Abstract
Disclosure: D.A. Gutierrez: None. M. Logtenberg: None. L. Schweitzer: None. A. Capili: None. R. LaRiviere: None. D. Huggins: None. R. Friedman: None. J. Donaghey: None. E. Sonya: None. S. Smith: None. D. Suryaprakash: None. Graves’ Disease (GD) and Thyroid Eye Disease (TED) are caused by autoantibodies stimulating the TSH receptor (TSHR) on thyrocytes and orbital fibroblasts, respectively. The mechanism of pathogenicity of TSHR stimulating autoantibodies has been elucidated over decades of research, enabling our ability to develop more effective and safe treatments for patients suffering from these diseases. With the goal of achieving specific and deep depletion of anti-TSHR autoantibodies in patients, we engineered MER511, a monomeric TSHR-IgG Fc fusion protein that combines the TSHR ectodomain with a mutated Fc domain that specifically increases affinity to the inhibitory and endocytic receptor, FcγRIIB, while abrogating affinity to activating FcγRs and complement component C1q. MER511 specifically binds and neutralizes patient-derived anti-TSHR autoantibodies, leading to clearance and degradation via FcγRIIB in LSECs while concurrently inhibiting antigen-specific B-cell function. Here, we demonstrate that MER511 can fully neutralize the pathogenicity of patient-derived stimulating anti-TSHR autoantibodies and that of polyclonal anti-TSHR autoantibodies in GD/TED patient samples. Additionally, MER511 does not bind TSH or impact TSH signaling, thus enabling the restoration of normal thyroid function in patients. In a humanized FcγR/FcRn mouse model, MER511 rapidly depleted passively transferred patient-derived monoclonal autoantibodies (M22), and unlike the FcRn drug class, this depletion was specific to TSHR autoantibodies, as total mouse IgG levels were not impacted. Pharmacokinetically, MER511 has an antibody-like half-life that is expected to enable a favorable dosing regimen for patients. In vitro and in vivo toxicology studies have shown a safe profile for MER511, with low risk for immunogenicity and no drug-related adverse events in preclinical models. These findings collectively support the clinical evaluation of MER511 as a promising therapeutic modality for the treatment of GD and TED. Presentation: Monday, July 14, 2025