Zanidatamab Beats Trastuzumab on OS in Phase 3, But Comparator Gap Clouds Market Access

Ziihera Demonstrates Superior Overall Survival in First-Line HER2+ GEA

Jazz Pharmaceuticals plc announced second interim top-line overall survival (OS) results from the Phase 3 HERIZON-GEA-01 trial. The study showed that Ziihera® (zanidatamab-hrii) in combination with chemotherapy achieved statistically significant and clinically meaningful improvements in OS compared with trastuzumab plus chemotherapy. This was observed in adults with HER2-positive (HER2+) locally advanced or metastatic gastroesophageal adenocarcinoma (GEA) as a first-line treatment. The OS hazard ratio for the Ziihera plus chemotherapy regimen improved compared to the first interim analysis, and its safety profile remained consistent with previously reported data, showing no new safety signals. These results reinforce Ziihera's position as a definitive HER2-targeted backbone therapy, following its recent FDA approval for first-line HER2+ advanced GEA.

• The Phase 3 HERIZON-GEA-01 trial demonstrated that Ziihera (zanidatamab-hrii) combined with chemotherapy achieved statistically significant and clinically meaningful improvements in overall survival (OS) compared to trastuzumab plus chemotherapy. This outcome establishes Ziihera as a superior HER2-targeted therapy for first-line HER2-positive locally advanced or metastatic gastroesophageal adenocarcinoma (GEA), setting a new benchmark for treatment efficacy in this aggressive cancer.
• With longer follow-up, the overall survival hazard ratio for both Ziihera plus chemotherapy and Ziihera plus tislelizumab and chemotherapy regimens further improved. These updated results reinforce Jazz Pharmaceuticals' confidence in Ziihera as the new HER2-targeted backbone therapy, replacing trastuzumab, and support the regimen of Ziihera plus tislelizumab and chemotherapy as the new standard of care in the United States for first-line HER2+ advanced GEA.
• The FDA recently approved Ziihera on August 25, 2026, for the first-line treatment of adults with HER2+ unresectable locally advanced or metastatic GEA. This approval covers two distinct regimens: Ziihera in combination with tislelizumab and chemotherapy for patients with HER2+ (IHC 3+ and IHC 2+/ISH+) disease, and Ziihera in combination with chemotherapy for patients with HER2+ (IHC 3+) disease, offering tailored treatment options.
• The safety profile of Ziihera in combination with chemotherapy was generally consistent with the known safety profile of each agent and previously reported data from HERIZON-GEA-01, with no new safety signals observed. Jazz Pharmaceuticals has submitted these updated OS data for presentation at a major medical meeting in the fourth quarter of 2026 and plans to submit them to global health authorities for further regulatory consideration.

The Persistent Challenges in Treating Advanced HER2+ GEA

Despite meaningful advances in HER2-targeted therapy for gastroesophageal adenocarcinoma, several biological and clinical obstacles continue to limit treatment durability and patient outcomes. The landscape is evolving rapidly, yet resistance, tumor heterogeneity, and the complexity of pathway biology remain central barriers to sustained efficacy.

• Intratumoral HER2 heterogeneity and poor prognosis: HER2 expression within individual tumors is often variable. Patients with HER2 heterogeneity — classified by immunohistochemistry on endoscopic biopsy specimens — demonstrated significantly worse outcomes on trastuzumab-based chemotherapy compared with homogeneous HER2 expressors: overall response rate 35.7% vs. 79.5% (P = 0.002), median progression-free survival 2.5 vs. 7.9 months (HR: 1.905; 95% CI: 1.109–3.268), and median overall survival 12.5 vs. 25.7 months (HR: 2.430; 95% CI: 1.389–4.273). Multivariate analysis identified IHC HER2 heterogeneity as an independent poor prognostic factor (HR: 3.115; 95% CI: 1.610–6.024).

• Acquired resistance to trastuzumab and T-DXd: Acquired resistance to HER2-targeting agents is frequent. Spatial transcriptomic analysis of patient-matched HER2-positive gastric cancers revealed multiple distinct resistance mechanisms to trastuzumab, including epithelial-mesenchymal transition (EMT) with PD-L1 and CCL2 upregulation in approximately one-third of patients, and activation of the endoplasmic reticulum-associated degradation (ERAD) pathway — including genes such as GOLM1 — in another third. Resistance to trastuzumab deruxtecan (T-DXd) was associated with HLA loss and increases in oxidative phosphorylation pathways. Increased expression of CLDN18.2 was also observed in trastuzumab-resistant tumors.

• PI3K-Akt pathway activation undermining HER2-targeted therapy: The PI3K-Akt pathway is a principal downstream signaling pathway of HER2. PIK3CA mutations and phosphate and tensin homolog (PTEN) inactivation cause over-activation of downstream signaling independent of upstream HER2 activation, with reported frequencies of 4–25% and 16–77%, respectively. HER2 overexpression has been found to be significantly correlated with pAkt expression in gastric cancer tissues, and pAkt expression was correlated with poor prognosis. PIK3CA mutations and/or PTEN inactivation may affect the effectiveness of HER2-targeting therapy. KLK10 (NES1) has also been identified as a driver of trastuzumab resistance through PI3K/AKT pathway activation in resistant cell lines.

• HER2 loss over time and tumor heterogeneity limiting sequential therapy: Resistance to anti-ERBB2 therapy in gastroesophageal adenocarcinoma is "mostly due to tumor heterogeneity with the existence of low expressing ERBB2 tumor clones and loss of ERBB2 over time." This dynamic HER2 expression complicates treatment sequencing and biomarker-guided decision-making across lines of therapy.

• Failure of multiple HER2-directed agents to improve upon trastuzumab-based therapy: Attempts to improve first-line outcomes by adding or substituting HER2-directed agents — including lapatinib, high-dose trastuzumab, pertuzumab, and trastuzumab emtansine — did not displace trastuzumab-based therapy, underscoring the difficulty of translating strategies successful in breast cancer to the gastroesophageal setting.

• Variable efficacy across antibody-drug conjugates and toxicity management: A systematic review and meta-analysis of ADCs in HER2-positive advanced gastric cancer (12 studies, 1,041 patients) reported a pooled overall response rate of 33.4% (95% CI: 26.3%–41.3%), with substantial variability among agents — T-DXd achieved an ORR of 42.5% while trastuzumab emtansine showed only 20.6%. The most common severe toxicities were grade ≥3 anemia (21.1%) and neutropenia (15.1%), requiring active management. Additionally, adjudicated drug-related interstitial lung disease or pneumonitis occurred in 13.9% of patients receiving T-DXd in the DESTINY-Gastric04 trial, representing a known and clinically significant risk requiring monitoring.

• Complexity of integrating immune checkpoint inhibition and defining optimal sequencing: The central clinical question has shifted to "which HER2-targeted platform should anchor first-line therapy, how immune checkpoint inhibition should be integrated, and how HER2 heterogeneity and biomarker evolution should guide treatment sequencing" — reflecting the absence of established frameworks for sequencing the expanding array of HER2-directed and immunotherapy combinations.

HERIZON-GEA-01: Unpacking Ziihera's Definitive OS Benefit

Several landmark and emerging trials have shaped the treatment landscape for HER2-positive gastric and gastroesophageal junction (GEJ) adenocarcinoma, spanning perioperative, first-line, and later-line settings. The studies below vary in phase, design, and primary endpoints, reflecting the evolving complexity of HER2-directed therapy in this indication.

The knowledge base does not have sufficient information on this aspect. Specifically, detailed design parameters and endpoints for HELOISE, LOGiC, JACOB, TyTAN, GATSBY, T-ACT, KEYNOTE-811, and DESTINY-Gastric01 — trials referenced by name in the retrieved literature — are not reported in the available data.

Ziihera: Setting a New Standard in First-Line HER2+ GEA

For more than a decade following the ToGA trial, trastuzumab plus platinum-fluoropyrimidine chemotherapy remained the only established first-line HER2-targeted standard for unresectable or metastatic HER2-positive gastric or gastroesophageal junction adenocarcinoma. That paradigm has since shifted substantially. The randomized, double-blind, phase III KEYNOTE-811 trial evaluated pembrolizumab added to trastuzumab and chemotherapy as first-line treatment, demonstrating improvements in response, progression-free survival, and overall survival, particularly in PD-L1-positive disease. Separately, the phase III HERIZON-GEA-01 trial showed that zanidatamab plus chemotherapy, with or without tislelizumab, improves progression-free survival compared with trastuzumab plus chemotherapy, and that zanidatamab plus tislelizumab and chemotherapy improves overall survival. These results signal that the central clinical question is shifting from whether HER2 blockade should be combined with chemotherapy to which HER2-targeted platform should anchor first-line therapy and how immune checkpoint inhibition should be integrated.

In the second-line setting, trastuzumab deruxtecan (T-DXd) — a novel antibody-drug conjugate composed of a monoclonal anti-HER2 antibody and a topoisomerase I inhibitor, DX-8951 derivative — has emerged as a pivotal agent. The phase II DESTINY-Gastric01 trial demonstrated that T-DXd produced an objective response in 51% of participants with pretreated HER2-positive advanced gastric cancer versus 14% with standard chemotherapy, and reduced the risk of death by 41%, with a median overall survival of 12.5 months versus 8.4 months. These results supported approval of T-DXd for HER2-positive pretreated advanced gastric cancer in Japan. The subsequent international, randomized, phase 3 DESTINY-Gastric04 trial then compared second-line T-DXd at 6.4 mg per kilogram of body weight against ramucirumab plus paclitaxel in patients with HER2-positive metastatic gastric cancer or gastroesophageal junction adenocarcinoma confirmed on tumor biopsy after progression on trastuzumab-based therapy. T-DXd demonstrated significantly longer overall survival (median 14.7 vs. 11.4 months; hazard ratio for death, 0.70; 95% confidence interval, 0.55 to 0.90; P = 0.004), along with significant improvements in progression-free survival (hazard ratio, 0.74; 95% CI, 0.59 to 0.92) and confirmed objective response (44.3% vs. 29.1%).

Safety profiles across these regimens warrant attention. In DESTINY-Gastric01, 10% of T-DXd-treated patients experienced interstitial lung disease or pneumonitis potentially related to treatment. In DESTINY-Gastric04, adjudicated drug-related interstitial lung disease or pneumonitis occurred in 13.9% of patients receiving T-DXd (grade 1 or 2 in 33 patients and grade 3 in 1), compared with 1.3% in the ramucirumab plus paclitaxel group. Drug-related adverse events of any grade were reported in 93.0% of T-DXd patients and 91.4% of those receiving ramucirumab plus paclitaxel, while grade 3 or higher events occurred in 50.0% and 54.1%, respectively. Beyond T-DXd, T-DXd is also moving from later-line therapy into first-line development through platinum-free combinations with fluoropyrimidine and immune checkpoint blockade, further underscoring the breadth of the evolving treatment landscape for HER2-positive gastroesophageal adenocarcinoma.

Ziihera's OS Data Reshapes First-Line HER2+ GEA

The recent announcement regarding Ziihera's (zanidatamab) overall survival (OS) data in first-line HER2-positive gastroesophageal adenocarcinoma (GEA) marks a significant inflection point in the treatment paradigm for this aggressive cancer. For over a decade, trastuzumab combined with platinum-fluoropyrimidine chemotherapy has been the foundational standard, a legacy of the ToGA trial. However, the HERIZON-GEA-01 trial's latest results, demonstrating statistically significant and clinically meaningful OS improvements for zanidatamab plus chemotherapy, signal a definitive shift. This dual HER2-targeted bispecific antibody is now poised to become a new backbone therapy, offering patients a superior efficacy profile compared to the long-established trastuzumab regimen.

This development is not without its complexities and strategic considerations. While the OS benefit is clear, the safety profile of zanidatamab-containing regimens, particularly the higher incidence of grade ≥3 diarrhea, will require careful management and patient education. Furthermore, the competitive landscape is rapidly evolving. The literature highlights that zanidatamab in combination with tislelizumab and chemotherapy previously showed an even more robust OS benefit in interim analyses, suggesting that the optimal zanidatamab-based regimen might involve additional agents. Moreover, the emergence of pembrolizumab-based chemoimmunotherapy, which has also demonstrated OS improvements, particularly in PD-L1-positive disease, means Ziihera will enter a dynamic market where differentiation and precise patient selection will be crucial.

The central clinical question for HER2-positive advanced gastric cancer is indeed shifting: it's no longer just about whether HER2 blockade should be combined with chemotherapy, but rather which HER2-targeted platform should anchor first-line therapy, and how immune checkpoint inhibition should be optimally integrated. Ziihera's success sets a new, higher bar for efficacy, pushing the boundaries of what's achievable for patients and driving further innovation in next-generation HER2-targeting strategies.