On 15 patients with rheumatoid arthritis (RA) who were treated with mizoribine, a novel immunosuppressive drug, we measured concentrations of mizoribine in the plasma and synovial fluid over time. The plasma and synovial fluid levels were reached to the maximum of 0.84 of the concentration ratio (synovial fluid/plasma) . Mizoribine transfer into the plasma and synovial fluid was delayed in RA patients with renal dysfunction. Also, drug absorption may be disturbed in cases having stomach disorders. As a whole, mizoribine can easily be transfered from plasma to synovial fluid.
