Inclusion Criteria:

          -  Patients must be at least 18 years of age;

          -  Patients must have either:

               -  AML (de novo or secondary, and any WHO 2008 classification excluding acute
                  promyelocytic leukemia) that has failed to achieve CR or CRi (IWG 2003) after at
                  least 1 cycle of induction chemotherapy, or has relapsed after any duration of CR
                  or CRi; or,

               -  MDS (RAEB-1 or RAEB-2 WHO 2008 classification) that has failed to achieve any
                  hematologic improvement (IWG 2006 criteria) after at least 4 cycles of induction
                  therapy (e.g., azacitidine, decitabine), or has relapsed after any duration of CR
                  or PR.;

          -  Patient performance status must be Eastern Cooperative Oncology Group (ECOG) 0, 1 or
             2;

          -  Patients must have a life expectancy of greater than 14 days;

          -  Patients must have total bilirubin ≤ 2;

          -  Patients must have serum AST and ALT levels ≤ 2.5 times upper limit of normal;

          -  Patients must have serum creatinine less than or equal to 2.5 times upper limit of
             normal;

          -  Patients must have PT/INR and PTT in normal range ± 25%;

          -  Women of child-bearing potential (i.e., women who are pre-menopausal or not surgically
             sterile) may participate, provided they meet the following conditions:

               -  Must agree to use physician-approved contraceptive methods (e.g., abstinence,
                  intrauterine device, oral contraceptive, double barrier device) throughout the
                  study and for three months following the last dose of OXi4503; and

               -  Must have a negative serum or urine pregnancy test within 7 days prior to
                  beginning treatment on this trial;

          -  Males with female partners of child-bearing potential must agree to use
             physician-approved contraceptive methods (e.g., abstinence, condoms, vasectomy)
             throughout the study and should avoid conceiving children for 6 months following the
             last dose of OXi4503;

          -  Written informed consent, willingness, and ability to comply with all study
             procedures.

        Exclusion Criteria:

          -  Acute promyelocytic leukemia (APL) with t(15;17);

          -  Absolute peripheral blood myeloblast count greater than 25,000/mm3;

          -  Uncontrolled hypertension, defined as blood pressure 140/90 mm Hg despite maximum
             medical intervention;

          -  History of congenital long QT syndrome or torsades de pointes;

          -  Pathologic bradycardia or heart block (excluding first degree heart block);

          -  Prolonged baseline QTc, defined as QTc interval > 470 msec in women and > 450 msec in
             men;

          -  History of ventricular arrhythmia (excluding premature ventricular contractions,
             PVCs);

          -  Major operative surgery within 28 days;

          -  Unstable angina pectoris within 28 days;

          -  Myocardial infarction and/or new ST elevation or depression or new Q wave on ECG
             within 28 days;

          -  Any history of hemorrhagic stroke;

          -  Symptomatic congestive heart failure Class III or greater (New York Heart Association
             Functional Classification);

          -  On full dose anti-coagulation defined as warfarin intended to raise the INR to 2-3, or
             enoxaparin 1 mg/kg twice a day or unfractionated heparin intended to raise the PTT to
             60-90 seconds;

          -  Major hemorrhagic event within 28 days requiring transfusion of packed red blood
             cells;

          -  Prior history of hypertensive crisis or hypertensive encephalopathy;

          -  Active, uncontrolled, clinical significant infection;

          -  Any open wound;

          -  Pregnant and nursing patients are excluded because the effects of OXi4503 on a fetus
             or nursing child are unknown.

          -  Treatment with any anticancer therapy (standard or investigational) within the
             previous 21 days prior to the first dose of study drug or less than full recovery
             (CTCAE grade 1) from the clinically significant toxic effects of that treatment. The
             use of hydroxyurea may be used for two weeks after dosing in Cycle 1 (e.g., Days 1-14
             dosed with hydroxyurea).

        Relative Exclusion Criteria:

          -  Patients on concurrent medications known to prolong the QTc interval may participate
             as long as their screening QTc interval meets eligibility criteria.