TEGAFUR IS AN ORALLY ACTIVE PRODRUG OF FLUOROURACIL. ALTHOUGH IT HAS BEEN GIVEN AS A SINGLE AGENT, IT IS MORE OFTEN USED WITH DRUGS THAT MODIFY ITS BIOAVAILABILITY AND TOXICITY. THESE INCLUDE URACIL AND GIMESTAT (5-CHLORODIHYDROPYRIMIDINE, CDHP), WHICH CAN INCREASE FLUOROURACIL CONCENTRATIONS BY INHIBITION OF DIHYDROPYRIMIDINE DEHYDROGENASE, THE ENZYME RESPONSIBLE FOR ITS FURTHER CATABOLISM, AND OXONIC ACID (OTASTAT), WHICH INHIBITS ANOTHER ENZYME, OROTATE PYRIMIDINE PHOSPHORIBOSYL TRANSFERASE, THOUGHT TO PLAY A ROLE IN THE GASTROINTESTINAL TOXICITY OF FLUOROURACIL AND ITS PRODRUGS.