Post-progression treatment options after CDK4/6 inhibitors in hormone receptor-positive, HER2-negative metastatic breast cancer

The combination of cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) and endocrine therapy (ET) is the standard first-line treatment for hormone receptor-positive (HR + ) and HER2-negative metastatic breast cancer (mBC). Despite their efficacy, resistance inevitably develops, necessitating alternative therapeutic strategies post-progression. This review explores current and emerging treatment options following progression on CDK4/6i, focusing on endocrine therapies, targeted therapies, combination approaches, and the continued use of CDK4/6i. Endocrine therapies, including fulvestrant and novel oral selective estrogen receptor degraders (SERDs) like elacestrant, show promise, especially in patients with ESR1 mutations. Targeted therapies such as PI3K/AKT/mTOR inhibitors, exemplified by alpelisib and capivasertib, offer potential by addressing downstream signaling pathways involved in resistance. Additionally, FGFR inhibitors like erdafitinib are under investigation for their role in overcoming specific resistance mechanisms. Combination strategies involving CDK4/6 inhibitors with immune checkpoint inhibitors or other targeted agents are also being explored, with early trials suggesting possible synergistic effects, although further validation is required. Continuation of CDK4/6 inhibitors beyond progression has shown potential benefits in selected patients, but the data are heterogeneous, and further studies are needed to clarify their role. While chemotherapy remains a standard option for patients who progress on these treatments, the goal is to delay its use through the effective utilization of endocrine and targeted therapies. Understanding resistance mechanisms and tailoring treatment to individual patient profiles is crucial for optimizing outcomes. Ongoing clinical trials are expected to provide deeper insights, guiding the development of more effective post-progression therapeutic strategies. This evolving landscape highlights the need for continuous research and individualized patient care to improve survival and quality of life in HR + mBC patients.