This study will evaluate the safety and efficacy of NK520 in the treatment of relapsed/refractory acute myeloid leukemia. NK520 will be administered by intravenous injection. The safety and efficacy of this treatment will be evaluated.

This open label, single-arm study aims to evaluate the efficacy and safety of allogenic NK cells in subjects with relapsed/refractory acute myeloid leukemia. Allogenic NK cells will be infused once a week. After infusion, the investigators will observe the characteristics of dose limited toxicity (DLT), and determine the maximum tolerable dose(MTD). To provide basis for the dosage and treatment plan of cell products in follow-up clinical trials.

Inclusion Criteria:

1. Participants must be between 18 and 75 years;

2. Diagnostic Criteria:

   Meet the 2016 World Health Organization (WHO) diagnostic criteria for AML, unsuitable for current treatments or patients with relapsed/refractory AML after ≥2 lines of therapy. The definition of relapsed/refractory acute myeloid leukemia is based on the 2017 Chinese Guidelines for Diagnosis and Treatment:

   1. Relapsed AML: Diagnosis is confirmed when leukemia cells reappear in the peripheral blood or bone marrow blast cells exceed 5% after complete remission (CR) (excluding reasons such as bone marrow regeneration post-consolidation chemotherapy) or there is extramedullary infiltration by leukemia cells;
   2. Refractory AML: Initial cases unresponsive after two cycles of standard regimen treatment; recurrence within 12 months after CR and consolidation therapy; recurrence beyond 12 months with ineffectiveness of conventional chemotherapy; those who have relapsed twice or more; or persistent extramedullary leukemia;

3. Eastern Cooperative Oncology Group (ECOG) performance status score of 0 to 2;

4. Expected survival of at least 12 weeks;

5. Normal Organ Function.

Exclusion Criteria:

1. Acute promyelocytic leukemia;

2. Severe bleeding tendency or coagulation disorders, or currently receiving thrombolytic therapy;

3. Active tuberculosis (TB), currently undergoing anti-TB treatment, or treated for TB within 1 year prior to the study;

4. HIV-infected individuals, or known active syphilis infection;

5. Use of immunosuppressive drugs within 1 week before the first dose, excluding topical, inhaled, or other locally administered glucocorticoids, or physiologic doses of systemic glucocorticoids (not exceeding 10 mg/day prednisone equivalent) for allergic reactions or for managing respiratory distress from asthma, COPD, etc;

6. Receipt of live attenuated vaccines within 2 weeks before the first dose or planned during the study;

7. Participation in another clinical trial and receipt of investigational drug within 4 weeks prior to the first dose;

8. Receipt of immune-modulating drugs (including thymosin, interferons, except for local use to control pleural or ascitic fluid) within 2 weeks prior to the first dose;

9. At screening, hepatitis B or C viral tests positive according to either:

   • HBsAg positive with serum HBV-DNA titer ≥1×10^3 copies/mL or above normal limits;
   • HCV antibody positive;

10. Any other condition or situation in which the investigator deems the patient unsuitable for participation in this study.