Background: TASQ is an oral immunomodulatory, antiangiogenic and antimetastatic agent that targets the tumor microenvironment. In a phase 2 trial in mCRPC, TASQ increased PFS in chemotherapy-naive pts vs placebo (PBO) (Pili et al. JCO 2011), but there was no OS benefit in a recent phase 3 trial (NCT01234311). This study assessed whether TASQ maintenance therapy improved disease control duration in mCRPC pts with response or stabilization on first-line docetaxel (NCT01732549). Methods: Pts with mCRPC not progressing after docetaxel (min. 6 cycles; RECIST criteria; no PSA rise for last 3 tests) were assigned (1:1) to receive within 6 wks of last docetaxel administration: oral TASQ qd (0.25 mg/d rising to 1.0 mg/d over 4 wks, if tolerated) or PBO, until progression or toxicity. Randomization was stratified by visceral metastases and opioid analgesic use. Primary endpoint: radiographic PFS (rPFS) per RECIST 1.1 and PCWG 2 criteria, assessed locally. Planned sample size: 140 pts to achieve 80% power to detect an rPFS hazard ratio (HR) of 0.588 with a 1-sided α error of 0.05. Secondary endpoints: other clinical outcome measures, OS and safety. Results: From Jan 2013 to Oct 2014, 144 pts were randomized (TASQ: n = 71; PBO: n = 73) at 44 sites in 11 countries. Baseline characteristics were similar in both groups – median age: 70 yrs; visceral metastases: 23%; ECOG PS 0–1: 97%; opioid use: 15%. Median follow up: 59 wks for TASQ; 53 wks for PBO. Median rPFS (locally assessed) was 31.7 wks [90% CI 24.3–53.7] with TASQ; 22.7 wks [90% CI 16.1–25.9] with PBO (HR 0.6 [90% CI 0.4–0.9]; p = 0.0162). OS data are immature (14 deaths). Median time to next anticancer therapy: 42.3 wks [90% CI 32.0–58.0] for TASQ; 29.0 wks [90% CI 23.1–39.1] for PBO (HR 0.7 [90% CI 0.5–1.0]). TEAEs ( ≥ Grade 3): TASQ (51%); PBO (26%). TEAEs with TASQ (all grades; > 20%): decreased appetite (37%), constipation (32%), fatigue (28%), nausea (27%), arthralgia (25%), asthenia (24%) and back pain (21%). Conclusions: We believe this is the first completed trial of a maintenance strategy with a novel agent in mCRPC, an appealing concept in pts not progressing after docetaxel therapy. Clinical trial information: NCT01732549