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Authors’ response: Importance of defining loss of response before therapeutic drug monitoring
  1. C Steenholdt,
  2. M A Ainsworth
  1. Department of Gastroenterology, Herlev Hospital, Herlev, Denmark
  1. Correspondence to Casper Steenholdt, Department of Gastroenterology, Herlev Hospital, Herlev Ringvej 75, Herlev DK-2730, Denmark; steenholdt{at}

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In his letter,1 Tan questions the use of Crohn's Disease Activity Index (CDAI) as a clinical outcome measure in our randomised controlled trial (RCT) of how to optimally handle Crohn's disease patients with symptomatic infliximab (IFX) treatment failure during ongoing IFX maintenance therapy.2 This study is hitherto the only published RCT of personalised antitumour necrosis factor-alpha (anti-TNFα) therapy where measurements of serum trough levels of drug and anti-drug antibodies (Abs) have been used to prospectively guide interventions according to proposed underlying immune-pharmacological basis for treatment failure. The study builds on our original hypothesis, which operates with IFX treatment failure caused by (1) drug immunogenicity with blockade of drug-mediated TNF-neutralisation by anti-IFX Abs and/or anti-IFX Ab-mediated increased drug clearance; (2) non-immune-mediated pharmacokinetic issues with low circulating IFX trough levels without anti-IFX Abs; or (3) in case of high circulating TNF-neutralising capacity pharmacodynamic …

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  • Contributors CS drafted the Letter. Both authors approved the letter.

  • Funding This research received no specific funding. Support for this study was provided by unrestricted grants from Aase and Ejnar Danielsen's Foundation, Beckett Foundation, Danish Biotechnology Program, Danish Colitis-Crohn Society, Danish Medical Association Research Foundation, Frode V. Nyegaard and Wife's Foundation, Health Science Research Foundation of Region of Copenhagen, Herlev Hospital Research Council, Lundbeck Foundation, P. Carl Petersen's Foundation, Ole Østergaard Thomsen’s Research Foundation, and Jørn Brynskov’s Research Foundation.

  • Competing interests CS has served as speaker for MSD, Abbvie and Pfizer, and as a consultant for MSD and Takeda Pharmaceutical Company.

  • Provenance and peer review Not commissioned; internally peer reviewed.

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