Preclinical and Clinical Development of Therapeutic Antibodies Targeting Functions of CD47 in the Tumor Microenvironment

Abstract

CD47 is a ubiquitously expressed cell surface glycoprotein that functions as a signaling receptor for thrombospondin-1 and as the counter-receptor for signal regulatory protein-α (SIRPα). Engaging SIRPα on macrophages inhibits phagocytosis, and CD47 thereby serves as a physiological marker of self. However, elevated CD47 expression on some cancer cells also protects tumors from innate immune surveillance and limits adaptive antitumor immunity via inhibitory SIRPα signaling in antigen presenting cells. CD47 also mediates inhibitory thrombospondin-1 signaling in vascular cells, T cells, and NK cells, and blocking inhibitory CD47 signaling on cytotoxic T cells directly increases tumor cell killing. Therefore, CD47 functions as an innate and adaptive immune checkpoint. These findings have led to the development of antibodies and other therapeutic approaches to block CD47 functions in the tumor microenvironment. Preclinical studies in mice demonstrated that blocking CD47 can limit the growth of hematologic malignancies and solid tumors and enhance the efficacy of conventional chemotherapy, radiation therapy, and some targeted cancer therapies. Humanized CD47 antibodies are showing promise in early clinical trials, but side effects related to enhanced phagocytic clearance of circulating blood cells remain a concern. Approaches to circumvent these include antibody preloading strategies, development of antibodies that recognize tumor-specific epitopes of CD47, SIRPα antibodies, and bivalent antibodies that restrict CD47 blockade to specific tumor cells. Preclinical and clinical development of antibodies and related biologics that inhibit CD47/SIRPα signaling are reviewed, including strategies to combine these agents with various conventional and targeted therapeutics to improve patient outcome for various cancers.

Keywords: bifunctional antibodies; humanized CD47 antibodies; immune checkpoint; immunotherapy; signal regulatory protein-α.

Figure 1

Antiphagocytic function of CD47 on red blood cells (RBCs). Young RBCs express ~25 000 copies of CD47, which inhibits their phagocytic clearance [5]. RBC aging [119], diseases that increase RBC rigidity [120], and exposure to function-blocking CD47 antibodies decrease the SIRPα-mediated “don’t eat me” signal and thereby increase erythrophagocytosis.

Figure 2

CD47 functions in the tumor microenvironment. A) CD47 on tumor cells induces inhibitory SIRPα signaling that prevents macrophage phagocytosis and antigen presentation. Thrombospondin-1 induces CD47 signaling in CD8 T cells and NK cells that inhibits lytic tumor cell killing [15, 113]. B) CD47 antibodies that block SIRPα binding relieve the inhibitory signal in macrophages and antigen-presenting cells, enabling pro-phagocytic signals from tumor-secreted calreticulin or tumor-specific antibodies to activate ADCP and ADCC. C) Bispecific CD47 antibodies enhance selective blocking of CD47 on tumor cells and induce ADCP and/orADCC.

Figure 3

Direct effects of CD47 antibodies on tumor cells. Several CD47 antibodies induce signaling that causes cancer cell death (1F7, AD22, CC2C6, and AO-176) or suppress cancer stem cells (B6H12). In contrast, blocking CD47 in nonmalignant cells preserves stem cells and can be cytoprotective [31, 121].

Figure 4

Structures of therapeutic bispecific CD47 antibodies. A) Conventional bispecific antibodies combine Fv regions from two antibodies and are monovalent for CD47 and a tumor-specific or immune cell antigen. B) A tandem bispecific that is bivalent for CD47 and PD-L1 [109]. C) A bispecific antibody containing scFv domains recognizing CD47 and CD20 [108]. D) A bispecific combining rituximab with the CD47 nanobody huNb1 [95].