Patients on stable treatment (for >3 months) with prednisone ≤ 10 mg, methotrexate≤ 20 mg/w, azathioprine ≤ 150 mg/d, mycophenolate mofetil ≤ 2g/d, or leflunomide ≤ 20 mg/d may be included in the study; the therapy must be maintained as background therapy.