Cohort 1 will receive 20 mg/m2/day of Clofarabine once daily for five consecutive days, Cohort 2 will receive 30 mg/m2/day, and Cohort 3 will receive 40 mg/m2/day. Patients will receive one cycle as a rule. However, if there is evidence of some hematologic response after one cycle of treatment with Clofarabine, patients may receive up to a maximum of three cycles. If patients fail to achieve CR or CRp after two cycles of treatment with Clofarabine, further dosing for such patients should be stopped.