Methotrexate (MTX) enters cells through the reduced folate carrier (RFC-1) and exerts part of its effects through polyglutamation to MTX polyglutamates (MTXPGs) and inhibition of 5-aminoimidazole-4-carboxamide ribonucleotide transformylase (ATIC) and thymidylate synthase (TS). We investigated the contribution of common genetic polymorphisms in RFC-1 (G80A), ATIC (C347G), and TS (28-bp tandem repeats located in the TS enhancer region [TSER*2/*3]) and of MTXPGs to the effect of MTX in patients with rheumatoid arthritis.
The study was cross-sectional. All patients received MTX for at least 3 months. The numbers of tender and swollen joints, the Visual Analog Scale (VAS) scores for the physician's global assessment of disease activity, and the modified Health Assessment Questionnaire scores were collected. Using the VAS score for the physician's assessment of patient's response to MTX, the population of patients was dichotomized into responders to MTX (VAS score < or =2 cm) and nonresponders to MTX (VAS score >2 cm). A pharmacogenetic index was calculated as the sum of homozygous variant genotypes (RFC-1 AA + ATIC 347GG + TSER *2/*2) carried by the patients. MTXPG concentrations were measured in red blood cells (RBCs) by high-performance liquid chromatography.
The dose of MTX was not associated with the effects of MTX (P > 0.05). In contrast, increased RBC long-chain MTXPG concentrations (median 40 nmoles/liter; range <5-131 nmoles/liter) and an increased pharmacogenetic index were associated with a lower number of tender and swollen joints (P < 0.05) and a lower score for the physician's global assessment of disease activity (P < or = 0.001). Patients with RBC MTXPG levels of >60 nmoles/liter and carriers of a homozygous variant genotype were 14.0-fold (95% confidence interval [95% CI] 3.6-53.8) and 3.7-fold (95% CI 1.7-9.1), respectively, more likely to have a good response to MTX (P <or = 0.01).
These data suggest that measuring RBC MTXPG levels and/or the common polymorphisms in the folate-purine-pyrimidine pathway may help in monitoring MTX therapy.