Randomized, Double-Blind, Phase 3 Trial to Compare the Efficacy of Ipilimumab vs Placebo in Asymptomatic or Minimally Symptomatic Patients with Metastatic Chemotherapy-Naïve Castration Resistant Prostate Cancer

INTERVENTION: Arm A: Ipilimumab ‐ 5 mg/ml solution, Intravenous, 10 mg/kg, Every 3 weeks for up to 4 doses in the Induction Phase. Every 12 weeks in the Maintenance Phase. Up to 24 weeks in the Induction Phase. Treatment in the Maintenance Phase continues until Treatment Stopping Criteria are met, withdrawal of consent, or study closure. Arm B: Placebo ‐ Solution, Intravenous, 0 mg, Every 3 weeks for up to 4 doses in the Induction Phase. Every 12 weeks in the Maintenance Phase. Up to 24 weeks in the Induction Phase. Treatment in the Maintenance Phase continues until Treatment Stopping Criteria are met, withdrawal of consent, or study closure biological/vaccine SP4.026.292.663 CONDITION: Metastatic Chemotherapy‐Naïve Castration Resistant Prostate Cancer ; C61 PRIMARY OUTCOME: To compare overall survival (OS) of subjects, defined as the time from the date of randomization until the date of death. For those subjects who have not died OS will be censored at the last date the subject was known to be alive. Assessed at each study visit while on treatment and every 12 weeks during follow‐up SECONDARY OUTCOME: Compare Progression Free Survival (PFS) by collecting tumor assessments every 12 weeks until protocol defined progression or initiation of subsequent therapy for prostate cancer [Time Frame: each study visit while on treatment, and every 12 weeks during follow up until progression of disease or initiation of subsequent therapy for prostate cancer]. Compare time to pain progression by collection of a Patient Pain Diary prior to each treatment visit and every 12 weeks during follow up until progression of disease or initiation of subsequent therapy for prostate cancer [Time Frame: each study visit while on treatment, and every 12 weeks during follow up until progression of disease or initiation of subsequent therapy for prostate cancer]. Compare time to subsequent non‐hormonal systemic therapy by collection of subsequent prostate cancer therapies during follow up [Time Frame: Every 12 weeks during follow up until initiation of subequent non‐hormonal systemic therapy for prostate cancer]. Characterize Safety Profile by collection of adverse event information and review of laboratory values at every study visit [Time Frame: Continuously throughout study and during follow up until all toxicities have resolved, returned to baseline or been deemed irreversible] INCLUSION CRITERIA: Metastatic prostate cancer. Asymptomatic or minimally symptomatic. Progression during hormonal therapy. ECOG Performance Status 0‐1