Additional Study Information: The purpose of this study is to find out the side effects of JNJ-87189401 given in combination with JNJ-78278343 to patients with metastatic castration-resistant prostate cancer, and to find which doses of the two study drugs when given in combination cause the least side effects. The study will also look at how long JNJ-87189401 and JNJ-78278343 stay in the body, how they act on the body, and how the body responds to them. JNJ-87189401 and JNJ-78278343 have not been approved by the Food and Drug Administration (FDA). This is the first time that JNJ-87189401 will be used in people. JNJ-78278343 has been given to about 90 people with prostate cancer in a different study. It has never been used in combination with JNJ-87189401. This is the first time that the combination will be given to people.