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Purpose: Ewing sarcoma (ES) is a rare and highly malignant cancer that occurs in the bone and surrounding tissue of children and adolescents. The EWS/ETS fusion transcription factor that drives ES pathobiology was previously demonstrated to modulate cyclin D1 expression. In this study, we evaluated abemaciclib, a small-molecule CDK4 and CDK6 (CDK4 and 6) inhibitor currently under clinical investigation in pediatric solid tumors, in preclinical models of ES. Experimental Design: Using Western blot, high-content imaging, flow cytometry, ELISA, RNA sequencing, and CpG methylation assays, we characterized the in vitro response of ES cell lines to abemaciclib. We then evaluated abemaciclib in vivo in cell line–derived xenograft (CDX) and patient-derived xenograft (PDX) mouse models of ES as either a monotherapy or in combination with chemotherapy. Results: Abemaciclib induced quiescence in ES cell lines via a G1 cell-cycle block, characterized by decreased proliferation and reduction of Ki-67 and FOXM1 expression and retinoblastoma protein (RB) phosphorylation. In addition, abemaciclib reduced DNMT1 expression and promoted an inflammatory immune response as measured by cytokine secretion, antigen presentation, and interferon pathway upregulation. Single-agent abemaciclib reduced ES tumor volume in preclinical mouse models and, when given in combination with doxorubicin or temozolomide plus irinotecan, durable disease control was observed. Conclusions: Collectively, our data demonstrate that the antitumor effects of abemaciclib in preclinical ES models are multifaceted and include cell-cycle inhibition, DNA demethylation, and immunogenic changes.

Purpose: Checkpoint kinase 1 (CHK1) inhibitors potentiate the DNA-damaging effects of cytotoxic therapies and/or promote elevated levels of replication stress, leading to tumor cell death. Prexasertib (LY2606368) is a CHK1 small-molecule inhibitor under clinical evaluation in multiple adult and pediatric cancers. In this study, prexasertib was tested in a large panel of preclinical models of pediatric solid malignancies alone or in combination with chemotherapy. Experimental Design: DNA damage and changes in cell signaling following in vitro prexasertib treatment in pediatric sarcoma cell lines were analyzed by Western blot and high content imaging. Antitumor activity of prexasertib as a single agent or in combination with different chemotherapies was explored in cell line–derived (CDX) and patient-derived xenograft (PDX) mouse models representing nine different pediatric cancer histologies. Results: Pediatric sarcoma cell lines were highly sensitive to prexasertib treatment in vitro, resulting in activation of the DNA damage response. Two PDX models of desmoplastic small round cell tumor and one malignant rhabdoid tumor CDX model responded to prexasertib with complete regression. Prexasertib monotherapy also elicited robust responses in mouse models of rhabdomyosarcoma. Concurrent administration with chemotherapy was sufficient to overcome innate resistance or prevent acquired resistance to prexasertib in preclinical models of neuroblastoma, osteosarcoma, and Ewing sarcoma, or alveolar rhabdomyosarcoma, respectively. Conclusions: Prexasertib has significant antitumor effects as a monotherapy or in combination with chemotherapy in multiple preclinical models of pediatric cancer. These findings support further investigation of prexasertib in pediatric malignancies.

Purpose: Checkpoint kinase 1 (CHK1) is a key regulator of the DNA damage response and a mediator of replication stress through modulation of replication fork licensing and activation of S and G2–M cell-cycle checkpoints. We evaluated prexasertib (LY2606368), a small-molecule CHK1 inhibitor currently in clinical testing, in multiple preclinical models of pediatric cancer. Following an initial assessment of prexasertib activity, this study focused on the preclinical models of neuroblastoma. Experimental Design: We evaluated the antiproliferative activity of prexasertib in a panel of cancer cell lines; neuroblastoma cell lines were among the most sensitive. Subsequent Western blot and immunofluorescence analyses measured DNA damage and DNA repair protein activation. Prexasertib was investigated in several cell line–derived xenograft mouse models of neuroblastoma. Results: Within 24 hours, single-agent prexasertib promoted γH2AX–positive double-strand DNA breaks and phosphorylation of DNA damage sensors ATM and DNA–PKcs, leading to neuroblastoma cell death. Knockdown of CHK1 and/or CHK2 by siRNA verified that the double-strand DNA breaks and cell death elicited by prexasertib were due to specific CHK1 inhibition. Neuroblastoma xenografts rapidly regressed following prexasertib administration, independent of starting tumor volume. Decreased Ki67 and increased immunostaining of endothelial and pericyte markers were observed in xenografts after only 6 days of exposure to prexasertib, potentially indicating a swift reduction in tumor volume and/or a direct effect on tumor vasculature. Conclusions: Overall, these data demonstrate that prexasertib is a specific inhibitor of CHK1 in neuroblastoma and leads to DNA damage and cell death in preclinical models of this devastating pediatric malignancy. Clin Cancer Res; 23(15); 4354–63. ©2017 AACR.

CHK1 is a serine/threonine protein kinase essential for S-phase and G2/M cell cycle checkpoint regulation following DNA damage. Targeted inhibition of CHK1 in several tumor types increases DNA damage and replication stress, culminating in cell death through mitotic catastrophe. Recent studies have identified CHK1 as a therapeutic target in several pediatric tumor types. We evaluated the antitumor efficacy of LY2606368 mesylate monohydrate (“LY”), a checkpoint kinase 1 (CHK1)/CHK2 inhibitor currently in early phase clinical trials for adult solid cancers, in a panel of pediatric tumor cell lines and mouse models of embryonal tumors and pediatric sarcoma. In vitro effects of LY were assessed via Cell Titer Glo, immunoblotting, and cell cycle analysis by flow cytometry. For in vivo studies, mice bearing cell-derived (CDX) or patient-derived xenografts (PDX) of several pediatric tumor types were treated with four weekly cycles of 10 mg/kg LY BID for 3 consecutive days, followed by a 4 day dosing holiday. Tumor volume and body weight were measured 2x weekly. Xenograft tumor health following LY, chemotherapy, or combination treatment was evaluated by fluorescent immunohistochemistry (IHC) for a panel of markers for cell proliferation (Ki67), apoptosis (TUNEL), and angiogenesis (CD31, smooth muscle actin [SMA], MECA32). Single digit nanomolar sensitivity to LY was observed in the majority of pediatric cancer cell lines evaluated in vitro. A more detailed analysis of LY-treated neuroblastoma and pediatric sarcoma cell lines showed increased DNA damage, CHK1 phosphorylation, and MAPK pathway activation. Significant single agent LY activity was observed in mouse models of neuroblastoma and pediatric sarcoma, but not in models of hepatoblastoma or retinoblastoma. Acquired resistance to LY was observed in the ST162 and SJCRH30 models of alveolar rhabdomyosarcoma. Interestingly, more stroma was observed following LY single agent treatment as measured by CD31, SMA, and MECA32 IHC staining; co-treatment with chemotherapy reduced the amount of SMA expressing-cells. Overall, our data demonstrate that LY is highly effective as a single agent in murine in vivo models of human neuroblastoma and several pediatric sarcoma subtypes. Current studies include further evaluation of the LY mechanism of action; investigation into the mechanism of intrinsic and acquired resistance; and identification of possible biomarkers for LY sensitivity. Citation Format: Caitlin D. May, Richard Beckmann, Wayne Blosser, Michele Dowless, Alle VanWye, Teresa Burke, Gerard Oakley, Jennifer Stephens, Julie Stewart, Beverly Falcon, Louis Stancato. Targeting checkpoint kinase 1 (CHK1) with the small molecule inhibitor LY2606368 mesylate monohydrate in models of high-risk pediatric cancer yields significant antitumor effects. [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 2458.