Introduction: Eculizumab, a terminal complement protein C5 inhibitor, is approved in Japan for the treatment of patients with anti-acetylcholine receptor antibody-positive (AChR Ab+) generalized myasthenia gravis (gMG) that is difficult to control with plasmapheresis or high-dose intravenous immunoglobulin therapy. Methods: This analysis of mandatory postmarketing surveillance in Japan assessed the safety and effectiveness of eculizumab in patients with AChR Ab+ gMG who had completed case-report forms at 26 wk after eculizumab initiation up to the cutoff date of April 2021. Changes from baseline were assessed for Myasthenia Gravis-Activities of Daily Living (MG-ADL) and Quantitative Myasthenia Gravis (QMG) total scores overall, and MG-ADL scores in patient subgroups according to sex, age at diagnosis and baseline, and baseline disease severity. The change in concomitant corticosteroid use was also evaluated. Results: Data were available for 134 adults (67.2% female; mean age 51.9 y); the effectiveness-analysis set comprised 126 patients. After 26 wk, 78% of patients were continuing eculizumab treatment. Adverse drug reactions were reported by 49 patients (37%) (most frequently headache [n = 10]). Improvements in MG-ADL scores were seen regardless of sex; age at diagnosis (<50/≥50 y); baseline age (18 to <40/≥40 to <65/≥65 y); Myasthenia Gravis Foundation of America disease classification (IIa/IIb/IIIa/IIIb/IVa/IVb/V); or baseline MG-ADL score (<6/≥6). Of patients receiving corticosteroids, the proportion receiving low doses (average ≤5 mg/d) increased from 7.0% before eculizumab initiation to 26.4% by Week 52. Conclusion: Eculizumab was well tolerated and effective in treating AChR Ab+ gMG across a broad spectrum of adult Japanese patients with difficult-to-control gMG.
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