Fluorodeoxyuridine triphosphate

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.

FU-treated cells, both fluorodeoxyuridine triphosphate (FdUTP) and deoxyuridine triphosphate (dUTP) (the substrate that accumulates behind the blocked TS reaction) incorporate into DNA in place of the depleted physiological nucleotide, TTP. Presumably, the incorporation ofdUTP and/or FdUTP into DNA activates the excision-repair process. This can lead to DNA strand breakage because DNA repair requires TTP, which is lacking because of TS inhibition. 5-FU incorporation into RNA also causes toxicity because of effects on both the processing and functions ofRNA. 

FIGURE 51-5 Activation pathways for 5 fluorouracil (5-FU) and 5-floxuridine (FUR). FUDP, floxuridine diphosphate FUMP, floxuridine monophosphate FUTP, floxuridine triphosphate FUdR, fluorodeoxyuridine FdUDP fluorodeoxyuridine diphosphate FdUMP fluorodeoxyuridine monophosphate FdUTP fluorodeoxyuridine triphosphate PRPP 5-phosphoribosyl-1 -pyrophosphate. 

Mechanism of action Fluorouracil is an analogue of uracil. After entry into cells it undergoes conversion to active metabolites fluorodeoxyuridine monophosphate (FdUMP), fluorodeoxyuridine triphosphate (FdUTP) and fluorouridine triphosphate (FUTP). FdUMP directly inhibits thymidy-late synthetase, reducing the availability of thymidine nucleotides, which are required for DNA synthesis, until new enzyme can be synthesized. FUTP is incorporated into RNA and causes impaired RNA processing and functioning, which disrupts cellular metabolism and viability [71 ]. 

Thymidylate synthase belongs to a class of enzymes required for DNA replication, and its activity is higher in rapidly proliferating cells. In particular, thymidylate synthase is responsible for methylation of deoxyuridine monophosphate (dUMP, 21) to deoxythymidine monophosphate (dTMP, 22) with the use of 5,10-methylenetet-rahydrofolate (23) as a cofactor (Scheme 2) [12], With fluorodeoxyuridine monophosphate, a slowly-reversible ternary complex 24 is formed instead. Inhibition of thymidylate synthase leads to deoxyribonucleotide imbalance, and hence to interference with DNA synthesis and repair. Alternative mechanism of DNA-directed Fluorouracil effect is misincorporation of fluorodeoxyuridine triphosphate (20) into DNA. Analogously, fluorouridine triphosphate (17) is extensively incorporated into different RNA species, disrapting their normal processing and function [7,8,11]. 

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).

Flucytosine is taken up by fungal cells via the enzyme cytosine permease. It is converted intracellularly first to 5-FU and then to 5-fluorodeoxyuridine monophosphate (FdUMP) and fluorouridine triphosphate (FUTP), which inhibit DNA and RNA synthesis, respectively (Figure 48-1). Human cells are unable to convert the parent drug to its active metabolites, resulting in selective toxicity. 

ALL, acute lymphoblastic leukemia AML, acute myelogenous leukemia CLL, chronic lymphocytic leukemia CML, chronic myelogenous leukemia DHFR, dihydrofolate reductase dNTP, deoxyribonucleotide triphosphate FdUTP, 5-fluorodeoxyuridine-5 -triphosphate FUTP, 5-fluorouridine-5 -triphosphate TS, thymidine synthase. 

Fluorouracil (5-FU) is inactive in its parent form and requires activation via a complex series of enzymatic reactions to ribosyl and deoxyribosyl nucleotide metabolites. One of these metabolites, 5-fluoro-2 -deoxyuridine-5 -monophosphate (FdUMP), forms a covalently ternary complex with the enzyme thymidylate synthase and the reduced folate 5,10-methylenetetrahydrofolate, a reaction critical for the de novo synthesis of thymidylate. This results in inhibition of DNA synthesis through "thymineless death." 5-FU is converted to 5-fluorouridine-5 -triphosphate (FUTP), which is then incorporated into RNA, where it interferes with RNA processing and mRNA translation. 5-FU is also converted to 5-fluorodeoxyuridine-5 -triphosphate (FdUTP), which can be incorporated into cellular DNA, resulting in inhibition of DNA synthesis and function. Thus, the cytotoxicity of 5-FU is thought to be the result of combined effects on both DNA- and RNA-mediated events. 

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... 

Fig. 14. Effect of 5-fluorouracil (5-FU) and 5-fluorodeoxyuridine (5-FdUMP) on the synthesis of RNA and DNA. 5-FU is incorporated into RNA whereas 5-FdUMP inhibits thymidilate synthetase competitively and blocks DNA synthesis. The conversion of thymidine (dTr) into thymidilate (dTMP) by means of thymidine kinase is only a subsidary routine to thymidilate which can become important, however, under certain conditions such as in developing pollen (page 185). U = uridine triphosphate, Tr = thymidine, TMP = thymidine monophosphate = thymidilate, TDP = thymidine diphosphate, TTP = thymidine triphosphate, 5-F = 5-fluoro, d = deoxy, 1 = thymidine kinase, 2 = thymidilate synthetase.

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