Bispecific Antibody – Definition and Applications

What is a bispecific antibody?

A bispecific antibody (BsAb or BiAb) is a genetically engineered antibody molecule designed to recognize and bind two different target structures (antigens) at the same time. Unlike natural antibodies, which have two identical binding sites directed at a single antigen, bispecific antibodies are constructed so that each binding arm targets a different molecule. This unique property makes them a powerful and versatile tool in modern medicine, particularly in immuno-oncology and the treatment of autoimmune diseases.

Mechanism of Action

The mechanism of action of bispecific antibodies depends on their design and therapeutic objective. The main principles include:

• Cell redirection: The bispecific antibody simultaneously binds a tumor cell (e.g., via the antigen CD19 or BCMA) and an immune cell (e.g., a T lymphocyte via CD3), physically bringing the immune cell into direct contact with the tumor cell and triggering its destruction.
• Dual signal blockade: Two different signaling pathways or soluble mediators are inhibited simultaneously, which can be advantageous in autoimmune diseases and inflammatory conditions.
• Immune cell co-stimulation: By binding two surface molecules on immune cells simultaneously, bispecific antibodies can enhance immune cell activation.

Structure and Formats

Bispecific antibodies are produced in a wide variety of formats. The choice of format affects the size, stability, serum half-life, and mode of action of the molecule. Key classes include:

• IgG-like formats: These resemble a natural antibody in their basic structure and have a long half-life in the bloodstream. Examples include the CrossMab and Knob-into-Hole formats.
• Bispecific T-cell Engager (BiTE): Very small molecules without an Fc region that directly bridge T cells and tumor cells. A well-known example is blinatumomab, approved for the treatment of acute lymphoblastic leukemia (ALL).
• DART (Dual-Affinity Re-Targeting): Similar to BiTEs but with an additional disulfide bond for increased stability.
• TandAb formats: Larger tetravalent molecules with four binding arms.

Medical Applications

Bispecific antibodies are currently used or under clinical investigation in several medical fields:

Oncology (Cancer Therapy)

This is the most important area of application. Bispecific antibodies can harness the body's own immune system to specifically attack cancer cells. Approved examples include:

• Blinatumomab (Blincyto®): Approved for the treatment of B-cell acute lymphoblastic leukemia (B-ALL).
• Amivantamab (Rybrevant®): Approved for certain forms of non-small cell lung cancer (NSCLC) with EGFR mutations.
• Teclistamab (Tecvayli®): Approved for relapsed or refractory multiple myeloma.
• Mosunetuzumab (Lunsumio®): Approved for follicular lymphoma.

Hematology

In blood disorder therapy, bispecific antibodies are used to bridge missing clotting factors. Emicizumab (Hemlibra®) binds simultaneously to factor IXa and factor X, mimicking the function of the missing clotting factor VIII. It is approved for the treatment of hemophilia A.

Autoimmune Diseases and Inflammation

By simultaneously blocking two inflammatory mediators, bispecific antibodies show promise in conditions such as rheumatoid arthritis, systemic lupus erythematosus, and inflammatory bowel disease.

Advantages Over Conventional Antibodies

• Simultaneous targeting of two different structures with a single molecule
• Potential to overcome resistance mechanisms against conventional therapies
• Directed recruitment of immune cells to target cells such as tumor cells
• Possible synergistic effects through dual inhibition

Side Effects and Safety

Like all biological therapies, bispecific antibodies can cause side effects. The most common include:

• Cytokine Release Syndrome (CRS): An excessive immune response characterized by fever, chills, low blood pressure, and potential organ complications. It occurs predominantly with T-cell-engaging formats and requires close medical monitoring.
• Neurological side effects (ICANS): Immune effector cell-associated neurotoxicity syndrome, with symptoms such as confusion, speech disorders, or tremor.
• Infections: Because the immune system is altered by the therapy, susceptibility to infections may increase.
• Infusion reactions: Allergic reactions during administration.

Treatment with bispecific antibodies is therefore always carried out under close medical supervision, often in an inpatient or specialized outpatient setting.

Manufacturing

Bispecific antibodies are produced using genetic engineering techniques. The DNA sequences encoding the two different binding domains are combined and expressed in suitable cell systems (e.g., CHO cells or Escherichia coli). Manufacturing is technically demanding, as correct pairing of the antibody chains must be ensured to avoid unwanted byproducts.

References

1. Labrijn AF, Janmaat ML, Reichert JM, Parren PWHI. Bispecific antibodies: a mechanistic review of the pipeline. Nature Reviews Drug Discovery. 2019;18(8):585-608.
2. European Medicines Agency (EMA): Product information for blinatumomab, emicizumab, teclistamab, and mosunetuzumab. Available at: https://www.ema.europa.eu
3. Spiess C, Zhai Q, Carter PJ. Alternative molecular formats and therapeutic applications for bispecific antibodies. Molecular Immunology. 2015;67(2 Pt A):95-106.