Sensitization of ara‐C‐resistant lymphoma cells by a pronucleotide analogue

Abstract

AbstractAdequate intracellular concentrations of ara‐CMP, the monophosphorylated derivative of ara‐C, are essential for its cytotoxicity. The critical step for ara‐CMP formation is intracellular phosphorylation of ara‐C by deoxycytidine kinase (dCK). A common nucleoside resistance mechanism is mutation affecting the expression or the specificity of dCK. We describe the ability of a tert‐butyl S‐acyl‐thioethyl (SATE) derivative of ara‐CMP (UA911) to circumvent ara‐C resistance in a dCK‐deficient human follicular lymphoma cell line (RL‐G). The RL‐G cell line was produced by continuous exposure to gemcitabine and displayed low dCK mRNA and protein expression that conferred resistance both to ara‐C (2,250‐fold) and to gemcitabine (2,092‐fold). RL‐G cells were able to take up the UA911 pronucleotide by diffusion and metabolize it to the corresponding ara‐CMP and ara‐CTP nucleotides, exhibiting a 199‐fold reduction in resistance ratios, and a similar cell cycle arrest to the parental RL‐7 cells. Exposures to 10, 50 or 100 μM concentrations of UA911 produced 160 ± 7, 269 ± 8 and 318 ± 62 pmol ara‐CTP/mg protein in RL‐7 cells, and 100 ± 12, 168 ± 10 and 217 ± 39 pmol ara‐CTP/mg protein in RL‐G cells, respectively. Exposure of RL‐G cells to underivatized, radiolabeled ara‐C produced no detectable amounts of the active triphosphate metabolites. We conclude that the UA911 pronucleotide is capable of overcoming dCK‐mediated resistance. This result can be attributed to the unique cellular metabolism of the SATE pronucleotides giving rise to the intracellular delivery of ara‐CMP to dCK‐deficient cells. © 2003 Wiley‐Liss, Inc.