Pharmacokinetics refers to what the body does to a drug, whereas pharmacodynamics (see Chapter 2) describes what the drug does to the body. Once administered through one of several available routes, four pharmacokinetic properties determine the speed of onset of drug action, the intensity of the drug’s effect, and the duration of drug action (Figure 1.1):
Figure 1.1. Schematic representation of drug absorption, distribution, metabolism, and elimination.
- Absorption: First, drug absorption from the site of administration permits entry of the therapeutic agent (either directly or indirectly) into plasma.
- Distribution: Second, the drug may then reversibly leave the bloodstream and distribute into the interstitial and intracellular fluids.
- Metabolism: Third, the drug may be biotransformed by metabolism by the liver, or other tissues.
- Elimination: Finally, the drug and its metabolites are eliminated from the body in urine, bile, or feces.
Pharmacokinetic parameters allow the clinician to design and optimize treatment regimens, including decisions as to the route of administration for a specific drug, the amount and frequency of each dose, and the duration of treatment.
Routes of Drug Administration
The route of administration is determined primarily by the properties of the drug (for example, water or lipid solubility, ionization) and by the therapeutic objectives (for example, the desirability of a rapid onset of action, the need for long-term treatment, or restriction of delivery to a local site). Major routes of drug administration include enteral, parenteral, and topical among others. Figure 1.2 illustrates the subcategories of these routes as well as other methods of drug administration.
Figure 1.2. Commonly used routes of drug administration.
IV = intravenous; IM = intramuscular; SC = subcutaneous.
Enteral administration, or administering a drug by mouth, is the safest and most common, convenient, and economical method of drug administration. When the drug is given in the mouth, it may be swallowed, allowing oral delivery, or it may be placed under the tongue (sublingual), facilitating direct absorption into the bloodstream.
Giving a drug by mouth provides many advantages to the patient. Oral drugs are easily self-administered and, compared to drugs given parenterally, have a low risk of systemic infections that could complicate treatment. Moreover, toxicities and overdose by the oral route may be overcome with antidotes, such as activated charcoal. On the other hand, the pathways involved in oral drug absorption are the most complicated, and the low pH of the stomach may inactivate some drugs. A wide range of oral preparations is available including enteric-coated and extended-release preparations.
An enteric coating is a chemical envelope that resists the action of the fluids and enzymes in the stomach but dissolves readily in the upper intestine. Such coating is useful for certain groups of drugs (for example, omeprazole) that are acid unstable. Enteric coatings protect the drug from stomach acid, delivering them instead to the less acidic intestine, where the coating dissolves and allows the drug to be released. Similarly, drugs that have an irritant effect on the stomach, such as aspirin, can be coated with a substance that will dissolve only in the small intestine, thereby protecting the stomach.
Extended-release medications have special coatings or ingredients that control how fast the drug is released from the pill into the body. Having a longer duration of action may improve patient compliance, because the medication does not have to be taken as often. Additionally, extended-release dosage forms may maintain concentrations within an acceptable therapeutic range over a long period of time, as opposed to immediate-release dosage forms, which may result in larger peaks and troughs in plasma concentrations. These extended-release formulations are advantageous for drugs with short half-lives.
For example, the half-life of morphine is 2 to 4 hours in adults. Oral morphine must be administered six times in 24 hours to obtain a continuous analgesic effect. However, only two doses are needed when controlled-release tablets are used. Unfortunately, many of the extended release formulations may have been developed to create a marketing advantage over conventional-release products, rather than because of documented clinical advantage.
Placement under the tongue allows a drug to diffuse into the capillary network and, therefore, to enter the systemic circulation directly. Sublingual administration of an agent has several advantages, including rapid absorption, convenience of administration, low incidence of infection, bypass of the harsh gastrointestinal (GI) environment, and avoidance of first-pass metabolism (the drug is absorbed into the superior vena cava). The buccal route (between cheek and gum) is similar to the sublingual route.
The parenteral route introduces drugs directly across the body’s barrier defenses into the systemic circulation. Parenteral administration is used for drugs that are poorly absorbed from the GI tract (for example, heparin) and for agents that are unstable in the GI tract (for example, insulin). Parenteral administration is also used for treatment of unconscious patients and under circumstances that require a rapid onset of action.
In addition, these routes have the highest bioavailability and are not subject to first-pass metabolism or harsh GI environments. Parenteral administration provides the most control over the actual dose of drug delivered to the body. However, these administrations are irreversible and may cause pain, fear, local tissue damage, and infections. The three major parenteral routes are intravascular (intravenous or intra-arterial), intramuscular, and subcutaneous (see Figure 1.2). Each route has advantages and drawbacks.
IV injection is the most common parenteral route. For drugs that are not absorbed orally, such as the neuromuscular blocker atracurium, there is often no other choice. IV delivery permits a rapid effect and a maximum degree of control over the circulating levels of the drug. When injected as a bolus, the full amount of a drug is delivered to the systemic circulation almost immediately. The same dose also may be administered as an IV infusion during a longer time, resulting in a decrease in the peak plasma concentration and an increase in the time the drug is present in the circulation. IV injection is advantageous for administering chemicals that may cause irritation when administered via other routes, because the substance is rapidly diluted by the blood.
However, unlike drugs in the GI tract, those that are injected cannot be recalled by strategies, such as by binding to activated charcoal. IV injection may inadvertently introduce bacteria and other infective particles through contamination at the site of injection. It may also precipitate blood constituents, induce hemolysis, or cause other adverse reactions by the too-rapid delivery of high concentrations of a drug to the plasma and tissues. Therefore, patients must be carefully monitored for unfavorable drug reactions, and the rate of infusion must be carefully controlled.
Drugs administered IM can be in aqueous solutions, which are absorbed rapidly (Figure 1.3), or in specialized depot preparations, which are absorbed slowly. Depot preparations often consist of a suspension of the drug in a nonaqueous vehicle such as polyethylene glycol. As the vehicle diffuses out of the muscle, the drug precipitates at the site of injection. The drug then dissolves slowly, providing a sustained dose over an extended period of time. Examples of sustained-release drugs are haloperidol (see p. 165) and depot medroxyprogesterone (see p. 323). These drugs produce extended neuroleptic and contraceptive effects, respectively.
A. Schematic representation of subcutaneous and intramuscular injection. B. Plasma concentrations of midazolam after intravenous and intramuscular injection.
This route of administration, like IM injection, requires absorption via simple diffusion and is somewhat slower than the IV route. SC injection minimizes the risks of hemolysis or thrombosis associated with IV injection and may provide constant, slow, and sustained effects. This route should not be used with drugs that cause tissue irritation, because severe pain and necrosis may occur. [Note: Minute amounts of epinephrine are sometimes combined with a drug administered subcutaneously to restrict its area of action.
Epinephrine acts as a local vasoconstrictor and decreases removal of a drug, such as lidocaine, from the site of administration.] Other examples of drugs given via SC administration include solids, such as a single rod containing the contraceptive etonogestrel that is implanted for long-term activity (see p. 325), and programmable mechanical pumps that can be implanted to deliver insulin in diabetic patients.
Inhalation routes, both oral and nasal (see below), provide rapid delivery of a drug across the large surface area of the mucous membranes of the respiratory tract and pulmonary epithelium, producing an effect almost as rapidly as does IV injection. This route of administration is used for drugs that are gases (for example, some anesthetics) and those that can be dispersed in an aerosol. This route is particularly effective and convenient for patients with respiratory complaints (such as asthma or chronic obstructive pulmonary disease), because the drug is delivered directly to the site of action, thereby minimizing systemic side effects. Examples of drugs administered via this route include bronchodilators, such as albuterol, and corticosteroids, such as fluticasone.
This route involves administration of drugs directly into the nose. Agents include nasal decongestants, such as oxymetazoline, and anti-inflammatory corticosteroids such as mometasone furoate. Desmopressin is administered intranasally in the treatment of diabetes insipidus. Salmon calcitonin, a peptide hormone used in the treatment of osteoporosis, is also available as a nasal spray.
The blood-brain barrier (see p. 10) typically delays or prevents the absorption of drugs into the central nervous system (CNS). When local, rapid effects are needed, it is necessary to introduce drugs directly into the cerebrospinal fluid. For example, intrathecal amphotericin B is used in treating cryptococcal meningitis (see p. 430).
Topical application is used when a local effect of the drug is desired. For example, clotrimazole is applied as a cream directly to the skin in the treatment of dermatophytosis.
This route of administration achieves systemic effects by application of drugs to the skin, usually via a transdermal patch (Figure 1.4). The rate of absorption can vary markedly, depending on the physical characteristics of the skin at the site of application as well as the lipid solubility of the drug. This route is most often used for the sustained delivery of drugs, such as the antianginal drug nitroglycerin, the antiemetic scopolamine, and nicotine transdermal patches, which are used to facilitate smoking cessation.
A. Schematic representation of a transdermal patch. B. Transdermal nicotine patch applied to arm.
Because 50 percent of the drainage of the rectal region bypasses the portal circulation, the biotransformation of drugs by the liver is minimized with rectal administration. Like the sublingual route of administration, the rectal route has the additional advantage of preventing the destruction of the drug by intestinal enzymes or by low pH in the stomach. The rectal route is also useful if the drug induces vomiting when given orally, if the patient is already vomiting, or if the patient is unconscious. [Note: The rectal route is commonly used to administer antiemetic agents.] On the other hand, rectal absorption is often erratic and incomplete, and many drugs irritate the rectal mucosa. Figure 1.5 summarizes the characteristics of the common routes of administration.
Figure 1.5. The absorption pattern, advantages, and disadvantages of the most common routes of administration.
Absorption of Drugs
Absorption is the transfer of a drug from its site of administration to the bloodstream via one of several mechanisms. The rate and efficiency of absorption depend on both factors in the environment where the drug is absorbed and the drug’s chemical characteristics and route of administration (which influence its bioavailability). For IV delivery, absorption is complete. That is, the total dose of drug administered reaches the systemic circulation (100% bioavailability). Drug delivery by other routes may result in only partial absorption and, thus, lower bioavailability.
Mechanisms of absorption of drugs from the GI tract
Depending on their chemical properties, drugs may be absorbed from the GI tract by passive diffusion, facilitated diffusion, active transport, or endocytosis.
The driving force for passive absorption of a drug is the concentration gradient across a membrane separating two body compartments. In other words, the drug moves from a region of high concentration to one of lower concentration. Passive diffusion does not involve a carrier, is not saturable, and shows a low structural specificity. The vast majority of drugs gain access to the body by this mechanism. Water-soluble drugs penetrate the cell membrane through aqueous channels or pores, whereas lipid-soluble drugs readily move across most biologic membranes due to their solubility in the membrane lipid bilayers (Figure 1.6A).
Figure 1.6. Schematic representation of drugs crossing a cell membrane.
ATP = adenosine triphosphate; ADP = adenosine diphosphate.
Other agents can enter the cell through specialized transmembrane carrier proteins that facilitate the passage of large molecules. These carrier proteins undergo conformational changes, allowing the passage of drugs or endogenous molecules into the interior of cells and moving them from an area of high concentration to an area of low concentration. This process is known as facilitated diffusion. It does not require energy, can be saturated, and may be inhibited by compounds that compete for the carrier (Figure 1.6B).
This mode of drug entry also involves specific carrier proteins that span the membrane. A few drugs that closely resemble the structure of naturally occurring metabolites are actively transported across cell membranes using these specific carrier proteins. Energy-dependent active transport is driven by the hydrolysis of adenosine triphosphate (Figure 1.6C). It is capable of moving drugs against a concentration gradient, from a region of low drug concentration to one of higher drug concentration.
The process shows saturation kinetics for the carrier, much in the same way that an enzyme-catalyzed reaction shows a maximal velocity at high substrate levels where all the active sites are filled with substrate.1 Active transport systems are selective and may be competitively inhibited by other cotransported substances.
Endocytosis and exocytosis
These types of drug delivery systems transport drugs of exceptionally large size across the cell membrane. Endocytosis involves engulfment of a drug molecule by the cell membrane and transport into the cell by pinching off the drug-filled vesicle (Figure 1.6D). Exocytosis is the reverse of endocytosis and is used by cells to secrete many substances by a similar vesicle formation process. Vitamin B12 is transported across the gut wall by endocytosis, whereas certain neurotransmitters (for example, norepinephrine) are stored in intracellular membrane-bound vesicles in the nerve terminal and are released by exocytosis.