Evaluation of Biomarkers for HER3-targeted Therapies in Cancer

HER3 signaling and targeted therapy in cancer

ERBB family members including epidermal growth factor receptor (EGFR) also known as HER1, ERBB2/HER2/Neu, ERBB3/HER3 and ERBB4/HER4 are aberrantly activated in multiple cancers and hence serve as drug targets and biomarkers in modern precision therapy. The therapeutic potential of HER3 has long been underappreciated, due to impaired kinase activity and relatively low expression in tumors. However, HER3 has received attention in recent years as it is a crucial heterodimeric partner for other EGFR family members and has the potential to regulate EGFR/HER2-mediated resistance. Upregulation of HER3 is associated with several malignancies where it fosters tumor progression via interaction with different receptor tyrosine kinases (RTKs). Studies also implicate HER3 contributing significantly to treatment failure, mostly through the activation of PI3K/AKT, MAPK/ERK and JAK/STAT pathways. Moreover, activating mutations in HER3 have highlighted the role of HER3 as a direct therapeutic target. Therapeutic targeting of HER3 includes abrogating its dimerization partners’ kinase activity using small molecule inhibitors (lapatinib, erlotinib, gefitinib, afatinib, neratinib) or direct targeting of its extracellular domain. In this review, we focus on HER3-mediated signaling, its role in drug resistance and discuss the latest advances to overcome resistance by targeting HER3 using mono- and bispecific antibodies and small molecule inhibitors.

HER3 as a Therapeutic Target in Cancer

Targeting members of the human epidermal growth factor receptor family, especially EGFR and HER2, has been an established strategy for the treatment of tumors with abnormally activated receptors due to overexpression, mutation, ligand-dependent receptor dimerization and ligand-independent activation. Less attention has been paid to the oncogenic activity of HER3, although there is growing evidence that it mediates resistance to EGFR and HER2 pathway directed therapies. The main caveat for the development of effective HER3 targeted therapies is the absence of a strong enzymatic activity to target, as well as the limited potential for single-agent activity. In this review, we highlight the role of HER3 in cancer and, more specifically, in lung cancer. The basis for HER3 involvement in HER2 resistance and EGFR inhibition is discussed, as well as current pharmacologic strategies to combat HER3 inhibition.