BACKGROUNDBone damage around the joints is one of the major pathophysiological mechanisms that leads to rheumatoid arthritis (RA) chronic disability. Serum tartrate-resistant acid phosphatase 5b (TRACP-5b) is secreted by osteoclasts, its activity can be used as a clinically relevant bone resorption marker. The aim of this study was to test whether the measurement of serum levels of TRACP-5b in patients with RA would correlate with measures of disease activity and with responses to therapy.

METHODSFifty-six patients were randomly assigned to receive recombinant human cytotoxic tlymphocyte-associated antigen-4 immunoglobulin (RhCTLA4-Ig), infliximab or methotrexate (MTX). The clinical and serologic indicators of RA activity were evaluated at baseline and at 24 weeks. Serum TRACP-5b was measured by Enzyme-linked Immunosorbent Assay (ELISA) at 0, 12 and 24 weeks. Hand X-rays were obtained at baseline.

RESULTSAt baseline, the levels of TRACP-5b correlated with the severity of X-ray damage, disease duration (r = 0.332, P = 0.012), and tender joint count (r = 0.408, P = 0.002). The 24 weeks values of TRACP-5b for RhCTLA4-Ig group and infliximab group differed significantly from the baseline values in each group (P < 0.05; P < 0.05), whereas only the value for RhCTLA4-Ig group differed significantly from the 24 weeks value for the MTX group (P < 0.01). Considering the two biologics-treated groups together, the TRACP-5b levels at 24 weeks differed significantly from the baseline values only in those patients who reached an ACR70 level (P < 0.05).

CONCLUSIONSMeasurement of serum TRACP-5b in RA patients reflects clinical and radiological measures of disease activity, treatment with certain biologics, and degree of response to therapy. TRACP-5b should be investigated further as a potential biomarker to predict response to therapy, including slowing of radiographic progression.