**1. Introduction**

tems for the delivery of small molecules, proteins, peptides, oligonucleotides and genes. How these systems overcome challenges offered by biological barriers to drug absorption

**Ali Demir Sezer**

Haydarpaşa, Istanbul, Turkey

Department of Pharmaceutical Biotechnology, Faculty of Pharmacy, Marmara University,

and drug targeting will also be described.

VIII Preface

Lipids are amphiphilic molecules, where one part of the molecule is water-loving (hydrophilic) and the other water-hating (hydrophobic). When lipids are placed in contact with water, the unfavorable interactions of the hydrophobic segments of the molecule with the solvent result in the self assembly of lipids, often in the form of liposomes. Liposomes consist of an aqueous core surrounded by a lipid bilayer, much like a membrane, separating the inner aqueous core from the bulk outside. They were first discovered by Bangham and his co-workers in 1961 [1] and described as swollen phospholipid systems [2]. In the following years, a variety of enclosed phospholipid bilayer structures were defined which were initially called bangosomes and then liposomes, which was derived by the combination of two Greek words, "lipos" meaning fat and "soma" meaning body.

Liposomes have been used to improve the therapeutic index of new or established drugs by modifying drug absorption, reducing metabolism, prolonging biological half-life or reducing toxicity. Drug distribution is then controlled primarily by properties of the carrier and no longer by physico-chemical characteristics of the drug substance only.

Lipids forming liposomes may be natural or synthetic, and liposome constituents are not exclusive of lipids, new generation liposomes can also be formed from polymers (sometimes referred to as polymersomes). Whether composed of natural or synthetic lipids or polymers, liposomes are biocompatible and biodegradable which make them suitable for biomedical research. The unique feature of liposomes is their ability to compartmentalize and solubilize both hydrophilic and hydrophobic materials by nature. This unique feature, coupled with biocompatibility and biodegradability make liposomes very attractive as drug delivery vehicles.

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Hydrophobic drugs place themselves inside the bilayer of the liposome and hydrophilic drugs are entrapped within the aqueous core or at the bilayer interface. Liposomal formulations enhance the therapeutic efficiency of drugs in preclinical models and in humans compared to conventional formulations due to the alteration of biodistribution. Liposome binding drugs, into or onto their membranes, are expected to be transported without rapid degradation and minimum side effects to the recipient because generally liposomes are composed of biode‐ gradable, biologically inert and non-immunogenic lipids. Moreover, they produce no pyro‐ genic or antigenic reactions and possess limited toxicity [3-5]. Consequently, all these properties as well as the ease of surface modification to bear the targetable properties make liposomes more attractive candidates for use as drug-delivery vehicles than other drug carrying systems such as nanoparticles [6, 7] and microemulsions [8, 9]. In the 1970s [1, 10-13], liposomes were introduced as drug delivery vehicles but the initial clinical results were not satisfactory due to their colloidal and biological instability and their inefficient encapsu‐ lation of drug molecules.

Liposomes represent versatile and advanced nanodelivery systems for a wide range of biologically active compounds [16]. The final amount of the encapsulated drug is affected by a selection of an appropriate preparation method providing a preparation of liposomes of various size, lamellarity and physicochemical properties [17]. The entrapment of the drugs, both hydrophilic and hydrophobic, into the liposomes is used to bypass the frequent generic toxicity associated with the drug as often seen in cancer drugs [18]. Thus, it represents a very effective route that enhances the drug therapeutic effect. The modification of liposomes permits a passive or active targeting of the tumor site. This effect enables an efficient drug payload into the malignant cell of tumors, while the non-malignant cells become minimally

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In some of the first demonstrations of the improved in vivo activity of liposome-encapsulated drugs in animal models, the anti-cancer drug cytosine arabinoside used which showed a significant increase in the survival times of the mice bearing leukemia [19, 20] which became a popular model for testing the effects of a wide range of liposome characteristics on thera‐ peutic outcomes. Following experiments include liposomal amphotericin B [21] and liposomal doxorubicin [22] that finally led to the first clinical trials of liposomal drugs. Nowadays; the liposomal products (as a suspension, as an aerosol or in a semi-solid form such as a gel, cream, or powder) in the market still include mostly anticancer preparations as well as antifungal and antibacterial preparations and cosmetics. In addition, liposomes are recently used as thera‐ peutic agents to treat a disease because increased gene transfer efficiencies have been obtained

The benefits and limitations of liposome drug carriers critically depend and based on physi‐ cochemical and colloidal characteristics such as size, composition, loading efficiency and stability, as well as their biological interaction with the cell membranes. There are four major interactions between liposomes and cells [23]. The predominant interaction among them is either simple adsorption or subsequent endocytosis. Adsorption occurs when the attractive forces exceed the repulsive ones and obviously this type of interaction depends on the surface properties of liposomes. In the delivery through endocytosis, liposome and its contents indirectly place themselves in the cytoplasm. Fusion with cell membranes, delivery of the liposomal content directly into the cell through the merge of liposome lipids into the mem‐ brane, is much rarer. The last possible interaction is the lipid exchange which is a long-range interaction that involves the exchange of bilayer constituents, such as lipids, cholesterol, and membrane bound molecules with components of cell membranes. Upon entering into the body, the delivered liposomes via one of these interaction types trigger the response of the immune system and the encapsulated material may become inactive. Therefore; substantial researches have been carried out in the development of the biocompatible and nonrecognizable

Various types of liposomes can be prepared by different preparation methods, depending on the required application. In this chapter, these methods are summarized to give a general understanding of the relationship between structure and functionality of liposomes. As one of the advantages of liposomal formulations is the encapsulation ability of both hydrophobic and hydrophilic drugs, incorporation methods are shortly visited. Physical properties of liposomes

impacted.

liposomal surfaces.

via liposomal gene vectors in gene therapy.

Subsequent research on their stability and drug interactions resulted in several commercial liposome products in the market in the 1980s and early 1990s [14]. A schematic representation of liposomal drug delivery is given in Figure 1.

Liposomes represent versatile and advanced nanodelivery systems for a wide range of biologically active compounds [16]. The final amount of the encapsulated drug is affected by a selection of an appropriate preparation method providing a preparation of liposomes of various size, lamellarity and physicochemical properties [17]. The entrapment of the drugs, both hydrophilic and hydrophobic, into the liposomes is used to bypass the frequent generic toxicity associated with the drug as often seen in cancer drugs [18]. Thus, it represents a very effective route that enhances the drug therapeutic effect. The modification of liposomes permits a passive or active targeting of the tumor site. This effect enables an efficient drug payload into the malignant cell of tumors, while the non-malignant cells become minimally impacted.

Hydrophobic drugs place themselves inside the bilayer of the liposome and hydrophilic drugs are entrapped within the aqueous core or at the bilayer interface. Liposomal formulations enhance the therapeutic efficiency of drugs in preclinical models and in humans compared to conventional formulations due to the alteration of biodistribution. Liposome binding drugs, into or onto their membranes, are expected to be transported without rapid degradation and minimum side effects to the recipient because generally liposomes are composed of biode‐ gradable, biologically inert and non-immunogenic lipids. Moreover, they produce no pyro‐ genic or antigenic reactions and possess limited toxicity [3-5]. Consequently, all these properties as well as the ease of surface modification to bear the targetable properties make liposomes more attractive candidates for use as drug-delivery vehicles than other drug carrying systems such as nanoparticles [6, 7] and microemulsions [8, 9]. In the 1970s [1, 10-13], liposomes were introduced as drug delivery vehicles but the initial clinical results were not satisfactory due to their colloidal and biological instability and their inefficient encapsu‐

Subsequent research on their stability and drug interactions resulted in several commercial liposome products in the market in the 1980s and early 1990s [14]. A schematic representation

**Figure 1.** Structural and design considerations for liposomal drug delivery. Liposomes can be surface functionalized to endow stealth through PEGylation and to promote receptor-mediated endocytosis by using targeting ligands such as antibodies, peptides, proteins, carbohydrates, and various other small molecules. PEGylation extends liposomal circu‐ lation half-life in vivo by reducing clearance, immune recognition, and the non-specific absorption of serum proteins. Polyethylene glycol (PEG) density determines its structure at the liposome surface, with densities below 9% adopting a mushroom-like globular structure and those above 9% adopting a more rigid, extended, brush-like morphology. Che‐ motherapeutics or diagnostics can be encapsulated into the aqueous lumen, incorporated into the lipid bilayer, or

conjugated to the liposome surface. Abbreviation: siRNA, small interfering RNA [15].

lation of drug molecules.

2 Application of Nanotechnology in Drug Delivery

of liposomal drug delivery is given in Figure 1.

In some of the first demonstrations of the improved in vivo activity of liposome-encapsulated drugs in animal models, the anti-cancer drug cytosine arabinoside used which showed a significant increase in the survival times of the mice bearing leukemia [19, 20] which became a popular model for testing the effects of a wide range of liposome characteristics on thera‐ peutic outcomes. Following experiments include liposomal amphotericin B [21] and liposomal doxorubicin [22] that finally led to the first clinical trials of liposomal drugs. Nowadays; the liposomal products (as a suspension, as an aerosol or in a semi-solid form such as a gel, cream, or powder) in the market still include mostly anticancer preparations as well as antifungal and antibacterial preparations and cosmetics. In addition, liposomes are recently used as thera‐ peutic agents to treat a disease because increased gene transfer efficiencies have been obtained via liposomal gene vectors in gene therapy.

The benefits and limitations of liposome drug carriers critically depend and based on physi‐ cochemical and colloidal characteristics such as size, composition, loading efficiency and stability, as well as their biological interaction with the cell membranes. There are four major interactions between liposomes and cells [23]. The predominant interaction among them is either simple adsorption or subsequent endocytosis. Adsorption occurs when the attractive forces exceed the repulsive ones and obviously this type of interaction depends on the surface properties of liposomes. In the delivery through endocytosis, liposome and its contents indirectly place themselves in the cytoplasm. Fusion with cell membranes, delivery of the liposomal content directly into the cell through the merge of liposome lipids into the mem‐ brane, is much rarer. The last possible interaction is the lipid exchange which is a long-range interaction that involves the exchange of bilayer constituents, such as lipids, cholesterol, and membrane bound molecules with components of cell membranes. Upon entering into the body, the delivered liposomes via one of these interaction types trigger the response of the immune system and the encapsulated material may become inactive. Therefore; substantial researches have been carried out in the development of the biocompatible and nonrecognizable liposomal surfaces.

Various types of liposomes can be prepared by different preparation methods, depending on the required application. In this chapter, these methods are summarized to give a general understanding of the relationship between structure and functionality of liposomes. As one of the advantages of liposomal formulations is the encapsulation ability of both hydrophobic and hydrophilic drugs, incorporation methods are shortly visited. Physical properties of liposomes such as stability, storage and sterilization are discussed along with the characterization techniques for size, charge, etc. Clinical applications of liposomes are a vast area of research where cancer therapy is the area of highest impact. Different clinical applications of liposomes and most recent advances in cancer therapy are summarized. New generation involving constituents other than conventional ones such as phospholipids prove to be a growing field in nanotechnology. A brief list of different types of new generation liposomes are given with short descriptions at the end of this chapter.

**2.2. Solvent evaporation**

between bilayers [30].

*2.2.2. Solvent evaporation*

mixing.

*2.2.1. Solubilization of the lipid*

In general this method consists of four major steps; first is the solubilization of the lipid (and a hydrophobic compound) in an organic solvent; second is solvent evaporation; third is hydration with a buffer (and the hydrophilic compound) and if need the fourth often involves

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The aqueous volume enclosed within these lipid membranes is very small proportion of total volume used for preparation (5-10%). Consequently, large amount of water soluble drug is wasted during the preparation. On the other hand, lipid soluble drug can be encapsulated with 100% efficiency, providing that they are not present in quantities which overwhelm the structural components of the membrane [28]. The volume of entrapment can be significantly increased by the usage of negatively charged lipids in the membrane which tend to push the bilayers apart from each other. The same effect can also be achieved in the presence of neutral lipids by freezing and thawing repeatedly the obtained liposomes. 30% volume of entrapment can be achieved, which can further be increased at higher lipid concentrations [29]. The freezethaw protocol results in a dramatic change in liposome morphology followed by freezefracture electron micrographs. Before freeze-thawing, the samples exhibit the tightly packed "onions skin" arrangements of concentric bilayers normally associated with liposomal systems. After a few freeze-thaw steps, however, new structures are observed where the interlamellar spaces are much increased, and where closed lamellar systems can be intercalated

The starting point of this liposome preparation method is to prepare an organic solution of membrane lipids in order to ensure complete and homogenous mixing of all the components as they are required in the final membrane preparation. Compounds to be incorporated which are lipid soluble will be added to the organic solution, while compounds to be entrapped in the aqueous compartment of liposomes will be dissolved in the aqueous environment. In this method, phospholipids are first dissolved in an organic solvent along with lipid soluble compounds (if any) to be incorporated in the liposome to ensure complete and homogenous

The next step is the evaporation of the organic solvent. The simplest is to allow the solvent to evaporate in a glass container. A better method is evaporating the solvent using a rotary evaporator connected to a vacuum pump to obtain a thin film of the lipid on the walls of a round bottom flask. In order to increase encapsulation, it is recommended to start with a large volume round bottom flask so that the lipids will be dried down onto a large surface area possible to form a very thin film. The evaporator is detached from vacuum pump and introduced to nitrogen. The container is then removed from the evaporator and fixed to a

lyophilizer or exposed to high vacuum overnight to remove the residual solvent.

obtaining unilamellar liposomes from the obtained multilamellar ones.
