Background: Interleukin-12 (IL12) is a cytokine that induces antitumor immune response and immune memory by activation of NK cells, induction of IFNg, and maturation of DC cells in the tumor microenvironment (TME). However, prior clinical experience with IL12 has been limited by toxicities including cytokine release syndrome (CRS) and hepatotoxicity. We developed a novel tumor-targeted IL12 gene (ttIL12) that encodes the p35 and p40 VNTANST fusion subunits that targets cell surface vimentin (CSV). Vimentin is expressed intracellularly in many normal and neoplastic mesenchymal cells but our group discovered that intracellular vimentin is often flipped to the cell surface (referred to as CSV) across multiple tumor types.2 Preclinical studies demonstrated efficacy and induction of long-term memory of ttIL12 against metastatic osteosarcoma with reduced toxicity in immune competent models.3 However, ttIL12 had limited efficacy in treating large volume tumors and did not completely eliminate toxic peripheral cytokines. To address this, our team generated a membrane anchored ttIL12 to arm T cells known as attIL12-T cell therapy. We hypothesize attIL12 will minimize toxic cytokines in peripheral tissues but promote induction of INFg/TNFa in the TME to decrease the toxicity and boost efficacy. Methods: This is a phase 1 trial of attIL12-T cells in patients (pts) with locally advanced or metastatic soft tissue or bone sarcomas to assess safety, maximal tolerated dose (MTD) and recommended phase 2 dose (RP2D). Pts in part A (dose finding) will be enrolled using a Bayesian optimal interval (BOIN) design in up to 6 dose levels. Part B (dose expansion) will evaluate preliminary efficacy in 10 additional pts with osteosarcoma treated at the RP2D. Treatment:Pts will undergo leukapheresis to collect peripheral blood mononuclear cells (PBMCs) for generation of attIl12-T cells. Bridging therapy is permitted but not required. Pts will receive cyclophosphamide (Cy) IV prior to planned T cell administration. attIL12 T cells will be administered at the specified dose level IV in a single infusion (dose level 1 and 2) or 2 serial doses (dose level 3-6) on day 0 and day 14. Pts will be monitored for toxicity and response evaluation. Pts will also undergo pre-treatment/on-treatment tumor biopsies for translational and correlative analysis. Eligibility:Pts must be ≥ 12 years of age, have a confirmed diagnosis of locally advanced or metastatic sarcoma, evaluable disease, received at least 1 prior line of standard systemic therapy, ECOG PS 0-1, and adequate organ function to tolerate immune-effector cell therapy. Pts with known sensitivity to Cy, active/prior autoimmune disease, uncontrolled CNS disease, or other uncontrolled comorbidities that pose potential safety risk will be excluded. Enrollment to cohort 1 began in Dec 2023. Clinical trial information: NCT05621668.