INTERFERENCE OF CIRCULATING AZATHIOPRINE BUT NOT METHOTREXATE OR SULFASALAZINE WITH MEASUREMENTS OF INTERLEUKIN-6 BIOACTIVITY

Bioassays are currently used to measure the presence of functionally a ctive cytokines in biological fluids. These assays may be influenced b y the presence of other substances, either cytokine specific or not, i n such fluids. In the present study, we analyzed whether some currentl y used disease-modifying antirheumatic drugs (DMARDs) could interfere with the measurements of circulating interleukin-6 (IL-6) bioactivity in the B9 hybridoma assay. When sera from healthy controls and patient s treated with various DMARDs, such as azathioprine (AZA), methotrexat e (MTX), intramuscular gold, and sulfasalazine (SASP), were tested in the IL-6 bioassay, an inhibitory effect was observed only with sera fr om patients treated with AZA. Addition of exogenous AZA, 6-mercaptopur ine (6-MP), and MTX to the IL-6 bioassay resulted in a dose-dependent inhibition of the B9 cell proliferation induced by IL-6, AZA being mos t potent on a molar basis. Concentrations of AZA and 6-MP compatible w ith serum concentrations achieved in RA patients were able to inhibit the bioassay, but this was not the case for MTX. Exogenous SASP and it s metabolites did not modify the IL-6-induced B9 cell proliferation. T his study shows that circulating AZA (or its metabolites) exert an inh ibitory effect in the IL-6 bioassay. This method is therefore not suit able to measure IL-6 concentrations in patients treated with AZA. Inte rference of drugs must be ruled out when bioassays are used to evaluat e cytokine levels in biological fluids.