Written by: Denise Yardley, MD
Sponsored by: Daiichi-Sankyo
The landscape of breast cancer treatment is advancing with innovative discoveries challenging the traditional HER2 classification. The identification of HER2-low and HER2-ultralow subtypes offers a more nuanced understanding of tumor biology, paving the way for personalized therapies that promise better outcomes for a wider range of patients. This fresh perspective reshapes clinical practice, indicating a new era in the fight against breast cancer where precision medicine leads the charge.
Breast cancer remains an extremely heterogeneous disease, with a number of subtypes characterized by distinct molecular and clinical features. One key classifying marker is the human epidermal growth factor receptor 2 (HER2). The expression of HER2 or lack thereof traditionally serves to categorize tumors as either HER2 positive or HER2 negative, based on the detected level of HER2 protein expression. The emergence of new antibody conjugate drugs challenged the reliance of conventional HER2 targeted therapies on 3+ HER2 expression by immunohistochemistry (IHC) or gene amplification, thereby establishing HER2 low as a novel breast cancer entity that stands to benefit from HER2 targeted therapies. As a result of DESTINY-Breast 04 in 2022, ASCO CAP guidelines for HER2 testing, affirming the importance of accurately identifying with HER2 low to ensure optimal patient selection for HER2 targeted therapies, were revised in 2023 to include advancements in diagnostic techniques that have led to the identification of two additional subtypes: HER2-low and HER2-ultralow breast cancer. Identifying these novel HER2 subtypes have significant implications for treatment and prognosis.
The identification of the HER2 prototype oncogene served as a ripe therapeutic target in breast cancer as well as other cancers. The focus of these therapeutic interventions were on the small 15-20% group of tumors demonstrating HER2 protein overexpression as a function of HER2 gene amplification. The development and subsequent efficacy of the targeted monoclonal anti-HER2 antibody trastuzumab led to its approval for metastatic breast cancer in 1998 followed by the approval of pertuzumab in 2012. Trastuzumab worked by binding the HER2 protein receptor, inhibiting HER2 homodimerization thus preventing HER2-mediated signalling while pertuzumab inhibited HER2 heterodimerization with HER3, a related growth factor receptor. However, a low or moderate expression of the HER2 target without gene amplification failed to benefit from conventional anti-HER2 agents (NSABP B-47).
Extensive pathology training efforts and quality assurance programs followed to reliably achieve high concordance for identifying and characterizing tumors denoted as HER2+ with the intent of identifying tumors most likely to benefit from classical HER2 targeted agents. The HER2 testing algorithm resulted in a binary categorization of tumors that were either HER2 negative or HER2 positive, on the basis on an IHC score of 3+ or IHC 2+ with in situ hybridization (ISH) positive. HER2 negative was a catchall that included tumors that were completely devoid of the HER2 protein (IHC 0) as well as those that had low to moderate expression labelled as IHC 1+ and IHC 2+ but ISH negative. This distinction however was not clinically meaningful, and the two groups were combined and were not eligible for HER2 therapies. It was not at all clear if there was a distinct tumor biology associated with lower level HER2 expression. The monoclonal anti-HER2 antibodies were ineffective in HER2 low tumors because their activity relies mainly on the blockade of aberrant HER2 signaling via dimerization inhibition, HER2 internalization, and/or antibody dependent cellular cytotoxicity (ADCC). Since these therapies bind the extracellular domain of the HER2 receptor, effective efficacy hinged on HER2 receptor overexpression which facilitates ADCC.
Despite the impact of the success of these agents in improving outcomes in HER2+ overexpressing tumors, a subgroup of patients failed to respond or experience disease recurrence, creating a robust pathway for the development of more effective and well tolerated second line therapies. Antibody drug conjugates (ADC), already a mainstay in hematologic malignancies, functioned as tumoral antibody specific antibodies connected via a linker to a potent cytotoxic payload. Trastuzumab emtansine (T-DM1) soon emerged, incorporating the antitumor properties of trastuzumab joined via a noncleavable linker with the cytotoxic activity of the microtubule inhibitory agent DM1. Use of this ADC allowed for targeted receptor binding and transport of cytotoxic chemotherapy, specifically into cancer cells, with subsequent disruption of the intracellular signaling pathways. The results of the EMILIA trial moved trastuzumab emtansine as a new standard in the second line setting of HER2+ MBC, following the demonstration of improved PFS and OS coupled with a more favorable toxicity profile than lapatinib and capecitabine.
Given these advances, refractoriness in classical HER2+ breast cancers developed to trastuzumab emtansine fostering the development of the third generation ADC trastuzumab deruxtecan, with a monoclonal anti-HER2 antibody linked to a topoisomerase payload through a tetrapeptide cleavable linker. This ADC had a higher drug to antibody ratio of 8 and was effective in trastuzumab emtansine insensitive HER2+ breast tumors. A series of trials referred to as DESTINY trials, evaluated this third generation ADC with DESTINY Breast 01, 02, and 03 in HER2+ breast cancer while DESTINY Breast 04 and 06 looked at HER2 low breast cancer, embracing the 80% of breast cancers assessed as HER2 negative and historically not candidates for anti-HER2 therapy. The DAISY trial was deigned to evaluate trastuzumab deruxtecan according to HER2 expression levels showing the greatest response in the HER2 overexpressing tumors defined by IHC 3+ or ISH positive followed by cohort 2 consisting of HER2 low tumors defined as IHC2+/ISH negative or HER2 nonexpressing tumors or IHC 0. This suggested that very low levels of HER2 expression could allow for receptor binding of trastuzumab deruxtecan and furthermore, that the definition of HER2 low needed to be expanded to include HER2 ultralow, cases that show faintly perceptible HER2 staining that is greater than 0% and < 10% (currently considered IHC 0). What emerged was that HER2 expression is now increasingly perceived as a continuum that defies the former classical dichotomous distinction of HER2 positive and HER2 negative cancers that traditionally guided treatment decision making. While monoclonal antibodies were ineffective in HER2 low tumors because their activity relies mainly on binding of the extracellular domain of the HER2 receptor and is more effective if the receptor is overexpressed facilitating ADCC. This is in stark contrast to the ADC trastuzumab deruxtecan, which can overcome some of these monoclonal antibody limitations by also being able to release a cytotoxic payload that can be internalized by surrounding cells that do not express HER2 (bystander effect). With the introduction of the third generation ADC therapies in what was previously collectively classified as HER2 negative tumors, a paradigm shift in the treatment of tumors without conventionally defined HER2 overexpression or HER2 gene amplification has occurred.
The identification of HER2-low and HER2-ultralow breast cancer represents a significant advancement in the field of breast cancer research and treatment. HER2 IHC scoring, nomenclature, testing modalities and current treatment protocols are evolving and the reproducibility of pathologists truly being able to separate IHC HER2 0 and HER2 1+ persists. Alternative assays and/or testing modalities to better discriminate low levels of HER2 protein expression may lead to future algorithms but at present, ongoing research and clinical trials are essential to further understand the biological behavior and clinical significance of HER2-low and HER2-ultralow breast cancer. As an example, the majority of HER2-low and HER2-ultralow breast cancers are hormone receptor-positive (HR+) which has important implications for treatment with the combinations of hormone therapy with HER2-targeted therapy and the potential to further improve outcomes for patients with HR+/HER2-low and HR+/HER2-ultralow breast cancer. Of overriding importance, is the need for additional studies to also evaluate the long-term outcomes of patients with these HER2 subtypes and to identify additional HER2-targeted therapies that may be effective. The DESTINY Breast 04 and 06 trials highlight the need for refining the diagnostic techniques, and to develop standardized testing protocols, to ensure accurate classification of HER2 status. By embracing personalized treatment approaches, clinicians can improve outcomes for patients with HER2-low and HER2-ultralow breast cancer and provide more effective and targeted care. The traditional dichotomy of HER2 status has now been supplanted by the expanding spectrum of HER2 positivity in breast cancer. Comprehensive characterization of the evolving spectrum of HER2 tumors, to further define their clinical and molecular features, is of paramount importance.
