Abstract
This narrative review explores the mechanisms underlying resistance to cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) in hormone receptor (HR)-positive metastatic breast cancer (MBC), a critical challenge in contemporary oncology. Despite the proven efficacy of CDK4/6i in improving clinical outcomes, both intrinsic and acquired resistance remain substantial challenges. We discuss clinical data that underscore pivotal molecular alterations associated with resistance, including mutations in the Retinoblastoma gene (RB1), germline variants, aberrations in the PIK3CA gene that activate the phosphatidylinositol 3-kinase/protein kinase B (AKT)/mammalian target of rapamycin signaling cascade, and modifications in fibroblast growth factor receptor signaling. Additional resistance mechanisms-such as the loss of the FAT1 tumor suppressor gene and the dysregulation of cell cycle regulators like cyclin E and CDK2-are also explored. The role of circulating tumor DNA analysis in tracking genomic changes during therapy is also considered. Furthermore, the review assesses emerging therapeutic strategies, particularly combination therapies that target alternative pathways to counteract resistance mechanisms. By synthesizing current evidence and providing actionable insights, this review aims to enhance our understanding of endocrine resistance mechanisms among clinical oncologists and gives them some future perspectives to expand strategies to overcome this challenge.