Overview

The purpose of this study is to identify the recommended phase 2 combination dose (RP2CD) of Pasritamig in combination with JNJ-86974680 in Part 1 (Dose finding) of the study and to determine how safe and tolerable the RP2CD is for treatment of participants with advanced prostate cancer in Part 2 (Dose expansion) of study.

Eligibility

Inclusion Criteria:

• Histologically confirmed adenocarcinoma of the prostate. Primary small cell carcinoma, carcinoid tumor, neuroendocrine (NE) carcinoma, or large cell NE carcinoma arising in the prostate are not allowed; however, adenocarcinomas with NE features (for example \[e.g.\], immunohistochemistry \[IHC\] with both androgen receptor \[AR\]- and NE-marker positivity) are allowed
• Metastatic castration-resistant prostate cancer (mCRPC) that is metastatic either to bone, any lymph node, or both without clear evidence of metastasis to visceral organs. Local-regional invasion (rectum, bladder) and bone disease with soft tissue component can be included
• Prior orchiectomy or medical castration (for example, must be receiving ongoing androgen deprivation therapy with a gonadotropin-releasing hormone \[GnRH\] analog \[agonist or antagonist\] prior to the first dose of study drug and must continue this therapy throughout the treatment phase)
• Prostate-specific antigen (PSA) greater than or equal to (\>=) 2 nanograms per milliliters (ng/mL) at screening
• Measurable or evaluable disease
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

Exclusion criteria:

• Toxicity related to prior anticancer therapy that has not returned to grade less than or equal to (\<=) 1 or baseline levels (except for alopecia, neuropathy \[Grade 2\] and vitiligo)
• Known allergies, hypersensitivity, or intolerance to any of the components (for example, excipients) of pasritamig or JNJ-86974680
• Active infection or condition that requires treatment with systemic antibiotics within 7 days prior to the first dose of study treatment. Antibiotic or antiviral prophylaxis is allowed
• Have leptomeningeal disease or brain metastases, except participants with definitively, locally treated brain metastases that are clinically stable and asymptomatic \>2 weeks, and who are off corticosteroid treatment for at least 2 weeks prior to first dose of study treatment
• Any serious underlying medical conditions or other issue that would impair the ability of the participant to receive or tolerate the planned treatment at the investigational site to understand the informed consent, or any condition for which, in the opinion of the investigator, participation would not be in the best interest of the participant or that could prevent, limit, or confound the protocol-specified assessments