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