Drug-induced hepatotoxicity or drug-induced liver injury (DILI) is an acute or chronic response to a natural or manufactured compound. DILI can be classified based on clinical presentation (hepatocellular, cholestatic, or mixed), mechanism of hepatotoxicity, or histological appearance from a liver biopsy. The true incidence is difficult to estimate, yet it has become the leading cause of acute liver failure (ALF) in the United States. The two mechanisms of hepatotoxicity are intrinsic, which is dose-dependent, and idiosyncratic, which is more unpredictable. Most cases of DILI are asymptomatic; however, the most common sign is jaundice. Laboratory tests in hepatocellular injury will have elevation in aminotransferases, while in cholestatic injury, alkaline phosphatase (ALP) is elevated. Liver biopsy is not mandatory for diagnosis but could exclude other causes of liver disease. Treatment begins with the removal of the offending agent, and the prognosis for recovery is usually favorable after discontinuation of the drug.
There are patient risk factors associated with the development of DILI, which include female sex, older age, and increased body mass index (BMI). More than 1000 medications and herbal compounds are known to cause hepatotoxicity and can be found on a searchable database.
Intrinsic DILI is most commonly caused by acetaminophen, while it is less often seen in aspirin, tetracycline, and vitamin A .
The true incidence of DILI is difficult to determine as it is often underreported and there are several different diagnostic criteria used. In the US and worldwide, DILI annual incidence in the general population is under 15-20 per 100,000. In the US, it is the most common cause of acute liver failure episodes (13-16%), yet it is important to note that it is still a much less common etiology of acute liver injury overall. The incidence of idiosyncratic DILI is higher in women (59%) when compared to men which may be due to hormonal interactions with immunomodulating drugs or differing pharmacokinetics. Patients more than 50 years old are more likely to develop DILI, possibly due to increased prescription use.
The pathogenesis of DILI can be divided into two mechanisms: intrinsic and idiosyncratic. The intrinsic mechanism is both predictable and reproducible from drugs that are known to cause liver injury in a dose-dependent manner with a short latency period. For example in acetaminophen toxicity, the metabolism of the drug leads to the production of reactive metabolites that excessively accumulate causing apoptosis and necrosis of the hepatocytes. The idiosyncratic mechanism of DILI is characterized by a more unpredictable course and is not reproducible. It occurs in susceptible patients usually independent of the drug dose, with a variable latency period generally beginning 7 to 14 days after first ingestion. Although the exact mechanism is unknown, it is thought to be due to a combination of host, drug, and environmental factors. The host factors are patient age, gender, genetic polymorphisms, immune status, and metabolism. Drug factors include the dose, duration, weight, and degree of lipophilicity. Environmental factors consist of concomitant alcohol use, diet, tobacco, and toxins. Idiosyncratic DILI mechanisms can be further divided into immune-mediated (allergic) liver injury from hypersensitivity or non-immune mediated metabolic (non-allergic) mechanisms from mitochondrial injury.
Although liver biopsy is not required for diagnosis, drug-induced hepatotoxicity can be classified based on histologic findings. The histological appearance can also assist in identifying an etiology, excluding other diagnoses, and thus drive management. It is also a tool that can assist in prognostication.
Clinical presentations in drug-induced hepatotoxicity are varied and, most times, can be asymptomatic. DILI manifests most commonly as jaundice, then weakness, abdominal pain, dark stools or urine, nausea, and pruritis (usually seen in cholestatic liver injury). DILI can present as acute or chronic liver failure, which makes it very difficult to distinguish from other liver conditions. In immune-mediated hepatotoxicity, patients may present with fever, rash, eosinophilia, and even Stevens-Johnsons syndrome. The patient’s history will reveal signs or symptoms that develop within 3 to 6 months of exposure to the offending agent; thus, a thorough medication is history is of paramount importance. On physical exam, there may be jaundice, presence of right upper quadrant tenderness, and sometimes hepatomegaly when liver injury leads to chronic liver disease.
Diagnosis begins with a compatible history, known drugs, and excluding underlying liver disease. Subsequently, a high index of suspicion is required, as there are no specific tests available to diagnose DILI. Testing should include alanine aminotransaminase (ALT), aspartate transferase (AST), alkaline phosphatase (ALP), gamma-glutamyl transpeptidase (GGT), total bilirubin, albumin, prothrombin time (PT), and international normalized ratio (INR). Other laboratory tests that could assist in diagnosis are complete blood count (CBC), electrolytes, viral serologies, and autoantibodies. The pattern of liver test abnormalities can distinguish between the types of liver injury. In hepatocellular injury, there is a marked elevation of transaminases (typically 3 times the upper limit) in comparison to the ALP level. In cholestatic injury or cholestasis, ALP is typically 3 times the upper limit of normal (ULN) in comparison to transaminases. There also exists a mixed pattern of injury where both the aminotransferases and ALP are 3 times the ULN. In ALF, coagulopathy may be observed, indicating impaired liver function. Imaging studies such as abdominal ultrasound or MRI can be helpful in cholestatic injury to exclude other biliary tract pathology. Again, liver biopsy is not necessary for diagnosis but can be useful in exclusion, especially if other causes of liver disease are suspected. Emerging data exists on potential biomarkers to aid in the diagnosis or predict the progression of DILI. It has been shown that micro-RNAs are found in increased quantity in DILI, suggesting its release from damaged cells. Though this is not specific, it can be used to assess liver injury.
Several clinical prediction scores have also been developed to develop an objective diagnostic tool for DILI. Among them is the Roussel Uclaf Causality Assessment (RUCAM), which includes risk factors, course of liver enzymes, suspected offending agents, and alternative causes into the scoring system. Using the calculated “R” score, the type of liver injury can be classified, and subsequently, the likelihood of DILI can be determined.
The principal treatment for drug-induced hepatotoxicity is the removal of the offending agent. N-acetyl-cysteine (NAC) is the treatment for intrinsic DILI secondary to acetaminophen toxicity, as this promotes the regeneration of glutathione leading detoxification of the toxic metabolite. The other specific therapy that is available is L-carnitine for valproic acid overdose. Glucocorticoid therapy is usually used when the histological appearance of DILI resembles that of autoimmune hepatitis. For this reason, it has a limited role and usually does not change the course of recovery. Symptomatic therapies such as bile acid sequestrants for cholestatic DILI or antihistamines for pruritis can be used with some efficacy. Hospital admission is required for patients with signs or symptoms of DILI progression or ALF. If ALF is suspected, early liver transplant consideration is essential, because there is high mortality with ALF. An important additional aspect of management is reporting cases of DILI to regulatory bodies to evaluate if the suspected drug needs to be withdrawn from the market.
Generally, patients have a favorable prognosis after discontinuation of the offending drug, as 90% recover. Mortality risk can be predicted through “Hy’s law,” which include criteria the following criteria that are associated with a poorer prognosis: ALT/AST more than 3 times ULN, total bilirubin up to 2 times ULN in the absence of obstruction, and no other explanation of the mentioned laboratory values. Individuals more than 65 years old had a higher incidence of Hy’s law; however, mortality does not differ. Hepatocellular injury is more likely than cholestatic injury to have a 10% to 50% higher risk of mortality or to require a liver transplant. In general, about 10% of patients progress to requiring liver transplantation. Following liver transplantation, survival is 66%.
Prevention of DILI can begin with patient education on their medications, which include over the counter drugs, prescription medications, and HDS. If patients should experience signs and symptoms of liver toxicity or ALF, further evaluation is required.
More than 1000 medications and herbal compounds can cause drug-induced hepatotoxicity. Patients can present with symptoms that manifest similarly to other hepatobiliary disorders. Thus, it is important to perform a thorough drug history when drug-induced hepatotoxicity is suspected. Once DILI is suspected as the likely etiology of hepatotoxicity, remove the offending agent, and monitor for signs of ALF as transplantation evaluation is necessary. A multidisciplinary approach with clinicians, pharmacists, and nursing is required for diagnosis and to report suspected drugs to regulatory organizations to assess its potential removal from the market. A web-based resource can aid healthcare providers in this. It contains a database of offending compounds with a description of its liver toxicity, and there is also a case submission registry that allows users to report DILI cases directly; this can then be forwarded to the U.S. Food and Drug Administration's Event Reporting System. [Level 2]
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