Enhancing endocrine response with novel targeted therapies: Why have the clinical trials to date failed to deliver on the preclinical promise?

Acquired resistance to endocrine therapies has severely limited their long-term effectiveness in breast cancer. In recent years a clear rationale has developed for combining signal transduction inhibitors (STIs) with endocrine therapies to delay the emergence of acquired resistance and enhance endocrine responsiveness. A variety of biologic agents have been developed to target key proteins along the EGFR, HER2, MAPK, and P13K/Akt signal transduction cascades. While several of these agents have shown early promise in selected breast cancer models, translating these data into convincing clinical results has been generally disappointing to date. By applying more rigorous trial design and tumor selection criteria to future trials, it is much more likely that adding the new generation of targeted therapies can fulfill its promise in enhancing endocrine responsiveness and our ability to treat breast cancer patients.