Chemical Structure and Mechanism of Action
Polyene molecules have a complex structure based on a polyunsaturated macrocyclic lactone ring. They are amphipathic, consisting of both lipophilic and hydrophilic regions.
Their mechanism unfolds at the fungal cytoplasmic membrane:
- The lipophilic region binds to ergosterol, the primary structural sterol of the fungal membrane (acting as the functional analog of human cholesterol).
- The hydrophilic sections of multiple molecules align to form a transmembrane channel (pore).
- Through these newly formed pores, the cell rapidly loses low-molecular-weight metabolites and ions ($K^+$, $Mg^{2+}$).
The loss of intracellular contents leads to pathogen death, producing a fungicidal effect. Selectivity of action is explained by the fact that mammalian cell membranes contain cholesterol, for which polyenes have a significantly lower affinity.
Amphotericin B: The Primary Systemic Agent
This antibiotic has a very high affinity: its binding capacity for ergosterol is 500 times greater than for human cholesterol. In addition to pore formation, amphotericin B generates toxic free radicals, further destabilizing the membrane.
- Spectrum of Activity: Administered via intravenous infusion as a life-saving treatment for severe systemic mycoses (aspergillosis, cryptococcosis, blastomycosis, histoplasmosis, coccidioidomycosis, and systemic candidiasis).
- Non-Antifungal Activity: The agent is lethal to certain protozoa. It is utilized in the treatment of visceral leishmaniasis and primary amoebic meningoencephalitis (caused by Naegleria fowleri).
- Pharmacokinetics: It is not absorbed from the GI tract and penetrates the blood-brain barrier poorly (intrathecal administration may be used for Coccidioides infections). It is eliminated by the kidneys. With chronic administration, the half-life increases from 24–48 hours to up to 15 days due to drug accumulation in tissues.
Toxicity and Lipid Formulations
Amphotericin B carries high toxicity. Adverse reactions are divided into immediate and delayed (organ-specific):
- "Cytokine Storm": An acute reaction occurring during the first hours of infusion (chills, fever, hypotension) caused by the release of TNF-$\alpha$ and IL-1 from immune cells. Managed by slowing the infusion rate and premedication (paracetamol, NSAIDs, hydrocortisone).
- Nephrotoxicity: The primary adverse effect. Constriction of afferent arterioles causes ischemia and proximal tubular injury. Manifests as hypokalemia, hypomagnesemia, and acidosis.
- Hematotoxicity: Thrombocytopenia and anemia (secondary to decreased renal erythropoietin synthesis).
- Neurotoxicity: Headache, seizures, tremor, polyneuropathy, and visual disturbances.
To reduce toxicity, lipid formulations have been developed (liposomal Ambisome, lipid complex Abelcet, colloidal dispersion Amphocil). In these preparations, the antibiotic is incorporated inside a lipid carrier and released preferentially upon contact with the fungus. This reliably protects the patient's kidneys while maintaining full clinical efficacy, though treatment costs are substantially higher.
Topical Agents
Due to high systemic toxicity and a lack of gastrointestinal absorption, certain polyenes (nystatin, levorin) are used exclusively for topical or local application. They are most active against yeast-like fungi of the genus Candida.
Indications for Use:
- Treatment of superficial candidiasis of the skin, mucous membranes (oral cavity, pharynx, vagina), and the intestines.
- Prophylaxis: Prevention of candidiasis during prolonged broad-spectrum antibiotic therapy.
When taken orally, these drugs act only within the lumen of the gastrointestinal tract. Potential adverse effects are limited to allergic reactions and gastrointestinal distress.