This is an open label phase 1 study of fixed dose ATO (Arsenic Trioxide) and escalating doses of cyclophosphamide using a standard 3+3 dose escalation design. All subjects will be treated with sequential cycles of 3 days of ATO at 0.15 mg/kg/d IV followed by Cyclophosphamide as a single IV dose on day 4 along with mesna at a dose equal to the cyclophosphamide (for doses ≥1000 mg/m2) and hydration for a maximum of 6 cycles. ATO and Cyclophosphamide will be repeated every 28-42 days. Treatment will be given inpatient for the first cycle, with the option of outpatient treatment for subsequent cycles. Subjects may remain on study in the absence of disease progression or unacceptable toxicity for a maximum six cycles. Toxicity assessments will be performed continuously; DLT determination will be made based on adverse events (AEs) that occur during cycle 1 (day 1-28). An expansion cohort of ten subjects at the maximum tolerated dose will occur at the conclusion of dose escalation.