People in this study must: - Be men who are 18 years old or older. - Have a serious kind of prostate cancer called metastatic castration-resistant prostate cancer (mCRPC), which means the cancer has spread and does not respond to hormone treatment. - Have cancer that has spread to bones or soft tissues, shown by special scans like bone scans, CT scans, or MRIs. - Be willing to have a small sample taken from the cancer area before and during treatment, if it is safe. - Have already tried a kind of medicine called ARPI (like abiraterone or enzalutamide) and the cancer kept growing. - Be on hormone treatment for at least 6 months or have had surgery to remove both testicles, with very low testosterone levels. - Have had chemotherapy before for prostate cancer. - Have signs that the cancer is getting worse. - If joining a special part of the study, be able to eat a high-fat meal and not eat for 12 hours before.