Genetic Factors

Genetic determinants predispose to drug-induced liver disease,¹³ as they do for other types of drug reaction, such as penicillin allergy. Atopic patients have been thought to have an increased risk of some types of drug hepatitis, but this increase in risk has not been proved. Genetic factors determine the activity of drug-activating and antioxidant pathways, encode pathways of canalicular bile secretion, and modulate the immune response, tissue stress responses, and cell death pathways (see Chapter 72). Documented examples of drugs associated with a familial predisposition to adverse hepatic drug reactions are few and include valproic acid and phenytoin.^5,8,13 Inherited mitochondrial diseases are a risk factor for valproic acid–induced hepatotoxicity.¹⁴ Some forms of drug-induced liver disease, particularly drug-induced hepatitis and granulomatous reactions, can be associated with the reactive metabolite syndrome (see later).

Weak associations have been reported between specific human leukocyte antigen (HLA) haplotypes and some types of drug-induced liver disease. Andrade and colleagues¹³ found positive associations between the class II HLA haplotype and cholestatic or mixed liver damage for some drugs. They suggested that no specific HLA allele predisposed to the overall risk of drug-induced liver disease but that the pattern of liver injury could be influenced by these genetic determinants. Other investigators have found stronger associations between the HLA haplotype and cholestatic reactions to amoxicillin-clavulanic acid and ticlopidine (see later).^13,15

Age

Most hepatic drug reactions are more common in adults than in children. Exceptions include valproic acid hepatotoxicity, which is most common in infants younger than three years of age and rare in adults, and Reye’s syndrome, in which salicylates play a key role.^16,17 As discussed later, both may be examples of mitochondrial toxicity.¹⁴ In adults, the risk of isoniazid-associated hepatotoxicity is greater in persons older than 40 years of age. Similar observations have been made for nitrofurantoin, halothane, etretinate, diclofenac, and troglitazone.^1,4,5,10 The increased frequency of adverse drug reactions in older subjects is largely the result of increased exposure, the use of multiple agents, and altered drug disposition.^1,4,5,10 In addition, clinical severity of hepatotoxicity increases strikingly with age, as exemplified by fatal reactions to isoniazid and halothane.

Gender

Women are particularly predisposed to drug-induced hepatitis, a difference that cannot be attributed simply to increased exposure. Examples include toxicity caused by halothane, nitrofurantoin, sulfonamides, flucloxacillin, minocycline, and troglitazone.^5,6 Drug-induced chronic hepatitis caused by nitrofurantoin, diclofenac, or minocycline has an even more pronounced female preponderance.⁶ Conversely, equal sex frequency or even male preponderance is common for some drug reactions characterized by cholestasis, as for amoxicillin-clavulanic acid. Azathioprine-induced liver disease is more likely to develop in male renal transplant recipients than in female recipients.¹⁸

Concomitant Exposure to Other Agents

Patients who are taking multiple drugs are more likely to experience an adverse reaction than those who are taking one agent.^5,9,10 The mechanisms include enhanced cytochrome P450 (CYP)-mediated metabolism of the second drug to a toxic intermediate (see later). Examples discussed later include toxicity caused by acetaminophen, isoniazid, valproic acid, other anticonvulsants, and anticancer drugs. Alternatively, drugs may alter the disposition of other agents by reducing bile flow or competing with canalicular pathways for biliary excretion (phase 3 drug elimination) (see later). This mechanism may account for apparent interactions between oral contraceptive steroids and other drugs to produce cholestasis. Drugs or their metabolites may also interact in mechanisms of cellular toxicity and cell death that involve mitochondrial injury, intracellular signaling pathways, activation of transcription factors, and regulation of hepatic genes involved in controlling the response to stress and injury that triggers pro-inflammatory and cell death processes.^19,20

Previous Drug Reactions

A history of an adverse drug reaction generally increases the risk of reactions to the same drug and also to some other agents (see later). Nevertheless, instances of cross-sensitivity to related agents in cases of drug-induced liver disease are surprisingly uncommon. Examples of cross-sensitivity between drugs (or drug classes) include the haloalkane anesthetics (see Chapter 87), erythromycins, phenothiazines and tricyclic antidepressants, isoniazid and pyrazinamide, sulfonamides and other sulfur-containing compounds (e.g., some clyclooxygenase-2 [COX-2] inhibitors), and some NSAIDs. A crucial point is that a previous reaction to the same drug is a major risk factor for an increase in the severity of drug-induced liver injury.⁶

Alcohol

Chronic excessive alcohol ingestion decreases the dose threshold for, and enhances the severity of, acetaminophen-induced hepatotoxicity and increases the risk and severity of isoniazid hepatitis, niacin (nicotinic acid, nicotinamide) hepatotoxicity, and methotrexate-induced hepatic fibrosis.

Nutritional Status

Obesity is strongly associated with the risk of halothane hepatitis (see Chapter 87) and appears to be an independent risk factor for NASH and hepatic fibrosis in persons taking methotrexate or tamoxifen. Fasting also predisposes to acetaminophen hepatotoxicity,²¹ and a role for undernutrition has been proposed in isoniazid hepatotoxicity.²²

Preexisting Liver Disease

In general, liver diseases such as alcoholic cirrhosis and cholestasis do not predispose to adverse hepatic reactions. Exceptions include toxicity to some anticancer drugs, niacin, pemoline, and hycanthone. Preexisting liver disease is a critical determinant of methotrexate-induced hepatic fibrosis (discussed later). Patients with chronic HBV infection¹² and possibly those with chronic HCV infection or HIV/AIDS appear to be at heightened risk of liver injury during anti-tuberculosis or HAART therapy,²³ after exposure to ibuprofen and possibly other NSAIDs, after myeloablative therapy in preparation for bone marrow transplantation (resulting in sinusoidal obstruction syndrome [see later]),²⁴ and possibly after taking antiandrogens, such as flutamide and cyproterone acetate.²⁵ A particularly strong association has been observed between HCV infection (present in 33% of patients with HIV/AIDS) and the risk of liver injury during HAART; the risk may be increased 2- to 10-fold.^26–30

Other Diseases

Rheumatoid arthritis appears to increase the risk of salicylate hepatotoxicity, and a curious, unexplained observation is that hepatitis associated with sulfasalazine appears to be more common in patients with rheumatoid arthritis than in those with inflammatory bowel disease.^8–10,31,32 Diabetes mellitus, obesity, and chronic kidney disease predispose to methotrexate-induced hepatic fibrosis, whereas HIV/AIDS confers a heightened risk of sulfonamide hypersensitivity.^31–33 A retrospective cohort study of five health maintenance organizations found that the age- and sex-standardized incidence of drug-induced acute liver failure in patients with diabetes mellitus was 0.08 to 0.15 per 1000 person-years, irrespective of the therapeutic agent used (the number using troglitazone was small); the incidence was highest (approximately 0.3 per 1000) during the first six months of exposure.³² Renal transplantation is a risk factor for azathioprine-associated vascular injury, whereas kidney disease predisposes to tetracycline-induced fatty liver.⁶ Finally, sinusoidal obstruction syndrome induced by anticancer drugs is more common after bone marrow transplantation²⁴ and in persons with HCV infection.^{5,6,8–10,26}

Cytochrome P450

Most type 1 reactions are catalyzed by microsomal drug oxidases, the key component of which is a hemoprotein of the CYP gene superfamily. The apparent promiscuity of drug oxidases toward drugs, environmental toxins, steroid hormones, lipids, and bile acids results from the existence of multiple closely related CYP proteins. More than 20 CYP enzymes are present in the human liver.³⁴

The reaction cycle involves binding of molecular oxygen to the iron in the heme prosthetic group, with subsequent reduction of oxygen by acceptance of an electron from nicotinamide-adenine dinucleotide phosphate (NADPH) cytochrome P450 reductase, a flavoprotein reductase. The resulting “activated oxygen” is incorporated into the drug or another lipophilic compound. Reduction of oxygen and insertion into a drug substrate (“mixed function oxidation”) can result in formation of chemically reactive intermediates, including free radicals, electrophilic “oxy-intermediates” (e.g., unstable epoxides, quinone imines), and reduced (and therefore reactive) oxygen species (ROS). The quintessential example is the CYP2E1-catalyzed metabolite of acetaminophen, N-acetyl-p-benzoquinone imine (NAPQI), an oxidizing and arylating metabolite that is responsible for liver injury associated with acetaminophen hepatotoxicity. Other quinone compounds are potential reactive metabolites of troglitazone, quinine, and methyldopa. Epoxide metabolites of diterpenoids may be hepatotoxic products of the hepatic metabolism of some plant toxins (see Chapter 87).³⁵ ROS have broad significance in the production of tissue injury, particularly by contributing to the production of oxidative stress and triggering tissue stress responses and cell death pathways, as discussed later.

The hepatic content of CYP proteins is higher in acinar zone 3 (see Chapter 71). Localization of CYP2E1 is usually confined to a narrow rim of hepatocytes 1 to 2 cells thick around the terminal hepatic venule. This finding explains in part the zonality of hepatic lesions produced by drugs and toxins, such as acetaminophen and carbon tetrachloride, which are converted to reactive metabolites.

Genetic and Environmental Determinants of Cytochrome P450 Enzymes

Other Pathways of Drug Oxidation

In addition to CYP enzymes, electron transport systems of mitochondria may lead to the generation of tissue-damaging reactive intermediates during the metabolism of some drugs. Examples of such reactive intermediates include nitroradicals from nitrofuran derivatives (nitrofurantoin, cocaine). Subsequent electron transfer by flavoprotein reductases into molecular oxygen generates superoxide and other ROS. Some anticancer drugs such as doxorubicin and the imidazole antimicrobial agents can participate in other oxidation-reduction (redox) cycling reactions that generate ROS.

Phase 2 (Conjugation) Reactions

Phase 2 reactions involve formation of ester links to the parent compound or a drug metabolite. The responsible enzymes include glucuronosyl (or glucuronyl) transferases, sulfotransferases, glutathione-S transferases, and acetyl and amino acid N-transferases. The resulting conjugates are highly water soluble and can be excreted readily in bile or urine. Conjugation reactions can be retarded by depletion of their rate-limiting cofactors, such as glucuronic acid and inorganic sulfate, and the relatively low capacity of these enzyme systems restricts the efficacy of drug elimination when substrate concentrations exceed enzyme saturation. In general, drug conjugates are nontoxic, and phase 2 reactions are considered to be detoxification reactions, with exceptions. For example, some glutathione conjugates can undergo cysteine S-conjugate β-lyase-mediated activation to highly reactive intermediates. Little is known about the regulation of such enzymes or their potential significance for drug-induced liver disease or hepatocarcinogenesis.

Phase 3 Pathways

Several transporters secrete drugs, drug metabolites, or their conjugates into bile, and this mechanism is often referred to as phase 3 of hepatic drug elimination. These pathways involve ATP-binding cassette (ABC) proteins, which derive the energy for their transport functions from hydrolysis of ATP. ABC transport proteins are widely distributed in nature and include the cystic fibrosis transmembrane conductance regulator (CFTR) (see Chapter 76) and the canalicular and intestinal copper transporters (see Chapter 75). The role of ABC transport proteins in secretion of bile has been reviewed (see Chapter 64).^37,38,41,42

Multidrug resistance protein 1 (MDR1) is highly expressed on the apical (canalicular) plasma membrane of hepatocytes, where it transports cationic drugs, particularly anticancer agents, into bile. Another family of ABC transporters, the multidrug resistance-associated proteins (MRPs), is also expressed in the liver. At least two members of this family excrete drug (and other) conjugates from hepatocytes: MRP-3 on the basolateral surface facilitates passage of drug conjugate into the sinusoidal circulation, and MRP-2, expressed on the canalicular membrane, pumps endogenous compounds (e.g., bilirubin diglucuronide, leukotriene-glutathionyl conjugates, glutathione) and drug conjugates into bile. The bile salt export pump (BSEP) and MDR3 (in humans) and Mdr2 (in mice) are other canalicular transporters concerned, respectively, with bile acid and phospholipid secretion into bile. Genetic polymorphisms of these genes are associated with human cholestatic liver diseases (see Chapters 64 and 76). BSEP interacts with several drugs.⁴²

Regulation of the membrane expression and activity of these drug elimination pathways is complex. The possibility that their altered expression or impaired activity (by competition between agents, changes in membrane lipid composition, or damage from reactive metabolites or covalent binding) could lead to drug accumulation, impairment of bile flow, or cholestatic liver injury has been demonstrated for estrogens,^43,44 troglitazone,⁴⁵ terbinafine,⁴⁶ and flucloxacillin⁴⁷ and may have wider mechanistic importance for drug-induced cholestasis and other forms of liver injury.⁴²

Direct Hepatotoxins and Reactive Metabolites

Highly hepatotoxic chemicals injure key subcellular structures, particularly mitochondria and the plasma membrane. The injury arrests energy generation, dissipates ionic gradients, and disrupts the physical integrity of the cell. This type of overwhelming cellular injury does not apply to currently relevant hepatotoxins, most of which require metabolic activation to mediate damage to liver cells. The resulting reactive metabolites can interact with critical cellular target molecules, particularly those with nucleophilic substituents such as thiol-rich proteins and nucleic acids. Together with ROS, they act as oxidizing species within the hepatocyte to establish oxidative stress, a state of imbalance between pro-oxidants and antioxidants. ROS are also key signaling molecules that mediate biological responses to stress, as discussed later. Alternatively, reactive metabolites bind irreversibly to macromolecules, particularly proteins and lipids. Such covalent binding may produce injury by inactivating key enzymes or by forming protein-drug adducts that could be targets for immunodestructive processes that cause liver injury.

Oxidative Stress and the Glutathione System

The liver is exposed to oxidative stress by the propensity of hepatocytes to reduce oxygen, particularly in mitochondria and also in microsomal electron transport systems (such as CYP2E1), and by NADPH-oxidase-catalyzed formation of ROS and nitroradicals in Kupffer cells, endothelial cells, and stimulated polymorphs and macrophages. To combat oxidative stress, the liver is well-endowed with antioxidant mechanisms, including micronutrients, such as vitamin E and vitamin C, thiol-rich proteins (e.g., metallothionein, ubiquinone), metal-sequestering proteins (e.g., ferritin), and enzymes that metabolize reactive metabolites (e.g., epoxide hydrolases), ROS (e.g., catalase, superoxide dismutase), and lipid peroxides (e.g., glutathione peroxidases). Glutathione (l-gamma-glutamyl-l-cyteine-glycine) is the most important antioxidant in the mammalian liver.¹⁹

Hepatocytes are the exclusive site of glutathione synthesis. Hepatic levels of glutathione are high (5 to 10 mmol/L) and can be increased by enhancing the supply of cysteine for glutathione synthesis; this mechanism is the cornerstone of thiol antidote therapy for acetaminophen poisoning. Hepatocyte glutathione synthesis increases in response to pro-oxidants, as occurs when CYP2E1 is overexpressed as a result of signaling via the redox-sensitive transcription factor Nrf.^19,20,48,49 Glutathione is a critical cofactor for several antioxidant pathways, including thiol-disulfide exchange reactions and glutathione peroxidase. Glutathione peroxidase has a higher affinity for hydrogen peroxide than does catalase, and it disposes of lipid peroxides, free radicals, and electrophilic drug metabolites. Reduced glutathione is a cofactor for conjugation reactions catalyzed by the glutathione S-transferases. Other reactions proceed nonenzymatically. In turn, the products include glutathione-protein mixed disulfides and oxidized glutathione. The latter can be converted back to glutathione by proton donation catalyzed by glutathione reductase.

Normally, most glutathione within the hepatocyte is in the reduced state, indicating the importance of this pathway for maintenance of the redox capacity of the cell. The reduced form of NADPH is an essential cofactor for glutathione reductase; NADPH formation requires ATP, thereby illustrating a critical link between the energy-generating capacity of the liver and its ability to withstand oxidative stress. Glutathione is also compartmentalized within the hepatocyte, with the highest concentrations found in the cytosol. Adequate levels of glutathione are essential in mitochondria, where ROS are constantly being formed as a minor by-product of oxidative respiration and in response to some drugs or metabolites that interfere with the mitochondrial respiratory chain. Mitochondrial glutathione is maintained by active uptake from the cytosol, a transport system that is altered by chronic ethanol exposure, and is, therefore, another potential target of drug toxicity.¹⁹

Biochemical Mechanisms of Cellular Injury

Mechanisms once thought to be central to hepatotoxicity, such as covalent binding to cellular enzymes and peroxidation of membrane lipids, are no longer regarded as exclusive pathways of cellular damage. Rather, oxidation of proteins, phospholipid fatty acyl side chains (lipid peroxidation), and nucleosides appear to be components of the biochemical stress that characterizes toxic liver injury. Secondary reactions also may play a role; these reactions include post-translational modification of proteins via adenosine diphosphate (ADP) ribosylation or protease activation, cleavage of DNA by activation of endogenous endonucleases, and disruption of lipid membranes by activated phospholipases.²⁰ Some of these catabolic reactions could be initiated by a rise in the cytosolic ionic calcium concentration [Ca²⁺]_i, as a result of increased Ca²⁺ entry or release from internal stores in the endoplasmic reticulum and mitochondria.^19,20

The concept that hepatotoxic chemicals cause hepatocyte cell death by a biochemical final common pathway (e.g., activation of catalytic enzymes by a rise in [Ca²⁺]_i) has proved inadequate to explain the diverse processes that can result in lethal hepatocellular injury. Rather, a variety of processes can damage key organelles, thereby causing intracellular stress that activates signaling pathways and transcription factors. In turn, the balance between these factors can trigger the onset of cell death or facilitate protection of the cell, as discussed in the following sections.

Types of Cell Death

Apoptosis

Apoptosis is an energy-dependent, genetically programmed form of cell death that typically results in controlled deletion of individual cells. In addition to its major roles in developmental biology, tissue regulation, and carcinogenesis, apoptosis is important in toxic, viral, and immune-mediated liver injury.^50–53 The ultrastructural features of apoptosis are cell and nuclear shrinkage, condensation and margination of nuclear chromatin, plasma membrane blebbing, and ultimately fragmentation of the cell into membrane-bound bodies that contain intact mitochondria and other organelles. Engulfment of these apoptotic bodies by surrounding epithelial and mesenchymal cells conserves cell fragments that contain nucleic acid and intact mitochondria. These fragments are then digested by lysosomes and recycled without release of bioactive substances. As a consequence, apoptosis in it purest form (usually found only in vitro) does not incite an inflammatory tissue reaction. The cellular processes that occur in apoptosis are often mediated by caspases, a family of proteolytic enzymes that contain a cysteine at their active site and cleave polypeptides at aspartate residues; non–caspase-mediated programmed cell death has also been described in experimental hepatotoxicity (see also Chapter 72).

Apoptosis rarely, if ever, is the sole form of cell death in common forms of liver injury, such as ischemia-reperfusion injury, cholestasis, and toxic liver injury, all of which are typically associated with a hepatic inflammatory response. Whether or not activation of pro-death signals causes cell death depends on several factors, including pro-survival signals, the rapidity of the process, the availability of glutathione and ATP, and the role of other cell types. Some of these issues are discussed briefly here and are reviewed in more detail elsewhere.^20,50–53

Hepatocytes undergo apoptosis when pro-apoptotic intracellular signaling pathways are activated, either because of toxic biochemical processes within the cell (intrinsic pathway) or because cell surface receptors are activated to transduce cell death signals (external pathway). Pro-apoptotic receptors are members of the tumor necrosis factor-α (TNF-α) receptor superfamily, which possess a so-called death domain. These receptors include Fas, for which the cognate ligand is Fas-ligand (Fas-L), TNF-R1 receptor (cognate ligand is TNF), and TNF-related apoptosis-inducing ligand (TRAIL) receptors (cognate ligand is TRAIL). In addition to model hepatotoxins such as the quinone, menadione, and hydrogen peroxide, some drugs (e.g., acetaminophen, plant diterpenoids) have been shown to be converted into pro-oxidant reactive metabolites, thereby initiating the following sequence: CYP-mediated metabolism to form reactive metabolites → glutathione depletion → mitochondrial injury with release of cytochrome c and operation of the mitochondrial membrane permeability transition (MPT) → caspase activation → apoptosis.

Mitochondria play a pivotal role in pathways that provoke or oppose apoptosis.^50,51,53 In the external pathway, activation of the death domain of pro-apoptotic receptors recruits adapter molecules—Fas-associated death domain (FADD) and TNF receptor-associated death domain (TRADD)—which bind and activate procaspase 8 to form the death-inducing signaling complex (DISC). In turn, caspase 8 cleaves Bid, a pro-apoptotic member of the B cell lymphoma/leukemia (Bcl-2) family, to tBid. tBid causes translocation of Bax to the mitochondria, where it aggregates with Bak to promote permeability of the mitochondria.⁵⁰ Release of cytochrome c and other pro-death molecules, including Smac (which binds caspase inhibitor proteins, such as inhibitor of apoptosis proteins [IAPs]) and apoptosis-inducing factor (AIF, also known as Apaf)⁵¹ allows formation of the “aptosome,” which activates caspase 9 and eventually caspase 3 to execute cell death (Fig. 86-1). Intracellular stresses in various sites release other mitochondrial permeabilizing proteins (e.g., Bmf from the cytoskeleton and Bim from the endoplasmic reticulum), whereas members of the Bcl-2 family, Bcl-2 and Bcl-X_L, antagonize apoptosis and serve as survival factors by regulating the integrity of mitochondria; the protective mechanism is poorly understood. Stress-activated protein kinases, particularly c-jun N-terminal kinase (JNK) may also be pro-apoptotic⁵² by phosphorylating and inactivating the mitochondrial protective protein Bcl-X_L.

Figure 86-1. Apoptosis and necrosis pathways in mammalian cells. See text for details. Bcl, B-cell lymphoma/leukemia family (Bax, Bid, and Bcl-xL are members); DISC, death-inducing signaling complex; FADD, Fas-associated death domain; FLIP, FLICE-inhibitory proteins; IAP, inhibitor of apoptosis proteins; MPT, mitochondrial permeability transition; RIP, receptor-interacting protein; TNF, tumor necrosis factor; TNF-R1, TNF receptor-1; TRADD, TNF receptor-associated death domain; TRAF2, TNF receptor-associated factor-2; TRAIL, TNF-related apoptosis ligand.

Execution of cell death by apoptosis usually occurs via activation of caspase 3, but more than one caspase-independent pathway of programmed cell death has been described.⁵³ Stresses to the endoplasmic reticulum can bypass mitochondrial events by activation of caspase 12, which in turn activates caspase 9 independently of the apoptosome. The final steps of programmed cell death are energy dependent. Therefore, depletion of ATP abrogates the controlled attempt at “cell suicide,” resulting instead in necrosis (see later) or an overlapping pattern that has been designated as “apoptotic necrosis” or “necraptosis.”^54,55 Furthermore, when apoptosis is massive, the capacity for rapid phagocytosis can be exceeded, and “secondary” necrosis can occur.⁵⁵

Intracellular processes and activation of pro-apoptotic death receptors are not mutually exclusive pathways of cell death in toxic liver injury. In fact, drug toxicity could predispose the injured hepatocyte to apoptosis mediated by TNF-R or Fas-operated pathways by several mechanisms, including blockade of nuclear factor-kappa B (NF-κB), which usually is a hepatoprotective transcription factor in hepatocytes, and inhibition of purine and protein synthesis. Furthermore, activation of Kupffer cells (e.g., by endotoxin) and recruitment of activated inflammatory cells can increase production of TNF-α. Apoptosis-initiating pathways have broad relevance in several areas of hepatology.

Inhibition of caspases is an important protective mechanism against cell death. Such anti-apoptotic pathways include chemical blockade of the cysteine thiol group by nitric oxide (NO) or ROS and cellular depletion of glutathione.²⁰ Protein inhibitors include IAP family members, heat shock proteins (HSPs), and FLICE (caspase-8)-inhibitory proteins (FLIP).^50–52 FLIP inhibit caspase-8 activation as a decoy for FADD binding. Bcl-2 and Bcl-X_L inhibit mitochondrial permeability, whereas phosphatidylinositol 3-kinase/Akt phosphorylates caspase 9 and activates NF-κB.

Role of Hepatic Nonparenchymal Cells and the Innate Immune Response

In addition to migratory cells, activation of nonparenchymal liver cell types is likely to play an important role in drug and toxin-induced liver injury. Kupffer cells function as resident macrophages and antigen-presenting cells. Some of the toxic effects of activated Kupffer cells, as well as of recruited leukocytes, may be mediated by release of cytokines, such as TNF and Fas-L, which under some circumstances can induce cell death in hepatocytes by apoptosis or necrosis.⁵⁵ In addition, activated Kupffer cells release ROS, nitroradicals, leukotrienes, and proteases.

Endothelial cells of the hepatic sinusoids or terminal hepatic veins are vulnerable to injury by some hepatotoxins because of their low glutathione content. Such hepatotoxins include the pyrrolizidine alkaloids, which are an important cause of the sinusoidal obstruction syndrome (hepatic veno-occlusive disease).⁵⁶ Other types of drug-induced vascular injury may be caused primarily by involvement of the sinusoidal endothelial cells (see Chapter 83).

Hepatic stellate cells (formerly fat-storing or Ito cells) are the principal liver cell type involved in matrix deposition in hepatic fibrosis. Stellate cells are activated in methotrexate-induced hepatic fibrosis. The possibility that vitamin A, ROS, or drug metabolites can transform stellate cells into collagen-synthesizing myofibroblasts is of considerable interest.

Reactive Metabolite Syndrome

Drugs implicated as a cause of RMS include sulfonamides, aminopenicillins, fluoroquinolones, clozapine, anticonvulsants (phenytoin, lamotrigine, phenobarbital, carbamazepine), minocycline, protease inhibitors (nevirapine, abacavir), some NSAIDs, and Chinese herbal medicines.⁶² Risk factors for RMS include a family history of an affected first-degree relative (increases the risk to 1 in 4). Use of other drugs, such as glucocorticoids or valproic acid, at the time the new agent is started increases the risk 4- to 10-fold. The presence of a disorder associated with immune dysregulation (e.g., systemic lupus erythematosus) increases the risk 10-fold, whereas HIV/AIDS increases the risk 100-fold.

The illness characteristically begins between 1 and 12 weeks (typically 2 to 4 weeks) after the drug is started; “sentinel symptoms” include fever, pharyngitis, malaise, periorbital edema, headache or otalgia, rhinorrhea, and mouth ulcers. A severe drug rash is an essential feature. Erythematous reactions are usual and may evolve to toxic epidermal necrolysis or erythema multiforme, often with mucositis (Stevens-Johnson syndrome). Early abnormalities on blood testing include neutrophilia and elevated levels of acute-phase reactants; atypical lymphocytosis and eosinophilia may be noted later. Hepatic reactions are found in about 13% of cases. Findings include cholestasis, acute hepatitis, and granulomas. Other features include lymphadenopathy (16%), nephritis (6%), pneumonitis (6%), and more severe hematologic abnormalities (5%).

Latent Period to Onset

For idiosyncratic reactions, a latent period occurs between the commencement of drug intake and the onset of clinical and laboratory abnormalities. The duration of the latent period is commonly 2 to 8 weeks for immunoallergic types of drug hepatitis (such as the RMS) and often 6 to 20 weeks or longer for agents such as isoniazid, dantrolene, and troglitazone. Occasionally, liver injury may become evident after discontinuation of the causative agent; for oxypenicillins and amoxicillin-clavulanate, the onset of hepatotoxicitiy may occur as long as two weeks after the end of therapy. In other cases, hepatotoxicity is rare after the first exposure to a drug but more frequent and more severe after subsequent courses. Examples include halothane, nitrofurantoin, and dacarbazine. A history of a previous reaction to the drug in question (inadvertent rechallenge) may, therefore, be the key to the diagnosis of drug-induced liver disease.

Dechallenge and Rechallenge

Another aspect of the temporal relationship between ingestion of a drug and hepatotoxicity is the response to discontinuation of the drug, or dechallenge. Dechallenge should be accompanied by discernible and progressive improvement within days or weeks of stopping the incriminated agent. Exceptions occur with ketoconazole, troglitazone, coumarol, etretinate, and amiodarone; with these agents, reactions may be severe, and clinical recovery may be delayed for months. Although some types of drug-induced cholestasis also can be prolonged, failure of jaundice to resolve in a suspected drug reaction most often is indicative of an alternative diagnosis. Rarely, deliberate rechallenge may be used to confirm the diagnosis of drug-induced liver disease or to prove the involvement of one particular agent when the patient has been exposed to several drugs; however, this approach is potentially hazardous and should be undertaken only with a fully informed and consenting (in writing) patient and preferably with the approval of an institutional ethics committee.

General Nature, Frequency, and Predisposing Factors

Acetaminophen (paracetamol) is a widely used analgesic available without prescription. It is safe when taken in the recommended therapeutic dose of 1 to 4 g daily, but hepatotoxicity produced by self-poisoning with acetaminophen has been recognized since the 1960s. Despite the effectiveness of thiol-based antidotes, acetaminophen remains the most common cause of drug-induced liver injury in most countries and an important cause of acute liver failure.^7,73 Parasuicide and suicide are the usual reasons for overdose.^73,74 Although controversial,^75,76 hepatologists and pediatricians see cases of acetaminophen poisoning that have arisen through what Zimmerman and Maddrey termed therapeutic misadventure.⁷⁷ This occurrence is especially common in persons who habitually drink alcohol to excess and has also been recognized after daily ingestion of moderate therapeutic doses (10 to 20 g over three days) of acetaminophen in adults and children who are fasting or malnourished²¹ or who are taking drugs that interact with the metabolism of acetaminophen.⁷⁷

Single doses of acetaminophen that exceed 7 to 10 g (140 mg/kg body weight in children) may cause liver injury, but this outcome is not inevitable. Severe liver injury (serum ALT level greater than 1000 U/L) or fatal cases usually involve doses of at least 15 to 25 g, but because of interindividual variability, survival is possible even after ingestion of a massive single dose of acetaminophen (greater than 50 g).⁷⁸ Among persons with an untreated acetaminophen overdose, severe liver injury occurred in only 20%, and among those with severe liver injury, the mortality rate was 20%.⁷⁸ Conversely, among heavy drinkers, daily acetaminophen doses of 2 to 6 g have been associated with fatal hepatotoxicity.^75–78

Risk factors for acetaminophen-induced hepatotoxicity are summarized in Table 86-3. Children are relatively resistant to acetaminophen-induced hepatotoxicity,⁷⁹ possibly because of their tendency to ingest smaller doses, greater likelihood of vomiting, or biological resistance. Therapeutic misadventure after multiple doses, especially during fasting and when weight-based recommendations have been exceeded, has a high mortality rate. By contrast, the presence of underlying liver disease does not predispose to acetaminophen hepatotoxicity.

Table 86-3 Risk Factors for Acetaminophen-Induced Hepatotoxicity

Self-poisoning with acetaminophen is most common in young women, but fatalities are most frequent in men, possibly because of alcoholism and late presentation.^73–75 The time of presentation is critical because thiol therapy given within 12 hours of acetaminophen poisoning virtually abolishes significant liver injury (see later). Therapeutic misadventure is also associated with a worse outcome.⁷⁶ Concomitant use of agents such as phenobarbital, phenytoin, isoniazid, and zidovudine is another risk factor for acetaminophen hepatotoxicity. These drugs may promote the oxidative metabolism of acetaminophen to NAPQI by inducing CYP2E1 (for isoniazid) or CYP3A4 (for phenytoin) or by competing with glucuronidation pathways (for zidovudine). Alcohol and fasting have dual effects by enhancing expression of CYP2E1 and by depleting hepatic glutathione. Fasting also may impair acetaminophen conjugation by depleting cofactors for the glucuronidation and sulfation pathways.²¹

Acetaminophen hepatotoxicity produces zone 3 hepatic necrosis, with extension into submassive (bridging) or panacinar (massive) necrosis in severe cases. Inflammation is minimal, and recovery is associated with complete resolution without fibrosis. The zonal pattern of acetaminophen-induced necrosis is related to the mechanism of hepatotoxicity, particularly the role of CYP2E1, which is expressed in zone 3, and to lower levels of glutathione in zone 3 hepatocytes than in hepatocytes in the other zones.

Clinical Course, Outcomes, and Prognostic Indicators

In the first two days after acetaminophen self-poisoning, features of liver injury are not present. Nausea, vomiting, and drowsiness are often caused by concomitant ingestion of alcohol and other drugs. After 48 to 72 hours, serum ALT levels may be elevated, and symptoms such as anorexia, nausea and vomiting, fatigue, and malaise may occur. Hepatic pain may be pronounced. In severe cases, the course is characterized by repeated vomiting, jaundice, hypoglycemia, and other features of acute liver failure, particularly coagulopathy and hepatic encephalopathy. The liver may shrink as a result of severe necrosis. Serum levels of ALT are often between 2000 and 10,000 U/L. These high levels, together with high levels of other intracellular proteins (ferritin, glutathione S-transferases), may provide a clue to the diagnosis in complex settings, as may occur with alcoholic patients and those with viral hepatitis.⁷⁷

Indicators of a poor outcome^73–76 include grade IV hepatic coma, acidosis, severe and sustained impairment of coagulation factor synthesis, renal failure, and a pattern of falling serum ALT levels in conjunction with a worsening prothrombin time (see also Chapter 93). Renal failure reflects acute tubular necrosis or the hepatorenal syndrome. Myocardial injury also has been attributable to acetaminophen toxicity.⁷⁸ Death occurs between 4 and 18 days after the overdose and generally results from cerebral edema and sepsis complicating hepatic and multiorgan failure. A majority of patients recover completely. Cases of apparent chronic hepatotoxicity rarely have been attributed to continued ingestion of acetaminophen (2 to 6 g/day), usually in a susceptible host, such as a heavy drinker or a person with preexisting, unrecognized liver disease.^5,6 Rare cases of acetaminophen hypersensitivity, typically involving skin or lung, have been reported in association with liver injury.^80,81

Management

In patients who present within four hours of ingesting an excessive amount of acetaminophen, the stomach should be emptied with a wide-bore gastric tube. Osmotic cathartics or binding agents have little if any role in management. Charcoal hemoperfusion has no established role. The focus of management is on identifying patients who should receive thiol-based antidote therapy and, in those with established severe liver injury, assessing the patient’s candidacy for liver transplantation.

Blood levels of acetaminophen should be measured at the time of presentation. Because of delayed gastric emptying, however, blood levels within four hours of ingestion may underestimate the extent of exposure. After four hours, acetaminophen blood levels give a reliable indicator of the risk of liver injury in patients with an acute overdose (not in those with a therapeutic misadventure). The risk of liver injury is then estimated by reference to the Prescott nomogram (Fig. 86-2).⁷⁸ Indications for antidote therapy include a reliable history of major poisoning (more than 10 g) or blood acetaminophen levels in the moderate or high-risk bands on the monogram, or both.^74,78 At-risk patients should be hospitalized for monitoring.

Figure 86-2. Acetaminophen toxicity nomogram. The risk of hepatotoxicity correlates with the plasma acetaminophen level and the time after ingestion.

(From Smilkstein MJ, Knapp GL, Kulig KW, et al. Efficacy of oral N-acetylcysteine in the treatment of acetaminophen overdose. Analysis of the National Multicenter Study [1976-1985]. N Engl J Med 1988; 319:1557-62.)

Hepatic necrosis occurs only when glutathione concentrations fall below a critical level, thereby allowing NAPQI to produce liver injury. Administration of cysteine donors stimulates hepatic synthesis of glutathione. Many cysteine precursors or thiol donors could be used, but NAC has become the agent of choice. Oral administration is preferred in the United States,^73,78 with a loading dose of 140 mg/kg followed by administration of 70 mg/kg every 4 hours for 72 hours. This regimen is highly effective, despite the theoretical disadvantage that delayed gastric emptying and vomiting may reduce intestinal absorption of NAC. In Europe and Australia, NAC is administered by slow bolus intravenous injection followed by infusion (150 mg/kg over 15 minutes in 200 mL of 5% dextrose, with a second dose of 50 mg/kg 4 hours later, if the blood acetaminophen levels indicate a high risk of hepatoxicity, and a total dose over 24 hours of 300 mg/kg).⁷⁸ The intravenous route may be associated with a higher rate of hypersensitivity reactions because of the higher systemic blood levels achieved.⁵ Adverse reactions to NAC may be severe, with rash, angioedema, and shock, which occasionally is fatal.⁵ Therefore, NAC must be administered under close supervision and only for appropriate indications. In patients known to be sensitized to NAC, methionine is probably just as effective but is not available in a commercial preparation; it must be made up fresh and often causes vomiting.⁷⁸

Cases of acetaminophen-induced severe liver injury are virtually abolished if NAC is administered within 12 hours and possibly within 16 hours of acetaminophen ingestion.^73,74,78 After 16 hours, thiol donation is unlikely to affect the development of liver injury because oxidation of acetaminophen to NAPQI with consequent oxidation of thiol groups is complete and mitochondrial injury and activation of cell death pathways are likely to be established. Nevertheless, NAC has been reported to decrease the mortality associated with acetaminophen-induced hepatotoxicity when administered 16 to 36 hours after self-poisoning,^73,74,78 possibly because NAC stabilizes vascular reactivity in patients with liver failure. Therefore, administration of NAC is recommended for patients with a late presentation after acetaminophen overdose. Other strategies to protect the liver against acetaminophen poisoning, such as inhibition of CYP-dependent metabolism through the use of cimetidine or administration of prostaglandin analogs, which are efficacious in rats, have not been established as clinically useful. The constitutive androstane receptor (CAR) has been identified as a regulator of acetaminophen metabolism and hepatotoxicity in mice.⁸² Inhibition of CAR activity by administration of androstanol one hour after acetaminophen dosing blocks liver injury.⁸³

Liver transplantation has been advocated as a therapeutic option for select patients in whom liver failure develops after acetaminophen poisoning.^73,74 The selection of cases is based on the prognostic indicators discussed earlier and is strongly influenced by the prospects for successful psychological rehabilitation (see Chapter 95).⁷⁴ In several series, about 60% of listed patients have been transplanted, and survival rates have exceeded 70%.⁷⁴

Prevention

Safe use of acetaminophen involves adherence to the recommended maximum dose for healthy adults and children and education about the risk factors that lower the toxic dose threshold. Acetaminophen doses of more than 2 g a day are contraindicated in heavy drinkers, in those taking other medications (particularly phenytoin, zidovudine, and isoniazid), and during fasting. Prolonged use of acetaminophen requires caution in patients with severe cardiorespiratory disease or advanced cirrhosis. Use of acetaminophen for self-poisoning continues despite attempts at public education about the risks involved. The chances of harm from a suicidal gesture may be reduced by the sale of acetaminophen in smaller package sizes and in blister packs, which hamper ready access to the tablets or capsules.^84,85

Niacin (Nicotinic Acid)

Hepatotoxicity associated with use of niacin, or nicotinic acid (3-pyridinecarboxylic acid), has been noted since the 1960s. When used to treat hypercholesterolemia, niacin has been an important cause of liver injury.⁸⁶ It is a dose-dependent hepatotoxin; liver injury usually occurs at doses that exceed 2 g/day, but in rare instances, low-dose (500 mg/day) sustained-release niacin has been implicated in fulminant hepatic failure.⁸⁷ Patients who are taking sulfonylurea drugs and those with preexisting liver disease, particularly alcoholic hepatitis, are at increased risk. No association with age, diet, or insulin-managed diabetes mellitus has been recognized. The symptoms of niacin hepatotoxicity begin as early as one week to as long as four years after the drug is started. The clinicopathologic spectrum encompasses mild and transient increases in serum ALT levels, jaundice, acute hepatitis, and cholestasis. Liver injury resolves completely when the drug is stopped. Liver biopsy specimens show hepatic necrosis and centrilobular cholestasis. Well-documented cases of fulminant hepatitis, some necessitating liver transplantation, also have been attributed to niacin. Substitution of one niacin preparation for another without a dose adjustment should be avoided; switching from immediate- to sustained-release preparations requires a 50% to 70% reduction in the dose of niacin.

Valproic Acid (Sodium Valproate)

Valproic acid-associated hepatic injury occurs almost exclusively in children, particularly those younger than three years of age. Also at risk are persons with a family history of a mitochondrial enzyme deficiency (particularly involving the urea cycle or long-chain fatty acid metabolism), Friedreich’s ataxia, or Reye’s syndrome or with a sibling affected by valproic acid hepatotoxicity. Another risk factor is multiple drug therapy. Cases in adults have been described rarely. The overall risk of liver injury among persons taking valproic acid varies from 1 per 500 persons exposed among high-risk groups (children under age 3, polypharmacy, genetic defects of mitochondrial enzymes) to less than 1 in 37,000 in low-risk groups.⁸⁸

No relationship exists between valproic acid toxicity and dose, but blood levels of valproic acid tend to be high in one half of affected persons. The metabolite 4-en-valproic acid, produced by CYP-catalyzed metabolism of valproic acid, is a dose-dependent hepatotoxin in animals and in vitro. The concept has emerged that valproic acid is an occult dose-dependent toxin in which accumulation of a hepatotoxic metabolite (favored by coexposure to CYP-inducing antiepileptic agents) produces mitochondrial injury in a susceptible host (e.g., young children, especially those with partial deficiencies of mitochondrial enzymes).⁸⁹

Symptoms begin 4 to 12 weeks after treatment with valproic acid is started and are often nonspecific, including lethargy, malaise, poor feeding, somnolence, worsening seizures, muscle weakness, and facial swelling. In typical cases, features of hepatotoxicity follow, including anorexia, nausea, vomiting, abdominal discomfort over the liver, and weight loss.^88,89 When jaundice ensues, hypoglycemia, ascites, coagulation disorders, and encephalopathy indicate liver failure with imminent coma and death. In some cases, a neurologic syndrome characterized by ataxia, mental confusion, and coma predominates, with little evidence of hepatic involvement. In other cases, fever and tender hepatomegaly suggestive of Reye’s syndrome may be present (see later); such cases tend to have a better prognosis. Additional extrahepatic features may include alopecia, hypofibrinogenemia, thrombocytopenia, and pancreatitis. The terminal phase is often indicated by renal failure, hypoglycemia, metabolic acidosis, and severe bacterial infection.

Laboratory features include modest elevations of serum bilirubin and aminotransferase levels; the aspartate aminotransferase (AST) level is usually higher than the ALT level. A profound decrease in clotting factor levels, hypoalbuminemia, and raised serum ammonia levels are common. A small liver with increased echogenicity suggestive of steatosis or extensive necrosis is seen on hepatic imaging. Histologic examination of the liver shows submassive or massive hepatic necrosis in two thirds of cases with either zonal or generalized microvesicular steatosis.⁸⁹ Ultrastructural studies indicate conspicuous abnormalities of the mitochondria.

Treatment is supportive. Prevention depends on careful adherence to prescribing guidelines, including the avoidance of valproic acid in combination with other drugs in the first three years of life and in children who may have mitochondrial enzyme abnormalities. Elevations in liver biochemical test levels develop in at least 40% of patients who take valproic acid and therefore are an unreliable predictor of severe hepatotoxicity. Warning patients and parents about the need to report any adverse symptoms during the first six months of valproic acid therapy is most important.

Antiretroviral Agents

Elevated liver biochemical test levels and clinical evidence of liver disease are common in patients with HIV/AIDS. Reasons for this finding include HBV or HCV infection, other hepatobiliary infections, lymphoma and other tumors, and possibly effects of HIV infection itself. The frequency of hepatic injury with HAART (which often includes three or four agents) is at least 10%.^23,27,28,90 The agents used can be broadly categorized as nucleoside (or nucleotide) reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, and protease inhibitors. Because HIV co-infection with HBV or HCV increases the risk of toxicity, all patients should be screened for viral hepatitis before starting HAART.²⁸

Aspirin

Aspirin occasionally has been associated with major increases in serum ALT levels suggestive of drug hepatitis, but hepatotoxicity occurs only when blood salicylate concentrations exceed 25 mg/100 mL.¹⁰⁶ In addition, individual susceptibility factors include hypoalbuminemia, active juvenile rheumatoid arthritis, and systemic lupus erythematosus. Most cases of aspirin-induced hepatotoxicity have been identified by biochemical testing, rather than clinical features. If present, symptoms usually begin within the first few days or weeks of high-dose aspirin therapy. Acute liver failure and fatalities have been rare. Resolution occurs rapidly after drug withdrawal, and salicylates can be reintroduced at a lower dose. All salicylates appear to carry hepatotoxic potential so there is no advantage to replacing aspirin with another salicylate. Liver biopsy specimens reveal a nonspecific focal hepatitis with hepatocellular degeneration and hydropic changes. Steatosis is not usually present, and the absence of steatosis distinguishes aspirin hepatotoxicity from Reye’s syndrome.

Reye’s syndrome has been linked with use of aspirin in febrile children. Although Reye’s syndrome is not simply a form of drug-induced liver disease, aspirin plays an important role in its multifactorial pathogenesis. Reye’s syndrome usually occurs between three and four days after an apparently minor viral infection. It is characterized by acute encephalopathy and hepatic injury, the latter documented by a three-fold or greater rise of serum aminotransferase or ammonia levels and by characteristic histologic findings. Because of effective public health campaigns against the use of aspirin in young febrile children, the incidence of Reye’s syndrome has declined markedly.¹⁰⁷

Patients with juvenile rheumatoid arthritis (Still’s disease) or systemic lupus erythematosus appear to be at particular risk of Reye’s syndrome. Clinical and laboratory features of chronic liver disease do not develop in affected persons, and features of drug allergy are not present. Management requires suspecting the correct diagnosis and reduction in the dose (or discontinuation) of aspirin. Recovery is usually rapid. Aspirin can be used again in lower doses, but other NSAIDs have displaced the use of high-dose aspirin for most conditions.

The incidence of Reye’s syndrome in the United States, Great Britain, and elsewhere has fallen dramatically since aspirin use has been avoided in children with a viral illness,^16,17 although misdiagnosis in the past of cases that subsequently were shown to be caused by inborn errors of metabolism that mimic Reye’s syndrome may account in part for the apparent decline in the incidence of Reye’s syndrome.¹⁰⁷

Other Drugs

l–asparaginase is an antileukemic drug that often causes hepatotoxicity, which usually is reversible but can result in liver failure associated with diffuse microvesicular steatosis.¹⁰⁸

Amodiaquine, a 4-aminoquinolone antimalarial agent, has been associated with fatal hepatotoxicity, as well as with agranulocytosis.¹⁰⁹ Toxicity may be related to the total dose of the drug. Amodiaquine should be reserved for active treatment of chloroquine-resistant falciparum malaria, and dose recommendations should be strictly observed.

Hycanthone is an antischistosomal agent that causes dose-dependent hepatoxicity. Risk factors for hepatotoxicity include concomitant administration of phenothiazines or estrogens, preexisting liver injury, and bacterial infection, but the most important risk factor is dose.¹¹⁰

The hepatotoxic effects of environmental toxins, illicit drug abuse substances, and metals are discussed in Chapter 87.

Nitrofurantoin

Nitrofurantoin, a synthetic furan-based compound, is a urinary antiseptic agent that continues to lead to cases of hepatic injury.¹¹² The frequency of nitrofurantoin hepatic injury ranges from 0.3 to 3 cases per 100,000 exposed persons.^113,114 The risk increases with age, particularly after the age of 64. Two thirds of acute cases occur in women, and the female-to-male ratio is 8 : 1 for chronic hepatitis.^113,114 The range of liver diseases associated with nitrofurantoin includes acute hepatitis, occasionally with features of cholestasis, hepatic granulomas, chronic hepatitis with autoimmune phenomena, acute liver failure, and cirrhosis.^113,114 Causality has been proved by rechallenge, and no relationship to dose has been observed; cases have even been described after ingestion of milk from a nitrofurantoin-treated cow.¹¹⁵

The relative frequencies of hepatocellular and cholestatic or mixed reactions and of acute and chronic hepatitis caused by nitrofurantoin have been the subject of debate. The nature of the adverse reactions covers a spectrum of biochemical and histologic features that have no apparent relevance to the patient’s clinical outcome. Chronicity depends mostly on the duration of drug ingestion, which has been less than six weeks in acute cases but more than six months in 90% of chronic cases.^113,114 Patients with chronic hepatitis often have continued to take nitrofurantoin despite symptoms attributable to an adverse drug effect or have been exposed to another course of the drug after previous toxicity. The mortality rate for chronic nitrofurantoin hepatitis is 20%, compared with 5% to 10% for acute hepatitis.¹¹³

The latent period between initial exposure to the drug and the onset of liver disease ranges from a few days to six weeks. Early symptoms may be nonspecific (e.g., fever, myalgia, arthralgia, fatigue, malaise, anorexia, and weight loss) and are followed by more specific features of hepatitis, such as nausea and vomiting, hepatic pain or discomfort, dark urine, jaundice, and, occasionally, pruritus. Rash occurs in 20% of affected persons, and lymphadenopathy may be present. Pneumonitis, which may be complicated by pulmonary fibrosis, can develop concurrently with hepatitis in 20% of cases and is suggested by cough and dyspnea. Rarely, liver failure develops, with ascites, coagulopathy, and encephalopathy. In patients with chronic hepatitis, clinical findings (such as spider angiomata, hepatosplenomegaly, muscle wasting, and ascites) may suggest chronic liver disease.

Liver biochemical testing may show pronounced elevation of serum ALT levels, but more often the pattern is mixed, with an increase in the serum alkaline phosphatase level as well. In other cases, the results suggest cholestasis. Serum bilirubin levels tend to be increased in proportion to the severity of the reaction. In contrast to most types of acute drug hepatitis, serum albumin concentrations often are low. Hypergammaglobulinemia is more likely in patients with chronic hepatitis than in those with acute hepatitis.¹¹³ Eosinophilia occurs in 33% of cases. Autoantibodies (antinuclear antibodies and smooth muscle antibodies) are present in some patients with acute hepatitis and in 80% of those with chronic hepatitis. The presence of autoantibodies can make differentiation of nitrofurantoin-induced fulminant hepatitis from autoimmune hepatitis challenging.¹¹⁶ In contrast to idiopathic autoimmune hepatitis, the frequency of the human leukocyte antigens HLA-B8 and -DRw3 is not increased.^113,114

No specific treatment for nitrofurantoin hepatitis exists. Glucocorticoids have no role—even in patients with chronic hepatitis with autoimmune features. Recovery is rapid after nitrofurantoin is discontinued. Monitoring liver biochemical test levels in users of nitrofurantoin is unlikely to be useful or cost-effective.

Other Drugs

Methyldopa was one of the first drugs reported to cause immunoallergic drug hepatitis. Cases are now rare because better antihypertensive agents are available, except for pregnancy-related cases.¹¹⁷ Hepatic reactions to methyldopa vary from abnormal liver biochemical test levels, severe acute hepatitis, granuloma formation, and cholestasis to chronic hepatitis with bridging necrosis and cirrhosis. The female predilection, clinical and laboratory changes, course, and extrahepatic features of drug allergy are similar to those for nitrofurantoin.

Phenytoin causes severe acute drug hepatitis in less than one per 10,000 persons exposed.¹¹⁸ Incidence rates are equal in men and women, and cases can occur in childhood. Blacks may be more often affected than whites. Rash, fever, eosinophilia, lymphadenopathy, a pseudomononucleosis syndrome, and other allergic features are common. The clinical features are suggestive of immunoallergy as part of RMS. An individual or familial enzymatic defect that leads to reduced disposal of phenytoin arene oxide has been detected among patients with phenytoin reactions.¹¹⁸ This finding implicates a possible metabolic factor in the pathogenesis of phenytoin toxicity.

The mortality rate is 10% to 40%. Some deaths are caused by liver failure, whereas others are the result of severe systemic hypersensitivity, bone marrow suppression, exfoliative dermatitis, or vasculitis involving the skin and kidney. Rarer hepatic associations with phenytoin reactions include cholestatic hepatitis and bile duct injury. The most common association with phenytoin therapy is an adaptive response of the liver with induction of microsomal enzymes; at least two thirds of patients have raised serum GGTP levels, and one third exhibit raised serum alkaline phosphatase levels. On histopathologic examination, ground-glass cytoplasm, which represents hypertrophied smooth endoplasmic reticulum, is usually present in hepatocytes.

Barbiturates, including phenobarbital, rarely are associated with acute hepatitis. Described cases have been similar to phenytoin reactions; fever and rash are usual, and the rate of mortality as a result of liver failure is high.¹¹⁹ Among newer antiepileptic drugs, felbamate¹²⁰ and topiramate¹²¹ have been associated with acute liver failure.

Sulfonamides are a cause of drug hepatitis that is relatively common with combination drugs such as co-trimoxazole (sulfamethoxazole and trimethoprim).¹²² Trimethoprim alone has been associated with some cases of cholestatic hepatitis; the estimated risk is 1.4 cases per 100,000 exposed persons.¹²² Reactions to co-trimoxazole resemble those associated with trimethoprim more closely than they resemble sulfonamide reactions; cholestasis or cholestatic hepatitis is more common than is hepatitis. Patients with HIV/AIDS are predisposed to sulfonamide hypersensitivity. Some other drugs have a sulfa moiety that differs from that of sulfonamides but that may increase the risk of cross-sensitivity reactions; for example, celecoxib, a COX-2 inhibitor, has been observed to cause severe hepatitis in two women with a history of sulfonamide sensitivity.¹²³ Likewise, sulfonylureas, such as gliclazide, rarely have been associated with drug hepatitis with features of immunoallergy.¹²⁴

The latent period between exposure to the drug and the onset of sulfonamide hepatitis is 5 to 14 days, and clinical features often include fever, rash, mucositis (Stevens-Johnson syndrome), lymphadenopathy, and vasculitis (that is, features of RMS). Reactions may be severe, and deaths have occurred. The serum ALT level is usually increased to a greater degree than the serum alkaline phosphatase level, but mixed or cholestatic reactions occur. A few cases of hepatic granuloma formation and of chronic hepatitis also have been associated with sulfonamides.

Sulfasalazine (salicylazosulfapyrine, salazopyrine) has been associated with rare cases of often severe acute hepatitis.¹²⁵ Although the sulfonamide moiety has been associated to be responsible, this assumption has been challenged by the report of one patient in whom hepatitis recurred after exposure to mesalamine (mesalazine, 5-aminosalicylate).¹²⁶ This finding implicates the salicylate moiety, and like salicylate hepatitis (discussed earlier) sulfasalazine hepatotoxicity appears to be more common in patients with rheumatoid arthritis than in those with inflammatory bowel disease. A case of mesalamine-induced chronic hepatitis with autoimmune features was diagnosed after 21 months of treatment with the drug.¹²⁷ Granulomatous hepatitis has been reported with mesalamine.¹²⁸

Minocycline and other tetracyclines used in conventional low doses are a rare but important cause of drug hepatitis,^129,130 including cases that have resulted in acute liver failure requiring liver transplantation.¹³¹ Minocycline is one of the few agents in current use that can lead to drug-induced autoimmune hepatitis, as discussed later.

Disulfiram (Antabuse) rarely has been associated with acute hepatitis, occasionally leading to liver failure.¹³² Disulfiram hepatitis usually is easy to distinguish from alcoholic hepatitis by the ten-fold or greater elevation of serum ALT activity.

β-adrenergic blocking agents rarely have been incriminated in hepatotoxicity. Acebutolol,¹³³ carvedilol,¹³⁴ labetalol,¹³⁵ and metoprolol¹³⁶ each have been associated with cases of acute hepatitis; some cases were proved by rechallenge. Reactions were hepatocellular and severe. Data are insufficient to determine whether or not immunoallergy is likely.

The calcium channel blockers, nifedipine,¹³⁷ verapamil,¹³⁸ diltiazem,¹³⁹ and amlodipine¹⁴⁰ have good safety records, but rare cases of acute hepatitis with a short incubation period (five days to six weeks) and other features of immunoallergy have been reported.

Of the angiotensin II receptor blockers, irbesartan has been linked to two reports of cholestasis.^141,142 In both patients, jaundice developed within one month of the start of therapy. Liver biochemical testing showed predominant cholestasis, and findings on ultrasonography were normal. Histologic examination revealed marked cholestatic features in both patients, with an inflammatory infiltrate and eosinophils in one patient. Clinical resolution occurred when the medication was stopped; however, liver biochemical abnormalities persisted for more than one year in one patient. Biliary ductopenia is a rare complication. Other angiotensin II receptor blockers implicated in cases of acute hepatitis or cholestatic hepatitis include losartan, valsartan, and candesartan.^143–145

Angiotensin-converting enzyme inhibitor-induced liver disease is a rare but important complication of this widely prescribed class of drugs. The incidence has been estimated to be 9 per 100,000 patients treated.¹⁴⁶ Reactions to captopril (the oldest and possibly most hepatotoxic representative) and enalapril usually manifest as cholestatic hepatitis, but hepatocellular or mixed hepatocellular reactions can occur.^147–149 Features of hypersensitivity, such as fever, skin rashes, and eosinophilia have been observed in patients with captopril hepatotoxicity.¹⁴⁷ Histologic examination reveals marked centrilobular cholestasis with eosinophilic portal infiltrates.¹⁴⁷ Liver biochemical abnormalities usually resolve after withdrawal of the drug, but resolution may take up to six months in some cases. Fulminant hepatic failure has been attributed to lisinopril,¹⁵⁰ whereas fosinopril has been associated with bland cholestasis.¹⁵¹ Ramipril has been implicated in three cases of cholestatic liver injury, one of which progressed to biliary cirrhosis.¹⁵² Biliary ductopenia also has been observed with enalapril.¹⁵³

Hydroxymethylglutaryl-coenzyme A reductase inhibitors (“statins”), as a class of drugs, are not strongly associated with important hepatic injury, although literature reports and data contributed to drug safety surveillance authorities appear to be discordant. Use of statins has increased greatly as new guidelines have lowered the target level for low-density lipoprotein (LDL) cholesterol, thereby resulting in use of higher doses of statins. A dose-related rise in serum aminotransferase levels develops in 1% to 3% of people who take statins.¹⁵⁴ A minor (less than two-fold normal) rise in the serum ALT and AST levels (without symptoms) is the most common manifestation of liver injury with these compounds. These elevations usually reverse rapidly with discontinuation of the statin and also reverse if therapy is not interrupted. Lovastatin,¹⁵⁵ pravastatin,¹⁵⁶ atorvastatin,¹⁵⁷ simvastatin,¹⁵⁸ and rosuvastatin¹⁵⁹ have been implicated in a few reports of acute hepatitis or cholestatic hepatitis. Use of some members of this group of lipid-lowering agents in combination with gemfibrozil is associated with an increased rate of myositis but apparently not with an increased rate of drug-induced liver injury.

Prescribing guidelines for statins invariably warn about the risk of liver injury when these agents are prescribed to persons with preexisting liver abnormalities. Nevertheless, evidence that fatty liver (or steatohepatitis), hepatitis C, or other common liver disorders predispose to drug-induced liver disease is lacking. Moreover, a controlled trial of high-dose pravastatin in patients with preexisting liver disease has demonstrated the safety of statins in this setting.¹⁶⁰ Similar conclusions were reached in the Dallas Heart Study, in which statin users were no more likely to exhibit serum ALT elevations than non-users.¹⁶¹ Likewise, statin hepatotoxicity has been unrelated to the serum ALT level at baseline in other retrospective studies^162,163 and has even been shown to reduce overall progression of hepatic fibrosis.¹⁶³ Monitoring serum amionotransferase levels is recommended, but this approach is unlikely to predict toxicity¹⁶⁴ or to be cost-effective.¹⁶⁵ In the Air Force Texas Coronary Atherosclerosis Prevention Study (AFCAPS/TEXCAPS) increases of more than three times the upper limit of normal were observed in only 18 of 100,000 aminotransferase determinations in users of lovastatin, and in no case did hepatitis develop.¹⁶⁵

Etretinate, a synthetic retinoid, is useful for treating several skin diseases. Unlike vitamin A (see Chapter 87), synthetic retinoids are not predictable hepatotoxins, but etretinate has been associated with elevated liver biochemical test levels in 10% to 25% of treated patients.¹⁶⁶ Levels may normalize with a reduction in drug dose, thereby suggesting partial dose dependency. Approximately 10 cases of severe hepatitis have been attributed to etretinate, and some have been proved by rechallenge.¹⁶⁶ Most patients were women older than age 50; two cases were associated with chronicity, and one patient appeared to respond to glucocorticoids. Because etretinate has a half-life of 100 days, monitoring serum ALT levels is recommended in users of the drug. A progressive increase in the serum ALT level to values above twice the upper limit of normal are an indication to stop the etretinate or to perform a liver biopsy.¹⁶⁶ Acitretin is another synthetic retinoid that has been associated with a few instances of acute hepatitis, including cases associated with bile duct injury and progressive hepatic fibrosis.^167,168

Gastric acid suppression drugs have an excellent safety record, although rare adverse hepatic reactions have been reported. The histamine H₂ receptor antagonist oxmetidine was removed from clinical trials because of hepatotoxicity, and subsequently, ebrotidine was withdrawn because of many cases of liver injury.¹⁶⁹ Cimetidine,¹⁷⁰ ranitidine,¹⁷¹ and famotidine¹⁷² have been associated with cases of acute hepatitis, mostly mild and often with cholestatic features. Some cases have been proved by rechallenge. Features of immunoallergy were present in some of the cimetidine reactions. Cases of hepatotoxicity have been attributed to the proton pump inhibitors omeprazole, lansoprazole, and pantoprazole; these reports have been isolated and causality was not established unequivocally in some of the cases.^173,174

Zafirlukast, a leukotriene receptor antagonist effective against asthma, has been reported to cause severe liver injury, with several instances of acute liver failure.^175,176 Montelukast has been implicated in three cases of acute hepatitis or cholestatic hepatitis.¹⁷⁷

Ticlopidine, an antiplatelet agent, has been associated with more than 30 reports of hepatotoxicity. Examination of liver histology has shown bland cholestasis in most cases and occasionally microvesicular steatosis; cholestatic hepatitis with bile duct injury also has been reported.¹⁷⁸ An association with a specific HLA haplotype (A*3303) was observed in Japanese patients with ticlopidine hepatotoxicity.¹⁷⁹ Currently, clopidogrel is preferred to ticlopidine, but clopidogrel also can cause hepatocellular or mixed hepatocellular-cholestatic liver injury.¹⁸⁰

Isoniazid

Isoniazid-induced liver injury has been characterized since the 1970s, but deaths still occur.^181–184 Hepatitis develops in about 21 per 1000 persons exposed to isoniazid; 5% to 10% of cases are fatal. The risk and severity of isoniazid hepatitis increase with age; the risk is 0.3% in the third decade of life and increases to 2% or higher after age 50.^181,182 Overall frequency rates are the same in men and women, but 70% of fatal cases are in women; black and Hispanic women may be at particular risk.^181,182 The risk of toxicity is not related to the dose or blood level of isoniazid. The role of genetic factors has been controversial. Associations have been described with specific genes that code for enzymes involved in aspects of drug metabolism or detoxification (CYP2E1, N-acetyltransferase, glutathione-S-transferase), but data are conflicting.^185–187 Chronic excessive alcohol intake increases the frequency and severity of isoniazid hepatotoxicity,^181,182 as may rifampin and pyrazinamide.¹⁸⁸ Concomitant use of pyrazinamide or acetaminophen have been associated with several cases that were fatal or led to liver transplantation.¹⁸⁹ Some studies have found that the risk of liver injury from isoniazid and other anti-tuberculosis drugs is increased among persons with chronic HBV infection, but reports are conflicting.¹⁸⁹ Malnutrition may play a role in isoniazid hepatotoxicity in some countries. Likewise, in patients with HCV or HIV infection (or both), the risk of significant serum ALT elevations during anti-tuberculosis treatment has been reported to be increased several-fold; successful antiviral treatment of hepatitis C allowed the safe reintroduction of anti-tuberculosis drugs in four patients.

Serum ALT levels increase in 10% to 36% of persons taking isoniazid in the first 10 weeks. The elevations typically are minor and resolve spontaneously. In persons in whom hepatitis develops, the latent period from exposure to disease ranges from 1 week to more than 6 months, with a median of approximately 8 weeks and, in severe cases, 12 weeks.^181,182 Re-exposure to isoniazid may be associated with an accelerated onset, although the experience in India is that gradual reintroduction of isoniazid and rifampin therapy can be achieved in a majority of cases after drug hepatitis has resolved. Prodromal symptoms occur in one third of patients and include malaise, fatigue, and early symptoms of hepatitis, such as anorexia, nausea, and vomiting. Jaundice appears several days later and is the only feature in approximately 10% of cases. Fever, rash, arthralgias, and eosinophilia are uncommon.

Liver biochemical testing indicates hepatocellular injury; serum AST levels exceed serum ALT levels in one half of patients. Serum bilirubin levels usually are elevated; values that are increased more than ten-fold indicate a poor prognosis. In one study,¹⁸¹ one third of patients had a prolonged prothrombin time, and 60% of these cases were fatal. Liver biopsy samples generally show hepatocellular injury, which is focal in approximately 50% of cases, often with marked hydropic change in residual hepatocytes. In the remaining cases, hepatocellular necrosis is zonal, submassive, or massive, with inflammation confined to the portal tracts. Cholestasis and lobular regeneration suggestive of early cirrhosis is a rare feature.

Cases with a fatal outcome have been associated with a longer duration of treatment with isoniazid or continued ingestion of isoniazid after the onset of symptoms.^181,182 Therefore, most deaths from isoniazid hepatitis could be prevented if patients report symptoms early in the course and isoniazid is discontinued. In the United States, isoniazid hepatotoxicity is second only to acetaminophen as an indication for liver transplantation for drug-induced liver injury.¹⁸⁹ Children are less susceptible than adults, but serious hepatotoxicity can occur in children; over a 10-year period (1987-1997), eight children required liver transplantation for isoniazid hepatotoxicity in the United States (0.2% of pediatric liver transplants).¹⁹⁰

Recovery is rapid if isoniazid is discontinued before severe liver injury is established. Management of liver failure is supportive (see Chapter 93); liver transplantation is indicated in the most severe cases. Prevention is the most appropriate way to prevent isoniazid hepatotoxicity, and determining whether the risks of isoniazid preventive therapy outweigh those of latent tuberculosis is critical. The optimal approach to monitoring a patient for isoniazid toxicity is uncertain; every-other-week or monthly monitoring of serum ALT levels will not always prevent the rapid onset of severe hepatotoxicity. Effective prevention depends on awareness of early symptoms, no matter how nonspecific.

Other Drugs

Drugs Used for Neurologic Disorders

Several neuroleptic agents have been associated with drug hepatitis. Some reactions appear to be immunoallergic, whereas others conform to the pattern of apparent metabolic idiosyncrasy, depending on the structure of the drug. Such reactions have been reported for commonly used antidepressants, such as fluoxetine,^249,250 aroxetine,²⁵¹ venlafaxine,²⁵² trazodone,²⁵³ tolcapone,²⁵⁴ and nefazodone²⁵⁵; the last two drugs also have been implicated in several cases of acute liver failure.