Novel Strategies in Hormone Receptor-Positive Advanced Breast Cancer: Overcoming Endocrine Resistance

Resistance to standard endocrine therapies in hormone receptor-positive advanced breast cancer represents a significant clinical challenge. Different intracellular signaling pathways mediate independent activation of the estrogen receptor (ER), promoting tumor cell proliferation despite anti-hormonal treatment. Recently, the inhibition of cell cycle regulators, CDK4 and CDK6, has demonstrated to significantly enhance the effectiveness of endocrine therapy by overcoming or delaying resistance to estrogen blockade. Strategies such as inhibition of the PI3K/mTOR pathway or epigenetic modulation of ER-related gene expression are closely following the trail of CDK inhibitors. Here, we seek to review the most recent efforts to improve outcomes in these patients, in an attempt to extend endocrine treatment and defer the need for cytotoxic regimens. We also discuss future directions to be considered in the treatment of ER-positive disease, as mechanisms of resistance to these new agents arise.