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To determine the potential for additive or synergistic effects of combination therapy with the selective anti–tumor necrosis factor α agent etanercept and the anti–interleukin-1 agent anakinra. Two hundred forty-four patients in whom rheumatoid arthritis (RA) was active despite methotrexate therapy were treated with subcutaneous etanercept only (25 mg twice weekly), full-dosage etanercept (25 mg twice weekly) plus anakinra (100 mg/day), or half-dosage etanercept (25 mg once weekly) plus anakinra (100 mg/day) for 6 months in a double-blind study at 41 centers in the US. Patients had never previously received anticytokine therapy. Patient response was measured with the American College of Rheumatology (ACR) core set criteria, a health-related quality-of-life questionnaire, and the Disease Activity Score. Safety was assessed by the number of adverse events and clinical laboratory values. Plasma concentrations of both agents and antibody formation against both agents were also assessed. Combination therapy with etanercept plus anakinra provided no treatment benefit over etanercept alone, regardless of the regimen, but was associated with an increased safety risk. Thirty-one percent of the patients treated with full-dosage etanercept plus anakinra achieved an ACR 50% response, compared with 41% of the patients treated with etanercept only. This result was not statistically significant (P = 0.914). The incidence of serious infections (0% for etanercept alone, 3.7–7.4% for combination therapy), injection-site reactions, and neutropenia was increased with combination therapy. Combination therapy had no effect on the pharmacokinetics or immunogenicity of either agent. Combination therapy with etanercept and anakinra provides no added benefit and an increased risk compared with etanercept alone and is not recommended for the treatment of patients with RA. [ABSTRACT FROM AUTHOR]