Abstract 6357: Generation of a mutant SIRPa fusion protein with highly-improved affinity and favorable safety profile
Authors: Hu Yingying , Fu Yayuan , Liu Lei , Li Wenjing , Xue Liting , Yu Zhiyong , Zhang Yun , Gao Meijuan , Tan Yixin , Wang Fudong , Wu Yadan , Li Jie , Li Zhenzhen , Zhou Feng , Yang Wenqing , Cao Zhuoxiao , Tang Renhong 
Published: June 2023

Background/Purpose: CD47 has been validated to be expressed on various tumor types including acute myeloid leukemia, myelodysplastic syndrome and other hematologic or solid tumors. As primary ‘don’t eat me’ signal, high expression of CD47 on tumor cells interacts with SIRPα negatively regulating the phagocytosis level. However, poor safety profile as well as insufficient combinatory effect remain to be the issues limiting the clinical outcomes of CD47-targeting molecules. Herein, we used rational design strategies to generate SCR9168 with potent affinity improvement, and maintained favorable safety profiles in different animal species, including cynomolgus monkeys.

Procedures: Mutagenesis was introduced to the residues critical for CD47/SIRPα binding interface. Affinity was measured by SPR analysis. In vitro activities were determined by biochemical- or cell-based binding, blocking and antibody-dependent cellular phagocytosis (ADCP) assays. The efficacy in vivo was assessed with the OE19 xenograft model. Pharmacokinetics (PK) and safety were monitored in both mice and cynomolgus monkeys.

Results: The affinity of SCR9168 achieves 178 pM for human CD47 which is nearly 50-fold increase in comparison with wild-type SIRPα. Enhanced affinity has also been confirmed in binding to OE19 or DLD1 cell lines, as well as in blocking human SIRPα binding to Raji cells. SCR9168 remains strong binding potency to monkey, mouse or rat CD47 which allows the PK and safety assessment using these animal species. The combined effects of SCR9168 and multiple therapeutic antibodies targeting tumor-associated antigens (TAAs) have been evaluated with ADCP assays using human monocyte-derived macrophages. SCR9168 markedly increases the ratio of phagocytic cells combining with antibodies, such as cetuximab. However, due to the inert function of Fc fragment, no phagocytosis of red blood cells or platelets was caused after the incubation in vitro. In addition, SCR9168 treatment in combination with trastuzumab leads to significantly improved suppression of tumor growth in a dose-dependent manner. Moreover, administration of two doses at day 1 and day 11 in cynomolgus monkeys results in no toxicity events related to SCR9168 based on the data from hematological analysis.

Conclusion: SCR9168 demonstrates the best-in-class potential among SIRPα mutein molecules. It elicits improved and dose-dependent efficacy in phagocytosis or tumor suppression combining with therapeutic antibodies, such as trastuzumab or cetuximab. Favorable safety profile with no phagocytosis of RBC or platelets in vitro as well as no hematological toxicity observation in cynomolgus monkeys allows broader dose range exploration in early clinical phase. SCR9168 is currently in development stage and IND enabling will be expected in the end of 2023.

Citation Format: Yingying Hu, Yayuan Fu, Lei Liu, Wenjing Li, Liting Xue, Zhiyong Yu, Yun Zhang, Meijuan Gao, Yixin Tan, Fudong Wang, Yadan Wu, Jie Li, Zhenzhen Li, Feng Zhou, Wenqing Yang, Zhuoxiao Cao, Renhong Tang. Generation of a mutant SIRPa fusion protein with highly-improved affinity and favorable safety profile [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6357.