A Liquid Chromatography-Tandem Mass Spectrometry Method for the Quantification of Cystic Fibrosis Drugs (Caftors) in Plasma and Its Application for Therapeutic Monitoring

Cystic fibrosis (CF) is a life-threatening disorder caused by mutations in the CFTR gene, leading to defective chloride ion transport and thickened mucus in the respiratory and gastrointestinal systems. CFTR modulators, including ivacaftor, lumacaftor, tezacaftor, and elexacaftor, have improved patient outcomes, but interindividual pharmacokinetic variability and potential drug-drug interactions require therapeutic drug monitoring (TDM) for optimal efficacy and safety. In this context, a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method has been developed and validated for the simul-taneous quantification of CFTR modulators and their major active metabolites in human plasma to support pharmacokinetic studies and routine TDM. The multiplex LC-MS/MS assay was established using plasma protein precipitation, followed by chromatographic separation on an Xselect HSS T3 column and positive electrospray ionization mode detec-tion. The method was validated based on FDA and EMA guidelines for specificity, linear-ity, accuracy, precision, matrix effects, and stability, demonstrating robust performance with excellent precision and accuracy. International interlaboratory comparisons con-firmed the reliability of the assay. The developed method has been applied for monitoring caftors plasma levels for patient TDM and also to research studies in alternative matrices such as breast milk. Future studies will help at characterizing caftors pharmacokinetic variability in patients and drug-drug interactions to further refine personalized dosing strategies.