Background: Tumor-infiltrating lymphocyte (TIL) cell therapy has shown promising efficacy in patients with solid tumor malignancies. Interleukin-2 (IL-2) administration after TIL infusion supports persistence and survival of infused TIL. Aldesleukin is a synthetic IL-2 with a short half-life administered every 8–12 hours, to support the expansion and persistence of the TIL in vivo. IOV-3001 is a modified dimeric IL-2 fused to palivizumab that has a longer half-life and potentially better safety profile, compared with aldesleukin. Here, we describe the preclinical activity, pharmacokinetics (PK), pharmacodynamics (PD), and in vivo safety profile of IOV-3001. Methods: Binding of IOV-3001 and aldesleukin to the IL-2 receptor (IL-2R), IL-2R-mediated activation via STAT5 phosphorylation, and T cell proliferation were assessed. Pharmacological activities of IOV-3001 and aldesleukin were evaluated in B16-F10 mice infused with pmel-1 T cells. Safety after a single dose of IOV-3001 (0.01–9.0 mg/kg) was assessed in cynomolgus monkeys across 3 independent studies. PK and PD parameters, clinical pathology, hematology, and histopathology were assessed. Results: Both IOV-3001 and aldesleukin induced comparable STAT5 phosphorylation and proliferation of T-cell subsets. Mice infused with pmel-1 T cells and subsequently treated with IOV-3001 or aldesleukin showed similar reductions in tumor burden. The half-life of IOV-3001 in cynomolgus monkeys ranged from 5–8 hours. IOV-3001 was well tolerated in monkeys across the dose range studied except for 1 animal administered IOV-3001 at the highest tested dose level, with recovery by Day 29. Inflammatory cytokines (IL-12 p40, IL-6, MCP-1) increased from 4 hours to ≤3 days after dosing and returned to baseline by Day 29. Fibrinogen, bilirubin, and triglycerides increased on Day 3 and returned to baseline by Day 29. No signs of capillary leak were observed. Increased numbers of hematopoietic cells were found in the bone marrow, spleen, and lymph nodes 3 days post dosing. During the 4-week recovery, these changes diminished or resolved, while bone marrow showed differential dose-dependent effects. IL-2–induced proinflammatory PD effects were observed, including >10-fold peak CD8+ T cell expansion in peripheral blood mononuclear cells at Day 5 versus preacclimation in most cynomolgus monkeys treated with IOV-3001. Conclusions: IOV-3001 exhibited a similar mechanism of action (MoA) to that of aldesleukin in vitro and has a longer half-life in vivo. PD effects were consistent with the MoA of IL-2. IOV-3001 showed a favorable preclinical safety profile. These results suggest a potentially improved safety profile for IOV-3001 with less frequent dosing compared with aldesleukin. These features of IOV-3001 strongly advocate for its development in TIL cell therapy regimens.