RADIANT: A Study of Radium-223 Dichloride (Xofigo) vs Enzalutamide or Abiraterone (ARPIs, Androgen Receptor Pathway Inhibitors) in Patients With Prostate Cancer That Has Spread to the Bones (mCRPC, Metastatic Castration-Resistant Prostate Cancer)

Researchers in this study want to compare how well drug radium-223 dichloride (Xofigo) and androgen receptor pathway inhibitor (ARPI) therapy work in participants with prostate gland cancer which has spread to the bone and progressed on or after one line of ARPI therapy. Meanwhile researchers want to compare the safety of radium-223 dichloride and ARPI therapy. Radium-223 dichloride is known as a radioactive drug that is taken up by bones after it is injected into the body. It works by giving off a type of radioactivity that travels a very short distance and kills the tumor cells that have spread to the bone without major effects to the healthy cells. It has been approved in many countries for the treatment of patients with prostate cancer which has spread to the bone. The ARPI drugs used in this study will be either abiraterone acetate (Zytiga) (plus prednisone/prednisolone) or enzalutamide (Xtandi). Both of them are standard approved medications which are used in the treatment of advanced prostate cancer. Participants in this study will receive either Radium-223 dichloride or a ARPI therapy. Radium-223 dichloride will be given as an infusion into one of the veins on Day 1 of each 4-week cycle for a total of up to 6 cycles. Oral ARPI therapy will be given per the standard approved dose once daily until the disease has progressed. Participants will visit the hospital or clinic every 2 weeks for the first 6 cycles, and only on the first day of each cycle from cycle 7 and onwards. Observation for each participant will last for about 2 years in total. Blood and urine samples will be collected from the participants and participants will be asked to complete questionnaires about the well-being and the pain.