Outcomes for children with relapsed/refractory (R/R) acute lymphoblastic leukemia (ALL) are dismal. Therefore, the investigators performed this multicenter, phase II study to evaluate the efficacy and , safety and pharmacokinetic of clofarabine in Chinese pediatric patients with R/R ALL

Inclusion Criteria:

• Pediatric patients (younger than 21 years old) with acute lymphoblastic leukemia confirmed by histology
• Refractory or relapsed acute lymphocytic leukemia who had received at least two drugs treatment
• No prior chemotherapy within 2 weeks before entry and resolution of toxic effects from prior therapy
• Normal cardiac function, adequate hepatic function [total bilirubin ≤1.5× upper limit of normal (ULN), aspartate aminotransferase (AST) alanine aminotransferase (ALT) ≤ 3ULN] and renal function (serum creatinine ≤ 2 ULN)
• Eastern Cooperative Oncology Group (ECOG) performance status 0-2
• Life expectancy more than 3 months

Exclusion Criteria:

• AEs not recovered from prior therapy
• Within 3 months from allogeneic or autologous stem cell transplantation
• With central nervous involvement or uncontrolled infection
• Patients who used clofarabine before or allergic to fludarabine or cladribine