A phase Ia study of safety and clinical activity of atezolizumab (atezo) in patients (pts) with metastatic castration-resistant prostate cancer (mCRPC).

187Background: In the past decade, several therapies have been approved for mCRPC. However, most pts develop resistance and experience disease progression. Thus, there remains a high unmet need. Atezo (anti–PD-L1) blocks the interaction between PD-L1 and its receptors, PD-1 and B7.1, thereby restoring anti-tumor immunity. Atezo has demonstrated clinical efficacy in many tumor types. Here we report the clinical safety and activity of atezo in pts with mCRPC from a Phase Ia study (PCD4989g; NCT01375842). Methods: Eligible pts previously received enzalutamide and/or sipuleucel-T for mCRPC. Pts also had PSA or radiographic progression prior to enrollment. Atezo 1200mg was administered IV q3w. CT and bone scans were performed q6w for 24w and q12w thereafter (q6w if treatment beyond disease progression). Primary objectives were safety and tolerability; OS was an exploratory objective. mCRPC cohort–specific objectives included PSA response, radiographic progression per PCWG2 criteria, soft tissue response per RE...