Hepatotoxicity

Hepatotoxicity

Alternative names
Toxic liver disease Toxin induced liver disease Drug induced liver disease Drug induced liver damage Hepatogenous poisoning
Subordinate terms
Toxic hepatitis Toxin induced hepatitis Drug induced hepatitis Drug-induced hepatic necrosis Drug induced hepatic fibrosis Drug induced hepatic granuloma Toxic liver disease with hepatitis Toxic liver disease with cholestasis

Hepatotoxicity (from hepatic toxicity) implies chemical-driven liver damage.

The liver plays a central role in transforming and clearing chemicals and is susceptible to the toxicity from these agents. Certain medicinal agents, when taken in overdoses and sometimes even when introduced within therapeutic ranges, may injure the organ. Other chemical agents, such as those used in laboratories and industries, natural chemicals (e.g., microcystins) and herbal remedies can also induce hepatotoxicity. Chemicals that cause liver injury are called hepatotoxins.

More than 900 drugs have been implicated in causing liver injury^[1] and it is the most common reason for a drug to be withdrawn from the market. Chemicals often cause subclinical injury to liver which manifests only as abnormal liver enzyme tests. Drug-induced liver injury is responsible for 5% of all hospital admissions and 50% of all acute liver failures.^[2][3]

Drug metabolism in liver

Drug metabolism in liver: transferases are : glutathione, sulfate, acetate, glucoronic acid. P-450 is cytochrome P-450 enzymes. 3 different pathways are depicted for Drugs A, B and C.

The human body identifies almost all drugs as foreign substances (i.e. xenobiotics) and subjects them to various chemical processes (i.e. metabolism) to make them suitable for elimination. This involves chemical transformations to (a) reduce fat solubility and (b) to change biological activity. Although almost all tissues in the body have some ability to metabolize chemicals, smooth endoplasmic reticulum in the liver is the principal "metabolic clearing house" for both endogenous chemicals (e.g., cholesterol, steroid hormones, fatty acids, proteins) and exogenous substances (e.g., drugs, alcohol).^[4] The central role played by liver in the clearance and transformation of chemicals makes it susceptible to drug-induced injury.

Drug metabolism is usually divided into two phases: phase 1 and phase 2. Phase 1 reaction is thought to prepare a drug for phase 2. However many compounds can be metabolized by phase 2 directly. Phase 1 reaction involves oxidation, reduction, hydrolysis, hydration and many other rare chemical reactions. These processes tend to increase water solubility of the drug and can generate metabolites which are more chemically active and potentially toxic. Most of phase 2 reactions take place in cytosol and involve conjugation with endogenous compounds via transferase enzymes. Chemically active phase 1 products are rendered relatively inert and suitable for elimination by this step.

A group of enzymes located in the endoplasmic reticulum, known as cytochrome P-450, is the most important family of metabolizing enzymes in the liver. Cytochrome P-450 is the terminal oxidase component of an electron transport chain. It is not a single enzyme, but rather consists of a closely related family of 50 isoforms; six of them metabolize 90% of drugs.^[5][6] There is a tremendous diversity of individual P-450 gene products, and this heterogeneity allows the liver to perform oxidation on a vast array of chemicals (including almost all drugs) in phase 1. Three important characteristics of the P450 system have roles in drug-induced toxicity:

1. Genetic diversity:

Each of the P-450 proteins is unique and accounts (to some extent) for the variation in drug metabolism between individuals. Genetic variations (polymorphism) in P-450 metabolism should be considered when patients exhibit unusual sensitivity or resistance to drug effects at normal doses. Such polymorphism is also responsible for variable drug response among patients of differing ethnic backgrounds.

Cytochrome P-450 enzyme induction and inhibition^[6][7][8]

Potent inducers	Potent inhibitors	Substrates
Rifampicin, Carbamazepine, Phenobarbital, Phenytoin, (St John's wort),	Amiodarone, cimetidine, ciprofloxacin, fluconazole, fluoxetine, erythromycin, isoniazid, diltiazem	Caffeine, clozapine, omeprazole, losartan, theophylline

2. Change in enzyme activity:

Many substances can influence the P-450 enzyme mechanism. Drugs interact with the enzyme family in several ways.^[9] Drugs that modify cytochrome P-450 enzyme are referred to as either inhibitors or inducers. Enzyme inhibitors block the metabolic activity of one or several P-450 enzymes. This effect usually occurs immediately. On the other hand, inducers increase P-450 activity by increasing its synthesis. Depending on the inducing drug's half life, there is usually a delay before enzyme activity increases.^[6]

3. Competitive inhibition:

Some drugs may share the same P-450 specificity and thus competitively block their bio transformation. This may lead to accumulation of drugs metabolized by the enzyme. This type of drug interaction may also reduce the rate of generation of toxic substrate.

Mechanism of liver damage

Factors influencing drug induced hepatotoxicity[10]
Age Ethnicity and race Gender Nutritional status Underlying liver disease Renal function Pregnancy Duration and dosage of drug Enzyme induction Drug-to-drug interaction

Drugs continue to be taken off the market due to late discovery of hepatotoxicity. Due to its unique metabolism and close relationship with the gastrointestinal tract, the liver is susceptible to injury from drugs and other substances. 75% of blood coming to the liver arrives directly from gastrointestinal organs and then spleen via portal veins which bring drugs and xenobiotics in near-undiluted form. Several mechanisms are responsible for either inducing hepatic injury or worsening the damage process. Many chemicals damage mitochondria, an intracellular organelle that produce energy. Its dysfunction releases excessive amount of oxidants which, in turn, injure hepatic cells. Activation of some enzymes in the cytochrome P-450 system such as CYP2E1 also lead to oxidative stress.^[11] Injury to hepatocyte and bile duct cells lead to accumulation of bile acid inside liver. This promotes further liver damage.^[12] Non-parenchymal cells such as Kupffer cells, fat storing stellate cells, and leukocytes (i.e. neutrophil and monocyte) also have role in the mechanism.

Adverse drug reactions

Drug-induced hepatitis with granulomata. Other causes were excluded with extensive investigations. Liver biopsy. H&E stain.

Adverse drug reactions are classified as type A (intrinsic or pharmacological) or type B (idiosyncratic).^[13] Type A drug reaction accounts for 80% of all toxicities.^[14]

Drugs or toxins that have a pharmacological (type A) hepatotoxicity are those that have predictable dose-response curves (higher concentrations cause more liver damage) and well characterized mechanisms of toxicity, such as directly damaging liver tissue or blocking a metabolic process. As in the case of acetaminophen overdose, this type of injury occurs shortly after some threshold for toxicity is reached.

Idiosyncratic (type B) injury occurs without warning, when agents cause non-predictable hepatotoxicity in susceptible individuals which is not related to dose and has a variable latency period.^[15] This type of injury does not have a clear dose-response nor temporal relationship, and most often does not have predictive models. Idiosyncratic hepatotoxicity has led to the withdrawal of several drugs from market even after rigorous clinical testing as part of the FDA approval process; Troglitazone (Rezulin) and trovafloxacin (Trovan) are two prime examples of idiosyncratic hepatotoxins pulled from market.

When used orally, ketoconazole has been associated with hepatic toxicity, including some fatalities.^[16]

Patterns of injury

Patterns of drug-induced liver disease

Type of injury:	Hepatocellular	Cholestatic	Mixed
ALT	≥ Twofold rise	Normal	≥ Twofold rise
ALP	Normal	≥ Twofold rise	≥ Twofold rise
ALT: ALP ratio	High, ≥5	Low, ≤2	2-5
Examples[17]	Acetaminophen Allopurinol Amiodarone HAART NSAID	Anabolic steroid Chlorpromazine Clopidogrel Erythromycin Hormonal contraception	Amitryptyline, Enalapril Carbamazepine Sulfonamide Phenytoin

Chemicals produce a wide variety of clinical and pathological hepatic injury. Biochemical markers (e.g. alanine transferase, alkaline phosphatase and bilirubin) are often used to indicate liver damage. Liver injury is defined as a rise in either (a) ALT level more than three times of upper limit of normal (ULN), (b) ALP level more than twice ULN, or (c) total bilirubin level more than twice ULN when associated with increased ALT or ALP.^[17][18] Liver damage is further characterized into hepatocellular (predominantly initial Alanine transferase elevation) and cholestatic (initial alkaline phosphatase rise) types. However they are not mutually exclusive and mixed types of injuries are often encountered.

Specific histo-pathological patterns of liver injury from drug-induced damage are discussed below.

Zonal Necrosis

This is the most common type of drug-induced liver cell necrosis where the injury is largely confined to a particular zone of the liver lobule. It may manifest as a very high level of ALT and severe disturbance of liver function leading to acute liver failure.

Causes include:

Paracetamol, carbon tetrachloride

Hepatitis

In this pattern, hepatocellular necrosis is associated with infiltration of inflammatory cells. There can be three types of drug-induced hepatitis. (A) viral hepatitis is the most common, where histological features are similar to acute viral hepatitis. (B) in focal or non-specific hepatitis, scattered foci of cell necrosis may accompany lymphocytic infiltration. (C) chronic hepatitis is very similar to autoimmune hepatitis clinically, serologically, and histologically.

Causes:

(a) Viral hepatitis: Halothane, isoniazid, phenytoin

(b) Focal hepatitis: Aspirin

(c) Chronic hepatitis: Methyldopa, diclofenac

Cholestasis

Liver injury leads to impairment of bile flow and cases are predominated by itching and jaundice. Histology may show inflammation (cholestatic hepatitis) or it can be bland (without any parenchymal inflammation). On rare occasions, it can produce features similar to primary biliary cirrhosis due to progressive destruction of small bile ducts (Vanishing duct syndrome).

Causes:

(a) Bland: Oral contraceptive pills, anabolic steroid, androgens

(b) Inflammatory: Allopurinol, co-amoxiclav, carbamazepine

(c) Ductal: Chlorpromazine, flucloxacillin

Steatosis

Hepatotoxicity may manifest as triglyceride accumulation which leads to either small droplet (microvesicular) or large droplet (macrovesicular) fatty liver. There is a separate type of steatosis where phospholipid accumulation leads to a pattern similar to the diseases with inherited phospholipid metabolism defects (e.g. Tay-Sachs disease)

Causes:

(a) Microvesicular: Aspirin (Reye's syndrome), ketoprofen, tetracycline (especially if expired)

(b) Macrovesicular: Acetamenophen, methotrexate

(c) Phospholipidosis: Amiodarone, total parenteral nutrition

Granuloma

Drug-induced hepatic granulomas are usually associated with granulomas in other tissues and patients typically have features of systemic vasculitis and hypersensitivity. More than 50 drugs have been implicated.

Causes:

Allopurinol, phenytoin, isoniazid, quinine, penicillin, quinidine

Vascular lesions

These result from injury to the vascular endothelium.

Causes:

Venoocclusive disease: Chemotherapeutic agents, bush tea

Peliosis hepatis: Anabolic steroids

Hepatic vein thrombosis: Oral contraceptives

Neoplasm

Neoplasms have been described with prolonged exposure to some medications or toxins. Hepatocellular carcinoma, angiosarcoma, and liver adenomas are the ones usually reported.

Causes:

Vinyl chloride, combined oral contraceptive pill, anabolic steroid, arsenic, thorotrast

Clinical assessment

Algorithm for suspected drug induced hepatic toxicity

This remains a challenge in clinical practice due to a lack of reliable markers.^[19] Many other conditions lead to similar clinical as well as pathological pictures. To diagnose hepatotoxicity, a causal relationship between the use of the toxin or drug and subsequent liver damage has to be established, but might be difficult, especially when idiosyncratic reaction is suspected.^[20] Simultaneous use of multiple drugs may add to the complexity. As in acetaminophen toxicity, well established, dose-dependent, pharmacological hepatotoxicity is easier to spot. Several clinical scales such as CIOMS/RUCAM scale and Maria and Victorino criteria have been proposed to establish causal relationship between offending drug and liver damage. CIOMS/RUCAM scale involves a scoring system which categorizes the suspicion into "definite or highly probable" (score > 8), “probable” (score 6-8), “possible” (score 3-5), “unlikely” (score 1-2) and “excluded” (score ≤ 0). In clinical practice, physicians put more emphasis on the presence or absence of similarity between the biochemical profile of the patient and known biochemical profile of the suspected toxicity (e.g., cholestatic damage in amoxycillin-clauvonic acid ).^[19]

Treatment and prognosis

In most cases, liver function will return to normal if the offending drug is stopped early. Additionally, the patient may require supportive treatment. In acetaminophen toxicity, however, the initial insult can be fatal. Fulminant hepatic failure from drug-induced hepatotoxicity may require liver transplantation. In the past, glucocorticoids in allergic features and ursodeoxycholic acid in cholestatic cases had been used, but there is no good evidence to support their effectiveness.

An elevation in serum bilirubin level of more than 2 times ULN with associated transaminase rise is an ominous sign. This indicates severe hepatotoxicity and is likely to lead to mortality in 10% to 15% of patients, especially if the offending drug is not stopped (Hy's Law).^[21][22] This is because it requires significant damage to the liver to impair bilirubin excretion, hence minor impairment (in the absence of biliary obstruction or Gilbert syndrome) would not lead to jaundice. Other poor predictors of outcome are old age, female sex, high AST.^[23][24]

Drugs withdrawn for hepatotoxicity

The following therapeutic drugs were withdrawn from the market primarily because of hepatotoxicity: Troglitazone, bromfenac, trovafloxacin, ebrotidine, nimesulide, nefazodone, ximelagatran and pemoline.^[19][25][26]

Specific drug or toxin

Acetaminophen

Main article: Acetaminophen toxicity

Acetaminophen (3D structure) overdose is the most common cause of drug-induced liver disease

Acetaminophen (paracetamol, also known by the brand name Tylenol and Panadol) is usually well tolerated in prescribed dose, but overdose is the most common cause of drug-induced liver disease and acute liver failure worldwide.^[10] Damage to the liver is not due to the drug itself but to a toxic metabolite (N-acetyl-p-benzoquinone imine NAPQI, or NABQI) which is produced by cytochrome P-450 enzymes in the liver.^[27] In normal circumstances, this metabolite is detoxified by conjugating with glutathione in phase 2 reaction. In an overdose, a large amount of NAPQI is generated which overwhelms the detoxification process and leads to liver cell damage. Nitric oxide also plays role in inducing toxicity.^[28] The risk of liver injury is influenced by several factors including the dose ingested, concurrent alcohol or other drug intake, interval between ingestion and antidote, etc. The dose toxic to the liver is quite variable from person to person and is smaller in chronic alcoholics.^{[citation needed]} Measurement of blood level is important in assessing prognosis, higher levels predicting a worse prognosis. Administration of Acetylcysteine, a precursor of glutathione, can limit the severity of the liver damage by capturing the toxic NAPQI. Those who develop acute liver failure can still recover spontaneously, but may require transplantation if poor prognostic signs such as encephalopathy or coagulopathy is present (see King's College Criteria).

Nonsteroidal anti-inflammatory drugs (NSAIDs)

Although individual analgesics rarely induce liver damage due to their widespread use, NSAIDs have emerged as a major group of drugs exhibiting hepatotoxicity. Both dose-dependent and idiosyncratic reactions have been documented.^[29] Aspirin and phenylbutazone are associated with intrinsic hepatotoxicity; idiosyncratic reaction has been associated with ibuprofen, sulindac, phenylbutazone, piroxicam, diclofenac and indomethacin.

Glucocorticoids

Glucocorticoids are so named due to their effect on the carbohydrate mechanism. They promote glycogen storage in the liver. An enlarged liver is a rare side effect of long-term steroid use in children.^[30] The classical effect of prolonged use both in adult and paediatric population is steatosis.^[31]

Isoniazid

Isoniazide (INH) is one of the most commonly used drugs for tuberculosis; it is associated with mild elevation of liver enzymes in up to 20% of patients and severe hepatotoxicity in 1-2% of patients.^[32]

Natural products

Examples include many amanita mushrooms (particularly the destroying angels), and aflatoxins.^{[citation needed]}

Industrial toxin

Examples include arsenic, carbon tetrachloride, and vinyl chloride.

Herbal and alternative remedies

Examples include Ackee fruit, Bajiaolian, Camphor, Copaltra???, Cycasin, Kava,^[33] pyrrolizidine alkaloids, Horse chestnut leaf, Valerian, Comfrey.^[34]

• Chinese herbal remedies:

Jin Bu Huan, Ma-huang, Sho-wu-pian

• 

  Ackee fruit

See also

• Hepatoprotection
• Reye's syndrome

References

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