Zymeworks reported that its Phase 3 HERIZON‑GEA‑01 trial met its primary endpoints, with the combination of Ziihera (zanidatamab‑hrii) plus tislelizumab (Tevimbra) and chemotherapy producing a median overall survival of 26.4 months versus 19.2 months for the trastuzumab‑plus‑chemotherapy control. Median progression‑free survival was 12.4 months for the Ziihera arms and 8.1 months for the control, confirming a clinically meaningful benefit in both survival and disease control.
The safety profile was consistent with expectations for HER2‑directed therapy and PD‑1 inhibition. Grade ≥ 3 treatment‑related adverse events occurred in 71.8 % of patients receiving Ziihera, tislelizumab, and chemotherapy, 59.0 % of those receiving Ziihera and chemotherapy, and 59.6 % of the trastuzumab arm. Diarrhea was the most common Grade ≥ 3 event, affecting 24.5 % of the triple‑combination group, 20.0 % of the dual‑combination group, and 12.9 % of the control. Discontinuation due to adverse events was 11.9 % for the triple‑combination, 8.5 % for the dual‑combination, and 2.3 % for trastuzumab.
Under existing agreements with Jazz Pharmaceuticals and BeOne Medicines, Zymeworks is eligible for up to $440 million in milestone payments upon regulatory approval in gastroesophageal adenocarcinoma. The payments are structured as $250 million in the United States, $100 million in Europe, $75 million in Japan, and $15 million in China, providing a substantial near‑term cash infusion that can offset the company’s ongoing operating losses.
Kenneth Galbraith, Chair and CEO, said the data “underscore the promise of zanidatamab to advance care for patients with HER2‑positive disease and validate our partnership‑driven approach.” He added that the results “further establish Ziihera as the new HER2‑targeted agent of choice in first‑line locally advanced or metastatic HER2‑positive GEA, with the potential to redefine standard care.”
Investors reacted cautiously, with some profit‑taking after earlier gains and concerns about the higher rates of Grade ≥ 3 adverse events and diarrhea. The market’s tempered response reflects the balance between the clinical benefit and the safety profile that will be scrutinized by regulators and payers.
The trial’s success positions Ziihera as a potential new standard of care in a market estimated at $700 million annually. The milestone payments and anticipated royalty revenue from future indications strengthen Zymeworks’ cash runway into the second half of 2027 and support its transition to a royalty‑driven business model. The data also enhance the company’s competitive standing in oncology, providing a differentiated product that could capture market share from trastuzumab and other HER2‑targeted therapies.
Zymeworks will present the full dataset at the 2026 ASCO‑GI Symposium on January 8, 2026, where regulators and payers will assess the implications for clinical practice and reimbursement. The company’s focus on partnership‑driven development and milestone monetization positions it to capitalize on the positive trial outcomes and accelerate the commercialization of Ziihera in multiple indications.
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