Abstract LB171: The differentiated CD47 monoclonal antibody AO-176 exhibits significant in vivo activity against xenograft models of pediatric acute lymphoblastic leukemia (ALL)

Introduction: CD47 is a cell surface marker that interacts with signal regulatory protein alpha (SIRPα) on macrophages and dendritic cells to negatively regulate phagocytosis. This interaction elicits a “don't eat me” signal preventing phagocytosis by immune cells, enabling cell survival. CD47 is expressed on tumor cells, including acute leukemia cells. AO-176 is a highly differentiated monoclonal antibody that targets CD47, exerts potent in vivo activity in mouse xenograft models, has an established safety profile in cynomolgus monkeys, and is being evaluated in clinical trials. Given the broad expression of CD47 in malignant hematopoietic cells, it was of interest for the Pediatric Preclinical Testing Consortium to evaluate AO-176 in vivo against patient-derived xenograft (PDX) models of pediatric ALL. Methods: CD47 mRNA and protein expression were quantified by RNAseq (https://pedcbioportal.org) and flow cytometry (relative fluorescence intensity, RFI), respectively. AO-176 was tested in vivo against 3 T-ALL (ALL-8, ALL-30, ALL-121) and one early T-cell precursor (ETP) ALL (ETP-2) PDXs in NSG mice at 25 mg/kg intraperitoneally once per week for 4 weeks. Engraftment was evaluated by enumerating the proportion of human CD45+ cells in the peripheral blood (%huCD45+) at weekly intervals. Events were defined as the %huCD45+ ≥ 25%. At Day 28 post-treatment initiation or event (whichever occurred first), 3 mice/group were humanely killed to assess leukemia infiltration of hematolymphoid organs. Drug efficacy was assessed by event-free survival (EFS) of treated (T) and control (C) groups by T-C, T/C, and stringent objective response measures (Houghton et al, Pediatr Blood Cancer 49:928-40, 2007). Results: CD47 mRNA expression was significantly higher in a panel of T-lineage ALL PDXs (n=25; mean + SD FPKM, 149 + 57.9) compared with B-ALL PDXs (n=65; 65.3 + 18.7 FPKM) (P