C5a is a small anaphylatoxin of 74 amino acids, released from the N-terminus of the α-chain. Free C5a adopts a four-helix bundle structure in solution. Its C-terminal region is directly responsible for interacting with the receptors C5aR1 and C5L2, chemotaxis of neutrophils and monocytes, induction of degranulation, and enhancement of vascular permeability.

C5b is the initiator of MAC assembly. Newly generated C5b exposes a binding site for C6, followed by sequential recruitment of C7, C8, and multiple C9 molecules, forming a transmembrane pore in the target cell membrane. MAC-mediated lysis is critical for defense against pathogens such as Neisseria.

Potently attracts neutrophils, monocytes, and macrophages to migrate to the site of infection/injury, rapidly recruiting immune cells and initiating local immune defense.

Stimulates mast cells and basophils to degranulate, releasing histamine and leukotrienes, which cause vasodilation, increased permeability, local redness, swelling, edema, and pain.

Activates neutrophils and macrophages, promoting the release of oxygen free radicals and pro-inflammatory cytokines, thereby amplifying inflammation; also participates in regulating adaptive immunity and autoimmune responses.

MAC inserts into the cell membrane of bacteria, virus-infected cells, and abnormal cells, forming a transmembrane pore. This leads to efflux of intracellular fluid, ion imbalance, and ultimately cell lysis and death, eliminating pathogens and abnormal cells.

Kills bacteria, fungi, viruses, and tumor cells.

Attacks normal self-cells, leading to hemolysis, glomerular injury, thrombosis, atypical hemolytic uremic syndrome, paroxysmal nocturnal hemoglobinuria, etc.

Overactivation of C5 is involved in various autoimmune and inflammatory diseases.

In paroxysmal nocturnal hemoglobinuria (PNH), due to the lack of CD55 and CD59 on the surface of blood cells, the alternative pathway is continuously activated, leading to MAC deposition on red blood cells and chronic intravascular hemolysis.

In atypical hemolytic uremic syndrome (aHUS), mutations in complement alternative pathway regulators cause uncontrolled C5 convertase activity, resulting in microvascular endothelial injury and thrombosis.

In animal models of experimental autoimmune uveitis and myasthenia gravis, blocking C5 significantly reduces tissue damage.

Fig 2 Simplified diagram of the complement pathway

The U.S. FDA has currently approved two classes of C5 inhibitors with different mechanisms of action.

The first class uses Complement C5 antibodies to directly block C5 cleavage. Eculizumab and Ravulizumab are representative drugs; both bind to the α-chain of C5, sterically hindering the cleavage by C5 convertase. The discovery of such antibodies has benefited from well-established antibody discovery platforms, among which Phage Display Technology is widely used to screen for high-affinity candidate molecules. These drugs can both inhibit C5a-mediated inflammation and prevent C5b from initiating MAC assembly. They have been used to treat PNH and aHUS. However, the use of such drugs increases the risk of infection by encapsulated bacteria; therefore, vaccination against meningococcus is mandatory before treatment.

The second class is Avacincaptad pegol, which has a completely different molecular form. It is not an antibody but a single-stranded oligonucleotide obtained from a random nucleic acid library through aptamer selection, followed by PEGylation to enhance in vivo stability. This drug is specifically used for the treatment of geographic atrophy. Avacincaptad pegol binds to C5 and blocks its cleavage, thereby preventing MAC formation, while preserving the anti-infective and immunomodulatory functions of upstream molecules such as C3a. In two Phase III clinical trials, intravitreal injection of 2 mg of this drug monthly reduced the growth rate of geographic atrophy lesions by approximately 30% compared to the sham group after 12 months.

Fig 3 The complement cascade and complement inhibitors approved by the U.S. FDA, European EMA, and Australia

As a terminal effector molecule of the complement cascade, C5's cleavage products C5a and the Membrane Attack Complex (MAC) respectively dominate the amplification of inflammatory signals and the lysis/clearance of target cells. Intervention strategies targeting C5, whether based on antibody-mediated steric blockade or aptamer-based nucleic acid drugs, have demonstrated clear therapeutic value in both preclinical and clinical studies.

To support drug discovery and mechanistic research targeting C5, Alpha Lifetech has accumulated extensive technical expertise and can provide high-purity, high-activity C5 recombinant proteins to meet needs ranging from molecular interaction to cellular functional validation. In addition, the company has established a mature Peptide Library Screening and Phage Display Technology Platform that can efficiently screen for specific antibodies, peptides, and alternative scaffold molecules against C5 or its cleavage interface, facilitating the development of next-generation complement inhibitors. Leveraging these specialized capabilities, Alpha Lifetech is committed to providing a one-stop solution from tools to screening for translational medicine research in the complement field.

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