5- fluorouridine triphosphate

Orotate phosphoribosyltransferase (OPRT) catalyses the conversion of 5-FU to fluorouridine monophosphate (FUMP), which is subsequently phosphorylated to activated fluorouridine triphosphate (FUTP). FUTP incorporates into RNA and thereby compromises RNA processing and function (Fig. 1). 

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. 

There is sequence-dependent synergy between fluorouracil and methotrexate. Pretreatment with methotrexate enhances the formation of fluorouridine monophosphate and hence fluorouridine triphosphate and RNA-directed toxicity. In studies in which methotrexate has been given 1 hour before fluorouracil, response rates did not differ significantly. However, when it was given 4 hours or more before, there were significantly better response rates (147). 

Fluorouracil is anabolized to its 2 -deoxyribosc mo phosphate, a potent inhibitor of thymidylate synthetiiic also is converted into fluorouridine triphosphate, whiilii incorporated into RNA and DNA." There is cell Ou phase-specificity for the S phase. 

All susceptible fungi are capable of deaminating flucytosine to 5-fluorouracil, a potent antimetabolite that is used in cancer chemotherapy. Fluorouracil is metabolized fast to 5-fluorouracil-ribose monophosphate (5-FUMP) by the enzyme uracil phosphodbosyi transferase (UPRTase, also called uridine monophosphate pyrophosphorylase). As in mammalian cells, 5-FUMP then is either incorporated into RNA (via synthesis of 5-fluorouridine triphosphate) or metabolized to 5-fluoro-2 -5 deoxyuridine-5-monophos-phate (5-FdUMP), a potent inhibitor of thymidylate synthetase. DNA synthesis is impaired as the ultimate inhibition of this latter reaction. The selective action of flucytosine is due to the lack or low levels of cytosine deaminase in mammalian cells, which prevents metabolism to fluorouracil. 

FIGURE 48-2 Action of flucytosine in fungi. 5-Flucytosine is transported by cytosine permease into the fungal cell, where it is deaminated to 5-fluorouracil (5-FU). The 5-FU is then converted to 5-fluorouracil-ribose monophosphate (5-FUMP) and then is either converted to 5-fluorouridine triphosphate (5-FUTP) and incorporated into RNA or converted by ribonucleotide reductase to 5-fluoro-2 -deoxyuridine-5 -monophosphate (5-FdUMP), which is a potent inhibitor of thymidylate synthase. 5-FUDP, 5-fluorouridine-5 -diphosphate dUMP, deoxyuridine-5 -monophosphate dTMP, deoxyuridine-5 -monophosphate UPRTase, uracil phosphoribosyl transferase. 

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 ]. 

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... 

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]. 

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