Zanidatamab Plus Chemo With/Without Tislelizumab Yields Unprecedented PFS/OS Outcomes in HER2+ Gastroesophageal Adenocarcinoma

Zanidatamab-hrii (Ziihera) in combination chemotherapy with or without tislelizumab-jsgr (Tevimbra) represents a key development in the frontline management of HER2-positive locally advanced, unresectable, or metastatic gastroesophageal adenocarcinoma, after findings from the phase 3 HERIZON-GEA-01 trial (NCT05152147) showed unprecedented progression-free survival (PFS) and overall survival (OS) data in this patient population, according to Elena Elimova, MD.

Findings from the primary analysis of HERIZON-GEA-01 presented during the 2026 Gastrointestinal Cancers Symposium (ASCO GI) revealed that patients who received the triplet (n = 302) achieved a median PFS of 12.4 months (95% CI, 9.8-18.5) compared with 8.1 months (95% CI, 7.0-8.9) among those who received trastuzumab (Herceptin) plus chemotherapy (HR, 0.63; 95% CI, 0.51-0.78; P < .0001).¹ The median OS was 26.4 months (95% CI, 21.5-30.3) vs 19.2 months (95% CI, 16.8-21.8), respectively (HR, 0.72; 95% CI, 0.57-0.90; P = .0043).

Among patients treated with zanidatamab plus chemotherapy alone (n = 304), the median PFS was 12.4 months (95% CI, 9.8-14.5; HR vs trastuzumab plus chemotherapy, 0.65; 95% CI, 0.52-0.81; P < .0001). The median OS in this group was 24.4 months (95% CI, 20.4-30.0; HR vs trastuzumab plus chemotherapy, 0.80; 95% CI, 0.64-1.01; P = .0564).

“This was the first study [in this patient population] to show a median PFS of more than 1 year [and] a median OS of more than 2 years,” Elimova said in an interview with OncLive®. “I never thought I'd be in a place where I'd be talking about those kinds of survival [figures]. I hope that patients will [now be able to] live longer and better; those are the 2 reasons that we give chemotherapy. We strive to think of novel treatment options, and this option seemed to work for this patient population.”

In the interview, Elimova, an associate professor in the Department of Medicine at the University of Toronto and a staff medical oncologist at Princess Margaret Hospital, discussed the current treatment landscape for patients with HER2-positive gastroesophageal adenocarcinoma, the design of HERIZON-GEA-01, and remaining questions with zanidatamab-based therapy in this setting.

OncLive: What are some of the challenges with current treatment approaches for patients with advanced HER2-positive gastroesophageal adenocarcinoma?

Elimova: It’s important to recognize that approximately 20% of patients with gastric, esophageal, and gastroesophageal junction [GEJ] cancers have HER2-positive disease.² Historically, many studies have included patients with gastric and GEJ tumors while excluding those with esophageal adenocarcinoma. That exclusion has largely been historical rather than biologically driven.

Our study included patients with esophageal adenocarcinoma, which is particularly important. In Canada, if a study does not include a specific patient population, we cannot prescribe that drug for those patients. That creates a real challenge in clinical practice. It is critical that clinical trials reflect the full spectrum of patients we see in the clinic so that the results can be applied appropriately.

When I look at how treatment has evolved for metastatic esophageal adenocarcinoma, we have certainly made progress. However, with current standards of care, median PFS remains less than 1 year, and median OS is still under 2 years. Clearly, both medical oncologists and our patients want—and need—to do better.

What was the rationale for examining this combination in this patient population?

Zanidatamab is a HER2-targeted bispecific antibody that binds to ECD2 and ECD4 through a biparatopic binding mechanism. What is particularly interesting about this agent is its ability to cross-link neighboring HER2 molecules, leading to increased receptor clustering on the cell surface. This enhanced clustering activates multiple downstream mechanisms of action.

Tislelizumab is a high-affinity anti–PD-1 agent. Prior to this study, we conducted [a pair of] phase 2 trials in the first-line setting evaluating zanidatamab plus chemotherapy, with tislelizumab [NCT04276493] or without tislelizumab [NCT03929666]. Those studies demonstrated very promising activity, which ultimately led to the design of the phase 3 HERIZON-GEA-01 trial.

What were the key design characteristics of HERIZON-GEA-01?

This phase 3, global, randomized registration trial enrolled patients with previously untreated, locally advanced unresectable, or metastatic gastroesophageal adenocarcinoma. In total, 914 patients were included, all of whom had centrally confirmed HER2-positive disease. Patients who had received prior HER2-directed therapy or anti–PD-1 therapy in any setting were excluded.

Patients were [randomly assigned] in a 1:1:1 fashion to receive trastuzumab plus chemotherapy, which was the standard of care at the time the study was designed; zanidatamab plus chemotherapy; or zanidatamab plus tislelizumab and chemotherapy. It is important to note that patients in the zanidatamab-containing arms received antidiarrheal prophylaxis, based on findings from our phase 2 studies.

Patients were stratified by geographic region and HER2 status, but not by PD-L1 status, as the importance of PD-L1 was not fully appreciated at the time the study was designed. The primary end point was a dual end point of PFS and OS.

What were the notable efficacy data from the study presented during ASCO GI?

What was particularly striking was that in both zanidatamab-containing arms, median PFS exceeded 12 months, establishing a new benchmark in this disease—something we have not seen previously. There was also a more than 4-month improvement in PFS compared with the control arm, which is unprecedented in gastroesophageal adenocarcinoma. The HR for PFS was less than 0.7 [for both experimental regimens vs control], underscoring the magnitude of benefit observed.

Regarding OS, statistical testing was performed first for the zanidatamab plus tislelizumab and chemotherapy arm vs the trastuzumab plus chemotherapy arm, based on the study’s fixed-sequence testing plan. In that comparison, median OS improved by more than 7 months, reaching 26.4 months [in the zanidatamab plus tislelizumab and chemotherapy arm]—again, a result that has never been reported previously in metastatic gastroesophageal adenocarcinoma. The HR [for OS for zanidatamab plus tislelizumab and chemotherapy vs trastuzumab plus chemotherapy] was 0.72, and the P-value was statistically significant [P = .0043].

When we examined OS for zanidatamab plus chemotherapy vs trastuzumab plus chemotherapy, this analysis represented only the first interim look, with 2 additional analyses still planned. Although the result did not cross the prespecified boundary for statistical significance, there was a clear trend favoring zanidatamab, with a median OS of 24.4 months vs 19.2 months. We look forward to seeing how these data mature over time.

What were the safety profiles of the combinations?

Diarrhea was the most common treatment-related adverse effect across all 3 arms of the study. Interestingly, even in the trastuzumab plus chemotherapy arm, the rate of diarrhea was higher than what has been reported in prior phase 3 trials.

As expected, diarrhea occurred more frequently in the zanidatamab-containing arms, which is why antidiarrheal prophylaxis was incorporated into the protocol. Importantly, when diarrhea did occur, it typically happened during cycle 1, resolved within 3 weeks, and rarely led to treatment discontinuation. Fewer than 5% of patients [in either experimental arm] discontinued zanidatamab due to diarrhea. This suggests that with appropriate prophylaxis and management strategies, patients are able to remain on this highly active HER2-directed therapy, as well as anti–PD-1 therapy when applicable.

What are the remaining questions in terms of which patients may benefit most from this combination, or from the addition of tislelizumab?

What was surprising in this study—and I’m again going to say this was not stratified by PD-L1—was that approximately 30% of the patients [across all 3 arms] had a PD-L1 tumor area positivity [TAP] score of less than 1. Patients with a PD-L1 TAP score less than 1 and patients with TAP scores greater than or equal to 1 benefited [from the experimental regimens]. And the question is, why? Zanidatamab is a different type of molecule. It’s a bispecific antibody, but this is different than what other studies have shown. Other studies have shown that anti–PD-1 therapies work mainly in PD-L1–positive patients. I believe [our findings] are intriguing, and the degree of benefit favored both of those groups. I would like a biological explanation for that. Given the fact that the subgroup analysis will be so large, representing approximately 30% of patients, it is meaningful and something important to look at [in the future].

References

1. Elimova E, Young Rha S, Shitara K, et al. Zanidatamab + chemotherapy (CT) ± tislelizumab for first-line (1L) HER2-positive (HER2+) locally advanced, unresectable, or metastatic gastroesophageal adenocarcinoma (mGEA): primary analysis from HERIZON-GEA-01. J Clin Oncol. 2025;44(suppl 2):LBA285. doi:10.1200/JCO.2026.44.2_suppl.LBA285
2. Kunz PL, Mojtahed A, Fisher GA, et al. HER2 expression in gastric and gastroesophageal junction adenocarcinoma in a US population: clinicopathologic analysis with proposed approach to HER2 assessment. Appl Immunohistochem Mol Morphol. 2012;20(1):13-24. doi:10.1097/PAI.0b013e31821c821c