This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Sommadossi, J.-P. Articles by Diasio, R. B. Search for Related Content PubMed PubMed Citation Articles by Sommadossi, J.-P. Articles by Diasio, R. B.

Modulation of 5-Fluorouracil Catabolism in Isolated Rat Hepatocytes with Enhancement of 5-Fluorouracil Glucuronide Formation¹

Departments of Medicine and Pharmacology, Medical College of Virginia, Richmond, Virginia 23298 [D. A. G., D. S. C., I. D. G.], Laboratoire de Pharmacocinétique et Toxicocinétique, INSERM SC 16, 13385 Marseille Cedex 5, France [J-P. S., J-P. C.], and Department of Medicine and Pharmacology, University of Alabama in Birmingham, Birmingham, Alabama 35294 [R. B. D.]

The catabolism of 5-fluorouracil (FUra), which accounts for 90% of the elimination of this antimetabolite in vivo, has recently been characterized in freshly isolated rat hepatocytes in suspension using a highly specific high-performance liquid chromatographic methodology. The present study evaluates the effect of thymine and uracil, which are thought to be catabolized by the same enzymes as FUra, on the metabolism and transmembrane distribution of FUra in isolated rat hepatocytes. Following simultaneous exposure of cells for 5 min to 30 µM [6-³H]FUra and increasing concentrations of either thymine or uracil, dihydrofluorouracil (FUH₂) levels decreased in a concentration-dependent manner, and the concentration determined for 50% inhibition of FUra catabolism was 8.0 ± 0.3 (S.D.) and 67.8 ± 15.6 µM for thymine and uracil, respectively. Analysis of intracellular and extracellular ³H from 1 min to 2 hr after simultaneous incubation of the hepatocytes with 30 µM FUra and thymine (or uracil) in a 1:7 molar ratio resulted in a decrease of intracellular and extracellular FUH₂ and -fluoro-ß-alanine (FBAL), while -fluoro-ß-ureidopropionic acid (FUPA) was enhanced. Unmetabolized FUra (not detected in the absence of thymine or uracil) was detected intracellularly in the presence of thymine or uracil 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 µM in the presence of thymine or uracil, respectively, within 1 hr. This metabolite, which penetrates the cell membrane only slowly, accounted for approximately 60% of the intracellular ³H in the presence of 300 µM FUra and 2 mM thymine, whereas FUra catabolism was inhibited by more than 99% under these conditions. The formation of FUra anabolites was insignificant in the presence of thymine and uracil, 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. These studies indicate that thymine is a more potent modulator of FUra catabolism in hepatocytes than is uracil. 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.

¹ Presented in part at the 74th Annual Meeting of the American Association for Cancer Research, San Diego, CA, May 25 to 28, 1983 (25). This study was supported by USPHS Grants CA-23412 and CA-16906 and by a grant from the "Association pour le Développement de la Recherche sur le Cancer."

² Supported in part by National Cancer Institute, Scientist Exchange Program G 50111 (United States-France Cancer Program).

³ To whom requests for reprints should be addressed, at Box 186, University Station, University of Alabama in Birmingham, Birmingham, AL 35294.

Received 5/21/84. Accepted 10/ 3/84.