The main purpose of this study is to see if ONCT-534 is safe and useful for treating people with R/R mCRPC, when other treatments are not working. ONCT-534 has not been approved by the Food and Drug Administration (FDA). This is a first in human study. We will ask you to have blood drawn, biopsy, physical exam, and tumor imaging such as CT scan, MRI, and bone scan. You may be in this study up to 2 years. Are you Eligible? (Inclusion Criteria) Must be 18 years old or older - No history of seizure disorder(s) - No major surgery within 30 days prior to start of study drug