Introduction

The term bispecific antibody (bsAb) is used to describe a large family of molecules designed to recognize two different epitopes or antigens. BsAbs come in many formats, ranging from relatively small proteins, merely consisting of two linked antigen-binding fragments, to large immunoglobulin G (IgG) like molecules with additional domains attached.


(Figure 1:Bispecific antibody mechanism of action ; Source: Genscript)


An attractive bsAb feature is their potential for novel functionalities, that is activities that do not exist in mixtures of the parental or reference antibodies. In these so-called obligate bsAbs, the physical linkage of the two binding specificities creates a dependency that can be temporal, with binding events 




occurring sequentially, or spatial, with binding events occurring simultaneously, such as in linking an effector to a target cell. To date, more than 20 different commercialized technology platforms are available for bsAb creation and development 2 bsAbs are marketed and over 85 are in clinical development. [1]


The Power of Dual Targeting

Dual targeting strategies using bispecific antibodies can be divided into two types: 

  1. those that directly act on target structures. e.g., cell surface receptors or soluble factors and 
  2. those that use dual targeting for delivery (retargeting) of a therapeutically active moiety. e.g., effector molecules and effector cells. [2]

(Figure 2: Bispecific antibody formats, Source: https://doi.org/10.4161/mabs.4.2.19000)


Direct actions include binding and neutralization of two ligands or two receptors, neutralization of a receptor and a ligand, activation of two receptors, activation of one receptor and neutralization of another receptor or a soluble factor, but also neutralization by binding to different epitopes of one receptor or ligand. Indirect actions include ADCC and CDC mediated by an Fc region, retargeting of immune effector cells through a further binding site, targeting of an effector molecule, e.g., a toxin, a cytokine or a 

prodrug-converting enzyme and targeting of drug-loaded nanoparticles. [3]

Direct and indirect actions can be combined within one molecule to further improve efficacy.


Overcoming Challenges in Design and Engineering

Much progress has been made in bispecific antibody development over the last decade. There are, however, remaining challenges that researchers are endeavoring to address, such as:

  • Optimizing target antigen selection to achieve the desired synergistically therapeutic effect

  • Developing improved methods that ensure correct matching of heavy and light chains in the antigen-binding arms of the BsAb

  • Optimizing dosing protocols to minimize the potential for adverse inflammatory responses, some of which can be life-threatening.

  • Designing BsAbs on-co-therapies that address perennial barriers such as tumor heterogeneity, mutational burden, and intractable tumor microenvironment. [4]

Researchers are using biophysical data in tandem with functional assays to tackle these challenges.






The Role of Fc in bsAbs


In bispecific antibodies (bsAbs), the Fc region plays a crucial role by enabling effector functions like antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and enhancing the antibody's stability, solubility, and serum half-life through interaction with Fc receptors, essentially allowing the bsAbs to recruit immune cells

to target cells more effectively while maintaining a longer presence in the bloodstream; however, depending on the desired mechanism of action, sometimes the Fc region is engineered to be modified or removed entirely to avoid unwanted immune responses. [5]


The Importance of Molecular Geometry

The three dimensional spatial arrangement of BsAbs is a significant factor in determining their therapeutic potential. The position of the antigen-binding modules could decide whether the bsAb will be able to bind its targets in a beneficial way or not. For example, the inter-paratopic distance the distance between the two binding sites could decide whether the bsAb can bridge two cells or position an enzyme and its substrate correctly. Researchers are increasingly utilizing structural examinations of receptor-antibody complexes to inform the rational design of bsAbs having optimal geometries for specific therapeutic applications. [6]



Symmetric vs. Asymmetric bsAbs


BsAbs can be classified into symmetric and asymmetric way. Symmetric bsAbs which adhere to the HC2LC2 Structure are relatively easier to produce and purify making them a popular choice for many therapeutic applications. 



(Figure 4: bsAb formats categorized according to molecular  

configuration; Source: National Library of Medicine_articles no-

PMC10850309/#s6)


However, they are limited in their flexibility as the antigen-binding domains always appear in pairs. In contrast, asymmetric bsAbs offer greater flexibility in valency and specificity but are more challenging to produce due to the risk of chain mispairing and the formation of impurities. [7]


Developability Considerations

In addition to their therapeutic potential bsAbs must also possess favorable drug-like qualities such as high expression, good biophysical stability and low aggregational efficiency. Developability screening must be performed early to

choose BsAbs with the best prospects for success in clinical development. 


(Figure: 5 Developability workflow for numerous candidates. Jarasch, 2015) 


Advances in silico predictive software and high throughput assays are helping researchers to screen for developability liabilities early in the drug development process and reducing the risk of investing in antibodies with poor prospects for success. Advances in in silico predictive tools and high throughput assays are helping researchers screen for developability liabilities early in the drug development process, reducing the risk of investing in antibodies that are unlikely to succeed. [8]


Conclusion

Bispecific antibodies are a futureistic therapeutic class of molecules that could potentially change the design of treatment for a broad array of diseases. Their capacity to target two disparate targets at once holds unprecedented promise for synergistic therapeutic action, yet also poses special challenges in engineering and design. As scientists increasingly anticipate the design and manufacture of bsAbs, these novel molecules stand ready to become a mainstay of contemporary medicine. which holding out new promise for patients with complex and hard-to-treat illnesses like cancer. [9]






References: 

[1] Kontermann, R. E., & Brinkmann, U. (2015). Bispecific antibodies. Drug Discovery Today, 20(7), 838-847.

[2] Wu, Z., Liu, Y., & Zhu, X. (2021). The development of bispecific antibodies. Annual Review of Medicine, 72, 209-227.

[3] Moore, G. L., Kan, D., & Bobrowicz, P. (2020). Antibody engineering for improved therapeutic efficacy. Nature Reviews Drug Discovery, 19(3), 178-197.

[4] Todorovic, V., Schwaneberg, U., & Schmid, F. X. (2018). Engineering of antibody-antigen interactions: A roadmap to improved therapeutics. Journal of Molecular Biology, 430(21), 4533-4550.

[5] Tuse, D., Singh, R., & Tripp, R. (2022). Role of Fc engineering in bispecific antibody design. BioDrugs, 36(1), 55-72.

[6] Muda, M., Gross, A. W., & Dawson, J. P. (2016). Engineering the geometry of bispecific antibodies. Molecular Immunology, 78, 102-113.

[7] Chames, P., & Baty, D. (2009). Bispecific antibodies for cancer therapy: The light at the end of the tunnel? MAbs, 1(6), 539-547.

[8] Jain, T., Sun, T., Durand, S., Hall, A., & Yu, Y. (2017). Biophysical properties of the clinical-stage antibody landscape. Proceedings of the National Academy of Sciences, 114(5), E840-E848.

[9] Garcia, J., & Martinez, C. (2021). Advances in bispecific antibody development: Therapeutic strategies and challenges. Expert Opinion on Drug Discovery, 16(3), 243-256.