Inclusion Criteria:

- Diagnosis of acute myeloid leukemia (except AML FAB-M3 or acute promyelocytic
leukemia [APL]) based on the World Health Organization (WHO) classification of ≥ 20%
blasts in the bone marrow or peripheral blood at initial diagnosis (prior to start of
standard chemotherapy)

- ECOG performance status of 0 or 1

- Subjects with newly-diagnosed AML or subjects with relapsed AML after at least 2
chemotherapy regimens.

- Adequate liver and renal function

- Signed informed consent

Exclusion Criteria:

- Anticancer therapy including chemotherapeutic, biologic, or investigative agents
within 30 days of XL999 treatment

- Hematopoietic stem cell transplantation within the previous 6 weeks

- Immunosuppressive therapy (eg, cyclosporine, steroids, tacrolimus) for
graft-versus-host disease (GvHD) within 30 days prior to the start of XL999

- The subject has not recovered to grade ≤ 1 or to within 10% of baseline from adverse
events due to investigational or chemotherapeutic drugs or stem cell transplantation
which were administered > 4 weeks prior to study enrollment

- Uncontrolled and/or concomitant illness

- Pregnant or breastfeeding females

- Known HIV