To evaluate the hematological responses based on the response assessment criteria when AMG531 is subcutaneous (SC)-administered with anti-human thymocyte immunoglobulin (ATG) + ciclosporin A (CsA) therapy for 6 months in patients with aplastic anemia (AA) who were previously untreated with immunosuppressive therapy.

Inclusion Criteria:

1. Voluntary signed informed consent to participate in the study;
2. A diagnosis of AA confirmed by blood and bone-marrow examinations, etc.;
3. Considered to require new treatment with ATG and CsA provided that NSAA must be platelet or erythrocyte transfusion-dependent.
4. An Eastern Cooperative Oncology Group (ECOG) performance status (PS) score of 0 to 1at screening

Exclusion Criteria:

1. Previously treated with ATG, CsA, or Alemtuzumab;

2. Diagnosed as having congenital AA (Fanconi anemia, congenital dyskeratosis, etc.);

3. Diagnosed as having AML or chronic myelomonocytic leukemia;

4. Concurrent thrombocytopenia of other etiologies (e.g., MDS, ITP, cirrhosis);

5. Concurrent active infection not adequately responding to appropriate therapy;

6. Concurrent clinically significant illness(es) items which are deemed by the Investigator to be likely to affect the study conduct and assessments.

7. Having active malignancies, or having a history of treatment of malignancies within 5 years prior to informed consent.

8. Concurrent PNH

9. Having Grade 2 or higher bone marrow reticulin based on Bone Marrow Pathology (2nd edition) ;

10. History of chromosome aberrations discovered in bone marrow cells.

11. Having blast cells > 2% in bone marrow;

12. Positive for anti-human immunodeficiency virus (HIV) antibody;

13. Receiving prophylactic or therapeutic treatment for hepatitis type B

14. Positive for hepatitis C virus (HCV) antibody, and HCV infection being confirmed

15. Planned hematopoietic stem cell transplantation during the study;

16. Systemic treatment with any of the following medication for the treatment of AA within 4 weeks before Day 1, however, excluding their use as premedication:

    • Anabolic steroids
    • Corticosteroids;

17. Pregnant or breastfeeding women, or women willing to become pregnant;

18. Other conditions unsuitable for participation in the study in the opinion of the Investigator.