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Enhancement of 1-ß-D-Arabinofuranosylcytosine Accumulation within L1210 Cells and Increased Cytotoxicity following Thymidine Exposure¹

Departments of Medicine and Pharmacology, Section of Medical Oncology, Yale School of Medicine, New Haven, Connecticut 06510

The effect of prior exposure to thymidine (dThd) on the intracellular accumulation of deoxycytidine (dCyd) and 1-ß-D-arabinofuranosylcytosine (ara-C), as well as intracellular deoxyribonucleotide pools, was studied in suspension cultures of L1210 cells during logarithmic growth. Pretreatment with 0.1 mM dThd for 5 hr resulted in an increase in intracellular dCyd incorporation from 30 pmol dCyd per 10⁶ cells to 220 pmol dCyd per 10⁶ cells following 1 hr exposure to 5 µM [³H]dCyd. Under the same conditions, the intracellular incorporation of ara-C was increased from 32 pmol ara-C per 10⁶ cells to 120 pmol ara-C per 10⁶ cells following 1 hr exposure to 5 µM [³H]-ara-C. Utilization of a 1-hr dThd exposure time, as well as dThd concentrations of 0.01 and 2 mM, resulted in less intracellular enhancement of dCyd and ara-C incorporation. Following a 5-hr exposure to 0.1 mM dThd, the incorporation of [³H]ara-C into RNA increased from 0.21 to 0.48 pmol ara-C per µg D-ribose; DNA incorporation increased from 0.70 to 3.50 pmol ara-C per µg deoxyribose. In addition, dThd pretreatment was associated with an 8-fold increase in 1-ß-D-arabinofuranosylcytosine 5'-triphosphate production.

The maximum reduction of intracellular deoxycytidine 5'-triphosphate pools, from 106 pmol/10⁶ cells to 25 pmol/10⁶ cells, occurred after a 5-hr exposure of 0.1 mM dThd. This reduction in the deoxycytidine triphosphate pool was inversely correlated with the magnitude of the enhancement of intracellular ara-C incorporation. The sequence of 0.1 mM dThd for 5 hr followed by 5 µM ara-C for 1 hr resulted in a reduction in L1210 colony formation to 3% of control values, which represented synergistic cell killing relative to either drug given alone or the reverse drug sequence. Thus, dThd enhances the intracellular and nucleic acid incorporation of ara-C, most probably as a consequence of a reduction in deoxycytidine triphosphate pools, and this is the probable biochemical explanation for the enhanced in vitro cytotoxicity.

¹ Supported by National Cancer Institute Grants CA 24187 and CA 09200 and a Swebilius Cancer Award from the Yale Comprehensive Cancer Center.

² To whom requests for reprints should be addressed.

Received 9/24/79. Accepted 2/ 5/80.