Verapamil as a culprit of palbociclib toxicity

A promising drug, palbociclib, received accelerated approval as a first line treatment when used with the aromatase inhibitor, letrozole, for postmenopausal women with hormone receptor positive advanced or metastatic breast cancer. We report a case of a patient who presented with febrile neutropenia, grade 3 stomatitis with lip swelling, periorbital edema, and transaminitis while on palbociclib and verapamil. Labs normalized upon discontinuation of verapamil and our patient was able to continue treatment with palbociclib and letrozole. Verapamil's inhibition of both permeability-glycoprotein (P-gp) and CYP3A4 is suspected to have led to the adverse side effects seen in our patient.