Tumor-Agnostic Landscape with HER2 Amplification in Japan: Real-World Prevalence and Implications for Targeting HER2

Limited data exist on the prevalence of human epidermal growth factor receptor 2 (ERBB2/HER2) amplification in patients with all types of solid tumors. This retrospective, observational study (UMIN ID: UMIN000057382) analyzed the prevalence of HER2 amplification across all solid tumors using comprehensive genomic profiling (CGP) data from the Center for Cancer Genomics and Advanced Therapeutics database in Japan. We analyzed 89,374 eligible patients with solid tumors: HER2 amplification was detected in 5119 patients (5.7%). The highest rates of HER2 amplification were observed in patients with tumors of the esophagus/stomach (12.9%), followed by tumors of the bladder/urinary tract (10.6%), breast (9.5%), biliary tract (8.4%), and uterus (8.4%). Among the five assay platforms, FoundationOne CDx accounted for 69.7% of all CGP tests and had an HER2 amplification detection rate of 7.4%, compared to the other four platforms (range: 1.9-15.4% of all CGP tests [detection rates: 1.5-2.3%]). Substantial differences were observed in the mutation frequencies of multiple genes between HER2 amplified and HER2 non-amplified tumors. The results highlight that HER2 amplification extends beyond conventional tumor types and enables the identification, via CGP testing, of non-traditional tumor subsets (cancers other than breast and gastric cancer), including rare cancers, that could be candidates for HER2-targeting therapy such as trastuzumab deruxtecan in Japan.