All drugs eventually must be eliminated from the body to terminate their effect and to prevent excessive accumulation of the drug. The body can usually eliminate the drug by biotransformation—chemically altering the original compound so that it is no longer active—by excreting the active form of the drug, or by a combination of biotransformation and excretion.
Eliminating a drug and terminating its effects after it is no longer needed are often essential. For instance, the effects of general and local anesthetics must eventually wear off, allowing the patient to resume normal functioning. Although termination of drug activity can occur when the active form of the drug is excreted from the body via organs such as the kidneys, excretory mechanisms are often too slow to effectively terminate any activity within a reasonable time period. If excretion were the only way to terminate drug activity, some compounds would continue to exert their effects for several days or even weeks. Drug biotransformation into an inactive form usually occurs within a matter of minutes or hours, thus reducing the chance for toxic effects caused by drug accumulation or prolonged drug activity. Excretion can then remove any metabolic by-products that remain after biotransformation, further reducing the risk that these by-products might accumulate and cause toxicity.
Biotransformation and excretion are therefore essential in eliminating specific drugs from the body. This is important, considering the large number of patients who have altered liver function, kidney function, or other physiological changes that affect drug elimination. This chapter will introduce you to the ways that drugs are normally eliminated and how drug metabolism and excretion can be influenced by disease, age, genetics, and other factors. This information will set the stage for subsequent chapters that address how specific drugs are eliminated after being administered to treat various conditions.
Biotransformation, or drug metabolism, refers to chemical changes that take place in the drug following administration. Biotransformation typically results in an altered version of the original compound, known as a metabolite, which is usually inactive or has a greatly reduced level of pharmacological activity. Occasionally, the metabolite has a higher level of activity than the original compound. In these cases, the drug may be given in an inactive, or “prodrug,” form that will activate via biotransformation following administration. However, after it has exerted its pharmacological effect, drug termination is the primary function of biotransformation.1
Cellular Mechanisms of Drug Biotransformation
The chemical changes that occur during drug metabolism are usually caused by oxidation, reduction, hydrolysis, or conjugation of the original compound.1 Examples of each type of reaction are listed in Table 3-1. Enzymes that are located within specific tissues are responsible for catalyzing changes in the drug's structure and subsequently altering the drug's pharmacological properties. The type of reaction and the enzymes catalyzing the reaction include the following: