130 Dihydropyrimidine dehydrogenase as a pivotal target for FU biomodulation. Role of 5-ethynyluracil

Abstract

Dihydropyrimidine dehydrogenase (DPD) is the rate-limiting enzyme of fluorouracil (FU) catabolism. Ethynyluracil (776C) is a very potent, mechanism-based irreversible DPD inhibitor that improves the antitumor efficacy and the therapeutic index of FU in laboratory animals. We tested the cytotoxic effects of the FU-776C combination on a panel of 12 human cancer cell lines (4 breast, 4 head and neck, 3 colon, I duodenum). Basal DPD activity (radioenzymatic assay) and FU sensitivity (FU IC50, MTT test) were determined. The FU potentiation by 776C was calculated from the ratio (F) of FU IC50 without 776C divided by FU IC50 with 776C. 776C was not cytotoxic to any of the cell lines tested. On CAL51 cell line, expressing a high basal DPD activity, FU enhancement by 776C was a saturable phenomenon related to the 776C concentration; the inhibition of DPD increased between 10 −12 to 10 −6 M of 776C. For the following studies, 776C was tested at 10 −6 M. FU IC50 varied from IS to 7770 μM among cell lines (median 390 μM). Basal DPD activity ranged from not detectable P = 0.019). Enhancement of FU cytotoxicity by 776C occurred only in the 6 cell lines expressing the greatest basal DPD activity (> 50 p mol/min/mg p rot, F ranging between 1.7 and 5.2), whereas 776C did not modify FU cytotoxicity in the remaining cell lines expressing the lowest DPD activity « 50 pmol/min/mg prot, F ranging between 0.7 and 1.4); F was significantly different between these 2 groups of cell lines ( P = 0.005). These results justify clinical trials with DPD inhibitors like 776C.