Thymidilate synthetase

Flucytosine is converted into the anti metabolite 5-fluorouracil that inhibits thymidilate synthetase, thereby disrupting DNA synthesis. It also interferes with protein synthesis by incorporation of fluorouracil into RNA in place of uracil. Although active against most Candida species, its spectrum of antifungal activity, overall, is narrow. Since resistance can develop rapidly it is usually coadministered with another agent and its main value is that it facilitates a reduction in the dose (and, presumably, the toxic effect) of amphotericin when co-prescribed in this way. The main adverse effects are marrow aplasia and hepatotoxicity. 

The concept that oestrogens stimulate cell proliferation directly arises mainly from the observation that physiological concentrations of oestrogens stimulate both the de novo and salvage pathways of DNA synthesis as well as inducing a number of enzymes intimately involved in DNA synthesis and including DNA polymerase, thymidine and uridine kinases, thymidilate synthetase and dihydrofolate reductase. There is evidence that some of these enzymes may be regulated at the transcrip-... 

B-7) 5-fluorouracil (5-FU) is an antitumor drug that acts at the same map area as does methotrexate in pyrimidine biosynthesis, but acts by a different mechanism. Dirough its resemblance to uracil, it inhibits thymidilate synthetase at this step and prevents the formation of thymidine, a pyrimidine. In a similar vein, cytarabine is an antitumor agent that acts through its resemblance to cytidine, another pyrimidine. 

Reduced folates are co-factors for the 5-fluorodeoxy-uridine monophosphate-thymidilate synthetase reaction. Leucovorin (calcium fohnate) therefore potentiates the toxicity of 5-fluorouracil, and fatal adverse effects have been reported in patients over 65 years of age receiving high-dose treatment with leucovorin simultaneously with fluorouracil. This has led some groups to recommend that initial dose levels of fluorouracil should be lowered by 20% and that therapy be stopped temporarily at the first sign of distal gastrointestinal adverse effects (SEDA-15, 414). 

With the aid of cytosine permease, flucytosine reaches the fungal cell where it is converted by cytosine deaminase into 5-fluorouracil [51-21-8]. Cytosine deaminase is not present in the host, which explains the low toxicity of 5-FC. 5-Fluorouracil is then phosphorylated and incorporated into RNA and may also be converted into 5-fluorodeoxyuridine monophosphate, which is a potent and specific inhibitor of thymidilate synthetase. As a result, no more thymidine nucleotides are formed, which in turn leads to a disturbance of the DNA-synthesis. These effects produce an inhibition of the protein synthesis and cell replication (1,23,24). 5-Fluorouracil cannot be used as an antimycotic. It is poorly absorbed by the fungus to begin with and is also toxic for mammalian cells. 

Flucytosine (Ancoben) Deaminated to 5-fluoro-uradl by the fungus. Incorporated into RNA. Metabolized to 5-fluorodeoxyuridine (5-FdURD) which inhibits thymidilate synthetase. Leukopenia, nausea, diarrhea, T LFTs, bone marrow depression. 

Competitive Inhibition of enzymes. In the case of competitive inhibition, the structural analogs compete with the natural substrates for the active site of enzymes. The enzymes of nucleic acid, and protein, synthesis can also be inhibited in this way. Thus, the enzyme thymidilate synthetase is competitively inhibited by 5-fiuorodeoxyuridine, a derivative of 5-fluorouracil (Fig. 14). Thymidilate synthetase supplies d-thymidine-5 -phosphate (-thymidilate), a substance which is important to the synthesis of DNA. Inhibition of this enzyme thus brings DNA synthesis to a stop and, with it, all further development. We will discuss DNA synthesis and also thymidilate synthetase in more detail later (p. 165). 

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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