The mode of action of cytokine inhibitors.

Tumor necrosis factor-alpha (TNF-alpha) and interleukin 1 (IL-1) are important mediators of inflammation and tissue damage in animal models of inflammatory arthritis and in patients with active rheumatoid arthritis (RA). Several inhibitors of these cytokines are now available for RA treatment, each having a different mode of action. Etanercept is a recombinant fusion protein of the soluble type II TNF receptor on a human IgG1 backbone, whereas infliximab is a chimeric anti-TNF-alpha monoclonal antibody containing a murine TNF-alpha binding region and human IgG1 backbone. Both agents potently and selectively bind TNF-alpha in the cellular microenvironment, thereby preventing TNF-alpha from interacting with membrane-bound TNF receptors on target cells. In comparison, anakinra is a recombinant human IL-1 receptor antagonist (IL-1Ra) that binds avidly to type 1 IL-1 receptors but does not stimulate any intracellular responses. Studies of these agents in animal models of inflammatory arthritis suggest that TNF-alpha plays a more important role in promoting inflammation, whereas IL-1 is more important in causing cartilage and bone destruction. However, these differential actions have not been borne out in clinical trials, where TNF-alpha blockers and anakinra similarly reduce clinical signs and symptoms of RA as well as slow radiographic evidence of disease progression.