New Drug Shows Promise in Treating Advanced Breast Cancer

A new drug, trastuzumab rezetecan, has shown significant promise in treating patients with advanced ERBB2-low metastatic breast cancer, according to findings from the phase 3 clinical trial HORIZON-Breast06 published in JAMA. The study indicates that the drug offers a substantial progression-free survival benefit compared to standard chemotherapy.

The trial, which involved 551 female patients across 73 hospitals in China, randomly assigned participants to receive either trastuzumab rezetecan intravenously every three weeks or physician's choice of chemotherapy. The study period spanned from June 2023 to August 2025.

Results showed that the median time without tumor growth was 16.4 months for patients receiving trastuzumab rezetecan, significantly longer than the 6.8 months observed for those treated with chemotherapy. Patients taking the new drug were also 73% less likely to experience cancer progression or death during the study period compared to those on chemotherapy. Furthermore, tumors shrank in 75.5% of patients treated with trastuzumab rezetecan, versus 41.2% in the chemotherapy group. At the 12-month mark, 90.6% of patients on trastuzumab rezetecan were alive, compared to 81.4% in the chemotherapy group.

The drug demonstrated effectiveness in patients with both hormone receptor-positive and hormone receptor-negative breast cancer. Researchers concluded that trastuzumab rezetecan provided a "significant progression-free survival benefit over physician’s choice of chemotherapy in patients with ERBB2-low advanced breast cancer."

Loren Rourke, a breast surgical oncologist not involved in the study, described the results as "incredibly encouraging." She highlighted that the extended period without cancer progression is crucial for patients living with advanced breast cancer, allowing more time to enjoy life. The treatment's benefit across different breast cancer subtypes was also noted as particularly exciting.

While the drug appeared to delay cancer progression, it was associated with more frequent serious side effects. Over 84% of patients receiving the new drug experienced serious side effects, compared to nearly 68% of those on chemotherapy. These side effects often required dose modification, though researchers described them as "generally manageable." Common issues included anemia and other blood-related problems, which can lead to fatigue and an increased risk of infection. Rourke emphasized the need to understand these side effects and their impact on quality of life.

Additional research is needed to fully understand the long-term implications of trastuzumab rezetecan, including whether it will ultimately help patients live longer. Experts agree that while the findings represent meaningful progress, further questions need to be addressed regarding survival rates and the management of side effects.