Polyenes (amphotericin formulations)

Mechanism of Action - Pore-Forming Punishers

• Binds directly to ergosterol, the principal sterol in fungal cell membranes, altering their structure.
• This binding creates aggregates that form transmembrane pores or channels.
• These pores disrupt membrane integrity, causing rapid leakage of essential intracellular cations (K⁺, Mg²⁺) and macromolecules, leading to fungal cell death (fungicidal).

📌 AmphoTERicin TEARS holes in the fungal membrane.

⭐ The drug's major toxicity, nephrotoxicity, arises from its ability to also bind to cholesterol in mammalian cell membranes, particularly in the renal tubules. Liposomal formulations help mitigate this by selectively targeting fungal cells.

Formulations & PK - Fancy Lipid Packages

• Conventional (C-AMB): Amphotericin B deoxycholate

  • Micellar suspension; notorious for toxicity.
  • Binds to both fungal ergosterol and human cholesterol, causing significant nephrotoxicity and infusion reactions.

• Lipid Formulations (L-AMB, ABLC, ABCD)

  • Package AmB in lipid carriers to ↓ toxicity.
  • Higher affinity for fungal ergosterol, sparing human cells.
  • PK: All IV. Large volume of distribution (Vd); sequestered in reticuloendothelial system (liver, spleen), reducing free drug in plasma and thus kidney exposure.

⭐ The primary advantage of lipid formulations is a significant reduction in nephrotoxicity compared to conventional amphotericin B.

Spectrum & Clinical Use - Fungal Foe Fighters

• Broad Spectrum: Active against a wide range of pathogenic fungi, but NOT dermatophytes.
• Key Fungal Targets:
  • Candida spp. (including many azole-resistant strains)
  • Cryptococcus neoformans
  • Aspergillus spp.
  • Endemic Dimorphic Fungi (Histoplasma, Blastomyces, Coccidioides)
  • Mucorales (agent of choice)
• Primary Clinical Use: Reserved for severe, life-threatening systemic mycoses.
  • Initial/induction therapy for cryptococcal meningitis (with flucytosine).
  • Empiric treatment for febrile neutropenia unresponsive to antibiotics.

⭐ Amphotericin B is often used as initial induction therapy to rapidly reduce fungal burden before transitioning to a less toxic azole for maintenance.

Adverse Effects - Ampho-Terrible's Toll

• Immediate (Infusion-Related)

  • Fever, chills, rigors ("shake and bake"), headache, hypotension.
  • Management: Pre-medicate with NSAIDs, antihistamines, and hydrocortisone.

• Cumulative (Dose-Limiting)

  • Nephrotoxicity: Major toxicity. Binds to cholesterol in the distal renal tubule, creating pores that leak electrolytes.
  • Causes afferent arteriolar vasoconstriction, reducing GFR.
  • 📌 Mnemonic: "Ampho-tears-a-hole" in the renal tubule membrane.

  ⭐ Causes Type 1 (distal) Renal Tubular Acidosis → severe hypokalemia & hypomagnesemia. Mitigate with pre-infusion saline loading.

• Hematologic

  • Normocytic, normochromic anemia from suppressed erythropoietin production by damaged kidneys.

• Other

  • Thrombophlebitis at infusion site (less with central lines).

High‑Yield Points - ⚡ Biggest Takeaways

• Binds ergosterol in the fungal cell membrane, creating pores that lead to cell death.
• The drug of choice for many life-threatening, systemic mycoses like Cryptococcus, Aspergillus, and Mucor.
• Major toxicities include infusion-related reactions ("shake and bake") and severe nephrotoxicity.
• Lipid formulations (e.g., liposomal amphotericin B) significantly reduce renal toxicity.
• Causes predictable electrolyte wasting, leading to hypokalemia and hypomagnesemia.
• Always pre-medicate and ensure adequate hydration to mitigate adverse effects.