5-Fluorouracil from uracil

Fluorouracil from uracil, obtained by replacement of H with F gives rise to the formation of an extremely therapeutically potent antineoplastic drug ... 

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... 

Fluorouracil and its derivatives are still important dmgs in the chemotherapy of numerous cancers (doxifluridine, tegafur, carmofur) (cf. Chapter 8). Studies on the mechanism of action of 5-FU had great influence on the development of other antitumor drugs that are derived from pyrimidine and purine. 5-FU is industrially prepared by fluorination of uracil with elemental fluorine (cf. Figure 2.6, Chapter 2). 

As active transport uses a carrier system, it is normally specific for a particular substance or group of substances. Thus, the chemical structure of the compound and possibly even the spatial orientation are important. This type of transport is normally reserved for endogenous molecules such as amino acids, required nutrients, precursors, or analogues. For example, the anticancer drug 5-fluorouracil (Fig. 3.6), an analogue of uracil, is carried by the pyrimidine transport system. The toxic metal lead is actively absorbed from the gut via the calcium transport system. Active uptake of the toxic herbicide paraquat into the lung is a crucial part of its toxicity to that organ (see chap. 7). Polar and nonionized molecules as well as lipophilic molecules may be transported. As active transport may be saturated, it is a zero-order rate process in contrast to passive diffusion (Fig. 3.3). 

Nishimoto S, Hatta H, Ueshima H, Kagiya T (1992) 1-(5 -Fluoro-6 -hydroxy-5, 6 -dihydrouracil-5 -yl)-5-fluorouracil,a novel N(1)-C(5) linked dimer that releases 5-fluorouracil by radiation activation under hypoxic conditions. J Med Chem 35 2711-2712 Norman ROC, Storey PM, West PR (1970) Electron spin resonance studies, part XXV. Reactions of the sulphate radical anion with organic compounds. J Chem Soc (B) 1087-1095 Novais HM, Steenken S (1986) ESR studies of electron and hydrogen adducts of thymine and uracil and their derivatives and of 4,6-dihydroxypyrimidines in aqueous solution. Comparison with data from solid state. The protonation at carbon of the electron adducts. J Am Chem Soc 108 1-6... 

All of the direct fluorinations reported appear to be addition-elimination processes with solvent involvement (Scheme 42). A study of the mechanism and stereochemistry of uracil and cytosine fluorination using fluorine and acetyl hypofluorite has implicated a radical-cation mechanism (86JOC1466). The effect of acetate ion on the products proved to be important. In its absence both m-isomers (49) and trans-isomers (50) were observed in the reaction mixture, but only 50 [and 5-fluorouracil (51)] in its presence. The process has been rationalized in terms of the reaction diagram shown in Scheme 43. NMR studies have revealed that the acetate from the solution containing acetate ion, rather than the residue from acetyl hypofluorite, binds to the 6-position of uracil to form the intermediates (49 and 50). Acetate is a sufficiently strong base to induce trans-elimination of acetic acid from the cis-isomer (49). 5-Fluorouracil (51) was obtained in 45% yield from these reaction sequences (86CJC424). 

A similar model was employed to predict the DDIs between 5-fluorouracil and sorivudine (124). Sorivudine is converted by gut flora to (E)-5-(2-bromovinyl) uracil, which inactivates dihydropyrimidine dehydrogenase and impairs the metabolism of 5-fluorouracil by this enzyme. This interaction led to 15 deaths in Japan from 5-fluorouracil toxicity due to elevated exposure to the drug. Using a fcE of 0.00018 min, a fivefold increase in the AUC of 5-fluorouracil was predicted after administration of sorivudine (150 mg/day for 5 days), which was close to the observed data in patients. 

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. 

FIGURE 28. Energy-level diagram showing the four highest MOs in uracil, 5-fluorouracil, thymine, 5-(trifluoromethyl)uracil and l-methyl-5-(trifluoromethyl)uracil. Reproduced by permission of IUPAC from Reference 259... 

As in the case of electrochemical reduction, the photochemical transformation of 5-fluorouracil derivatives differs from that of the other 5-halogeno uracils. The primary photoproduct of 5-fluorouracil, its glycosides and poly(5-FU) is the photohydrate. However, at shorter wavelengths of irradiation, e.g. 254 nm where the photohydrate exhibits absorption, there is elimination of HF from the 5,6 bond and formation of barbituric acid 129 13I>. There is also some evidence for acetone photosensitized formation of cyclobutane dimers of 5-fluorouracil132), as well as dimer formation in irradiated poly(5-FU)133>. 

In the assay by Lu et al. (1992), the catabolites of uracil, thymine, or 5-fluorouracil were separated from the respective parent compounds by chromatography on two 5 /xm Hypersil columns (from Jones Chromatography, Littleton, CO) used in tandem. The mobile phase contained 1.5 mM potassium phosphate (pH 8.0 for 5-fluorouracil, pH 8.4 for thymine and uracil) and 5 mM tetrabutylammonium hydrogen sulfate. The column effluent was monitored at an appropriate wavelength in the UV region. 

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