Fig. 11.1  Some examples of nanoparticles currently under investigation for use as ocular drug delivery vehicles: (a) albumin nanoparticles, (b) chitosan nanoparticles, (c) polyamidoamine (PAMAM) dendrimers, (d) poly (lactic-co-glycolic) acid (PLGA) nanoparticles, and (e) polyethylene glycol (PEG)-coated liposomes

11.2  Nanoparticles

Disparate nanoparticles from those that mimic cellular structures, lipid-based delivery carriers, to engineered branched structures such as dendrimers have been designed to enhance the efficacy of specific therapeutic agents. US FDA already approved some nanosystems such as Doxil® (a liposomal formulation of doxorubicin) and Abraxane® (albumin-bound paclitaxel) due to their enhanced drug retention at the target sites, compared to drug alone. Depending on the drug properties, target properties, and the purpose of the carrier, different synthetic materials and techniques are available for designing specific nanoparticles. The following sections describe the synthetic techniques and uses for polymer nanoparticles, liposomes (lipid-based carriers), micelles (amphiphilic molecule-based, self-assembling systems), carbohydrate (chitosan) nanoparticles, protein (albumin) nanoparticles, branched nanoparticles (dendrimers) and other nanosystems consisting of multiple nanoparticle structures.

11.2.1  Polymer Nanoparticles

Polymers are large molecular weight compounds consisting of systematic or random repeat units; for example, polyethylene glycol (PEG), polylactic acid (PLA), polyglycolic acid (PGA), and polylactic-co-glycolic acid (PLGA) that are currently in clinical use in the form of drug delivery systems or surgical sutures are all polymers of repeating units. Polymers can be classified based on their structure (e.g., polyesters, polyanhydrides), stability (e.g., biodegradable, nondegradable), charge (e.g., cationic, anionic), lipophilicity (e.g., hydrophobic, hydrophilic, amphiphilic), origin (e.g., synthetic, natural, semi-synthetic), architecture (e.g., linear, branched, crosslinked), and nature of repeating units (e.g., homopolymers, copolymers, block copolymers, random copolymers). Further, polymers can be formed in various supramolecular architectures (e.g., interpenetrating and noninterpenetrating networks, micelles) (Qiu and Bae 2006). Also, polymers can be designed for sensitivity to various stimuli including pH and temperature. Given the versatility of polymer design, polymer-based nanosystems are expected to be the mainstay of nanotechnology-­based drug delivery systems. Depending on their chemical makeup and architecture, polymeric delivery systems can be designed to have certain properties such as biodegradation, sustained release, increased gene transfection efficiency, and controlled release by actuated or stimuli-sensitive physicochemical changes.

Polymers such as PLA and PLGA are widely used in the design of a delivery system since these compounds are biodegradable, biocompatible, and well tested in humans. PLA and PLGA particles are degraded by autocatalysis (Dunne et al. 2000). PLGA degradation can range from days to months depending on the molecular weight of the polymer, lactide:glycolide ratio, and the size and shape of the delivery system. In 1987, HV Maulding characterized the release profiles of PLGA microparticles (~45 kDa) ranging from 45 to 177 mm and determined that it takes 70 days for 100% loss of the molecular weight of PLGA (Maulding 1987).

Drug release from nanoparticles can be more rapid compared to microparticles, leading to less prolonged release. Kompella et al. demonstrated that PLA microparticles (3.6 mm) containing budesonide, a glucocorticoid used to treat inflammation, were able to sustain drug levels within the retina, vitreous, cornea and lens at similar levels between day 1 and 14 days, whereas tissue drug levels for PLA nanoparticles (345 nm) decreased by several fold in 7 days, with the levels being below detection limits by 14 days in retina, vitreous, cornea, and lens (Kompella et al. 2003). Thus, in addition to the polymer nature, the size of the delivery system influences drug release and hence, delivery in vivo.

Polymers also have great potential as nonviral vectors due to their relative safety compared to viral vectors. PLA and PLGA nanoparticles have been used for oligonucleotide or gene delivery to retinal pigment epithelial (RPE) cells and were proven to be more effective than traditional transfection methods due to their ability to transfect the cells without adverse effects such as cell toxicity (Aukunuru et al. 2003; Bejjani et al. 2005). These nanoparticles have also been shown to protect the encapsulated plasmids from degradation by lysosomal nucleases (Hedley et al. 1998). Other polymers have demonstrated preferential binding to genetic components by

electrostatic interactions. The positively charged polymer, polyethylenimine, is able to electrostatically interact with nucleic acid drugs to create a polymer–drug complex (Boussif et al. 1995). Due to the composition of the polymer, it acts as a proton sponge in the presence of highly acidic environments such as the lysosomes. This allows for the nucleic acid drugs to stay active while in the endosome after endocytosis and for the drug to be released from the polymer complex after the polymer accepts protons. The versatile nature of polymers is largely attributed to the wide range of materials that can be used to synthesize polymer structures. Using polymers as gene transfection agents demonstrates only one of the many possible applications for polymers.

Polymers can be designed to undergo structural changes upon activation to release their drug components. Polymers containing cinnamic acid groups spontaneously form into linear, spiral, tube, or corkscrew structures depending on the UV wavelength applied and whether the polymer is irradiated on both sides or only one side (Lendlein et al. 2005). For instance, irradiation of an elongated polymer at >260 nm for 60 min on one side of the polymer spontaneously forms to a corkscrew structure. However, if the polymer is subjected to irradiation at >260 nm for 60 min on both sides, a spiral conformation is obtained. This technique may be applied to the development of drug delivery systems to allow for actuated release from polymer structures or formation of unique drug delivery systems upon irradiation. Thermosensitive materials that exhibit temperature-dependent physicochemical properties offer attractive opportunities in designing delivery systems. Such materials are useful in preparing delivery systems that can be injected as solutions to form gels in the body (Zhang et al. 2002). Synthesis of a two-component thermosensitive polymer was reported by Lendlein et al. (Lendlein et al. 2005). The first component is a molecular switch that conforms to a temporary shape at a given temperature. This polymer is grafted on a permanent polymer network to form a particular architecture. Polymer grafting will vary depending on the network composition desired. For example, one could copolymerize n-butylacrylate (BA), hydroxyethyl methacrylate (HEMA), and ethyleneglycol-1-acrylate-2-CA (HEA-CA) with the crosslinker poly(propylene glycol)-dimethylcrylate. The elasticity of the polymer is dependent on the amount of ethyleneglycol-1-acrylate-2-CA (HEA-CA) added. The second component is synthesized by making a permanent network of n-butylacrylate (BA) with the crosslinker, poly(propylene glycol)-dimethacrylate. Star-poly(ethylene glycol) containing cinnamylidene acetic acid (CAA) in 10% chloroform solution is then added to the network to functionalize the polymer with cinnamic acid (CA) groups. Such polymers can potentially be transformed into different architectures.

The complexity of polymer particle preparation is entirely dependent on the properties desired and materials to be used. Solid polymeric particles containing a specific drug can be synthesized using an oil-in-water (o/w) emulsion technique (Fig. 11.2) (Kompella et al. 2001, 2003). The polymer and drug contents (e.g., PLGA and budesonide) are dissolved in an appropriate organic solvent (e.g., dichloromethane) and then emulsified in an aqueous medium containing an emulsifier (e.g., polyvinyl alcohol, PVA). The mixture is further agitated with a probe sonicator

Mix and sonicate at 50 W for 3 min to obtain an O/W emulsion

While stirring add drop-wise to an aqueous PVA (0.5 or 2% w/v) solution with or without budesonide. Continue to stir at 200 rpm for 4- 6 hours to obtain a nanosuspension

Ultracentrifuge to obtain nanoparticles pellet

Wash the pellet twice with de-ionized water and reconstitute the pellet in deionized water

Lyophilize the reconstituted contents for 48 hours to obtain a dry product

Fig. 11.2  Scheme describing the emulsion-solvent evaporation method for preparing budesonideloaded polylactide (PLA) nanoparticles (Kompella et al. 2001)

to obtain an o/w emulsion with smaller droplet size. The emulsion is then added drop-wise to excess of aqueous medium containing PVA, while stirring overnight at room temperature to allow the evaporation of the organic solvent. Upon removal of organic solvent, the polymer precipitates along with the drug from emulsion droplets, resulting in fine nanoparticles. The final preparation may contain nanoparticles as well as microparticles, depending on the materials and energy used. Particles can be further segregated in different size ranges using ultracentrifugation at different speeds. An alternative approach to prepare PLGA nanoparticles is supercritical fluid extraction of emulsions (Mayo et al. 2010). This technique allows high encapsulation efficiencies for hydrophilic drugs in a polymer matrix. Further, it reduces organic solvent content in polymeric particles to detection limits or a few parts per million.