Zanidatamab Expands First-Line HER2-Positive GI Care

What the authorization changes

The FDA decision covers adults with unresectable locally advanced or metastatic HER2-positive adenocarcinoma arising in the stomach, gastroesophageal junction, or esophagus. Treatment must be first-line in the advanced-disease setting. The authorization does not extend to resectable disease, earlier perioperative treatment, or esophageal squamous cell carcinoma.

The common element is zanidatamab-hrii, a bispecific HER2-directed antibody, combined with fluoropyrimidine- and platinum-containing chemotherapy. Clinicians may use that backbone alone or add the programmed cell death protein 1 inhibitor tislelizumab-jsgr. The FDA therefore authorized two distinct combinations rather than one regimen with an optional drug added without supporting evidence.

The wording matters. “With or without tislelizumab” describes two evidence-based strategies, but it does not establish that the immune-checkpoint inhibitor has no incremental benefit or that the regimens are interchangeable for every patient. Selection requires consideration of the relevant labels, contraindications, expected toxicities, comorbidities, disease burden, and patient priorities.

HER2 status is also an eligibility requirement, not a descriptive subgroup identified after treatment. Testing should use an FDA-approved method and an adequate tumor specimen, with attention to the heterogeneous HER2 expression that can occur in upper gastrointestinal adenocarcinomas. The applicable prescribing information and companion-diagnostic requirements should determine the accepted testing definition.

Evidence supporting the decision

The registrational evidence came from HERIZON-GEA-01, a randomized, open-label phase 3 study listed on ClinicalTrials.gov with planned enrollment of approximately 918 participants. It evaluated previously untreated, unresectable locally advanced or metastatic HER2-positive gastric, gastroesophageal junction, and esophageal adenocarcinoma.

Participants were assigned to zanidatamab plus fluoropyrimidine-platinum chemotherapy, zanidatamab plus tislelizumab and the same chemotherapy class, or the active control of trastuzumab plus chemotherapy. The control reflected established HER2-directed first-line treatment rather than placebo, making the central question whether replacing trastuzumab with zanidatamab, with or without checkpoint blockade, improved outcomes.

The trial assessed progression-free survival and overall survival, with tumor-response outcomes and safety providing additional evidence. According to the FDA regulatory decision, the findings supported both authorized zanidatamab combinations. The agency’s posted review and current prescribing information remain the controlling sources for the final analysis population, effect estimates, confidence intervals, data cutoff, follow-up duration, and adverse-reaction frequencies; those numerical details were not included in the source extract supplied for this explainer and should not be reconstructed from preliminary disclosures.

The randomized design supports causal comparisons between each experimental arm and the trastuzumab-based control, subject to the prespecified analysis plan. The open-label design can influence treatment decisions, reporting of subjective symptoms, and post-progression management, although blinded independent review of imaging can reduce bias in progression assessments.

Importantly, a three-arm trial does not automatically establish a statistically powered comparison between the two zanidatamab regimens. Unless the regulatory review specifies a direct, multiplicity-controlled comparison, clinicians should not infer the size of tislelizumab’s incremental contribution by informally comparing results across the experimental arms.

How the two strategies fit into practice

The authorization broadens the first-line HER2-directed menu across three related anatomic sites. It may be especially consequential for esophageal adenocarcinoma, which is often discussed separately from gastric and gastroesophageal junction disease even when systemic-treatment principles overlap. Histology remains essential: this approval concerns adenocarcinoma, not squamous disease.

Before treatment, the practical sequence is to confirm advanced unresectable or metastatic disease, verify adenocarcinoma histology and primary site, document qualifying HER2 positivity, and establish that the patient has not received systemic therapy for advanced disease. Prior perioperative therapy does not necessarily answer the first-line question by itself; eligibility may depend on the label’s definitions, treatment-free interval, and the trial criteria reflected in the regulatory review.

Choosing whether to include tislelizumab requires a separate clinical assessment. Checkpoint blockade adds the possibility of immune-mediated toxicities involving the lungs, liver, bowel, endocrine organs, kidneys, skin, and other systems. Active autoimmune disease, prior transplantation, immunosuppression, organ function, and the need for rapid disease control may affect the risk-benefit discussion. No cross-arm inference should replace the FDA labels or the trial’s prespecified evidence.

Zanidatamab also has treatment-specific monitoring considerations, while fluoropyrimidine-platinum chemotherapy contributes hematologic, gastrointestinal, renal, neurologic, and other toxicities that vary by the selected agents. Baseline cardiac assessment, infusion-reaction planning, pregnancy counseling, laboratory surveillance, and review of drug-specific warnings should follow the approved prescribing information. The authorization itself is not a rationale to improvise doses or schedules.

The decision also has operational implications. Pathology services need validated HER2 testing and timely reporting, infusion centers must accommodate a multidrug regimen, and payers must recognize that both combinations fall within the authorized population. Access to molecular testing and specialist care may determine whether the regulatory expansion translates into equitable use.

Limits and unresolved questions

This is a regulatory decision built on an industry-sponsored, open-label registrational trial. Randomization strengthens internal validity, but patients enrolled in phase 3 oncology studies may have better performance status, organ function, and access to monitoring than many patients seen in routine practice. Frail adults, people with major autoimmune disease, and those with substantial cardiac, renal, or hepatic impairment may be underrepresented.

Longer follow-up is needed to characterize the durability of survival benefit, late toxicities, treatment after progression, and outcomes in clinically important subgroups. The relative value of the two authorized approaches also remains uncertain without a powered direct comparison. Real-world evidence will be important for older adults, patients with heterogeneous or borderline HER2 expression, and populations with limited access to repeat biopsy or comprehensive testing.

Questions clinicians ask

Who is eligible for the expanded indication?

Eligible patients are adults receiving first-line therapy for unresectable locally advanced or metastatic HER2-positive adenocarcinoma of the stomach, gastroesophageal junction, or esophagus. The indication does not cover squamous histology, resectable disease, or tumors without qualifying HER2 positivity; the current label governs testing and prior-treatment details.

What are the two FDA-authorized combinations?

One strategy combines zanidatamab-hrii with fluoropyrimidine- and platinum-containing chemotherapy. The second adds tislelizumab-jsgr to that zanidatamab-chemotherapy backbone. Both are authorized for the same disease setting, but the approval does not mean that adding checkpoint blockade is appropriate for every eligible patient.

Is PD-L1 positivity required to add tislelizumab?

The indication described by the FDA is anchored to HER2-positive disease and the specified first-line setting. Clinicians should check the final prescribing information for any biomarker qualification, because PD-L1 subgroup findings and a formal companion-diagnostic requirement are not equivalent and should not be inferred from other gastric or esophageal cancer indications.

Can the two zanidatamab regimens be compared directly?

Not necessarily. Although both were evaluated within the same three-arm phase 3 study, the key regulatory comparisons were against trastuzumab plus chemotherapy. A numerical difference between experimental arms should not be treated as proof of superiority unless the statistical plan included an adequately powered, multiplicity-controlled direct comparison.

References

1. FDA approves zanidatamab-hrii and tislelizumab-jsgr for HER2-positive gastric and gastroesophageal junction adenocarcinomas — US Food and Drug Administration, 2026
2. A Study of Zanidatamab in Combination With Chemotherapy With or Without Tislelizumab in Participants With HER2-Positive Gastroesophageal Adenocarcinoma — ClinicalTrials.gov, 2026
3. Drugs@FDA: FDA-Approved Drugs — US Food and Drug Administration, 2026