Comparison of cytotoxicity of 2-chloro- 2'-arabino-fluoro-2'-deoxyadenosine (clofarabine) with cladribine in mononuclear cells from patients with acute myeloid and chronic lymphocytic leukemia.

Background and Objectives: Clofarabine (CAFdA), one of the newer nucleoside drugs is undergoing a phase II clinical trial for the treatment of pediatric refractory/relapsed acute myeloid and lymphocytic leukemia. Although CAFdA is structurally similar to the clinically established analogs fludarabine and cladribine (CdA), its metabolism and mechanism of actions are significantly different. The present study investigates the in vitro cytotoxicity of CAFdA and CdA in mononuclear cells isolated from 52 patients with chronic lymphocytic (CLL) and acute myeloid leukemia (AML).
Design and Methods: We incubated the leukemic cells with drugs for 48 hours and cytotoxicity was then evaluated by the MTT dye assay. We also determined the levels of deoxycytidine and deoxyguanosine kinase with radio-chemical substrate-based assays and used a high performance liquid chromatographic method to measure cellular nucleotides in leukemia cells after 2 hours' incubation.
Results: Using equimolar concentrations of CAFdA and CdA, the in vitro cytotoxicity for the population was significantly higher with CAFdA than with CdA (median EC50 for CAFdA 0.12 microM and for CdA 0.15 microM, p<0.001). From the individual estimates the difference in cytotoxicity between CAFdA and CdA was more pronounced in cells from CLL patients (median EC50 for CAFdA 0.08 microM and for CdA 0.16 microM p<0.001) than in those from AML patients. We also found that CAFdA was phosphorylated more efficiently than CdA. No correlations were detected in this study between the levels of CdA and CAFdA nucleotides, enzymes levels and the in vitro responses.
Interpretation and Conclusions: The greater in vitro cytotoxicity and cell metabolism of CAFdA compared to CdA confirm the high activity of CAFdA and encourage clinical trials with CAFdA in leukemic patients.