Evaluation of the pan-class I phosphoinositide 3-kinase (PI3K) inhibitor copanlisib in the Pediatric Preclinical Testing Consortium in vivo models of osteosarcoma

Copanlisib is a pan-class I phosphoinositide 3-kinase (PI3K) inhibitor, with activity against all four PI3K class I isoforms (PI3Kα, PI3Kβ, PI3Kγ, and PI3Kδ). Whole-genome and RNA sequencing data have revealed several PI3K aberrations in osteosarcoma tumor samples. The in vivo anticancer effects of copanlisib were assessed in a panel of six osteosarcoma models. Copanlisib induced prolonged event-free survival in five of six osteosarcoma models; however, all models demonstrated progressive disease suggesting minimal activity. While copanlisib did not result in tumor regression, more data are needed to fully explore the role of the PI3K pathway in the pathogenesis of osteosarcoma.

Abstract 1086: PEGylated talazoparib enhances therapeutic window of its combination with temozolomide in Ewing sarcoma

Introduction: The Ewing family of sarcomas is the fourth most common highly malignant childhood cancer. Current standard therapy is relatively ineffective for relapsed and metastatic Ewing sarcoma predominantly due to development of resistance to chemotherapeutics. Nanoparticle-formulated drugs are reported to increase uptake into tumor tissue, while reducing drug access to normal tissues, because of reduced permeability of normal vasculature. This potentially leads to lower dose exposure in normal tissues and reduced toxicity. The purpose of this study was to evaluate the antitumor activity of the PEGylated poly(ADP) ribose polymerase 1/2 (PARP1/2) inhibitor talazoparib combined with the DNA alkylating agent temozolomide (TMZ) in Ewing sarcoma xenograft models and other pediatric cancers.

Method:. We evaluated the preclinical efficacy of PEGylated talazoparib (PEG~TLZ), alone or in combination with TMZ, in Ewing sarcoma and glioblastoma patient derived xenograft (PDX) models. Additional solid tumor models, such as rhabdomyosarcoma, Wilms tumor, malignant rhabdoid tumor, osteosarcoma, and synovial sarcoma were evaluated using a novel single mouse testing approach. PEG~TLZ was administered as a single administration intraperitoneally to mice bearing subcutaneous tumors at a single dose of 5 - 20 µmol/kg alone, or at 10 µmol/kg on day 1 combined with TMZ administered orally at 40 mg/kg starting on day 3 or 4 for 5 consecutive days.

Results: Mice bearing Ewing sarcoma and MGMT-deficient glioblastoma xenografts tolerated PEG~TLZ+TMZ with minimal toxicity and achieved maintained complete response (MCR). The time-separated drug administration schedule of the single dose of PEG~TLZ followed by the 5-day TMZ treatment demonstrated objective responses in Ewing sarcoma, rhabdoid tumor, rhabdomyosarcoma, and glioblastoma PDX models.

Conclusion: We evaluated the range of PEG~TLZ+TMZ activity among pediatric solid tumor panels using conventional and single-mouse testing approaches, and demonstrated that PEG~TLZ combined with delayed TMZ administration enhances the therapeutic window of the treatment compared to free TLZ. From the clinical standpoint, the single intravenous administration of PEG~TLZ could be advantageous for treating infants and young children.

Citation Format: Vanessa Del Pozo, Andrew J. Robles, Shaun Fontaine, Qianqian Liu, Joel E. Michalek, Peter J. Houghton, Raushan Kurmasheva. PEGylated talazoparib enhances therapeutic window of its combination with temozolomide in Ewing sarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1086.

Abstract LB061: Evaluation of the in vivo efficacy of the B7-H3 targeting antibody-drug conjugate (ADC) DS7300a: A report fro the Pediatric Preclinical In Vivo Resting (PIVOT) program

Introduction: B7-H3 (encoded by the CD276 gene) is suggested to act as an immune checkpoint molecule and is highly expressed in some pediatric solid tumors. Monoclonal antibodies targeting B7-H3 (8H9 and MGA271) are well-tolerated but have demonstrated limited success in clinical trials. DS-7300a is a B7-H3 targeting ADC with a payload of DXd (an exatecan derivative that inhibits DNA topoisomerase I). DS-7300a has a drug-to-antibody ratio of 4 and it has shown promising early clinical activity in adults with advanced solid cancers (Johnson, Annals of Oncology 2021; 32:S583-S585).

Methods: Xenograft models for several pediatric cancer types were selected for preclinical testing of DS-7300a based on the high rates of B7-H3 expression in these cancers. Models were dosed at 10 mg/kg administered intravenously every other week for two doses (Q2wk x 2). Different experimental designs were used for efficacy testing. For rhabdomyosarcoma, Ewing sarcoma, Wilms tumor, and other pediatric solid tumors a single mouse trial (SMT) design with 40 models was used. A cohort design (N=10) was used for efficacy testing in 6 osteosarcoma models, 3 patient-derived orthotopic xenograft glioblastoma (PDOX) models, and 3 PDOX ependymoma models. For neuroblastoma models, 10 models (2 mice per model) were tested. To evaluate treatment efficacy, objective response measures based on changes in relative tumor volume were used (Houghton, Pediatr Blood Cancer 2007;49:928-940).

Results: As a single agent, DS-7300a induced a statistically significant prolongation of survival in 2 of 3 orthotopic glioblastoma and 1 of 3 ependymal tumor models. DS-7300a demonstrated high efficacy (maintained complete response, complete response, and partial response) in most non-CNS models tested: 17 of 21 rhabdomyosarcomas, 5 of 7 osteosarcomas, 7 of 10 neuroblastomas and 2 of 2 Wilms tumors. For Ewing sarcoma, most models were classified as progressive disease (9 of 15). For osteosarcoma models, the log cell kill per dose values ranged from 0.95 to 2.76 indicating high activity of the agent in these models. The activity for DS-7300a followed the general pattern of protein and RNA expression levels for B7-H3/CD276 in the models, with Ewing sarcoma models showing lower expression compared to the other models.

Conclusions: DS-7300a shows tumor-regressing anticancer activity across a wide range of pediatric solid tumor models. The maintained complete remissions observed for osteosarcoma models are noteworthy, as this level of response is uncommon for these models and as osteosarcoma shows the highest B7-H3 expression among pediatric cancers. The high level of preclinical activity observed for DS-7300a combined with the promising early clinical activity observed for adult patients provide strong rationale for studying DS-7300a in children with B7-H3 expressing solid tumors.

Citation Format: Richard Gorlick, E. Anders Kolb, Yifei Wang, Peter Houghton, Raushan Kurmasheva, Yael Mosse, John Maris, Matthew Tsang, David Groff, Kateryna Krytska, Xiao-Nan Li, Yuchen Du, Jun Hasegawa, Nanae Izumi, Steven Neuhauser, Anuj Srivastava, Tim Stearns, Vivek Philip, Emily L. Jocoy, Jeff Chuang, Carol J. Bult, Beverly Teicher, Malcolm Smith. Evaluation of the in vivo efficacy of the B7-H3 targeting antibody-drug conjugate (ADC) DS7300a: A report fro the Pediatric Preclinical In Vivo Resting (PIVOT) program [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB061.

Abstract 3038: Evaluation of ROR1-targeted antibody-drug conjugates against ROR1-expressing pediatric preclinical models - a report from the pediatric preclinical testing consortium (PPTC)

Introduction: ROR1 is an orphan-receptor tyrosine kinase-like surface antigen that is expressed primarily during development but is not expressed on normal adult tissues. ROR1 is expressed on multiple adult hematologic and solid cancers, making it a target for antibody-based therapies. UC-961 (cirmtuzumab), a humanized IgG1 monoclonal antibody, binds with high affinity to a specific extracellular epitope of human ROR1 and rapidly internalizes and traffics to lysosomes. VLS-101 is an ADC developed by conjugating UC-961 to a cleavable linker with an MMAE payload. VLS-211 was generated by conjugating UC-961 with a PNU payload. VLS-101 is in clinical development for adults with hematologic and solid cancers and has received FDA Fast Track and Orphan Drug designation for the treatment of patients with mantle cell lymphoma. To better define the utility of ROR1 as an immuno-oncology target for pediatric cancers, we evaluated VLS-101 and VLS-211 against pediatric preclinical models selected based on ROR1 expression.

Methods: Models were selected based on RNA-seq transcript levels for ROR1 with subsequent confirmation by flow cytometry for acute lymphoblastic leukemia (ALL) models. For the ALL models, UC-961 (5 mg/kg), VLS-101 (2.5 and 5.0 mg/kg), and VLS-211 (0.25 and 0.5 mg/kg) were each administered weekly x 3. For the Ewing sarcoma (EWS) models, the dose of VLS-211 was 0.5 and 1 mg/kg. Four models were treated using a twice-weekly or every 4-day treatment schedule while 2 models were treated weekly.

Results: Elevated ROR1 gene expression among PPTC models was most notable among the ALL and EWS panels. Leukemia cell surface expression of ROR1 correlated with transcript levels. ALL models with elevated ROR1 expression [TCF3-HLF (1), TCF3-PBX1 (2), ETV6-RUNX1 (2), and BCR-ABL1 (1)] as well as a non-expressing model were tested against the ROR1 ADCs. Activity of each ADC was ROR1 expression-level dependent, and for VLS-211 dose-dependency was noted. At the highest doses of each ADC studied, 2 of 6 (VLS-101) and 3 of 6 (VLS-211) ROR1-expressing models showed objective responses. For most models, median time to event was longer for VLS-211 versus VLS-101. Five EWS models were studied, and expression-level dependent activity was observed for both ADCs. VLS-101 induced tumor regressions in 2 of 5 models, while VLS211 induced regressions in 3 of 5 models. Time to event was generally longer for VLS-211 compared to VLS-101. No objective responses were observed to UC-961 for the ALL and EWS models.

Conclusions: VLS-101 and VLS-211 showed expression-level dependent activity against ALL and EWS models. These results support ROR1 as a relevant immuno-oncology target for the subset of B-ALL and Ewing sarcoma cases with elevated ROR1 expression. Further research is required to identify the optimal payload for ROR1 ADCs for use against pediatric cancers.

Citation Format: Richard B. Lock, Kathryn Evans, Narimanne El-Zein, Brian J. Lannutti, Katti A. Jessen, Eric J. Earley, Stephen W. Erickson, Malcolm A. Smith, Raushan Kurmasheva, Peter J. Houghton. Evaluation of ROR1-targeted antibody-drug conjugates against ROR1-expressing pediatric preclinical models - a report from the pediatric preclinical testing consortium (PPTC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3038.

Testing of B7-H3 targeting antibody-drug conjugate (ADC) MGC018 in models of pediatric solid tumors by the Pediatric Preclinical Testing Consortium (PPTC)

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Background: B7-H3 (encoded by the CD276 gene) is an immunoregulatory molecule that is widely expressed in pediatric embryonal tumors and sarcomas, while expression is limited for normal tissues. Among PPTC models, osteosarcoma models showed highest CD276 transcript levels followed by Wilms tumor, neuroblastoma, rhabdomyosarcoma, and Ewing sarcoma. Protein expression by IHC generally followed expression at the RNA level, with high expression by IHC observed across a range of models. MGC018 is a duocarmycin-based humanized ADC targeting B7-H3 that shows selective cytotoxicity for B7-H3 expressing cancer cells, robust in vivo activity against a range of adult cancer preclinical models, a favorable pharmacokinetic and safety profile in cynomolgus monkeys, and has entered clinical testing for adults with cancer. Herein we report the in vivo antitumor activity of MGC018 against preclinical models of pediatric solid tumors. Methods: MGC018 was tested at 6 mg/kg administered intraperitoneally (IP) on Day 1. SYD988, an anti-CD20 ADC with the same linker and payload as MGC018, was used as a control arm at 6 mg/kg IP on Day 1. Osteosarcoma models, due to their slower growth kinetics and lower rates of tumor regression, were tested with n = 10 mice per arm, while the sarcoma and neuroblastoma models were tested with n of 1 or n of 2 designs, respectively. Activity was evaluated using response criteria mirroring clinical criteria (progressive disease (PD), stable disease (SD), partial response (PR), complete response (CR), and maintained CR (MCR)], by minimum relative tumor volume across all models, and by event-free survival (EFS) comparisons between treatment groups. Results: For neuroblastoma models (n = 9), 3 and 2 neuroblastoma models showed objective responses (PR/CR/MCR) or SD, respectively, to MGC018, while none showed an objective response or SD to SYD988. For osteosarcoma, results are mature for 1 of 5 models, with this model showing an MCR response to MGC018 and a PD response to SYD988. For Ewing sarcoma (n = 5), a PR and MCR were observed to MGC018 with the remaining models showing PD; for SYD988 a single CR was observed with the remaining 4 models showing PD. For rhabdomyosarcoma (n = 3) 2 MCR and 1 PD were noted for MGC018, while only PD was noted for SYD988. EFS duration exceeding 100 days to a single dose of MGC018 was observed for 2 rhabdomyosarcoma, 1 Ewing sarcoma, and 1 neuroblastoma model. Evaluation of the relationship between B7-H3 expression and response to MGC018 will be performed when results from all studies are completed. Conclusions: B7-H3 is an important target for immuno-oncology agents for childhood cancers. Our results provide proof-of-principle for the ability of MGC018 to produce profound antitumor activity in select pediatric solid tumor models in a B7-H3 specific manner.

Recent Developments in Nanomedicine for Pediatric Cancer

Cancer is the second biggest cause of death in children in the US. With the development of chemotherapy, there has been a substantial increase in the overall survival rate in the last 30 years. However, the overall mortality rate in children with cancer remains 25%, and many survivors experience a decline in overall quality of life and long-term adverse effects caused by treatments. Although cancer cells share common characteristics, pediatric cancers are different from adult cancers in their prevalence, mutation load, and drug response. Therefore, there is an urgent unmet need to develop therapeutic approaches specifically designed for children with cancer. Nanotechnology can potentially overcome the deficiencies of conventional methods of administering chemotherapy and ultimately improve clinical outcomes. The nanoparticle-based drug delivery systems can decrease the toxicity of therapy, provide a sustained or controlled drug release, improve the pharmacokinetic properties of loading contents, and achieve a targeted drug delivery with achievable modifications. Furthermore, therapeutic approaches based on combining nanoformulated drugs with novel immunotherapeutic agents are emerging. In this review, we discussed the recently developed nanotechnology-based strategies for treating blood and solid pediatric cancers.

The B7-H3-Targeting Antibody-Drug Conjugate m276-SL-PBD Is Potently Effective Against Pediatric Cancer Preclinical Solid Tumor Models

PURPOSE

Patients with relapsed pediatric solid malignancies have few therapeutic options, and many of these patients die of their disease. B7-H3 is an immune checkpoint protein encoded by the CD276 gene that is overexpressed in many pediatric cancers. Here, we investigate the activity of the B7-H3-targeting antibody-drug conjugate (ADC) m276-SL-PBD in pediatric solid malignancy patient-derived (PDX) and cell line-derived xenograft (CDX) models.

EXPERIMENTAL DESIGN

B7-H3 expression was quantified by RNA sequencing and by IHC on pediatric PDX microarrays. We tested the safety and efficacy of m276-SL-PBD in two stages. Randomized trials of m276-SL-PBD of 0.5 mg/kg on days 1, 8, and 15 compared with vehicle were performed in PDX or CDX models of Ewing sarcoma (N = 3), rhabdomyosarcoma (N = 4), Wilms tumors (N = 2), osteosarcoma (N = 5), and neuroblastoma (N = 12). We then performed a single mouse trial in 47 PDX or CDX models using a single 0.5 m/kg dose of m276-SL-PBD.

RESULTS

The vast majority of PDX and CDX samples studied showed intense membranous B7-H3 expression (median H-score 177, SD 52). In the randomized trials, m276-SL-PBD showed a 92.3% response rate, with 61.5% of models showing a maintained complete response (MCR). These data were confirmed in the single mouse trial with an overall response rate of 91.5% and MCR rate of 64.4%. Treatment-related mortality rate was 5.5% with late weight loss observed in a subset of models dosed once a week for 3 weeks.

CONCLUSIONS

m276-SL-PBD has significant antitumor activity across a broad panel of pediatric solid tumor PDX models.

PCAT: an integrated portal for genomic and preclinical testing data of pediatric cancer patient-derived xenograft models

Although cancer is the leading cause of disease-related mortality in children, the relative rarity of pediatric cancers poses a significant challenge for developing novel therapeutics to further improve prognosis. Patient-derived xenograft (PDX) models, which are usually developed from high-risk tumors, are a useful platform to study molecular driver events, identify biomarkers and prioritize therapeutic agents. Here, we develop PDX for Childhood Cancer Therapeutics (PCAT), a new integrated portal for pediatric cancer PDX models. Distinct from previously reported PDX portals, PCAT is focused on pediatric cancer models and provides intuitive interfaces for querying and data mining. The current release comprises 324 models and their associated clinical and genomic data, including gene expression, mutation and copy number alteration. Importantly, PCAT curates preclinical testing results for 68 models and 79 therapeutic agents manually collected from individual agent testing studies published since 2008. To facilitate comparisons of patterns between patient tumors and PDX models, PCAT curates clinical and molecular data of patient tumors from the TARGET project. In addition, PCAT provides access to gene fusions identified in nearly 1000 TARGET samples. PCAT was built using R-shiny and MySQL. The portal can be accessed at http://pcat.zhenglab.info or http://www.pedtranscriptome.org.

Dose-response effect of eribulin in preclinical models of osteosarcoma by the pediatric preclinical testing consortium

The pediatric preclinical testing program previously demonstrated activity of eribulin in osteosarcoma patient-derived xenograft (PDX) models. The phase 2 trial in patients with relapsed osteosarcoma failed to meet response endpoints. Eribulin was evaluated in the original and an expanded set of PDX models and tested at multiple dose levels and schedules to evaluate dose-response. Maximal response was observed at the highest dose, consistent with prior results. The alternative schedule generated similar responses. We demonstrate steep dose-response for eribulin in osteosarcoma PDX models, implying that any deviation from achievement of effective concentrations may have a significant impact on activity.

Abstract LB-217: Preclinical evaluation of trastuzumab deruxtecan (T-DXd; DS-8201a), a HER2 antibody-drug conjugate, in pediatric solid tumors by the Pediatric Preclinical Testing Consortium (PPTC)

Introduction: HER2 is expressed in a subset of pediatric solid tumors and is a target of interest for innovative immune therapies including CAR-T cells and antibody-drug conjugates (ADCs). We evaluated preclinical efficacy of T-DXd, a humanized monoclonal HER2 antibody conjugated to a topoisomerase 1 inhibitor payload, DXd, in solid tumor patient derived xenograft (PDX) models. Methods: 7 osteosarcoma (OS), 2 rhabdoid tumor (RT) and 3 Wilms tumor (WT) PDX models with varying HER2 expression were tested using 10 mice per group. ERBB2 mRNA expression was determined using RNA seq. T-DXd or vehicle control was administered IV to mice harboring flank tumors at a dose of 5mg/kg on Day 1. Standard PPTC statistical methods were employed for EFS for treatment (T) and control (C) groups, minimum relative tumor volume (minRTV) and objective response measure. Results: Among PPTC solid tumor models ERBB2 mRNA expression was observed across multiple histologies, with highest expression observed for WT (median = 22 FPKM), followed by RT, OS, and Ewing sarcoma. The relationship between HER2 expression by IHC and by RNAseq was inconsistent. Mice tolerated T-DXd with minimal toxicity. T-DXd significantly prolonged EFS in 5/7 OS, 2/2 RT and 3/3 WT PDX models (see table). Complete response (CR) or maintained CR were observed for 4 of 5 WT and RT models, while stable disease was the best response among OS models. Conclusions: T-DXd exhibits single agent anti-tumor activity in models of pediatric solid tumors. Single mouse testing experiments will be used to extend knowledge of the range of activity of T-DXd for pediatric solid tumors. Correlations between ERBB2 gene expression, HER2 expression by IHC, and genetic changes associated with camptothecin sensitivity are ongoing and will be presented. Clinical trials assessing efficacy of a HER2-directed ADC in pediatric patients with HER2-expressing tumors should be considered.

EFS T - C(days)EFS T/C Ratiop-value Gehan-WilcoxonminRTVmean±SDminRTVp-valueObjective Response MeasureOS-120.31.85p < 0.0011.152±0.090p = 0.023PD1OS-230.12.76p < 0.0010.815±0.183p < 0.001PD2OS-94.11.14p = 0.1031.334±0.203p = 1.000PD1OS-1734.01.97p = 0.0091.172±0.179p = 0.017PD1OS-3111.91.84p < 0.0011.676±0.395p = 0.015PD1OS-3361.34.87p < 0.0010.486±0.220p < 0.001SDOS-603.81.11p = 0.0161.084±0.103p = 0.007PD1BT29 (RT)> 58.4> 3.28p < 0.0010.019±0.041p < 0.001MCRRBD2 (RT)> 97.3> 13.57p < 0.0010.021±0.066p < 0.001CRKT-10 (WT)> 85.1> 7.57p < 0.0010.000±0.000p < 0.001MCRKT-11 (WT)18.32.62p < 0.0010.954±0.486p < 0.001PD2KT-13 (WT)> 71.5> 3.69p < 0.0010.019±0.060p < 0.001MCR

Citation Format: Pooja Hingorani, Wendong Zhang, Raushan Kurmasheva, Zhongting Zhang, Yifei Wang, Zhaohui Xu, Michael Roth, Jonathan Gill, Douglas Harrison, Stephen Erickson, Edward A. Kolb, Malcolm Smith, Peter Houghton, Richard Gorlick. Preclinical evaluation of trastuzumab deruxtecan (T-DXd; DS-8201a), a HER2 antibody-drug conjugate, in pediatric solid tumors by the Pediatric Preclinical Testing Consortium (PPTC) [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr LB-217.

Abstract LB-060: A very long-acting poly(ADP-ribose) polymerase inhibitor

Purpose: The purpose of this work was to prepare and characterize the anti-tumor properties of a prodrug for a very long-acting of poly(ADP-ribose) polymerase (PARP) inhibitor. Background: Four PARP inhibitors (PARPi) have been approved for QD oral use in treatment of human cancers. It is believed that PARP requires continuous inhibition for optimal anti-tumor effects, but, as with any drug with a short t1/2, daily administered PARPi exhibit high Cmax values and peak-to-trough excursions. We speculated that the prolonged exposure and lower Cmax and Cmax/Cmin of a long-acting PARPi might provide a more effective, less toxic therapeutic. In choosing which PARPi to target, a major consideration was whether the carrier has capacity to deliver sufficient levels of the drug over a long dosing interval. Talazoparib (TLZ) was chosen for this study because it is the most potent of the PARP inhibitors, requiring only 1 mg/day in adults compared to hundreds of mg/day for other PARPi's. Experimental procedures: We prepared a long-acting prodrug of TLZ by attaching it to a PEG40kDa carrier by a β-eliminative releasable linker. The chemistry was achieved by a novel alkylation of TLZ at the poorly acidic 2-NH of the phthalazinone moiety with an O-azidoalkyl-N-alkyl-N-chloromethyl carbamate, followed by coupling to PEG-cyclooctyne. Daily PO doses of TLZ or a single IP injection of the PEG-TLZ conjugate were administered to xenografts in mice possessing defects in homologous recombination - either a PALB2 mutation in the KT-10 Wilms tumor, or a BRCA1-deficient MX-1 triple-negative breast cancer. New data: PEG~TLZ was highly effective in treating both KT-10 and MX-1 xenografts. Although the t1/2 of TLZ in the mouse is only ~3 hr, tumor growth in animals treated with PEG~TLZ was suppressed for about one month. The EFS T/C values - the ratio of the median time to event between treated and control groups - of single injections of ~5 mg TLZ/kg as PEG~TLZ in either tumor was more than 4, indicating the drug is a highly active agent at low doses. The amount of TLZ in a single efficacious dose of the PEG~TLZ conjugate was equivalent to the same amount of free TLZ administered in divided daily doses for 4 or more weeks. Although we did not investigate scheduling, dosing PEG~TLZ once every 3 to 4 weeks should be sufficient to suppress tumor growth for extended periods. Conclusion: We developed a novel method of conjugating linkers to the 2N of the phthalazinone moiety of PARPi. We prepared a cleavable PEG~TLZ that releases TLZ with a t1/2 of 160 hr at pH 7.4. In mouse xenografts of tumors with defective HR, single non-toxic doses of PEG~TLZ suppresses tumor growth for ~1 month, and are equi-effective to QD administration of TLZ over that period. We posit that the long lasting effect is due to the long t1/2 of the prodrug, increased sensitivity of the tumor upon continued exposure to TLZ, tumor accumulation of the 15 nm nanomolecule, and counteracting drug resistance by efflux pumps.

Citation Format: Shaun D. Fontaine, Gary W. Ashley, Peter J. Houghton, Raushan Kurmasheva, Morgan Diolati, Alan Ashworth, Daniel V. Santi. A very long-acting poly(ADP-ribose) polymerase inhibitor [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr LB-060.

Abstract A23: Novel approaches to Ewing sarcoma therapy

Ewing family of sarcomas (EwS) comprises the fourth most common highly malignant childhood cancer. The driver translocation in these tumors, EWSR1-FLI1, introduces DNA damage and replication stress. These dysregulations render EwS cells reliant on the ATR/CHK1 axis to maintain genome stability. High sensitivity of EwS cells to PARP1 inhibition (PARPi) was reported and attributed to a positive PARP1/EWSR1-FLI1 feedback loop. Further, it has been shown that with EWSR1-FLI1 expression, BRCA1 is trapped with the active transcription complex unable to go to DNA breaks. PARPi impairs the repair of single-strand DNA breaks and alkylated bases leading to double-strand DNA breaks, which cannot be repaired efficiently in BRCA-deficient cancers (synthetic lethality) (1). Several groups including ours previously identified the combination of PARP1i talazoparib (TLZ) with DNA-damaging agent, temozolomide (TMZ), to cause regressions of ~50% of EwS xenograft models. However, both in mice and in children’s trial, the combination causes toxicity and necessitates reduction of the dose of TMZ to ~13% of its single-agent maximum tolerated dose. Combined inhibition of PARP1 and ATR has been shown to synergize in suppressing growth of BRCA-mutant patient-derived xenografts models (2). The long-term goal of the proposed study is to develop more effective EwS therapy based on EWSR1-FLI1-induced vulnerabilities. EwS cells represent heterogeneity in expressing EWSR1-FLI1 fusion protein. As sequencing is generally performed in “bulk” with results being interpreted as average of gene expression patterns derived from vast populations of cells, the biologically relevant differences between cells may not be represented. Single-cell RNA-seq therefore offers new possibilities of overcoming such challenges. By studying single cells, their transcripts and generating sequencing libraries of separate cells, we are able to study fundamental biologic properties of cell populations and biologic systems at a single-cell level. Our studies demonstrate positive correlation between the expression of EWSR1-FLI1 and DNA replication genes at single-cell resolution. Single-cell studies may provide deep understanding of time-dependent behaviors of individual cells in response to dynamic changes of EWSR1-FLI1 expression. We propose that EWSR1-FLI1-induced DNA replication stress leads to reliance of EwS cells on PARP1 and ATR DNA repair pathways in order to maintain genome stability. We have selected number of metal-conjugated antibodies to identify changes in DNA damage response genes in subpopulations of EwS cells using CyTOF. Latest findings on changes of DNA damage/repair genes upon downregulation of EWSR1-FLI1 at a single-cell level will be presented.

Citation Format: Dauren Alimbetov, Yidong Chen, Peter Houghton, Raushan Kurmasheva. Novel approaches to Ewing sarcoma therapy [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr A23.

Nanoformulation of Talazoparib Increases Maximum Tolerated Doses in Combination With Temozolomide for Treatment of Ewing Sarcoma

The Pediatric Preclinical Testing Program previously identified the PARP inhibitor talazoparib (TLZ) as a means to potentiate temozolomide (TMZ) activity for the treatment of Ewing sarcoma. However, the combination of TLZ and TMZ has been toxic in both preclinical and clinical testing, necessitating TMZ dose reduction to ~15% of the single agent maximum tolerated dose. We have synthesized a nanoparticle formulation of talazoparib (NanoTLZ) to be administered intravenously in an effort to modulate the toxicity profile of this combination treatment. Results in Ewing sarcoma xenograft models are presented to demonstrate the utility of this delivery method both alone and in combination with TMZ. NanoTLZ reduced gross toxicity and had a higher maximum tolerated dose than oral TLZ. The dose of TMZ did not have to be reduced when combined with NanoTLZ as was required when combined with oral TLZ. This indicated the NanoTLZ delivery system may be advantageous in decreasing the systemic toxicity associated with the combination of oral TLZ and TMZ.

Abstract C003: Initial testing of m276-PBD CD276 antibody-drug conjugate in preclinical models of pediatric cancers by the Pediatric Preclinical Testing Consortium (PPTC)

Purpose: CD276 (B7-H3) is an immunoregulatory molecule that is reported to be widely expressed in pediatric embryonal tumors, pediatric sarcomas, and tumor infiltrating blood vessels. CD276 protein is expressed at low levels on several normal tissues, including cerebral cortex, liver and germinal lymph node. m276 is a fully-human IgG1 that binds with similar affinity to both mouse CD276 (24 nM kD) and human CD276 (29 nM kD) (Seaman et al., Cancer Cell, 2017). To generate an antibody-drug conjugate, m276 was site-specifically conjugated to the DNA damaging agent pyrrolobenzodiazepine (PBD) via a cleavable valine-alanine linker, providing m276-PBD with a Drug-to-Antibody Ratio (DAR) of 2. Here we examined the antitumor activity of m276-PBD against preclinical xenograft models of pediatric solid tumors. Experimental Procedures: Expression of CD276 across PPTC xenograft models (>200) representing leukemias, brain tumors and solid tumors was determined by RNA seq, and additionally in neuroblastoma models by IHC. Xenograft experiments were undertaken in heterotopic models using standard methods of the PPTC. Response criteria were tumor regression (PR, CR, maintained CR [at 6 weeks]) and Event-Free Survival (EFS). m276-PBD was administered by intraperitoneal injection at a dose of 0.5 mg/kg, once weekly x 3 consecutive weeks. Results: CD276 expression was high in most solid tumors (median 41 FPKM) with highest expression in osteosarcoma. Neuroblastoma, rhabdomyosarcoma, Wilms tumor and embryonal brain tumor models had similar levels of expression, whereas ALL models showed low expression. In vivo efficacy studies are ongoing, but data to date are available for 5 osteosarcoma, 4 rhabdomyosarcoma, 2 Ewing sarcoma and 2 Wilms tumors. Maintained Complete Response (MCR) at 6 weeks was attained in 2/5 osteosarcoma, 3/4 rhabdomyosarcoma and 1/2 Ewing sarcoma. CR was achieved in 1/2 Wilms tumor, 1/2 rhabdomyosarcoma, and 2/5 osteosarcoma models. Body weight loss (<3%) was noted in only one study. Conclusions: Expression of CD276 was high in most PPTC solid tumor models. mCD276-PBD was highly active against most models tested inducing long-lasting CR’s. There was no toxicity, suggesting that this agent has an effective therapeutic window in these models. Mature results (100 days observation) and additional results for neuroblastoma models will be presented.

Citation Format: Raushan Kurmasheva, E. Anders Kolb, Malcolm A. Smith, Beverly A. Teicher, Stephen W. Erickson, John M. Maris, Yael P. Mosse, Kateryna Krytska, David Groff, Matthew Tang, Yifei Wang, Brad St. Croix, Richard Gorlick, Peter J. Houghton. Initial testing of m276-PBD CD276 antibody-drug conjugate in preclinical models of pediatric cancers by the Pediatric Preclinical Testing Consortium (PPTC) [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C003. doi:10.1158/1535-7163.TARG-19-C003

Abstract 3848: In vivo evaluation of the LSD1 inhibitor SP-2577 against Ewing sarcoma and rhabdomyosarcoma preclinincal models: A report from the Pediatric Preclinical Testing Consortium (PPTC)

Background: Lysine-specific demethylase 1 (LSD1/KDM1A) acts as a transcriptional co-regulator and utilizes flavinadenine dinucleotide (FAD) as a cofactor to remove mono- and di-methyl groups from histone H3 lysine-4 (H3K4) and lysine-9 (H3K9). SP-2577, in contrast to other LSD1 inhibitors in clinical evaluation, is a reversible inhibitor of LSD1. SP-2509 (a tool compound for SP-2577) reversed the EWSR1-FLI1 mediated transcription profile and oncogenic phenotype. To follow-up on these results, the PPTC evaluated SP-2577 against a set of Ewing sarcoma and rhabdomyosarcoma xenografts. Methods: SP-2577 was administered at a dose of 100 mg/kg intraperitoneally (IP) for 28 days and was tested against 7 Ewing sarcoma and 4 alveolar and 1 embryonal rhabdomyosarcoma xenografts. Standard PPTC methods for assessing time to event (EFS T/C = ratio of median time to event for treated versus control animals). Objective response measures similar to clinical measures were applied (Houghton, Pediatr Blood Cancer 2007;49:928-940). Results: SP-2577 was generally well tolerated during the efficacy testing phase. Among 120 animals treated with SP-2577 in efficacy testing, 9 (7.5%) experienced toxic death, and the median maximum weight loss was 1.8% (range 0.0% to 13.7%). As a single agent, SP-2577 induced a statistically significant prolongation in time to event in 3 of 7 Ewing sarcoma models and 4 of 5 rhabdomyosarcoma models. The extent of prolongation of time to event was small for most of the models for which there was a statistically significant prolongation in time to event, with EFS T-C values ranging from 1.9 to 14 days and with EFS T/C values ranging from 1.15 to 2.79. None of the models tested showed objective responses, but rather showed progressive disease as their best response. The minimum relative tumor volumes were significantly smaller compared to control for 4 of 7 Ewing sarcoma models and for 2 of 5 rhabdomyosarcoma models. However, the mean minimum relative tumor volumes were all greater than 1.0, indicating the lack of tumor regression. Pharmacodynamic effects of SP-2577 that were studied included c-Myc, N-Myc, HMOX1, histone H3, and histone H3(K4) dimethyl levels. There were no consistent changes in global histone H3(K4) dimethyl levels, and there was no obvious decrease in either c-Myc or N-Myc, whereas HMOX1 was slightly increased by treatment in two Ewing sarcoma models. Conclusions: SP-2577 was adequately tolerated in the Ewing sarcoma and rhabdomyosarcoma xenografts treatment cohorts that were studied. While a number of the models studied showed statistically significant effects on tumor growth rate, the effect size was generally small and tumor regression was not observed. (Supported by CA199297 and CA199222)

Citation Format: Peter J. Houghton, Steve W. Erickson, Beverly Teicher, Malcolm A. Smith, Ruolan Han, Raushan Kurmasheva. In vivo evaluation of the LSD1 inhibitor SP-2577 against Ewing sarcoma and rhabdomyosarcoma preclinincal models: A report from the Pediatric Preclinical Testing Consortium (PPTC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3848.

Abstract 3835: In vivo evaluation of the Menin inhibitor VTP-50469 against Ewing sarcoma preclinical models: A report from the Pediatric Preclinical Testing Consortium (PPTC)

Background: Menin and MLL-containing trithorax complexes play important roles in developmental transcription programs by promoting gene expression through depositing H3K4me3 marks on gene promoters. Menin and MLL1, (lysine methyltransferase 2A, KMT2A) are reported to be overexpressed in Ewing sarcoma. Further, inhibition of the menin-MLL interaction by MI-503 resulted in loss of menin and MLL1, resulting in inhibition of proliferation and tumorigenicity of Ewing sarcoma cells (Svoboda et al., Oncotarget, 2017). VTP-50469 is a small molecule that disrupts menin-MLL1 interactions and is highly active against MLL-rearranged leukemia xenograft models. To extend these results, the PPTC evaluated VTP-50469 in vivo against a set of Ewing sarcoma xenografts.

Methods: VTP-50469 was administered twice daily (BID) at a dose of 120 mg/kg or 100 mg/kg orally (PO) for 28 days, a dose and schedule that is highly effective in leukemia MLL-rearranged models. Standard PPTC methods for assessing time to event (EFS T/C = ratio of median time to event for treated versus control animals). Objective response measures similar to clinical measures were applied (Houghton, Pediatr Blood Cancer 2007;49:928-940).

Results: VTP-50469 was generally well tolerated during the efficacy testing phase. Among 70 animals treated with VTP-50469 in efficacy testing, 3 (4.3%) experienced toxic death. As a single agent, VTP-50469 statistically significantly prolonged time to event in 4 of 7 Ewing sarcoma models. In models for which the time to event was significantly prolonged, the extent of prolongation was modest, with EFS T-C values ranging from 3.6 to 7.8 days and with EFS T/C values ranging from 1.24 to 1.74. None of the models tested showed objective responses, but rather showed progressive disease as their best response. The minimum relative tumor volumes were significantly smaller compared to control for 2 of 7 Ewing sarcoma models. However, the mean minimum relative tumor volumes were all greater than 1.0, indicating the absence of tumor regression.

Conclusions: VTP-50469 was adequately tolerated in the Ewing sarcoma xenograft treatment cohorts that were studied. While a number of the models studied showed statistically significant effects on tumor growth rate, the effect size was generally small and tumor regression was not observed. As this dose and schedule of VTP-50469 is highly effective against MLL-rearranged leukemia xenograft models, these results suggest that the menin-MLL1 interaction may play a less critical role in maintenance of Ewing sarcoma. (Supported by CA199297 and CA199222)

Citation Format: Peter J. Houghton, Raushan Kurmasheva, Gerard M. McGeehan, Steve W. Erickson, Beverly Teicher, Malcolm Smith. In vivo evaluation of the Menin inhibitor VTP-50469 against Ewing sarcoma preclinical models: A report from the Pediatric Preclinical Testing Consortium (PPTC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3835.

Evaluation of the multi-kinase inhibitor regorafenib in the Pediatric Preclinical Testing Consortium osteosarcoma, rhabdomyosarcoma, and Ewing sarcoma in vivo models

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Background: Regorafenib is a multi-kinase inhibitor, developed by adding a fluorine atom to the phenyl ring of sorafenib. Regorafenib inhibits multiple kinases including BRAF, FGFR1, KIT, PDGFRB, RAF, RET, and VEGFR1-3, many at a higher potency than sorafenib. Prior studies within the Pediatric Preclinical Testing Consortium (PPTC) demonstrated sorafenib exhibited intermediate activity for tumor growth inhibition in more than 50% of the sarcoma models tested at a dose of 60mg/kg by oral gavage daily (5 days/wk for 6 consecutive weeks). The in vivo effects of regorafenib were studied in the PPTC osteosarcoma (OS), rhabdomyosarcoma (Rh) and Ewing (EW) sarcoma xenograft models. Methods: The in vivo anticancer effects of regorafenib were assessed in a panel of 6 osteosarcoma models (OS2, OS9, OS31, OS33, OS36, OS60), two rhabdomyosarcoma models (Rh30, Rh41), and one Ewing sarcoma model (EW5). Regorafenib was administered by oral gavage at a dose of 30 mg/kg/day given daily for 21 consecutive days. Time to event and tumor volume responses were defined and analyzed utilizing standard PPTC statistical methods. Results: Regorafenib induced significant improvements in event-free survival (EFS) compared to control in 100% (9/9) of sarcoma models tested. Most models showed pronounced slowing of tumor growth compared to control during the 21 days of regorafenib treatment, with tumor growth generally approximating control rates soon after completion of regorafenib treatment. Three out of 8 sarcoma models demonstrated EFS T/C values > 2 (1/6 OS, 2/2 Rh, 0/1 EW). Minimum relative tumor volumes ranged from 0.74 to 1.60, with no models meeting criteria for objective response. Conclusions: Regorafenib induced modest inhibition of tumor growth in the PPTC sarcoma models evaluated. The overall pattern of response to the multi-kinase inhibitor regorafenib against the PPTC sarcoma models appears similar to that of the kinase inhibitor sorafenib, with pronounced slowing of tumor growth in some models that is limited to the period of agent administration being the primary treatment effect.

Abstract 5872: Pediatric Preclinical Testing Consortium (PPTC) of eribulin in osteosarcoma (OS) patient-derived xenograft (PDX) models

Introduction: Eribulin is a synthetic analogue of halichondrin B and inhibits cancer cell proliferation via blockade of microtubule function. It is FDA approved for patients with breast cancer with two prior chemotherapy regimens for the treatment of metastatic disease and for previously treated patients with inoperable or metastatic liposarcoma. In vivo testing of eribulin against osteosarcoma (OS) Pediatric Preclinical Testing Program (PPTP) PDX models, developed from primary tumors, demonstrated significant single-agent activity. However, a Children's Oncology Group (COG) phase 2 trial of eribulin in patients with relapsed OS did not demonstrate significant antitumor activity of single-agent eribulin. In the current study the efficacy of eribulin in PDX models generated from primary and relapsed tumors, as well as dose-response, were assessed.

Methods: Eribulin was evaluated in 6 PPTC models, 4 from primary OS specimens (OS46, OS51, OS55, OS56) and 2 from relapsed specimens (OS39R, OS60). Eribulin was administered via intraperitoneal (IP) injection at a dose of 1mg/kg on days 1 and 4, repeated every 21 days. Response to treatment was determined based on PPTP-established endpoints (1). In addition, 3 dose levels of eribulin were evaluated in 3 PPTP models developed from primary tumors (OS1, OS17, OS33) and 1 model developed from a relapsed tumor (OS60). In the dose-response studies, eribulin was administered IP at 1mg/kg, 0.5mg/kg and 0.25mg/kg on days 1 and 4, repeated every 21 days with follow-up through day 42.

Results: Eribulin at a dose level of 1mg/kg induced stable disease (SD) or partial response (PR) in 4/9 and 3/9 OS xenografts, respectively. SD/PR responses were observed in PDX models generated from primary tumors (5/7) as well as PDX models generated from relapsed tumors (2/2). Eribulin demonstrated maximal antitumor activity at the highest dose level, 1mg/kg (SD/PR in 3/4). At 0.5 mg/kg 1/4 models achieved SD/PR, and at 0.25mg/kg all 4 models showed progressive disease within the 6-week testing period. Minimum relative tumor volumes also showed a dose-response effect from 0.25 to 1.0 mg/kg.

Conclusions: Eribulin demonstrated antitumor activity against OS PDX models generated from primary and relapsed tumors at a dose level of 1mg/kg. Eribulin showed a clear dose-response effect in terms of its ability to induce SD/PR responses, highlighting the importance of dose selection. This dose-response effect suggests that an inability to achieve comparable serum drug concentrations in human clinical trials may explain the limited tumor responses observed in the COG phase 2 trial of eribulin. Pharmacokinetic studies from the COG phase 2 study are ongoing in an effort to understand the disparate tumor responses seen in the in vivo studies and the phase 2 clinical trial.

Reference: 1. Houghton PJ et al. Pediatr Blood Cancer 2007;49:928-40.

Supported by NCI Grants: CA199222, CA199221.

Citation Format: Wendong Zhang, E. Anders Kolb, Richard Gorlick, Michael Roth, Jonathan Gill, Sudie Rowshan, Stephen Erickson, Raushan Kurmasheva, Peter Houghton, Beverly Teicher, Malcolm A. Smith. Pediatric Preclinical Testing Consortium (PPTC) of eribulin in osteosarcoma (OS) patient-derived xenograft (PDX) models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5872.

Abstract 3100: Nanoformulated Talazoparib and Olaparib for enhanced delivery

Introduction: PARP inhibitors such as Talazoparib and Olaparib exploit deficiencies in DNA repair pathways, making them attractive candidates for treatment of a number of different cancers. These drugs are particularly effective when used in combination with other DNA damaging agents such as chemotherapeutics and radiation therapy. Combination trials, however, have resulted in severe toxicities, necessitating either dose reduction or delay. Dose reduction leads to suboptimal dosing and provides little therapeutic benefit compared to monotherapy. Systemically administered nanoparticles offer a more effective way to selectively accumulate drugs in tumors and bypass toxicities associated with oral delivery. We have developed nanoparticle delivery systems for both Olaparib and Talazoparib in order to improve tumor accumulation while bypassing the toxicity associated with oral administration.

Methods: Lipid nanoformulations of Olaparib and Talazoparib have been developed and characterized in regard to size, surface charge, drug loading, release, and stability. NanoTalazoparib has been tested in vitro in breast cancer cell lines including W0069, W780, and HCC1937 which exhibit BRCA1 and 2 mutations, and NanoOlaparib in the lung cancer cell line Calu-6 which also has a defective FA-BRCA pathway. Mice have been treated with NanoOlaparib and NanoTalazoparib alone and in combination with radiation or temozolomide in order to evaluate toxicity. Therapeutic efficacy studies are currently underway.

Results: The nanoformulations have been formulated to encapsulate a clinically relevant dose of either Talazoparib or Olaparib and release at 37°C over a period of days, while remaining stable during storage at 4°C. In vitro, both nanoformulations show the same activity as free drug with IC50s in the nanomolar range for these cell lines with varying deficiencies in the BRCA pathway. Mice have shown no appreciable weight loss during treatment with either nanoformulation alone or in combination with other treatment modalities.

Conclusion: Nanoformulations of Talazoparib and Olaparib have been developed and characterized to demonstrate activity in vitro and tolerable doses in vivo. We have found that mice tolerate NanoTalazoparib at higher doses when combined with Temozolomide than when given oral Talazoparib. The sustained release from the nanoparticles allows for the nanoformulation to be administered less often than the daily administration for oral drug and the improved tolerability opens the door for combination therapy with both chemotherapeutics and radiation therapy. Therapeutic efficacy studies are underway and we expect that as a monotherapy NanoTalazoparib will be more effective at lower doses than oral Talazoparib, based on the longer circulation time and more selective accumulation in tumors. We also anticipate that combination therapy will be more effective with the nanoformulation, as the maximum tolerated dose is higher than that of the oral drug.

Citation Format: Paige Baldwin, Rajiv Kumar, Edward Favours, Karen Liby, Raushan Kurmasheva, David Kozono, Srinivas Sridhar. Nanoformulated Talazoparib and Olaparib for enhanced delivery [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 3100. doi:10.1158/1538-7445.AM2017-3100

Abstract 5054: Nanoformulation of talazoparib to increase efficacy when combined with temozolomide for the treatment of Ewing sarcoma xenografts

Ewing Family of tumors (EFT) comprises the fourth most common highly malignant childhood cancer. Although sustained event-free survival (EFS) can be achieved with intensive chemo-radiation therapy for patients with local-regional disease, this therapy is relatively ineffective in the treatment of metastatic disease with EFS of 12% at 5 years. Ewing sarcoma is characterized by a reciprocal translocation between chromosomes 11 and 22 that encodes a chimeric oncoprotein resulting from the fusion of EWSR1 to the FLI1 transcription factor in ~85% of tumors. Therapy for patients with EFT comprises surgery, intensive use of cytotoxic agents and radiation therapy. Dose intensification and dose compression has resulted in some improvement in outcome, but patents with advanced or metastatic disease at diagnosis still represent a challenge. Further, patients alive at 5 years from diagnosis still have a high probability of subsequent relapse. Further, long-term consequences of treatment included cardiac dysfunction, and secondary malignancies. Thus, more effective and less toxic therapies are required to treat patients with advanced disease.

Our studies, as part of the Pediatric Preclinical Testing Program (PPTP), identified the combination of the PARP inhibitor, talazoparib, with the DNA damaging agent temozolomide, as being highly synergistic in xenograft models of Ewing sarcoma, but not against other tumor types. In this study 5 of 10 Ewing tumor xenografts models showed dramatic regressions to the combination, while administered as single agents neither talazoparib or temozolomide were active. We have studied the talazoparib-temozolomide synergy in vitro, and results indicate that in models where there is no synergy as xenografts, the cell lines have either intrinsic resistance to talazoparib, temozolomide or both drugs. In mice, and in the clinical trial (NCT02116777), the talazoparib-temozolomide combination is toxic requiring a reduction in temozolomide dose to ~15% of its single maximum-tolerated dose.

We are exploring the use of nanoparticle-formulated talazoparib (npTLZ) developed by Nanomaterials Synthesis Laboratory at Northeastern University without tumor targeting or with antibody-mediated targeting to increase the tumor-drug delivery, reduce normal tissue toxicity (mainly thrombocytopenia), and potentially allow escalation of temozolomide dose. In the PPTP study, the MTD for temozolomide combined with free talazoparib (0.25 mg/kg PO BID daily x 5) was 12 mg/kg. Our recent data showed no toxicity of temozolomide at 66 mg/kg (PO daily x 5) in mice treated with npTLZ (0.5 mg/kg IV daily x 5), suggesting that npTLZ does not potentiate TMZ toxicity to normal mouse tissues. We anticipate that nanoparticle delivery of talazoparib combined with temozolomide will allow reduced toxicity while increasing the response rate for this combination in preclinical models of Ewing sarcoma.

Citation Format: Nabeela Baig, Rostislav Likhotvorik, Paige Baldwin, Srinivas Sridhar, Raushan Kurmasheva. Nanoformulation of talazoparib to increase efficacy when combined with temozolomide for the treatment of Ewing sarcoma xenografts [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 5054. doi:10.1158/1538-7445.AM2017-5054

Abstract 2098: Understanding the synergy between temozolomide and PARP inhibition in Ewing sarcoma

Ewing sarcoma is the fourth most common highly malignant childhood cancer; it is defined by a tumor-specific chromosomal translocation. In approximately 85% of all tumors, the EWSR1 gene on chromosome 22 is fused to a member of E26 transformation-specific sequence family of transcription factors, the FLI1 gene on chromosome 11. DNA damage induced by expression of EWSR1-FLI1 fusion gene is potentiated by PARP1 inhibition in Ewing cells, where EWSR1-FLI1 genes act in a positive feedback loop to maintain the expression of PARP1. As single agents, PARP inhibitors have shown promising activity in vitro, but only modest activity in in vivo models. In our previous work (in PPTP), we showed that low level damage to DNA by temozolomide (TMZ) can be potentiated up to 40-fold through inhibition of PARP by BMN-673, leading to dramatic tumor regressions in 5 of the 10 Ewing sarcoma xenograft models. Therefore, we were aimed to determine the differences between the tumors that respond to treatment with BMN-673 and those intrinsically resistant to it, and to understand the underlying mechanisms of such resistance, with the ultimate goal to develop more effective therapy for Ewing sarcoma.

We have used the same cell lines to investigate biochemical differences between those Ewing sarcoma lines where there is synergy in xenograft models in mice, and those where the combination is inactive. Our studies using cell lines show that, 1) in vitro BMN-673 and TMZ are synergistic in all cell lines, irrespective of their response as xenografts; 2) In vivo synergy correlates with intrinsic sensitivity to BMN-673 and TMZ in vitro. For Ewing lines, where there is no in vivo synergy when they are grown as xenografts, they are ∼9-fold resistant to BMN-673, and ∼15-fold resistant to TMZ relative to lines where there is synergy as xenografts.

The resistance of the Ewing sarcoma xenografts to the synergy of the combination of BMN-673 and TMZ in mice is a consequence of intrinsic resistance to either or both of the drugs.

The combination of BMN-673 and TMZ in Ewing sarcoma xenografts is the most dramatic synergy between two drugs to be reported. The synergy in vivo is restricted to Ewing sarcoma, yet only half of the models respond to combination treatment with the other half completely resistant. Studies proposed in this application will potentially identify biomarkers that will allow identification of patients likely to benefit from such treatment, and spare toxicity for those unlikely to respond.

Citation Format: Ruth Carlson, Raushan Kurmasheva. Understanding the synergy between temozolomide and PARP inhibition in Ewing sarcoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2098.

Abstract A137: Initial testing (Stage 1) of EPZ-6438 (tazemetostat), a novel EZH2 inhibitor, by the pediatric preclinical testing program (PPTP)

Background: EZH2 is the functional histone methyltransferase component of the multiprotein complex known as Polycomb Repressive Complex 2 (PRC2). PRC2 catalyzes mono-, di- and tri-methylation of H3K27. Activating mutations in EZH2 produce altered substrate specificity, and mutant EZH2 acts in concert with wild-type EZH2 to produce increased H3K27 tri-methylation, abnormal repression of PRC2 targets, and tumorigenesis. Rhabdoid tumors are associated with loss of SMARCB1, a component of the SWI/SNF chromatin remodeling complex. Tumorigenesis in SMARCB1 deficient mice is blocked by inactivation of EZH2 (Cancer Cell 2010;18:316-28), and an EZH2 inhibitor induced regression in a rhabdoid tumor xenograft (PNAS 2013;110:7922-7).

EPZ-6438 is a potent, specific and SAM-competitive EZH2 inhibitor that demonstrates favorable pharmacological properties and in vivo activity in multiple xenograft tumor models following oral dosing. It has entered clinical evaluation, and objective responses were noted in patients with NHL and rhabdoid tumor with twice daily oral dosing during phase 1 dose escalation studies. The aim of this preclinical study was to assess the antitumor activity of EPZ-6438 against pediatric solid tumor models with a focus on rhabdoid tumors of the kidney and CNS.

Procedures: EPZ-6438 was evaluated using a dose of 400 mg/kg (mono-HBr salt, 350 mg/kg of active compound) administered twice-daily by oral gavage for 28 days. Standard PPTP measures of in vivo antitumor activity were utilized.

Results: EPZ-6438 induced significant differences in event-free survival (EFS) distribution compared to control in 9 of 30 (30%) xenografts studied, with significant differences observed in 5 of 7 (71%) rhabdoid tumor xenograft lines compared to 4 of 23 (17%) non-rhabdoid lines (χ2 test p = 0.006). For the EFS T/C activity measure, intermediate activity requires an EFS T/C value > 2.0, and high activity additionally requires a reduction in final tumor volume compared to starting tumor volume. EPZ-6438 demonstrated intermediate or high EFS T/C activity in 2 of 26 (8%) and 1 of 26 (4%) xenografts evaluable for this measure, respectively. Intermediate/high activity for the EFS T/C metric was observed exclusively among rhabdoid tumor xenografts (3 of 5 rhabdoid tumor lines versus 0 of 21 non-rhabdoid tumor lines, χ2 test p<0.001). For the objective response metric, 1 of 7 rhabdoid tumor xenografts (G401) showed stable disease. PD2 (Progressive Disease 2) response indicates progressive disease with growth delay (EFS T/C >1.5) and was observed in an additional 5 of 7 rhabdoid tumor xenografts, but in only 3 of 23 non-rhabdoid tumor xenografts. For two rhabdoid tumors (G401 and KT-16), delayed reductions in tumor size to EPZ-6438 were noted following 1-2 weeks of tumor growth. Pharmacodynamic evaluation of H3K27 methylation status in treated and control tumors is ongoing.

Conclusions: EPZ-6438 showed antitumor activity against rhabdoid tumor models, but showed no consistent activity against any other histology. The pattern of delayed tumor response observed for two rhabdoid tumor xenografts will need to be considered when developing clinical trials for EPZ-6438. Further preclinical testing using rhabdoid tumor lines and evaluating EPZ-6438 in combination with other anticancer agents may provide guidance for its pediatric clinical development.

Citation Format: Raushan Kurmasheva, Kat Cosmopoulos, Melissa Sammons, Edward Favours, Jianwrong Wu, Peter Houghton, Malcolm Smith. Initial testing (Stage 1) of EPZ-6438 (tazemetostat), a novel EZH2 inhibitor, by the pediatric preclinical testing program (PPTP). [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 A137.

Abstract 1615: Initial testing (stage 1) of the Curaxin, CBL0137, by the Pediatric Preclinical Testing Program (PPTP)

The Curaxin, CBL0137, is a member of a new class of small molecules that simultaneously activate p53 and inhibit cancer-associated stress response pathways such as NFκB and Heat Shock Factor 1. CBL0137 sequesters the FACT (FAcilitates Chromatin Transcription) complex on chromatin, inhibiting its function. CBL0137 has shown antitumor activity against a broad range of tumor models. Because of its novel mechanism of action CBL0137 was evaluated for its antitumor activity against the PPTP in vitro and in vivo models.

Methods: CBL0137 was tested against the PPTP's in vitro cell line panel (n = 23) at concentrations ranging from 1.0 nM to 10.0 μM using the PPTP's standard 96 hour exposure period. CBL0137 was tested against the PPTP solid tumor xenografts using a dose of 50 mg/kg administered by the intravenous route weekly for 4 weeks. Standard PPTP time to event and objective response metrics were used to assess CBL0137 in vivo activity.

Results: The median relative IC50 (rIC50) value for the PPTP cell lines was 0.28 μM, with a range from 0.13 μM to 0.80 μM. There were no significant differences in rIC50 values by histotype. The median rIC50 for the neuroblastoma cell lines exceeded that of the non-neuroblastoma cell lines (0.51 μM vs 0.26 μM, respectively, p = 0.16), while the median rIC50 for the ALL cell lines was less than that of the non-ALL cell lines (0.20 μM vs 0.31 μM, respectively, p = 0.06). All PPTP cell lines showed a pronounced cytotoxic effect, with T/C Ymin% values approaching 0% and with Relative I/O% values approaching -100% for all cell lines at the higher concentrations tested.

CBL0137 was generally well tolerated in vivo with a 3.1% toxicity rate in the treated groups compared to a 0.8% rate for control animals. CBL0137 induced significant differences in EFS distribution compared to control in 9 of 30 (30%) of the solid tumor xenografts evaluable for this measure and in 8 of 8 (100%) evaluable ALL xenografts. Significance differences in EFS distributon were observed in 4 of 6 osteosarcoma lines, 2 of 2 rhabdoid tumor lines, and 2 of 6 rhabdomyosarcoma lines. No objective responses were observed among the 30 solid tumor xenografts. For the ALL panel, 1 xenograft achieved CR and 4 achieved PR.

In summary, CBL0137 showed potent cytotoxic activity in vitro without clear histotype selectivity. In vivo the most consistent activity for CBL0137 was observed against the ALL xenografts, with most solid tumor xenograft lines showing limited response to single agent CBL0137. In considering clinical translation of these testing results, it will be important to relate the drug levels in SCID (soid tumor models) or NOD/SCID (ALL models) mice at 50 mg/kg to those tolerated in humans at the CBL0137 recommended phase 2 dose.

Citation Format: Malcolm A. Smith, Min Kang, Patrick C. Reynolds, Richard B. Lock, Hernan Carol, Richard Gorlick, Anders E. Kolb, John M. Maris, Stephen T. Keir, Catherine A. Billups, Raushan Kurmasheva, Peter J. Houghton. Initial testing (stage 1) of the Curaxin, CBL0137, by the Pediatric Preclinical Testing Program (PPTP). [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 1615. doi:10.1158/1538-7445.AM2015-1615

Abstract 1617: Analysis of single mouse tumor response results from the Pediatric Preclinical Testing Program (PPTP)

Traditional approaches to evaluating antitumor agents using human tumor xenograft models have usually used 8-10 mice in control and treatment groups against a limited panel of tumor models. However, with increasing evidence of molecular/genetic subsets of tumors within a histotype, there is a need to establish panels of tumors that accurately reflect such heterogeneity. The consequences of evaluating agents in a large number of models is that it is highly resource consuming. An alternative approach is to use fewer animals per tumor line, allowing a greater number of models to be evaluated that capture greater molecular/genetic heterogeneity of the cancer type.

We analyzed 67 agents evaluated by the PPTP to determine whether a single mouse, chosen by a random number generating routine in Excel, predicted the median response for groups of mice (solid tumors n = 10; acute lymphoblastic leukemias n = 8). Agents evaluated included standard cytotoxic agents, tyrosine kinase inhibitors, receptor-binding antibodies, and anti-angiogenic agents. Data were analyzed to determine predictive value for 79 xenograft models, and to identify classes of antitumor agent where predictive value of single mouse experiments was poor.

The individual tumor response was compared to the group median response using response criteria developed in the PPTP. A total of 2106 comparisons were made. The single tumor response accurately predicted the group median response in 1684 comparisons (79.34%). Models had a range for accurate prediction (0.583 - 0.918). Allowing for mis-prediction of + or - one response category, the overall prediction rate increased to 95.63%. Single tumor results accurately predicted objective response rates (ORR) determined by group response, in 66 of 67 studies. There was a single study where single mouse data incorrectly predicted an agent as ‘active’ (>20% ORR).

For most tumor models the single mouse appears to give results similar to groups of 8-10 mice. Importantly, even allowing for slight inaccuracy (± 1 response category), the single mouse experiment identified all active agents, and in the worse case over-predicted activity in one tumor histotype. The advantage of the single mouse experimental design is that it allows for inclusion of models that encompass greater molecular heterogeneity, thus recapitulating the clinical heterogeneity more accurately. Potentially, this design also reduces the numbers of mice used for experimentation. However, to generate these patient derived xenograft models, particularly solid tumors at diagnosis and relapse will require national and international coordination through organizations such as the Children's Oncology Group.

Citation Format: Brendan Murphy, Han Yin, John M. Maris, Anders E. Kolb, Richard Gorlick, Patrick C. Reynolds, Min Kang, Stephen T. Keir, Raushan Kurmasheva, Igor Dvorchik, Jianrong Wu, Catherine Billups, Malcolm A. Smith, Peter J. Houghton. Analysis of single mouse tumor response results from the Pediatric Preclinical Testing Program (PPTP). [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 1617. doi:10.1158/1538-7445.AM2015-1617

Abstract 3969: Pediatric preclinical testing program (PPTP) evaluation of BMN 673, an inhibitor of poly-adp ribose polymerase (PARP), with temozolomide (TMZ)

Introduction: BMN 673 is a potent and selective inhibitor of PARP1/2. Inhibitors of PARP such as BMN 673 show clinical activity against cancers lacking homologous repair through mutations in BRCA1 and 2. PARP inhibitors are of particular interest for Ewing sarcoma (ES) given reports of ES cell lines being preferentially sensitive to PARP inhibitors. However, BMN 673 and other PARP inhibitors have shown limited single agent activity against Ewing sarcoma xenografts.

Methods: BMN 673 (10 nM) was evaluated in combination with TMZ and topotecan against the 23 cell lines of the PPTP in vitro panel using 96 hour exposure. For in vivo combination studies, two dose levels of BMN 673 (0.25 & 0.1 mg/kg BID x 5 days) given with TMZ [12 & 30 mg/kg/day x 5 days, respectively] were evaluated against ES xenografts. The combination regimen using the lower dose (LD) of TMZ and higher dose of BMN 673 was studied against other histotypes. Standard PPTP measures of in vivo antitumor activity were employed to assess response.

Results: BMN 673 markedly potentiated TMZ activity in vitro, with the TMZ rIC50 reduced by a median of 10-fold for PPTP cell lines, with ES cell lines showing a median 50-fold reduction. BMN 673 potentiated topotecan to a lesser degree (median 2.8-fold reduction in rIC50), with no differential sensitivity by histotype.

In vivo, 10 ES xenografts showed little or no response to 5 days of treatment with single agent TMZ (30 mg/kg) or BMN 673 (0.25 mg/kg BID), but 8 of 10 showed complete responses (CR) or maintained CR (MCR) to BMN 673 and TMZ (LD), and 5 of 10 showed CR or MCR to BMN 673 and higher-dose TMZ. The 10 ES xenografts could be separated into 2 groups based on duration of response to the combinations, with 5 showing delayed growth or brief objective responses (median time to event ≤ 30 days), and with the other 5 showing prolonged time to regrowth (median > 77 days). Among 5 neuroblastoma xenografts, all showed significant treatment effects to BMN 673 plus TMZ(LD), with 1 achieving CR. Among 4 osteosarcoma xenografts, each showed significant tumor growth delay, but no objective responses were noted. Among 3 glioblastoma xenografts, 2 of 3 achieved maintained CRs (MCR) to the BMN 673 plus TMZ(LD) combination. Excessive toxicity was observed for ALL xenografts in NOD-SCID mice treated with the combination.

Conclusions: While BMN 673 shows limited single agent activity against PPTP solid tumor and ALL models with responses limited to models also highly sensitive to cisplatin, the BMN 673 plus TMZ(LD) combination shows dramatic activity for a subset of Ewing sarcoma xenografts and for selected additional models. Based on these results a pediatric phase 1 trial of BMN 673 plus TMZ with a phase 2 expansion for Ewing sarcoma is proceeding. Efforts are ongoing to relate genomic alterations identified through exome sequencing to responsiveness of ES xenografts to BMN 673 plus TMZ. (Supported by NO1-CM-42216)

Citation Format: Malcolm Smith, Min Kang, Patrick Reynolds, Richard Gorlick, Anders Kolb, John Maris, Richard Lock, Hernan Carol, Stephen Keir, Catherine Billups, Raushan Kurmasheva, Peter Houghton. Pediatric preclinical testing program (PPTP) evaluation of BMN 673, an inhibitor of poly-adp ribose polymerase (PARP), with temozolomide (TMZ). [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3969. doi:10.1158/1538-7445.AM2014-3969

Abstract C206: Pediatric Preclinical Testing Program (PPTP) evaluation of BMN 673, an inhibitor of Poly-ADP Ribose Polymerase (PARP), alone and with Temozolomide (TMZ)

Introduction: BMN 673 is a potent and selective inhibitor of PARP1/2. Inhibitors of PARP such as BMN 673 show clinical activity against cancers lacking homologous repair through mutations in BRCA1 and 2. For Ewing sarcoma (ES), the EWS/FLI1 chimeric transcription factor increases PARP expression, and PARP appears to facilitate EWS/FLI1 function. Two reports have indicated that ES cell lines are more sensitive to PARP inhibitors than most other cell lines.

Methods: BMN 673 was evaluated as a single agent and in combination with TMZ against the 23 cell lines of the PPTP in vitro panel using 96 hour exposure. In single-agent studies, daily oral BMN 673 administration was tested against the PPTP solid tumor xenografts (SCID mice) and acute lymphoblastic leukemia (ALL; NOD-SCID mice) panels using a dose of 0.16 mg/kg BID x 5 and 0.33 mg/kg QD on weekends for up to 28 days. In combination studies, two dose levels of BMN 673 (0.25 & 0.1 mg/kg BID x 5 days) given with TMZ (12 & 30 mg/kg/day x 5 days, respectively) were evaluated against ES xenografts. Standard PPTP measures of in vivo antitumor activity were employed to assess response.

Results: The median relative IC50 (rIC50) for BMN 673 against the PPTP cell lines was 28 nM, with a range from 4 nM to >1000 nM. There was a trend for lower rIC50 values for the ES cell line panel. In combination with a fixed concentration of BMN 673 (10 nM), the TMZ rIC50 was markedly reduced for some PPTP cell lines, with ES cell lines showing up to a 60-fold reduction. In vivo, BMN 673 was well tolerated with only a 1.9% toxicity rate in the treated groups. BMN 673 induced significant improvements in event-free survival (EFS) distribution compared to control in 18 of 35 (51%) of the evaluable solid tumor xenografts, but in 0 of 8 ALL models. Only 2 of 34 (6%) evaluable solid tumor xenografts and no ALL models showed EFS T/C values > 2. Complete responses (CR) were observed for a Wilms tumor and a medulloblastoma model.

BMN 673 in combination with TMZ induced CRs that were maintained through week 12 for 2 of 4 ES xenografts (TMZ dosed at either 12 or 30 mg/kg/day). Among an additional 6 ES xenografts that have recently initiated testing, 3 have ongoing CRs to the combination at Weeks 4 to 5. None of the ES xenografts responded to 5 days of treatment with single agent TMZ (30 mg/kg) or BMN 673 (0.5 mg/kg/day).

Conclusions: Single agent BMN 673 shows limited activity against the PPTP solid tumor and ALL models, with no single agent activity against ES xenografts in vivo, despite ES cell lines demonstrating differential sensitivity in vitro. BMN 673 as a single agent induced CRs in 2 of 43 models, both of which are also highly responsive to cisplatin. Dramatic activity for the BMN 673 plus TMZ combination was observed for 5 of 10 ES models, with maintained CRs noted in vivo at TMZ doses as low as 12 mg/kg. Based on these results a pediatric phase 1 trial of BMN 673 plus TMZ is planned. (Supported by NO1-CM-42216)

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C206.

Citation Format: Malcolm Smith, Min Kang, Patrick Reynolds, Richard Gorlick, Anders Kolb, John Maris, Richard Lock, Hernan Carol, Stephen Keir, Catherine Billups, Raushan Kurmasheva, Peter Houghton. Pediatric Preclinical Testing Program (PPTP) evaluation of BMN 673, an inhibitor of Poly-ADP Ribose Polymerase (PARP), alone and with Temozolomide (TMZ). [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C206.

Abstract C207: Pediatric Preclinical Testing Program (PPTP) stage 1 evaluation of Glembatumumab Vedotin

Introduction: Glembatumumab vedotin is an antibody-auristatin conjugate that targets cells expressing the transmembrane glycoprotein NMB (GPNMB, also known as osteoactivin, dc-HIL, and HGFIN). Because of the restrictive expression of GPNMB, testing was limited to the PPTP osteosarcoma (OS) xenograft panel as well as two non-GPNMB expressing rhabdomyosarcoma (RMS) xenografts.

Methods: Glembatumumab vedotin was administered intravenously in 0.9% saline at a dose of 2.5 mg/kg using a weekly x 3 schedule. GPNMB was assessed at the RNA level (Agilent SurePrint3) and by IHC.

Results: All of the six OS xenografts against which glembatumumab vedotin was tested expressed GPNMB at the RNA level as evaluated using Agilent gene expression arrays. By contrast, none of the RMS xenografts expressed GPNMB at the RNA level. Most OS xenografts showed GPNMB expression detectable by IHC (6/7), while none of the RMS xenografts showed tumor staining for GPNMB (0/6).

Glembatumumab vedotin was well tolerated, and all 8 tested xenograft models were considered evaluable for efficacy. For the OS xenografts, glembatumumab vedotin induced significant differences in EFS distribution compared to control in each of the 6 models studied. Three of 6 OS xenografts showed maintained complete responses (MCR) to glembatumumab vedotin. Two other xenografts showed progressive disease with growth delay (PD2), with one of these showing near stable disease during the 6 weeks of treatment and observation. The final OS xenograft showed progressive disease with no growth delay (PD1). As expected, the two RMS xenografts showed limited responses to glembatumumab vedotin (PD1 and PD2), although one did show significant tumor growth inhibition with an EFS T/C value of 2.6.

Two of the OS xenografts with MCR showed the highest GPNMB expression at the RNA level. Conversely, the xenograft with the lowest GPNMB gene expression had the poorest response to glembatumumab vedotin. For GPNMB protein expression assessed by IHC, 2 of the xenografts with MCR showed the highest expression level and highest percent tumor expression, but the other responding line showed GPNMB expression on only 5% of tumor cells, despite exhibiting one of the highest expression levels of GPNMB by mRNA. The two OS xenograft lines least responsive to glembatumumab vedotin were also the least responsive OS xenografts to vincristine and eribulin.

Conclusions: Glembatumumab vedotin showed high-level activity against 3 of 6 OS xenografts. Response of OS xenografts to glembatumumab vedotin may be related to both GPNMB expression and to the intrinsic susceptibility of xenografts to tubulin inhibitors. RMS xenografts showed PD1 or PD2 responses to glembatumumab vedotin despite having known sensitivity to tubulin-binding agents such as eribulin. Glembatumumab vedotin is the first antibody-drug conjugate with documented in vivo activity against OS. (Supported by NO1-CM-42216)

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C207.

Citation Format: Richard Gorlick, Anders Kolb, Jainrong Wu, Raushan Kurmasheva, Thomas Hawthorne, Peter Houghton, Malcolm Smith. Pediatric Preclinical Testing Program (PPTP) stage 1 evaluation of Glembatumumab Vedotin. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C207.

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

Abstract 2758: Pediatric preclinical testing program (PPTP) stage 1 evaluation of MLN0128, a potent TOR kinase inhibitor

Introduction: The PI3K-TORC1 pathway links extracellular (growth factors) and intracellular (nutrient sensing) to cell cycle progression and proliferation. This pathway is dysregulated in many adult cancers, but less is known regarding childhood malignancies. MLN0128 is a novel orally-available small molecule inhibitor of both the TORC1 and TORC2 complexes, key components of the PI3K/mTOR signaling pathway.

Methods: MLN0128 was evaluated against the 23 cell lines of the PPTP in vitro panel using 96 hour exposure at concentrations from 0.1 nM to 1.0 μM. MLN0128 was tested against the PPTP solid tumor xenografts (SCID mice) and acute lymphoblastic leukemia (ALL; NOD-SCID mice) panels using a dose of 1 mg/kg administered by the P.O. route daily for 21 days.

Results: In vitro the median relative IC50 value for the PPTP cell lines was 19 nM, with a range from 2 nM to 102 nM. There was a trend for lower median rIC50 values for the rhabdomyosarcoma and Ewing sarcoma cell line panels (8 nM and 5 nM, respectively). The median rIC50 value for the ALL cell lines (68 nM) was significantly greater than that for the non-ALL cell lines.

In vivo MLN0128 was well tolerated, with only a 1.4% toxicity rate in the treated groups, compared to a 0.3% toxicity rate in control animals. All 38 tested xenograft models were considered evaluable for efficacy. MLN0128 induced significant differences in EFS distribution compared to control in 24 of 31 (77%) of the evaluable solid tumor xenografts, but did not induce significant differences in EFS distribution in any of the 7 evaluable ALL xenografts. MLN0128 induced tumor growth inhibition meeting criteria for intermediate EFS T/C activity (EFS T/C > 2) in 6 of 30 (20%) evaluable solid tumor xenografts. Intermediate activity for the EFS T/C metric occurred in the rhabdoid tumor panel (2 of 3) and in single xenografts in four other panels. Objective responses were not observed for the solid tumor or for the ALL xenograft panels. Pharmacodynamic studies to determine TOR inhibition are planned.

Conclusions: The activity observed for MLN0128 against the PPTP preclinical models is similar to that previously reported by the PPTP for another TOR kinase inhibitor (Houghton PJ, et al. Pediatr Blood Cancer. 58:191-9, 2012). When combined with PPTP results reported for PI3K and AKT inhibitors (Reynolds CP, et al. Pediatr Blood Cancer. 2012. Epub 2012/09/25 and Gorlick R, et al. Pediatr Blood Cancer. 59:518-24, 2012), the available data suggest that kinase inhibitors targeting the PI3K pathway produce limited single agent activity for the tumor types represented by the PPTP models. (Supported by award NO1-CM-42216 from the NCI).

Citation Format: Peter Houghton, Min H. Kang, Patrick Reynolds, Richard Lock, Hernan Carol, Richard Gorlick, Anders Kolb, John Maris, Stephen Keir, Catherine Billups, Raushan Kurmasheva, Malcolm Smith. Pediatric preclinical testing program (PPTP) stage 1 evaluation of MLN0128, a potent TOR kinase inhibitor. [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 2758. doi:10.1158/1538-7445.AM2013-2758

Abstract 2308: ΔNp63 promotes neuroblastoma by regulating tumor angiogenesis

Background: The tumor suppressor gene p63 and its family members p53/p73 have been described as critical determinants of tumorigenesis. ΔNp63, a splice variant of p63 lacking the N-terminal transactivation domain, is thought to antagonize the transcriptional regulation of the p53, p63 and p73 target genes and blocks their tumor suppressor activity. Overexpression of ΔNp63 has been observed in a number of adult human cancers, suggesting a role of this isoform in tumor formation. While some studies have proposed that ΔNp63 might simply inhibit p53 function within cells, leading to malignant transformation, we found that this model is challenged by the lack of consistent correlation between p53 mutation and ΔNp63 expression suggesting that ΔNp63 might possess a gain of function.

Methods: Since expression of ΔNp63 isoforms in primary tumors has been shown to correlate with poor prognosis, we attempted to analyze their relationships with tumor angiogenesis. To test whether overexpression of oncogenic ΔNp63 in HEK-293T cells can stimulate tube formation in endothelial cells using HUVEC tube formation assay. Further, we performed gene affymetrix as well as protein micro array analysis in primary cells compared to cells overexpressed ΔNp63 isoforms. In addition, we used siRNA techniques to verify any potential role of ΔNp63 isoforms in tumor angiogenesis in vitro as well as in vivo experiments.

Results: We found that overexpression of the oncogenic isoform of p53 family member ΔNp63 in HEK-293T cells resulted in stimulation of tubular structures in HUVEC cells. As 293T cells lack functional p53, these results strongly suggest a gain of function for ΔNp63. In addition, we found that ΔNp63 is overexpressed in neuroblastoma and ΔNp63 overexpression results in increasing secretion of the inflammatory cytokines interleukin-6 (IL-6) and -8 (IL-8) leading to elevated phosphorylation of STAT-3. Our further analysis showed that elevated phosphorylation of STAT-3 induced stabilization of HIF-1α protein resulting in VEGF secretion.

Conclusions: We found that ΔNp63 is overexpressed in neuroblastoma and resulted in elevated secretion of the inflammatory cytokines IL-6 and -8 leading to increased STAT3 activation. Our further studies showed that elevated ΔNp63 induced stabilization of HIF-1α protein resulting in VEGF secretion in a p53 independent manner. Importantly, we show in vitro and in vivo experiments that depletion of ΔNp63 isoform by specific siRNAs significantly reduced angiogenic potential of Neuroblastoma. In summary, our study revealed the underlying molecular mechanism how ΔNp63, as a master transcription factor, modulates tumor angiogenesis in Neuroblastoma.

Citation Format: Hemant K. Bid, Maren Cam, Antony Audino, Raushan Kurmasheva, Jiayuh Lin, Peter Houghton, Hakan Cam. ΔNp63 promotes neuroblastoma by regulating tumor angiogenesis. [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 2308. doi:10.1158/1538-7445.AM2013-2308

Abstract LB-354: Pediatric Preclinical Testing Program (PPTP) stage 1 evaluation of the XPO1/CRM1 inhibitor KPT-330

Introduction: KPT-330 is an oral Selective Inhibitor of Nuclear Export (SINE) that binds covalently to XPO1 at Cys528 resulting in its irreversible inactivation. The nuclear export of over 200 proteins with specific nuclear export sequences (NES) is mediated via XPO1. Amongst the client proteins are many tumor suppressor and growth regulatory proteins (e.g., FOXO, IκB, pRb, p53, p73, p21, and p27).

Methods: KPT-330 was tested against the PPTP's in vitro cell line panel at concentrations ranging from 1.0 nM to 10.0 μM using the PPTP's standard 96 hour exposure period. It was tested against the PPTP solid tumor xenografts using a dose of 10 mg/kg administered by the oral route thrice weekly (M-W-F) for 4 weeks with a total treatment/observation period of 6 weeks.

Results: The median relative IC50 (rIC50) value for the PPTP cell lines was 125 nM, with a range from 13 nM to greater than 10 μM. There were no significant differences in rIC50 values by histotype, although there was a trend for greater sensitivity for the Ewing sarcoma cell lines (median rIC50 = 57 nM) and lesser sensitvity for the neuroblastoma cell lines (median rIC50 = 235 nM). Most cell lines showed Relative I/O% values between -75% and -100%, consistent with a prominent cytotoxic effect for KPT-330.

KPT-330 was well tolerated in vivo. It induced significant differences in EFS distribution compared to control in 29 of 37 (78%) solid tumor xenografts and in 5 of 8 (63%) ALL xenografts. For those xenografts with a significant difference in EFS distribution between treated and control groups, an EFS T/C value of greater than 2.0 indicates a substantial agent effect in slowing tumor growth. KPT-330 induced this level of effect in 11 of 32 (34%) solid tumor xenografts, most frequently for the Wilms tumor (2 of 3) and the Ewing sarcoma (4 of 5) panels. Objective responses were observed in 3 of 38 (4%) solid tumor xenografts, including a maintained complete response (MCR) for a Wilms tumor xenograft, a CR for a medulloblastoma xenograft, and a CR for a slow-growing ependymoma xenograft. For the ALL panel, 2 of 8 (25%) xenografts achieved either CR (ALL-8, T-cell ALL) or MCR (ALL-19, B-precursor ALL).

Conclusions: KPT-330 shows potent in vitro activity against many PPTP cell lines, consistent with the activation of multiple tumor suppressor proteins across diverse tumor genotypes. KPT-330 shows tumor regressing activity against selected PPTP solid tumor and ALL xenografts, and shows tumor growth inhibition for a larger number of models. Defining the relationship between KPT-330 systemic exposure in mice and humans will be important in assessing the clinical relevance of the PPTP in vivo results. Planned PD testing may identify biomarkers associated with response of pediatric preclinical models to KPT-330. KPT-330 is in phase 1 clinical trials in adults with advanced solid or hematological malignancies (NCT01607905 and NCT01607892). (Supported by NCI NO1-CM-42216)

Citation Format: Peter Houghton, Min Kang, Patrick Reynolds, Richard Gorlick, Anders Kolb, John Maris, Stephen Keir, Hernan Carol, Richard Lock, Catherine Billups, Raushan Kurmasheva, Yosef Landesman, Sharon Shacham, Michael Kauffman, Malcolm A. Smith. Pediatric Preclinical Testing Program (PPTP) stage 1 evaluation of the XPO1/CRM1 inhibitor KPT-330. [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 LB-354. doi:10.1158/1538-7445.AM2013-LB-354

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

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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.

Abstract LB-318: Pediatric Preclinical Testing Program (PPTP) evaluation of the JAK inhibitor AZD1480

Background: AZD1480 is a potent, competitive small molecule inhibitor of JAK1/2 kinase that has entered clinical evaluation. JAK inhibition is of particular pediatric interest given the activating JAK1/2 mutations observed in a subset of pediatric ALL cases. The activity of AZD1480 was evaluated against the PPTP's in vitro and in vivo panels. Methods: AZD1480 (provided by AstraZeneca) was tested in vitro at concentrations from 1.0 nM to 10.0 µM. It was evaluated against solid tumor xenografts at 60 mg/kg administered by oral gavage daily x 5 for 3 weeks, with a total treatment and observation period of 6 weeks. For the ALL panel (using NOD-SCID mice), the maximum tolerated dose was lower, and a twice daily schedule was utilized: 10 mg/kg BID (with a single daily dose of 15 mg/kg on weekends). Standard PPTP measures of in vivo antitumor activity were employed to assess response to AZD1480. Results: The median relative IC50 (rIC50) for AZD1480 against the PPTP cell lines was 1.5 µM, with a range from 0.3 µM to 5.9 µM. AZD1480 induced significant differences in EFS distribution compared to control in 25 of 27 (93%) evaluable solid tumor xenografts. AZD1480 induced tumor growth inhibition meeting criteria for intermediate or high EFS T/C activity in 11 of 26 (42%) solid tumor xenografts evaluable for this measure. Both Wilms tumor xenografts tested showed EFS T/C > 2, as did 2 of 4 GBM xenografts and 2 of 4 neuroblastoma xenografts. An objective response was observed for 1 solid tumor xenograft, a Wilms tumor xenograft KT-10 that achieved a maintained complete response (MCR). Many solid tumor xenografts show phospho-STAT3 expression, but this marker showed no discernible relationship with response to AZD1480. For the ALL panel, 5 JAK mutated xenografts (3 JAK2 and 2 JAK1) were selected for testing to determine whether AZD1480 shows high activity in models in which the JAK-STAT pathway is activated by mutation. Additionally, 4 non-JAK mutated xenografts were evaluated. Models with JAK mutations show phospho-STAT5 as evidence of JAK-STAT signaling. However, the only ALL xenograft with EFS T/C > 2 was a JAK2 mutant (R867Q) xenograft, TGT-20, and no models showed objective responses (PR or CR). Conclusions: AZD1480 showed tumor growth inhibitory activity against most of the solid tumor xenografts, and induced an objective response (an MCR) in a single Wilms tumor xenograft. Genomic sequencing is being undertaken to determine whether this xenograft has a genomic alteration(s) in a JAK family kinase or another kinase that may explain its favorable response. No objective responses were noted for the ALL panel, even among xenografts with JAK mutations. Our results suggest that inhibition of JAK signaling alone may not be sufficient for clinical activity against JAK-mutated ALL. (Supported by NCI NO1CM42216)

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 LB-318. doi:1538-7445.AM2012-LB-318

Abstract LB-317: Pediatric Preclinical Testing Program (PPTP) evaluation of volasertib (BI 6727), a Polo-like kinase (PLK) inhibitor

Background: Volasertib is a first in class, selective and potent cell cycle kinase inhibitor that induces mitotic arrest and apoptosis by targeting PLK. Genomic screens have identified PLK1 as a potential therapeutic target for several pediatric cancers, including rhabdomyosarcoma and neuroblastoma. Methods: Volasertib (provided by Boehringer Ingelheim) was tested in vitro at concentrations from 0.1 nM to 1.0 µM. Volasertib was tested against the PPTP solid tumor xenografts using a dose of 30 mg/kg administered intravenously weekly x 3. For the ALL panel (using NOD-SCID mice), the MTD was 15 mg/kg, and this dose was used for efficacy testing. The total planned treatment period was 3 weeks with an additional 3 weeks observation. Two measures of antitumor activity were primarily used: 1) an objective response measure modeled after the clinical setting; and 2) a time to event (4-fold increase in tumor volume) measure based on the median event-free survival (EFS) of treated (T) and control (C) animals for each xenograft. Intermediate activity requires EFS T/C > 2, with high activity additionally requiring regression at the end of the observation period. Results: The median relative IC50 (rIC50) value for the PPTP cell lines was 14.1 nM, with a range from 6.0 nM to 135 nM. The median rIC50 values were lowest for the ALL cell line panel compared to the remaining cell lines (11.9 versus 16.0 nM, respectively), but this difference was not significant, and overall there were no differences in rIC50 by histotype. Against the PPTP in vivo panels volasertib induced significant differences in EFS distribution compared to control in 19 of 32 (59%) evaluable solid tumor xenografts and in 2 of 4 (50%) evaluable ALL xenografts. Volasertib induced tumor growth inhibition meeting criteria for intermediate EFS T/C activity in 11 of 30 (37%) evaluable solid tumor xenografts. Intermediate activity for the EFS T/C metric was most consistently observed in the neuroblastoma (4 of 6) and glioblastoma (2 of 3) panels. For the ALL panel, 2 of 4 (50%) xenografts met criteria for intermediate activity. Objective responses were observed for 4 of 32 solid tumor and 1 of 4 ALL xenografts. Two of 6 neuroblastoma xenografts demonstrated CRs, as did 1 of 3 glioblastoma and 1 of 5 rhabdomyosarcoma xenografts evaluable for this response measure. Conclusions: Volasertib showed low nanomolar in vitro potency against the PPTP cell lines with no histotype selectivity. Volasertib induced regressions in 5 of 36 evaluable PPTP xenografts with the neuroblastoma panel showing the most consistent pattern of responsiveness to volasertib. Given available pharmacokinetic data showing that mice tolerate higher systemic exposure to volasertib than humans, it is unlikely that the PPTP in vivo results are under-estimating the potential clinical activity of volasertib against the childhood cancer types evaluated here. (Supported by NCI NO1CM42216)

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 LB-317. doi:1538-7445.AM2012-LB-317

Abstract 2535: Regulation of Chk1 by the mTOR pathway is essential for cancer cells to complete DNA replication in response to replication stress

Oncogenic signaling leads to DNA replication stress, DNA damage and consequently DNA damage response (DDR), a barrier for tumorigenesis. For survival and proliferation, cancer cells must overcome this oncogene-induced DDR, but the molecular mechanism by which this is achieved is largely unknown. Here we report that the mTOR pathway prevents either spontaneous or drug induced DNA damage by sustaining ATR-Chk1 checkpoint in pediatric rhabdomyosarcoma (RMS) both in vivo and in vitro. Pharmacological inhibition of mTOR signaling by the selective TOR kinase inhibitor AZD8055 lead to spontaneous DNA damage and apoptosis as demonstrated by the induction of H2AX phosphorylation and PARP1 cleavage both in cell culture and tumor xenografts of RMS. Treatment RMS Rh30 cells with AZD8055 enhanced hydroxyurea induced H2AX phosphorylation and attenuated ATR-Chk1 checkpoint activation, as shown by decreased levels of phospho-Chk1(S345). We further found that mTOR signaling controls CHK1 expression at the transcriptional level via enhancing the activity of cyclin dependent kinases (CDKs). Consistent with its essential role for cells to survive DNA replication stress, siRNA-mediated knockdown of Chk1, or inhibition of Chk1 function by a Chk1 kinase inhibitor resulted in H2AX phosphorylation, while ectopic increase of Chk1 attenuated the DNA damage induced by mTOR kinase inhibition. Most importantly, the mTOR kinase inhibitor prevented the slow S-phase progression following DNA replication stress, which was accompanied with a decrease of Chk1 protein levels and phospho-Chk1(S345). Our data suggest that the TORC1/TORC2 pathway promotes RMS tumorigenesis by enhancing the cell survival functions of the ATR-Chk1 checkpoint in response to DNA replication stress. Our findings discovered an important unexpected functional linkage between TOR signaling and the ATR-Chk1 checkpoint signaling cascade and may impact targeting these two prominent cancer cell signaling pathways. Supported by CA77776.

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 2535. doi:1538-7445.AM2012-2535

Developing new agents for the treatment of childhood cancer

The development of new drugs for the treatment of childhood cancer presents novel challenges, and pediatric clinical trials of a new agent are often initiated many years after testing in adults. It is estimated that there are over 400 new entities being developed as cancer treatments, although few of these have been developed specifically for the treatment of childhood malignancies. This raises the question of how agents can be prioritized for pediatric clinical testing. In this review, the molecular characteristics of childhood cancers that may be valuable in steering choices for rational or molecularly targeted treatments are described, and the Pediatric Preclinical Testing Program, a National Cancer Institute initiative to identify agents that should be prioritized for clinical evaluation against childhood cancer, is presented.