Cytosine permease

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 thymidylate 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 repHcation (1,23,24). 5-Fluorouracil caimot be used as an antimycotic. It is poorly absorbed by the fungus to begin with and is also toxic for mammalian cells. 

Flucytosine-resistant strains can develop very rapidly. These mutants may have a disturbed 5-FC-metabohsm, or a compensatory mechanism for the disturbed nucleic acid functions. No cytosine permease was found in a resistant Cyptococcus neoformans strain, whereas cytosine deaminase was absent in resistant C. albicans strains. A deficiency of uridine monophosphate pyrophosphorylase occurs frequently in resistant C. albicans strains (1). 

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. 

Flucytosine is an antimetaboUte of the fluoropyrimidme type. The principle of selectivity of flucytosine for the fungal cell is dual it depends on an enzyme in order to penetrate the cell (fungal cytosine permease) and a fungal enzyme that deaminates flucytosine to the active antimetabolite 5-fluorouracil, which is metabolized to 5-fluor-ouridine. Replacement of 5-fluorouracil in RNA results in the disruption of protein sjmthesis in the fungus. Flucytosine has selective activity against pathogenic... 

One other NP-derived molecule that is also in clinical use predominately against yeasts is the modified pyrimidine nucleoside, 5-fluorocytosine (flucytosine) (35, Figure 8), which although made synthetically, can be considered to be derived from a NP. Following transport into Candida or Cryptococcus a cytosine permease,... 

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. 

The most common mechanism of intrinsic resistance is a decreased uptake of flucytosine by the fungus by a reduction in cytosine permease activity, which results in reduced concentrations of dmg entering into the cell. The development of resistance to flucytosine during therapy may be a result of reduced expression of, or a deficiency in, an enzyme at any step in the intracellular metabolism of flucytosine. However, the most common resistance mechanisms observed in clinical isolates are due to either reduced cytosine deaminase or URPTase activity. 

Resistance arising during therapy (secondary resistance) is an important cause of therapeutic failure when flucytosine is used alone for cryptococcosis and candidiasis it can result from loss of the permease necessary for cytosine transport or decreased activity of UPRTase or cytosine deaminase (Figure 48-2). 

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