Two Decades of Trastuzumab: From HER2 Targeting to a Cornerstone in Antibody Therapeutics Research

Inhibition of HER2-driven signaling

Binding of trastuzumab disrupts receptor dimerization within the HER family, resulting in suppression of key downstream pathways, including PI3K/Akt/mTOR and Ras/Raf/MAPK. This leads to reduced proliferation and induction of apoptosis in HER2-amplified cells. This mechanism also serves as a central functional readout in cell-based assays evaluating anti-HER2 antibodies.

Fc-mediated immune effector functions

The Fc region of trastuzumab engages FcγRIIIa on NK cells, triggering robust antibody-dependent cellular cytotoxicity (ADCC). Additionally, activation of the classical complement pathway contributes to complement-dependent cytotoxicity (CDC).

Additional mechanisms

These include inhibition of HER2 extracellular domain shedding, promotion of receptor internalization and degradation, and induction of G1 cell cycle arrest.

Establishment of humanized antibody development paradigms

As one of the earliest widely used humanized antibodies, trastuzumab employed a CDR grafting strategy combined with human framework substitution, effectively reducing immunogenicity. This design approach laid the groundwork for subsequent humanized and fully human antibody development. In parallel, studies on immunogenicity assessment, in vitro/in vivo activity comparison, and pharmacokinetics helped define standardized preclinical evaluation workflows.

Standardization of functional evaluation systems

At the cellular level, trastuzumab-based assays—such as HER2-positive cell proliferation inhibition, binding affinity measurement, and ADCC/CDC activity assays—have become widely adopted platforms for antibody characterization.
At the in vivo level, HER2-amplified xenograft models (established with HER2-overexpressing cell lines) have provided standardized approaches for efficacy evaluation and tissue distribution analysis. Today, trastuzumab is routinely used as a benchmark positive control in anti-HER2 antibody development.

Advancing research on resistance mechanisms

Extensive investigation into both primary and acquired resistance to trastuzumab has identified multiple mechanisms, including HER2 structural alterations, aberrant activation of downstream signaling pathways, compensatory bypass signaling, and immunosuppressive tumor microenvironments. These insights have provided critical theoretical foundations for next-generation antibody design and resistance-targeting strategies. Corresponding resistant cell lines and animal models have also become essential tools in oncology research.

Predictive biomarkers

To date, no biomarker beyond HER2 overexpression/amplification has been prospectively validated for predicting trastuzumab response. Candidate biomarkers—including ESR1 expression, stromal tumor-infiltrating lymphocytes (sTILs), PIK3CA/PTEN mutations, and circulating tumor DNA (ctDNA)—have shown inconsistent results and lack robust clinical validation.

Antibody engineering optimization

Key areas of active research include Fc glycoengineering, affinity maturation, bispecific antibody design, and the development of antibody-drug conjugate (ADC) platforms derived from trastuzumab scaffolds.

Deeper understanding of resistance biology

While much work has focused on tumor-intrinsic mechanisms, resistance driven by the tumor microenvironment, clonal evolution, and epithelial–mesenchymal transition (EMT) remains insufficiently characterized and represents an important frontier.