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Interleukin-6 (IL-6) is a monomeric protein that binds to either soluble or membrane-bound IL-6 receptors (IL-6R)1 ,2 Tocilizumab, a humanised anti-IL-6R monoclonal antibody, binds to soluble IL-6R (sIL-6R) and membrane-bound IL-6R, blocking signal transduction pathways through competitive inhibition of IL-6 binding.3 We have previously demonstrated that baseline plasma tumour necrosis factor (TNF) levels are associated with the clinical response to infliximab, anti-TNF monoclonal antibody binding to soluble and membrane-bound TNF.4 Therefore, it is tempting to speculate that baseline serum levels of sIL-6R, rather than those of IL-6, are associated with clinical response to tocilizumab in patients with rheumatoid arthritis (RA). To test this hypothesis, we analysed serum levels of IL-6 and sIL-6R before tocilizumab treatment in our institution and evaluated their association with clinical remission.
Consecutive patients with RA in our institution who commenced 8 mg/kg tocilizumab treatment every 4 weeks as the first biologic agent between March 2010 and April 2012 were included. At baseline, serum levels of IL-6 and sIL-6R were measured by electrochemiluminescence assay with the Ultra-Sensitive Kit (Meso Scale …