of Rh18 xenografts followed by regrowth of all tumors with the median event time of 89.6 days compared 9.5 days for control or RG7388 treated tumors (P = 0.0287). The combination of XRT with RG7388 on either schedule induced complete regressions with no tumor regrowth (19 weeks observation). Thus, RG7388 given on either schedule significantly potentiated XRT (P<0.0003) for both schedules). RG7388 slightly potentiated XRT against Rh30 xenografts where the cumulative tumor volumes at the end of the period of observation (day 134) was significantly smaller for the combination group in Schedule 1 (P = 0.0005) and Schedule 2 (P = 0.0029) when compared with tumors receiving 30 Gy alone. Local XRT-induced skin toxicity was not enhanced by RG7388.

Tumor samples were derived from untreated tumors, or tumors following 2, 4 or 6 Gy XRT and 24 and 48 Hr after the last XRT fraction with or without RG7388, or at the same time points from mice treated for 3 days with RG7388 alone (80 mg/kg). RG7388/XRT increased p21 levels over the 3 days of treatment, to a greater extent than XRT alone whereas p21 induction in tumors treated with RG7388 alone was lowest. Both XRT and combination treatments induced PARP cleavage over the first 48 hr.

Although additional models should be examined to see whether the synergistic activity of XRT combined with RG7388 occurs frequently, these initial results suggest that the combination may provide more effective local control for pediatric soft tissue sarcoma.

Citation Format: Peter J. Houghton, Doris A. Phelps, Kathryn Bondra, Star Seum, Christopher Chronowski, Justin Leasure, Raushan T. Kurmasheva, Stephen Middleton, Dian Wang, Xiaokui Mo. Inhibition of MDM2 by RG7388 confers hypersensitivity to X-radiation in xenograft models of childhood sarcoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1614. doi:10.1158/1538-7445.AM2015-1614

FANCD2 is a potential therapeutic target and biomarker in alveolar rhabdomyosarcoma harboring the PAX3-FOXO1 fusion gene

PURPOSE

Alveolar rhabdomyosarcoma that harbors the PAX3-FOXO1 fusion gene (t-ARMS) is a common and lethal subtype of this childhood malignancy. Improvement in clinical outcomes in this disease is predicated upon the identification of novel therapeutic targets.

EXPERIMENTAL DESIGN

Robust mouse models were used for in vivo analysis, and molecular studies were performed on xenografts treated in parallel. Two independent patient sets (n = 101 and 124) of clinically annotated tumor specimens were used for analysis of FANCD2 levels and its association with clinical and molecular characteristics and outcomes.

RESULTS

Our xenograft studies reveal a selective suppression of FANCD2 by m-TOR kinase inhibition and radiosensitization of the t-ARMS line only. In the initial patient set, we show that FANCD2 transcript levels are prognostic in univariate analysis, and are significantly associated with metastatic disease and that the copresence of the translocation and high expression of FANCD2 is independently prognostic. We also demonstrate a significant and nonrandom enrichment of mTOR-associated genes that correlate with FANCD2 gene expression within the t-ARMS samples, but not within other cases. In the second patient set, we show that on a protein level, FANCD2 expression correlates with PAX3-FOXO1 fusion gene and is strongly associated with phospho-P70S6K expression in cases with the fusion gene.

CONCLUSIONS

Our data demonstrate that FANCD2 may have a significant role in the radiation resistance and virulence of t-ARMS. Indirectly targeting this DNA repair protein, through mTOR inhibition, may represent a novel and selective treatment strategy.

Using NanoDot dosimetry to study the RS 2000 X-ray biological irradiator

PURPOSE

To use NanoDot dosimeters to study the RS 2000 X-ray Biological Irradiator dosimetry characteristics and perform in vivo dosimetry for cell or small animal experiments.

METHODS AND MATERIALS

We first calibrated the Landauer NanoDot(™) Reader by irradiating some NanoDot dosimeters with a set of known doses at specific positions defined by the irradiator. A group of five NanoDot dosimeters were placed at five specific positions where the dose rates were known and provided by the irradiator. Each group was irradiated for a set of times respectively. By correlating the readings of dosimeters with the given irradiated doses, we established the dose-reading relationship for the irradiator under the specific running condition. The established calibration curve was validated by exposing arbitrary known doses to a set of dosimeters, using the Landauer NanoDot(™) Reader to measure the doses, and then making the comparison between the two doses. To study the dose gradient of the X-ray inside the irradiated target (dose variation/cm), we placed dosimeters under different thicknesses of water-equivalent bolus and irradiated them, then measured the doses to determine the dose gradient.

RESULTS

Using the method described above, we were able to calibrate the Landauer InLight NanoDot(™) Reader and use NanoDot dosimeters to measure the actual doses delivered to the targets for the cell/small animal experiments that use the RS 2000 X-ray Biological Irradiator.

CONCLUSIONS

NanoDots are ideal dosimeters to use for in vivo dosimetry for cell/small animal irradiation experiments. The dose decrease inside the animal tissue is about 20% per cm.

Abstract 4439: Targeting FANCD2 as a radiosensitizer in pediatric rhabdomyosarcoma

Purpose. Alveolar rhabdomyosarcoma that harbors the PAX3/FOXO1 translocation (t-ARMS) is the most common and lethal subtype of this childhood malignancy. In this report we identify FANCD2 as a potential mediator of radiation resistance and biomarker of increased virulence in t-ARMS.

Methods. After the observation that knockdown of mTOR resulted in a loss of FANCD2 expression, we postulated that pharmacologic inhibition of TORC1/TORC2 would result in a similar suppression of this DNA-repair protein and increase cell-kill after radiotherapy. Therefore, an initial set of mice bearing t-ARMS and embryonal rhabdomyosarcoma (ERMS) xenografts were subjected to a dose escalation study, followed by a standard dosing study which tested the concurrent administration of AZD8055 (an mTOR kinase inhibitor) and radiotherapy versus either treatment alone. Molecular studies, focusing on drug action and resultant FANCD2 expression and function were also performed on xenografts treated in parallel. We also performed clonogenic survival assays in RMS cell lines with and without FANCD2 knockdown to determine the impact of this protein on radiation sensitivity in these lines. Lastly, we tested the prognostic significance of FANCD2 in the tumors from two independent patient sets.

Results. In a dose escalation study of 30 mice, we first observed the selective radiosensitization within the t-ARMS xenograft line versus the ERMS line. In a second set of 80 mice, subjected to standardized treatment arms, validated this selective radiosensitization. To verify the desired drug action, we analyzed xenografts from mice treated in parallel (collected at various points in treatment) and found that FANCD2 expression and activity is significantly suppressed by the addition of the drug in the sensitized, t-ARMS, but much less so in the ERMS xenograft line. We further observed that direct FANCD2 suppression (via siRNA knockdown) confers sensitivity to radiation therapy in both of these lines in vitro. In the initial patient set (n=108), we show that, on a protein level, FANCD2 expression correlates with PAX3/FOXO1 translocation and that the presence of this marker is additive to the poor prognosis of a translocated status. The co-existence of these two biomarkers is independently prognostic from other known clinical variables and the PAX3/FOXO1 translocation itself. In our second patient set (n=101), we show that based on the mRNA expression of FANCD2, that quartile-FANCD2 levels are prognostic in a univariate analysis, significantly associated with metastatic (Stage 4) disease and that, again, the co-presence of the translocation and high expression of FANCD2 is independently prognostic.

Conclusion. Our data demonstrate that FANCD2 may have a significant role in the radiation resistance and virulence of t-ARMS, and indirectly targeting this DNA repair protein, through mTOR inhibition, may represent a novel and selective treatment strategy.

Citation Format: Mamata Singh, Justin Leasure, Christopher Chronowski, Ning Li, Kathryn Bondra, Wenrui Duan, Miguel Villalona, Anthony Vergis, Lauren Hensley, Rita Kaplon, Brian Geier, Raushan Kurmasheva, Gary Woods, Sue Hammond, Peter Houghton, Christopher E. Pelloski. Targeting FANCD2 as a radiosensitizer in pediatric rhabdomyosarcoma. [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 4439. doi:10.1158/1538-7445.AM2013-4439

The application of radiation therapy to the Pediatric Preclinical Testing Program (PPTP): results of a pilot study in rhabdomyosarcoma

BACKGROUND

The Pediatric Preclinical Testing Program (PPTP) has been successfully used to determine the efficacy of novel agents against solid tumors by testing them within a mouse-flank in vivo model. To date, radiation therapy has not been applied to this system. We report on the feasibility and biologic outcomes of a pilot study using alveolar and embryonal rhabdomyosarcoma xenograft lines.

PROCEDURES

We developed a high-throughput mouse-flank irradiation device that allows the safe delivery of radiotherapy in clinically relevant doses. For our pilot study, two rhabdomyosarcoma xenograft lines from the PPTP, Rh30 (alveolar) and Rh18 (embryonal) were selected. Using established methods, xenografts were implanted, grown to appropriate volumes, and were subjected to fractionated radiotherapy. Tumor response-rates, growth kinetics, and event-free survival time were measured.

RESULTS

Once optimized, the rate of acute toxicity requiring early removal from study in 93 mice was only 3%. During the optimization phase, it was observed that the alveolar Rh30 xenograft line demonstrated a significantly greater radiation resistance than embryonal Rh18 in vivo. This finding was validated within the standardized 30 Gy treatment phase, resulting in overall treatment failure rates of 10% versus 60% for the embryonal versus alveolar subtype, respectively.

CONCLUSIONS

Our pilot study demonstrated the feasibility of our device which enables safe, clinically relevant focal radiation delivery to immunocompromised mice. It further recapitulated the expected clinical radiobiology.

The application of radiotherapy to the pediatric preclinical testing program: Results of a pilot study

9544

Background: The Pediatric Preclinical Testing Program (PPTP) has been successfully utilized to determine the efficacy of novel agents by testing via its mouse-flank in vivo model. We report on the feasibility and biologic outcomes of a pilot study using rhabdomyosarcoma (RMS) xenograft lines treated with radiotherapy (RT) alone and concurrently with the mTOR tyrosine kinase inhibitor, AZD8055, using the PPTP model. Methods: We developed a mouse flank irradiation device for daily delivery of RT in clinically relevant doses (2 Gy per fraction up to 40 Gy).Two RMS xenograft lines of the PPTP, Rh30 (alveolar) and Rh18 (embryonal), were implanted into SCID mice, grown to appropriate volumes and were subjected to fractionated RT. In a second study, daily co-administration of AZD8055 (5-20 mg/Kg, gavage) with RT was performed. Cure rates (durable complete response >12 weeks post-treatment) and RT dose densities (given dose / initial xenograft volume, Gy/cc) were compared between groups. Results: With RT alone at mean dose-densities of 59-60 Gy/cc, cure was achieved in only 4/18 (22%) of the Rh30-bearing mice and 9/12 (75%) of the Rh18-bearing mice (p=0.006). Profiling data revealed higher levels of Fanconi anemia pathway gene expression in Rh30 compared to the more sensitive Rh18. Since recent data showed conditional knockout of mTOR resulted in the loss of FANCD2 gene expression, we postulated that blockade of TORC1/TORC2 with AZD8055 would reduce FANCD2 and increase the RT-sensitivity of Rh30. The addition of AZD8055 to RT resulted in a selective sensitization of the Rh30 line. With a mean RT dose-density of 27 Gy/cc, the cure rate in Rh30-bearing mice improved to 11/15 (73%). For the Rh18 group, the cure rate was 7/15 (46%) at a mean dose density of 44 Gy/cc. Western blot analysis showed the co-administration of AZD8055 abrogated the brisk increase in mTOR signaling and FANCD2 expression after the first several 2 Gy fractions of RT; most strikingly in Rh30. Conclusions: This study demonstrates the feasibility of applying RT to the PPTP model. It recapitulated the expected clinical radiobiology and demonstrated its utility in preclinical testing and the discovery of novel mechanisms of RT resistance in pediatric tumors.