Baseline lymphopenia as prognostic factor in patients with metastatic breast cancer treated with palbociclib.

Cyclin-dependent-kinase 4-6 inhibitors (CDK4/6i) have improved the management of hormone receptor (HR)/human epidermal growth factor receptor (HER)2 metastatic breast cancer (mBC). Currently, there are no valid prognostic factors for response to CDK4/6i. Baseline lymphopenia is reported as a prognostic factor in several types of cancer.