Pharmacology of Amphotericin B

Introduction/Overview

Amphotericin B represents a cornerstone polyene macrolide antibiotic in the therapeutic armamentarium against severe, life-threatening systemic fungal infections. First isolated in 1955 from Streptomyces nodosus, it has maintained its status as a gold-standard antifungal agent for decades, often described as “amphoterrible” due to its significant toxicity profile, yet indispensable for its broad-spectrum and fungicidal activity. Its clinical relevance remains paramount, particularly for the treatment of invasive fungal infections in immunocompromised hosts, including patients with hematological malignancies, organ transplant recipients, and those with advanced HIV infection. The development of lipid-based formulations has been a pivotal advancement, mitigating some toxicities while preserving efficacy, thereby expanding its therapeutic window. Understanding the pharmacology of amphotericin B is essential for clinicians to balance its potent antifungal effects against its considerable adverse reaction potential.

Learning Objectives

• Describe the chemical classification of amphotericin B and the rationale behind the development of lipid-based formulations.
• Explain the detailed molecular mechanism of action, including its primary fungicidal effect and secondary immunomodulatory properties.
• Analyze the pharmacokinetic profile, highlighting differences between conventional deoxycholate and lipid-based formulations.
• Identify the major clinical indications, including empirical, targeted, and prophylactic uses in systemic mycoses.
• Evaluate the spectrum of adverse effects, management strategies for infusion-related reactions and nephrotoxicity, and significant drug interactions.

Classification

Amphotericin B is definitively classified as a polyene macrolide antifungal antibiotic. The term “polyene” refers to its chemical structure, which contains a large macrolide lactone ring with multiple conjugated double bonds (typically three to seven; amphotericin B possesses seven). This hydrophobic polyene chain is linked to a hydrophilic polyol region containing multiple hydroxyl groups, conferring the molecule’s amphipathic nature—a property central to its mechanism of action. The molecule is practically insoluble in water, necessitating formulation with a solubilizing agent for intravenous administration.

Formulation Categories

The clinical utility of amphotericin B is intrinsically linked to its formulations, which are categorized as follows:

• Amphotericin B deoxycholate (AMB-d): The conventional formulation, a colloidal dispersion solubilized by the bile salt sodium deoxycholate. This formulation is associated with the highest incidence of acute and chronic toxicities.
• Lipid-Based Formulations: Developed to reduce toxicity while maintaining antifungal efficacy. These include:
  • Liposomal Amphotericin B (L-AMB): Amphotericin B incorporated into small, unilamellar bilayer liposomes composed of phospholipids and cholesterol.
  • Amphotericin B Lipid Complex (ABLC): A ribbon-like complex of amphotericin B and phospholipids.
  • Amphotericin B Colloidal Dispersion (ABCD): A discoid-shaped complex with cholesteryl sulfate (less commonly used).

These lipid formulations alter the pharmacokinetics and tissue distribution, leading to a modified toxicity profile, notably reduced nephrotoxicity, but at a substantially higher financial cost.

Mechanism of Action

The primary mechanism of action of amphotericin B is its binding to ergosterol, the principal sterol component of fungal cell membranes. This interaction underlies its potent fungicidal activity against a broad range of pathogenic fungi.

Molecular and Cellular Mechanisms

The amphipathic structure of amphotericin B allows it to integrate into the fungal cell membrane. The lipophilic polyene segment interacts with the ergosterol, while the hydrophilic polyol region faces the aqueous environment. This integration leads to the formation of transmembrane pores or channels. These pores are aggregates of several amphotericin B molecules that create an aqueous channel through the lipid bilayer. The formation of these channels disrupts the membrane’s integrity, leading to the uncontrolled leakage of intracellular ions (primarily potassium and magnesium) and small molecules. This efflux results in:

1. Collapse of the electrochemical gradient essential for nutrient transport and cellular homeostasis.
2. Ultimate cell lysis and death, characterizing its fungicidal effect.

The drug’s selective toxicity towards fungal over mammalian cells is relative, not absolute. It arises from a higher affinity for ergosterol (K_a ≈ 10⁶ M^-1) compared to cholesterol (K_a ≈ 10⁴ M^-1), the major sterol in human cell membranes. However, this differential affinity is insufficient to prevent significant interaction with human cells, particularly renal tubular cells and erythrocytes, which accounts for its major toxicities.

Secondary Mechanisms and Immunomodulatory Effects

Beyond direct membrane disruption, amphotericin B exhibits several secondary pharmacological effects that may contribute to its clinical efficacy:

• Oxidative Damage: The drug can act as an oxidative catalyst, promoting the generation of reactive oxygen species within the fungal cell, which contributes to cellular damage.
• Immune System Modulation: Amphotericin B can stimulate the host immune response. It has been shown to:
  • Act as an immunoadjuvant, enhancing macrophage and neutrophil phagocytosis and killing of fungi.
  • Stimulate the production of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and interleukin-1β (IL-1β), which may explain some infusion-related reactions.
  • Augment the oxidative burst in phagocytes.
• Binding to Other Sterols: At high concentrations, it may bind to cholesterol in human membranes, leading to toxicity.

Pharmacokinetics

The pharmacokinetics of amphotericin B are complex, highly dependent on the formulation administered, and characterized by extensive tissue binding and a prolonged terminal half-life.

Absorption, Distribution, Metabolism, and Excretion

Absorption: Amphotericin B is not absorbed from the gastrointestinal tract. Oral formulations are used only for localized gut infections (e.g., oral candidiasis). For systemic therapy, it must be administered intravenously. Intrathecal administration is rarely used for refractory meningeal infections.

Distribution: The drug is highly protein-bound (>90%) and distributes widely throughout the body. However, distribution varies significantly between formulations:

• AMB-d: Achieves high concentrations in the liver, spleen, lungs, and kidneys. Penetration into the cerebrospinal fluid (CSF), vitreous humor, and amniotic fluid is poor (<5% of plasma levels).
• Lipid Formulations: Are preferentially taken up by the mononuclear phagocyte system (reticuloendothelial system) in the liver, spleen, and lungs. This “stealth” delivery results in lower circulating free drug and consequently lower renal concentrations. Liposomal amphotericin B (L-AMB) achieves higher concentrations in infected tissues, including the brain, compared to AMB-d, due to enhanced delivery via inflamed endothelium.

The volume of distribution (V_d) is large, often exceeding total body water, reflecting extensive tissue sequestration.

Metabolism: The metabolic pathway of amphotericin B is not fully elucidated. It appears to undergo minimal hepatic metabolism. There is no evidence of cytochrome P450-mediated metabolism, which limits certain metabolic drug interactions.

Excretion: Elimination is biphasic and very slow. The drug is excreted primarily via non-renal pathways. Only a small fraction (2-5%) of the administered dose appears unchanged in the urine over several days. The majority is believed to be slowly eliminated via the bile and feces, with a portion being degraded in situ. Renal clearance does not correlate with creatinine clearance, and dose adjustments in renal impairment are not based on pharmacokinetics but on toxicity monitoring.

Half-life and Dosing Considerations

The terminal elimination half-life (t_1/2) is prolonged, approximately 15 days for AMB-d, and can be even longer for lipid formulations due to slow release from deep tissue stores. This long half-life supports once-daily dosing. Steady-state concentrations are typically reached after 7-10 days of therapy.

Dosing is based on the specific formulation, indication, and patient tolerance:

• AMB-d: A test dose (1 mg over 20-30 minutes) was historically recommended but is of limited predictive value. Therapeutic doses range from 0.5-1.5 mg/kg/day, infused over 2-6 hours. Higher doses (up to 1.5 mg/kg/day) are used for severe infections like mucormycosis.
• Liposomal Amphotericin B (L-AMB): Standard dosing is 3-5 mg/kg/day for most invasive fungal infections, but doses of 10 mg/kg/day may be used for mucormycosis. It is infused over 2 hours.
• ABLC: Dosed at 5 mg/kg/day, infused over 2 hours.

Therapeutic drug monitoring of serum concentrations is not routinely performed in clinical practice, as efficacy correlates more with dose and susceptibility of the pathogen, and toxicity is managed clinically.

Therapeutic Uses/Clinical Applications

Amphotericin B remains a first-line or important alternative agent for numerous serious systemic fungal infections due to its broad spectrum, fungicidal nature, and low incidence of microbial resistance.

Approved Indications

• Invasive Candidiasis: Particularly for critically ill patients, those with Candida krusei or fluconazole-resistant Candida glabrata infections, and in hemodynamically unstable patients. Lipid formulations are often preferred in patients with or at risk for nephrotoxicity.
• Cryptococcal Meningitis: A cornerstone of induction therapy, especially in HIV-associated disease, typically combined with flucytosine. AMB-d or L-AMB are used for at least the first two weeks.
• Invasive Aspergillosis: Voriconazole is first-line, but amphotericin B (particularly lipid formulations) is a primary alternative for salvage therapy or when triazoles are contraindicated.
• Mucormycosis (Zygomycosis): High-dose lipid-based amphotericin B (L-AMB or ABLC) is the first-line pharmacological therapy, combined with aggressive surgical debridement.
• Endemic/Systemic Mycoses: First-line for severe, disseminated, or meningeal forms of histoplasmosis, blastomycosis, coccidioidomycosis, and paracoccidioidomycosis. Often used in the acute, severe phase before stepping down to an oral azole.
• Empirical Antifungal Therapy: In persistently febrile neutropenic patients unresponsive to broad-spectrum antibiotics, lipid-based amphotericin B is an effective option, though echinocandins and voriconazole are also used.
• Visceral Leishmaniasis: Amphotericin B is highly effective against the protozoan Leishmania donovani complex. Liposomal amphotericin B is the drug of choice in many regions due to its high efficacy with a short course and reduced toxicity.

Off-Label and Specialized Uses

• Antifungal Prophylaxis: In very high-risk hematopoietic stem cell transplant recipients, low-dose L-AMB may be used.
• Topical/Bladder Irrigation: AMB-d solutions are used for fungal cystitis via bladder irrigation and for topical application in fungal keratitis.
• Candiduria in Renal Failure: Bladder irrigation with AMB-d may be considered when systemic therapy is contraindicated.

Adverse Effects

The adverse effect profile of amphotericin B is substantial and often dose-limiting. Adverse reactions can be categorized as infusion-related, chronic, and metabolic.

Common Side Effects

Infusion-Related Reactions: Occur during or within 1-3 hours of infusion, mediated largely by prostaglandins and cytokines (especially TNF-α). Symptoms include:

• Fever, chills, rigors (often severe)
• Nausea, vomiting, headache
• Hypotension, tachycardia, tachypnea
• Generalized malaise

These reactions are most common with the initial doses and often diminish with continued therapy. Premedication with acetaminophen, diphenhydramine, and/or hydrocortisone is standard. Meperidine can be effective for severe rigors. Slowing the infusion rate may also help. Lipid formulations, particularly L-AMB, induce these reactions less frequently.

Nephrotoxicity: The most significant chronic toxicity. Mechanisms include:

1. Vasoconstriction of the afferent renal arteriole, reducing glomerular filtration rate (GFR).
2. Direct tubular cell toxicity from membrane interaction, leading to potassium and magnesium wasting, and distal renal tubular acidosis.

Manifestations include a rise in serum creatinine, hypokalemia, hypomagnesemia, and renal tubular acidosis. Nephrotoxicity is dose-dependent and often reversible upon discontinuation, but permanent damage can occur. Risk is highest with AMB-d. Preventive strategies include sodium loading (administration of 500-1000 mL of normal saline before infusion), avoiding concurrent nephrotoxins (e.g., aminoglycosides, cyclosporine), and using lipid formulations.

Electrolyte Disturbances: Hypokalemia and hypomagnesemia are nearly universal and can be profound, requiring aggressive oral or intravenous replacement.

Serious/Rare Adverse Reactions

• Anaphylaxis: Rare but can occur. A true anaphylactic reaction is a contraindication to further use.
• Cardiotoxicity: Rapid infusion can cause arrhythmias, including ventricular fibrillation. This is thought to be related to electrolyte disturbances or a direct effect. Infusions should never be given as a bolus.
• Hematologic Toxicity: Normocytic, normochromic anemia is common due to suppression of erythropoietin production. Leukopenia and thrombocytopenia are rare.
• Hepatotoxicity: Mild elevations in liver enzymes can occur; severe hepatitis is uncommon.
• Neurotoxicity: Can occur with intrathecal administration (arachnoiditis, seizures) or very high systemic doses.

There are no specific black box warnings for amphotericin B, but its severe potential toxicities are prominently featured in its prescribing information.

Drug Interactions

Amphotericin B has several clinically significant drug interactions, primarily pharmacodynamic rather than pharmacokinetic, given its lack of CYP450 metabolism.

Major Drug-Drug Interactions

• Other Nephrotoxic Agents: Concurrent use with aminoglycosides, cyclosporine, tacrolimus, cisplatin, vancomycin, or foscarnet produces additive or synergistic nephrotoxicity. This combination should be avoided if possible, or renal function must be monitored with extreme vigilance. Dose reduction of the concomitant agent or substitution with a lipid-based amphotericin B formulation may be necessary.
• Diuretics: Loop diuretics (e.g., furosemide) and thiazides can exacerbate amphotericin B-induced hypokalemia and hypomagnesemia. Potassium-sparing diuretics may be preferable, but careful monitoring of electrolytes is mandatory.
• Corticosteroids: Concurrent use of corticosteroids (e.g., hydrocortisone used for premedication) can potentiate hypokalemia, especially with fludrocortisone-like activity.
• Cardiac Glycosides (Digoxin): Amphotericin B-induced hypokalemia and hypomagnesemia increase the risk of digitalis toxicity (arrhythmias). Serum potassium and magnesium levels must be maintained in the high-normal range.
• Neuromuscular Blocking Agents: Hypokalemia can potentiate the effects of both depolarizing (succinylcholine) and non-depolarizing (vecuronium, rocuronium) agents.
• Flucytosine (5-FC): This combination is synergistic against Cryptococcus and some Candida species. However, amphotericin B can impair renal excretion of flucytosine, leading to elevated 5-FC levels and increased risk of bone marrow toxicity (leukopenia, thrombocytopenia). When used together, flucytosine doses should be reduced, and serum levels monitored if available.

Contraindications

Absolute contraindications are limited to a documented history of severe hypersensitivity or anaphylaxis to amphotericin B itself. Cross-reactivity between the deoxycholate and lipid formulations is possible but not absolute; a patient with a severe reaction to one formulation may sometimes tolerate another under close supervision. Relative contraindications include severe pre-existing renal impairment (where a lipid formulation is strongly preferred) and severe hypokalemia or hypomagnesemia, which should be corrected prior to initiation if possible.

Special Considerations

Use in Pregnancy and Lactation

Pregnancy (FDA Category B): Animal reproduction studies have not demonstrated teratogenicity, but controlled human studies are lacking. Amphotericin B crosses the placenta poorly. It is generally considered the antifungal of choice for treating life-threatening systemic fungal infections during pregnancy because its known risks (maternal toxicity) are often outweighed by the benefit of treating a serious infection, and the teratogenic potential of alternative azoles (especially fluconazole in high doses) is a greater concern. Maternal electrolyte disturbances and renal function require close monitoring.

Lactation: It is not known whether amphotericin B is excreted in human milk. Given its poor oral bioavailability, systemic absorption by a nursing infant from ingested milk is likely negligible. However, caution is typically advised, and the decision to breastfeed should weigh the importance of the drug to the mother.

Pediatric and Geriatric Considerations

Pediatrics: Amphotericin B is used extensively in pediatric patients for the same indications as in adults. Dosing is weight-based (mg/kg). Children may tolerate the drug better than adults with respect to nephrotoxicity but are equally susceptible to infusion reactions and electrolyte disturbances. Close monitoring of renal function and electrolytes is essential. Lipid formulations are often used in neonates and infants to minimize renal risk.

Geriatrics: Older patients often have reduced renal reserve and are more susceptible to nephrotoxicity and electrolyte imbalances. They may also be on multiple medications that interact with amphotericin B (e.g., diuretics, digoxin). Lower initial doses, aggressive sodium loading, preferential use of lipid formulations, and meticulous monitoring of renal function and electrolytes are paramount. Volume status must be carefully assessed to avoid fluid overload with pre-hydration.

Renal and Hepatic Impairment

Renal Impairment: Dose adjustment is not required based on pharmacokinetics. However, the risk of worsening renal function is high. In patients with pre-existing renal impairment (serum creatinine >2.5 mg/dL), a lipid-based formulation is strongly recommended to minimize further nephrotoxicity. If AMB-d must be used, dosing may be altered to an every-other-day schedule at a higher dose, though this is less common now. The cornerstone of management is prevention: sodium loading, avoiding concurrent nephrotoxins, and vigilant monitoring.

Hepatic Impairment: No specific dose adjustments are required. Amphotericin B is not primarily metabolized by the liver, and significant hepatotoxicity is uncommon. However, liver function tests should be monitored periodically during prolonged therapy.

Summary/Key Points

• Amphotericin B is a polyene macrolide antifungal agent with a broad spectrum and fungicidal activity, remaining critical for treating life-threatening systemic mycoses.
• Its mechanism involves binding to ergosterol in fungal cell membranes, forming pores that lead to cell death. Secondary immunomodulatory effects may contribute to efficacy.
• Pharmacokinetics are formulation-dependent. Conventional amphotericin B deoxycholate (AMB-d) causes significant nephrotoxicity, while lipid-based formulations (L-AMB, ABLC) alter tissue distribution to reduce renal exposure and toxicity, albeit at higher cost.
• Major clinical indications include invasive candidiasis, cryptococcal meningitis, mucormycosis (first-line), invasive aspergillosis (salvage), endemic mycoses, and visceral leishmaniasis.
• The adverse effect profile is dominated by infusion-related reactions (fever, chills, rigors) and dose-limiting nephrotoxicity, accompanied by almost universal hypokalemia and hypomagnesemia.
• Significant drug interactions are primarily pharmacodynamic, involving additive nephrotoxicity with other agents (aminoglycosides, calcineurin inhibitors) and exacerbation of electrolyte disturbances with diuretics and digoxin.
• Special populations require careful management: lipid formulations are preferred in renal impairment and pregnancy (for serious infections), while pediatric and geriatric patients need meticulous electrolyte and renal function monitoring.

Clinical Pearls

• Premedicate for infusion reactions (acetaminophen ± diphenhydramine ± hydrocortisone). Meperidine is effective for severe rigors.
• Always administer a liter of normal saline pre-hydration before AMB-d to reduce nephrotoxicity, unless contraindicated by volume status.
• Monitor serum creatinine, potassium, and magnesium at least twice weekly during therapy. Aggressively replace potassium and magnesium.
• When combining with flucytosine, reduce the flucytosine dose and monitor for hematologic toxicity.
• In a patient with deteriorating renal function on AMB-d, consider switching to a lipid-based formulation rather than discontinuing antifungal therapy entirely.
• The high cost of lipid formulations is often justified by reduced nephrotoxicity, shorter hospital stays, and avoidance of dialysis in high-risk patients.

References

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