Background: Hu5F9-G4 (5F9) is an antibody targeting CD47, a macrophage immune checkpoint and “don’t eat me” signal on cancers. CD47 blockade induces tumor phagocytosis and eliminates leukemia stem cells (LSC) in AML models. Azacitidine (AZA) synergizes with 5F9 by inducing “eat me” signals on AML, enhancing phagocytosis. This trial explored the safety/efficacy of 5F9 alone or with AZA in AML/MDS patients (pts). Methods: This Phase 1b treated: r/r AML/MDS pts with 5F9; and untreated AML (induction chemo ineligible) and higher risk MDS pts with 5F9+AZA. A 5F9 priming/intrapatient dose escalation regimen (1-30 mg/kg weekly) was used to mitigate on target anemia. Results: 10 (6 AML, 4 MDS) r/r pts received 5F9 (median 2 prior therapies (range 1-6). 24 untreated pts (15 AML, 9 MDS) received 5F9+AZA. In total, median age was 73, 62% of AML pts were intermediate or poor cytogenetic risk (38% unknown), all MDS pts were intermediate or high risk by IPSS-R. 5F9 alone or with AZA was well-tolerated with no MTD reached. 5F9 did not potentiate AZA toxicities. Treatment-related AEs ( > 10% of pts) for 5F9+AZA were anemia (25%), thrombocytopenia (20%), and infusion reactions (15%). In 25 efficacy evaluable pts, 8/15 (53%) untreated AML/MDS pts had a CR/CRi to 5F9+AZA (5/10 (50%) in AML, 3/5 (60%) in MDS). 1/10 (10%) r/r AML/MDS pts had a response (MLFS) to 5F9 alone. LSC frequency was reduced/eliminated in most 5F9+aza responders; 50% of responders were MRD negative by flow cytometry. 4/10 (40%) AML pts became RBC transfusion independent and 4/5 (80%) MDS pts had hematologic improvement. Time to response was more rapid (median 1.9 mos) than expected for AZA alone. As of Jan 2019, no responder has relapsed (median follow-up of 3.4 mos (range 1.1 – 6.8 mos). 2 pts had successful allogeneic transplant. Conclusions: 5F9+AZA is a novel immunotherapy blocking a key macrophage checkpoint. It has been well tolerated with robust activity in AML/MDS pts with rapid CRs and MRD negativity. Adding 5F9 to cytotoxic agents may be a promising treatment strategy. An expansion cohort is ongoing. Funded by Forty Seven and the California Institute for Regenerative Medicine. Clinical trial information: NCT03248479