LAUSR

e16504 Background: Sipuleucel-T is an autologous cellular immunotherapy for asymptomatic or minimally symptomatic metastatic castration-resistant prostate cancer. In IMPACT (NCT00065442), a phase 3 trial, sipuleucel-T-induced immune responses against target Ag prostatic acid phosphatase (PAP) or PA2024, a recombinant protein consisting of PAP and granulocyte macrophage colony stimulating factor, correlated with OS (Sheikh 2013). Additionally, sipuleucel-T-induced immunoglobulin G (IgG) responses against secondary, non-target Ag (i.e. Ag spread) and the breadth of Ag spread also correlated with improved OS (GuhaThakurta 2015). Here we assessed Ag spread and OS in real-world patients from PRIME (NCT01727154), an immune monitoring sub-study of sipuleucel-T trials Methods: IgG levels in pre and wk 6 (2 wk post sipuleucel-T completion) sera from PRIME (n = 100) were quantified using Luminex xMAP. IgG responses to secondary Ag (LGALS3, PSA, KLK2, LGALS8, K-Ras, E-Ras) were defined as ≥ 1.5-fold increase over baseline. OS associations with individual Ag responses and total number of Ag per patient were assessed using the Kaplan-Meier method. Hazard ratios (HR) were estimated using a Cox Proportional Hazard Model. Results: IgG responses to ≥1 secondary Ag were observed in ≥72% of patients. Individual IgG responses to LGALS3, K-Ras, and LGALS8 at wk 6 were significantly associated with OS. Furthermore, breadth of Ag spread positively correlated with OS (Table); as the total number of Ag responses per patient increased, OS improved compared to patients with no secondary IgG responses. Conclusions: The results presented are consistent with findings from the prior phase 3 trial IMPACT. Secondary Ag responses were generated in real-world patients treated with sipuleucel-T, and these responses correlated with OS. Furthermore, breadth of Ag spread also correlated with improved OS. Clinical trial information: NCT01727154. [Table: see text]