Received for publication December 7, 2005.
Revised March 27, 2006.
Accepted for publication April 14, 2006.

The role of human nucleoside transporters in cellular uptake of 4'-thio--D-arabinofuranosylcytosine and -D-arabinosylcytosine

Abstract

4'-Thio--D-arabinofuranosyl cytosine (TaraC) is in Phase I development for treatment of cancer. In hENT1-containing CEM cells, initial rates of uptake (10 µM; pmol/µl cell water/s) of [³H]TaraC and [³H]1--D-arabinofuranosyl cytosine (araC) were low (0.007 ± 003 and 0.034 ± 0.003, respectively) compared to that of [³H]uridine (0.317 ± 0.048), a high-activity hENT1 permeant. In hENT1- and hENT2-containing HeLa cells, initial rates of uptake (10 µM; pmol/cell/s) of [³H]TaraC, [³H]araC and [³H]deoxycytidine were low (0.30 ± 0.003, 0.42 ± 0.03, and 0.51 ± 0.11, respectively) and mediated primarily by hENT1 (~74%, ~65%, and ~61%, respectively). In HeLa cells with recombinant hCNT1 or hCNT3 and pharmacologically blocked hENT1 and hENT2, transport of 10 µM [³H]TaraC and [³H]araC was not detected. The apparent affinities of recombinant transporters (produced in yeast) for a panel of cytosine-containing nucleosides yielded results that were consistent with the observed low permeant activities of TaraC and araC for hENT1/2 and negligible permeant activities for hCNT1/2/3. During prolonged drug exposures of CEM cells with hENT1 activity, araC was more cytotoxic than TaraC, whereas co-exposures with nitrobenzylthioinosine (to pharmacologically block hENT1) yielded identical cytotoxicities for araC and TaraC. The introduction by gene transfer of hENT2 and hCNT1 activities, respectively, into nucleoside-transport defective CEM cells increased sensitivity to both drugs moderately and slightly. These results demonstrated that nucleoside transport capacity (primarily via hENT1, to a lesser extent by hENT2 and possibly by hCNT1) is a determinant of pharmacologic activity of both drugs.

Key words: Nucleoside/Nucleotide, Nucleoside/Nucleotide derivatives, Membrane targets

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