A targeted drug shrank tumours in more than three-quarters of patients with advanced breast cancer in a phase three trial, while significantly delaying the point at which the disease began to progress.
The treatment, trastuzumab rezetecan, was tested in patients with ERBB2-low metastatic breast cancer – a form in which tumours contain low levels of the HER2 protein, which can help cancer cells grow.
Researchers compared the drug with standard chemotherapy in 551 women treated at 73 hospitals across China. Participants were randomly assigned to receive either trastuzumab rezetecan through an intravenous infusion every three weeks or a chemotherapy regimen chosen by their doctor.
Trastuzumab rezetecan delays cancer progression
The findings, published in JAMA, showed that patients given trastuzumab rezetecan went a median of 16.4 months without their cancer worsening, compared with 6.8 months among those who received chemotherapy.
Patients treated with the drug were 73 per cent less likely to experience disease progression or die during the study period. The benefit was seen in patients with both hormone receptor-positive and hormone receptor-negative breast cancer.
Tumours reduced in size in 75.5 per cent of patients in the trastuzumab rezetecan group, compared with 41.2 per cent of those given chemotherapy.
After 12 months, 90.6 per cent of patients receiving the targeted treatment were alive, compared with 81.4 per cent of those treated with chemotherapy. Researchers cautioned, however, that the overall survival data was not yet mature enough to determine whether the drug extends life over the longer term.
The trial included 449 patients who had not previously received chemotherapy for metastatic disease. The study ran from June 2023 to August 2025, with the final progression-free survival analysis based on follow-up data collected until March 20, 2026.
Trastuzumab rezetecan is an antibody-drug conjugate. It is designed to attach to cancer cells carrying the ERBB2 protein and deliver a powerful chemotherapy payload directly into them.
Loren Rourke, a breast surgical oncologist in Texas who was not involved in the research, described the results as “incredibly encouraging”.
“When you’re talking about women living with advanced breast cancer, those months matter,” she told Fox News Digital. “They represent more time when the cancer is being held in check and potentially more opportunities to enjoy life without the disease getting worse.”
She said the results were particularly notable because the treatment appeared to work across different breast cancer subtypes.
“It’s another important step forward, and it gives us genuine reason to be hopeful,” she added. “A treatment that can keep cancer from progressing for a longer period has the potential to make an enormous difference in a patient’s life.”
Side effects remain a concern
The trial also highlighted significant toxicity. Serious treatment-related side effects occurred in 84.1 per cent of patients given trastuzumab rezetecan, compared with 67.9 per cent of those receiving chemotherapy.
The most common problems included reductions in neutrophil and white blood cell counts, as well as anaemia. Dose reductions because of side effects were needed in half of the patients who received the targeted drug.
Rourke warned that delaying progression did not automatically mean patients experienced a better quality of life.
“We need to better understand the side effects, particularly the significant decreases in blood counts that can lead to fatigue, anaemia and increased risk of infection,” she said.
The researchers said the adverse effects were generally manageable, although they occurred more frequently than with chemotherapy and often required changes to the dose.
They also noted that the trial was conducted solely in China, meaning further multinational research will be needed to assess how widely the results apply. Longer follow-up is also required to establish whether trastuzumab rezetecan improves overall survival.
“We know this treatment delayed cancer progression, but we don’t yet know whether it will help patients live longer,” Rourke said. “That’s an important distinction.”
