NCLEX: Pharmacokinetics

Pharmacokinetics: DRUG CLEARANCE THROUGH METABOLISM

Focus topic: Pharmacokinetics

Once a drug enters the body, the process of elimination begins. The three major routes of elimination are hepatic metabolism, biliary elimination, and urinary elimination. Together, these elimination processes decrease the plasma concentration exponentially. That is, a constant fraction of the drug present is eliminated in a given unit of time. Most drugs are eliminated according to first-order kinetics, although some, such as aspirin in high doses, are eliminated according to zero-order or nonlinear kinetics. Metabolism leads to production of products with increased polarity, which allows the drug to be eliminated. Clearance (CL) estimates the amount of drug cleared from the body per unit of time. Total CL is a composite estimate reflecting all mechanisms of drug elimination and is calculated as follows:

CL = 0 693 × V t1 2 . / d /

where t1/2 is the elimination half-life, Vd is the apparent volume of distribution, and 0.693 is the natural log constant. Drug half-life is often used as a measure of drug CL, because, for many drugs, Vd is a constant.

A. Kinetics of metabolism

Focus topic: Pharmacokinetics

  • First-order kinetics: The metabolic transformation of drugs is catalyzed by enzymes, and most of the reactions obey Michaelis- Menten kinetics.
  • Zero-order kinetics: With a few drugs, such as aspirin, ethanol,
    and phenytoin, the doses are very large.

The enzyme is saturated by a high free drug concentration, and the rate of metabolism remains constant over time. This is called zero-order kinetics (also called nonlinear kinetics). A constant amount of drug is metabolized per unit of time. The rate of elimination is constant and does not depend on the drug concentration.

Pharmacokinetics

B. Reactions of drug metabolism

Focus topic: Pharmacokinetics

The kidney cannot efficiently eliminate lipophilic drugs that readily cross cell membranes and are reabsorbed in the distal convoluted tubules. Therefore, lipid-soluble agents are first metabolized into more polar (hydrophilic) substances in the liver via two general sets of reactions, called phase I and phase II.

Pharmacokinetics

1. Phase I: Phase I reactions convert lipophilic drugs into more polar molecules by introducing or unmasking a polar functional group, such as –OH or –NH2. Phase I reactions usually involve reduction, oxidation, or hydrolysis. Phase I metabolism may increase, decrease, or have no effect on pharmacologic activity.

  • Phase I reactions utilizing the P450 system: The phase I reactions most frequently involved in drug metabolism are catalyzed by the cytochrome P450 system (also called microsomal mixed-function oxidases). The P450 system is important for the metabolism of many endogenous compounds (such as steroids, lipids) and for the biotransformation of exogenous substances (xenobiotics). Cytochrome P450, designated as CYP, is a super family of heme-containing isozymes that are located in most cells, but primarily in the liver and GI tract.

[1] Nomenclature: The family name is indicated by the Arabic number that follows CYP, and the capital letter designates the subfamily, for example, CYP3A. A second number indicates the specific isozyme, as in CYP3A4.

[2] Specificity: Because there are many different genes that encode multiple enzymes, there are many different P450 isoforms. These enzymes have the capacity to modify a large number of structurally diverse substrates. In addition, an individual drug may be a substrate for more than one isozyme. Four isozymes are responsible for the vast majority of P450-catalyzed reactions. They are CYP3A4/5, CYP2D6, CYP2C8/9, and CYP1A2. Considerable amounts of CYP3A4 are found in intestinal mucosa, accounting for first-pass metabolism of drugs such as chlorpromazine and clonazepam.

[3] Genetic variability: P450 enzymes exhibit considerable genetic variability among individuals and racial groups. Variations in P450 activity may alter drug efficacy and the risk of adverse events. CYP2D6, in particular, has been shown to exhibit genetic polymorphism. CYP2D6 mutations result in very low capacities to metabolize substrates. Some individuals, for example, obtain no benefit from the opioid analgesic codeine, because they lack the CYP2D6 enzyme that activates the drug. Similar polymorphisms have been characterized for the CYP2C subfamily of isozymes. For instance, clopidogrel carries a warning that patients who are poor CYP2C19 metabolizers have a higher incidence of cardiovascular events (for example, stroke or myocardial infarction) when taking this drug. Clopidogrel is a prodrug, and CYP2C19 activity is required to convert it to the active metabolite. Although CYP3A4 exhibits a greater than 10-fold variability between individuals, no polymorphisms have been identified so far for this P450 isozyme.

[4] Inducers: The CYP450-dependent enzymes are an important target for pharmacokinetic drug interactions. One such interaction is the induction of selected CYP isozymes. Xenobiotics (chemicals not normally produced or expected to be present in the body, for example, drugs or environmental pollutants) may induce the activity of these enzymes. Certain drugs (for example, phenobarbital, rifampin, and carbamazepine) are capable of increasing the synthesis of one or more CYP isozymes. This results in increased biotransformation of drugs and can lead to significant decreases in plasma concentrations of drugs metabolized by these CYP isozymes, with concurrent loss of pharmacologi effect. For example, rifampin, an antituberculosis drug, significantly decreases the plasma concentrations of human immunodeficiency virus (HIV) protease inhibitors, thereby diminishing their ability to suppress HIV replication. St. John’s wort is a widely used herbal product and is a potent CYP3A4 inducer. Many drug interactions have been reported with concomitant use of St. John’s wort. lists some of the more important inducers for representative CYP isozymes. Consequences of increased drug metabolism include 1) decreased plasma drug concentrations, 2) decreased drug activity if the metabolite is inactive, 3) increased drug activity if the metabolite is active, and 4) decreased therapeutic drug effect.

[5] Inhibitors: Inhibition of CYP isozyme activity is an important source of drug interactions that lead to serious adverse events. The most common form of inhibition is through competition for the same isozyme. Some drugs, however, are capable of inhibiting reactions for which they are not substrates (for example, ketoconazole), leading to drug interactions. Numerous drugs have been shown to inhibit one or more of the CYP-dependent biotransformation pathways of warfarin. For example, omeprazole is a potent inhibitor of three of the CYP isozymes responsible for warfarin metabolism. If the two drugs are taken together, plasma concentrations of warfarin increase, which leads to greater anticoagulant effect and increased risk of bleeding. [Note: The more important CYP inhibitors are erythromycin, ketoconazole, and ritonavir, because they each inhibit several CYP isozymes.] Natural substances may also inhibit drug metabolism. For instance, grapefruit juice inhibits CYP3A4 and leads to higher levels and/or greater potential for toxic effects with drugs, such as nifedipine, clarithromycin, and simvastatin, that are metabolized by this system.

  • Phase I reactions not involving the P450 system: These include amine oxidation (for example, oxidation of catecholamines or histamine), alcohol dehydrogenation (for example, ethanol oxidation), esterases (for example, metabolism of aspirin in the liver), and hydrolysis (for example, of procaine).

2. Phase II: This phase consists of conjugation reactions. If the metabolite from phase I metabolism is sufficiently polar, it can be excreted by the kidneys. However, many phase I metabolites are still too lipophilic to be excreted. A subsequent conjugation reaction with an endogenous substrate, such as glucuronic acid, sulfuric acid, acetic acid, or an amino acid, results in polar, usually more water-soluble compounds that are often therapeutically inactive. A notable exception is morphine-6-glucuronide, which is more potent than morphine. Glucuronidation is the most common and the most important conjugation reaction. [Note: Drugs already possessing an –OH, –NH2, or –COOH group may enter phase II directly and become conjugated without prior phase I metabolism.] The highly polar drug conjugates are then excreted by the kidney or in bile.

Pharmacokinetics

Pharmacokinetics

Pharmacokinetics

Pharmacokinetics: DRUG CLEARANCE BY THE KIDNEY

Focus topic: Pharmacokinetics

Drugs must be sufficiently polar to be eliminated from the body. Removal of drugs from the body occurs via a number of routes, the most important being elimination through the kidney into the urine. Patients with renal dysfunction may be unable to excrete drugs and are at risk for drug accumulation and adverse effects.

A. Renal elimination of a drug

Focus topic: Pharmacokinetics

Elimination of drugs via the kidneys into urine involves the processes of glomerular filtration, active tubular secretion, and passive tubular reabsorption.

  • Glomerular filtration: Drugs enter the kidney through renal arteries, which divide to form a glomerular capillary plexus. Free drug (not bound to albumin) flows through the capillary slits into the Bowman space as part of the glomerular filtrate. The glomerular filtration rate (GFR) is normally about 125 mL/min but may diminish significantly in renal disease. Lipid solubility and pH do not influence the passage of drugs into the glomerular filtrate. However, variations in GFR and protein binding of drugs do affect this process.
  • Proximal tubular secretion: Drugs that were not transferred into the glomerular filtrate leave the glomeruli through efferent arterioles, which divide to form a capillary plexus surrounding the nephric lumen in the proximal tubule. Secretion primarily occurs in the proximal tubules by two energy-requiring active transport systems: one for anions (for example, deprotonated forms of weak acids) and one for cations (for example, protonated forms of weak bases). Each of these transport systems shows low specificity and can transport many compounds. Thus, competition between drugs for these carriers can occur within each transport system. [Note: Premature infants and neonates have an incompletely developed tubular secretory mechanism and, thus, may retain certain drugs in the glomerular filtrate.]
  • Distal tubular reabsorption: As a drug moves toward the distal convoluted tubule, its concentration increases and exceeds that of the perivascular space. The drug, if uncharged, may diffuse out of the nephric lumen, back into the systemic circulation. Manipulating the urine pH to increase the fraction of ionized drug in the lumen may be done to minimize the amount of back diffusion and increase the clearance of an undesirable drug. As a general rule, weak acids can be eliminated by alkalinization of the urine, whereas elimination of weak bases may be increased by acidification of the urine. This process is called “ion trapping.” For example, a patient presenting with phenobarbital (weak acid) overdose can be given bicarbonate, which alkalinizes the urine and keeps the drug ionized, thereby decreasing its reabsorption.
  • Role of drug metabolism: Most drugs are lipid soluble and, without chemical modification, would diffuse out of the tubular lumen when the drug concentration in the filtrate becomes greater than that in the perivascular space. To minimize this reabsorption, drugs are modified primarily in the liver into more polar substances via phase I and phase II reactions (described above).

Pharmacokinetics

Pharmacokinetics: CLEARANCE BY OTHER ROUTES

Focus topic: Pharmacokinetics

Drug clearance may also occur via the intestines, bile, lungs, and breast milk, among others. Drugs that are not absorbed after oral administration or drugs that are secreted directly into the intestines or into bile are eliminated in the feces. The lungs are primarily involved in the elimination of anesthetic gases (for example, isoflurane). Elimination of drugs in breast milk may expose the breast-feeding infant to medications and/or metabolites being taken by the mother and is a potential source of undesirable side effects to the infant. Excretion of most drugs into sweat, saliva, tears, hair, and skin occurs only to a small extent. Total body clearance and drug half-life are important measures of drug clearance that are used to optimize drug therapy and minimize toxicity.

A. Total body clearance

Focus topic: Pharmacokinetics

The total body (systemic) clearance, CLtotal, is the sum of all clearances from the drug-metabolizing and drug-eliminating organs. The kidney is often the major organ of elimination. The liver also contributes to drug clearance through metabolism and/or excretion into the bile. Total clearance is calculated using the following equation:

CL CL CL CL CL total hepatic renal pulmonary other = + + +

where CLhepatic + CLrenal are typically the most important.

B. Clinical situations resulting in changes in drug half-life

Focus topic: Pharmacokinetics

When a patient has an abnormality that alters the half-life of a drug, adjustment in dosage is required. Patients who may have an increase in drug half-life include those with 1) diminished renal or hepatic blood flow, for example, in cardiogenic shock, heart failure, or hemorrhage; 2) decreased ability to extract drug from plasma, for example, in renal disease; and 3) decreased metabolism, for example, when a concomitant drug inhibits metabolism or in hepatic insufficiency, as with cirrhosis. These patients may require a decrease in dosage or less frequent dosing intervals. In contrast, the half-life of a drug may be decreased by increased hepatic blood flow, decreased protein binding, or increased metabolism. This may necessitate higher doses or more frequent dosing intervals.

VIII. DESIGN AND OPTIMIZATION OF DOSAGE REGIMEN

Focus topic: Pharmacokinetics

To initiate drug therapy, the clinician must select the appropriate route of administration, dosage, and dosing interval. Selection of a regimen depends on various patient and drug factors, including how rapidly therapeutic levels of a drug must be achieved. The regimen is then further refined, or optimized, to maximize benefit and minimize adverse effects.

A. Continuous infusion regimens

Focus topic: Pharmacokinetics

To initiate drug therapy, the clinician must select the appropriate route of administration, dosage, and dosing interval. Selection of a regimen depends on various patient and drug factors, including how rapidly therapeutic levels of a drug must be achieved. The regimen is then further refined, or optimized, to maximize benefit and minimize adverse effects.

A. Continuous infusion regimens

Focus topic: Pharmacokinetics

Therapy may consist of a single dose of a drug, for example, a sleep inducing agent, such as zolpidem. More commonly, drugs are continually administered, either as an IV infusion or in oral fixed-dose/ fixed-time interval regimens (for example, “one tablet every 4 hours”). Continuous or repeated administration results in accumulation of the drug until a steady state occurs. Steady-state concentration is reached when the rate of drug elimination is equal to the rate of drug administration, such that the plasma and tissue levels remain relatively constant.

  • Plasma concentration of a drug following IV infusion: With continuous IV infusion, the rate of drug entry into the body is constant. Most drugs exhibit first-order elimination, that is, a constant fraction of the drug is cleared per unit of time. Therefore, the rate of drug elimination increases proportionately as the plasma concentration increases. Following initiation of a continuous IV infusion, the plasma concentration of a drug rises until a steady state (rate of drug elimination equals rate of drug administration) is reached, at which point the plasma concentration of the drug remains constant.

a. Influence of the rate of infusion on steady-state concentration:
The steady-state plasma concentration (Css) is directly proportional to the infusion rate. For example, if the infusion rate is doubled, the Css is doubled (Figure 1.21). Furthermore, the Css is inversely proportional to the clearance of the drug. Thus, any factor that decreases clearance, such as liver or kidney disease, increases the Css of an infused drug (assuming Vd remains constant). Factors that increase clearance, such as increased metabolism, decrease the Css.

b. Time required to reach the steady-state drug concentration:
The concentration of a drug rises from zero at the start of the infusion to its ultimate steady-state level, Css (Figure 1.21). The rate constant for attainment of steady state is the rate constant for total body elimination of the drug. Thus, 50% of Css of a drug is observed after the time elapsed, since the infusion, t, is equal to t1/2, where t1/2 (or half-life) is the time required for the drug concentration to change by 50%. After another half-life, the drug concentration approaches 75% of Css. The drug concentration is 87.5% of Css at 3 half-lives and 90% at 3.3 half-lives. Thus, a drug reaches steady state in about four to five half-lives.

The sole determinant of the rate that a drug achieves steady state is the half-life (t1/2) of the drug, and this rate is influenced only by factors that affect the half-life. The rate of approach to steady state is not affected by the rate of drug infusion.

Pharmacokinetics

Pharmacokinetics

Pharmacokinetics

B. Fixed-dose/fixed-time regimens

Focus topic: Pharmacokinetics

Administration of a drug by fixed doses rather than by continuous infusion is often more convenient. However, fixed doses of IV or oral medications given at fixed intervals result in time-dependent fluctuations in the circulating level of drug, which contrasts with the smooth ascent of drug concentration observed with continuous infusion.

  • Multiple IV injections: When a drug is given repeatedly at regular intervals, the plasma concentration increases until a steady state is reached. Because most drugs are given at intervals shorter than five half-lives and are eliminated exponentially with time, some drug from the first dose remains in the body when the second dose is administered, some from the second dose remains when the third dose is given, and so forth. Therefore, the drug accumulates until, within the dosing interval, the rate of drug elimination equals the rate of drug administration and a steady state is achieved.

a. Effect of dosing frequency: With repeated administration at regular intervals, the plasma concentration of a drug oscillates about a mean. Using smaller doses at shorter intervals reduces the amplitude of fluctuations in drug concentration. However, the Css is affected by neither the dosing frequency (assuming the same total daily dose is administered) nor the rate at which the steady state is approached.

b. Example of achievement of steady state using different dosage regimens: Curve B of Figure 1.23 shows the amount of drug in the body when 1 unit of a drug is administered IV and repeated at a dosing interval that corresponds to the half-life of the drug. At the end of the first dosing interval, 0.50 units of drug remain from the first dose when the second dose is administered. At the end of the second dosing interval, 0.75 units are present when the third dose is given. The minimal amount of drug remaining during the dosing interval progressively approaches a value of 1.00 unit, whereas the maximal value immediately following drug administration progressively approaches 2.00 units. Therefore, at the steady state, 1.00 unit of drug is lost during the dosing interval, which is exactly matched by the rate of administration. That is, the “rate in” equals the “rate out.” As in the case for IV infusion, 90% of the steady-state value is achieved in 3.3 half-lives.

  • Multiple oral administrations: Most drugs that are administered on an outpatient basis are oral medications taken at a specific dose one, two, or three times daily. In contrast to IV injection, orally administered drugs may be absorbed slowly, and the plasma concentration of the drug is influenced by both the rate of absorption and the rate of elimination.

C. Optimization of dose

Focus topic: Pharmacokinetics

The goal of drug therapy is to achieve and maintain concentrations within a therapeutic response window while minimizing toxicity and/ or side effects. With careful titration, most drugs can achieve this goal. If the therapeutic window of the drug is small (for example, digoxin, warfarin, and cyclosporine), extra caution should be taken in selecting a dosage regimen, and monitoring of drug levels may help ensure attainment of the therapeutic range. Drug regimens are administered as a maintenance dose and may require a loading dose if rapid effects are warranted. For drugs with a defined therapeutic range, drug concentrations are subsequently measured, and the dosage and frequency are then adjusted to obtain the desired levels.

Pharmacokinetics

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  • Maintenance dose: Drugs are generally administered to maintain a Css within the therapeutic window. It takes four to five half-lives for a drug to achieve Css. To achieve a given concentration, the rate of administration and the rate of elimination of the drug are important. The dosing rate can be determined by knowing the target concentration in plasma (Cp), clearance (CL) of the drug from the systemic circulation, and the fraction (F) absorbed (bioavailability):
  • Loading dose: Sometimes rapid obtainment of desired plasma levels is needed (for example, in serious infections or arrhythmias). Therefore, a “loading dose” of drug is administered to achieve the desired plasma level rapidly, followed by a maintenance dose to maintain the steady state (Figure 1.25). In general, the loading dose can be calculated as

Loading dose = (Vd) × (desired steady-state plasma concentration)/F

For IV infusion, the bioavailability is 100%, and the equation becomes

Loading dose = (Vd) × (desired steady-state plasma concentration)

Loading doses can be given as a single dose or a series of doses. Disadvantages of loading doses include increased risk of drug toxicity and a longer time for the plasma concentration to fall if excess levels occur. A loading dose is most useful for drugs that have a relatively long half-life. Without an initial loading dose, these drugs would take a long time to reach a therapeutic value that corresponds to the steady-state level.

  • Dose adjustment: The amount of a drug administered for a given condition is estimated based on an “average patient.” This approach overlooks interpatient variability in pharmacokinetic parameters such as clearance and Vd, which are quite significant in some cases. Knowledge of pharmacokinetic principles is useful in adjusting dosages to optimize therapy for a given patient. Monitoring drug therapy and correlating it with clinical benefits provides another tool to individualize therapy.

When determining a dosage adjustment, Vd can be used to calculate the amount of drug needed to achieve a desired plasma concentration. For example, assume a heart failure patient is not well controlled due to inadequate plasma levels of digoxin. Suppose the concentration of digoxin in the plasma is C1 and the desired target concentration is C2, a higher concentration. The following calculation can be used to determine how much additional digoxin should be administered to bring the level from C1 to C2.

(Vd)(C1) = Amount of drug initially in the body

(Vd)(C2) = Amount of drug in the body needed to achieve the desired plasma concentration

The difference between the two values is the additional dosage needed, which equals Vd (C2 − C1).

Pharmacokinetics

Pharmacokinetics

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