Bispecific antibodies (BsAbs), Antibody Discovery and Therapeutics
Luke McLaughlin, Biotech Digital Marketer, Business Developer and Life Science Content Creator
Bispecific antibodies (BsAbs) are a class of artificial proteins that are engineered to recognize two different antigens or epitopes. This capability allows them to simultaneously engage two different types of targets, such as cancer cells and immune cells, which can enhance the body's immune response against tumors or address complex diseases that require the modulation of multiple biological pathways. Bispecific antibodies represent a significant advancement in therapeutic antibody engineering and offer promising strategies for the treatment of various diseases, including cancer, autoimmune disorders, and infectious diseases.
Mechanism of Action
BsAbs can work through several mechanisms, depending on their design:
Bispecific antibodies (BsAbs) are engineered to have two different antigen-binding sites, allowing them to simultaneously interact with two different targets. This dual specificity can confer unique therapeutic advantages over traditional monoclonal antibodies (mAbs) that recognize only a single antigen. The mechanisms of action (MoA) of bispecific antibodies are diverse and tailored to the therapeutic goals of each antibody, ranging from cancer immunotherapy to the treatment of autoimmune diseases. Here are some of the key mechanisms through which bispecific antibodies exert their effects:
1. Redirecting Immune Cells to Tumor Cells
One of the most prominent applications of bispecific antibodies is in cancer immunotherapy, where they can redirect immune cells to tumor cells, leading to targeted cell killing. A common example is T-cell redirecting bispecific antibodies that bind to CD3 on T-cells with one arm and a tumor-associated antigen (TAA) on cancer cells with the other. This dual binding activates the T-cells and directs their cytotoxic activity towards the cancer cells, leading to tumor cell lysis. This approach is particularly used in hematologic malignancies, with CD19/CD3 bispecific antibodies showing significant clinical benefits.
2. Dual Antigen Targeting for Improved Specificity and Efficacy
BsAbs can bind two different antigens on the same cell or different cells, enhancing therapeutic specificity and efficacy. By simultaneously targeting two pathways involved in disease progression, BsAbs can overcome resistance mechanisms or achieve synergistic therapeutic effects. For example, in cancer therapy, a bispecific antibody might target both a growth factor receptor and an immune checkpoint molecule on tumor cells, thereby inhibiting tumor growth while enhancing immune-mediated tumor cell killing.
3. Blocking Multiple Pathways
In diseases where multiple signaling pathways are dysregulated, such as in certain cancers and autoimmune diseases, BsAbs can simultaneously block two targets involved in these pathways. This can prevent the disease from bypassing the blockade of a single pathway, a common mechanism of resistance to targeted therapies. For instance, a bispecific antibody that blocks both VEGF and PDGF can be more effective in inhibiting angiogenesis and tumor growth than targeting either pathway alone.
4. Heterodimerization or Crosslinking of Receptors
BsAbs can induce the heterodimerization of receptors or crosslink them in a way that modulates their activity. This can lead to the activation or inhibition of signaling pathways critical for disease pathogenesis. For example, bispecific antibodies that crosslink two receptors on the surface of immune cells can modulate the immune response in autoimmune diseases, leading to therapeutic effects.
5. Delivery of Therapeutic Agents
Some bispecific antibodies are designed to deliver therapeutic agents, such as toxins, drugs, or radioactive compounds, directly to targeted cells. One arm of the antibody binds to a specific antigen on the target cell, while the other is conjugated to the therapeutic agent. This approach allows for precise delivery of the therapeutic agent to the target cells, minimizing systemic exposure and reducing side effects.
Advantages and Therapeutic Potential
The ability of bispecific antibodies to engage two targets offers several advantages over traditional monoclonal antibodies, including improved specificity, the ability to engage and activate immune cells, the potential to overcome resistance mechanisms, and the capability to modulate complex disease pathways. As research and development in this area continue to advance, bispecific antibodies are expected to play an increasingly significant role in the treatment of a wide range of diseases, offering new hope for patients with conditions that are difficult to treat with existing therapies.
Development and Producers
The development of bispecific antibodies has accelerated in the past decade, with multiple formats being explored, such as single-chain variable fragments (scFvs), bispecific T-cell engagers (BiTEs), and others. As of my last update in April 2023, several bispecific antibodies have been approved for clinical use, particularly in oncology, and many more are in various stages of development.
Producing bispecific antibodies (BsAbs) involves complex biotechnological processes that differ significantly from the production of conventional monoclonal antibodies (mAbs). The complexity arises from the need to create an antibody that can simultaneously bind to two different antigens. There are several methods for producing BsAbs, each with its own advantages and challenges. Below, I outline the most commonly used techniques:
1. Quadroma (Hybrid-Hybridoma) Technology
This is one of the earliest methods used for producing bispecific antibodies. It involves the fusion of two different hybridoma cells, each producing a different monoclonal antibody. The resulting hybrid-hybridoma, or quadroma, is capable of producing bispecific antibodies alongside a mixture of the two parental monoclonal antibodies. The main challenge with this approach is the separation and purification of the bispecific antibodies from the mixture.
2. Genetic Engineering
This is the most common method for producing BsAbs today, leveraging recombinant DNA technology to create bispecific molecules with precise control over the antibody architecture. There are several strategies within this category:
Knobs-into-Holes (KiH): A technology where one antibody heavy chain is engineered to have a "knob" structure that fits into a "hole" on the heavy chain of the other antibody. This approach facilitates the correct assembly of the bispecific antibody.
Single-chain variable fragment (scFv) fusion: In this approach, two single-chain variable fragments (scFvs) from different antibodies are linked together, either directly or through a flexible linker, to form a bispecific molecule.
CrossMab: A technique where the heavy and light chain domains are swapped between the two antibodies that are being combined. This method also facilitates correct assembly and improves stability.
Dual-variable-domain (DVD) Ig: This involves the fusion of two variable domains on top of each other within the same chain, allowing the creation of a bispecific antibody that can bind to two different antigens.
3. Chemical Conjugation
This method involves the chemical linking of two monoclonal antibodies or antibody fragments. Although this approach can be used to generate bispecific antibodies with high yields, it often results in heterogeneous products that require complex purification processes. The conjugation points and the stability of the linkers can also affect the efficacy and safety of the final product.
4. Microbial Systems
Recent advances have explored the use of microbial systems, such as bacteria or yeast, to produce bispecific antibodies or fragments thereof. These systems can offer cost advantages and simpler scaling-up processes but may face challenges in terms of post-translational modifications and yields.
Challenges in Production
Producing bispecific antibodies poses several challenges, including:
Complexity of Design: Designing an effective bispecific antibody that has the desired specificity, affinity, and functionality for both targets.
Expression and Yield: Achieving high levels of expression and proper assembly of the bispecific antibodies in host cells, which is crucial for commercial production.
Purification: Separating the bispecific antibodies from monospecific antibodies, other bioproducts, and host cell proteins can be challenging and impacts the purity and quality of the final product.
Stability and Pharmacokinetics: Ensuring that the bispecific antibodies are stable and have suitable pharmacokinetics for therapeutic use.
Statistics and Current Development
As of early 2023:
There were over 100 bispecific antibodies in clinical development, with a significant number in Phase I and II trials.
The majority of candidates target oncology indications, but there is a growing interest in other areas such as autoimmune diseases and infectious diseases.
The market for bispecific antibodies is expected to grow significantly, with forecasts suggesting it could reach billions of dollars in annual sales, driven by the approval of new therapies and the expansion of indications for existing ones.
The field of bispecific antibodies is rapidly evolving, with continuous improvements in design, efficacy, and safety. These therapies hold great promise for addressing unmet medical needs, particularly in areas where conventional monoclonal antibodies or small molecule drugs have fallen short.