Study Evaluating the Addition of Pembrolizumab to Radium-223 in mCRPC

This research study is a Phase II clinical trial. Phase II clinical trials test the safety and effectiveness of an investigational intervention to learn whether the intervention works in treating a specific disease. "Investigational" means that the intervention is being studied. The FDA (the U.S. Food and Drug Administration) has approved radium-223 by itself as a treatment option for the participant's disease. The FDA has not approved pembrolizumab for the participant's specific disease, but it has been approved for other uses, such as a type of skin cancer called melanoma. In this research study, the investigators are evaluating the immune response, safety and tolerability of the combination of pembrolizumab (a type of immunotherapy drug) plus radium-223. Pembrolizumab works to block the PD-1 pathway, which plays an important role in lessening the activity of one's immune system to fight cancer. Pembrolizumab is therefore referred to as a PD-1 inhibitor, and acts by stimulating the patient's T cells, which are important immune cells, to attack tumors and treat cancer. Radium-223 targets cancer that exists in the bone directly. Radium-223 binds to minerals in the bone to deliver radiation directly to the cancer that has spread to the bones while limiting damage to the surrounding body tissues. Part of this study is to look at whether the investigators may more effectively control the participant's prostate cancer by combining these drugs. Radium-223 may kill cancer cells and release proteins specific to the tumor. The participant's immune system can then use those proteins to teach the T cells what the cancer looks like, and identify it for attack. Pembrolizumab can increase the number and activity of these immune cells, building a T cell "army" specialized to recognize and attack the cancer. The way these two drugs work on the cancer and the immune system may result in better control of the tumor than just radium-223 alone but needs to be investigated.