A Phase 1/2 study to evaluate the safety and efficacy of ARV-766 given by mouth alone or in combination with abiraterone in men with metastatic prostate cancer.

Inclusion Criteria:

Part A,B,C and D:

• Histological, pathological, or cytological confirmed diagnosis of adenocarcinoma of the prostate.
• Ongoing androgen deprivation therapy (ADT) with a gonadotropin releasing hormone analog or inhibitor, or orchiectomy (surgical or medical castration).
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

Part A:

• Progression on at least 2 prior approved systemic therapies for metastatic prostate cancer (at least one must be a second-generation androgen inhibitor, e.g., abiraterone, enzalutamide, darolutamide, apalutamide).
• Progressive mCRPC

Part B:

• Participants must have received at least one but no more than three prior second generation anti-androgen agents (e.g., enzalutamide or abiraterone).
• Participants must have received no more than two prior chemotherapy regimens.
• Progressive mCRPC

Part C & D:

• Metastatic castration resistant or sensitive prostate cancer with radiographic evidence of metastatic disease

Exclusion Criteria:

Part A and B:

• Known symptomatic brain metastases requiring steroids (above physiologic replacement doses).
• Active inflammatory gastrointestinal disease, chronic diarrhea, known diverticular disease, or previous gastric resection or lap band surgery.
• Radiation therapy within 4 weeks of first dose of study drug or prior irradiation to >25% of the bone marrow.
• Receipt of an investigational drug(s) within 4 weeks prior to anticipated first dose
• Systemic anti-cancer therapy within 2 weeks of first dose of study drug (except agents to maintain castrate status). For bicalutamide, mitomycin C, or nitrosoureas the exclusion period must be 6 weeks and for abiraterone 4 weeks.

Part C and D

• Prior treatment with a second generation NHA