HER2 discordance in upper gastrointestinal cancers in a US-based analysis.

436 Background: HER2 overexpression is present in 20 to 30% of advanced upper gastrointestinal (UGI) cancers and can be targeted in the first-line setting by combining chemotherapy with trastuzumab +/- pembrolizumab. However, the persistence of HER2 overexpression in progressive cases is unknown. Despite this, HER2 overexpression is often targeted in the second-line or more-advanced settings with trastuzumab-deruxtecan based on the initial pathology. Small international series (n=48 and n=7) demonstrated that up to 43% of UGI cancer patients lose HER2 overexpression in progressive tissue. We sought to characterize HER2 overexpression loss in subsequent biopsies in a US-based population. Methods: This study queried the US-based, electronic health record-derived, deidentified Flatiron Health Research Database from January 2011 to June 2025, which has longitudinal patient-level data from approximately 280 cancer clinics. Eligible patients had HER2 assessed prior to initiation of first-line therapy and a second assessment at least 7 weeks or later after first biopsy. HER2 positivity was defined by the ASCO/CAP guidelines. Demographics and baseline clinical and laboratory variables were extracted. We describe proportions with 95% confidence intervals (CI) using the Clopper-Pearson method. We used univariate logistic regressions to estimate odds ratios (ORs) for the binary outcome of loss of HER2 overexpression. Subgroup analyses were stratified by baseline HER2 status (IHC 3+ vs IHC 2+ with fluorescence in situ hybridization [FISH] amplification). Results: A total of 17,305 individuals were assessed for inclusion. In total, 696 patients had at least one HER2 assessment at ≥7 weeks from the initial assessment and 333 patients met criteria for inclusion. Of these, 83 patients (25%) were classified as HER2+ at baseline. HER2+ were more frequently male (73%), former or active smokers (70%), and were more likely to receive HER2-directed therapies in the front line (59%). The initial biopsy site was commonly the primary site (88%), as was the site of second biopsy (67%). Overall, HER2 overexpression was lost on subsequent biopsy 36% (95% CI 25.9-47.4%) of the time. Regardless of therapy received, loss of HER2 overexpression occurred in 83% (51.6-97.9%) of patients with baseline IHC2+/FISH amplified compared to 28% (18.1-40.1%) in patients with baseline IHC3+ (OR 12.8, 95% CI 2.6-63.4, p=0.002). No other clinical or laboratory factors were associated with loss of HER2 overexpression on logistic regression. Conclusions: Loss of HER2 overexpression is a common occurrence in UGI cancers, particularly amongst patients with baseline HER2 IHC2+/FISH amplification. This is possibly due to clonal pressure from targeting HER2, tumoral heterogeneity, or other unknown mechanisms. Greater consideration should be given for repeat biopsies and biomarker assessments in these patients prior to targeting HER2 in the progressive setting.