Oxidation fluorouracil

Ketofucosyl nucleosides (38a and 38b) of 5-fluorouracil and 6-azauracil have been obtained39 from the partially protected L-fucosyl-pyrimidines 37a and 37b by oxidation with Me2SO-DCC and Ru04-CHC13, respectively. 

Tetrahydrofolate cofactors participate in one-carbon transfer reactions. As described above in the section on vitamin B12, one of these essential reactions produces the dTMP needed for DNA synthesis. In this reaction, the enzyme thymidylate synthase catalyzes the transfer of the one-carbon unit of N 5,N 10-methylenetetrahydrofolate to deoxyuridine monophosphate (dUMP) to form dTMP (Figure 33-2, reaction 2). Unlike all of the other enzymatic reactions that utilize folate cofactors, in this reaction the cofactor is oxidized to dihydrofolate, and for each mole of dTMP produced, one mole of tetrahydrofolate is consumed. In rapidly proliferating tissues, considerable amounts of tetrahydrofolate can be consumed in this reaction, and continued DNA synthesis requires continued regeneration of tetrahydrofolate by reduction of dihydrofolate, catalyzed by the enzyme dihydrofolate reductase. The tetrahydrofolate thus produced can then reform the cofactor N 5,N 10-methylenetetrahydrofolate by the action of serine transhydroxy- methylase and thus allow for the continued synthesis of dTMP. The combined catalytic activities of dTMP synthase, dihydrofolate reductase, and serine transhydroxymethylase are often referred to as the dTMP synthesis cycle. Enzymes in the dTMP cycle are the targets of two anticancer drugs methotrexate inhibits dihydrofolate reductase, and a metabolite of 5-fluorouracil inhibits thymidylate synthase (see Chapter 55 Cancer Chemotherapy). 

The mixture of methotrexate sodium with cytarabine, fluorouracil, hydrocortisone sodium, and prednisolone sodium phosphate produces precipitate upon storage, although it is not observed immediately. A concentration-dependent photodegradation was reported for methotrexate and considered high in the presence of bicarbonate ions and unprotected polybutadiene tubings.226 Methotrexate wastes may be disposed of by oxidation with potassium permanganate and sulfuric acid or by oxidation with aqueous alkaline hypochlorite. 

These ideas have been illustrated in a recent study of the co-crystalline complex of l-MethylCytosine 5-FluoroUracil [34], Using model calculations it was shown how the hydrogen bonding network of the crystal is able to sustain a proton shuttle which leads to the selective formation of certain radicals. Calculations were able to predict that the site of reduction would be the cytosine base (yielding the N3 protonated cytosine anion C(N3+H) ), while the uracil base would be the site of oxidation (yielding the N1 deprotonated uracil cation U(N1-H) ). These are indeed the primary radiation induced species observed experimentally [34, 88], The results also nicely agree with the model proposed for radical trapping by Bernhard [11],... 

The treatment of cancer frequently involves drugs that interfere with DNA synthesis. For example, 5-fluorouracil (5-FU) prevents the conversion of dUMP to dTMP, reducing the level of thymine nucleotides required for DNA synthesis. Methotrexate prevents formation of tetrahy-drofolate from its more oxidized precursors. As a result, the formation of both thymine for DNA synthesis and the purines for DNA and RNA synthesis is inhibited. 

Fig. 40.5. Transfer of a one-carbon unit from N, methhylene FH4 to dUMP to form dTMP. FH4 is oxidized to FH2 (dihydrofolate) in this reaction. FH2 is reduced to FH4 by dihydrofolate reductase and FH4 is converted to N, methylene FH4 using serine as a carbon donor. Shaded bars indicate the steps at which the antimetabolites 5-fluorouracil (5-FU) and methotrexate act. 5-FU inhibits thymidylate synthase. Methotrexate inhibits dihydrofolate reductase.

With the advent of good specific ion electrodes, methods were developed to liberate the bound fluorine and directly measure the fluoride concentration. The reagent sodium-biphenyl followed by oxidation with hydrogen peroxide is used to liberate the organically bound fluorine in fluorouracil. A fluoride specific ion electrode is used, in conjunction with a high-ionic-strength-buffer solution, for direct measurement of the liberated fluoride (22). 

While a cytokine mixture (interleukin-ip/inter-feron-y) increased the production of NO by stomach cancer cells (NCI-N87) in a concentration- and time-dependent manner, pre-treatment with 5-fluorouracil reduced the expression of iNOS and thus inhibited nitric oxide production (Jung et al. 2002). 5-Fluorouracil stabilized IxBa and inactivated IxB kinase. 

All the prodrugs of Fluorouracil discussed above contained the fragment of 1 in their structure their transformation to 1 included hydrolysis reaction as the key step. On the contrary, 5-fluoro-2-pyrimidinone (5-FP, 38) which has been studied in Phase I clinical trials [45] is activated through oxidative process. In particular, pyrimidine 38 is transformed to 1 by aldehyde oxidase, which is present in high concentrations in the human livers but not in the gastrointestinal tract [46]. 

In smdies of a human breast carcinoma cell hue and a mouse melanoma cell line resistant to the chemotherapeutic agents 5-fluorouracil and doxorubicin increased not only by increasing hypoxia but also after inhibition of iNOS. Consequently, the authors hypothesized that some of the effects of hypoxia could be lack of nitric oxide in the tumor tissue (Matthews et al. 2001). 

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