Selection and Characterization of FD164, a High-Affinity Signal Regulatory Protein α Variant with Balanced Safety and Effectiveness, from a Targeted Epitope Mammalian Cell-Displayed Antibody Library

Phagocytic resistance plays a key role in tumor-mediated immune escape, so phagocytosis immune checkpoints are a potential target for cancer immunotherapy. CD47 is one of the important phagocytosis immune checkpoints; thus, blocking the interaction between CD47 and signal regulatory protein alpha (SIRP alpha) may provide new options for cancer treatment. Using computer-aided targeted epitope mammalian cell-displayed antibody library, we screened and obtained an engineered SIRP alpha variant fragment crystallizable fusion protein, FD164, with higher CD47-binding activity than wild-type SIRP alpha. Compared with wild-type SIRP alpha, FD164 has approximately 3-fold higher affinity for binding to CD47, which further enhanced its phagocytic effect in vitro and tumor suppressor activity in vivo. FD164 maintains the similar antitumor activity of the clinical research drug Hu5F9 in the mouse xenograft model. Furthermore, FD164 combined with rituximab can significantly improve the effect of single-agent therapy. On the other hand, compared with Hu5F9, FD164 does not cause hemagglutination, and its ability to bind to red blood cells or white blood cells is weaker at the same concentration. Finally, it was confirmed by computer structure prediction and alanine scanning experiments that the N-45, E-47, (TEVYVK58)-T-52, K-60, (EVTELTRE122)-E-115, and E-124 residues of CD47 are important for SIRP alpha or FD164 recognition. Briefly, we obtained a high-affinity SIRP alpha variant FD164 with balanced safety and effectiveness. SIGNIFICANCE STATEMENT Up to now, few clinically marketed drugs targeting CD47 have been determined to be effective and safe. FD164, a potential signal regulatory protein a variant fragment crystallizable protein with balanced safety and effectiveness, could provide a reference for the development of antitumor drugs.

Automated summary

Phagocytic resistance is crucial in tumor immune escape, with CD47 being a key immune checkpoint. In this study, an engineered SIRP alpha variant Fc fusion protein, FD164, with higher CD47-binding affinity, was developed, showing promising potential for cancer treatment with improved phagocytic effects and tumor suppression activities. The discovery of FD164 provides a reference for the development of effective and safe anti-tumor drugs targeting CD47.