Zymeworks’ zanidatamab (Ziihera®) produced statistically significant and clinically meaningful improvements in progression‑free survival in the Phase 3 HERIZON‑GEA‑01 trial versus trastuzumab plus chemotherapy in first‑line HER2‑positive gastroesophageal adenocarcinoma (GEA). Adding PD‑1 inhibitor tislelizumab to zanidatamab and chemotherapy showed statistically significant overall survival benefits at the first OS interim analysis. Jazz Pharmaceuticals, which holds regional rights, and development partners announced topline data supporting zanidatamab as a potential new standard of care in first‑line HER2+ GEA. Jazz indicated plans to submit a supplemental Biologics License Application in the first half of 2026 based on the results. The trial readout reconfigures partner expectations and commercial strategy: zanidatamab showed consistent PFS gains across both combination regimens, and the arm with PD‑1 blockade delivered measurable OS benefit. Partners and investors will watch regulatory filings and subsequent OS analyses to assess the magnitude of label expansion and market positioning.