title: Methotrexate polyglutamates in erythrocytes are associated with lower disease activity in patients with rheumatoid arthritis.
abstract: OBJECTIVE
  To investigate if erythrocyte-methotrexate-polyglutamate (MTX-PG) concentrations in patients with rheumatoid arthritis (RA) are associated with disease activity or adverse events.
  
  
  METHODS
  We used a longitudinal study design with two cohorts. The derivation cohort included 102 and the validation cohort included 285 patients with RA on MTX. We measured erythrocyte-MTX-PG with 1-5 glutamate residues at 3 months, 6 months and 9 months after MTX start with a liquid chromatography (LC)-mass spectrometry (MS)/MS assay. Outcomes were disease activity score in 28 joints (DAS28) and adverse events. Longitudinal associations of MTX-PG concentrations after 3 months, 6 months and 9 months with DAS28 were tested with a linear mixed model adjusted for age, gender, baseline DAS28, MTX dose and comedication.
  
  
  RESULTS
  In the derivation cohort, mean DAS28 decreased from 4.26 (SE=0.14) at baseline to 2.72 (SE=0.13) after 9 months. Thirty per cent of patients in the derivation cohort experienced more than three adverse events after 3 months, which decreased to 18% after 9 months. In the validation cohort, DAS28 and adverse events were comparable with the derivation cohort. In the derivation cohort, MTX-PG1 (ß=-0.005), MTX-PG2 (ß=-0.022), MTX-PG3 (β=-0.007) and total MTX-PG (ß=-0.004) were associated (p<0.05) with lower DAS28 over 9 months. In the validation cohort, MTX-PG2 (ß=-0.015), MTX-PG3 (ß=-0.010), MTX-PG4 (ß=-0.008) and total MTX-PG (ß=-0.003) were associated with lower DAS28 over 9 months. None of the MTX-PGs was associated with adverse events.
  
  
  CONCLUSIONS
  In this first longitudinal study, we showed that an increase in erythrocyte-MTX-PG concentration was associated with a decreased DAS28 over 9 months in two cohorts, and is therefore a potential tool for therapeutic drug monitoring of MTX in RA.
authors.affiliation: Department of Clinical Chemistry, Erasmus University Medical Center, Rotterdam, Netherlands.
authors.forename: Maurits C F J
authors.initials: MC
authors.lastname: de Rotte
authors.affiliation: Department of Clinical Chemistry, Erasmus University Medical Center, Rotterdam, Netherlands.
authors.forename: Ethan
authors.initials: E
authors.lastname: den Boer
authors.affiliation: Department of Rheumatology, Erasmus University Medical Center, Rotterdam, Netherlands.
authors.forename: Pascal H P
authors.initials: PH
authors.lastname: de Jong
authors.affiliation: Department of Pediatric Hemato-Oncology, Erasmus University Medical Center, Rotterdam, Netherlands.
authors.forename: Saskia M F
authors.initials: SM
authors.lastname: Pluijm
authors.affiliation: Department of Pediatric Immunology, University Medical Center Utrecht, Wilhelmina Children's hospital, Utrecht, Netherlands.
authors.forename: Maja Bulatović
authors.initials: MB
authors.lastname: Ćalasan
authors.affiliation: Department of Rheumatology, Maasstad Hospital, Rotterdam, Netherlands.
authors.forename: Angelique E
authors.initials: AE
authors.lastname: Weel
authors.affiliation: Interne Specialismen Rijnmond Noord, Sint Franciscus Hospital, Rotterdam and IJsselland Hospital, Capelle aan de IJssel, Netherlands.
authors.forename: A Margriet
authors.initials: AM
authors.lastname: Huisman
authors.affiliation: Department of Rheumatology, Vlietland Hospital, Schiedam, Netherlands.
authors.forename: Andreas H
authors.initials: AH
authors.lastname: Gerards
authors.affiliation: Department of Rheumatology, Albert Schweitzer Hospital, Dordrecht, Netherlands.
authors.forename: Barbara
authors.initials: B
authors.lastname: van Schaeybroeck
authors.affiliation: Department of Pediatric Immunology, University Medical Center Utrecht, Wilhelmina Children's hospital, Utrecht, Netherlands.
authors.forename: Nico M
authors.initials: NM
authors.lastname: Wulffraat
authors.affiliation: Department of Clinical Chemistry, Erasmus University Medical Center, Rotterdam, Netherlands.
authors.forename: Jan
authors.initials: J
authors.lastname: Lindemans
authors.affiliation: Department of Rheumatology, Erasmus University Medical Center, Rotterdam, Netherlands.
authors.forename: Johanna M W
authors.initials: JM
authors.lastname: Hazes
authors.affiliation: Department of Clinical Chemistry, Erasmus University Medical Center, Rotterdam, Netherlands.
authors.forename: Robert
authors.initials: R
authors.lastname: de Jonge
mesh_terms: D018501:Antirheumatic Agents / Q000627:therapeutic use*; D001172:Arthritis, Rheumatoid / Q000188:drug therapy*; D002853:Chromatography, Liquid; D015331:Cohort Studies; D004912:Erythrocytes / Q000737:chemistry*; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008727:Methotrexate / Q000031:analogs & derivatives* / Q000032:analysis / Q000627:therapeutic use*; D008875:Middle Aged; D011099:Polyglutamic Acid / Q000031:analogs & derivatives* / Q000032:analysis; D053719:Tandem Mass Spectrometry
chemical_list: D018501:Antirheumatic Agents; D011099:Polyglutamic Acid; C014085:methotrexate polyglutamate; D008727:Methotrexate
keywords: DMARDs (synthetic); Pharmacokinetics; Rheumatoid Arthritis