This study tests FG-3246 for men with metastatic castration-resistant prostate cancer (mCRPC). Metastatic means the cancer has spread, and castration-resistant means it continues to grow despite low testosterone levels. FG-3246 is a special kind of medicine called an antibody-drug conjugate (ADC). An ADC is a medicine that links an antibody, which seeks out cancer cells, with another drug to help kill those cells.

Participants must have proven prostate cancer and must have tried a specific type of cancer medicine called an androgen receptor signaling inhibitor (ARSI) before but not taxane chemotherapy. They need to have a tumor sample for testing and normal organ function. They can't join if they've had certain treatments recently or have other specific health issues.

• The study involves multiple visits, including tests and treatments.
• Participants will be monitored for safety and how well the drug works.
• There may be risks from new treatment, such as side effects.