Background: Lutetium (¹⁷⁷Lu) vipivotide tetraxetan (¹⁷⁷Lu-PSMA-617) plus protocol-permitted standard of care (SoC) significantly improved radiographic progression-free survival (rPFS) and overall survival (OS) in patients with prostate-specific membrane antigen (PSMA)-positive metastatic castration-resistant prostate cancer, previously treated with at least one androgen receptor pathway inhibitor (ARPI) and one or two taxanes. Androgen deprivation therapy (ADT) and ARPIs may alter PSMA expression and radiosensitivity. PSMAddition will assess the efficacy and safety of ¹⁷⁷Lu-PSMA-617 plus SoC versus SoC alone in adults with metastatic hormone-sensitive prostate cancer (mHSPC). Methods: PSMAddition (NCT04720157) is an international, prospective, open-label, randomized, phase 3 trial, with crossover, in adults with mHSPC. Eligible patients are treatment-naive or minimally treated candidates for hormonal therapy, with PSMA-positive disease (determined by ⁶⁸Ga-PSMA-11 PET/CT), ECOG performance status of 0 to 2 and adequate major organ function. Patients are excluded if they have rapidly progressing tumors that require chemotherapy. Approximately 1126 patients will be randomized 1:1 to receive ¹⁷⁷Lu-PSMA-617 (7.4 GBq every 6 weeks for a maximum of 6 cycles) plus SoC or SoC alone (an ARPI and ADT). Stratification factors are tumor volume (high/low), age (≥ 70/ < 70 years), and previous/planned prostatectomy or radiation treatment of the primary prostate tumor (yes/no). Recruitment for a China extension cohort (40–60 patients) will follow after completion of main study recruitment. The primary endpoint is rPFS, as assessed by blinded independent centralized review. Upon centrally confirmed radiographic progression, participants in the SoC-only arm can cross over to the ¹⁷⁷Lu-PSMA-617 arm. The planned sample size provides 95% power to detect a hazard ratio of 0.7 for rPFS after 418 events with an overall one-sided significance level of 0.025. The key secondary endpoint is OS, others include the proportion of patients with a prostate-specific antigen (PSA) decline of ≥ 90% from baseline, time to development of metastatic castration-resistant prostate cancer, composite PFS (radiographic, clinical or PSA progression), safety and tolerability, and health-related quality of life. Imaging and biospecimen (tissue and blood) collection is embedded for correlative analysis. Participants who have received ¹⁷⁷Lu-PSMA-617will have the option to enroll in a separate long-term safety follow-up study. Updated from previous presentation at the 2021 European Society for Medical Oncology Congress, FPN 3035, with permission. Clinical trial information: NCT04720157.