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.

For never-smokers with HER2 mutations, immunotherapy is largely ineffective and risks severe immune-related adverse events when the patient is later switched to the correct TKI. This paradigm mirrors the approach for EGFR and ALK mutations, where targeted therapy is the standard upfront, even with high PD-L1 expression.

Despite both being options, experts favor the TKI zongertinib for second-line treatment of HER2-mutant NSCLC. This preference is driven by zongertinib's higher response rates (~71%) and longer progression-free survival in this setting, coupled with a better side effect profile compared to the ADC trastuzumab-deruxtecan (TDXD).

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.

NGS testing is revealing that acquired HER2 kinase domain mutations, not amplifications, are an emerging resistance mechanism in ER+ lobular breast cancer. This creates a targetable population for HER2 TKIs like neratinib or tucatinib, offering a new line of targeted therapy.

For HER2-mutant NSCLC, Zongertinib (a TKI) is now the first-line choice. While TKIs show activity after ADCs like TDXD, the effectiveness of using an ADC after TKI failure remains an unproven but critical clinical question for oncologists.

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.

The standard approach to reducing cancer drug toxicity is narrowing the target to specific mutations (e.g., HER2, KRAS). While this improves safety, it drastically shrinks the addressable patient population for each new therapy. This puts immense pressure on the pharmaceutical business model, where development costs average $2.5 billion per drug.

The new treatment paradigm for HER2-positive lung cancer will likely involve sequencing a TKI like zongertinib first, followed by an antibody-drug conjugate (ADC). Early data suggests that the efficacy of TKIs is significantly reduced when used after an ADC, making the TKI-first approach critical for maximizing patient outcomes.