A Trial of the FMS-like Tyrosine Kinase 3 (FLT3) Inhibitor Gilteritinib Administered as Maintenance Therapy Following Allogeneic Transplant for Patients With FLT3/Internal Tandem Duplication (ITD) Acute Myeloid Leukemia (AML)

Participant is considered a suitable candidate for HCT and has an acceptable source of allogeneic donor stem cells, as defined per institutional practice (allogeneic HCT for any donor source \[matched sibling, unrelated donor (URD), mismatched URD, related haploidentical, or umbilical cord blood\] and any graft source \[umbilical cord, BM, peripheral blood (PB)\], and any conditioning \[myeloablative conditioning (MAC), reduced intensity conditioning (RIC), or non-myeloablative conditioning (NMA)\] will be permitted).

Participant is considered a legal adult by local regulation at the time of signing informed consent form (ICF).

Participant consents to allow access to diagnostic BM aspirate or PB sample and/or the DNA derived from that sample, if available, that may be used to validate a companion diagnostic that is being developed in parallel with gilteritinib.

Participant has confirmed, morphologically documented AML in CR1. For the purposes of registration, CR1 will be defined as \< 5% blasts in the BM with no morphologic characteristics of acute leukemia (e.g., Auer Rods) in the BM with no evidence of extramedullary disease such as central nervous system involvement or granulocytic sarcoma.

* Participant has not received more than 2 cycles of induction chemotherapy to achieve CR1. The induction cycles can be the same regimen or different regimens. The regimen(s) may contain conventional agents, investigational agents, or a combination of both.

* Participants with CR with incomplete count recovery (CRp or CRi) are allowed. Incomplete platelet recovery (CRp) is defined as CR with platelet count \< 100 x 109/L. Incomplete blood count recovery (CRi) is defined as CR with residual neutropenia \< 1 x 109/L with or without complete platelet recovery. Red blood cell count (RBC) and platelet transfusion independence is not required.

* The maximum time allowed from establishment of CR1 to registration is 12 months.

Participant has presence of the FLT3/ITD activating mutation in the BM or PB as determined by the local institution at diagnosis.

Participant must meet the following criteria as indicated on the clinical laboratory tests:

* Serum creatinine within normal range, or if serum creatinine outside normal range, then glomerular filtration rate (GFR) \> 40 mL/min/1.73m2 as calculated with the Cockcroft-Gault equation with adjustment if total body weight is ≥ 125% of ideal body weight.

* Total bilirubin (TBL) ≤ 2.5 mg/dL, except for participants with Gilbert's syndrome.

* Serum AST and/or alanine aminotransferase (ALT) \< 3 x institutional upper limit of normal (ULN).

Participant has left ventricular ejection fraction at rest ≥ 40%.

Participant has diffusing capacity of the lung for carbon monoxide (DLCO) (corrected for hemoglobin) ≥ 50% predicted and/or forced expiratory volume in 1 second (FEV1) ≥ 50% predicted.

Female participants must either:

* Be of non-childbearing potential:

* postmenopausal (defined as at least 1 year without menses) prior to screening or

* documented as surgically sterilized (at least 1 month prior to the screening visit)

Or, if of childbearing potential,

* Agree not to try to become pregnant during the study for 6 months after the final study drug administration

* And have a negative serum pregnancy test at screening

* And, if heterosexually active, agree to consistently use highly effective contraception per locally accepted standards in addition to a barrier method starting at screening and throughout the study period and for 6 months after the final study drug administration.

* For United Kingdom sites:

* Highly effective forms of birth control include:

* Consistent and correct usage of established hormonal contraceptives that inhibit ovulation

* Established intrauterine device (IUD) or intrauterine system (IUS)

Female participants must agree not to breastfeed or donate ova throughout the study drug treatment period and for 6 months after the final study drug administration.

Male participants (even if surgically sterilized), and partners who are women of childbearing potential must be using highly effective contraception in addition to a barrier method throughout the study drug treatment period and for 127 days after the final study drug administration.

* For United Kingdom sites:

* Highly effective forms of birth control include:

* Consistent and correct usage of established hormonal contraceptives that inhibit ovulation

* Established IUD or IUS

* Vasectomy (A vasectomy is a highly effective contraception method provided the absence of sperm has been confirmed. If not, an additional highly effective method of contraception should be used.)

* Male is sterile due to a bilateral orchiectomy

Male participants must not donate sperm throughout the study drug treatment period and for 127 days after the final study drug administration.

Participant is able to take an oral medication.

Participant agrees not to participate in another interventional study while on treatment.

Participant is ≥ 30 days and ≤ 90 days from hematopoietic cell infusion.

Participant has achieved engraftment. Engraftment is defined as ANC ≥ 500 cells/μL and platelets ≥ 20000/μL on 3 consecutive measurements (each occurring at least 1 day apart). The participant must not have had a platelet transfusion within 7 days prior to the first measurement.

Participant has confirmed ongoing morphologically documented AML in CR1. For the purposes of randomization, CR1 will be defined as \< 5% blasts with no morphologic characteristics of acute leukemia (e.g., Auer Rods) in the BM with no evidence of extramedullary disease such as central nervous system involvement or granulocytic sarcoma.

Participant meets the following criteria as indicated on the clinical laboratory tests:

* Serum creatinine within normal range, or if serum creatinine outside normal range, then GFR \> 40 mL/min/1.73m2 as calculated with the Cockcroft-Gault equation with adjustment if total body weight is ≥ 125% of ideal body weight.

* TBL \< 2.5 mg/dL, except for participants with Gilbert's syndrome.

* Serum AST and/or ALT \< 3 x institutional ULN.

* Serum potassium and magnesium ≥ the institutional lower limit of normal (LLN).

If the participant has developed overall grades II-IV acute GVHD, the following criteria must be met to be randomized:

* No requirement of \> 0.5 mg/kg of prednisone (or equivalent) daily dose within 1 week of randomization

* No escalation of systemic immunosuppression in terms of increase of corticosteroids or addition of new agent / modality within 2 weeks of randomization. (Note that increasing calcineurin inhibitors or sirolimus to achieve therapeutic trough levels is allowed.) Topical skin and topical gastrointestinal steroids are allowed.

Participant is able to take oral medication.

What this study is about