NK Cell Immunotherapy to Reduce Relapse after Haploidentical Transplant for High-Risk Pediatric and Young Adult AML

Background and Significance. The prognosis for children and young adult (YA) patients with relapsed AML after allogeneic hematopoietic cell transplantation (allo-HCT) is poor. Only patients eligible for a second HCT face a modest probability of long-term survival (30%). Indeed, relapsed AML accounts for > 50% of all childhood leukemia-related deaths. Novel treatments are needed to enhance the efficacy of allo-HCT and prevent relapse. Our group has developed a novel NK cell product with functional memory-like properties and enhanced anti-leukemia function (ML NK cells). ML NK cells are generated after brief in vitro stimulation with interleukins (IL) -12, -15, and -18. We have previously demonstrated promising clinical efficacy of ML NK cells for the treatment of relapsed/refractory AML in both children and adults.

Study Design and Methods. We are now conducting a new phase 1/2 study implementing the adoptive transfer of ML NK cells following TCRαβ T cell depleted haplo-HCT for pediatric and young adult patients with high-risk AML. The absence of pharmacological immunosuppression in this transplant platform creates an ideal environment for ML NK cells to proliferate and persist long-term. We hypothesize that the synergistic activities of ML NK cells with γδ T cells and conventional NK cells will mediate potent graft versus leukemia effects without causing graft-versus host disease.

In this institutional phase 1/2 study, pediatric and YA patients with high-risk AML undergo TCRαβ T cell depleted haplo-HCT followed by infusion of donor-derived ML NK cells. This study will enroll approximately 24 patients ≤ 30 years of age with high-risk AML. Patients with high-risk genetic features or poor treatment response are eligible in first complete remission (CR1). Patients in CR2 or subsequent, and recipients of one prior allo-HCT are also eligible. All patients must be in morphological CR prior to enrollment. Additional criteria include no active CNS AML, performance status ≥ 60%, and adequate liver, renal, lung, and heart function. Patients need to have a suitable single haplotype matched (≥ 4 of 8) familial donor, who must be available and willing to undergo one mobilized and one non-mobilized leukapheresis procedure.

Patients receive either myeloablative conditioning (MAC) or reduced intensity conditioning (RIC), based on treating physician preference. In both regimens, ATG is administered from day -9 to -7 with individualized dosing based on absolute lymphocyte count and patient weight. MAC consists of busulfan from days -6 to -4 targeting an AUC of 70-95 mg x h/L, fludarabine 30 mg/m2 from days -6 to -3 and thiotepa 5 mg/kg for two doses on day -2. RIC consists of fludarabine 30 mg/m2 from days -8 to -5, thiotepa 5 mg/kg for two doses on day -4, and melphalan 70 mg/m2 on days -2 and -1.

Donors undergo a standard hematopoietic progenitor cell (HPC) mobilization regimen consisting of G-CSF. The HPC graft is collected on day 5 of G-CSF stimulation via leukapheresis and subsequently undergoes TCRαβ and CD19⁺depletion using the Miltenyi CliniMACS device. The donor undergoes a second, non-mobilized leukapheresis on day +6 for the generation of ML NK cells. Fresh ML NK cells are infused on day +7. All ML NK cells generated are infused with minimum and maximum cell doses of 0.5x10⁶/kg and 20x10⁶/kg recipient weight, respectively. IL-2 (1 million units/m²) is administered subcutaneously every other day from Day +7 to +19 (7 doses total) to support ML NK cell expansion.

The primary objective of this clinical trial is to assess the safety and feasibility of this approach. Secondary objectives include the estimation of 1-year relapse-free survival (RFS) as well as defining donor ML NK cell expansion, phenotype, leukemia killing, and cellular interactions with other anti-leukemic immune populations. These data will be used to correlate specific NK cell parameters (number, activation state, NKG2A expression, etc.) with clinical outcomes. Patients will be followed for 5 years for extended monitoring. The study opened for enrollment in April 2024 and enrollment is expected to be completed by April 2027. The trial is registered at clinicaltrials.gov (NCT06158828).