Repeat exposure to incremental doses of acetaminophen provides protection against acetaminophen-induced lethality in mice: an explanation for high acetaminophen dosage in humans without hepatic injury

Role and Regulation of Hepatobiliary ATP-Binding Cassette Transporters during Chemical-Induced Liver Injury

Severity of drug-induced liver injury (DILI) ranges from mild, asymptomatic, and transient elevations in liver function tests to irreversible liver damage, often needing transplantation. Traditionally, DILI is classified mechanistically as high-frequency intrinsic DILI, commonly dose dependent or DILI that rarely occurs and is idiosyncratic in nature. This latter form is not dose dependent and has a pattern of histopathological manifestation that is not always uniform. Currently, a third type of DILI called indirect hepatotoxicity has been described that is associated with the pharmacological action of the drug. Historically, DILI was primarily linked to drug metabolism events; however, the impact of transporter-mediated rates of drug uptake and excretion has gained greater prominence in DILI research. This review provides a comprehensive view of the major findings from studies examining the contribution of hepatic ATP-binding cassette transporters as key contributors to DILI and how changes in their expression and function influence the development, severity, and overall toxicity outcome. SIGNIFICANCE STATEMENT: Drug-induced liver injury (DILI) continues to be a focal point in drug development research. ATP-binding cassette (ABC) transporters have emerged as important determinants of drug detoxification, disposition, and safety. This review article provides a comprehensive analysis of the literature addressing: (a) the role of hepatic ABC transporters in DILI, (b) the influence of genetic mutations in ABC transporters on DILI, and (c) new areas of research emphasis, such as the influence of the gut microbiota and epigenetic regulation, on ABC transporters.

CD5L deficiency attenuate acetaminophen-induced liver damage in mice via regulation of JNK and ERK signaling pathway

CD5 molecule like (CD5L), a member of the scavenger receptor cysteine-rich domain superfamily, plays a critical role in immune homeostasis and inflammatory disease. Acetaminophen (APAP) is a safe and effective antipyretic analgesic. However, overdose may cause liver damage or even liver failure. APAP hepatotoxicity is characterized by extensive necrotic cell death and a sterile inflammatory response, in which the role of CD5L remains to be investigated. In this study, we found that the expression of CD5L was increased in the livers of mice after APAP overdose. Furthermore, CD5L deficiency reduced the increase of alanine transaminase (ALT) level, histopathologic lesion area, c-Jun N-terminal kinase (JNK)/extracellular signal-regulated kinase (ERK) phosphorylation level, Transferase-Mediated dUTP Nick End-Labeling positive (TUNEL⁺) cells proportion, vascular endothelial cell permeability and release of inflammatory cytokines induced by excess APAP. Therefore, our findings reveal that CD5L may be a potential therapeutic target for prevention and treatment of APAP-induced liver injury.

Mechanistic insights into the protective effect of paracetamol against rotenone-induced Parkinson's disease in rats: Possible role of endocannabinoid system modulation

Parkinson's disease (PD) is a disabling progressive neurodegenerative disease. So far, PD's treatment remains symptomatic with no curative effects. Aside from its blatant analgesic and antipyretic efficacy, recent studies highlighted the endowed neuroprotective potentials of paracetamol (PCM). To this end: the present study investigated: (1) Possible protective role of PCM against rotenone-induced PD-like neurotoxicity in rats, and (2) the mechanisms underlying its neuroprotective actions including cannabinoid receptors' modulation. A dose-response study was conducted using three doses of PCM (25, 50, and 100 mg/kg/day, i.p.) and their effects on body weight changes, spontaneous locomotor activity, rotarod test, tyrosine hydroxylase (TH) and α-synuclein expression, and striatal dopamine (DA) content were evaluated. Results revealed that PCM (100 mg/kg/day, i.p.) halted PD motor impairment, prevented rotenone-induced weight loss, restored normal histological tissue structure, reversed rotenone-induced reduction in TH expression and striatal DA content, and markedly decreased midbrain and striatal α-synuclein expression in rotenone-treated rats. Accordingly, PCM (100 mg/kg/day, i.p.) was selected for further mechanistic investigations, where it ameliorated rotenone-induced oxidative stress, neuro-inflammation, apoptosis, and disturbed cannabinoid receptors' expression. In conclusion, our findings imply a multi-target neuroprotective effect of PCM in PD which could be attributed to its antioxidant, anti-inflammatory and anti-apoptotic activities, in addition to cannabinoid receptors' modulation.

Therapeutic Role of Inducible Nitric Oxide Synthase Expressing Myeloid-Derived Suppressor Cells in Acetaminophen-Induced Murine Liver Failure

Background

Acetaminophen (APAP) overdose is one of the major etiologies of liver failure. Hepatocyte necrosis induced by toxic metabolites of APAP can activate proinflammatory responses, including elastase-expressing neutrophils, to exacerbate liver injury. Myeloid-derived suppressor cells (MDSCs) increased in inflammation can inhibit proinflammatory responses. Our aim is to investigate the role of MDSC in APAP-induced liver failure and the possible therapeutic application.

Methods

BLAB/c mice were injected with a sublethal/lethal dose of APAP as the murine model of liver failure. MDSCs were defined as CD11b⁺Gr-1⁺ cells with the ability of T-cell suppression.

Results

A sublethal challenge of APAP could increase the intrahepatic MDSC and protect mice against subsequent lethal challenge of APAP, lipopolysaccharide (LPS)/D-galatosamine or concanavalin A. This protection was lost if MDSCs were depleted and inducible nitric oxide synthase (iNOS) was the key molecule in this MDSC-mediated protection. Taking advantage of these observations, different bone marrow-derived MDSCs (BM-MDSCs) were generated. Among different cytokine-treated BM-MDSCs, tumor necrosis factor alpha/LPS-primed MDSCs (TNF-α/LPS MDSCs) had the strongest liver-protection ability after adoptive transfer. Further mechanistic explorations showed, iNOS-expressing TNF-α/LPS MDSCs induced the apoptosis of activated neutrophil and decreased the intrahepatic infiltration of elastase-expressing neutrophil. Moreover, we generated MDSCs from human peripheral blood mononuclear cells (PBMCs) with similar phenotype.

Conclusion

We demonstrated the protective role of MDSCs and therapeutic effect of TNF-α/LPS MDSCs in APAP-induced liver failure. MDSC might protect against the APAP-induced liver failure by reducing the intrahepatic infiltration of activated neutrophil to limit inflammation. Therefore, a therapeutic role of MDSCs for APAP-induced liver failure was proposed.

Application of the DILIsym® Quantitative Systems Toxicology drug-induced liver injury model to evaluate the carcinogenic hazard potential of acetaminophen

In 2019, the California Office of Environmental Health Hazard Assessment (OEHHA) initiated a review of the carcinogenic hazard potential of acetaminophen. The objective of the analysis herein was to inform this review by assessing whether variability in patient baseline characteristics (e.g. baseline glutathione (GSH) levels, pharmacokinetics, and capacity of hepatic antioxidants) leads to potential differences in carcinogenic hazard potential at different dosing schemes: maximum labeled doses of 4 g/day, repeated doses above the maximum labeled dose (>4-12 g/day), and acute overdoses of acetaminophen (>15 g). This was achieved by performing simulations of acetaminophen exposure in thousands of diverse virtual patients scenarios using the DILIsym® Quantitative Systems Toxicology (QST) model. Simulations included assessments of the dose and exposure response for toxicity and mode of cell death based on evaluations of the kinetics of changes of: GSH, N-acetyl-p-benzoquinone-imine (NAPQI), protein adducts, mitochondrial dysfunction, and hepatic cell death. Results support that, at therapeutic doses, cellular GSH binds to NAPQI providing sufficient buffering capacity to limit protein adduct formation and subsequent oxidative stress. Simulations evaluating repeated high-level supratherapeutic exposures or acute overdoses indicate that cell death precedes DNA damage that could result in carcinogenicity and thus acetaminophen does not present a carcinogenicity hazard to humans at any dose.

Pre-treatment twice with liposomal clodronate protects against acetaminophen hepatotoxicity through a pre-conditioning effect

BACKGROUND AND AIM

Acetaminophen (APAP) overdose is a major cause of acute liver injury, but the role of macrophages in propagation of the hepatotoxicity is controversial. Early research revealed that macrophage inhibitors protect against APAP injury. However, later work demonstrated that macrophage ablation by acute pre-treatment with liposomal clodronate (LC) exacerbates the toxicity. To our surprise, during other studies, we observed that pre-treatment twice with LC seemed to protect against APAP hepatotoxicity, in contrast to acute pre-treatment. The aim of this study was to confirm that observation and to explore the mechanisms.

METHODS

We treated mice with empty liposomes (LE) or LC twice per week for 1 week before APAP overdose and collected blood and liver tissue at 0, 2, and 6 h post-APAP. We then measured liver injury (serum ALT activity, histology), APAP bioactivation (total glutathione, APAP-protein adducts), oxidative stress (oxidized glutathione [GSSG]), glutamate cysteine-ligase subunit c (Gclc) mRNA, and nuclear factor erythroid 2-related factor (Nrf2) immunofluorescence. We also confirmed ablation of macrophages by F4/80 immunohistochemistry.

RESULTS

Pre-treatment twice with LC dramatically reduced F4/80 staining, protected against liver injury, and reduced oxidative stress at 6 h post-APAP, without affecting APAP bioactivation. Importantly, Gclc mRNA was higher in the LC group at 0 h and total glutathione was higher at 2 h, indicating accelerated glutathione re-synthesis after APAP overdose due to greater basal glutamate-cysteine ligase. Oxidative stress was lower in the LC groups at both time points. Finally, total Nrf2 immunofluorescence was higher in the LC group.

CONCLUSIONS

We conclude that multiple pre-treatments with LC protect against APAP by accelerating glutathione re-synthesis through glutamate-cysteine ligase. Investigators using two or possibly more LC pre-treatments to deplete macrophages, including peritoneal macrophages, should be aware of this possible confounder.

Enhancement of acetaminophen-induced chronic hepatotoxicity in spontaneously diabetic torii (SDT) rats

Some patients encounter hepatotoxicity after repeated acetaminophen (APAP) dosing even at therapeutic doses. In the present study, we focused on the diabetic state as one of the suggested risk factors of drug-induced liver injury in humans and investigated the contribution of accelerated gluconeogenesis to the susceptibility to APAP-induced hepatotoxicity using an animal model of type 2 diabetes patients. Sprague-Dawley (SD) rats and spontaneously diabetic torii (SDT) rats were each given APAP at 0 mg/kg, 300 and 500 mg/kg for 35 days by oral gavage. Plasma and urinary glutathione-related metabolites, liver function parameters, and hepatic glutathione levels were compared between the non-APAP-treated SDT and SD rats and between the APAP-treated SDT and SD rats. Hepatic function parameters were not increased at either dose level in the APAP-treated SD rats, but were increased at both dose levels in the APAP-treated SDT rats. Increases in hepatic glutathione levels attributable to the treatment of APAP were noted only in the APAP-treated SD rats. There were differences in the profiles of plasma and urinary glutathione-related metabolites between the non-APAP-treated SD and SDT rats and the plasma/urinary endogenous metabolite profile after treatment with APAP in the SDT rats indicated that hepatic glutathione synthesis was decreased due to accelerated gluconeogenesis. In conclusion, SDT rats were more sensitive to APAP-induced chronic hepatotoxicity than SD rats and the high susceptibility of SDT rats was considered to be attributable to lowered hepatic glutathione levels induced by accelerated gluconeogenesis.

Estimation of potential risk of allyl alcohol induced liver injury in diabetic patients using type 2 diabetes spontaneously diabetic Torii-Leprfa (SDT fatty) rats

In order to estimate the potential risk of chemicals including drug in patients with type 2 diabetes mellitus (T2DM), we investigated allyl alcohol induced liver injury using SD rats and Spontaneously Diabetic Torii-Lepr^fa (SDT fatty) rats as a model for human T2DM. The diabetic state is one of the risk factors for chemically induced liver injury because of lower levels of glutathione for detoxification by conjugation with chemicals and environmental pollutants and their reactive metabolites. Allyl alcohol is metabolized to a highly reactive unsaturated aldehyde, acrolein, which is detoxified by conjugation with glutathione. Therefore, we used allyl alcohol as a model compound. Our investigations showed that SDT fatty rats appropriately mimic the diabetic state in humans. The profiles of glucose metabolism, hepatic function tests and glutathione synthesis in the SDT fatty rats were similar to those in patients with T2DM. Five-week oral dosing with allyl alcohol to the SDT fatty rats revealed that the allyl alcohol induced liver injury was markedly enhanced in the SDT fatty rats when compared with the SD rats and the difference was considered to be due to lower hepatic detoxification of acrolein, the reactive metabolite of allyl alcohol, by depleted hepatic glutathione synthesis. Taking all the results of the present study into consideration, the potential for allyl alcohol to induce liver injury is considered to be higher in diabetic patients than in healthy humans.

Montelukast Prevents Mice Against Acetaminophen-Induced Liver Injury

Acetaminophen (APAP) is a widely used over-the-counter antipyretic and analgesic drug. Overdose of APAP is the leading cause of hospital admission for acute liver failure. Montelukast is an antagonist of cysteinyl leukotriene receptor 1 (Cysltr1), which protects from inflammation and oxidative stress. However, the function of montelukast in APAP-induced hepatotoxicity remains unknown. In this study, we examined whether pharmacological inhibition of Cystlr1 could protect mice against APAP-induced hepatic damage. We found that APAP treatment upregulated messenger RNA and protein levels of Cysltr1 both in vitro and in vivo. Pharmacological inhibition of Cysltr1 by montelukast ameliorated APAP-induced acute liver failure. The hepatoprotective effect of montelukast was associated with upregulation of hepatic glutathione/glutathione disulfide level, reduction in c-Jun-NH2-terminal kinase activation and oxidative stress. In mouse primary hepatocytes, inhibition of Cysltr1 by montelukast ameliorated the expression of inflammatory-related genes and APAP-induced cytotoxicity. We conclude that montelukast may be used to treat APAP-induced acute hepatic injury.

Mass isotopomer-guided decluttering of metabolomic data to visualize endogenous biomarkers of drug toxicity

Metabolomics offers the opportunity to uncover endogenous biomarkers that can lead to metabolic pathways and networks and that underpin drug toxicity mechanisms. A novel protocol is presented and discussed that is applicable to drugs which generate urinary metabolites when administered to mice sensitive to its toxicity. The protocol would not apply to drugs that are not metabolized or eliminated by a different route. Separate stable isotope-labeled and unlabeled drug administration to mice is made together with collection of urines from control animals. Untargeted mass spectrometry-based metabolomic analysis of these three urine groups is conducted in addition to principal components analysis (PCA). In the case of unlabeled acetaminophen and [acetyl-²H₃]acetaminophen, each given at a hepatotoxic dose (400 mg/kg i.p.) to the sensitive mouse strain (wild-type 129), the PCA loadings plot showed a distribution of ions in the shape of a "fallen-Y" with the deuterated metabolites in one arm and the paired nondeuterated metabolites in the other arm of the fallen-Y. Ions corresponding to the endogenous toxicity biomarkers sat in the mouth of the fallen-Y. This protocol represents an innovative means to separate endogenous biomarkers from drug metabolites, thereby aiding the identification of biomarkers of drug toxicity. For acetaminophen, increased hepatic oxidative stress, mitochondrial damage, Ca²⁺ signaling, heme catabolism, and saturation of glucuronidation, together with decreased fatty acid β-oxidation and cellular energy dysregulation were all implied from the discovered biomarkers. The protocol can be applied to other drugs and may now be translated to clinical studies.

Syndecan-1 limits the progression of liver injury and promotes liver repair in acetaminophen-induced liver injury in mice

Accidental or intentional misuse of acetaminophen (APAP) is the leading cause of acute liver failure in the Western world. Although mechanisms that trigger APAP-induced liver injury (AILI) are well known, those that halt the progression of APAP liver disease and facilitate liver recovery are less understood. Heparan sulfate proteoglycans (HSPGs) bind to and regulate various tissue injury factors through their heparan sulfate (HS) chains, but the importance of HSPGs in liver injury in vivo remains unknown. Here, we examined the role of syndecan-1, the major cell-surface HSPG of hepatocytes, in AILI. Ablation of syndecan-1 in mice led to unopposed progression of liver injury upon APAP overdose. However, direct APAP hepatoxicity and liver injury at early times post-APAP overdose were unaffected by syndecan-1, suggesting that syndecan-1 influences later mechanisms that lead to liver repair. The exuberant liver injury phenotypes in syndecan-1 null (Sdc1^-/- ) mice were traced to a deficiency in protein kinase B (Akt) activation in hepatocytes, which led to a delayed increase in glycogen synthase kinase-3β (GSK-3β)-mediated hepatocyte apoptosis. Inhibition of Akt worsened, whereas inhibition of GSK-3β and caspases protected mice from AILI. Moreover, administration of purified syndecan-1, HS, or engineered heparan compounds containing 2-O-sulfate groups rescued Sdc1^-/- mice from AILI by potentiating Akt signaling and inhibiting GSK-3β-mediated apoptosis in hepatocytes. In addition, HS showed a significantly prolonged therapeutic efficacy as compared to N-acetylcysteine.

CONCLUSION

These results demonstrate that 2-O-sulfated domains in syndecan-1 HS halt disease progression and promote liver repair by enhancing hepatocyte survival in AILI. We propose that syndecan-1 is a critical endogenous factor that controls the balance between prosurvival signaling and apoptosis in hepatocytes in APAP liver disease. (Hepatology 2017;66:1601-1615).

Multiple microRNAs function as self-protective modules in acetaminophen-induced hepatotoxicity in humans

Acetaminophen (APAP) overdose is the leading cause of acute liver failure. Yet the mechanisms underlying adaptive tolerance toward APAP-induced liver injury are not fully understood. To better understand molecular mechanisms contributing to adaptive tolerance to APAP is an underpinning foundation for APAP-related precision medicine. In the current study, the mRNA and microRNA (miRNA) expression profiles derived from next generation sequencing data for APAP-treated (5 and 10 mM) HepaRG cells and controls were analyzed systematically. Putative miRNAs targeting key dysregulated genes involved in APAP hepatotoxicity were selected using in silico prediction algorithms, un-biased gene ontology, and network analyses. Luciferase reporter assays, RNA electrophoresis mobility shift assays, and miRNA pull-down assays were performed to investigate the role of miRNAs affecting the expression of dysregulated genes. Levels of selected miRNAs were measured in serum samples obtained from children with APAP overdose (58.6-559.4 mg/kg) and from healthy controls. As results, 2758 differentially expressed genes and 47 miRNAs were identified. Four of these miRNAs (hsa-miR-224-5p, hsa-miR-320a, hsa-miR-449a, and hsa-miR-877-5p) suppressed drug metabolizing enzyme (DME) levels involved in APAP-induced liver injury by downregulating HNF1A, HNF4A and NR1I2 expression. Exogenous transfection of these miRNAs into HepaRG cells effectively rescued them from APAP toxicity, as indicated by decreased alanine aminotransferase levels. Importantly, hsa-miR-320a and hsa-miR-877-5p levels were significantly elevated in serum samples obtained from children with APAP overdose compared to health controls. Collectively, these data indicate that hsa-miR-224-5p, hsa-miR-320a, hsa-miR-449a, and hsa-miR-877-5p suppress DME expression involved in APAP-induced hepatotoxicity and they contribute to an adaptive response in hepatocytes.

Fructose diet alleviates acetaminophen-induced hepatotoxicity in mice

Acetaminophen (APAP) is a commonly used analgesic and antipyretic that can cause hepatotoxicity due to production of toxic metabolites via cytochrome P450 (Cyp) 1a2 and Cyp2e1. Previous studies have shown conflicting effects of fructose (the major component in Western diet) on the susceptibility to APAP-induced hepatotoxicity. To evaluate the role of fructose-supplemented diet in modulating the extent of APAP-induced liver injury, male C57BL/6J mice were given 30% (w/v) fructose in water (or regular water) for 8 weeks, followed by oral administration of APAP. APAP-induced liver injury (determined by serum levels of liver enzymes) was decreased by two-fold in mice pretreated with fructose. Fructose-treated mice exhibited (~1.5 fold) higher basal glutathione levels and (~2 fold) lower basal (mRNA and activity) levels of Cyp1a2 and Cyp2e1, suggesting decreased bioactivation of APAP and increased detoxification of toxic metabolite in fructose-fed mice. Hepatic mRNA expression of heat shock protein 70 was also found increased in fructose-fed mice. Analysis of bacterial 16S rRNA gene amplicons from the cecal samples of vehicle groups showed that the fructose diet altered gut bacterial community, leading to increased α-diversity. The abundance of several bacterial taxa including the genus Anaerostipes was found to be significantly correlated with the levels of hepatic Cyp2e1, Cyp1a2 mRNA, and glutathione. Together, these results suggest that the fructose-supplemented diet decreases APAP-induced liver injury in mice, in part by reducing metabolic activation of APAP and inducing detoxification of toxic metabolites, potentially through altered composition of gut microbiota.

CSH guidelines for the diagnosis and treatment of drug-induced liver injury

Drug-induced liver injury (DILI) is an important clinical problem, which has received more attention in recent decades. It can be induced by small chemical molecules, biological agents, traditional Chinese medicines (TCM), natural medicines (NM), health products (HP), and dietary supplements (DS). Idiosyncratic DILI is far more common than intrinsic DILI clinically and can be classified into hepatocellular injury, cholestatic injury, hepatocellular-cholestatic mixed injury, and vascular injury based on the types of injured target cells. The CSH guidelines summarized the epidemiology, pathogenesis, pathology, and clinical manifestation and gives 16 evidence-based recommendations on diagnosis, differential diagnosis, treatment, and prevention of DILI.

From hepatoprotection models to new therapeutic modalities for treating liver diseases: a personal perspective

A variety of rodent models of hepatoprotection have been developed in which tolerance to acetaminophen-induced hepatotoxicity occurs. Autoprotection/heteroprotection is a phenomenon where prior exposure to a mildly toxic dose of toxicant confers protection against a subsequently administered higher dose of the same toxicant (as in the case of autoprotection) or to a different toxicant (referred to as heteroprotection). Multiple mechanisms regulate this adaptive response, including hepatocellular proliferation, proteostasis, enhanced expression of cytoprotective genes, and altered tissue immune response. In this review, we will discuss recent findings that highlight the complexity of these adaptive mechanisms and we also outline the usefulness of these findings to devise therapeutic and/or diagnostic tools for acetaminophen-induced liver damage in patients.

Hearing, reactive metabolite formation, and oxidative stress in cochleae after a single acute overdose of acetaminophen: an in vivo study

CONTEXT

Although the liver is the primary target organ in acetaminophen (APAP) toxicity, other organs are affected. Previous data suggested that chronic APAP abuse can be ototoxic and the mechanism involves APAP-induced oxidative stress and reactive metabolite (N-acetyl-p-benzoquinone imine, NAPQI)-induced endoplasmic reticulum stress. However, the effect of a single acute overdose on hearing has not been tested.

OBJECTIVES

To determine if a single acute APAP overdose causes hearing damage, and to explore possible mechanisms of APAP ototoxicity.

MATERIALS AND METHODS

Male C57BL/6 J mice were treated with a single human-relevant overdose of APAP (300 mg APAP per kg bodyweight). Blood, liver and cochleae were harvested at 0, 2, 6 and 24 h post-APAP. In some mice, auditory brainstem responses (ABRs) to a range of frequencies were measured at 24 h. The furosemide plus kanamycin (FS/K) model of drug ototoxicity was used as a positive control for hearing loss. NAPQI formation after APAP was assessed by measuring glutathione depletion and covalent protein binding, and oxidative stress was assessed by measuring glutathione disulfide.

RESULTS

There was no evidence of reactive metabolite formation or hearing loss after a single overdose of APAP at a clinically relevant dose. However, there was a transient increase in oxidative stress.

DISCUSSION

Although a single acute overdose was not ototoxic, there was evidence of oxidative stress which may support a role for oxidative stress in hearing loss due to chronic APAP abuse.

CONCLUSION

A single human-relevant acute overdose of APAP causes transient oxidative stress in cochleae but not hearing loss.

N-Acetyl cysteine does not prevent liver toxicity from chronic low-dose plus subacute high-dose paracetamol exposure in young or old mice

Paracetamol is an analgesic commonly used by people of all ages, which is well documented to cause severe hepatotoxicity with acute overexposures. The risk of hepatotoxicity from nonacute paracetamol exposures is less extensively studied, and this is the exposure most common in older adults. Evidence on the effectiveness of N-acetyl cysteine (NAC) for nonacute paracetamol exposures, in any age group, is lacking. This study aimed to examine the effect of long-term exposure to therapeutic doses of paracetamol and subacute paracetamol overexposure, in young and old mice, and to investigate whether NAC was effective at preventing paracetamol hepatotoxicity induced by these exposures. Young and old male C57BL/6 mice were fed a paracetamol-containing (1.33 g/kg food) or control diet for 6 weeks. Mice were then dosed orally eight times over 3 days with additional paracetamol (250 mg/kg) or saline, followed by either one or two doses of oral NAC (1200 mg/kg) or saline. Chronic low-dose paracetamol exposure did not cause hepatotoxicity in young or old mice, measured by serum alanine aminotransferase (ALT) elevation, and confirmed by histology and a DNA fragmentation assay. Subacute paracetamol exposure caused significant hepatotoxicity in young and old mice, measured by biochemistry (ALT) and histology. Neither a single nor double dose of NAC protected against this toxicity from subacute paracetamol in young or old mice. This finding has important clinical implications for treating toxicity due to different paracetamol exposure types in patients of all ages, and implies a need to develop new treatments for subacute paracetamol toxicity.

Adaptation to acetaminophen exposure elicits major changes in expression and distribution of the hepatic proteome

Acetaminophen overdose is the leading cause of acute liver failure. One dose of 10–15 g causes severe liver damage in humans, whereas repeated exposure to acetaminophen in humans and animal models results in autoprotection. Insight of this process is limited to select proteins implicated in acetaminophen toxicity and cellular defence. Here we investigate hepatic adaptation to acetaminophen toxicity from a whole proteome perspective, using quantitative mass spectrometry. In a rat model, we show the response to acetaminophen involves the expression of 30% of all proteins detected in the liver. Genetic ablation of a master regulator of cellular defence, NFE2L2, has little effect, suggesting redundancy in the regulation of adaptation. We show that adaptation to acetaminophen has a spatial component, involving a shift in regionalisation of CYP2E1, which may prevent toxicity thresholds being reached. These data reveal unexpected complexity and dynamic behaviour in the biological response to drug-induced liver injury.

Acetaminophen hepatotoxicity in mice: Effect of age, frailty and exposure type

Acetaminophen is a commonly used analgesic that can cause severe hepatotoxicity in overdose. Despite old age and frailty being associated with extensive and long-term utilization of acetaminophen and a high prevalence of adverse drug reactions, there is limited information on the risks of toxicity from acetaminophen in old age and frailty. This study aimed to assess changes in the risk and mechanisms of hepatotoxicity from acute, chronic and sub-acute acetaminophen exposure with old age and frailty in mice. Young and old male C57BL/6 mice were exposed to either acute (300 mg/kg via oral gavage), chronic (100 mg/kg/day in diet for six weeks) or sub-acute (250 mg/kg, t.i.d., for three days) acetaminophen, or saline control. Pre-dosing mice were scored for the mouse clinical frailty index, and after dosing serum and liver tissue were collected for assessment of toxicity and mechanisms. There were no differences with old age or frailty in the degree of hepatotoxicity induced by acute, chronic or subacute acetaminophen exposure as assessed by serum liver enzymes and histology. Age-related changes in the acetaminophen toxicity pathways included increased liver GSH concentrations, increased NQO1 activity and an increased pro- and anti-inflammatory response to acetaminophen in old age. Frailty-related changes included a negative correlation between frailty index and serum protein, albumin and ALP concentrations for some mouse groups. In conclusion, although there were changes in some pathways that would be expected to influence susceptibility to acetaminophen toxicity, there was no overall increase in acetaminophen hepatotoxicity with old age or frailty in mice.

Translational biomarkers of acetaminophen-induced acute liver injury

Acetaminophen (APAP) is a commonly used analgesic drug that can cause liver injury, liver necrosis and liver failure. APAP-induced liver injury is associated with glutathione depletion, the formation of APAP protein adducts, the generation of reactive oxygen and nitrogen species and mitochondrial injury. The systems biology omics technologies (transcriptomics, proteomics and metabolomics) have been used to discover potential translational biomarkers of liver injury. The following review provides a summary of the systems biology discovery process, analytical validation of biomarkers and translation of omics biomarkers from the nonclinical to clinical setting in APAP-induced liver injury.

Pharmacologic cholinesterase inhibition improves survival in acetaminophen-induced acute liver failure in the mouse

Background

Acetaminophen (APAP) is one of the most widely used analgesic and antipyretic pharmaceutical substances in the world and accounts for most cases of drug induced liver injury resulting in acute liver failure. Acute liver failure initiates a sterile inflammatory response with release of cytokines and innate immune cell infiltration in the liver. This study investigates, whether pharmacologic acetylcholinesterase inhibition with neostigmine diminishes liver damage in acute liver failure via the cholinergic anti-inflammatory pathway.

Methods

Acute liver failure was induced in BALB/c mice by a toxic dose of acetaminophen (APAP). Neostigmine and/or N-acetyl-cysteine (NAC) were applied therapeutically at set time points and the survival was investigated. Liver damage was assessed by serum parameters, histopathology and serum cytokine assays 12 h after initiation of acute liver failure.

Results

Serum parameters, histopathology and serum cytokine assays showed pronounced features of acute liver failure 12 h after application of acetaminophen (APAP). Neostigmine treatment led to significant reduction of serum liver enzymes (LDH (47,147 ± 12,726 IU/l vs. 15,822 ± 10,629 IU/l, p = 0.0014) and ALT (18,048 ± 4,287 IU/l vs. 7,585 ± 5,336 IU/l, p = 0.0013), APAP-alone-treated mice vs. APAP + neostigmine-treated mice), inflammatory cytokine levels (IL-1β (147 ± 19 vs. 110 ± 25, p = 0.0138) and TNF-α (184 ± 23 vs. 130 ± 33, p = 0.0086), APAP-alone-treated mice vs. APAP + neostigmine-treated mice) and histopathological signs of damage.

Animals treated with NAC in combination with the peripheral cholinesterase inhibitor neostigmine showed prolonged survival and improved outcome.

Conclusions

Neostigmine is an acetylcholinesterase inhibitor that ameliorates the effects of APAP-induced acute liver failure in the mouse and therefore may provide new treatment options for affected patients.

Tolerance to acetaminophen hepatotoxicity in the mouse model of autoprotection is associated with induction of flavin-containing monooxygenase-3 (FMO3) in hepatocytes

Acetaminophen (APAP) pretreatment with a hepatotoxic dose (400 mg/kg) in mice results in resistance to a second, higher dose (600 mg/kg) of APAP (APAP autoprotection). Recent microarray work by our group showed a drastic induction of liver flavin containing monooxygenase-3 (Fmo3) mRNA expression in our mouse model of APAP autoprotection. The role of liver Fmo3, which detoxifies xenobiotics, in APAP autoprotection is unknown. The purpose of this study was to characterize the gene regulation and protein expression of liver Fmo3 during APAP hepatotoxicity. The functional consequences of Fmo3 induction were also investigated. Plasma and livers were collected from male C57BL/6J mice over a period of 72 h following a single dose of APAP (400 mg/kg) to measure Fmo3 mRNA and protein expression. Although Fmo3 mRNA levels increased significantly following APAP treatment, protein expression changed marginally. In contrast, both Fmo3 mRNA and protein expression were significantly higher in APAP autoprotected livers. Unlike male C57BL/6J mice, female mice have ∼80-times higher constitutive Fmo3 mRNA levels and are highly resistant to APAP hepatotoxicity. Coadministration of APAP with the FMO inhibitor methimazole rendered female mice susceptible to APAP hepatotoxicity, with no changes in susceptibility detected in male mice. Furthermore, a human hepatocyte cell line (HC-04) clone over-expressing human FMO3 showed enhanced resistance to APAP cytotoxicity. Taken together, these findings establish for the first time induction of Fmo3 protein expression and function by xenobiotic treatment. Our results also indicate that Fmo3 expression and function plays a role in protecting the liver from APAP-induced toxicity. Although the mechanism(s) of this protection remains to be elucidated, this work describes a novel protective function for this enzyme.

Evaluation of human hepatocytes cultured by three-dimensional spheroid systems for drug metabolism

We investigated the utility of three-dimensional (3D) spheroid cultures of human hepatocytes in discovering drug metabolites. Metabolites of acetaminophen, diclofenac, lamotrigine, midazolam, propranolol and salbutamol were analyzed by liquid chromatography-tandem mass spectrometry (LC/MS/MS) to measure enzyme activities in this system cultured for 2 and 7 days. Sequential metabolic reactions by Phase I and then Phase II enzymes were found in diclofenac [CYP2C9 and UDP-glucuronyltransferases (UGTs)], midazolam (CYP3A4 and UGTs) and propranolol (CYP1A2/2D6 and UGTs). Moreover, lamotrigine and salbutamol were metabolized to lamotrigine-N-glucuronide and salbutamol 4-O-sulfate, respectively. These metabolites, which are human specific, could be observed in clinical studies, but not in conventional hepatic culture systems as in previous reports. Acetaminophen was metabolized to glucuronide and sulfate conjugates, and N-acetyl-p-benzo-quinoneimine (NAPQI) and its metabolites were not observed. In addition, mRNA of drug-metabolism enzymes [CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, UGT1A1, UGT2B7, sulfotransferase 1A1 (SULT1A1) and glutathione S-transferase pi 1 (GSTP1)], which were measured by qRT-PCR, were expressed in the human hepatocyte spheroids. In conclusion, these results suggest that human hepatocyte spheroids are useful in discovering drug metabolites.

Paracetamol impairs the profile of amino acids in the rat brain

In our experiment we investigated the effect of subcutaneous administration of paracetamol on the levels of amino acids in the brain structures. Male Wistar rats received for eight weeks paracetamol at two doses: 10 mg/kg b.w. (group P10, n=9) and 50 mg/kg b.w. per day s.c. (group P50, n=9). The regional brain concentrations of amino acids were determined in the prefrontal cortex, hippocampus, hypothalamus and striatum of control (Con, n=9) and paracetamol-treated groups using HPLC. Evaluation of the biochemical results indicated considerable decrease of the content of amino acids in the striatum (glutamine, glutamic acid, taurine, alanine, aspartic acid) and hypothalamus (glycine) between groups treated with paracetamol compared to the control. In the prefrontal cortex paracetamol increased the level of γ-aminobutyric acid (GABA). The present study demonstrated significant effect of the long term paracetamol treatment on the level of amino acids in the striatum, prefrontal cortex and hypothalamus of rats.

Protective effects of goldenseal (Hydrastis canadensis L.) on acetaminophen-induced hepatotoxicity through inhibition of CYP2E1 in rats

Background:

Goldenseal (Hydrastis canadensis L.) inhibits various cytochrome P450 (CYP) isoforms such as CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A in vitro. High doses of acetaminophen (APAP) generate the highly reactive intermediate, N-acetyl-p-benzoquinone imine (NAPQI), catalyzed mainly by CYP2E1. The aim of this study was to investigate the hepatoprotective effects of orally administrated goldenseal against APAP-induced acute liver failure (ALF) via inhibition of CYP2E1.

Materials and Methods:

Male Wistar rats were treated orally with goldenseal (300 and 1000 mg/kg) 2, 18, and 26 h before and 6 h after oral APAP (400 mg/kg) administration. Serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities as well as serum APAP concentration were evaluated.

Results:

Goldenseal extract inhibited CYP1A2, CYP2D6, CYP2E1, and CYP3A activity, and the inhibitory effect on CYP2E1 was the strongest (IC₅₀ 4.32 μg/mL). Treatment with goldenseal (300 mg/kg) significantly attenuated the APAP-induced increase in serum AST and ALT, and the hepatoprotective effect of goldenseal was stronger than that of silymarin (200 mg/kg). Moreover, serum APAP concentration was increased by goldenseal treatment, presumably as a result of the inhibitory effect of goldenseal on the metabolism of APAP to NAPQI.

Conclusion:

These results suggest that goldenseal ameliorates APAP-induced ALF and that this protection can likely be attributed to the inhibition of CYP2E1 activity, which generates the highly reactive intermediate of APAP.

An Update on Drug-induced Liver Injury

Idiosyncratic drug-induced liver injury (DILI) is an important cause of morbidity and mortality following drugs taken in therapeutic doses. Hepatotoxicity is a leading cause of attrition in drug development, or withdrawal or restricted use after marketing. No age is exempt although adults and the elderly are at increased risk. DILI spans the entire spectrum ranging from asymptomatic elevation in transaminases to severe disease such as acute hepatitis leading to acute liver failure. The liver specific Roussel Uclaf Causality Assessment Method is the most validated and extensively used for determining the likelihood that an implicated drug caused DILI. Asymptomatic elevation in liver tests must be differentiated from adaptation. Drugs producing DILI have a signature pattern although no single pattern is characteristic. Antimicrobial and central nervous system agents including antiepileptic drugs are the leading causes of DILI worldwide. In the absence of a diagnostic test or a biomarker, the diagnosis rests on the evidence of absence of competing causes such as acute viral hepatitis, autoimmune hepatitis and others. Recent studies show that antituberculosis drugs given for active or latent disease are still a major cause of drug-induced liver injury in India and the West respectively. Presence of jaundice signifies a severe disease and entails a worse outcome. The pathogenesis is unclear and is due to a mix of host, drug metabolite and environmental factors. Research has evolved from incriminating candidate genes to genome wide analysis studies. Immediate cessation of the drug is key to prevent or minimize progressive damage. Treatment is largely supportive. N-acetylcysteine is the antidote for paracetamol toxicity. Carnitine has been tried in valproate injury whereas steroids and ursodeoxycholic acid may be used in DILI associated with hypersensitivity or cholestatic features respectively. This article provides an overview of the epidemiology, the patterns of hepatotoxicity, the pathogenesis and associated risk factors besides its clinical management.

Effects of preemptive analgesia with celecoxib or acetaminophen on postoperative pain relief following lower extremity orthopedic surgery

Background:

Efficacy of preemptive analgesia with nonsteroidal antiinflammatory drugs (NSAIDs) in comparison with acetaminophen is controversial. The present study evaluates the preemptive analgesia efficacy of celecoxib and acetaminophen in comparison with placebo for postoperative pain relief in patients who underwent orthopedic surgery under general anesthesia.

Materials and Methods:

Ninetypatients eligible for elective distal extremity surgery were categorized in three groups: group C includedpatients who received oral celecoxib 200 mg 2 h before surgery; group A included those who received oral acetaminophen 320 mg 2 h before surgery; and group P included those who received oral placebo 2 h before surgery. Pain scores were recorded at 4, 12, and 24 h after operation.

Results:

The pain scores 4 h after operation was significantly less in group C than in groups A and P (4.7±1.7 vs. 5±1.5 vs. 6.8±1.7, respectively, P = 0.015). No significant difference was noted in pain scores at 12 h (4.6±2, 4.9±1.9, 4.3±1.4 in group A, group C, group P, respectively P > 0.05) and 24 h (3.1±1.7, 3.0±1.4, 3.3±1.7 in group A, group C, group P, respectively, P > 0.05) after operation among the three groups.

Conclusion:

Using oral celecoxib 200 mg 2 h before operation is better thanusing oral acetaminophen 320 mg 2 h before the beginning of surgery for control of postoperative pain in patients who underwent lower extremity orthopedic surgery under general anesthesia.

Acute and chronic acetaminophen use and renal disease: a case-control study using pharmacy and medical claims

BACKGROUND

Studies have examined the association between acetaminophen (APAP) use and renal disease; however, their interpretation is limited by a number of methodological issues.

OBJECTIVE

To study the association between acute and chronic prescription-acquired APAP use and renal disease.

METHODS

This was a retrospective case-control study of medical and pharmacy claims of a 10% random sample of the enrollees from the IMS LifeLink Health Plans commercial claims dataset for dates of service from January 1, 1997, through December 31, 2009. Subjects were continuously enrolled and aged 18 years or older. Cases had at least 1 incident claim of renal disease defined by ICD-9-CM codes in the primary diagnosis field. Controls were randomly selected from individuals without evidence of renal disease, liver disease, or asthma in medical claims and matched to cases in a 3-to-1 ratio based on 3 variables (age, gender, and geographic region). APAP exposure, dosage, and duration of use were measured in the 7 and 30 days (acute) and in the 1-year (chronic) look-back periods. Multivariable conditional logistic regression was used to estimate the risk of APAP exposure adjusted for comorbidities, use of other nephrotoxic drugs, and health system factors.

RESULTS

There were 4,724 cases and 14,172 controls with a mean (SD) age of 60.8 (17.8) years, and 52.6% were males; 10.9% of cases and 4.2% of controls had APAP exposure in the 30 days pre-index with mean potential maximum daily dosages of 3,846.5 mg and 3,190.8 mg, respectively. Acute APAP exposure was significantly associated with renal disease, and the risk decreased with longer look-back periods (7 days: adjusted odds ratio [OR] = 1.93, 95% CI = 1.61-2.30); 30 days: OR = 1.71, 95% CI = 1.48-1.97). Cumulative APAP dosage greater than 1 kg and APAP use for longer than 30 days in the pre-index year were not significantly associated with an increased risk of renal disease (both P values = 0.900).

CONCLUSIONS

Acute prescription-acquired APAP use was associated with renal disease, while chronic use was not. Because this study assessed APAP use in pharmacy claims, further research accounting for over-the-counter APAP use is warranted before the safety of chronic APAP consumption can be firmly established.

Acetaminophen enhances pruritus in a mouse model of contact dermatitis induced by suboptimal concentration of hapten

Acetaminophen (APAP) is one of the most commonly used drugs worldwide to reduce fever, particularly in children. It is generally considered to be a safe drug. However, a number of studies have shown that regular use of APAP increases the risk of developing allergic diseases. Nonetheless, no animal models have been used to investigate these findings. Therefore, we aimed to create an animal model of APAP-induced pruritus in mice. APAP (0.25% and 0.5%) was administered via drinking water daily from infancy, and a suboptimal concentration of 2,4,6-trinitrochlorobenzene (TNCB) was applied repeatedly to each ear three times a week for 7 weeks to evoke chronic allergic contact dermatitis. Neither 0.25% nor 0.5% APAP was overtly hepatotoxic after 73 days of daily administration. Repeated challenge with TNCB evoked increase in the number of scratching bouts compared to day 1. This increase in the number of scratching bouts was significant in 0.25% and 0.5% APAP groups but not in the group treated with TNCB alone. Daily administration of 0.5% APAP significantly increased in the number of scratching bouts compared to TNCB alone on day 29. This animal model will be useful for investigating the mechanism underlying the increased risk of development of eczema caused by regular APAP use and for examining safer and more effective therapy with APAP.

Recent developments in acute liver failure

Acute liver failure is a remarkably rare syndrome, the result of rapid hepatocyte injury occurring over days or a few weeks, and encompassing multiple etiologies, but all with a remarkably similar clinical picture. The clinical features of coagulopathy and encephalopathy characterize this severe and often fatal condition. To date, transplantation has been the only reliable form of rescue for many patients. Recent developments have included a clearer understanding of the different contributing etiologies, how to build a diagnosis and prognosis based on initial laboratory findings, a more aggressive approach to intensive care management and more detailed understanding of the role of transplantation in this setting. This review will provide an overview of standard practices and new research initiatives and findings for this interesting but vexing orphan disease. Particular attention will be paid to practical matters for clinicians to consider in approaching the ALF patient. Few controlled clinical trials have been possible because of the condition's rarity. Critical care of these rare patients is key to their survival and decisions must be made decisively, sometimes with inadequate information. Experience is helpful but experienced clinician managers are even rarer than the disease: few hepatologists or intensivists have in-depth experience with ALF patients.

3,5,5-trimethyl-hexanoyl-ferrocene diet protects mice from moderate transient acetaminophen-induced hepatotoxicity

Acetaminophen (APAP) overdose is the most frequent cause of adult acute liver failure. Susceptibility or resistance to APAP toxicity is most likely accounted for by the interplay of several factors. One factor important in multiple different chronic liver diseases that may play a role in APAP toxicity is elevated hepatic iron. Hereditary hemochromatosis is traditionally associated with hepatic iron overload. However, varying degrees of elevated hepatic iron stores observed in chronic hepatitis C and B, alcoholic liver disease and nonalcoholic fatty liver disease also have clinical relevance. We employed an animal model in which mice are fed a 3,5,5-trimethyl-hexanoyl-ferrocene (TMHF)-supplemented diet to evaluate the effect of elevated hepatic iron on APAP hepatotoxicity. Three hundred milligrams per kilogram APAP was chosen because this dosage induces hepatotoxicity but is not lethal. Since both excess iron and APAP induce oxidative stress and mitochondrial dysfunction, we hypothesized that the TMHF diet would enhance APAP hepatotoxicity. The results were the opposite. Centrilobular vacuolation/necrosis, APAP adducts, nitrotyrosine adducts, and a spike in serum alanine aminotransferase, which were observed in control mice treated with APAP, were not observed in TMHF-fed mice treated with APAP. Further analysis showed that the levels of CYP2E1 and CYP1A2 were not significantly different in TMHF-treated compared with control mice. However, the magnitude of depletion of glutathione following APAP treatment was considerably less in TMHF-treated mice than in mice fed a control diet. We conclude that a TMHF diet protects mice from moderate transient APAP-induced hepatotoxicity prior to the formation of APAP adducts, and one contributing mechanism is reduction in glutathione depletion.

Review article: Drug-induced liver injury--its pathophysiology and evolving diagnostic tools

BACKGROUND

Drug-induced liver injury (DILI) is a significant cause of morbidity and mortality accounting for at least 13% of acute liver failure cases in the US. It is the leading cause of acute liver failure among patients referred for liver transplantation and the most common reason that drugs in development do not obtain FDA approval. The incidence of DILI has been reported to be one in 10,000 to one in 100,000 patients; however, the actual incidence is probably higher due in part to the difficulty of diagnosis.

AIM

To present a review of the current literature on DILI with a focus on its pathophysiology and evolving diagnostic modalities.

METHODS

A PubMed literature search was conducted using the terms 'drug induced liver injury', 'pathophysiology', 'causality', 'diagnosis', 'toxicogenomics' and 'pharmacogenetics'.

RESULTS

Drug-induced liver injury is an area of ongoing research. From the time it was first recognised, our understanding of the pathophysiology, its classification, diagnosis and reporting by established national networks continues to challenge and evolve. Metabonomics, pharmacogenetics, proteomics and transcriptomics are more recent areas of study that have been applied to further the understanding of DILI.

CONCLUSIONS

Despite recent advances in our understanding of drug-induced liver injury, many aspects of its pathophysiology and clinical impact remain unclear. In addition, genomic-based studies are evolving concepts, which undoubtedly continue to contribute to our understanding of the underlying mechanisms of drug-induced liver injury.

Acetaminophen ototoxicity after acetaminophen/hydrocodone abuse: evidence from two parallel in vitro mouse models

OBJECTIVE

Acetaminophen/hydrocodone, a commonly used analgesic preparation, has been linked to rapidly progressing sensorineural hearing loss in human patients. The cellular and molecular mechanisms underlying the ototoxic effects of this drug combination are currently unknown, but are usually associated with high doses of hydrocodone. This study was aimed at identifying the specific agent responsible for hearing loss from toxic killing of cochlear sensory cells.

STUDY DESIGN

Dose-response study.

SETTING

University laboratory and private research facility.

SUBJECTS AND METHODS

Math1 green fluorescent protein neonatal mouse cochlear cultures as well as a mouse auditory cell line (HEI-OC1) were exposed in vitro to different concentrations of acetaminophen, hydromorphone (the active metabolite of hydrocodone), and the micronutrient L-carnitine, either alone or combined. Using fluorescent and light microscopy, we quantified the sensory hair cells from a 600-microm basal segment before and after treatment. Acetaminophen/hydrocodone-induced apoptosis of HEI-OC1 was evaluated by caspase 3-activation studies. Statistically significant cell survival was determined with Student t test and analysis of variance.

RESULTS

Cell death was associated mainly with exposure to acetaminophen, was slightly potentiated when combined with hydromorphone, and was partially prevented by L-carnitine. Exposure to hydrocodone or hydromorphone alone failed to kill either cochlear hair cells or HEI-OC1 cells.

CONCLUSION

Our findings point to acetaminophen, rather than hydrocodone, as the primary cytotoxic agent. Hydrocodone, however, may work synergistically with acetaminophen, increasing the damage to auditory cells. These findings are an important first step toward understanding the mechanism of acetaminophen/hydrocodone ototoxicity and may lead to future treatment strategies for hearing loss from ototoxic medications.

Alteration in metabolism and toxicity of acetaminophen upon repeated administration in rats

Our previous studies showed that administration of a subtoxic dose of acetaminophen (APAP) to female rats increased generation of carbon monoxide from dichloromethane, a metabolic reaction catalyzed mainly by cytochrome P450 (CYP) 2E1. In this study we examined the changes in metabolism and toxicity of APAP upon repeated administration. An intraperitoneal dose of APAP (500 mg/kg) alone did not increase aspartate aminotransferase, alanine aminotransferase, or sorbitol dehydrogenase activity in serum, but was significantly hepatotoxic when the rats had been pretreated with an identical dose of APAP 18 h earlier. The concentrations and disappearance of APAP and its metabolites in plasma were monitored for 8 h after the treatment. APAP pretreatment reduced the elevation of APAP-sulfate, but increased APAP-cysteine concentrations in plasma. APAP or APAP-glucuronide concentrations were not altered. Administration of a single dose of APAP 18 h before sacrifice increased microsomal CYP activities measured with p-nitrophenol, p-nitroanisole, and aminopyrine as probes. Expression of CYP2E1, CYP3A, and CYP1A proteins in the liver was also elevated significantly. The results suggest that administration of APAP at a subtoxic dose may result in an induction of hepatic CYP enzymes, thereby altering metabolism and toxicological consequences of various chemical substances that are substrates for the same enzyme system.

Toxicodynamics of subacute co-exposure to groundwater contaminant arsenic and analgesic-antipyretic drug acetaminophen in rats

Arsenic is an environmental contaminant, while acetaminophen is an extensively used nonsteroidal analgesic-antipyretic drug. We evaluated whether subacute co-exposure to arsenic and acetaminophen would produce more toxicity than that caused by exposure to either of the xenobiotics in rats. Toxicity was evaluated through changes in body weight, feed consumption, liver weight and microsomal drug-metabolizing enzymes, lipid peroxidation and antioxidants in liver. Arsenic had no effect on body weight and feed consumption. Acetaminophen-mediated decrease in body weight was attenuated in the co-exposed rats. Acetaminophen alone or its co-administration with arsenic decreased feed consumption. Arsenic reduced acetaminophen-mediated increase in the activities of drug-metabolizing enzymes. The co-exposure caused lesser lipid peroxidation than the individual exposure. Arsenic or acetaminophen given alone depleted GSH and decreased the activities of superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glutathione-S-transferase and these effects remained mostly unaffected after co-exposure. The results suggest that co-exposure to arsenic and acetaminophen may be less hazardous than their independent exposure in rats.

Current concepts of mechanisms in drug-induced hepatotoxicity

Drug-induced liver injury (DILI) has become a leading cause of severe liver disease in Western countries and therefore poses a major clinical and regulatory challenge. Whereas previously drug-specific pathways leading to initial injury of liver cells were the main focus of mechanistic research and classifications, current concepts see these as initial upstream events and appreciate that subsequent common downstream pathways and their attenuation by drugs and other environmental and genetic factors also have a profound impact on the risk of an individual patient to develop overt liver disease. This review summarizes current mechanistic concepts of DILI in a 3-step model that limits its principle mechanisms to three main ways of initial injury, i.e. direct cell stress, direct mitochondrial impairment, and specific immune reactions. Subsequently, initial injury initiates further downstream events, i.e. direct and death receptor-mediated pathways leading to mitochondrial permeability transition, which then results in apoptotic or necrotic cell death. For all mechanisms, mitochondria play a central role in events leading to apoptotic vs. necrotic cell death. New treatment targets consequently focus on interference with downstream pathways that mediate injury and therefore determine the ultimate outcome of DILI. Genome wide and targeted pharmacogenetic as well as metabonomic approaches are now used in order to reach the key goals of a better understanding of mechanisms in hepatotoxicity, and to develop new strategies for its prediction and treatment. However, the complexity of interactions between genetic and environmental risk factors is considerable, and DILI therefore currently remains unpredictable for most hepatotoxins.

Effect of repeated administration with subtoxic doses of acetaminophen to rats on enterohepatic recirculation of a subsequent toxic dose

Development of resistance to toxic effects of acetaminophen (APAP) was reported in rodents and humans, though the mechanism is only partially understood. We examined in rats the effect of administration with subtoxic daily doses (0.2, 0.3, and 0.6g/kg, i.p.) of APAP on enterohepatic recirculation and liver toxicity of a subsequent i.p. toxic dose of 1g/kg, given 24h after APAP pre-treatment. APAP and its major metabolite APAP-glucuronide (APAP-Glu) were determined in bile, urine, serum and liver homogenate. APAP pre-treatment was not toxic, as determined by serum markers of liver damage and neither induced oxidative stress as demonstrated by assessment of ROS generation in liver or glutathione species in liver and bile. APAP pre-treatment induced a partial shift from biliary to urinary elimination of APAP-Glu after administration with the toxic dose, and decreased hepatic content and increased serum content of this conjugate, consistent with a marked up-regulation of its basolateral transporter Mrp3 relative to apical Mrp2. Preferential secretion of APAP-glu into blood decreased enterohepatic recirculation of APAP, thus attenuating liver exposition to the intact drug, as demonstrated 6h after administration with the toxic dose. The beneficial effect of interfering the enterohepatic recirculation was alternatively tested in animals receiving activated charcoal by gavage to adsorb APAP of biliary origin. The data indicated decreased liver APAP content and glutathione consumption. We conclude that selective up-regulation of Mrp3 expression by APAP pre-treatment may contribute to development of resistance to APAP hepatotoxicity, at least in part by decreasing its enterohepatic recirculation.

Proteomic Study of Hepatic Nuclear Extracts in an Adaptive Acetaminophen Tolerance Model

Introduction

Variability in response to acetaminophen (APAP)-induced aseptic inflammation and tolerance to the impending hepatic damage has been described. To understand the mechanism of adaptive tolerance, we investigated the proteomic profiles of crude nuclear lysates in a mouse model. We hypothesized that pretreatment with low doses of APAP prior to a toxic dose results in differential protein expression.

Materials and Methods

Mice (BALB/C) were separated into three groups: the pretreated (PT) group received incremental doses of APAP while the last dose only (LD) and naïve groups were given saline vehicle. A toxic dose of APAP was administered on the seventh day to the PT and LD animals only and all groups were euthanized 3 h postdose. Total protein from crude hepatic nuclear lysates were applied to protein arrays and analyzed by immunoaffinity mass spectrometry.

Results and Discussion

Comparative data analyses of protein peaks revealed a protein that was significantly increased at m/z of 60,030 (p60) in the LD animals vs the other two groups. The closest match for the preliminary identification of the p60 protein based on a Swiss-Prot/TagIdent database search using the approximate isoelectric point and molecular weight information was Ccr4–Not complex subunit-2. This protein is a subunit of a multiprotein complex and serves as a transcriptional suppressor involved in controlling mRNA synthesis and degradation. Preliminary identification was also supported by Western blot analysis using anti-CNOT2 antibody.

Conclusion

Considering the APAP tolerance model, we conclude that toxicogenomic approaches such as nuclear profiling are useful tools in assessing differential expression of transcriptional factors involved in inflammatory response and adaptive tolerance to toxins.

Acetaminophen-related acute liver failure in the United States

Acetaminophen overdoses are the number one cause of acute liver failure (ALF) in the United States; they account for 50% of all cases of ALF and carry a 30% mortality. Nevertheless, acetaminophen is a highly successful product with sales easily exceeding a billion dollars annually. A narcotic-containing acetaminophen preparation is the number one generic drug sold in the United States. Historically, a strong association was claimed with alcohol as a co-factor and the accidental nature of ingestions was emphasized but the association with alcohol remains a matter of debate. Recently, a very reliable assay has been developed which detects a byproduct of the toxic reaction, acetaminophen-CYS adducts, protein-acetaminophen compounds that are released from damaged hepatocytes in similar fashion to aminotransferases. The US Acute Liver Failure Study Group is a multi-center network established to gather data prospectively on all forms of acute liver failure, a rare but often fatal condition. The group has data on more than 500 acetaminophen cases and described 275 of them in detail in a recent paper. The clinical pattern of presentation is somewhat different in the US compared with the UK, since nearly half are considered to be unintentional, the result of overuse of acetaminophen-containing compounds for pain relief, without suicidal intent. Despite the relatively good prognosis for acetaminophen overdoses, 30% die once acute liver failure occurs and this is the largest cause of death in the entire ALF registry.