The complement system: Biology, pathology, and therapeutic interventions

Targeting the innate immune complement system has long been pursued as a promising strategy for treating a wide spectrum of acute and chronic immune-mediated disorders. Nearly 2 decades since the approval of the first complement inhibitor targeting C5, the field of complement therapeutics has flourished. In just the past 5 years, 8 new complement-targeting drugs have entered the market. Traditionally dominated by monoclonal antibodies, the approved drug arsenal now includes an ever-expanding selection of macrocyclic peptides, low molecular weight compounds, and aptamers. Other novel approaches in clinical development include recombinant proteins, nanobodies, oligonucleotide therapeutics, and gene therapies. Alongside this remarkable diversification of drug modalities, the complement field is advancing toward deciphering and precisely targeting all pathways of the complement network. Although current approved therapies primarily benefit patients with rare diseases, there is a nascent shift towards addressing more prevalent conditions. In line with this unprecedented expansion in the field, we aim to provide a comprehensive overview of the complement system, its activation pathways, key effector mechanisms, and regulation. We discuss the consequences of unchecked complement activation leading to disease pathologies and highlight therapeutic intervention points within the cascade. We review the discovery and molecular pharmacology of clinically approved and late-phase complement therapeutics, bridging to their clinical pharmacokinetics, safety, and regulatory status for clinical indications. We particularly focus on the diversifying modalities of complement-targeting drugs and evaluate the unique opportunities and challenges these new developments present. With this knowledge, we forecast potential short-term and long-term development trends in the field. SIGNIFICANCE STATEMENT: Therapeutic targeting of the complement system has been widely pursued for treating autoimmune and inflammatory disorders, with the field recently seeing an exponential increase in clinical approvals (8 new drugs during 2022-2024). This timely comprehensive review presents an overview of the complement cascade, its effector mechanisms, and the molecular pharmacology and clinical utility of currently approved and late-phase complement-targeting drugs. We thus provide a valuable one-stop guide for existing and upcoming researchers working in the complement field.