The HER family of receptor tyrosine kinases, including EGF receptor (EGFR), HER2, HER3, and HER4, transduce growth-promoting signals in response to ligand binding to their extracellular domains (ECD). This family is deregulated in numerous cancers, with mutations in EGFR and HER2 often serving as "driver" events to activate key growth factor signaling pathways such as the RAS-ERK and PI3K-AKT pathways. Less attention has been paid to the oncogenic functions of HER3 due to its lack of intrinsic kinase activity. Recent work, however, has placed HER3 in the spotlight as a key signaling hub in several clinical contexts. First, HER3 has been shown to play a major role in mediating resistance to HER2 and phosphoinositide 3-kinase (PI3K) pathway-directed therapies due to its feedback regulation via AKT signaling. Second, activating mutations in HER3 have been identified in multiple cancer types, including gastric, colon, bladder, and non-small cell lung cancers. As a result, HER3 is now being examined as a direct therapeutic target. In the absence of a strong enzymatic activity to target, the focus has been on strategies to prevent HER3 activation including blocking its most relevant dimerization partner's kinase activity (erlotinib, gefitinib, and lapatinib), blocking its most relevant dimerization partner's ability to dimerize with HER3 (trastuzumab and pertuzumab), and directly targeting the HER3 ECD (MM-121, U3-1287, and LJM716). Although drugs targeting EGFR and HER2 have proven effective even as single agents, the preclinical and clinical data on the antibodies directly targeting HER3 suggest more limited potential for single-agent activity. Possible reasons for this include the lack of a suitable biomarker for activated HER3, the lack of potency of the antibodies, and the lack of relevance of HER3 for growth of some of the cancer types analyzed. Nevertheless, clear improvements in activity are being observed for many of these compounds when they are given in combination. In this snapshot, we will highlight the basis for HER3 activation in cancer, the different pharmacologic strategies being used, and opportunities for further development.
