Phase I / II Study on Infusion of Alloreactive or Ex Vivo il-15 Stimulated Natural Killer Cells after Haploidentical Stem Cell Transplantation in Pediatric Patients with Acute Leukemia (PHINK): A Study of the Spanish Hematopoietic Stem Cell Transplantation Group (GETH)

Background: Haploidentical hematopoietic stem cell transplantation (haplo-HSCT) is a highly complicated but effective procedure for high-risk pediatric leukemia in the absence of an HLA-identical donor. Post-transplant relapse is the main problem for patient survival. According to literature, Natural Killer (NK) cells play a potent role as inducers of the graft-versus-leukemia (GvL) effect in haplo-HSCT. The presence of alloreactive NK cells correlates with a lower relapse rate, improved engraftment, decreasing graft-versus-recipient disease (GvHD) and viral infection risk. In the absence of donor-recipient NK cell alloreactivity, other NK cell activation strategies are necessary. We propose a phase I/II clinical trial with dose escalation, multicenter, framed in the GETH, to determine dose-limiting toxicity (DLT) and the maximum tolerated dose (MTD) of a single infusion of alloreactive NK cells (Allo-NK) or NK cells stimulated ex vivo with IL-15 (IL15-NK) after haplo-HSCT in pediatric patients with high-risk leukemias. Methods: Eighteen pediatric patients with high-risk leukemia were recruited between September 16, 2020, and May 12, 2023, at 5 hospitals in Spain, Hospital Universitario La Paz, Hospital Universitario Gregorio Marañón, Hospital Universitario Niño Jesús, Hospital Universitario Central of Asturias and Hospital Santa Creu and Sant Pau. Patients were stratified in 2 treatment groups: Allo-NK (n=9) or IL15-NK (n=9), depending on the KIR disparity between donor and recipient. Our phase I/II clinical trial presents a dose-escalation design (3+3). The treatment cohorts are divided into cohort 1 (up to 2x10 7 cells/kg), cohort 2 (2-5x10 7 cells/kg) and cohort 3 (5-10x10 7 cells/kg). We have monitored infections reactivation, as well as, chimerism and engraftment after transplantation. In Allo-NK group, the KIR clone that provides alloreactivity in the NK cell was also tracked. The phenotypic profile and functional capacity of NK cells present in the patient up to 360 days after transplantation have also been analyzed. Results: Nine patients (4 in Allo-NK and 5 in IL-15NK) were included in cohort 1, 7 patients (4 in Allo-NK and 3 in IL-15NK) in cohort 2 and 2 patients (1 in Allo-NK and 1 in IL-15NK) in cohort 3. We were unable to complete the dose escalation design initially proposed due to the high weight of some patients and the unavailability of ex vivo expanded NK cells. In IL15-NK group patients presented a higher prevalence of B-ALL (p=0.018) a