Methotrexate accumulation in target intestinal mucosa and white blood cells differs from non‐target red blood cells of patients with Crohn's disease.

Background: Intracellular methotrexate polyglutamates (MTX‐PGs) concentrations are measurable in red blood cells (RBCs) during MTX treatment. MTX‐PG3 concentrations correlate with efficacy in patients with Crohn's disease (CD). Since RBCs are not involved in pathogenesis of CD and lack extended MTX metabolism, we determined MTX‐PGs accumulation in peripheral blood mononuclear cells (PBMCs: effector cells) and intestinal mucosa (target cells) and compared those with RBCs as a potential more precise biomarker. Methods: In a multicentre prospective cohort study, blood samples of patients with CD were collected during the first year of MTX therapy. Mucosal biopsies were obtained from non‐inflamed rectum and/or inflamed intestine. MTX‐PGs concentrations in mucosa, PBMCs and RBCs were measured by liquid chromatography–tandem mass spectrometry. Results: From 80 patients with CD, a total of 27 mucosal biopsies, 9 PBMC and 212 RBC samples were collected. From 12 weeks of MTX therapy onwards, MTX‐PG3 was the most predominant species (33%) in RBCs. In PBMCs, the distribution was skewed towards MTX‐PG1 (48%), which accounted for an 18 times higher concentration than in RBCs. Long‐chain MTX‐PGs were highly present in mucosa: 21% of MTX‐PGtotal was MTX‐PG5. MTX‐PG6 was measurable in all biopsies. Conclusions: MTX‐PG patterns differ between mucosa, PBMCs and RBCs of patients with CD. [ABSTRACT FROM AUTHOR]