Defining low-HER2 cancers using RNA quantification by next generation sequencing.

e13044 Background: The recent success of treatment with HER2-directed antibody-drug conjugate T-DXd (trastuzumab-deruxtecan) in treating low-HER2 expression breast cancer raises the possibility of similar success in other tumors that express similarly low levels of HER2. HER2 amplification has been reported in various types of tumors at relatively low frequency. However, “low-HER2” expression in various types of tumors remains undefined. In this study, we utilized next generation sequencing to explore the levels of ERBB2 (HER2), ERBB3, ERBB4, PIK3CA, and ERS1 mRNA in various types of cancers and attempted to set limits reflecting distinct biology of tumors based of level of ERBB2 expression. Methods: RNA was extracted from FFPE samples from patients with cancers of breast (N=203), ovary (N=95), endometrium (N=75), and esophagus/stomach (N=46). RNA was sequenced via a 1408 gene panel using a standard hybrid capture approach. Results: Based on previous data correlating gene amplification with gene expression (DOI: 10.1002/imed.1051), we defined ERBB2 expression as high, low, and very-low when ERBB2 expression measured >1500 TPM (transcripts per million), 800-1500 TPM, and <800 TPM, respectively. ERBB2 high, low, and very-low were detected in 20%, 32%, and 48% of breast cancer; 5%, 29%, and 65% of endometrial cancer; 9%, 26%, and 65% of ovarian cancer; 12%, 16%, and 72% of esophageal/stomach cancer; and 8%, 19%, and 73% of lung cancer. With minor exceptions, cases classified as ERBB2 high and low from the five tumor types showed no correlation with levels of PIK3CA, ESR1, or ERBB4 mRNA. In contrast, cases with relatively very-low ERBB2 showed significant correlation with PIK3CA, ESR1, and ERBB4 mRNA. Endometrial cancer was an exception and showed no correlation with PIK3CA mRNA in very-low ERBB2 cases. Activation of PIK3CA has been reported to be associated with resistance to anti-HER2 therapy. These findings suggest that cases showing high or low ERBB2 are independent of PIK3CA and likely less influenced by the negative effects of PIK3CA on response to HER2 inhibitors. Cases classified as ERBB2 high or low showed significantly higher (P<0.0001) PIK3CA mutations as compared with ERBB2 very-low cases (27% vs 14%). ERBB2 mutations were relatively rare, detected in only 7%, 8%, 14%, 1%, and 4% of breast, endometrial, esophageal/stomach, ovarian, and lung cancers, respectively. However, significantly (P=0.001) higher mutation rate is detected in ERBB2-high and -low as compared with very-low ERBB2 cases (8% vs 6%). Conclusions: ERBB2/HER2 RNA quantification in cancers can define a biologically distinct group of patients. Patients defined as very-low ERBB2 as classified in this approach appear to be biologically distinct and more likely to be resistant to anti-HER2 therapy. Clinical trials using this approach in selecting patients for treatment with antibody-drug conjugates are justified and needed.