Background: Anti-CD20 mAb therapy has provided survival advantage to patients (pts) with CLL; but pts invariably relapse after anti-CD20 therapy and new approaches are needed. The CLL cells of most pts express CD19, which are upregulated following anti-CD20 therapy. MEDI-551, an affinity-optimized anti-CD19 antibody, destroys malignant cells by Ab-dependent cellular cytotoxicity (ADCC) once bound to CD19. Methods: The activity and toxicity of single-agent MEDI-551 in CLL pts with prior rituximab administration was assessed in a phase 1/2, open-label, dose-escalation and expansion study (NCT00983619). Response was assessed using the 2008 Intl Working Group criteria. B-cell depletion was assessed with flow cytometry and confirmed with biomarker analyses (BAFF). Safety assessments included laboratory parameters and adverse events (AEs and serious AEs [SAEs]). Results: Of 91 pts with refractory B-cell malignancies included in the study, 26 had CLL. CLL pts had received a median of 6 prior therapies: 89% with chemotherapy, 27% with single-agent biologics. Within 3 cycles of MEDI-551 (3 mos), >60% of assessable pts achieved CD20⁺ B-cell depletion to <20 cell/uL. Decreases in circulating CD20⁺ and CD22⁺B cells were associated with concomitant increases in serum BAFF concentrations. Of 20 pts evaluable for response, 4 achieved partial response and 13 had stable disease. Commonly reported AEs were generally grade 1/2 and included infusion reactions (62%), nausea (23%), pyrexia (23%), and neutropenia (23%). Six SAEs were noted in 3 pts: 1 had infusion reaction and general health deterioration, another had subarachnoid hemorrhage (SAH), and a third had dyspnea, pyrexia, and back pain. Only infusion reaction was considered treatment related. Two treatment-unrelated events of general health deterioration and SAH resulted in death. Conclusions: Single-agent activity with a manageable toxicity profile was seen in CLL pts treated in this phase 1/2 study of MEDI-551. An ongoing phase 2 study of MEDI-551 in combination with bendamustine in relapsed CLL patients (NCT01466153) is evaluating clinical response to MEDI-551 and chemotherapy. This study was sponsored by MedImmune, LLC.