Inclusion Criteria:

          -  An established and confirmed diagnosis of AML by World Health Organization criteria,
             excluding acute promyelocytic leukemia (APL) (with promyelocytic leukemia -retinoic
             acid receptor alpha [PML-RARA])

          -  Patients with R/R AML, defined as:

               -  Relapsed: >= 5% bone marrow blasts by morphology, reappearance of peripheral
                  blood blasts, or development of extramedullary leukemia after achieving prior CR
                  or CRi. First or second relapse is eligible. First relapse is restricted to
                  participants with CR 1 duration of less than 9-12 months

               -  Refractory: no CR or CRi after two cycles of induction. Induction cycles include
                  regimens with the intent to achieve remission and can include high intensity
                  and/or low intensity regimens

          -  Eastern Cooperative Oncology Group (ECOG) performance status =< 1 (or Karnofsky >=
             60%)

          -  Serum bilirubin =< 1.5 institutional upper limit of normal (ULN) (For patients with
             hemolysis, Gilbert's syndrome or liver infiltration with leukemia, serum bilirubin =<
             3 x institutional ULN)

          -  Aspartate aminotransferase (AST)(serum glutamic-oxaloacetic transaminase
             [SGOT])/alanine aminotransferase (ALT)(serum glutamate pyruvate transaminase [SGPT])
             =< 3 x institutional ULN (For patients with liver infiltration with leukemia,
             AST[SGOT]/ALT[SGPT] =< 5 x institutional ULN)

          -  Glomerular filtration rate (GFR) >= 60 mL/min/1.73 m^2 using the Chronic Kidney
             Disease Epidemiology Collaboration (CKD-EPI) formula

          -  Patients must be medically eligible to receive MEC, including acceptable pre-study
             cardiac function (left ventricular ejection fraction of >= 45%) and lifetime
             anthracycline exposure (=< 360 mg/m^2 daunorubicin equivalents)

          -  Patients may have had prior allogeneic hematopoietic cell transplant at least 3 months
             prior to enrollment but should not have evidence of active graft versus host disease
             or require systemic immune suppression

          -  Patients must be willing to submit the blood sampling and bone marrow sampling for any
             mandatory PK and pharmacodynamics analyses and exploratory biomarkers

          -  Female patients with child bearing potential must have a negative serum pregnancy test
             within 72 hours prior to the first study drug administration and all patients must be
             willing to use effective methods of contraception during the treatment period and 3
             months after study completion

          -  Human immunodeficiency virus (HIV)-infected patients will be eligible for this trial
             if they are on effective antiretroviral regimens utilizing non-CYP-interacting agents,
             they have an undetectable viral load, they have a CD4 count > 350 cells/mm^3, and they
             have no history of acquired immune deficiency syndrome (AIDS)-defining opportunistic
             infections. If there is evidence of chronic hepatitis B virus (HBV) infection, HBV
             viral load must be undetectable on suppressive therapy, if indicated. If there is
             history of hepatitis C virus (HCV) infection, the patient must have been treated and
             have undetectable HCV viral load

          -  Patients with a prior or concurrent malignancy whose natural history or treatment does
             not have the potential to interfere with the safety or efficacy assessment of the
             investigational regimen are eligible for this trial

          -  Ability to understand and the willingness to sign a written informed consent document.
             Participants with impaired decision-making capacity (IDMC) who have a
             legally-authorized representative (LAR) and/or family member available will also be
             eligible

          -  All non-hematologic adverse events (AEs) of prior chemotherapy, surgery, or
             radiotherapy, except alopecia, must have resolved to National Cancer Institute (NCI)
             Common Terminology Criteria for Adverse Events (CTCAE) grade =< 1 prior to starting
             therapy

        Exclusion Criteria:

          -  Patients must not have had prior treatment with MEC

          -  Patients must not have documented active central nervous system (CNS) involvement by
             leukemia. Patients with known brain metastases should be excluded from this clinical
             trial because of their poor prognosis and because they often develop progressive
             neurologic dysfunction that would confound the evaluation of neurologic and other
             adverse events (AEs)

          -  Patients must not have received any other investigational or commercial agents or
             therapies administered with the intention to treat their leukemia within 14 days or 5
             elimination half-lives (whichever is shorter) of first receipt of study drug, with the
             exception of hydroxyurea and/or leukapheresis used to control white blood cell counts

          -  Patients who cannot discontinue concomitant medications or herbal supplements that are
             strong inhibitors or strong inducers of cytochrome P450 (CYP) isoenzymes CYP3A4/5,
             CYP2C9 and CYP2C19 during treatment with M3814. Concomitant use of CYP1A2, CYP2B6 and
             CYP3A4/5 substrates with a narrow therapeutic index are also excluded during treatment
             with M3814. Patients may confer with the study doctor to determine if alternative
             medications can be used. The following categories of medications and herbal
             supplements must be discontinued for at least the specified period of time prior to
             the first dose of M3814:

               -  Strong inducers of CYP3A4/5, CYP2C9 and CYP2C19: >= 3 weeks or 5 elimination
                  half-lives (whichever is shorter) prior to the first dose of M3814

               -  Strong inhibitors of CYP3A4/5, CYP2C9 and CYP2C19: >= 1 week or 5 elimination
                  half-lives (whichever is shorter) prior to the first dose of M3814

               -  Substrates of CYP1A2, CYP2B6 and CYP3A4/5 with a narrow therapeutic index: >= 1
                  day prior to the first dose of M3814

          -  Patients who cannot discontinue concomitant proton-pump inhibitors (PPIs). Patients
             may confer with the study doctor to determine if such medications can be discontinued.
             These must be discontinued >= 5 days or 5 elimination half-lives (whichever is
             shorter) prior to the first dose of M3814. Patients do not need to discontinue calcium
             carbonate or H2 blockers

          -  Patients receiving sorivudine or any chemically related analogues (such as brivudine)
             are excluded

          -  Patients who require oral or parenteral anticoagulants or thrombolytic agents for
             therapeutic purposes (including coumadin and warfarin), or who received such agents
             within 5 days of the first dose of M3814. Low and high-molecular weight heparins are
             permitted provided the platelets are maintained at greater than 30,000/mm^3

          -  Patients with ongoing active infection or who have received a live attenuated vaccine
             within 30 days of dosing with M3814

          -  Patients must not have known significant cardiopulmonary disease defined as:

               -  Unstable angina;

               -  Congestive heart failure (New York Heart Association [NYHA] class III or IV;

               -  Myocardial infarction (MI) within 6 months prior to first dose. Patients who had
                  ischemic heart disease such as acute coronary syndrome (ACS), MI, and/or
                  revascularization more than 6 months before screening and who are without cardiac
                  symptoms or those with a prior non-ST elevation MI (NSTEM) due to demand-supply
                  mismatch (NSTEM Type II) may enroll

          -  Patients should not have severe and/or uncontrolled medical conditions or other
             conditions that, in the opinion of the investigator, could affect their participation
             in the study

          -  Patients with psychiatric illness/social situations that would limit compliance with
             study requirements

          -  Patients should not be pregnant or breastfeeding

          -  A marked baseline prolongation of QT/QTc interval (e.g., repeated demonstration of a
             QTc interval > 480 milliseconds (ms) (CTCAE grade 1) using Fredericia's QT correction
             formula

          -  A history of additional risk factors for TdP (e.g., heart failure, hypokalemia, family
             history of Long QT syndrome)

          -  The use of concomitant medications that prolong the QT/corrected QT (QTc) interval

          -  Gastrointestinal disorders that may affect the M3814 absorption

          -  Patients will need to avoid any intake of grapefruit, grapefruit juice, Seville
             oranges, Seville orange marmalade, or other products containing grapefruit or Seville
             oranges within 7 days before the first administration of M3814 and throughout the
             duration of M3814 treatment