Rebound lymphocytosis in a patient with chronic lymphocytic leukemia after cessation of a CDK 4/6 inhibitor for concomitant breast cancer.

Herein, we present the case of a female with co-occurring chronic lymphocytic leukemia (CLL) and metastatic ER/PR-positive breast cancer, who when taken off palbociclib for severe neutropenia and infectious complications, experienced rebound lymphocytosis. Though this was ultimately a benign clinical outcome, it exemplifies how CDK4/6 inhibitors induce a cell cycle arrest by decreasing the proliferation of hematopoietic stem cells (HSC), in this case CLL cells, with recovery of counts once drug is stopped.