An investigation of the correlation between blood concentration of mizoribine and its efficacy in treatment of rheumatoid arthritis based on indices of drug survival and improvement in DAS28-CRP.

The efficacy of mizoribine (MZR) in treatment of rheumatoid arthritis (RA) was retrospectively investigated in terms of drug survival, improvement in Disease Activity Score-28 (DAS28)-C-reactive protein (CRP), and blood MZR concentration obtained 3 h after dosing (MZR-C3). To compare the efficacy of MZR administered via different regimens, the subjects were divided into 2 groups: those receiving a single dose of MZR at 100-150 mg every other day (group A) and those receiving 2 or 3 divided doses of the drug on consecutive days, which is the usual dosing method of the drug (group B). Group A had significantly higher MZR-C3 levels compared with group B, as well as significantly greater improvement in DAS28-CRP. Moreover, drug survival was significantly longer in group A. The primary regression equation suggested that the effective blood MZR concentration in RA treatment is MZR-C3 of 1.47 µg/mL or more. The results of the present study indicate that it is possible to increase the efficacy of MZR in a blood concentration-dependent manner, and also to control RA over a prolonged period, using single administration of MZR on alternate days at an increased dose.