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Stimulation of growth of some tumours
Activates blood clotting
Possible role in destruction of joints
Death of pancreatic islet cells induces insulin resistance
• severe side-effects usually accompany systemic administration of therapeutically relevant doses of this cytokine.
One such product has, however, been approved for general medical use. Beromun is the trade name given to a TNF-a based product produced by recombinant means in E. coli. The recombinant product is identical to the native human protein, with three 17.3 kDa, 157 amino
acid polypeptides forming the biologically active homotrimer. An overview of its manufacture is provided in Figure 5.12. Beromun is indicated for the treatment of soft tissue sarcoma in the limbs. It is administered by isolated limb perfusion (i.e. 3-4 mg of product is circulated through the limb over 90 min via a perfusion circuit divorced from general body circulation). As the product does not enter systemic circulation, toxic systemic side effects are avoided. Surgical removal of the tumour mass generally ensues but prior treatment with Beromun prevents/delays the need for whole limb amputation in most patients.
Most clinical interest in TNF now centres around neutralizing its biological effects in situations where overexpression of TNF induces negative clinical effects. TNF has been firmly implicated in mediating many of the adverse effects associated with dozens of diseases (Table 5.11). Administration of anti-TNF monoclonal antibodies or soluble forms of the TNF receptor should help reduce the severity of many of the symptoms of these diseases.
Enbrel is a product now approved for medical use which is based upon this strategy. The product is an engineered hybrid protein consisting of the extracellular domain of the TNF p75 receptor fused directly to the Fc (constant) region of human IgG (see Chapter 10, Box 10.2, for a discussion of antibody structure). The product is expressed in a CHO cell line from which it is excreted as a dimeric soluble protein of approximately 150 kDa. After purification and excipient addition (mannitol, sucrose and trometamol), the product is freeze-dried. It is indicated for the treatment of rheumatoid arthritis and is usually administered as a twice weekly s.c. injection of 25 mg product reconstituted in water for injections. Enbrel functions as a competitive inhibitor of TNF, a major pro-inflammatory cytokine. Binding of TNF to Enbrel prevents it from binding to its true cell surface receptors. Although commercial product literature makes little or no reference to the function of the antibody Fc component, its presence may hasten removal and degradation of the product-TNF complex.
More recently, an additional approach to preventing TNF toxicity has been proposed. Several metalloprotease inhibitors (most notably hydroxamic acid) prevent proteolytic processing (i.e. release) of TNF-a from producer cell surfaces. Such inhibitors may also prove useful in preventing TNF-induced illness. The extent to which TNF (and inhibitors of TNF) will serve as future therapeutic agents remains to be determined by future clinical trials.
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