Determinants of Red Blood Cell Methotrexate Polyglutamate Concentrations in Rheumatoid Arthritis Patients Receiving Long-Term Methotrexate Treatment

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 MTXGlu(n)]). There is large interpatient variability in MTXGlu(n) concentrations. This study was undertaken to determine nongenetic factors that influence red blood cell (RBC) MTXGlu(n) 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 MTXGlu(n) concentrations were measured by high-performance liquid chromatography. Univariate analysis was performed to determine variables influencing MTXGlu(n) concentrations. Backward stepwise multivariate regression analysis was done to determine variables that affect individual MTXGlu(n) concentrations; variables with P values of <0.1 in the univariate analysis for any MTXGlu(n) 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 MTXGlu(n) concentrations. Smokers had significantly lower concentrations of MTXGlu(3), MTXGlu(3-5), and MTXGlu(1-5). Sex, rheumatoid factor and anti-cyclic citrullinated peptide status, RBC folate level, and body mass index had no significant effect on MTXGlu(n) levels. Concomitant use of other DMARDs was associated with lower MTXGlu(2) levels, and treatment with nonsteroidal antiinflammatory drugs was associated with lower MTXGlu(3) and MTXGlu(1-5) concentrations. Multivariate regression analysis revealed that age, MTX dosage, and estimated GFR were the major determinants of MTXGlu(n) concentrations. Conclusion. Large interpatient variability in MTXGlu(n) 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 MTXGlu(n).