Three sample preparation methods for clinical determination of CDK4/6 inhibitors with endocrine therapy in breast cancer patient plasma using LC-MS: Cross-validation (red), ecological (green) and economical (blue) assessment.

Cyclin D-dependent kinase 4/6 inhibitors palbociclib, ribociclib and abemaciclib, in combination with aromatase inhibitors anastrozole and letrozole or oestrogen receptor degrader fulvestrant, are being assessed as candidates for therapeutic drug monitoring. An ideal bioanalytical method for their determination in patient plasma samples is therefore of high interest, as there is no routine reference method yet available in the clinical practice. In this work, three sample preparation approaches - dispersive liquid-liquid microextraction (DLLME), solid-phase extraction (SPE), and newly developed phospholipid removal (PLR) for LC-MS determination of these six drugs are comprehensively assessed. The methods are validated in the clinically relevant linear ranges with remarkable precision (RSD ≤6.9 %) and accuracy (bias -13.6 - 11.8 %). To compare the procedures in a real-world setting, they are applied on 38 samples from breast cancer patients. The differences between paired results are below 20 % for more than 92 % of the repeats and the RSD is ≤13.1 % between the corresponding results. Statistical comparison of the results reveals excellent overall agreement between the methods (Lin's concordance correlation coefficient ≥0.9969, maximal Bland-Altman bias 6.3 %). DLLME proved to be the most ecologically acceptable method due to the high degree of miniaturisation (AGREEprep score 0.44), PLR enabled very high sample throughput and cost-effectiveness (BAGI 72.5), while SPE showed the best analytical performance (redness score 100). All three methods are suitable for their designated purpose, and the choice of the ideal method can be made based on the scope of application, available funds and equipment and desired ecological footprint.