Is There a Benefit of HER2-Positive Breast Cancer Subtype Determination in Clinical Practice?

Breast cancer (BC) with increased expression of human epidermal growth factor receptor 2 with tyrosine kinase activity (HER2+) is a clinically and bio-logically heterogeneous dis-ease. In terms of gene expression, there are four major molecular subtypes - Luminal A, Luminal B, HER2-enriched (HER2-E), and Basal-like. The most common subtype is HER2-E (50- 60%). In hormone-dependent (HR+) HER2-positive tumors, the subgroup HER2-E represents 40- 50% of cases; others are luminal A and B subtypes.

The aim of this review is to provide information on the significance of the distribution of HER2-positive tumors accord-ing to subtype, which is considered a predictive parameter for guid-ing treatment decisions. For example, HER2-E subtype is characterized by a higher probability of achiev-ing complete pathological remission when treated with chemother-apy and antiHER2 ther-apy, and it is thought that it could be treated us-ing a dual HER2 blockade without chemother-apy. Currently, triple-positive tumors, a specific subtype of breast cancer characterized by HER2+ and HR+, are more often subjects of interest. Their unique bio-logical properties are due to complex interactions between HER2 and estrogen receptor (ER) signalling, which result in lower effectiveness of endocrine ther-apy in these patients than in HR+ and HER2-negative patients and, at the same time, the ER positivity in HER2+ tumors can result in resistance to antiHER2 ther-apy. This type of BC is a non-homogeneous group where the impacts of HER2 positivity on tumor malignant behavior and activity of the estrogen-driven signal-ing pathway are inconsistent. Current studies focus on test-ing new treatments such as dual HER2 block-ing or immunother-apy, in combination with antiHER2 targeted ther-apy with fulvestrant, aromatase inhibitors, cyclin dependent kinase 4/ 6 inhibitors, or inhibitors of the PI3K (phosphatidylinositol-3-kinase) pathway.

The distribution of HER2+ BC accord-ing to individual subtype provides information that can contribute to achiev-ing more accurate decisions about the most appropriate ther-apy. Key words breast cancer - subtype - HER2 - trastuzumab - HER2 positive - triple positive - HER2 enriched The authors declare they have no potential conflicts of interest concerning drugs, products, or services used in the study. The Editorial Board declares that the manuscript met the ICMJE recommendation for biomedical papers. Submitted: 27. 9. 2018 Accepted: 26. 11. 2018.