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 mediator released during complement activation which binds to and activates a G-protein-coupled receptor. Molecular cloning of the C3aR has facilitated studies to identify nonpeptide antagonists of the C3aR. A chemical lead that selectively inhibited the C3aR in a high throughput screen was identified and chemically optimized. The resulting antagonist, N 2 -[(2,2-diphenylethoxy)acetyl]-l-arginine (SB 290157), functioned as a competitive antagonist of 125 I-C3a radioligand binding to rat basophilic leukemia (RBL)-2H3 cells expressing the human C3aR (RBL-C3aR), with an IC 50 of 200 nM. SB 290157 was a functional antagonist, blocking C3a-induced C3aR internalization in a concentration-dependent manner and C3a-induced Ca 2+ mobilization in RBL-C3aR cells 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 was not solely limited to the human C3aR; SB 290157 also inhibited C3a-induced Ca 2+ mobilization of RBL-2H3 cells expressing the mouse and guinea pig C3aRs. It potently inhibited C3a-mediated ATP release from guinea pig platelets and inhibited C3a-induced potentiation of the contractile response to field stimulation of perfused rat caudal artery. Furthermore, in animal models, SB 290157, inhibited neutrophil recruitment in a guinea pig LPS-induced airway neutrophilia model and decreased paw edema in a rat adjuvant-induced arthritis model. This selective antagonist may be useful to define the physiological and pathophysiological roles of the C3aR.