HERIZON-GEA-01: The Future of HER2-Directed Therapy in Gastroesophageal Cancer

HER2+ esophageal cancers were traditionally treated with long established agents such as trastuzumab, a monoclonal antibody deigned to bind to the HER2 receptor. Now, data from the HERIZON-GEA-01 trial demonstrate that zanidatamab, a bispecific monoclonal HER2 receptor antibody, can be more effective in the first-line setting for patients with HER2+ gastroesophageal adenocarcinoma.

“For about 15 years now, the standard treatment for HER2+ gastric and gastroesophageal adenocarcinoma has been the use of trastuzumab plus chemotherapy. That was based on the TOGA trial,” Dr. Manish Shah, a medical oncologist at Weill Cornell, tells SurvivorNet Connect. “Zanidatamab is a new drug. It’s a bispecific drug that targets two portions of the HER2 protein.”

The advent of bispecific antibodies has led to the development of several monoclonal antibodies that bind to multiple epitopes, eliciting a stronger anti-tumor response. Zanidatamab binds to two separate and non-overlapping epitopes on the HER2 receptor.

The drug is already in use in previously treated, unresectable or metastatic HER2+ biliary tract cancer. HERIZON-GEA-01 was opened to build on the success in treating upper GI malignancies. It investigated the drug’s use in unresectable, locally advanced, recurrent, or metastatic HER2+ gastroesophageal adenocarcinomas — and findings were quite promising.

HERIZON-GEA-01’s Impact

HERIZON-GEA-01 represents a paradigm shift in the management of HER2+ GEA. For the first time, a HER2-targeted antibody has clearly outperformed trastuzumab in the first-line metastatic setting.

“We know very, very clearly that gastric cancer is a biomarker driven disease, and the biomarkers that we need to check are HER2, PDL1, mismatch repair deficiency … once you have that complete picture, you can then decide on treatment and certainly if you’re HER2+, you would go down HER2 treatment paradigm,” Dr. Shah explains.

In practical terms, HERIZON-GEA-01 moves HER2+ GEA closer to treatment paradigms seen in breast cancer, where optimized HER2 targeting drives durable survival gains rather than incremental improvements. It has become clear that new biologic agents with multiple targets will likely replace older monoclonal antibodies such as trastuzumab.

HERIZON-GEA-01: Breaking Down The Results

The study enrolled ~914 adults with unresectable, locally advanced, recurrent, or metastatic HER2+ GEA. HER2 positivity was defined as IHC 3+ or IHC 2+ with ISH positivity.

Patients were randomized 1:1:1 into three arms:

• Arm A (control): Trastuzumab + physician’s choice chemotherapy (CAPOX or 5-FU/cisplatin)
• Arm B: Zanidatamab + chemotherapy
• Arm C: Zanidatamab + PD-1 inhibitor tislelizumab + chemotherapy

Treatment continued until disease progression, unacceptable toxicity, or withdrawal. Chemotherapy could be stopped after ~6 cycles per protocol. The primary endpoints were progression-free survival (PFS) and overall survival (OS).

HERIZON-GEA-01 demonstrated that replacing trastuzumab with zanidatamab in the first-line setting meaningfully improves outcomes for patients with HER2-positive gastroesophageal adenocarcinoma. Both zanidatamab-based arms showed early and durable separation of survival curves, indicating more effective upfront disease control rather than a delayed benefit.

“The most potent regimen on the HERIZON study was the triplet combination of zanidatamab plus tislelizumab plus chemotherapy,” Dr. Shah explains.

Key efficacy findings include:

• Progression-free survival improved from ~8 months with trastuzumab-based therapy to ~12 months with zanidatamab-containing regimens
• Risk reduction for progression or death of approximately 35-37% versus control
• Overall survival exceeded two years in the zanidatamab arms, with the strongest benefit seen when combined with tislelizumab
• Higher and more durable response rates, suggesting deeper HER2 pathway suppression

Clinically, these results push first-line HER2+ GEA beyond the long-standing efficacy ceiling seen with trastuzumab plus chemotherapy.

Potential Toxicities

In the trial, the safety profile reflected a predictable layering of chemotherapy toxicity, enhanced HER2 blockade, and immune-related effects in the triplet arm. Importantly, no unexpected or prohibitive safety signals emerged.

Zanidatamab-associated toxicities include:

• Diarrhea was the most characteristic adverse event. It was usually grade 1-2, occurred early, and was manageable with supportive care.
• Infusion-related reactions, occurred most commonly during initial infusions. They were typically low grade, non-recurrent, and mitigated with premedication and slower infusion rates.

Chemotherapy-driven toxicities included:

• Cytopenias
• Fatigue
• Nausea
• Electrolyte abnormalities (e.g., hypokalemia)

These were similar in nature and frequency to standard fluoropyrimidine-platinum regimens.

Notes on cardiac safety include:

• Low rates of clinically significant LVEF decline
• Comparable to trastuzumab, despite stronger HER2 engagement

Immune-related toxicities (triplet arm) include:

• Thyroid dysfunction, hepatitis, pneumonitis, and dermatologic reactions
• Mostly low-grade and managed with standard immune-toxicity algorithms
• Higher overall grade ≥3 AE rates, but without excessive treatment discontinuation

Overall, toxicities were manageable and did not undermine treatment continuity or efficacy. It is important to discuss the potential side effects and impact on overall quality of life with your patients. This treatment regimen combines conventional chemotherapy with two biologically targeted agents all of which carry the risk of significant toxicity. As with many immunotherapies, tislelizumab carries the risk of developing immune related toxicities should be discussed with patients especially those with underlying autoimmune disorders as it may exacerbate these conditions. Zanidatamab is a Her2 directed therapy which carries the risk of cardiotoxicity and may not be appropriate for patients with low cardiac ejection fractions.

Limitations of This Trial

Despite its strengths, HERIZON-GEA-01 has several limitations worth acknowledging, including:

• Its open-label design may influence reporting of subjective adverse events.
• The three-arm structure complicates direct interpretation of the incremental benefit of immunotherapy versus optimized HER2 blockade alone.
• Global heterogeneity in chemotherapy backbone and supportive care practices
• Mature OS data for zanidatamab plus chemotherapy alone remains less definitive than for the triplet arm
• Biomarker granularity is limited (HER2 heterogeneity and dynamic loss were not deeply explored and predictors of benefit beyond HER2 status remain unclear).

These issues do not invalidate the findings but are relevant when translating results into practice and guideline recommendations. While the standard of care will continue to evolve even as the data in this trial mature, it is clear that biologically targeted agents such as zanidatamab will continue to be incorporated into traditional chemotherapy.