Bispecific Antibodies Targeting CD47 and CD19

Introduction of CD47

CD47, a transmembrane protein belonging to the immunoglobulin superfamily, is widely expressed on both normal and malignant cells. Encoded by the CD47 gene, it features a single extracellular IgV domain, a transmembrane domain, and a brief cytoplasmic tail. Serving as a crucial "don't eat me" signal, CD47 binds to its receptor SIRPα on macrophages and phagocytes, inhibiting their phagocytic activity. Beyond this, CD47 plays roles in cell adhesion, migration, proliferation, and survival. Overexpression of CD47 is observed in various cancers, contributing to immune evasion and tumor persistence. Consequently, CD47 emerges as a potential target for cancer immunotherapy.

Introduction of CD19

CD19, a transmembrane protein within the immunoglobulin superfamily, is specifically expressed on B cells and B cell-derived malignancies. Encoded by the CD19 gene, it possesses two extracellular Ig-like domains, a transmembrane domain, and a cytoplasmic tail with multiple tyrosine residues. Functioning as a coreceptor for the B cell receptor (BCR), CD19 forms a complex with CD21, CD81, and CD225, amplifying the BCR signaling pathway. Essential for B cell development, activation, differentiation, and antibody production, CD19 also serves as a well-established marker and target for B cell malignancies, including acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL), and non-Hodgkin lymphoma (NHL).

Signaling Pathways Involved in Bispecific Antibodies Targeting CD47 and CD19

Bispecific antibodies (bsAbs) represent engineered molecules with the unique ability to simultaneously bind to two distinct antigens or epitopes on either the same or different cells. Specifically designed bsAbs targeting CD47 and CD19 aim to disrupt the CD47-SIRPα interaction on tumor cells expressing CD19, promoting enhanced phagocytosis by macrophages and other phagocytes. Additionally, bsAbs targeting CD47 and CD19 may activate various immune effector cells, including natural killer (NK) cells, dendritic cells (DCs), and T cells, through mechanisms such as antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP), or antigen cross-presentation.

When a bsAb binds to CD47 on a tumor cell, it inhibits the binding of SIRPα on a macrophage, disrupting the inhibitory phosphatase SHP-1 activation and suppressing phagocytosis. This disruption allows the macrophage to recognize and engulf the tumor cell via its Fc receptor. Simultaneously, the bsAb binds to CD19 on the tumor cell, amplifying the B cell receptor (BCR) signaling pathway. This amplification increases the expression of pro-apoptotic molecules such as BIM and PUMA, ultimately inducing tumor cell death. Additionally, the bsAb may stimulate the release of cytokines and chemokines from the macrophage, including TNF-α, IL-1β, IL-6, IL-8, IL-10, IL-12, and CCL2. These substances can recruit and activate other immune cells within the tumor microenvironment. The bsAb also facilitates the presentation of tumor antigens by DCs to T cells, promoting T cell activation and proliferation. Furthermore, the bsAb induces ADCC by NK cells through their Fc receptor CD16.

By modulating these signaling pathways, bsAbs targeting CD47 and CD19 have the potential to trigger a robust anti-tumor immune response, overcoming some limitations associated with conventional monoclonal antibodies or other immunotherapeutic agents. However, challenges and risks, such as toxicity to normal cells, the antigen sink effect, immunogenicity, and tumor escape mechanisms, are associated with bsAbs targeting CD47 and CD19. Consequently, more preclinical and clinical studies are imperative to optimize the design, delivery, and combination of bsAbs targeting CD47 and CD19, evaluating their safety and efficacy across different cancer settings.

Clinic Status of Bispecific Antibodies Targeting CD47 and CD19

As of now, no bispecific antibodies (bsAbs) targeting CD47 and CD19 have received clinical approval. However, various bsAbs with this target are in active clinical development by different companies and research institutions. Notably, NI-1701 and TG-1801 are two promising candidates designed to address B cell malignancies by targeting CD47 and CD19. Developed by Novimmune SA and licensed to TG Therapeutics Inc., NI-1701 is a human IgG molecule featuring a high-affinity anti-CD19 arm and a low-affinity anti-CD47 arm. The design aims to selectively block CD47 on CD19-positive B cells, minimizing unintended CD47 blockade on other cell types, such as red blood cells and platelets. With a fully competent IgG1 Fc-domain, NI-1701 facilitates antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP) effector functions. Currently, NI-1701 is undergoing a phase I clinical trial for relapsed or refractory B cell non-Hodgkin lymphoma (NHL). Another human IgG molecule developed by TG Therapeutics Inc. in collaboration with Novimmune SA, TG-1801 shares a similar structure with NI-1701, featuring a high-affinity anti-CD19 arm and a low-affinity anti-CD47 arm. In addition to the shared mechanism of action with NI-1701, TG-1801 incorporates a mutation in the Fc-domain to enhance its binding to the activating Fc receptor CD16A, potentially increasing its ADCC activity. TG-1801 is currently in a phase I/II clinical trial for relapsed or refractory B cell NHL.

Table 1. Comparison of NI-1701 and TG-1801