Prostate cancer has the second highest incidence rate and is the fifth leading cause of cancer-related deaths among men worldwide. The purpose of this study is to assess safety, pharmacokinetics, and efficacy of ABBV-969 as a monotherapy.

ABBV-969 is an investigational drug being developed for the treatment of metastatic castration-resistant prostate cancer (mCRPC). There are parts to this study. Participants will receive ABBV-969 as a single agent at different doses. Approximately 230 adult participants will be enrolled in the study across sites worldwide.

In part 1 (dose escalation), ABBV-969 will be intravenously infused in escalating doses as a monotherapy. In part 2, multiple doses will be selected from Part 1 and mCRPC participants will be assigned to one of these doses in a randomized fashion to determine the recommended Phase 2 dose. The estimated duration of the study is up to 3 years.

There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic and may require frequent medical assessments, blood tests, and scans.

Inclusion Criteria:

• Histological, pathological, and/or cytological confirmation of adenocarcinoma of the prostate.
• Estimated life expectancy > 6 months.
• An Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
• Must have progressed on prior novel hormonal agents (NHAs) (e.g., abiraterone acetate and/or enzalutamide) for the treatment of metastatic prostate cancer and/or castration-resistant prostate cancer (CRPC). Determination of progression is done per local investigator according to Response Evaluation Criteria in Solid Tumors (RECIST), version 1.1 and/or Prostate Cancer Working Group 3 (PCWG3).
• Serum testosterone levels <= 50 ng/dL (<= 1.73 nmol/L) within the screening period and prior to the first dose of the study drug.
• Must have received at least one NHA (e.g., enzalutamide and/or abiraterone). Additionally, participants must have received at least one taxane for prostate cancer (or are intolerant to, or unable to get access to taxanes).
• Must have >= 1 metastatic lesion that is present on baseline computed tomography (CT), magnetic resonance imaging (MRI), or bone scan imaging obtained <= 28 days prior to beginning study therapy.
• Serum prostate specific antigen (PSA) level >= 1.0 ng/mL.
• Availability of representative baseline tumor tissue (most recent archived tumor tissue after any novel hormonal agent (NHA) and/or any Prostate-Specific Membrane Antigen (PSMA) targeted therapy or fresh biopsy collected during screening if collecting a fresh biopsy at screening is deemed safe in the judgment of the investigator) suitable for immunohistochemistry (IHC) testing.
• Laboratory values meeting the criteria laid out in the protocol.
• QT interval corrected for heart rate (QTc) <= 470 msec (using Fridericia's correction), no >= Grade 3 arrythmia, and no other clinically significant cardiac abnormalities.

Exclusion Criteria:

• Unresolved Grade 2 or higher toxicities related to previous anticancer therapy except alopecia.
• History of other active malignancy, as laid out in the protocol.
• History of interstitial lung disease (ILD) or pneumonitis that required treatment with systemic steroids, nor any evidence of active ILD or pneumonitis on screening chest CT scan.
• History of or active idiopathic pulmonary fibrosis, organizing pneumonia (e.g., bronchiolitis obliterans), drug-induced pneumonitis, or idiopathic pneumonitis.
• History of or active clinically significant, intercurrent lung-specific illnesses including, but not limited to those listed in the protocol.