Background: Abiraterone acetate (AA) improves OS and rPFS in first line mCRPC patients (pts). After AA progression D is commonly used as standard second line therapy. However, the value of maintaining AA in combination with D despite progression has not been tested beyond small exploratory studies (Tagawa ST, Eur Urol 2016) ABIDO is a randomized-phase II trial that evaluates efficacy and safety of D + AA vs D after first-line AA progression in mCRPC. Methods: Asymptomatic or minimally symptomatic mCRPC pts with no visceral metastases, ECOG PS 0-1, and adequate organ functions were included. The study has two stages: In stage I pts receive AA (1000 mg/d + prednisone (P) 10 mg qd) until radiological or unequivocal clinical progression. In stage II pts were randomized to D 75 mg/m2 q3wk in combination with AA 1000 mg/d (arm A) or without AA (arm B) The primary endpoint was rPFS and the secondary endpoints radiological response (RR), OS, PSA-response, PSA-PFS and safety. Results: 88 pts were randomized, (46 arm A, 42 Arm B). Median age was 69 y/o, 43% had ECOG 0 and 91%/11%/5% had bone, liver and lung metastases. Median rPFS was 11.4 months (m) in arm A vs 10.5 m in ARM B; 12-m rPFS was 43% vs 45%; Median PSA PFS was 6.2 vs 5.5 m and median OS was 17.3 vs 16.9 m. Twenty four pts (52%) in arm A and 19 (46%) in arm B achieve ≥50% PSA response. RR was achieved in 15% vs 7% of pts and disease control rate in 74% in both arms. No statistically significant differences were found in efficacy parameters. Half of pts received 10 cycles of D (median 7 and 8). D median dose intensity was 86% and 90% for each arm and 91% for AA. Eleven pts discontinued treatment due to non-hematological toxicity, 5 in arm A and 6 in arm B. Most frequent G3-4 toxicities per arm (A/B) were: neutropenia (57%/29%; P=0.027), febrile neutropenia (17%/10%), diarrhea (9%/7%), and asthenia (11%/10%). Conclusions: ABIDO trial was unable to demonstrate the significant clinical benefit of maintenance AA approach + D after AA first-line therapy. No differences were observed in RR, PSA PFS, rPFS and OS. In AA + D cohort, more frequent and severe hematological toxicity (neutropenia and febrile neutropenia) were reported. Clinical trial information: NCT02036060