title: Determinants of erythrocyte methotrexate polyglutamate levels in rheumatoid arthritis.
abstract: OBJECTIVE
  Low-dose methotrexate (MTX) is the anchor drug in the treatment for rheumatoid arthritis (RA). Response to MTX is related to the intracellular MTX-polyglutamate (MTX-PG) levels and little is known about its determinants. We aimed to define the determinants of erythrocyte MTX-PG concentrations in 2 prospective cohorts of patients with RA.
  
  
  METHODS
  Patients with RA treated with MTX from 2 longitudinal cohorts were included: 93 from the MTX-R study (Rotterdam, the Netherlands derivation cohort), and 247 from the treatment in Rotterdam Early Arthritis Cohort study (validation cohort). MTX-PG concentrations were measured at 3 months of treatment using liquid chromatography/mass spectrometry. The MTX-PG were used as outcome measure. Various sociodemographic, clinical, biochemical, and genetic factors were assessed at baseline. Associations with MTX-PG levels were analyzed using multivariate regression analysis.
  
  
  RESULTS
  Age was positively associated with MTX-PG1 (stβ 0.23, p=0.033) and total MTX-PG (stβ 0.23, p=0.018) in the derivation cohort, and with all MTX-PG in the validation cohort (MTX-PG1: stβ 0.13, p=0.04; MTX-PG2: stβ 0.21, p=0.001; MTX-PG3: stβ 0.22, p<0.001; MTX-PG4+5: stβ 0.25, p<0.001; and total MTX-PG: stβ 0.32, p<0.001). Erythrocyte folate levels were positively associated with MTX-PG3 (stβ 0.3, p=0.021) and total MTX-PG levels (stβ 0.32, p=0.022) in the derivation cohort, which was replicated for MTX-PG3 (stβ 0.15, p=0.04) in the validation cohort. Patients with the folylpolyglutamate synthase (FPGS) rs4451422 wild-type genotype had higher concentrations of MTX-PG3 (p<0.05), MTX-PG4+5 (p<0.05), and total MTX-PG (p<0.05) in both cohorts. In the combined cohort, MTX dose was positively associated with levels of MTX-PG3 (stβ 0.23, p<0.001), MTX-PG4+5 (stβ 0.30, p<0.001), and total MTX-PG (stβ 0.20, p=0.002), but negatively associated with MTX-PG2 levels (stβ -0.22, p<0.001).
  
  
  CONCLUSIONS
  Our prospective study shows that higher age, higher MTX dose, higher erythrocyte folate status, and the FPGS rs4451422 wild-type genotype are associated with higher MTX-PG concentrations. While only up to 21% of interpatient variability can be explained by these determinants, this knowledge may aid in the development of personalized treatment in RA.
authors.affiliation: From the Department of Clinical Chemistry; Department of Rheumatology; Department of Pediatric Hemato-Oncology; Erasmus University Medical Center, Rotterdam, the Netherlands.E. den Boer, MSc; M.C.J.F. de Rotte, PharmD; S.G. Heil, PhD; R. de Jonge, PhD, Department of Clinical Chemistry; J.M. Hazes, MD, PhD, Department of Rheumatology; S.M.F. Pluijm, PhD, Department of Pediatric Hemato-Oncology, Erasmus University Medical Center.
authors.forename: Ethan
authors.initials: E
authors.lastname: den Boer
authors.affiliation: From the Department of Clinical Chemistry; Department of Rheumatology; Department of Pediatric Hemato-Oncology; Erasmus University Medical Center, Rotterdam, the Netherlands.E. den Boer, MSc; M.C.J.F. de Rotte, PharmD; S.G. Heil, PhD; R. de Jonge, PhD, Department of Clinical Chemistry; J.M. Hazes, MD, PhD, Department of Rheumatology; S.M.F. Pluijm, PhD, Department of Pediatric Hemato-Oncology, Erasmus University Medical Center.
authors.forename: Maurits C J F
authors.initials: MC
authors.lastname: de Rotte
authors.affiliation: From the Department of Clinical Chemistry; Department of Rheumatology; Department of Pediatric Hemato-Oncology; Erasmus University Medical Center, Rotterdam, the Netherlands.E. den Boer, MSc; M.C.J.F. de Rotte, PharmD; S.G. Heil, PhD; R. de Jonge, PhD, Department of Clinical Chemistry; J.M. Hazes, MD, PhD, Department of Rheumatology; S.M.F. Pluijm, PhD, Department of Pediatric Hemato-Oncology, Erasmus University Medical Center.
authors.forename: Saskia M F
authors.initials: SM
authors.lastname: Pluijm
authors.affiliation: From the Department of Clinical Chemistry; Department of Rheumatology; Department of Pediatric Hemato-Oncology; Erasmus University Medical Center, Rotterdam, the Netherlands.E. den Boer, MSc; M.C.J.F. de Rotte, PharmD; S.G. Heil, PhD; R. de Jonge, PhD, Department of Clinical Chemistry; J.M. Hazes, MD, PhD, Department of Rheumatology; S.M.F. Pluijm, PhD, Department of Pediatric Hemato-Oncology, Erasmus University Medical Center.
authors.forename: Sandra G
authors.initials: SG
authors.lastname: Heil
authors.affiliation: From the Department of Clinical Chemistry; Department of Rheumatology; Department of Pediatric Hemato-Oncology; Erasmus University Medical Center, Rotterdam, the Netherlands.E. den Boer, MSc; M.C.J.F. de Rotte, PharmD; S.G. Heil, PhD; R. de Jonge, PhD, Department of Clinical Chemistry; J.M. Hazes, MD, PhD, Department of Rheumatology; S.M.F. Pluijm, PhD, Department of Pediatric Hemato-Oncology, Erasmus University Medical Center.
authors.forename: Johanna M
authors.initials: JM
authors.lastname: Hazes
authors.affiliation: From the Department of Clinical Chemistry; Department of Rheumatology; Department of Pediatric Hemato-Oncology; Erasmus University Medical Center, Rotterdam, the Netherlands.E. den Boer, MSc; M.C.J.F. de Rotte, PharmD; S.G. Heil, PhD; R. de Jonge, PhD, Department of Clinical Chemistry; J.M. Hazes, MD, PhD, Department of Rheumatology; S.M.F. Pluijm, PhD, Department of Pediatric Hemato-Oncology, Erasmus University Medical Center. r.dejonge@erasmusmc.nl.
authors.forename: Robert
authors.initials: R
authors.lastname: de Jonge
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D001172:Arthritis, Rheumatoid / Q000097:blood* / Q000188:drug therapy* / Q000503:physiopathology; D015331:Cohort Studies; D004305:Dose-Response Relationship, Drug; D004334:Drug Administration Schedule; D004912:Erythrocytes / Q000187:drug effects*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D016014:Linear Models; D008137:Longitudinal Studies; D008297:Male; D008727:Methotrexate / Q000031:analogs & derivatives* / Q000097:blood / Q000493:pharmacokinetics / Q000627:therapeutic use; D008875:Middle Aged; D015999:Multivariate Analysis; D009426:Netherlands; D011099:Polyglutamic Acid / Q000031:analogs & derivatives* / Q000097:blood / Q000493:pharmacokinetics / Q000627:therapeutic use; D011446:Prospective Studies; D015203:Reproducibility of Results; D018570:Risk Assessment; D012720:Severity of Illness Index; D018709:Statistics, Nonparametric; D013997:Time Factors; D016896:Treatment Outcome
chemical_list: D011099:Polyglutamic Acid; C014085:methotrexate polyglutamate; D008727:Methotrexate
keywords: ERYTHROCYTES; METHOTREXATE; PROSPECTIVE STUDIES; RHEUMATOID ARTHRITIS; SINGLE-NUCLEOTIDE POLYMORPHISM