Background: The acute and late effects of acute myeloid leukemia (AML) therapy in children can be devastating, and relapse rates remain high. Targeted therapies are needed to decrease toxicity and improve cure rates. Most AML patients express CD33 on their leukemic blasts. CD33 is a viable target with CD33 targeted therapies approved for AML. CD33 targeted CAR T cell therapies are being developed, but safety and efficacy remain a challenge. Safety concerns include cytokine release syndrome, neurotoxicity and aplasia. Efficacy is limited, with antigen escape and T cell exhaustion proposed as mechanisms of treatment failure. Dimerizing Agent Regulated ImmunoReceptor Complex (DARIC) is a next-generation technology that utilizes a split receptor design separating the antigen binding and intracellular signaling subunits (Leung, 2019). These subunits remain inactive until the administration of rapamycin which allows for dimerization of the subunits, enabling T cell activation when antigen is present. Modulating CAR T cell activation may aid hematopoietic recovery, enhance CAR T cell function and persistence, and mitigate toxicity. We developed a DARIC T cell product, SC-DARIC33, that couples the DARIC technology with a lentiviral construct that incorporates the use of a novel humanized camelid nanobody for targeting the C2 splice isoform within the membrane proximal domain of CD33. Methods: Up to 18 patients will be enrolled in this phase 1 study (NCT05105152). Patients must have early first relapse of AML (≤ 6 months from diagnosis), first relapse refractory to at least one reinduction attempt, second or greater relapse, or refractory de novo AML. The first 3 patients must be ≥18 years, all patients who meet eligibility criteria must be ≤28 years. The primary objectives of this Phase I study (NCT05105152) are assessment of safety/toxicity and feasibility of manufacturing of SC-DARIC33. Secondary objectives include assessment of efficacy, on/off rate of DARIC expression, engraftment, expansion and persistence of SC-DARIC33. Also, the effect of cessation of rapamycin on activation state of SC-DARIC 33 and on hematopoietic recovery will be evaluated. Subjects receive lymphodepletion prior to a single dose of SC-DARIC33 T cells on Day 0 followed by initiation of rapamycin on Day +2 with the purpose of activating the T cells. Rapamycin continues until Day 21 and bone marrow evaluation occurs on Day 28. Subsequent cycles of rapamycin are allowed for patients who respond. The trial will implement a BOIN design with three possible dose levels (1 x 10⁶, 5 x 10⁶, and 10 x 10⁶ DARIC⁺ cells/kg) to determine the maximum tolerated dose (MTD). The trial is open for enrollment at Seattle Children’s Hospital. Clinical trial information: NCT05105152.