Blockade of the C5a receptor fails to protect against experimental autoimmune encephalomyelitis in rats

Complement activation contributes to inflammation and tissue damage in human demyelinating diseases and in rodent models of demyelination. Inhibitors of complement activation ameliorate disease in the rat model antibody-dependent experimental autoimmune encephalomyelitis and rats unable to generate the membrane attack complex of complement develop inflammation without demyelination. The role of the highly active chemotactic and anaphylactic complement-derived peptide C5a in driving inflammation and pathology in rodent models of demyelination has been little explored. Here we have used a small molecule C5a receptor antagonist, AcF-[OPdChaWR], to examine the effects of C5a receptor blockade in rat models of brain inflammation and demyelination. C5a receptor antagonist therapy completely blocked neutrophil response to C5a in vivo but had no effect on clinical disease or resultant pathology in either inflammatory or demyelinating rat models. We conclude that C5a is not required for disease induction or perpetuation in these strongly complement-dependent disease models.

Potent cyclic antagonists of the complement C5a receptor on human polymorphonulcear leukocytes. Relationships between structures and activity

Human C5a is a plasma protein with potent chemoattractant and pro-inflammatory properties, and its overexpression correlates with severity of inflammatory diseases. C5a binds to its G protein-coupled receptor (C5aR) on polymorphonuclear leukocytes (PMNLs) through a high-affinity helical bundle and a low-affinity C terminus, the latter being solely responsible for receptor activation. Potent and selective C5a antagonists are predicted to be effective anti-inflammatory drugs, but no pharmacophore for small molecule antagonists has yet been developed, and it would significantly aid drug design. We have hypothesized that a turn conformation is important for activity of the C terminus of C5a and herein report small cyclic peptides that are stable turn mimics with potent antagonism at C5aR on human PMNLs. A comparison of solution structures for the C terminus of C5a, small acyclic peptide ligands, and cyclic antagonists supports the importance of a turn for receptor binding. Competition between a cyclic antagonist and either C5a or an acyclic agonist for C5aR on PMNLs supports a common or overlapping binding site on the C5aR. Structure-activity relationships for 60 cyclic analogs were evaluated by competitive radioligand binding with C5a (affinity) and myeloperoxidase release (antagonist potency) from human PMNLs, with 20 compounds having high antagonist potencies (IC50, 20 nM(-1) muM). Computer modeling comparisons reveal that potent antagonists share a common cyclic backbone shape, with affinity-determining side chains of defined volume projecting from the cyclic scaffold. These results define a new pharmacophore for C5a antagonist development and advance our understanding of ligand recognition and receptor activation of this G protein-coupled receptor.

Increased potency of a novel complement factor 5a receptor antagonist in a rat model of inflammatory bowel disease

We have previously shown that complement factor 5a(C5a) plays a role in the pathogenesis of 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis in rats by using the selective, orally active C5a antagonist AcF-[OP(D-Cha) WR]. This study tested the efficacy and potency of a new C5a antagonist, hydrocinnamate (HC)-[OP(D-Cha) WR], which has limited intestinal lumenal metabolism, in this model of colitis. Analogs of AcF-[OP(D-Cha) WR] were examined for their susceptibility to alimentary metabolism in the rat using intestinal mucosal washings. One metabolically stable analog, HC-[OP(D-Cha)WR], was then evaluated pharmacokinetically and investigated at a range of doses (0.03 - 10 mg/kg/ day p.o.) in the 8-day rat TNBS- colitis model, against the comparator drug AcF-[OP(D-Cha) WR]. Using various amino acid substitutions, it was determined that the AcF moiety of AcF-[OP(D-Cha) WR] was responsible for the metabolic instability of the compound in intestinal mucosal washings. The analog HC-[OP( D-Cha) WR], equiactive in vitro to AcF-[OP(D-Cha) WR], was resistant to intestinal metabolism, but it displayed similar oral bioavailability to AcF-[OP(D-Cha) WR]. However, in the rat TNBS- colitis model, HC-[OP(D-Cha) WR] was effective at reducing mortality, colon edema, colon macroscopic scores, and increasing food consumption and body weights, at 10- to 30- fold lower oral doses than AcF-[OP( D-Cha) WR]. These studies suggest that resistance to intestinal metabolism by HC-[OP(D-Cha) WR] may result in increased local concentrations of the drug in the colon, thus affording efficacy with markedly lower oral doses than AcF-[OP(D-Cha) WR] against TNBS-colitis. This large increase in potency and high efficacy of this compound makes it a potential candidate for clinical development against intestinal diseases such as inflammatory bowel disease.

Will the new CB1 cannabinoid receptor antagonist SR-147778 have advantages over rimonabant?

Obesity and alcoholism are two common modern-day diseases. The cannabinoid CB, receptor antagonist rimonabant is in Phase III clinical trial for the treatment of obesity with preliminary results showing that it decreases appetite and body weight. Animal studies have shown that rimonabant is effective in the treatment of alcoholism. SR-147778 is a new potent and selective CB1 receptor antagonist. In animals, SR-147778 has been shown to inhibit CB1 receptor-mediated hypothermia, analgesia and slowing of gastrointestinal transit. In rats trained to drink sucrose, the oral administration of SR-147778 3 mg/kg, before the presentation of sucrose, decreased the consumption of sucrose. SR-147778 3 mg/kg also reduced spontaneous feeding in rats deprived of food and also in non-deprived rats. In Sardinian alcohol-preferring (sP) rats, in the alcohol-naive state, SR-147778 slowed the development of a preference for alcohol. in alcohol-experienced sP rats SR-147778 (2.5, 5 and 10 mg/kg p.o.) reduced the alcohol intake. When alcohol-experienced sP rats are deprived of alcohol for 15 days, there is a large intake of alcohol on reintroduction of alcohol, and this response was almost abolished by treatment with SR-147778. From the preclinical studies published to date, there is no obvious major point of difference between rimonabant and SR-147778, and both are promising agents for the treatment of obesity and alcoholism.