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Target expression does not cleanly correlate with antibody-drug conjugate activity because receptor internalization matters more than surface presence alone. In lung cancer, HER2 tyrosine kinase domain mutations cause the receptor to internalize much faster than the native receptor, driving more efficient trafficking of the deruxtecan payload into the cell. Consequently, HER2 mutation status is a vastly superior predictor of T-DXD efficacy compared to HER2 immunohistochemistry overexpression.
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Effective treatment of HER2-driven NSCLC requires more than just identifying mutations. HER2 is a multiplexed biomarker where both genetic mutations (TKD and non-TKD) and protein overexpression (via IHC) are independently actionable. Comprehensive testing is crucial to ensure patients are eligible for the full range of available targeted therapies, including TKIs and ADCs.
The specific type of HER2 mutation significantly impacts TKI efficacy. YVMA exon 20 insertions show the highest response rates. Other TKD (tyrosine kinase domain) mutations perform moderately well, while non-TKD mutations respond poorly. This molecular nuance is critical for predicting treatment success and managing patient expectations.
Oncologists distinguish between HER2 amplification (the target for ADCs like TDXD) and activating mutations. A patient whose tumor loses amplification but retains a mutation is considered "HER2 mutated," not "HER2 positive," and is generally not a candidate for ADC therapy.
Unlike many targeted therapies, resistance to the antibody-drug conjugate Trastuzumab-Deruxtecan (TDXD) in HER2-mutant lung cancer is not caused by new HER2 mutations. Instead, resistance mechanisms are linked to the chemotherapy payload, such as the development of efflux pumps, which is a fundamentally different challenge for subsequent treatment strategies.
In NSCLC, "HER2 positive" can mean one of three largely separate conditions: rare exon 20 mutations (~2%), rare gene amplifications (~2%), or more common protein overexpression (20-30%). Understanding these distinctions is critical for accurate biomarker testing and selecting appropriate therapies.
Experts question if HER2 status truly predicts ADC efficacy in urothelial cancer. The benefit seen across low-expression levels suggests HER2's main role may be simply to target the chemo payload to cancer cells, rather than indicating a specific biological dependency.
HER2 in non-small cell lung cancer comprises two distinct conditions: genomic mutations (e.g., exon 20 insertions) treated with Tyrosine Kinase Inhibitors (TKIs), and protein overexpression detected by IHC, treated with Antibody-Drug Conjugates (ADCs). Confusing them leads to incorrect therapy, as TKIs are ineffective for overexpression alone.
Despite being "targeted therapies," multiple promising antibody-drug conjugates (ADCs) for small cell lung cancer (SCLC) show no correlation between the target protein's expression level and patient response. This suggests the payload or other factors are the primary drivers of efficacy, complicating biomarker development for patient selection.
The mechanism of antibody-drug conjugates (ADCs) changes the purpose of HER2 testing. Unlike older therapies needing HER2 amplification to interrupt a functional pathway, ADCs only need the HER2 protein as an "anchor" to deliver their payload. This means even low protein expression is relevant, and follow-up amplification tests like FISH are often unnecessary for determining ADC eligibility.
While most HER2 mutations in NSCLC occur in the tyrosine kinase domain (TKD), about 20% arise elsewhere. Current HER2 TKIs show high activity against TKD mutations but have significantly lower response rates (~30%) for non-TKD mutations, highlighting an area of unmet need for this patient subset.
