A Phase 1b Study of the Safety and Pharmacokinetics of Pivekimab Sunirine in Pediatric Subjects With Relapsed or Refractory Acute Myeloid Leukemia (AML)

Inclusion Criteria: * Must have histologically confirmed acute myeloid leukemia (AML) meeting one of the following disease criteria: * Second or greater relapse. OR * Disease refractory to second or subsequent line of therapy (defined as resistant disease after at least one cycle of each treatment regimen). * Must have myeloid leukemic blasts that are CD123-positive by flow cytometry as determined by the treating institution. * Has \>= 5% myeloid leukemic blasts in bone marrow at time of relapse or refractory disease and prior to Screening for this study. * Performance status by Lansky (\< 16 years old at evaluation) or Karnofsky (\>= 16 years old at evaluation) score \>= 50 or ECOG score \<= 2. * May have status of central nervous system (CNS)1, CNS2, or CNS3 disease without clinical signs or neurologic symptoms suggestive of CNS leukemia, such as facial nerve palsy, brain/eye involvement or hypothalamic syndrome. Participants receiving intrathecal therapy and no additional CNS-directed systemic therapy at study entry are eligible and may continue treatment as clinically indicated in accordance with institutional practice. * For those participants who have not reached the age of consent, parent or legal guardian with the willingness and ability to provide informed consent and participant willing and able to give assent, as appropriate for age and country. Exclusion Criteria: * Known clinically significant cardiac disease. * Down syndrome. * Acute promyelocytic leukemia (APL) or juvenile myelomonocytic leukemia (JMML). * Symptomatic central nervous system (CNS3) disease * Prior history of any severity veno-occlusive disease/sinusoidal obstructive syndrome (VOD/SOS) of the liver. * Prior history of hematopoietic stem cell transplant within 6 months prior to Screening without evidence of active GvHD at the time of screening and the participant is off medications to treat or prevent either post-transplant graft-versus-host disease (GvHD) or post-transplant rejection (except for a stable dose of corticosteroids). * Have received prior Chimeric Antigen Receptor T-cell (CAR-T) therapy. * Any other known current malignancy requiring therapy. * Currently receiving anticancer therapy with antineoplastic intent, including radiotherapy, systemic therapy small molecules, monoclonal antibodies, other investigational agents, or high-dose chemotherapy with the exception of intrathecal therapy.