Background: In mCRPC, data are limited regarding optimal combinatorial or sequential use of available treatments. P12-2 (STRIDE; NCT01981122) is an ongoing, randomized, open-label, phase 2 study evaluating concurrent vs sequential administration of the androgen receptor inhibitor, enz, with the autologous cellular immunotherapy, sip-T. Methods: Fifty-twopatients (pts) with asymptomatic or minimally symptomatic mCRPC were randomized 1:1 to receive 3 sip-T infusions with enz starting 2 wks before (n = 25, concurrent arm A) or 10 wks after (n = 27, sequential arm B) sip-T initiation. The primary endpoint is peripheral T cell proliferation response to PA2024, the sip-T immunizing antigen. Secondary endpoints include interferon (IFN)-γ ELISPOT and humoral immune responses to PA2024 and prostatic acid phosphatase (PAP), product release parameters (total nucleated cell count, CD54⁺ cell counts, and antigen presenting cell activation [as measured by CD54 upregulation]), cytokine production, and adverse events (AEs). Results through wk 26 are described. Results: PA2024-specific T cell proliferative response was significantly elevated at all post-baseline time points (p < 0.001) and was sustained through wk 26, including a > 10-fold increase at wk 20 in both arms. This PA2024-specific response was observed in nearly all pts, 95.8% in arm A vs 92.6% in arm B. Both arms showed a significant and sustained increase in humoral responses to PA2024 and PAP as well as IFN-γ ELISPOT response to PA2024. Sip-T product parameters were similar between arms. Cytokines indicative of immune activation (such as IFN-γ, interleukin-2, and tumor necrosis factor-α) were also elevated in both arms. AEs were observed in 88% (arm A) and 100% (arm B) of pts. The incidence of grade ≥ 3 AEs was similar between arms. Conclusions: Most mCRPC pts receivingenz concurrently with or subsequent to sip-T demonstrated significant and sustained peripheral T cell and humoral immune responses through wk 26. These interim data suggest that both schedules were safe, and enz did not impair sip-T production or subsequent immune responses. Clinical trial information: NCT01981122