Description
5-
Fluorocytosine (5-
FC), a fluorinated pyrimidine analog, is a synthetic antimycotic prodrug that is converted by cytosine deaminase to 5-
fluorouracil.
1 5-
Fluorouracil, a widely used cytotoxic drug, is further metabolized to fluorinated ribo-
and deoxyribonucleotides, resulting in the inhibition of DNA and protein synthesis, which has multiple effects including inhibition of
Candida species and
C. neoformans infections and cytotoxicity towards cancer cells.
1,2 In combination with a retroviral replicating vector carrying a cytosine deaminase prodrug-
activating gene, 5-
FC has been shown to selectively eliminate CT26 and Tu-
2449 tumor cells
in vitro (IC
50s = 4.2 and 1.5 μM, respectively) and to significantly improve survival and reduce tumor size (at a dose of 500 mg/kg) in two different syngeneic mouse glioma models.
3
Chemical Properties
White Crystalline Solid
Originator
Ancobon,Roche,US,1972
Uses
5-FC is a toxic antifungal/antimicrobial agent
Uses
antifungal and antimicrobial agent
Definition
ChEBI: An organofluorine compound that is cytosine that is substituted at position 5 by a fluorine. A prodrug for the antifungal 5-fluorouracil, it is used for the treatment of systemic fungal infections.
Indications
Flucytosine (Ancobon) is a synthetic, fluorinated pyrimidine that is structurally
related to fluorouracil (FU) and floxuridine. It can be fungistatic and
fungicidal. Although it is used more frequently in the treatment of systemic
infections caused by Candida and Cryptococcus, dermatologic indications
may include infections due to chromomycosis, sporotrichosis, Cladosporium,
and Sporothrix species. It is generally ineffective against Aspergillus species.
Manufacturing Process
The preparation of 5-fluorouracil is given under "Fluorouracil." As described in
US Patent 3,040,026, 5-fluorouracil is then subjected to the following steps to
give flucytosine.
Step 1: 2,4-Dichloro-5-Fluoropyrimidine - A mixture of 104 grams (0.8 mol)
of 5-fluorouracil, 1,472 grams (9.6 mols) of phosphorus oxychloride and 166
grams (1.37 mols) of dimethylaniline was stirred under reflux for 2 hours.
After cooling to room temperature, phosphorus oxychloride was removed by
distillation at 18 to 22 mm and 22° to 37°C. The residue was then poured into
a vigorously stirred mixture of 500 ml of ether and 500 gram of ice. After
separating the ether layer, the aqueous layer was extracted with 500 ml, then
200 ml of ether. The combined ether fractions were dried over sodium sulfate,
filtered, and the ether removed by vacuum distillation at 10° to 22°C. The
residue, a yellow solid melting at 37° to 38°C, weighed 120 grams
corresponding to a 90% yield. Vacuum distillation of 115 grams of this
material at 74° to 80°C (16 mm) gave 108 grams of white solid melting at
38° to 39°C corresponding to an 84.5% yield.
Step 2: 2-Chloro-4-Amino-5-Fluoropyrimidine - To a solution of 10.0 grams
(0.06 mol) of 2,4-dichloro-5-fluoropyrimidine in 100 ml of ethanol, 25 ml of
concentrated aqueous ammonia were slowly added. A slightly opalescent
solution resulted. The temperature gradually rose to 35°C. The solution was
then cooled in ice to 18°C and thereafter remained below 30°C. After three
hours, a Volhard titration showed that 0.0545 mol of chlorine was present in
ionic form. Storage in a refrigerator overnight resulted in some crystallization
of ammonium chloride. A white sludge, resulting from the evaporation of the
reaction mixture at 40°C, was slurried with 75 ml of water, filtered and
washed free of chloride. After drying in vacuo, the product melted at 196.5°
to 197.5°C, yield 6.44 grams. Evaporation of the mother liquors yielded a
second crop of 0.38 gram, raising the total yield to 6.82 grams (79.3%).
Step 3: 5-Fluorocytosine - A slurry of 34.0 grams (0.231 mol) of 2-chloro-4-
amino-5-fluoropyrimidine in 231 ml of concentrated hydrochloric acid was
heated in a water bath at 93° to 95°C for 125 minutes. The reaction was
followed by means of ultraviolet spectrophotometry using the absorption at
245, 285, and 300 mμ as a guide. The absorption at 300 mμ rose to a
maximum after 120 minutes and then dropped slightly. The clear solution was
cooled to 25°C in an ice bath, then evaporated to dryness under vacuum at
40°C. After slurrying with water three times and reevaporating, the residue
was dissolved in 100 milliliters of water. To this solution, cooled in ice, 29 ml
of concentrated ammonia were added dropwise. The resulting precipitate was
filtered, washed free of chloride with water, then with alcohol and ether. After
drying in vacuo at 65°C, the product weighed 22.3 grams. An additional 6.35
grams was obtained by evaporation of the mother liquor, thus yielding a total
of 28.65 grams (96.0%).
Brand name
Ancobon (Valeant).
Therapeutic Function
Antifungal
Antimicrobial activity
The spectrum of activity is restricted to Candida spp., Cryptococcus
spp. and some fungi causing chromoblastomycosis.
Acquired resistance
About 2–3 of Candida spp. isolates (more in some centers) are
resistant before treatment starts, and resistance may develop
during treatment. The most common cause of resistance
appears to be loss of the enzyme uridine monophosphate
pyrophosphorylase.
General Description
Chemical structure: nucleoside
Pharmaceutical Applications
A synthetic fluorinated pyrimidine available for intravenous
infusion or oral administration.
Biochem/physiol Actions
Nucleoside analog that has antifungal activities. 5-FC is deaminated by cytosine deaminase to product 5-fluorouracil, resulting in RNA miscoding. 5-Fluorocytosine inhibits DNA and RNA synthesis and interferes with ribosomal protein synthesis.
Mechanism of action
Flucytosine (5-flucytosine, 5-FC; Ancoban) is a fluorinated
pyrimidine analogue of cytosine that was originally
synthesized for possible use as an antineoplastic
agent. 5-FC is converted to 5-fluorouracil inside the
cell by the fungal enzyme cytosine deaminase. Subsequently,
5-FC metabolites interfere with fungal DNA
synthesis by inhibiting thymidylate synthetase. Incorporation
of these metabolites into fungal RNA may inhibit
protein synthesis.
Pharmacokinetics
Oral absorption: Complete
Cmax 25 mg/kg 6-hourly oral: 70–80 mg/L after 1–2 h
Plasma half-life: 3–6 h
Volume of distribution: 0.7–1 L/kg
Plasma protein binding c. 12%
Absorption is slower in persons with impaired renal function,
but peak concentrations are higher. Levels in the CSF
are around 75% of the simultaneous serum concentration.
More than 90% of a dose of flucytosine is excreted in the
urine in unchanged form. The serum half-life is much longer
in renal failure, necessitating modification of the dosage regimen:
for patients with a creatinine clearance below 40 mL/
min the dosage interval should be doubled to 12 h; in severe
renal failure the dosage interval should be further increased to
once daily or less, based on frequent serum drug concentration
measurements.
Pharmacology
5-FC is well absorbed orally, with greater than 90%
bioavailability. The serum half-life is 3 to 5 hours, with
serum levels peaking 4 to 6 hours after a single dose.The
drug is widely distributed in body fluids, with cerebrospinal
fluid levels 60 to 80% of serum levels.The drug
also penetrates well into urine, aqueous humor, and
bronchial secretions.Minimal serum protein binding allows
more than 90% of each dose to be excreted in the
urine; significant dosage reductions are required in the
presence of renal impairment. 5-FC can be removed by
both hemodialysis and peritoneal dialysis. 5-FC conversion
to toxic metabolites may occur in mammalian cells
to a limited extent, which accounts for 5-FC toxicity.
Clinical Use
5-Fluorocytosine, 5-FC, 4-amino-5-fluoro-2(1H)-pyrimidinone, 2-hydroxy-4-amino-5-fluoropyrimidine (Ancobon). 5-Fluorocytosine is an orally active antifungal agent with a very narrow spectrum of activity. It is indicated only for the treatment of serious systemic infections caused by susceptible strains of Candida and Cryptococcus spp.The mechanism of action of 5-fluorocytosine (5-FC)has been studied in detail.The drug enters the fungal cell by active transport onATPases that normally transport pyrimidines. Once insidethe cell, 5-fluorocytosine is deaminated in a reaction catalyzedby cytosine deaminase to yield 5-fluorouracil(5-FU). 5-Fluorouracil is the active metabolite of the drug.5-Fluorouracil enters into pathways of both ribonucleotideand deoxyribonucleotide synthesis. The fluororibonucleotidetriphosphates are incorporated into RNA, causingfaulty RNA synthesis. This pathway causes cell death. Inthe deoxyribonucleotide series, 5-fluorodeoxyuridinemonophosphate (F-dUMP) binds to 5,10-methylenetetrahydrofolicacid, interrupting the one-carbon pool substratethat feeds thymidylate synthesis. Hence, DNA synthesisis blocked.
Clinical Use
Candidosis (in combination with amphotericin B or fluconazole)
Cryptococcosis (in combination with amphotericin B or fluconazole)
Monitoring of flucytosine concentrations is desirable in all
patients, and mandatory in those with renal impairment.
Clinical Use
Flucytosine has significant antifungal activity against
C. albicans, other Candida spp., C. neoformans, and the
fungal organisms responsible for chromomycosis. Not
considered the drug of choice for these fungal infections,
5-FC does remain useful as part of combination
therapy for systemic candidiasis and cryptococcal
meningitis and as an alternative drug for chromomycosis.
When it is used as monotherapy, resistance and
clinical failure are common. Potential mechanisms for
drug resistance include decreased fungal cell membrane
permeability and reduced levels of fungal
cytosine deaminase. Combination therapy with amphotericin
B and flucytosine in the treatment of
cryptococcal meningitis and deep-seated Candida infections,
such as septic arthritis and meningitis, permits
reduced dosing of amphotericin B and prevents the
emergence of 5-FC resistance. When higher doses of
amphotericin B are used, combination therapy with
5-FC confers no additional clinical benefit except in
the treatment of Candida endophthalmitis, where tissue
penetration remains problematic.
Side effects
Nausea, vomiting, abdominal pain and diarrhea are common.
Serious side effects include myelosuppression and hepatic
toxicity; they occur more frequently when serum concentrations
exceed 100 mg/L.
The nephrotoxic effects of amphotericin B can result in
elevated blood concentrations of flucytosine, and levels of the
latter drug should be monitored when these compounds are
administered together.
Side effects
When 5-FC is prescribed alone to patients with normal
renal function, skin rash, epigastric distress, diarrhea,
and liver enzyme elevations can occur.When it is prescribed
to patients with renal insufficiency or to patients
receiving concurrent amphotericin B therapy, blood levels
of 5-FC may rise, and bone marrow toxicity leading
to leukopenia and thrombocytopenia is common. 5-FC
serum levels should be closely monitored in patients
with renal insufficiency. Because of baseline leukopenia,
5-FC is often not tolerated by end-stage HIVinfected
patients with disseminated fungal infection.
Synthesis
Flucytosine, 5-fluorocytosine (35.4.4), is synthesized from fluorouracil (30.1.3.3). Fluorouracil is reacted with phosphorous oxychloride in dimethylaniline to make 2,4-dichloro-5-fluoropyrimidine (35.4.2), which is reacted with ammonia to make a product substituted with chlorine at the fourth position of the pyrimidine ring—4-amino- 2-chloro-5-fluoropyrimidine (35.4.3). Hydrolysis of the chlorovinyl fragment of this compound in a solution of hydrochloric acid gives the desired flucytosine.
An alternative way of synthesis consists of making flucytosine from a precursor of fluorouracil—5-fluoro-2-methylthiouracil (30.1.3.2) using a somewhat analogous scheme. Treating 5-fluoro-2-methylthiouracil (30.1.3.2) with phosphorous pentachloride gives 4-chloro-5-fluoro-2-methylthiopyrimidine (35.4.5), which upon being reacted with ammonium is transformed into 4-amino-5-fluoro-2-methylthiopyrimidine (35.4.6). Hydrolysis of the methylthiovinyl fragment using concentrated hydrobromic acid gives the desired flucytosine.
Drug interactions
Potentially hazardous interactions with other drugs
Cytarabine: concentration of flucytosine possibly
reduced.
Metabolism
Flucytosine itself is not cytotoxic but, rather, is a pro-drug that is taken up by fungi and metabolized to 5-fluorouracil (5-FU) by fungal cytidine deaminase. Then, 5-FU is converted to 5-fluorodeoxyuridine, which as a thymidylate synthase inhibitor interferes with both protein and RNA biosynthesis. 5-Fluorouracil is cytotoxic and is employed in cancer chemotherapy. Human cells do not contain cytosine deaminase and, therefore, do not convert flucytosine to 5-FU. Some intestinal flora, however, do convert the drug to 5-FU, so human toxicity does result from this metabolism.
