Background: Despite significant advances in therapy for acute myeloid leukemia (AML), 30-40% of young patients will relapse, after which prognosis is poor. In young patients, curative-intent salvage therapy involves intensive re-induction followed by hematopoietic stem cell transplant. Recently, the COG Phase II study of CPX-351 (liposomal cytarabine:daunorubicin, Vyxeos™) in pediatric patients with AML in first relapse (NCT02642965) demonstrated a CR/CRi rate of 81.3%. Separately, our first-in-pediatrics CPX-351 Phase I (NCT01943682) showed 48% in a heavily pre-treated pediatric cohort with multiply relapsed and refractory (R/R) AML. Our integrated pilot study of single cell RNA sequencing (scRNA-seq) done before, during, and after CPX-351 showed p53 targets over time with enrichment for genes regulating apoptosis (ex.: FAS, BAX), suggesting blasts may be primed for apoptosis following CPX-351. Venetoclax is a small molecule inhibitor of the anti-apoptotic protein BCL-2, a regulator of apoptotic balance in some leukemias. Based on our preclinical data, we developed a Phase I study to investigate venetoclax with CPX-351 for the treatment of young patients with R/R acute leukemias. Methods: The V2 Trial (NCT03826992) is a single-institution Phase I study to evaluate the safety and tolerability of venetoclax with CPX-351 in patients ages 1-39 years with R/R acute leukemias. Inclusion diagnoses include AML, mixed phenotype acute leukemia (MPAL), KMT2A-rearranged acute lymphoblastic leukemia (ALL), and T-ALL. Exclusion criteria include CNS status 3, bone marrow failure syndromes, and prior cardiotoxic exposures above acceptable risk thresholds. Subjects receive a single course of CPX-351 at the FDA approved adult dose on Days 1, 3, 5 with concurrent daily venetoclax. In the dose exploration phase, venetoclax dosing is 400 mg daily (or allometrically-scaled equivalent) for 21 (Dose Level 0) or 14 days (Dose Level -1) using a rolling 6 design. Primary endpoints are determination of the recommended phase 2 dose of venetoclax in combination with CPX-351 and description of toxicities. Secondary endpoints include estimations of CR/CR_p/CR_i +/- MRD negativity in the context of a phase I study and evaluation of therapy-related cardiac dysfunction. Correlative studies include analysis of venetoclax pharmacokinetics with concomitant CPX-351. At the initial dose level, DLT were encountered and the study is now continuing enrollment at Dose Level -1. Clinical trial information: NCT03826992