Can you talk about the evolution and treatment of human epidermal growth factor receptor 2 (HER2)-low breast cancer?
Dr. Abdou: Until recently, HER2 status had been defined as a positive or negative result, but this convention has evolved, and now a newly defined population with low levels of HER2 expression has been identified. This HER2-low population accounts for about 55% of all breast cancers. Previously, low HER2 expression levels were considered HER2-negative in clinical practice because HER2-targeted therapies had been considered ineffective in this setting. Patients with HER2-low disease therefore had limited targeted treatment options after progression on their primary therapy.
Now, new studies and clinical trials have opened the door to effective treatments for this cohort of patients. The clinical trial DESTINY-Breast04, which was presented at ASCO 2022, led to the first FDA approval in August 2022 of a targeted therapy option for patients with HER2-low breast cancer subtypes, reclassifying this cohort as a new targetable subset in breast cancer.
DESTINY-Breast04 was the first randomized clinical trial to show that targeting HER2 provides clinically meaningful benefits for patients with HER2-low metastatic breast cancer, not only patients with HER2-positive disease. The phase 3 study enrolled about 557 patients with hormone receptor (HR)-negative or -positive breast cancer and centrally confirmed HER2-low expression who were previously treated with 1 or 2 prior lines of chemotherapy. Patients were randomized to receive either the antibody–drug conjugate trastuzumab deruxtecan or physician’s choice of standard chemotherapy. The risk of disease progression was about 50% lower and the risk of death was about 36% lower with trastuzumab deruxtecan compared with chemotherapy. 1
These impressive and practice-changing results opened the door to a new treatment option for a substantial group of patients with HER2-low disease and significantly expanded the population of patients who can benefit from HER2-targeted therapy.
What molecular characteristics do you take into consideration to help determine whether patients are eligible for these targeted treatment options?
Dr. Abdou: As we said earlier, HER2 status should no longer be recorded as a binary result of either HER2-positive or HER2-negative. It is important to start routinely testing for the level of HER2 expression in the tumor. Obtaining these levels is done through commonly used immunohistochemical (IHC) assays that allow direct visualization of the HER2 protein. Breast tumors considered to be HER2-low are classified as IHC1+ or as IHC2+ with in situ hybridization or FISH-negative status.
HER2-low breast cancer consists of a heterogeneous group of breast cancers, most of which are HR-positive tumors, whereas about 20% are HR-negative tumors. While these tumors may have distinct molecular profiles leading to clinicopathological and prognostic differences within these groups—HR-positive tumors represent more luminal subtypes and HR-negative tumors tend to be predominantly basal-like subtypes—these distinctions do not necessarily affect patient eligibility for targeted therapy. The benefit of trastuzumab deruxtecan was seen in both subgroups, although the HR-positive population was much more well represented in the DESTINY-Breast04 study.
Other than the HER2 expression status, I also take into consideration the presence of clinical comorbidities, particularly pulmonary comorbidities or prior lung injuries. Trastuzumab deruxtecan can cause a potentially serious type of lung toxicity called interstitial lung disease (ILD). In DESTINY-Breast04, ILD developed in about 12% of patients in the trastuzumab deruxtecan group, with 3 deaths as a result.
Therefore, it’s important for us to carefully select these patients and closely monitor them while they’re on treatment.
What is next in the treatment of HER2-low breast cancer, and what would you like to see in the future?
Dr. Abdou: The exciting new field of HER2-low breast cancer has really opened the door to novel studies and clinical trials, several of which are exploring the role of antibody–drug conjugates in patients with metastatic HER2-low disease and others that are studying early-stage HER2-low breast cancer. In early-stage HER2-low breast cancer, we may potentially see an even greater benefit with these drugs because the disease has not yet developed resistance to therapy. Other studies are examining the role of combination therapy in metastatic breast cancer, such as antibody–drug conjugates in combination with immunotherapy and other targeted agents. I look forward to results from those studies.
Also, importantly, as we start using these therapies more widely, I would like to see more accurate and sensitive ways of assessing the HER2 expression status. The current IHC assay, although widely available, fails to identify many women who have HER2 expression in their tumors. I think more sensitive tests may be able to identify even more women who can benefit from these targeted therapies.