Complement component C5 is a pro-inflammatory protein that is part of the classical complement pathway. It plays a crucial role in the activation of C3 and C5, contributing to the inflammatory response associated with conditions like age-related macular degeneration (AMD). In the terminal pathway distal to C3, C5 is the critical component. C5 has some structural similarities to C3: C5 contains macroglobulin domains (MG1-MG8), the CUB domain, the C5d domain (structurally homologous to the TED of C3), the C5a domain (also called anaphylatoxin) and an extended linker region. Native human C5 is a naturally glycosylated (1.6%) polypeptide containing two disulfide-linked chains. C5 is essential for formation of the membrane attack complex (MAC) and is activated by all three pathways of complement activation.
Eculizumab (Soliris) is a monoclonal antibody that binds C5 and blocks its proteolytic activation. Eculizumab was approved in 2007 for paroxysmal nocturnal hemoglobinuria (PNH), becoming the first complement inhibitor available to patients. With annual sales of more than US$ 6.53 bln in 2024, first and second generation C5 inhibitor antibodies Soliris and Ultomiris from AstraZeneca (acquired Alexion Pharmaceuticals) and Piasky have set the stage for next generation biosuperiors as well as biosimilars of a commercially successful and clinically effective treatment of complement C5 mediated diseases. New treatment modalities in development aim at improving the mode and frequency of administration as well as the scope of indications.
Complement C5-mediated diseases are a group of disorders where the C5 protein of the complement system is implicated in tissue damage, autoimmune processes, and inflammation, including PNH, Atypical Hemolytic Uremic Syndrome (aHUS), Sickle Cell Disease (SCD), Myasthenia Gravis (MG), and Neuromyelitis Optica Spectrum Disorder (NMOSD). These conditions can involve the formation of the Membrane Attack Complex (MAC) or the release of anaphylatoxins like C5a, leading to symptoms such as anemia, blood clots, and kidney damage.

