A Study to Assess Adverse Events, and How Intravenously (IV) Infused ABBV-969 Moves Through the Bodies of Adult Participants With Metastatic Castration-Resistant Prostate Cancer

* 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.

* 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.