title: Assessment of the relationship between methotrexate polyglutamates in red blood cells and clinical response in patients commencing methotrexate for rheumatoid arthritis.
abstract: OBJECTIVE
  Therapeutic drug monitoring in patients with rheumatoid arthritis (RA) receiving methotrexate (MTX, MTXGlu1) has not been established. In this study, we aim to explore the relationship between red blood cell (RBC) concentrations of MTX and its polyglutamate metabolites (MTXGlu(n); n = 2, 3, 4, 5) and clinical response in RA patients commencing MTX.
  
  
  METHODS
  The binding activity of MTXGlu(n) to three putative enzymes involved in the MTX mechanism of action—dihydrofolate reductase, thymidylate synthase, and 5-aminoimidazole-4-carboxamide ribonucleotide transformylase—was simulated. RBC MTXGlu(n) concentrations that gave the highest inhibition activity across all three enzymes were linked with the disease activity score DAS28-3v (C-reactive protein [CRP]). A population pharmacokinetic-pharmacodynamic model was developed to describe the relationship between RBC MTX polyglutamate concentrations and clinical response in 12 RA patients commencing MTX.
  
  
  RESULTS
  The highest inhibition activity was with RBC MTXGlu(3-5). These polyglutamates were further evaluated for their relationship with DAS28-3v (CRP). Three of the 12 patients had a high DAS28-3v (CRP) at baseline (mean = 6.1) and showed a delayed response to MTX treatment. The remaining nine patients with a lower DAS28-3v (CRP) baseline (mean = 3.6) showed an immediate response. The developed MTX pharmacokinetic-pharmacodynamic model provided an acceptable description of the observed DAS28-3v (CRP) across all patients.
  
  
  CONCLUSIONS
  The developed model describes a longitudinal relationship between RBC MTXGlu(3-5) concentrations and DAS28-3v (CRP) in patients with RA commencing MTX. Further work is required to determine whether measurement of RBC MTX polyglutamates might be useful for dose individualisation in patients with RA.
authors.affiliation: School of Pharmacy, University of Otago, Dunedin, New Zealand.
authors.forename: Shan
authors.initials: S
authors.lastname: Pan
authors.forename: Lisa K
authors.initials: LK
authors.lastname: Stamp
authors.forename: Stephen B
authors.initials: SB
authors.lastname: Duffull
authors.forename: Murray L
authors.initials: ML
authors.lastname: Barclay
authors.forename: Judith M
authors.initials: JM
authors.lastname: Dalrymple
authors.forename: Jill
authors.initials: J
authors.lastname: Drake
authors.forename: Mei
authors.initials: M
authors.lastname: Zhang
authors.forename: Julia
authors.initials: J
authors.lastname: Korell
mesh_terms: D000328:Adult; D000368:Aged; D018501:Antirheumatic Agents / Q000493:pharmacokinetics* / Q000627:therapeutic use; D001172:Arthritis, Rheumatoid / Q000097:blood* / Q000188:drug therapy; D004912:Erythrocytes / Q000378:metabolism; D005260:Female; D006801:Humans; D008297:Male; D008727:Methotrexate / Q000031:analogs & derivatives* / Q000493:pharmacokinetics / Q000627:therapeutic use; D008875:Middle Aged; D008954:Models, Biological; D011099:Polyglutamic Acid / Q000031:analogs & derivatives* / Q000493:pharmacokinetics / Q000627:therapeutic use; D016896:Treatment Outcome
chemical_list: D018501:Antirheumatic Agents; D011099:Polyglutamic Acid; C014085:methotrexate polyglutamate; D008727:Methotrexate