Effect of methotrexate on blood purine and pyrimidine levels in patients with rheumatoid arthritis

Objective. The mechanism of anti-inflammatory effects of methotrexate (MTX) at low dose may relate to a decrease in availability of the purine precurs or or it may depend on accumulation of 5-aminoimidazole-4-carboxamide (AICA R) and the anti-inflammatory nucleoside adenosine. The aim of this study wa s to evaluate the possible mechanism of action by analysis of changes in bl ood concentrations of purine and pyrimidine metabolites during MTX treatmen t. Methods. Venous blood samples were collected from rheumatoid arthritis pati ents before and at different times for up to 7 days after the start of MTX treatment. Whole blood concentrations of adenosine, uridine, hypoxanthine, uric acid and erythrocyte nucleotides were measured by HPLC. Results. The initial blood adenosine concentration was 0.073 +/- 0.013 mu M and no differences were observed during MTX treatment. However, a decrease in uric acid concentration was observed from 205.5 +/- 13.5 to 160.9 +/- 1 3.5 mu M (P < 0.05) within 24 h after MTX administration. The hypoxanthine concentration decreased in parallel with uric acid, while the uridine conce ntration decreased 48 h after MTX administration. Nb accumulation of AICAR- triphosphate (ZTP) was observed in the erythrocytes. Conclusions. MTX decreases circulating purine and pyrimidine concentrations , and their availability for DNA and RNA synthesis, which may affect immune cell proliferation and protein (cytokine) expression. The absence of adeno sine concentration changes and lack of ZTP formation is evidence against an AICAR/adenosine mechanism; although localized adenosine concentration chan ges cannot be excluded.