A Study of JNJ-78278343, a T-Cell-Redirecting Agent Targeting Human Kallikrein 2 (KLK2), for Advanced Prostate Cancer

Inclusion Criteria:

• Confirmed adenocarcinoma of the prostate which has spread to other body parts

• Part 1: Prior treatment with at least 1 prior novel androgen receptor (AR)-targetedtherapy or chemotherapy

• Measurable or evaluable disease

• Concurrent use of any other anticancer treatment must be discontinued for at least 2weeks before the first dose of study drug

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

• Prior surgical removal of testicles; or, for participants who have not undergonesurgical removal of testicles, must be receiving ongoing androgen deprivationtherapy (ADT) with a gonadotropin releasing hormone analog

Exclusion Criteria:

Disease conditions

• Active central nervous system (CNS) involvement

• Toxicity related to prior anticancer therapy has not adequately recovered

Prior/Concomitant Therapy

• Prior treatment with human kallikrein (KLK) 2-targeted therapy

• Received, or are receiving, medications that suppress the immune system within 3days prior to the first dose of study drug

• Received or plans to receive any live, attenuated vaccine within 4 weeks prior tothe first dose of study drug

Prior/Concurrent Medical Conditions

• Diagnosis of cancer other than prostate cancer within 2 years prior to the firstdose of study drug

• Solid organ or bone marrow transplantation

• Major clotting diseases within one month prior to the first dose of study drug

• Active autoimmune disease within 12 months prior to the first dose of study drug

• Active infection

• Major diseases of heart and blood vessels within 6 months prior to the first dose ofstudy drug

• Clinically significant lung diseases

• Active or chronic hepatitis B or hepatitis C infection

• Known positive test result for human immunodeficiency virus (unless stable onantiretroviral therapy with undetectable viral load)

• Any serious underlying medical conditions or other issue that would impair theability of the participant to receive or tolerate the planned treatment