Immune response with sipuleucel-T in patients (pts) with metastatic castration-resistant prostate cancer (mCRPC): Phase II ProACT study.

148 Background: Sipuleucel-T is an autologous cellular immunotherapy for asymptomatic or minimally symptomatic mCRPC. Sipuleucel-T prepared using 10μg/mL PA2024 (PAP-GM-CSF antigen) stimulates immune responses and prolongs overall survival (OS). ProACT is an ongoing phase 2 study to evaluate immune responses and OS of sipuleucel-T when manufactured using 3 different concentrations of PA2024. Here we report data from the FDA-approved concentration, 10μg/mL PA2024 (n=40). Methods: Pts with asymptomatic or minimally symptomatic mCRPC (n=120) received sipuleucel-T manufactured using 2, 5 or 10μg/mL PA2024 for a total of 3 infusions with 2-wk intervals. Peripheral immune responses were evaluated 2, 4 and 6 mo after 1st infusion by serum ELISA (n=40), and B cell activation phenotype was characterized during the manufacture of the product by flow cytometry (n=19). Results: During product manufacture both mature and memory B cells were activated; the percentage of B cells that had a mature phenotype (defined as C...