Abstract 1197: B7-H3-CAR T cells for the treatment of pediatric adrenocortical carcinoma

Abstract

Abstract Outcomes for pediatric patients with unresectable, metastatic, or relapsed adrenocortical carcinoma (ACC) are dismal despite aggressive management with multimodality therapies. CAR T cell therapy (CAR T) is a promising approach to improve outcomes for patients with this disease. However, progress is hindered by a lack of targetable surface antigens and limited availability of relevant preclinical models. B7-H3 is a promising immunotherapeutic target but little is known about its expression in pediatric ACC. Thus, we sought to i) evaluate B7-H3 expression in primary pediatric ACC samples and ii) assess B7-H3-CAR T activity in two novel pediatric patient-derived xenograft (PDX) models. First, we assessed B7-H3 expression in 44 pediatric adrenocortical tumors (ACTs) using a CLIA-approved immunohistochemistry (IHC) assay. ACTs had a range of B7-H3 positivity, including 24 (55%) with expression above the threshold for eligibility on our active B7-H3-CAR T clinical trial (H-score ≥ 100; NCT04897321). Univariate analyses demonstrated B7-H3 expression was correlated with the presence of germline TP53 mutations and poor prognostic factors including the inability to achieve complete resection, Cushing phenotype, and the absence of TP53 by IHC. Given a dearth of in vivo models, we next established two clinically relevant pediatric ACC PDX models. SJ-ACC4 was derived from a metastatic tumor and had somatic mutations in TP53 (p.C135Lfs*14) and CTNNB1 (p.S45F). SJACT030812_X1 (www.stjude.org/cstn) was derived from a primary tumor and had a germline TP53 mutation (p.R110Pfs*39). Both had partial ATRX deletions (exons 11-30; 3-28), uniparental disomy at chromosome 11p15, and were B7-H3-positive. Notably, SJ-ACC4 was resistant to a battery of chemotherapeutic agents. To evaluate B7-H3-CAR T anti-ACC activity in vivo, SJ-ACC4 tumor fragments were implanted subcutaneously in NOD SCID gamma (NSG) mice and treated with B7-H3-CAR or control T cells. Strikingly, B7-H3-CAR T eradicated 100% of SJ-ACC4 tumors and mice survived long term compared to controls that met endpoints within 35 days (n=8 per group, p< 0.0001). Finally, we asked if B7-H3-CAR T had antitumor activity in an orthotopic pediatric ACC PDX model. SJACT030812_X1 cells were injected into the para-adrenal region of NSG mice under ultrasound guidance and monitored by serial volumetric ultrasonography. B7-H3-CAR T again had significant antitumor activity, and mice survived long-term compared to controls that met endpoints within 42 days (n=5 per group, p< 0.01). In conclusion, B7-H3 is expressed in a majority of pediatric ACTs and B7-H3-CAR T have potent anti-ACC activity. These findings provide a strong rationale for clinically evaluating B7-H3-CAR T for pediatric patients with chemotherapy-resistant ACC. Additionally, the newly established pediatric ACC PDX models provide a valuable resource for investigating different aspects of this rare and aggressive disease. Citation Format: Rebecca Epperly, Emilia M. Pinto, Teresa Santiago, Heather Sheppard, Michael R. Clay, Christopher Morton, Phuong Nguyen, Yinmei Zhou, Mary Woolard, Carla O'Reilly, Julie Justice, Walter Akers, Melissa Johnson, Asa Karlstrom, Cheng Cheng, Gerard Zambetti, Andrew Davidoff, Raul Ribeiro, Christopher DeRenzo. B7-H3-CAR T cells for the treatment of pediatric adrenocortical carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1197.