Abstract A018-PR014: B7-H3-CAR T cells expressing 41BB ligand for pediatric solid tumors: Preliminary results of a phase 1 study

Abstract In this phase 1 dose escalation study (3CAR; NCT04897321) we are evaluating the safety and antitumor activity of a B7-H3-targeted chimeric antigen receptor (B7-H3-CAR) T cell strategy for pediatric patients with relapsed/refractory (r/r) solid tumors. These patients have poor outcomes despite multimodal therapy and need novel therapeutic approaches. B7-H3 is an immunomodulatory protein broadly expressed in solid tumors, with limited expression in healthy tissues, making it an attractive target for immunotherapy. We developed B7-H3-CAR T cells with a CD28. ζ signaling domain, which express 41BB ligand (41BBL) on the cell surface to provide additional costimulation. B7-H3-CAR T cells were generated from CD4+/CD8+ selected autologous apheresis products using a lentiviral vector encoding both the B7-H3-CAR and 41BBL. In this study we are evaluating a single systemic infusion of B7-H3-CAR T cells following fludarabine/cyclophosphamide lymphodepletion in pediatric patients with r/r solid tumors. B7-H3 expression was confirmed by immunohistochemistry on previously obtained tissues using a CLIA-approved assay, with an H-score ≥ 100 considered positive. Evaluated dose levels (DLs) include 3x105 (DL1), 1x106 (DL2), 3x106 (DL3), 1x107 (DL4), and 3x107 (DL5) CAR+ cells/kg. We have enrolled 23 participants for T cell collection, all of whom had successful B7-H3-CAR T cell products manufactured. As of July 2025, 16 participants received B7-H3-CAR T cells (DL1: n=4, DL2: n=3, DL3: n=3, DL4: n=5, DL5: n=1). The median age was 15. 1 years (range 4. 8-21. 3), and participants had a range of solid tumor diagnoses (osteosarcoma n=4, rhabdomyosarcoma n=3, Ewing sarcoma n=2, hepatoblastoma n=2, other soft tissue sarcomas n=5) with a median H-score of 173 (range 110-250). No dose-limiting toxicities were observed among 15 evaluable participants. Nine participants experienced cytokine release syndrome (grade 1: n=7, grade 2: n=2), characterized by early onset fevers starting within 12 hours post-infusion. No participants experienced neurotoxicity. Ten participants underwent tumor biopsy 2 weeks post-infusion. Post-treatment B7-H3 expression was retained in seven of eight quantifiable specimens (median H-score 190, range 30-220). Two participants had local inflammation at superficial tumor sites, with accompanying necrosis on post-treatment biopsies. Imaging-based disease response at 4-6 weeks post-infusion included: 8 stable disease, 7 progressive disease, and 1 not evaluable due to excisional biopsy of a solitary target lesion. B7-H3-CAR T cells were detected in the peripheral blood by qPCR consistently starting with DL2 and were also detected in the peritoneal fluid of a participant with sarcomatosis. B7-H3-CAR T cells can be safely infused in pediatric patients with r/r solid tumors with evidence of homing to tumor sites and antitumor activity. Enrollment is ongoing to establish a recommended CAR T cell dose for further evaluation. Citation Format: Rebecca Epperly, Teresa Santiago, Selene Koo, Lindsay Talbot, Elaine Harstead, Jennifer Wallace, Alexandra Boyd, Jennyfer Bran, Timothy Lockey, Catherine Willis, Faizan Malik, Ying Li, Leigh Poston, Sarah Schell, Polly Adams, Manjul Rana, Scott Perry, Jean-Yves Metais, Deanna Langfitt, Salem Akel, Frank Fazio, Renee Madden, Esther Obeng, Ali Suliman, M Paulina Velasquez, Amr Qudeimat, Caitlin Zebley, Senthil Bhoopalan, Akshay Sharma, Aimee C Talleur, Swati Naik, Brandon M Triplett, Michael Frett, Vinod Maller, Andrew Smith, Michael Bishop, Sara Federico, Alberto Pappo, Andrew Davidoff, Cheng Cheng, Beth McCarville, Stephen Gottschalk, Chris DeRenzo. B7-H3-CAR T cells expressing 41BB ligand for pediatric solid tumors: Preliminary results of a phase 1 study abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Discovery and Innovation in Pediatric Cancer— From Biology to Breakthrough Therapies; 2025 Sep 25-28; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2025;85 (18Suppl₂): Abstract nr A018-PR014.