The U.S. Food and Drug Administration (FDA) has approved two Ziihera® (zanidatamab-hrii)-containing regimens for the first-line treatment of adults with unresectable locally advanced or metastatic HER2-positive (HER2+) gastroesophageal adenocarcinoma (GEA). The August 25, 2026 approval, granted to Jazz Pharmaceuticals, is supported by Phase 3 data from the HERIZON-GEA-01 trial, which demonstrated the longest median overall survival (OS) reported in a Phase 3 trial in this setting. Ziihera now becomes the first and only bispecific HER2-targeted antibody combined with a PD-1 inhibitor and chemotherapy approved for first-line HER2+ advanced GEA.

Clinical Takeaway

Two distinct Ziihera-based regimens have received FDA approval in the first-line HER2+ GEA setting, with each regimen defined by specific HER2 expression criteria:

Drug Profile & Mechanism

  • Ziihera (zanidatamab-hrii) is a bispecific HER2-targeted antibody that simultaneously binds two distinct domains of the HER2 receptor, enabling dual HER2 blockade through a single agent.
  • When combined with tislelizumab-jsgr, a PD-1 immune checkpoint inhibitor, the regimen provides concurrent HER2-directed and immune-mediated antitumor activity alongside cytotoxic chemotherapy.
  • The U.S. Prescribing Information for Ziihera contains Boxed Warnings for diarrhea and embryo-fetal toxicity.

Target Population

  • Disease: Unresectable locally advanced or metastatic HER2-positive gastroesophageal adenocarcinoma (GEA), encompassing cancers of the stomach, gastroesophageal junction, and esophagus.
  • HER2 criteria: IHC 3+ or IHC 2+/ISH+ (for the triplet regimen); IHC 3+ only (for the doublet regimen).
  • PD-L1 status: Approval of the Ziihera + tislelizumab + chemotherapy regimen is independent of PD-L1 status.
  • Disease burden: GEA is the fifth most common cancer worldwide, with approximately 20% of patients harboring HER2+ disease.

Study Design

Efficacy Outcomes

  • Progression-Free Survival (PFS):
    • Both Ziihera-containing combinations significantly improved PFS in the overall HER2+ population.
    • Risk of disease progression or death reduced by 35% versus trastuzumab plus chemotherapy.
    • Median PFS: 12.4 months (Ziihera-based) vs. 8.1 months (trastuzumab plus chemotherapy).
  • Overall Survival (OS) — Ziihera + tislelizumab + chemotherapy:
    • Demonstrated a statistically significant and clinically meaningful OS benefit in the overall HER2+ population.
    • Risk of death reduced by 28% compared with trastuzumab plus chemotherapy.
    • Median OS: 26.4 months vs. 19.2 months, representing a greater than seven-month improvement.
    • This is the longest median OS reported in a Phase 3 trial in the HER2+ advanced GEA setting.
  • Subgroup consistency: PFS and OS benefits were generally consistent across major prespecified subgroups, including geographic region and PD-L1 status (both PD-L1–positive and PD-L1–negative tumors).

Safety

  • Ziihera-containing regimens demonstrated a manageable safety profile consistent with prior zanidatamab clinical studies.
  • Diarrhea was the most common adverse reaction, with predominantly early onset in the treatment course.
  • Loperamide prophylaxis was administered during the first cycle; diarrhea was managed with antidiarrheals and treatment modifications as needed.
  • Diarrhea management strategies resulted in few treatment discontinuations of Ziihera.
  • The U.S. Prescribing Information contains Boxed Warnings for diarrhea and embryo-fetal toxicity. Clinicians should consult the full Prescribing Information prior to use.

Key Clinical Implications

✔ Ziihera plus tislelizumab and chemotherapy is now the first and only bispecific HER2-targeted antibody combined with a PD-1 inhibitor approved in the first-line GEA setting, covering all HER2+ patients (IHC 3+ and IHC 2+/ISH+) regardless of PD-L1 expression.

✔ The HERIZON-GEA-01 trial achieved a median OS of 26.4 months — the longest ever reported in a Phase 3 first-line HER2+ GEA trial — representing a clinically meaningful improvement of more than seven months over trastuzumab-based therapy.

✔ PFS and OS benefits were consistent across PD-L1–positive and PD-L1–negative subgroups, underscoring that treatment selection need not be contingent on PD-L1 status for HER2+ GEA patients receiving the Ziihera triplet regimen.

✔ HER2 testing at diagnosis remains critical: clinicians should confirm HER2 status (IHC and ISH) for all patients with newly diagnosed advanced or metastatic GEA to identify those who may benefit from these newly approved regimens.

✔ Proactive diarrhea management — including loperamide prophylaxis in cycle 1 — is an important component of the Ziihera treatment algorithm given the Boxed Warning for diarrhea; clinicians should counsel patients and establish a management plan prior to initiation.

Bottom Line

The FDA approval of Ziihera (zanidatamab-hrii)-based regimens marks a significant shift in the first-line treatment landscape for HER2+ advanced GEA. Backed by the longest median OS ever demonstrated in a Phase 3 trial in this setting — 26.4 months with the Ziihera triplet — and efficacy benefits that extend across PD-L1 subgroups, these regimens establish a new standard of care for a patient population that has historically faced limited options and poor prognosis. Oncology teams should prioritize HER2 testing at diagnosis and familiarize themselves with the approved regimens, safety management strategies, and patient selection criteria outlined in the full Prescribing Information.

Sources: