Bender, Julia Lynne Glade

TPS10057 Background: Ewing sarcoma (EWS) is an aggressive tumor of the bone and soft tissue primarily affecting adolescents and young adults. For patients with recurrent disease, outcomes remain poor with 5-year survival rates of 10%–15%, highlighting the need for novel approaches. STEAP1 is highly expressed in EWS cell lines, xenograft models, and patient tumor samples 1 and is transcriptionally regulated by EWSR1::FLI1 , the main oncogenic driver in EWS. Xaluritamig, a humanized bispecific XmAb 2+1 T-cell engager targeting STEAP1 and CD3, has demonstrated potent, antigen-dependent T-cell–mediated lysis of STEAP1-expressing EWS cell lines 1 and induced robust in vivo antitumor activity in EWS xenograft models with concomitant CD8+ T-cell activation. In metastatic castration-resistant prostate cancer, another STEAP1-expressing solid malignancy, xaluritamig has shown meaningful antitumor activity and a manageable safety profile 2 supporting its clinical evaluation in R/R EWS. Methods: This ongoing, phase 1b, single-arm, open-label, two-part, multicenter study (NCT07297979) is evaluating xaluritamig in pediatric, adolescent, and adult patients with a histologic diagnosis of EWS and an EWSR1 :: ETS fusion gene locally confirmed by next-generation sequencing. Patients must have R/R EWS following ≥1 line of systemic therapy and Karnofsky or Lansky performance status ≥70. Part 1 (dose confirmation) consists of cohort 1 (age ≥12 years) and cohort 2 (age ≥2 to <12 years), and part 2 (dose expansion) includes patients aged ≥2 years with no upper age limit at enrollment. This study includes a 21-day screening period, a treatment period, a safety follow-up period, and a long-term follow-up period. Patients receive xaluritamig every 2 weeks once reaching the target dose until radiographic disease progression per RECIST v1.1, clinical disease progression, unacceptable toxicity, initiation of other anticancer therapy not permitted per protocol, withdrawal of consent, death, or end of study as determined by the sponsor, whichever is earlier. Primary endpoints are safety, tolerability, and determination of recommended dose(s) for expansion. Secondary endpoints are pharmacokinetics, preliminary antitumor activity, and immunogenicity. Biomarker evaluation as an exploratory endpoint includes assessment of serum cytokines, other pro-inflammatory markers, immune-cell subsets, circulating tumor DNA, STEAP1 protein expression, and tumor genomic profile and their association with safety and efficacy. CRS mitigation plan includes step-up dosing, steroid premedication, and administration of interleukin-6 inhibitors as needed. Site activation commenced in January 2026 and enrollment is ongoing. 1. Nolan-Stevaux O. Cancer Res. 2020;80(16_Suppl):DDT02–03. 2. Kelly WK, Danila DC, Lin CC, et al. Cancer Discov . 2024;14(1):76-89. Clinical trial information: NCT07297979 .

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 .