Analytical and Clinical Validation of the Plasma-Based Guardant360 CDx Test for Assessing HER2 (ERBB2) Mutation Status in Patients with Non-Small-Cell Lung Cancer for Treatment with Trastuzumab Deruxtecan in DESTINY-Lung01/02

This study demonstrates the analytical and clinical validity of the approved (United States and Japan) plasma- based Guardant360 companion diagnostic (CDx) test for selecting patients with human epidermal growth factor receptor 2 (HER2 [ERBB2])-mutated (HER2m) non-small-cell lung cancer (NSCLC) for trastuzumab deruxtecan (T-DXd) treatment. Concordance between the Guardant360 CDx test and the plasma-based AVENIO ctDNA Expanded Kit Assay (AVENIO), as well as the tissue-based clinical trial assays (CTAs) was investigated. Clinical utility was assessed by comparing T-DXd clinical efficacy results of patients in DESTINY-Lung01/02 who tested positive for HER2 mutations using the Guardant360 CDx test to benchmark efficacy results from DESTINY-Lung01/02. Finally, concordance between the Guardant360 CDx test and the tissue-based Oncomine Dx Target (ODxT) test was explored. High concordance was observed between the Guardant360 CDx test versus AVENIO [positive percent agreement (PPA), 98.8%; negative percent agreement (NPA), 91.5%] and CTAs (DESTINY-Lung01 Cohort 2-PPA, 91.0%; NPA, 100%; DESTINY-Lung02 arm 1-PPA, 86.0%; NPA, 100%). Confirmed objective response rates were similar in patients with HER2m NSCLC identified by the Guardant360 CDx test and by CTAs. There was a high level of agreement between the Guardant360 CDx test and the ODxT test. The Guardant360 CDx test demonstrated analytical and clinical validity for identifying patients with HER2m NSCLC for T-DXd therapy; results support plasma-based testing when tissue-based testing is not feasible. (J Mol Diagn 2025, 27: 119-129; https://doi.org/10.1016/j.jmoldx.2024.11.006)