Clofarabine (Clolar, Genzyme) has been studied in the treatment of various types of leukemia and is FDA approved for the treatment of childhood acute lymphoblastic leukemia. It is structurally related to fludarabine and cladribine, sharing some characteristics and avoiding others. Clofarabine exerts its antineoplastic activity through several mechanisms. The active metabolite of clofarabine is its triphosphate form. This molecule competes with deoxyadenosine triphosphate for the ribonucleotide reductase and DNA polymerase, which leads to decreased DNA synthesis and repair, inhibits DNA strand elongation and cell replication. In addition, pretreatment with clofarabine before cytarabine administration leads to increases in intracellular concentrations of cytarabine triphosphate, the active form of cytarabine. (