Flucytosine

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Cat Number

API2022857

CAS Number

2022-85-7

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Product Detail Description Scientific Support Customer Q&A

CAS Number

2022-85-7

EINECS

217-968-7

Storage

Store at room temperature

Synonyms

Fluorocytosine; Ancobon; Ancotil; 5-Fluorocystosine; 5-Fluorocytosin

Molecular Formula

C4H4FN3O

Molecular Weight

129.09

Smiles

C1=NC(=O)NC(=C1F)N

Appearance

White crystalline powder

Melting Point

298-300°C (dec.)

Relative Density

1.3990 (Estimate)

General Description

Flucytosine is a fluorinated pyrimidine analogue, specifically 5-fluorocytosine, which functions as an antimetabolite. The molecule is a white to off-white crystalline powder that is freely soluble in water. It is a synthetic compound that lacks intrinsic antifungal activity but acts as a prodrug that is converted intracellularly into its active form.

Mechanism of Action

Flucytosine is taken up by fungal cells via cytosine permease and then deaminated by cytosine deaminase to 5-fluorouracil. This active metabolite is further converted to 5-fluorodeoxyuridine monophosphate (FdUMP), which inhibits thymidylate synthase, and to 5-fluorouridine triphosphate (FUTP), which incorporates into RNA. Both actions disrupt DNA and protein synthesis, leading to fungal cell death.

Application

Flucytosine is indicated for the treatment of serious infections caused by susceptible strains of Candida and Cryptococcus, particularly cryptococcal meningitis. It is almost always used in combination with amphotericin B to enhance efficacy and reduce the emergence of resistance. Its use is limited by its narrow spectrum and the risk of bone marrow suppression.

In a phase 2 trial of 48 HIV-positive patients with cryptococcal meningitis, reduced-dose flucytosine (60 mg/kg/day for 10 days) with liposomal amphotericin B and fluconazole showed an early fungicidal activity of 0.28 log10 CFU/mL/day, significantly lower than the historical 0.41 with standard-dose 100 mg/kg/day (P=0.019). Eighteen-week survival was 77%. Rehospitalization (25%) and relapse (6.3%) were higher than expected. Reduced flucytosine dosing demonstrated inferior fungal clearance, supporting continued use of standard dosing.

Fig. 1 Spaghetti plot of individual log10 quantitative cryptococcal culture clearance over time for the FLOOR and historical cohorts. (Rutakingirwa MK, et al., 2026)

References

Rutakingirwa MK, et al. Evaluating the Use of Lower Dose Flucytosine for the Treatment of Cryptococcal Meningitis: A Clinical Trial. Clin Infect Dis. 2026;82(1):67-74.

Does Flucytosine require protection from light during long-term storage?

Yes, it is photosensitive. UV exposure can cause photodegradation and discoloration of the fluorinated pyrimidine ring. Store in light-resistant containers, preferably amber glass.

What is the recommended storage temperature for Flucytosine?

Store at controlled room temperature (15–25°C). Avoid excessive heat above 30°C, which can accelerate degradation.

Is Flucytosine stable in solution for intravenous and oral use?

Aqueous solutions are stable when protected from light and stored at room temperature.

How is the impurity 5-fluorouracil (a degradation product) monitored?

This degradation product is specifically quantified using a stability-indicating HPLC method, ensuring it remains within pharmacopoeial limits.