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DRUG METABOLISM - Frank J. Gonzalez and Robert H. Tukey
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HOW HUMANS COPE WITH EXPOSURE TO XENOBIOTICS
The ability of humans to metabolize and clear drugs is a natural process that involves the same enzymatic pathways and transport systems that are utilized for normal metabolism of dietary constituents. Humans come into contact with scores of foreign chemicals or xenobiotics (substances foreign to the body) through exposure to environmental contaminants as well as in our diets. Fortunately, humans have developed a means to rapidly eliminate xenobiotics so they do not cause harm. In fact, one of the most common sources of xenobiotics in the diet is from plants that have many structurally diverse chemicals, some of which are associated with pigment production and others that are actually toxins (called phytoallexins) that protect plants against predators. A common example is poisonous mushrooms that have many toxins that are lethal to mammals, including amanitin, gyromitrin, orellanine, muscarine, ibotenic acid, muscimol, psilocybin, and coprine. Animals must be able to metabolize and eliminate such chemicals in order to consume vegetation. While humans can now choose their dietary source, a typical animal does not have this luxury and as a result is subject to its environment and the vegetation that exists in that environment. Thus, the ability to metabolize unusual chemicals in plants and other food sources is critical for survival. Drugs are considered xenobiotics and most are extensively metabolized in humans. It is worth noting that many drugs are derived from chemicals found in plants, some of which had been used in Chinese herbal medicines for thousands of years. Of the prescription drugs in use today for cancer treatment, many derive from plant species (see Chapter 51); investigating folklore claims led to the discovery of most of these drugs. It is therefore not surprising that animals utilize a means for disposing of human-made drugs that mimics the disposition of chemicals found in the diet. This capacity to metabolize xenobiotics, while mostly beneficial, has made development of drugs very time consuming and costly due in large part to (1) interindividual variations in the capacity of humans to metabolize drugs, (2) drug-drug interactions, and (3) species differences in expression of enzymes that metabolize drugs. The latter limits the use of animal models in drug development. A large number of diverse enzymes have evolved in animals that apparently only function to metabolize foreign chemicals. As will be discussed below, there are such large differences among species in the ability to metabolize xenobiotics that animal models cannot be relied upon to predict how humans will metabolize a drug. Enzymes that metabolize xenobiotics have historically been called drug-metabolizing enzymes, although they are involved in the metabolism of many foreign chemicals to which humans are exposed. Dietary differences among species during the course of evolution could account for the marked species variation in the complexity of the drug-metabolizing enzymes. Today, most xenobiotics to which humans are exposed come from sources that include environmental pollution, food additives, cosmetic products, agrochemicals, processed foods, and drugs. In general, these are lipophilic chemicals, that in the absence of metabolism would not be efficiently eliminated, and thus would accumulate in the body, resulting in toxicity. With very few exceptions, all xenobiotics are subjected to one or multiple pathways that constitute the phase 1 and phase 2 enzymatic systems. As a general paradigm, metabolism serves to convert these hydrophobic chemicals into derivatives that can easily be eliminated through the urine or the bile. In order to be accessible to cells and reach their sites of action, drugs generally must possess physical properties that allow them to move down a concentration gradient into the cell. Thus, most drugs are hydrophobic, a property that allows entry through the lipid bilayers into cells where drugs interact with their target receptors or proteins. Entry into cells is facilitated by a large number of transporters on the plasma membrane (see Chapter 2). This property of hydrophobicity would render drugs difficult to eliminate, since in the absence of metabolism, they would accumulate in fat and cellular phospholipid bilayers in cells. The xenobiotic-metabolizing enzymes convert drugs and xenobiotics into compounds that are hydrophilic derivatives that are more easily eliminated through excretion into the aqueous compartments of the tissues. Thus, the process of drug metabolism that leads to elimination plays a major role in diminishing the biological activity of a drug. For example, (S)-phenytoin, an anticonvulsant used in the treatment of epilepsy, is virtually insoluble in water. Metabolism by the phase 1 cytochrome P450 isoenzymes (CYPs) followed by phase 2 uridine diphosphate-glucuronosyltransferase (UGT) enzymes produces a metabolite that is highly water soluble and readily eliminated from the body (Figure 3-1). Metabolism also terminates the biological activity of the drug. In the case of phenytoin, metabolism also increases the molecular weight of the compound, which allows it to be eliminated more efficiently in the bile. While xenobiotic-metabolizing enzymes are responsible for facilitating the elimination of chemicals from the body, paradoxically these same enzymes can also convert certain chemicals to highly reactive toxic and carcinogenic metabolites. This occurs when an unstable intermediate is formed that has reactivity toward other compounds found in the cell. Chemicals that can be converted by xenobiotic metabolism to cancer-causing derivatives are called carcinogens. Depending on the structure of the chemical substrate, xenobiotic-metabolizing enzymes produce electrophilic metabolites that can react with nucleophilic cellular macromolecules such as DNA, RNA, and protein. This can cause cell death and organ toxicity. Reaction of these electrophiles with DNA can sometimes result in cancer through the mutation of genes such as oncogenes or tumor suppressor genes. It is generally believed that most human cancers are due to exposure to chemical carcinogens. This potential for carcinogenic activity makes testing the safety of drug candidates of vital importance. Testing for potential cancer-causing activity is particularly critical for drugs that will be used for the treatment of chronic diseases. Since each species has evolved a unique combination of xenobiotic-metabolizing enzymes, nonprimate rodent models cannot be solely used during drug development for testing the safety of new drug candidates targeted for human diseases. Nevertheless, testing in rodent models such as mice and rats can usually identify potential carcinogens. THE PHASES OF DRUG METABOLISM Xenobiotic metabolizing enzymes have historically been grouped into the phase 1 reactions, in which enzymes carry out oxidation, reduction, or hydrolytic reactions, and the phase 2 reactions, in which enzymes form a conjugate of the substrate (the phase 1 product) (Table 3-1). The phase 1 enzymes lead to the introduction of what are called functional groups, resulting in a modification of the drug, such that it now carries an -OH, -COOH, -SH, -O- or NH2 group. The addition of functional groups does little to increase the water solubility of the drug, but can dramatically alter the biological properties of the drug.

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