Polyene Macrolide Antimicrobials for Use In Animals

The polyenes bind to sterol components in the phospholipid-sterol membranes of fungal cells to form complexes that induce physical changes in the membrane. The number of conjugated bonds and the molecular size of a particular polyene macrolide influence its affinity for different sterols in fungal cell membranes. While these agents primarily target fungal sterols, they have some affinity for host cell membrane cholesterol as well; as a result, polyenes have a low toxicity threshold and can create adverse effects in the patient.

Amphotericin B has a greater affinity for fungal ergosterol than for eukaryotic (host) cell membrane cholesterol, making it more suitable for intravenous use. The long polyene structure causes pore formation in the fungal cell membrane; subsequent potassium ion efflux and hydrogen ion influx cause internal acidification and a halt in enzymatic functions. Sugars and amino acids also eventually leak from an arrested fungal cell. Polyenes are most often fungistatic at clinical-use concentrations, but high drug concentrations and pH values between 6.0 and 7.3 in the surrounding medium may lead to fungicidal rather than fungistatic action.

In addition to these direct effects on susceptible yeasts and fungi, evidence suggests that amphotericin B may also act as an immunopotentiator (both humoral and cell mediated), thus enhancing the host’s ability to overcome mycotic infections.

Polyene macrolides are inherently resistant to dermatophytes. Acquired resistance to the polyene antifungal macrolides is rare both clinically and in vitro. Pythium, a pseudofungus, is less susceptible because it contains limited ergosterol (i.e., the drug target) in its cell membranes. Resistance has been documented for Candida spp, which are among the more rapidly growing fungal organisms. In general, resistance develops slowly and does not reach high levels, even after prolonged treatment.

This lower resistance potential is largely because polyenes do not require entry into the fungal cell in order to exert their activity. Therefore, the only method for development of fungal resistance to polyenes is via the modification of their binding target, ergosterol. Because ergosterol is essential for cell membrane integrity and fluidity, there is limited ability for the fungal cell to modify this target. The most common fungal mechanisms for development of resistance to polyene antifungals are via the absence of ergosterol in the fungal cell membrane (through mutations in ERG3 or ERG6), or via a decrease in the ergosterol content in fungal cells.

The polyene antimicrobials have broad antifungal activity against organisms ranging from yeasts to filamentous fungi and from saprophytic to pathogenic fungi; however, there are great differences between the susceptibilities of the various species and strains of fungi. Because of its broad range of activity and low rate of innate or acquired resistance, amphotericin B has become a mainstay of systemic antifungal treatment. Polyenes are ineffective against dermatophytes. In vitro polyene sensitivity profiles do not always correlate well with the clinical response, which suggests that host factors may also play a role in drug-pathogen interaction.

Amphotericin B is effective against yeasts (eg, Candida spp, Rhodotorula spp, Cryptococcus neoformans), dimorphic fungi (eg, H capsulatum, B dermatitidis, C immitis) and molds. It also has been administered successfully to treat disseminated sporotrichosis, pythiosis, and zygomycosis, although it may not always be effective. Nystatin is most commonly administered in the treatment of Candida spp; however, its spectrum of activity may also include C neoformans, Trichosporon spp, and Rhodotorula spp. Most dimorphic fungi, such as B dermatitidis, Paracoccidioides brasiliensis, Coccidioides spp,and H capsulatum, also demonstrate some sensitivity to nystatin.  Nystatin is inactive against Aspergillus spp and dermatophytes. The antimicrobial activity of pimaricin is similar to that of nystatin, although it is mainly administered for local treatment of candidiasis, trichomoniasis, and mycotic keratitis.

None of the amphotericin B formulations are FDA-approved for veterinary use, and therefore off-label use is common and recommended for many systemic fungal infections. Amphotericin B is available as an intravenous solution complexed to bile acids but also as several different preparations complexed to lipid mixtures. Newer preparations, such as liposomal amphotericin B or lipid-complexed amphotericin B are more desirable due to their minimization of hepatic and renal adverse effects. Reticuloendothelial cells phagocytize the lipid component of these preparations and thereby direct drug delivery to the site of fungal infection, reducing renal exposure to the amphotericin B. Prolonged antifungal activity (compared with nonliposomal preparations) has been documented.

Nystatin is available as part of a multidrug topical otic preparation for the treatment of Malassezia otitis in dogs and cats. Nystatin is also available as an FDA-approved medicated feed for the treatment of crop mycosis and mycotic diarrhea in growing turkeys. Nystatin is also often administered in the treatment of fungal keratitis via ophthalmic preparations. There are no FDA-approved veterinary formulations of natamycin, but natamycin is available as a 5% ophthalmic suspension on the human market.

Amphotericin B is usually administered intravenously or topically and occasionally used locally, intrathecally, or intraocularly. In general, amphotericin B has poor corneal penetration after topical or systemic administration, and is irritating to corneal tissues at concentrations greater than 3%. Amphotericin B 0.15% is generally well tolerated and unlikely to cause tissue damage. Nystatin and natamycin are usually applied topically or administered orally because parenteral formulations have been found to be toxic and generally lack efficacy. Nystatin is administered orally to treat intestinal candidiasis. Systemic absorption is minimal from sites of local application, and nystatin is not absorbed across intact mucosal surfaces.

Amphotericin B is widely distributed in the body after intravenous infusion. It associates with cholesterol in host cell membranes throughout the body and is subsequently released slowly into the circulation. Penetration into the CSF, saliva, aqueous humor, vitreous humor, and hemodialysis solutions is generally poor. Amphotericin B becomes highly bound to plasma lipoproteins (~95%). In humans, complexing amphotericin B with various lipid-based products has been shown to alter its distribution, resulting in higher plasma concentrations and a lower apparent volume of distribution when compared to noncomplexed products. These lipid formulations result in an accumulation in deep or protected tissue compartments (including resident macrophages) after repeated dosing, which does not result in increased toxicity.

The disposition of amphotericin B is not well described in companion animals. Approximately 5% of a total daily dose of amphotericin B is excreted unchanged in the urine. Over a 2-week period, ~20% of the drug may be recovered in the urine. The hepatobiliary system accounts for 20%–30% of excretion. The fate of the remainder of amphotericin B is unknown. Lipid-complexed or liposomal formulations lead to decreased renal and fecal clearance. This may be due to amphotericin B being sequestered within circulating liposomes that escape typical elimination routes or due to the changes in distribution that allow for elimination directly from the tissue compartment.

Amphotericin B has a biphasic elimination pattern. The initial phase lasts 24 hours, during which levels fall rapidly (70% for plasma and 50% for urine). The second elimination phase has a 15-day half-life, during which plasma concentrations decline very slowly. Amphotericin B is usually infused intravenously, every 48–72 hours, until the total cumulative dosage has been reached. The disposition of the various lipid-complexed amphotericin B products is variable. Because of its small size, amphotericin B liposomal injection (LAmB) is characterized by the slowest uptake by reticuloendothelial cells and thus the highest plasma drug concentrations of amphotericin B. However, the amount of free versus complexed amphotericin B is not clear. AmBisome also was able to achieve CNS concentrations and was associated with the least nephrotoxicity in human studies. AmBisome has been studied in Beagles. Achievable amphotericin concentrations were much higher at equivalent doses of AmBisome when compared with other products; further, dogs were able to well tolerate 4 mg/kg for 30 days. Amphotericin concentrations accumulate with multiple doses when administered as AmBisome.

Topical nystatin use does not result in systemic absorption. Similarly, no systemic absorption of nystatin was noted in poultry administered up to 1,400 mg/kg orally for 66 days;1 oral use of this drug is commonly employed to treat or prevent crop mycosis in turkeys. However, caution should be exercised in food-producing animals with a compromised gastrointestinal barrier, as systemic absorption could occur and produce undesirable drug residues in food products derived from these animals. Pharmacokinetics for intravenous nystatin have been described in rabbits; however, due to the high degree of toxicity after parenteral use of this drug, this route is not recommended for any species.

Oral administration of nystatin can lead to anorexia and GI disturbances. The intravenous infusion of amphotericin B can cause an anaphylactoid reaction due to direct mast cell degranulation. A pretest dose is recommended to detect this reaction, and pretreatment with H₁ antihistamines and short-acting glucocorticoids may be appropriate. Thrombophlebitis may occur with perivascular leakage. The primary toxicity associated with amphotericin B is nephrotoxicity. Within 15 minutes of intravenous administration of amphotericin B, renal arterial vasoconstriction occurs and lasts for 4–6 hours. This leads to diminished renal blood flow and glomerular filtration.

The administration of amphotericin B can lead to a number of adverse effects, including anorexia, nausea, vomiting, hypersensitivity reactions, drug fever, normocytic normochromic anemia, cardiac arrhythmias, hepatic dysfunction, and neurologic symptoms.

A number of adjuvant therapies are used to minimize adverse events of amphotericin B. Pretreatment with antiemetic and antihistaminic agents prevents the nausea, vomiting, and hypersensitivity reactions. Giving corticosteroids intravenously also limits severe hypersensitivity reactions. Mannitol (1 g/kg, IV), with each dose of amphotericin B, and sodium bicarbonate (2 mEq/kg, IV or PO, daily) may help prevent acidification defects, metabolic acidosis, and azotemia; however, clinical evidence of efficacy has not been proved. Saralasin (6–12 mcg/kg/minute, IV) and dopamine (7 mcg/kg/minute, IV) infusions have prevented oliguria and azotemia induced by amphotericin B in dogs. Administering intravenous fluids or furosemide before amphotericin B prevents pronounced decreases in renal blood flow and glomerular filtration rate.

Newer preparations in which amphotericin B is mixed with lipid or liposomal vehicles (particularly liposomes) are safer and have maintained efficacy. Amphotericin B has been administered topically (0.05%) or subconjunctivally for fungal keratitis but has been associated with mild to severe ocular tissue irritation. Adverse effects after topical or oral administration of nystatin and pimaricin are minimal due to the lack of systemic absorption, but parenteral forms of nystatin have been withdrawn from the market due to severe nephrotoxicity.

Amphotericin B may be combined with other antimicrobial agents with synergistic results. This often allows both the total dose of amphotericin B and the length of treatment to be decreased. Examples include combinations of 5-flucytosine and amphotericin B for treatment of cryptococcal meningitis, minocycline and amphotericin B for coccidioidomycosis, and imidazole and amphotericin B for several systemic mycotic infections. Rifampin may also potentiate the antifungal activity of amphotericin B.

Drugs that should be avoided during amphotericin B treatment include aminoglycosides (nephrotoxicity), digitalis drugs (increased toxicity), curarizing agents (neuromuscular blockade), mineralocorticoids (hypokalemia), thiazide diuretics (hypokalemia, hyponatremia), antineoplastic drugs (cytotoxicity), and cyclosporine (nephrotoxicity).

Treatment with polyene macrolide antimicrobials increases plasma bilirubin, CK, AST, ALT, BUN, eosinophil count, and urine protein, and decreases plasma potassium and platelet count.