Abstract 1645: Cell-based reporter bioassays for the development of Fc-functional and Fc-silent SIRPα/CD47 checkpoint inhibitors

Abstract CD47, a membrane glycoprotein commonly overexpressed in human cancers, interacts with its cognate receptor SIRPα on myeloid cells to deliver a “don't eat me” signal that inhibits phagocytosis. This SIRPα/CD47 interaction has emerged as a critical and potentially druggable immune checkpoint. Blockade of SIRPα/CD47 interaction promotes phagocytosis of tumor cells in vitro, and SIRPα/CD47 inhibitors exhibit anti-tumor activity, including synergistic activity with other tumor-targeted mAbs (e.g. rituximab), in vivo. These encouraging pre-clinical data have sparked widespread development of SIRPα/CD47 inhibitors and driven a need for robust functional assays to measure the activity of these therapeutic candidates. Biological activity of SIRPα/CD47 inhibitors is typically assessed via in vitro phagocytosis assays based on imaging or flow cytometry. These low-throughput assays often use primary monocyte-derived macrophages and are inherently prone to donor variability. Additionally, many SIRPα/CD47 inhibitors under development are designed to minimize Fc function to enhance safety and are thus incapable of driving phagocytosis directly. To overcome these limitations, we have developed a pair of reporter-based bioassays for measuring the biological activity of Fc-functional and Fc-silent SIRPα/CD47 inhibitors. These assays utilize a SIRPα-positive monocytic effector cell-line that expresses multiple Fc gamma receptors (FcγRs) and encodes a FcγR-responsive NanoLuc luciferase reporter that is inhibited by SIRPα/CD47 interaction upon co-culture of SIRPα effector cells with CD47-positive target cells. Addition of Fc-functional inhibitors simultaneously disrupts the SIRPα/CD47 interaction and engages FcγRs on the SIRPα effector cells, resulting in NanoLuc reporter activation. To enable testing of Fc-silent inhibitors, we have engineered a second CD47 target cell-line which provides a constitutive activating stimulus to SIRPα effector cells that is restricted by SIRPα/CD47 interaction. Addition of Fc-silent SIRPα/CD47 blockers releases inhibition by SIRPα/CD47, resulting in NanoLuc reporter activation. These newly developed reporter bioassays provide a robust, high-throughput platform to facilitate discovery and development of diverse SIRPα/CD47 checkpoint inhibitors. Citation Format: Jonathan Mitchell, Jamison Grailer, Jim Hartnett, Frank Fan, Mei Cong, Zhijie Jey Cheng. Cell-based reporter bioassays for the development of Fc-functional and Fc-silent SIRPα/CD47 checkpoint inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1645.