ALX148 Is a High Affinity Sirpα Fusion Protein That Blocks CD47, Enhances the Activity of Anti-Cancer Antibodies and Checkpoint Inhibitors, and Has a Favorable Safety Profile in Preclinical Models

CD47 is a widely expressed cell surface protein thought to function as an immune checkpoint in cancer. CD47 binds SIRPα on myeloid cells, suppressing tumor cell phagocytosis and other innate immune functions. CD47-SIRPα signaling has also been implicated in the suppression of adaptive antitumor responses, but the relevant cellular functions have yet to be elucidated. Abundant CD47 expression has been observed in tumors and may aid tumor cells in the evasion of immune surveillance.

Therapeutic blockade of the CD47 pathway may stimulate antitumor immunity and improve cancer therapy. To this end, the novel CD47-blocking molecule ALX148 was generated by fusing a modified SIRPα N-terminal D1 domain to an inactive Fc. Surface plasmon resonance analysis demonstrated that ALX148 binds human and murine CD47 with high affinity, with respective K_Ds of 0.14 nM and 12 nM. ALX148 also bound multiple CD47-expressing cell populations in flow cytometry assays. ALX148 blocks the interaction between CD47 and wild type SIRPα, enhancing antibody-dependent phagocytosis of hematologic tumor cell lines by daratumumab, rituximab, and obinutuzumab.

ALX148 increased the efficacy of antibodies targeting hematologic tumors, such as rituximab and obinutuzumab, in xenograft models of hematologic malignancies. Because ALX148 also binds murine CD47 with high affinity, syngeneic tumor models were used to examine the activity of ALX148 in the context of an intact immune system. ALX148 enhanced the antitumor activities of checkpoint inhibitors. Specifically, the immune response induced by ALX148 bridged innate and adaptive immune responses. ALX148 stimulated antitumor properties of innate immune cells, promoting dendritic cell activation and a shift of tumor associated macrophages toward a proinflammatory phenotype. ALX148 in combination with checkpoint inhibitors caused increased cytotoxic T cell infiltrates, with greater CD8 to Treg ratios, and greater amounts of interferon gamma production by T cells in both tumors and the spleen. These findings reveal multiple mechanisms by which adaptive immunity is stimulated by CD47 blockade.

Notably, ALX148 had no adverse effect on CD47-expressing red blood cells and platelets. In vitro, ALX148 did not induce erythrocyte hemagglutination. In non-human primate toxicology studies, ALX148 multi-dose administration was well tolerated and did not affect levels of circulating red blood cells, white blood cells, or platelets. This is in contrast to other published CD47-blocking molecules, which have caused anemia and thrombocytopenia in preclinical and clinical studies. Thus, ALX148 may have minimal hematologic toxicity in humans.

ALX148 enhances adaptive and innate immune responses, demonstrates enhanced preclinical anti-tumor activity in combination with multiple standard of care cancer therapies, and is well tolerated. It is currently being investigated in a first-in-patient, phase 1 dose escalation/expansion, multi-center study in patients with advanced solid tumors and lymphoma. (NCT03013218).