Taking on low, ultralow HER2 breast cancer
May 2025—Since the hunt began to identify low levels of HER2 in metastatic breast cancers, the action has revealed itself like the plot of a Henry James novel: Nothing much happens. Also, a lot happens. And each narrative thread is conveyed in hard-to-parse language. The goal has been to qualify patients for the antibody-drug conjugate trastuzumab deruxtecan (T-DXd), which was shown in the Destiny-Breast04 trial to significantly improve survival in so-called HER2-low cases. Immunohistochemistry assays were designed to identify strongly positive cases, however, and thus not useful for those at the 0 and 1+ end of the spectrum. Now, several years after the presentation of the D-B04 results at the ASCO 2022 annual meeting, which launched the low-end ship, pathologists and oncologists are adjusting to the implications of the most recent Destiny trial, D-B06. In late January, the FDA approved the drug Enhertu for HER2-low or HER2-ultralow cases, as well as the Ventana Pathway HER2 (4B5) companion diagnostic for assessing these lower levels of HER2 in patients with metastatic breast cancer.