Pharmacology_Katzung

Pharmacology_Katzung. Some of the chemically dissimilar P450 substrate drugs, on repeated administration, induce P450 expression by enhancing the rate of its synthesis or reducing its rate of degradation (Table 4–2). Induction results in accelerated substrate metabolism and usually in a decrease in the pharmacologic action of the inducer and also of co-administered drugs. However, in the case of drugs metabolically transformed to reactive metabolites, enzyme induction may exacerbate metabolite-mediated toxicity. Various substrates induce P450 isoforms having different molecular masses and exhibiting different substrate specificities and immunochemical and spectral characteristics.


Pharmacology_Katzung

Pharmacology_Katzung. Various substrates induce P450 isoforms having different molecular masses and exhibiting different substrate specificities and immunochemical and spectral characteristics. Environmental chemicals and pollutants are also capable of inducing P450 enzymes. Exposure to benzo[a]pyrene and other polycyclic aromatic hydrocarbons, which are present in tobacco smoke, charcoal-broiled meat, and other organic pyrolysis products, is known to induce CYP1A enzymes and to alter the rates of drug metabolism. Other environmental chemicals known to induce specific P450s include the polychlorinated biphenyls (PCBs), which were once used widely in industry as insulating materials and plasticizers, and 2,3,7,8-tetrachlorodibenzo-pdioxin (dioxin, TCDD), a trace byproduct of the chemical synthesis of the defoliant 2,4,5-T (see Chapter 56).


Pharmacology_Katzung

Pharmacology_Katzung. Increased P450 synthesis requires enhanced transcription and translation along with increased synthesis of heme, its prosthetic cofactor. A cytoplasmic receptor (termed AhR) for polycyclic aromatic hydrocarbons (eg, benzo[a]pyrene, dioxin) has been identified. The translocation of the inducer-receptor complex into the nucleus, followed by ligand-induced dimerization with Arnt, a closely related nuclear protein, leads to subsequent activation of regulatory elements of CYP1A genes, resulting in their induction. This is also the mechanism of CYP1A induction by cruciferous vegetables, and the proton pump inhibitor, omeprazole. A pregnane X receptor (PXR), a member of the steroid-retinoidthyroid hormone receptor family, has recently been shown to mediate CYP3A induction by various chemicals (dexamethasone, rifampin, mifepristone, phenobarbital, atorvastatin, and hyperforin, a constituent of St. John’s wort) in the liver and intestinal mucosa. A similar receptor, the constitutive androstane


Pharmacology_Katzung

Pharmacology_Katzung. rifampin, mifepristone, phenobarbital, atorvastatin, and hyperforin, a constituent of St. John’s wort) in the liver and intestinal mucosa. A similar receptor, the constitutive androstane receptor (CAR), has been identified for the relatively large and structurally diverse phenobarbital class of inducers of CYP2B6, CYP2C9, and CYP3A4. Peroxisome proliferator receptor α (PPAR-α) is yet another nuclear receptor highly expressed in liver and kidneys, which uses lipid-lowering drugs (eg, fenofibrate and gemfibrozil) as ligands. Consistent with its major role in the regulation of fatty acid metabolism, PPAR-α mediates the induction of CYP4A enzymes, responsible for the metabolism of fatty acids such as arachidonic acid and its physiologically relevant derivatives. It is noteworthy that on binding of its particular ligand, PXR, CAR, and PPAR-α each forms heterodimers with another nuclear receptor, the retinoid X-receptor (RXR). This heterodimer in turn binds to response elements within the


Pharmacology_Katzung

Pharmacology_Katzung. of its particular ligand, PXR, CAR, and PPAR-α each forms heterodimers with another nuclear receptor, the retinoid X-receptor (RXR). This heterodimer in turn binds to response elements within the promoter regions of specific P450 genes to induce gene expression.


Pharmacology_Katzung

Pharmacology_Katzung. P450 enzymes may also be induced by substrate stabilization, eg, decreased degradation, as is the case with troleandomycinor clotrimazole-mediated induction of CYP3A enzymes, the ethanol-mediated induction of CYP2E1, and the isosafrole-mediated induction of CYP1A2. Certain drug substrates inhibit cytochrome P450 enzyme activity (Table 4–2). Imidazole-containing drugs such as cimetidine and ketoconazole bind tightly to the P450 heme iron and effectively reduce the metabolism of endogenous substrates (eg, testosterone) or other co-administered drugs through competitive inhibition. Macrolide antibiotics such as troleandomycin, erythromycin, and erythromycin derivatives are metabolized, apparently by CYP3A, to metabolites that complex the cytochrome P450 heme iron and render it catalytically inactive. Another compound that acts through this mechanism is the inhibitor proadifen (SKF-525-A, used in research), which binds tightly to the heme iron and TABLE 4–1 Phase I reactions.


Pharmacology_Katzung

Pharmacology_Katzung. TABLE 4–1 Phase I reactions. Cytochrome P450-dependent oxidations: Acetanilide, propranolol, phenobarbital, phenytoin, phenylbutazone, amphetamine, warfarin, 17˜-ethinyl estradiol, naphthalene, benzpyrene Amobarbital, pentobarbital, secobarbital, chlorpropamide, ibuprofen, meprobamate, glutethimide, phenylbutazone, digitoxin RCH2CH3 RCH2CH2OH RCH2CH3 RCHCH3 OH N-Dealkylation Morphine, ethylmorphine, benzphetamine, aminopyrine, ca˜eine, theophylline O-Dealkylation Codeine, p-nitroanisole RNHCH3 RNH2 + CH2O ROCH3 ROH + CH2O 6-Methylthiopurine, methitural N-Oxidation Primary amines Aniline, chlorphentermine Secondary amines 2-Acetylamino°uorene, acetaminophen RNH2 RNHOH NH R1 R2 N R1 R2 OH Tertiary amines Nicotine, methaqualone S-Oxidation Thioridazine, cimetidine, chlorpromazine S R1 R2 S R1 R2 O TABLE 4–1 Phase I reactions. (Continued) Flavin monooxygenase (Ziegler’s enzyme) Chlorpromazine, amitriptyline, benzphetamineR3N+˜O – R3N R3N+OH H+


Pharmacology_Katzung

Pharmacology_Katzung. TABLE 4–1 Phase I reactions. (Continued) Flavin monooxygenase (Ziegler’s enzyme) Chlorpromazine, amitriptyline, benzphetamineR3N+˜O – R3N R3N+OH H+ Methimazole, propylthiouracil N N SH N N SOH N N SO2H Phenylethylamine, epinephrine Carbonyl reductions Metyrapone, methadone, naloxoneRCR˜ RCHR˜O OH Esters Procaine, succinylcholine, aspirin, cloÿbrate, methylphenidate R1COOR2 R1COOH + R2OH RCOOH + R1NH2 Procainamide, lidocaine, indomethacin quasi-irreversibly inactivates the enzyme, thereby inhibiting the metabolism of potential substrates.


Pharmacology_Katzung

Pharmacology_Katzung. Some substrates irreversibly inhibit P450s via covalent interaction of a metabolically generated reactive intermediate that may react with the P450 apoprotein or heme moiety or even cause the heme to fragment and irreversibly modify the apoprotein. The antibiotic chloramphenicol is metabolized by CYP2B1 to a species that modifies the P450 protein and thus also inactivates the enzyme. A growing list of such suicide inhibitors—inactivators that attack the heme or the protein moiety—includes certain steroids (ethinyl estradiol, norethindrone, and spironolactone); fluroxene; allobarbital; the analgesic sedatives allylisopropylacetylurea, diethylpentenamide, and ethchlorvynol; carbon disulfide; grapefruit furanocoumarins; selegiline; phencyclidine; ticlopidine and clopidogrel; ritonavir; and propylthiouracil. On the other hand, the barbiturate secobarbital is found to inactivate CYP2B1 by modification of both its heme and protein moieties. Other metabolically activated drugs whose P450


Pharmacology_Katzung

Pharmacology_Katzung. propylthiouracil. On the other hand, the barbiturate secobarbital is found to inactivate CYP2B1 by modification of both its heme and protein moieties. Other metabolically activated drugs whose P450 inactivation mechanism is not fully elucidated are mifepristone, troglitazone, raloxifene, and tamoxifen.


Pharmacology_Katzung

Pharmacology_Katzung. TABLE 4–2 Human liver P450s (CYPs), and some of the drugs metabolized (substrates), inducers, and selective inhibitors. Note: Some P450 substrates can be potent competitive inhibitors and/or mechanism-based inactivators.


Pharmacology_Katzung

Pharmacology_Katzung. Amiodarone, amlodipine, clopidogrel, clotrimazole, desipramine, disulfiram, doxorubicin, ethinyl estradiol, fluoxetine, fluvoxamine, isoflurane, ketoconazole, mestranol, methimazole, nefazodone, nelfinavir, orphenadrine, paroxetine, phencyclidine, sertraline, thiotepa, ticlopidine 2C18 Tolbutamide, phenytoin Phenobarbital 2C19 Aripiprazole, carisoprodol, citalopram, clomipramine, clopidogrel, clozapine, desipramine, diazepam, diphenhydramine, doxepin, escitalopram, fluoxetine, imipramine, lansoprazole, S-mephenytoin, methadone, moclobemide, naproxen, nelfinavir, nirvanol, olanzapine, omeprazole, pantoprazole, phenobarbital, phenytoin, proguanil, propranolol, rabeprazole, sertraline, thalidomide, voriconazole, R-warfarin 2D6 Amitriptyline, atomoxetine, bufuralol, bupranolol, carvedilol, chlorpheniramine, chlorpromazine, clomipramine, clozapine, codeine, debrisoquine, desipramine, dextromethorphan, dihydrocodeine, encainide, flecainide, fluoxetine, fluvoxamine, guanoxan, haloperidol,


Pharmacology_Katzung

Pharmacology_Katzung. chlorpheniramine, chlorpromazine, clomipramine, clozapine, codeine, debrisoquine, desipramine, dextromethorphan, dihydrocodeine, encainide, flecainide, fluoxetine, fluvoxamine, guanoxan, haloperidol, hydrocodone, imipramine, maprotiline, 4–methoxy-amphetamine, metoclopramide, metoprolol, mexiletine, nebivolol, nortriptyline, oxycodone, palonosetron, paroxetine, perhexiline, perphenazine, phenformin, propafenone, propoxyphene, propranolol, risperidone, selegiline (deprenyl), sparteine, tamoxifen, thioridazine, timolol, tolterodine, tricyclic antidepressants, tramadol, trazodone, venlafaxine 2E1 Acetaminophen, chlorzoxazone, dacarbazine, enflurane, ethanol (a minor pathway), halothane, isoflurane, isoniazid, sevoflurane, theophylline, trimethadione Aminoglutethimide, artemisinin, barbiturates, carbamazepine, phenytoin, primidone, rifampin, rifapentine, St. John’s wort


Pharmacology_Katzung

Pharmacology_Katzung. Ethanol, isoniazid N3-Benzylnirvanol, N3-benzylphenobarbital, chloramphenicol, cimetidine, clopidogrel, delavirdine, efavirenz, esomeprazole, felbamate, fluconazole, fluoxetine, fluvoxamine, isoniazid, moclobemide, modafinil, nootkatone, omeprazole, ticlopidine, voriconazole Bupropion, cinacalcet, chloroquine, diphenhydramine, fluoxetine, haloperidol, imatinib, paroxetine, propafenone, propoxyphene, quinidine, terbinafine, thioridazine Amitriptyline, chlorpromazine, cimetidine, clomethiazole, clotrimazole, clozapine, disulfiram, diethylthiocarbamate, diallyl sulfide, econazole, methimazole, methoxsalen, 4-methylpyrazole, miconazole, modafinil, ritonavir, selegiline, sildenafil, sulconazole, ticlopidine, tioconazole TABLE 4–2 Human liver P450s (CYPs), and some of the drugs metabolized (substrates), inducers, and selective inhibitors. Note: Some P450 substrates can be potent competitive inhibitors and/or mechanism-based inactivators. (Continued)


Pharmacology_Katzung

Pharmacology_Katzung. CYP Substrates Inducers Inhibitors 3A41 Aminoglutethimide, avasimibe, barbiturates, carbamazepine, efavirenz, glucocorticoids, nevirapine, pioglitazone, phenytoin, primidone, rifampin, rifa-pentine, St. John’s wort


Pharmacology_Katzung

Pharmacology_Katzung. Acetaminophen, alfentanil, alfuzosin, almotriptan, alprazolam, amiodarone, amlodipine, aprepitant, astemizole, atazanavir, atorvastatin, bepridil, bexarotene, bosentan, bromocriptine, budesonide, buspirone, carbamazepine, cisapride, clarithromycin, clonazepam, clopidogrel, cocaine, colchicine, conivaptan, cortisol, cyclosporine, dapsone, darunavir, dasatinib, delavirdine, dexamethasone, diazepam, dihydroergotamine, dihydropyridines, diltiazem, disopyramide, doxorubicin, droperidol, dutasteride, ebastine, efavirenz, eletriptan, eplerenone, ergotamine, erlotinib, erythromycin, estazolam, eszopiclone, ethinyl estradiol, ethosuximide, etoposide, everolimus, exemestane, felodipine, fentanyl, finasteride, flurazepam, fluticasone, fosamprenavir, galantamine, gefitinib, gestodene, granisetron, halofantrine, ifosfamide, imatinib, indinavir, irinotecan, isradipine, itraconazole, ixabepilone, lapatinib, lidocaine, loperamide, lopinavir, loratadine, lovastatin, macrolides, maraviroc, mefloquine,


Pharmacology_Katzung

Pharmacology_Katzung. ifosfamide, imatinib, indinavir, irinotecan, isradipine, itraconazole, ixabepilone, lapatinib, lidocaine, loperamide, lopinavir, loratadine, lovastatin, macrolides, maraviroc, mefloquine, methadone, methylprednisolone, miconazole, midazolam, mifepristone, modafinil, nefazodone, nevirapine, nicardipine, nifedipine, nimodipine, nisoldipine, paclitaxel, paricalcitol, pimozide, pioglitazone, praziquantel, prednisolone, prednisone, progesterone, quetiapine, quinacrine, quinidine, quinine, ranolazine, rapamycin, repaglinide, rifabutin, ritonavir, saquinavir, sibutramine, sildenafil, simvastatin, sirolimus, solifenacin, spironolactone, sufentanil, sulfamethoxazole, sunitinib, tacrolimus, tadalafil, tamoxifen, tamsulosin, teniposide, terfenadine, testosterone, tetrahydrocannabinol, tiagabine, tinidazole, tipranavir, tolvaptan, topiramate, triazolam, troleandomycin, vardenafil, verapamil, vinblastine, vincristine, ziprasidone, zolpidem, zonisamide, zopiclone Amprenavir, azamulin, boceprevir,


Pharmacology_Katzung

Pharmacology_Katzung. tipranavir, tolvaptan, topiramate, triazolam, troleandomycin, vardenafil, verapamil, vinblastine, vincristine, ziprasidone, zolpidem, zonisamide, zopiclone Amprenavir, azamulin, boceprevir, clarithromycin, conivaptan, diltiazem, erythromycin, fluconazole, grapefruit juice (furanocoumarins), indinavir, itraconazole, ketoconazole, lopinavir, mibefradil, nefazodone, nelfinavir, posaconazole, ritonavir, saquinavir, telaprevir, telithromycin, troleandomycin, verapamil, voriconazole 1CYP3A5 has similar substrate and inhibitor profiles but, except for a few drugs, is generally less active than CYP3A4.


Pharmacology_Katzung

Pharmacology_Katzung. Parent drugs or their phase I metabolites that contain suitable chemical groups often undergo coupling or conjugation reactions with an endogenous substance to yield drug conjugates (Table 4–3). In general, conjugates are polar molecules that are readily excreted and often inactive. Conjugate formation involves high-energy intermediates and specific transfer enzymes. Such enzymes (transferases) may be located in microsomes or in the cytosol. Of these, uridine 5′-diphosphate (UDP)-glucuronosyl transferases (UGTs) are the most dominant enzymes (Figure 4–4). These microsomal enzymes catalyze the coupling of an activated endogenous substance (such as the UDP derivative of glucuronic acid) with a drug (or endogenous compound such as bilirubin, the end product of heme metabolism). Nineteen UGT genes (UGTA1 and UGT2) encode UGT proteins involved in the metabolism of drugs and xenobiotics. Similarly, 11 human sulfotransferases (SULTs) catalyze the sulfation of substrates using


Pharmacology_Katzung

Pharmacology_Katzung. Nineteen UGT genes (UGTA1 and UGT2) encode UGT proteins involved in the metabolism of drugs and xenobiotics. Similarly, 11 human sulfotransferases (SULTs) catalyze the sulfation of substrates using 3′-phosphoadenosine 5′-phosphosulfate (PAPS) as the endogenous sulfate donor. Cytosolic and microsomal glutathione (GSH) transferases (GSTs) are also engaged in the metabolism of drugs and xenobiotics, and in that of leukotrienes and prostaglandins, respectively. Chemicals containing an aromatic amine or a hydrazine moiety (eg, isoniazid) are substrates of cytosolic N-acetyltransferases (NATs), encoded by NAT1 and NAT2 genes, which utilize acetyl-CoA as the endogenous cofactor.


Pharmacology_Katzung

Pharmacology_Katzung. S-Adenosyl-l-methionine (SAMe; AdoMet)-mediated O-, N-, and S-methylation of drugs and xenobiotics by methyltransferases (MTs) also occurs. Finally, endobiotic, drug, and xenobiotic epoxides generated via P450-catalyzed oxidations can also be hydrolyzed by microsomal or cytosolic epoxide hydrolases (EHs). Conjugation of an activated drug such as the S-CoA derivative of benzoic acid, with an endogenous substrate, such as glycine, also occurs. Because the endogenous substrates originate in the diet, nutrition plays a critical role in the regulation of drug conjugations. Phase II reactions are relatively faster than P450-catalyzed reactions, thus effectively accelerating drug biotransformation.


Pharmacology_Katzung

Pharmacology_Katzung. Phase II reactions are relatively faster than P450-catalyzed reactions, thus effectively accelerating drug biotransformation. Drug conjugations were once believed to represent terminal inactivation events and as such have been viewed as “true detoxification” reactions. However, this concept must be modified, because it is now known that certain conjugation reactions (acyl glucuronidation of nonsteroidal anti-inflammatory drugs, O-sulfation of N-hydroxyacetylaminofluorene, and N-acetylation of isoniazid) may lead to the formation of reactive species responsible for the toxicity of the drugs. Furthermore, sulfation is known to activate the orally active prodrug minoxidil into a very efficacious vasodilator, and morphine-6-glucuronide is more potent than morphine itself.


Pharmacology_Katzung

Pharmacology_Katzung. FIGURE 4–4 Relative contributions of various cytochrome P450 isoforms (A) and different phase II pathways (B) to metabolism of drugs in clinical use. Many drugs are metabolized by two or more of these pathways. Note that two pathways, CYP3A4/5 and UGT, are involved in the metabolism of more than 75% of drugs in use. DPYD, dihydropyrimidine dehydrogenase; GST, glutathione-S-transferase; NAT, N-acetyltransferase; SULT, sulfotransferase; TPMT, thiopurine methyltransferase; UGT, UDP-glucuronosyltransferase. (Reproduced, with permission, from Brunton LL, Chabner BA, Knollman BC: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 12th ed. McGraw-Hill, 2011. Copyright © The McGraw-Hill Companies, Inc.) TABLE 4–3 Phase II reactions.


Pharmacology_Katzung

Pharmacology_Katzung. Metabolism of drugs and other foreign chemicals may not always be an innocuous biochemical event leading to detoxification and elimination of the compound. Indeed, as previously noted, several compounds have been shown to be metabolically transformed to reactive intermediates that are toxic to various organs. Such toxic reactions may not be apparent at low levels of exposure to parent compounds when alternative detoxification mechanisms are not yet overwhelmed or compromised and when the availability of endogenous detoxifying cosubstrates (GSH, glucuronic acid, sulfate) is not limited. However, when these resources are exhausted, the toxic pathway may prevail, resulting in overt organ toxicity or carcinogenesis. The number of specific examples of such drug-induced toxicity is expanding rapidly. An example is acetaminophen (APAP; paracetamol)-induced hepatotoxicity (Figure 4–5). Acetaminophen, an analgesic antipyretic drug, is quite safe in therapeutic doses (1.2 g/d for an adult). It


Pharmacology_Katzung

Pharmacology_Katzung. An example is acetaminophen (APAP; paracetamol)-induced hepatotoxicity (Figure 4–5). Acetaminophen, an analgesic antipyretic drug, is quite safe in therapeutic doses (1.2 g/d for an adult). It normally undergoes glucuronidation and sulfation to the corresponding conjugates, which together make up 95% of the total excreted metabolites. The alternative P450-dependent GSH conjugation pathway accounts for the remaining 5%. When acetaminophen intake far exceeds therapeutic doses, the glucuronidation and sulfation pathways are saturated, and the P450-dependent pathway becomes increasingly important. Little or no hepatotoxicity results as long as hepatic GSH is available for conjugation. However, with time, hepatic GSH is depleted faster than it can be regenerated, and a reactive, toxic metabolite accumulates. In the absence of intracellular nucleophiles such as GSH, this


Pharmacology_Katzung

Pharmacology_Katzung. FIGURE 4–5 Metabolism of acetaminophen (top center) to hepatotoxic metabolites. GSH, glutathione; SG, glutathione moiety. reactive metabolite (N-acetylbenzoiminoquinone) not only reacts with nucleophilic groups of cellular proteins resulting in direct hepatocellular damage, but also participates in redox cycling, thereby generating reactive O2 species (ROS) and consequent oxidative stress that greatly enhance acetaminophen-induced hepatotoxicity. The chemical and toxicologic characterization of the electrophilic nature of the reactive acetaminophen metabolite has led to the development of effective antidotes—cysteamine and N-acetylcysteine (NAC; Acetadote; Mucomyst). Administration of N-acetylcysteine (the safer of the two) within 8–16 hours after acetaminophen overdosage has been shown to protect victims from fulminant hepatotoxicity and death (see Chapter 58). Administration of GSH is not effective because it does not cross cell membranes readily.


Pharmacology_Katzung

Pharmacology_Katzung. The dose and frequency of administration required to achieve effective therapeutic blood and tissue levels vary in different patients because of individual differences in drug distribution and rates of drug metabolism and elimination. These differences are determined by genetic factors as well as nongenetic variables, such as commensal gut microbiota, age, sex, liver size, liver function, circadian rhythm, body temperature, and nutritional and environmental factors such as concomitant exposure to inducers or inhibitors of drug metabolism. The discussion that follows summarizes the most important of these variables. Individual differences in metabolic rate depend on the nature of the drug itself. Thus, within the same population, steady-state plasma levels may reflect a 30-fold variation in the metabolism of one drug and only a twofold variation in the metabolism of another.


Pharmacology_Katzung

Pharmacology_Katzung. Genetic factors that influence enzyme levels account for some of these differences, giving rise to “genetic polymorphisms” in drug metabolism (see also Chapter 5). The first examples of drugs found to be subject to genetic polymorphisms were the muscle relaxant succinylcholine, the antituberculosis drug isoniazid, and the anticoagulant warfarin. A true genetic polymorphism is defined as the occurrence of a variant allele of a gene at a population frequency of ≥ 1%, resulting in altered expression or functional activity of the gene product, or both. Well-defined and clinically relevant genetic polymorphisms in both phase I and phase II drug-metabolizing enzymes exist that result in altered efficacy of drug therapy or adverse drug reactions (ADRs). The latter frequently necessitate dose adjustment (Table 4–4), a consideration particularly crucial for drugs with low therapeutic indices. A. Phase I Enzyme Polymorphisms


Pharmacology_Katzung

Pharmacology_Katzung. A. Phase I Enzyme Polymorphisms Genetically determined defects in the phase I oxidative metabolism of several drugs have been reported (Table 4–4; see also Chapter 5). These defects are often transmitted as autosomal recessive traits and may be expressed at any one of the multiple metabolic transformations that a chemical might undergo. Human liver P450s 3A4, 2C9, 2D6, 2C19, 1A2, and 2B6 are responsible for about 75% of all clinically relevant phase I drug metabolism (Figure 4–4), and thus for about 60% of all physiologic drug biotransformation and elimination. Thus, genetic polymorphisms of these enzymes, by significantly influencing phase I drug metabolism, can alter their pharmacokinetics and the magnitude or the duration of drug response and associated events.


Pharmacology_Katzung

Pharmacology_Katzung. Three P450 genetic polymorphisms have been particularly well characterized, affording some insight into possible underlying molecular mechanisms, and are clinically noteworthy, as they require therapeutic dosage adjustment. The first is the debrisoquin-sparteine oxidation type of polymorphism, which TABLE 4–4 Some examples of genetic polymorphisms in phase I and phase II drug metabolism. TABLE 4–4 Some examples of genetic polymorphisms in phase I and phase II drug metabolism. (Continued) 1Observed or predictable. ADR, adverse drug reaction; EM, extensive metabolizer; PM, poor metabolizer; UM, ultrarapid metabolizer.


Pharmacology_Katzung

Pharmacology_Katzung. apparently occurs in 3–10% of Caucasians and is inherited as an correlates with a higher risk of relapse in patients with breast autosomal recessive trait. In affected individuals, the CYP2D6-cancer treated with tamoxifen, an anticancer drug that relies on its dependent oxidations of debrisoquin and other drugs (Table 4–2; CYP2D6-dependent metabolic activation to endoxifen for its effi-Figure 4–6) are impaired. These defects in oxidative drug metabo-cacy. More recently, however, another polymorphic genotype has lism are probably co-inherited. The precise molecular basis for the been reported that results in ultrarapid metabolism of relevant defect appears to be faulty expression of the P450 protein due to drugs due to the presence of CYP2D6 allelic variants with up to either defective mRNA splicing or protein folding, resulting in 13 gene copies in tandem. This ultrarapid metabolizer (UM) gen-little or no isoform-catalyzed drug metabolism and thereby con-otype is most common in


Pharmacology_Katzung

Pharmacology_Katzung. mRNA splicing or protein folding, resulting in 13 gene copies in tandem. This ultrarapid metabolizer (UM) gen-little or no isoform-catalyzed drug metabolism and thereby con-otype is most common in Ethiopians and Saudi Arabians, populaferring a poor metabolizer (PM) phenotype. This PM phenotype tions that display it in up to one third of individuals. As a result, –2.0 –1.0 0 1.0 2.0 Log10 metabolic ratio


Pharmacology_Katzung

Pharmacology_Katzung. FIGURE 4–6 Genetic polymorphism in debrisoquin 4-hydroxylation by CYP2D6 in a Caucasian population. The semilog frequency distribution histogram of the metabolic ratio (MR; defined as percent of dose excreted as unchanged debrisoquin divided by the percent of dose excreted as 4-hydroxydebrisoquin metabolite) in the 8-hour urine collected after oral ingestion of 12.8 mg debrisoquin sulfate (equivalent to 10 mg free debrisoquin base). Individuals with MR values >12.6 were phenotyped as poor metabolizers (PM, red bars), and those with MR values <12.6 but >0.2 were designated as extensive metabolizers (EM, blue bars). Those with MR values <0.2were designated as ultrarapid metabolizers (URM, green bars) based on the MR values (0.01–0.1) of individuals with documented multiple copies of CYP2D6 allelic variants resulting from inherited amplification of this gene. (Data from Woolhouse et al: Debrisoquin hydroxylation polymorphism among Ghanians and Caucasians. Clin Pharmacol Ther


Pharmacology_Katzung

Pharmacology_Katzung. of CYP2D6 allelic variants resulting from inherited amplification of this gene. (Data from Woolhouse et al: Debrisoquin hydroxylation polymorphism among Ghanians and Caucasians. Clin Pharmacol Ther 1979;26:584.) these subjects require twofold to threefold higher daily doses of nortriptyline (an antidepressant and a CYP2D6 substrate) to achieve therapeutic plasma levels. The poor responsiveness to antidepressant therapy of the UM phenotype also clinically correlates with a higher incidence of suicides relative to that of deaths due to natural causes in this patient population. Conversely, in these UM populations, the prodrug codeine (another CYP2D6 substrate) is metabolized much faster to morphine, often resulting in undesirable adverse effects of morphine, such as abdominal pain. Indeed, intake of high doses of codeine by a mother of the ultrarapid metabolizer type was held responsible for the morphine-induced death of her breast-fed infant.


Pharmacology_Katzung

Pharmacology_Katzung. The second well-studied genetic drug polymorphism involves the stereoselective aromatic (4)-hydroxylation of the anticonvulsant mephenytoin, catalyzed by CYP2C19. This polymorphism, which is also inherited as an autosomal recessive trait, occurs in 3–5% of Caucasians and 18–23% of Japanese populations. It is genetically independent of the debrisoquin-sparteine polymorphism. In normal “extensive metabolizers” (EMs) (S)-mephenytoin is extensively hydroxylated by CYP2C19 at the 4 position of the phenyl ring before its glucuronidation and rapid excretion in the urine, whereas (R)-mephenytoin is slowly N-demethylated to nirvanol, an active metabolite. PMs, however, appear to totally lack the stereospecific (S)-mephenytoin hydroxylase activity, so both (S)and (R)-mephenytoin enantiomers are N-demethylated to nirvanol, which accumulates in much higher concentrations. Thus, PMs of mephenytoin show signs of profound sedation and ataxia after doses of the drug that are well tolerated by normal


Pharmacology_Katzung

Pharmacology_Katzung. to nirvanol, which accumulates in much higher concentrations. Thus, PMs of mephenytoin show signs of profound sedation and ataxia after doses of the drug that are well tolerated by normal metabolizers. Two defective CYP2C19 variant alleles (CYP2C19*2 and CYP2C19*3), the latter predominant in Asians, are largely responsible for the PM genotype. The molecular bases include splicing defects resulting in a truncated, nonfunctional protein. CYP2C19 is responsible for the metabolism of various clinically relevant drugs (Table 4–4). Thus, it is clinically important to recognize that the safety of each of these drugs may be severely reduced in persons with the PM phenotype. On the other hand, the PM phenotype can notably increase the therapeutic efficacy of omeprazole, a proton-pump inhibitor, in gastric ulcer and gastroesophageal reflux diseases (see Chapter 5 for additional discussion of the CYP2C19 polymorphism).


Pharmacology_Katzung

Pharmacology_Katzung. Another CYP2C19 variant allele (CYP2C19*17) exists that is associated with increased transcription and thus higher CYP2C19 expression and even higher functional activity than that of the wild type CYP2C19-carrying EMs. Individuals carrying this CYP2C19*17 allele exhibit higher metabolic activation of pro-drugs such as the breast cancer drug tamoxifen, the antimalarial chlorproguanil, and the antiplatelet drug clopidogrel. The former event is associated with a lower risk of breast cancer relapse, and the latter event with an increased risk of bleeding. Carriers of the CYP2C19*17 allele are also known to enhance the metabolism and thus the elimination of drugs such as the antidepressants escitalopram and imipramine, as well as the antifungal voriconazole. This consequently impairs the therapeutic efficacy of these drugs, thus requiring clinical dosage adjustments.


Pharmacology_Katzung

Pharmacology_Katzung. The third relatively well-characterized genetic polymorphism is that of CYP2C9. Two well-characterized variants of this enzyme exist, each with amino acid mutations that result in altered metabolism. The CYP2C9*2 allele encodes an Arg144Cys mutation, exhibiting impaired functional interactions with POR. The other allelic variant, CYP2C9*3, encodes an enzyme with an Ile359Leu mutation that has lowered affinity for many substrates. For example, individuals displaying the CYP2C9*3 phenotype have greatly reduced tolerance for the anticoagulant warfarin. The warfarin clearance in CYP2C9*3-homozygous individuals is about 10% of normal values, and these people have a much lower tolerance for the drug than those who are homozygous for the normal wild type allele. These individuals also have a much higher risk of adverse effects with warfarin (eg, bleeding) and with other CYP2C9 substrates such as phenytoin, losartan, tolbutamide, and some nonsteroidal anti-inflammatory drugs (Table 4–4).


Pharmacology_Katzung

Pharmacology_Katzung. much higher risk of adverse effects with warfarin (eg, bleeding) and with other CYP2C9 substrates such as phenytoin, losartan, tolbutamide, and some nonsteroidal anti-inflammatory drugs (Table 4–4). Note, however, that despite the predominant role of CYP2C9 in warfarin clearance (particularly that of its pharmacologically more potent S-isomer), warfarin maintenance doses are largely dictated by polymorphisms in the VKORC1 gene responsible for the expression of vitamin K epoxide reductase, the specific cellular target of warfarin, rather than by CYP2C9*2/*3 polymorphisms alone (see Chapter 5).


Pharmacology_Katzung

Pharmacology_Katzung. Allelic variants of CYP3A4 have also been reported, but their contribution to the well-known interindividual variability in drug metabolism apparently is limited. On the other hand, the expression of CYP3A5, another human liver isoform, is markedly polymorphic, ranging from 0% to 100% of the total hepatic CYP3A content. This CYP3A5 protein polymorphism is now known to result from a single nucleotide polymorphism (SNP) within intron 3, which enables normally spliced CYP3A5 transcripts in 5% of Caucasians, 29% of Japanese, 27% of Chinese, 30% of Koreans, and 73% of African Americans. Thus, it can significantly contribute to inter-individual differences in the metabolism of preferential CYP3A5 substrates such as midazolam. Two other CYP3A5 allelic variants that result in a PM phenotype are also known.


Pharmacology_Katzung

Pharmacology_Katzung. Polymorphisms in the CYP2A6 gene have also been recently characterized, and their prevalence is apparently racially linked. CYP2A6 is responsible for nicotine oxidation, and tobacco smokers with low CYP2A6 activity consume less and have a lower incidence of lung cancer. CYP2A6 1B allelic variants associated with faster rates of nicotine metabolism have been recently discovered. It remains to be determined whether patients with these faster variants will fall into the converse paradigm of increased smoking behavior and lung cancer incidence.


Pharmacology_Katzung

Pharmacology_Katzung. Additional genetic polymorphisms in drug metabolism are being discovered. Of these, the gene for CYP2B6 has become noteworthy as one of the most polymorphic P450 genes, with a 20to 250-fold variation in interindividual CYP2B6 expression. Despite its low (1–5%) contribution to the total liver P450 content, these CYP2B6 polymorphisms may have a significant impact on the CYP2B6-dependent metabolism of several clinically relevant drugs such as cyclophosphamide, S-methadone, efavirenz, nevirapine, bupropion, selegiline, and propofol. Of clinical relevance, women (particularly Hispanic-American women) express considerably higher hepatic levels of CYP2B6 protein than men.


Pharmacology_Katzung

Pharmacology_Katzung. Studies of theophylline metabolism in monozygotic and dizygotic twins that included pedigree analysis of various families have revealed that a distinct polymorphism may exist for this drug and may be inherited as a recessive genetic trait. Genetic drug metabolism polymorphisms also appear to occur for aminopyrine and carbocysteine oxidations. Regularly updated information on human P450 polymorphisms is available at http://www.cypalleles.ki.se/.


Pharmacology_Katzung

Pharmacology_Katzung. Although genetic polymorphisms in drug oxidations often involve specific P450 enzymes, such genetic variations can also occur in other enzymes. Recently, genetic polymorphisms in POR, the essential P450 electron donor, have been reported. In particular, an allelic variant (at a 28% frequency) encoding a POR A503V mutation has been reported to result in impaired CYP17dependent sex steroid synthesis and impaired CYP3A4and CYP2D6-dependent drug metabolism in vitro. Its involvement in clinically relevant drug metabolism, while predictable, remains to be established. Descriptions of a polymorphism in the oxidation of trimethylamine, believed to be metabolized largely by the flavin monooxygenase (Ziegler’s enzyme), result in the “fishodor syndrome” in slow metabolizers, thus suggesting that genetic variants of other non-P450-dependent oxidative enzymes may also contribute to such polymorphisms. B. Phase II Enzyme Polymorphisms


Pharmacology_Katzung

Pharmacology_Katzung. Succinylcholine is metabolized only half as rapidly in persons with genetically determined deficiency in pseudocholinesterase (now generally referred to as butyrylcholinesterase [BCHE]) as in persons with normally functioning enzyme. Different mutations, inherited as autosomal recessive traits, account for the enzyme deficiency. Deficient individuals treated with succinylcholine as a surgical muscle relaxant may become susceptible to prolonged respiratory paralysis (succinylcholine apnea). Similar pharmacogenetic differences are seen in the acetylation of isoniazid. The defect in slow acetylators (of isoniazid and similar amines) appears to be caused by the synthesis of less of the NAT2 enzyme rather than of an abnormal form of it. Inherited as an autosomal recessive trait, the slow acetylator phenotype occurs in about 50% of blacks and whites in the USA, more frequently in Europeans living in high northern latitudes, and much less commonly in Asians and Inuit (Eskimos). The slow


Pharmacology_Katzung

Pharmacology_Katzung. phenotype occurs in about 50% of blacks and whites in the USA, more frequently in Europeans living in high northern latitudes, and much less commonly in Asians and Inuit (Eskimos). The slow acetylator phenotype is also associated with a higher incidence of isoniazid-induced peripheral neuritis, drug-induced autoimmune disorders, and bicyclic aromatic amine-induced bladder cancer.


Pharmacology_Katzung

Pharmacology_Katzung. A clinically important polymorphism of the TPMT (thiopurine S-methyltransferase) gene is encountered in Europeans (frequency, 1:300), resulting in a rapidly degraded mutant enzyme and consequently deficient S-methylation of aromatic and heterocyclic sulfhydryl compounds including the anti-cancer thiopurine drugs 6-mercaptopurine, thioguanine, and azathioprine, required for their detoxification. Patients inheriting this polymorphism as an autosomal recessive trait are at high risk of thiopurine drug-induced fatal hematopoietic toxicity.


Pharmacology_Katzung

Pharmacology_Katzung. Genetic polymorphisms in the expression of other phase II enzymes (UGTs and GSTs) also occur. Thus, UGT polymorphisms (UGT1A1*28) are associated with hyperbilirubinemic diseases (Gilbert’s syndrome) as well as toxic effects due to impaired drug conjugation and/or elimination (eg, the anticancer drug irinotecan). Similarly, genetic polymorphisms (GSTM1) in GST (mu1 isoform) expression can lead to significant adverse effects and toxicities of drugs dependent on its GSH conjugation for elimination. C. Role of Pharmacogenomic Testing in Clinically Safe & Effective Drug Therapy


Pharmacology_Katzung

Pharmacology_Katzung. Despite our improved understanding of the molecular basis of pharmacogenetic defects in drug-metabolizing enzymes, their impact on drug therapy and ADRs, and the availability of validated pharmacogenetic biomarkers to identify patients at risk, this clinically relevant information has not been effectively translated to patient care. Thus, the much-heralded potential for personalized medicine, except in a few instances of drugs with a relatively low therapeutic index (eg, warfarin), has remained largely unrealized. This is so even though 98% of US physicians are apparently aware that such genetic information may significantly influence therapy. This is partly due to the lack of adequate training in translating this knowledge to medical practice, and partly due to the logistics of genetic testing and the issue of cost-effectiveness. Severe ADRs are known to contribute to 100,000 annual US deaths, about 7% of all hospital admissions, and an increased average length of hospital stay.


Pharmacology_Katzung

Pharmacology_Katzung. testing and the issue of cost-effectiveness. Severe ADRs are known to contribute to 100,000 annual US deaths, about 7% of all hospital admissions, and an increased average length of hospital stay. Genotype information could greatly enhance safe and efficacious clinical therapy through dose adjustment or alternative drug therapy, thereby curbing much of the rising ADR incidence and its associated costs. (See Chapter 5 for further discussion.)


Pharmacology_Katzung

Pharmacology_Katzung. It is increasingly recognized that the human gut microbiome can also significantly influence drug responses. It thus serves as another relevant source of therapeutic misadventures and adverse drug-drug interactions. More than 1000 species of intestinal microorganisms have been identified, including obligate anaerobic bacteria and various yeasts that coexist in a dynamic, often symbiotic, ecological equilibrium. Their biotransformation repertoire is nonoxidative, albeit highly versatile, extending from predominantly reductive and hydrolytic reactions to decarboxylation, dehydroxylation, dealkylation, dehalogenation, and deamination. Notably, such bacterially mediated reduction of the cardiac drug digoxin significantly contributes to its metabolism and elimination. Co-treatment with antibiotics such as erythromycin or tetracycline increases digoxin serum levels twofold, increasing the risk of cardiotoxicity. Similarly, drugs that are primarily glucuronidated in the liver are excreted


Pharmacology_Katzung

Pharmacology_Katzung. such as erythromycin or tetracycline increases digoxin serum levels twofold, increasing the risk of cardiotoxicity. Similarly, drugs that are primarily glucuronidated in the liver are excreted into the gut via the bile, whereupon they are subjected to de-glucuronidation by gut microbial β-glucuronidases (hydrolases). The pharmacologically active parent aglycone is subsequently reabsorbed into the portal circulation with consequent extension of its pharmacologic action and hepatic phase II reconjugation and subsequent enterohepatic recycling. Thus, if the parent drug is dosage limited or has a low therapeutic index, this may mean increased toxicity. For example, under normal dosage, the analgesic acetaminophen is largely metabolized via glucuronidation and sulfation, as discussed earlier, and eliminated into the hepatic sinusoidal plasma. However, upon overdosage, the increased production of these metabolites is quite likely to saturate their normal excretory transport process. Their


Pharmacology_Katzung

Pharmacology_Katzung. and eliminated into the hepatic sinusoidal plasma. However, upon overdosage, the increased production of these metabolites is quite likely to saturate their normal excretory transport process. Their consequently enhanced biliary excretion would subject a greater fraction of the acetaminophen-glucuronide to de-glucuronidation by intestinal microbial β-glucuronidases, which may further contribute to the toxic acetaminophen burden. This possibility is even more relevant for glucuronides of parent drugs of noted gastrointestinal toxicity. Accordingly, selective inhibition of microbial β-glucuronidases has been documented to alleviate the gastrointestinal toxicity of anticancer drugs such as irinotecan, as well as the enteropathies induced by nonsteroidal anti-inflammatory drugs (NSAIDs) such as indomethacin, ketoprofen or diclofenac, that incur substantial enterohepatic circulation. This possibility has fueled the pharmaceutical design and development of even more selective inhibitors


Pharmacology_Katzung

Pharmacology_Katzung. as indomethacin, ketoprofen or diclofenac, that incur substantial enterohepatic circulation. This possibility has fueled the pharmaceutical design and development of even more selective inhibitors targeted against microbial β-glucuronidases.


Pharmacology_Katzung

Pharmacology_Katzung. Diet and environmental factors contribute to individual variations in drug metabolism. Charcoal-broiled foods and cruciferous vegetables are known to induce CYP1A enzymes, whereas grapefruit juice is known to inhibit the CYP3A metabolism of co-administered drug substrates (Table 4–2; also see below). Cigarette smokers metabolize some drugs more rapidly than nonsmokers because of enzyme induction (see previous section). Industrial workers exposed to some pesticides metabolize certain drugs more rapidly than unexposed individuals. Such differences make it difficult to determine effective and safe doses of drugs that have narrow therapeutic indices.


Pharmacology_Katzung

Pharmacology_Katzung. Increased susceptibility to the pharmacologic or toxic activity of drugs has been reported in very young and very old patients compared with young adults (see Chapters 59 and 60). Although this may reflect differences in absorption, distribution, and excretion, differences in drug metabolism also play a role. Slower metabolism could be due to reduced activity of metabolic enzymes or reduced availability of essential endogenous cofactors. Sex-dependent variations in drug metabolism have been well documented in rats but not in other rodents. Young adult male rats metabolize drugs much faster than mature female rats or prepubertal male rats. These differences in drug metabolism have been clearly associated with androgenic hormones. Clinical reports suggest that similar sex-dependent differences in drug metabolism also exist in humans for ethanol, propranolol, some benzodiazepines, estrogens, and salicylates.


Pharmacology_Katzung

Pharmacology_Katzung. Many substrates, by virtue of their relatively high lipophilicity, are not only retained at the active site of the enzyme but remain non-specifically bound to the lipid endoplasmic reticulum membrane. In this state, they may induce microsomal enzymes, particularly after repeated use. Acutely, depending on the residual drug levels at the active site, they also may competitively inhibit metabolism of a simultaneously administered drug.


Pharmacology_Katzung

Pharmacology_Katzung. Enzyme-inducing drugs include various sedative-hypnotics, antipsychotics, anticonvulsants, the antitubercular drug rifampin, and insecticides (Table 4–5). Patients who routinely ingest barbiturates, other sedative-hypnotics, or certain antipsychotic drugs may require considerably higher doses of warfarin to maintain a therapeutic effect. On the other hand, discontinuance of the sedative inducer may result in reduced metabolism of the anticoagulant and bleeding—a toxic effect of the ensuing enhanced plasma levels of the anticoagulant. Similar interactions have been observed in individuals receiving various combinations of drug regimens such as rifampin, antipsychotics, or sedatives with contraceptive agents, sedatives with anticonvulsant drugs, and even alcohol with hypoglycemic drugs (tolbutamide). One inducer of note is St. John’s wort, a popular over-the-counter herbal medicine ingested as treatment for mild to severe depression. Because of its marked induction of hepatic CYP3A4


Pharmacology_Katzung

Pharmacology_Katzung. One inducer of note is St. John’s wort, a popular over-the-counter herbal medicine ingested as treatment for mild to severe depression. Because of its marked induction of hepatic CYP3A4 and, to a lesser extent, CYP2C9 and CYP2C19, St. John’s wort has been linked to a large number of DDIs. Most of such DDIs stem from P450 induction by St. John’s wort and entail accelerated P450-dependent metabolism of the co-ingested drug (eg, alprazolam, contraceptive estrogens, warfarin, lovastatin, delavirdine, ritonavir). In contrast, St. John’s wort-mediated CYP2C19 induction may enhance the activation of the antiplatelet prodrug clopidogrel by accelerating its conversion to the active metabolite. Finally, some St. John’s wort-elicited DDIs may entail decreased P450-dependent metabolism due to competitive inhibition and consequently increased plasma levels and clinical effect (eg, meperidine, hydrocodone, morphine, oxycodone). Other DDIs entail synergistic increases in serotonin levels (due to


Pharmacology_Katzung

Pharmacology_Katzung. inhibition and consequently increased plasma levels and clinical effect (eg, meperidine, hydrocodone, morphine, oxycodone). Other DDIs entail synergistic increases in serotonin levels (due to monoamine oxidase inhibition) and correspondingly increased serotonergic tone and adverse effects (eg, paroxetine, sertraline, fluoxetine, fenfluramine).


Pharmacology_Katzung

Pharmacology_Katzung. It must also be noted that an inducer may enhance not only the metabolism of other drugs but also its own metabolism. TABLE 4–5 Partial list of drugs that enhance drug TABLE 4–6 Partial list of drugs that inhibit drug metabolism in humans. metabolism in humans. 1Secobarbital is an exception. See Table 4–6 and text. 2With chronic (repeated) administration; acutely, ritonavir is a potent CYP3A4 inhibitor/inactivator. 3For a more comprehensive list of drugs whose metabolism is enhanced by St. John’s wort, see Rahimi and Abdollahi, 2012; Russo et al, 2014; and Tsai et al, 2012. Thus, continued use of some drugs may result in a pharmacokinetic type of tolerance—progressively reduced therapeutic effectiveness due to enhancement of their own metabolism.


Pharmacology_Katzung

Pharmacology_Katzung. Conversely, simultaneous administration of two or more drugs may result in impaired elimination of the more slowly metabolized drug and prolongation or potentiation of its pharmacologic effects (Table 4–6). Both competitive substrate inhibition and irreversible substrate-mediated enzyme inactivation may augment plasma drug levels and lead to toxic effects from drugs with narrow therapeutic indices. Indeed, such acute interactions of terfenadine (a second-generation antihistamine) with a CYP3A4 substrate-inhibitor (ketoconazole, erythromycin, or grapefruit juice) resulted in fatal cardiac arrhythmias (torsades de pointes) requiring its withdrawal from the market. Similar DDIs with CYP3A4 substrate-inhibitors (such as the antibiotics erythromycin and clarithromycin, the antidepressant nefazodone, the antifungals itraconazole and ketoconazole, and the HIV protease inhibitors indinavir and ritonavir) and consequent cardiotoxicity


Pharmacology_Katzung

Pharmacology_Katzung. Troleandomycin Theophylline, methylprednisolone 1While some inhibitors are selective for a given P450 enzyme, others are more general and can inhibit several P450s concurrently. 2Active components in grapefruit juice include furanocoumarins such as 6′, 7′-dihydroxybergamottin (which inactivates both intestinal and liver CYP3A4) as well as other unknown components that inhibit P-glycoprotein-mediated intestinal drug efflux and consequently further enhance the bioavailability of certain drugs such as cyclosporine. For a more comprehensive list of drugs whose metabolism is inhibited by grapefruit juice furanocoumarins, see Bailey et al, 2013.


Pharmacology_Katzung

Pharmacology_Katzung. led to withdrawal or restricted use of the 5-HT4 agonist cisapride. Similarly, allopurinol both prolongs the duration and enhances the chemotherapeutic and toxic actions of mercaptopurine by competitive inhibition of xanthine oxidase. Consequently, to avoid bone marrow toxicity, the dose of mercaptopurine must be reduced in patients receiving allopurinol. Cimetidine, a drug used in the treatment of peptic ulcer, has been shown to potentiate the pharmacologic actions of anticoagulants and sedatives. The metabolism of the sedative chlordiazepoxide has been shown to be inhibited by 63% after a single dose of cimetidine; such effects are reversed within 48 hours after withdrawal of cimetidine.


Pharmacology_Katzung

Pharmacology_Katzung. Impaired metabolism may also result if a simultaneously administered drug irreversibly inactivates a common metabolizing enzyme. These inhibitors, in the course of their metabolism by cytochrome P450, inactivate the enzyme and result in impairment of their own metabolism and that of other cosubstrates. This is the case of the furanocoumarins in grapefruit juice, eg, 6′,7′-dihydroxybergamottin and bergamottin, which inactivate CYP3A4 in the intestinal mucosa and consequently enhance its proteolytic degradation. This impairment of intestinal first-pass CYP3A4-dependent metabolism significantly enhances the bioavailability of drugs such as ergotamine, felodipine, nifedipine, terfenadine, verapamil, ethinylestradiol, lovastatin, saquinavir, and cyclosporine A and is associated with clinically relevant DDIs and food-drug interactions. The list of drugs subject to DDIs involving grapefruit juice is extensive and includes many drugs with a very narrow therapeutic index and a high potential


Pharmacology_Katzung

Pharmacology_Katzung. relevant DDIs and food-drug interactions. The list of drugs subject to DDIs involving grapefruit juice is extensive and includes many drugs with a very narrow therapeutic index and a high potential for lethal adverse reactions. However, it must be borne in mind that not all commercially available grapefruit juices are equally potent, as the CYP3A4 inactivation potency is totally dependent on the amount of furanocoumarins extracted into the juice from the zest (highest), pith, and pulp of the grapefruit. Furthermore, recovery from these interactions is dependent on CYP3A4 resynthesis and thus may be slow.


Pharmacology_Katzung

Pharmacology_Katzung. Some drugs require conjugation with endogenous substrates such as GSH, glucuronic acid, or sulfate for their inactivation. Consequently, different drugs may compete for the same endogenous substrates, and the faster-reacting drug may effectively deplete endogenous substrate levels and impair the metabolism of the slower-reacting drug. If the latter has a steep dose-response curve or a narrow margin of safety, potentiation of its therapeutic and toxic effects may result.


Pharmacology_Katzung

Pharmacology_Katzung. Acute or chronic diseases that affect liver architecture or function markedly affect hepatic metabolism of some drugs. Such conditions include alcoholic hepatitis, active or inactive alcoholic cirrhosis, hemochromatosis, chronic active hepatitis, biliary cirrhosis, and acute viral or drug-induced hepatitis. Depending on their severity, these conditions may significantly impair hepatic drug-metabolizing enzymes, particularly microsomal oxidases, and thereby markedly affect drug elimination. For example, the half-lives of chlordiazepoxide and diazepam in patients with liver cirrhosis or acute viral hepatitis are greatly increased, with a corresponding increase in their effects. Consequently, these drugs may cause coma in patients with liver disease when given in ordinary doses.


Pharmacology_Katzung

Pharmacology_Katzung. Some drugs are metabolized so readily that even marked reduction in liver function does not significantly prolong their action. However, cardiac disease, by limiting blood flow to the liver, may impair disposition of those drugs whose metabolism is TABLE 4–7 Rapidly metabolized drugs whose hepatic clearance is blood flow-limited. flow-limited (Table 4–7). These drugs are so readily metabolized by the liver that hepatic clearance is essentially equal to liver blood flow. The impaired enzyme activity or defective formation of enzymes associated with heavy metal poisoning or porphyria also results in reduced hepatic drug metabolism. Pulmonary disease may also affect drug metabolism, as indicated by the impaired hydrolysis of procainamide and procaine in patients with chronic respiratory insufficiency and the increased half-life of antipyrine (a P450 functional probe) in patients with lung cancer.


Pharmacology_Katzung

Pharmacology_Katzung. Although the effects of endocrine dysfunction on drug metabolism have been well explored in experimental animal models, corresponding data for humans with endocrine disorders are scanty. Thyroid dysfunction has been associated with altered metabolism of some drugs and of some endogenous compounds as well. Hypothyroidism increases the half-life of antipyrine, digoxin, methimazole, and some β blockers, whereas hyperthyroidism has the opposite effect. A few clinical studies in diabetic patients indicate no apparent impairment of drug metabolism, although impairment has been noted in diabetic rats. Malfunctions of the pituitary, adrenal cortex, and gonads markedly reduce hepatic drug metabolism in rats. On the basis of these findings, it may be supposed that such disorders could significantly affect drug metabolism in humans. However, until sufficient evidence is obtained from clinical studies in patients, such extrapolations must be considered tentative.


Pharmacology_Katzung

Pharmacology_Katzung. Finally, the release of inflammatory mediators, cytokines, and nitric oxide associated with bacterial or viral infections, cancer, or inflammation are known to impair drug metabolism by inactivating P450s and enhancing their degradation. Bailey DG, Dresser G, Arnold JMA: Grapefruit and medication interactions: Forbidden fruit or avoidable consequences? Can Med Assoc J 2013;185:309. Benowitz NL: Pharmacology of nicotine: Addiction, smoking-induced disease, and therapeutics. Annu Rev Pharmacol Toxicol 2009;49:57. Clayton TA et al: Pharmacometabonomic identification of a significant hostmicrobiome metabolic interaction affecting human drug metabolism. Proc Natl Acad Sci USA 2009;106:14728. Correia MA: Human and rat liver cytochromes P450: Functional markers, diagnostic inhibitor probes and parameters frequently used in P450 studies. In: Ortiz de Montellano P (editor): Cytochrome P450: Structure, Mechanism and Biochemistry, 3rd ed. Kluwer Academic/Plenum Press, 2005.


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Pharmacology_Katzung. Correia MA, Hollenberg PF: Inhibition of cytochrome P450 enzymes. In: Ortiz de Montellano P (editor): Cytochrome P450: Structure, Mechanism and Biochemistry, 4th ed. Springer International, 2015. Correia MA, Ortiz de Montellano P: Inhibition of cytochrome P450 enzymes. In: Ortiz de Montellano P (editor): Cytochrome P450: Structure, Mechanism and Biochemistry, 3rd ed. Kluwer Academic/Plenum Press, 2005. Daly AK: Pharmacogenetics and human genetic polymorphisms. Biochem J 2010;429:435. Guengerich FP: Human cytochrome P450 enzymes. In: Ortiz de Montellano P (editor): Cytochrome P450: Structure, Mechanism and Biochemistry, 4th ed. Springer International, 2015. Guengerich FP: Role of cytochrome P450 enzymes in drug-drug interactions. Adv Pharmacol 1997;43:7. Hustert E et al: The genetic determinants of the CYP3A5 polymorphism. Pharmacogenetics 2001;11:773. Ingelman-Sundberg M: Pharmacogenetics: An opportunity for a safer and more efficient pharmacotherapy. J Intern Med 2001;250:186.


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Pharmacology_Katzung. Ingelman-Sundberg M: Pharmacogenetics: An opportunity for a safer and more efficient pharmacotherapy. J Intern Med 2001;250:186. Ingelman-Sundberg M et al: Influence of cytochrome P450 polymorphisms on drug therapies: Pharmacogenetic, pharmacoepigenetic and clinical aspects. Pharmacol Ther 2007;116:496. Ingelman-Sundberg M, Sim SC: Pharmacogenetic biomarkers as tools for improved drug therapy; emphasis on the cytochrome P450 system. Biochem Biophys Res Commun 2010;396:90. Kang MJ et al: The effect of gut microbiota on drug metabolism. Expert Opin Drug Metab Toxicol 2013;9:1295. Kroemer HK, Klotz U: Glucuronidation of drugs: A reevaluation of the pharmacological significance of the conjugates and modulating factors. Clin Pharmacokinet 1992;23:292. Kuehl P et al: Sequence diversity in CYP3A promoters and characterization of the genetic basis of polymorphic CYP3A5 expression. Nat Genet 2001; 27:383.


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Pharmacology_Katzung. Kuehl P et al: Sequence diversity in CYP3A promoters and characterization of the genetic basis of polymorphic CYP3A5 expression. Nat Genet 2001; 27:383. Lindenbaum J et al: Inactivation of digoxin by the gut flora: Reversal by antibiotic therapy. N Engl J Med 1981;305:789. Lown KS et al: Grapefruit juice increases felodipine oral availability in humans by decreasing intestinal CYP3A protein expression. J Clin Invest 1997;99:2545. Meyer UA: Pharmacogenetics—Five decades of therapeutic lessons from genetic diversity. Nat Rev Genet 2004;5:669. Morgan ET et al: Regulation of drug-metabolizing enzymes and transporters in infection, inflammation, and cancer. Drug Metab Dispos 2008;36:205. Nelson DR et al: The P450 superfamily: Update on new sequences, gene mapping, accession numbers, and nomenclature. Pharmacogenetics 1996;6:1. Nelson DR et al: Updated human P450 sequences. http://drnelson.uthsc.edu/ cytochromeP450.html.


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Pharmacology_Katzung. Nelson DR et al: Updated human P450 sequences. http://drnelson.uthsc.edu/ cytochromeP450.html. Pirmohamed M: Drug-grapefruit juice interactions: Two mechanisms are clear but individual responses vary. Br Med J 2013;346:f1. Posadzki P, Watson L, Ernst E: Herb-drug interactions: An overview of systematic reviews. Br J Clin Pharmacol 2013;75:603. Rahimi R, Abdollahi M: An update on the ability of St. John’s wort to affect the metabolism of other drugs. Expert Opin Drug Metab Toxicol 2012;8:691. Rieder MJ et al: Effect of VKORC1 haplotypes on transcriptional regulation and warfarin dose. N Engl J Med 2005;352:2285. Russo E et al: Hypericum perforatum: Pharmacokinetic, mechanism of action, tolerability, and clinical drug-drug interactions. Phytother Res 2014;28:643. Saitta KS et al: Bacterial beta-glucuronidase inhibition protects mice against enteropathy induced by indomethacin, ketoprofen or diclofenac: Mode of action and pharmacokinetics. Xenobiotica 2014;44:28.


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Pharmacology_Katzung. Sueyoshi T, Negishi M: Phenobarbital response elements of cytochrome P450 genes and nuclear receptors. Annu Rev Pharmacol Toxicol 2001;41:123. Thummel KE, Wilkinson GR: In vitro and in vivo drug interactions involving human CYP3A. Annu Rev Pharmacol Toxicol 1998;38:389. Tsai HH et al: Evaluation of documented drug interactions and contraindications associated with herbs and dietary supplements: A systematic literature review. Int J Clin Pract 2012;66:1056. Wallace BD et al: Structure and inhibition of microbiome beta-glucuronidases essential to the alleviation of cancer drug toxicity. Chem Biol 2015;22:1238. Wang L, McLeod HL, Weinshilboum RM: Genomics and drug response. N Engl J Med 2011;364:1144. Williams SN et al: Induction of cytochrome P450 enzymes. In: Ortiz de Montellano P (editor): Cytochrome P450. Structure, Mechanism, and Biochemistry. Kluwer Academic/Plenum Press, 2005; and references therein.


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Pharmacology_Katzung. Willson TM, Kliewer SA: PXR, CAR and drug metabolism. Nat Rev Drug Discov 2002;1:259. Wilson ID, Nicholson JK: The role of gut microbiota in drug response. Curr Pharm Des 2009;15:1519. Xu C et al: CYP2A6 genetic variation and potential consequences. Adv Drug Delivery Rev 2002;54:1245.


Pharmacology_Katzung

Pharmacology_Katzung. Acetaminophen (APAP) is a relatively safe drug, provided it is taken at the recommended therapeutic doses. As discussed in the text, at normally ingested dosages, 95% of APAP is converted by phase II enzymes into much less toxic and more water-soluble APAP-glucuronide and APAP-sulfate, both of which are eliminated in the urine (Figure 4–5). Five percent of parent APAP is converted by phase I P450 enzymes into a reactive toxic product that is conjugated by GSH, excreted in the urine, and thus detoxi-fied. However, APAP’s safety may be greatly compromised in mixed drug overdoses, ie, when ingested with other drugs such as hydrocodone, duloxetine, and carisoprodol, which compete with APAP for phase II-dependent elimination or for cellular cofactors (GSH, UDPGA, PAPS) involved in these processes. Accordingly, more APAP is diverted into its hepatotoxic reactive metabolite pathway, resulting in liver cell damage. Moreover, HCV infection could indeed have further compromised liver function


Pharmacology_Katzung

Pharmacology_Katzung. Accordingly, more APAP is diverted into its hepatotoxic reactive metabolite pathway, resulting in liver cell damage. Moreover, HCV infection could indeed have further compromised liver function including drug metabolism. APAP’s half-life is 2 hours, and therapeutic and toxic blood levels are 15 mcg/mL and > 300 mcg/mL, respectively (Chapter 3). Given that at 48 hours after inges-tion (ie, 24 half-lives later), the patient’s APAP blood level is 75 mcg/mL, it is obvious that her initial APAP levels were dangerously above the toxic range, and thus upon ED admission, her liver function tests are consistent with ongoing liver failure. She should be given N-acetylcysteine, the APAP-specific antidote (Acetadote, Mucomyst; see Chapter 58) and continuous intravenous glucose infusion to provide the precursor (glucose) for generating the UDPGA cofactor required for APAP glucuronidation, as well as the fluid to induce urine output and accelerate APAP-metabolite elimination.


Pharmacology_Katzung

Pharmacology_Katzung. Jennifer E. Hibma, PharmD, & Kathleen M. Giacomini, PhD A 35-year-old male with newly diagnosed human immu-nodeficiency virus (HIV) infection was prescribed an anti-retroviral regimen, which included the protease inhibitor atazanavir 300 mg to be taken by mouth once daily, along with ritonavir, a pharmacokinetic enhancer, and two nucleo-side analog antiretroviral agents. Liver function and renal function were normal. After 1 year of treatment, the patient experienced visible yellow discoloration of the skin and eyes. Blood samples were drawn, and grade 4 hyperbilirubinemia was documented. When atazanavir was discontinued and the antiretroviral regimen was modified to include lopinavir, the plasma levels of bilirubin returned to the normal range, and skin and eye color were cleared. Could a UGT1A1*28polymorphism have led to the adverse effects?


Pharmacology_Katzung

Pharmacology_Katzung. Pharmacogenomics, the study of genetic factors that underlie variation in drug response, is a modern term for pharmacogenetics. Pharmacogenomics implies a recognition that more than one genetic variant may contribute to variation in drug response. Historically, the field began with observations of severe adverse drug reactions in certain individuals, who were found to harbor genetic variants in drug-metabolizing enzymes. As a scientific field, pharmacogenomics has advanced rapidly since the sequencing of the human genome. In the last decade, powerful genome-wide association (GWA) studies, in which hundreds of thousands of genetic variants across the genome are tested for association with drug response, led to the discovery of many other important polymorphisms that underlie variation in both therapeutic and adverse drug response. In addition to polymorphisms in genes that encode drug-metabolizing enzymes, it is now known that polymorphisms in genes that encode transporters, human


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Pharmacology_Katzung. in both therapeutic and adverse drug response. In addition to polymorphisms in genes that encode drug-metabolizing enzymes, it is now known that polymorphisms in genes that encode transporters, human leukocyte antigen (HLA) loci, cytokines, and various other proteins are also predictive of variation in therapeutic and adverse drug responses. In addition to the new discoveries that have been made, the past decade has ushered in “precision medicine,” also known as “stratified or personalized medicine,” in which genetic information is used to guide drug and dosing selection for subgroups of patients or individual patients in medical practice. The Clinical Pharmacogenetics Implementation Consortium (CPIC) published a series of guidelines for using genetic information in selecting medications and in dosing. These highly informative guidelines are being used by practitioners in prescribing drugs to more effectively treat patients. In this chapter, we begin with a case study and then


Pharmacology_Katzung

Pharmacology_Katzung. and in dosing. These highly informative guidelines are being used by practitioners in prescribing drugs to more effectively treat patients. In this chapter, we begin with a case study and then describe genetic variants that are determinants of drug response. Where appropriate, CPIC recommendations are included to provide information on how to use genetic variant data appropriately in therapeutic medicine.


Pharmacology_Katzung

Pharmacology_Katzung. The description in this chapter of DNA sequence variations in germline DNA involves a number of terms that describe the nature of the variations and their locations within the genome. A glossary of commonly used terms is presented in the Glossary Table. Some of the more common and important variations are described in the text that follows.


Pharmacology_Katzung

Pharmacology_Katzung. As described in Chapter 4, biotransformation reactions mediated by P450 phase I enzymes typically modify functional groups (−OH, −SH, −NH2, −OCH3) of endogenous and xenobiotic compounds, resulting in an alteration of the biological activity of the compound. Phase I enzymes are involved in the biotransformation of over 75% of prescription drugs; therefore, polymorphisms in these enzymes may significantly affect blood levels, which in turn may alter response to many drugs. Polymorphisms in drug-metabolizing enzymes dominated the field of pharmacogenomics for many years, and for some years, metabolic phenotypes such as extensive metabolizer (EM), reflecting an individual’s metabolic rate of a particular drug that is a known substrate of a specific enzyme, were used to describe genetic effects on drug metabolism. After genotypic information became available, a new nomenclature was used to characterize an individual’s metabolic rate. In particular, diplotypes, consisting of one maternal


Pharmacology_Katzung

Pharmacology_Katzung. on drug metabolism. After genotypic information became available, a new nomenclature was used to characterize an individual’s metabolic rate. In particular, diplotypes, consisting of one maternal and one paternal allele, using star (*) allele nomenclature, have been used. Each star (*) allele is defined by specific sequence variation(s) within the gene locus, eg, single nucleotide polymorphisms (SNPs), and may be assigned a functional activity score when the functional characterization is known, eg, 0 for nonfunctional, 0.5 for reduced function, and 1.0 for fully functional. Some genes, such as CYP2D6, are subject to whole gene deletions, eg, CYP2D6*5, and whole gene duplications or multiplications, eg, *1xN, *2xN, where N is the number of copies. If more than one copy of the gene is detected, the activity score is then multiplied by the number of copies observed. Enzyme activity is generally a co-dominant or additive trait. For example, if an individual carries one normal function


Pharmacology_Katzung

Pharmacology_Katzung. the activity score is then multiplied by the number of copies observed. Enzyme activity is generally a co-dominant or additive trait. For example, if an individual carries one normal function allele and one nonfunctional allele, he will have an intermediate metabolic activity or be considered an intermediate metabolizer (IM). The sum of allelic activity scores typically ranges between 0 and ≥ 3.0 and is most often used to define phenotypes as follows: 0 = PM (poor metabolizer), 0.5 = IM, 1.0–2.0 = EM, and ≥ 2.0 = UM (ultra-rapid metabolizer).


Pharmacology_Katzung

Pharmacology_Katzung. As described in Chapter 4, cytochrome P450 2D6 is involved in the metabolism of up to one quarter of all drugs used clinically, including predominantly basic compounds such as β blockers, antidepressants, antipsychotics, and opioid analgesics. Among the CYP enzymes, CYP2D6 is responsible for metabolism of about 20% of clinically used drugs. Similar to other polymorphic enzymes, four clinically defined metabolic phenotypes, ie, PMs, IMs, EMs, and UMs, are used to predict therapeutic and adverse responses following the administration of CYP2D6 substrates.


Pharmacology_Katzung

Pharmacology_Katzung. The gene encoding CYP2D6 is highly polymorphic, with over 100 alleles defined (www.cypalleles.ki.se/cyp2d6.htm); however, greater than 95% of phenotypes can be accounted for with just nine alleles, ie, CYP2D6 alleles *3, *4, *5, and *6 are nonfunctional; alleles *10, *17, and *41 have reduced function; and alleles *1 and *2 are fully functional. As with many polymorphisms, allele frequencies vary across populations (Table 5–1). Some genetic variants are shared among populations at similar allele frequencies, whereas others vary considerably. For example, the most common nonfunctional allele, CYP2D6*4, is observed at TABLE 5–1 Major alleles and frequencies in African, Asian, and European populations.


Pharmacology_Katzung

Pharmacology_Katzung. TABLE 5–1 Major alleles and frequencies in African, Asian, and European populations. TABLE 5–1 Major alleles and frequencies in African, Asian, and European populations. (Continued) 1The Single Nucleotide Polymorphism Database (dbSNP) is an online public repository of genomic variation established by the National Center for Biotechnology Information (NCBI), https://www.ncbi.nlm.nih.gov/SNP/.


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Pharmacology_Katzung. a frequency of approximately 20% in Europeans and is nearly absent (< 1%) in Asians (Table 5–1). Based on Hardy-Weinberg principles (see Glossary), the percentage of Europeans who are homozygous for the CYP2D6*4 allele, ie, who carry the *4 allele on both maternal and paternal chromosomes, would be 4%, whereas that of those who are heterozygotes would be 32%. This parallels the lower number of PMs (defined as having two nonfunctional alleles, eg, PMs are homozygous for *3, *4, *5, *6, or any combination of nonfunctional alleles such as *4/*5 ), observed in Asian populations (∼1%) compared with European populations (∼5–10%) (Table 5–1). In contrast, the *5 gene deletion is found at similar frequencies (∼3–5%) across European, African, and Asian populations, suggesting that this mutation likely took place prior to the separation of the three major races more than 100,000 years ago. Clinically, since some genotyping platforms are specific to a single ethnicity, it is important to ensure


Pharmacology_Katzung

Pharmacology_Katzung. took place prior to the separation of the three major races more than 100,000 years ago. Clinically, since some genotyping platforms are specific to a single ethnicity, it is important to ensure alleles applicable to the patient population being treated are tested. Of note, rare or previously undiscovered variants are typically not included in commercial tests, and thus novel or rare polymorphisms, which may exhibit altered function, will be missed.


Pharmacology_Katzung

Pharmacology_Katzung. Example: Codeine is a phenanthrene derivative prodrug opioid analgesic indicated for the management of mild to moderately severe pain (Chapter 31). Codeine, like its active metabolite morphine, binds to μ-opioid receptors in the central nervous system (CNS). Morphine is 200 times more potent as an agonist than codeine, and conversion of codeine into morphine is essential for codeine’s analgesic activity. The enzyme responsible for the O-demethylation conversion of codeine into morphine is CYP2D6. Patients with normal CYP2D6 activity (ie, EMs) convert sufficient codeine to morphine (∼5–10% of an administered dose) to produce the desired analgesic effect. PMs and IMs are more likely to experience insufficient pain relief, while UMs are at an increased risk for side effects, eg, drowsiness and respiratory depression, due to higher systemic concentrations of morphine. Interestingly, gastrointestinal adverse effects, eg, constipation, are decreased in PMs, whereas the central side effects,


Pharmacology_Katzung

Pharmacology_Katzung. respiratory depression, due to higher systemic concentrations of morphine. Interestingly, gastrointestinal adverse effects, eg, constipation, are decreased in PMs, whereas the central side effects, eg, sedation and dizziness, do not differ between PMs and EMs. The antitussive properties associated with codeine are not affected by CYP2D6 activity. According to CPIC guidelines, standard starting doses are recommended in EMs and IMs with close monitoring, especially in IMs; and CPIC recommends use of an alternative agent in PMs and UMs (see Table 5–2).


Pharmacology_Katzung

Pharmacology_Katzung. Cytochrome P450 CYP2C19 is known to preferentially metabolize acidic drugs including proton-pump inhibitors, antidepressants, antiepileptics, and antiplatelet drugs (Chapter 4). Four clinical phenotypes related to CYP2C19 activity (PM, IM, EM, and UM) are closely associated with genetic biomarkers that may assist in guiding individualized therapeutic dosing strategies. The gene that encodes CYP2C19 is highly polymorphic, with over 30 alleles defined (www.cypalleles.ki.se/cyp2c19.htm), yet just four alleles can account for the majority of phenotypic variability, ie, CYP2C19 allele *2 and *3 are nonfunctional, CYP2C19 allele *1 is fully functional, and CYP2C19*17 has increased function. Phenotypes range from PMs who have two deficient alleles, eg, *2/*3, *2/*2, or *3/*3, to UMs who have increased hepatic expression levels of the CYP2C19 protein, due to *1/*17 or *17/*17 alleles (see Table 5–2). Of note, the *17 increased function allele is unable to fully compensate for nonfunctional


Pharmacology_Katzung

Pharmacology_Katzung. hepatic expression levels of the CYP2C19 protein, due to *1/*17 or *17/*17 alleles (see Table 5–2). Of note, the *17 increased function allele is unable to fully compensate for nonfunctional alleles, and therefore, the presence of a *17 allele in combination with a nonfunctional allele would be considered an IM phenotype (see Table 5–2). The PM phenotype is more common in Asians (∼16%) than in Europeans and Africans (∼2–5%), which can be expected based on the inheritance patterns of variant alleles across populations, eg, the most common nonfunctional allele, ie, CYP2C19*2, is observed approximately twice as frequently in Asians (∼30%) compared with Africans and Europeans (∼15%), while the apparent gain-of-function *17 allele is observed rarely in Asians (< 3%) but more frequently in Europeans and Africans (16–21%) (see Table 5–1).


Pharmacology_Katzung

Pharmacology_Katzung. Example: Clopidogrel is a thienopyridine antiplatelet pro-drug indicated for the prevention of atherothrombotic events. Active metabolites selectively and irreversibly inhibit adenosine diphosphate-induced platelet aggregation (Chapter 34). Clopidogrel is metabolized in the body via one of two main mechanisms; approximately 85% of an administered dose is rapidly hydrolyzed by hepatic esterases to its inactive carboxylic acid derivative, while the remaining ∼15% is converted via two sequential CYP-mediated oxidation reactions (predominantly CYP2C19) to the active thiol metabolite responsible for antiplatelet activity.


Pharmacology_Katzung

Pharmacology_Katzung. Genetic polymorphisms in the CYP2C19 gene that decrease active metabolite formation and consequently reduce the drug’s antiplatelet activity are associated with variability in response to clopidogrel. Carriers of the reduced function CYP2C19 *2 alleles taking clopidogrel are at increased risk for serious adverse cardiovascular events, particularly in acute coronary syndrome managed with percutaneous coronary intervention (PCI); the hazard ratios (HR) are 1.76 for *2/*2 genotype and 1.55 for *2 heterozygotes compared to noncarriers. The risk associated with stent thrombosis is even greater (HR 3.97 for *2/*2 genotype and 2.67 for *2 heterozygotes compared to *1 homozygotes). However, for other indications, eg, atrial fibrillation and stroke, the effects of the CYP2C19*2 allele are less dramatic. Thus, current clinical recommendations from CPIC are specific for acute coronary syndrome with PCI: Standard starting doses are recommended in EMs and UMs, and CPIC recommends use of an


Pharmacology_Katzung

Pharmacology_Katzung. dramatic. Thus, current clinical recommendations from CPIC are specific for acute coronary syndrome with PCI: Standard starting doses are recommended in EMs and UMs, and CPIC recommends use of an alternative antiplatelet agent, eg, prasugrel or ticagrelor, in PMs and IMs (Table 5–2). The US Food and Drug Administration (FDA)-approved label for clopidogrel recommends alternative antiplatelet drugs for patients who are poor metabolizers of clopidogrel.


Pharmacology_Katzung

Pharmacology_Katzung. Dihydropyrimidine dehydrogenase (DPD, encoded by the DPYD gene) is the first and rate-limiting step in pyrimidine catabolism, as well as a major elimination route for fluoropyrimidine chemotherapy agents (Chapter 54). Considerable intergroup and intragroup variation exists in DPD enzyme activity. Many of the alleles identified in the DPYD gene either are too rare to sufficiently characterize or have shown conflicting associations with DPD activity. Three nonfunctional alleles have been identified, ie, DPYD *2A, *13, and rs67376798. All three of these variants are rare; however, the *2A allele is the most commonly observed allele and is often the only variant tested in commercial genotyping platforms (see National Institutes of Health Genetic Testing Registry, http://www.ncbi.nlm.nih.gov/gtr/conditions/C2720286/ or http://www.ncbi.nlm.nih.gov/gtr/conditions/CN077983/). Frequencies of the *2A allele range from less than 0.005 in most European, African, and Asian populations to 3.5% in a


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Pharmacology_Katzung. or http://www.ncbi.nlm.nih.gov/gtr/conditions/CN077983/). Frequencies of the *2A allele range from less than 0.005 in most European, African, and Asian populations to 3.5% in a Swedish population (see Table 5–1).


Pharmacology_Katzung

Pharmacology_Katzung. Example: Three fluoropyrimidine drugs are used clinically, namely 5-fluorouracil (5-FU), capecitabine, and tegafur (only approved in Europe). 5-FU is the pharmacologically active compound of each drug, and all are approved to treat solid tumors including colorectal and breast cancer (Chapter 54). 5-FU must be administered intravenously, while both capecitabine and tegafur are oral prodrugs that are rapidly converted to 5-FU in the body. Only 1–3% of an administered dose of the prodrug is converted to the active cytotoxic metabolites, ie, 5-fluorouridine 5′-monophosphate (5-FUMP) and 5-fluoro-2′-deoxyuridine-5′monophosphate (5-FdUMP), which effectively target rapidly dividing cancer cells and inhibit DNA synthesis. The majority of an administered dose (∼80%) is subjected to pyrimidine catabolism via DPD and is excreted in the urine. Complete or TABLE 5–2 Gene-based dosing recommendations for selected drugs.


Pharmacology_Katzung

Pharmacology_Katzung. TABLE 5–2 Gene-based dosing recommendations for selected drugs. Fluoropy-*1/*1 Normal • Standard dose. CPIC rimidines *1/*2A, *1/*13, Reduced • Reduce initial dose 50% and titrate based *1/rs67376798A activity on toxicity or on pharmacokinetic test results (if available). *2A/*2A, *2A/*13, Complete • Different non-fluoropyrimidine anticancer agent. *13/*13, rs67376798A/ deficiency rs67376798A Irinotecan *1/*1, *1/*28 Normal • Standard starting dose. *28/*28 Reduced • Reduce starting dose by at least one dose level. Or, Drug label • Dose > 250 mg/m2: Reduce starting dose 30% DPWG3 and increase in response to neutrophil count. Dose = 250 mg/m2: No dose adjustment. Atazanavir *1/*1, *1/*36, Normal No reason to avoid prescribing atazanavir. Inform CPIC *36/*36, patient of risks. Based on this genotype, there is a rs887829 C/C less than 1 in 20 chance of stopping atazanavir for jaundice.


Pharmacology_Katzung

Pharmacology_Katzung. *1/*28, *1/*37, Intermediate No reason to avoid prescribing atazanavir. Inform *36/*28, *36/*37, patient of risks. Based on this genotype, there is a rs887829 C/T, *1/*6 less than 1 in 20 chance of stopping atazanavir for jaundice. *28/*28, *28/*37, Reduced Consider alternative agent. Based on this *37/*37, rs887829 T/T genotype, there is a high (20–60%) likelihood of (*80/*80), *6/*6 developing jaundice that will result in discontinuation of atazanavir. Thiopurines *1/*1 Normal, high • Standard starting dose. CPIC activity *1/*2, *1/*3A, Intermediate • Start at 30–70% of target dose and titrate *1/*3B, *1/*3C, activity every 2–4 weeks with close clinical monitoring of *1/*4 tolerability, eg, white blood cell counts and liver function tests.


Pharmacology_Katzung

Pharmacology_Katzung. TABLE 5–2 Gene-based dosing recommendations for selected drugs. (Continued) 1Diplotypes are shown as the two members of a chromosome pair, eg, *1/*1 indicates both chromosomes contain the *1 allele for that gene, whereas *1/*17 denotes a heterozygote with one *1 allele and one *17 allele. 2CPIC: Clinical Pharmacogenetics Implementation Consortium: Full drug-specific recommendations are available online at http://www.pharmgkb.org/page/cpic. 3DPWG: Dutch Pharmacogenetics Working Group: Full drug-specific recommendations are available online https://www.pharmgkb.org/page/dpwg. 4SVR: sustained viral response. 5IWPG: International Warfarin Pharmacogenetics Consortium.


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Pharmacology_Katzung. partial deficiency of DPD can lead to dramatically reduced clearances of 5-FU, increased levels of toxic metabolites 5-FUMP and 5-FdUMP, and consequently an increased risk for severe dose-dependent fluoropyrimidine toxicities, eg, myelosuppression, mucositis, neurotoxicity, hand-and-foot syndrome, and diarrhea. In a recent genotype-driven dosing study of over 1600 patients treated with fluoropyrimidine-based chemotherapy, including 18 carriers of DPYD*2A who were treated with 50% of the normal dose, the incidence of severe toxicity was significantly reduced from 73% (historical controls) to 28%. CPIC recommendations for therapeutic regimens are shown in Table 5–2.


Pharmacology_Katzung

Pharmacology_Katzung. As described in Chapter 4, phase II enzyme biotransformation reactions typically conjugate endogenous molecules, eg, sulfuric acid, glucuronic acid, and acetic acid, onto a wide variety of substrates in order to enhance their elimination from the body. Consequently, polymorphic phase II enzymes may diminish drug elimination and increase risks for toxicities. In this section, we describe key examples of polymorphic phase II enzymes and the pharmacologic consequence for selected prescription drugs.


Pharmacology_Katzung

Pharmacology_Katzung. The uridine 5′-diphospho(UDP) glucuronosyltransferase 1A1 (UGT1A1) enzyme, encoded by the UGT1A1 gene, conjugates glucuronic acid onto small lipophilic molecules, eg, bilirubin and a wide variety of therapeutic drug substrates so that they may be more readily excreted into bile (Chapter 4). The UGT1A1 gene locus has over 30 defined alleles, some of which lead to reduced or completely abolished UGT1A1 function. Most reduced function polymorphisms within the UGT1A1 gene locus are quite rare; however, the *28 allele is common across three major ethnic groups (Table 5–1). Approximately 10% of European populations are homozygous carriers of the *28 allele, ie, UGT1A1 *28/*28 genotype, and are recognized clinically to have Gilbert’s syndrome. The *28 allele is characterized by an extra TA repeated in the proximal promoter region and is associated with reduced expression of the UGT1A1 enzyme. Clinically, Gilbert’s syndrome is generally benign; however, affected individuals may have 60–70%


Pharmacology_Katzung

Pharmacology_Katzung. in the proximal promoter region and is associated with reduced expression of the UGT1A1 enzyme. Clinically, Gilbert’s syndrome is generally benign; however, affected individuals may have 60–70% increased levels of circulating unconjugated bilirubin due to a ∼30% reduction in UGT1A1 activity. Individuals with the UGT1A1*28/*28 genotype are thus at an increased risk for adverse drug reactions with UGT1A1 drug substrates due to reduced biliary elimination.


Pharmacology_Katzung

Pharmacology_Katzung. Example: Irinotecan is a topoisomerase I inhibitor prodrug and is indicated as first-line chemotherapy in combination with 5-FU and leucovorin for treatment of metastatic carcinoma of the colon or rectum (Chapter 54). Irinotecan is hydrolyzed by hepatic carboxylesterase enzymes to its cytotoxic metabolite, SN-38, which inhibits topoisomerase I and eventually leads to termination of DNA replication and cell death. The active SN-38 metabolite is responsible for the majority of therapeutic action as well as the dose-limiting bone marrow and gastrointestinal toxicities. Inactivation of SN-38 occurs via the polymorphic UGT1A1 enzyme, and carriers of the UGT1A1*6 and UGT1A1*28 polymorphisms are consequently at increased risk for severe life-threatening toxicities, eg, neutropenia and diarrhea, due to decreased clearance of the SN-38 metabolite.


Pharmacology_Katzung

Pharmacology_Katzung. Thiopurine S-methyltransferase (TPMT) covalently attaches a methyl group onto aromatic and heterocyclic sulfhydryl compounds and is responsible for the pharmacologic deactivation of thiopurine drugs (Chapter 4). Genetic polymorphisms in the gene encoding TPMT may lead to three clinical TPMT activity phenotypes, ie, high, intermediate, and low activity, which are associated with differing rates of inactivation of thiopurine drugs and altered risks for toxicities. While the majority (86–97%) of the population inherits two functional TPMT alleles and has high TPMT activity, around 10% of Europeans and Africans inherit only one functional allele and are considered to have intermediate activity. Furthermore, about 0.3% of Europeans inherit two defective alleles and have very low to no TPMT activity (Table 5–1). Over 90% of the phenotypic TPMT variability across populations can be accounted for with just three point mutations that are defined by four non-functional alleles, ie, TPMT*2, *3A,


Pharmacology_Katzung

Pharmacology_Katzung. (Table 5–1). Over 90% of the phenotypic TPMT variability across populations can be accounted for with just three point mutations that are defined by four non-functional alleles, ie, TPMT*2, *3A, *3B, and *3C (Table 5–2). Most commercial genotyping platforms test for these four common genetic biomarkers and are therefore able to identify individuals with reduced TPMT activity.


Pharmacology_Katzung

Pharmacology_Katzung. Example: Three thiopurine drugs are used clinically, ie, azathioprine, 6-mercaptopurine (6-MP), and 6-thioguanine (6-TG). All share similar metabolic pathways and pharmacology. Azathioprine (a prodrug of 6-MP) and 6-MP are used for treating immunologic disorders, while 6-MP and 6-TG are important anticancer agents (Chapter 54). 6-MP and 6-TG may be activated by the salvage pathway enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) to form 6-thioguanine nucleotides (TGNs), which are responsible for the majority of therapeutic efficacy as well as bone marrow toxicity. Alternatively, 6-MP and 6-TG may be inactivated by enzymes such as polymorphic TPMT and xanthine oxidase, leaving less available substrate to be activated by HGPRTase. The TPMT gene is a major determinant of thiopurine metabolism and exposure to cytotoxic 6-TGN metabolites and thiopurine-related toxicities. See Table 5–2 for recommended dosing strategies. Recent GWA studies have also implicated variants in


Pharmacology_Katzung

Pharmacology_Katzung. metabolism and exposure to cytotoxic 6-TGN metabolites and thiopurine-related toxicities. See Table 5–2 for recommended dosing strategies. Recent GWA studies have also implicated variants in the enzyme NUDT15, which catalyzes the hydrolysis of nucleotide diphosphates, as being associated with thiopurine intolerance in children from Japan, Singapore, and Guatemala.


Pharmacology_Katzung

Pharmacology_Katzung. Glucose 6-phosphate dehydrogenase (G6PD) is the first and rate-limiting step in the pentose phosphate pathway and supplies a significant amount of reduced NADPH in the body. In red blood cells (RBCs), where mitochondria are absent, G6PD is the exclusive source of NADPH and reduced glutathione, which play a TABLE 5–3 Classification of G6PD deficiency (WHO Working Group, 1989).


Pharmacology_Katzung

Pharmacology_Katzung. TABLE 5–3 Classification of G6PD deficiency (WHO Working Group, 1989). critical role in the prevention of oxidative damage. Under normal conditions, G6PD in RBCs is able to detoxify unstable oxygen species while working at just 2% of its theoretical capacity. Following exposure to exogenous oxidative stressors, eg, infection, fava beans, and certain therapeutic drugs, G6PD activity in RBCs increases proportionately to meet NADPH demands and ultimately to protect hemoglobin from oxidation. Individuals with G6PD deficiency, defined as less than 60% enzyme activity, according to World Health Organization classification (Table 5–3), are at increased risk for abnormal RBC destruction, ie, hemolysis, due to reduced antioxidant capacity under oxidative pressures.


Pharmacology_Katzung

Pharmacology_Katzung. The gene that encodes the G6PD enzyme is located on the X chromosome and is highly polymorphic, with over 180 genetic variants identified that result in enzyme deficiency. Greater than 90% of variants are single-base substitutions in the coding region that produce amino acid changes, which result in unstable proteins with reduced enzyme activity. As with most X-linked traits, males with one reference X chromosome and females with two reference X chromosomes will have equivalent “normal” G6PD activity. Similarly, hemizygous-deficient males (with a deficient copy of the G6PD gene on their single X chromosome) and homozygous-deficient females (with two deficient copies) express reduced activity phenotypes (Table 5–1). However, for heterozygous females (with one deficient allele and one normal allele), genotype-to-phenotype predictions are less reliable due to the X-chromosome mosaicism, ie, where one X chromosome in each female cell is randomly inactivated, leading to G6PD activity that


Pharmacology_Katzung

Pharmacology_Katzung. allele), genotype-to-phenotype predictions are less reliable due to the X-chromosome mosaicism, ie, where one X chromosome in each female cell is randomly inactivated, leading to G6PD activity that may range from fully functional to severely deficient. G6PD enzyme activity phenotype estimations for heterozygous females therefore may be improved with complementary G6PD activity testing.


Pharmacology_Katzung

Pharmacology_Katzung. G6PD enzyme deficiency affects over 400 million people worldwide, and the World Health Organization has categorized G6PD activity into five classes (Table 5–3). The majority of polymorphic G6PD-deficient genotypes are associated with class II for severe deficiency (< 10% enzyme activity) and class III for moderate deficiency (10–60% enzyme activity). Most individuals with reduced function alleles of G6PD have ancestries in geographical areas of the world corresponding to areas with high malaria prevalence. Polymorphic alleles gained in frequency over time as they offered some benefit against death from malaria. The estimated frequency of G6PD deficiency is approximately 8% in malaria endemic countries, with the milder G6PD-A(–) allele prevalent in Africa, and the more severe G6PD-Mediterranean allele widespread across western Asia (Saudi Arabia and Turkey to India). There is a much more heterogeneous distribution of variant alleles in East Asia and Asia Pacific, which complicates G6PD


Pharmacology_Katzung

Pharmacology_Katzung. allele widespread across western Asia (Saudi Arabia and Turkey to India). There is a much more heterogeneous distribution of variant alleles in East Asia and Asia Pacific, which complicates G6PD risk predictions; however, the most frequently identified forms in Asia include the more severe class II alleles, eg, Mediterranean, Kaiping, and Canton, as well as some class III alleles, eg, Mahidol, Chinese-5, and Gaohe (Table 5–1).


Pharmacology_Katzung

Pharmacology_Katzung. Example: Rasburicase, a recombinant urate-oxidase enzyme, is indicated for the initial management of high uric acid levels in cancer patients receiving chemotherapy. Rasburicase alleviates the uric acid burden that often accompanies tumor-lysing treatments by converting uric acid into allantoin, a more soluble and easily excreted molecule. During the enzymatic conversion of uric acid to allantoin, hydrogen peroxide, a highly reactive oxidant, is formed. Hydrogen peroxide must be reduced by glutathione to prevent free radical formation and oxidative damage. Individuals with G6PD deficiency receiving rasburicase therapy are at greatly increased risk for severe hemolytic anemia and methemoglobinemia. The manufacturer recommends that patients at high risk (individuals of African or Mediterranean ancestry) be screened prior to the initiation of therapy and that rasburicase not be used in patients with G6PD deficiency (Table 5–2).


Pharmacology_Katzung

Pharmacology_Katzung. Plasma membrane transporters, located on epithelial cells of many tissues, eg, intestinal, renal, and hepatic membranes, mediate selective uptake and efflux of endogenous compounds and xenobiotics including many drug products. Transporters, which often work in concert with drug-metabolizing enzymes, play important roles in determining plasma and tissue concentrations of drugs and their metabolites. Genetic differences in transporter genes can dramatically alter drug disposition and response and thus may increase risk for toxicities. In this section, a key example of a polymorphic uptake transporter and its pharmacologic impact on statin toxicity are described.


Pharmacology_Katzung

Pharmacology_Katzung. The OATP1B1 transporter (encoded by the SLCO1B1 gene) is located on the sinusoidal membrane (facing the blood) of hepatocytes and is responsible for the hepatic uptake of mainly weakly acidic drugs and endogenous compounds, eg, statins, methotrexate, and bilirubin. Over 40 nonsynonymous variants (nsSNPs) have been identified in this transporter, some of which result in decreased transport function. A common reduced function polymorphism, rs4149056, has been shown to reduce transport of OATP1B1 substrates in vitro as well as to alter pharmacokinetic and clinical outcomes in vivo. The variant results in an amino acid change, Val174Ala, and is associated with reduced membrane expression, likely as a result of impaired trafficking capability. Allele *5 is relatively rare (rs4149056 alone; ∼1%), but various other reduced function alleles (*15 and *17; haplotypes containing rs4149056) are common in most European and Asian populations (between 5% and 15%) (Table 5–1).