| Summary: | Targeted therapies against human epidermal growth factor receptor 2 (HER2) are associated with increased interstitial lung disease (ILD). Trastuzumab, lapatinib, pertuzumab, and trastuzumab emtansine have markedly extended HER2 breast cancer survival but current knowledge on how these HER2-targeted agents induce interstitial lung disease is still poorly defined due to limited cases in the literature. Physicians mostly managed this complication by dose interruption, dose de-escalation, or discontinuation with success. In 2019, the FDA had granted accelerated approval on trastuzumab deruxtecan (T-Dxd) in HER2 breast cancer in the late line setting. Severe ILD incidence rate was over ten percent and led to fatal outcomes in 2.2% of patients in the T-Dxd trial. Searching for biomarkers to detect ILD incidence before it becomes clinically fulminant or for treatment response monitoring is of high clinical value.A Case of life-threatening trastuzumab-induced ILD was encountered in our facility. The ILD was confirmed to be antineutrophil cytoplasmic antibody (ANCA) pulmonary capillaritis. The biomarker of neutrophil extracellular traps (NETs), serum MPO-DNA complex, showed a good correlation with the clinical severity. Soon after B cell depleting agent rituximab usage, the serum MPO-DNA outperformed ANCA autoantibody and maintained its correlation with clinical severity. In addition to the trastuzumab-induced ILD case, a prospective cohort in our facility also confirmed the usefulness of MPO-DNA in monitoring vasculitis activity. We postulated that upfront testing with biomarkers of vasculitis during HER2 targeted treatment with high ILD incidence may be beneficial in the future.
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