Prostate cancer has the second highest incidence rate and is the fifth leading cause of cancer-related deaths among men worldwide. The purpose of this study is to assess safety, pharmacokinetics, and efficacy of ABBV-969 as a monotherapy. ABBV-969 is an investigational drug being developed for the treatment of metastatic castration-resistant prostate cancer (mCRPC). There are parts to this study. Participants will receive ABBV-969 as a single agent at different doses. Approximately 230 adult participants will be enrolled in the study across sites worldwide. In part 1 (dose escalation), ABBV-969 will be intravenously infused in escalating doses as a monotherapy. In part 2, multiple doses will be selected from Part 1 and mCRPC participants will be assigned to one of these doses in a randomized fashion to determine the recommended Phase 2 dose. The estimated duration of the study is up to 3 years. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic and may require frequent medical assessments, blood tests, and scans.