Bispecific T-Cell Engagers for Cancer Immunotherapy

Introduction

What are Bispecific Antibodies?

Bispecific antibodies are engineered molecules that can bind to two different antigens simultaneously. Unlike traditional monoclonal antibodies that target a single antigen, BsAbs are designed to recognize and attach to two distinct targets. This dual-targeting capability allows BsAbs to bring two different molecules into close proximity, facilitating a range of therapeutic functions, including the redirection of immune cells to cancer cells. In recent years, 11 new bsAbs have been approved by regulatory agencies within three years, with 9 approved TCE for cancer treatment (amivantamab, tebentafusp, mosunetuzumab, cadonilimab, teclistamab, glofitamab, epcoritamab, talquetamab, elranatamab) and 2 for non-cancer indications (faricimab, ozoralizumab)^[1].

Format of Bispecific antibody

There are several types of bispecific antibodies, each with unique structures and mechanisms ^[2]. Some common types include:

• Fc-less bispecific antibody: An example is the bispecific tandem scFv format, where two scFv are connected by a linker. The first BiTE molecule, Blincyto® (blinatumomab), was approved for treating refractory B-cell precursor acute lymphoblastic leukemia (ALL) in 2014.
• IgGs like with asymmetric architecture: This format is similar to natural IgG, offering better half-life and stability compared to scFv-based BsAbs. The challenge lies in resolving the pairing of heavy and light chains. Various strategies have been developed, such as Fc engineering like Knob-in-Hole, CrossMab, and post-assembly purification of two monoclonal antibodies (Duobody).
• Bispecific antibody with symmetric architecture: These antibodies achieve bivalent binding at each binding site by connecting an additional scFv to either the heavy or light chain. A classic example is DVD-Ig ^[3]. We will discuss more details of how different format affect the BsAb in future articles.

How TCEs Work

T-Cell Engagers (TCEs) are a subtype of bispecific antibodies that play a crucial role in cancer immunotherapy. The primary function of TCEs is to bridge T cells (a type of immune cell) and cancer cells, facilitating direct T-cell mediated killing of the cancer cells ^[4].

TCEs are designed to bind to an antigen on the surface of T cells, such as CD3, and another tumor associate antigen (TAA) on cancer cells, such as CD19 or CD20. By physically linking these two cell types, TCEs form a synapse that allows the T cell to release cytotoxic molecules directly into the cancer cell, leading to its destruction.

The binding of TCEs to T cells and cancer cells triggers the following sequence of events:

• Activation of T cells: The TCEs stimulate T cells, enhancing their cytotoxic activity.
• Formation of an immunological synapse: This close contact allows the T cell to release perforin and granzymes, which penetrate and kill the cancer cell.
• Elimination of cancer cells: The targeted cancer cells are efficiently destroyed by the activated T cells.

Development of Bispecific Antibody T-Cell Engagers

The development of BsAb TCEs is a complex and multi-step process that requires careful planning and execution. There are some key steps involved:

• TAA selection: The first step in developing BsAb TCEs is selecting appropriate target antigens. The chosen antigens must be highly expressed on cancer cells but minimally present on normal cells to reduce off-target effects and toxicity.
• Antibody engineering: Once the target antigens are identified, the next step is engineering the bispecific antibodies. This involves creating antibody fragments that can bind to the chosen antigens. Techniques such as phage display and hybridoma technology are commonly used to generate these fragments.
• In vitro testing: After engineering the antibodies, they undergo extensive in vitro testing to evaluate their binding affinity, specificity, and cytotoxicity. These tests ensure that the BsAb TCEs can effectively engage T cells and cancer cells.
• Preclinical trials: Successful in vitro candidates are then tested in preclinical models, such as mouse xenograft models, to assess their efficacy and safety in vivo. These trials provide crucial data on the pharmacokinetics, biodistribution, and potential side effects of the BsAb TCEs.

Challenges and Solutions in Development

Developing BsAb TCEs is fraught with challenges, including:

• Binding affinity: Ensuring that BsAbs have high binding affinity for TAA antigens and an appropriate affinity for CD3. Although there is a trend towards using weaker CD3 to reduce the potential for cytokine release syndrome (CRS) from activated T cells, there is no consensus on the ideal CD3 affinity ^[5].
• Specificity: Avoiding off-target effects by finding the optimal binding affinity balance between the two targets, thereby minimizing toxicity to normal cells.
• Stability: Engineering BsAbs with sufficient stability and half-life to be effective in vivo, ensuring the best therapeutic window.
• Innovative solutions, such as optimizing antibody fragments and using advanced protein engineering techniques, are employed to overcome these challenges.

Case Studies and Clinical Trials

Several BsAb TCEs have shown promising results in preclinical and clinical trials. Here are a few notable examples:

• Blinatumomab: Blinatumomab (Blincyto) is a BiTE that targets CD3 on T cells and CD19 on B-cell malignancies. It was the first BsAb TCE approved by the FDA for treating relapsed or refractory B-cell acute lymphoblastic leukemia (B-ALL). Clinical trials demonstrated that Blinatumomab significantly improved survival rates in patients with B-ALL.
• Mosunetuzumab: Mosunetuzumab is a second-generation BsAb TCE targeting CD3 and CD20. Unlike Blinatumomab, Mosunetuzumab is a full-length antibody with improved pharmacokinetic properties. It is currently being tested in clinical trials for various lymphoma indications and has shown promising efficacy and safety profiles.
• BsAb TCEs for solid tumor: T cell-based immunotherapies for solid tumors have not achieved the same clinical success as seen in hematological malignancies, partly due to the immunosuppressive effects of the tumor microenvironment. However, in May 2024, the FDA approved IMDELLTRA™ (tarlatamab-dlle), the first and only T-cell engager therapy for treating extensive-stage small cell lung cancer, bringing new hope for TCEs in solid tumors. More TCE clinical study results are expected to be released in 2025 ^[6].

Innovations and Future Directions

The field of BsAb TCE development is rapidly evolving, with several innovations and future directions on the horizon:

• New targets and sombination therapies: Researchers are exploring new targets for BsAb TCEs, including novel tumor antigens and immune checkpoints. Additionally, combination therapies involving BsAb TCEs and other immunotherapies, such as immune checkpoint inhibitors, are being investigated to enhance treatment efficacy.
• Future prospects: Ongoing research aims to improve the pharmacokinetics and biodistribution of BsAb TCEs, reduce their immunogenicity, and enhance their ability to penetrate solid tumors. These efforts are expected to lead to the development of more effective and versatile BsAb TCEs.

Conclusion

Bispecific antibody T-Cell Engagers (BsAb TCEs) represent a significant advancement in cancer immunotherapy. Their ability to bridge T cells and cancer cells offers a powerful tool for targeting and eliminating cancer cells. While the development of BsAb TCEs presents challenges, ongoing research and innovation are paving the way for more effective and versatile therapies. The future of BsAb TCEs in cancer treatment is promising, and continued efforts in this field hold the potential to revolutionize cancer therapy.

References

[1] Surowka M, Klein C. A pivotal decade for bispecific antibodies? MAbs. 2024 Jan-Dec;16(1):2321635. doi: 10.1080/19420862.2024.2321635. Epub 2024 Mar 11. Erratum in: MAbs. 2024 Jan-Dec;16(1):2335597. doi: 10.1080/19420862.2024.2335597. PMID: 38465614; PMCID: PMC10936642.

[2] Goebeler ME, Bargou RC. T cell-engaging therapies - BiTEs and beyond. Nat Rev Clin Oncol. 2020 Jul;17(7):418-434. doi: 10.1038/s41571-020-0347-5. Epub 2020 Apr 2. PMID: 32242094.

[3] Brinkmann U, Kontermann RE. The making of bispecific antibodies. MAbs. 2017 Feb/Mar;9(2):182-212. doi: 10.1080/19420862.2016.1268307. PMID: 28071970; PMCID: PMC5297537.

[4] Haber, L., Olson, K., Kelly, M.P. et al. Generation of T-cell-redirecting bispecific antibodies with differentiated profiles of cytokine release and biodistribution by CD3 affinity tuning. Sci Rep. 2021 Jul 13;11(1):14397. doi: 10.1038/s41598-021-93842-0.

[5] Shanshal M, Caimi PF, Adjei AA, Ma WW. T-Cell Engagers in Solid Cancers-Current Landscape and Future Directions. Cancers (Basel). 2023 May 18;15(10):2824. doi: 10.3390/cancers15102824.

[6] Vafa O, Trinklein ND. Perspective: Designing T-Cell Engagers With Better Therapeutic Windows. Front Oncol. 2020 Apr 15;10:446. doi: 10.3389/fonc.2020.00446.