Drugs are most often eliminated by biotransformation and/or excretion into the urine or bile. The process of metabolism transforms lipophilic drugs into more polar readily excretable products. The liver is the major site for drug metabolism, but specific drugs may undergo biotransformation in other tissues, such as the kidney and the intestines.
[Note: Some agents are initially administered as inactive compounds (pro-drugs) and must be metabolized to their active forms
A. Kinetics of metabolism
1. First-order kinetics:
The metabolic transformation of drugs is catalyzed by enzymes, and most of the reactions obey Michaelis-Menten kinetics:
In most clinical situations, the concentration of the drug, [C], is much less than the Michaelis constant, Km, and the Michaelis-Menten equation reduces to,
That is, the rate of drug metabolism is directly proportional to the concentration of free drug, and first-order kinetics are observed
(Figure Effect of drug dose on the rate of metabolism.)
This means that a constant fraction of drug is metabolized per unit of time.
2. Zero-order kinetics:
With a few drugs, such as aspirin, ethanol, and phenytoin, the doses are very large. Therefore [C] is much greater than Km, and the velocity equation becomes
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 (sometimes referred to clinically as nonlinear kinetics). A constant amount of drug is metabolized per unit of time.
B. Reactions of drug metabolism
The kidney cannot efficiently eliminate lipophilic drugs that readily cross cell membranes and are reabsorbed in the distal tubules. Therefore, lipid-soluble agents must first be metabolized in the liver using two general sets of reactions, called Phase I and Phase II
(Figure : The biotransformation of drugs.)
1. Phase I :
Phase I reactions function to convert lipophilic molecules into more polar molecules by introducing or unmasking a polar functional group, such as OH or NH2. Phase I metabolism may increase, decrease, or leave unaltered the drug's pharmacologic activity.
a. 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 oxidase): The oxidation proceeds by the drug binding to the oxidized form of cytochrome P450, and then oxygen is introduced through a reductive step, coupled to NADPH:cytochrome P450 oxidoreductase.
b. Summary of the P450 system: The P450 system is important for the metabolism of many endogenous compounds (steroids, lipids, etc.) and for the biotransformation of exogenous substances (xenobiotics). Cytochrome P450, designated as CYP, is composed of many families of heme-containing isozymes that are located in most cells but are primarily found in the liver and GI tract. The family name is indicated by an arabic number followed by a capital letter for the subfamily (for example, CYP3A). Another number is added to indicate the specific isozyme (CYP3A4). There are many different genes, and many different enzymes; thus, the various P450s are known as isoforms. Six isozymes are responsible for the vast majority of P450-catalyzed reactions: CYP3A4, CYP2D6, CYP2C9/10, CYP2C19, CYP2E1, and CYP1A2. The percentages of currently available drugs that are substrates for these isozymes are 60, 25, 15, 15, 2, and 2 percent, respectively.
[Note: An individual drug may be a substrate for more than one isozyme.] Considerable amounts of CYP3A4 are found in intestinal mucosa, accounting for first-pass metabolism of drugs such as chlorpromazine and clonazepam. As might be expected, these enzymes exhibit considerable genetic variability, which has implications for individual dosing regimens, and even more importantly, as determinants of therapeutic responsiveness and the risk of adverse events. CYP2D6, in particular, has been shown to exhibit genetic polymorphism.5 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 enzyme that O-demethylates and activates the drug. This reaction is CYP2D6-dependent. The frequency of this polymorphism is in part racially determined, with a prevalence of five to ten percent in European Caucasians as compared to less than two percent of Southeast Asians. Similar polymorphisms have been characterized for the CYP2C subfamily of isozymes. Although CYP3A4 exhibits a greater than ten-fold interindividual variability, no polymorphisms have been identified for this P450 isozyme.
Figure Some representative P450 isozymes.
c. Inducers: The cytochrome P450 dependent enzymes are an important target for pharmacokinetic drug interactions. One such interaction is the induction of selected CYP isozymes. Certain drugs, most notably phenobarbital, rifampin, and carbamazepine, are capable of increasing the synthesis of one or more CYP isozymes. This results in increased biotransformations of drugs and can lead to significant decreases in plasma concentrations of drugs metabolized by these CYP isozymes, as measured by AUC, with concurrent loss of pharmacologic effect. For example, rifampin, an antituberculosis drug , significantly decreases the plasma concentrations of human immunodeficiency virus (HIV) protease inhibitors,6 diminishing their ability to suppress HIV virion maturation.in figure above 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 metabolite is inactive,
3) increased drug activity if metabolite is active,
and 4) decreased therapeutic drug effect. In addition to drugs, natural substances and pollutants can also induce CYP isozymes. For example, polycyclic aromatic hydrocarbons (found as air pollutants) can induce CYP1A. This has implications for certain drugs; for example, amitriptyline and warfarin are metabolized by P4501A2. Polycyclic hydrocarbons induce P4501A2, which decreases the therapeutic concentrations of these agents
d. Inhibitors: Inhibition of CYP isozyme activity is an important source of drug interactions that leads 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 inhibition of coagulation and risk of hemorrhage and other serious bleeding reactions.
[Note: The more important CYP inhibitors are erythromycin, ketoconazole, and ritonavir, because they each inhibit several CYP isozymes.] Cimetidine blocks the metabolism of theophylline, clozapine, and warfarin. Natural substances such as grapefruit juice may inhibit drug metabolism. Grapefruit juice inhibits CYP3A4 and, thus, drugs such as amlodipine, clarithromycin, and indinavir, which are metabolized by this system, have greater amounts in the systemic circulationâ leading to higher blood levels and the potential to increase therapeutic and/or toxic effects of the drugs. Inhibition of drug metabolism may lead to increased plasma levels over time with long-term medications, prolonged pharmacological drug effect, and increased drug-induced toxicities.
e. 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 pravastatin in 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 too lipophilic to be retained in the kidney tubules. 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 most 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. Neonates are deficient in this conjugating system, making them particularly vulnerable to drugs such as chloramphenicol, which is inactivated by the addition of glucuronic acid .
[Note: Drugs already possessing an OH, HN2, or COOH group may enter Phase II directly and become conjugated without prior Phase I metabolism.] The highly polar drug conjugates may then be excreted by the kidney or bile.
3. Reversal of order of the phases: Not all drugs undergo Phase I and II reactions in that order. For example, isoniazid is first acetylated (a Phase II reaction) and then hydrolyzed to isonicotinic acid (a Phase I reaction).