44 Background: The addition of trastuzumab to chemotherapy improves survival in first line treatment of advanced HER2 overexpressing EGC. However, the response rate is under 50% in this molecularly heterogeneous group of cancers. Genomic alterations (GA) associated with lack of benefit from trastuzumab are understudied. Methods: We prospectively analyzed clinical samples from patients with EGC using hybrid-capture based comprehensive genomic profiling (CGP). Pre-specified literature review was used to determine GA associated with de-novo trastuzumab resistance. Results: From 2,245 gastroesophageal junction (GEJ) adenocarcinomas and 1,883 gastric adenocarcinomas (GC) we identified 395 HER 2-amplified GEJ (18%) and 132 HER 2-amplified (HER2amp) GC (7.0%) cases. Median HER 2 copy number was 19 in GEJ and 16 in GC samples. PIK3CA GA and METamp were observed in ~9% and ~5% of both HER 2amp and non- HER2amp EGC cases; however, co-amplification of cell-cycle mediators CDK6 and CCCNE1 were each significantly enriched in HER 2amp cases (Table). MYCamp and deleterious SMAD4 GA were also significantly enriched in cases with Her2amp. CDKN2A GA were common regardless of HER 2amp, particularly in GEJ . In cases with >100 estimated HER 2 copies, only PIK3CA GA were significantly less frequent than in those with 6-99 copies (2.3% vs. 9.8%, P =0.04). All HER 2amp cases were microsatellite stable. A clinically annotated HER 2amp cohort will be presented. Conclusions: GA predicted to decrease trastuzumab sensitivity exist in a significant portion of HER 2amp EGC, although not all are enriched relative to non- HER 2amp cases. CGP may provide additive information to traditional HER2 testing and identify patients less likely to benefit from therapy. Larger clinical datasets are needed to validate our observations. [Table: see text]
Significance of HER 2/neu in Gastric Adenocarcinomas, A Clinicopathological correlation
