Week 2, April 2025: Antibody-Targeted Therapies: A Look at FcRn Inhibitors and Fc Engineering

Just this week, Merida Biosciences emerged from stealth with a $121 million Series A financing to develop a new class of precision therapeutics that eliminate pathogenic antibodies in autoimmune and allergic diseases. With programs targeting Graves’ disease, primary membranous nephropathy, and severe allergies, their approach is designed to selectively bind and remove only disease-causing antibodies, sparing the rest of the immune system. Built on a novel Fc protein engineering platform, their antibody-like drugs are tailored for deep, durable, and highly selective depletion of both autoantibodies and the B cells that produce them.

In parallel, the therapeutic targeting of the neonatal Fc receptor (FcRn) is gaining momentum in the field of antibody-mediated autoimmune diseases. A recent comprehensive review synthesizes clinical evidence from multiple trials, showing that FcRn inhibitors significantly reduce circulating IgG levels by blocking their interaction with FcRn, thereby promoting their lysosomal degradation. This mechanism is particularly impactful in treating IgG-mediated conditions such as myasthenia gravis (MG), immune thrombocytopenia (ITP), and chronic inflammatory demyelinating polyneuropathy (CIDP). The study underscores the broad potential of FcRn inhibitors across various IgG-driven diseases, while also suggesting the need for better targeting to minimize side effects like hypoalbuminemia.

Complementing this clinical perspective, a mechanistic study led by researchers from the University of Southampton dives into the cellular and molecular dynamics of FcRn antagonism, specifically addressing how these therapies affect albumin homeostasis. While FcRn's primary role is in IgG recycling, it also transports serum albumin, meaning that blocking FcRn can inadvertently reduce albumin levels. This study identifies two major mechanisms for this side effect: enhanced degradation of FcRn itself and competitive binding between albumin and therapeutic antagonists. These insights highlight the importance of designing next-generation FcRn-targeting drugs with improved specificity to reduce adverse metabolic outcomes.

Further advancing our understanding of this therapeutic class, research on Johnson & Johnson's nipocalimab (a high-affinity monoclonal antibody against FcRn) demonstrates its ability to selectively deplete IgG without disrupting other immune functions. Structural analyses reveal that nipocalimab binds to a unique epitope on FcRn, maintaining strong interaction across both extracellular and intracellular pH environments. In vitro and in vivo models confirm its dose- and time-dependent action on FcRn occupancy and IgG reduction. This selectivity makes it a promising candidate for treating both autoantibody- and alloantibody-mediated diseases, positioning it at the forefront of antibody precision medicine.

Beyond autoimmune disorders, antibody engineering is also opening new frontiers in infectious disease therapy. In a recent study on tuberculosis (TB), researchers from the Ragon Institute of Mass General Brigham, MIT, and Harvard engineered Fc variants of an antibody targeting the Mycobacterium tuberculosis capsule to explore how Fc effector functions influence pathogen control. Among 52 antibody variants tested, several enhanced Mtb restriction in a neutrophil-dependent manner. Single-cell RNA sequencing showed that these Fc-engineered antibodies promote neutrophil survival and activation of antimicrobial programs. This work illustrates the therapeutic potential of harnessing antibody Fc design not only to neutralize pathogens but to modulate immune cell behavior for enhanced disease control.