Establishment of an analytical method for simultaneous quantitation of CDK4/6 inhibitors, aromatase inhibitors, and an estrogen receptor antagonist in human plasma using LC-ESI-MS/MS

•A simple, highly sensitive, and specific LC/ESI-MS/MS method was established.•CDK4/6 inhibitors, aromatase inhibitors, and an estrogen receptor antagonist were quantitated simultaneously.•Palbociclib, abemaciclib, letrozole, and fulvestrant in plasma of breast cancer patients were analyzed. Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors (palbociclib, abemaciclib, and ribociclib) are used to treat human epithelial growth factor receptor (HER)-2 negative and hormone receptor (HR) positive advanced breast cancer in combination with aromatase inhibitors (letrozole, anastrozole) or an estrogen receptor antagonist (fulvestrant). Administration of these drugs frequently causes severe side effects, such as neutropenia and diarrhea. Therefore, therapeutic drug monitoring (TDM) of CDK4/6 inhibitors, aromatase inhibitors, and the estrogen receptor antagonist is considered important for ensuring the efficacy and safety of these drugs. In this study, we describe a simple, highly sensitive, and specific liquid chromatography/electrospray ionization tandem mass spectrometry method for simultaneous quantitation of the concentrations of palbociclib, abemaciclib, ribociclib, letrozole, anastrozole, and fulvestrant. In addition, we analyzed plasma samples from patients with HER2-negative and HR-positive advanced breast cancer treated with these compounds using this novel method. In our method, the intra-assay relative error (RE) values ranged from −12.8% to 12.9%, the inter-assay RE values ranged from −4.8% to 6.2%, and the coefficient of variation (CV) values for intra- and inter-assay were ≤8.6% and ≤13.3%, respectively. The analytes showed good stability with RE values ranging from −13.5% to 13.6% and CV values