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e19003 Background: Preclinical data indicate that anti-PD-1 agents can facilitate activity of bispecific T-cell engager (BiTE) molecules. Here we assess the combination of the anti-CD19 BiTE molecule blinatumomab with the anti-PD-1 antibody AMG 404 in adults with R/R ALL (NCT04524455). Methods: Eligible adults with R/R ALL (Ph+ disease included) received 2-5 treatment cycles. Each cycle was 42 days, consisting of 4 weeks of cIV blinatumomab and a 2-week treatment-free interval as per label. In Cohort 1, AMG 404 was dosed IV at 240 mg every 4 weeks (Q4W); first dose was Day (D) 11 of Cycle (C) 1. Primary endpoints were dose-limiting toxicities (DLTs) and other adverse events (AEs). Results: As of 20 Dec 2021, patients (pts) from Cohort 1 (n=8) had median age of 57 (range: 24-73) y, 6/8 male, 6/8 Caucasian, 1 with extramedullary disease (duodenum), 2 with prior blinatumomab, and a median of 5 (2-15) prior treatment lines. Two pts remain on study and 2 completed the study; 4 pts discontinued the study due to death (n=3) or consent withdrawn (n=1). No DLTs were reported for the 3 evaluable pts. Of the 5 pts not evaluable for DLTs, in C1, 4 had disease progression and 1 an unrelated fatal pneumonia. Treatment-related grade (Gr) ≥3 and/or serious AEs in all 8 pts included cytokine release syndrome (CRS) (1 pt Gr 2, 1 pt Gr 1 and 3), Gr 3 increases in ALT and AST, Gr 3 fever, Gr 3-4 neutropenia, Gr 4 neutropenia/Gr 3-4 thrombocytopenia (same pt), Gr 2 sensorimotor polyneuropathy, Gr 3 hypertension, Gr 3 encephalopathy, and in 1 pt Gr 3-4 decreases in white blood cells, lymphocytes, and neutrophils. One pt developed Gr 3 SARS-COV-2 pneumonia on C2D25, resolving without clinical sequelae. C3 start was delayed by 12 days but protocol treatment resumed uneventfully. All 3 DLT-evaluable pts had a complete response (CR) or CR with partial hematologic recovery (CRh), 2/3 without measurable residual disease (MRD), within 2 cycles; the 3rd pt had an MRD response at the end of C3. Preliminary pharmacokinetic results for the combination of blinatumomab and AMG 404 demonstrated that their exposures were consistent with those observed for each as monotherapy and did not indicate any drug-drug interactions. To date, all samples tested for anti-blinatumomab antibodies have been negative. Conclusions: In this ongoing phase 1b study, the combination of blinatumomab with AMG 404 was tolerated with a manageable safety profile. No DLTs were reported. Enrollment continues in Cohort 2 in which AMG 404 is dosed Q4W at 480 mg starting on C1D1, 48 hours prior to blinatumomab. Clinical trial information: NCT04524455. [Table: see text]