Background: The life prognosis of elderly patients with myeloperoxidase–anti-neutrophil cytoplasmic antibodies-associated vasculitis (MPO-AAV) has been improved by reducing the corticosteroid or cyclophosphamide dose to avoid opportunistic infection. However, many elderly MPO-AAV patients experience recurrence and renal death. An effective and safer maintenance treatment method is necessary to improve the renal prognosis of MPO-AAV. Methods: Patients with MPO-AAV who reached complete or incomplete remission after induction therapy were prospectively and randomly divided into mizoribine (MZR; n = 25) and control (n = 28) groups. The primary endpoint was relapse of MPO-AAV. The patients’ serum MZR concentration was measured before (C0) and 3 h after taking the MZR. The maximum drug concentration (Cmax) and the serum MZR concentration curves were determined using population pharmacokinetics parameters. We also assessed the relationship between the MZR concentrations and adverse events. The observation period was 12 months. Results: Fifty-eight MPO-AAV patients from 16 hospitals in Japan were enrolled. Ten patients relapsed (MZR group, n = 6; control group, n = 4; a nonsignificant between-group difference). Changes in the serum MZR concentration could be estimated for 22 of the 25 MZR-treated patients: 2 of the 11 patients who reached a Cmax of 3 μg/mL relapsed, whereas 4 of the 11 patients who did not reach this Cmax relapsed. The treatment of one patient with C0 > 1 μg/mL was discontinued due to adverse events. No serious adverse events occurred. Conclusion: There was no significant difference in the recurrence rate of MPO-AAV between treatment with versus without MZR.
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