ESMO 2026

An experimental cancer treatment developed by AstraZeneca has significantly extended survival among patients with advanced stomach cancer in a late-stage clinical trial, raising hopes for people whose disease has progressed despite earlier treatment.

The medicine, called sonesitatug vedotin, or Sone-Ve, was evaluated in the Phase III CLARITY-Gastric01 trial. Among patients who had already received at least two previous treatments, median overall survival reached 10 months with the experimental drug, compared with five months among patients receiving their doctors’ choice of existing treatment.

The findings, presented at the European Society for Medical Oncology (ESMO) Congress in Madrid on October 25, 2026, showed a 53% reduction in the relative risk of death in this heavily treated patient group. In the broader trial population, which included patients receiving second-line or later treatment, the medicine reduced the relative risk of death by 42%. Median overall survival in that group was 10.6 months, compared with 6.4 months for patients receiving the comparison treatments.

The results could help expand the options available to people with advanced gastric cancer and related tumours. However, the findings remain clinical-trial results: Sone-Ve is experimental, and the study did not establish a statistically significant improvement in its independently assessed progression-free survival endpoint across the overall trial population.

How the Experimental Treatment Works

Sone-Ve belongs to a class of medicines known as antibody-drug conjugates, or ADCs. These treatments combine an antibody that recognises a particular target on cancer cells with a potent drug designed to damage and kill those cells.

The AstraZeneca medicine targets a protein called claudin 18.2, commonly abbreviated as CLDN18.2. This protein can be present on the surface of certain stomach cancer cells and is being investigated as a target for precision cancer treatment.

After the antibody binds to the target, the drug is designed to deliver its cytotoxic payload to the cancer cell. The aim is to concentrate treatment activity around cells carrying the target rather than relying entirely on chemotherapy that circulates throughout the body.

Sone-Ve contains an antibody directed against CLDN18.2, a specialised linker and a cell-killing compound known as monomethyl auristatin E, or MMAE.

Although this approach offers a potential advantage, targeted delivery does not eliminate the possibility of side effects. The medicine must still be assessed for its safety, effectiveness and suitability for different groups of patients.

What the Phase III Trial Found

The CLARITY-Gastric01 study evaluated Sone-Ve against treatments selected by doctors for patients with advanced or metastatic gastric cancer, gastroesophageal junction cancer, or oesophageal adenocarcinoma expressing CLDN18.2.

The trial included patients whose tumours showed the target protein in at least 25% of tumour cells. This threshold is important because it could allow the treatment to be studied in a broader population than therapies that require higher levels of CLDN18.2 expression.

The principal findings included:

• Median overall survival in the third-line or later group: 10 months with Sone-Ve versus five months with the comparison treatments.

• Relative risk of death in that group: Reduced by 53%.

• Median overall survival across the broader second-line or later population: 10.6 months versus 6.4 months.

• Relative risk of death across the broader population: Reduced by 42%.

These figures describe different analyses of the trial and should not be combined. The five-month versus 10-month comparison applies to patients treated in the third-line or later setting, while the 10.6-month versus 6.4-month comparison applies to the broader study population.

Overall survival measures the time patients live after a defined point in a study. Median overall survival is the point at which half of the patients in a group have died and half remain alive. It does not mean that every patient receiving the medicine will live for 10 months, nor does it predict an individual's outcome.

The results are nevertheless notable because patients whose cancer progresses after several treatments often have fewer effective options remaining.

An Important Limitation: Progression-Free Survival

Despite the overall survival improvement, Sone-Ve did not achieve statistical significance for the trial's other dual primary endpoint: progression-free survival, assessed by an independent central review in the broader study population.

Progression-free survival measures how long patients live without their disease worsening or death occurring, according to the study's definition. It is a different measure from overall survival and provides additional information about how a treatment affects the course of the disease.

The absence of a statistically significant result for this endpoint means the trial did not demonstrate a definitive improvement in independently assessed progression-free survival across that population.

An analysis based on investigators' assessments showed a more pronounced trend in favour of Sone-Ve, but that does not replace the result of the prespecified independent assessment.

The distinction matters when comparing experimental medicines. A treatment may extend overall survival without showing a statistically significant benefit on every other measure. Researchers and regulators must consider the complete evidence, including the study design, efficacy results, adverse effects and the needs of the patients being treated.

What the Safety Results Showed

Safety is another important consideration for patients who have already undergone multiple rounds of cancer treatment.

In the reported trial data, serious or severe treatment-related adverse events were less frequent in the Sone-Ve group than in the comparison group, at 36.4% and 44.8%, respectively, according to the reported safety measure.

However, stomach-related side effects were more common with Sone-Ve. These events were reported in 11.1% of patients receiving the experimental medicine, compared with 6.3% in the comparison group.

These findings do not establish that the experimental treatment is safer for every patient. Individual risks can differ depending on a person's health, cancer characteristics, previous treatments and other medicines.

Further assessment will be needed to understand how the treatment's benefits and risks compare with available alternatives in different clinical settings.

Could More Patients Become Eligible for Targeted Treatment?

One potentially important feature of the study is its CLDN18.2 eligibility threshold.

The trial included patients whose tumours expressed the protein in at least 25% of tumour cells. Some existing CLDN18.2-targeted treatment approaches use a higher expression threshold, which can restrict eligibility to a narrower group of patients.

AstraZeneca has said the lower threshold could expand the population potentially eligible for Sone-Ve. The company estimates that CLDN18.2 expression at the trial's threshold could be found in a substantial proportion of patients with gastric and gastroesophageal junction cancers.

Eligibility would still depend on the final evidence, regulatory decisions, approved testing criteria and an individual patient's clinical circumstances. The trial findings do not mean that every person with stomach cancer would benefit from the medicine.

Testing tumour tissue for relevant biomarkers could become increasingly important as more targeted medicines enter cancer research and clinical practice.

What the Results Could Mean for Patients

Advanced stomach cancer remains difficult to treat, particularly when the disease returns or progresses after initial therapy. At that stage, doctors must weigh the potential benefits of additional treatment against its side effects and the patient's overall condition.

If future studies and regulatory reviews support the results, Sone-Ve could become another treatment option for appropriately selected patients whose tumours express CLDN18.2.

The potential impact would depend on several factors, including confirmation of the clinical benefit, the final safety profile, regulatory approval, availability of biomarker testing and the cost of treatment.

The findings could also encourage further research into antibody-drug conjugates targeting specific proteins on gastrointestinal cancer cells. Researchers are investigating whether this approach can improve outcomes in other tumour types and whether combining targeted medicines with other treatments can produce additional benefits.

AstraZeneca is also evaluating Sone-Ve in research involving earlier treatment settings. Results from those studies will be important in determining whether the medicine could help patients before their disease has progressed through several treatment regimens.

The Road Ahead

The CLARITY-Gastric01 results represent a promising development in the treatment of advanced stomach cancer, particularly for patients who have already received multiple therapies.

The doubling of median overall survival in the third-line or later group is an important finding, but it should be interpreted alongside the trial's limitations, including the lack of statistical significance for independently assessed progression-free survival in the broader population.

Sone-Ve remains an experimental medicine, and the reported results do not establish that it is approved or available as standard treatment in every country. Regulatory authorities will need to assess the full evidence before determining whether it should be authorised for use.

For now, the study provides further evidence that biomarker-directed treatments may have a growing role in advanced gastric cancer. Its ultimate significance will depend on the complete clinical evidence and whether the benefits can be confirmed and translated into routine patient care.