Dose Dependent Inhibition Of Uridine Phosphorylase (Up) By Eniluracil (Eu): Was The Clinical Inferiority Of The Eu/5-Fluorouracil (5-Fu) Phase Iii Trials Due To An Unrecognized Inhibition Of 5-Fu Anabolism?

2058 Background: Eniluracil (EU) is an irreversible inactivator of dihydropyrimidine dehydrogenase (DPD). EU inhibits DPD dependent 5-fluorouracil (5-FU) catabolism, allowing for oral 5-FU administration with essentially complete bioavailability. Although earlier murine studies suggested an increased antitumor effect when EU was administered before 5-FU, clinical studies used co-administered EU and 5-FU (ratio: 10 EU:1 5-FU) (b.i.d.). These phase III trials demonstrated inferiority compared to a regimen of 5-FU/leukovorin, leading to discontinuation of EU development. We hypothesize that the clinical failure of this EU/5-FU schedule may have resulted from competitive inhibition of 5-FU anabolic (ANA) activation by EU. In this study we examined whether EU could competitively inhibit uridine phosphorylase (UP) or orotate phosphoribosyl transferase (OPRT), the primary ANA enzymes of 5-FU. Methods: The cytoplasmic fraction of human embryonic kidney cells (HEK-293) was used as the source of UP and OPRT. Reverse phase HPLC with radioactivity detection was used to quantify [2-14C]-uracil formation from [2-14C]-uridine (UP activity) and to quantify [6-14C]-FUMP formation from [6-14C]-5-FU (OPRT activity). For UP activity, reaction mixtures consisted of increasing concentrations of EU in phosphate buffer containing 150 μM [2-14C]-uridine. For OPRT activity, reaction mixtures consisted of increasing concentrations of EU in phosphate buffer containing 5 μM [6-14C]-FU and 100 μM benzylacyclouridine (UP inhibitor). Reactions were initiated with addition of UP/OPRT enzyme source and allowed to proceed for 30 min at 37°C. Results: EU displayed dose-dependent competitive inhibition of UP activity (IC50 = 0.375 mM). In particular, the EU/5-FU ratio of 2.5:1 produced approximately 50% inhibition of UP activity. In contrast, EU did not inhibit OPRT activity. Conclusions: This study demonstrates that EU competitively inhibits UP, an important enzyme in 5-FU ANA activation, and suggests that changing the dose ratio and/or increasing the interval between EU and 5-FU administration may be useful in optimizing 5-FU antitumor efficacy. [Table: see text]