C3 activation is inhibited by analogs of compstatin but not by serine protease inhibitors or peptidyl α-ketoheterocycles

C3 convertase is a key enzyme in the complement cascade and is an attractive therapeutic target for drug design. Recent studies have demonstrated that this enzyme is inhibited by compstatin (Morikis, D., Assa-Munt, N., Sahu, A., Lambris, J.D., 1998. Solution structure of Compstatin, a potent complement inhibitor. Protein Sci. (7) 619–627; Sahu, A., Kay, B.K., Lambris, J.D., 1996. Inhibition of human complement by a C3-binding peptide isolated from a phage-displayed random peptide library. J. Immunol. (157) 884–891), a 13 amino acid cyclic peptide that binds to C3. Since the enzyme exhibits some homology to serine proteases, substrate-based design could be another avenue for drug design. In this study, we confirm the activity of compstatin using different sources of enzyme and different assay systems. We also tested the activity of substituted compstatin analogs and compared the selectivity and toxicity of these compounds to peptidyl α-ketoheterocyclic compounds. Our work confirms the activity of compstatin in both alternative and classical complement pathways, describes 11 new active analogs of this cyclic peptide, and provides evidence for key segments of the peptide for activity. Compstatin and related active analogs showed little or no inhibition of clotting or key enzymes in the clotting cascade nor did they appear to have significant cytotoxicity. The characteristics of compstatin suggest that this peptide and its analogs could be attractive candidates for further clinical development. By contrast, known serine protease inhibitors, including peptidyl α-ketoheterocycles, did not inhibit C3 convertase illustrating the atypical nature of this enzyme.