5-Fluorouracil metabolism

Parisot D, MC Malet-Martino, P Crasnier, R Martino (1989) nuclear magnetic resonance analysis of 5-fluorouracil metabolism in wild-type and 5-fluorouracil-resistant Nectria haematococca. Appl Environ Microbiol 55 2474-2479. 

Kanamitsu SI, Ito K, Okuda H, et al. Prediction of in vivo drug-drug interactions based on mechanism-based inhibition from in vitro data inhibition of 5-fluorouracil metabolism by (E)-5-(2-bromovinyl)uracil. Drug Metab Dispos 2000 28(4) 467 t74. 

Parisot, D., M.C. Malet-Martino, R. Martino, and P. Crasnier. 1991.19F nuclear magnetic resonance analysis of 5-fluorouracil metabolism in four differently pigmented strains of Nectria haematococca. Appl. Environ. Microbiol. 57 3605-3612. 

The rate-limiting step in fluorouracil metabolism is mediated by dihydropyrimidine dehydrogenase (DPD) which degrades fluorouracil to dihydrofluorouracil (DHFU) in the liver [2 70 ]. 

Susceptibility factors Genetic Polymorphisms of orotate phosphoribosyltransferase (OPRT), related to fluorouracil metabolism, predict more severe adverse reactions, including grade 3/4 diarrhea [110 ]. 

The key steps in Fluorouracil metabolism are shown in Scheme 1. Up to 80 % of 1 administered as injection is transformed to dihydrofluorouracil (DHFU, 13) by dihydropyrimidine dihydrogenase (mostly in liver tissues). However, this metabolite is not involved into antineoplastic activity instead, 13 itself and its further metabolites are responsible for most of the toxic effects of 1. The main mechanism of activation of Fluorouracil is conversion to fluorouridine monophosphate (FUMP, 14), either directly by orotate phosphoribosyltransferase, or via fluorouridine (FUR, 15) through the sequential action of uridine phosphorylase and uridine kinase. 14 is then phosphorylated to give fluorouridine diphosphate (FUDP, 16), which can be either phosphorylated again to the active metabolite fluorouridine triphosphate (ITJTP, 19), or reduced to fluorodeoxyuridine diphosphate (FdUDP, 18) by ribonucleotide reductase. In turn, 18 can either be dephosphorylated or phosphorylated to generate... 

Semmler W, Bacher-Baumann P and Guckel F (1990) Real time follow-up of 5-fluorouracil metabolism in the liver of tumor patients by means of F-19 MR spectroscopy. Radiology 174 141-145. 

Fluorouracil (5-fluorouracil, 5-FU, Fig. 5) represents an early example of rational drag design in that it originated from the observation that tumor cells, especially from gut, incorporate radiolabeled uracil more efficiently into DNA than normal cells. 5-FU is a fluorinated pyrimidine analog that must be activated metabolically. In the cells 5-FU is converted to 5-fluoro-2>deoxyuridine-monophosphate (FdUMP). This metabolite inhibits thymidilate synthase which catalyses the conversion of uridylate (dUMP) to thymidilate (dTMP) whereby methylenetetrahydrofo-late plays the role of the carbon-donating cofactor. The reduced folate cofactor occupies an allosteric site of... 

The antimetabolites interfere with various metabolic functions of cells, thereby disrupting normal cell functions. They inactivate enzymes or alter the structure of DNA, changing the DNA s ability to replicate These drag are most effective in the treatment of rapidly dividing neoplastic cells. Examples of the antimetabolites include methotrexate and fluorouracil (Adrucil). 

Substantial attention has been devoted to the metabolism of 5-fluorouracil and related compounds. For example, F NMR was used successfully both in cell extracts and in whole mycelia to elucidate anabolic reactions involving pyrimidine nucleotides and degradation to a-fluoro-p-alanine in the fungus Nectria haematococca (Parisot et al. 1989,1991). 

Fluoro-2 -deoxyuridine has been extensively used in studies of the mechanism of action of thymidylate synthase, and 5-fluorouracil is an anticancer drug that has provided a lead to the development of others. The metabolism of 5-fluorouracil by the ascomycete fungus Nectria haematococca has been studied using F NMR (Parisot et al. 1991). a-Fluoro-P-alanine (2-fluoro-3-aminopropionate) was produced (Figure 10.27), while 5-fluorouridine-5 -mono-, di-, and triphosphate were found in acid extracts of the mycelia, and the 2 - and 3 -monophosphates were recovered from RNA. 

Fluorouracil (5-FU) acts as a false pyrimidine, inhibiting the formation of the DNA base thymidine.26,35 The main mechanism by which it accomplishes this is by inhibiting the enzyme thymidylate synthase, the rate-limiting step in thymidine formation. 5-FU first must be metabolized to its active metabolite (F-dUMP). Additionally, metabolites of 5-FU may incorporate into RNA, inhibiting its synthesis. 

Smith P, Mirabelli C, Fondacaro J, Ryan F, Dent J (1988) Intestinal 5-fluorouracil absorption Use of Ussing chambers to assess transport and metabolism. Pharm Res 5 598-603. 

The antitumor agent 5-fluorouracil (4.236) is rapidly metabolized to 2-l luoro-/ -alanine (4.237) according to the sequence depicted in Fig. 4.10 [150][151]. The degradation of 5-fluorouracil occurs in all tissues, but tumor tissues contain very small amounts of dihydropyrimidine aminohydrolase. 

D. J. Sweeney, S. Barnes, R. B. Diasio, Formation of Conjugates of 2-Fluoro-/3-alanine and Bile Acids during the Metabolism of 5-Fluorouracil and 5-Fluoro-2-deoxyuridine in the Isolated Perfused Liver , Cancer Res. 1988, 48, 2010-2014. 

Fig. 3. Metabolism of the fluoropyrimidines dTMP = deoxythymidine monophosphate, dUMP = deoxyuridine monophosphate, FdUDP = fluorodeoxyuridine diphosphate, FdUMP - fluoro-deoxyuridine monophosphate, FdUTP = fluorodeoxyuridine triphosphate, FU-DNA= fluorouracil-deoxyribonucleic acid, FUDP = fluorouracil diphosphate, FUMP = fluorouracil monophosphate, FU-RNA = fluorouracil-ribonucleic acid, FUTP = fluorouracil triphosphate.

Although FdUrd produces only DNA-medicated cyotoxicity, 5-FU can also be metabolized to fluorouracil monophosphate (FUMP) and ultimately to fluorouracil triphosphate (FUTP), which can be incorporated into RNA in place of uridine triphosphate (UTP). In other words, incorporation of 5-FU into RNA mimics uracil de novo synthesis and affects the production of ribosomal RNAs (rRNAs) (16,17). 5 -FU also affects several aspects of messenger RNA (mRNA) function, including transcription (18), translation (19), and slicing (20). 

Parker WB, Cheg YC. Metabolism and mechanism of action of 5-fluorouracil. Pharmacol Ther 1990 48 381-395. 

Huber BE, Austin EA, Richards CA, et al. Metabolism of 5-fluorocytosine to 5-fluorouracil in human colorectal tumor cells transduced with the cytosine deaminase gene Significant antitumor effects when only a small percentage of tumor cells express cytosine deaminase. Proc Natl Acad Sci USA 1994 91 8302-8306. 

Drugs that require metabolic activation for antitumor activity, such as the antimetabolites 5-fluorouracil and 6-mercaptopurine, may be ineffective if a tumor is deficient in the required activating enzymes. Alter-... 

There is no evidence that 5-fluorouracil becomes unable to penetrate tumor cells. There may be an increase in P-glycoprotein, but this is not usually associated with 5-fluorouracil. There may be an induction in the drug metabolism for some antineoplastic drugs, but this does not appear to be the case for 5-fluorouracil. Increased metallothionein content has been associated with resistance in the case of cisplatin but not 5-fluorouracil. 

The drug has been administered orally, but absorption by this route is erratic. The plasma half-Ufe of 5-fluorouracil after intravenous injection is 10 to 20 minutes. It readily enters CSF. Less than 20% of the parent compound is excreted into the urine, the rest being largely metabolized in the liver. 

Floxuridine (FUDR) is the nucleoside of 5-fluo-rouracil that is readily converted into 5-fluorouracil in vivo. It has similar pharmacological effects but is preferred to 5-fluorouracil for hepatic arterial infusions because it is more extensively metabolized in the liver than 5-fluorouracil, with less systemic toxicity. 

Fig. 1. 5-FU metabolism. Abbreviations.- 5-FU (5-fluorouracil) FdUMP (fluorodeoxyuridine monophosphate) IS (thymidylate synthase) FUMP (fluorouridme monophosphate) DPD (dihydropyrimidine dehydrogenase) FUTP (fluorouridine triphosphate) FU H2 (dihydrofluorouracil) FBAL (fluoro-p-alanine).

El Sayed YM, Sadee W. Metabolic activation of R,S-l-(tetrahydro-2-furanyl)-5-fluorouracil (ftorafur) to 5-fluorouracil by soluble enzymes. Cancer Res 1983 43 4039 044. 

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