Background: Follicular lymphoma (FL) is generally considered incurable with patients (pts) often experiencing multiple relapses requiring varying lines of subsequent treatments. Abexinostat (Abx) is a novel potent oral pan- Histone Deacetylase Inhibitor (HDACi) with a pharmacokinetic profile that allows maintenance of sufficient drug concentrations for anti-tumor activity with twice daily (BID) dosing. In prior phase 1/2 studies, Abx was shown to be well-tolerated with significant clinical activity and durable responses in patients with Relapsed/Refractory (R/R) FL. Methods: This open label, single arm study is being conducted to assess the efficacy and safety of Abx in pts with R/R FL. Adults with histologically confirmed grade 1, 2 or 3a FL who have previously received at least 2 lines of therapies, and ECOG PS of 0-2 are being recruited. Abx is administered orally at 80 mg BID 4 hours apart in “one week on, one week off” schedule (Days 1 to 7 & 15 to 21 of a 28-day cycle). Pts undergo efficacy assessment by enhanced CT/MRI every 8 weeks for the first 24 weeks, and every 12 weeks thereafter, and PET-CT at weeks 12 and 24 and to confirm a complete response, in accordance with the Lugano 2014 criteria. The primary endpoint is overall response rate (ORR) assessed by independent review committe (IRC), defined as the % of pts who achieve complete response (CR) or partial response (PR). Secondary endpoints include duration of response (DOR), progression-free survival (PFS), overall survival (OS), and safety. In a planned interim futility analysis conducted with the first 37 pts, if <12/37 (32%) responded (CR or PR), the study was to be terminated. Results: Between June 17, 2020 and Jan 28, 2022, 41 pts received Abx. 37 pts underwent at least one post baseline tumor assessment. Median age was 55 (range 34 – 79), 46% of pts were male, 70% had stage IV disease and 24% had >3 FLIPI-2. Pts had a median of 3 prior lines of therapy (range 2-6), and 22% were refractory to the last prior treatment. As of the data cutoff date on Jan 31, 2022, of 37 pts evaluable for efficacy, the ORR was 70% (26/37 pts), 16% CR (6/37) and the disease control rate was 92% (34/37). The median time to response was 10.8 weeks. The study has met the pre-defined stage I criteria and has entered stage II, aiming to enroll up to 81 evaluable pts. Of 41 pts evaluable for safety, the most common treatment emergent adverse events (TEAEs) (≥ 30%) were thrombocytopenia (85%), diarrhea (61%), neutropenia (54%), leukopenia (49%), asthenia (39%), nausea (37%), and anemia (34%). Grade ≥ 3 TEAEs (≥ 5%) included thrombocytopenia (41%), neutropenia (27%), leukopenia (7%), lymphopenia (7%), prolonged QT (7%), and anemia (5%). One pt discontinued treatment due to AEs. Conclusions: Oral Abx demonstrated promising efficacy and was well tolerated in patients with R/R FL. Clinical trial information: NCT03934567.