Epidemiology

Drug-induced liver injury (DILI) is known to have occurred when any liver injury, presenting on a spectrum of liver failure to elevation of liver enzymes or bilirubin, and is identified and suspected to be related to a prescription or non-prescription drug or herbal and dietary supplement [1]. DILI is more common in drugs that undergo greater than 50% of their metabolism in the liver [2]. DILI plays an important role in drug development and contributes to 15% of primary non-approvals due to safety concerns [3]. DILI may manifest in at least 12 different clinical phenotypes [4••]. At least a couple of these presentations are characterized by fat infiltration in hepatocytes—nonalcoholic fatty liver and acute fatty liver with lactic acidosis. When DILI manifests on liver histology or by imaging as hepatic steatosis or steatohepatitis, it is stated to be drug-induced fatty liver disease (DIFLD). Henceforth, we will use the term DIFLD to describe drug-induced steatosis or steatohepatitis.

Hepatic steatosis occurs when there is increased uptake or reduced transport out of the cell or excess synthesis of fat in hepatocytes. Fatty infiltration of hepatocytes is a characteristic histopathologic feature of subjects with nonalcoholic and alcoholic fatty liver disease. With a rapidly growing prevalence of metabolic syndrome, we are seeing a similar increase in prevalence of nonalcoholic fatty liver disease (NAFLD). One in three adults in the in the US population have NAFLD based on magnetic resonance spectroscopy [5••]. Hepatic steatosis is defined as lipid droplet accumulation in ≥ 5% of the hepatocytes [6]. NAFLD is commonly associated with features of metabolic syndrome. In rare circumstances, infiltration of fat in hepatocytes can occur as a response to drug-induced liver injury.

Clinical Presentation

Patients presenting with DILI present with non-specific symptoms prompting laboratory evaluation when DILI is identified. Alternatively, patients may be asymptomatic and face an incidental diagnosis when laboratory markers are tested. DIFLD encompasses the drugs that cause DILI and manifest steatosis or steatohepatitis on histopathology. The clinical presentation varies widely depending on the implicated drug. Drugs which undergo greater than 50% of their metabolism in the liver, present with hepatocellular pattern of liver enzyme elevation [2]. Drugs that undergo biliary excretion have a higher chance of presenting with jaundice [2]. All drugs with the potential to cause DILI do not do so in all individuals who are exposed to it. This suggests that in addition, the characteristics of implicated drug itself, there is a component of DILI risk that is attributable to the host and the interaction of the drug with the host [7, 8]. DILI may be caused by certain drugs due to intrinsic properties of their chemical structure, in which case, it is called intrinsic DILI [9]. Intrinsic DILI has a dose-response effect, where most of the individuals exposed to the drug progress to develop DILI if dose requirements are met [10•, 11]. Contrary to this, DILI may also be seen in a small proportion of individuals exposed to the drug, mostly attributable to host-related factors rather than to drug or its dose, in which case, it is defined as idiosyncratic DILI [12]. Adaptive immune responses are believed to play a major role in idiosyncratic DILI [13,14,15]. Most of the drugs that cause DIFLD do so owing to their structure and or interaction with host features. DIFLD can present clinically as acute liver failure (acute fatty liver) or as nonalcoholic fatty liver disease.

Acute Fatty Liver

Fatty infiltration of liver can be rarely seen along with signs of acute liver injury. A classic description of such a presentation is Reye’s syndrome. Reye’s syndrome is a classic presentation of acute fatty liver associated with use of a drug, in this case classically in children treated with aspirin. Clinical presentation is characterized by viral prodrome followed by rapid clinical decline and altered mental status secondary to acute liver failure and cerebral edema. Laboratory data typically presents with mild elevations in alanine and aspartate aminotransferases and markedly elevated ammonia levels. This is associated with a high fatality rate. Subjects presenting with Reye’s syndrome are commonly found to have urea cycle disturbances, such as ornithine transcarbamylase (OTC) deficiency [16]. Liver biopsy usually reveals minimal microvesicular hepatic steatosis. Management primarily involves stopping the use of aspirin and initiating lactulose in cases of hyperammonemia and supportive care. When cerebral edema sets in, administration of dexamethasone and mannitol is advised and in rare circumstances, exchange transfusions have also been administered [16, 17]. A rapid onset of liver failure with lactic acidosis can be seen in certain nucleoside reverse transcriptase inhibitors (NRTI), such as fialuridine. Phase 2 study of this agent in patients with chronic hepatitis B infection, who received at least 9 weeks of treatment, was terminated when rapid onset of hepatic failure with histologic evidence of steatosis, cholestasis, lactic acidosis, and pancreatitis in at least 7 of the 15 subjects [18•]. It is important to understand from this study that this clinical picture was not seen in pilot studies where the drug was administered only for 2–4 weeks, suggesting the role of duration of drug use and its risk of hepatotoxicity.

Table 1 lists a few selected drugs that have been commonly associated with drug-induced fatty liver disease. We will describe the characteristics of DIFLD associated with a few of these medications in detail below.

  1. A.

    Amiodarone, an iodinated benzofuran, is a widely used class III anti-arrhythmic agent. It is used intravenously in acute settings and orally in other clinical scenarios. DILI from amiodarone manifests differently in both routes of administration. Acute DILI from amiodarone occurs within 24 h of initiating IV formulation of this medication, attributed to the vehicle polysorbate-80. DILI from oral amiodarone use is classically associated with DIFLD. Elevated liver enzymes are seen in a quarter of all subjects who take amiodarone [19•]. Liver histology was available and evaluable in nine subjects, where all subjects showed a combination of Mallory bodies, steatosis, ballooning, and mild to moderate fibrosis [19•]. Studies have also shown lysosomes engorged with phospholipids in hepatocytes of subjects exposed to amiodarone [19•, 20, 21]. Amiodarone manifests with DIFLD owing to several mechanisms. Amiodarone and dronedarone, which are cationic amphiphilic drugs, which allows these chemicals to permeate cellular membranes efficiently and accumulate intracellularly. Once inside the cell, these compounds interrupt several processes, such as inhibition of phospholipases [22], inhibition of fatty acid transport into the mitochondria [23], and inhibition of fatty acid β-oxidation [24]. Long-term use of such agents results in cumulative toxicity, understandably cripples hepatocytes, and explains the effects of advanced fibrosis and cirrhosis in patients who stay on amiodarone for several years. Amiodarone also inhibits carnitine palmitoyltranferase-1 (CPT-1), thus causing a subsequent downstream inhibition of β-oxidation [23].

  2. B.

    Methotrexate is used commonly in psoriasis and rheumatoid arthritis. Methotrexate can cause acute liver enzyme elevations and chronic hepatotoxicity that encompasses steatosis, fibrosis, and cirrhosis. It is believed to act through methotrexate polyglutamates that inhibit the enzyme aminoimidazole carboxamide adenosine ribonucleotide transformylase, resulting in inhibition of adenosine and adenosine monophosphate (AMP) deminases, with accumulation of adenosine [25,26,27]. In a retrospective analysis of 24 subjects on methotrexate (mean cumulative dose of 5813 mg) and two available liver biopsies, 88% (21 of 24) subjects witnessed progression of liver disease by histopathology (worsening fibrosis (76%) or steatosis or necroinflammation). It is interesting to note that 71% (17 of 24) of those diagnosed with hepatotoxicity with methotrexate demonstrated features of fatty liver disease, i.e., DIFLD, while the rest of exposed subjects demonstrated fibrosis. Among those presenting with DIFLD with methotrexate, a majority (76%) have co-existing risk factors that predispose them to NAFLD (obesity, diabetes mellitus). These data suggest that methotrexate causes DIFLD in subjects with co-existing metabolic risk factors for NAFLD, likely a synergistic effect [28]. Methotrexate is a folate antagonist, which is believed to play a role in adenosine release and inhibition of transmethylation reactions [25]. Due to the co-existence of metabolic features in subjects with methotrexate-related liver injury, these subjects are advised to optimize control of metabolic features. To minimize risk factors that could lead to hepatic steatosis, these subjects are recommended to avoid alcohol. There is also a concern of hepatitis B reactivation in subjects receiving methotrexate. When liver enzyme elevations are seen in subjects on methotrexate, it is prudent to check hepatitis B serology and DNA to rule out hepatitis B reactivation [29]. A Cochrane database review of all randomized controlled trials where subjects were treated with folate concurrently with methotrexate has shown a 77% relative risk reduction in terms of liver enzyme elevation [30]. In subjects with established steatosis, liver biopsies at regular intervals have been advised. Although, a recent meta-analysis of studies on subjects taking methotrexate for psoriasis, revealed a sensitivity and specificity of 80% for vibration controlled transient elastography, results must be taken with caution due to low quality of the included data [31]. One may choose annual non-invasive studies to monitor for fibrosis progression, although these have not been validated in this population.

  3. C.

    Valproic acid is an anticonvulsant which is being increasingly used for psychiatric indications. It is a fatty acid by chemical structure, which explains its ability to disrupt lipid metabolism. One in 37,000 subjects exposed to valproate develop idiosyncratic liver toxicity, with a much higher incidence in children 0–2 years of age [32]. Valproic acid-induced DIFLD may present clinically in three distinct forms ranging from acute fatty liver similar to Reye’s syndrome, hepatocellular, or mixed liver enzyme pattern with jaundice or isolated hyperammonemia without liver enzyme elevations or obvious hepatic injury. Liver histopathology reveals macrovesicular steatosis, cholestasis, inflammation, and even fibrosis (if prolonged liver injury) [4••, 33, 34•]. In a cross-sectional study of epileptic subjects, 61% of subjects on valproic acid had hepatic steatosis on imaging ultrasound, and these subjects also had higher BMI compared to subjects who did not have hepatic steatosis. Valproic acid use leads to hepatic steatosis through several mechanisms. Valproic acid acts through sequestration of Acyl CoA. This not only results in making Acetyl CoA unavailable for β-oxidation [35], but valproic acid-CoA inhibits malonyl-CoA binding to CPT-1 and palmitoyl-CoA, thus decreasing fatty acid transport into the mitochondria through a relative carnitine deficiency [36]. This results in accumulation of long chain fatty acids in the cytoplasm and eventually to weight gain. Recent study from drug-induced liver injury network (DILIN) has shown that heterozygous variants of POLG, which codes mitochondrial DNA polymerase γ is associated with valproate-induced hepatotoxicity (odds ratio = 23.6) [34•]. Prompt intravenous carnitine supplementation improves survival in valproate toxicity [4•, 37].

  4. D.

    Tamoxifen is a widely used chemotherapeutic agent and is a non-steroidal anti-estrogenic drug used in patients with hormone responsive breast cancer. Tamoxifen hepatotoxicity can present rarely as acute mixed liver injury and peliosis hepatis [38]. In this excerpt, we will focus on DIFLD manifestation of tamoxifen hepatotoxicity. In a retrospective case series, 2% of breast cancer patients exposed to tamoxifen developed NASH (including cirrhosis in 11% of the cases) compared to 0.3% in those who did not, with most of the cases presenting with NASH within first 2 years of initiating tamoxifen. Most patients (88%) with tamoxifen-related fatty liver disease, showed resolution of transaminase elevation after discontinuation of tamoxifen [39]. Tamoxifen is a cationic amphiphilic drug that allows accumulation in mitochondria. Inside the mitochondria, it disrupts topoisomerase-mediated relaxation of DNA, resulting in depletion of mitochondrial DNA products, such as respiratory chain proteins. We see this in upstream slowing down/inhibition of β-oxidation and subsequent accumulation of fatty acid substrates, manifesting as fatty infiltration of the liver [40]. As DIFLD with tamoxifen is indolent in presentation and resolves with discontinuation of the agent, routine liver enzyme monitoring is recommended. Progression of fibrosis may be monitored using non-invasive testing and or liver biopsy. If progression of fibrosis is seen, benefit of continuing treatment should be weighed against the risk of progression.

  5. E.

    Mipomersen is approved for use in familial hypercholesterolemia. It is an oligonucleotide inhibitor of apolipoprotein B-100 synthesis, which results in decreased excretion of apolipoprotein with subsequent reduction in VLDL and LDL secretion form liver and hence levels in serum [41]. A direct consequence of the mechanism of action of mipomersen is accumulation of triglycerides in hepatocytes, thus resulting in fatty liver. A meta-analysis of six published randomized controlled trials using mipomersen shows that 17% of all subjects exposed to mipomersen had an ALT elevation greater than three times upper limit of normal [42, 43]. Based on the limited number of liver biopsies performed in this population, steatosis with no evidence of fibrosis was seen [44].

  6. F.

    Lomitapide is an oral microsomal triglyceride transfer protein (MTP) inhibitor which decreases the assembly of apolipoprotein B containing lipoproteins in the intestine and liver. This causes a direct accumulation of triglycerides in hepatocytes, thus decreasing serum LDL levels [45,46,47]. Based on phase III extension study data, median fat content in the liver increased at 3 years of being on lomitapide and transaminases were found to be elevated in a quarter subjects, which responded to dose reduction [47]. Due to high risk of liver toxicity, lomitapide is approved by FDA for use as part of REMS (risk evaluation and mitigation strategy) with close monitoring of liver enzymes [48].

  7. G.

    Chemotherapy-associated steatohepatitis (CASH) is a term given to DIFLD associated with chemotherapeutic agents. In addition to tamoxifen and methotrexate detailed above, other commonly implicated agents include irinotecan, oxaliplatin, cituximab, bevacizumab, and 5-flurouracil, which are used in treatment of metastatic colorectal cancer [49,50,51,52,53]. A meta-analysis of all studies in patients treated for metastatic colorectal cancer showed that irinotecan-based regimens were associated with DIFLD compared to oxaliplatin-based regimens which were found to be associated with sinusoidal injury [54]. Animal studies confirm ability of irinotecan to cause histopathologic steatosis and liver enzyme elevations [49]. Asparaginase has also been found to be associated with DIFLD, sometimes causing a severe liver injury associated with jaundice [55, 56].

Table 1 Selected drugs associated with DIFLD and their selected characteristics
Full size table

Pathogenesis

Accumulation of lipids in hepatocytes can occur whenever lipid homeostasis is disturbed. This may occur due to excess substrate burdening the mitochondrial respiratory chain (MRC) or the pathways that feed the MRC, such as fatty acid oxidation or citric acid cycle. The accumulation of substrate could be due to dietary excess, or enhanced uptake of substrate into the hepatocyte or from disruption of the downstream pathways or disturbing the factors regulating lipid homeostasis. Accumulation of fat in hepatocytes may also occur if the mitochondrial DNA that encodes proteins involved in the above pathways is disrupted. Although a majority of mitochondrial proteins are encoded by cellular DNA, 13 of them are encoded in the mitochondrial DNA, which is susceptible to reactive oxygen species due to their proximity to the electron transport chain in the inner membrane and lack of histone proteins. In some instances of DIFLD, phospholipid accumulation intracellularly can mimic hepatic steatosis. When this is caused by drugs, it is called drug-induced phospholipidosis (DIP). Most common agents that cause DIP, do so by disrupting the electrostatic charge-based interaction between lysosomal hydrolases and anionic phospholipids. Most of the drugs that cause DIP are weak bases containing aromatic with hydrophobic aliphatic main domain and a hydrophilic cationic secondary or tertiary amine side chain, which allows permeability into cellular membranes, including lysosomes. Hence, these drugs are called cationic amphiphilic drugs or CADs. Once inside a lysosome, these drugs get protonated and hence trapped inside the lysosomal membrane. This may eventually decrease the pH of the lysosome, thus interfering with hydrolase function of enzymes, especially phospholipases, resulting in accumulation of phospholipids. Amiodarone, tamoxifen, and perhexiline are a few examples of drugs that cause phospholipidosis [19•]. Whole genome transcription using rat models of DIFLD identified nine differentially expressed genes that could predict DIFLD, most of which belonged to lipogenesis, lipid droplet growth, lipid transport, fatty acid oxidation, and glucose metabolism [57•]. Below, we provide a brief overview of pathogenesis of DIFLD.

  1. G.

    Mitochondria

    1. a.

      Increased uptake of fatty acids: Medications that disrupt the electron transport chain may be associated with increased fatty acids, which accumulate AMP, thus activating adenosine monophosphate kinase (AMPK). AMPK shuts down the ATP consuming processes and stimulates ATP synthesizing catabolic pathways, which leads to increased fatty acid uptake through fatty acid transporter/cluster of differentiation 36 (FAT/CD36) [58].

    2. b.

      Decreased intake into mitochondria: Certain other drugs, such as troglitazone, leads to accumulation of triglycerides in the cytoplasm by inhibiting acyl Co-A-synthase [59].

    3. c.

      Inhibition of fatty acid β-oxidation: Amiodarone, dronedarone, tamoxifen, and valproic acid inhibit β-oxidation, which remains a crucial mechanism in DIFLD process.

    4. d.

      Disruption of oxidative phosphorylation: Steatosis is seen when drugs cause both uncoupling of oxidative phosphorylation and inhibit β-oxidation, such as amiodarone and perhexiline. Drugs are able to quench protons when they reach the intermembrane space of mitochondria, utilizing the electric gradient to cross the inner membrane, thus hijacking the ATP synthase step and hence depleting ATP in the mitochondria [60, 61]. Disrupting of the electron transport chain results in accumulation of free electrons that go on to bind to proteins, oxygen, and water leading to free radical formation. In addition, there is an accumulation of the reduced NADH and FADH2 leaving no NAD and FAD available to get reduced, as is crucially needed in the tri-carboxylic acid cycle and fatty acid oxidation, leading to accumulation of lactate and fatty acids respectively in hepatocytes [62, 63]. Nucleoside reverse transcriptase inhibitors (NRTI) such as stavudine, zidovudine, and didanosine can cause hepatic steatosis and lactic acidosis, by inhibiting mitochondrial respiratory chain complex I, III, IV, and V [64].

    5. e.

      Chelating mitochondrial DNA: Agents that prevent the expression of mitochondrial genes, cause disruption of all pathways in the mitochondria that depend on these proteins. Most commonly, the agents that are used as part of chemotherapy are capable of chelating mitochondria DNA. Chemotherapeutic agents and NRTI that are associated with drug-induced DIFLD are commonly known to do so by this mechanism [18•, 65,66,67]. Irinotecan, tamoxifen, and methotrexate intercalate with mitochondrial DNA resulting in reduced proteins which are needed for the functioning of the electron transport chain. NRTI, by design are incorporated into DNA [40]. When this mitochondrial toxicity is widespread and continued beyond a short course, this can lead to fulminant hepatic failure and lactic acidosis, similar to that seen in Rye’s syndrome [18•].

    6. f.

      Inhibition of secretion of lipids: Microsomal transfer protein (MTP) helps triglycerides to attach to apo-B to assist in secretion of very low density lipoprotein (VLDL) and accumulation of lipids inside the cell. Several drugs have been implicated in disrupting MTP, such as amiodarone, perhexiline, tetracycline, lomitapide, and mipomersen.

    7. g.

      Combination of above: It is also possible that a few drugs are able to interrupt several of the above metabolic pathways in the mitochondria. Amiodarone is one example of a drug that causes steatohepatitis through wide spread effects on mitochondria.

  2. H.

    Affecting regulators of lipid homeostasis

    • Altering leptin levels: Medications such as didanosine and stavudine are known to cause lipodystrophy, hence decreasing leptin secretion, which can lead to increased lipid synthesis and insulin resistance [68,69,70]. Leptin concentrations were elevated in patients with depression who were on amitriptyline and mirtazapine, beyond the increase that could be explained by change in weight alone, which is highly suggestive of leptin resistance [71]. Additional anti-psychotic drugs, such as olanzapine, quetiapine, gabapentin, and risperidone among others are associated with significant weight gain [72•].

Other Modifying Factors that May Increase Susceptibility to Drug-Induced Fatty Liver Injury

Genetic Factors

Single nucleotide polymorphisms (SNP), patatin-like phospholipase domain containing 3 (PNPLA3) (I148M), and transmembrane 6 superfamily member 2 (TM6SF2) (E167K) are known to be associated with steatosis, steatohepatitis, and even fibrosis progression [73,74,75,76,77]. Recent studies have shown that these very SNPs increased predisposition to DIFLD as well. Using a candidate gene approach in participants from two phase 2 studies where diabetic subjects were treated with glucagon receptor antagonist (LY2409021), elevated ALT and hepatic fat fraction on MRI where associated with PNPLA3 I148M and TM6SF2 E167K variants [78••]. Further studies are needed to investigate the generalizability of this finding to other agents associated with DIFLD. Heterozygous variants of POLG, encoding mitochondrial DNA polymerase γ has been shown to be associated with valproate DIFLD (OR = 23.6) [34•].

Diet

Research has shown that in mice fed with a diet rich in trans fats in addition to anabolic steroids predisposed them to steatohepatitis, with high thiobarbituric acid reactive substances (TBARS) and protein carbonyls (lipid peroxidation and protein oxidation products), compared to mice receiving standard diet alone or in combination with anabolic steroids or those of high fat diet alone [79]. But there are no human studies demonstrating a relationship between diet and the risk for DIFLD.

Fatty Liver Disease at Baseline

Pre-existing fatty liver disease has been shown to be a risk factor for DIFLD. It is thought that hepatocytes with fat deposition are vulnerable to toxicity from chemicals or drugs due to dampened compensatory machinery [80, 81].

Diagnosis and Management of Patients with DIFLD

As steatohepatitis has several inciting factors, diagnosing DIFLD requires a thorough history regarding all possible factors, such as features of metabolic syndrome, rapid weight loss, current list of drugs and duration of intake of suspect drugs. Due to high prevalence of nonalcoholic fatty liver disease in the USA, it is highly likely that patients have multiple causes for steatosis/steatohepatitis.

  1. a.

    One should unequivocally recommend that if there are signs of acute fatty liver, the suspected culprit drug is to be stopped immediately.

  2. b.

    In a nonacute fatty liver DIFLD, it is advisable that when possible, the drug will be stopped, in an attempt to minimize risk factors even if there are other associated risk factors for steatohepatitis in addition to the inciting drug. This also requires an understanding of the several other factors of the disease for which the drug is being used, host liver health at baseline and characteristics of the drug (known to cause DIFLD) that is being used.

  3. c.

    As DIFLD, like every other DILI, has an iatrogenic component of injury, it is very important to discuss the risks of hepatotoxicity for the commonly associated drugs and educate patients and health care team regarding avoidance of the implicated agent in the future. This approach seems reasonable in most cases, where the duration of intended drug use is limited and one hopes that after cessation of drug use, DIFLD will be resolve. But, in drugs where duration of intended use is longer, one can never exclude the possibility of disease progression. In subjects who are on chemotherapy for a malignancy, risk of DIFLD progression must be weighed against the benefits of treating the primary disease. The best approach in most DIFLD cases involves a multi-disciplinary approach where the hepatologist seeks to understand the disease factors from the doctor managing the disease and applies them to the patient’s risk factors based on baseline liver disease to provide a thoughtful recommendation.

  4. d.

    Once a decision has been taken to continue inciting DIFLD agents, recommendations to minimize other risk factors for fatty liver should be offered. Patients should be advised to pursue a healthy diet (with low carbohydrate load) and increase physical activity as much as possible. Avoidance of alcohol is recommended to curb any additional risk factors for fatty liver disease. Minimizing chronic prednisone use is advisable. Supplementation with folate may be beneficial in subjects on methotrexate. L-carnitine supplementation is recommended in valproate DIFLD to improve survival. In cases where DIFLD agent is used for long duration, monitoring for progression of fibrosis must be pursued. Traditionally, liver biopsies have been used to do so, although, in absence of clinical and lab features of disease progression, one may consider using non-invasive modalities, such as vibration-controlled transient elastography.

Conclusion

DIFLD, an important phenotype of DILI, is caused by a growing number of drugs with the ability to interfere with carbohydrate and lipid metabolism. It is crucial to discern the acute fatty liver phenotype of DIFLD, as it is associated with extremely high mortality and morbidity, while stopping the implicated drug is associated with limiting of progressive damage. In all other cases of DIFLD, co-existing liver disease, duration of intended DIFLD drug use, critical nature of disease being treated by DIFLD agent should be considered during management.