Background: Six months of a GnRH agonist with SRT is a standard of care for patients with unfavorable features and a detectable PSA post-RP. FORMULA-509 was designed to evaluate whether adding six months of AAP and Apa to this regimen could improve outcomes. Methods: FORMULA-509 (NCT03141671) is an investigator-initiated, multi-center, open-label, randomized trial. Patients had PSA≥0.1 post-RP and one or more unfavorable features (Gleason 8-10, PSA>0.5, pT3/T4, pN1 or radiographic N1, PSA doubling time <10 months, negative margins, persistent PSA, gross local/regional disease, or Decipher High Risk). All patients received SRT plus 6 months of GnRH agonist and randomization was to concurrent bicalutamide 50 mg or AAP 1000mg/5mg + Apa 240mg QD. Radiation to pelvic nodes was required for pN1 and optional for pN0. The primary endpoint was PSA progression-free survival (PFS) and secondary endpoint was metastasis-free survival (MFS) determined by conventional imaging. The study was powered to detect a HR of 0.50 for PFS and a HR of 0.30 for MFS, each with 80% power and one-sided type I error of 0.05. Stratification was by PSA at study entry (>0.5 vs.≤0.5) and pN0 vs pN1. Analyses within these subgroups were pre-planned. Results: 345 participants (332 evaluable) from 9 sites were randomized from 11/24/2017 to 3/25/2020 (172 bicalutamide, 173 AAP/Apa). Median follow-up was 34 (6-53) months; 29% were pN1 and 31% had PSA >0.5 ng/mL. The HR for PFS was 0.71 (90% CI 0.49-1.03), stratified one-sided log-rank p=0.06 (3-year PFS was 68.5% bicalutamide vs 74.9% AAP/Apa). The HR for MFS was 0.57 (90% CI 0.33-1.01), stratified one-sided log rank p=0.05 (3-year MFS was 87.2% bicalutamide vs 90.6% AAP/Apa). In a pre-planned analysis by stratification factors, AAP/Apa was significantly superior for patients with PSA >0.5 for PFS [HR 0.50, (90% CI 0.30-0.86), p=0.03 (2-sided); 3-year PFS 46.8% bicalutamide vs. 67.2% AAP/Apa] and for MFS [HR 0.32 (90% CI 0.15-0.72), p=0.01 (2-sided); 3-year MFS 66.1% bicalutamide vs. 84.3% AAP/Apa.] No statistically significant benefit was detected in pre-planned analyses of stratification subgroups defined by PSA≤0.5, pN0, or pN1. Adverse events were consistent with the known safety profiles of the agents being studied, with more rash and hypertension in the AAP/Apa arm. Conclusions: Although this primary analysis did not meet the pre-specified threshold for statistical significance, it does strongly suggest that the addition of AAP/Apa to SRT+6 months of ADT may improve PFS and MFS, particularly in the subgroup of patients with PSA>0.5 where a pre-planned subgroup analysis by stratification factors observed a statistically significant benefit for both PFS and MFS. Clinical trial information: NCT03141671.