Abstract IA13: Preclinical development of a humanized anti-CD47 antibody targeting AML stem cells

Human acute myeloid leukemia (AML) is organized as a cellular hierarchy initiated and maintained by rare self-renewing leukemia stem cells (LSC), which must be eliminated in order to cure the patient. We originally identified increased expression of CD47 on human AML LSC compared to their normal counterparts. CD47 is a widely expressed cell surface protein that functions as a regulator of phagocytosis mediated by cells of the innate immune system, such as macrophages and dendritic cells. CD47 serves as the ligand for a receptor on these innate immune cells, SIRP-alpha, which in turn delivers an inhibitory signal for phagocytosis. In this way, CD47 essentially functions as a “don't eat me” signal. We previously found increased expression of CD47 on primary human acute myeloid leukemia (AML) stem cells, and demonstrated that blocking monoclonal antibodies directed against CD47 enabled the phagocytosis and elimination of AML, non-Hodgkin's lymphoma (NHL), and many solid tumors in xenograft models. These results established CD47 as a promising clinical therapeutic antibody target for human AML and other cancers. Towards this goal, we have now developed a novel humanized anti-CD47 antibody with potent efficacy and favorable toxicokinetic properties as a candidate human therapeutic. A novel monoclonal anti-human CD47 antibody, 5F9, was generated, and antibody humanization was carried out by grafting its complementarity determining regions (CDRs) onto a human IgG4 format. The resulting humanized 5F9 antibody (Hu5F9-G4) bound monomeric human CD47 with an 8 nM affinity. Hu5F9-G4 induced potent macrophage-mediated phagocytosis of primary human AML cells in vitro and completely eradicated human AML in vivo, leading to long-term disease-free survival of patient-derived xenografts. Moreover, Hu5F9-G4 synergized with rituximab to eliminate NHL engraftment and cure xenografted mice. Finally, toxicokinetic studies in non-human primates showed that Hu5F9-G4 could be safely administered intravenously at doses able to achieve potentially therapeutic serum levels. Thus, Hu5F9-G4 is actively being developed for clinical trials in human cancers including AML. Citation Format: Ravi Majeti. Preclinical development of a humanized anti-CD47 antibody targeting AML stem cells. [abstract]. In: Proceedings of the AACR Special Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(17 Suppl):Abstract nr IA13.