Background: Ra-223, a bone-targeted alpha therapy, is a well-tolerated treatment option that prolongs survival in patients (pts) with symptomatic mCRPC to bone. Docetaxel targets microtubule trafficking improving survival in the mCRPC and metastatic hormone-sensitive settings. We hypothesized that simultaneously targeting the tumor and bone compartment yields superior outcomes than targeting either alone. We previously determined the dose and schedule of co-administering Ra-223 + docetaxel in a randomized phase I/IIa trial. The combination appeared to have superior declines in prostate specific antigen (PSA) and bone markers, delayed PSA progression, and was better tolerated (with adjusted dose/schedule) relative to standard docetaxel alone. We are now conducting a phase III study.