Amphotericin B

Overview

Amphotericin B is a potent polyene antifungal agent reserved for the treatment of severe, life-threatening systemic fungal infections in dogs and cats. Because of its narrow therapeutic margin and high potential for toxicity, it is generally used only when disease is progressive, refractory to safer antifungal agents (such as azoles), or when antifungal susceptibility results specifically indicate its use.

Amphotericin B must be administered parenterally to achieve effective systemic concentrations. It is not absorbed adequately from the gastrointestinal tract and therefore is given by intravenous infusion or, in selected protocols, as a diluted subcutaneous infusion. Therapy is typically performed in a hospital setting due to the need for careful drug handling, fluid administration, and frequent laboratory monitoring.

Several formulations of amphotericin B are available and are not interchangeable. These include the conventional deoxycholate formulation and newer lipid-based formulations (lipid complex and liposomal). The lipid-based products penetrate tissues more effectively and are significantly less nephrotoxic than the conventional formulation, but they are substantially more expensive and have different preparation, dilution, and dosing requirements.

Mechanism of Action (MOA): Amphotericin B binds to ergosterol in fungal cell membranes, creating pores that increase membrane permeability and lead to leakage of intracellular electrolytes, particularly potassium, resulting in fungal cell death. Mammalian cell membranes contain cholesterol rather than ergosterol, but partial binding explains the drug’s significant toxicity, especially to renal tubular cells.

The primary dose-limiting adverse effect of amphotericin B in dogs and cats is nephrotoxicity. Renal function monitoring is essential throughout therapy, and treatment protocols are often adjusted or discontinued based on changes in blood urea nitrogen, creatinine, and electrolyte concentrations.

Indications

Amphotericin B is indicated in dogs and cats for the treatment of severe, life-threatening systemic fungal infections when rapid antifungal activity is required, when disease is progressive, or when there is inadequate response to less toxic antifungal agents such as azoles.

• Refractory systemic mycoses: Used when infections fail to respond to first-line antifungals (eg, fluconazole, itraconazole) or when rapid disease control is critical.
• Severe or disseminated fungal infections: Including infections with organisms such as Blastomyces, Histoplasma, Coccidioides, Cryptococcus, Aspergillus, Sporothrix, and Mucor species, particularly when systemic involvement is present.
• Life-threatening disease: Indicated when fungal infection poses an immediate risk to life or vital organ function and aggressive antifungal therapy is required.
• Azole-intolerant patients: Used when adverse effects, drug interactions, or resistance limit the use of azole antifungals.
• Induction therapy: May be used as an initial induction agent to rapidly reduce fungal burden, followed by transition to a less nephrotoxic antifungal for long-term maintenance therapy.
• Susceptibility-guided therapy: Reserved for cases in which antifungal susceptibility testing supports amphotericin B as the most appropriate option.

Due to its potential for significant toxicity, amphotericin B should not be used for mild or localized fungal infections in dogs and cats and is best reserved for carefully selected cases under close veterinary supervision.

Dosage (Reference)

Because of its narrow safety margin, cost, and need for parenteral administration, amphotericin B is reserved for serious, life-threatening systemic fungal infections in dogs and cats. Dosage regimens vary widely depending on formulation, route of administration, patient response, and development of renal toxicity. Cumulative dose is a critical consideration.

Dog

In dogs, amphotericin B is administered either intravenously or via diluted subcutaneous infusion. Adequate hydration and close renal monitoring are mandatory throughout therapy.

Cat

Cats appear more sensitive to the nephrotoxic effects of amphotericin B than dogs. Lower initial doses and strict renal monitoring are essential.

Warnings & Precautions

Amphotericin B is a highly potent antifungal agent with a narrow safety margin. Its use in dogs and cats requires careful case selection, strict dosing accuracy, and intensive monitoring, particularly of renal function.

• Nephrotoxicity: Renal toxicity is the most significant and common adverse concern, especially with the conventional deoxycholate formulation. Amphotericin B can cause renal vasoconstriction and direct tubular epithelial damage, leading to decreased glomerular filtration rate.
• Pre-existing renal disease: Use cautiously in dogs and cats with renal insufficiency. Although not absolutely contraindicated, amphotericin B should only be used when safer antifungal options have failed, and renal parameters must be closely monitored.
• Formulation-specific precautions: Conventional (deoxycholate), lipid complex, and liposomal formulations are not interchangeable. Each formulation has different reconstitution methods, dilution requirements, dosing protocols, and toxicity profiles. Medication errors can result in severe toxicity.
• Hydration status: Adequate hydration before and during therapy is essential to reduce nephrotoxicity. Care must be taken to avoid fluid overload, particularly in cats or patients with cardiac disease.
• Dose verification: Due to the high toxicity risk, all dose calculations and drug preparations should be double-checked prior to administration.
• Electrolyte disturbances: Amphotericin B may cause hypokalemia, hypomagnesemia, and renal tubular acidosis. Electrolytes should be monitored regularly during treatment.
• Infusion-related reactions: Slow IV infusion is recommended to minimize adverse effects. Rapid administration increases the risk of hypotension, arrhythmias, and systemic reactions.
• Formulation selection: Lipid complex and liposomal formulations are preferred in dogs and cats that develop renal toxicity with the conventional formulation or that have pre-existing renal compromise, as they are significantly less nephrotoxic.
• SC administration: Subcutaneous infusion techniques have been used in dogs and cats to mimic slow IV delivery and potentially reduce nephrotoxicity, but injection-site reactions and sterile abscess formation may occur if concentrations are too high.
• Use limitation: Amphotericin B should be reserved for severe, life-threatening, or refractory systemic fungal infections and should not be used for mild or localized disease.

Drug Interactions

Amphotericin B has numerous clinically important drug interactions, primarily related to its nephrotoxic potential, effects on electrolytes (especially potassium and magnesium), and high plasma protein binding. Concurrent use with the following drugs requires careful risk–benefit assessment and close monitoring in dogs and cats.

• Nephrotoxic drugs (e.g., aminoglycosides, cyclosporine, cisplatin):
  Concurrent or sequential use may result in additive renal toxicity. Avoid combinations when possible, or monitor renal function closely.
• Corticosteroids (e.g., prednisolone, dexamethasone):
  May exacerbate amphotericin B–induced potassium loss and increase the risk of hypokalemia.
• Potassium-depleting diuretics (e.g., loop or thiazide diuretics):
  Increased risk of hypokalemia, which may worsen cardiac and neuromuscular complications.
• Digoxin:
  Amphotericin B–induced hypokalemia may increase the risk of digoxin toxicity.
• Neuromuscular blocking agents:
  Hypokalemia caused by amphotericin B may potentiate neuromuscular blockade.
• Flucytosine:
  In vitro synergy has been reported against some fungal organisms, but concurrent use may increase flucytosine toxicity; careful monitoring is required.
• Highly protein-bound drugs (e.g., azole antifungals, NSAIDs, furosemide):
  Theoretically may be displaced from plasma protein binding sites, causing transient increases in free drug concentrations; clinically significant effects are uncommon but monitoring is advised.
• Zidovudine (AZT):
  Concurrent use may increase the risk of myelotoxicity or nephrotoxicity.

Side Effects & Overdose

Side Effects

Amphotericin B is associated with a high incidence of adverse effects, particularly related to renal toxicity. Adverse reactions are dose-dependent and more common with the conventional deoxycholate formulation compared with lipid complex or liposomal formulations. Cats may be more sensitive to nephrotoxic effects than dogs.

• Nephrotoxicity:
  The most significant and common adverse effect. Caused by renal vasoconstriction and direct tubular epithelial cell toxicity, leading to decreased glomerular filtration rate.
• Electrolyte abnormalities:
  Hypokalemia and hypomagnesemia are common and may be associated with renal tubular acidosis or nephrogenic diabetes insipidus.
• Azotemia:
  Elevations in BUN and serum creatinine may occur during therapy; renal changes are often reversible but may take weeks to months to resolve.
• Gastrointestinal signs:
  Anorexia and vomiting may be observed during treatment.
• Infusion-related reactions:
  Lethargy, restlessness, fever, tachycardia, tachypnea, hypotension, and cardiac arrhythmias may occur during or shortly after administration.
• Hematologic effects:
  Nonregenerative anemia has been reported with prolonged therapy.
• Local reactions:
  Phlebitis with IV administration and sterile abscess formation with SC administration, particularly at higher concentrations.
• Neurologic signs (cats):
  Cats with CNS fungal infections may show transient worsening of neurologic signs early in therapy, likely due to an inflammatory response to dying organisms.

Overdose

Information regarding acute overdose of amphotericin B in dogs and cats is limited. Toxicity is expected to primarily involve severe renal and electrolyte disturbances.

• Severe nephrotoxicity:
  Marked azotemia, renal tubular damage, and electrolyte derangements may occur with excessive dosing.
• Electrolyte imbalances:
  Profound hypokalemia and hypomagnesemia may lead to cardiac arrhythmias and neuromuscular dysfunction.
• Systemic effects:
  Severe lethargy, weight loss, dehydration, and cardiovascular instability may be observed.
• Management:
  Treatment is supportive and includes aggressive IV fluid therapy, correction of electrolyte abnormalities, and close monitoring of renal parameters.
• Adjunctive measures:
  Administration of IV fluids and osmotic diuretics (e.g., mannitol) may help reduce renal injury following accidental overdose.
• Consultation:
  Veterinary toxicology consultation is recommended in suspected overdose cases.

Key Notes

Practical clinical pearls to optimize the safe and effective use of amphotericin B in dogs and cats:

• Reserved for severe disease:
  Amphotericin B should be reserved for life-threatening or progressive systemic fungal infections when safer antifungal agents are ineffective or contraindicated.
• Formulation matters:
  Conventional (deoxycholate), lipid complex, and liposomal formulations are NOT interchangeable; dosing, dilution, and toxicity profiles differ significantly.
• Cumulative dose is critical:
  Clinical response and toxicity are more closely related to cumulative dose rather than single-dose administration.
• Lipid formulations preferred:
  Lipid complex and liposomal formulations are preferred in patients with pre-existing renal compromise or those that develop renal toxicity during therapy.
• Hydration strategy:
  Adequate hydration before, during, and after administration is an essential component of therapy and may help mitigate renal injury.
• Slow administration:
  Slow IV infusion or diluted SC infusion protocols are commonly used to improve tolerability and allow outpatient management in selected cases.
• Monitoring drives decisions:
  Dose adjustments, treatment pauses, or discontinuation should be based on trends in renal parameters rather than a single laboratory value.
• Not a first-line antifungal:
  Azole antifungals are generally preferred as initial therapy; amphotericin B is typically introduced when response is inadequate or rapid fungal killing is required.
• Client communication:
  Owners should be counseled about the seriousness of the disease, the need for frequent monitoring, and the potential for treatment interruption due to toxicity.