title: Incomplete inhibition by eculizumab: mechanistic evidence for residual C5 activity during strong complement activation.
abstract: Eculizumab inhibits the terminal, lytic pathway of complement by blocking the activation of the complement protein C5 and shows remarkable clinical benefits in certain complement-mediated diseases. However, several reports suggest that activation of C5 is not always completely suppressed in patients even under excess of eculizumab over C5, indicating that residual C5 activity may derogate the drug's therapeutic benefit under certain conditions. By using eculizumab and the tick-derived C5 inhibitor coversin, we determined conditions ex vivo in which C5 inhibition is incomplete. The degree of such residual lytic activity depended on the strength of the complement activator and the resulting surface density of the complement activation product C3b, which autoamplifies via the alternative pathway (AP) amplification loop. We show that at high C3b densities required for binding and activation of C5, both inhibitors reduce but do not abolish this interaction. The decrease of C5 binding to C3b clusters in the presence of C5 inhibitors correlated with the levels of residual hemolysis. However, by employing different C5 inhibitors simultaneously, residual hemolytic activity could be abolished. The importance of AP-produced C3b clusters for C5 activation in the presence of eculizumab was corroborated by the finding that residual hemolysis after forceful activation of the classical pathway could be reduced by blocking the AP. By providing insights into C5 activation and inhibition, our study delivers the rationale for the clinically observed phenomenon of residual terminal pathway activity under eculizumab treatment with important implications for anti-C5 therapy in general.
authors.affiliation: Institute of Pharmacology of Natural Products and Clinical Pharmacology, Ulm University, Ulm, Germany.
authors.forename: Markus J
authors.initials: MJ
authors.lastname: Harder
authors.affiliation: Lehrstuhl für Biologische Chemie, Technische Universität München, Freising, Germany.
authors.forename: Nadine
authors.initials: N
authors.lastname: Kuhn
authors.affiliation: Institute of Transfusion Medicine, University of Ulm, Ulm, Germany.|Institute of Clinical Transfusion Medicine and Immunogenetics Ulm, German Red Cross Blood Transfusion Service and University Hospital of Ulm, Ulm, Germany; and.
authors.forename: Hubert
authors.initials: H
authors.lastname: Schrezenmeier
authors.affiliation: Institute of Transfusion Medicine, University of Ulm, Ulm, Germany.|Institute of Clinical Transfusion Medicine and Immunogenetics Ulm, German Red Cross Blood Transfusion Service and University Hospital of Ulm, Ulm, Germany; and.
authors.forename: Britta
authors.initials: B
authors.lastname: Höchsmann
authors.affiliation: Institute of Transfusion Medicine, University of Ulm, Ulm, Germany.|Institute of Clinical Transfusion Medicine and Immunogenetics Ulm, German Red Cross Blood Transfusion Service and University Hospital of Ulm, Ulm, Germany; and.
authors.forename: Inge
authors.initials: I
authors.lastname: von Zabern
authors.affiliation: Institute of Transfusion Medicine, University of Ulm, Ulm, Germany.|Institute of Clinical Transfusion Medicine and Immunogenetics Ulm, German Red Cross Blood Transfusion Service and University Hospital of Ulm, Ulm, Germany; and.
authors.forename: Christof
authors.initials: C
authors.lastname: Weinstock
authors.affiliation: Institute of Pharmacology of Natural Products and Clinical Pharmacology, Ulm University, Ulm, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Simmet
authors.affiliation: Institute of Clinical Transfusion Medicine and Immunogenetics Ulm, German Red Cross Blood Transfusion Service and University Hospital of Ulm, Ulm, Germany; and.
authors.forename: Daniel
authors.identifier: 0000-0001-6140-0233
authors.initials: D
authors.lastname: Ricklin
authors.affiliation: Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA.
authors.forename: John D
authors.initials: JD
authors.lastname: Lambris
authors.affiliation: Lehrstuhl für Biologische Chemie, Technische Universität München, Freising, Germany.
authors.forename: Arne
authors.initials: A
authors.lastname: Skerra
authors.affiliation: Institute of Transfusion Medicine, University of Ulm, Ulm, Germany.|Institute of Clinical Transfusion Medicine and Immunogenetics Ulm, German Red Cross Blood Transfusion Service and University Hospital of Ulm, Ulm, Germany; and.
authors.forename: Markus
authors.initials: M
authors.lastname: Anliker
authors.affiliation: Institute of Pharmacology of Natural Products and Clinical Pharmacology, Ulm University, Ulm, Germany.
authors.forename: Christoph Q
authors.initials: CQ
authors.lastname: Schmidt
mesh_terms: D061067:Antibodies, Monoclonal, Humanized / Q000494:pharmacology* / Q000627:therapeutic use; D003167:Complement Activation / Q000187:drug effects*; D003179:Complement C3b / Q000276:immunology; D003182:Complement C5 / Q000037:antagonists & inhibitors* / Q000276:immunology; D003170:Complement Pathway, Alternative / Q000187:drug effects; D006457:Hemoglobinuria, Paroxysmal / Q000188:drug therapy / Q000276:immunology; D006461:Hemolysis / Q000187:drug effects; D006801:Humans
chemical_list: D061067:Antibodies, Monoclonal, Humanized; D003182:Complement C5; D003179:Complement C3b; C481642:eculizumab