Bispecific antibodies (bsAbs) are engineered molecules capable of simultaneously binding two distinct antigens or epitopes. This property enables mechanisms of action not achievable with conventional monoclonal antibodies, including immune cell redirection, dual pathway inhibition, and targeted payload delivery. As a result, bsAbs have become a major focus in therapeutic antibody development, particularly in oncology.
The field has expanded rapidly over the past decade. More than 200 bsAbs are currently in preclinical or clinical development, and multiple molecules have received regulatory approval. A significant proportion of these are immune cell engagers, particularly T cell-redirecting constructs. In parallel, newer formats targeting tumor-associated antigens, immune checkpoints, or combinations thereof are being actively explored.
This rapid expansion has been accompanied by increasing structural diversity. BsAbs can be broadly classified into IgG-like formats containing an Fc region and fragment-based formats lacking an Fc region, each with distinct functional and manufacturability profiles. While this diversity enables tailored therapeutic mechanisms, it also introduces significant challenges in expression, assembly, and downstream processing.
More than 100 distinct bsAb formats have been described, including BiTEs, DARTs, TandAbs, dual-variable-domain immunoglobulins (DVD-Ig), and IgG-scFv fusions. Each format differs in domain organization, valency, and molecular weight, which directly influences expression behavior.
IgG-like bsAbs benefit from Fc-mediated stability, longer half-life, and compatibility with established purification workflows. However, their structural complexity, typically involving four or more polypeptide chains, creates significant challenges in achieving correct assembly. In contrast, fragment-based bsAbs are composed of smaller antibody fragments such as single-chain variable fragments (scFvs) and can often be expressed from fewer polypeptide chains. This simplifies production and can enable expression in non-mammalian systems, but these formats frequently exhibit reduced stability and shorter circulation times due to the absence of Fc-mediated recycling.
Structural heterogeneity is therefore a central determinant of expression performance. Differences in domain architecture can lead to variability in folding efficiency, solubility, and aggregation propensity. Consequently, format selection is not only a functional decision but also a key determinant of manufacturability.
One of the most significant barriers in bsAb expression is ensuring correct chain pairing. In IgG-like bsAbs, correct assembly requires both accurate heavy chain heterodimerization and proper pairing of each heavy chain with its corresponding light chain.
Early production methods, such as quadroma technology, highlighted the scale of this problem. Co-expression of two different heavy chains and two different light chains can theoretically produce up to 16 different combinations, with only one representing the desired bispecific molecule. This leads to low yields and substantial product heterogeneity.
To address these issues, several engineering strategies have been developed. The knobs-into-holes (KiH) approach introduces complementary mutations in the CH3 domains to promote preferential heterodimerization of heavy chains. CrossMab technology reduces light chain mispairing by exchanging domains between heavy and light chains. The common light chain strategy eliminates light chain diversity by using a single light chain compatible with both heavy chains.
Although these approaches significantly improve pairing fidelity, they do not fully eliminate mispairing. Additional optimization is often required at both the molecular design and expression levels to achieve acceptable yields and purity.
The complexity of bsAb structures frequently results in lower expression yields compared to monoclonal antibodies. Mammalian expression systems, particularly Chinese hamster ovary (CHO) cells, are typically required to support proper folding, disulfide bond formation, and post-translational modifications.
However, inefficient assembly and intracellular quality control mechanisms can lead to retention and degradation of misfolded or mispaired chains, reducing overall productivity. Achieving balanced expression of multiple chains is another critical factor, as imbalanced expression can exacerbate mispairing and aggregation.
Fragment-based bsAbs offer an alternative approach, as they can often be expressed in simpler systems and from fewer chains, resulting in higher yields and reduced production complexity. Nevertheless, these formats may require additional engineering to compensate for reduced stability and half-life.
Given these constraints, manufacturability considerations must be incorporated early in the design phase. Format selection, vector design, and expression system optimization are interdependent factors that collectively determine production efficiency.
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The increased number of domains and interfaces in bsAbs introduces additional folding challenges. Improper folding can result in aggregation, reduced biological activity, and increased immunogenicity risk.
Certain early formats, such as diabodies, were associated with structural instability, prompting the development of more stable constructs such as TandAbs and DARTs. Engineering strategies, including linker optimization, domain swapping, and stabilization mutations are commonly applied to improve folding efficiency and structural integrity.
However, these modifications must be carefully evaluated. Changes that improve stability may alter binding affinity, specificity, or functional activity. As a result, optimization of bsAbs requires a balance between biophysical properties and biological function.
The selection of appropriate target antigens is a critical determinant of bsAb design. In solid tumors, antigen heterogeneity and the immunosuppressive tumor microenvironment complicate target selection and reduce therapeutic efficacy.
To address antigen escape and tumor heterogeneity, there is increasing interest in multispecific antibodies capable of targeting multiple antigens simultaneously. While this approach may improve therapeutic outcomes, it further increases molecular complexity, exacerbating challenges in expression and assembly.
Functional requirements also influence molecular design. For example, bsAbs that engage immune cells must facilitate the formation of an immunological synapse, which imposes constraints on geometry, affinity, and valency. These requirements can limit the range of viable formats and complicate expression optimization.
BsAbs, particularly Fc-containing formats, are subject to post-translational modifications such as glycosylation. Variability in glycosylation patterns can affect Fc-mediated effector functions, stability, and pharmacokinetics.
In addition to glycosylation heterogeneity, bsAbs often exhibit product-related impurities arising from mispaired chains, partially assembled intermediates, and degradation products. This heterogeneity complicates both process development and quality control.
Controlling post-translational modifications requires careful optimization of cell culture conditions and, in some cases, cell line engineering. Consistency in product quality is essential for clinical development and regulatory approval.
The presence of multiple closely related species presents significant challenges for purification. Standard Protein A affinity chromatography, commonly used for monoclonal antibodies, may not sufficiently resolve correctly assembled bsAbs from mispaired variants.
Additional purification steps, such as ion exchange chromatography and size exclusion chromatography, are often required to achieve the desired purity. These steps increase process complexity and can reduce overall yield.
Furthermore, different bsAb formats may require customized purification strategies, limiting the applicability of platform processes and increasing development timelines.
Comprehensive analytical characterization is essential to confirm correct assembly and ensure product quality. BsAbs require more sophisticated analytical approaches than monoclonal antibodies due to their structural complexity.
Techniques such as mass spectrometry, capillary electrophoresis, and multi-dimensional chromatography are commonly employed to assess chain pairing, molecular integrity, and heterogeneity. These methods must be capable of distinguishing closely related species and detecting low-abundance impurities.
The increased analytical burden contributes to longer development timelines and adds complexity to regulatory submissions.
Advances in protein engineering and expression technologies have enabled significant improvements in bsAb manufacturability. Strategies to address expression challenges include molecular design approaches such as KiH, CrossMab, and common light chain technologies to improve pairing fidelity.
Fc engineering can enhance stability and half-life, while linker design and domain optimization can improve folding and solubility. Selection of appropriate formats based on both functional requirements and manufacturability is critical.
In addition, optimized expression platforms that enable balanced chain expression and efficient assembly are increasingly important. Integrated approaches that combine molecular design, cell line development, and process optimization can substantially improve yields and product quality. Access to specialized expression systems and expertise can facilitate efficient production of complex bispecific formats, particularly during early-stage development when multiple design iterations are required.
Expression challenges have direct implications for the clinical translation of bsAbs. Low yields, complex purification processes, and product heterogeneity can increase manufacturing costs and delay development timelines.
Emerging formats, including trispecific antibodies, bispecific antibody-drug conjugates, and cytokine-mimetic bsAbs, are expected to further expand the therapeutic potential of this class. However, these innovations will also introduce additional expression and manufacturing challenges.
Future progress in the field will depend on the ability to co-optimize biological function and manufacturability. Early integration of developability considerations into bsAb design, supported by advanced expression technologies, will be essential to enable efficient translation from discovery to clinical application.
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References:
Li, H., Er Saw, P., & Song, E. (2020). Challenges and strategies for next-generation bispecific antibody-based antitumor therapeutics. Cellular & molecular immunology, 17(5), 451–461. https://doi.org/10.1038/s41423-020-0417-8
Gu, Y., & Zhao, Q. (2024). Clinical Progresses and Challenges of Bispecific Antibodies for the Treatment of Solid Tumors. Molecular diagnosis & therapy, 28(6), 669–702. https://doi.org/10.1007/s40291-024-00734-w
Goebeler, M. E., Stuhler, G., & Bargou, R. (2024). Bispecific and multispecific antibodies in oncology: Opportunities and challenges. Nature Reviews Clinical Oncology, 21(7), 539-560. https://doi.org/10.1038/s41571-024-00905-y
Klein, C., Brinkmann, U., Reichert, J. M., & Kontermann, R. E. (2024). The present and future of bispecific antibodies for cancer therapy. Nature Reviews Drug Discovery, 23(4), 301-319. https://doi.org/10.1038/s41573-024-00896-6
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