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Licensed Unlicensed Requires Authentication Published by De Gruyter October 20, 2021

Isotope dilution LC-MS/MS quantification of the cystic fibrosis transmembrane conductance regulator (CFTR) modulators ivacaftor, lumacaftor, tezacaftor, elexacaftor, and their major metabolites in human serum

Katharina Habler ORCID logo , Anne-Sophie Kalla , Michael Rychlik , Mathias Bruegel , Daniel Teupser , Susanne Nährig , Michael Vogeser and Michael Paal ORCID logo EMAIL logo



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.


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.


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.


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.

Corresponding author: Michael Paal, Institute of Laboratory Medicine, University Hospital, LMU Munich, Marchioninistr. 15, 81377 Munich, Germany, Phone: +49 89 4400 73200, Fax +49 89 4400 73240, E-mail:

Funding source: Friedrich-Baur-Stiftung

  1. Research funding: Michael Paal received a research grant (award number 62/20) from the Friedrich-Baur-Stiftung. The funding source had no involvement in the research.

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Competing interests: Authors state no conflict of interest.

  4. Informed consent: Informed consent was obtained from all individuals included in this study.

  5. Ethical approval: Research related to human use has been complied with all the relevant national regulations, institutional policies and in accordance the tenets of the Helsinki Declaration (as revised in 2013), and has been approved by the institutional review board of the Ludwig-Maximilians-Universität (protocol code 20-999).


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Supplementary Material

The online version of this article offers supplementary material (

Received: 2021-06-22
Accepted: 2021-09-07
Published Online: 2021-10-20
Published in Print: 2022-01-26

© 2021 Walter de Gruyter GmbH, Berlin/Boston

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