Liposome-encapsulated aminoglycosides in pre-clinical and clinical studies

Raymond Schiffelersa,b,*, Gert Stormb and Irma Bakker-Woudenberga

a Department of Medical Microbiology & Infectious Diseases, Erasmus University Medical Center Rotterdam (EMCR), PO Box 1738, 3000 DR Rotterdam; b Department of Pharmaceutics, Utrecht Institute for Pharmaceutical Sciences (UIPS), Utrecht University, Utrecht, The Netherlands


    Abstract
 Top
 Abstract
 Introduction
 In vitro data
 Local application
 Intravenous administration
 Concluding remarks
 Acknowledgements
 References
 
Liposome-encapsulated amikacin has recently entered clinical trials. The rationale for liposome encapsulation of aminoglycosides is the possibility to increase the therapeutic index of this class of antibiotics by increasing aminoglycoside concentrations at the site of infection and/or by reducing the toxicity of these drugs. Three approaches can be distinguished: the use of liposomes as a depot formulation for local drug administration; targeting of (relatively) short circulating conventional liposomes to the cells of the mononuclear phagocyte system (MPS) for treating intracellular bacterial infections; and targeting of long-circulating liposomes to infectious foci localized outside the MPS. This review discusses the pre-clinical and clinical data in connection with recent developments in liposome technology.


    Introduction
 Top
 Abstract
 Introduction
 In vitro data
 Local application
 Intravenous administration
 Concluding remarks
 Acknowledgements
 References
 
Aminoglycosides

After the introduction of streptomycin in 1944, aminoglycosides developed into an important class of antibiotics. Their broad antimicrobial activity, post-antibiotic effect, synergy with ß-lactam antibiotics, rapid, concentration-dependent bactericidal activity and low cost contributed to their success, as well as a low frequency of resistance to them.14 However, they require parenteral administration. Moreover, dose-related adverse effects on kidneys and audio-vestibular apparatus make it necessary for the plasma concentrations to be maintained within a narrow range.58 Therefore, aminoglycosides are currently used for the treatment of severe (nosocomial) Gram-negative and Gram- positive infections, especially in immunocompromised patients, and for the treatment of mycobacterial infections.912

A drug delivery system that helps to increase the therapeutic index of the aminoglycosides by increasing the concentration of the drug at the site of infection and/or reducing the nephro- and ototoxicity would attract considerable interest, and liposomal encapsulation of aminoglycosides may provide this.

Liposomes

Liposomes are spherical vesicles, with particle sizes ranging from 30 nm to several micrometres, consisting of one or more lipid bilayers surrounding aqueous spaces.13,14 Hydrophilic drugs, such as aminoglycosides, can be encapsulated in the internal aqueous compartment, whereas hydrophobic drugs may bind to or are incorporated in the lipid bilayer.13,15 The bilayers are usually composed of natural or synthetic phospholipids and cholesterol, but the incorporation of other lipids or their derivatives, as well as proteins, is also possible.1315 The physicochemical characteristics of the liposome, like particle size, surface charge, sensitivity to pH changes and bilayer rigidity, can be manipulated.14 Manipulation of these characteristics can have marked effects on the in vivo behaviour of liposomes and therefore have a major impact on therapeutic success. Liposomes have also been studied as model membranes regarding the interaction of aminoglycosides with phospholipids in relation to aminoglycoside toxicity.1619 The present review will focus exclusively on liposomes as a drug delivery system for aminoglycosides.


    In vitro data
 Top
 Abstract
 Introduction
 In vitro data
 Local application
 Intravenous administration
 Concluding remarks
 Acknowledgements
 References
 
Extracellular bacteria

The earliest publications on liposome-encapsulated aminoglycosides appeared some 20 years ago. Variable results were reported on the antibacterial activity of liposomal antibiotics against extracellular bacteria. It was generally shown that the concentrations of the liposome-encapsulated aminoglycoside necessary to obtain growth inhibition and killing needed to be substantially higher compared with the free drug.2022 Encapsulation of the antibiotic reduces its antibacterial activity because the bacteria are separated from it by the liposomal bilayer. Variability of the in vitro data is probably the consequence of the variations in the liposome lipid compositions used, resulting in the encapsulated agents having various release profiles.

In contrast to this general observation, Beaulac et al.23 and Sachetelli et al.24 reported that a liposome formulation composed of dipalimitoylphosphatidylcholine and dimyristoylphosphatidylglycerol encapsulating tobramycin showed a considerable antibacterial effect against a range of Gram-positive and Gram-negative bacteria at concentrations below the MIC of the free antibiotic in vitro. They argued that the enhanced antibacterial effect may be due to a fusion mechanism of this liposome formulation with bacteria.24

Intracellular bacteria

In vitro studies using intracellularly infected phagocytic cells demonstrated that the phagocytosis of aminoglycoside-loaded liposomes yielded therapeutic intracellular drug concentrations,25 and consequently enhanced killing of intracellular microorganisms such as Staphylococcus aureus,26,27Escherichia coli,28 Brucella abortus,2931 Brucella canis30 and Mycobacterium avium complex (MAC).3235 A recent report addressed the possibility of further improving liposomal drug efficacy towards infected cells. Liposomes encapsulating gentamicin composed of pH-sensitive bilayers based on dioleoylphosphatidylethanolamine showed an improved antibacterial effect against intracellular Salmonella typhimurium and Listeria monocytogenes in murine macrophage-like J774A cells when compared with non-pH-sensitive liposome formulations.36 It is believed that the pH sensitivity of the liposomes promotes drug release in the acidic environment of the lysosomes after phagocytosis by the infected cells.


    Local application
 Top
 Abstract
 Introduction
 In vitro data
 Local application
 Intravenous administration
 Concluding remarks
 Acknowledgements
 References
 
Local application of large, multilamellar aminoglycoside-containing liposomes exploits the possibility of using liposomes as a reservoir from which the encapsulated drug can be released slowly, resulting in therapeutically active drug concentrations that are present at the site of infection for prolonged periods of time. Research in this area has focused on intravitreal or subconjunctival injection or topical application of liposomes for treatment of bacterial endophthalmitis or keratitis.3743 All studies reported prolonged presence of therapeutic aminoglycoside concentrations compared with administration of the free drug, offering the opportunity of reducing the number of injections necessary for successful treatment. In addition, systemic drug levels remained low. Research has been carried out mainly in rabbits but a single study reported excellent therapeutic results in eye infections affecting AIDS patients.44

Similar results to those obtained in the ophthalmic studies were reported after the prophylactic local application of aminoglycoside-loaded liposomes in models of soft tissue infection, burn wounds, prosthetic vascular grafts or surgical wound infections,4550 and after intrabronchial/ intratracheal administration of liposomal aminoglycosides in rodents.5154 Following intrabronchial administration, liposome-encapsulated tobramycin was shown to eradicate mucoid Pseudomonas aeruginosa in a model of chronic pulmonary infection.53 Interestingly, treatment results were dependent on the lipid composition of the liposomal formulation. Free tobramycin as well as tobramycin encapsulated in liposomes with rigid lipid bilayers showed no bactericidal effect, whereas tobramycin in liposomes composed of fluid lipid bilayers was able to eliminate the bacteria. These data are in agreement with data from in vitro experiments that have shown that fluid liposomes tend to release encapsulated aminoglycosides faster compared with their rigid counterparts.54


    Intravenous administration
 Top
 Abstract
 Introduction
 In vitro data
 Local application
 Intravenous administration
 Concluding remarks
 Acknowledgements
 References
 
Conventional liposomes

Circulation kinetics and tissue distribution. Extensive research on liposome behaviour after iv administration has shown that many liposome types rapidly accumulate in the cells of the mononuclear phagocyte system (MPS), particularly in the liver and spleen.5557 It is believed that the relatively rapid clearance of the liposomes is the result of opsonization in the bloodstream facilitating MPS recognition and uptake.58,59 Such liposomes are generally termed ‘conventional’ liposomes. The rate at which conventional liposomes are taken up by the MPS can be manipulated by controlling the liposome dose, but also by variation of liposomal characteristics such as charge, size and lipid composition. Generally, large, charged liposomes composed of fluid lipid bilayers tend to accumulate in the MPS more rapidly than small, neutral, rigid liposomes.60 With the objective of reducing the MPS uptake of conventional liposomes, it has been shown that by increasing the liposome dose, the proportion of liposomes that remains in the circulation can be increased because of saturation of MPS uptake.61 However, saturation of the MPS should be avoided as it will impair the body's ability to clear microorganisms from the circulation, which is an important defence mechanism in patients with severe infections.62,63

The pharmacokinetics of intravenously administered conventional liposome-encapsulated aminoglycosides generally show that plasma half-lives are prolonged compared with the free drug.6468 The blood levels reported in some representative studies of (liposomal) aminoglycosides are shown in Figure 1Go. Free and liposome-encapsulated drug were administered at equivalent doses. It is important to realize that when injected in the free form the aminoglycoside is completely active therapeutically, while after injection of the liposome-encapsulated form only the released portion is expected to show antimicrobial activity. The tissue distribution of aminoglycosides is greatly changed by liposomal encapsulation, as is illustrated in Figure 2Go. Free and liposome-encapsulated drug were again administered at equivalent doses. Renal concentrations of aminoglycosides are approximately similar after administration of either the free or the liposome-encapsulated forms, whereas much higher concentrations were observed in the liver and spleen after the injection of the liposome-encapsulated aminoglycosides. The absolute uptake of the liver exceeds that of the spleen when their respective weights are taken into consideration. Swenson et al.66 reported measurable gentamicin levels in the liver and spleen up to 2 and 15 weeks, respectively, after injection of a single liposomal gentamicin dose of 20 mg/kg. Concentrations in other organs achieved with these conventional liposomes are generally insignificant, although a few reports indicated increased concentrations in the lung.65,68 Interestingly, Ladigina & Vladimirsky65 showed that in the lungs of mice infected with Mycobacterium tuberculosis, a six-fold increase was seen in the amount of drug localizing in the infected lungs. However, absolute drug concentrations remained low.



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Figure 1. Circulation kinetics of conventional liposome encapsulated aminoglycosides (closed symbols) and free aminoglycosides (open symbols). Aminoglycoside concentrations at indicated time-points after injection of a single dose of gentamicin 20 mg/kg in rats (triangles),66 amikacin 40 mg/kg in mice (circles)68 or gentamicin 5.1 mg/kg in AIDS patients (squares).80

 


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Figure 2. Tissue distribution of conventional liposome (CL)-encapsulated aminoglycosides and free aminoglycosides. Concentrations in tissues (, spleen; , liver; , kidney) at 24 h after injection of a single dose of gentamicin 20 mg/kg (G) in rats66 and amikacin 40 mg/kg (A) in mice.68

 
It has been suggested that after liposome uptake and processing by the MPS cells, the drug may be released into the blood, prolonging drug blood levels. Bermudez et al.69 showed that substantial urinary excretion of amikacin continued for up to 7 days after injection of 50 mg/kg liposomal amikacin, whereas mice that received an equivalent dose of the free drug excreted most of the administered dose within the first day and had an undetectable level in the urine by day 4. Similar results were obtained by Swenson et al.,66 showing cumulative gentamicin urinary excretion continuing up to 10 days after injection of liposomal gentamicin 20 mg/kg. Even at that time point, only 80% of the injected dose was excreted cumulatively.

Safety. Considering the prolonged presence of aminoglycosides in the body, it is unfortunate that studies on nephro- or ototoxicity of ‘conventional’ liposomal formulations of aminoglycosides are lacking. There are, however, reports comparing the acute toxicity (characterized by convulsions or death as a result of neuromuscular blockade) of free versus liposome-encapsulated aminoglycosides in mice. Without exception all studies showed a substantial reduction in acute toxicity for the liposome-encapsulated drug.67,6971

Therapeutic efficacy. Generally, because of their hydrophilic nature, aminoglycosides are not the drug of choice for treating intracellular infections inside phagocytic cells. However, conventional liposomes readily accumulate in the MPS.7274 Therefore, aminoglycoside-loaded conventional liposomes were initially studied using in vivo models of intracellular infections inside the MPS cells. An overview of treatment results achieved with conventional liposome formulations is presented in Table 1Go.


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Table 1. Clinical and preclinical therapeutic efficacy of aminoglycosides in conventional liposomes
 
Promising results are reported regarding a bactericidal effect in the liver and spleen in intracellular infections caused by Mycobacterium spp., Salmonella spp. and Brucella spp.30,6771,7589 A pH-sensitive liposome formulation further increased therapeutic efficacy in liver and spleen in a murine intracellular S. typhimurium infection.88 In some studies a reduced bacterial load in lungs, blood and/or kidneys was also reported, but the antibacterial effects in these organs were always less pronounced and were only achieved at higher dosages. These results illustrate both the strengths and weaknesses of conventional liposomes as carrier systems for antibiotics. On the one hand, liposome-encapsulated aminoglycosides are very efficiently transported into the MPS cells in liver and spleen and consequently high intracellular concentrations can be achieved, resulting in good therapeutic efficacy as shown by prolonged survival and the opportunity to increase the dosing interval. On the other hand, owing to the relatively fast and efficient uptake of the liposomes by the MPS cells, relatively low levels of active drug are seen in organs outside the liver and spleen, and thus only moderate therapeutic effects are observed in these organs.

A limited number of reports describe the therapeutic efficacy of conventional liposomes encapsulating aminoglycosides directed against foci of infection outside the cells of the MPS. The prolonged presence of drug in the body after administration of conventional liposomeencapsulated aminoglycosides has been the rationale behind studying their prophylactic activity against extracellular bacterial infections. Swenson et al.66 showed that the dose of liposome-encapsulated gentamicin needed for protection against a lethal ip infection caused by K. pneumoniae or E. coli was substantially lower than for the free drug, when administered from 7 up to 2 days before bacterial inoculation. This result is not surprising, since the free drug is almost completely excreted within 24 h after injection. In a single dose study in a murine model of K. pneumoniae infection, a single dose of liposomeencapsulated gentamicin 20 mg/kg was more effective than an 80 mg/kg dose of free drug.89 The prolonged residence time of gentamicin in the body by liposome-encapsulation is probably responsible for the enhanced efficacy.

Long-circulating liposomes (LCLs)

Circulation kinetics and tissue distribution. To enable the liposomes to reach infectious sites outside the major MPS-organs, such as the liver and spleen, it is necessary to decrease the rate of uptake of the liposomes by the phagocytic cells. One way to achieve this is by preparing small, neutral vesicles with a rigid bilayer. Using this approach, NeXstar Pharmaceuticals (currently Gilead Sciences Inc.) have developed MiKasome, a small (c. 50 nm) unilamellar liposome formulation containing amikacin. This formulation is currently in clinical trials. Another approach to prolonging the circulation time of liposomes is the incorporation of poly(ethylene glycol) (PEG) coupled to phosphatidylethanolamine in the liposome bilayers. It is believed that the hydrophilic PEG provides a layer of steric hindrance around the liposome reducing liposome opsoniza-tion and thereby rapid recognition and uptake by the MPS cells. These liposomes are therefore termed ‘sterically stabilized liposomes’ (SSLs). The low MPS uptake of the SSLs is to a high degree irrespective of liposome lipid composition, which is an important advantage when tuning the liposome lipid composition for optimal targeting, retention and release.9097 Using this approach in our laboratory, we have developed a long-circulating SSL formulation containing gentamicin.98 Such flexibility in tailoring the liposome characteristics does not apply, for example, to MiKasome, as the lipid composition of MiKasome is restricted to a rigid membrane structure to retain its long half-life.

Studies with aminoglycosides encapsulated in both types of LCL show that drug plasma half-lifes are markedly prolonged. Blood levels obtained for MiKasome and SSLgentamicin are shown in Figure 3Go. Studies in rats receiving MiKasome 50 mg/kg demonstrated that the AUC in plasma is increased approximately 130-fold compared with the AUC of an equivalent dose of free amikacin.99 Similar findings were also seen in rabbits, dogs, rhesus monkeys and humans.100,101 In man, the mean plasma half-life of MiKasome was 114 h. After 1 week of daily dosing with 2.5 or 5 mg/kg/day mean plasma concentrations were 120 and 215 mg/L, respectively. One week later, plasma concentrations still amounted to 10–20 mg/L. Yet, the concentrations of free amikacin released from the liposome never exceeded 4 mg/L. Our experimental studies with SSL-gentamicin showed a similar picture in rats, with 70- to 130-fold increase in AUC compared with the free drug.98



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Figure 3. Circulation kinetics of LCL-encapsulated aminoglycosides (closed symbols) and free aminoglycosides (open symbols). Aminoglycoside concentrations at indicated time-points after injection of a single dose gentamicin 5 mg/kg in rats (circles)98 or amikacin 50 mg/kg in rats (squares).99

 
The tissue distribution of aminoglycosides is greatly changed after administration in the liposome-encapsulated form of both types of LCL to rats, as is illustrated in Figure 4Go. Equivalent doses of free and liposome-encapsulated drug were administered. Relatively high tissue concentrations are seen in the liver and spleen compared with free drug. In addition, higher drug concentrations are observed in other organs such as bone marrow, lungs, intestine, lymph nodes, skin and heart. MiKasome has been recovered from microvacuolated macrophages in most tissues after injection, which indicates that phagocytic cells could serve as a depot of amikacin. The urinary recovery of unchanged amikacin after injection in the MiKasome formulation is dramatically reduced compared with that in case of the free drug. Whereas practically all amikacin is excreted within 24 h after injection of the free drug, MiKasome showed less than 40% recovery in urine by day 10.100



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Figure 4. Tissue distribution of LCL-encapsulated aminoglycosides and free aminoglycosides. Concentrations in tissues at 24 h after injection of a single dose of gentamicin 5 mg/kg (G) in rats (data are based on liposome distribution, note the increased liposome levels in the infected lung tissue; organs shown are the only organs investigated)98 or amikacin 50 mg/kg (A) in rats.99 Key: , heart; , skin; , duodenum; , bone marrow; , kidney; , lung; , liver; , spleen.

 
In addition to the reduced affinity of LCL for the MPS and increased localization in other organs, it was demonstrated in our laboratory by Bakker-Woudenberg et al.102 that in a rat model of a unilateral pneumonia caused by K. pneumoniae, the localization of SSL in the infected left lung was approximately four-fold higher than the localization in the contralateral non-infected right lung. In the same model, 10-fold lower levels of localization in the infected lung were observed when liposomes with a relatively short circulation time were used.103 Recent studies indicate that a prolonged liposomal circulation time is essential for substantial localization in the target site. An increase in AUC of the liposome formulation, achieved by tuning the lipid composition, was reflected by a proportional increase in localization at the infectious focus.97 Similar findings of selective liposome localization at the target site in models of inflammation such as adjuvant arthritis, osteomyelitis, intra-abdominal abscesses, colitis, allergic encephalomyelitis, focal thigh infection and contact hypersensitivity have been reported.104113 The selectivity of the localization of LCL at the site of infection or inflammation is mediated by the locally increased capillary permeability as a result of the inflammatory response.114116 The nature of the inflammatory stimulus seems not important since instillation of 0.1 M hydrochloric acid or lipopolysaccharide into the lung also induced increased capillary permeability and localization of the liposomes.116 A contribution of infiltrating inflammatory cells to selective target site localization of liposomes has been suggested by some authors.105,115 Studies in the animal model using unilateral K. pneumoniae pneumonia indicate that the contribution of infiltrating inflammatory cells is not required for substantial target site localization of liposomes, as the degree of localization was similar in leucopenic rats as well as in immunocompetent rats.116 This is an important observation as these results would indicate that targeted liposomal drug delivery could also be beneficial to immunocompromised patients, who suffer from severe infections and have a higher risk of failure of their treatment.

Safety. Much work has been done on the safety of MiKasome. Parameters tested in a 1 month study with daily or every third day injection of MiKasome in Beagle dogs were based on clinical chemistry, haematology, urine analysis and coagulation together with body weights, clinical observations and vital signs. Gross necropsy and histopathologic examination of tissues was performed at the end of the study period.100 Daily doses of 20 mg/kg or every third day doses of 60 mg/kg were not associated with the occurrence of adverse effects despite mean steady state plasma concentrations above 750 mg/L and pre-dose levels >600 mg/L. Surprisingly, kidney concentrations above 1 mg/g did not lead to elevation of blood urea nitrogen or creatinine concentrations. The study shows that the ratio of cortical to medullary amikacin was substantially reduced by liposome encapsulation compared with the free drug. Therefore, it appears that liposome encapsulation results in a different kidney localization, preventing aminoglycoside-induced nephrotoxicity.100

A clinical study of safety in HIV-positive patients showed that after 1 week of daily dosing of 2.5 or 5 mg/kg, plasma levels were approximately 120 and 215 mg/L, respectively. Plasma amikacin levels of 10–20 mg/L persisted for 2 weeks after the last dose. However, no renal or audiovestibular toxicity was noted in any of the subjects participating in the study.100

Administration of gentamicin in rats showed acute toxicity after a single dose of 40 mg/kg, characterized by convulsions. A similar dose of SSL-gentamicin showed no acute toxicity.117

Therapeutic efficacy. Results of the treatment studies with aminoglycosides encapsulated in LCL are shown in Table 2Go.98,117122 The majority of studies report that the efficacy of LCL-encapsulated aminoglycosides is superior to that of the free aminoglycosides. Most studies relate to the use of MiKasome. The long half-life of LCL in the circulation allows for prolonged dosing intervals or even single dose treatments. A clinical trial in urinary tract infection patients shows that a single dose of MiKasome 40 mg/kg produced a high cure rate and the efficacy was comparable to seven daily infusions of 10 mg/kg.118 In two rabbit models of endocarditis, it was shown that single daily doses of MiKasome improved survival and were as efficient in reducing bacterial numbers as twice daily doses of the free drug, which is probably related to the prolonged residence time in the body of the liposomal formulation.119,120 In contrast, the rate of vegetation sterilization was higher in the animals treated with the free drug, probably as a result of the short-lasting, but high peak-levels of the free drug in the circulation. In the endocarditis models, treatments were combined with suboptimal doses of oxacillin to take advantage of the documented synergy between aminoglycosides and ß-lactams. The studies do not show whether differences in strength of the synergic interaction exist between free amikacin or MiKasome. A recent study reported that liposomal-co-encapsulation of gentamicin and ceftazidime resulted in a synergic interaction of both drugs against a (resistant) K. pneumoniae pneumonia in rats, in contrast to combination of the free drugs.123 This study shows that liposomal formulation does not inhibit and may even promote synergic drug interactions.


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Table 2. Clinical and preclinical therapeutic efficacy of aminoglycosides in LCLs
 
In immunocompromised mice, the relatively high tissue concentrations of MiKasome are probably responsible for the enhanced prophylactic activity of the liposomal drug in prolonging survival and reduction in bacterial numbers (both outside and within the liver and spleen).121 The studies related to SSL-gentamicin demonstrated in a K. pneumoniae pneumonia model in rats that the therapeutic efficacy was clearly superior to the free drug in a single dose schedule.98 Evaluation of its efficacy in a multidose schedule in leucopenic rats showed that addition of a single dose of SSL-gentamicin to free gentamicin treatment showed complete survival, using a seven-fold lower cumulative amount of gentamicin compared with treatment with free gentamicin alone. In leucopenic rats infected with K. pneumoniae having a low susceptibility to gentamicin, free gentamicin at the maximum tolerated dose did not result in survival. Addition of SSL-gentamicin was needed for therapeutic success. Complete survival was obtained by adding an SSL-gentamicin formulation with a fluid lipid bilayer, whereas adding a rigid SSL-gentamicin formulation showed only 50% survival. The increased gentamicin release from the fluid liposomes presumably improved rat survival, thus showing the importance of liposome lipid composition for therapeutic efficacy.117

Only one single study failed to show a superior effect of LCL-encapsulated aminoglycoside compared with conventional liposomal drug in the treatment of MAC infection.122 Unfortunately, the preparations used in this study were not characterized with respect to their circulation time as well as their tissue distribution, so the underlying cause of the results cannot be traced.


    Concluding remarks
 Top
 Abstract
 Introduction
 In vitro data
 Local application
 Intravenous administration
 Concluding remarks
 Acknowledgements
 References
 
Liposome-encapsulated aminoglycosides offer possibilities for increasing the therapeutic index of this class of antibiotics. Local application of liposomes may provide a reservoir that prolongs therapeutic drug concentrations at the site of infection. Readily accessible infected tissues such as in the eye, wounds and lungs could benefit from this local administration. In order to optimize therapeutic efficacy it is important to balance drug release from and retention in the liposome. Specific liposome compositions may enhance bacterial killing by interacting with the infectious organism.

Conventional liposomes are mostly taken up by the MPS after iv administration, the targeted delivery of drugs to MPS cells in the liver and spleen seems to be the most relevant application of this liposome type. Treatment of intracellular infections in the MPS cells may benefit from the high amounts of aminoglycosides that can be delivered intracellularly. By making liposomes pH-sensitive, the therapeutic availability of the liposome-encapsulated drug that is phagocytosed may even be increased. Research is needed on the nephro- and ototoxicity of conventional liposomal aminoglycosides, with respect to their prolonged presence in the body. This research should also include the potential danger of promoting microbial resistance as a result of the prolonged exposure of the resident microbial flora to the drug.

In case the infectious focus is located outside the MPS, conventional liposomes are of limited value. Therefore, research has been aimed at decreasing the MPS uptake of liposomes and consequently increasing their circulation time. LCLs were the result of these efforts. Intravenously administered LCLs potentially offer drug targeting to sites of infection not restricted to the MPS. A number of reports have demonstrated enhanced therapeutic efficacy of LCL-encapsulated aminoglycosides compared with free drugs or conventional liposomes. Unfortunately however, most studies with liposome-encapsulated aminoglycosides have, up to now, been performed in animal models with an intact host defence and infected with bacteria susceptible to the antibiotic. Treatment failure in clinical practice, however, particularly occurs in patients with impaired host defences or in patients infected with bacteria of low susceptibility. A single study addressed both issues in determining the efficacy of SSL-gentamicin.117 These issues should be incorporated more in animal models to demonstrate the value of liposomes in clinically relevant settings. So far, MiKasome has shown an excellent safety profile. Yet, similar to the conventional liposome formulations, the effects that the prolonged tissue drug concentrations have on development of resistance need to be addressed. The results that have been reviewed indicate promising prospects for liposome-encapsulated aminoglycosides and warrant further clinical investigations into the use of these formulations for the treatment of severe infections.


    Acknowledgements
 Top
 Abstract
 Introduction
 In vitro data
 Local application
 Intravenous administration
 Concluding remarks
 Acknowledgements
 References
 
R.S. is supported by grant 902-21-161 of the Dutch Organisation for Scientific Research.


    Notes
 
* Correspondence address. Department of Pharmaceutics Z 7.19, Utrecht Institute for Pharmaceutical Sciences (UIPS), Utrecht University, PO Box 80082, 3508 TB Utrecht, The Netherlands. Tel:+31-30-2539392; Fax:+31-30-2517839; E-mail: r.m.schiffelers{at}pharm.uu.nl Back


    References
 Top
 Abstract
 Introduction
 In vitro data
 Local application
 Intravenous administration
 Concluding remarks
 Acknowledgements
 References
 
1 . Lacy, M. K., Nicolau, D. P., Nightingale, C. H. & Quintiliani, R. (1998). The pharmacodynamics of aminoglycosides. Clinical Infectious Diseases 27, 23–7.[ISI][Medline]

2 . Lortholary, O., Tod, M., Cohen, Y. & Petitjean, O. (1995). Aminoglycosides. Medical Clinics of North America 79, 761–87.[ISI][Medline]

3 . Begg, E. J. & Barclay, M. L. (1995). Aminoglycosides—50 years on. British Journal of Clinical Pharmacology 39, 597–603.[ISI][Medline]

4 . Zembower, T. R., Noskin, G. A., Postelnick, M. J., Nguyen, C. & Peterson, L. R. (1998). The utility of aminoglycosides in an era of emerging drug resistance. International Journal of Antimicrobial Agents 10, 95–105.[ISI][Medline]

5 . Kumana, C. R. & Yuen, K. Y. (1994). Parenteral aminoglycoside therapy. Selection, administration and monitoring. Drugs 47, 902–13.[ISI][Medline]

6 . Molitoris, B. A. (1997). Cell biology of aminoglycoside nephrotoxicity: newer aspects. Current Opinion in Nephrology and Hypertension 6, 384–8.[ISI][Medline]

7 . Bagger-Sjoback, D. (1997). Effect of streptomycin and gentamicin on the inner ear. Annals of the New York Academy of Sciences 830, 120–9.[ISI][Medline]

8 . Hammett-Stabler, C. A. & Johns, T. (1998). Laboratory guidelines for monitoring of antimicrobial drugs. National Academy of Clinical Biochemistry. Clinical Chemistry 44, 1129–40.[Abstract/Free Full Text]

9 . Cometta, A. & Glauser, M. P. (1996). The use of aminoglycosides in neutropenic patients. Schweizerische Medizinische Wochenschrift Supplementum 76, 21S–7S.[Medline]

10 . Maertens, J. & Boogaerts, M. A. (1998). Anti-infective strategies in neutropenic patients. Acta Clinica Belgica 53, 168–77.[ISI][Medline]

11 . Maschmeyer, G., Hiddemann, W., Link, H., Cornely, O. A., Buchheidt, D., Glass, B. et al. (1997). Management of infections during intensive treatment of hematologic malignancies. Annals of Hematology 75, 9–16.[ISI][Medline]

12 . Quinn, J. P. (1998). Clinical strategies for serious infection: a North American perspective. Diagnostic Microbiology and Infectious Disease 31, 389–95.[ISI][Medline]

13 . Vemuri, S. & Rhodes, C. T. (1995). Preparation and characterization of liposomes as therapeutic delivery systems: a review. Pharmacautica Acta Helvitae 70, 95–111.

14 . Jones, M. N. (1995). The surface properties of phospholipid liposome systems and their characterization. Advances in Colloid and Interface Science 54, 93–128.[ISI][Medline]

15 . Gregoriadis, G. & Florence, A. T. (1993). Liposomes in drug delivery. Clinical, diagnostic and ophthalmic potential. Drugs 45, 15–28.[ISI][Medline]

16 . Swan, S. K. (1997). Aminoglycoside nephrotoxicity. Seminars in Nephrology 17, 27–33.[ISI][Medline]

17 . Carrier, D., Bou Khalil, M. & Kealey, A. (1998). Modulation of phospholipase. A2 activity by aminoglycosides and daptomycin: a Fourier transform infrared spectroscopic study. Biochemistry 37, 7589–97.[ISI][Medline]

18 . van Bambeke, F., Mingeot-Leclercq, M. P., Brasseur, R., Tulkens, P. M. & Schanck, A. (1996). Aminoglycoside antibiotics prevent the formation of non-bilayer structures in negatively-charged membranes. Comparative studies using fusogenic (bis(beta-diethylaminoethylether)hexestrol) and aggregating (spermine) agents. Chemistry and Physics of Lipids 79, 123–35.[ISI][Medline]

19 . Gurnani, K., Khouri, H., Couture, M., Bergeron, M. G., Beauchamp, D. & Carrier, D. (1995). Molecular basis of the inhibition of gentamicin nephrotoxicity by daptomycin; an infrared spectroscopic investigation. Biochimica et Biophysica Acta 1237, 86–94.[ISI][Medline]

20 . Antos, M., Trafny, E. A. & Grzybowski, J. (1995). Antibacterial activity of liposomal amikacin against Pseudomonas aeruginosa in vitro. Pharmacological Research 32, 85–7.[ISI][Medline]

21 . Omri, A., Ravaoarinoro, M. & Poisson, M. (1995). Incorporation, release and in-vitro antibacterial activity of liposomal aminoglycosides against Pseudomonas aeruginosa. Journal of Antimicrobial Chemotherapy 36, 631–9.[Abstract]

22 . Omri, A. & Ravaoarinoro, M. (1996). Comparison of the bactericidal action of amikacin, netilmicin and tobramycin in free and liposomal formulation against Pseudomonas aeruginosa. Chemotherapy 42, 170–6.[ISI][Medline]

23 . Beaulac, C., Sachetelli, S. & Lagace, J. (1998). In-vitro bactericidal efficacy of sub-MIC concentrations of liposome-encapsulated antibiotic against gram-negative and gram-positive bacteria. Journal of Antimicrobial Chemotherapy 41, 35–41.[Abstract]

24 . Sachetelli, S., Khalil, H., Chen, T., Beaulac, C., Senechal, S. & Lagace, J. (2000). Demonstration of a fusion mechanism between a fluid bactericidal liposomal formulation and bacterial cells. Biochimica et Biophysica Acta 1463, 254–66.[ISI][Medline]

25 . Dees, C. & Schultz, R. D. (1990). The mechanism of enhanced intraphagocytic killing of bacteria by liposomes containing antibiotics. Veterinary Immunology and Immunopathology 24, 135–46.[ISI][Medline]

26 . Bonventre, P. F. & Gregoriadis, G. (1978). Killing of intraphagocytic Staphylococcus aureus by dihydrostreptomycin entrapped within liposomes. Antimicrobial Agents and Chemotherapy 13, 1049–51.[ISI][Medline]

27 . MacLeod, D. L. & Prescott, J. F. (1988). The use of liposomally-entrapped gentamicin in the treatment of bovine Staphylococcus aureus mastitis. Canadian Journal of Veterinary Research 52, 445–50.[ISI][Medline]

28 . Stevenson, M., Baillie, A. J. & Richards, R. M. (1983). Enhanced activity of streptomycin and chloramphenicol against intracellular Escherichia coli in the J774 macrophage cell line mediated by liposome delivery. Antimicrobial Agents and Chemotherapy 24, 742–9.[ISI][Medline]

29 . Dees, C., Fountain, M. W., Taylor, J. R. & Schultz, R. D. (1985). Enhanced intraphagocytic killing of Brucella abortus in bovine mononuclear cells by liposomes-containing gentamicin. Veterinary Immunology and Immunopathology 8, 171–82.[ISI][Medline]

30 . Fountain, M. W., Weiss, S. J., Fountain, A. G., Shen, A. & Lenk, R. P. (1985). Treatment of Brucella canis and Brucella abortus in vitro and in vivo by stable plurilamellar vesicle-encapsulated aminoglycosides. Journal of Infectious Diseases 152, 529–35.[ISI][Medline]

31 . Vitas, A. I., Diaz, R. & Gamazo, C. (1996). Effect of composition and method of preparation of liposomes on their stability and interaction with murine monocytes infected with Brucella abortus. Antimicrobial Agents and Chemotherapy 40, 146–51.[Abstract]

32 . Bermudez, L. E., Wu, M. & Young, L. S. (1987). Intracellular killing of Mycobacterium avium complex by rifapentine and liposome-encapsulated amikacin. Journal of Infectious Diseases 156, 510–3.[ISI][Medline]

33 . Kesavalu, L., Goldstein, J. A., Debs, R. J., Duzgunes, N. & Gangadharam, P. R. (1990). Differential effects of free and liposome encapsulated amikacin on the survival of Mycobacterium avium complex in mouse peritoneal macrophages. Tubercle 71, 215–7.[ISI][Medline]

34 . Ashtekar, D., Duzgunes, N. & Gangadharam, P. R. (1991). Activity of free and liposome encapsulated streptomycin against Mycobacterium avium complex (MAC) inside peritoneal macrophages. Journal of Antimicrobial Chemoterapy 28, 615–7.[ISI][Medline]

35 . Majumdar, S., Flasher, D., Friend, D. S., Nassos, P., Yajko, D., Hadley, W. K. et al. (1992). Efficacies of liposome-encapsulated streptomycin and ciprofloxacin against Mycobacterium avium–M. intracellulare complex infections in human peripheral blood monocyte/macrophages. Antimicrobial Agents and Chemotherapy 36, 2808–15.[Abstract]

36 . Lutwyche, P., Cordeiro, C. & Wiseman, D. J. (1998). Intracellular delivery and antibacterial activity of gentamicin encapsulated in pH-sensitive liposomes. Antimicrobial Agents and Chemotherapy 42, 2511–20.[Abstract/Free Full Text]

37 . Barza, M., Baum, J. & Szoka, F., Jr (1984). Pharmacokinetics of subconjunctival liposome-encapsulated gentamicin in normal rabbit eyes. Investigative Ophthalmology and Visual Science 25, 486–90.[Abstract]

38 . Fishman, P. H., Peyman, G. A. & Lesar, T. (1986). Intravitreal liposome-encapsulated gentamicin in a rabbit model. Prolonged therapeutic levels. Investigative Ophthalmology and Visual Science 27, 1103–6.[Abstract]

39 . Barza, M., Stuart, M. & Szoka, F., Jr (1990). Effect of size and lipid composition on the pharmacokinetics of intravitreal liposomes. Investigative Ophthalmology and Visual Science 28, 893–900.[Abstract]

40 . Kim, E. K. & Kim, H. B. (1990). Pharmacokinetics of intravitreally injected liposome-encapsulated tobramycin in normal rabbits. Yonsei Medical Journal 31, 308–14.[Medline]

41 . Assil, K. K., Frucht-Perry, J., Ziegler, E., Schanzlin, D. J., Schneiderman, T. & Weinreb, R. N. (1991). Tobramycin liposomes. Single subconjunctival therapy of pseudomonal keratitis. Investigative Ophthalmology and Visual Science 32, 3216–20.[Abstract]

42 . Frucht-Perry, J., Assil, K. K., Ziegler, E., Douglas, H., Brown, S. I., Schanzlin, D. J. et al. (1992). Fibrin-enmeshed tobramycin liposomes: single application topical therapy of pseudomonal keratitis. Cornea 11, 393–7.[ISI][Medline]

43 . Zeng, S., Hu, C., Wei, H., Lu, Y., Zhang, Y., Yang, J. et al. (1993). Intravitreal pharmacokinetics of liposome-encapsulated amikacin in a rabbit model. Ophthalmology 100, 1640–4.[ISI][Medline]

44 . Peyman, G. A., Charles, H. C., Liu, K. R., Khoobehi, B. & Niesman, M. (1988). Intravitreal liposome-encapsulated drugs: a preliminary human report. International Ophthalmology 12, 175–82.[ISI][Medline]

45 . Price, C. I., Horton, J. W. & Baxter, C. R. (1989). Enhanced effectiveness of intraperitoneal antibiotics administered via liposomal carrier. Archives of Surgery 124, 1411–5.[Abstract]

46 . Price, C. I., Horton, J. W. & Baxter, C. R. (1990). Topical liposomal delivery of antibiotics in soft tissue infection. Journal of Surgical Research 49, 174–8.[ISI][Medline]

47 . Price, C. I., Horton, J. W. & Baxter, C. R. (1992). Liposome delivery of aminoglycosides in burn wounds. Surgery, Gynecology and Obstetrics 174, 414–8.

48 . Price, C. I., Horton, J. W. & Baxter, C. R. (1994). Liposome encapsulation: a method for enhancing the effectiveness of local antibiotics. Surgery 115, 480–7.[ISI][Medline]

49 . Grayson, L. S., Hansbrough, J. F., Zapata-Sirvent, R., Roehrborn, A. J., Kim, T. & Kim, S. (1995). Soft tissue infection prophylaxis with gentamicin encapsulated in multivesicular liposomes: results from a prospective, randomized trial. Critical Care Medicine 23, 84–91.[ISI][Medline]

50 . Huh, J., Chen, J. C., Furman, G. M., Malki, C., King, B., Kafie, F. et al. (1998). Local treatment of prosthetic vascular graft infection with multivesicular liposome-encapsulated amikacin. Journal of Surgical Research 74, 54–8.[ISI][Medline]

51 . Demaeyer, P., Akodad, E. M., Gravet, E., Schietecat, P., Van Vooren, J. P., Drowart, A. et al. (1993). Disposition of liposomal gentamicin following intrabronchial administration in rabbits. Journal of Microencapsulation 10, 77–88.[ISI][Medline]

52 . Omri, A., Beaulac, C., Bouhajib, M., Montplaisir, S., Sharkawi, M. & Lagace, J. (1994). Pulmonary retention of free and liposome-encapsulated tobramycin after intratracheal administration in uninfected rats and rats infected with Pseudomonas aeruginosa. Antimicrobial Agents and Chemotherapy 38, 1090–5.[Abstract]

53 . Beaulac, C., Clement-Major, S., Hawari, J. & Lagace, J. (1996). Eradication of mucoid Pseudomonas aeruginosa with fluid liposome-encapsulated tobramycin in an animal model of chronic pulmonary infection. Antimicrobial Agents and Chemotherapy 40, 665–9.[Abstract]

54 . Beaulac, C., Clement-Major, S., Hawari, J. & Lagace, J. (1997). In vitro kinetics of drug release and pulmonary retention of microencapsulated antibiotic in liposomal formulations in relation to the lipid composition. Journal of Microencapsulation 14, 335–48.[ISI][Medline]

55 . Schroit, A. J., Madsen, J. & Nayar, R. (1986). Liposome–cell interactions: in vitro discrimination of uptake mechanism and in vivo targeting strategies to mononuclear phagocytes. Chemistry and Physics of Lipids 40, 373–93.[ISI][Medline]

56 . Senior, J. (1987). Fate and behaviour of liposomes in vivo. CRC Critical Reviews in Therapeutic Drug Carrier Systems 3, 123–93.[ISI][Medline]

57 . Gregoriadis, G., Kirby, C. & Senior, J. (1983). Optimization of liposome behaviour in vivo. Biology of the Cell 47, 11–8.[ISI]

58 . Szebeni, J. (1998). The interaction of liposomes with the complement system. Critical Reviews in Therapeutic Drug Carrier Systems 15, 57–88.[ISI][Medline]

59 . Patel, H. M. (1992). Serum opsonins and liposomes: their interaction and opsonophagocytosis. Critical Reviews in Therapeutic Drug Carrier Systems 9, 39–90.[ISI][Medline]

60 . Gregoriadis, G. (1988). Fate of injected liposomes: observations on entrapped solute retention vesicle clearance and tissue distribution in vivo. In Liposomes as Drug Carriers, Recent Trends and Progress, (Gregoriadis, G., Ed.), pp. 3–18. John Wiley and Sons, Chichester.

61 . Allen, T. M. (1988). Interactions of liposomes and other drug carriers with the mononuclear phagocyte system. In Liposomes as Drug Carriers, Recent Trends and Progress, (Gregoriadis, G., Ed.), pp. 37–50. John Wiley and Sons, Chichester.

62 . Van Etten, E. W., ten Kate, M. T., Snijders, S. V. & Bakker-Woudenberg, I. A. (1998). Administration of liposomal agents and blood clearance capacity of the mononuclear phagocyte system. Antimicrobial Agents and Chemotherapy 42, 1677–81.[Abstract/Free Full Text]

63 . Storm, G., ten Kate, M. T., Working, P. K. & Bakker-Woudenberg, I. A. (1998). Doxorubicin entrapped in sterically stabilized liposomes: effects on bacterial blood clearance capacity of the mononuclear phagocyte system. Clinical Cancer Research 4, 111–5.[Abstract]

64 . Morgan, J. R. & Williams, K. E. (1980). Preparation and properties of liposome-associated gentamicin. Antimicrobial Agents and Chemotherapy 17, 544–8.[ISI][Medline]

65 . Ladigina, G. A. & Vladimirsky, M. A. (1986). The comparative pharmacokinetics of 3H-dihydrostreptomycin in solution and liposomal form in normal and Mycobacterium tuberculosis infected mice. Biomedical Pharmacotherapy 40, 416–20.

66 . Swenson, C. E., Stewart, K. A., Hammett, J. L., Fitzsimmons, W. E. & Ginsberg, R. S. (1990). Pharmacokinetics and in vivo activity of liposome-encapsulated gentamicin. Antimicrobial Agents and Chemotherapy 34, 235–40.[ISI][Medline]

67 . Tadakuma, T., Ikewaki, N., Yasuda, T., Tsutsumi, M., Saito, S. & Saito, K. (1985). Treatment of experimental salmonellosis in mice with streptomycin entrapped in liposomes. Antimicrobial Agents and Chemotherapy 28, 28–32.[ISI][Medline]

68 . Cynamon, M. H., Swenson, C. E., Palmer, G. S. & Ginsberg, R. S. (1989). Liposome-encapsulated-amikacin therapy of Mycobacterium avium complex infection in beige mice. Antimicrobial Agents and Chemotherapy 33, 1179–83.[ISI][Medline]

69 . Bermudez, L. E., Yau-Young, A. O., Lin, J. P., Cogger, J. & Young, L. S. (1990). Treatment of disseminated Mycobacterium avium complex infection of beige mice with liposome-encapsulated aminoglycosides. Journal of Infectious Diseases 161, 1262–8.[ISI][Medline]

70 . Vladimirsky, M. A. & Ladigina, G. A. (1982). Antibacterial activity of liposome-entrapped streptomycin in mice infected with Mycobacterium tuberculosis. Biomedical Pharmacotherapy 36, 375–7.

71 . Fierer, J., Hatlen, L., Lin, J. P., Estrella, D., Mihalko, P. & Yau-Young, A. (1990). Successful treatment using gentamicin liposomes of Salmonella dublin infections in mice. Antimicrobial Agents and Chemotherapy 34, 343–8.[ISI][Medline]

72 . Kirsh, R. & Poste, G. (1986). Liposome targeting to macrophages: opportunities for treatment of infectious diseases. Advances in Experimental Medical Biology 202, 171–84.

73 . Karlowsky, J. A. & Zhanel, G. G. (1992). Concepts on the use of liposomal antimicrobial agents: applications for aminoglycosides. Clinical Infectious Diseases 15, 654–67.[ISI][Medline]

74 . Bakker-Woudenberg, I. A., Storm, G. & Woodle, M. C. (1994). Liposomes in the treatment of infections. Journal of Drug Targeting 2, 363–71.[ISI][Medline]

75 . Duzgunes, N., Perumal, V. K., Kesavalu, L., Goldstein, J. A., Debs, R. J. & Gangadharam, P. R. (1988). Enhanced effect of liposome-encapsulated amikacin on Mycobacterium avium–M. intracellulare complex infection in beige mice. Antimicrobial Agents and Chemotherapy 32, 1404–11.[ISI][Medline]

76 . Klemens, S. P., Cynamon, M. H., Swenson, C. E. & Ginsberg, R. S. (1990). Liposome-encapsulated-gentamicin therapy of Mycobacterium avium complex infection in beige mice. Antimicrobial Agents and Chemotherapy 34, 967–70.[ISI][Medline]

77 . Gangadharam, P. R., Ashtekar, D. A., Ghori, N., Goldstein, J. A., Debs, R. J. & Duzgunes, N. (1991). Chemotherapeutic potential of free and liposome encapsulated streptomycin against experimental Mycobacterium avium complex infections in beige mice. Journal of Antimicrobial Chemoterapy 28, 425–35.[Abstract]

78 . Duzgunes, N., Ashtekar, D. R., Flasher, D. L., Ghori, N., Debs, R. J., Friend, D. S. et al. (1991). Treatment of Mycobacterium avium–intracellulare complex infection in beige mice with free and liposome-encapsulated streptomycin: role of liposome type and duration of treatment. Journal of Infectious Diseases 164, 143–51.[ISI][Medline]

79 . Cynamon, M. H., Klemens, S. P. & Swenson, C. E. (1992). TLC G-65 in combination with other agents in the therapy of Mycobacterium avium infection in beige mice. Journal of Antimicrobial Chemoterapy 29, 693–9.[Abstract]

80 . Nightingale, S. D., Saletan, S. L., Swenson, C. E., Lawrence, A. J., Watson, D. A., Pilkiewicz, F. G. et al. (1993). Liposomeencapsulated gentamicin treatment of Mycobacterium avium– Mycobacterium intracellulare complex bacteremia in AIDS patients. Antimicrobial Agents and Chemotherapy 37, 1869–72.[Abstract]

81 . Wiley, E. L., Perry, A., Nightingale, S. D. & Lawrence, J. (1994). Detection of Mycobacterium avium–intracellulare complex in bone marrow specimens of patients with acquired immunodeficiency syndrome. American Journal of Clinical Pathology 101, 446–51.[ISI][Medline]

82 . Ehlers, S., Bucke, W., Leitzke, S., Fortmann, L., Smith, D., Hansch, H. et al. (1996). Liposomal amikacin for treatment of M. avium infections in clinically relevant experimental settings. Zentralblatt für die Bakteriologie 284, 218–31.

83 . Leitzke, S., Bucke, W., Borner, K., Muller, R., Hahn, H. & Ehlers, S. (1998). Rationale for and efficacy of prolonged-interval treatment using liposome-encapsulated amikacin in experimental Mycobacterium avium infection. Antimicrobial Agents and Chemotherapy 42, 459–61.[Abstract/Free Full Text]

84 . Tomioka, H., Saito, H., Sato, K. & Yoneyama, T. (1991). Therapeutic efficacy of liposome-encapsulated kanamycin against Mycobacterium intracellulare infection induced in mice. American Reviews on Respiratory Diseases 144, 575–9.

85 . Khalil, R. M., Murad, F. E., Yehia, S. A., El-Ridy, M. S. & Salama, H. A. (1996). Free versus liposome-entrapped streptomycin sulfate in treatment of infections caused by Salmonella enteritidis. Pharmazie 51, 182–4.[ISI][Medline]

86 . Vitas, A. I., Diaz, R. & Gamazo, C. (1997). Protective effect of liposomal gentamicin against systemic acute murine brucellosis. Chemotherapy 43, 204–10.[ISI][Medline]

87 . Hernandez-Caselles, T., Vera, A., Crespo, F., Villalain, J. & Gomez-Fernandez, J. C. (1989). Treatment of Brucella melitensis infection in mice by use of liposome-encapsulated gentamicin. American Journal of Veterinary Research 50, 1486–8.[ISI][Medline]

88 . Cordeiro, C., Wiseman, D. J., Lutwyche, P., Uh, M., Evans, J. C., Finlay, B. B. et al. (2000). Antibacterial efficacy of gentamicin encapsulated in pH-sensitive liposomes against an in vitro Salmonella enterica serovar typhimurium intracellular infection model. Antimicrobial Agents and Chemotherapy 44, 533–9.[Abstract/Free Full Text]

89 . Ginsberg, R. S., Mitilenes, G. M. & Lenk, R. P. (1988). The impact of liposome encapsulation of gentamicin on the treatment of extracellular gram-negative bacterial infections. UCLA Symposium on Molecular Cell Biology New Series 89, 205–14.

90 . Torchilin, V. P. (1998). Polymer-coated long-circulating microparticulate pharmaceuticals. Journal of Microencapsulation 15, 1–19.[ISI][Medline]

91 . Woodle, M. C. & Lasic, D. D. (1992). Sterically stabilized liposomes. Biochimica et Biophysica Acta 1113, 171–99.[ISI][Medline]

92 . Woodle, M. C. (1993). Surface-modified liposomes: assessment and characterization for increased stability and prolonged blood circulation. Chemistry and Physics of Lipids 64, 249–62.[ISI][Medline]

93 . Woodle, M. C., Newman, M. S. & Working, P. K. (1995). Biological properties of sterically stabilized liposomes. In Stealth Liposomes, (Lasic, D. & Martin, F., Eds), pp. 103–18. CRC Press, Boca Raton, FL.

94 . Storm, G. & Woodle, M. C. (1998). Long circulating liposome: from concept to clinical reality. In Long Circulating Liposomes: Old Drugs, New Therapeutics, (Woodle, M.C. & Storm, G., Eds), pp. 3–16. Springer Verlag, Berlin.

95 . Litzinger, D. C., Buiting, A. M., van Rooijen, N. & Huang, L. (1994). Effect of liposome size on the circulation time and intraorgan distribution of amphipathic poly(ethylene glycol)-containing liposomes. Biochimica et Biophysica Acta 1190, 99–107.[ISI][Medline]

96 . Schiffelers, R. M., Bakker-Woudenberg, I. A., Snijders, S. V. & Storm, G. (1999). Localization of sterically stabilized liposomes in Klebsiella pneumoniae-infected rat lung tissue: influence of liposome characteristics. Biochimica et Biophysica Acta 1421, 329–39.[ISI][Medline]

97 . Schiffelers, R. M., Bakker-Woudenberg, I. A. & Storm, G. (2000). Localization of sterically stabilized liposomes in Klebsiella pneumoniae-infected rat lung tissue: dependence on circulation kinetics and presence of poly(ethylene) glycol coating. Biochimica et Biophysica Acta 1468, 253–61.[ISI][Medline]

98 . Bakker-Woudenberg, I. A., ten Kate, M. T., Stearne-Cullen, L. E. & Woodle, M. C. (1995). Efficacy of gentamicin or ceftazidime entrapped in liposomes with prolonged blood circulation and enhanced localization in Klebsiella pneumoniae-infected lung tissue. Journal of Infectious Diseases 171, 938–47.[ISI][Medline]

99 . Fielding, R. M., Lewis, R. O. & Moon-McDermott, L. (1998). Altered tissue distribution and elimination of amikacin encapsulated in unilamellar, low-clearance liposomes (MiKasome®). Pharmaceutical Research 15, 1775–81.[ISI][Medline]

100 . Fielding, R. M., Mukwaya, G. & Sandhaus, R. A. (1998). Clinical and preclinical studies with low-clearance liposomal amikacin (MiKasome®). In Long Circulating Liposomes: Old Drugs, New Therapeutics, (Woodle, M. C. & Storm, G., Eds), pp. 213–26. Springer Verlag, Berlin.

101 . Fielding, R. M., Moon-McDermott, L., Lewis, R. O. & Horner, M. J. (1999). Pharmacokinetics and urinary excretion of amikacin in low-clearance unilamellar liposomes after a single or repeated intravenous administration in the rhesus monkey. Antimicrobial Agents and Chemotherapy 43, 503–9.[Abstract/Free Full Text]

102 . Bakker-Woudenberg, I. A., Lokerse, A. F., ten Kate, M. T., Mouton, J. W., Woodle, M. C. & Storm, G. (1993). Liposomes with prolonged blood circulation and selective localization in Klebsiella pneumoniae-infected lung tissue. Journal of Infectious Diseases 168, 164–71.[ISI][Medline]

103 . Bakker-Woudenberg, I. A., Lokerse, A. F., ten Kate, M. T., Melissen, P. M., van Vianen, W. & van Etten, E. W. (1992). Enhanced localization of liposomes with prolonged blood circulation time in infected lung tissue. Biochimica et Biophysica Acta 1138, 318–26.[ISI][Medline]

104 . Oyen, W. J., Boerman, O. C., Dams, E. T., Storm, G., van Bloois, L., Koenders, E. B. et al. (1997). Scintigraphic evaluation of experimental colitis in rabbits. Journal of Nuclear Medicine 38, 1596–600.[Abstract]

105 . Rousseau, V., Denizot, B., Le Jeune, J. J. & Jallet, P. (1999). Early detection of liposome brain localization in rat experimental allergic encephalomyelitis. Experimental Brain Research 125, 255–64.[ISI][Medline]

106 . Goins, B., Klipper, R., Rudolph, A. S., Cliff, R. O., Blumhardt, R. & Phillips, W. T. (1993). Biodistribution and imaging studies of technetium-99m-labeled liposomes in rats with focal infection. Journal of Nuclear Medicine 34, 2160–8.[Abstract]

107 . Boerman, O. C., Storm, G., Oyen, W.J., van Bloois, L., van der Meer, J. W., Claessens, R. A. et al. (1995). Sterically stabilized liposomes labeled with indium-111 to image focal infection. Journal of Nuclear Medicine 36, 1639–44.[Abstract]

108 . Klimuk, S. K., Semple, S. C., Scherrer, P. & Hope, M. J. (1999). Contact hypersensitivity: a simple model for the characterization of disease-site targeting by liposomes. Biochimica et Biophysica Acta 1417, 191–201.[ISI][Medline]

109 . Dams, E. T., Reijnen, M. M., Oyen, W. J., Storm, G., Laverman, P., Kok, P. J. et al. (1999). Imaging experimental intraabdominal abscesses with 99mTc-PEG liposomes and 99mTc-HYNIC IgG. Annals of Surgery 229, 551–7.[ISI][Medline]

110 . Awasthi, V., Goins, B., Klipper, R., Loredo, R., Korvick, D. & Phillips, W. T. (1998). Imaging experimental osteomyelitis using radiolabeled liposomes. Journal of Nuclear Medicine 39, 1089–94.[Abstract]

111 . Boerman, O. C., Oyen, W. J., van Bloois, L., Koenders, E. B., van der Meer, J. W., Corstens, F. H. et al. (1997). Optimization of technetium-99m-labeled PEG liposomes to image focal infection: effects of particle size and circulation time. Journal of Nuclear Medicine 38, 489–93[Abstract]

112 . Love, W. G., Amos, N., Kellaway, I. W. & Williams, B. D. (1990). Specific accumulation of cholesterol-rich liposomes in the inflammatory tissue of rats with adjuvant arthritis. Annals of Rheumatic Disease 149, 611–4.

113 . Oyen, W. J., Boerman, O. C. & Storm, G. (1996). Detecting infection and inflammation with technetium-99m-labeled Stealth liposomes. Journal of Nuclear Medicine 37, 1392–7.[Abstract]

114 . Huang, S. K., Martin, F. J. & Friend, D. S. (1995). Mechanism of Stealth® liposome accumulation in some pathological tissues. In Stealth Liposomes, (Lasic, D. & Martin, F., Eds), pp. 103–18. CRC Press, Boca Raton, FL.

115 . Oyen, W. J., Boerman, O. C. & van der Laken, C. J. (1996). The uptake mechanisms of inflammation- and infection-localizing agents. European Journal of Nuclear Medicine 23, 459–65.[ISI][Medline]

116 . Schiffelers, R. M., Storm, G. & Bakker-Woudenberg, I. A. (2001). Host factors influencing the preferential localization of sterically stabilized liposomes in Klebsiella pneumoniae-infected rat lung tissue. Pharmaceutical Research, 18, 780–7.[ISI][Medline]

117 . Schiffelers, R. M., Storm, G., ten Kate, M. T. & Bakker-Woudenberg, I. A. (2001). Therapeutic efficacy of liposomeencapsulated gentamicin in rat Klebsiella pneumoniae pneumonia in relation to low bacterial susceptibility and impaired host defense. Antimicrobial Agents and Chemotherapy 45, 464–70.[Abstract/Free Full Text]

118 . Krieger, J., Childs, S. & Klimberg, I. (1999). UTI treatment using liposomal amikacin (MiKasome®). In Program and Abstracts of the Ninth European Congress of Clinical Microbiology and Infectious Diseases, Berlin, 1999. Clinical Microbiology and Infection 5S3, Abstract P194, p. 136.

119 . Xiong, Y. Q., Kupferwasser, L. I., Zack, P. M. & Bayer, A. S. (1999). Comparative efficacies of liposomal amikacin (MiKasome®) plus oxacillin versus conventional amikacin plus oxacillin in experimental endocarditis induced by Staphylococcus aureus: microbiological and echocardiographic analyses. Antimicrobial Agents and Chemotherapy 43, 1737–42.[Abstract/Free Full Text]

120 . Xiong, Y. Q., Adler-Moore, J. & Zack, P. (1997). Efficacy of MiKasome® (a liposomal amikacin formulation) vs free amikacin in experimental endocarditis due to Pseudomonas aeruginosa. In Program and Abstracts of the Ninety-seventh General meeting American Society for Microbiology, Miami Beach, CA, 1997. Abstract A30, p. 6. American Society for Microbiology, Washington, DC.

121 . Eng, E. T. (1996). Prophylactic and therapeutic treatment of gram-negative septicemia with liposomal and non-liposomal encapsulated amikacin in immunocompromised mice. Thesis presented to California State Polytechnic University, Pomona, CA.

122 . Gangadharam, P. R., Ashtekar, D. R., Flasher, D. L. & Duzgunes, N. (1995). Therapy of Mycobacterium avium complex infections in beige mice with streptomycin encapsulated in sterically stabilized liposomes. Antimicrobial Agents and Chemotherapy 39, 725–30.[Abstract]

123 . Schiffelers, R. M., Storm, G., ten Kate, M. T., Stearne-Cullen, L. E. T., den Hollander, J. G., Verbrugh, H. A. et al. (2001). In vivo synergistic activity of liposome-co-encapsulated gentamicin and ceftazidime. Journal of Pharmacology and Experimental Therapeutics 298, 369–75.[Abstract/Free Full Text]