MODULATION OF 5-FLUOROURACIL CATABOLISM IN ISOLATED RAT HEPATOCYTES WITH ENHANCEMENT OF 5-FLUOROURACIL GLUCURONIDE FORMATION

  • 1 January 1985
    • journal article
    • research article
    • Vol. 45  (1) , 116-121
Abstract
The catabolism of 5-fluorouracil (FUra), which accounts for 90% of the elimination of this antimetabolite in vivo, was recently characterized in freshly isolated rat hepatocytes in suspension using a highly specific high-performance liquid chromatographic methodology. The effect of thymine [T] and uracil [U], which are thought to be catabolized by the same enzymes as FUra, on the metabolism and transmembrane distribution of FUra was examined in isolated rat hepatocytes. Following simultaneous exposure of cells for 5 min to 30 .mu.M [6-3H]FUra and increasing concentrations of either T or U dihydrofluorouracil (FUH2) levels decreased in a concentration-dependent manner and the concentration determined for 50% inhibition of FUra catabolism was 8.0 .+-. 0.3 (SD) and 67.8 .+-. 15.6 .mu.M for T and U, respectively. Analysis of intracellular and extracellular 3H from 1 min-2 h after simultaneous incubation of the hepatocytes with 30 .mu.M FUra and T (or U) in a 1:7 molar ratio resulted in a decrease of intracellular and extracellular FUH2 and .alpha.-fluoro-.beta.-alanine (FBAL), while .alpha.-fluoro-.beta.-ureidopropionic acid (FUPA) was enhanced. Unmetabolized FUra (not detected in the absence of T or U) was detected intracellularly in the presence of T or U and was accompanied by the appearance of a novel metabolite, preliminarily identified as a glucuronide of the FUra base which reached intracellular levels of 44 .+-. 9.76 and 27.45 .+-. 1.35 .mu.M in the presence of T or U, respectively, within 1 h. This metabolite, which penetrates the cell membrane only slowly, accounted for .apprx. 60% of the intracellular 3H in the presence of 300 .mu.M FUra and 2 mM T, whereas FUra catabolism was inhibited by > 90% under these conditions. The formation of FUra anabolites was insignificant in the presence of T and U and incorporation of FUra into RNA was not enhanced. The lack of anabolism of FUra in isolated hepatocytes exposed to either high initial concentrations of FUra or high intracellular FUra concentrations resulting from modulation (inhibition) of FUra catabolism is consistent with the clinical observation of minimal hepatotoxicity with FUra, despite exposure of the liver to high blood levels. T evidently is a more potent modulator of FUra catabolism in hepatocytes than is U. Further studies are needed to clarify the biological importance of the glucuronide of the base FUra which accumulates intracellularly as the concentration of FUra increases within the hepatocytes. [The role of FUra as an antineoplastic drug was discussed.].