Crohn's Disease (CD) is a chronic inflammatory disease of the gastrointestinal tract, with a low percentage of patients remaining in clinical remission despite the long-term use of many drugs (steroids, immunosuppressants, aminosalicylates, antibiotics). As a consequence of the limited efficacy and significant toxicity of current therapy, there is widespread interest in the development of novel drugs for the clinical management of CD.
In the early 1990s, the pathogenetic mechanisms of inflammatory bowel diseases (IBDs) were clarified. Greater knowledge of systemic and local inflammatory cascade of CD (Tl-helper-driven inflammation)  allowed identification of key mediators of inflammation, and pharmacologic research was addressed towards specific biologic drugs interfering with this cascade. Proinflammatory cytokines play a central role in amplification of the inflammatory process, inducing release of other cytokines and increasing the expression of adhesion molecules . Tumour necrosis factor (TNF)-a and interleukin (IL)-l are released at the beginning of the inflammatory cascade and contribute to recruitment and activation of inflammatory cells (macrophages, neutrophils, monocytes) by increasing mucosal intestinal permeability and stimulating the release of adhesion molecules. Anti-inflammatory cytokines play an important role in avoidance of an abnormal immune response. In particular, IL-10 controls TNF production, major histocompatibility complex (MHC)-II expression and release of other inflammatory mediators whereas IL-12 induces the release of interferon gamma (INF-y) by macrophages and natural-killer lymphocytes .
The term "biological agent" means any therapeutic agent derived from a living organism. Therefore, biological therapy may include : (1) modified biological compounds (i.e. vaccine), hormones, etc.; (2) recombinant peptides or proteins (i.e. growing factors, growth hormones, etc.); (3) monoclonal antibodies or fusion proteins; (4) antisense oligonu-
cleotides. Many molecules have been used in controlled or open trials, mainly in fistulising or steroid-dependent CD (Table 1). Monoclonal antibodies and antisense nucleotides interfering with messenger ribonucleic acid (mRNA) are the drugs most frequently tested in clinical trials.
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