Boosting long-term immune responses to sipuleucel-T (sip-T) by retreatment of patients (pts) with metastatic castration-resistant prostate cancer (mCRPC).

196Background: Sip-T, an autologous cellular immunotherapy for asymptomatic/minimally symptomatic mCRPC, induces immune responses to target antigens prostatic acid phosphatase (PAP) and PA2024, a recombinant protein of PAP and granulocyte macrophage colony-stimulating factor. Immune responses with sip-T correlate with overall survival. PROTECT (phase 3 NCT00779402) assessed sip-T in biochemically recurrent androgen-dependent PC. PROTECT pts developing mCRPC were eligible for P10-1 (phase 2 NCT01338012). Methods: PROTECT pts (sip-T arm) were retreated with sip-T in mCRPC. Antigen presenting cell (APC) activation (CD54 molecule ratio after and before culture with PA2024), cellular (PA2024/PAP ELISPOT; T cell proliferation) and humoral responses were assessed and compared with treatment-naive mCRPC pts (IMPACT NCT00065442; STAMP NCT01487863; STRIDE NCT01981122). Results: Caucasian men (n=8), median follow-up 9.2 y and age 74 y, had an ECOG PS of 0 (75%) or 1. Median APC activation with sip-T infusion 1 was ~...