A Novel C5 Complement Inhibitor Protects Against Sepsis-Induced Activation of Complement, Coagulation and Inflammation and Provides Survival Benefit in E. coli Sepsis

Bacterial sepsis triggers robust early activation of the complement system that leads to the formation of the C5b-9 terminal complex (TCC) and potent proinflammatory anaphylatoxins, C3a and C5a, which significantly contribute to organ failure and death. Complement inhibition at C3 level provides organ protection in low bacteremia experimental sepsis but may impair the clearance of the bacteria. Here we used a baboon model of E. coli sepsis and in vitro whole-blood assays to test the protective effects of a novel C5 inhibitor (RA101295, a 2036 Da cyclic peptide). In vitro whole blood assays were performed to study the effects of RA101295 on sepsis-induced complement activation, inflammation, oxidative stress and phagocytosis of the bacteria. Lepirudin-anticoagulated blood was incubated with 5x10⁷ cfu/ml E. coli or 500 µg/ml lipopolysaccharide (LPS) for 30-240 min in the presence/absence of the inhibitor. Results showed that RA101295 strongly inhibited E. coli and LPS–dependent C5b-9 and C5a generation. Similarly, RA101295 inhibited the E. coliand LPS induced oxidative burst in neutrophils and monocytes by 40-50% with minimal decrease (<15%) on their phagocytic function.

Next, we used a baboon model of E. coli sepsis to investigate the in vivo effects of RA101295. E. coli challenge (1-2x10¹⁰ cfu/kg; LD₁₀₀) in baboons strongly induced the generation of soluble C5b-9 complexes. Treatment with RA101295 C5 inhibitor (four subcutaneous 10 mg/kg doses at 8-12 hrs intervals) fully inhibited complement activation. RA101295 inhibitor remarkably improved the clinical parameters of all treated animals. Two of three baboons survived the lethal dose; while the survival time of the third animal was increased by 300%. Animals treated with complement inhibitor displayed better heart and lung performance and had lower febrile response than non-treated septic baboons.

RA101295 treatment of E. coli sepsis led to significantly lower consumptive coagulopathy as reflected by the APTT/PT, fibrinogen consumption and fibrin/fibrinogen degradation products. These data highlight the tight crosstalk between the coagulation and complement systems and suggest that the observed protective effects are due in part to dampening of sepsis-induced coagulopathy. The treatment during the bacteremic stage had no negative effects on bacterial clearance. Similar to the in vitro data, these results suggest that C5 inhibition with this peptide does not increase the risk of secondary infections and reoccurrence of sepsis, probably because is not interfering with the generation of C3b opsonin. Histologic analysis of organs confirmed that treatment with RA101295 provided substantial organ protection. In contrast to the non-treated group, RA101295 treated animals showed no obvious signs of thrombosis and capillary leak in the lungs; no follicular necrosis in the spleen and no tubular necrosis and glomerular thrombosis in the kidneys.

Overall, our data demonstrate that RA101295 strongly inhibits LPS and E. coli induced complement activation and TCC without affecting the bacterial clearance. These results suggest that complement inhibition at C5 level represents a promising therapeutic target in the fight against sepsis-induced MOF.