Efficacy of MLN9708 (ixazomib) in experimental autoimmune myasthenia gravis and in anti-AChR producing primary thymic cell cultures from myasthenia gravis patients

Marina Mané Damas ^1,2 Abhishek Saxena ^2,3 Gisela Nogales Gadea ^2,4 Jo Stevens ^2,5 Shannen Vincken ^2,6 Maarten van Beek ^7,8 Nynke Jolein van den Hoogen ^7,9 Bert Joosten ⁷ Nick Willcox ¹⁰ Hans Duimel ¹¹ Jos G. Maessen ¹² Peter Cornelis Molenaar ² Marc H De Baets ² Mario Losen ² Pilar Martinez Martinez ^2*

• ¹ Maastricht University, Maastricht, Netherlands
• ² Department of Psychiatry and Neuropsychology, School for Mental Health and Neuroscience, Faculty of Health, Medicine and Life Sciences, Maastricht University, Maastricht, Netherlands, Netherlands
• ³ Advanced Biologics Design Group, Center for Translational Research, Shenzhen Bay Laboratory, Shenzhen, China, Shenzhen, China
• ⁴ Neuromuscular and Neuropediatric Diseases Research Group, Neuroscience department, Institut d’Investigació en Ciencies de la Salut Germans Trias i Pujol, Universitat Autónoma de Barcelona, Badalona, Spain
• ⁵ Antibody Development, Roche, Munich, Germany, Munich, Germany
• ⁶ Lonza Netherlands, Geleen, Netherlands
• ⁷ Department of Anesthesiology, Mental Health and Neuroscience Research Institute, Faculty of Health, Medicine and Life Sciences, Maastricht University, Maastricht, Netherlands
• ⁸ B Braun group, Rotterdam, Netherlands
• ⁹ Department of Pediatrics, Hotchkiss Brain Institute, University of Calgary, Calgary, Canada
• ¹⁰ Department of Clinical Neurosciences, Weatherall Institute for Molecular Medicine, University of Oxford, Oxford, United Kingdom
• ¹¹ Microscope CORE Lab, Maastricht University, Maastricht, Netherlands
• ¹² Department of Cardiothoracic Surgery, Maastricht UMC+, Maastricht, Netherlands

Proteasome inhibitors can eliminate malignant, alloreactive, or autoreactive plasma cells. These cells are key players in antibody-mediated autoimmune disorders and thus suitable therapeutic targets for these drugs. However, certain proteasome inhibitors cause toxic peripheral neuropathy in patients. Ixazomib (MLN9708, Ninlaro), an oral proteasome inhibitor, has a more favorable safety profile in multiple myeloma patients. Here we tested its efficacy in preventing and treating experimental autoimmune myasthenia gravis (EAMG).Female Lewis rats were treated with two subcutaneous doses of 0.35 mg/kg of ixazomib per week, starting either 4 weeks before or at disease onset; which both substantially lowered final total IgG and rat acetylcholine receptor (AChR) autoantibody levels. Interestingly, two weekly doses of 0.20 mg/kg of ixazomib for the last 4 weeks did not reduce autoantibody levels. A single dose of 0.50 mg/kg was acutely toxic in rats. In cultures of thymic cells from early-onset myasthenia gravis (EOMG) patients, 30 nM ixazomib or higher almost completely eliminated plasma cells and halted their IgG and AChR antibody production. We conclude that proteasome inhibition with ixazomib effectively depletes plasma cells from MG patients in vitro and in a rat model in vivo. These results encourage further investigations into therapeutic plasma cell targeting for MG patients.