Ziihera Demonstrates Superior Efficacy in Advanced Gastroesophageal Junction Cancer Trial

Zymeworks Inc. (ZYME) has announced compelling efficacy results from its Phase 3 HERIZON-GEA-01 trial evaluating Ziihera (zanidatamab-hrii) for treating gastroesophageal junction cancer and related malignancies. The breakthrough findings have triggered a significant market response, with ZYME shares climbing over 38.82% to $27.51 in premarket trading on Monday.

## Clinical Trial Success Reshapes Treatment Landscape

The HERIZON-GEA-01 study compared Ziihera combined with chemotherapy against current standard regimens in HER2-positive malignancies affecting the stomach, gastroesophageal junction, and esophagus. When administered as a first-line therapy, the Ziihera-chemotherapy combination achieved statistically significant improvements in progression-free survival (PFS) relative to the comparator arm. More notably, the triple combination—Ziihera plus the PD-1 inhibitor Tevimbra (tislelizumab) and chemotherapy—demonstrated statistically significant benefits in both overall survival (OS) and PFS metrics, positioning the drug as a potential new care standard for this patient population.

## Regulatory Milestone and Commercial Expansion Plans

Ziihera's regulatory trajectory has accelerated globally. The therapy received accelerated approval in the United States for previously treated, unresectable or metastatic HER2-positive (IHC 3+) biliary tract cancer, secured conditional approval in China, and obtained conditional marketing authorization from the European Commission. This momentum extends to gastroesophageal indications, as Jazz Pharmaceuticals and BeOne Medicines—Zymeworks' commercialization partners—are preparing a supplemental Biologics License Application (sBLA) submission in H1 2026 to support Ziihera's use in first-line gastroesophageal adenocarcinoma treatment.

## Market Perspective

ZYME has traded between $9.03 and $20.17 over the preceding 12 months, with the current premarket surge marking a significant valuation inflection. The Phase 3 efficacy readout validates the clinical hypothesis underlying the gastroesophageal junction cancer indication strategy.