Abstract Background: CD37 is an internalizing transmembrane antigen highly expressed by normal B cells and on most of B-cell malignancies, and represents an interesting therapeutic target for the treatment of B-cell NHL. 177Lu-DOTA-HH1 (Betalutin®) is a novel CD37-targeting antibody radionuclide conjugate in clinical development. It consists of a CD37-binding murine IgG1 antibody HH1 labelled with the short-ranged beta-emitter lutetium-177 (T½ = 6.7 days) chelated to DOTA. 177Lu-DOTA-HH1 is delivered in a ready-to-use formulation. Efficacy and safety data of patients (pts) receiving 177Lu-DOTA-HH1 with HH1 pre-dosing, as well as new efficacy and safety data from pts receiving 177Lu-DOTA-HH1 without HH1 pre-dosing will be presented. Methods: Pts with relapsed incurable CD37 positive NHL of follicular grade I-IIIA, marginal zone, mantle cell, lymphoplasmacytic and small lymphocytic subtypes and with platelet counts ≥ 150 x109/l were eligible for inclusion in the study. In a 3+3 study design pts received rituximab (375 mg/m2) day 1 and 8 in order to deplete normal B cells. On day 29 pre-dosing with HH1 (50 mg, cold CD37 antibody) was administered before 177Lu-DOTA-HH1 injection (Arm 1). In Arm 2 177Lu-DOTA-HH1 was administered without HH1 pre-dosing on day 29. The starting doses for Arm 1 and 2 were 10 MBq/kg b.w. and 15 MBq/kg b.w, respectively. Pts enrolment has been completed (n=13) in Arm 1 with the dose-limiting toxicity (DLT) observed at 20 MBq/kg bw and a dose expansion cohort is currently open for enrollment at 15 MBq/kg with HH1 pre-dosing. Arm 2 is currently open for enrollment. Tumour response was assessed by FDG PET/CT scans (Cheson 2007), and pts will be followed for 5 years. Results: Arm 1:A total of13 (M/F 11/2) pts, median age 68 years, follicular lymphoma (n=12), and mantle cell lymphoma (n=1) have been enrolled since the study start in December 2012. The range of prior therapies was 1 to 8, where 5 of 13 pts were refractory to rituximab. The most common toxicities observed were hematologic and all DLTs were reversible and manageable. At 20 MBq/kg (n=3) G 3/4 neutropenia and/or thrombocytopenia were observed in all pts and platelet transfusions were required in 2 pts. At 15 MBq/kg (n=6) DLTs were: 1 G 3 thrombocytopenia lasting >14 days and 1 G 4 neutropenia/ thrombocytopenia lasting >7 days. The median time to nadir for platelets and neutrophils was 40 and 49 days, respectively. No pts experienced febrile neutropenia. Serious AEs were reported in 5 pts: at 10 MBq/kg pneumonia (possibly related) and pulmonary embolism (PE) unrelated, in the same pt, with history of PE; thrombocytopenia requiring platelet transfusions (2 pts) and epistaxis in 1 of them (20 MBq/kg), possibly related; transient atrial fibrillation (2 pts) at 15 MBq/kg, possibly related. No secondary malignancies or other long term events have been observed. Best overall tumor response observed across all dose levels were 4 complete and 3 partial remissions, 2 stable disease and 4 progression of disease (one pt had confirmed transformed lymphoma at 3 months). The duration of response (complete and partial remissions) ranged from 6 to more than 21 months. One patient is still in remission after 2 years. The median response duration has not yet been reached. Arm 2: Inclusion in this arm is ongoing. Data on efficacy and safety will be presented and compared with the pts receiving pre-dosing. Conclusions: 177Lu-DOTA-HH1, which is a single dose ready-to-use formulation, has a predictable and manageable safety profile. Most AEs were hematological in nature, all transient and reversible. Promising efficacy and durable responses have been observed. 177Lu-DOTA-HH1 has the potential to be a novel therapy for B-cell malignancies. Disclosures Kolstad: Nordic Nanovector ASA: Membership on an entity's Board of Directors or advisory committees, Research Funding. Bolstad:Nordic Nanovector ASA: Employment. Bruland:Nordic Nanovector ASA: Equity Ownership. Dahle:Nordic Nanovector ASA: Employment, Equity Ownership. Hartvig Larsen:Nordic Nanovector ASA: Equity Ownership, Membership on an entity's Board of Directors or advisory committees.