Enhancing the efficacy of hormonal agents with selected targeted agents

Several selected targeted agents are being investigated in combination with endocrine therapy for patients with breast cancer in an attempt to overcome or prevent endocrine resistance. The role of type I growth factor receptors epidermal growth factor receptor (EGFR) and HER2 in cross-talk with estrogen receptor (ER) signaling has been confirmed in preclinical studies in which various inhibitors have yielded additive or synergistic effects when combined with endocrine agents. Recently, several results from clinical trials investigating this concept have been reported. In ER-positive/HER-positive advanced breast cancer, the addition of trastuzumab to the aromatase inhibitor anastrozole, or the tyrosine kinase inhibitor (TKI) lapatinib to letrozole, both have significantly improved progression-free survival (PFS). The EGFR TKI gefitinib combined with tamoxifen as first-line therapy for ER-positive metastatic disease improved PFS (but not objective response rate) for patients with no previous endocrine therapy or completion of previous adjuvant therapy. A second study in a similar setting showed significant improvement in PFS for gefitinib plus anastrozole. Although it is encouraging that this approach could delay resistance, only a small proportion of patients benefit. Attempts to identify likely responders have been made in the neoadjuvant setting, with pre- and post-treatment biopsies being used to study biomarker changes. A recent preoperative study of letrozole with or without the mammalian target of rapamycin (mTOR) inhibitor everolimus reported greater tumor shrinkage for the combination, with changes in proliferation being predictive for response together with strong expression of protein S6 kinase, a downstream marker of activated mTOR. Key aspects that need to be addressed in future trials include understanding the mechanisms of action for each novel agent, designing the best trial and endpoints to demonstrate added benefit, and ensuring appropriately stratified populations based on previous endocrine exposure and/or sensitivity.