Ipilimumab is an antibody (proteins that can find and destroy foreign molecules such as those on bacteria and viruses) against CTLA-4 (a molecule that controls a part of the immune system by shutting it down). It is approved by the U.S. Food and Drug Administration (FDA) to treat patients with late-stage melanoma, skin cancer. The use of ipilimumab in patients with CRPC has not been approved by the FDA.