Abstract The tumor-directed delivery of therapeutics using monoclonal antibodies specific to a tumor-associated antigen promises to accumulate large doses of the delivered payload at the tumor site while sparing healthy organs. The antibody-based delivery of interleukin-2 (IL-2) to extracellular targets expressed in the easily accessible tumor vasculature has shown promising results in animal models of solid tumors and hematological malignancies. In xenograft and immunocompetent murine models of acute myeloid leukemia (AML), IL-2-based vascular targeting antibody fusions have recently demonstrated potent anti-leukemic activity, especially when used in combination with cytarabine. Here, we report our experiences in four patients with relapsed AML after allogeneic hematopoietic stem cell transplantation (allo-HSCT), who were treated with the immunocytokine F16-IL2, consisting of a human monoclonal antibody specific to spliced large isoforms of tenascin-C fused to human IL-2, in combination with very low dose cytarabine (5 mg subcutaneously twice daily for 10 days). Clinical evidence of anti-leukemia efficacy was shown in all patients. One patient with rapidly progressing disseminated extramedullary AML lesions achieved a complete metabolic response in PET/CT, which lasted three months. Two out of three patients with bone marrow relapse achieved a blast reduction with transient molecular negativity (NPM1). One of the two enjoyed a short complete remission before AML relapse occurred two months after the first infusion of F16-IL2. The other patient did not regenerate neutrophil and thrombocyte counts and showed progressive disease after completion of the first cycle. In line with a site-directed delivery of the cytokine, F16-IL2 led to an extensive infiltration of immune effector cells (natural killer cells, CD8+ T cells, γδ T cells) in the bone marrow. Grade 2 fevers were the only non-hematological side effects in two patients. Grade 3 cytokine-release syndrome developed in the other two patients, required hospitalization, but was manageable in both cases with systemic glucocorticoids. No non-hematological grade 4 toxicities were observed. The concept of specifically targeting IL-2 to the leukemia-associated stroma using armed antibodies deserves further evaluation in clinical trials, especially in patients who relapse after allo-HSCT. Disclosures Off Label Use: In this report, the antibody-cytokine fusion protein F16-IL2 has been used in a compassionate use setting in individual patients presenting with AML relapse after allogeneic stem cell transplantation. F16-IL2 is currently being evaluated in phase I/II studies in patients with solid cancer.. Neri:Philogen SpA: Employment, Equity Ownership.