5- Fluorocytosine

5- FLUOROCYTOSINE

This antifungal agent inhibits DNA synthesis at the early stages involving production of the nucleotide thymidylic acid (TMP). 5-Fluorocytosine (5-FC) is converted by a deaminase inside fungi to 5-fluorouracil, then to the corresponding nucleoside triphosphate, 5-fluorodeoxyuridine monophosphate (5-F-dUMP) which then acts as an inhibitor of thymidylate synthetase (Figure 12.6). This enzyme normally produces thymidylic acid (TMP) from deoxyuridine monophosphate (dUMP) by addition of a methyl group (supplied by a folate cofactor, section 4.1.1) to the 5 position of the uracil ring. As this position is blocked by the fluoro group in 5-FC, the phosphate acts as an inhibitor of the enzyme. 5-FC can be considered as a prodrug; it has the value of being taken up by fungi as the nucleoside, whereas the active triphosphate produced inside the cells would not be taken up because of its negative charge. Although 5-FC is an important antifungal agent in the treatment of life-threatening infections, resistance can occur due to active efflux of the drug from the cells before it can inhibit DNA synthesis.