Increased sensitivity of hydroxyurea-resistant leukemic cells to gemcitabine

Tumor cell resistance to certain chemotherapeutic agents may result in cros s-resistance to related antineoplastic agents. To study cross-resistance am ong inhibitors of ribonucleotide reductase, we developed hydroxyurea-resist ant (HU-R) CCRF-CEM cells. These cells were 6-fold more resistant to hydrox yurea than the parent hydroxyurea-sensitive (HU-S) cell line and displayed an increase in the mRNA and protein of the R2 subunit of ribonucleotide red uctase. We examined whether HU-R cells were cross-resistant to gemcitabine, a drug that blocks cell proliferation by inhibiting ribonucleotide reducta se and incorporating itself into DNA, Contrary to our expectation, HU-R cel ls had an increased sensitivity to gemcitabine. The IC,, of gemcitabine was 0.061 +/- 0.03 mu M for HU-R cells versus 0.16 +/- 0.02 mu M for HU-S cell s (P = 0.005), The cellular uptake of [H-3]gemcitabine and its incorporatio n into DNA were increased in HU-R cells. Over an 18-h incubation with radio labeled gemcitabine (0.25 ELM), gemcitabine uptake was 286 +/- 373 fmol/10( 6) cells for HU-R cells and 128 +/- 8.8 fmol/10(6) cells for HU-S cells (P = 0.03), The incorporation of gemcitabine into DNA was 75 +/- 6.7 fmol/10(6 ) cells for HU-R cells versus 22 +/- 0.6 fmol/10(6) cells for HU-S cells (P < 0.02), Our studies suggest that the increased sensitivity of HU-R cells to gemcitabine results from increased drug uptake by these cells, This, in turn, favors the incorporation of gemcitabine into DNA, resulting in enhanc ed cytotoxicity. The increased sensitivity of malignant cells to gemcitabin e after the development of hydroxyurea resistance may be relevant to the de sign of chemotherapeutic trials with these drugs.