Optimizing outcomes in HER2-positive breast cancer: the molecular rationale

The epidermal growth factor (EGF) receptor HER2 is a transmembrane receptor tyrosine kinase that plays a crucial role in the regulation of cell proliferation and survival. The overexpression of HER2 correlates strongly with prognosis in breast cancer. The targeted blockade of HER2 activity with monoclonal antibodies (e.g., trastuzumab [Herceptin]) and small-molecule tyrosine kinase inhibitors (e.g., lapatinib) results in the inhibition of tumor growth in HER2-positive cancers. Anti-HER2 therapies have also shown efficacy in combination with chemotherapy in clinical trials in patients with HER2-positive breast cancer. Their efficacy may, however, be limited by molecular mechanisms that compensate for HER2 suppression (e.g., activity of EGF receptor) or mechanisms of resistance (e.g., loss of PTEN). HER2 continues, however, to be overexpressed by the cancer cells, and the continued suppression of HER2 may be required for maximum antitumor effect. It should be noted that in the absence of definitive data from randomized trials showing an absence or presence of benefit, the use of anti-HER2 agents such as trastuzumab in multiple sequential regimens has become the standard of care. Combining HER2 blockers with agents that overcome the compensatory or resistance mechanisms may increase the efficacy of anti-HER2 therapies. In addition, anti-HER2 therapies can have synergy with common chemotherapy regimens and remain effective through multiple lines of therapy. Optimizing the use of therapies that target HER2 signaling will lead to further advances in the treatment of breast cancer.