HER2-low cancers express levels of human epidermal growth factor receptor 2 that are below standard thresholds for HER2-positive immunohistochemistry. In 2022, results from the DESTINY-Breast04 trial showed T-DXd (Enhertu, AstraZeneca) to be an effective second-line chemotherapy in patients with HER2-low metastatic breast cancer.
The highly awaited new findings, from the manufacturer-sponsored, open-label Phase 3 DESTINY-Breast06 trial, were presented at the annual meeting of the American Society of Clinical Oncology (ASCO) in Chicago, Illinois.
The findings not only definitively establish a role for T-DXd earlier in the treatment sequence for HER2-low cancers, they also suggest benefit in a group of patients designated for the purposes of this trial to be HER2-ultralow. These patients have cancers with only faintly detectable HER2 expression on currently used assays (J Clin Oncol 42, 2024 [suppl 17; abstr LBA 1000]).
In a separate set of findings also presented at ASCO, from the randomized phase 1B open-label study, DESTINY-Breast07, T-Dxd showed efficacy in previously untreated HER2-positive metastatic breast cancer patients both alone and in combination with the monoclonal antibody pertuzumab (Perjeta, Genentech).
DESTINY-Breast06 Methods and Results
The DESTINY-Breast06 findings were presented by lead investigator Guiseppe Curigliano, MD, PhD, of the University of Milan and European Institute of Oncology. Dr. Curigliano and his colleagues randomized 866 patients with metastatic breast cancer: 436 to intravenous T-Dxd and 430 to the investigator’s choice of capecitabine, nab-paclitaxel, or paclitaxel chemotherapy. The investigators chose capecitabine 60% of the time.
Most patients had cancers classed as HER2 low (immunohistochemistry 1+ or 2+), while 153 had cancers classed by investigators as HER2-ultralow (IHC 0 with membrane staining or IHC under 1+). Patients enrolled in the study were those whose disease had progressed after endocrine therapy with or without targeted therapy. Patients’ median age was between 57 and 58, and all were chemotherapy-naive in the metastatic breast cancer setting.
The main outcome of the study was median progression-free survival in the HER2-low group. T-Dxd was seen improving progression-free survival, with median 13.2 months vs. 8.1 months (hazard ratio, 0.62; 95% confidence interval, 0.51-0.74; P < .0001). In the intention-to-treat population, which included the HER2 ultralow patients, the benefit was the same (HR, 0.63; 95% CI, 0.53-0.75; P < .0001). This suggested that T-DXd is also effective in these patients, and it will be extremely important going forward to identify the lowest level of HER2 expression in metastatic breast cancers that can still benefit from therapy with T-DxD, Dr. Curigliano said.
Overall survival could not be assessed in the study cohort because complete data were not yet available, Dr. Curigliano said. However, trends pointed to an advantage for T-DXd, and tumor response rates were markedly higher with T-DXd: 57% compared with 31% for standard chemotherapy in the full cohort.
Serious treatment-emergent adverse events were more common in the T-Dxd–treated patients, with 11% of that arm developing drug-related interstitial lung disease, and three patients dying of it. Five patients in the T-DXd arm died of adverse events deemed treatment-related, and none died from treatment-related adverse events in the standard chemotherapy arm. Altogether 11 patients died in the T-DXd arm and 6 in the chemotherapy arm.