New targeted drug more than doubles time before HER2-low breast cancer grows

A phase 3 clinical trial of 551 women with advanced HER2-low breast cancer, carried out at 73 hospitals in China, found that a targeted drug called trastuzumab rezetecan kept the cancer from growing for a median of 16.4 months, compared with 6.8 months for women given standard chemotherapy. The results of the trial, named HORIZON-Breast06, were published in the medical journal JAMA on October 23, 2026, and presented at the European Society for Medical Oncology (ESMO) Congress 2026. Serious side effects were more common with the new drug, and the researchers said overall survival data are still immature.

Breast cancer is one of the most common cancers among women. Doctors group breast cancers partly by a protein called HER2 on the surface of cancer cells. Cancers with high levels are called HER2-positive and have long had targeted treatments. Many others have only low levels of HER2, called HER2-low. Until recent years, these were treated like cancers with no HER2 at all, mostly with chemotherapy. A newer class of drugs, called antibody-drug conjugates, has changed that.

An antibody-drug conjugate works like a guided missile. An antibody, a protein designed to find a specific target such as HER2, is linked to a powerful cancer-killing drug. The antibody carries the drug to the cancer cells, so more of it reaches the tumour. Trastuzumab rezetecan is one such drug, developed in China. In this trial, it more than doubled the median time before the cancer started growing again, a measure called progression-free survival, compared with chemotherapy chosen by doctors.

The results come with clear limits. Serious side effects were more frequent with the new drug: 84.1% of women had grade 3 or worse treatment-related side effects, compared with 67.9% on chemotherapy. Grade 3 means a side effect severe enough to need medical care or to limit daily activities. Overall survival, the most important measure of whether women live longer, is not yet final; the early 12-month estimates favour the drug but are described as descriptive only. The trial was carried out entirely in China, so results may not apply the same way to every population, and the drug's approval status differs from country to country.

For women and families facing advanced breast cancer, this is a research result, not a treatment recommendation. Whether this drug or a similar one is suitable depends on the type of cancer, previous treatment and overall health, which only a treating oncologist can judge.

Short-term: The results add to evidence that antibody-drug conjugates can outperform chemotherapy for HER2-low breast cancer. Regulators and doctors will study the full JAMA paper, including side-effect management.

Long-term: If overall survival results confirm the benefit, the drug could become an option for women with HER2-low advanced breast cancer where it is approved, adding to the choices beyond chemotherapy.

Who is affected: Women with advanced HER2-low breast cancer and their families; cancer specialists; and health systems deciding which new cancer drugs to approve and fund.

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