Chapter 1

### LEARNING OBJECTIVES

After completing this chapter, the reader should be able to:

• ► List the types and locations of drug targets.

• ► List common dosage forms and the nature of the drug within the listed dosage forms.

• ► Describe common routes of drug administration, including the administration locations and the absorption sites.

• ► Define bioavailability and describe important physiological and physicochemical factors that influence a drug's bioavailability.

• ► Describe the following physicochemical factors that can influence the fate of the drug in the body: solid state properties, ionization, solubility and dissolution, partition coefficient, mass transport and membrane passage, complexation and protein binding.

• ► Briefly describe each parameter in the LADME acronym.

• ► Describe the general roles of drug transporters (with examples) in the ADME processes of a drug's disposition in the body.

• ► Describe, in general, how drugs are liberated from their dosage forms.

• ► Describe gastrointestinal drug absorption into the systemic circulation, including the influences of a drug's physicochemical properties, and the steps taken by a dissolved drug in the small intestines to its entry into enteroctytes, hepatocytes, and finally the bloodstream.

• ► Describe enterohepatic cycling, possible rate-limiting steps in drug absorption, and first-pass metabolism.

• ► Describe the distribution of a drug from the plasma to the interstitia of tissues, including the influences of a drug's physicochemical properties, and of binding to plasma proteins and to tissue components.

• ► List other distribution sites, including the CNS, fat, and placenta.

• ► Define the apparent volume of drug distribution and relate it to the distribution of body water.

• ► Give an overview of the necessity, phases, and locations of drug metabolism.

• ► Describe renal drug excretion, including the influences of a drug's physicochemical properties.

• ► List nonrenal excretory pathways.

• ► Define and describe clearance and half-life, and briefly relate them to their importance for drug dosing.

### INTRODUCTION

Biopharmaceutics can be defined as the study of the physical and chemical properties of drugs and their proper dosage as related to the onset, duration, and intensity of drug action, or it can be defined as the study of the effects of physicochemical properties of the drug and the drug product, in vitro, on the bioavailability of the drug, in vivo, to produce a desired therapeutic effect. Both definitions imply the relationship between the physicochemical properties of the drug, the drug's biological fate in the body after its administration, and the resulting pharmacological action of the drug. Most of this textbook is focused on the details of important physicochemical properties. This chapter introduces fundamental concepts and processes related to the fate of the drug in the body in order to help provide a framework for many of the concepts in subsequent chapters. As a background to discussing the fate (or disposition) of drugs, a brief review of their targets, their dosage forms, and their routes of administration is given next.

#### Drug Targets

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