Clinical trials of intracellular signal transductions inhibitors for breast cancer — a strategy to overcome endocrine resistance

Abstract

Acquired resistance to endocrine therapy in breast cancer is associated with an increase in peptide growth factor signaling that results in cross-talk activation of the estrogen receptor (ER). Small molecule signal transduction inhibitors (STIs) can target components of these intracellular pathways, and may prove effective in anticancer therapy. However, early phase II clinical trials with various STIs as monotherapy in advanced breast cancer have shown only a modest level of efficacy for these intracellular inhibitors. Preclinical data suggest that combinations of tamoxifen with STIs may provide significantly greater growth inhibition than either therapy alone, and, furthermore, may delay the emergence of endocrine resistance. There are now several trials assessing the efficacy of combinations of small molecule tyrosine kinase inhibitors (TKIs), such as gefitinib and lapatinib, with either tamoxifen or aromatase inhibitors both in the second-line, endocrine-resistant and first-line, hormone-sensitive setting. Similar trials continue with both farnesyltransferase inhibitors (FTIs) and mTOR antagonists, where there are strong preclinical data to suggest additive or synergistic effects for either of these agents in combination with endocrine therapies. Biomarker studies in the presurgical setting are also being utilized as an alternative approach to investigate whether combined endocrine/STI therapy is an effective clinical strategy. This article reviews some of the preclinical evidence supporting this strategy, together with the current status of clinical trials in this area.