Relationships among Plasma [2-c]uracil Concentrations, Breath

Dihydropyrimidine dehydrogenase (DPD), the first enzyme in the sequential metabolism of pyrimidine, regulates blood concentrations of 5-fluorouracil and is deeply involved in its toxicity. This study was designed to examine the effects of a DPD inhibitor on blood concentrations of [2-C]uracil ([C]uracil) and CO2 concentration ( C) expired in breath after oral or intravenous administration of [C]uracil to DPD-suppressed dogs prepared by pretreatment with 5-(trans-2-bromovinyl)uracil (BVU), a DPD inhibitor. Area under the curve (AUCt) of [ C]uracil after oral administration at 20 mol/kg to dogs pretreated with BVU at 2, 5, and 40 mol/kg were 37-, 88and 120-fold higher than those of the control dogs, respectively. In contrast, breath AUCt values of C were reduced to 0.88-, 0.47and 0.13-fold the control values, respectively. Upon intravenous administration of [C]uracil at 20 mol/kg to dogs pretreated with BVU at 0.5, 2, and 40 mol/kg, blood AUCt values of [C]uracil were 1.4-, 4.2-, and 13-fold higher than those of the control group, respectively, whereas breath AUCt values were reduced to 1.0-, 0.83-, and 0.07-fold the respective control values. DPD activities in the liver cytosol of dogs pretreated with BVU at 0.5, 2, 5, and 40 mol/kg were decreased to 0.71-, 0.12-, 0.06-, and 0.04-fold those of the control dogs, respectively. These findings demonstrate that breath output ( C) is a good marker of hepatic DPD activity in vivo. 5-Fluorouracil (5-FU) is a metabolic antagonist of pyrimidine that was first synthesized by Duschinsky et al. (1957). 5-FU and its prodrugs (tegafur, carmofur, doxifluridine, capecitabine, etc.) are anticancer agents that are widely used in the management of several common malignancies, including cancer of the colon, breast, and skin. Due to the structural similarity of 5-FU to the pyrimidine base uracil, it is incorporated into the metabolic cycles in vivo through the same routes as uracil, exhibits antagonistic action against anabolic metabolism (e.g., inhibition of DNA synthesis and RNA function), and kills tumor cells (Pinedo and Peters, 1988; Diasio and Harris, 1989). In addition to tumors cells, however, host cells that rapidly incorporate precursors for nucleic acids selectively uptake 5-FU to cause cytotoxicity. As a result, unusually severe adverse drug reactions (e.g., diarrhea, stomatitis, mucositis, myelosuppression, and neurotoxicity) occur. The major metabolizing organ for 5-FU is the liver (Ho et al., 1986), which accounts for approximately 80% or more of the total systemic clearance (Woodcock et al., 1980; Heggie et al., 1987). 5-FU is first converted by dihydropyrimidine dehydrogenase (DPD) to 5-fluorodihydrouracil, then by dihydropyrimidinase to -fluoroureidopropionic acid, and finally by -ureidopropionase to -fluoro-alanine, ammonia, and carbon dioxide (Fig. 1). These final metabolites are excreted in urine and breath. Administration of 5-FU or a prodrug to patients with abnormal pyrimidine metabolism such as DPD deficiency or concomitant use with a DPD inhibitor probably causes abnormally high blood concentrations of 5-FU, which result in serious adverse drug reactions (Lyss et al., 1993; Morrison et al., 1997). In a particularly notable example, the combined use of sorivudine (an antiviral drug) and 5-FU resulted in a large number of deaths in 1993 in Japan (Okuda et al., 1997, 1998). The mechanism of this severe toxicity was attributed to the generation of 5-(trans-2-bromovinyl)uracil (BVU) from sorivudine by gut flora (Nakayama et al., 1997). BVU has strong DPD inhibitory activity. BVU, a structural analog of 5-FU, has been conclusively determined to inhibit DPD via an irreversible (i.e., covalent-binding) and mechanism-based inhibition (Desgranges et al., 1986; Nishiyama et al., 2000). Given this background, diagnostic methods able to predict and prevent adverse drug reactions to 5-FU in patients with pyrimidine metabolism disorders have been actively sought. Although several methods are now available, including quantification of urinary pyrimidine (Sumi et al., 1995, 1998), measurement of DPD activity in Article, publication date, and citation information can be found at http://dmd.aspetjournals.org. doi:10.1124/dmd.104.001032. ABBREVIATIONS: 5-FU, 5-fluorouracil; DPD, dihydropyrimidine dehydrogenase; BVU, 5-(trans-2-bromovinyl)uracil; LC-MS/MS, liquid chromatography-tandem mass spectrometry; IRMS, isotope ratio-mass spectrometry; CV, coefficient of variation; AUCt, area under the plasma concentration-time curve from time 0 to t; AUC , area under the plasma concentration-time curve from time 0 to infinity. 0090-9556/05/3303-381–387$20.00 DRUG METABOLISM AND DISPOSITION Vol. 33, No. 3 Copyright © 2005 by The American Society for Pharmacology and Experimental Therapeutics 1032/1196378 DMD 33:381–387, 2005 Printed in U.S.A. 381 at A PE T Jornals on A ril 2, 2017 dm d.aspurnals.org D ow nladed from