Hormone receptor positive, HER2 negative metastatic breast cancer: A systematic review of the current treatment landscape

Endocrine therapy for the treatment of hormone receptor positive, HER2 negative, metastatic breast cancer is continually evolving. We systematically reviewed phase 2 and 3 randomized controlled trials (RCTs) of agents used in this setting to assess the effectiveness and safety of these agents for postmenopausal women. Across the 32 studies in more than 10,000 patients, the greatest improvement in progression-free survival (PFS) was seen with the addition of a cyclin-dependent kinase (CDK)4/6 inhibitor to standard endocrine therapy. Treatment with a mammalian target of rapamycin (mTOR) inhibitor, phosphoinositol-3-kinase (Pi3K) inhibitor, vascular endothelial growth factor (VEGF) inhibitor and with a selective estrogen receptor degrader (SERD) also showed benefit in PFS for selected trials. Overall survival (OS) improved with the use of mTOR inhibitors and a SERD; however, studies were not powered for an OS endpoint. Encouraging results from early studies of histone deacetylase (HDAC) and B-cell lymphoma (BCL2) inhibitors are yet to be confirmed in phase III clinical trials. Study discontinuation rates and toxicity-related deaths were highest with VEGF inhibitors in combination with endocrine therapy, limiting their use in hormone receptor positive breast cancer. CDK4/6 inhibitors and mTOR inhibitors appeared to have activity in both first and second line settings, but required additional monitoring for common toxicities. The activity of epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors was limited to the first-line setting and treatment discontinuation rates were higher than with mTOR inhibitors and SERDs. Overall, PFS benefit appears to be greatest when agents acting on CDK4/6, mTOR and Pi3K pathways, and SERDs are added to standard endocrine therapy. If these early results persist in further studies, these data are likely to change the way we treat hormone receptor positive, HER2 negative metastatic breast cancer. In the follow-up article to this review, we will consider the potential future treatment options for these patients.