This is an exploratory therapeutic study (according to the terminology of the "ICH Harmonised Tripartite Guideline Topic E8. General Considerations for Clinical Trials". EMEA, March 1998. CPMP/ICH/291/95), open-label, non-randomized, multicenter study. It is considered phase I/II since the safety and efficacy of the infusion of allogeneic haploidentical NK cells in combination with chemotherapy and/or radiotherapy in the treatment of pediatric, adolescent and young adult patients with refractory sarcoma will be sought.

Metastatic or relapsed sarcoma in children, adolescents and young adults has a 5-year survival rate of less than 20% and current therapies, consisting of radical surgery and neoadjuvant chemotherapy, are ineffective and new therapeutic strategies are needed.

Natural Killer (NK) cell therapy is a new immunotherapy in development for cancer treatment. We propose a phase I/II clinical trial with the aim of determining the safety and efficacy of infusion of activated and expanded NK cells (NKAE) from a haploidentical donor in children suffering from sarcoma refractory to conventional therapy. The study will be carried out at Hospital Universitario La Paz (Madrid), Hospital Universitario Virgen de la Arrixaca (Murcia) and Hospital Universitario Cruces (Bilbao).

The results of this study are expected to have a direct influence on the approach to metastasis and refractoriness in pediatric solid tumors in future medical therapies.

It is expected to recruit 10 patients who, having received conventional treatment and/or salvage, continue to present metastatic disease or progression. Each patient will receive:

1. One cycle of lymphoablative chemotherapy: cyclophosphamide 60 mg/kg iv on day -6 and fludarabine 25 mg/m2/day iv on days -5 to -1.
2. Irradiation (2Gy) localized to the tumor and/or its metastases, ideally within 48h prior to NK cell infusion.
3. NK cells: Two infusions of cells separated by at least 4 days are established. The first infusion, day 0, will infuse up to 5x107/kg cells with NK and NKT immunophenotype (CD56+CD3- and CD3+). The first infusion will ideally be performed 24-48 h after completion of immunoablative chemotherapy. The second infusion, starting at day +4, up to 5x108/kg, provided that there has been no toxicity attributable to the infusion of the cell product in the previous cycle. In no case will more than 1x107/kg immunophenotype T cells (CD56-CD3+) be infused.
4. Cytokine IL-2: Starting on day -1 IL-2 will be administered at a dose of 1x106 IU/m2 subcutaneously every 48 hours, a total of 6 doses.

Inclusion Criteria:

1. Patients aged between 0 and 30 years diagnosed with malignant sarcoma, who at the end of conventional treatment still have detectable residual disease (based on imaging techniques) or in case of stable disease or minimal disease when there is an absence of clinical benefit from chemotherapy (poor tolerance due to adverse effects related to chemotherapy).
2. Lansky/Karnofsky index > 60%.
3. Mild-moderate (<4) organ functional impairment (hepatic, renal, respiratory), according to National Cancer Institute criteria (NCI CTCAE v5.0).
4. Left ventricular ejection fraction >39%.
5. Adult subjects who voluntarily signed informed consent prior to the first study intervention.
6. Minor subjects whose representative/legal guardian voluntarily signed the informed consent prior to the first intervention of the study.
7. In the case of mature minors (12 - 17 years of age), in addition to the consent signed by the legal guardian, the minor's assent will be obtained.
8. Women of childbearing capacity must have a negative pregnancy test at the time of inclusion and must agree to use highly effective contraceptive methods (diaphragms plus spermicide or male condom plus spermicide, oral contraceptive combined with a second method of contraceptive implant, injectable contraceptive, permanent intrauterine device, sexual abstinence or partner with vasectomy) during their participation in the study and within 30 days of the last visit.
9. Presence of a compatible haploidentical donor (father or mother or sibling).

Exclusion Criteria:

1. Patients with a history of poor therapeutic compliance.

2. Patients who, after a psycho-social evaluation, are censored as unsuitable for the procedure.

   • Socio-familial situation that makes proper participation in the study impossible.
   • Patients with emotional or psychological problems secondary to the disease, such as post-traumatic stress disorder, phobias, delirium, psychosis, requiring specialist support.
   • Evaluation of the involvement of family members in the patient's health.
   • Impossibility to understand information about the trial.

3. Severe functional organ impairment (hepatic, renal, respiratory) (4), according to the criteria of the National Cancer Institute (NCI CTCAE 5.0).

4. Contraindications, interactions, precautions for use and dose reductions indicated in the corresponding data sheets must be considered.

5. Subjects who have been administered other investigational drugs in the 90 days prior to inclusion.

There is no masking in this study