Amphotericin B colloidal dispersion

Amphotericin B colloidal dispersion (ABCD) is a colloidal dispersion of a stable complex of amphotericin B with cholesteryl sulphate in a 1:1 proportion, forming uniform disk-shaped particles. ABCD is associated with less nephrotoxicity than conventional amphotericin B deoxycholate. Infusion-related adverse events are more frequent in patients receiving ABCD than in patients receiving liposomal amphotericin B or amphotericin B deoxycholate. ABCD has been shown in a randomised, double-blind study, to be an effective alternative to amphotericin B deoxycholate for empirical treatment of patients with fever and neutropenia. ABCD is active in the treatment of invasive Candida spp. and Aspergillus spp. infections in immunocompromised hosts, however most of the data supporting its use for these types of infections is derived from non-comparative open-label clinical trials of patient refractory to or intolerant of conventional antifungal therapy. ABCD is approved by the US FDA for the treatment of invasive aspergillosis in patients where renal impairment or unacceptable toxicity precludes the use of amphotericin B deoxycholate in effective doses, and in patients with invasive aspergillosis where prior amphotericin B deoxycholate therapy has failed. Two other lipid formulations of amphotericin B, amphotericin B lipid complex and liposomal amphotericin B, are available and, like ABCD, are associated with reduced nephrotoxicity as compared to amphotericin B deoxycholate. The role of ABCD in comparison with these other lipid formulations of amphotericin B is discussed herein. High cost remains an issue with all lipid formulations of amphotericin B.     

Amphotericin B colloidal dispersion

Amphotericin B colloidal dispersion (ABCD) is a colloidal dispersion of a stable complex of amphotericin B with cholesteryl sulphate in a 1:1 proportion, forming uniform disk-shaped particles. ABCD is associated with less nephrotoxicity than conventional amphotericin B deoxycholate. Infusion-related adverse events are more frequent in patients receiving ABCD than in patients receiving liposomal amphotericin B or amphotericin B deoxycholate. ABCD has been shown in a randomised, double-blind study, to be an effective alternative to amphotericin B deoxycholate for empirical treatment of patients with fever and neutropenia. ABCD is active in the treatment of invasive Candida spp. and Aspergillus spp. infections in immunocompromised hosts, however most of the data supporting its use for these types of infections is derived from non-comparative open-label clinical trials of patient refractory to or intolerant of conventional antifungal therapy. ABCD is approved by the US FDA for the treatment of invasive aspergillosis in patients where renal impairment of unacceptable toxicity precludes the use of amphotericin B deoxycholate in effective doses, and in patients with invasive aspergillosis where prior amphotericin B deoxycholate therapy has failed. Two other lipid formulations of amphotericin B, amphotericin B lipid complex and liposomal amphotericin B, are available and, like ABCD, are associated with reduced nephrotoxicity as compared to amphotericin B deoxycholate. The role of ABCD in comparison with these other lipid formulations of amphotericin B is discussed herein. High cost remains an issue with all lipid formulations of amphotericin B.     

Lipid-based formulations of amphotericin B

Amphotericin B remains the drug of choice for the treatment of invasive fungal infections and visceral leishmaniasis. However, both the dose-dependent nephrotoxicity and the low response rates (10-80%) associated with amphotericin B limit its clinical use. The first marketed formulation of amphotericin B with deoxycholate, Fungizone, remains the "gold standard" in spite of its renal toxicity. Several investigations have been made to reduce the nephrotoxicity of amphotericin B by formulation strategies. Lipid-based formulations of amphotericin B were found to reduce toxicity and to increase tolerance and therapeutic efficacy. Three lipid formulations are now available in most countries: liposomal amphotericin B (AmBisome), amphotericin B lipid complex and amphotericin B colloidal dispersion. Amphotericin B colloidal dispersion was less nephrotoxic, but immediate reactions to this formulation were as frequent and severe as those to amphotericin B. Amphotericin B lipid complex appeared to be as effective as amphotericin B, with improved general and renal tolerability. Several comparative studies have confirmed that AmBisome has similar or superior efficacy relative to amphotericin B in various fungal infections, in visceral leishmaniasis and also in the empirical treatment of febrile neutropenia. Renal and general tolerability is excellent. A significant drawback to the newer, less toxic, commercial lipid-based formulations is their cost. There is a need to develop more affordable lipid-based formulations of amphotericin B.     

The lipid formulations of amphotericin B

Amphotericin B spectrum covers most of the fungal pathogens involved in human diseases. Its use is limited by infusion-related effects and nephrotoxicity. As a result of strong lipophilic properties, encapsulation in liposomes or binding to lipid complexes led to the development of lipid formulations in an attempt to increase both efficacy and safety. Three lipid formulations of amphotericin B are commercially available: a liposomal preparation, a lipid complex and a colloidal dispersion. They differ in their lipid composition, shape, pharmacokinetic behaviour and clinical effects. The nephrotoxicity of these formulations is significantly decreased compared to their parent compound. Infusion-related events are lowest with liposomal amphotericin B. Increased efficacy of the lipid formulations over conventional amphotericin B, however, still has to be demonstrated. These formulations are mainly indicated for the treatment of documented fungal infections in patients failing conventional amphotericin B or with renal impairment. Liposomal amphotericin B is also indicated for empirical therapy of suspected fungal infections in febrile neutropenic patients giving this compound an advantage over the two other formulations. Lipid formulations of amphotericin B are extremely expensive. Whether the increase in cost translates into a long-term benefit for the patient is still unknown.     

Lipid-based amphotericin B for the treatment of fungal infections

The frequency of life-threatening fungal infections has increased dramatically over the past few decades. For more than 30 years amphotericin B has been the standard treatment for systemic and deep-seated fungal infections, primarily because of its broad spectrum of activity. Its usefulness is limited by a relatively high frequency of significant adverse events including infusion-related reactions and nephrotoxicity. In an effort to overcome these side effects, a number of lipid-based formulations were developed, each with its own composition and pharmacokinetic behavior. The clinical significance of these differences is unknown. Available clinical data suggest the formulations have a reduced propensity for causing nephrotoxicity. However, considering limited efficacy data, they should be reserved as second-line therapy for patients who cannot tolerate or fail an adequate trial of conventional amphotericin B or cannot benefit from other antifungal agents.     

Determination of the Relative Toxicity of Amphotericin B Formulations: A Red Blood Cell Potassium Release Assay

Amphotericin B remains the drug of choice for the treatment of life-threatening, systemic fungal infections. Nevertheless, the use of amphotericin B in the traditional deoxycholate formulation (d-AmB) is limited by severe toxic side effects. Several relatively new formulations have recently received regulatory approval: amphotericin B colloidal dispersion (ABCD), amphotericin B lipid complex (ABLC), and liposomal amphotericin B (AmBisome). We quantitate the level of reduction of intrinsic toxicity of these formulations over d-AmB using a red blood cell potassium release assay, employing a variety of blood sources and incubation times. We also examine the practice of diluting d-AmB in the parenteral nutrition product Intralipid. Overall, the propensity in each formulation of amphotericin B to partition into the red cell membrane during incubation is measured by determining the concentration of amphotericin B required to achieve 50% potassium release. The concentrations leading to 50% potassium release were in the order AmBisome ABLC ABCD >> d-AmB d-AmB/Intralipid and reflect inversely the degree to which each formulation makes amphotericin B available to mammalian, cholesterol-containing membranes. To evaluate intrinsic efficacy in an analogous way we investigated the availability of amphotericin B to partition into ergosterol-containing membranes by carrying out potassium release experiments using Candida albicans fungal cells instead of red blood cells. In contrast to the results obtained with red cells, the concentrations required to achieve potassium release from fungal cells were essentially identical for all formulations. It can be concluded that the intrinsic, physicochemical component of therapeutic index improvement in these formulations follows the order AmBisome ABLC ABCD >> d-AmB d-AmB/Intralipid.     

What is the current and future status of conventional amphotericin B?

Amphotericin B deoxycholate has been the 'gold standard' treatment for invasive fungal infections for over 40 years. Driven to improve on the renal toxicity of amphotericin B deoxycholate, extensive pharmaceutical research has led to the development of several new antifungals including lipid formulations of amphotericin B, broad-spectrum azoles and echinocandins. Compared with amphotericin B deoxycholate, the lipid formulations of amphotericin B (amphotericin B lipid complex, amphotericin B colloidal dispersion and liposomal amphotericin B) share distinct advantages in improved drug safety, in particular reduced incidence and severity of amphotericin B deoxycholate-related nephrotoxicity. However, the lipid formulations of amphotericin B are significantly more expensive than amphotericin B deoxycholate and, as for many of these new antifungals, there are as yet insufficient published studies to guide clinicians. This paper examines aspects of safety, efficacy, and health economic data for the lipid formulations of amphotericin B in particular, in order to provide a rationale to justify substituting amphotericin B deoxycholate with the lipid formulations of amphotericin B.     

Toxicity of solubilized and colloidal amphotericin B formulations to human erythrocytes

The toxicity of a number of solubilized or colloidal amphotericin B formulations to human erythrocytes has been studied in-vitro. All the solubilized formulations studied, using poloxamer F127 or L92, or sodium deoxycholate as solubilizing agents, showed similar toxicity, erythrocyte lysis being greater than 90% for amphotericin B concentrations between 4 to 8 micrograms mL-1. Emulsion formulations stabilized by poloxamers showed reduced toxicity, while those stabilized by egg lecithin showed less than 5% erythrocyte lysis up to an amphotericin B concentration of 200 micrograms mL-1. The mechanisms of the differential toxicity is considered to be due to the differences in the equilibrium concentration of free amphotericin B in the aqueous phase.     

Amphotericin B lipid complex for the treatment of invasive fungal infections

Amphotericin B lipid complex (ABLC; Abelcet^®, Enzon Pharmaceuticals) has become the dominant marketed lipid amphotericin B compound to emerge since the approval of these agents from the mid-1990s onwards. This agent is a 1:1 combination of amphotericin B and a lipid moiety consisting of dimyristoyl phosphatidylcholine and dimyrisoyl phosphatidylcholine, which exists in a ribbon-like molecular structure. ABLC undergoes rapid reticuloendothelial uptake from the circulation and achieves significantly higher tissue concentrations in the liver, spleen and lung compared to comparably dosed conventional amphotericin B. ABLC is approved by the FDA for all mycoses in amphotericin B-intolerant or -refractory infection. Randomised, controlled trials of amphotericin B have shown comparable efficacy in candidiasis and an improved outcome in invasive aspergillosis versus historical controls. ABLC has demonstrated a reduced incidence of nephrotoxicity and infusion reactions versus amphotericin B. Comparative studies against other lipid formulations are quite limited and have shown variable differences in infusion toxicity, nephrotoxicity, hepatotoxicity and clinical efficacy. Postapproval experience has shown substantial efficacy for less common mycotic pathogens including zygomycosis. The precise position of ABLC versus both other lipid formulations and expanding formulary of new antifungal agents is in flux. Future studies which examine its clinical efficacy, role in combination therapy, toxicity and cost-effectiveness in these complex patients are needed.     

Amphotericin B lipid complex for the treatment of invasive fungal infections

Amphotericin B lipid complex (ABLC; Abelcet, Enzon Pharmaceuticals) has become the dominant marketed lipid amphotericin B compound to emerge since the approval of these agents from the mid-1990s onwards. This agent is a 1:1 combination of amphotericin B and a lipid moiety consisting of dimyristoyl phosphatidylcholine and dimyrisoyl phosphatidylcholine, which exists in a ribbon-like molecular structure. ABLC undergoes rapid reticuloendothelial uptake from the circulation and achieves significantly higher tissue concentrations in the liver, spleen and lung compared to comparably dosed conventional amphotericin B. ABLC is approved by the FDA for all mycoses in amphotericin B-intolerant or -refractory infection. Randomised, controlled trials of amphotericin B have shown comparable efficacy in candidiasis and an improved outcome in invasive aspergillosis versus historical controls. ABLC has demonstrated a reduced incidence of nephrotoxicity and infusion reactions versus amphotericin B. Comparative studies against other lipid formulations are quite limited and have shown variable differences in infusion toxicity, nephrotoxicity, hepatotoxicity and clinical efficacy. Postapproval experience has shown substantial efficacy for less common mycotic pathogens including zygomycosis. The precise position of ABLC versus both other lipid formulations and expanding formulary of new antifungal agents is in flux. Future studies which examine its clinical efficacy, role in combination therapy, toxicity and cost-effectiveness in these complex patients are needed.     

Amphotericin B formulations and drug targeting

Amphotericin B is a low-soluble polyene antibiotic which is able to self-aggregate. The aggregation state can modify its activity and pharmacokinetical characteristics. In spite of its high toxicity it is still widely employed for the treatment of systemic fungal infections and parasitic disease and different formulations are marketed. Some of these formulations, such as liposomal formulations, can be considered as classical examples of drug targeting. The pharmacokinetics, toxicity and activity are clearly dependent on the type of amphotericin B formulation. New drug delivery systems such as liposomes, nanospheres and microspheres can result in higher concentrations of AMB in the liver and spleen, but lower concentrations in kidney and lungs, so decreasing its toxicity. Moreover, the administration of these drug delivery systems can enhance the drug accessibility to organs and tissues (e.g., bone marrow) otherwise inaccessible to the free drug. During the last few years, new AMB formulations (AmBisome, Abelcet, and Amphotec) with an improved efficacy/toxicity ratio have been marketed. This review compares the different formulations of amphotericin B in terms of pharmacokinetics, toxicity and activity and discusses the possible drug targeting effect of some of these new formulations.     

Fatal amphotericin B overdose due to administration of nonlipid formulation instead of lipid formulation

The toxicity of amphotericin B deoxycholate has led to increased preference for lipid formulations with more favorable safety profiles. However, the primary use of lipid formulations is cost prohibitive, and many hospital formularies list both lipid and nonlipid formulations. A dispensing and administration error that caused amphotericin B deoxycholate to be given instead of liposomal amphotericin B related in a fatality. Measures to prevent confusion and aid in understanding the differences between lipid and nonlipid formulations of amphotericin B should be implemented.     

Variability in polyene content and cellular toxicity among deoxycholate amphotericin B formulations

STUDY OBJECTIVE: To evaluate the toxicity of amphotericin B deoxycholate formulations. DESIGN: In vitro experiment. SETTING: University research center. MATERIAL: Human mononuclear THP-1 cells. INTERVENTION: The human mononuclear cells were exposed in vitro for 2 hours to the following deoxycholate formulations of amphotericin B, in 2.5- and 5-micro/ml concentrations: Apothecon, Pharmacia, Sigma, Gensia, Pharma-Tek, and VHA. MEASUREMENTS AND MAIN RESULTS: Toxicity of the amphotericin B formulations were assessed by measuring interleukin (IL)-1beta expression in an in vitro model. Amphotericin B content was measured by enzyme-linked immunosorbent assay (ELISA), and amphotericin A and B contents were assessed by spectrophotometry. Endotoxin contamination was evaluated in all reagents. Expression of IL-1beta from Sigma, Pharmacia, and Pharma-Tek formulations was increased approximately 250%, 50%, and 25%, respectively, compared with amphotericin A. Amphotericin B content of Sigma, Pharmacia, Pharma-Tek, and Gensia formulations, as measured by ELISA, was increased approximately 450%, 200%, 200%, and 100%, respectively, compared with Apothecon. This variation could not be explained by differences in amphotericin A or B content as measured by spectrophotometry. CONCLUSION: Amphotericin B is obtained from a fermentation plant and manufactured as a pharmaceutical at different facilities. Both previous clinical observations and the current in vitro evaluation revealed significant differences among the formulations. Likely, other polyenes or pyrogenic toxins in differing amounts are in these formulations, thus explaining the variability in toxicity observed among the formulations.     

Liposomal and lipid formulations of amphotericin B. Clinical pharmacokinetics.

Amphotericin B remains a very important drug for the treatment of fungal infections despite its toxicity. Encapsulation of amphotericin B into liposomes appears to reduce the toxic effects and to improve the clinical efficacy, allowing higher dosages to be given. The exact mechanism behind the reduced toxicity is not yet known. Amphotericin B is widely distributed after intravenous administration as the deoxycholate solubilisate. The highest concentrations are found in the liver, spleen and kidney. Protein binding and binding to the tissues is very high. The fate of the drug in the body is not known in detail. Renal and biliary excretion are both low and no metabolites have been identified. The drug is still detectable in the liver, spleen and kidney for as long as 1 year after stopping therapy. The pharmacokinetics of the different liposomal amphotericin B or lipid complexes of amphotericin B, which were recently developed, are quite diverse. A number of these preparations, such as amphotericin B lipid complex (ABLC), 'AmBisome' and amphotericin B colloidal dispersion (ABCD) are in clinical development. Their pharmacokinetics depend to a large extent on the composition and particle size of the liposomes or lipid complexes. Relatively large structures such as ABLC are rapidly taken up by the mononuclear phagocyte system, whereas smaller liposomes remain in the circulation for prolonged periods. In all studies only the total amphotericin B (both free and liposome- or lipid-associated) concentrations were determined. There is a need for studies correlating clinical efficacy and tolerability of liposomal amphotericin B with the pharmacokinetic properties of these formulations.     

Potential role of aerosolized amphotericin B formulations in the prevention and adjunctive treatment of invasive fungal infections

The incidence of invasive fungal infections (IFIs) continues to increase, largely due to the steady rise in the number of at-risk patients and the increased use of aggressive immunosuppressant agents. Many available treatments are often limited by concerns about efficacy, safety, drug interactions, and/or cost. Owing to the poor treatment outcomes of immunosuppressed patients with IFIs, new preventative and treatment strategies are being investigated. Among these are the aerosolized formulations of amphotericin B. Published experience with the use of aerosolized amphotericin B deoxycholate (AmBd) in the prevention of IFIs has raised concerns regarding challenges in drug administration and tolerability. However, evolving data regarding administration of lipid-based formulations of amphotericin B indicate potential advantages over AmBd in the prevention and adjunctive treatment of IFIs.     

Lipid-based antifungal agents: current status

Immunocompromised patients are well known to be predisposed to developing invasive fungal infections. These infections are usually difficult to diagnose and more importantly, the resulting mortality rate is high. The limited number of antifungal agents available and their high rate of toxicity are the major factors complicating the issue. However, the development of lipid-based formulations of existing antifungal agents has opened a new era in antifungal therapy. The best examples are the lipid-based amphotericin B preparations, amphotericin B lipid complex (ABLC; Abelcet), amphotericin B colloidal dispersion (ABCD; Amphotec or Amphocil), and liposomal amphotericin B (AmBisome). These formulations have shown that antifungal activity is maintained while toxicity is reduced. This progress is followed by the incorporation of nystatin into liposomes. Liposomal nystatin formulation is under development and studies of it have provided encouraging data. Finally, lipid-based formulations of hamycin, miconazole, and ketoconazole have been developed but remain experimental. Advances in technology of liposomes and other lipid formulations have provided promising new tools for management of fungal infections.     

Lipid-based amphotericin B formulations: from animals to man

Amphotericin B has been the mainstay of systemic antifungal therapy for over 30 years, despite its serious side-effects, and, although numerous alternative antifungal agents have been developed, none to date has matched the efficacy of amphotericin B. However, modern drug delivery technology has improved the safety of amphotericin B by incorporating it into lipid-based delivery systems, including liposomes. Three such formulations, based on the natural affinity of amphotericin B for lipids, are currently marketed. All increase the therapeutic index of amphotericin B, thereby allowing more aggressive treatment than is possible with the conventional product. However, they differ in structure, side-effect profiles and evidence of proven efficacy as discussed in this review.     

Clinical pharmacokinetics of systemically administered antimycotics

Systemic fungal infections are a major threaten for immunocompromised patients. Beside the antimycotic spectrum, the pharmacokinetic properties of an antifungal drug are crucial for its clinical efficacy. Since patients with systemic mycoses frequently present with a significant co-morbidity, pharmacokinetics under special conditions such as renal insufficiency, renal replacement therapy or impaired liver function have to be considered. Amphotericin B is eliminated unchanged by the liver and the kidney. Its plasma protein binding accounts for 95 to 99 percent. Conventional amphotericin B deoxycholate has a remarkable infusion related and renal toxicity. Therefore, lipid formulations have been developed. By now, three lipid formulations are therapeutically used: liposomal amphotericin B, amphotericin B colloidal dispersion and amphotericin B lipid complex. Striking differences in their plasma pharmacokinetics have been found. These differences can be attributed to the diverse disposition of the lipid moieties, while liberated amphotericin B displays a pharmacokinetic behavior which is independent from the lipid-formulation applied. The highest amphotericin B tissue concentrations have been found in the liver and in the spleen, followed by lung, kidney and heart. Concentrations in brain tissue are very low. Flucytosine has no relevant protein binding and is eliminated by glomerular filtration. Fluconazole, itraconazole, voriconazole, posaconazole and ravuconazole are triazoles, used for treatment of systemic fungal infections. Significant drug interactions have to be considered during therapy with triazoles, particularly in patients dependent on immunosuppression. These interactions are caused by the metabolism of triazoles in the liver where the cytochrome P450 (CYP) system is involved at a different extend as well as by their mechanisms of action. Triazoles display a favorable tissue distribution with high penetration into the central nervous system. Echinocandins such as caspofungin and micafungin are rapidly taken up by peripheral tissues, particularly by the liver. In the first 24 hours this uptake appears to be the main route of elimination from plasma. Enzymatic degradation takes place, but is independent of CYP. Thus, drug interactions are a minor problem during echinocandin treatment. The highest tissue levels of caspofungin and micafungin have been measured in the liver. Moderate concentrations are achieved in lung, spleen and kidney. Penetration into the brain is relatively poor.     

The experience is CLEAR

Conventional amphotericin B has been the 'gold standard' for antifungal efficacy, but nephrotoxicity problems limit its clinical utility. In the late 1990s, three lipid-based formulations of amphotericin B were introduced, all of which offered comparable efficacy and reduced renal complications. However, to date, robust safety and comparative efficacy data have been sparse. This paper briefly reviews the available clinical data on amphotericin B lipid complex (ABLC). Also, in detail, it reviews the findings of Collaborative Exchange of Antifungal Research (CLEAR), the most extensive dataset on systemic antifungal treatment with the lipid-based agent ABLC.     

The reformulation of Amphotericin B for oral administration to treat systemic fungal infections and visceral leishmaniasis

Amphotericin B (AmB) is a parenterally administered broad-spectrum antifungal and leishmanicidal drug that has been on the market for over sixty years. Unfortunately, significant infusion-related side effects and renal toxicity often accompany treatment, limiting its clinical applications. Lipid-based formulations have somewhat ameliorated the associated toxicity, but the increased cost of formulations restricts widespread use. AmB is amphipathic and exhibits low solubility and permeability, resulting in negligible absorption when administered orally. Advances in drug delivery systems have overcome some of the solubility issues that prevent oral bioavailability and new formulations are currently in development. The existence of an effective, safe and inexpensive oral formulation of amphotericin B would have significant applications for the treatment of disseminated fungal infections and would dramatically expand access to treatment of visceral leishmaniasis by introducing a readily available highly tolerated oral formulation of a drug with known efficacy.