LAUSR

e16541 Background: PSA is a widely used biomarker for monitoring outcome in mCRPC. Current treatment guidelines do not consider PSA progression before at least 12 weeks of treatment, and recommend treatment continuation in pts experiencing progression by PSA only, without radiographic or clinicall progression. We aimed to evaluate the prognostic value of a PSA progression in mCRPC pts treated with first-line therapy. Methods: We analyzed the value of a PSA progression (PSAProg) at cycle 5 in pts treated in the COU-AA-302 trial. PSAProg was defined as an increase ≥ 25% and ≥ 2 ng/mL from baseline, confirmed by a second reading. Radiographic progression (RadProg) was defined as per PCWG2 criteria for bone scan or RECIST progression. Survival and radiographic progression-free survival (rPFS) from the time of PSAProg was calculated using Kaplan-Meier estimates. Cox-regression models were used to evaluate the impact of PSAProg and treatment arm on survival. Results: 1088 pts were randomized in the COU-AA-302 trial. 908 pts (83.5%) had valid baseline and cycle 5 PSA values. Of these, 222 (24.4%) pts experienced PSAProg, which was confirmed in 195 (21.5%) pts; 45/479 (9.4%) of abiraterone and 150/429 (35%) of placebo-treated pts. A confirmed PSAProg was associated with shorter survival (37.8 vs 26m; HR: 1.8; p < 0.001) and rPFS (13.9 vs 5.6m; HR:2.1; p < 0.001). Median survival from the time of PSAProg was 22.2m (95%CI:18.9-25.4). Abiraterone-treated pts had a significantly longer survival from the time of PSAProg (23.1 vs 17.4m; HR: 0.64; p = 0.018). 111 pts (56,9%) experienced PSAprog without RadProg; in pts with PSAprog only, median time to RadProg was 7.4m (95%CI:7.1-12.9). No differences in rPFS from the time of PSAprog were observed in abiraterone vs placebo-treated pts (HR: 0.99; p = 0.963). Conclusions: 9.4% of abiraterone-treated pts in the COU-AA-302 trial experienced PSAProg at 12 weeks (cycle 5 day 1). PSAProg was associated with significantly worse survival. Abiraterone increased survival from the time of PSAProg, which supports its continued use in pts with PSA progression only at 12 weeks. This study was carried out under YODA Project #2018-3011.