Metabolic Response to <scp>CDK4</scp> /6 Inhibition in <scp>ER</scp> + Breast Cancer Creates a Therapeutic Vulnerability in Drug‐Tolerant Persister Cells

Although endocrine therapies prevent recurrence and progression of estrogen receptor alpha (ER)-positive breast cancer, approximately one-third of patients experience recurrent disease that is rarely cured in the advanced/metastatic setting. A subpopulation of endocrine-tolerant breast cancer cells persists as residual disease that confers risk for the eventual emergence of drug resistance. An analysis of persisters that continue to proliferate despite endocrine therapy revealed the activation of pathways related to metabolism and E2F transcription factor signaling. E2F signaling is driven by cyclin-dependent kinases 4 and 6 (CDK4/6), and CDK4/6 inhibitors (CDK4/6i) are used clinically to prevent and manage endocrine resistance. CDK4/6i slowed the cycling of endocrine-tolerant persisters. Analyzing metabolic alterations induced by CDK4/6i, we found that CDK4/6i-tolerant persisters had upregulation of mitochondrial content, mitochondrial membrane potential, respiration, and reactive oxygen species (ROS). Inhibition of mitochondrial complex I further increased ROS levels and enhanced growth inhibition in both endocrine-sensitive and -resistant cell lines and patient-derived xenografts. These findings collectively offer mitochondrial respiration as a therapeutic target in CDK4/6-tolerant persister breast cancer cells to help eradicate residual disease.