Trastuzumab deruxtecan has shown efficacy in patients with previously treated human epidermal growth factor receptor 2 (HER2)-positive advanced or metastatic breast cancer. The efficacy and safety of trastuzumab deruxtecan in patients with no previous therapy for HER2-positive advanced or metastatic breast cancer are unclear.

We conducted a phase 3 trial involving patients with HER2-positive advanced or metastatic breast cancer and no previous chemotherapy or HER2-directed therapy for metastatic disease. Patients were randomly assigned in a 1:1:1 ratio to receive trastuzumab deruxtecan plus pertuzumab; trastuzumab deruxtecan plus placebo; or a taxane, trastuzumab, and pertuzumab (THP). The primary end point was progression-free survival as assessed by blinded independent central review. Secondary end points included objective response, duration of response, and safety.

For this prespecified interim analysis, data for trastuzumab deruxtecan plus pertuzumab and for THP are reported; data for trastuzumab deruxtecan plus placebo remain blinded until the final analysis of progression-free survival. At the data-cutoff date (February 26, 2025), the median progression-free survival was 40.7 months with trastuzumab deruxtecan plus pertuzumab (383 patients) and 26.9 months with THP (387 patients) (hazard ratio for progression or death, 0.56; 95% confidence interval [CI], 0.44 to 0.71; P<0.00001 [P-value boundary for superiority, 0.00043]). The incidence of a confirmed response was 85.1% with trastuzumab deruxtecan plus pertuzumab and 78.6% with THP (complete responses in 15.1% and 8.5%, respectively), with a median duration of response of 39.2 months and 26.4 months. Safety was consistent with the known profiles of the individual treatments. The incidence of grade 3 or higher adverse events was 63.5% with trastuzumab deruxtecan plus pertuzumab and 62.3% with THP; the most common were neutropenia, hypokalemia, and anemia with trastuzumab deruxtecan plus pertuzumab and neutropenia, leukopenia, and diarrhea with THP. Adjudicated drug-related interstitial lung disease or pneumonitis occurred in 12.1% of patients receiving trastuzumab deruxtecan plus pertuzumab (grade 1 or 2 in 44 patients and grade 5 [death] in 2 patients) and in 1.0% of those receiving THP (all grade 1 or 2).

Trastuzumab deruxtecan plus pertuzumab led to a significantly lower risk of progression or death than THP when used as first-line treatment for HER2-positive advanced or metastatic breast cancer, with no new safety signals. (Funded by AstraZeneca and Daiichi Sankyo; DESTINY-Breast09 ClinicalTrials.gov number, NCT04784715.).