Background: Sip-T is an FDA-approved immunotherapy for treating patients (pts) with asymptomatic or minimally symptomatic metastatic castration-resistant prostate cancer (mCRPC). It is manufactured from autologous peripheral blood mononuclear cells (PBMCs) cultured with PA2024, a fusion of prostatic acid phosphatase (PAP) and granulocyte macrophage colony-stimulating factor. Survival of sip-T–treated mCRPC pts correlates with immune responses to PA2024 and/or PAP. PA2024- or PAP-specific CD4+ and CD8+ T cell proliferation and cytokine production and release were assessed to better understand sip-T–induced T cell responses. Methods: Pts with biochemical recurrence or mCRPC were from sip-T trials (NCT01431391, NCT01981122). PBMCs collected at baseline through 6 mo post–sip-T were cultured in vitro and stimulated with PA2024 or PAP. CD4+ and CD8+ T cells were assessed (n=19) for proliferation and intracellular IL-2 and IFN-γ. The cytokine profile was confirmed in supernatant with a meso scale discovery assay. P<0.10 was statistically significant. Results: Compared with baseline, PA2024-specific proliferating CD4+ and CD8+ T cells had increased intracellular IL-2 and IFN-γ levels at wk 6 and mo 6, with a similar trend for PAP-specific proliferating T cells (Table 1). Compared with unstimulated controls, a significant >2-fold increase in PA2024-stimulated IL-2 and IFN-γ in supernatant was observed at wk 6 and mo 6 over baseline (p<0.001). PAP-stimulated IL-2 and IFN-γ supernatant levels increased over baseline and were significantly elevated for IFN-γ at wk 6 (p<0.10). Conclusions: Sip-T therapy generated a de novo PA2024-specific T cell response, as indicated by the cytokine release profile. The PAP-stimulated cytokine profile suggests that pre-existing immunity with terminally differentiated T cells are expanded. Thus, sip-T reactivated an anti-PAP response in memory T cells, thereby overcoming immunosuppressive mechanisms in PC. Clinical trial information: NCT01431391; NCT01981122

ns = not significant