The purpose of this study is to evaluate the safety, pharmacokinetics (PK), and preliminary activity of GDC-1261 in participants with advanced or metastatic prostate cancer. It's also to identify a recommended dose(s) and regimen for GDC-1261 for subsequent studies.

Inclusion Criteria:

• Eastern Cooperative Oncology Group (ECOG) Performance Status of <=1
• Life expectancy is >= 3 months
• Histologically or cytologically confirmed prostate adenocarcinoma
• Disease progression during or following the direct prior line of therapy
• Ongoing androgen deprivation therapy (ADT) with gonadotropin-releasing hormone (GnRH) agonist or antagonist, or have had bilateral orchiectomy
• Metastatic disease
• Adequate end organ function

Exclusion Criteria:

• Treatment with any approved systemic anti-cancer therapy within 14 days or 5 drug elimination half-lives
• Structurally unstable bone lesions suggest an impending fracture
• Untreated central nervous system (CNS) metastases or leptomeningeal disease
• Uncontrolled pain
• History of malignancy within 5 years
• Infection requiring systemic IV antibiotics within 14 days or oral antibiotics within 7 days prior to screening, or any evidence of current infection
• Any medical condition or abnormal clinical laboratory finding that, in the investigator's judgment, would preclude the individual's safe participation in and completion of the study or could affect the interpretation of the results

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing