**1. Introduction**

Drug delivery systems (DDSs) are pharmaceutical formulations or devices that help in achieving targeted delivery and/or controlled release (CR) of therapeutic agents in our body [1]. Following administration, the DDSs liberate the active ingredients, and subsequently, the bioactive molecules are transported across various biological barriers to reach the site of action. The scientists have contributed substantially to understand the role of different physiological barriers in efficient transport of drugs in the circulatory system and drug movement through cells and tissues. In addition, their significant contribution to the development of a number of new modes of drug delivery has entered clinical practice. Despite a significant advancement in the process of new drug design and discovery, many drugs have unacceptable side effects due to interaction of the drug with parts of the body that are not the target for the drug. Sometimes, side effects occur depending on the medication, the mode of delivery, and response from our body. The buildup of high blood plasma drug concentration due to accumulation of drug from repeated administration of conventional DDS may lead to untoward side effects. Hence, the attempts must be made to afford better patient compliance effect from the reduction in the number and frequency of doses needed to maintain the desired therapeutic responses. These side effects can vary greatly from person to person in type and severity. The method by which the drug is delivered can have a significant impact on its efficacy.

It is necessary to develop suitable DDS for all drugs to allow their effective and safe application to the patient. Indeed, DDSs control the drug release rate and drug absorption and ultimately the therapeutic effects along with side effects of the drug. Ideal DDSs ensure that the active drug is available at the site of action according to the need of patient for an intended duration. The drug concentration at the appropriate site should remain in the therapeutic

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© 2016 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,

**Figure 1.** The drug plasma levels after single oral administration of a drug from (a) IR dosage form, (b) SR dosage form, and (c) CR dosage form.

window, that is, between minimal effective concentration (MEC) and minimal toxic concentration (MTC). This concept is illustrated in **Figure 1**.

The maintenance of drug concentration in therapeutic range depends on the frequency of dosing, the drug clearance rates, the route of administration, and the DDS employed. Some drugs have an optimum concentration range within which maximum benefit is derived, and concentrations above or below this range can be toxic or produce no therapeutic benefit at all. DDS can be classified according to their physical state, site/route of administration, and the rate of drug release. The dosage form may be gaseous (e.g., anesthetics), liquid (e.g., solutions, emulsions, and suspensions), semisolid (e.g., creams, ointments, and gels), and solid dosage (e.g., tablets and capsules) [2]. Drugs can be administered directly into the body through injection or infusion termed parenteral drug delivery. Depending on the site of administration, one can differentiate among intravenous, intramuscular, subcutaneous, intradermal, and intraperitoneal administration. Mostly semisolid dosage forms including creams, ointments, and gels are applied onto the skin to enter into the body. However, the liquid dosage forms, such as emulsions, or solid dosage forms, such as transdermal patches, can also be used. Dosage forms can be classified into immediate release (IR) and modified release (MR). IR dosage forms allow the drug to dissolve in the gastrointestinal contents, without delaying or prolonging the dissolution or absorption of the drug. In MR dosage forms, the time course and/or location of drug release is chosen to accomplish therapeutic or convenience objectives not offered by conventional dosage forms. MR dosage forms include both delayed and extended release drug products.

Delayed release dosage forms release the active ingredient at a time other than immediately after administration, for example, enteric-coated dosage forms or colon-specific dosage forms. These systems delay the release of drug until the dosage form reaches the small intestine. In this way, they can protect the drug from degradation in the low-pH environment of the stomach or protect the stomach from irritation by the drug. The dosage form is coated with polymer that dissolves and releases the drug at higher pH during its travel from low-pH environment of the stomach to the high-pH environment of the small intestine. Once this occurs, the release is again immediate, and the resulting plasma concentration versus time curve is similar to the one for IR dosage forms. Extended release products (sustained release (SR) and controlled release systems) are formulated to make the drug available over an extended period after ingestion. This allows a reduction in dosing frequency compared to the drug presented as a conventional dosage form. IR dosage forms are designed to achieve quicker onset of drug action than that achieved by delayed or extended release dosage forms, which are often desirable to increase the stability, safety, and efficacy of the drug, to improve the therapeutic outcome of the drug treatment, and/or to increase patient compliance and convenience of administration.

The extended release DDS can be of either sustained release (SR) or controlled release (CR) dosage forms. The polymer-based matrix or reservoir sustained release systems maintain the rate of drug release over a longer period and reduce the frequency of dosing. Conversely, CR DDSs are designed to predict constant plasma drug concentrations regardless of the biological environment of the application site. Therefore, CR systems actually control the drug concentration in the body, whereas SR systems just regulate the release of the drug from the dosage form [3, 4]. Further, SR systems are basically restricted to oral formulations, while CR systems can be administered through various routes, including transdermal, oral, and vaginal administration.

window, that is, between minimal effective concentration (MEC) and minimal toxic concen-

**Figure 1.** The drug plasma levels after single oral administration of a drug from (a) IR dosage form, (b) SR dosage form,

The maintenance of drug concentration in therapeutic range depends on the frequency of dosing, the drug clearance rates, the route of administration, and the DDS employed. Some drugs have an optimum concentration range within which maximum benefit is derived, and concentrations above or below this range can be toxic or produce no therapeutic benefit at all. DDS can be classified according to their physical state, site/route of administration, and the rate of drug release. The dosage form may be gaseous (e.g., anesthetics), liquid (e.g., solutions, emulsions, and suspensions), semisolid (e.g., creams, ointments, and gels), and solid dosage (e.g., tablets and capsules) [2]. Drugs can be administered directly into the body through injection or infusion termed parenteral drug delivery. Depending on the site of administration, one can differentiate among intravenous, intramuscular, subcutaneous, intradermal, and intraperitoneal administration. Mostly semisolid dosage forms including creams, ointments, and gels are applied onto the skin to enter into the body. However, the liquid dosage forms, such as emulsions, or solid dosage forms, such as transdermal patches, can also be used. Dosage forms can be classified into immediate release (IR) and modified release (MR). IR dosage forms allow the drug to dissolve in the gastrointestinal contents, without delaying or prolonging the dissolution or absorption of the drug. In MR dosage forms, the time course and/or location of drug release is chosen to accomplish therapeutic or convenience objectives not offered by conventional dosage forms. MR dosage forms include both delayed and extended release drug products.

tration (MTC). This concept is illustrated in **Figure 1**.

and (c) CR dosage form.

2 Advanced Technology for Delivering Therapeutics

Ideally, the release rate from the dosage form should be the rate-determining step for drug absorption and in fact for the drug concentration in the plasma and target site. The resulting plasma concentration versus time curves become increasingly flatter from IR to extended release dosage forms, indicating the prolonged maintenance of drug in the therapeutic range after a single administration of the dosage form. Controlled DDSs have been introduced to overwhelm the drawback of fluctuating drug levels associated with conventional dosage forms [5]. Controlled drug release and subsequent biodegradation are important for developing successful formulations. The release mechanisms involve desorption of surface-bound/ adsorbed drugs; diffusion through the carrier matrix or polymer membrane surrounding drug core; matrix erosion; combination of erosion/diffusion process; and responsiveness to stimuli such as light, changes in pH, or temperature.

The formulation scientist must optimize the bioavailability of the drug. To achieve this goal, the delivery systems should allow the drug to reach the systemic circulation, more importantly to the target site in the body to avoid side effects by preventing the exposure of drug to the nontarget sites. In addition, the drug must be physically and chemically compatible with the formulation excipients in the dosage forms and stable microbiologically. The delivery systems should be designed in such a way that it can improve the patient compliance. One can design an oral dosage form instead of parenteral formulations for the drug, which can allow self-administration of the dosage forms. Moreover, the pharmaceutical quality of the delivery systems must be ensured in accordance with the regulatory specifications to facilitate reproducible drug release from the system and minimize the influence of the body such as food effects on drug release. It is also necessary to investigate the feasibility of the developed DDS to be scaled up from the laboratory to the production scale.

However, controlled release systems do not exclusively deliver the drug to the target organ. For this reason, the target-specific drug delivery systems must be designed in order to control biodistribution of the drug. Consequently, various novel concepts have been emerged to meet the specific needs of an ideal drug delivery system. This chapter introduces a brief description of targeted drug delivery mechanism along with some of the novel-targeted drug delivery options.
