What are the different types of drugs available for Bispecific T-cell Engager (BiTE)?

Introduction to Bispecific T-cell Engagers (BiTEs)

Definition and Mechanism of Action
Bispecific T-cell engagers (BiTEs) are a class of synthetic, bispecific antibodies designed to physically link cytotoxic T cells to tumor cells. Typically constructed by fusing two single-chain variable fragments (scFvs) from different monoclonal antibodies via a short flexible peptide linker, one scFv is specific for the CD3 component of the T-cell receptor (TCR) complex on T cells, while the other recognizes a tumor-associated antigen (TAA) on cancer cells. By simultaneously binding these two antigens, BiTEs form a “cytolytic synapse” between T cells and tumor cells, thereby bypassing the requirement for major histocompatibility complex (MHC) recognition and co-stimulatory signals. This innovative design allows even resting, polyclonal T cells to be activated directly and to mediate tumor cell lysis through the release of perforin, granzymes, and pro-inflammatory cytokines.

Role in Cancer Immunotherapy
The ability of BiTEs to directly recruit and activate T cells against tumor cells has established them as a vital tool in cancer immunotherapy. In hematological malignancies, BiTEs have already demonstrated impressive clinical outcomes such as converting minimal residual disease–positive states to complete responses. Moreover, by redirecting endogenous, polyclonal T cells to tumor cells, BiTEs amplify the natural immune response, often overcoming hurdles such as antigen escape and T cell exhaustion seen with other modalities. Their “off‐the‐shelf” nature also provides a logistical advantage over personalized approaches like CAR T‐cell therapy, making BiTEs an attractive option for both blood cancers and, increasingly, solid tumors despite ongoing challenges in tumor penetration and microenvironment suppression.

Classification of BiTE Drugs

Approved BiTE Drugs
Currently, the field of BiTE therapy is exemplified by a few key products that have reached regulatory approval. The most prominent example is blinatumomab—a CD19×CD3 bispecific T-cell engager that was the first BiTE drug approved by the U.S. Food and Drug Administration (FDA) and the European Medicines Agency (EMA) for the treatment of relapsed/refractory B-cell acute lymphoblastic leukemia (B-ALL). Blinatumomab’s approval marked a significant milestone by validating the BiTE concept and providing proof-of-principle that simultaneous engagement of T cells and tumor cells leads to effective cell-mediated cytotoxicity. In addition to its use in hematological cancers, there has been progress in the solid tumor space. For instance, tebentafusp-tebn—another bispecific molecule that targets a melanoma-associated antigen in the context of HLA-A∗02:01—has been approved for uveal melanoma, illustrating that the BiTE paradigm can be extended beyond hematologic indications. These approved BiTE drugs set the stage for emerging candidates by demonstrating that the concept is both clinically feasible and impactful.

BiTE Drugs in Clinical Trials
Beyond the approved agents, a wide array of BiTE drugs is under active clinical investigation as investigators seek to expand the targets and improve the properties of BiTEs. Many candidates are being evaluated across both hematological malignancies and solid tumors. For example, in multiple myeloma, BiTEs targeting B-cell maturation antigen (BCMA) (such as AMG 420 and AMG 701) are undergoing early phase trials with promising efficacy data. Other clinical candidates are designed to target antigens such as CD20, CD33, EGFRvIII, and PSMA—each addressing the distinct biology of their respective tumor types. In lymphomas, next-generation BiTEs like epcoritamab, glofitamab, and odronextamab are being explored in phase I/II trials, where they are being administered intravenously or subcutaneously with step-up dosing regimens to manage side effects like cytokine release syndrome (CRS). Importantly, novel formats such as DNA-encoded BiTEs or mRNA-based BiTE constructs are also being developed to extend in vivo expression duration and overcome the typically short half-life of the conventional BiTE molecules. Moreover, innovative designs that incorporate an Fc fragment to extend serum half-life or employ nanobodies (VHHs) instead of scFvs, termed Nb-BiTEs, are in preclinical evaluation, with the aim of improving tissue penetration and reducing immunogenicity. Collectively, these clinical trials and preclinical developments represent a broad spectrum of BiTE drug candidates that vary in their target antigens, molecular formats, methods of delivery, and pharmacokinetic enhancements.

Mechanisms and Targets of BiTE Drugs

Common Targets in BiTE Therapy
The targets of BiTE drugs differ according to the type of cancer being treated and reflect the antigenic landscape of malignant cells:

1. Hematological Malignancies:
- CD19: With expression confined largely to the B-cell lineage, CD19 has been the foremost target for B-ALL treatment using BiTEs such as blinatumomab.
- CD20: Given its role as a differentiation antigen in B-cell lymphomas, several BiTE candidates target CD20 in hopes of delivering effective anti-lymphoma activity.
- BCMA (B-cell maturation antigen): Predominantly expressed in multiple myeloma cells, BCMA is targeted by multiple BiTEs in clinical trials, such as AMG 420 and AMG 701, aiming to address the unmet need in relapsed/refractory multiple myeloma.
- CD33 and FLT3: These antigens serve as targets in acute myelogenous leukemia (AML) and are being evaluated in early-stage studies with BiTE molecules aimed at directing cytotoxic T cells to myeloid blasts.

2. Solid Tumors:
- EGFRvIII: Mutated forms of EGFR, particularly in glioblastoma multiforme (GBM), have been engaged by BiTEs to overcome the challenges of targeting a heterogeneous antigen in solid tumors.
- PSMA (Prostate-Specific Membrane Antigen): This target is exploited by BiTEs such as pasotuxizumab, which are designed to direct T cells against prostate cancer cells.
- SSTR2 (Somatostatin Receptor 2): Emerging BiTE formats, like those targeting neuroendocrine tumors, employ SSTR2 as an antigen to mediate T-cell engagement against the tumor.
- Other Targets: Novel targets such as CD105 have been explored particularly in the context of a nanobody-based BiTE (Nb-BiTE), opening new avenues for targeting the tumor vasculature and stroma in solid tumors.

Mechanisms of Action and Efficacy
BiTE drugs work by utilizing their bispecific nature to simultaneously bind T cells and tumor cells. This bridging action leads to the formation of an immunological synapse that closely mimics the natural TCR-MHC interaction, triggering T cell activation, proliferation, and the release of cytotoxic granules, including perforin and granzymes, which result in selective tumor cell lysis. Despite the simplicity of this concept, the efficacy and potency are modulated by several factors:

- Binding Affinities: The affinity of each scFv component toward its respective target is critical. The anti-CD3 arm is often engineered to have a lower affinity than the tumor-specific arm to reduce off–target or systemic T cell activation while still permitting effective synapse formation at the tumor site.
- Molecular Size and Format: Traditional BiTEs composed solely of scFv domains are relatively small (around 55 kDa) which allows for rapid tissue penetration but also leads to a short half-life (usually around 2 hours), necessitating continuous infusion. To overcome this, modifications such as Fc-fusion, PEGylation, or binding to serum albumin are being explored to extend the half-life.
- Dose and Administration Strategies: Owing to their short half-life, approved BiTE drugs like blinatumomab are administered through continuous intravenous (IV) infusion with a step-up dosing strategy to mitigate adverse events, such as CRS and neurological toxicities.
- Redirection of Polyclonal T Cells: A major advantage of BiTE drugs is that they recruit and activate endogenous T cells without needing these cells to be genetically modified. This mode of action mobilizes a broad immune response, potentially leading to the generation of an immunologic memory against tumor cells over time.
- Efficacy in the Tumor Microenvironment: While BiTEs have shown potent preclinical activity, their efficacy in vivo—especially in solid tumors—can be limited by factors such as poor drug penetration, an immunosuppressive tumor microenvironment (TME), and antigen heterogeneity. Novel designs, including nanobody-based BiTEs and gene-encoded BiTEs, are being developed to improve delivery and function in these challenging contexts.

Challenges and Developments in BiTE Therapy

Current Challenges in BiTE Development
Though the promise of BiTE therapy is substantial, several hurdles remain that influence both their clinical efficacy and safety:

- Pharmacokinetic Limitations: The small molecular size of conventional BiTEs results in rapid renal clearance and a short serum half-life. This necessitates continuous IV infusion regimens, which can be inconvenient for patients and limits broader clinical application.
- On-target, Off-tumor Toxicity: Many tumor-associated antigens targeted by BiTEs are not exclusively expressed on malignant cells. This can lead to on-target off-tumor effects causing adverse events such as cytokine release syndrome (CRS) and neurotoxicity, as observed in clinical studies with blinatumomab.
- Tumor Microenvironment Barriers: The immunosuppressive TME, especially in solid tumors, can inhibit T-cell activation and impede BiTE function. Dense stroma, hypoxia, and inhibitory cytokines in the TME collectively reduce the infiltration and cytotoxic function of T cells, limiting the efficacy of BiTE drugs.
- Manufacturing Complexity and Stability Issues: The production of BiTEs requires precise recombinant expression systems to ensure proper folding and stability. Aggregation tendencies and low yield are potential obstacles, and strategies to optimize biophysical properties are continuously being refined.
- Antigen Escape and Heterogeneity: Tumor cells can downregulate or lose the expression of targeted antigens over time, leading to resistance against BiTE therapy. This phenomenon is a significant obstacle in hematological malignancies—for example, the emergence of CD19-negative relapses after blinatumomab treatment—and poses an even greater challenge in the antigenically heterogeneous environment of solid tumors.

Future Directions and Innovations
In response to these challenges, researchers and pharmaceutical companies are exploring multiple innovative approaches to enhance the performance and broaden the applications of BiTE drugs:

- Extended Half-life Formats: Incorporating Fc domains or other half-life extension modules (such as albumin-binding domains or PEGylation) into the BiTE structure can significantly prolong serum persistence, thus reducing the frequency of administration and improving patient convenience.
- Nanobody-based BiTEs (Nb-BiTEs): The use of nanobody (VHH) domains in place of traditional scFvs provides distinct advantages, including a smaller size (14–18 kDa), increased tissue penetration, enhanced stability, and lower immunogenic potential. Nb-BiTEs targeting antigens like CD105 have shown superior performance in preclinical models and may ultimately lead to improved outcomes in solid tumors and other difficult-to-treat cancers.
- Gene-encoded BiTEs and mRNA-based Approaches: Advances in gene therapy allow for the in vivo production of BiTEs following administration of plasmid DNA or mRNA. These approaches can lead to sustained expression of the BiTE molecule, potentially overcoming pharmacokinetic limitations associated with protein-based therapies.
- Combination Therapies: A promising strategy for overcoming resistance and enhancing efficacy involves combining BiTE drugs with checkpoint inhibitors (such as anti-PD-1 or anti-PD-L1 therapies), other targeted agents, or even cellular therapies like CAR T-cell therapy. Such combinations have the potential to synergistically activate immune responses and counteract the immunosuppressive mechanisms in the TME.
- Multi-specific Engagers Beyond BiTEs: Researchers are also expanding the concept into bispecific killer cell engagers (BiKEs) and trispecific killer cell engagers (TriKEs), which specifically target and activate NK cells in addition to T cells. These platforms aim to broaden the immune effector population and are under active investigation in both hematological and solid tumors.
- Optimized Dosing Regimens and Administration Routes: Exploring subcutaneous dosing and step-up dosing regimens to mitigate adverse events while achieving sufficient therapeutic concentrations is another focus. Studies are fine-tuning the infusion rates, dose schedules, and combination protocols to enhance safety and efficacy profiles.
- Precision Medicine Approaches: With advancements in genomics and tumor profiling, identifying patient-specific antigen expression patterns and immunologic profiles can inform the design of personalized BiTE therapies. This precision medicine approach will help in selecting the optimal BiTE candidate for a given patient, thereby increasing the chance of a durable response.

Conclusion
In summary, the landscape of Bispecific T-cell engager (BiTE) drugs is both rich and evolving, encompassing a diverse array of molecules developed for different cancer indications. Starting from the pioneering blinatumomab—which remains the gold standard for targeting CD19 in B-ALL—to newer agents such as tebentafusp-tebn approved for uveal melanoma, the field has expanded tremendously. Clinical trials now encompass BiTEs designed for hematological malignancies (e.g., those targeting CD20, BCMA, and CD33) as well as for solid tumors (targeting antigens such as EGFRvIII, PSMA, SSTR2, and even employing nanobody-based formats to improve tissue penetration).

Mechanistically, BiTEs redirect polyclonal T cells toward tumor cells by forming immunologic synapses via simultaneous binding to CD3 on T cells and specific TAAs, thereby triggering potent cytotoxic responses. Nevertheless, challenges remain, including short serum half-life, potential on-target off-tumor toxicity, and barriers posed by the tumor microenvironment. The current research is addressing these issues through advanced engineering methods such as Fc fusion, DNA/mRNA delivery, and the design of nanobody-based constructs. Moreover, combination strategies with checkpoint inhibitors and multi-specific engagers like BiKEs/TriKEs further extend the range and potential of these therapies.

The future of BiTE therapy seems promising, as efforts converge on optimizing pharmacokinetics, enhancing tumor penetration, and safely expanding the range of targetable antigens. With continuous innovation and integration of precision medicine, the next generation of BiTE drugs will likely demonstrate higher efficacy while minimizing adverse effects, ultimately contributing to a more effective and personalized approach in cancer immunotherapy.

This comprehensive overview from multiple perspectives—including molecular design, clinical application, pharmacokinetic improvements, and combinatorial strategies—illustrates that the therapeutic range of BiTE drugs is broad and multifaceted. As clinical trials progress and innovative modifications are adopted, BiTEs are set to play an ever-increasing role in transforming cancer treatment modalities for a wide spectrum of malignancies.