Targeting the Neonatal Fc Receptor in Autoimmune Diseases: Pipeline and Progress.

Autoimmune diseases are highly prevalent and affect people at all ages, women more often than men. The most prominent immunological manifestation is the production of antibodies directed against self-antigens. In many cases, these antibodies (Abs) drive the pathogenesis by attacking the body's own healthy cells, causing serious health problems that may be life threatening. Most autoantibodies are of the immunoglobulin G (IgG) isotype, which has a long plasma half-life and potent effector functions. Thus, there is a need for specific treatment options that rapidly eliminate these pathogenic IgG auto-Abs. In this review, we discuss how the neonatal Fc receptor (FcRn) acts as a regulator of the high levels of not only IgG Abs, but also albumin, by rescuing both these soluble proteins from cellular catabolism, and how a molecular and cellular understanding of this complex biology has spurred an intense interest in the development of FcRn-targeting strategies for the treatment of IgG-driven autoimmune diseases. We find that this emerging therapeutic class demonstrates efficacy within several autoimmune diseases with distinct pathophysiology. This offers hope for both new therapeutic avenues for highly prevalent diseases currently treated by other means, and rare diseases with no approved therapies to date. In addition, we elaborate on studies that have led to approval of the first FcRn antagonists, the clinical progress and structural design of molecules in the pipeline, their position in the overall therapeutic landscape of autoimmunity, the design of next-generation antagonists as well as the use of this receptor-targeting principle for other therapeutic applications.