Sipuleucel-T to modify the B7-H3 immune checkpoint in men with castrate resistant prostate cancer.

273Background: B7-H3 is a T cell costimulatory molecule whose expression correlates with increased prostate cancer stage and mortality. We measured serum B7-H3 (sB7-H3) levels in men undergoing sipuleucel-T (sip-T) immunotherapy for castrate-resistant prostate cancer (CRPC) to determine if sip-T modified B7-H3 levels. Methods: We analyzed 41 banked serum specimens before and after sip-T treatment for CRPC. An ELISA test for sB7-H3 was used to measure serum sB7-H3 levels using a standard curve consisting of B7-H3 fusion protein, and a 4 parameter logistic model fit to back-calculate sB7-H3 levels. Each sample was measured in duplicate and the average of these two measurements used. Generalized additive and segmented regression models were used to model the relationship between sB7-H3 level and time to (and from) sip-T treatment. Results: We found that the sB7-H3 assay had a high degree of reproducibility with a Pearson correlation coefficient of 0.9 between the two measurement occasions. Importantly, sB7-H...