This clinical trial is for adult men who have metastatic prostate cancer. This trial has multiple arms, or portions. Participants will only take part in one portion of the study.
   
The purpose of the first portion of this study is to assess the safety and tolerability (good and bad effects) and how prostate cancer responds to two different doses of the investigational study drug ARV-766 in men with metastatic prostate cancer who have progressed on prior approved systemic therapies for disease like theirs.
   
In the second portion of the study, the safety and tolerability of ARV-766 will be assessed in combination with another drug called abiraterone in men with metastatic prostate cancer who have not received prior novel hormonal therapies.
   
ARV-766 is investigational, which means it has not yet been approved by the U.S. Food and Drug Administration (FDA). Abiraterone is an approved systemic therapy for metastatic prostate cancer.
   
Each study treatment cycle lasts 28 days. In the first portion of the study, participants will be taking a certain dose of ARV-766 by mouth once each day with food. Treatment cycles will continue as long as the participant's disease does not progress and cause them worsening symptoms and they do not have unacceptable side effects.
   
In the second portion of the study, study treatment cycles also last 28 days. Participants will take ARV-766 by mouth once each day with food. Participants will take the abiraterone on an empty stomach with water. Participants will also take a corticosteroid with the abiraterone and ADT. ADT (LHRH agonist/antagonist) stops the body from producing testosterone and is given as regular shots: once a month, once every 3, 4, or 6 months or once per year, depending on what the participant and their study doctor choose. This will continue as long as the participant's disease does not progress and cause them worsening symptoms and they do not have unacceptable side effects to the study treatment. If the participant has been on combination study treatment for 12 months and their cancer is responding with no detectable disease, their study doctor may decide to stop ARV-766 and continue with abiraterone, corticosteroid, and ADT.