Flucytosine

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Mechanism of Action - Fungal DNA Faker

  • Selective Uptake: Enters fungal cells via cytosine-specific permease.
  • Lethal Conversion: Inside the fungus, it's converted to its active forms.
  • DNA Block: 5-FdUMP inhibits thymidylate synthase, blocking dTMP synthesis and halting DNA production.
  • RNA Disruption: 5-FUTP is incorporated into fungal RNA, disrupting protein synthesis.

Selective Toxicity: Human cells lack cytosine deaminase, preventing the conversion of Flucytosine to toxic 5-FU, which is a key reason for its antifungal selectivity.

Antifungal mechanisms of action on fungal cell

📌 Mnemonic: Flucytosine → Fake Uracil → Faulty DNA/RNA.

Clinical Spectrum & Use - The Amphotericin Ally

  • Narrow-spectrum fungistatic; resistance develops rapidly when used as monotherapy.
  • Primary Targets:
    • Cryptococcus neoformans
    • Some Candida spp.
    • Molds causing chromoblastomycosis.
  • Key Strategy: Used synergistically with Amphotericin B.
    • Amphotericin B creates pores in the fungal membrane, which ↑ flucytosine entry.
    • This combination allows for lower, less toxic doses of Amphotericin B.
  • Main Indication:
    • Severe systemic infections, particularly cryptococcal meningitis.

⭐ The synergy is key: Amphotericin B opens the "door" (cell membrane pores), letting Flucytosine enter the fungal cell to inhibit DNA/RNA synthesis.

Pharmacokinetics - Body's Drug Tour

  • Absorption: Well-absorbed orally (>90% bioavailability).
  • Distribution:
    • Distributes widely into body tissues and fluids.
    • Excellent penetration into the CSF (~75% of serum levels).
    • Low plasma protein binding.
  • Metabolism: Minimal host metabolism. Gut flora can convert it to 5-fluorouracil (5-FU), contributing to toxicity.
  • Excretion:
    • Primarily renal; >90% excreted unchanged in urine.
    • ⚠️ Dose reduction is critical in renal insufficiency.

⭐ Its ability to penetrate the CNS makes it vital for treating fungal meningitis, especially cryptococcal, typically in combination with Amphotericin B.

Adverse Effects - Bone Marrow Blues

  • Primary Toxicity: Dose-dependent bone marrow suppression.
  • Mechanism: Intestinal flora convert flucytosine to the antimetabolite 5-fluorouracil (5-FU), a potent myelosuppressant.
    • Inhibits thymidylate synthase, disrupting DNA and RNA synthesis in hematopoietic stem cells.
  • Manifestations:
    • Anemia (↓ RBCs)
    • Leukopenia (↓ WBCs) → risk of infection
    • Thrombocytopenia (↓ Platelets) → risk of bleeding

High-Yield: Toxicity is markedly increased in patients with renal insufficiency due to drug accumulation. Always monitor plasma levels and renal function.

Flucytosine mechanism of action and toxicity

Resistance - Fungal Escape Plan

  • Altered uptake: Mutation in fungal cytosine permease reduces drug entry.
  • Impaired metabolism: Deficient cytosine deaminase or UMP pyrophosphorylase prevents conversion to active metabolites (5-FU, FdUMP).

⭐ Resistance develops rapidly with monotherapy. Always combine, usually with Amphotericin B for synergistic effect.

High‑Yield Points - ⚡ Biggest Takeaways

  • Converted to 5-fluorouracil (5-FU) in fungal cells, which inhibits DNA and RNA synthesis by blocking thymidylate synthase.
  • Almost always used in combination with Amphotericin B, especially for severe systemic mycoses like cryptococcal meningitis.
  • Amphotericin B enhances flucytosine uptake into fungal cells, creating a synergistic effect.
  • Rapid resistance develops when used as monotherapy.
  • Main adverse effect is bone marrow suppression due to systemic conversion to 5-FU.

Practice Questions: Flucytosine

Test your understanding with these related questions

You are taking care of a patient with renal failure secondary to anti-fungal therapy. The patient is a 66-year-old male being treated for cryptococcal meningitis. This drug has a variety of known side effects including acute febrile reactions to infusions, anemia, hypokalemia and hypomagnesemia. What is the mechanism of action of this drug?

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Flashcards: Flucytosine

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Which antifungal inhibits fungal DNA and RNA synthesis?_____

TAP TO REVEAL ANSWER

Which antifungal inhibits fungal DNA and RNA synthesis?_____

Flucytosine

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