The Exciting New Field of HER2-Low Breast Cancer Treatment

SIMPLE SUMMARY: Breast cancer can express, at varied levels, a protein named HER2, commonly responsible for making it grow and send distant metastases. In the past, patients affected by the so called HER2-positive breast cancer had lower probabilities of cure and survival, though with the advent of drugs that target HER2, three decades ago, their prognosis has greatly improved. So far, only patients with strong HER2 expression on their tumour can be treated with these benefitial drugs, like trastuzumab, though recently stronger drugs have also been shown capable of eliminating breast cancer cells with lower levels of HER2 expression (HER2-low). Sooner or later, these new drugs, like trastuzumab-deruxtecan, may be available for treating such patients. Therefore, the aim of this narrative review of the literature is to provide an outline of what is going on on this specific field of research, and what could be expected in the future in the clinic. ABSTRACT: Since human epidermal growth factor receptor-2 (HER2) characterization, going through clinical research and regulatory approval of HER2-targeted therapies, much has elapsed and is still unfolding. Hitherto, only breast cancer (BC) patients with HER2 immunohistochemistry 3+ or with HER2 gene fluorescence in-situ hybridization (FISH) amplification (a.k.a., HER2-positive BC) have benefited from anti-HER2 agents. In recent years, however, much of the research effort has been expanded, with positive outcomes being reached for formerly known HER2-negative BC that yet express HER2 to some degree (HER2 immunohistochemistry 1+ or 2+, but FISH negative) and are currently being classified as HER2-low BC for the purpose of trial enrollment. In this sense, our aim is to review the body of evidence of HER2-low BC that led to the study of first-generation anti-HER2 agents, like trastuzumab, and how they have failed to achieve any clinical applicability in this setting. In addition, we review new data that is leading to the growing success of the new generation of drugs, especially the promising HER2-directed antibody–drug conjugates. A narrative review is also performed regarding the rationale behind the consolidated and ongoing clinical trials studying anti-HER2 agents in combination with unrelated agents, such as immunotherapy, endocrine therapy, and CDK4/6 inhibitors. Hopefully, all this ongoing research effort will be able to extend the survival benefits seen with anti-HER2 agents in HER2-positive disease, at least to some degree, to the greater proportion of patients with HER2-low BC.