Background: INCB050465 is a selective PI3Kδ inhibitor with no preclinical hepatotoxicity at clinically relevant doses. We report emerging safety and efficacy data from a phase 1/2 study of INCB050465 in patients (pts) with r/r B-cell malignancies (NCT02018861). Methods: The protocol was initiated with a single patient cohort, treated with INCB050465 5 mg QD PO. Subsequent cohorts used a 3+3 design and evaluated doses of 10–45 mg QD. Based on PK/PD, the 20 and 30 mg QD cohorts were expanded. Responses were assessed Q9W by the Lugano Classification or International Working Group on Chronic Lymphocytic Lymphoma (CLL) criteria. Results: As of the data cutoff (Nov 1, 2016), 52 pts were treated (median age, 65 y [range, 30–88]; baseline tumors: diffuse large B-cell lymphoma [DLBCL], n=14; follicular lymphoma [FL], n=10; Hodgkin lymphoma [HL], n=9; marginal zone lymphoma [MZL], n=8; CLL, n=6; mantle cell lymphoma [MCL], n=5; 62% had >3 prior systemic regimens). Median duration of therapy was 3.3 mo (range, 0.6–13.4); no DLTs were identified. 67% of pts discontinued therapy (disease progression, 31%; AEs, 25%); 33% had dose interruption; 4% had reduction. Most common nonhematologic AEs (all grade [Gr]; Gr ≥3): nausea (38%; 0%), diarrhea (31%; 6%), vomiting (25%; 0%); Gr ≥3 hematologic AEs: neutropenia (21%), lymphopenia (17%), thrombocytopenia (10%), anemia (4%). 40% of pts had serious AEs, most frequently colitis, diarrhea, hypotension (all n=3). 1 pt had Gr 3 pneumonitis; none had Pneumocystis jirovecii pneumonia (PJP) or Gr ≥2 elevated transaminase. Objective responses (ORs) occurred at all doses (Table), except 5 mg QD; 90% were observed at first assessment. Conclusions: INCB050465 demonstrated manageable toxicities with no clinically meaningful transaminitis/PJP. OR rates were generally high, with 90% observed at first assessment. Different dosing regimens/schedules, long-term safety, and disease-specific cohorts are being evaluated. Clinical trial information: NCT02018861.N^aOR, n (%)CR,^b nPR,^b nAll pts4522 (49)1012    NHL3119 (61)109        DLBCL145 (36)32        FL97 (78)25        MZL44 (100)22        MCL4^c3 (75)30    CLL6^c2 (33)02    HL81 (13)01