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Major considerations in the design of a drug product include the therapeutic objective, the application site, and systemic drug absorption from the application site. If the drug is intended for systemic activity, the drug should ideally be completely and consistently absorbed from the application site. In contrast, if the drug is intended for local activity, then systemic absorption from the application should be minimal to prevent systemic drug exposure and possible systemic side effects. For extended-release drug products, the drug product should remain at or near the application site and then slowly release the drug for the desired period of time. The systemic absorption of a drug is dependent on (1) the physicochemical properties of the drug, (2) the nature of the drug product, and (3) the anatomy and physiology of the drug absorption site.

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In order to develop a drug product that elicits the desired therapeutic objective, the pharmaceutical scientist must have a thorough understanding of the biopharmaceutic properties of the drug and drug product and the physiologic and pathologic factors affecting drug absorption from the application site. Pharmacists must also understand the relationship of drug dosage to therapeutic efficacy and adverse reactions and the potential for drug–drug and drug–nutrient interactions. This chapter will focus on the anatomic and physiologic considerations for the systemic absorption of a drug, whereas Chapter 14 will focus on the biopharmaceutic aspects of the drug and drug-product design including considerations in manufacturing and performance tests. Since the major route of drug administration is the oral route, major emphasis in the chapter will be on gastrointestinal drug absorption.

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Drugs may be given by parenteral, enteral, inhalation, intranasal, transdermal (percutaneous), or intranasal route for systemic absorption. Each route of drug administration has certain advantages and disadvantages. Some characteristics of the more common routes of drug administration are listed in Table 13-1. The systemic availability and onset of drug action are affected by blood flow at the administration site, the physicochemical characteristics of the drug and the drug product, and by any pathophysiologic condition at the absorption site. After a drug is systemically absorbed, drug distribution and clearance follow normal physiological conditions of the body. Drug distribution and clearance are not usually altered by the drug formulation but may be altered by pathology, genetic polymorphism, and drug–drug interactions, as discussed in other chapters.

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Table 13-1 Common Routes of Drug Administration

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