Discovery and Preclinical Characterization of CC-95251, an Anti-SIRPα Antibody That Enhances Macrophage-Mediated Phagocytosis of Non-Hodgkin Lymphoma (NHL) Cells When Combined with Rituximab

Introduction: The transmembrane protein CD47 is ubiquitously expressed on normal cells and often overexpressed on cancer cells, including NHL. Binding of CD47 to signal regulatory protein-α (SIRPα) on macrophages induces an anti-phagocytic signal enabling tumor cells to escape the innate immune response. Macrophage antitumor activity can be restored by simultaneously blocking the CD47-SIRPα signaling axis and inducing a pro-phagocytic signal with the use of tumor-opsonizing antibodies. Targeting SIRPα on monocytes and macrophages rather than the ubiquitously expressed CD47 may overcome some toxicities associated with anti-CD47 therapies. Here, we report the discovery and characterization of a fully human anti-SIRPα antibody and its preclinical activity in combination with the opsonizing antibody rituximab in CD20+ diffuse large B-cell lymphoma (DLBCL) cell lines.

Methods: A total of ~ 10 ¹⁰ fully human immunoglobulin G antibodies were screened for binding to the extracellular domain of recombinant human SIRPα using a yeast display platform. Surface plasmon resonance was used to determine CC-95251 binding coverage across SIRPα haplotypes. The ability of CC-95251 to block CD47-SIRPα interaction was measured using Octet ^® and Biacore™ assays. We determined the crystal structure of SIRPα in complex with the CC-95251 Fab to characterize its epitope and to define the structural basis for CD47-SIRPα interaction blockade. To identify tumor types likely susceptible to CD47-SIRPα axis disruption, expression levels of CD47-SIRPα and CD163 were assessed in bulk tumor samples using The Cancer Genome Atlas (TCGA) data. The antitumor effects of CC-95251 in combination with rituximab were examined by measuring the percentage of phagocytic macrophages in co-culture experiments of differentiated macrophages and CD20+ DLBCL cell lines (OCI-Ly3, RIVA, Pfeiffer, and Karpas 422). To confirm CC-95251 binding to monocytes, immunophenotyping of peripheral blood mononuclear cells from healthy donors and cynomolgus monkeys was performed using multiparameter flow cytometry. Lastly, pharmacokinetics and hematologic effects were analyzed in cynomolgus monkeys after treatment with 10, 30, or 100 mg/kg CC-95251.

Results: Initial screening by yeast display yielded ~ 350 candidates. The top 24 clones were characterized fully and CC-95251 was selected as the lead monoclonal antibody exhibiting high binding affinity across the 6 most prevalent SIRPα human haplotypes. CC-95251 potently blocked CD47-SIRPα binding in a dose-dependent manner, with a concentration of 100 nM inhibiting CD47 binding almost completely. Co-crystallization modeling showed that CC-95251 engages SIRPα in a region overlapping the CD47 binding site, demonstrating a mechanism for CD47-SIRPα blockade.

DLBCL was identified as a suitable tumor type for CC-95251 treatment based on CD47-SIRPα expression and macrophage infiltration. Co-culture experiments of donor macrophages and several DLBCL cell lines showed that CC-95251 monotherapy had weak-to-moderate antitumor activity. However, when combined with rituximab, the levels of phagocytic macrophages were markedly increased in a CC-95251 dose-dependent manner, suggesting that inhibition of the CD47-SIRPα anti-phagocytic axis with CC-95251 and activation of pro-phagocytic signaling with rituximab provide an enhanced antitumor effect in DLBCL cell lines.

CC-95251 predominantly bound to cells of myeloid origin, including monocytes and, to a lesser extent, myeloid dendritic cells, whereas no binding to natural killer cells was observed. Toxicology studies in cynomolgus monkeys showed safe intravenous delivery of CC-95251 at therapeutic doses, with no evidence of white blood cell, monocyte, lymphocyte, or red blood cell depletion.

Conclusions: CC-95251 is a novel, high-affinity, fully human monoclonal anti-SIRPα antibody that blocks the binding of CD47 to SIRPα. When combined with the therapeutic opsonizing antibody rituximab, CC-95251 enhances macrophage phagocytic activity against DLBCL cell lines in co-culture models. These results support the clinical evaluation of CC-95251 + rituximab for relapsed or refractory NHL. A phase 1 dose-escalation and -expansion study of CC-95251 for the treatment of advanced solid and hematologic malignancies is underway (NCT03783403).