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 (Table 1); based on other studies of sipuleucel-T, ELISPOT responses are not generally present prior to the first treatment (Beer. Clin Cancer Res 2011; Sheikh. Cancer Immunol Immunother 2013). Conclusions: This is the first trial to report the feasibility of sipuleucel-T retreatment following treatment in an earlier stage of prostate cancer. These data indicate the presence of existing immunological memory to the immunizing antigen several years after initial treatment. In addition, retreatment with sipuleucel-T appeared to boost product potency compared with prior treatment. Clinical trial information: NCT01338012. [Table: see text]
Development of sipuleucel-T: Autologous cellular immunotherapy for the treatment of metastatic castrate resistant prostate cancer