Mass spectrometry (MS) is a versatile analytical tool used in various fields such as biochemistry, pharmacology, omics, and clinical analysis for determining and quantifying compounds based on their molecular mass and structure through the mass-to-charge ratio. While MS offers high specificity and selectivity, it encounters challenges including matrix effects, in-source fragmentation, and other interferences caused by natural isotopic abundance, as well as isomeric and isobaric compounds. These challenges can impede accurate qualitative and quantitative analysis. Visual aids such as graphical illustrations can help elucidate the chemical differences and similarities among isotopes, isomers, and isobaric compounds.

Objectives: Cystic fibrosis (CF) transmembrane conductance regulator (CFTR) modulators have revolutionized the therapeutic landscape in CF treatment. These vital drugs are extensively metabolized via CYP3A, so caution must be exercised in multimodal CF therapy because of the risk of adverse drug interactions. Our goal was to develop a highly sensitive assay for the purpose of therapeutic drug monitoring (TDM) in diagnostic laboratories. Methods: After protein precipitation, the CFTR modulators ivacaftor, lumacaftor, tezacaftor, elexacaftor, and their metabolites ivacaftor-M1, ivacaftor-M6, and tezacaftor-M1 were separated with a two-dimensional chromatography setup within 5 min, and quantified with stable isotope-labeled internal standards. The method was validated according to the European Medicines Agency (EMA) guideline on bioanalytical method validation and applied to CF patient samples. Results: Inaccuracy was ≤7.0% and the imprecision coefficient of variation (CV) was ≤8.3% for all quality controls (QCs). The method consistently compensated for matrix effects, recovery, and process efficiency were 105–115 and 96.5–103%, respectively. Analysis of CF serum samples provided concentrations comparable to the pharmacokinetic profile data reported in the EMA assessment report for the triple combination therapy Kaftrio. Conclusions: We hereby present a robust and highly selective isotope dilution liquid chromatography tandem mass spectrometry (ID-LC-MS/MS) assay for the simultaneous quantification of the so far approved CFTR modulators and their metabolites in human serum. The assay is suitable for state-of-the-art pharmacovigilance of CFTR modulator therapy in CF patients, in order to maximize safety and efficacy, and also to establish dose-response relationships in clinical trials.

Background/Objectives: Elexacaftor, tezacaftor, and ivacaftor (ETI) have significantly improved lung function in people with cystic fibrosis (pwCF). Despite exceptional improvements in most cases, treatment-related inter-subject variability and drug–drug interactions that complicate modulator therapy have been reported. Methods: This retrospective analysis presents data on the serum concentration of ETI in our pwCF with full or reduced dosage from August 2021 to December 2023 via routine therapeutic drug monitoring (TDM). The data were compared with the maximum drug concentrations (Cmax) from the pharmaceutical company’s summary of product characteristics. Results: A total of 786 blood samples from 155 pwCF (41% female, 59% male) were analyzed. The examinations were divided into four groups: full dose within the given tmax (38.5% of all measurements), full dose outside the tmax (29%), reduced dose within the tmax (19.2%), and reduced dose outside the tmax (13.2%). In pwCF receiving the full dose and blood taken within the tmax, 45.3% of serum concentrations of elexacaftor, 51.1% of serum concentrations of ivacaftor, and 8.9% of serum concentrations of tezacaftor were found to be above the Cmax, respectively. For those on reduced doses within the tmax, 24.5% had a serum concentration of elexacaftor, 23.2% had a serum concentration of ivacaftor, and 2.5% had a serum concentration of tezacaftor above the Cmax, respectively. Conclusions: Many pwCF under ETI therapy have Cmax values for elexacaftor and ivacaftor above the recommended range, even on reduced doses or before the tmax was reached. This highlights the value of a TDM program. Further pharmacokinetic studies are necessary.