7535 Background: Follicular lymphoma (FL) is the most common indolent non-Hodgkin lymphoma (iNHL) subtype, yet treatment options in the relapsed/refractory (r/r) setting are limited. Copanlisib is a pan-Class I phosphatidylinositol 3-kinase (PI3K) inhibitor with predominant PI3K-α and PI3K-δ activity. We report results from the FL subset of a large phase II study (n=141) in iNHL patients (pts) (NCT01660451, part B). Methods: A total of 104 pts with indolent FL (grade 1-3a) relapsed/refractory to ≥2 prior lines of treatment were treated with copanlisib (60 mg IV infusion) administered on days 1, 8 and 15 of a 28-day cycle. The primary endpoint was objective tumor response rate (ORR) per independent radiologic review (Cheson et al., JCO 20:579, 2007). Results: Of the 104 pts treated, 62% were refractory; median prior lines 3 (range 2-8), median time from progression 8 wks (range 1-73 wks). 52% were male, 83% white, median age 62 yrs, and 62% ECOG 0. At the time of primary analysis the ORR was 58.7%, comprising 15 pts (14.4%) with complete response (CR) and 46 (44.2%) with partial response. Stable disease was observed in 35 (33.7%) pts and progression of disease as best response in 2 pts. The median duration of response was 370 days (range 0-687), with 43 responders censored at data cut-off. Median duration of treatment was 22 wks (range 1-105); 33 (32%) pts remained on treatment. For all pts, the most common treatment-emergent AEs occurring in >25% of pts included (all grade/grade 3+): diarrhea (34%/5%), reduced neutrophil count (30%/24%), fatigue (30%/2%), and fever (25%/4%). Hyperglycemia (50%/41%) and hypertension (30%/24%) were transient. The incidence of pneumonitis (8%/1.4%), hepatic enzymopathy (AST 28%/1.4%; ALT 23%/1.4%), opportunistic infection (1.4%) and colitis (0.7%) were low. Six deaths were observed, 3 of which were attributed to copanlisib: one lung infection, one respiratory failure, and one thromboembolic event. Conclusions: Copanlisib was highly active as a single agent in heavily pretreated r/r FL pts and resulted in durable responses in the majority of pts. Toxicities were manageable, with a low incidence of severe AEs associated with other PI3K inhibitors, especially hepatic enzymopathy, opportunistic infections, and colitis. Clinical trial information: NCT01660451.