Circulating soluble tumor necrosis factor receptors, interleukin‐2 receptors, tumor necrosis factor α, and interleukin‐6 levels in rheumatoid arthritis.

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Abstract
Objective. To assess whether circulating concentrations of soluble tumor necrosis factor receptors (sTNFR; p55 and p75), soluble interleukin‐2 receptors (sIL‐2R), tumor necrosis factor α (TNFα), and interleukin‐6 (IL‐6) reflect clinical response and whether changes are dependent on the drug used in rheumatoid arthritis (RA) patients taking methotrexate (MTX) or azathioprine (AZA).Methods. These cytokines and soluble receptors were assessed in 20 control subjects and serially for up to 48 weeks in 61 RA patients, by bioassay (IL‐6) and immunoassays (sTNFR, sIL‐2R, TNFα, and IL‐6).Results. Concentrations of p55 and p75, sIL‐2R, and TNFα (but not IL‐6) were significantly higher in RA patients than in controls. Significant decreases in sIL‐2R and p55 concentrations were associated with clinical improvement and were observed in patients treated with MTX, but not AZA. Both treatments induced decreases in IL‐6 concentrations, but circulating AZA (or its metabolites) appears to interfere with the measurement of IL‐6 bioactivity. TNFα and p75 levels did not show significant changes.Conclusion. Measurement of circulating sIL‐2R, p55, and IL‐6 may be useful in the evaluation of RA disease activity and response to therapy. Interference by circulating levels of drugs must be ruled out when bioassays are used to evaluate cytokine levels.