Rituximab in the treatment of MuSK antibody-positive myasthenia gravis

Mechanisms underlying B cell immune dysregulation and autoantibody production in MuSK myasthenia gravis

Pathogenic autoantibodies to muscle-specific tyrosine kinase (MuSK) can be found in patients with myasthenia gravis (MG) who do not have detectable antibodies to the acetylcholine receptor. Although the autoantibody-mediated pathology is well understood, much remains to be learned about the cellular immunology that contributes to autoantibody production. To that end, our laboratory has investigated particular components associated with the cellular immunopathology of MuSK MG. First, we found that B cell tolerance defects contribute to the abnormal development of the naive repertoire, which indicates that dysregulation occurs before the production of autoantibodies. Second, both the naive and antigen-experienced memory B cell repertoire, which we examined through the application of high-throughput adaptive immune receptor repertoire sequencing, include abnormalities not found in healthy controls. This highlights a broad immune dysregulation. Third, using complementary approaches, including production of human monoclonal antibodies, we determined that circulating plasmablasts directly contribute to the production of MuSK-specific autoantibodies in patients experiencing relapse following B cell depletion therapy. These collective findings contribute to defining a mechanistic model that describes MuSK MG immunopathogenesis.

Construction of an miRNA-regulated drug-pathway network reveals drug repurposing candidates for myasthenia gravis

Myasthenia gravis (MG) is a rare debilitating autoimmune neuromuscular disorder. Many studies have focused on the mechanism and treatment strategies of MG. However, the exact pathogenesis of MG and effective treatment strategies remain unclear. Recent studies have indicated that microRNAs (miRNAs or miRs) can regulate the pathological pathways of MG, suggesting their potential role in novel treatments. In the present study, we created a comprehensive catalog of experimentally confirmed MG risk genes and miRNAs by manually mining published literature and public databases. Based on these genes and miRNAs, we identified 41 MG risk pathways and 105 approved drugs that can affect these pathways. Some important MG-related pathways, such as hsa04060 (cytokine-cytokine receptor interaction) and hsa05200 (pathway in cancer), were found to be regulated by MG risk miRNAs and drugs. Furthermore, we constructed an miRNA-regulated drug-pathway network and identified miRNAs and drugs that synergistically regulate key MG pathways and biological processes. We developed a drug repurposing strategy to identify 25 drug repurposing candidates for MG; several of these drugs, such as rituximab, adalimumab, sunitinib, and muromonab, have the potential to be novel MG treatment drugs. This study provides novel insight into the pathogenesis of MG and potential drug candidates for MG were identified.

[A case of refractory generalized myasthenia gravis with anti-acetylcholine receptor antibodies treated with rituximab]

We report a case of a 57-year-old woman with thymoma-associated generalized myasthenia gravis (MG) showing severe bulbar and respiratory symptoms, moderate weakness of the neck muscles, and mild weakness of extremity muscles. Corticosteroid treatment with various types of immunosuppressive agents, such as cyclosporine, tacrolimus, and azathioprine, did not improve her symptoms. Plasma exchange transiently improved her symptoms, and she was required to undergo plasmapheresis every 4 weeks. At first, cyclophosphamide pulse therapy was administered, which improved her symptoms transiently. Thereafter, rituximab (RTX) was administered. Six months after RTX administration, respiratory distress and dysphagia improved gradually, and reduction in the dosage of corticosteroids from 30 mg/day to 10 mg/day did not result in symptom deterioration. Therefore, the interval between successive plasmapheresis treatments was increased from 4 to 9 weeks 19 months after the first RTX administration. During a 26-month period from the first administration of RTX, the number of CD20+ B cells in peripheral blood decreased and remained at 0% to 26% of that before RTX treatment. The titer of anti-acetylcholine receptor antibodies did not change during the first course of treatment (0.6-0.9 nmol/l). The clinical symptom worsened with the increase of the number of CD20+ B cells in peripheral blood in the 27 month after 1st RTX administration. Therefore, RTX was administered a second time, after which the patient's clinical symptoms again improved gradually. The titer of anti-acetylcholine receptor antibodies came to be stable with 0.5-0.7 nmol and low level during the 2nd course. Corticosteroids could be discontinued in the 16th month. The findings suggest that RTX can be one of the choices for pharmacological therapy in patients with intractable MG accompanied by the presence of anti-acetylcholine receptor antibodies.

Myasthenia gravis: an update for the clinician

This paper provides a thorough overview of the current advances in diagnosis and therapy of myasthenia gravis (MG). Nowadays the term 'myasthenia gravis' includes heterogeneous autoimmune diseases, with a postsynaptic defect of neuromuscular transmission as the common feature. Myasthenia gravis should be classified according to the antibody specificity [acetylcholine, muscle-specific receptor tyrosine kinase (MuSK), low-density lipoprotein receptor-related protein 4 (LRP4), seronegative], thymus histology (thymitis, thymoma, atrophy), age at onset (in children; aged less than or more than 50 years) and type of course (ocular or generalized). With optimal treatment, the prognosis is good in terms of daily functions, quality of life and survival. Symptomatic treatment with acetylcholine esterase inhibition is usually combined with immunosuppression. Azathioprine still remains the first choice for long-term immunosuppressive therapy. Alternative immunosuppressive options to azathioprine include cyclosporin, cyclophosphamide, methotrexate, mycophenolate mofetil and tacrolimus. Rituximab is a promising new drug for severe generalized MG. Emerging therapy options include belimumab, eculizumab and the granulocyte- macrophage colony-stimulating factor. One pilot study on etanercept has given disappointing results. For decades, thymectomy has been performed in younger adults to improve non-paraneoplastic MG. However, controlled prospective studies on the suspected benefit of this surgical procedure are still lacking. In acute exacerbations, including myasthenic crisis, intravenous immunoglobulin, plasmapheresis and immunoadsorption are similarly effective.

[Proposal for rituximab treatment in patients with myasthenia gravis]

Rituximab (RTX) is a chimeric murine/human monoclonal antibody directed against the CD20 surface antigen of B cells. RTX has been for the treatment of non-Hodikin lymphoma. Recently, RTX was shown to be effective in treating patients with myasthenia gravis (MG) who did not show improvement with other immunomodulation treatments. However, the evidence of its efficacy is mostly limited to a few case series or open-label trials. At the present stage, RTX may be permitted to be used in patients who understand the risk of its side effects and provide consent for its use in treatment. In future, it will be necessary to establish an effective and safe medication protocol of RTX in Japan.

Management challenges in muscle-specific tyrosine kinase myasthenia gravis

Myasthenia gravis with antibodies to muscle-specific tyrosine kinase (MuSK-MG) is generally considered a severe disease because of the associated weakness distribution with prevalent involvement of bulbar muscles and a rapidly progressive course and early respiratory crises. Its treatment can be unrewarding, owing to poor response to acetylcholinesterase inhibitors in most patients, disease relapses in spite of high-dose immunosuppression, and development of permanent bulbar weakness. High-dose prednisone plus plasma exchange is the recommended approach for treating rapidly progressive bulbar weakness. In the disease management, oral steroids proved effective, plasma exchange produced marked, albeit short-term, improvement, while conventional immunosuppressants were comparatively less effective. Rituximab is a promising treatment for refractory MuSK-MG; in uncontrolled studies, nearly all treated patients achieved significant improvement with substantial decrease of medication. It is yet to be clarified whether the early use of rituximab could prevent the permanent bulbar weakness, which constitutes a relevant disability in these patients.

Rituximab in severe seronegative juvenile myasthenia gravis: review of the literature

Myasthenia gravis is an autoimmune neuromuscular disorder caused by circulating antibodies that block acetylcholine receptors at the postsynaptic neuromuscular junction. A wide range of symptomatic therapy with acetylcholinesterase inhibitors and immunotherapy such as corticosteroids, azathioprine, cyclosporine, mycophenolate mofetil, plasmapheresis, and intravenous immunoglobulin has been used in the treatment of myasthenia gravis, with variable responses. However, most modalities of treatment involve delayed onset of action. We describe a child with severe, life-threatening seronegative myasthenia gravis who repeatedly failed extubation and responded dramatically to rituximab. She achieved complete and sustained remission for more than 9 months, with gradual reduction in steroid dose without any side effects. Advances in the treatment of myasthenia gravis have reduced mortality and morbidity and improved the quality of life in these patients.

Rituximab used successfully in the treatment of anti-NMDA receptor encephalitis

We report the case of a young woman with anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis, without tumor, who was successfully treated with rituximab. Because conventional immunotherapy, including corticosteroids, immunoglobulin (IVIg), and plasma exchange showed little improvement in our patient, we introduced another treatment using rituximab. A week after the first administration of rituximab, her symptoms improved gradually and significantly. This case provides in vivo evidence that rituximab is an effective agent for treating anti-NMDAR encephalitis, even in those cases where conventional immunotherapies have been ineffective. Rituximab should be regarded as a beneficial therapeutic agent for this disease.