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Amphotericin B

Amphotericin B is a polyene antifungal antibiotic derived from the bacterium Streptomyces nodosus, widely recognized for its broad-spectrum activity against serious and life-threatening fungal and certain protozoal infections in humans.^[1] It functions by binding to ergosterol in the fungal cell membrane, forming pores that disrupt membrane integrity, leading to leakage of cellular contents, oxidative damage, and ultimately fungal cell death.^[1] This mechanism provides fungicidal effects against a range of pathogens, including Aspergillus spp., Candida spp., Cryptococcus neoformans, and dimorphic fungi like Histoplasma capsulatum.^[2] First isolated in 1955 and introduced clinically in the late 1950s, it remains a cornerstone therapy for invasive mycoses, particularly in immunocompromised patients, despite the development of alternative antifungals.^[2] Amphotericin B is available in several formulations to balance efficacy and tolerability, with the conventional deoxycholate form (Fungizone®) approved by the FDA in 1965, followed by lipid-based versions like liposomal amphotericin B (AmBisome®) in 1997 and amphotericin B lipid complex (Abelcet®) in 1995.^[2] These lipid formulations encapsulate the drug in liposomes or lipid complexes, reducing nephrotoxicity—the most significant adverse effect of the conventional form, which affects up to 80% of patients—while maintaining antifungal potency.^[1] Indications include empirical therapy for febrile neutropenia, treatment of disseminated candidiasis, cryptococcal meningitis, and aspergillosis, often as first-line or salvage therapy when other agents fail.^[3] Resistance remains rare, primarily due to alterations in ergosterol biosynthesis, but emerging concerns with species like Candida auris highlight the need for judicious use.^[2] Despite its efficacy, amphotericin B is reserved for severe infections due to potential toxicities, including acute infusion-related reactions (fever, chills, rigors) occurring in approximately 80% of patients with initial doses and electrolyte imbalances like hypokalemia.^[1] Renal monitoring is essential, with lipid formulations preferred in patients at risk for kidney injury.^[3] Over six decades of use, amphotericin B has saved countless lives in critical care settings, and ongoing research explores less toxic oral or nanoparticle delivery systems to expand its accessibility.^[2]

Medical Uses

Fungal Infections

Amphotericin B is a first-line or salvage therapy for severe systemic fungal infections, particularly those caused by dimorphic fungi and molds in immunocompromised patients.^[1] It is indicated for aspergillosis when voriconazole is not feasible, invasive candidiasis, cryptococcosis, disseminated histoplasmosis, severe blastomycosis, coccidioidomycosis, mucormycosis, and severe sporotrichosis.^[4]^[5]^[6]^[7]^[8]^[9]^[10]^[11] In invasive aspergillosis, lipid formulations of amphotericin B serve as an alternative initial therapy for patients unable to tolerate voriconazole, with evidence supporting its use in salvage settings for refractory cases.^[4] For invasive candidiasis, including candidemia and deep-seated infections, amphotericin B deoxycholate or lipid formulations are recommended in cases of azole intolerance or resistance, particularly for Candida krusei or glabrata.^[5] Cryptococcosis treatment, especially meningitis, involves liposomal amphotericin B at 3-4 mg/kg daily combined with flucytosine for induction in high-income settings.^[6] For endemic mycoses, amphotericin B is preferred for severe or disseminated histoplasmosis, with lipid formulations at 3-5 mg/kg daily for initial therapy in immunocompromised patients.^[7] In blastomycosis, it is used for life-threatening pulmonary or disseminated disease, followed by step-down to itraconazole.^[8] For severe disseminated coccidioidomycosis, lipid formulation amphotericin B is recommended initially; for meningitis, azoles are first-line, with amphotericin B (IV or intrathecal) as alternative for severe cases or azole failure.^[9] Mucormycosis guidelines strongly recommend high-dose liposomal amphotericin B (5-7.5 mg/kg daily) as first-line therapy due to its broad activity against Mucorales species.^[10] Severe sporotrichosis, such as disseminated or osteoarticular forms, requires lipid formulations of amphotericin B at 3-5 mg/kg daily until clinical improvement, followed by step-down to itraconazole.^[11] Standard dosing for systemic fungal infections typically involves amphotericin B deoxycholate at 0.5-1.5 mg/kg/day intravenously, while lipid formulations range from 3-5 mg/kg/day, adjusted higher (up to 10 mg/kg/day) for central nervous system involvement or mucormycosis.^[1] Therapy duration varies by infection site and response, often 2-6 weeks initially before transitioning to oral azoles.^[1] Amphotericin B, particularly the liposomal formulation at 3 mg/kg/day, plays a key role in empirical antifungal therapy for high-risk febrile neutropenia patients with persistent fever despite broad-spectrum antibiotics, as endorsed by IDSA guidelines for presumed invasive fungal infection.^[1]^[12]

Protozoal Infections

Amphotericin B is indicated as a first-line treatment for visceral leishmaniasis (also known as kala-azar) in regions with significant resistance to pentavalent antimonials, such as parts of the Indian subcontinent where treatment failure rates were historically up to 60% but are currently around 30-45% in areas like Bihar (as of 2024).^[13]^[14] For cutaneous leishmaniasis, it is reserved for select cases, including complicated, severe, or antimony-resistant infections, particularly those caused by species like Leishmania braziliensis or L. panamensis in the New World.^[13]^[15] The liposomal formulation of amphotericin B is preferred for visceral leishmaniasis due to its improved safety profile and efficacy, typically administered intravenously at 3 mg/kg/day for 5–7 days, achieving total doses of 15–21 mg/kg with cure rates exceeding 95% in endemic areas like India.^[13]^[16] In cutaneous leishmaniasis, liposomal amphotericin B dosing ranges from 2–3 mg/kg/day for a total of 20–40 mg/kg over 10–20 doses, offering effective resolution in complex cases with minimal relapse.^[13] While conventional deoxycholate amphotericin B remains an option at 0.75–1 mg/kg every other day for 15–20 doses, its use is limited by higher toxicity, making liposomal forms the standard in resource-limited settings.^[13] Combination therapies incorporating amphotericin B enhance outcomes for visceral leishmaniasis by improving efficacy and shortening treatment duration; for example, a single 5 mg/kg dose of liposomal amphotericin B combined with miltefosine (100 mg/day for 7–8 days) or paromomycin (15 mg/kg/day for 10–11 days) yields cure rates of 98–99% in antimony-resistant regions of India.^[13] Such regimens reduce the risk of relapse and are particularly beneficial in high-burden areas.^[13] The World Health Organization endorses amphotericin B, especially the liposomal form, as a key therapy for antimony-unresponsive visceral leishmaniasis cases, recommending it in elimination programs for the Indian subcontinent and other endemic foci to address resistance and achieve high cure rates.^[13] Recent WHO guidelines (2022) for VL/HIV co-infection emphasize liposomal amphotericin B-based regimens. In India, intensified elimination efforts have reduced VL incidence dramatically, with fewer than 1,000 cases reported annually as of 2023, though vigilance for resistance continues.^[17]^[18]^[19] These guidelines emphasize its role in combination strategies to optimize resource use and patient outcomes in global control efforts.^[13]

Spectrum of Activity

Amphotericin B demonstrates a broad spectrum of antifungal activity, primarily targeting pathogenic fungi by binding to ergosterol in their cell membranes, which disrupts membrane integrity and leads to cell death. It is highly effective against a range of yeasts, including most Candida species (such as C. albicans, C. glabrata, and C. parapsilosis) and Cryptococcus neoformans, with minimum inhibitory concentrations (MICs) typically in the susceptible range. The drug also shows strong activity against molds, notably Aspergillus species (e.g., A. fumigatus and A. flavus) and members of the Mucorales order (e.g., Rhizopus and Mucor spp.), where it exhibits fungicidal effects at low concentrations. Additionally, amphotericin B is active against dimorphic fungi such as Histoplasma capsulatum and Blastomyces dermatitidis, which can cause systemic infections in immunocompromised hosts. Beyond fungi, it possesses notable activity against the protozoan parasite Leishmania species, making it a key therapeutic option for visceral leishmaniasis. Susceptibility testing for amphotericin B is guided by established breakpoints from organizations like the European Committee on Antimicrobial Susceptibility Testing (EUCAST). For most Candida species, isolates are considered susceptible at MICs ≤1 μg/mL and resistant at >1 μg/mL; similar breakpoints apply to Aspergillus species (susceptible ≤1 μg/mL, resistant >1 μg/mL). The Clinical and Laboratory Standards Institute (CLSI) does not provide formal clinical breakpoints but uses epidemiological cutoff values (ECVs) to distinguish wild-type from non-wild-type populations, typically around 1–2 μg/mL for Candida and 2–4 μg/mL for Aspergillus species. These thresholds help predict clinical efficacy, though in vivo outcomes can vary due to host factors and infection site. Despite its broad activity, amphotericin B has limitations against certain pathogens. It shows poor clinical utility and relatively weak in vitro activity against dermatophytes (e.g., Trichophyton rubrum), with MICs comparable to ketoconazole but inferior to terbinafine. The drug lacks activity against bacteria such as Nocardia species, which are treated with sulfonamides or other antibacterials rather than antifungals. Emerging resistance is a concern with Candida auris, where many isolates exhibit elevated MICs (often >1 μg/mL) and reduced killing despite appearing susceptible by breakpoint criteria. Susceptibility to amphotericin B is primarily influenced by the ergosterol content in the target organism's cell membranes, as the drug's binding affinity is higher for ergosterol than for mammalian cholesterol, conferring selectivity. Organisms with altered ergosterol biosynthesis or reduced membrane ergosterol levels may show decreased susceptibility. Typically, there is no cross-resistance with azoles, which inhibit ergosterol synthesis upstream, allowing amphotericin B to remain effective against azole-resistant strains in most cases.

Formulations and Administration

Deoxycholate Amphotericin B

Deoxycholate amphotericin B, also known as conventional or Fungizone amphotericin B, is the original intravenous formulation of the antifungal agent, introduced in 1959 as a groundbreaking therapy for systemic fungal infections.^[20] It consists of amphotericin B complexed with sodium deoxycholate to enhance water solubility, as the parent compound is poorly soluble at physiological pH. Each 50 mg vial typically contains 50 mg of amphotericin B, 41 mg of sodium deoxycholate, and 20.2 mg of sodium phosphates (mono- and dibasic) as a buffer, with the vial headspace filled with nitrogen to prevent oxidation.^[21] For preparation, the lyophilized powder is reconstituted with 10 mL of sterile water for injection to yield a 5 mg/mL colloidal suspension, which is then further diluted in 5% dextrose injection (not saline, to maintain stability) to a final concentration of 0.1 mg/mL for administration; bacteriostatic agents or electrolytes must be avoided to prevent precipitation.^[21] Administration requires a slow intravenous infusion over 2 to 6 hours to minimize infusion-related reactions, with an initial test dose of 1 mg infused in 20 mL of 5% dextrose over 20 to 30 minutes while monitoring vital signs.^[1] Dosing typically begins at 0.25 mg/kg/day and is gradually increased to 0.5 to 1 mg/kg/day based on tolerance, with a maximum of 1.5 mg/kg/day; for equivalence to lipid formulations, 1 mg/kg of deoxycholate amphotericin B is approximately comparable to 3 to 5 mg/kg of liposomal amphotericin B in terms of therapeutic effect, though direct substitution requires clinical judgment.^[21]^[22] Premedication with acetaminophen, diphenhydramine, or hydrocortisone 30 to 60 minutes prior is often recommended to attenuate acute reactions.^[1] As the historical standard for over five decades, deoxycholate amphotericin B offered broad-spectrum efficacy against life-threatening mycoses at a low cost, making it accessible in resource-limited settings.^[23] However, its significant toxicity profile, including a high risk of nephrotoxicity affecting up to 80% of patients, has led to its gradual replacement by safer lipid-based alternatives in many developed healthcare systems, though it remains a viable option where cost is a barrier.^[1]^[24]

Liposomal Amphotericin B

Liposomal amphotericin B is a lipid-encapsulated formulation designed to improve the safety profile of amphotericin B by reducing its toxicity, particularly to the kidneys. The most widely used product, AmBisome, consists of amphotericin B intercalated into small unilamellar liposomes formed by a phospholipid bilayer comprising hydrogenated soy phosphatidylcholine, cholesterol, distearoylphosphatidylglycerol, and alpha-tocopherol.^[25] This structure limits the drug's distribution to renal tissues, thereby minimizing direct tubular exposure and nephrotoxic effects.^[26] Administration of liposomal amphotericin B is exclusively intravenous, with the lyophilized powder reconstituted in sterile water and diluted in 5% dextrose for infusion over approximately 60 to 120 minutes using an in-line filter with a pore diameter of at least 1.0 micron.^[25] Unlike the conventional deoxycholate formulation, no premedication or test dose is required prior to infusion, facilitating easier clinical use.^[1] Dosing regimens vary by indication but generally range from 3 to 5 mg/kg body weight per day for empirical therapy or systemic fungal infections such as invasive aspergillosis or candidiasis.^[25] For visceral leishmaniasis, a total cumulative dose of up to 21 mg/kg is administered, typically as 3 mg/kg daily on days 1 through 5, 14, and 21 in immunocompetent patients, or higher and more frequent dosing in immunocompromised individuals.^[25] Adjustments may be made based on renal function, though the formulation's tolerability allows continuation in patients with mild impairment.^[26] The primary clinical advantage of liposomal amphotericin B is its substantially reduced nephrotoxicity, with studies reporting 50% to 60% lower rates of renal adverse events compared to amphotericin B deoxycholate, including fewer instances of serum creatinine doubling or discontinuation due to kidney injury. This makes it the preferred option for high-risk patients, such as those with preexisting renal impairment or requiring prolonged therapy, where it demonstrates better overall tolerability and allows for higher cumulative doses without escalating toxicity.^[1] Infusion-related reactions, such as fever or chills, can occur but are generally milder than with other formulations.^[26]

Lipid Complex Amphotericin B

Lipid complex formulations of amphotericin B, such as amphotericin B lipid complex (ABLC) and amphotericin B colloidal dispersion (ABCD), consist of the antifungal agent complexed with lipids to form large, ribbon-like or disk-shaped structures that enhance tolerability while maintaining efficacy against systemic fungal infections.^[27] ABLC, marketed as Abelcet, incorporates amphotericin B in a 1:1 molar ratio with a phospholipid mixture of dimyristoyl phosphatidylcholine (DMPC) and dimyristoyl phosphatidylglycerol (DMPG) at a 7:3 ratio, resulting in multilamellar ribbon-like complexes approximately 1.6–11 μm in size.^[28] In contrast, ABCD, previously available as Amphotec, features amphotericin B complexed with cholesteryl sulfate in a 1:1 molar ratio, forming uniform disk-shaped particles around 0.12 μm in diameter upon reconstitution.^[29] These colloidal dispersions differ from liposomal formulations by their aggregate, non-vesicular architecture, which influences tissue distribution and reduces accumulation in renal tissues.^[20] Administration of lipid complex amphotericin B occurs via intravenous infusion, typically over 1–2 hours to minimize infusion-related reactions, with precautions similar to those for the deoxycholate formulation, including premedication with antipyretics or antihistamines and close monitoring for hypersensitivity.^[28] For ABLC, the infusion rate is 2.5 mg/kg per hour, while ABCD is infused at 1 mg/kg per hour initially, potentially shortening to 2 hours for subsequent doses if tolerated.^[29] The standard dosing for systemic fungal infections is 5 mg/kg per day for ABLC and 3–4 mg/kg per day for ABCD in adults and children, allowing higher cumulative doses than conventional amphotericin B without proportional toxicity increases.^[27] These regimens are particularly suited for patients intolerant to or failing deoxycholate therapy, with adjustments based on renal function and response.^[28] The primary advantages of lipid complex formulations include a reduced risk of nephrotoxicity compared to conventional amphotericin B deoxycholate, with incidence rates approximately 30–50% lower, enabling safer use in patients with renal impairment or those requiring prolonged therapy.^[27] This improvement stems from decreased renal uptake and tubular damage, as the lipid complexes preferentially target the reticuloendothelial system over the kidneys.^[20] Additionally, these formulations serve as a cost-effective alternative to liposomal amphotericin B, offering similar efficacy against invasive mycoses at a lower price point while exhibiting an intermediate toxicity profile.^[30] Note that ABCD has been discontinued in many markets since 2011 due to higher rates of infusion-related adverse events, leaving ABLC as the predominant lipid complex option.^[20]

Oral Amphotericin B

Oral amphotericin B has historically been limited by its negligible systemic absorption following oral administration, with bioavailability approaching zero, which confines its effects to the gastrointestinal tract.^[1] This poor absorption has restricted its use primarily to topical applications or as part of selective digestive decontamination (SDD) regimens in critically ill patients, where it is administered orally in combination with other nonabsorbable antimicrobials like polymyxin E and tobramycin to prevent endogenous infections by targeting fungal overgrowth in the gut.^[31] In SDD protocols, oral amphotericin B is typically given as a suspension multiple times daily, providing local antifungal coverage without significant systemic exposure.^[32] Recent advancements have focused on investigational oral formulations designed to enhance bioavailability while minimizing toxicity. One prominent example is MAT2203, an oral lipid nanocrystal (LNC) formulation of amphotericin B developed by Matinas BioPharma, which encapsulates the drug in cochleate structures to improve gastrointestinal absorption and targeted delivery to fungal cells.^[33] This formulation is administered as oral capsules and has been evaluated in clinical trials for systemic fungal infections where intravenous therapy is challenging. In a Phase 2 trial for cryptococcal meningitis (NCT04031833), MAT2203 combined with flucytosine demonstrated noninferiority to standard intravenous amphotericin B plus flucytosine in terms of early fungicidal activity, with similar survival rates and reduced nephrotoxicity, based on data from 2023 involving 141 HIV-positive patients.^[34] As of 2025, oral amphotericin B formulations like MAT2203 remain primarily investigational and are not approved by the FDA for systemic indications, though they hold promise for enabling outpatient therapy and improving access in resource-limited settings where intravenous administration is logistically difficult.^[35] The potential to shift treatment from hospital-based intravenous regimens to oral options could reduce healthcare burdens, particularly for conditions like cryptococcal meningitis in low-income regions, pending successful Phase 3 outcomes and regulatory approval.^[36]

Adverse Effects

Nephrotoxicity

Amphotericin B induces nephrotoxicity primarily through renal vasoconstriction and direct damage to renal tubular cells, resulting from its binding to cholesterol in mammalian cell membranes and subsequent disruption of cellular integrity.^[1] This leads to reduced renal blood flow and impaired tubular function, often manifesting as acute kidney injury (AKI).^[37] The incidence of nephrotoxicity is notably high with conventional deoxycholate amphotericin B, affecting over 80% of patients, and is dose- and duration-dependent, with risks escalating at cumulative doses exceeding 1 mg/kg.^[38] In contrast, lipid-based formulations, such as liposomal amphotericin B, significantly lower this rate to around 15%, due to reduced exposure of renal tissues to the active drug.^[39] For example, in patients with hematologic malignancies, conventional deoxycholate was associated with AKI in approximately 36% of cases, often emerging within days of initiation.^[40] Key risk factors include pre-existing renal impairment, concurrent administration of other nephrotoxic agents such as cyclosporine or aminoglycosides, and dehydration, which can amplify vasoconstrictive effects and tubular toxicity.^[41] Concurrent nephrotoxic drugs like cyclosporine have been shown to synergistically increase the risk of severe AKI.^[42] Monitoring involves regular assessment of serum creatinine and electrolytes, including potassium and magnesium, with daily checks initially and then weekly, to detect early elevations in creatinine (e.g., ≥0.3 mg/dL or 50% from baseline).^[1] Preventive measures include pre-infusion hydration with 1 L of normal saline to mitigate vasoconstriction, alongside potential dose adjustments if renal function declines.^[43] Infusion-related reactions (IRRs) to amphotericin B occur acutely during or shortly after intravenous administration and are primarily attributed to the release of pro-inflammatory cytokines triggered by the drug's interaction with host cells. These reactions are mediated by amphotericin B deoxycholate stimulating cytokine genes in vitro and in vivo, leading to symptoms such as fever, chills, rigors, nausea, vomiting, back pain, and arthralgias.^[44]^[45] Fever and chills are common on the first day of therapy with conventional amphotericin B deoxycholate, occurring in over 50% of patients.^[46] The incidence of IRRs is notably higher with amphotericin B deoxycholate, ranging from 30% to 72% across patient populations, compared to reduced rates with lipid formulations. Premedication with agents such as acetaminophen, diphenhydramine, and hydrocortisone administered 30 minutes prior to infusion can significantly mitigate these reactions by suppressing cytokine-mediated inflammation.^[46]^[45] Additionally, infusing the drug over 2 to 6 hours, rather than more rapidly, helps minimize symptom severity by allowing gradual exposure.^[1] Tolerance to IRRs often develops with repeated administration, typically within the first few doses as the patient's immune response adapts during therapy. Management protocols emphasize premedication and controlled infusion rates, with symptoms generally becoming less frequent after initial exposures.^[47] Among formulations, conventional amphotericin B deoxycholate exhibits the highest IRR rates, while liposomal amphotericin B shows the lowest, with significantly reduced incidence of fever and chills compared to conventional forms. Lipid complex formulations fall intermediately, offering a balance between efficacy and tolerability. Recent studies as of 2023 on single high-dose liposomal amphotericin B indicate further reductions in infusion-related adverse events.^[48]^[30]

Other Adverse Effects

Amphotericin B therapy is frequently associated with electrolyte disturbances, particularly hypokalemia and hypomagnesemia. Hypokalemia occurs in 30% to 50% of patients receiving liposomal amphotericin B, often necessitating potassium supplementation and close monitoring to prevent complications such as cardiac arrhythmias.^[49] Hypomagnesemia, resulting from renal tubular loss, is also common and typically accompanies hypokalemia, with incidences reported up to 50% in treated patients; it can exacerbate hypokalemia and requires magnesium replacement.^[1] Hematologic adverse effects of amphotericin B include normocytic, normochromic anemia, which develops in up to 25% of patients during prolonged therapy due to suppression of erythropoietin production and is generally reversible upon discontinuation.^[50] Thrombocytopenia is a rarer manifestation, observed in fewer than 10% of cases and often linked to underlying hematologic conditions or concurrent myelotoxic therapies rather than the drug alone.^[51] Hepatic effects are usually mild and transient, with elevations in alanine aminotransferase (ALT) and aspartate aminotransferase (AST) occurring in up to 20% of patients, typically resolving after therapy cessation.^[52] Cholestasis is uncommon but has been reported in isolated cases, presenting as hyperbilirubinemia with minimal transaminase elevation, particularly in critically ill individuals.^[52] Neurological adverse effects are rare and generally limited to prolonged use, including isolated reports of seizures in patients with predisposing factors such as AIDS.^[53] Peripheral neuropathy has been documented infrequently (0.1% to 1% of cases), sometimes manifesting as demyelinating encephalopathy in vulnerable populations like bone marrow transplant recipients.^[1]

Drug Interactions

Nephrotoxic Drug Interactions

Amphotericin B exhibits significant interactions with other nephrotoxic agents, leading to enhanced renal impairment when used concurrently. These interactions primarily involve additive or synergistic effects on renal tubular cells and glomerular function, resulting in elevated serum creatinine levels and reduced glomerular filtration rate. Common culprits include aminoglycosides such as gentamicin, vancomycin, cyclosporine, and tacrolimus, which are frequently co-administered in patients with severe infections or undergoing immunosuppression. This risk is lower with lipid formulations of amphotericin B.^[54] The interaction with aminoglycosides, exemplified by gentamicin, arises from shared mechanisms of proximal tubular toxicity, where both agents disrupt lysosomal integrity and induce oxidative stress in renal epithelial cells, leading to cellular necrosis. Similarly, vancomycin potentiates amphotericin B's nephrotoxicity through additive damage to the renal tubules, with studies reporting an odds ratio of 5.21 for acute kidney injury when combined.^[55] Cyclosporine and tacrolimus, calcineurin inhibitors used in transplant settings, exacerbate amphotericin B-induced vasoconstriction of renal arterioles and direct tubular injury, further impairing renal blood flow and solute reabsorption. Concurrent administration of these agents can lead to greater elevations in serum creatinine, reflecting accelerated tubular dysfunction and potential for acute renal failure. Such combinations significantly increase the incidence of nephrotoxicity, particularly when multiple nephrotoxins are involved, as evidenced in patients receiving empirical antifungal therapy alongside antibiotics.^[56] This heightened risk underscores the need for vigilant assessment in high-risk populations, such as those with hematologic malignancies or post-transplant. To mitigate these interactions, renal function—including serum creatinine, electrolytes, and estimated glomerular filtration rate—should be monitored at least twice weekly during therapy, with more frequent checks if baseline impairment exists. Dose reductions of amphotericin B (e.g., to 0.3–0.5 mg/kg/day) or the interacting agent may be necessary if creatinine rises by 0.5 mg/dL or more, alongside hydration protocols to maintain urine output above 2 L/day. In severe cases, switching to lipid-formulated amphotericin B or alternative antifungals like voriconazole is recommended to minimize cumulative renal insult.

Other Pharmacological Interactions

Amphotericin B exhibits additive or synergistic effects when combined with flucytosine in the treatment of cryptococcal meningitis, leading to improved survival rates compared to amphotericin B monotherapy.^[57] This combination enhances fungal clearance and is recommended as first-line induction therapy for at least two weeks in severe cases.^[58] In contrast, concurrent use with azoles such as fluconazole often results in antagonistic interactions against Candida and Cryptococcus species, as azoles reduce ergosterol availability in fungal membranes, thereby diminishing amphotericin B's binding and fungicidal activity.^[59]^[60] Corticosteroids, while potentially exacerbating underlying fungal infections by suppressing immune responses, can mitigate amphotericin B-associated infusion-related reactions when used as premedication, such as hydrocortisone, to attenuate cytokine release.^[61] However, their concurrent administration requires careful monitoring due to the risk of compounded electrolyte imbalances.^[62] Hypokalemia induced by amphotericin B can potentiate digoxin toxicity by increasing cardiac sensitivity to the glycoside, potentially leading to arrhythmias; potassium supplementation and electrocardiographic monitoring are essential in patients receiving both agents.^[1] Therapeutic drug monitoring of amphotericin B plasma levels may be beneficial in select cases, such as pediatric or transplant patients, to optimize efficacy and minimize non-renal toxicity, though it is not routinely required.^[63] As amphotericin B is a substrate of P-glycoprotein (P-gp), co-administration with strong P-gp inducers (e.g., rifampin) or inhibitors (e.g., verapamil) should be avoided or closely monitored to prevent altered drug exposure and efficacy.^[64]^[65]

Pharmacology

Mechanism of Action

Amphotericin B exerts its antifungal and antiprotozoal effects primarily by interacting with sterol components in the cell membranes of target organisms. The drug, a polyene macrolide, binds selectively to ergosterol, the predominant sterol in fungal and protozoal membranes, through hydrophobic and hydrogen-bonding interactions. This binding facilitates the aggregation of amphotericin B molecules into transmembrane channels or pores, typically composed of 7–10 drug molecules forming a barrel-like structure with embedded sterol molecules. These pores span the lipid bilayer, creating aqueous conduits that disrupt membrane integrity.^[1]^[66] The formation of these ion channels leads to uncontrolled leakage of essential ions, including potassium (K⁺), sodium (Na⁺), and protons, from the cell interior. This ion efflux causes rapid membrane depolarization, osmotic imbalance, and subsequent cell lysis, rendering amphotericin B fungicidal and leishmanicidal at therapeutic concentrations. Additionally, the pores promote the generation of reactive oxygen species (ROS), which induce oxidative damage to cellular components, further contributing to pathogen death. The drug's activity is independent of the fungal cell cycle, allowing it to target both proliferating and quiescent cells effectively.^[1]^[67]^[66] Amphotericin B's selectivity for fungal and protozoal cells over mammalian ones stems from its approximately 10- to 50-fold higher affinity for ergosterol compared to cholesterol, the primary sterol in human membranes. Ergosterol's rigid structure and higher membrane concentration in pathogens enhance pore stability and ion conductance, amplifying the drug's potency against these organisms. Although amphotericin B can bind cholesterol and form less stable pores in host cells, this differential affinity underlies its therapeutic window. Resistance to amphotericin B is rare and typically arises from mutations in ergosterol biosynthesis genes (e.g., ERG3 or ERG11), altering sterol composition, rather than common mechanisms like efflux pump overexpression seen with azoles.^[68]^[67]^[66]

Pharmacokinetics

Amphotericin B exhibits negligible oral bioavailability, estimated at less than 5%, due to its poor aqueous solubility and instability in gastrointestinal conditions, necessitating parenteral administration for systemic therapy.^[69] Intravenous infusion of the conventional deoxycholate formulation at typical doses of 0.5–1 mg/kg achieves peak plasma concentrations of approximately 1–2 μg/mL.^[1] Lipid-based formulations, such as liposomal amphotericin B, yield substantially higher peak levels, up to 58 μg/mL at a 5 mg/kg dose, owing to their larger particle size and prolonged circulation.^[70] The drug demonstrates a large volume of distribution, approximately 4 L/kg, reflecting extensive tissue penetration and binding, with over 90% accumulation in organs such as the liver, spleen, and kidneys.^[1] Plasma protein binding exceeds 90%, further contributing to its wide distribution.^[71] Penetration into the cerebrospinal fluid is limited, reaching only 2–4% of simultaneous plasma concentrations, which often requires intrathecal administration for central nervous system infections.^[1] Lipid formulations exhibit formulation-specific distribution patterns, with liposomal variants showing a smaller apparent volume of distribution (around 0.22–0.42 L/kg) due to uptake by mononuclear phagocytic cells in the liver, spleen, and lungs, while achieving lower kidney concentrations (about 15% of those in hepatic tissues).^[70] Metabolism of amphotericin B is minimal and does not involve hepatic cytochrome P450 enzymes, with the parent drug remaining largely unchanged in circulation.^[71] Elimination occurs slowly, primarily through renal excretion of unchanged drug (2–5% over several days for conventional formulations), with minor biliary contributions; lipid formulations show even lower urinary output (<10% over 7 days) and favor fecal elimination.^[1] The initial plasma half-life is 24–48 hours for the deoxycholate form, extending to a terminal phase of about 15 days due to gradual release from tissue depots, leading to accumulation in patients with renal impairment.^[71] In contrast, lipid formulations prolong the terminal half-life significantly, up to 7 days for liposomal amphotericin B, enabling less frequent dosing and reduced peak exposures.^[70]

Toxicity

Mechanism of Toxicity

Amphotericin B exerts its toxicity primarily through binding to cholesterol in mammalian cell membranes, forming barrel-shaped pores that disrupt membrane integrity and lead to uncontrolled ion leakage, particularly potassium and magnesium, causing cellular dysfunction and death.^[1] This interaction is especially pronounced in renal tubular cells, where the high cholesterol content facilitates pore formation and subsequent ion imbalance, contributing to direct cytotoxicity in the proximal tubules.^[72] Additionally, amphotericin B induces afferent arteriole vasoconstriction in the kidney via stimulation of thromboxane A2 production, reducing renal blood flow and exacerbating ischemic damage to tubular epithelium.^[73] Systemically, amphotericin B triggers the release of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and interleukin-1 (IL-1) from monocytes and macrophages, leading to infusion-related reactions including fever and chills.^[74] In erythrocytes, the drug promotes oxidative stress by generating reactive oxygen species, which damage lipid membranes and induce hemolysis, resulting in normocytic anemia.^[75] Lipid-based formulations of amphotericin B, such as liposomal amphotericin B, mitigate toxicity by encapsulating the drug in lipid structures that limit the availability of free amphotericin B to bind host cholesterol, thereby reducing pore formation and associated cellular damage while preserving antifungal efficacy.^[76] This approach preferentially targets fungal membranes due to lower free drug concentrations in plasma.^[77]

Toxicity Management

Management of amphotericin B toxicity focuses on preventive measures to minimize nephrotoxicity and electrolyte disturbances, alongside prompt intervention when adverse effects occur. Aggressive hydration is a cornerstone of prevention, with administration of 1 liter of normal saline containing 20 mEq potassium chloride over 2 hours before each infusion recommended to reduce the risk of renal impairment.^[78] Lipid formulations, such as liposomal amphotericin B, are preferred over conventional deoxycholate due to their significantly lower nephrotoxic potential, allowing for safer use in patients at high risk for kidney injury.^[38] Alternate-day dosing may be considered after initial therapy to further mitigate cumulative toxicity once renal function stabilizes.^[78] For treatment of established toxicity, discontinuation is advised in cases of severe renal impairment, such as when serum creatinine rises to ≥2 times baseline and persists despite interventions, or in overdose scenarios where supportive care is provided but dialysis offers limited removal of the drug.^[1] Supportive measures include electrolyte replacement, with intravenous potassium chloride (up to 40 mEq) and oral supplements for hypokalemia (serum K+ <3.3 mmol/L), alongside magnesium supplementation (e.g., 250 mg tablets twice daily) to address common deficiencies.^[78] If toxicity is severe, switching to alternative antifungals like echinocandins is recommended to avoid further deterioration.^[1] Monitoring protocols are essential for early detection, involving baseline and 2–3 times weekly assessments of serum creatinine, potassium, magnesium, and hemoglobin, particularly during the second week of therapy when toxicity risks peak.^[78] Daily monitoring of fluid intake, output, and weight is also advised, especially in hospitalized patients.^[79] In special populations, elderly patients require heightened vigilance due to age-related declines in renal function, potentially necessitating dose reductions or more frequent monitoring to prevent exacerbation of toxicity.^[1] For those with hepatic disease, no routine dose adjustment is needed as amphotericin B elimination is primarily renal, but close observation for any emerging complications is warranted.^[1]

Biosynthesis

Microbial Production

Amphotericin B is primarily produced by the actinomycete bacterium Streptomyces nodosus, a soil-dwelling microorganism first identified as the key producer of this polyene macrolide antifungal.^[80] The biosynthesis occurs through a complex polyketide pathway involving modular polyketide synthase (PKS) enzymes that assemble a 37-carbon polyene chain from acetate, propionate, and butyrate-derived units, forming the macrolactone core.^[81] Subsequent post-PKS modifications, including glycosylation with the amino sugar mycosamine at the C19 position, confer the molecule's amphoteric properties and enhance its solubility and activity.^[82] This pathway is encoded by a large gene cluster spanning approximately 135 kb, which includes PKS modules and accessory genes for tailoring reactions such as hydroxylations and epoxidations.^[80] Industrial production of amphotericin B relies on submerged aerobic fermentation of S. nodosus in nutrient-rich media, typically composed of glucose as the primary carbon source and soybean flour or soy-based hydrolysates providing nitrogen and additional nutrients.^[83] The process involves seed culture preparation followed by inoculation into large-scale fermenters, where controlled aeration and agitation maintain dissolved oxygen levels above 20-30% to support polyketide chain elongation and oxidation steps.^[84] Yields have been optimized through strain engineering, such as gene cluster overexpression and precursor supplementation, achieving titers up to 5 g/L in fed-batch cultures, a significant improvement over wild-type production of around 1-2 g/L.^[85] Biosynthesis is tightly regulated by environmental factors, including carbon catabolite repression mediated by glucose, which suppresses PKS gene expression when glucose levels are high, delaying secondary metabolite production until later growth phases.^[86] Oxygen availability plays a critical role in macrolide formation, as it is required for cytochrome P450-mediated hydroxylations and the incorporation of molecular oxygen into the polyene structure; suboptimal aeration reduces precursor flux and overall yield.^[85] These regulatory mechanisms ensure efficient resource allocation, with industrial processes often employing fed-batch strategies to mitigate repression and maintain aerobic conditions for maximal output.^[84]

Chemical Synthesis

Amphotericin B presents formidable challenges in chemical synthesis owing to its intricate structure as a heptaene polyene macrolide, featuring 20 stereogenic centers, a 38-membered macrocycle, and a sensitive conjugated polyene moiety prone to degradation by light and oxygen.^[87] These factors demand rigorous control over stereochemistry, efficient fragment assembly, and protection strategies to prevent isomerization or oxidation during multi-step processes.^[88] Semi-synthetic routes typically start from the natural product isolated via microbial fermentation and involve targeted modifications to generate analogs with altered properties. A classic example is the preparation of N-iodoacetyl amphotericin B, formed by acylation of the mycosamine amino group, which facilitated X-ray crystallographic analysis and served as a precursor for further derivatization.^[89] Contemporary semi-syntheses focus on toxicity mitigation; for instance, the analog AM-2-19 is produced through a two-step process from amphotericin B, entailing C2' epimerization to install a methyl ester configuration that disrupts cholesterol binding, followed by C16 amidation with a serinol side chain to enhance ergosterol selectivity.^[90] This modification yields a compound with retained broad-spectrum antifungal activity but significantly reduced nephrotoxicity, as demonstrated in vitro and in murine models.^[90] Efforts toward total synthesis have been pursued to enable structural diversification, with the landmark achievement by Nicolaou and coworkers in 1987 marking the first enantioselective route to amphotericin B. This multi-step endeavor, exceeding 70 linear steps, assembled key fragments including the polyol chain and aglycone via stereocontrolled aldol reactions and macrolactonization, culminating in glycosidation with a mycosamine derivative; notably, Suzuki-Miyaura cross-coupling was employed to construct the sensitive polyene segment of the aglycone.^[88] The overall yield remained below 1%, underscoring the inefficiency of such routes for large-scale production.^[87] These chemical and semi-chemical syntheses primarily support research into less toxic amphotericin B derivatives, such as those targeting ergosterol-specific ion channels, but are not commercially viable compared to fermentation-based production due to high costs, low yields, and handling complexities.^[91]^[87]

History

Discovery and Early Development

Amphotericin B was isolated in 1955 from a soil sample collected in the Orinoco River region of Venezuela by researchers at the Squibb Institute for Medical Research. The compound was produced through fermentation of the actinomycete Streptomyces nodosus, which had been identified in the sample.^[92] It was named amphotericin B in 1955 due to its amphoteric chemical properties, capable of reacting as both an acid and a base.^[93] The initial isolation efforts were led by a team including R. Donovick, W. Gold, J.F. Pagano, and H.A. Stout, who reported its broad-spectrum antifungal activity against various pathogenic fungi in vitro.^[92] Early studies in the mid-1950s confirmed amphotericin B's potent antifungal effects, particularly against yeasts and dimorphic fungi, positioning it as a promising agent for systemic infections previously lacking effective treatments.^[20] The first reported human use occurred in 1957, when it was administered to a patient with disseminated histoplasmosis, marking a critical step in its transition from laboratory to clinical application.^[94] Subsequent pre-clinical evaluations highlighted its efficacy in animal models of fungal infections, though challenges with solubility and toxicity were immediately evident, requiring formulation with sodium deoxycholate for intravenous administration.^[95] The U.S. Food and Drug Administration approved amphotericin B in 1965 under the trade name Fungizone, formulated as a deoxycholate complex, based on emerging clinical evidence of its life-saving potential.^[20] Pivotal early trials demonstrated its efficacy in treating severe cases of coccidioidomycosis, with significant improvements in patient survival rates compared to prior therapies.^[96] These trials, conducted in the late 1950s, underscored the drug's role in managing disseminated fungal diseases, while also noting early concerns over renal toxicity and infusion-related reactions that would influence future management strategies.^[97]

Formulation Advancements

The development of lipid-based formulations of amphotericin B in the 1990s marked a significant advancement in mitigating the drug's nephrotoxicity while preserving its antifungal efficacy. These formulations, including amphotericin B lipid complex (ABLC; Abelcet), approved by the FDA in November 1995 for treating invasive fungal infections in patients refractory to or intolerant of conventional amphotericin B, encapsulated the drug in lipid structures to reduce renal exposure. Similarly, amphotericin B cholesteryl sulfate complex (ABCD; Amphotec) received FDA approval in November 1996 for aspergillosis in patients intolerant to conventional therapy, and liposomal amphotericin B (L-AmB; AmBisome) was approved in August 1997 for empirical therapy in febrile neutropenia and cryptococcal meningitis in HIV-infected patients. These innovations shifted amphotericin B from a last-resort option to a more tolerable standard for severe systemic mycoses. The push for these lipid formulations stemmed from clinical trials in the 1980s demonstrating substantial reductions in nephrotoxicity, with early studies showing up to 50% lower incidence of renal impairment compared to the deoxycholate form, alongside cost-benefit analyses highlighting improved patient outcomes and reduced hospitalization costs despite higher upfront expenses. For instance, lipid complexes were found to allow higher cumulative doses with fewer discontinuations due to toxicity, balancing efficacy against the economic burden of renal support therapies. These advancements were driven by the need to address the conventional formulation's 50-80% nephrotoxicity rate in prolonged use. Recent investigational progress has focused on oral and less toxic analogs to further enhance accessibility and safety. The oral lipid nanocrystal formulation MAT2203 completed Phase 2 trials in 2023, demonstrating noninferior early fungicidal activity to intravenous amphotericin B for cryptococcal meningitis, with significantly lower toxicity, including reduced anemia and renal effects, in a cohort of HIV-associated cases in South Africa. Preclinically, the amphotericin B analog AM-2-19, engineered in 2023 to selectively target fungal ergosterol over human cholesterol, has shown potent antifungal activity against Candida and Aspergillus species in mouse models with markedly reduced kidney damage, positioning it as a promising candidate for clinical translation as of 2025. As a cornerstone antifungal, amphotericin B, particularly its liposomal form, remains on the WHO Model List of Essential Medicines for treating visceral leishmaniasis and other severe infections; however, access remains limited in low-income countries. For example, as of 2021, only 17.5% of clinics in Africa reported availability of liposomal amphotericin B, exacerbated by high costs, with no-profit pricing increasing from $16.25 to $23 per vial in 2024, cold-chain requirements, and supply shortages for leishmaniasis control in regions like South Asia and East Africa.^[98]

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Amphotericin B (AmB) is a polyene derived from Streptomyces nodosus. This compound was discovered in the mid-1950s and remains a first-line agent for the ...
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Coccidioidomycosis and its treatment with amphotericin B - PubMed
Coccidioidomycosis and its treatment with amphotericin B. Am J Med. 1958 Apr;24(4):568-92. doi: 10.1016/0002-9343(58)90297-3. Authors. M L LITTMAN, P L ...Missing: pivotal trials
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Amphotericin B - ScienceDirect
It was originally isolated from a soil culture from the Orinoco River region, Venezuela. Used topically as a cream, or parenterally as a Na-desoxycholate ...