Synovial and serum levels of methotrexate during methotrexate therapy of rheumatoid arthritis.

Methotrexate (MTX) levels were studied following intravenous MTX in both serum and synovial fluid (SF) of rheumatoid arthritis patients. Two hours after injection serum MTX levels were higher than those of SF. At 24 hours SF levels of MTX exceeded those of the serum, while at 72 hours both blood and SF concentrations were undetectable. The localization of parenteral MTX in the SF may have importance in the understanding of its mechanism and site of action in rheumatoid arthritis.