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

Adequate 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 microM 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.