We reported previously that
5-fluorouracil (FUra) efficacy could be enhanced by increasing tumoral
thymidine phosphorylase (TP) activity. Potentiated TP yield was achieved by either transfecting cells with human TP gene (A. Evrard et al., Br. J.
Cancer, 80: 1726-1733, 1999) or associating FUra with
2'-deoxyinosine (d-Ino), a modulator providing the
tumors with TP cofactor
deoxyribose 1-phosphate (J. Ciccolini et al., Clin.
Cancer Res., 6: 1529-1535, 2000). The purpose of the present work was to study the effects of a combined modulation (TP gene transfer + use of d-Ino) on the sensitivity to FUra of the LS174T human colorectal cell line. Results showed a near 4000 times increase of cell sensitivity in vitro after double (genetic + biochemical) modulation. This potentiation of
tumor response was accompanied by a total change in the FUra anabolic pathway with a 5000% increase of cytosolic
fluorodeoxyuridine monophosphate, a stronger and longer inhibition of
thymidylate synthase, and 300% augmentation of DNA damage. Besides, whereas
thymidine failed to inhibit FUra cytotoxicity in LS174T wild-type cells, the potentiation of the antitumor activity observed in the modulating regimen was partly reversed by
thymidine, indicative of
thymidylate synthase as the main
drug target. The impact of this double modulation was next investigated in xenograft-bearing nude mice. Results showed that whereas FUra alone was completely ineffective on wild-type
tumor growth, the size of TP-transfected
tumors in animals treated with the FUra/d-Ino combination was reduced by 80% (P < 0.05). Our results suggest that FUra exhibits stronger antiproliferative activity when activated via TP through the
DNA pathway and that high tumoral TP activity therefore leads to enhanced sensitivity to fluoropyrimidines.