Beyond trastuzumab and lapatinib: new options for HER2-positive breast cancer

HER2-positive breast cancer (BC) constitutes a molecular subtype of the disease with an aggressive biologic behavior. Trastuzumab revolutionized the treatment of this disease, changing its natural history. Lapatinib is active in the metastatic setting, approved for patients who were pretreated with trastuzumab. However, resistance to anti-HER2 agents is a major clinical issue, occurring in both early-stage and advanced disease, and new treatment options are clearly needed. An abundance of HER2-targeted agents are being clinically developed: monoclonal antibodies, small molecule inhibitors, and antibody drug conjugates (ADC). Combining HER2-targeted agents in regimens of dual HER2 blockade has already reached clinical practice in the metastatic setting, confirming the preclinical efficacy of enhanced HER2 inhibition. Promising results have been generated in the neoadjuvant setting, and large randomized trials are seeking evidence for dual HER2 blockade in the adjuvant setting. ADC represent another hope for improved treatment outcomes of HER2-positive BC, as exemplified by the positive results of clinical trials employing trastuzumab-DM1 (trastuzumab emtansine, T-DM1). Moreover, an understanding of the molecular mechanisms mediating resistance to HER2 blockade has opened new therapeutic avenues, with several targeted agents entering clinical trials. This paper presents the clinical data of the HER2-targeted agents under development, as well as an overview of the biologic rationale for the development of agents aimed at circumventing anti-HER2 resistance.