Antibody-Based Drug Mechanisms
Understand how monoclonal antibodies exert therapeutic effects through diverse mechanisms.
## Introduction
Monoclonal antibodies (mAbs) are the fastest-growing drug class, with over 100 FDA-approved products. Their large molecular weight (~150 kDa), exquisite target specificity, and long plasma half-life (typically 1–4 weeks due to FcRn recycling) distinguish them from small-molecule drugs. mAbs act through multiple mechanisms, often simultaneously.
## Primary Mechanisms of Action
### Ligand Neutralization
Antibodies bind soluble ligands in the extracellular space, preventing them from engaging their receptors. **Adalimumab** and **infliximab** neutralize TNF-alpha, a key pro-inflammatory cytokine, reducing downstream NF-kB signaling. **Bevacizumab** sequesters VEGF-A, starving tumors of their angiogenic blood supply.
### Receptor Blockade
Antibodies bind cell-surface receptors, sterically blocking ligand interaction or preventing receptor dimerization. **Trastuzumab** blocks HER2 homodimerization, inhibiting proliferative PI3K/AKT signaling in HER2-amplified breast cancer. **Nivolumab** and **pembrolizumab** block the PD-1 immune checkpoint, restoring cytotoxic T-cell anti-tumor immunity.
### Immune Effector Functions (Fc-Mediated)
The Fc (crystallizable fragment) region of IgG antibodies engages innate immune effector mechanisms to destroy target cells:
- **ADCC**: Fc binds Fc-gamma-RIIIA on NK cells, triggering perforin/granzyme-mediated target lysis. Critical for rituximab's efficacy against CD20+ lymphomas.
- **CDC**: Fc activates complement protein C1q, initiating the classical complement cascade and membrane attack complex.
- **ADCP**: Fc-gamma receptor engagement on macrophages promotes phagocytic engulfment of opsonized target cells.
### Antibody-Drug Conjugates (ADCs)
ADCs link a cytotoxic payload to a targeting antibody via a chemical linker (cleavable or non-cleavable). The antibody delivers the payload selectively to antigen-expressing cells, minimizing systemic toxicity. **Trastuzumab deruxtecan** (T-DXd) carries a topoisomerase I inhibitor with a bystander effect—released payload kills neighboring antigen-negative tumor cells.
## Engineering Strategies
- **Humanization**: Grafting CDRs onto human framework reduces immunogenicity (-zumab suffix)
- **Fc engineering**: Afucosylation enhances ADCC 50-fold; LALA mutations silence effector functions
- **Bispecifics**: Simultaneously engage two targets (blinatumomab: CD3 x CD19 for ALL)
## Key Takeaways
- Monoclonal antibodies act through neutralization, blockade, and immune effector recruitment
- Fc-mediated ADCC, CDC, and ADCP contribute significantly to oncology efficacy
- ADCs combine antibody targeting specificity with cytotoxic payload delivery
- Fc engineering and bispecific formats continue to expand therapeutic possibilities