**3. Novel delivery modalities**

To prevent chemical degradation, harmful side effects, and improve drug bioavailability and accumulation in the desired site, various drug delivery and drug targeting systems are currently under development. The delivery carriers can be made slowly degradable, stimuli-responsive (e.g., pH, ionic strength, temperature, ultrasound, light, electricity, enzymes), and even targeted (e.g., by conjugating them with specific ligands). Over last two decades, nanotechnology has shown potential benefits in improving drug delivery and targeting properties and therefore opens up new markets for pharmaceutical and drug delivery companies. The drug delivery systems are also designed to overcome some physical barriers, such as the blood-brain barrier (BBB) for better location and effectiveness of the drug at the target site. Due to their small size, the NPs can pass through certain biological barriers.

Polymeric NPs are colloidal particles with a size range of 10–1000 nm, which are fabricated using biodegradable synthetic polymers, such as poly (lactide-*co*-glycolide), polyacrylates, and polycaprolactones; nonbiodegradable synthetic polymers, such as poly (methyl methacrylate), polyacrylamide, poly (vinyl alcohol), and poly (ethylene glycol); or natural polymers, such as albumin, gelatin, alginate, gellan gum, and chitosan [12]. Sometimes, blends or graft copolymers of natural and synthetic polymers are also used. In recent years, biodegradable polymeric NPs have attracted considerable attention in the fabrication of potential drug delivery devices due to easy removal of degraded fragments from the body via normal metabolic pathways.

Various methods, such as solvent evaporation, spontaneous emulsification, solvent diffusion, salting out/emulsification-diffusion, and polymerization, have been used to prepare the NPs [13]. Depending upon the method of preparation of NPs, the drug is confined to a cavity surrounded by a polymer membrane (nanocapsules) or dispersed physically and uniformly in the polymer matrix (nanospheres). The drug is loaded via hydrophobic interactions between drugs and nanocarriers. The drug can also be conjugated to polymeric carriers via covalent chemistry.

An important feature of targeted particle delivery system is the ability to simultaneously carry a high amount of drug while displaying ligands on the surface of particles. The overall binding strength of NPs to target is a function of both the affinity of the ligand-target interaction and the number of targeting ligands present on the particle surface [14].

The drug-loaded particles are internalized into cells in determining their biological activity. The particles of as large as 500nm size can be internalized by nonphagocytic cells via energy-dependent process. The particles with <200 nm diameter are internalized via clathrin-coated pits, but larger ones are taken up by cells via caveolae membrane invaginations [15]. However, the internalization of particles can be mediated independent of both clathrin and caveolae pathways. To facilitate efficient internalization, NPs have been targeted against internalizing receptors, and an increased therapeutic activity has been observed in some tumor models [16, 17].

Targeting ligands include any molecule that recognizes and binds to target antigen or receptors overexpressed or selectively expressed by particular cells or tissue components. The antibodies or their fragments, peptides, glycoproteins, vitamins, or carbohydrates are the common class of ligands. The NPs are made long circulating by making their surface hydrophilic after coupling or coating poly(ethylene glycol) (PEG). Functionality could also be introduced by incorporating PEG with functional end groups for coupling to target ligands.

There has been a considerable progress in the field of gene delivery using polymeric NPs. For gene delivery, the plasmid DNA is introduced into the target cells, and the genetic information is ultimately translated into the corresponding protein [18]. To achieve this, an efficient vector that possesses high transfection efficiency, biodegradability, targeting ability, DNA protecting ability, stimuli sensitivity, and low cytotoxicity for delivering a target gene to specific tissues or cells must be selected to cure both the genetic and acquired diseases of human [19]. Despite more gene transfection efficiency, viral vectors may pose a significant risk to patients, while nonviral carriers are inherently safer than viral carriers [20]. Furthermore, the nonviral carriers are expected to be less immunogenic with a possible versatile surface modification [21]. The nonviral vectors are usually made of lipids or polymers with/without using other inorganic materials. The NPs can protect genes against nuclease degradation and improve their stability [22]. Furthermore, they can be used for targeted delivery purpose. Because the biopolymers are non-toxic, biodegradable, and biocompatible, the biopolymerbased non-viral vectors are also being tested for safe and efficient gene delivery.

Polymeric NPs are colloidal particles with a size range of 10–1000 nm, which are fabricated using biodegradable synthetic polymers, such as poly (lactide-*co*-glycolide), polyacrylates, and polycaprolactones; nonbiodegradable synthetic polymers, such as poly (methyl methacrylate), polyacrylamide, poly (vinyl alcohol), and poly (ethylene glycol); or natural polymers, such as albumin, gelatin, alginate, gellan gum, and chitosan [12]. Sometimes, blends or graft copolymers of natural and synthetic polymers are also used. In recent years, biodegradable polymeric NPs have attracted considerable attention in the fabrication of potential drug delivery devices due to easy removal of degraded fragments from the body via normal metabolic

Various methods, such as solvent evaporation, spontaneous emulsification, solvent diffusion, salting out/emulsification-diffusion, and polymerization, have been used to prepare the NPs [13]. Depending upon the method of preparation of NPs, the drug is confined to a cavity surrounded by a polymer membrane (nanocapsules) or dispersed physically and uniformly in the polymer matrix (nanospheres). The drug is loaded via hydrophobic interactions between drugs and nanocarriers. The drug can also be conjugated to polymeric carriers via covalent

An important feature of targeted particle delivery system is the ability to simultaneously carry a high amount of drug while displaying ligands on the surface of particles. The overall binding strength of NPs to target is a function of both the affinity of the ligand-target interac-

The drug-loaded particles are internalized into cells in determining their biological activity. The particles of as large as 500nm size can be internalized by nonphagocytic cells via energy-dependent process. The particles with <200 nm diameter are internalized via clathrin-coated pits, but larger ones are taken up by cells via caveolae membrane invaginations [15]. However, the internalization of particles can be mediated independent of both clathrin and caveolae pathways. To facilitate efficient internalization, NPs have been targeted against internalizing receptors, and an

Targeting ligands include any molecule that recognizes and binds to target antigen or receptors overexpressed or selectively expressed by particular cells or tissue components. The antibodies or their fragments, peptides, glycoproteins, vitamins, or carbohydrates are the common class of ligands. The NPs are made long circulating by making their surface hydrophilic after coupling or coating poly(ethylene glycol) (PEG). Functionality could also be introduced by incorporat-

There has been a considerable progress in the field of gene delivery using polymeric NPs. For gene delivery, the plasmid DNA is introduced into the target cells, and the genetic information is ultimately translated into the corresponding protein [18]. To achieve this, an efficient vector that possesses high transfection efficiency, biodegradability, targeting ability, DNA protecting ability, stimuli sensitivity, and low cytotoxicity for delivering a target gene to specific tissues or cells must be selected to cure both the genetic and acquired diseases of human [19]. Despite more gene transfection efficiency, viral vectors may pose a significant risk to patients, while nonviral carriers are inherently safer than viral carriers [20]. Furthermore,

tion and the number of targeting ligands present on the particle surface [14].

increased therapeutic activity has been observed in some tumor models [16, 17].

ing PEG with functional end groups for coupling to target ligands.

pathways.

6 Advanced Technology for Delivering Therapeutics

chemistry.

The liposomes are the most clinically established nanosystems for drug delivery. They are self-assembled spherical vesicles of bilayer structures of phospholipids and cholesterol surrounding an aqueous core, and their size can be controlled as small as 50–100 nm. The vesicles are biocompatible and biodegradable and confer the ability to entrap both hydrophilic and hydrophobic drugs. The variation in composition of lipid membrane and surface chemistry, the liposome properties, such as size, surface charge, and functionality can be easily manipulated. The incorporation of polyethylene glycol (PEG) prevents interactions with plasma proteins, retards recognition by the RES [23], and thus enhances the liposome circulation lifetime, that is, stealth liposomes. Liposomes can also be conjugated with active-targeting ligands, such as antibodies or folate for target-specific drug delivery. Their efficacy has been demonstrated in reducing systemic effects and toxicity, as well as in attenuating drug clearance [24]. Despite potential advantages, the liposomes as targeted drug delivery carriers are associated with some major drawbacks like poor control over drug release rate, leakage of drug into the blood, low encapsulation efficiency, industrial scale-up, and poor storage stability [25, 26].

Recently, extensive work and experiments with solid lipids resulted in the invention of lipid-based solid particles in the submicron range (10–1000 nm). These NPs are made up of biocompatible and biodegradable lipids with potential application in drug delivery. They possess a solid lipid core matrix that can solubilize lipophilic molecules for enhancing bioavailability. The physiologically similar lipid core of triglycerides or fatty acids or waxes is stabilized by surfactants (emulsifiers). All classes of emulsifiers (with respect to charge and molecular weight) can be used to stabilize the lipid dispersion. It has been found that an emulsifier combination may prevent particle agglomeration more efficiently [27]. The lipid NPs combine the advantages of lipid emulsion and polymeric NPs while overcoming the temporal and *in vivo* stability issues that trouble the conventional and nanoscale delivery approaches [28]. A variety of materials can be used to engineer solid NPs for targeting tissues by either passive or active targeting.

Lipid-polymer hybrid NPs are core-shell structures comprising polymer cores and lipid shells, which exhibit complementary characteristics of both polymeric NPs and liposomes, particularly in terms of their physical stability, biocompatibility, and *in vivo* cellular delivery efficacy [29]. In core-shell-type lipid-polymer hybrid NPs, a biodegradable polymeric core is surrounded by a shell composed of phospholipid layers. The hybrid architecture can provide advantages such as controllable particle size, surface functionality, high loading of multiple drugs, tunable drug release profile, and good serum stability [30].

Several drugs do not have adequate physiochemical characteristics such as high lipid solubility, low molecular size, and positive charge, to traverse blood-brain barrier and deliver drug into the brain [31]. Therefore, the delivery of drugs to central nervous system (CNS) is a challenge for treating neurological disorders. The drugs may be administered directly into the CNS or administered systematically for targeted action in the CNS. The osmotic and chemical opening of the blood-brain barrier as well as the transport/carrier systems constitutes some of the widely reported strategies to promote the permeation of blood-brain barrier (BBB) and delivery of drugs in brain. In conjunction with the net delivery of drug, the access to the intended target site within the CNS is also important. To serve this purpose, the drugs may be conjugated with various nanostructures such as liposomes and NPs and a suitable route of administration can be sought. It has been postulated that nanoscale drug carriers possess a great potential for improving the delivery of drugs through nasal routes to deliver drugs to the brain. Among other mucosal sites, nasal delivery is especially attractive for brain-targeted drug delivery, as the nasal epithelium is characterized by its relatively high permeability, vuscularized mucosa, and low enzymatic activity. If a nasal drug formulation is delivered deep and high enough into the nasal cavity, the olfactory mucosa may be reached and drug transport into the brain and/or cerebrospinal fluid (CSF) via the olfactory receptor neurons may occur [32].

Transdermal systems in the form of patches deliver the drugs across the skin barrier for systemic effects at a predetermined and controlled rate. Due to concentration gradient between the transdermal patch and blood, the drug will continue diffusing into the blood for prolonged period of time and maintain constant drug concentration in the blood flow. Transdermal drug delivery avoids problems such as gastrointestinal irritation, metabolism, pH-dependent variation in delivery rate, and interference with gastric emptying due to presence of food. However, slow penetration rates, lack of dosage flexibility and/or precision, and a restriction to relatively low dosage drugs are the major limitations. The stratum corneum of the skin forms a formidable barrier against uptake, and thus transdermal delivery is difficult to achieve. The substances having molecular weight greater than 500 Da [33] and hydrophilic characteristics encounter the difficulty in absorption through skin. Penetration enhancers often have to be added to the delivery system to improve delivery into or through the skin. Recently, it has been demonstrated that atmospheric-pressure argon microplasma irradiation (AAMI) can improve skin permeability of drugs without the need of injection needles and skin damages [34]. AAMI can be a promising alternative to promote drug delivery through the skin and simultaneously minimize the pain from other manipulations related to skin penetration enhancement. However, the feasibility of atmospheric microplasma irradiation is still under investigation for enhancing percutaneous absorption of drugs.

The delivery of drug to a specific target in the body is comparable to the magic bullet principle applied in nuclear medicine. Nuclear medicine may advance drug development by visualizing biodistribution and site of action [35]. The biodistribution and release kinetics of drug from the novel formulations can be quantified by radiolabeling with γ-emitting radionuclide. Many nuclear medicine departments have participated in the assessment of drug performance and toxicity in contributing data to clinical trials. The application of nuclear medicine techniques to the evaluation of pharmaceutical formulations has been an interesting area of work. Scintigraphy can be used to determine the position of drug release and assess site-specific absorption of orally administered drugs, for example, the evaluation of controlled release formulation designed to release the drug specifically in colon [36]. Hence, the importance of nuclear medicine in drug delivery application has been described in detail in this book.
