Background: Magrolimab (Hu5F9-G4) is an antibody blocking CD47, a macrophage immune checkpoint and don’t eat me signal on cancers. It induces tumor phagocytosis and eliminates leukemia stem cells. Azacitidine (AZA) synergizes with magrolimab by inducing eat me signals on leukemic cells, enhancing phagocytosis. We report Ph1b data including a potential MDS registration cohort. Methods: Magrolimab+AZA was given to untreated intermediate to very high risk IPSS-R MDS and intensive chemo unfit AML patients. A magrolimab priming/intrapatient dose escalation regimen (1-30 mg/kg QW, Q2W Cycle 3+) was used. AZA was dosed 75mg/m² days 1-7. Efficacy was assessed by IWG 2006 (MDS) and ELN 2017 (AML) criteria. Results: 68 patients (39 MDS, 29 AML) with a median age of 72 were treated with magrolimab+AZA. 19% were intermediate cytogenetic risk with 68% poor risk (13% unknown). 27% were TP53 mutant. The combo was well-tolerated with safety similar to AZA alone. Common treatment-related AEs were anemia (38%), fatigue (21%), neutropenia (19%), thrombocytopenia (18%) and infusion reaction (16%). Treatment-related febrile neutropenia was 1.5%. Only 1 patient (1.5%) discontinued due to an AE. In RBC transfusion dependent patients, 58% of MDS and 64% of AML patients became transfusion independent. 30/33 (91%) efficacy evaluable MDS patients had an objective response (42% CR, 24% marrow CR (4/8 also with HI), 3% PR, 21% HI alone, 9% SD). MDS patient responses deepened on study, with a 56% CR rate in patients with ≥ 6 mo follow-up. In AML, 16/25 (64%) responded (40% CR, 16% CRi, 4% PR, 4% MFLS, 32% SD, 4% PD). In 12 TP53 mutant AML patients, 75% had a CR+CRi (42% CR, 33% CRi, 17% SD, 8% PD). Cytogenetic CR was seen in 35% and 50% of responding MDS and AML patients. 22% of MDS and 50% of AML patients with CR/CRi/marrow CR were MRD negative by flow cytometry. Median duration of response is not reached in either MDS or AML, including TP53 mutant AML, with a median follow-up of 5.8, 8.8 and 9.4 mos, respectively (range: 1.9 – 16.8 mos). 91% of MDS and 100% of AML responding patients are in response at 6 mos. The 6 mo overall survival estimate is 100% in MDS and 91% in TP53 mutant AML patients. Conclusions: Magrolimab is a macrophage targeting immunotherapy that with AZA is well tolerated with durable efficacy in MDS, AML, particularly TP53 mutant, a poor prognostic group. A potential registration single arm MDS cohort is ongoing (NCT03248479). ENHANCE, a randomized Ph3 MDS trial is planned. Additional patients/analyses will be reported. Funded by Forty Seven and CIRM. Clinical trial information: NCT03248479.