Evaluation of cytarabine against Ewing sarcoma xenografts by the pediatric preclinical testing program

Treatment with the nucleoside analog cytarabine has been shown to mimic changes in gene expression associated with downregulation of the EWS-FLI1 oncogene in Ewing sarcoma cell lines, selectively inhibit their growth in vitro, and cause tumor regression in athymic nude mice. For this report cytarabine was studied in vitro against a panel of 23 pediatric cancer cell lines and in vivo against 6 Ewing sarcoma xenografts. Acute lymphoblastic leukemia cell lines were the most sensitive to cytarabine in vitro (median IC₅₀ 9 nM), while Ewing sarcoma cell lines showed intermediate sensitivity (median IC₅₀ 232 nM). Cytarabine at a dose of 150 mg/kg administered daily 5× failed to significantly inhibit growth of five xenograft models, but reduced growth rate of the A673 xenograft by 50%. Cytarabine shows no differential in vitro activity against Ewing sarcoma cell lines and is ineffective in vivo against Ewing sarcoma xenografts at the dose and schedule studied.