Background: Positron emission tomography of radiolabeled prostate-specific membrane antigen (PSMA-PET) is more sensitive and specific than conventional imaging for the detection of lesions in patients with BCR prostate cancer. PRIMORDIUM (NCT04557059) includes standardized PSMA-PET imaging and enrolls PSMA-PET positive patients (≥1 locoregional lesion; interventional cohort) and PSMA-PET negative patients (no lesion; observational cohort); all patients are non-metastatic by conventional imaging at screening. Methods: We presented study methods at ESMO 2021 (Poster 649TiP). All patients enrolled in PRIMORDIUM have high-risk BCR (prostate specific antigen [PSA] doubling time ≤12 months or Gleason score ≥8) after RP. PSMA-PET-positive patients are randomized 1:1 to either the control arm (whole pelvic salvage radiotherapy [RT] + 6 months of luteinizing hormone-releasing hormone agonist [LHRHa]) or the interventional arm (RT + LHRHa + apalutamide 240 mg/day for 180 days); stereotactic body RT (SBRT) to ≤3 distant lesions is optional. PSMA-PET-negative patients are managed per routine clinical practice. Results: Key baseline characteristics of 198 enrolled patients at data cutoff (12-Jan-2023) are summarized in the table. Conclusions: The PRIMORDIUM study aims to evaluate the intensification of treatment with apalutamide for patients assessed with PSMA-PET. From 198 enrolled patients with high-risk BCR, positive PSMA-PET was documented in 44% of patients at a median of 39 months after RP and a median PSA of 0.51 ng/mL, while negative PSMA-PET was observed in 56% of patients at a median of 26 months after RP and a median PSA of 0.35 ng/mL. In this analysis, all PSMA-PET-positive patients had locoregional lesion(s) and 10% also had distant lesion(s) on PSMA-PET. Clinical trial information: NCT04557059.

Data are median [25^th; 75^th percentile] or non-missing n (%). *n=87 for PSMA-PET positive; †n=109 for PSMA-PET negative; ‡Includes values ≤0.