Solid lipid nanoparticles: Production, characterization and applications

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Abstract

Solid lipid nanoparticles (SLN) have attracted increasing attention during recent years. This paper presents an overview about the selection of the ingredients, different ways of SLN production and SLN applications. Aspects of SLN stability and possibilities of SLN stabilization by lyophilization and spray drying are discussed. Special attention is paid to the relation between drug incorporation and the complexity of SLN dispersions, which includes the presence of alternative colloidal structures (liposomes, micelles, drug nanosuspensions, mixed micelles, liquid crystals) and the physical state of the lipid (supercooled melts, different lipid modifications). Appropriate analytical methods are needed for the characterization of SLN. The use of several analytical techniques is a necessity. Alternative structures and dynamic phenomena on the molecular level have to be considered. Aspects of SLN administration and the in vivo fate of the carrier are discussed.

Introduction

In recent years it has become more and more evident that the development of new drugs alone is not sufficient to ensure progress in drug therapy. Exciting experimental data obtained in vitro are very often followed by disappointing results in vivo. Main reasons for the therapy failure include:

  • Insufficient drug concentration due to poor absorption, rapid metabolism and elimination (e.g. peptides, proteins). Drug distribution to other tissues combined with high drug toxicity (e.g. cancer drugs)

  • Poor drug solubility which excludes i.v. injection of aqueous drug solution

  • High fluctuation of plasma levels due to unpredictable bioavailability after peroral administration, including the influence of food on plasma levels (e.g. cyclosporine)

A promising strategy to overcome these problems involves the development of suitable drug carrier systems. The in vivo fate of the drug is no longer mainly determined by the properties of the drug, but by the carrier system, which should permit a controlled and localized release of the active drug according to the specific needs of the therapy. The size of the carrier depends on the desired route of administration and ranges from few nanometers (colloidal carriers), to the micrometer range (microparticles) and to several millimeters (implants). For parenteral administration, it is highly desirable to use biodegradable materials, which avoid surgery to remove the implant after complete drug release and which make the administration of micro- and nanoparticles feasible. The concept has been realized in several commercial products. Implants and microparticles based on biodegradable polyesters permit a controlled drug release over a period of weeks to months after s.c. or i.m. implantation/injection. Commercially available systems have been developed for the treatment of prostate cancer and other GnRH-related diseases [1]. An example of the concept of localized drug release is the development of biodegradable implants for the treatment of gliomas, which ensure very high drug concentrations in the brain and minimize drug concentration in other tissues, including bone marrow [2]. Implants and microparticles are too large for drug targeting and intravenous administration. Therefore, colloidal carriers have attracted increasing attention during recent years. Investigated systems include nanoparticles, nanoemulsions, liposomes, nanosuspensions, micelles, soluble polymer–drug conjugates.

The existence of different colloidal carrier systems raises the question as to which of them might be the most suitable for the desired purpose. Of course, there is no simple answer to this question. Aspects to consider include:

  • Drug loading capacity

  • Possibility of drug targeting

  • In vivo fate of the carrier (interaction with the biological surrounding, degradation rate, accumulation in organs)

  • Acute and chronic toxicity

  • Scaling up of production

  • Physical and chemical storage stability

  • Overall costs

Polymers from natural [3], [4] and synthetic sources [5] have been used. Polymer based systems in the submicron size range include water soluble polymer–drug conjugates [6], polymer nanocapsules [7], [8] and nanospheres. A certain advantage of polymer systems is the wealth of possible chemical modifications, including the synthesis of block- and comb-polymers.

Problems of polymer based nanoparticles derive from residues from organic solvents used in the production process, polymer cytotoxicity [9] and the scaling up of the production processes. In most production processes, the concentration of nanoparticles is low and does not exceed 2%. Polymer hydrolysis during storage has to be taken into account and lyophilization is often required to prevent polymer degradation.

Liposomes are spherical vesicles composed of one or more phospholipid bilayers (in most cases phosphatidylcholine). Lipophilic drugs can be incorporated into the lipid bilayers while hydrophilic drugs are solubilized in the inner aqueous core [10], [11]. Drug release, in vivo stability and biodistribution are determined by size, surface charge, surface hydrophobicity and membrane fluidity [12]. Membrane permeability can be adapted by the selection of the phospholipids and the incorporation of additives (e.g. cholesterol). It is possible to prevent a rapid reticuloendothelial uptake of the liposomes by the incorporation of natural compounds (e.g. gangliosides) or by the use of chemical modified polyethylene glycols [13], [14], [15], [16], [17], [18], [19]. The development of such sterically stabilized systems (‘stealth liposomes’) permits the practical realization of drug targeting strategies (e.g. by incorporation of specific antibodies) [20], [21]. Liposome based drug carriers also permit the intravenous injection of lipophilic drugs with very low water solubility, e.g. amphotericin B (AmBisome®) [22]. The toxicity of the liposome system is 1/10 compared to a commercial micelle-based amphotericin formulation. Chemical and physical stability problems might lead to liposome aggregation and drug degradation during storage and compromise the performance of liposome based drug carriers [23].

Nanosuspensions are colloidal particles which are composed of the drug and the emulsifier only. Possible production procedures include ball milling [24] or high pressure homogenization [25], [26].

Lipid nanoemulsions were introduced during the 50s for the purpose of parenteral nutrition. Fatty vegetable oils (e.g. soy oil) or middle chain triglycerides are used for the lipid phase, which amounts to typically 10–20% of the emulsion. Further ingredients include phospholipids (stabilizers, 0.6–1.5%) and glycerol (osmolarity regulation, 2.25%). During recent years it has been recognized that these systems might also be used as drug carriers for lipophilic drugs and several formulations are commercialized [27], [28], [29], [30], [31], [32], [33], [34], [35]. Examples include etomidate (Etomidat-Lipuro®) and diazepam (Diazepam-Lipuro®) [36], [37], [38]. In comparison to previous, solubilization-based formulations of these drugs, a reduction of the local and systemic side effects (e.g. pain during injection) has been found. The hemolytic activity of sodium oleate is decreased in lipid emulsions because the lytic agent is restricted at the interface and in the lipophilic core and so the direct contact with erythrocyte membranes is hindered [39].

The possibility of controlled drug release from nanoemulsions is limited due to the small size and the liquid state of the carrier. For most drugs, a rapid release of the drug will be observed [40], [41], [42]. It has been estimated, that retarded drug release requires very lipophilic drugs, the octanol/water partition coefficient should be larger than 1 000 000:1 [43]. Advantages of nanoemulsions include toxicological safety and a high content of the lipid phase as well as the possibility of large scale production by high pressure homogenization.

The use of solid lipids instead of liquid oils is a very attractive idea to achieve controlled drug release, because drug mobility in a solid lipid should be considerably lower compared with an liquid oil. Solid lipids have been used for several years in the form of pellets in order to achieve a retarded drug release after peroral administration (e.g. Mucosolvan® Retard Capsules). In the beginning of the 80s, Speiser and coworkers developed solid lipid microparticles (by spray drying) [44] and ‘Nanopellets for peroral administration’ [45].

Nanopellets developed by Speiser [45] were produced by dispersing of melted lipids with high speed mixers or ultrasound. The products obtained by this procedure often contain relatively high amounts of microparticles. This might not be a serious problem for peroral administration, but it excludes an intravenous injection. Higher concentrations of the emulsifier result in a reduction of the particle size, but also increase the risk of toxic side effects. Similar systems have been described by Domb as ‘Lipospheres’ [46], [47], [48]. They are also produced by means of high shear mixing or ultrasound and often contain considerable amounts of microparticles, too.

In the following years, it has been demonstrated that high pressure homogenization is a more effective method for the production of submicron sized dispersions of solid lipids compared to high shear mixing or ultrasound [49], [50], [51]. Dispersions obtained in this way are called solid lipid nanoparticles (SLN™). Most SLN dispersions produced by high pressure homogenization (HPH) are characterized by an average particle size below 500 nm and a low microparticle content. Other production procedures are based on the use of organic solvents (HPH/solvent evaporation) [52] or on dilution of microemulsions [53], [54].

Section snippets

Aims of solid lipid nanoparticles

It has been claimed that SLN combine the advantages and avoid the disadvantages of other colloidal carriers [55]. Proposed advantages include:

  • Possibility of controlled drug release and drug targeting

  • Increased drug stability

  • High drug payload

  • Incorporation of lipophilic and hydrophilic drugs feasible

  • No biotoxicity of the carrier

  • Avoidance of organic solvents

  • No problems with respect to large scale production and sterilization

General ingredients

General ingredients include solid lipid(s), emulsifier(s) and water. The term lipid is used here in a broader sense and includes triglycerides (e.g. tristearin), partial glycerides (e.g. Imwitor), fatty acids (e.g. stearic acid), steroids (e.g. cholesterol) and waxes (e.g. cetyl palmitate). All classes of emulsifiers (with respect to charge and molecular weight) have been used to stabilize the lipid dispersion. It has been found that the combination of emulsifiers might prevent particle

Defining the goals

An adequate characterization of the solid lipid nanodispersion is a necessity for the control of the quality of the product. The characterization methods should be sensitive to the key parameters of SLN performance and should avoid artifacts. However, characterization of SLN is a serious challenge due to the colloidal size of the particles and the complexity of the system, which includes also dynamic phenomena. One statement of Laggner about lipids should always be kept in mind [107]:

“Lipids

Possible problems in SLN preparation and SLN performance

SLN offer several advantages compared to other systems (easy scaling up, avoidance of organic solvents, high content of nanoparticles). These advantages have been discussed in a variety of papers. However, less attention has been paid to the detailed and appropriate investigation of the limitations of this carrier system. Therefore, these aspects will be discussed in the following part of the article. Points to consider include high pressure-induced drug degradation, the coexistence of

Drug incorporation and drug release

A large number of drugs with a great variety of lipophilicity and the general structure have been studied with regard to their incorporation into SLN [147], e.g. oxazepam, diazepam, cortisone, betamethasone valerate, retinol, prednisolone, retinol, menadione, ubidecarenone [148], timolol [149], [150], desoxycortisone [71], pilocarpine [151], progesterone [152], hydrocortisone [152], idarubicin [53], doxorubicin [53], thymopentin [153], [D-Trp-6]LHRH [154], gadolinium (III) complexes [155],

Storage stability

SLN and nanoemulsions have remarkable similarities with respect to their composition and production methods. However, SLN cannot simply be regarded as colloidal lipid dispersions with solidified droplets. The problems connected with the presence of additional colloidal structures (micelles, mixed micelles, liposomes) exist for both carrier systems. However, SLN have additional features (supercooled melts, different modifications, non-spherical shapes) which are contributing to or determining

Toxicity aspects and in vivo fate

One can anticipate that SLN are well tolerated in living systems because they are made from physiological compounds and therefore, metabolic pathways exist. Of course, the toxicity of the emulsifiers has to be considered, but their potential toxicity is relevant for other carrier systems, too. No problems should be observed for peroral or transdermal administration and i.m. or s.c. injection if appropriate surfactants are used. The particle size is not a very critical issue for these

Administration routes and in vivo fate

The in vivo fate of the SLN particles will depend mainly on the following points:

(a) administration route

(b) interactions of the SLN with the biological surroundings including:

(b1) distribution processes (adsorption of biological material on the particle surface and desorption of SLN components into the biological surrounding)

(b2) enzymatic processes (e.g. lipid degradation by lipases and esterases)

SLN are composed of physiological or physiologically related lipids or waxes. Therefore, pathways

Summary and outlook

Solid lipid nanoparticles do not, as proposed, “combine the advantages of other colloidal drug carriers and avoid the disadvantages of them”. They cannot simply be regarded as nanoemulsions with a solid core. Clear advantages of SLN include the composition (physiological compounds), the rapid and effective production process including the possibility of large scale production, the avoidance of organic solvents and the possibility to produce high concentrated lipid suspensions. Disadvantages

Acknowledgements

The authors would like to thank Susan Liedtke and Katja Jores for their support in the preparation of this manuscript.

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