Anti-HER2 therapies: when more is more

Human epidermal growth factor receptor 2 (HER2) is a member of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases (RTKs), which includes EGFR (HER1), c-erbB2 (HER2), c-erbB3 (HER3), and c-erbB4 (HER4) (1). These four receptors share an extracellular domain in N-terminal position corresponding to the ligand binding site, a transmembrane domain, and an intracellular domain in C-terminal position having tyrosine kinase (TK) activity, except for HER3, whose kinase domain is inactive. Apart for HER2, for which no ligand has been identified, for other members of the HER family, ligands bind to the receptor leading to homo- or hetero-dimerisation, and self-activation of tyrosine residues in the C terminal domain, and transduction of signal via the Ras/Raf/MEK/ERK and the PI3K/AKT/mTOR pathways (2-3) involved in cell survival, migration, apoptosis and proliferation.