Last updated: May 8, 2026
Roughly 15–22% of gastric and GEJ adenocarcinomas overexpress HER2 (ERBB2), and that one biomarker opens an entirely separate treatment pathway with its own approved drugs and its own clinical trial pipeline. If you're HER2-positive, the trials worth looking at are different from the ones for HER2-negative or CLDN18.2-positive disease.
HER2 testing required: IHC 3+, or IHC 2+ with positive ISH/FISH for gene amplification. Almost all HER2-targeted trials require this confirmation. If your tumor is IHC 1+ or IHC 0, you're "HER2-low" or "HER2-zero" and a separate set of trials applies (especially for trastuzumab deruxtecan in HER2-low expansion cohorts).
HER2 testing in gastric cancer is a two-step process. The pathologist starts with immunohistochemistry (IHC) on a tumor biopsy and assigns one of four scores based on how strongly HER2 protein stains on the cell membrane:
For IHC 2+ tumors, an in situ hybridization (ISH/FISH) test counts copies of the HER2 gene. A HER2:CEP17 ratio ≥ 2.0 (or HER2 copy number ≥ 6) confirms amplification — equivalent to HER2-positive for trial eligibility. IHC 2+/ISH-negative is HER2-low.
Re-testing matters. HER2 status can be heterogeneous within a single tumor and between primary and metastatic sites — published series suggest 10–20% of patients show HER2 status changes between the primary tumor and a metastasis, and similar shifts can occur after first-line chemotherapy. If your initial biopsy was HER2-negative and you've progressed, asking your oncologist about a fresh biopsy of a metastatic lesion is worth doing — some trials specifically allow this re-testing for eligibility.
Sample requirements vary by trial. Most HER2-targeted trials accept either primary tumor or metastatic biopsy material; a few specify "fresh" biopsy taken within 6 months of enrollment. Your trial coordinator can tell you what's acceptable.
Outside a clinical trial, here's what your oncologist would currently offer for HER2-positive gastric/GEJ adenocarcinoma. Trial decisions usually compare against (or build on) these standards:
Approval status varies by country. The above reflects FDA + EMA labels as of May 2026. Local availability and reimbursement differ.
Standard of care (2026): First-line metastatic HER2+ — trastuzumab + fluoropyrimidine/platinum chemo + pembrolizumab (the KEYNOTE-811 regimen, median OS ~20 months). Second-line and beyond — trastuzumab deruxtecan (T-DXd, DESTINY-Gastric series). HER2-low: T-DXd has activity in expansion cohorts. Locally advanced/perioperative: adding HER2-targeted therapy to FLOT or SOX is being tested in multiple Phase 3 trials.
Building on the KEYNOTE-811 standard — testing whether HER2 ADCs or new HER2 antibodies improve on trastuzumab + chemo + pembrolizumab:
After progression on first-line trastuzumab-based therapy — next-gen ADCs are the most active area:
Dedicated HER2-low gastric trials are still mostly Phase 1/2 in 2026 — the field is establishing whether T-DXd (and other HER2 ADCs designed with bystander killing) work in lower-expression tumors. No Phase 3 HER2-low gastric registration trial is recruiting yet. The most relevant active studies:
If your IHC is 1+ or 2+/ISH-negative, also look at solid-tumor basket trials with HER2-low expansion arms (T-DXd has multiple cross-tumor cohorts) and broader gastric trials that include HER2-low subgroups. Eligibility differs from HER2-positive trials.
No dedicated Phase 3 perioperative HER2-targeted gastric trial is recruiting in 2026 — this is a real gap. Most perioperative trials don't pre-select for HER2, and HER2-targeted arms in this setting are still early-phase. For broader gastric perioperative trials, see the gastric cancer trials page; check eligibility for "HER2-positive cohort" or "HER2 stratification" language.
Showing selected notable HER2-specific trials. View all recruiting HER2+ gastric cancer trials on ClinicalTrials.gov (~150 total).
What does HER2-positive mean in gastric cancer?
HER2-positive gastric cancer means the tumor overexpresses HER2 (ERBB2) protein, defined as IHC 3+ or IHC 2+ with a positive ISH/FISH test confirming gene amplification. Roughly 15–22% of gastric cancers are HER2-positive, and HER2 status is required for trastuzumab-based and HER2 ADC trials.
What is the standard first-line treatment for HER2+ metastatic gastric cancer in 2026?
Trastuzumab + fluoropyrimidine/platinum chemotherapy + pembrolizumab (the KEYNOTE-811 regimen). This combination has shifted median overall survival from ~13 months on chemo + trastuzumab alone to roughly 20 months.
What clinical trials are available for HER2+ gastric cancer?
Approximately 150 recruiting interventional trials for HER2-positive gastric cancer are available in 2026, including trastuzumab deruxtecan (T-DXd) earlier-line combinations, disitamab vedotin, next-generation HER2 ADCs (BL-M07D1, SHR-A1811, IAH0968, HLX22), and HER2 + immunotherapy combinations.
Can my tumor be tested again if HER2 was negative the first time?
Yes, and it's worth doing. HER2 status can be heterogeneous (different parts of the tumor can score differently), and re-testing on a new biopsy — especially after progression on first-line therapy — sometimes reveals HER2 positivity that was missed initially. Some trials specifically allow repeat testing for eligibility.
What about HER2-low gastric cancer?
HER2-low (IHC 1+ or IHC 2+/ISH-) is increasingly being studied as a separate group, especially for trastuzumab deruxtecan. Several trials have HER2-low expansion cohorts. If your IHC is 1+ or 2+/ISH-negative, ask your oncologist specifically about HER2-low trials — eligibility differs from HER2-positive.
I have brain metastases — am I eligible for HER2-targeted trials?
Many HER2 ADC trials now allow patients with stable, treated brain metastases (off corticosteroids for a defined period). Trastuzumab deruxtecan in particular has documented intracranial activity, and several T-DXd trials specifically include patients with brain mets. Trials with newer HER2 antibodies and bispecifics vary — check the specific protocol's CNS eligibility language. Untreated or actively symptomatic brain mets are usually still excluded.
I've already had trastuzumab. Can I still join a HER2 ADC trial?
Yes, in most cases. Most second-line HER2 ADC trials (T-DXd, BL-M07D1, SHR-A1811) explicitly require prior trastuzumab-based therapy and progression on it — that's the whole point. First-line trials testing HER2 ADCs head-to-head against the KEYNOTE-811 regimen typically exclude patients who've already received trastuzumab for metastatic disease (but allow trastuzumab as part of prior adjuvant or perioperative therapy if completed >6–12 months earlier). Always check the specific trial's washout period.
What does monitoring look like during HER2 ADC therapy?
Two specific safety considerations beyond routine labs and imaging:
Cardiac function — trastuzumab and trastuzumab-based ADCs (T-DXd, BL-M07D1) can cause asymptomatic decline in left ventricular ejection fraction (LVEF). Most trials require a baseline echocardiogram or MUGA scan and repeat assessments every 12–16 weeks.
Interstitial lung disease (ILD) — T-DXd specifically carries a known risk of ILD/pneumonitis (~10–12% incidence, occasionally fatal). Trials require baseline chest CT, monitoring for new respiratory symptoms (cough, dyspnea), and prompt CT scans if symptoms appear. Patients with significant baseline lung disease may be excluded.
Standard infusion reactions and routine bloodwork (CBC, liver, kidney) round out the typical schedule.
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