Co-targeting estrogen receptor and HER2 pathways in breast cancer.

The estrogen steroid hormone receptor (ER) and human epithelial growth factor receptor 2 membrane tyrosine kinase growth factor receptor (HER2) are the mediators of two key pathways involved in breast carcinogenesis, invasive behavior and cell growth. Co-expression of these receptors results in specific biological features that are not fully understood, but include relative resistance to hormonal therapy and chemotherapy as well as better long-term outcome imparted by ER and worse outcome by HER2 expression. The ER and HER2 signaling pathways interact with each other as do many biological networks, and this creates opportunities for therapeutic co-targeting with agents that modulate these respective pathways. However, relatively few studies have been conducted to test concurrent manipulation of ER and HER2. The avoidance of chemotherapy side effects is an attractive feature that has further spurred explorations in this strategy. Still, the only dually targeted strategy approved by some regulatory agencies is the combination of hormonal therapy using aromatase inhibition and the HER2 kinase inhibitor lapatinib. Other dual combinations have also demonstrated a benefit, although most of the testing has compared hormonal therapy with or without HER2-directed agents and not the other way around, limiting the applicability of this concept in routine clinical practice, especially when chemotherapy is also used. Newer generation signal transduction inhibitors can augment the efficacy of hormonal therapy, with one such example of mTOR blockade using everolimus now in the clinic. The logical extension of ER and HER2 co-targeting is the discovery and clinical testing of "synthetic lethal" combinations attacking diverse pathways that produce quantum improvements over either therapy alone. Molecular annotation of human cancers can further inform personalized combinatorial regimens based on the unique circuitry of an individual patient's tumor, with the potential to yield much more than incremental gains in survival.