Combining Ipilimumab With Abiraterone Acetate Plus Prednisone in Chemotherapy and Immunotherapy-naïve Patients With Progressive Metastatic Castration-resistant Prostate Cancer

Inclusion Criteria: Chemotherapy- and immunotherapy-naïve patients with progressive metastatic CRPC are eligible. * Age 18 or older, and be willing and able to provide informed consent. * Histologically or cytologically confirmed adenocarcinoma of the prostate at either MSKCC or at the participating site. * Castrate serum testosterone level, ≤ 1.73 nmol/L (50 ng/dL), at the Screening visit. * Ongoing androgen deprivation therapy with a GnRH analogue or bilateral orchiectomy (ie, surgical or medical castration). * Metastatic disease on imaging (e.g., bone scan, CT, MRI). Patients whose disease spread is limited to regional pelvic lymph nodes are not eligible. If lymph node metastasis is the only evidence of metastasis, it must be ≥ 2 cm in diameter. * Progressive disease at study entry defined by PSA and/or radiographic criteria according to the PCWG2. * Karnofsky performance status of ≥80-100, and estimated life expectancy of ≥ 6 months. * Toxicities related to prior therapy must either have returned to ≤ Grade 1 or baseline or been deemed irreversible and in the opinion of the Investigator not worsened. * Able to swallow the study drug and comply with study requirements. Exclusion Criteria: * History of another malignancy within the previous 5 years other than nonmelanomatous skin cancer. * Absolute neutrophil count \< 1,500/μL, or platelet count \< 75,000/μL, or hemoglobin \< 5.6 mmol/L (9 g/dL) at the Screening visit. (NOTE: patients may not have received any growth fa…