SCHEDULE DEPENDENCE OF SENSITIVITY TO 2',2'-DIFLUORODEOXYCYTIDINE (GEMCITABINE) IN RELATION TO ACCUMULATION AND RETENTION OF ITS TRIPHOSPHATE IN SOLID TUMOR-CELL LINES AND SOLID TUMORS

2',2'-Difluorodeoxycytidine (Gemcitabine, dFdC) is a relatively new de oxycytidine antimetabolite, with established activity against ovarian cancer and non-small-cell lung cancer. dFdC is assumed to exert its an titumour effect mainly by incorporation of the triphosphate dFdCTP int o DNA. We determined the sensitivity to dFdC of six cell lines derived from solid tumours; two ovarian carcinoma (A2780 and OVCAR-3), two co lon carcinoma (WiDr and C26-10) and two squamous cell carcinoma cell l ines (UM-SCC-14C and UM-SCC-22B). In vitro sensitivity to dFdC was str ongly time dependent. Under all conditions A2780 was the most sensitiv e cell line with an IC50 (the concentration of dFdC causing 50% growth inhibition) of 31 and 0.6 nM at 1 and 48 hr exposure, respectively. W iDr and C26-10 cells were relatively insensitive, with IC(50)s Of 468 and 1133 nM, respectively, at 1 hr exposure, but of 11 and 6 nM at 48 hr exposure. Accumulation of the triphosphate dFdCTP was also time dep endent. After 4 hr exposure to 10 mu M dFdC, A2780, WiDr and C26-10 ce lls accumulated 223, 136 and 267 pmol/10(6) cells, respectively; after 24 hr exposure they accumulated 1045, 619 and 617 pmol/10(6) cells, r espectively. A2780 cells retained the high dFdCTP concentration longer than 24 hr. For comparison purposes we also studied dFdCTP kinetics i n the corresponding solid tumours, showing the same sensitivity patter n as the cell lines. In general, sensitivity to dFdC in vitro related with dFdCTP accumulation and retention, but in vivo this relation was less clear. Unexpectedly, remarkable in vitro and in vivo changes were observed in the ribonucleotide pools. The most predominant in vitro c ell line dependent changes were a decrease in CTP concentrations, acco mpanied by an increase in UTP and GTP concentrations. In vivo CTP, UTP and GTP pools increased in all tumours. In conclusion, in this study we demonstrate that dFdCTP is accumulated and retained in solid tumour s and cell lines. dFdCTP is not only important as a DNA precursor, but also appears to interfere with normal ribonucleotide metabolism.