Human Epidermal Growth Factor Receptor 2 “Low” in Breast Cancer in 2023

Human epidermal growth factor receptor 2 (HER2) immunohistochemistry (IHC) guidelines were first published by the American Society of Clinical Oncology (ASCO)–College of American Pathologists (CAP) in 2007,¹  followed by updates as deemed necessary in 2014² and 2018.³  Since the recent release of the DESTINY-Breast04⁴ (DB-04) trial results in June 2022, the pathology community has been eagerly awaiting another update. In this trial, patients with previously treated metastatic breast cancer that were classified as “HER2-low,” defined as IHC HER2 1+ or 2+ with negative in situ hybridization (ISH), showed significant improvement in both progression-free and overall survival when treated with trastuzumab deruxtecan (T-DXd) versus the oncologist’s choice of chemotherapy. The trial revolutionized the current binary HER2 paradigm of being positive (overexpression/amplification) or negative, to creating a new third category known as HER2-low. The trial’s practice-changing impact generated a 5-minute standing ovation at the ASCO meeting and an almost immediate endorsement by the National Comprehensive Cancer Network and the US Food and Drug Administration for treating patients with metastatic breast cancer.

See also Wolff AC, Somerfield MR, Dowsett M, et al. Human Epidermal Growth Factor Receptor 2 Testing in Breast Cancer: ASCO?College of American Pathologists Guideline Update.

This led to a rising momentum of requests from oncologists to pathologists to identify patients who may be eligible for treatment with T-DXd, especially on historic specimens. The updated CAP synoptic HER2 biomarker template⁵  facilitates identification of these patients by providing a footnote with formal definition of HER2-low and its therapeutic significance. In their most recent update, Wolff et al⁶  refrain from using HER2-low terminology because they state that HER2-low as an entity is neither “new nor reproducibly defined” and lacks “prognostic or predictive value.” They add that it was “artifactually created” for eligibility criteria for T-DXd. We agree that HER2-low is not a new or specific subtype of cancer, as seen by the multitude of studies in the recent literature that have consistently demonstrated that HER2-low tumors comprise a heterogeneous group of tumors at a clinical, morphological, immunohistochemical, and molecular level.⁷  Nevertheless, HER2-low provides a unique biomarker status for a specific therapeutic regimen (T-DXd) associated with a favorable prognosis, and for that it deserves an exclusive identity. Instead, the authors⁶  recommend inserting a footnote in the report, which distinguishes HER2 tumors that are overexpressed/amplified from those with low expression. While this footnote clearly goes beyond the dichotomous classification of HER2, it does not commit to a specific HER2-low label, which would help the treating physician to easily identify eligible patients. Ironically, the HER2-low terminology has already been unofficially assumed and adopted in the literature, in the updated CAP synoptic biomarker templates,⁵  and at various multidisciplinary breast discussions and conferences at a local, national, and global level.

It is true that the adoption of HER2-low terminology could be problematic since it would require changing the reporting schema for IHC 2+ results, which are currently reported as equivocal and reflexed to ISH. Thus, the standard IHC semiquantitative scoring designations of 0, 1+, 2+, and 3+ are currently prescribed. But this is more problematic due to the limitation of our current IHC assays that were originally calibrated to primarily identify HER2-positive disease (overexpression/amplification), with less well-defined discrimination in the lower end of the spectrum. When scoring the HER2 IHC slide, the pathologist’s primary role is to scrutinize it for overexpression (3+), failing which a search for equivocal staining (2+) is done to determine the need for reflex ISH testing for HER2 amplification. ASCO-CAP guidelines lump 0 and 1+ as negative, thus providing no incentive for pathologists to distinguish between these values. The DB-04 trial results impacted pathologists’ practices to now provide detailed IHC scores for determining eligibility for T-DXd therapy. While we appreciate the mandate by Wolff et al⁶  to report the semiquantitative IHC score in future reports, redefining the negative value to strictly 0 (excluding 1+) would have served better to streamline IHC reports, particularly because the authors do not support using HER2-low terminology.

This becomes even more relevant with the introduction of the term “ultralow,” defined as HER2 1+ ≤10%, which would be HER2-negative by current ASCO-CAP guidelines.⁸  In the DAISY trial,⁹  up to 30% of patients that were either IHC 0 or HER2-ultralow had a positive response to T-DXd. It is unknown whether the favorable results were due to the strength of T-DXd’s drug delivery, tumor heterogeneity, or limitations of pathologic detection. It raises the question of whether the trial’s IHC results were true or false negatives, the latter due to fixation artifact and or insensitivity of the IHC assay in the ultralow ranges of HER2.

For now, IHC is our current primary mode of HER2 detection, and we support the authors’⁶  best practices from utilizing controls with a range of protein expression to paying careful attention to preanalytic conditions in both primary and metastatic tumor samples, since DB-04 utilized whichever specimen was more accessible. Due to temporal heterogeneity, it is prudent to use as many different specimens as necessary to establish the HER2-low threshold. At a microscopic level, it is incumbent upon the pathologist to confirm the lower HER2 IHC scores at high power, since the magnitude of the score is inversely related to the magnification required. Consensus should be obtained in borderline cases where the critical 10% cutoffs affect the final determination of the case. Pathologists can improve HER2 evaluation through ongoing education via proficiency testing, webinars, practical guides, and utilization of quality assurance and control in order to develop clearer and more reproducible definitions of IHC scoring. Despite best efforts, there is still high interobserver variability due to the semiquantitative and observer-dependent nature of IHC interpretation.¹⁰  This has led to a demand for more novel sensitive quantitative assays such as HER2 automated image analysis, antibody bound fluorescent tags, reverse transcription polymerase chain reaction, and targeted mass spectrometry, which are all currently being developed and tested.⁷ 

The definition of HER2 breast cancer will continue to evolve as the results of future clinical trials mature. For example, the preliminary results of the DAISY trial⁹  provoke the question of whether T-DXd is actually effective in HER2-null (0) tumors. If proven true, we may have to revert to a binary classification of HER2, positive or negative, the reason for yet another potential ASCO-CAP HER2 IHC update!