Mini Reviews in Medicinal Chemistry, 2001, 1, 5-16 5 Modifying TNFα for Therapeutic Use: A Perspective on the TNF Receptor System Akihiro Hasegawa, Watrau Takasaki**, Mark I. Greene and Ramachandran Murali * Department of Pathology and Laboratory Medicine, 252 John Morgan Building, University of Pennsylvania School of Medicine, 36th and Hamilton Walk, Philadelphia, PA 19104, U.S.A. Abstract: TNFα is an inflammatory mediator that is relevant to several autoimmune diseases. Macromolecular inhibitors of TNFα have proven therapeutically useful in some preliminary studies. We have developed small molecule TNFα antagonist based on the crystal structure of TNF receptor complex. The TNFα inhibitor is specific and mediates biological function similar to the inhibitory soluble TNF receptor. This review focuses on development of small molecule anti-TNFα mimetics by us and current status of other agents. INTRODUCTION Several biochemical and biological properties of TNFα have been elucidated since the mid-1980s when TNFα was cloned, sequenced and purified [4-7]. The major source of TNFα is the activated monocytes/macrophages. TNFα is synthesized as a 26 kDa soluble TNFα molecule and observed as homotrimer under physiological conditions [8]. The main function of immune system is to eradicate foreign organisms such as viruses or bacteria. Defense against foreign organisms is mediated by innate immunity and by specific (or adaptive) immunity. The effector phases of both innate and specific immunity are large part mediated by protein hormones called cytokines. In innate immunity, the effector cytokines are mostly produced by mononuclear phagocytes and are therefore called monokines. Phagocytes accumulate at the site of infection and secrete monokines that include interleukin-1 (IL-1), interleukin-6 (IL-6), interleukin- 8 (IL-8), interleukin-12 (IL-12) and tumor necrosis factor- α (TNF α ). Most of these molecules are pleiotropic (i.e. affect different biological functions) and have effects on immunological processes such as inflammation and cellular responses such as apoptosis. Most of the cellular actions of TNFα have been attributed to the activities of two distinct receptor molecules TNF receptor I (TNFRI, p55) and TNFRII (p75) [9] which are expressed ubiquitously. TNFRI-knock out mice are resistant to endotoxin shock but succumb to infection, indicating that TNFRI plays an important role in defense against microorganisms [10]. The extracellular portions of both TNF receptors can also be shed from the cell surface through proteolytic cleavage and exist in soluble form and moreover soluble receptors retain the ability to bind TNFα and thus may act as physiological modulators of TNF activity in vivo [11,12]. In the last century, Coley [1] observed beneficial inflammatory effects in the terminally ill cancer patients. Much later, in 1985, Old identified a protein in the serum of endotoxin-treated rabbits that was responsible for the hemorrhagic necrosis of tumors [2]. It was named tumor necrosis factor (TNF) for its ability to trigger necrosis and involution of transplantable tumors, and later named TNFα after the discovery of lymphotoxin or TNFβ. TNFα was highly toxic to both humans and animals [3]. In unrelated experiments, cachectin isolated from waste body fluids of animal and human with chronic disease were found to be identical to the necrosis factor. Finally the study of lipopolysaccharide (LPS) induced biological functions led to the conclusion that TNFα is mediator of the shock, disseminated coagulation, metabolic acidosis and end-organ damage brought about by LPS. Binding of TNFα to its membrane-bound receptors induces diverse effects in different organs and tissues. Recently several TNF receptor-associated proteins have been cloned. The cytoplasmic domains of TNF receptors do not have any intrinsic enzymatic activity, and hence they signal by inducing aggregation of intracellular adaptor molecules. The cytoplasmic domains of TNFRI bear a motif termed as ‘death domain’ (DD). The DD is a protein-protein interaction motif that allows two proteins with DD to bind to each other. Binding of TNF to TNFRI induces recruitment of the DD-containing protein TRADD to the DD of TNFRI [13]. Overexpression of TRADD also induces TNF-regulated responses apoptosis and activation of the transcription factors NF-κB and Jun kinase [14]. The group of TNF receptor-associated factors (TRAF) also interact with members of the TNFR family [15]. Most of TRAF proteins interact with receptor molecules either directly, or indirectly through binding to other TRAF, or through binding to TRADD. TNFRII contains cytoplasmic TRAF binding motifs and is able to bind directly to TRAF proteins. Because TRAF2 can bind to TRADD, which in turn can *Address correspondence to this author at the Department of Pathology and Laboratory Medicine, 252 John Morgan Building, University of Pennsylvania School of Medicine, 36th and Hamilton Walk, Philadelphia, PA 19104, U.S.A; Tel: 215-573-9256; Fax: 215-898-2401; E-mail: murali@xray.med.upenn.edu ** Current Address: 2-58 Hiromachi, 1-Chome, Shinagawa-Ku, Tokyo 140, Japan. 1389-5575/00 $20.00+.00 © 2001 Bentham Science Publishers, Ltd.