Abstract

HER2 is a well-known prognostic and predictive biomarker in breast cancer, and it is assessed by immunohistochemistry (IHC) and reflex in situ hybridization (ISH) for clinical decision-making. In particular, the HER2-positive phenotype (i.e. IHC score 3+ or score 2+ with a positive ISH) allows for the selection of patients eligible for targeted therapy with anti-HER2 drugs. Lately, novel HER2-directed antibody-drug conjugates have demonstrated significant antitumor activity in breast cancers with low levels of HER2 expression. These tumors are defined by IHC score 1+ or score 2+ without gene amplification. Both primary and acquired resistance to anti-HER2 therapies remains a challenge in the treatment of breast cancers according to the HER2 positivity continuum. Thus, the ability to precisely discriminate between HER2-negative (i.e. IHC score 0), HER2-low, and HER2-positive breast cancers is no longer a mere academic exercise. Here, we summarize the possible mechanisms of resistance to anti-HER2 agents emphasizing the need for refreshing HER2 testing criteria, guidelines, and quality controls, and highlight the best approach to overcome the main pre-analytical and analytical variables that might trouble the sensitivity and reproducibility of this test.