A Ph Ib Study of REGN5678 Plus Cemiplimab in Patients With mCRPC

This study is testing REGN5678 (a bispecific antibody that targets both PSMA on prostate cancer cells and CD28 on immune cells) combined with cemiplimab (an immunotherapy drug) in men with metastatic prostate cancer that is no longer responding to hormone therapy. **You may be eligible if...** - You are 18 or older and male - You have metastatic castration-resistant prostate cancer (mCRPC — prostate cancer that has spread and is not responding to standard hormone therapy) - Your cancer has progressed despite prior treatments - You have measurable disease or rising PSA - Your organ function and blood counts are within acceptable ranges **You may NOT be eligible if...** - You have active autoimmune disease - You have significant brain metastases - You have received prior immunotherapy targeting PSMA - You are on chronic immunosuppressive steroids - You have active serious infections Talk to your doctor to see if this trial is right for you.