The FDA has approved trastuzumab deruxtecan for adults with HR-positive, HER2-low or ultra-low metastatic or unresectable breast cancer whose disease has progressed after at least one endocrine therapy, the agency recently announced.

At the same time, the agency approved Ventana PATHWAY anti-HER-2 (4B5) Rabbit Monoclonal Primary Antibody assay as a companion diagnostic device for identifying patients with HER2-ultralow expression who may be good candidates for trastuzumab deruxtecan.

FDA officials approved trastuzumab deruxtecan in this earlier line of therapy based on the phase 3 randomized DESTINY-Breast06 trial, an open-label study that included 866 patients with advanced or metastatic HR-positive, HER2-low or ultralow breast cancer. All patients had their diagnoses confirmed using the Ventana assay.

In a study published in The New England Journal of Medicine, the researchers assigned patients to either the study treatment or physician’s choice of chemotherapy at a 1:1 ratio. The main outcome was PFS in patients with HER2-low cancer.

Among 713 patients with HER2-low breast cancer, those who were assigned to trastuzumab deruxtecan had a significantly improved PFS, the researchers reported, with a median PFS of 13.2 months, compared with 8.1 months for those assigned to chemotherapy.

This improvement in PFS held true for the overall population as well. The researchers reported that median PFS in the overall population was 13.2 months for the trastuzumab group and 8.1 months for the chemotherapy group. Data on OS was immature at the time of the final PFS analysis, the FDA said in its press release; however, 39% (n = 335) of patients across both groups had died.

The confirmed objective response rate was 65.7% in the trastuzumab deruxtecan group and 30.8% in the chemotherapy group, according to the FDA.

Common adverse events occurring in at least 20% of patients included decreased neutrophils, hemoglobin, white blood cell counts, platelets, lymphocytes, and blood potassium, among others.

Trastuzumab deruxtecan is administered intravenously. The recommended dose is 5.4 mg/kg of bodyweight once every 3 weeks until unacceptable toxicity or disease progression.

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Disclosures: Bardia is a consultant for AstraZeneca and Daiichi, which make trastuzumab deruxtecan.
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