There are a large number of phase 2 conjugating enzymes, all of which are considered to be synthetic in nature since they result in the formation of a metabolite with an increased molecular mass. Phase 2 reactions also normally terminate the biological activity of the drug, although for drugs like morphine and minoxidil, glucuronide and sulfate conjugates, respectively, are more pharmacologically active than the parent. The contributions of different phase 2 reactions to drug metabolism are shown in Figure 6–3B. Two of the phase 2 reactions, glucuronidation and sulfation, result in the formation of metabolites with a significantly increased water-to-lipid partition coefficient, resulting in hydrophilicity and facilitating metabolite accumulation in the aqueous compartments of the cell and the body. While sulfation and acetylation terminate the biological activity of drugs, the solubility properties of these metabolites are altered through minor changes in the overall charge of the molecule. Characteristic of the phase 2 reactions is the dependency on the catalytic reactions for cofactors, (or more correctly, co-substrates) such as UDP-glucuronic acid (UDP-GA) and 3′-phosphoadenosine-5′-phosphosulfate (PAPS), for UDP-glucuronosyltransferases (UGT) and sulfotransferases (SULT), respectively, which react with available functional groups on the substrates. The reactive functional groups are often generated by the phase 1 CYPs, although there are many drugs (e.g., acetaminophen) where glucuronidation and/or sulfation occur directly without prior oxidative metabolism. All of the phase 2 reactions are carried out in the cytosol of the cell, with the exception of glucuronidation, which is localized to the luminal side of the endoplasmic reticulum. The catalytic rates of phase 2 reactions are significantly faster than the rates of the CYPs. Thus, if a drug is targeted for phase 1 oxidation through the CYPs, followed by a phase 2 conjugation reaction, usually the rate of elimination will depend upon the initial (phase 1) oxidation reaction. Since the rate of conjugation is faster and the process leads to an increase in hydrophilicity of the drug, phase 2 reactions are generally considered to assure the efficient elimination and detoxification of most drugs.
Glucuronidation Among the more important of the phase 2 reactions in the metabolism of drugs is that catalyzed by UDP-glucuronosyltransferases (UGTs) (Figure 6–3B). These enzymes catalyze the transfer of glucuronic acid from the cofactor UDP-glucuronic acid to a substrate to form β-D-glucopyranosiduronic acids (glucuronides), metabolites that are sensitive to cleavage by β-glucuronidase. The generation of glucuronides can be formed through alcoholic and phenolic hydroxyl groups, carboxyl, sulfuryl, and carbonyl moieties, as well as through primary, secondary, and tertiary amine linkages. Examples of glucuronidation reactions are shown in Table 6–2 and Figure 6–5. The structural diversity in the many different types of drugs and xenobiotics that are processed through glucuronidation assures that most clinically efficacious therapeutic agents will be excreted as glucuronides.
There are 19 human genes that encode the UGT proteins. Nine are encoded by the UGT1 locus and 10 are encoded by the UGT2 family of genes. Both families of proteins are involved in the metabolism of drugs and xenobiotics, while the UGT2 family of proteins appears to have greater specificity for the glucuronidation of endogenous substances such as steroids. The UGT2 proteins are encoded by unique genes on chromosome 4 and the structure of each gene includes six exons. The clustering of the UGT2 genes on the same chromosome, with a comparable organization of the regions encoding the open reading frames, is evidence that gene duplication has occurred, a process of natural selection that has resulted in the multiplication and eventual diversification of the potential to detoxify the plethora of compounds that are targeted for glucuronidation.
The nine functional UGT1 proteins are all encoded by the UGT1 locus (Figure 6–6), which is located on chromosome 2. The UGT1 locus spans nearly 200 kb, with over 150 kb of a tandem array of cassette exonic regions that encode ~280 amino acids of the amino terminal portion of the UGT1A proteins. Four exons are located at the 3′ end of the locus that encode the carboxyl 245 amino acids that combine with one of the consecutively numbered array of first exons to form the individual UGT1 gene products. Since exons 2-5 encode the same sequence for each UGT1A protein, the variability in substrate specificity for each of the UGT1A proteins results from the significant divergence in sequence encoded by the exon 1 regions. The 5′ flanking region of each first-exon cassette contains a fully functional promoter capable of initiating transcription in an inducible and tissue-specific manner.
Organization of the UGT1A Locus. Transcription of the UGT1A genes commences with the activation of PolII, which is controlled through tissue-specific events. Conserved exons 2-5 are spliced to each respective exon 1 sequence, resulting in the production of unique UGT1A sequences. The UGT1A locus encodes nine functional proteins.
From a clinical perspective, the expression of UGT1A1 assumes an important role in drug metabolism, since the glucuronidation of bilirubin by UGT1A1 is the rate-limiting step in assuring efficient bilirubin clearance, and this rate can be affected by both genetic variation and competing substrates (drugs). Bilirubin is the breakdown product of heme, 80% of which originates from circulating hemoglobin and 20% from other heme-containing proteins such as the CYPs. Bilirubin is hydrophobic, associates with serum albumin, and must be metabolized further by glucuronidation to assure its elimination. The failure to efficiently metabolize bilirubin by glucuronidation leads to elevated serum levels and a clinical symptom called hyperbilirubinemia or jaundice. There are >50 genetic lesions in the UGT1A1 gene that can lead to inheritable unconjugated hyperbilirubinemia. Crigler-Najjar syndrome type I is diagnosed as a complete lack of bilirubin glucuronidation; Crigler-Najjar syndrome type II is differentiated by the detection of low amounts of bilirubin glucuronides in duodenal secretions. Types I and II Crigler-Najjar syndrome are rare, and result from genetic polymorphisms in the open reading frames of the UGT1A1 gene, resulting in abolished or highly diminished levels of functional protein.
Gilbert's syndrome is a generally benign condition that is present in up to 10% of the population; it is diagnosed clinically because circulating bilirubin levels are 60-70% higher than those seen in normal subjects. The most common genetic polymorphism associated with Gilbert's syndrome is a mutation in the UGT1A1 gene promoter, identified as the UGT1A1*28 allele, which leads to reduced expression levels of UGT1A1. Subjects diagnosed with Gilbert's syndrome may be predisposed to ADRs (Table 6–3) that result from a reduced capacity of UGT1A1 to metabolize drugs. If a drug undergoes selective metabolism by UGT1A1, competition for drug metabolism with bilirubin glucuronidation will exist, resulting in pronounced hyperbilirubinemia as well as reduced clearance of the metabolized drug. Tranilast [N-(3′4′-demethoxycinnamoyl)-anthranilic acid] is an investigational drug used for the prevention of restenosis in patients who have undergone transluminal coronary revascularization (intracoronary stents). Tranilast therapy in patients with Gilbert's syndrome can to lead to hyperbilirubinemia, as well as hepatic complications resulting from elevated levels of tranilast.
Gilbert's syndrome also alters patient responses to irinotecan. Irinotecan, a prodrug used in chemotherapy of solid tumors (see Chapter 61), is metabolized to its active form, SN-38, by serum carboxylesterases (Figure 6–5). SN-38, a potent topoisomerase inhibitor, is inactivated by UGT1A1 and excreted in the bile (Figures 6–7 and 6–8). Once in the lumen of the intestine, the SN-38 glucuronide undergoes cleavage by bacterial β-glucuronidase and reenters the circulation through intestinal absorption. Elevated levels of SN-38 in the blood lead to bone marrow toxicities characterized by leukopenia and neutropenia, as well as damage to the intestinal epithelial cells (Figure 6–8), resulting in acute and life-threatening diarrhea. Patients with Gilbert's syndrome who are receiving irinotecan therapy are predisposed to the hematological and GI toxicities resulting from elevated serum levels of SN-38.
While most of the drugs that are metabolized by UGT1A1 compete for glucuronidation with bilirubin, Gilbert's patients who are HIV-positive and on protease inhibitor therapy with atazanavir (reyataz) develop hyperbilirubinemia because the drug inhibits UGT1A1 function. Although atazanavir is not a substrate for glucuronidation, severe hyperbilirubinemia can develop in Gilbert's patients who have inactivating mutations in their UGT1A3 and UGT1A7 genes. Clearly, drug-induced side effects attributed to the inhibition of the UGT enzymes can be a significant concern and can be complicated in the presence of gene inactivating polymorphisms.
The UGTs are expressed in a tissue-specific and often inducible fashion in most human tissues, with the highest concentration of enzymes found in the GI tract and liver. Based upon their physicochemical properties, glucuronides are excreted by the kidneys into the urine or through active transport processes through the apical surface of the liver hepatocytes into the bile ducts where they are transported to the duodenum for excretion with components of the bile. Most of the bile acids that are conjugated are reabsorbed from the gut back to the liver via enterohepatic recirculation; many drugs that are glucuronidated and excreted in the bile can re-enter the circulation by this same process. The β-D-glucopyranosiduronic acids are targets for β-glucuronidase activity found in resident strains of bacteria that are common in the lower GI tract, thus liberating the free drug into the intestinal lumen. As water is reabsorbed into the large intestine, the free drug can then be transported by passive diffusion or through apical transporters back into the intestinal epithelial cells, from which the drug can then re-enter the circulation. Through portal venous return from the large intestine to the liver, the reabsorption process allows for the reentry of drug into the systemic circulation (Figures 3–7 and 3–8).
Table 6-3Drug Toxicity and Gilbert's Syndrome ||Download (.pdf) Table 6-3 Drug Toxicity and Gilbert's Syndrome
|PROBLEM ||FEATURE |
|Gilbert's syndrome ||UGT1A1*28 (main variant in Caucasians) |
|Established toxicity reactions || |
|UGT1A1 substrates (potential risk?) || |
Simvastatin, atorvastatin, cerivastatin†
Routes of SN-38 transport and exposure to intestinal epithelial cells. SN-38 is transported into the bile following glucuronidation by liver UGT1A1 and extrahepatic UGT1A7. Following cleavage of luminal SN-38 glucuronide (SN-38G) by bacterial β-glucuronidase, reabsorption into epithelial cells can occur by passive diffusion (indicated by the dashed arrows entering the cell) as well as by apical transporters. Movement into epithelial cells may also occur from the blood by basolateral transporters. Intestinal SN-38 can efflux into the lumen through P-glycoprotein (P-gp) and multidrug resistance protein 2 (MRP2) and into the blood by means of MRP1. Excessive accumulation of the SN-38 in intestinal epithelial cells and bone marrow, resulting from reduced glucuronidation, can lead to the cellular damage and toxicity depicted in Figure 6-8 (Reproduced with permission from Tukey RH et al. Pharmacogenetics of human UDP-glucuronosyltransferases and irinotecan toxicity. Mol Pharmacol, 2002, 62:446–450. Copyright © 2002 The American Society for Pharmacology and Experimental Therapeutics.).
Cellular targets of sn-38 in the blood and intestinal tissues. Excessive accumulation of SN-38 can lead to bone marrow toxicities such as leukopenia and neutropenia, as well as damage to the intestinal epithelium. These toxicities are pronounced in individuals that have reduced capacity to form the SN-38 glucuronide, such as patients with Gilbert's syndrome. Note the different body compartments and cell types involved (Reproduced with permission from Tukey RH et al. Pharmacogenetics of human UDP-glucuronosyltransferases and irinotecan toxicity. Mol Pharmacol, 2002, 62:446–450. Copyright © 2002 The American Society for Pharmacology and Experimental Therapeutics.)
Sulfation. The sulfotransferases (SULTs) arecytosolic and conjugate sulfate derived from 3′-phosphoadenosine-5′-phosphosulfate (PAPS) to the hydroxyl and, less frequently, amine groups of aromatic and aliphatic compounds. Like all of the xenobiotic metabolizing enzymes, the SULTs metabolize a wide variety of endogenous and exogenous substrates.
In humans, 13 SULT isoforms have been identified, and based on sequence comparisons, have been classified into the SULT1 (SULT1A1, SULT1A2, SULT1A3/4, SULT1B1, SULT1C1, SULT1C2, SULT1C4, SULT1E1), SULT2 (SULT2A1, SULT2B1a, SULT2B1b), SULT4 (SULT4A1), and SULT6 (SULT6A1) families. SULTs play an important role in normal human homeostasis. For example, SULT2B1b is a predominant form expressed in skin, carrying out the catalysis of cholesterol. Cholesterol sulfate is an essential metabolite in regulating keratinocyte differentiation and skin development. SULT2A1 is very highly expressed in the fetal adrenal gland, where it produces the large quantities of dehydroepiandrosterone sulfate that are required for placental estrogen biosynthesis during the second half of pregnancy. SULT1A3 is highly selective for catecholamines, while estrogens (in particular 17β-estradiol) are sulfated by SULT1E1. In humans, significant fractions of circulating catecholamines, estrogens, iodothyronines, and DHEA exist in the sulfated form.
Some human SULTs display great fidelity to unique substrate specificities; other SULTs are shockingly promiscuous in their activities. The SULT1 family isoforms are considered to be the major forms involved in xenobiotic metabolism, with SULT1A1 being quantitatively and qualitatively the most important in the liver. It displays extensive diversity in its capacity to catalyze the sulfation of a wide variety of structurally heterogeneous xenobiotics with high affinity. SULT1 isoforms have been recognized as phenol SULTs, since they catalyze the sulfation of phenolic molecules such as acetaminophen, minoxidil, and 17α-ethinyl estradiol. SULT1B1 is similar to SULT1A1 in that it can sulfate a very wide range of compounds, although it is much more abundant in the intestine than the liver. While three SULT1C isoforms exist in humans, little is known about their substrate specificity toward drugs or other compounds. In rodents, SULT1C enzymes are capable of sulfating the hepatic carcinogen N-OH-2-acetylaminofluorene, and are responsible for the bioactivation of this and related carcinogens. Their role in this pathway in humans is not clear. SULT1C enzymes are expressed abundantly in human fetal tissues, yet decline in abundance in adults. SULT1E catalyzes the sulfation of endogenous and exogenous steroids, and has been found localized in liver, as well as in hormone-responsive tissues such as the testis, breast, adrenal gland, and placenta. In the upper GI tract, SULT1A3 and SULT1B1 are particularly abundant.
The conjugation of drugs and xenobiotics is considered primarily a detoxification step, assuring that these compounds enter the aqueous compartments of the body and are targeted for elimination. However, drug metabolism through sulfation often leads to the generation of chemically reactive metabolites, where the sulfate is electron withdrawing and may be heterolytically cleaved, leading to the formation of an electrophilic cation. Most examples of the generation by sulfation of a carcinogenic or toxic response in animal or test mutagenicity assays have been documented with chemicals derived from the environment or from heterocyclic arylamine food mutagens generated from well-cooked meat. Thus, it is important to understand whether genetic linkages can be made by associating known human SULT polymorphisms to cancers that are believed to originate from environmental sources. Since SULT1A1 is the most abundant SULT form in human tissues and displays broad substrate specificity, the polymorphic profiles associated with this gene and their associations with various human cancers is of considerable interest. Recently, gene copy number polymorphisms within the SULT1A1, SULT1A3, and SULT1A4 genes have been indentified, which may help explain much of the inter-individual variation in the expression and activity of these enzymes. Knowledge of the structure, activities, regulation, and polymorphisms of the SULT superfamily will aid in understanding of the linkages between sulfation and cancer susceptibility, reproduction, and development. Sulfation is a major xenobiotic metabolizing system during human development, with levels of many enzymes higher in the fetus than the adult.
The SULTs from the SULT1 and SULT2 families were among the first xenobiotic-metabolizing enzymes to be crystallized, and the data indicated a highly conserved catalytic core (Negishi et al., 2001). Crystal structures of the different SULTs indicate that while conservation in the PAPS binding region is maintained, the organization of the substrate binding region differs, helping to explain the observed differences in catalytic potential displayed with the different SULTs.
Glutathione Conjugation. The glutathione-S-transferases (GSTs) catalyze the transfer of glutathione to reactive electrophiles, a function that serves to protect cellular macromolecules from interacting with electrophiles that contain electrophilic heteroatoms (-O, -N, and -S) and in turn protects the cellular environment from damage (Hayes et al., 2005). The co-substrate in the reaction is the tripeptide glutathione, which is synthesized from γ-glutamic acid, cysteine, and glycine (Figure 6–9). Glutathione exists in the cell as oxidized (GSSG) or reduced (GSH) forms, and the ratio of GSH:GSSG is critical in maintaining a cellular environment in the reduced state. In addition to affecting xenobiotic conjugation with GSH, a severe reduction in GSH content can predispose cells to oxidative damage, a state that has been linked to a number of human health issues.
Glutathione (GSH) as a co-substrate in the conjugation of a drug or xenobiotic (X) by glutathione-S-transferase (GST).
In the formation of glutathione conjugates, the reaction generates a thioether linkage with drug or xenobiotic to the cysteine moiety of the tripeptide. Characteristically, all GST substrates contain an electrophilic atom and are hydrophobic, and by nature will associate with cellular proteins. Since the concentration of glutathione in cells is usually very high, typically ∼7 μmol/g of liver, or in the 10 mM range, many drugs and xenobiotics can react non-enzymatically with glutathione. However, the GSTs have been found to occupy up to 10% of the total cellular protein concentration, a property that assures efficient conjugation of glutathione to reactive electrophiles. The high concentration of GSTs also provides the cells with a sink of cytosolic protein, a property that facilitates non-covalent and sometimes covalent interactions with compounds that are not substrates for glutathione conjugation. The cytosolic pool of GSTs, once identified as ligandin, has been shown to bind steroids, bile acids, bilirubin, cellular hormones, and environmental toxicants, in addition to complexing with other cellular proteins.
Over 20 human GSTs have been identified and divided into two subfamilies: the cytosolic and the microsomal forms. The major differences in function between the microsomal and cytosolic GSTs reside in the selection of substrates for conjugation; the cytosolic forms have more importance in the metabolism of drugs and xenobiotics, whereas the microsomal GSTs are important in the endogenous metabolism of leukotrienes and prostaglandins. The cytosolic GSTs are divided into seven classes termed alpha (GSTA1 and 2), mu (GSTM1 through 5), omega (GSTO1), pi (GSTP1), sigma (GSTS1), theta (GSTT1 and 2), and zeta (GSTZ1). Those in the alpha and mu classes can form heterodimers, allowing for a large number of active transferases to form. The cytosolic forms of GST catalyze conjugation, reduction, and isomerization reactions.
The high concentrations of GSH in the cell and the plenitude of GSTs mean that few reactive molecules escape detoxification. However, while there appears to be an overcapacity of enzyme and reducing equivalents, there is always concern that some reactive intermediates will escape detoxification, and by nature of their electrophilicity, will bind to cellular components, and cause toxicity. The potential for such an occurrence is heightened if GSH is depleted or if a specific form of GST is polymorphic. While it is difficult to deplete cellular GSH levels, reactive therapeutic agents that require large doses for clinical efficacy have the greatest potential to lower cellular GSH levels. Acetaminophen, which is normally metabolized by glucuronidation and sulfation, is also a substrate for oxidative metabolism by CYP2E1 and CYP3A4, which generate the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI) that, under normal dosing, is readily neutralized through conjugation with GSH. However, an overdose of acetaminophen can lead to depletion of cellular GSH levels, thereby increasing the potential for NAPQI to interact with other cellular components resulting in toxicity and cell death (see Figure 4–5). Acetaminophen toxicity is associated with increased levels of NAPQI and hepatic necrosis.
Like many of the enzymes involved in drug and xenobiotic metabolism, all of the GSTs have been shown to be polymorphic. The mu (GSTM1*0) and theta (GSTT1*0) genotypes express a null phenotype; thus, individuals that are polymorphic at these loci are predisposed to toxicities by agents that are selective substrates for these GSTs. For example, the mutant GSTM1*0 allele is observed in 50% of the Caucasian population and has been linked genetically to human malignancies of the lung, colon, and bladder. Null activity in the GSTT1 gene has been associated with adverse side effects and toxicity in cancer chemotherapy with cytostatic drugs; the toxicities result from insufficient clearance of the drugs by GSH conjugation. Expression of the null genotype can be as high as 60% in Chinese and Korean populations. GST polymorphisms may influence efficacies and severity of adverse side effects of drugs.
While the GSTs play an important role in cellular detoxification, their activities in cancerous tissues have been linked to the development of drug resistance toward chemotherapeutic agents that are both substrates and nonsubstrates for the GSTs. Many anticancer drugs are effective because they initiate cell death or apoptosis, which is linked to the activation of mitogen-activated protein (MAP) kinases such as JNK and p38. Investigational studies have demonstrated that overexpression of GSTs is associated with resistance to apoptosis and the inhibition of MAP kinase activity. In a variety of tumors, GSTs are overexpressed, leading to a reduction in MAP kinase activity and reduced efficacy of chemotherapy. Taking advantage of the relatively high levels of GST in tumor cells, inhibition of GST activity has been exploited as a therapeutic strategy to modulate drug resistance by sensitizing tumors to anticancer drugs. TLK199, a glutathione analog, serves as a prodrug that undergoes activation by plasma esterases to a GST inhibitor, TLK117, which potentiates the toxicity of different anticancer agents (Figure 6–10). Alternatively, the elevated GST activity in cancer cells has been utilized to develop pro-drugs that can be activated by the GSTs to form electrophilic intermediates. For example, TLK286 is a substrate for GST that undergoes a β-elimination reaction, forming a glutathione conjugate and a nitrogen mustard (Figure 6–11) capable of alkylating cellular nucleophiles, resulting in anti-tumor activity.
Activation of TLK199 by cellular esterases to the glutathione-S-transferase (GST) inhibitor TLK117. (For additional information, see Townsend and Tew, 2003.)
Generation of the reactive alkylating agent following the conjugation of glutathione to TLK286. GST interacts with the prodrug and GSH analog, TLK286, via a tyrosine in the active site of GST. GSH portion is shown in red. The interaction promotes β-elimination and cleavage of the prodrug to a vinyl sulfone and an active alkylating fragment. (See Townsend and Tew, 2003.)
N-Acetylation. The cytosolic N-acetyltransferases (NATs) are responsible for the metabolism of drugs and environmental agents that contain an aromatic amine or hydrazine group. The addition of the acetyl group from the cofactor acetyl-coenzyme A often leads to a metabolite that is less water soluble because the potential ionizable amine is neutralized by the covalent addition of the acetyl group. NATs are among the most polymorphic of all the human xenobiotic drug-metabolizing enzymes.
The characterization of an acetylator phenotype in humans was one of the first hereditary traits identified, and was responsible for the development of the field of pharmacogenetics (see Chapter 7). Following the discovery that isonicotinic acid hydrazide (isoniazid, INH) could be used in the cure of tuberculosis, a significant proportion of the patients (5-15%) experienced toxicities that ranged from numbness and tingling in their fingers to CNS damage. After finding that isoniazid was metabolized by acetylation and excreted in the urine, researchers noted that individuals suffering from the toxic effects of the drug excreted the largest amount of unchanged drug and the least amount of acetylated isoniazid. Pharmacogenetic studies led to the classification of "rapid" and "slow" acetylators, with the "slow" phenotype being predisposed to toxicity. Purification and characterization of N-acetyltransferase and the eventual cloning of its RNA provided sequence characterization of the gene, revealing polymorphisms that correspond to the "slow" acetylator phenotype. There are two functional NAT genes in humans, NAT1 and NAT2. Over 25 allelic variants of NAT1 and NAT2 have been characterized, and in individuals in whom acetylation of drugs is compromised, homozygous genotypes for at least two variant alleles are required to predispose a patient to lowered drug metabolism. Polymorphism in the NAT2 gene, and its association with the slow acetylation of isoniazid, was one of the first completely characterized genotypes shown to affect drug metabolism, thereby linking pharmacogenetic phenotype to a genetic polymorphism. Although nearly as many mutations have been identified in the NAT1 gene as the NAT2 gene, the frequency of the slow acetylation patterns are attributed mostly to the polymorphism in the NAT2 gene.
Drugs that are subject to acetylation and their known toxicities are listed in Table 6–4. The therapeutic relevance of NAT polymorphisms is in avoiding drug-induced toxicities. The adverse drug response in a slow acetylator resembles a drug overdose; thus, reducing the dose or increasing the dosing interval is recommended. Aromatic amine or a hydrazine groups exist in many classes of clinically used drugs, and if a drug is known to be metabolized through acetylation, determining an individual's phenotype can be important in maximizing outcome in subsequent therapy. For example, hydralazine, a once popular orally active antihypertensive (vasodilator) drug, is metabolized by NAT2; the administration of therapeutic doses of hydralazine to a slow acetylator can result in extreme hypotension and tachycardia. Several drugs, such as the sulfonamides, that are known targets for acetylation have been implicated in idiosyncratic hypersensitivity reactions; in such instances, an appreciation of a patient's acetylating phenotype is particularly important. Sulfonamides are transformed into hydroxylamines that interact with cellular proteins, generating haptens that can elicit autoimmune responses. Individuals who are slow acetylators are predisposed to drug-induced autoimmune disorders.
Table 6-4Indications and Unwanted Side Effects of Drug Metabolized by N-Acetyltransferases ||Download (.pdf) Table 6-4 Indications and Unwanted Side Effects of Drug Metabolized by N-Acetyltransferases
|DRUG ||INDICATION ||MAJOR SIDE EFFECTS |
|Acebutolol ||Arrhythmias, hypertension ||Drowsiness, weakness, insomnia |
|Amantadine ||Influenza A, parkinsonism ||Appetite loss, dizziness, headache, nightmares |
|Aminobenzoic acid ||Skin disorders, sunscreens ||Stomach upset, contact sensitization |
|Aminoglutethimide ||Adrenal cortex carcinoma, breast cancer ||Clumsiness, nausea, dizziness, agranulocytosis |
|Aminosalicylic acid ||Ulcerative colitis ||Allergic fever, itching, leukopenia |
|Amonafide ||Prostate cancer ||Myelosuppression |
|Amrinone ||Advanced heart failure ||Thrombocytopenia, arrhythmias |
|Benzocaine ||Local anesthesia ||Dermatitis, itching, rash, methemoglobinemia |
|Caffeine ||Neonatal respiratory distress syndrome ||Dizziness, insomnia, tachycardia |
|Clonazepam ||Epilepsy ||Ataxia, dizziness, slurred speech |
|Dapsone ||Dermatitis, leprosy, AIDS-related complex ||Nausea, vomiting, hyperexcitability, methemoglobinemia, dermatitis |
|Dipyrone, metamizole ||Analgesic ||Agranulocytosis |
|Hydralazine ||Hypertension ||Hypotension, tachycardia, flush, headache |
|Isoniazid ||Tuberculosis ||Peripheral neuritis, hepatotoxicity |
|Nitrazepam ||Insomnia ||Dizziness, somnolence |
|Phenelzine ||Depression ||CNS excitation, insomnia, orthostatic hypotension, hepatotoxicity |
|Procainamide ||Ventricular tachyarrhythmia ||Hypotension, systemic lupus erythematosus |
|Sulfonamides ||Antibacterial agents ||Hypersensitivity, hemolytic anemia, fever, lupus-like syndromes |
Tissue-specific expression patterns of NAT1 and NAT2 have a significant impact on the fate of drug metabolism and the potential for eliciting a toxic episode. NAT1 is ubiquitously expressed among most human tissues, whereas NAT2 is found predominantly in liver and the GI tract. Both NAT1 and NAT2 can form N-hydroxy–acetylated metabolites from bicyclic aromatic hydrocarbons, a reaction that leads to the non-enzymatic release of the acetyl group and the generation of highly reactive nitrenium ions. Thus, N-hydroxy acetylation is thought to activate certain environmental toxicants. In contrast, direct N-acetylation of bicyclic aromatic amines is stable and leads to detoxification. Individuals who are NAT2 fast acetylators are able to efficiently metabolize and detoxify bicyclic aromatic amines through liver-dependent acetylation. Slow acetylators (NAT2 deficient), however, accumulate bicyclic aromatic amines, which then become substrates for CYP-dependent N-oxidation. These N-OH metabolites are eliminated in the urine. In tissues such as bladder epithelium, NAT1 is highly expressed and can efficiently catalyze the N-hydroxy acetylation of bicyclic aromatic amines, a process that leads to de-acetylation and the formation of the mutagenic nitrenium ion, especially in NAT2-deficient subjects. Epidemiological studies have shown that slow acetylators are predisposed to bladder cancer if exposed environmentally to bicyclic aromatic amines.
Methylation. In humans, drugs and xenobiotics can undergo O-, N-, and S-methylation. The identification of the individual methyltransferase (MT) is based on the substrate and methyl conjugate. Humans express three N-methyltransferases, one catechol-O-methyltransferase (COMT), a phenol-O-methyltransferase (POMT), a thiopurine S-methyltransferase (TPMT), and a thiol methyltransferase (TMT). These MTs exist as monomers and use S-adenosyl-methionine (SAM; AdoMet) as the methyl donor. With the exception of a signature sequence that is conserved among the MTs, there is limited conservation in sequence, indicating that each MT has evolved to display a unique catalytic function. Although the common theme among the MTs is the generation of a methylated product, substrate specificity is high and distinguishes the individual enzymes.
Nicotinamide N-methyltransferase (NNMT) methylates serotonin and tryptophan as well as pyridine-containing compounds such as nicotinamide and nicotine. Phenylethanolamine N-methyltransferase (PNMT) is responsible for the methylation of the neurotransmitter norepinephrine, forming epinephrine; the histamine N-methyltransferase (HNMT) metabolizes drugs containing an imidazole ring such as that found in histamine. COMT methylates the ring hydroxyl groups of neurotransmitters containing a catechol moiety, such as dopamine and norepinephrine, in addition to drugs such as methyldopa and drugs of abuse such as ecstasy (MDMA; 3, 4-methylenedioxymethamphetamine).
From a clinical perspective, the most important MT may be thiopurine S-methyltransferase (TPMT), which catalyzes the S-methylation of aromatic and heterocyclic sulfhydryl compounds, including the thiopurine drugs azathioprine (AZA), 6-mercaptopurine (6-MP); and thioguanine. AZA and 6-MP are used for the management of inflammatory bowel disease (see Chapter 47), as well as autoimmune disorders such as systemic lupus erythematosus and rheumatoid arthritis. Thioguanine is used in the treatment of acute myeloid leukemia, and 6-MP is used worldwide for the treatment of childhood acute lymphoblastic leukemia (see Chapters 61, 62, 63). Because TPMT is responsible for the detoxification of 6-MP, a genetic deficiency in TPMT can result in severe toxicities in patients taking these drugs. When given orally at clinically established doses, 6-MP serves as a prodrug that is metabolized by hypoxanthine guanine phosphoribosyl transferase (HGPRT) to 6-thioguanine nucleotides (6-TGNs), which become incorporated into DNA and RNA, resulting in arrest of DNA replication and cytotoxicity. The toxic side effects arise when a lack of 6-MP methylation by TPMT causes a buildup of 6-MP, resulting in the generation of toxic levels of 6-TGNs. The identification of the inactive TPMT alleles and the development of a genotyping test to identify homozygous carriers of the defective allele have now made it possible to identify individuals who may be predisposed to the toxic side effects of 6-MP therapy. Simple adjustments in the patient's dosage regiment have been shown to be a life-saving intervention for those with TPMT deficiencies.