Summary

Quizartinib is an experimental drug. It is not approved for regular use. It can only be
used in medical research.

Children or young adults with a certain kind of blood cancer (FLT3-ITD AML) might be able
to join this study if it has come back after remission or is not responding to treatment.

Objectives

The medical condition being investigated is relapsed or refractory AML in participants
aged =1 month to =21 years with Feline McDonough Sarcoma (FMS)-like tyrosine kinase 3
(FLT3)-internal tandem duplication (ITD) mutations (FLT3-ITD AML), following failure of
front-line intensive chemotherapy.

The trial will be conducted in multiple phases. An independent data monitoring committee
(DMC) will protect the rights, safety, and well-being of participants by monitoring the
progress and results. The DMC will comprise qualified physicians and scientists who are
not Investigators in the study and not otherwise directly associated with the Sponsor and
will be convened at the end of Phase 1.

A. Dose Escalation/De-escalation Phase:

Number of participants is determined by age group. Participants will be enrolled by
dose-level to determine the recommended Phase 2 dose (RP2D) of quizartinib for pediatric
participants that provides similar exposure to adult patients treated at the target adult
dose of 60 mg orally once daily.

B. Dose-Expansion Phase:

Participants will receive the RP2D of quizartinib for their respective age group.

During both dose escalation and dose expansion phases, participants will receive:

Re-Induction Therapy

- Intrathecal (IT) triple chemotherapy prophylaxis prior to and between cycles

- In re-induction Cycles 1 and 2, fludarabine/cytarabine (FLA) followed by quizartinib
as a single agent

Allogeneic Hematopoietic Stem Cell Transplantation (HSCT) Period:

After re-induction therapy, participants will be evaluated for eligibility to undergo
allogeneic hematopoietic stem cell transplant (HSCT). Eligible participants may receive a
single 28-day cycle of consolidation therapy (standard of care chemotherapy with or
without quizartinib) if an allogeneic HSCT is not available immediately. The options for
consolidation therapy are as follows:

- High intensity chemotherapy with quizartinib, or

- Low intensity chemotherapy alone, or

- Low intensity therapy with quizartinib as a single agent

Continuation Therapy:

Participants in remission after HSCT, or who are not eligible for HSCT but achieve at
least a partial remission (PR) after re-induction, will receive up to 12 continuous
28-day cycles of quizartinib continuation therapy at the same dose received during
re-induction in the dose expansion phase.

Long-term Follow-up:

The long-term follow-up phase begins upon completion of 12 cycles of quizartinib
Continuation Therapy or permanent discontinuation of quizartinib at any time. After
completion of the 30-day safety follow-up visit, subsequent visits will occur at the
following frequencies to assess survival and anti-leukemic treatments:

- every 3 months for the first 2 years, and then

- once a year thereafter until the last participant enrolled has been followed for
three years from the date of enrollment

Eligibility

1. Inclusion Criteria: Participants must meet all of the following criteria to be eligible for enrollment into the study: - Has diagnosis of AML according to the World Health Organization (WHO) 2008 classification with =5% blasts in bone marrow, with or without extramedullary disease - In first relapse or refractory to first-line high-dose chemotherapy with no more than 1 attempt (1 to 2 cycles of induction chemotherapy) at remission induction - prior HSCT is permitted - Has presence of the FLT3-ITD activating mutation in bone marrow or peripheral blood as defined in the protocol - Is between 1 month and 21 years of age at the time the Informed Consent/Assent form is signed - Has protocol-defined adequate performance status score - Has fully recovered from the acute clinically significant toxicity effects of all prior chemotherapy, immunotherapy, or radiotherapy, per protocol guidelines - Has protocol-defined adequate renal, hepatic and cardiac functions - If of reproductive potential, is permanently sterile or agrees to use highly effective birth control upon enrollment, during the period of therapy, and for 6 months following the last dose of quizartinib, etoposide, fludarabine, methotrexate, or cytarabine, whichever is later - If female of child-bearing potential, tests negative for pregnancy and agrees not to breast feed - Male participants must be surgically sterile or willing to use highly effective birth control during the treatment period, and for 6 months following the last dose of quizartinib, etoposide, fludarabine, methotrexate, or cytarabine, whichever is later. - Participant/legal representative is capable of understanding the investigational nature of the study, potential risks, and benefits, and the patient (and/or legal representative) signs a written assent/informed consent Exclusion Criteria: Participants who meet any of the following criteria will be disqualified from entering the study: - Has been diagnosed with isolated central nervous system relapse, acute promyelocytic leukemia (APL), juvenile myelomonocytic leukemia, French-American-British classification M3 or WHO classification of APL with translocation, or with myeloid proliferations related to Down syndrome - Has uncontrolled or pre-defined significant cardiovascular disease as detailed in the protocol - Has systemic fungal, bacterial, viral or other infection that is exhibiting ongoing signs/symptoms related to the infection without improvement despite appropriate antibiotics or other treatment. The patient must be off vasopressors and have negative blood cultures for at least 48 hours prior to the start of systematic protocol therapy. - Has known active clinically relevant liver disease (e.g., active hepatitis B or active hepatitis C) - Has known history of human immunodeficiency virus (HIV) - Has history of hypersensitivity to any of the study medications or their excipients - Is receiving or is anticipated to receive concomitant chemotherapy, radiation, or immunotherapy other than as specified in the protocol - Has any significant concurrent disease, illness, psychiatric disorder or social issue that would compromise subject safety or compliance, interfere with consent/assent, study participation, follow up, or interpretation of study results - Is currently participating in another investigative interventional procedure (observational or long-term interventional follow-up is allowed) - Is otherwise considered inappropriate for the study by the Investigator