What is flucytosine used to treat?

Fluorocytosine was initially synthesized at Hoffmann-La Roche Pharmaceuticals as an antimetabolite. Flucytosine has a narrow in vitro antimycotic activity spectrum that includes yeasts, Cryptococcus neoformans, as well as some Aspergillus strains and some chromomycetes. 

Because the antimycotic action of flucytosine is antagonized by purines, pyrimidines, peptides, and some amino acids in vitro, only media free from these antagonists, e.g., yeast – nitrogen base medium containing dextrose, can be used for in vitro testing. Of the fungus strains affected by flucytosine, more than 10 % can be expected to show primary resistance. The development of secondary resistance by sensitive fungi during treatment also occurs, especially in cases of underdosage or prolonged treatment. In vitro fungicidal effects can be obtained with > 10 times the MIC and a contact time of > 150 h.

Mechanism of Action

The mechanism of action of flucytosine is based on the incorporation of 5-fluorouracil, which is relatively selectively formed in fungal cells by deamination of flucytosine by the fungal cytosine-desaminase, into the messenger RNA and ribosomal RNA of the fungal cell. Fungal RNA containing 5- fluorouracil-riboside increasingly inhibits fungal protein biosynthesis. 

Indications

Flucytosine has proved very effective in the treatment of Candida fungemia and systemic candidoses (but not mucocutaneous candidiasis) in combination with amphotericin B. Also, Cryptococcus infections, including Cryptococcus neoformans meningoencephalitis in immunocompromized patients (e.g., AIDS patients), can be treated with flucytosine in combination with amphotericin B (and in the case of Cryptococcus neoformans associated infections, additionally fluconazole). Further, flucytosine is indicated in the therapy of chromomycoses. There can also be some activity of flucytosine in disseminated aspergilloses. Because of the rapid development of resistance, the substance should only be given in combination with other antimycotic agents, mostly amphotericin B.