Zymeworks and Jazz Pharmaceuticals announced positive topline results from the Phase 3 HERIZON‑GEA‑01 trial evaluating zanidatamab (Ziihera) in first‑line HER2‑positive locally advanced or metastatic gastroesophageal adenocarcinoma. Zanidatamab plus chemotherapy produced highly statistically significant and clinically meaningful progression‑free survival gains versus trastuzumab plus chemo; the triplet with PD‑1 inhibitor tislelizumab also showed statistically significant overall survival and PFS improvements. Jazz and partner BeOne Medicines plan regulatory submissions, with Jazz indicating a supplemental BLA filing for the U.S. market in 1H‑2026. The data position zanidatamab as a potential new HER2‑targeted agent‑of‑choice in a setting long dominated by trastuzumab‑based regimens. The results could reshape first‑line HER2 treatment algorithms in gastroesophageal cancer and accelerate commercialization plans between Zymeworks and Jazz.