Therapeutic drug monitoring in breast cancer therapy – LC-MS/MS method for quantification of the CDK4/6 inhibitors abemaciclib, palbociclib, ribociclib, and major metabolites abemaciclib M20 and M2 in human serum

The cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors palbociclib, ribociclib, and abemaciclib were approved by the U.S. Food and Drug Administration (FDA) and European Medicine Agency for the treatment of breast cancer between 2015 and 2018. Oral tumor therapeutics extend the options for cancer therapy, but also challenge physicians and patients. The aim of the present work was to establish a semi-automated liquid-chromatography tandem mass spectrometry (LC-MS/MS) method for the simultaneous quantification of abemaciclib, its active metabolites abemaciclib M20 and M2, palbociclib, and ribociclib in human serum. Detuning of ribociclib enabled the development of a simultaneous quantification method for abemaciclib, M20, M2, palbociclib, and ribociclib in the respective relevant concentration ranges based on semi-automated sample preparation with isotope dilution LC-MS/MS. The method was validated according to the guidance of the FDA. The LC-MS/MS method was successfully validated according to FDA and showed inaccuracies ≤ 10.7% and imprecisions ≤ 8.51%. Linearity was given from 20 to 800 ng/mL for abemaciclib, 15–600 ng/mL for M20, 10–400 ng/mL for M2 and palbociclib, and 100–4000 ng/mL for ribociclib. Normalized matrix factors and process efficiency showed no significant matrix effects regardless of the analytes. To demonstrate the applicability of the method, authentic samples were also analyzed. This novel semi-automated LC-MS/MS method covering all previously approved CDK4/6 inhibitors as well as the similarly pharmacologically active metabolites in human serum simultaneously was developed for potential future use in routine analysis in order to improve personalized therapy, patient safety, and treatment success.