Zanidatamab + chemotherapy (CT) ± tislelizumab for first-line (1L) HER2-positive (HER2+) locally advanced, unresectable, or metastatic gastroesophageal adenocarcinoma (mGEA): Primary analysis from HERIZON-GEA-01.

LBA285 Background: HERIZON-GEA-01 (NCT05152147) is a global, open-label, phase 3 trial of zanidatamab (dual HER2-targeted bispecific antibody) + CT ± tislelizumab (anti–PD-1) vs trastuzumab (tras) + CT in 1L HER2+ mGEA. Methods: Eligible patients (pts) with previously untreated HER2+ mGEA, regardless of PD-L1 status, were randomized (1:1:1) to zanidatamab (1800 mg [10% of pts in either zanidatamab-containing arm were diarrhea, hypokalemia, and anemia; the tras + CT arm were diarrhea, anemia, neutrophil count decreased, and platelet count decreased. HER2-targeted therapy was discontinued for related AEs in 11.9% of pts with zanidatamab + tislelizumab + CT, 8.5% with zanidatamab + CT, and 2.3% with tras + CT. Conclusions: Both zanidatamab-containing regimens demonstrated a clinically meaningful and statistically significant prolongation of PFS (mPFS >12 mo) vs tras + CT. Zanidatamab + tislelizumab + CT also provided a statistically significant and clinically meaningful OS benefit (mOS >26 mo). The trial is ongoing with additional OS analyses planned for zanidatamab + CT. No new safety signals were observed for zanidatamab or tislelizumab. These results support zanidatamab as a new standard in HER2-targeting agents, potentially replacing tras, as well as the use of tislelizumab in 1L HER2+ mGEA. Clinical trial information: NCT05152147 . Tras + CT(n = 308) Zanidatamab + CT (n = 304) Zanidatamab + Tislelizumab + CT (n = 302) mPFS (95% CI), mo 8.1 (7.0, 8.9) 12.4 (9.8, 14.5) 12.4 (9.8, 18.5) Hazard ratio (95% CI) – 0.65 (0.52, 0.81); P