Inclusion Criteria:

          1. 1. Willing and able to provide written informed consent for the trial.

          2. Patients 18 years of age or older, with relapsed/refractory AML by World Health
             Organization classification, T-cell leukemia (T-ALL), bi-lineal leukemia (BLL), or
             mixed phenotypic acute leukemia (MPAL) and who have exhausted available therapy.

          3. ECOG Performance Status 0 to 2.

          4. 12-lead ECG with no clinically unacceptable findings; adequate cardiac function/NYHA
             Class 0 to 2.

          5. Adequate hepatic function (unless deemed to be related to underlying leukemia).

               1. Direct bilirubin ≤ 2 x ULN

               2. ALT ≤ 3 x ULN

               3. AST ≤ 3 x ULN

          6. Adequate renal function as documented by creatinine clearance ≥ 50 mL/min based on the
             Cockcroft-Gault equation.

          7. In the absence of rapidly proliferative disease, the interval from prior
             leukemia-directed therapy to first dose of study drug will be at least 7 days for
             cytotoxic or non-cytotoxic (immunotherapy) agents. Use of supportive care measures per
             institution's standard of care is permitted at any time.

          8. The effects of brequinar on the developing human fetus are unknown. For this reason,
             women of child-bearing potential and men must agree to use adequate contraception
             (hormonal or barrier method of birth control; abstinence) prior to study entry and for
             the duration of study participation. Should a woman become pregnant or suspect she is
             pregnant while she or her partner is participating in this study, she should inform
             her treating physician immediately. Men treated or enrolled on this protocol must also
             agree to use adequate contraception prior to the study, for the duration of study
             participation, and for 90 days after completion of brequinar administration.

          9. Male subjects must agree to refrain from sperm donation from initial study drug
             administration until 90 days after the last dose of study drug.

        Exclusion Criteria:

          1. Patients in need of immediate leukapheresis.

          2. Any concurrent uncontrolled clinically significant medical condition, laboratory
             abnormality, or psychiatric illness that could place the participant at unacceptable
             risk of study treatment.

          3. QTc interval using Fridericia's formula (QTcF) ≥ 470 msec. Participants with a bundle
             branch block and prolonged QTc interval may be eligible after discussion with the
             medical monitor.

          4. Pre-existing liver disease.

          5. The use of other chemotherapeutic agents or anti-leukemic agents is not permitted
             during study with the following exceptions:

             a. Intrathecal chemotherapy for prophylactic use or maintenance of controlled CNS
             leukemia.

          6. Presence of graft versus host disease (GVHD) which requires an equivalent dose of ≥
             0.5 mg/kg/day of prednisone or therapy beyond systemic corticosteroids (e.g.
             cyclosporine or other calcineurin inhibitors or other immunosuppressive agents used
             for GVHD).

          7. Active cerebrospinal involvement of AML, T-cell leukemia (T-ALL), bi-lineal leukemia
             (BLL), or mixed phenotypic acute leukemia (MPAL).

          8. Diagnosis of acute promyelocytic leukemia (APL)

          9. Clinically active hepatitis B (HBV) or hepatitis C (HCV) infection.

         10. Severe gastrointestinal or metabolic condition that could interfere with the
             absorption of oral study medication.

         11. Prior malignancy, unless it has not been active or has remained stable for at least 2
             years. Participants with treated non-melanoma skin cancer, in situ carcinoma or
             cervical intraepithelial neoplasia, regardless of the disease-free duration, are
             eligible if definitive treatment for the condition has been completed. Participants
             with organ-confined prostate cancer with no evidence of recurrent or progressive
             disease are eligible if at the active surveillance stage, hormonal therapy has been
             initiated, or the malignancy has been surgically removed or treated with definitive
             radiotherapy.

         12. Nursing women or women of childbearing potential (WOCBP) with a positive pregnancy
             test.

         13. Documented hemoglobinopathy.