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

C3 convertase is a key enzyme in the complement cascade and is an attractiv e 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 complem ent 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 fro m a phage-displayed random peptide library. J. Immunol. (157) 884-891), a 1 3 amino acid cyclic peptide that binds to C3. Since the enzyme exhibits som e homology to serine proteases, substrate-based design could be another ave nue for drug design. In this study, we confirm the activity Of compstatin u sing different sources of enzyme and different assay systems. We also teste d the activity of substituted compstatin analogs and compared the selectivi ty and toxicity of these compounds to peptidyl alpha-ketoheterocyclic compo unds. Our work confirms the activity of compstatin in both alternative and classical complement pathways, describes 11 new active analogs of this cycl ic peptide, and provides evidence for key segments of the peptide for activ ity. Compstatin and related active analogs showed little or no inhibition o f clotting or key enzymes in the clotting cascade nor did they appear to ha ve significant cytotoxicity. The characteristics of compstatin suggest that this peptide and its analogs could be attractive candidates for further cl inical development. By contrast, known serine protease inhibitors, includin g peptidyl alpha-ketoheterocycles, did not inhibit C3 convertase illustrati ng the atypical nature of this enzyme. (C) 2000 Elsevier Science B.V. All r ights reserved.