Bispecific antibodies

Bispecific antibodies (bsAbs) are a transformative class of biologic therapeutics that simultaneously bind to two distinct epitopes or antigens. Unlike conventional monoclonal antibodies (mAbs), which target a single antigen, bsAbs are engineered to engage two different targets, offering new mechanisms for enhancing therapeutic efficacy, especially in cancer immunotherapy, autoimmune diseases, and infectious disease.

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1. Definition and Rationale

Bispecific antibodies are engineered immunoglobulins capable of recognizing two different antigens or epitopes. This dual-targeting approach allows bsAbs to:

• Redirect immune cells (e.g., T cells) toward tumor cells

• Simultaneously inhibit two pathogenic pathways

• Overcome tumor heterogeneity and antigen escape

• Enhance specificity and therapeutic index

Their unique functionality has led to widespread development across multiple therapeutic areas, particularly hematologic malignancies and solid tumors.

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2. Structural Classification and Formats

There are over 100 different formats of bsAbs, but they broadly fall into two main categories:

A. IgG-like Bispecific Antibodies

• Resemble native immunoglobulin structure with Fc region

• Extended half-life due to neonatal Fc receptor (FcRn) recycling

• Allow effector functions (ADCC, CDC)

Examples:

• Full-length symmetric bsAbs (knob-into-hole, CrossMAb)

• Fc-engineered bsAbs

B. Non-IgG-like Bispecific Antibodies (Fragment-based)

• Lacks Fc region

• Smaller, simpler molecules (e.g., BiTEs)

• Require continuous infusion due to short half-life

Examples:

• BiTEs (e.g., blinatumomab)

• DARTs (Dual-Affinity ReTargeting molecules)

• TandAbs (tandem diabodies)

Common Engineering Strategies:

• Knobs-into-holes (KIH) for heterodimerization

• CrossMAb technology

• Dock-and-lock (DNL)

• Dual-variable domain (DVD-Ig)

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3. Mechanisms of Action

A. Immune Cell Engagement

• One arm binds CD3 (on T cells) and the other a tumor-associated antigen (TAA)

• Promotes T-cell mediated cytotoxicity (e.g., BiTEs like blinatumomab)

B. Dual Checkpoint Blockade

• Targets PD-1 and CTLA-4 or PD-L1 and LAG-3 simultaneously

• Enhances antitumor T-cell activation (e.g., FS118)

C. Receptor Co-blockade

• Inhibits two growth factor receptors (e.g., EGFR + HER2)

• Reduces tumor cell proliferation and survival

D. Receptor Clustering or Agonism

• Simultaneously targets receptor and co-receptor

• Enhances signaling or immune activation

E. Tumor-specific Dual Targeting

• Binds two tumor-specific antigens for selective cytotoxicity

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4. FDA-Approved Bispecific Antibodies

1. Blinatumomab (Blincyto)

• Target: CD19 × CD3

• Indication: Relapsed/refractory B-cell ALL, MRD-positive B-ALL

• Type: BiTE (non-IgG-like)

• Dosing: Continuous IV infusion due to short half-life

2. Teclistamab (Tecvayli)

• Target: BCMA × CD3

• Indication: Relapsed/refractory multiple myeloma

• Format: IgG-like bispecific with Fc

• Subcutaneous administration

3. Mosunetuzumab (Lunsumio)

• Target: CD20 × CD3

• Indication: Relapsed/refractory follicular lymphoma

• Format: IgG-like bispecific with step-up dosing

4. Epcoritamab (Epkinly)

• Target: CD20 × CD3

• Indication: Relapsed/refractory diffuse large B-cell lymphoma (DLBCL)

• Subcutaneous injection

5. Glofitamab (approved in EU; US pending)

• Target: CD20 × CD3

• Dosing: IV with step-up regimen

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5. Agents in Late-Stage Clinical Trials

• REGN1979 (Odronextamab): CD20 × CD3 – DLBCL, FL

• AMG 701: BCMA × CD3 – Multiple myeloma

• Zenocutuzumab: HER2 × HER3 – NRG1 fusion-positive solid tumors

• FS118: PD-L1 × LAG-3 – Solid tumors

• MCLA-158: EGFR × LGR5 – Colon and head & neck cancers

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6. Clinical Indications

Hematological Malignancies:

• B-cell precursor ALL

• Diffuse large B-cell lymphoma

• Follicular lymphoma

• Mantle cell lymphoma

• Multiple myeloma

• Acute myeloid leukemia (under development)

Solid Tumors (experimental/early-phase trials):

• Non-small cell lung cancer (NSCLC)

• Prostate cancer

• Breast cancer

• Colorectal cancer (CEA-positive)

• Glioblastoma (EGFRvIII)

Other Therapeutic Areas:

• Autoimmune diseases (e.g., dual cytokine blockade)

• Infectious diseases (e.g., dual targeting of viral proteins)

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7. Dosing Strategies

Dosing and administration depend on the structural class:

BiTEs (e.g., blinatumomab):

• Continuous IV infusion due to rapid clearance

• Stepwise dose escalation to mitigate CRS

IgG-like bsAbs (e.g., teclistamab, epcoritamab):

• Weekly to monthly subcutaneous or IV injections

• Long half-life (~10–20 days)

• Step-up priming doses to minimize CRS

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8. Adverse Effects

Immune-Related Toxicities:

Cytokine Release Syndrome (CRS):

• Fever, hypotension, hypoxia, elevated cytokines

• Common in CD3-engaging bsAbs

• Managed with tocilizumab and steroids

• Grade-based interruption protocols are used

Immune Effector Cell-Associated Neurotoxicity Syndrome (ICANS):

• Confusion, seizures, encephalopathy

• Requires supportive care, corticosteroids

Other Adverse Events:

• Hypogammaglobulinemia (especially B-cell depleting agents)

• Hematologic toxicities: neutropenia, anemia

• Infections: viral, bacterial, fungal

• Injection site reactions (subcutaneous forms)

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9. Contraindications

• Known hypersensitivity to the agent or excipients

• Life-threatening CRS or ICANS history (for re-challenge)

• Severe uncontrolled infections

• Pre-existing severe CNS disease (for neurotoxic agents)

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10. Precautions and Monitoring

• Baseline blood counts, liver enzymes, renal function

• Monitor for symptoms of CRS and ICANS after initial doses

• Premedication with corticosteroids or antipyretics as per protocol

• Avoid live vaccines during and shortly after therapy

• Evaluate immunoglobulin levels during and after treatment (e.g., for infection risk)

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11. Pharmacokinetics

• IgG-like bsAbs:

  • Long half-life (days to weeks)

  • Suitable for intermittent dosing

  • Retain FcRn-mediated recycling

• Non-IgG-like bsAbs (e.g., BiTEs):

  • Short half-life (~2 hours)

  • Require continuous infusion

  • Rapid systemic clearance

Strategies for Optimization:

• Fc-fusion for longer half-life

• PEGylation to improve stability

• Subcutaneous formulations for ease of use

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12. Resistance Mechanisms

• Downregulation/loss of target antigens (e.g., CD19-negative relapse)

• T-cell exhaustion or senescence

• Suppressive tumor microenvironment (TME)

• Increased expression of checkpoint proteins (e.g., PD-L1)

• Neutralizing anti-drug antibodies (ADAs)

Combination Approaches to Overcome Resistance:

• bsAb + immune checkpoint inhibitors

• bsAb + lenalidomide (T-cell activator)

• Dual-targeting bsAbs (e.g., CD19 × CD22)

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13. Advantages Over Monoclonal Antibodies

Feature	Monoclonal Ab	Bispecific Ab
Target Specificity	Single	Dual
T-cell Redirection	No	Yes (if CD3-based)
Tumor Selectivity	Lower	Higher (dual antigens)
Resistance to Escape	Limited	Better (dual targets)
Half-life (IgG-like)	Long	Comparable
Half-life (non-IgG-like)	Long	Short (requires CIV)

14. Future Directions

• Trispecific antibodies targeting three antigens (e.g., CD3 × TAA × checkpoint)

• Dual-arm BiTEs for serial tumor engagement and immune modulation

• Checkpoint bispecifics (e.g., PD-1 × LAG-3) for exhausted T-cell rescue

• BsAbs in autoimmune diseases (e.g., TNF-α × IL-17 blockade)

• Bispecific NK cell engagers (BiKEs) using CD16A instead of CD3

• Gene therapy vectors encoding bsAb constructs (e.g., AAVs)