Successful treatment with liposomal amphotericin B in two patients with persisting fungemia

Antifungal pharmacodynamics: review of the literature and clinical applications

Invasive fungal infections are seen with growing frequency, likely due to increases in numbers of patients at risk of infection. Optimal selection and dosing of antifungal agents are important, as these infections are often refractory to available therapy. In contrast to antibacterials, studies examining the pharmacodynamic properties of antifungals and their application in treating invasive disease often are lacking. Agents administered for invasive infections are amphotericin B, flucytosine, and azole antifungals. Several drugs are under investigation, such as posiconazole, voriconazole, and the echinocandins, and preliminary pharmacodynamic data likely will help shape dosing regimens. Clinical trials that investigated dosage and administration, as well as the potential benefits of combination and sequential therapy, are addressed. In addition, antifungal susceptibility and animal models of infection are discussed.

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.

Development of liposomal amphotericin B formulation

A considerable effort has been spent in the past three decades to investigate various aspects of liposomes as novel drug delivery systems. In 1990, the first amphotericin B (AmB) liposomal preparation (L-AmB) under the brand name AmBisome was introduced into the market by Vestar. The successful marketing of the product moved liposomes out of the stage of experimental obscurity to the realistic stage of clinical utility. The launch of AmBisome sparked off the introduction of other lipid-based AmB products marketed by Liposome Technology (Amphocil) and The Liposome Co. (Abelcet). The drive behind the development of a modified formulation of AmB was to improve the therapeutic index of this drug with respect to its major drawback associated with both acute and chronic toxic effects. In a 30-year-long experience with AmB, several reports were recorded in the literature of acute adverse effects, such as fever, rigors, vomiting, cardiotoxicity and hypotension occurring during infusion; while long-term therapy was reported to be associated with hypokalemia, renal dysfunction and hematological abnormalities. Another serious problem encountered with the drug had been the poor response obtained in immunocompromised patients like those with AIDS, neutropenia and cancer patients on chemotherapy. The encapsulation of amphotericin B in liposomal vesicles was hence targeted not only to obtain an improvement in the therapeutic index but also to see if it was useful in eradicating deep-seated fungal infections in immunocompromised patients. The liposomal AmB was found to have a better therapeutic index and lower toxicity than the commercial AmB preparations. The LD50 of AmBisome in mouse was 175 mg/kg compared with 3.7 mg/kg for Fungizone, the commercial preparation of AmB. Additionally, L-AmB has prolonged circulation time, and extravasates into the site of infection and delivers the drug directly to the site, with no nephrotoxicity and neurotoxicity as experienced with AmB. This review traces the course of development of L-AmB and discusses the rationale behind the development of its liposomal preparation. The results in in vitro, in vivo and clinical studies, mechanism of action, biodistribution, and formulation considerations of L-AmB are described. The clinical experience with the marketed preparation is reviewed.

Alternative modalities of administering amphotericin B: current issues

Numerous studies performed in the field of antifungal therapy during the last decade have resulted in major developments in new modalities of administering amphotericin B including liposomes or other lipid vehicles. Current data available are very encouraging and several preparations are already commercially available in some countries. An improved therapeutic index has been shown in humans but large comparative trials are still needed to establish the definite role and indications of the various preparations as well as the optimal therapeutic regimens. These studies will make a significant contribution to improving the prognosis of patients predisposed to life threatening invasive fungal infections.

Clinical experience with multilamellar liposomal amphotericin B in patients with proven and suspected fungal infections

Over a 3-year period, an unsonicated multilamellar vesicle preparation containing a low ratio of amphotericin B (5 mole %) was used as a routine alternative to amphotericin B-deoxycholate in treating 17 patients with a variety of systemic fungal infections representative of those commonly encountered on a tertiary care centre infectious disease service. Patient acceptability and convenience of administration were noteworthy. In 6/7 patients who had been given the liposomal drug after experiencing severe side effects (primarily hypokalemia and marked elevation of serum creatinine) on the non-liposomal form, the problems that had led to institution of the liposomal drug were reversed during treatment. However, multilamellar liposomal amphotericin B at conventional dosage was not without detectable toxicity in this patient population. Three transplant patients receiving cyclosporin at the same time as liposomal amphotericin B experienced a rise in serum creatinine, and 4 patients became hypokalemic during treatment: none of these effects was severe or required discontinuation of therapy. One or more liver enzymes rose measurably in 7 patients during treatment with liposomal amphotericin B, but remained unchanged or actually decreased in the remaining patients.

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.

Protein flux across the membrane of single secretion granules

We have applied, for the first time to our knowledge, X-ray microscopy to measure the mass of protein contained in single sub-cellular membrane-bound structures and to make high resolution, time-resolved observations on them. Using this method we have been able to follow the flux of protein out of secretion (zymogen) granules isolated from the acinar cells of the exocrine pancreas. The results provide direct visual and quantitative confirmation of the hypothesis that the membrane enclosing this object is permeable to its various contained proteins, although the mechanism remains unknown.

Interaction of positively-charged liposomes with blood: implications for their application in vivo

Liposomes with positively-charged lipid components have previously demonstrated efficacy in animal models for human diseases, and are currently being evaluated in human clinical studies. Cationic lipids can improve entrapment efficiency of drugs and other substances which are negatively charged, and facilitate penetration of biological membranes in vitro, e.g. in transfection. However, toxic effects have also been reported for positively-charged liposomes containing stearylamine. In this report we have examined gross interactions between plasma components or erythrocytes with cholesterol-rich SUV composed of PC or DPPC and having 0-50 mol% of phospholipid replaced with positively-charged stearylamine, DOTMA, or BisHOP. Plasma interactions observed included increased turbidity of the usually clear stroma and/or formation of a clot-like mass. At plasma concentrations of 0.25 mumol/ml or more, the extent of plasma interactions depended upon the concentration of positive charge, the charge density of cationic lipid initially present in the liposomes, and to a lesser degree, the nature of the lipid providing the positive charge. At liposomal positive charge concentrations of greater than 0.5 mumol/ml plasma, stearylamine provoked a strong increase in plasma turbidity, whereas liposomes incorporating DOTMA or BisHOP provoked a strong clotting response. Some hemolysis of erythrocytes in vitro occurred on interaction with cationic liposomes where positive charge was contributed by DOTMA or stearylamine, but not BisHOP. Implications for the clinical use of liposomes containing cationic lipids, is discussed.

Successful treatment of hepatosplenic candidiasis with a liposomal amphotericin B preparation

The case of a granulocytopenic patient with acute undifferentiated leukaemia and hepatosplenic candidiasis who was refractory to conventional deoxycholate amphotericin B (AmpB) and 5-flucytosine therapy is reported. He experienced severe AmpB-related side-effects, and was subsequently successfully treated with a pharmaceutical preparation of AmpB (5.7 g) entrapped in sonicated liposomes, composed of lecithin, cholesterol and stearylamine in a molar ratio of 4:3:1. Three months later, during maintenance chemotherapy, liposomal AmpB (5.1 g) was reinstituted due to the finding of biopsies positive for Candida albicans at bronchoscopy. After healing of the patient's fungal infection a left upper lobe resection was performed, which showed advanced fibrosis with signs of inflammation, but no evidence of fungal disease. Since no acute side-effects and only moderate hypokalaemia were observed, it appears that liposomal AmpB is superior to conventional AmpB treatment in granulocytopenic patients with hepatosplenic candidiasis and unbearable therapy-related side-effects.