"A new LC-MS/MS method for the simultaneous quantification of abemaciclib, its main active metabolites M2 and M20, and letrozole for therapeutic drug monitoring".

Abemaciclib (ABEMA) is the last CDKi approved for the treatment of breast cancer. Adverse reactions to this drug are not experienced in the same manner by the entire patient population but in case of severe toxicity dose reductions and therapy discontinuation are required, suggesting that a TDM-guided treatment could be beneficial for these patients. ABEMA is extensively metabolized by the liver. The most abundant active metabolites are M2 and M20. This CDKi is administered together with anti-estrogen drugs, such as letrozole (LETRO). The aim of this work was to develop and validate a LC-MS/MS method for the simultaneous quantification of ABEMA, M2, M20, and LETRO. The chromatographic separation of the analytes was obtained using a SIL-20AC XR auto-sampler coupled to LC-20AD UFLC Prominence XR pumps (Shimadzu, Tokyo, Japan). The chromatographic column employed was an XTerra MS C18, (3,5 µm, 125 Å, 50x2.1 mm) coupled with a Security Guard Cartridge (MS C18, 125 Å, 3.9x5 mm) provided by Waters. Detection was performed by an API 4000 QTrap (SCIEX) mass spectrometer. The presented analytical method was fully validated according to EMA and FDA guidelines on bioanalytical method validation. Linearity was confirmed on 10 independent tests (R2 within 0.997-1.000) over the concentration ranges of 40-800 ng/mL for ABEMA, 10-200 ng/mL for M2 and M20, 20-400 ng/mL for LETRO. The method was applied to analyze plasma samples from patients enrolled in a clinical trial, collected at Cmin. Incurred sample reanalysis was performed on a set of 30 samples, confirming the reproducibility of the analytical method.