Interleukin‐6 (IL‐6) is a pleiotropic cytokine that regulates the immune response, inflammation, and hematopoiesis. Overproduction of IL‐6 plays pathologic roles in rheumatoid arthritis (RA), and the blockade of IL‐6 may be therapeutically effective for the disease. This study was undertaken to evaluate the safety and efficacy of a humanized anti–IL‐6 receptor antibody, MRA, in patients with RA.

In a multicenter, double‐blind, placebo‐controlled trial, 164 patients with refractory RA were randomized to receive either MRA (4 mg/kg body weight or 8 mg/kg body weight) or placebo. MRA was administered intravenously every 4 weeks for a total of 3 months. The clinical responses were measured using the American College of Rheumatology (ACR) criteria.

Treatment with MRA reduced disease activity in a dose‐dependent manner. At 3 months, 78% of patients in the 8‐mg group, 57% in the 4‐mg group, and 11% in the placebo group achieved at least a 20% improvement in disease activity according to the ACR criteria (an ACR20 response) (P < 0.001 for 8‐mg group versus placebo). Forty percent of patients in the 8‐mg group and 1.9% in the placebo group achieved an ACR50 response (P < 0.001). The overall incidences of adverse events were 56%, 59%, and 51% in the placebo, 4‐mg, and 8‐mg groups, respectively, and the adverse events were not dose dependent. A blood cholesterol increase was observed in 44.0% of the patients. Liver function disorders and decreases in white blood cell counts were also observed, but these were mild and transient. There was no increase in antinuclear antibodies or anti‐DNA antibodies. Anti‐MRA antibodies were detected in 2 patients.

Treatment with MRA was generally well tolerated and significantly reduced the disease activity of RA.