title: Determinants of red blood cell methotrexate polyglutamate concentrations in rheumatoid arthritis patients receiving long-term methotrexate treatment.
abstract: OBJECTIVE
  Methotrexate (MTX) is the most commonly used disease-modifying antirheumatic drug (DMARD) in the management of rheumatoid arthritis (RA). MTX is transported into cells, where additional glutamate moieties are added and it is retained as MTX polyglutamates (MTXGlu [referred to as a group as MTXGlun]). There is large interpatient variability in MTXGlun concentrations. This study was undertaken to determine nongenetic factors that influence red blood cell (RBC) MTXGlun concentrations in patients receiving long-term stable low-dose oral MTX.
  
  
  METHODS
  One hundred ninety-two patients receiving long-term oral MTX for the treatment of RA were recruited. Trough MTXGlun concentrations were measured by high-performance liquid chromatography. Univariate analysis was performed to determine variables influencing MTXGlun concentrations. Backward stepwise multivariate regression analysis was done to determine variables that affect individual MTXGlun concentrations; variables with P values of <0.1 in the univariate analysis for any MTXGlun were included.
  
  
  RESULTS
  Univariate analysis revealed that increased age, lower estimated glomerular filtration rate (GFR), higher MTX dosage, longer duration of MTX treatment, and use of prednisone were associated with significantly higher MTXGlun concentrations. Smokers had significantly lower concentrations of MTXGlu3, MTXGlu3-5, and MTXGlu1-5. Sex, rheumatoid factor and anti-cyclic citrullinated peptide status, RBC folate level, and body mass index had no significant effect on MTXGlun levels. Concomitant use of other DMARDs was associated with lower MTXGlu2 levels, and treatment with nonsteroidal antiinflammatory drugs was associated with lower MTXGlu3 and MTXGlu1-5 concentrations. Multivariate regression analysis revealed that age, MTX dosage, and estimated GFR were the major determinants of MTXGlun concentrations.
  
  
  CONCLUSIONS
  Large interpatient variability in MTXGlun concentrations can be explained, at least in part, by a combination of factors, particularly age, MTX dosage, and renal function. There are complex interactions between smoking, RBC folate levels, and concentrations of MTXGlun.
authors: Department of Mediicne, University of Otago, Christchurch, New Zealand. Lisa.stamp@cdhb.govt.nz
authors: Lisa K
authors: LK
authors: Stamp
authors: John L
authors: JL
authors: O'Donnell
authors: Peter T
authors: PT
authors: Chapman
authors: Mei
authors: M
authors: Zhang
authors: Christopher
authors: C
authors: Frampton
authors: Jill
authors: J
authors: James
authors: Murray L
authors: ML
authors: Barclay
mesh_terms: D000284:Administration, Oral; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D018501:Antirheumatic Agents / Q000032:analysis / Q000493:pharmacokinetics*; D001172:Arthritis, Rheumatoid / Q000188:drug therapy* / Q000378:metabolism; D003430:Cross-Sectional Studies; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004359:Drug Therapy, Combination; D004912:Erythrocytes / Q000737:chemistry / Q000378:metabolism*; D005260:Female; D006801:Humans; D008297:Male; D008727:Methotrexate / Q000031:analogs & derivatives / Q000032:analysis / Q000378:metabolism / Q000493:pharmacokinetics*; D008875:Middle Aged; D011099:Polyglutamic Acid / Q000031:analogs & derivatives / Q000032:analysis / Q000378:metabolism; D012907:Smoking; D055815:Young Adult
chemical_list: D018501:Antirheumatic Agents; D011099:Polyglutamic Acid; C014085:methotrexate polyglutamate; D008727:Methotrexate