Flucytosine is a fluorinated derivative of pyrimidine. Its spectrum of activity is narrower than that of amphotericin B. However, it exhibits a synergetic effect when used in combination with amphotericin B. In sensitive fungi, flucytosine is transformed into 5-fluorouracil, which in turn is turned into 5-fluorodeoxyuracilic acid, an inhibitor of thymidylate synthetase, and correspondingly, DNA synthesis. 5-Fluorouracil triphosphate, which causes the formation of defective RNA, may also be involved in this process. The mechanism is highly selective because mammalian cells are not able to turn a large amount of flucytosine into 5-fluorouracil.
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