Effectiveness and Safety of Radium‐223 for Bone‐Metastatic Castration‐Resistant Prostate Cancer: The KYUCOG ‐1901 Study

ABSTRACT Radium‐223 dichloride (Ra‐223) improves survival in bone‐metastatic castration‐resistant prostate cancer (mCRPC). However, prospective real‐world data are limited, particularly regarding treatment outcomes, predictors of completing six cycles, and integration with subsequent therapies. The KYUCOG‐1901 study was a prospective multicenter observational study at 19 Japanese institutions. Patients with mCRPC and ≥ 2 bone metastases received up to six cycles of Ra‐223. Effectiveness was assessed by PSA, alkaline phosphatase (ALP), time to visceral metastasis, time to cytotoxic chemotherapy, radiographic progression‐free survival (PFS), and overall survival (OS). Safety was evaluated using CTCAE v5.0. Of 101 enrolled, 93 patients were analyzed. Median follow‐up was 25.2 months. Early discontinuation was associated with high baseline PSA, ALP, LDH, and symptomatic disease. Subsequent therapies, including taxanes and androgen receptor signaling inhibitors (ARSIs), were administered in most patients. Maximum PSA and ALP declines of ≥ 30% were achieved in 16 (17.2%) and 39 (41.9%) patients, respectively. Median time to visceral metastasis, time to cytotoxic chemotherapy, radiographic PFS (rPFS), and OS were 32.9, 13.7, 8.8, and 23.0 months, respectively. Grade ≥ 3 adverse events occurred in 36.5%. No treatment‐related deaths were reported. Ra‐223 was effective and well tolerated in Japanese mCRPC patients. Early initiation in less symptomatic patients with lower disease burden may maximize benefit, and integration with subsequent therapies appears feasible. Trial Registration: University Hospital Medical Information Network Clinical Trials Registry UMIN000040358