In rheumatoid arthritis (RA) treatment, the concomitant use of methotrexate has 
been shown to reduce the incidence of antibodies to infliximab (ATI), on the 
other hand, it is unclear whether azathioprine can reduce ATI production. We 
enrolled a total of 10 Japanese adult patients with RA who were treated with 
infliximab concomitantly with methotrexate or azathioprine. Serum concentrations 
of infliximab and ATI of these patients were measured. The mean serum infliximab 
concentrations was 1.6±1.3 μg/ml in patients with methotrexate and 1.0±0.5 μg/ml 
in patients with azathioprine. Serum ATI concentrations were below the limit of 
quantitation in 4 of 5 patients in each group. The results from the present 
study suggest that azathioprine suppresses ATI production.