Bispecific Antibodies: Expanding the Frontiers of Targeted Immunotherapy

Introduction to Bispecific Antibodies

Bispecific antibodies (BsAbs) represent a significant advancement in antibody-based therapeutics, allowing simultaneous binding to two distinct antigens or epitopes. Unlike traditional monoclonal antibodies (mAbs), which target a single antigen, bispecific antibodies can engage two different targets on the same cell or bridge two separate cell types. This dual-targeting capability has opened new avenues in cancer treatment, particularly in hematologic malignancies, with emerging potential for solid tumors.

Bispecific antibodies can be broadly categorized into two main functional classes:

1. T cell engager (TCE) bispecific antibodies, which connect tumor cells with effector T cells to induce tumor cell lysis.

2. Receptor-blocking bispecific antibodies, which target two distinct tumor antigens to disrupt cancer cell signaling and enhance immune-mediated killing.

T Cell Engager (TCE) Bispecific Antibodies

Mechanism of Action

TCE bispecific antibodies are designed to link a cancer cell with a T cell, typically by binding CD3 on T cells and a tumor-associated antigen on cancer cells. This interaction induces T cell activation, leading to the release of cytotoxic granules and cytokines that mediate tumor cell destruction.

The first TCE bispecific antibody, blinatumomab, was approved in 2014 for B-cell precursor acute lymphoblastic leukemia (B-ALL). Blinatumomab employs a bispecific T cell engager (BiTE) format, consisting of two single-chain variable fragments (scFvs): one targeting CD19 on B cells and the other binding CD3 on T cells. This compact 54-kDa molecule represents the minimal structure necessary for T cell activation and has served as the basis for many subsequent TCE bispecific designs.

Regulatory Approvals and Expanding Indications

Following the success of blinatumomab, multiple TCE bispecific antibodies have gained regulatory approval, particularly in hematologic malignancies:

• CD20xCD3 bispecifics: Mosunetuzumab, epcoritamab, and glofitamab for B-cell lymphomas.

• BCMAxCD3 bispecifics: Teclistamab and elranatamab for multiple myeloma.

• GPRC5DxCD3 bispecifics: Talquetamab for multiple myeloma.

Despite these successes, the approval of TCE bispecific antibodies for solid tumors has been more challenging. However, a recent Phase I trial of a DLL3-targeting TCE bispecific antibody for small-cell lung cancer (SCLC) has shown promising results, suggesting that the efficacy barriers in solid tumors may soon be overcome.

TCE Bispecific Antibody Formats

Different pharmaceutical companies have developed proprietary TCE bispecific formats to optimize therapeutic efficacy and safety. Some of the most well-known include:

• BiTE (Bispecific T cell Engager) – Amgen (e.g., blinatumomab)

• DuoBody – Genmab

• DART (Dual-Affinity Re-Targeting) – MacroGenics

• XmAb – Xencor

• ImmTAC (Immune Mobilizing Monoclonal TCR Against Cancer) – Immunocore (e.g., tebentafusp)

Tebentafusp, an ImmTAC bispecific antibody targeting gp100 in melanoma, demonstrated improved overall survival in metastatic uveal melanoma and received FDA approval in 2022.

Bispecific Antibodies Targeting Two Antigens on the Same Cell

Receptor-Blocking Bispecific Antibodies

Unlike TCE bispecific antibodies, receptor-blocking bispecific antibodies do not engage immune effector cells. Instead, they target two distinct antigens or epitopes on the same cancer cell to maximize anti-tumor activity through multiple mechanisms:

1. Blocking multiple proliferative signaling pathways – preventing tumor growth.

2. Enhancing immune cell-mediated tumor clearance – through mechanisms such as antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP), and complement-dependent cytotoxicity (CDC).

Approved Receptor-Blocking Bispecific Antibodies

One of the most well-known receptor-blocking bispecific antibodies is amivantamab, which simultaneously targets EGFR and MET in non-small cell lung cancer (NSCLC) with exon 20 insertion mutations. This dual blockade is more potent than combining two separate monoclonal antibodies against EGFR and MET. Additionally, amivantamab’s engineered IgG1 Fc domain enhances its ADCC activity via FcγRIIIa binding, boosting NK cell-mediated tumor killing.

Another promising example is zanidatamab, a bispecific antibody targeting two distinct HER2 epitopes. By binding HER2 subdomains 2 and 4—recognized by pertuzumab and trastuzumab, respectively—zanidatamab promotes HER2 clustering, receptor downregulation, and increased CDC-mediated tumor cell death. In clinical trials, zanidatamab has demonstrated strong efficacy in HER2-positive breast and gastric cancers.

Fc Engineering in Bispecific Antibodies

Enhancing Half-Life and Pharmacokinetics

Many first-generation bispecific antibodies, including blinatumomab, lack an Fc domain, resulting in rapid renal clearance and a short half-life (~2 hours). Consequently, continuous intravenous infusion is required, increasing treatment burden. To improve pharmacokinetics, most newly approved bispecific antibodies incorporate an Fc domain that binds the neonatal Fc receptor (FcRn), allowing antibody recycling and prolonging half-life.

For example:

• Mosunetuzumab, teclistamab, epcoritamab, glofitamab, and elranatamab incorporate IgG1, IgG2, or IgG4 Fc domains to extend half-life, permitting weekly dosing.

• Fc modifications (e.g., low-fucose IgG1 Fc in amivantamab) enhance FcγRIIIa binding, boosting ADCC activity.

• Fc silencing mutations (e.g., N297G in mosunetuzumab; P329G, L234A, L235A in glofitamab) prevent unintended immune cell crosslinking and T cell depletion.

Toxicities and Management of Bispecific Antibodies

Cytokine Release Syndrome (CRS) and Neurotoxicity

TCE bispecific antibodies can trigger excessive immune activation, leading to cytokine release syndrome (CRS)—a systemic inflammatory response characterized by fever, hypotension, and multi-organ dysfunction. Neurotoxicity, including confusion, tremors, and speech disturbances, has also been reported.

CRS risk correlates with tumor burden, as rapid T cell activation leads to high cytokine levels (e.g., IL-6, IL-1, IFNγ). Strategies to mitigate CRS include:

• Step-up dosing regimens – gradually increasing the bispecific antibody dose to minimize T cell overstimulation.

• Prophylactic corticosteroids or IL-6 inhibitors (tocilizumab) – to dampen excessive immune activation.

• Tumor debulking chemotherapy prior to bispecific therapy – to reduce target antigen load and cytokine surge.

In contrast, receptor-blocking bispecific antibodies do not directly activate T cells and therefore do not induce CRS or neurotoxicity. Their adverse effects resemble those of traditional monoclonal antibodies, such as skin rash (amivantamab, EGFR inhibitors) or cardiotoxicity (zanidatamab, HER2 inhibitors).

Emerging Bispecific Antibody Formats

Next-Generation TCE Bispecifics

To improve efficacy while minimizing toxicity, novel bispecific formats are being developed:

• 2:1 binding formats (e.g., glofitamab, xaluritamig) – enhance tumor targeting by increasing avidity to tumor antigens while maintaining a single CD3-binding site for controlled T cell activation.

• IgM-based bispecifics (e.g., imvotamab) – leverage the pentameric IgM structure to achieve a 10:1 tumor-to-CD3 binding ratio, enhancing tumor cell killing while reducing cytokine secretion.

Bispecific Immune Checkpoint Blockers

Beyond TCE bispecifics, bispecific antibodies are being engineered to simultaneously block two immune checkpoints, such as:

• PD-1xLAG-3 and PD-1xCTLA-4 – enhance T cell activation beyond what is achieved with single-agent checkpoint inhibitors.

• PD-L1xCTLA-4 – being evaluated in clinical trials for synergistic immune activation.

Conclusion

Bispecific antibodies have revolutionized cancer immunotherapy, particularly in hematologic malignancies. The development of TCE bispecifics, receptor-blocking bispecifics, and novel Fc-engineered formats continues to expand their clinical utility. With ongoing advances, bispecific antibodies are poised to overcome current limitations in solid tumors and enhance the precision of targeted immunotherapies.

Related: Applications of Bispecific Antibodies in Therapeutics

References:

1. Paul, S., Konig, M. F., Pardoll, D. M., Bettegowda, C., Papadopoulos, N., Wright, K. M., Gabelli, S. B., Ho, M., Van Elsas, A., & Zhou, S. (2024). Cancer therapy with antibodies. Nature Reviews Cancer, 24(6), 399-426. https://doi.org/10.1038/s41568-024-00690-x