Safety and Efficacy Study of Sivelestat in Neuromyelitis Optica Spectrum Disorder

Inclusion Criteria: 1. Patients diagnosed with anti-AQP4 antibody-positive NMOSD according to the international diagnostic criteria for NMOSD (Wingerchuk, Neurology 2015). 2. Patients experiencing a relapse including any of the following, with the relapse occurring within 14 days of obtaining consent: i. Unilateral or bilateral optic neuritis ii. Myelitis 3. Patients whose FS domain has worsened by at least 1 point due to relapse. 4. Patients with one or more relapse lesions identified on MRI (however, if the relapse is considered to have occurred in the same location as an existing MRI lesion neurologically, identification of a new relapse lesion is not required). 5. Patients aged 18 years or older at the time of obtaining consent. 6. Female patients of childbearing potential who agree to use appropriate contraception from the time of obtaining consent until 180 days after the end of investigational product administration. 7. Male patients who agree to use appropriate contraception until 90 days after the end of investigational product administration. 8. Patients who can provide written informed consent. Exclusion Criteria: 1. Patients with multi-organ dysfunction involving 4 or more organs. 2. Patients with severe chronic respiratory disease. 3. Patients with autoimmune diseases other than NMOSD that are expected to require additional treatment during the study period. 4. Patients with active systemic bacterial, viral, or fungal infections. 5. Patients who have received either or both of the following prior treatments after an NMOSD relapse: i. Two or more courses of steroid pulse therapy ii. Plasmapheresis iii. High-dose immunoglobulin therapy 6. Patients with severe hepatic dysfunction. 7. Patients with alcohol dependence, drug dependence, or psychiatric disorders that would interfere with study participation. 8. Patients who have received other investigational drugs within 3 months prior to obtaining consent. 9. Pregnant women, women suspected of being pregnant, or breastfeeding women. 10. Patients with allergies to the investigational product or concomitant medications. 11. Patients with severe allergies or a history of severe allergies. 12. Patients with suicidal tendencies meeting any of the following criteria: i. Within 1 month prior to the screening assessment, there was suicidal behavior or ideation corresponding to "Yes" for Item 4 (Active suicidal ideation -some intent to act, but no specific plan) or Item 5 (Active suicidal ideation -specific plan and intent) of the Columbia-Suicide Severity Rating Scale (C-SSRS). (For subjects who only met Items 1-3, inclusion may be permitted at the discretion of the principal investigator or sub-investigator.) ii. Any suicidal behavior based on Item 6 of the C-SSRS occurred within the past 3 months. 13. Other patients judged inappropriate by the principal investigator or sub-investigator.