CDK4/6 Inhibitors in Breast Cancer Therapy: Mechanisms, Resistance, and Emerging Opportunities

Area of Science:

• Oncology
• Molecular Biology
• Genetics

Background:

• Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors are a cornerstone in treating hormone-receptor-positive (HR+), human epidermal growth factor receptor 2 (HER2)-negative (HER2-) breast cancer.
• These inhibitors, combined with endocrine therapy, enhance progression-free survival and, in some cases, overall survival.

Purpose of the Study:

• To review the mechanisms of resistance to CDK4/6 inhibitors in HR+/HER2- breast cancer.
• To discuss the evolving treatment landscape and emerging therapeutic strategies for patients progressing on CDK4/6 inhibitors.

Main Methods:

• Review of clinical trial data and preclinical studies on CDK4/6 inhibitors in HR+/HER2- breast cancer.
• Analysis of identified resistance mechanisms, including genetic alterations and pathway activations.
• Evaluation of current and investigational treatment options post-CDK4/6 inhibition.

Main Results:

• CDK4/6 inhibitors significantly improve outcomes in HR+/HER2- breast cancer, but resistance is a major clinical challenge.
• Identified resistance mechanisms include RB1 loss, CDK6 amplification, cyclin E-CDK2 signaling activation, bypass pathway upregulation (PI3K/AKT/mTOR/FGFR), and ESR1 mutations.
• Circulating tumor DNA (ctDNA) assays aid in detecting resistance mechanisms and guiding therapy modifications.

Conclusions:

• Despite resistance, CDK4/6 inhibitors have transformed HR+/HER2- breast cancer treatment.
• The post-CDK4/6 inhibitor landscape is expanding with novel agents and strategies, including PI3K-AKT pathway inhibitors, selective estrogen receptor degraders, and antibody-drug conjugates.
• Ongoing research into CDK2 inhibitors, ctDNA-guided therapy, and immunotherapy combinations promises to further optimize treatment sequencing and improve long-term outcomes.