Acquired gene alterations potentially related to resistance to anti-HER2 therapy in HER2-positive metastatic colorectal cancer (mCRC).

277 Background: While HER2-targeted therapies benefit patients (pts) with advanced HER2-positive mCRC, the development of resistance remains inevitable. This study aims to explore the landscape and frequency of genetic alterations putatively associated with acquired resistance to anti-HER2 therapy in HER2-positive mCRC. Methods: In this retrospective-multicenter international study involving Mayo Clinic, Duke Cancer Center and National Cancer Center Hospital East (Japan), we descriptively compared comprehensive genomic analyses from tissue or blood samples of pts with HER2-positive CRC pre-treatment and post-treatment with HER2-targeting agents. Results: A total of 36 pts were included in this study. 29 (80%) had HER2-positive mCRC as confirmed by immunohistochemistry (IHC) and fluorescent in situ hybridization (ISH), the remaining pts had HER2 amplification detected on blood or tissue . 34/36 (94%) pts were administered their first anti-HER2 agent after progressing on ≥2 prior lines of treatment. The most common anti-HER2 regimen administered was trastuzumab/pertuzumab (50%, 18/36), followed by trastuzumab-tyrosine kinase inhibitors (TKIs) (tucatinib or neratinib) (30%, 11/36) and trastuzumab deruxtecan (14%, 5/36). Putative genomic mechanisms of acquired resistance were detected in 72% (26/36), while the remaining pts lacked new genomic alterations that might explain resistance to anti-HER2 therapy, suggesting alternative escape mechanisms. Acquired alterations in downstream or alternate bypass pathways were identified in 72% (26/36) of pts; most commonly in the RAS/RAF pathway and in EGFR (34.6%, 9/26 each), followed by 27% (7/26) of pts with MET point mutations (mts) and amplification; activating muts in PI3K/AKT and in 27% (7/26), most commonly CDK12 (60%, 4/7) . HER2 receptor alterations were detected in 3 pts (8.3%), all of whom received trastuzumab-tucatinib and included HER2 L755S, HER2 V777L, HER2 D769Y, HER2 G727A and HER2 S310F, which are known to cause resistance to , however, information about tucatinib resistance is not yet available. Other altered pathways included AR, RB, NOTCH and HRD. Two pts lost HER2 expression on blood and tissue NGS post anti-HER2 therapy. Conclusions: The emergence of resistance might be in part related to acquired alterations that constitutively activate signaling pathways parallel or downstream of HER2, bypassing HER2 inhibition. Additional histologic studies, deep mutational scanning, larger populations and correlative analysis are needed to validate these findings.