Beyond Immune Checkpoint Inhibition: Bispecific T-cell Engagers

The field of cancer immunotherapy has witnessed remarkable advancements in recent years, with immune checkpoint inhibitors (ICIs) at the forefront of this revolution. However, as the limitations of ICIs become increasingly apparent, particularly in solid tumors, the scientific community has turned its attention to novel approaches that can potentially overcome these challenges. One such promising strategy is the development of bispecific T-cell engagers (BiTEs), which represent a paradigm shift in cancer immunotherapy.

BiTEs are engineered bispecific antibodies designed to simultaneously bind to a tumor-associated antigen (TAA) on cancer cells and the CD3ε subunit of the T-cell receptor complex on T cells. This dual-binding capability allows BiTEs to bring cytotoxic T cells into close proximity with cancer cells, facilitating targeted tumor cell lysis without the need for antigen presentation or costimulatory signals.

This article explores the current landscape of BiTE therapy, focusing on molecules in clinical development, emerging preclinical data, and the research tools being employed to further elucidate their potential.

BiTEs function through a unique mechanism that bypasses several limitations of conventional T-cell activation:

1. Direct T-cell Engagement: By binding to CD3ε, BiTEs can activate T cells without the need for antigen presentation by major histocompatibility complex (MHC) molecules.
2. MHC-Independent Tumor Recognition: The tumor-binding arm of BiTEs allows for recognition of cancer cells regardless of their MHC expression status, overcoming a common immune evasion mechanism.
3. Polyclonal T-cell Activation: BiTEs can engage and activate a broad range of T-cell subsets, including both CD4+ and CD8+ T cells, potentially leading to a more robust anti-tumor response.
4. Lower Activation Threshold: The high-affinity binding of BiTEs to both T cells and tumor cells can trigger T-cell activation at lower antigen densities compared to natural T-cell receptor (TCR) interactions.

Several BiTEs are currently in various stages of clinical development, targeting a range of hematological malignancies and solid tumors:

1. Blinatumomab (Blincyto®): Approved for the treatment of B-cell acute lymphoblastic leukemia (ALL), blinatumomab targets CD19 on B cells and remains the only FDA-approved BiTE to date.
2. AMG 420 (BI 836909): Targets B-cell maturation antigen (BCMA) for the treatment of multiple myeloma. Phase 1 trials have shown promising results in heavily pretreated patients.
3. AMG 701: Another BCMA-targeting BiTE for multiple myeloma, currently in phase 1/2 trials.
4. AMG 160: Prostate-specific membrane antigen (PSMA)-targeting BiTE for prostate cancer, undergoing phase 1 evaluation.
5. AMG 757: Targets delta-like ligand 3 (DLL3) for the treatment of small cell lung cancer (SCLC), currently in phase 1 trials.
6. Solitomab (AMG 110, MT110): An EpCAM-targeting BiTE that has been evaluated in various solid tumors, including colorectal cancer.
7. GBR 1342: A CD38/CD3 BiTE being investigated for multiple myeloma and other CD38-positive malignancies.

These clinical-stage BiTEs represent only a fraction of the molecules currently under investigation, with many more in preclinical development targeting various TAAs such as CEA, EGFR, and HER2.

Despite their promise, BiTEs face several challenges that researchers are actively working to address:

1. Short Half-life: Most BiTEs have a relatively short serum half-life, necessitating continuous infusion. Next-generation BiTEs incorporating an Fc domain or employing alternative scaffolds aim to extend pharmacokinetics.
2. On-target, Off-tumor Toxicity: Targeting antigens that are also expressed on normal tissues can lead to undesirable side effects. Strategies to mitigate this include the development of tumor-selective BiTEs and the incorporation of safety switches.
3. Cytokine Release Syndrome (CRS): Rapid and potent T-cell activation can lead to severe CRS. Dose-escalation strategies and prophylactic measures are being explored to manage this potentially life-threatening complication.
4. Solid Tumor Penetration: While BiTEs have shown remarkable efficacy in hematological malignancies, their performance in solid tumors has been less impressive. Researchers are investigating combination therapies and novel BiTE designs to enhance tissue penetration and overcome the immunosuppressive tumor microenvironment.

To further investigate the potential of BiTEs, researchers employ various tools and models, including:

• Anti-mouse CD3ε Antibodies:
  • Clone 145-2C11: Widely used for in vivo T-cell depletion and activation studies.
  • Clone 17A2: An alternative anti-mouse CD3ε antibody used for flow cytometry and functional studies.
  • Clone 500A2: Another anti-mouse CD3ε antibody used for in vivo T-cell activation and depletion.
  • Anti-CD4 (Clone GK1.5): Used for CD4+ T-cell depletion studies.
  • Anti-CD8 (Clone 2.43): Employed for CD8+ T-cell depletion experiments.
  • Anti-NK1.1 (Clone PK136): Used to deplete NK cells in certain mouse strains.
  • Anti-CD20 (Clone 5D2): A murine surrogate for rituximab, used in B-cell depletion studies.
  • Anti-PD-1 (Clone RMP1-14): Used to block PD-1 in combination therapy studies with BiTEs.
  • Anti-CTLA-4 (Clone 9H10): Employed to block CTLA-4 in immune checkpoint studies related to BiTEs.
  • Anti-CD40 (Clone FGK45): Used to activate antigen-presenting cells in BiTE combination studies.
• Humanized Mouse Models:
  • NOD scid gamma (NSG) mice engrafted with human immune cells provide a platform for evaluating human-specific BiTEs in vivo.
• Syngeneic Mouse Models:
  • These models allow for the evaluation of murine surrogates of BiTEs in immunocompetent mice, providing insights into the interactions between BiTEs and an intact immune system.
• BiTE-mimetic Constructs:
  • Researchers often generate murine-specific BiTEs or BiTE-like molecules to study the mechanism of action in fully immunocompetent mouse models.
• In Vitro Co-culture Systems:
  • Advanced 3D co-culture systems incorporating tumor cells, T cells, and other components of the tumor microenvironment allow for high-throughput screening and mechanistic studies of BiTEs.

The field of BiTE therapy is rapidly evolving, with several exciting developments on the horizon:

1. Multi-specific Engagers: Molecules targeting more than two antigens are being developed to enhance specificity and efficacy.
2. Conditionally Activated BiTEs: These next-generation molecules are designed to become active only in the tumor microenvironment, potentially improving safety profiles.
3. Combination Therapies: Ongoing trials are exploring the potential synergy between BiTEs and other immunotherapies, targeted therapies, and conventional treatment modalities.
4. BiTE-armed Cell Therapies: The integration of BiTE technology with adoptive cell therapies, such as CAR-T cells, represents a promising avenue for enhancing efficacy and overcoming resistance mechanisms.

Bispecific T-cell engagers represent a promising frontier in cancer immunotherapy, offering a mechanistically distinct approach to harnessing the power of the immune system against cancer. As our understanding of BiTE biology deepens and clinical experience grows, these molecules are poised to play an increasingly important role in the treatment of both hematological malignancies and solid tumors. Ongoing research aimed at addressing current limitations and exploring novel applications will undoubtedly shape the future landscape of BiTE therapy, potentially revolutionizing cancer treatment paradigms in the years to come.