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The FDA has approved the first targeted therapy for unresectable/metastatic breast cancer in the newly defined low-HER2 subtype.

The approval for trastuzumab deruxtecan (T-DXd, Enhertu) recognizes the recent expansion of the patient population with breast cancer eligible for treatment with HER2-targeted therapy. Historically, about 80% of breast cancers have been considered HER2 negative. With recognition of the low-HER2 subtype, about 60% of the former HER2-negative population can be reclassified as HER2 low, .

"Today's approval highlights the FDA's commitment to be at the forefront of scientific advances, making targeted cancer treatment options available for more patients," said Richard Pazdur, MD, of FDA's Oncology Center of Excellence, in a statement. "Having therapies that are specially tailored to each patient's cancer subtype is a priority to ensure access to safe and innovative treatments."

Primary support for the approval came from the DESTINY-breast04 randomized trial. Approximately 500 patients with unresectable/metastatic HER2-low, hormone receptor (HR)-negative or HR-positive breast cancer were randomized to T-DXd or physician's choice of chemotherapy.

Patients treated with T-DXd had a median progression-free survival of 9.9 months versus 5.1 months for the chemotherapy group (HR 0.50, P<0.001). T-DXd demonstrated superiority in both the HR-positive (10.1 vs 5.4 months) and HR-negative (8.5 vs 2.9 months) subgroups.

Median overall survival (OS) was 23.4 months with T-DXd versus 16.8 months with chemotherapy (HR 0.64, P=0.001). OS was superior with T-DXd in both the HR-positive and HR-negative subgroups.

The most common adverse events in T-DXd-treated patients in DESTINY-breast04 were nausea, fatigue, alopecia, vomiting, constipation, decreased appetite, musculoskeletal pain, and diarrhea. Prescribing information for T-DXd includes a boxed warning about a risk of interstitial lung disease and embryo-fetal toxicity. T-DXd is not recommended for use during pregnancy.

Reporting the DESTINY-breast04 results at the recent American Society of Clinical Oncology meeting, Shanu Modi, MD, of Memorial Sloan Kettering Cancer Center in New York City, said, "Overall, these results establish HER2-low metastatic breast cancer as a targetable population of breast cancer, with trastuzumab deruxtecan as a new standard of care in this setting."

The approval comes less than 2 weeks after T-DXd received from the FDA. Clinical development of T-DXd occurred as a global collaborative venture between AstraZeneca and Daiichi Sankyo.

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