Citations

10030 Background: CABO is a multi-targeted kinase inhibitor of VEGFR2, c-MET, AXL and RET and has shown clinical evidence of activity in two prior studies of adult and pediatric patients with advanced OS. We report results of a feasibility cohort of CABO + MAP chemotherapy in patients with newly diagnosed high-risk OS. Methods: Patients <40 years, body surface area ≥ 0.8 m 2 and new diagnosis of high-grade OS with metastatic disease and resectable primary tumor were eligible. Patients were enrolled to two CABO dose levels (Dose Level 1 (DL1) 25 mg/m 2 /day, maximum 40 mg/day; Dose Level 2 (DL2) 35 mg/m 2 /day; maximum 60 mg/day) using a modified IQ 3+3 design. CABO was orally administered concomitantly with six 35-day cycles of MAP (cisplatin 60 mg/m 2 on days 1-2 (cycles 1-4), doxorubicin 37.5 mg/m 2 on days 1-2 (with dexrazoxane), methotrexate 12 g/m 2 on days 22, 29) and for 24 weeks as maintenance monotherapy. CABO was held after 7 weeks and resumed during 4 th MAP cycle to allow for primary tumor resection and metastatic surgeries. Dose-limiting toxicities (DLT) were assessed during weeks 1-6 of treatment; additional monitoring for post-surgical complications was conducted through completion of 4 th MAP cycle. The primary endpoint for this cohort was feasibility of combination therapy. Results: Ten patients (median age 11.5 years, 70% male) were enrolled and treated with CABO + MAP (DL1, n=4; DL2, n=6). Median cumulative doses of all MAP agents were maintained throughout treatment with no dose reductions. Concomitant dosing of CABO with MAP was tolerated with no protocol-specified DLTs occurring during the evaluation period. DL2 (35 mg/m 2 ) was established as the recommended dose for further study of combination therapy. Commonly reported adverse events included hematologic toxicities, febrile neutropenia, and alanine/aspartate aminotransferase (AST/ALT) elevation. Interruption of CABO was required in 6 patients for Grade 3+ AST/ALT elevation following methotrexate (n=3), wound complications (n=3), and Grade 3 pneumothorax (n=1). Wounds included two Grade 3 port site events and one Grade 1 primary tumor resection site; two patients later resumed CABO after healing without wound recurrence. Dose reduction of CABO was required in one patient for Grade 4 amylase elevation. Conclusions: The combination of CABO + MAP is feasible with no protocol-specified DLTs during the evaluation period. Generalizability is limited based on cohort size. Further evaluation of CABO + MAP is ongoing in a phase II/III randomized efficacy study in newly diagnosed standard and high-risk patients using CABO dose of 35 mg/m 2 /day (max 60 mg), with delayed initiation of CABO in patients with port placement and early safety monitoring for wound-related events, as well as full study monitoring of dose delivery, wound complications and systemic toxicity. Clinical trial information: NCT05691478 .

Abstract Introduction: Ewing sarcoma (EwS) is a FET::ETS family member-driven primary bone cancer demonstrating vast heterogeneity. Between a patient’s primary diagnosis and disease progression, tumor cell state adaptation, microenvironment changes, and the landscape of evolving therapeutic vulnerabilities remain poorly understood. To address these gaps, the Sean Karl Cohort was established in 2025 to conduct the largest single-cell transcriptomic analysis of retrospective paired tumor samples from patients with EwS. Here, we present data from the five analytic teams, and through shared SOPs now expand this cohort to Europe. Methods: Common sample processing SOPs were established to isolate cells from FFPE material from paired patient EwS samples. Single-cell (sc) RNAseq was generated using the GEM-X Flex Gene Expression protocol (10x Genomics). Alex’s Lemonade Stand Foundation (ALSF) Data Lab established a common data processing and integration pipeline with an EwS-specific cell annotation workflow to create a harmonized dataset for downstream analyses. Analytic teams were created to define tumor cell subpopulations and their therapeutic vulnerabilities and to characterize the tumor microenvironment. Data: We demonstrate the feasibility of generating high-quality scRNAseq profiles from retrospective FFPE-preserved EwS tumors. To date, 116 samples have yielded ∼800,000 cells. Integrated analysis reveals reproducible EwS tumor cell states shared across patients with regulatory network analyses nominating candidate therapeutic vulnerabilities. The scale of this international cohort enables saturation analysis for rare cell populations. Paired and longitudinal samples further allow correlation of emergent cell states with therapy resistance and metastatic progression. For example, immune-focused analyses show increases in T cell and macrophage populations in post-therapy samples. A novel, standardized EwS-specific cell annotation workflow has been developed to harmonize analyses and findings. These data will be openly shared with the community through the ALSF Single-cell Pediatric Cancer Atlas Portal. Conclusion: Large-scale international collaborative sample sharing, standardized processing pipelines, and harmonized EwS-specific annotations now enable deep single-cell analyses to characterize tumor heterogeneity in this rare pediatric and adolescent cancer. An in vivo expansion of the Sean Karl Cohort is underway to assess conservation of EwS tumor cell states and regulatory vulnerabilities identified in human tumors pre- and post-therapy in preclinical models, further supporting translation of these findings toward therapeutic strategies. Citation Format: Abbe Pannucci, Elina Mukherjee, Jessica D. Daley, Shireen Sita Ganapathi, Elissa Boguslawski, Lea Surrey, Lauren Gutstein, Patrick Azar, Emily Stockfisch, Azfar Neyaz, Ivy John, Jennifer Picarsic, Yutaro Tanaka, Byron Butaney, Riaz Gillani, Brian D. Crompton, Katherine A. Janeway, Jaclyln Taroni, Jessica Davis, Damon Reed, Adam Shlien, Theodore W. Laetsch, Rajen Mody, Clémence Henon, Thomas G. Grünewald, Elizabeth R. Lawlor, Filemon Dela Cruz, Patrick J. Grohar, Jovana Pavisic, Ally Hawkins, Anthony R. Cillo, Kelly M. Bailey. The Sean Karl Cohort: An international single-cell RNAseq study of paired patient Ewing sarcoma specimens [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 643.

Abstract Purpose: Mouse models demonstrate a role for RANK and its ligand, RANKL, in osteosarcoma. The primary objective of this single-arm, open-label phase 2 trial was to determine whether denosumab, a RANKL mAb, improved disease control in recurrent osteosarcoma relative to benchmarks derived from historic Children’s Oncology Group clinical trial data. Patients and Methods: Skeletally mature patients ages 11 to 49 years old with measurable disease were eligible for cohort 1, and those with complete surgical resection of all sites of disease were eligible for cohort 2. Patients received denosumab 120 mg subcutaneously every 4 weeks with calcium and vitamin D supplementation. The primary endpoints were RECIST response and remaining event-free for 4 months for cohort 1 and remaining event-free for 12 months for cohort 2. Toxicity, pharmacokinetics, and pharmacodynamic effects were additional endpoints. Results: Fifteen patients in cohort 1 and 38 in cohort 2 were eligible and evaluable for the primary endpoint. One of 15 cohort 1 patients remained event-free at 4 months. There were no objective responses. Ten of 38 cohort 2 patients were event-free at 12 months. The predefined efficacy criteria were not met in either cohort. The most common ≥ grade 3 adverse events were hypocalcemia and hypophosphatemia (8% and 11%, respectively). At steady state, mean serum denosumab trough concentrations were 23.7 to 31 μg/mL in cycles 2 to 7. Serum c-telopeptide and urine n-telopeptide decreased on treatment. Conclusions: Denosumab was well-tolerated with anticipated side effects and pharmacokinetic and pharmacodynamic parameters but had insufficient activity for further development in osteosarcoma.

PURPOSE Human epidermal growth factor receptor 2 (HER2), a transmembrane receptor with kinase activity involved in cell signaling through the RAS pathway, is expressed in osteosarcoma (OST). Trastuzumab deruxtecan (T-DXd) is an antibody-drug conjugate with a HER2-targeting monoclonal antibody, trastuzumab, linked to a topoisomerase I inhibitor. We report results of a phase II study of T-DXd in adolescents and young adults with HER2-positive relapsed, unresectable OST (ClinicalTrials.gov identifier: NCT04616560 ). METHODS Patients age 12-39 years with relapsed, unresectable OST were eligible for centrally evaluated HER2 expression screening by immunohistochemistry. Patients with >10% of OST cells with cytoplasmic or membranous HER2 expression of any intensity were eligible to receive T-DXd at 5.4 mg/kg intravenously once every 3 weeks for up to 2 years. The study used a 9 + 15 Simon's optimal two-stage design with a primary end point of event-free survival at 24 weeks. RESULTS Fifty patients were screened for HER2, and 41 met the inclusion criteria of 10% of OST cells having membranous or cytoplasmic HER2 expression. Nine eligible patients, two younger than 18 years, were enrolled and received T-DXd. With a single patient having more than 24 weeks of stable disease (SD) of the nine planned in the first stage, this study did not meet criteria to progress to stage II. The estimated SD rate at 24 weeks is 11.1%. The median (range) number of cycles of therapy was 2 (2-12). Grade 3 and 4 possibly related adverse events occurred in three of nine patients and included nausea, vomiting, tumor hemorrhage, cytopenias (n = 3), wound infection, and hypertension. CONCLUSION T-DXd did not demonstrate sufficient activity to expand enrollment to the planned second stage in patients with OST. No new safety signals were observed.

10035 Background: Ewing sarcoma (EwS) is a rare malignancy of children, adolescents and young adults for which outcomes have progressively improved through intensification of conventional chemotherapy, including interval compression of cycles. Prior cooperative group reports of relapsed EwS included patients treated less intensively and with fewer treatment options at relapse. Here, we report overall survival (OS) and prognostic factors post-first recurrence in patients with EwS treated with frontline interval-compressed chemotherapy. Methods: We included patients treated on the last three phase 3 COG EwS studies treated with interval compressed chemotherapy, which accrued from 2001-2019. Patients with initially localized disease (L-EwS) were treated on AEWS0031 arm B and AEWS1031. Patients with initially metastatic disease (M-EwS) were treated on AEWS1221. The primary outcome was OS from first relapse. Demographic and clinical data from original diagnosis and from relapse were analyzed as potential prognostic factors. Kaplan-Meier survival curves were constructed and groups compared with log-rank tests. Results: 366 patients experienced disease recurrence as first event and were included in this analysis (AEWS0031 arm B, n = 69, AEWS1031, n = 115, for a total of 184 with L-EwS; AEWS1221, n = 182, all with M-EwS). Median age at relapse was 16 years. Median time from initial enrollment to first relapse was 1.52 years and was shorter for patients with M-EwS (Kruskall-Wallis, p < 0.001). First relapses were isolated local, isolated distant, and combined in 24%, 70%, and 6.4% of patients, respectively. Metastatic stage at initial diagnosis was associated with increased risk of post-first recurrence OS event (p < 0.0001). Two-year OS post-first recurrence (OS 2y ) was 25.6% for patients with M-EwS and 48.5% for patients with L-EwS. Time to first recurrence was also a predictor of post-recurrence survival (p < 0.0001): patients with initial L-EwS and relapse < 2 years from diagnosis had OS 2y of 31.4% vs. 70.8% for later relapses. Patients with initial M-EwS and relapse < 2 years from diagnosis had OS 2y of 18.9% vs. 71.2%. Patients with combined relapses had higher risk of post-relapse death compared to other patterns of failure (p = 0.003), an effect that was largely driven by the one seen in patients with initial L-EwS. Conclusions: Overall survival of patients with first recurrent EwS after having received modern era therapy with interval compressed chemotherapy on recent COG trials remains poor, particularly for M-EwS, early relapse and combined relapses. These data will inform the design of trials for this relapsed population and provide critical data to help counsel patients about goals of care at first relapse.

10032 Background: Identifying clinical and biological factors associated with outcomes in localized Ewing sarcoma (ES) will enable risk-stratified clinical trials with the goal of improved outcomes for high-risk patients and decreased treatment toxicity for low-risk patients. The specific aims of this analysis in localized ES patients were 1) to classify extraosseous (EO) primary tumors with subdivisions into deep (viscera, glands, body cavities, muscle, nerves) and superficial (cutaneous and subcutaneous) sites, and 2) to understand the relationship between baseline clinical factors (age, sex, primary tumor site, size (maximum dimension and volume)) and event-free survival (EFS). Methods: The analytic cohort included ES patients treated with ICC on AEWS0031 and AEWS1031. Primary tumor sites were defined as pelvic, non-pelvic, and EO (deep vs. superficial). Post-enrollment EFS was the primary endpoint. Univariate analyses used the Kaplan–Meier method (logrank test). Multivariable analyses used Cox proportional hazards models. To assess the impact of tumor volume as a continuous variable, a subgroup analysis was conducted using AEWS1031 only, as tumor volumes were collected prospectively on this study. Visual exploration of effects of continuous variables on EFS event hazard used restricted cubic splines with 3 knots. Tests were performed at the 5% level. Results: AEWS0031 (n = 628) and AEWS1031 Regimen B (n = 283) yielded 911 patients. In univariate analyses, difference in risk of EFS event was observed between tumor sites ( P = 0.03). EO tumors had the highest estimated EFS compared to pelvic and non-pelvic (5 year EFS 84.7% vs. 72.4% and 75.9%). Superficial EO appeared to be a very low-risk group, albeit interpretations are limited by the small group size and singular event (one second malignant neoplasm among 15 patients). In multivariable analysis of the combined cohort, sex, race, and ethnicity were not prognostic. EO tumors may be associated with decreased hazard compared with non-pelvic bone primaries (HR 0.58, P = 0.07), and tumors ≥200 mL with increased hazard (HR 1.56, P < 0.01). In the subgroup analysis, tumor volume and age were prognostic and in visualizations age was non-linearly related to the hazard of EFS event, increasing until approximately 15 years. Tumor volume was non-linearly related to the hazard of EFS event and increased until approximately 400 mL. Conclusions: Patients with ES and primary tumors ≥200 mL continue to be at higher risk of an EFS event when treated with ICC, and EO tumors may be lower risk compared with other sites. Risk of an event appears to remain constant in ES patients ≥15 years or with primary tumors ≥400 mL. These findings should be validated in prospective trials and tumor biology integrated with clinical factors to improve risk stratification for localized ES.

AbstractPatients with resectable Ewing sarcoma (ES) in select sites, such as the pelvis or chest wall, have a higher risk of positive microscopic margins following surgery. AEWS1031 was the first North American/Australasia ES cooperative group trial allowing standardized preoperative radiotherapy, intended to minimize the morbidity of combined modality local tumor control as well as improve the likelihood of clear surgical margins. In this study, we found that the use of low‐dose preoperative radiation to a smaller target volume resulted in a high rate of R0 resection. The robust pathologic response rate strengthens existing evidence illustrating the radiosensitivity of Ewing sarcoma.

Trends in diagnostic biopsy sample collection approaches for primary bone sarcomas have shifted in the past 2 decades. Although open/incisional biopsies used to be the predominant approach to obtain diagnostic material for Ewing sarcoma and osteosarcoma, image-guided core needle biopsies have increased in frequency and are safe for patients. These procedures are less invasive and reduce recovery times but have potential limitations. The quantity and quality of tissue obtained through these procedures vary between institutions. Acquired viable tissue volumes can be low, limiting the conduct of downstream expanded clinical workup, molecular analyses, and research. Patients with advanced Ewing sarcoma and osteosarcoma continue to have overall poor outcomes despite dose-intensive cytotoxic chemotherapy. The biology of treatment resistance is not currently well understood, partly due to limited availability of relevant tissue to study. There is a need for access to quality tumor specimens for molecular and other analyses to identify high-risk tumor subsets and drive discovery to improve patient outcomes. Given broad variability in bone tumor tissue procurement and processing across member institutions, the Children’s Oncology Group Bone Tumor Committee convened a multidisciplinary group of experts to outline the current and near-future tissue needs for optimal clinical care and access to research platforms. The goal of this working group was to provide high-level guidance on biopsy practices that safely meet these evolving needs. Harmonizing tissue collection practices is paramount to improving the care of children, adolescents, and young adults diagnosed with Ewing sarcoma and osteosarcoma.

PURPOSE The National Cancer Institute-Children's Oncology Group Pediatric Molecular Analysis for Therapy Choice trial assigned patients age 1-21 years with relapsed or refractory solid tumors, lymphomas, and histiocytic disorders to phase II treatment arms of molecularly targeted therapies on the basis of genetic alterations detected in their tumor. Patients with tumors that harbored prespecified genomic alterations in the cyclinD-CDK4/6-INK4a-Rb pathway with intact Rb expression were assigned and treated with the cdk4/6 inhibitor palbociclib. METHODS Patients received palbociclib orally once daily for 21 days of 28-day cycles until disease progression, intolerable toxicity, or up to 2 years. The primary end point was objective response rate; secondary end points included safety/tolerability and progression-free survival. RESULTS Twenty-three patients (median age, 15 years; range, 8-21) were enrolled; 20 received protocol therapy and were evaluable for toxicity and response. Of the evaluable patients, the most common diagnoses were osteosarcoma (n = 9) and rhabdomyosarcoma (n = 6). A single actionable gene amplification was found in 19 tumors ( CDK4 , n = 11, CDK6 , n = 2, CCND3 , n = 6), with one tumor harboring two amplifications ( CDK4 and CCND2 ). Hematologic toxicities were the most common treatment-related events. No objective responses were seen. Two patients with tumors harboring CDK4 amplifications (neuroblastoma and sarcoma) had best response of stable disease for six and three cycles. Six-month progression was 10% (95% CI, 1.7 to 27.2). CONCLUSION The CDK4/6 inhibitor palbociclib at 75 mg/m 2 orally daily was tolerable in this heavily pretreated cohort. No objective responses were observed in this histology-agnostic biomarker-selected population with treatment-refractory solid tumors, demonstrating that pathway alteration alone is insufficient in pediatric cancers to generate a response to palbociclib monotherapy.

PURPOSE Patients age 1-21 years with relapsed or refractory solid and CNS tumors were assigned to phase II studies of molecularly targeted therapies on the National Cancer Institute-Children's Oncology Group (NCI-COG) Pediatric Molecular Analysis for Therapy Choice (MATCH) trial. Patients whose tumors harbored predefined genetic alterations in the phosphoinositide 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) pathway and lacked mitogen-activated protein kinase pathway activating alterations were treated with the PI3K/mTOR inhibitor samotolisib. METHODS Patients received samotolisib twice daily in 28-day cycles until disease progression or unacceptable toxicity. A rolling 6 limited dose escalation was performed as, to our knowledge, this was the first pediatric study of samotolisib. The primary end point was the objective response rate; secondary end points included progression-free survival (PFS) and the recommended phase II dose and toxicity of samotolisib in children. RESULTS A total of 3.4% (41/1,206) of centrally tested patients were matched to this arm. Seventeen patients were treated. Among treated patients, the most common diagnoses included osteosarcoma (n = 6) and high-grade glioma (n = 5) harboring alterations in phosphatase and tensin homolog (n = 6), PIK3CA (n = 5), and tuberous sclerosis complex 2 (n = 3). No objective responses or prolonged stable disease were observed. Three-month PFS was 12% (95% CI, 2 to 31). Two patients experienced dose-limiting toxicities (mucositis and pneumonitis). Dose level 2 (115 mg/m 2 /dose twice daily) was determined to be the recommended phase II dose of samotolisib in children. CONCLUSION This nationwide study was successful at identifying patients and evaluating the efficacy of molecularly targeted therapy for rare molecular subgroups of patients in a histology-agnostic fashion. Unfortunately, there was no activity of samotolisib against tumors with PI3K/mTOR pathway alterations. Prospective trials such as the NCI-COG Pediatric MATCH are necessary to evaluate the efficacy of molecularly targeted therapies given their increasing use in clinical practice.