This study will evaluate the safety and efficacy of NK520 in the treatment of pediatric 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 pediatric 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 6 and 18 years;
2. Diagnostic Criteria: Meet the 2022 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: a. 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; b. 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. For participants under 16 years old, Lansky performance status must be ≥50%; for participants aged 16 or older, Karnofsky performance status must be ≥50%;
4. Expected survival of at least 12 weeks;
5. Normal Organ Function.

Exclusion Criteria:

1. Acute promyelocytic leukemia, chronic myeloid leukemia, acute mixed lineage leukemia, or known central nervous system leukemia;

2. AML associated with congenital syndromes, such as Down syndrome, Fanconi anemia, Bloom syndrome, Kostmann syndrome, or congenital aplastic anemia;

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

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

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

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

7. Receipt of immune-modulatory drugs (including thymosin, interferons, except for local use to manage conditions like pleural or ascites fluid) within 2 weeks before the first dose;

8. At screening, positive hepatitis B or C viral markers as follows:

   • HBsAg positive with serum HBV-DNA level ≥1×10^3 copies/mL or above normal range;
   • Positive for HCV antibodies;

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