Current state and potential applications of neonatal Fc receptor (FcRn) inhibitors in hematologic conditions.

The neonatal fragment crystallizable (Fc) receptor (FcRn) transports IgG across mucosal surfaces and the placenta and protects IgG from degradation. Numerous clinical trials are investigating therapeutic FcRn inhibition for various immune-mediated neuromuscular and rheumatologic conditions; however, FcRn inhibition also represents a potential therapy for IgG-mediated hematologic conditions (e.g., immune thrombocytopenia, autoimmune hemolytic anemia, immune thrombotic thrombocytopenic purpura, acquired hemophilia, red blood cell/platelet alloimmunization). Current evidence derived from both in vitro and in vivo studies suggests that FcRn inhibitors effectively reduce total IgG levels without impacting its production or altering the levels of other immunoglobulin isotypes. Moreover, the risk of serious adverse events, including serious infections, appears to be lower than that seen with other commonly used immunomodulatory/immunosuppressive therapies, albeit in the setting of limited clinical trial data. Ultimately, additional clinical trials that include varied patient populations are required prior to incorporating these agents into standard treatment algorithms for most hematologic conditions. However, based on the pathophysiology of IgG-mediated hematologic disorders and the mechanism of action of FcRn inhibitors, these agents may represent a future novel therapeutic strategy for patients with hematologic conditions caused by IgG antibodies.