Open-label, multicenter study of sipuleucel-T in men with metastatic castrate-resistant prostate cancer (mCRPC) previously treated with sipuleucel-T: Evaluation of antigen presenting cell (APC) activation and ELISPOT data.

5053^ Background: P10-1 (NCT01338012) is a study of sipuleucel-T, an autologous cellular immunotherapy, in men with mCRPC previously treated with sipuleucel-T in PROTECT (NCT00779402). This preliminary analysis of P10-1 evaluates APC activation (a measure of product potency) and immune responses in men retreated with sipuleucelET. Methods: Men who received ≥1 infusion of sipuleucel-T in PROTECT and progressed to mCRPC were retreated with 3 infusions of sipuleucel-T. APC activation was assessed by CD54 upregulation. T cell responses to prostatic acid phosphatase (PAP) and PA2024 (PAP-GM-CSF) antigens were assessed by IFN-γELISPOT assay. Results: As of October 23, 2012, 7 men were enrolled and received ≥1 infusion. Median time between the third PROTECT infusion and first P10-1 infusion was 9.2 (range: 7.8–10.0) years. APC activation was greater at the first P10-1 treatment vs the last PROTECT treatment (Table). PA2024 and PAP ELISPOT responses were present prior to retreatment, indicating long-term memory (...