Pergamon Lruhrmro Research Vol. 21, No. 3, 183-188. lYY7. pp.

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REVIEW

DO VARIATIONS IN MOLECULAR STRUCTURE AFFECT THE CLINICAL EFFICACY AND SAFETY OF LIPID-BASED

AMPHOTERICIN B PREPARATIONS?

Jayesh Mehta Leukaemia Unit, Royal Marsden Hospital, Sutton, Surrey, U.K.

(Received 24 June 1996. Accepted 2 July 1996)

Abstract-While amphotericin B is the drug of choice in the treatment of most fungal infections, systemic adverse reactions and dose-dependent nephrotoxicity associated with its use frequently render the conventional preparation of amphotericin unsuitable. A number of lipid-based formulations of amphotericin have been developed to overcome the limitations of the parent compound. These preparations, amphotericin B lipid complex (ABLC), amphotericin B colloid dispersion (ABCD) and liposomal amphotericin differ from one another significantly in their molecular structure. They are also considerably more expensive than the parent compound. Clinical data on their appropriate use are limited. This review looks at the differences in molecular structure of these preparations, and at the available clinical safety and efficacy data. 0 1997 Elsevier Science Ltd. All rights reserved.

Key words: amphotericin B colloid dispersion, amphotericin B lipid complex, antifungal therapy, fungal infections, liposomal amphotericin, nephrotoxicity.

Introduction

The incidence of life-threatening invasive fungal infec- tions has increased dramatically over the past few decades [l]. This is largely attributable to the increased number of immunocompromised patients which include hematopoietic stem cell and organ transplant recipients, patients infected with the human immunodeficiency virus, and patients receiving intensive chemotherapy for malignant diseases [l-5].

The early diagnosis of systemic fungal infection in immunocompromised or neutropenic patients remains difficult. In this population of patients, antifungal treatment is usually administered empirically for presumptive fungal infections. Much less frequently, it may be given when a fungal pathogen has been identified.

Abbreviations: ABCD, amphotericin B colloid dispersion; ABLC, amphotericin B lipid complex; AUC, area under the curve.

Correspondence to: Jayesh Mehta, M.D., Division of Hematology/Oncology, University of Arkansas for Medical Sciences, 4301 West Markham, Mail Slot 508, Little Rock, AR 722059985, U.S.A.

For over 30 years, amphotericin B has remained the primary treatment for systemic fungal infections in immunocompromised patients because of its activity against a wide range of pathogenic and opportunistic mycoses [6]. However, amphotericin is a toxic drug, and its use is limited by a variety of adverse effects. The major problem with amphotericin is its dose-related nephrotoxicity, which severely compromises the drug’s efficacy in treating systemic fungal infections and restricts its prophylactic use [7, 81.

Lipid-based Formulations of Amphotericin

The poor water solubility and excellent lipid solubi- lity associated with amphotericin make this agent an ideal candidate for drug-lipid interaction studies [9]. Recently, a number of lipid-based formulations of amphotericin have become available commercially. These include amphotericin B lipid complex (ABLC, Abelcet, The Liposome Company), amphotericin B colloidal dispersion (ABCD, Amphocil, Zeneca Phar- maceuticals), and liposomal amphotericin B (AmBi- some, NeXstar Pharmaceuticals).

These agents have been developed specifically to

183

184 J. Mehta

Table 1. Molecular structure of the currently available lipid-based amphotericin compounds

Formulation Liposome category

Particle size Structure Lipids (molar ratio)

ABLC (lipid complex) Multilamellar vesicle fraction

2-5 pm Lipid ribbon Dimyristoyl phosphatidylcholine:dimyristoyl phosphatidylglycerol (7:3)

Liposomal amphotericin (liposomes)

Small unilamellar vesicles 80 nm Liposome Hydrogenated soy phosphatidylcholine: cholesterol: Distearoyl phosphatidylglycerol (10:5:4)

ABCD (lipid complex) Multilamellar vesicle 122 nm Lipid disc fraction (f48 nm)

Cholesterol sulfate

improve the tissue distribution of amphotericin and to reduce the toxicity of conventional amphotericin. This allows higher doses of the drug to be administered with less adverse effects, resulting in a potentially safer and more effective alternative. However, there are major logistic difficulties in demonstrating the relative effica- cies of the new agents. Additionally, while the problems associated with conventional amphotericin therapy cannot be underestimated, the high cost of the new formulations needs to be considered carefully.

All three agents differ substantially in the way amphotericin interacts with lipid, which in turn affects their individual structure and pharmacological profile. This review assesses available data to see if such differences (Table 1) result in variations in safety and efficacy between these agents in clinical practice.

Molecular structure Amphotericin B lipid complex is derived from a

liposomal formulation of amphotericin B originally developed by Lopez-Berestein et al., consisting of dimyristoyl-phosphatidylcholine and dimyristoyl-phos- phatidylglycerol in a 7:3 molar ratio [lo]. Unlike the original liposomal formulation, ABLC has a ribbon-like appearance. Further molecular studies with ABLC suggested that the ribbons arise from amphotericin- induced disruption of the lipid bilayer which resulted in a specific reorientation of both lipid and amphotericin, forming a more stable structure which has a particle diameter of 2-5 pm [9, 111.

Amphotericin B colloid dispersion consists of am- photericin and sodium cholesteryl sulfate in a 1:l molar ratio. It is a colloidal dispersion and has a disc-like structure. The individual discs have diameters of approximately 122 nm and a thickness of 4 nm [12].

Liposomal amphotericin is a lyophilized formulation of amphotericin incorporated into unilamellar liposomes which are made up of hydrogenated soy phosphatidyl- choline, cholesterol and distearoyl phosphatidylglycerol at a molar ratio of 10:5:4. Unlike the lipid complex

forms of amphotericin, liposomal amphotericin is a small spherical structure, approximately 80 nm in diameter [ 131.

Table 1 summarizes the structural differences be- tween the three lipid-based formulations of amphoter- icin.

Pharmacokinetics Human pharmacokinetic studies with ABLC have

shown an area under the curve (AUC)o_Zd which is five- fold lower than that for conventional amphotericin. In addition, the volume of distribution and clearance are three- and seven-fold higher, respectively [9]. Fielding et al. [14] showed lower peak plasma levels following ABCD than after conventional amphotericin in a rat model. At 5 mg/kg, ABCD resulted in a two-fold reduction in AUC values. The volumes of distribution and clearance were also greater with ABCD. In contrast to AESLC and ABCD, single-dose liposomal amphoter- icin studies in rats and mice reported high peak plasma concentrations, a prolonged elimination half-life and a large AUC compared with conventional amphotericin WI.

The results of multiple-dose animal tissue distribution studies with ABLC have demonstrated a dose-dependent increase in mean amphotericin concentrations in the liver, spleen and lung. Concentrations in the kidneys were comparable to conventional amphotericin, but plasma levels were consistently lower [9]. With ABCD, marked differences were also seen in the pattern of tissue distribution, with the highest concentrations of amphotericin being detected in the liver, spleen and bone marrow. In contrast, concentrations of amphoter- icin in other organs, such as the kidney, lung and heart, were comparable to those seen following conventional amphotericin [ 161. Multiple-dose studies with liposomal amphotericin reported higher mean concentrations of amphotericin in the spleen and liver with concentrations of drug in the kidney and lung similar to those seen following conventional amphotericin [15].

Lipid-based amphotericin preparations 185

Table 2. Pharmacokinetic characteristics of the currently available lipid-based amphotericin compounds

Formulation

C peak plasma Tissue amphotericin concentrations compared to compared to conventional amphotericin conventional amphotericin Liver Lungs Kidneys

ABLC Liposomal amphotericin ABCD

Lower Higher Lower

Higher Higher Higher

Higher Similar Similar

Similar Similar Similar

The pharmacological profile of each lipid-based amphotericin preparation can be related directly to its molecular structure. The ABLC, which is made up of relatively large lipid structures, is taken up rapidly in significant quantities by the mononuclear phagocytes of the reticuloendothelial system. This results in a lower circulating amphotericin concentration and enhanced tissue penetration, as reflected by the large volume of distribution and total clearance values. Pharmacokinetic data suggest that a large proportion of the amphotericin administered as ABCD undergoes rapid and extensive uptake by the liver through phagocytosis. It has been proposed that the liver acts as a reservoir of lipid- complexed amphotericin from which the drug is slowly released [14]. Since AEXD consists of small disc-like structures which pass through the pulmonary vascular bed, no increase in amphotericin concentration is observed in lung tissue [ 171. The small size of liposomal amphotericin particles results in low uptake and reduced distribution of amphotericin by the reticuloendothelial system, as reflected by the large AUC values. Uptake of amphotericin, which is mirrored by a slowly falling serum concentration, occurs through adsorption of the intact liposomes into the liver [15]. The pharmacokinetic characteristics of the three available lipid-based ampho- tericin agents are shown in Table 2.

Clinical eficacy Amphotericin B lipid complex. ABLC has been

compared with conventional amphotericin for the treatment of hematogenous and invasive candidiasis in a prospective, randomized fashion [18]. In a phase III multicenter study, 153 patients were assigned to ABLC (5 mg/kg/day for a median of 14 days) and 78 to conventional amphotericin (0.6-0.8 mg/kg/day for a median of 14 days for non-neutropenic patients and 0.6-1.0 mg/kg/day for a median of 14 days for neutro- penic patients). The overall response rate for 124 evaluable patients treated with ABLC was 65% compared with 61% for 70 evaluable patients treated with conventional amphotericin. On an intent-to-treat basis, response rates were 54% for ABLC and 56% for amphotericin (P = 0.876).

In patients who were infected with non-albicans Candida spp., the response rate was 64% for those treated with ABLC compared with 57% for those treated with conventional amphotericin. In patients with C. albicans infections, the response rate was 68% for those treated with ABLC and 64% for those treated with conventional amphotericin. Among patients with multi- ple Candida infections, the response rate was 50% for those treated with ABLC whereas those treated with conventional amphotericin did not respond at all. Mycological eradication rates were 88% for patients treated with ABLC and 87% for patients treated with conventional amphotericin. Median survival was 134 days for ABLC-treated patients as compared with 61 days for patients who received conventional ampho- tericin (P = 0.275).

ABLC was compared with conventional amphotericin in a historically controlled study in 178 patients with invasive aspergillosis. These patients were treated in two emergency-use protocols and one small, prospective, single-arm study [19]. The historical control group included patients treated for aspergillosis with conven- tional amphotericin as first-line therapy during the same period at four institutions. Of the 178 patients, 111 were considered evaluable for response. Overall, patients treated with ABLC showed higher response rates (40 versus 23%, P = 0.002).

In an open-label study which included 228 adult and pediatric patients who had either failed previous conventional amphotericin (total dose at least 15 mg/ kg or 100 mg) or other antifungal agents [20], had renal disease precluding conventional amphotericin therapy (serum creatinine >265 pmol/l), or had developed amphotericin-induced nephrotoxicity (serum creatinine >220 pmol/l in adults and >132 pmol/l in children), ABLC at the daily dose of 5 mg/kg for 4 weeks produced overall clinical and mycological response rates of 78% (126/183) and 55% (61/110), respectively WI-

ABLC was also investigated in a group of immuno- compromised and/or neutropenic patients with hemato- logical malignancies who had proven or presumed systemic fungal disease, and who were intolerant of

186 J. Mehta

conventional amphotericin or had pre-existing renal disease [21]. Forty-two patients received 46 evaluable (>four-dose) courses of 5 mg/kg ABLC comprising from four to 58 doses (median 10.5). Fifteen courses of therapy for proven fungal infections resulted in eight complete responses, two almost complete responses, one partial response and four clinical failures (73% response rate). Administration of 31 empiric courses of ABLC resulted in 13 complete responses, six partial responses and 12 clinical failures (61% response rate). The overall response rate was 65% [21].

Amphotericin B colloid dispersion. In a combined analysis of data from three clinical trials, Cannon et al. (221 reported that ABCD resulted in an overall clinical response rate of 50%. In patients with Candida infections, the response rate was 63% (41165). Clinical responses against Aspergillus spp. and Cryptococcus spp. were seen in 35 and 46% of patients, respectively P21.

In a multicenter study which included 168 patients with confirmed fungal infections, the majority of whom were immunocompromised, the clinical response rate was 49% for candidiasis and 16% for aspergillosis [23].

Another study of ABCD in 60 bone marrow transplant recipients with invasive fungal infections treated with escalating drug doses (0.5-7.5 mg/kg/day) for up to 6 weeks showed a complete or partial response in 34 patients (57%) [24]. The results were better for hepatosplenic candidiasis than for aspergillosis with overall response rates of 87 and 54%, respectively [24].

Liposomal amphotericin. In a multicenter study [25], 126 patients with 137 episodes of suspected or confirmed fungal infection were treated with liposomal amphotericin at the dose of 0.5-5 mg/kg (median 2.2 mg/kg) for a median of 18 days (range l-97 days). Clinical cure occurred in 58% of 64 patients with proven invasive fungal infections. Good clinical results were seen in candidiasis (mycological eradication in 83%), and the overall mycological response in aspergillosis was 41% [25].

Coker et al. [26] administered liposomal amphotericin (3 mg/kg/day) to 23 AIDS patients with cryptococcosis. A complete clinical response was seen in 18/23 patients.

A randomized, placebo-controlled trial of prophylac- tic liposomal amphotericin (1 mglkgiday) in 69 allo- geneic and 15 autologous bone marrow transplant recipients showed a reduction in fungal colonization in amphotericin-treated patients (33 vs 62%, P = O.OS), but the incidence of suspected or proven fungal infections was comparable in the two groups [27].

In a single center study of 116 neutropenic patients not tolerating or not responding to conventional amphotericin, liposomal amphotericin produced a clin- ical response in 13/21 patients (62%) with confirmed invasive aspergillosis [28]. The overall mycological

response rate in aspergillosis was 61% (14/21). The average daily dose of liposomal amphotericin adminis- tered was 2 mg/kg/day, which was increased to 5 mglkgi day in patients failing to respond to the lower dose. The median duration of treatment was 11 days.

Clinical safety Amphotericin B lipid complex. A prospective, rando-

mized, multicenter study in 231 patients with hemato- genous and invasive candidiasis reported significantly lower nephrotoxicity in the ABLC-treated patients compared with patients receiving conventional ampho- tericin [ 181. Baseline serum creatinine levels doubled in 47% (36176) of patients treated with conventional amphotericin (0.1-l mg/kg/day) compared with 28% (41/145) of ABLC patients (5 mg/kg/day) (P = 0.007). Actuarial analysis showed a significantly longer period of time to doubling of serum creatinine in ABLC-treated patients (X2 days compared with 19 days for conven- tional amphotericin; P = 0.028). The most common reason for withdrawal from the study was nephrotoxi- city, with 19% (15178) of patients in the conventional amphotericin group discontinuing therapy compared with 8% (121153) of patients in the ABLC group (P = 0.016).

The main adverse events noted in an open-label, non- randomized study employing 5 mg/kg/day ABLC were infusion-related [20], and included fever, chills, and nausea or vomiting. The ABLC appeared to spare renal function in patients with renal insufficiency and, as a group, patients presenting with amphotericin-induced nephrotoxicity showed an improvement in renal function whilst on ABLC.

Amphotericin B colloid dispersion. The commonest adverse event is usually fever, noted in approximately 50% of patients. In a multicenter study of 87 patients with confirmed mycoses, 123 adverse events related to the use of ABCD were seen, with 53% of patients experiencing fevers and chills [29]. Administration of ABCD at an infusion rate of not higher than 1 mglkgih has been recommended to avoid infusion-related side- effects [30].

An analysis of combined data from three trials indicated that ABCD at the mean dose of 3 mg/kg/day was not associated with dose-dependent renal toxicity P21.

Liposomal amphotericin. The most common side- effects reported in a multicenter, compassionate use study in 126 patients receiving liposomal amphotericin (mean cumulative dose of 2615 mg; mean duration 21 days) were raised liver enzyme levels (56%) and hypokalemia (18%). Increases in serum creatinine levels occurred in 11 patients. In 17 cases, initially elevated creatinine levels returned to normal by the end of therapy [31].

Lipid-based amphotericin preparations 187

In a study of 84 patients receiving placebo (n = 42) or 1 mg/kg/day liposomal amphotericin (n = 42) for pro- phylaxis of fungal infections after bone marrow transplantation, seven (16.7%) patients in the liposomal amphotericin group experienced adverse reactions, three of which were infusion-related allergic reactions [27]. Four additional patients reported possibly related adverse events which resolved after the cessation of liposomal amphotericin. The liposomal amphotericin group had significantly higher urea (P = 0.02) and serum creatinine (P = 0.001) after treatment [27].

Safety data from a single center study of 116 patients revealed a high incidence of hypernatremia associated with liposomal amphotericin (15%) [28]. In addition, liver function abnormalities were seen in 23 patients (17%).

Discussion

Could the pharmacological variations between the lipid-based amphotericin preparations translate into differences in efficacy and safety in clinical practice? In patients presenting with systemic mycoses, the liver, the spleen and the lungs are often the sites of deep fungal infections such as candidiasis and aspergillosis [8]. It would be logical to assume that targeted drug delivery to these sites would be beneficial for eradica- tion of the infection from them. However, in practice, only prospective studies can provide this information.

ABLC has usually been used at the dose of 5 mg/kg in most published studies, providing equivalent or im- proved efficacy and improved safety compared with conventional amphotericin [18-211. For liposomal amphotericin and ABCD, the manufacturers recommend a starting dose of 1 mg/kg, which is then increased stepwise to 3 mg/kg as required. Doses of up to 5 mg/kg have been used in some clinical studies with liposomal amphotericin to obtain response [28].

The current indications for these preparations include failure of conventional amphotericin, pre-existing or amphotericin-induced nephrotoxicity, and intolerance of conventional amphotericin. Aspergillosis is a difficult infection to eradicate with conventional therapy in immunocompromised patients, and lipid-based drugs may allow delivery of high doses of amphotericin over relatively short periods of time safely [19, 201. Thus, aspergillosis may be an indication for lipid formulations of amphotericin, irrespective of whether conventional amphotericin has been tried or not, and whether renal function is normal or not. Data supporting the use of lipid-based formulations of amphotericin as first-line therapy of non-aspergillus infections are limited; the only controlled study in this setting suggests that ABLC is an effective first-line treatment of candidiasis, and is

as effective as and less nephrotoxic than conventional amphotericin [ 181.

Further data from randomized, comparative trials are required to define the role of these agents in the treatment of fungal infections as safer and more effective alternatives to conventional amphotericin. In view of the growing incidence of systemic fungal infections in immunocompromised patients [l], and the limited usefulness of fluconazole and itraconazole in the first-line and prophylactic settings, especially for organ- isms such as C. krusei and Aspergillus Spp. [32,33], it is likely that the indications for the use of these agents will continue to expand.

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