No abstract

e23517 Background: Targeted therapy development in sarcoma has been burdened by the heterogeneity of this rare group of tumors. B7H3 is an immune checkpoint molecule in the same family as PD-L1 (also known as B7H1). Its limited expression in non-cancerous tissues and over-expression in numerous tumor types makes it an attractive target for cancer immunotherapy. Available data suggests high expression of B7H3 in certain sarcoma subtypes, including rhabdomyosarcoma, osteosarcoma, liposarcoma, Ewing sarcoma, synovial sarcoma and chondrosarcoma. This study was conducted to further explore B7H3 expression patterns across sarcoma subtypes and the relationship, if any, with PD-1 and PD-L1 expression. Methods: This single-institution, retrospective analysis evaluated soft tissue sarcoma Formalin-Fixed Paraffin-Embedded tumor specimens in patients with a variety of different subtypes. Specimens were evaluated for expression of B7H3, vessel positivity for B7H3, B7H3 staining pattern, and expression of PD-L1 and PD-1. The association between B7H3 expression and baseline and tumor factors was tested using the Mantel Haenszel test or two-sample T-test. Overall survival was estimated using the Kaplan-Meier method where survival time was calculated from diagnosis to death (or last follow-up for those last known to be alive). Results: 156 sarcoma specimens encompassing 16 subtypes were included. B7H3 was broadly expressed across sarcoma subtypes, with some degree of expression found in 91% of samples and 61% demonstrating high levels of B7H3 expression. All tumor samples expressing B7H3 exhibited the biomarker in at least 80% of tumor cells per sample (with a mean of 92.34% of cells testing positive for B7H3). All samples positive for B7H3 showed both cytoplasmic and membranous expression, and B7H3 vessel positivity was found in 90% (137/152) of samples. There were no significant differences in expression or level of expression when stratified by sarcoma subtype, prior treatment(s), tumor size, tumor grade or patient age. No significant difference in overall survival was seen regardless of B7H3 expression or expression levels across sarcoma subtypes. There was no correlation between B7H3 expression and expression of PD-L1 or PD-1 in tumor cells. Conclusions: This is the most comprehensive analysis of B7H3 expression in sarcoma to date. This data shows high levels of B7H3 expression across sarcoma subtypes, suggesting its feasibility as a therapeutic target for future sarcoma treatments. Future clinical trials are needed to evaluate its numerous actions and if manipulation of B7H3 can be employed to improve outcomes in sarcoma. [Table: see text]