As of the March 2019 data cutoff, safety was evaluated in 81 patients. The most common treatment-emergent adverse events (AEs) were pyrexia (83%), cytokine release syndrome (CRS; 57%), chills (54%), infections and infestations (49%), increased C-reactive protein (38%), fatigue (38%), anemia (36%) and thrombocytopenia (30%). Six patients experienced Grade 3 or higher CRS (7%). The incidence and severity of CRS declined through optimized pre-medication, even with REGN1979 dose escalation. Grade 3 or higher AEs that occurred in at least 10% of patients were anemia (21%), lymphopenia (20%), neutropenia (17%), infections and infestations (15%), thrombocytopenia (14%) and hypophosphatemia (11%). Four patients discontinued due to AEs, which included Grade 3 hemolysis, fatigue, pneumonia and neck abscess (n=1 each). Fifty-two patients discontinued, 27 due to disease progression/recurrence, and 10 due to death. Deaths were caused by progressive disease (n=6, one with an AE of Grade 5 multi-organ failure) as well as cardiac arrest, gastric perforation, lung infection and pneumonia (n=1 each).