Identification of a selective nonpeptide antagonist of the anaphylatoxin C3a receptor that demonstrates antiinflammatory activity in animal models

The anaphylatoxin C3a Is a potent chemotactic peptide and inflammatory medi ator released during complement activation which binds to and activates a G -protein-coupled receptor. Molecular cloning of the C3aR has facilitated st udies to identify nonpeptide antagonists of the C3aR. A chemical lead that selectively inhibited the C3aR in a high throughput screen was identified a nd chemically optimized. The resulting antagonist, N-2-[(2,2-diphenylethoxy )acetyl]-L-arginine (SB 290157), functioned as a competitive antagonist of I-125-C3a radioligand binding to rat basophilic leukemia (RBL)-2H3 cells ex pressing the human C3aR (RBL-C3aR), with an IC50 of 200 nM. SB 290157 was a functional antagonist, blocking C3a-induced C3aR internalization in a conc entration-dependent manner and C3a-induced Ca2+ mobilization in RBL-C3aR ce lls and human neutrophils with IC(50)s of 27.7 and 28 nM, respectively. SB 290157 was selective for the C3aR in that it did not antagonize the C5aR or six other chemotactic G protein-coupled receptors. Functional antagonism w as not solely limited to the human C3aR; SB 290157 also inhibited C3a-induc ed Ca2+ mobilization of RBL-2H3 cells expressing the mouse and guinea pig C 3aRs. It potently inhibited C3a-mediated ATP release from guinea pig platel ets and inhibited C3a-induced potentiation of the contractile response to f ield stimulation of perfused rat caudal artery. Furthermore, in animal mode ls, SB 290157, inhibited neutrophil recruitment in a guinea pig LPS-induced airway neutrophilia model and decreased paw edema in a rat adjuvant-induce d arthritis model. This selective antagonist may be useful to define the ph ysiological and pathophysiological roles of the C3aR.