Novel Complement Modulators for Paroxysmal Nocturnal Hemoglobinuria: Peptide and Protein Inhibitors of C3 Convertase Prevent Both Surface C3 Deposition and Subsequent Hemolysis of Affected Erythrocytes in Vitro

Abstract 370

Paroxysmal nocturnal hemoglobinuria (PNH) is a complex hematological disorder characterized by the expansion of blood cells deficient in the surface complement inhibitors CD55 and CD59; affected erythrocytes suffer from uncontrolled complement activation on their surface, and subsequent membrane attack complex (MAC)-mediated intravascular hemolysis. The anti-C5 antibody eculizumab has proven effective in controlling intravascular hemolysis in vivo, leading to remarkable clinical benefit in almost all PNH patients. Yet, we have demonstrated that persistent C3 activation occurring during eculizumab treatment may lead to progressive C3 deposition on affected erythrocyte and subsequent C3-mediated extravascular hemolysis, possibly limiting the hematological benefit of anti-C5 treatment (Risitano et al, Blood 2009). Thus, upstream inhibition of the complement cascade seems an appropriate strategy to improve the results of current anti-complement treatment; indeed, we have recently documented that the CD21/factor H (FH) fusion protein TT30 efficiently prevents both hemolysis and C3 deposition of PNH erythrocytes (Risitano et al, Blood 2012). Here we used the same in vitro model to evaluate two novel complement inhibitors that both act at the level of C3 convertases. Cp30 is an analog of the peptidic inhibitor compstatin, which is a 13-residue disulphide-bridged peptide that selectively binds to C3 and its activate fragment C3b. Compstatin and its analogues thereby prevent the initiation, amplification and terminal damage of the complement cascade via all its major pathways (classical, alternative, and mannose/lectin). Cp30 is one of the analogues developed to increase potency and stability of compstatin. Mini-FH, on the other hand, is an engineered 43kDa protein that combines the regulatory and surface-recognition activities of FH while showing increased affinity for the opsonins C3b, iC3b and C3d. Indeed, mini-FH retained both convertase decay acceleration and cofactor activities typical of endogenous human FH, resulting in a potent and selective inhibition of activation and amplification of the complement alternative pathway, without affecting the classical and the mannose/lectin pathway. Erythrocytes from PNH patients were washed and incubated in ABO-matched sera and exposed to pH-lowering to activate the alternative pathway, both in absence and presence of Cp30, mini-FH, and appropriate controls. Assessment of hemolysis and of C3 activation and deposition on PNH erythrocytes was performed by flow cytometry analyses of erythrocytes using anti-C3 and anti-CD59 antibodies, as previously described (Risitano et al Blood 2012). In absence of inhibitors, >90% of PNH erythrocytes lysed within 24 hours of incubation. Cp30 demonstrated a dose-dependent inhibition of hemolysis, with an IC₅₀ of 4 μM and full inhibition at 8 μM. Cp30 also prevented deposition of any C3 fragment on the surface of surviving PNH erythrocyte. Similarly, mini-FH also showed dose-dependent inhibition of hemolysis, with an IC₅₀ of 0.05 μM and full inhibition at 0.1 μM. Notably, both full-length fH and fH SCR1-4 were much less efficient in preventing hemolysis and C3 deposition (IC₅₀ ∼ 0.5 μM; full inhibition >1 μM), supporting the higher potency of the engineered protein mini-FH. As expected, mini-FH also prevented surface deposition of C3 fragments on PNH erythrocytes. In conclusion, we confirm that inhibition of early phases of complement activation efficiently prevents hemolysis of PNH erythrocytes and their opsonization with C3 fragments in vitro. This effect may be obtained using either broad or pathway-specific inhibitors of C3 convertase, namely Cp30 and mini-FH, respectively. Thus, both strategies promise to prevent in vivo both MAC-mediated intravascular and C3-mediated extravascular hemolysis; however, according to their effect on specific complement pathways, they likely entail distinct patterns of potential risks. Our study provides the rationale for future translational plans to investigate the risk-to-benefit of these novel complement modulators in PNH.