Bispecific Antibodies Targeting CD3 and CD33

Introduction to CD3

CD3 (cluster of differentiation 3) is a protein complex expressed on T cells and consists of four different subunits: CD3γ, CD3δ, CD3ε and CD3ζ. The gene encoding CD3 is located on human chromosome 11. The structure of CD3 belongs to the immunoglobulin superfamily. Its extracellular part contains two immunoglobulin domains, and its intracellular part contains immunoreceptor tyrosine-based activation motifs (ITAMs), which are involved in signal transduction. CD3 binds to the T cell receptor (TCR) to form a TCR-CD3 complex, which jointly participates in the recognition and activation of antigen by T cells. CD3 plays an important role in a variety of immune-related diseases, such as autoimmune diseases, transplant rejection, and tumor immunotherapy.

Introduction to CD33

CD33 (cluster of differentiation 33) is a transmembrane receptor expressed on myeloid cells, also known as Siglec-3 (sialic acid-binding Ig-like lectin 3). The gene encoding CD33 is located on human chromosome 19. The structure of CD33 belongs to the SIGLEC family. Its extracellular part contains a V-type and a C2-type immunoglobulin domain, which can bind sialic acid substances; its intracellular part contains two ITIMs, which can inhibit cell activity. CD33 is mainly involved in intercellular signal transduction and adhesion, regulating immune response and inflammatory response. CD33 is highly expressed in acute myeloid leukemia (AML) and is an important tumor-associated antigen.

Signaling Pathways Involved in Bispecific Antibodies Targeting CD3 and CD33

Bispecific antibodies (BsAbs) targeting CD3 and CD33 represent a novel immunotherapy strategy that simultaneously recognizes CD3 on T cells and CD33 on tumor cells. This brings the two into close proximity, triggering T cell mediation, inducing cytotoxicity, and effectively killing tumor cells. This treatment modality is highly specific and can overcome the limitations of traditional monoclonal antibody treatments, such as drug tolerance, dose dependence, and side effects. BsAbs targeting CD3 and CD33 primarily involve the signaling pathway of the TCR-CD3 complex, a key link in T cell antigen recognition and activation. When BsAbs connect T cells and tumor cells, they provide an antigen stimulation signal, activating the TCR-CD3 complex. This triggers the activation or inhibition of downstream signaling molecules, including ZAP70, LAT, PLCγ1, IP3, DAG, PKC, Ca2+, NFAT, NF-κB, AP-1, etc. These signaling molecules ultimately cause T cells to release various cytokines, such as IL-2, IFN-γ, TNF-α, etc., and express various effector molecules, such as FasL, TRAIL, Perforin, Granzyme B, etc., inducing tumor cell apoptosis, death or necrosis.

Clinical Status of Bispecific Antibodies Targeting CD3 and CD33

Bispecific antibodies (BsAbs) targeting CD3 and CD33 present a promising immunotherapeutic strategy, effectively eliminating CD33-expressing myeloid malignant cells, such as acute myeloid leukemia (AML). Currently, no BsAbs targeting CD3 and CD33 have been approved for marketing, but several are in various stages of clinical trials. These BsAbs, primarily small molecule BsAbs based on BiTE (bispecific T cell engager) technology, include AMG 330 and AMG 673. AMG 330 can effectively activate T cells to kill AML cells in a dose-dependent manner. In contrast to AMG 330, AMG 673 uses a new scFv format called the "2+1" format, comprising two identical CD3 binding domains and one CD33 binding domain. This format improves antibody stability and affinity while reducing side effects.

Table 1. Bispecific antibodies targeting CD3 and CD33 in clinical trials.

In summary, BsAbs targeting CD3 and CD33 are an emerging immunotherapy approach that can utilize T cell-mediated cytotoxicity to eliminate malignant tumor cells expressing CD33 or other myeloid-related antigens. Currently, these BsAbs are still in the clinical trial stage and have not yet been approved for marketing, but they have shown certain effects and potential. In the future, the design, dosage, administration, and combination therapy of these BsAbs need to be further optimized to improve their efficacy, safety, and tolerability and provide better treatment options for patients with myeloid malignancies.

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