Abstract LB061: Evaluation of the in vivo efficacy of the B7-H3 targeting antibody-drug conjugate (ADC) DS7300a: A report fro the Pediatric Preclinical In Vivo Resting (PIVOT) program

Abstract Introduction: B7-H3 (encoded by the CD276 gene) is suggested to act as an immune checkpoint molecule and is highly expressed in some pediatric solid tumors. Monoclonal antibodies targeting B7-H3 (8H9 and MGA271) are well-tolerated but have demonstrated limited success in clinical trials. DS-7300a is a B7-H3 targeting ADC with a payload of DXd (an exatecan derivative that inhibits DNA topoisomerase I). DS-7300a has a drug-to-antibody ratio of 4 and it has shown promising early clinical activity in adults with advanced solid cancers (Johnson, Annals of Oncology 2021; 32:S583-S585). Methods: Xenograft models for several pediatric cancer types were selected for preclinical testing of DS-7300a based on the high rates of B7-H3 expression in these cancers. Models were dosed at 10 mg/kg administered intravenously every other week for two doses (Q2wk x 2). Different experimental designs were used for efficacy testing. For rhabdomyosarcoma, Ewing sarcoma, Wilms tumor, and other pediatric solid tumors a single mouse trial (SMT) design with 40 models was used. A cohort design (N=10) was used for efficacy testing in 6 osteosarcoma models, 3 patient-derived orthotopic xenograft glioblastoma (PDOX) models, and 3 PDOX ependymoma models. For neuroblastoma models, 10 models (2 mice per model) were tested. To evaluate treatment efficacy, objective response measures based on changes in relative tumor volume were used (Houghton, Pediatr Blood Cancer 2007;49:928-940). Results: As a single agent, DS-7300a induced a statistically significant prolongation of survival in 2 of 3 orthotopic glioblastoma and 1 of 3 ependymal tumor models. DS-7300a demonstrated high efficacy (maintained complete response, complete response, and partial response) in most non-CNS models tested: 17 of 21 rhabdomyosarcomas, 5 of 7 osteosarcomas, 7 of 10 neuroblastomas and 2 of 2 Wilms tumors. For Ewing sarcoma, most models were classified as progressive disease (9 of 15). For osteosarcoma models, the log cell kill per dose values ranged from 0.95 to 2.76 indicating high activity of the agent in these models. The activity for DS-7300a followed the general pattern of protein and RNA expression levels for B7-H3/CD276 in the models, with Ewing sarcoma models showing lower expression compared to the other models. Conclusions: DS-7300a shows tumor-regressing anticancer activity across a wide range of pediatric solid tumor models. The maintained complete remissions observed for osteosarcoma models are noteworthy, as this level of response is uncommon for these models and as osteosarcoma shows the highest B7-H3 expression among pediatric cancers. The high level of preclinical activity observed for DS-7300a combined with the promising early clinical activity observed for adult patients provide strong rationale for studying DS-7300a in children with B7-H3 expressing solid tumors. Citation Format: Richard Gorlick, E. Anders Kolb, Yifei Wang, Peter Houghton, Raushan Kurmasheva, Yael Mosse, John Maris, Matthew Tsang, David Groff, Kateryna Krytska, Xiao-Nan Li, Yuchen Du, Jun Hasegawa, Nanae Izumi, Steven Neuhauser, Anuj Srivastava, Tim Stearns, Vivek Philip, Emily L. Jocoy, Jeff Chuang, Carol J. Bult, Beverly Teicher, Malcolm Smith. Evaluation of the in vivo efficacy of the B7-H3 targeting antibody-drug conjugate (ADC) DS7300a: A report fro the Pediatric Preclinical In Vivo Resting (PIVOT) program [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB061.