Inclusion Criteria:
1. Male or female aged ≥18 years.
2. Diagnosis of myasthenia gravis (MG) confirmed by:
1. positive serology for anti-acetylcholine receptor (AChR) antibody at screening; and
2. any one of the following: abnormal neuromuscular transmission confirmed by repetitive nerve stimulation; history of positive anticholinesterase test (e.g., neostigmine test); or improvement in MG signs after oral cholinesterase inhibitors as assessed by the treating physician.
3. Body weight between 40 kg and 100 kg (inclusive) at screening.
4. MGFA clinical classification Class II to IVa at screening.
5. MG-ADL total score ≥5 at screening, with more than 50% of the total score from non-ocular symptoms.
6. For participants receiving immunosuppressive therapy (IST) (e.g., azathioprine \[AZA\], mycophenolate mofetil \[MMF\], methotrexate \[MTX\], cyclophosphamide \[CY\]): treatment for ≥6 months prior to screening and stable dose for ≥2 months (calculated as 30 days per month).
7. For participants receiving other IST (e.g., cyclosporine \[CsA\], tacrolimus \[TAC\]): treatment for ≥3 months prior to screening and stable dose for ≥1 month.
8. For participants receiving corticosteroid therapy at enrollment: stable dose (not exceeding 30 mg/day prednisone acetate or equivalent) for ≥28 days prior to screening.
9. For participants receiving cholinesterase inhibitor therapy: stable dose for ≥14 days prior to screening.
10. At screening, laboratory tests must meet: (a) Alanine aminotransferase (ALT) and Aspartate aminotransferase (AST) \<1.5×ULN; (b) serum total bilirubin \<1.5×ULN (\<3×ULN for confirmed Gilbert's syndrome); (c) Hemoglobin ≥100 g/dL; (d) Platelet count \>75×10⁹/L; (e) International normalized ratio (INR) and activated partial thromboplastin time (aPTT) \<1.5×ULN; (f) serum creatinine \<1.5×ULN and eGFR \>90 mL/min/1.73m²; (g) basically normal immune function as assessed by the investigator (lymphocyte count ≥1×LLN, IgG ≥1×LLN).
11. To reduce the risk of meningococcal infection (Neisseria meningitidis): participants must be vaccinated at least 14 days prior to the first dose of study drug if not vaccinated within the valid coverage period; if vaccinated within 14 days before dosing, antibiotic prophylaxis must be provided until 2 weeks post-vaccination (penicillin V potassium recommended; cefaclor or other antibiotics may be used if penicillin is unavailable or the participant is allergic; fluoroquinolones and macrolides are not recommended).
12. To reduce the risk of pneumococcal infection: vaccination against Streptococcus pneumoniae is required; if previously vaccinated, vaccination may be waived during screening; if not previously vaccinated, vaccination must be given at least 14 days prior to the first dose (antibiotic prophylaxis as described in item 11).
13. For females of childbearing potential: serum β-HCG pregnancy test must be negative at screening; effective and reliable contraception must be used during the study and for at least 6 months after discontinuation of study intervention (women of childbearing potential are defined as premenopausal women who have not undergone sterilization; menopause is defined as amenorrhea for ≥12 months without alternative medical measures; FSH \>40 mIU/mL confirms menopause).
14. Male participants must agree to use effective contraceptive measures (vasectomy, abstinence, condom use) throughout the study period (from screening to 6 months after study completion); female participants of childbearing potential must have negative blood pregnancy tests at screening and baseline, and female participants, male participants, and their sexual partners must agree to use contraceptive measures during the study and for 6 months after completion (both pharmacological and non-pharmacological methods are acceptable).
15. Capable of understanding the study procedures and methods, willing to sign the Informed Consent Form (ICF), and able to strictly adhere to the clinical study protocol to complete the study.
Exclusion Criteria:
1. Presence of untreated thymic epithelial tumors (including all types of thymoma or thymic carcinoma), extrathymic germ cell tumor, or other malignant mediastinal masses at the screening visit; or, as judged by the investigator, presence of thymic cysts or other space-occupying lesions requiring immediate intervention.
2. History of thymectomy or any other thymic surgery within 12 months prior to screening, or planned thymectomy during the trial period.
3. Prior history of thymic tumor is permitted for enrollment only if the subject meets all criteria in either of the following risk groups:
Low recurrence risk group: Subjects with histopathologically confirmed thymoma of Masaoka-Koga stages I-II (WHO types A, AB, B1, or B2) who meet all of the following criteria are eligible for enrollment in this study:
1. Curative treatment (e.g., R0 surgical resection) completed ≥ 12 months prior to the screening visit;
2. No clinical evidence of recurrence within 12 months prior to screening;
3. No radiological evidence of recurrence confirmed by contrast-enhanced chest CT or MRI within 6 months prior to randomization.
High recurrence risk group: Subjects with histopathologically confirmed thymic carcinoma (WHO type C) or Masaoka-Koga stages III-IV thymoma (including WHO type B3) who meet all of the following criteria are eligible for enrollment:
1. Curative treatment (surgery with or without radiotherapy/chemotherapy and other combined-modality therapy) completed \> 5 years prior to the screening visit;
2. No clinical evidence of recurrence within 5 years;
3. No evidence of recurrence or distant metastasis confirmed by contrast-enhanced chest CT or MRI within 6 months prior to randomization.
\[Note: If a participant cannot provide complete original histopathology reports or Masaoka-Koga staging records, they must be evaluated according to the high recurrence risk group criteria (i.e., treatment completed \> 5 years with no evidence of recurrence).\]
4. Weakness involving only ocular or periorbital muscles (MGFA Class I).
5. Occurrence of MG crisis (MGFA Class V) within 6 months prior to screening.
6. Known positive serology for muscle-specific receptor tyrosine kinase (MuSK) or lipoprotein receptor-related protein 4 (LRP4), or negative results for both anti-AChR and anti-MuSK antibodies.
7. Pregnant, lactating, or planning to become pregnant during the study period.
8. Any systemic bacterial infection or other infection deemed clinically significant by the investigator, or requiring intravenous antibiotic therapy within 28 days prior to the first dose.
9. Positive for hepatitis C virus (HCV) antibody at screening (except for those with negative HCV RNA); positive for human immunodeficiency virus (HIV) antibody; positive for anti-Treponema pallidum antibody (TP-Ab) (except for those with negative RPR or TRUST); or positive for hepatitis B virus (HBV) surface antigen (HBsAg) and/or HBV core antibody (HBcAb) with HBV-DNA above the upper limit of detection.
10. History of Neisseria meningitidis infection or unresolved meningococcal disease within 6 months prior to screening up to first dose.
11. Body temperature ≥38°C within 7 days prior to first treatment.
12. Use of intravenous immunoglobulin (IVIg) within 4 weeks prior to first treatment.
13. Use of plasma exchange or immunoadsorption within 4 weeks prior to first treatment.
14. Prior use of complement inhibitors (e.g., eculizumab, ravulizumab, crovalimab, or other complement inhibitors).
15. Use of telitacicept or other B-cell stimulating factor inhibitors within 3 months or 5 half-lives prior to screening (whichever is longer); use of rituximab, ocrelizumab, or other B-cell depletion therapies within 6 months prior to screening.
16. Use of human neonatal Fc receptor (FcRn) inhibitors within a period less than 5 half-lives prior to the first dose of study drug.
17. History of suicide attempt within the past 12 months, or suicidal ideation or behavior at screening, as assessed by the Columbia-Suicide Severity Rating Scale (C-SSRS).
18. Participation in any other investigational drug study or exposure to other investigational drugs, devices, or procedures within 30 days prior to screening (investational drugs for complement inhibitors, B-cell depletion therapies, and FcRn inhibitors are governed by exclusion criteria 12, 13, and 14, respectively).
19. Suspected allergy to any excipient of the product.
20. Any medical condition that, in the opinion of the investigator, may interfere with the participant's participation in the study, pose any additional risk to the participant, or interfere with the assessment of the participant.
