HER2-directed TKIs (like zongertinib) and ADCs (trastuzumab-deruxtecan/TDXD) have different mechanisms, and clinical data shows they are effective when used sequentially. Zongertinib works post-TDXD, and vice-versa. However, there is no evidence to support using one HER2 TKI after another has failed.

Historically, HER2-mutated lung cancer was treated with cytotoxic chemotherapy, unlike other oncogene-driven cancers that used targeted therapies upfront. Zongertinib's approval as a first-line oral TKI marks a significant philosophical shift, aligning its treatment strategy with the biomarker-driven care standard in lung oncology.

Unlike broader-spectrum tyrosine kinase inhibitors, zungertinib is highly selective for HER2 and avoids targeting wild-type EGFR. This specific mechanism is crucial as it leads to a better toxicity profile, particularly reducing the common EGFR-related side effects of rash and diarrhea, improving patient tolerability.

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).

Despite both Zongertinib and Sevabirtinib showing high efficacy in HER2-mutant NSCLC, Zongertinib's selective nature results in a much better safety profile. Sevabirtinib's dual EGFR/HER2 inhibition leads to significant GI and skin toxicities, making tolerability, not just efficacy, the key factor for clinical preference.

An expert oncologist indicated a willingness to use the TKI zongertinib off-label for stage III HER2-mutant NSCLC patients after chemoradiation, instead of the standard-of-care immunotherapy (durvalumab). This reflects a strong conviction that for driver mutation-positive cancers, targeted therapy is superior, even ahead of definitive clinical trial data in this specific setting.

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.

Zongertinib gained both second-line and frontline FDA approval for HER2-mutant NSCLC based on impressive single-arm study data. This unusual path, forgoing traditional randomized Phase III trials, highlights the FDA's willingness to accelerate access to highly effective drugs in areas of high unmet medical need.

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.