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TYPES OF NANOPARTICLES IN DRUG DELIVERY

Nanoparticulate systems can be classified into two broad categories based on the integrity ofthe particles, such as nonrigid nanoparticles, which encompass liposomes and solid lipid nanoparticles, and rigid nanoparticles, which encompass polymeric nanoparticles and inorganic nanoparticles (Fig. 6.1).

Nonrigid Nanoparticles

Unlike rigid nanoparticles, the nonrigid nanoparticles are known to consist of relatively soft structures that can be easily disrupted by external force. The lipid-based nanoparticles such as liposomes and solid lipid nanoparticles are generally categorized as nonrigid nanoparticles.

Liposomes

The term liposome is derived from two Greek words “lipo” and “soma,” which translates to “fat” and “body,” respectively. The concept of liposomes was first described by Dr. Alec Bangham in 1964. The breakthrough in research involving liposomes resulted in the approval of some formulations such as Doxil, DaunoXome, ThermoDox, and Marqibo since mid-1990s [4,6,23]. The liposomes can be effectively used to deliver water-soluble drugs (hydrophilic), water-insoluble drugs (hydrophobic), peptides, and nucleic acids.

Liposomes are considered to be biologically inert and biocompatible as they are simplified models of biological membranes and contain amphiphilic phospholipid layers (either natural or synthetic). The phospholipid molecules contain a hydrophilic “head” and a hydrophobic “tail” (hydrocarbon chain). Liposomes are spontaneously formed when a phospholipid layer is disrupted by external force such as sonication or stirring or when it comes in contact with an aqueous phase. The formation ofliposomes involves the self-association of phospholipid layers, which makes it spontaneous [24]. The core of

The major categories of nanoparticles in drug delivery

Figure 6.1 The major categories of nanoparticles in drug delivery.

Nanoparticulate Systems for Therapeutic and Diagnostic Applications 109

Typical bilamellar liposome structure

Figure 6.2 Typical bilamellar liposome structure.

Types of liposomes based on the size and number of layers

Figure 6.3 Types of liposomes based on the size and number of layers.

the liposome is hydrophilic and the space between the two layers is hydrophobic. This unique feature enables us to load either hydrophilic drugs within the core or hydrophobic drugs between the two phospholipid layers (Fig. 6.2).

Liposomes can be either unilamellar or multilamellar with varying size. The unilamellar vesicles can be further classified as small unilamellar vesicles (SUVs) and large unilamellar vesicles (LUVs) based on their size [25] (Fig. 6.3). Liposomes with a size range of <400 nm are known to preferentially accumulate in the tumor tissue by a phenomenon called enhanced permeability and retention effect where the nanosized particles escape through the leaky vasculature [26,27]. However, these conventional liposomes are rapidly eliminated by the mononuclear phagocyte system (MPS), formerly known as the reticuloendothelial system [13]. A popular modification of traditional liposomes, called stealth liposomes, is surface coated with a hydrophilic polymer such as polyethylene glycol and possesses the ability to evade uptake and elimination by the MPS [13,28].

 
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