We present the case of a 66-year-old man with metastatic prostate cancer (adenocarcinoma with a Gleason score of 4 + 5, initial prostate-specific antigen [PSA] level of 267 ng/mL, clinical stage T3bN0M1b) who was initially treated with doublet therapy, including apalutamide and leuprorelin, leading to a reduction in circulating levels of PSA < 0.001 ng/mL. Here, KLK3 is linked to metastatic prostate carcinoma.