MSACL 2023 Abstract

Introduction
With 1.7 million new cases per year, breast cancer is the third most common cancer worldwide. The cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors palbociclib, ribociclib, and abemaciclib were approved in recent years by the U.S. Food and Drug Administration (FDA) and European Medicine Agency (EMA) for the treatment of breast cancer. Generally, a fixed dosing regimen of oral tumor therapeutics (OTT) is used. However, several described side effects and pharmacokinetic interactions (CYP3A4 inducers and inhibitors) may affect appropriate drug levels and therapeutic success. OTT extend the options for cancer therapy in the home environment, but also pose challenges for physicians and patients.

Objectives
The aim of our work was the development of a robust liquid-chromatography tandem mass spectrometry (LC-MS/MS) method for all previously approved CDK4/6 inhibitors and to verify adequate serum levels of OTT in routine diagnostics.

Methods
We developed a semi-automated LC-MS/MS method for the simultaneous quantification of abemaciclib, its active metabolites abemaciclib M20 and M2, palbociclib, and ribociclib in human serum. Detuning and measurement of a natural isotopologue of ribociclib enabled measurement in the respective relevant concentration ranges. For chromatographic separation a biphenyl column and acidified mobile phases were used. The method was validated according to the guidance of the FDA and applied to authentic samples.

Results
The LC-MS/MS method was successfully validated according to FDA and showed inaccuracies ≤10.7% and imprecisions ≤8.51%. To demonstrate the applicability of the method, authentic samples were also analyzed of a single individual treated with abemaciclib under a compassionate use authorization. We also assessed pharmacokinetic data of abemaciclib in human serum.

Conclusion
This semi-automated LC-MS/MS method covers all previously approved CDK4/6 inhibitors as well as the similarly pharmacologically active metabolites in human serum simultaneously and was developed for potential future use in routine clinical testing.