N-acetyl-p-benzoquinone Imine Research Articles

Cysteine-modification of Kv7 Channels as Analgesic Mechanism of Action of Acetaminophen

N-acetyl-p-benzoquinone imine (NAPQI), an active metabolite of acetaminophen, decreases excitability in first- and second-order neurons of the nociceptive pathway, which involves an activation of Kv7 channels. NAPQI modifies cysteine residues and both alkylation and oxidative modification of cysteine residues in Kv7 channels were shown to increase M-currents. The aim of the study is to determine, if modification of cysteine residues in Kv7 channels by NAPQI is responsible for the reported increase of Kv7 currents. Heterologously expressed Kv7.1 to Kv7.5 channels were recorded in the perforated voltage clamp mode and clamped to −30 mV. Once every 15 s, the cells were hyperpolarized to −80 mV to elicit a deactivation current. Activation curves were recorded between −150 mV and +50 mV in 10 mV increments. Current levels at +60 mV were evaluated. Activation curves were recorded before, after three and ten minutes of NAPQI application. The effects of NAPQI were tested on wt and mutated versions of the respective Kv7 channel subtypes. While currents through Kv7.3 channels remained unchanged and Kv7.1-currents were reduced, deactivation currents through heterologously expressed Kv7.2, Kv7.4 and Kv7.5 channels increased about two-fold after ten minutes of NAPQI application. All currents remained unchanged during a subsequent five-minute wash out phase. Maximal current levels of Kv7.2, 7.4, and Kv7.5 homomeric channels were increased but only Kv7.2 and Kv7.4 also showed a left shift in half-maximal channel activation. A triple-cysteine stretch in the S2-S3 linker region was identified as responsible for the NAPQI-mediated increase of Kv7 currents.This module of three subsequent cysteine residues was previously shown to mediate the current increase induced by oxidative modification. We conclude that covalent modification of this stretch of three cysteine residues is responsible for the increase of Kv7 currents in response to NAPQI application.

Studies on the oxidative degradation of paracetamol by a μ-oxo-diiron(III) complex

In higher organisms, metalloenzymes like cytochrome P450, containing a Fe(III) metal center, play an active role in metabolism of paracetamol (APAP). Here, we have chosen a mimicking μ-oxo-diiron complex, [Fe(III)2(μ-O)(phen)4(H2O)2]4+(1, phen = 1,10-phenanthroline), to study spectrophotometrically the kinetics of the redox interactions with APAP. In acidic buffer media (pH = 3.4–5.1), APAP quantitatively reduces 1 following first-order reaction kinetics. Each molecule of 1 accepts two electrons from APAP and is reduced to ferroin [Fe(phen)3]2+. On oxidation, APAP produces N-acetyl-p-benzoquinone imine (NAPQI), which on hydrolysis results in a mixture of benzoquinone, quinone oxime, acetamide, and acetic acid. In reaction media due to successive deprotonations, 1 exists in equilibrium with the species [Fe(III)2(μ-O)(phen)4(H2O)(OH)]3+(1a) and [Fe(III)2(μ-O)(phen)4(OH)2]2+(1b) (pKa= 3.71 and 5.28, respectively). The kinetic analyses suggest for an unusual reactivity order as 1 < 1a ≫ 1b. The mechanistic possibilities suggest that although 1 is reduced by concerted electron transfer (ET) – proton transfer (PT) mechanism, 1a and 1b may be reduced by a concerted PT–ET mechanism where a slow proton-abstraction step is followed by a rapid ET process. It seems that the initial activation of the bridging μ-oxo group by a proton-abstraction results in the higher reactivity of 1a.

What are the Potential Sites of DNA Attack by N-Acetyl-p-benzoquinone Imine (NAPQI)?

Metabolic bioactivation of small molecules can produce electrophilic metabolites that can covalently modify proteins and DNA. Paracetamol (APAP) is a commonly used over-the-counter analgesic, and its hepatotoxic side effects have been postulated to be due to the formation of the electrophilic metabolite N-acetylbenzoquinone imine (NAPQI). It has been established that NAPQI reacts to form covalent bonds to the side-chain functional groups of cysteine, methionine, tyrosine, and tryptophan residues. While there have been scattered reports that APAP can form adducts with DNA the nature of these adducts have not yet been fully characterised. Here the four deoxynucleosides, deoxyguanosine (dG), deoxyadenosine (dA), deoxycytidine (dC), and deoxythymidine (dT) were reacted with NAPQI and the formation of adducts was profiled using liquid chromatography–mass spectrometry with positive-ion mode electrospray ionisation and collision-induced dissociation. Covalent adducts were detected for dG, dA, and dC and tandem mass spectrometry (MS/MS) spectra revealed common neutral losses of deoxyribose (116 amu) arising from cleavage of the glyosidic bond with formation of the modified nucleobase. Of the four deoxynucleosides, dC proved to be the most reactive, followed by dG and dA. A pH dependence was found, with greater reactivity being observed at pH 5.5. The results of density functional theory calculations aimed at understanding the relative reactivities of the four deoxynucleosides towards NAPQI are described.

Chrysin Effect in Prevention of Acetaminophen-Induced Hepatotoxicity in Rat.

Acetaminophen is a commonly used analgesic drug that induces hepatotoxicity at high doses and produces the acetaminophen metabolite N-acetyl-p-benzoquinone imine (NAPQI) through oxidase isoenzyme system. The antioxidant and anti-inflammatory activity of flavonoid chrysin has been reported in different studies. The present study was conducted to investigate the protective effect of chrysin on acute acetaminophen-induced hepatotoxicity. The cytotoxicity of chrysin on fibroblast cells was evaluated using MTT assay, and then, 54 rats were divided into nine groups of six, and acetaminophen (1500 mg/kg) was administered in all groups except for the control group, second and the seventh groups (40 mg/kg), and all groups were treated with chrysin for 14 days. Liver enzymes, inflammatory factors TNF-α and IL-2, and total antioxidant activity were measured in serum while liver tissue was histopathologically examined. Based on the MTT assay results, 31.25, 62.5, 125, 250, and 500 μg/mL chrysin had no adverse effects on healthy fibroblast cells (P < 0.05). Chrysin decreased the level of liver enzymes (ALT, AST, and ALP), which were previously increased after the use of acetaminophen (p < 0.05). The hepatoprotective effect and total antioxidant capacity increased in a dose-dependent manner and the effect of the highest concentration of chrysin was equal to the effect of silymarin (P < 0.05). TNF-α in groups 4 to 6 decreased in a dose-dependent manner (P = 0.04), and chrysin did not show any significant reducing effect on IL-2 compared to silymarin. Chrysin prevents the necrosis and injury of acute acetaminophen-induced hepatotoxicity by decreasing liver enzymes and TNF-α and increasing total antioxidant capacity and protecting the liver tissue.

2484 Intravenous Acetaminophen Associated With Hepatotoxicity in an Orthotopic Liver Transplant Patient

INTRODUCTION: Intravenous acetaminophen (IV APAP) is a commonly used post-operative analgesic. In adults, IV APAP administered at therapeutic doses of less than 4 g over 24 hours is considered safe. Five to ten percent of APAP is metabolized into N-acetyl-p-benzoquinone imine (NAPQI), the toxic metabolite responsible for APAP hepatotoxicity. NAPQI is inactivated when conjugated to tripeptide glutathione. CASE DESCRIPTION/METHODS: A 69 year old male with a history of secondary biliary liver cirrhosis status post orthotopic liver transplantation in 2006 and newly diagnosed prostate adenocarcinoma was admitted to the Urology service for planned robotic-assisted prostatectomy. He underwent uncomplicated robotic-assisted prostatectomy, requiring no fluid resuscitation, vasopressor support or blood products. Post-operatively, vital signs remained normal with no episodes of hypotension. He was started on IV APAP 1000 mg every 6 hours for 36 hours, followed by PO APAP 650 mg every 6 hours for post-operative pain. On post-operative day 2, AST was 2751 IU/L, ALT 3447 IU/L, TBili 1.9 mg/dL and INR 1.5 (Figure 1). All orders for acetaminophen were stopped. Liver Doppler ultrasound was normal. A serum acetaminophen level drawn 10 hours after the last IV APAP dose was &lt;15 ug/mL. Work up for infectious etiologies was negative. The patient was started on IV NAC therapy. Following completion of IV NAC therapy, AST was 986 IU/L, ALT 2340 IU/L, TBili 1.9 mg/dL, and INR 1.4. The patient was discharged home on post-operative day 4. Labs during his outpatient Hepatology follow-up appointment 4 days after discharge were AST 62 IU/L, ALT 426 IU/L, TBili 1.3 mg/dL, and INR 1.1. DISCUSSION: This is the first case of acute liver toxicity in a post-OLT patient who received therapeutic doses of IV APAP. There was no evidence of hypotension or APAP overdose in the electronic medical records. All other etiologies for acute liver toxicity were ruled out, including shock liver, infection and vascular compromise. It is possible APAP metabolism is altered in transplant livers compared to native livers. A study on 13 liver transplant patients concluded APAP metabolism was transiently altered in orthotopic livers following liver transplantation, resulting in enhanced NAPQI production. It is possible transplant livers carry less glutathione stores due to cold storage preservation and post-transplant reperfusion, rendering them more susceptible to hepatotoxicty from NAPQI concentrations that a native liver would tolerate.

3186 Fatal Case of Acetaminophen Toxicity in a Young Female Patient

INTRODUCTION: Acetaminophen is the most common cause of acute liver failure (ALF) in the United States. We report a fatal case of acetaminophen overdose after both delayed presentation for treatment and transplant evaluation in a non-liver transplant academic center. CASE DESCRIPTION/METHODS: An 18-year-old woman presented to the emergency department (ED) after attempting suicide by overdosing on acetaminophen. She ingested 48 of the 500 mg tablets (24-gram total dose) 18 hours prior to arrival to the ED. Her only symptom was back pain. Her medical history was significant for depression and three prior suicide attempts by acetaminophen overdose. Her mental status declined over the next few hours to the point of becoming non-verbal and mildly drowsy. On physical exam, patient was tachycardic and had self-inflicted scars to her forearms. Initial labs were significant for white blood cell count of 22,300 cells/mm3, bicarbonate of &lt;5 mmol/L (21-31), glucose 59 mg/dL (70-110), alkaline phosphatase 91 units/L (42-121), aspartate aminotransferase 479 units/L (5-34), alanine aminotransferase acetaminophen 228 mcg/mL (&lt;30), PT 27 seconds (9.4-12.5 sec), INR 2.5, and ammonia 147 mcmol/L (18-72). Venous blood gas revealed pH 6.819, pCO2 29 mmHg (39-51), pO2 51 mmHg (30-50), calculated bicarbonate of 4 mmol/L, and lactate of 14 mmol/L (0.5-2.20). Table 1 highlights changes in laboratory values during admission. The patient was admitted to the ICU, started on N-acetylcysteine (NAC) and bicarbonate infusions, and had urgent renal and general gastroenterology consultations. 48 hours after admission the patient was intubated due to deteriorating mental status and seizure activity. It was suspected that she was having cerebral edema and was emergently transferred to a liver transplant center. Ultimately, she did not undergo transplant and died from complications of ALF. DISCUSSION: Acetaminophen overdose carries a high risk of ALF and mortality. Survival depends on early treatment as high doses can overwhelm liver metabolism resulting in hepatotoxicity by N-acetyl-p-benzoquinoneimine (NAPQI). Treatment from time of ingestion was delayed in this case due to the patient's delay in seeking care. Transfer to transplant center was delayed due to lack of insurance, history of multiple attempts that affected her liver transplant candidacy, and under-appreciating risk of developing ALF. This case highlights the fatal complications of acetaminophen overdose, and underscores the importance of emergent transfer to transplant center.

Влияние окислительного стресса на нейротоксический эффект ацетаминофена

Paracetamol (acetaminophen, APAP) is a commonly-used antipyretic and analgesic. However, there have been reports indicating possible link between its use in pregnancy and impaired neuropsychic development in children. A number of prospective studies of the possible negative effect of acetaminophen on the development of a child after his mother took this drug during pregnancy, as well as the results of studies on glioma cells and neurons in murine cortex, may indicate presence of the neurotoxic effect of acetaminophen. It is currently unclear if paracetamol itself being pharmacologically active neurotropic substance, or its metabolites, one of which – NAPQI (N-acetyl-p-benzoquinone imine) known by its toxic effects in mitochindria, play the most significant role in proposed neurotoxicity. Therefore it seems important to study each metabolite separately. The ability of acetaminophen(paracetamol) in concentrations of 1 mg/ml and 2 mg/ml to reduce cell viability was shown on cells of the PC12 neuronal line using MTT-method, which is based on the ability of mitochondria of viable cells to restore formazan 3-(4,5-dimethylthiazole)-2,5-diphenyl-2-tetrazolium bromide (MTT). Concentrations of 0.125 mg/ml, 0.25 mg/ml and 0.5 mg/ml had no similar impact on cell culture viability. In addition, the impact of hydrogen peroxide (as an inducer of oxidative stress) on the neurotoxic effect of acetaminophen was studied. We demonstrated that in the presence of 0.3 mM or 0.5 mM hydrogen peroxide and acetaminophen in concentrations of 1 mg/ml and 2 mg/ml reliably reduced the percentage of surviving cells. We showed that the decrease of the viability of the cells of the PC12 neuronal line is obvious only after exposure to high concentrations of acetaminophen, especially in the presence of hydrogen peroxide, which means that neurotoxic effect is not likely to occur in vivo.

Guanabenz Attenuates Acetaminophen-Induced Liver Toxicity and Synergizes Analgesia in Mice.

Endoplasmic reticulum (ER) stress has been shown to be involved in the hepatotoxicity of acetaminophen (APAP). Guanabenz (GA), a widely known antihypertensive drug, is reported to exhibit an anti-ER stress effect. In this study, we investigated the potential of GA as an antidote against APAP-induced hepatotoxicity. The underlying biochemical mechanisms for the hepatoprotective effect of GA were explored. Here we found that treatment of mice with GA (10 mg/kg) before APAP overdose dramatically prevented APAP-induced liver enzyme elevation and resultant toxicity in mice, as indicated by suppression of elevated serum alanine aminotransferase (ALT) levels and liver histological analysis. Importantly, delayed administration of GA within 6 h after APAP overdose also showed an almost equivalent protective effect against APAP liver toxicity. Mechanistically, several pathways are involved in the protective effect of GA against APAP-induced live toxicity, including attenuation of ER stress and oxidative stress, increased levels of nontoxic phase I and II metabolites of APAP, decrease in the formation of toxic N-acetyl-p-benzoquinone imine (NAPQI), and its subsequent protein binding. Importantly, combination of GA with APAP exhibited synergistic interaction in the latter's analgesic activity, while sparing its antipyretic action. These findings provide the preclinical evidence of GA as a promising antidote for treatment of APAP-induced liver toxicity and raise a possibility of its combination with APAP in clinical settings.

Comparison of toxic responses to acetaminophen challenge in ICR mice originating from different sources

Acetaminophen (APAP) is the most common antipyretic analgesic worldwide. However, APAP overdose causes severe liver injury, especially centrilobular necrosis, in humans and experimental animals. At therapeutic dosage, APAP is mainly metabolized by sulfation and glucuronidation, and partly by cytochrome P450–mediated oxidation. However, APAP overdose results in production of excess reactive metabolite, N-acetyl-p-benzoquinone imine (NAPQI), by cytochromes P450; NAPQI overwhelms the level of glutathione (GSH), which could otherwise detoxify it. NAPQI binds covalently to proteins, leading to cell death. A number of studies aimed at the prevention and treatment of APAP-induced toxicity are underway. Rats are more resistant than mice to APAP hepatotoxicity, and thus mouse models are mainly used. In the present study, we compared the toxic responses induced by APAP overdose in the liver of ICR mice obtained from three different sources and evaluated the usability of the Korl:ICR stock established by the National Institute of Food and Drug Safety Evaluation in Korea. Administration of APAP (300 mg/kg) by intraperitoneal injection into male ICR mice enhanced CYP2E1 protein expression and depleted hepatic GSH level 2 h after treatment accompanied with significantly increased level of hepatic malondialdehyde, a product of lipid peroxidation. Regardless of the source of the mice, hepatotoxicity, as evidenced by activity of serum alanine aminotransferase, increased from 8 h and peaked at 24 h after APAP treatment. In summary, hepatotoxicity was induced after the onset of oxidative stress by overdose of APAP, and the response was the same over time among mice of different origins.

Liquid Chromatography-Tandem Mass Spectrometry Analysis of Acetaminophen Covalent Binding to Glutathione S-Transferases.

Acetaminophen (APAP)-induced hepatotoxicity is the most common cause of acute liver failure in the Western world. APAP is bioactivated to N-acetyl p-benzoquinone imine (NAPQI), a reactive metabolite, which can subsequently covalently bind to glutathione and protein thiols. In this study, we have used liquid chromatography-tandem mass spectrometry (LC-MS/MS) to characterize NAPQI binding to human glutathione S-transferases (GSTs) in vitro. GSTs play a crucial role in the detoxification of reactive metabolites and therefore are interesting target proteins to study in the context of APAP covalent binding. Recombinantly-expressed and purified GSTs were used to assess NAPQI binding in vitro. APAP biotransformation to NAPQI was achieved using rat liver microsomes or human cytochrome P450 Supersomes in the presence of GSTA1, M1, M2, or P1. Resulting adducts were analyzed using bottom-up proteomics, with or without LC fractionation prior to LC-MS/MS analysis on a quadrupole-time-of-flight instrument with data-dependent acquisition (DDA). Targeted methods using multiple reaction monitoring (MRM) on a triple quadrupole platform were also developed by quantitatively labeling all available cysteine residues with a labeling reagent yielding isomerically-modified peptides following enzymatic digestion. Seven modified cysteine sites were confirmed, including Cys112 in GSTA1, Cys78 in GSTM1, Cys115 and 174 in GSTM2, as well as Cys15, 48, and 170 in GSTP1. Most modified peptides could be detected using both untargeted (DDA) and targeted (MRM) approaches, however the latter yielded better detection sensitivity with higher signal-to-noise and two sites were uniquely found by MRM.

Synthesis of Acetaminophen Analogues Containing α-Amino Acids and Fatty Acids for Inhibiting Hepatotoxicity

Acetaminophen is a popular antipyretic analgesic medicine that has weaker anti-inflammatory properties and lower incidence of side effects than nonsteroidal anti-inflammatory drugs (NSAIDs). However, acetaminophen causes hepatotoxicity due to the reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI). We have obtained acetaminophen analogues in 57–99% yields by using aniline derivatives with protected α-amino acids and fatty acids via the corresponding mixed carbonic carboxylic anhydrides in aqueous MeCN. We have also succeeded in synthesizing AM404 analogues in 76–97% yields, which are expected to be promising candidates for reducing hepatotoxicity.

Mangiferin ameliorates acetaminophen-induced hepatotoxicity through APAP-Cys and JNK modulation

An overdose of the most popular analgesic, acetaminophen (APAP), is one of the leading causes of acute liver failure. It is well established that glutathione is exhausted by APAP-reactive intermediate N‑acetyl‑p‑benzoquinone-imine (NAPQI). This leads to elevated phosphorylated-c-Jun N-terminal kinase (p-JNK), which further activates reactive oxygen species (ROS), initiates an inflammatory response, and finally leads to severe hepatic injury. The present study was conducted to investigate the protective role of mangiferin (MAN), a naturally occurring xanthone and anti-oxidant, on APAP-induced hepatotoxicity. C57BL/6 mice were pretreated with or without MAN at 1 h prior to APAP challenge. MAN was administered at a dose of 12.5−50 mg/kg along with APAP at a dose of 400 mg/kg. According to the ALT/AST ratio, 25 mg/kg MAN was the most potent dose for further experiments. Serum ALT and AST depletion were observed in APAP + MAN (25 mg/kg)-treated mice at 6, 12, and 24 h. Early (1 h after APAP treatment) GSH depletion by APAP overdose was restored by MAN treatment, which reduced APAP-Cys adduct formation and promoted protection. p-JNK downregulation and AMPK activation were observed in MAN-treated mice, which could mechanistically reduce oxidative stress and inflammation. MAN up-regulated liver GSH and SOD and reduced lipid peroxidation. HO-1 protein and p47 phox mRNA expression indicated that MAN regulated oxidative stress along with JNK deactivation. The expression of inflammatory response genes TNF-α, IL-6, MCP-1, CXCL-1, and CXCL-2 reached the basal levels after MAN treatment. mRNA, protein, and serum levels of IL-1β were reduced, and NF-κB expression was similar to that of the MAN-treated APAP mice. MAN post-treatment (1 h after APAP treatment) also protected the mice from hepatotoxicity. In conclusion, MAN had a protective and therapeutic role in APAP-induced hepatotoxicity by improving the metabolism of acetaminophen and APAP-Cys adduct formation followed by JNK-mediated oxidative stress and inflammation.

Electrochemical Derivatization of Acetaminophen for Indirect Determination of Eflornithine Using β‐CD Modified Glassy Carbon Electrode

AbstractIn the present work, the oxidation of acetaminophen in the absence and presence of eflornithine was electrochemically investigated by means of cyclic voltammetry at a glassy carbon electrode (GCE). Our results indicate that N‐acetyl‐p‐benzoquinone imine (NAPQI) produced from two‐electron electrochemical oxidation of acetaminophen participates in a Michael addition reaction with eflornithine via an ECE mechanism. This fact was used for the determination of eflornithine using differential pulse voltammetry (DPV) technique on the surface of β‐Cyclodextrin modified glassy carbon (β‐CD/GC) electrode. β‐CD/GC electrode was prepared through an electrodeposition procedure and characterized by Fourier‐transform infrared spectroscopy (FT‐IR), Cyclic Voltammetry (CV), Field Emission Scanning Electron Microscopy (FESEM) and Energy‐dispersive X‐ray spectroscopy (EDS) techniques. Under optimum conditions, the β‐CD/GC electrode showed a good linearity as a function of the eflornithine concentration over the range from 5 to 100 μM with detection limit and quantification limit of 1.94 and 5.8 μM, respectively. Finally, the proposed protocol was confirmed to be successful in determination of eflornithine in human urine samples with good recovery, ranging from 97.2 % to 104.8 %.

Quercetin reduced the formation of N-acetyl-p-benzoquinoneimine, a toxic metabolite of paracetamol in rats and isolated rat hepatocytes.

N-acetyl-p-benzoquinoneimine (NAPQI) is toxic metabolite of paracetamol formed primarily by cytochrome P4502E1 (CYP2E1) metabolic pathway when administered at therapeutic doses or overdose. The influence of quercetin (flavonoid) on the bioactivation of paracetamol to NAPQI was investigated using rat liver microsomes and rats in vivo. Paracetamol (80mg/kg) was administered orally without or with silymarin (100mg/kg), a known inhibitor of CYP2E1, CYP3A4 and quercetin (10 and 20mg/kg) to rats for 15 consecutive days. Area under the plasma concentration-time curve (AUC0-∞ ) and the peakplasma concentration (Cmax ) of paracetamol were dose-dependently increased with quercetin (10 and 20mg/kg) compared to paracetamol control group (p<0.001). On the other hand, the AUC0-∞ and Cmax of NAPQI were decreased significantly with quercetin. The same results were observed with silymarin also. The elevated liver and kidney functional enzymes/compounds were significantly reduced by quercetin and silymarin compared to paracetamol control group. The formation of NAPQI was reduced in the incubation samples in presence of quercetin in experiment using isolated rat hepatocytes. The presentstudy results revealed that quercetin might be inhibited the CYP2E1-mediated metabolism of paracetamol; thereby decreased the formation of NAPQI and protected the liver and kidney.

Effect of chrysin on the formation of N-acetyl-p-benzoquinoneimine, a toxic metabolite of paracetamol in rats and isolated rat hepatocytes

Paracetamol (N-acetyl-para amino phenol) is the most commonly used analgesic and antipyretic around the world. Its causes hepatotoxicity and nephrotoxicity at overdose or even at therapeutic doses. It is primarily metabolized by glucuronidation and sulfate conjugation. It is also metabolized by cytochrome-P450 system (CYP2E1, CYP1A2 and CYP 3A4), leading to the formation of N-acetyl-p-benzoquinoneimine (NAPQI). The present study was planned to investigate the influence of chrysin (known CYP2E1 and CYP3A4 inhibitor) on the bioactivation of paracetamol to NAPQI using rat liver microsomes in vitro and rats in vivo. Paracetamol (80 mg/kg) was administered orally without or with silymarin (100 mg/kg), a known CYP2E1 inhibitor and chrysin (100 and 200 mg/kg) to rats for 15 consecutive days. The area under the plasma concentration–time curve (AUC0-∞) and the peak plasma concentration (Cmax) of paracetamol were dose-dependently increased with chrysin (100 and 200 mg/kg) compared to paracetamol control group. On the other hand, the AUC0-∞ and Cmax of NAPQI were decreased significantly with chrysin (100 and 200 mg/kg). The elevated liver and kidney function markers were significantly reduced by chrysin and silymarin compared to paracetamol control group (P < 0.01). Histopathological studies of liver and kidney also well correlated with liver and kidney function tests. Chrysin also reduced the formation of NAPQI in the incubation samples of rat hepatocytes. The present study (both in vivo and in vitro) results revealed that chrysin might be inhibited the CYP2E1, CYP1A2 and CYP3A4-mediated metabolism of paracetamol; thereby decreased the formation of NAPQI and protected the liver and kidney.

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Although acetaminophen (ApAP) is one of the most commonly used medicines worldwide, hepatotoxicity is a risk with overdose or in patients with compromised liver function. ApAP overdose is the most common cause of acute fulminant hepatic failure. Oxidation of ApAP to N-acetyl-p-benzoquinone imine (NAPQI) is the mechanism for hepatotoxicity. 1 is a non-hepatotoxic, metabolically unstable lipophilic ApAP analog that is not antipyretic. The newly synthesized 3 is a non-hepatotoxic ApAP analog that is stable, lipophilic, and retains analgesia and antipyresis. Intraperitoneal or po administration of the new chemical entities (NCEs), 3b and 3r, in concentrations equal to a toxic dose of ApAP did not result in the formation of NAPQI. Unlike livers from NCE-treated mice, the livers from ApAP-treated mice demonstrated large amounts of nitrotyrosine, a marker of mitochondrial free radical formation, and loss of hepatic tight junction integrity. Given the widespread use of ApAP, hepatotoxicity risk with overuse, and the ongoing opioid epidemic, these NCEs represent a novel, non-narcotic therapeutic pipeline.

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Although acetaminophen (ApAP) is one of the most commonly used medicines worldwide, hepatotoxicity is a risk with overdose or in patients with compromised liver function. ApAP overdose is the most common cause of acute fulminant hepatic failure. Oxidation of ApAP to N-acetyl-p-benzoquinone imine (NAPQI) is the mechanism for hepatotoxicity. 1 is a non-hepatotoxic, metabolically unstable lipophilic ApAP analog that is not antipyretic. The newly synthesized 3 is a non-hepatotoxic ApAP analog that is stable, lipophilic, and retains analgesia and antipyresis. Intraperitoneal or po administration of the new chemical entities (NCEs), 3b and 3r, in concentrations equal to a toxic dose of ApAP did not result in the formation of NAPQI. Unlike livers from NCE-treated mice, the livers from ApAP-treated mice demonstrated large amounts of nitrotyrosine, a marker of mitochondrial free radical formation, and loss of hepatic tight junction integrity. Given the widespread use of ApAP, hepatotoxicity risk with overuse, and the ongoing opioid epidemic, these NCEs represent a novel, non-narcotic therapeutic pipeline.

The paracetamol metabolite N-acetylp-benzoquinone imine reduces excitability in first- and second-order neurons of the pain pathway through actions on KV7 channels.

Paracetamol (acetaminophen, APAP) is one of the most frequently used analgesic agents worldwide. It is generally preferred over nonsteroidal anti-inflammatory drugs because it does not cause typical adverse effects resulting from the inhibition of cyclooxygenases, such as gastric ulcers. Nevertheless, inhibitory impact on these enzymes is claimed to contribute to paracetamols mechanisms of action which, therefore, remained controversial. Recently, the APAP metabolites N-arachidonoylaminophenol (AM404) and N-acetyl-p-benzoquinone imine (NAPQI) have been detected in the central nervous system after systemic APAP administration and were reported to mediate paracetamol effects. In contrast to nonsteroidal anti-inflammatory drugs that rather support seizure activity, paracetamol provides anticonvulsant actions, and this dampening of neuronal activity may also form the basis for analgesic effects. Here, we reveal that the APAP metabolite NAPQI, but neither the parent compound nor the metabolite AM404, reduces membrane excitability in rat dorsal root ganglion (DRG) and spinal dorsal horn (SDH) neurons. The observed reduction of spike frequencies is accompanied by hyperpolarization in both sets of neurons. In parallel, NAPQI, but neither APAP nor AM404, increases currents through KV7 channels in DRG and SDH neurons, and the impact on neuronal excitability is absent if KV7 channels are blocked. Furthermore, NAPQI can revert the inhibitory action of the inflammatory mediator bradykinin on KV7 channels but does not affect synaptic transmission between DRG and SDH neurons. These results show that the paracetamol metabolite NAPQI dampens excitability of first- and second-order neurons of the pain pathway through an action on KV7 channels.

Phloretin cytoprotection and toxicity

Phloretin (Phl) is a dihydrochalcone flavonoid with significant cytoprotective properties; e.g., free radical trapping, electrophile scavenging. Based on this, it has been suggested that Phl might be useful in the treatment of pathogenic processes and prevention of drug toxicities. Therefore, we determined the ability of Phl to provide route- and dose-dependent hepatoprotection in a mouse model of acetaminophen (APAP) overdose. Intraperitoneal (i.p.) administration of Phl produced a bimodal effect; i.e., the highest dose (2.40 mmol/kg) did not prevent APAP-induced lethality, whereas lower doses (0.2–0.4 mmol/kg) afforded modest hepatoprotection. When given alone, the highest i.p. Phl dose was lethal within 24 h, whereas the lower doses were not toxic. Oral Phl (0.40–2.40 mmol/kg) did not prevent APAP-induced hepatotoxicity. The highest oral dose given alone (2.4 mmol/kg) produced 64% lethality, whereas lower doses were not lethal. This toxicity profile was reflected in a study using APAP-exposed isolated mouse hepatocytes, which showed that the Phl pharmacophores, 1,3,5-trihydroxyacetophenone (PG) and 2′,4′,6′-trihydroxyacetophenone (THA) where protective. Corroborative cell free studies showed that polyphenol protectants prevented glutathione loss mediated by the APAP metabolite, N-acetyl-p-benzoquinone imine (NAPQI). Thus, in spite of possessing cytoprotective attributes, Phl was generally toxic in our APAP models. These and earlier findings suggest that Phl is not a candidate for drug design. In contrast, we have found that the enol-forming pharmacophores, THA and PG, are potential platforms for pharmacotherapeutic development.

Paracetamol-induced liver injury modelled in Xenopus laevis embryos

IntroductionFailure to predict drug-induced liver injury (DILI) remains a major contributing factor to lead compound drop-out during drug development. Xenopus embryos are amenable for early stage medium throughput small molecule screens and so have the potential to be used in pre-clinical screens. To begin to assess the usefulness and limitations of Xenopus embryos for safety assessment in the early phases of drug development, paracetamol was used as a model hepatotoxin. Paracetamol overdose is associated with acute liver injury. In mammals, the main mechanism of paracetamol-induced acute liver injury is an increased amount of the reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI) combined with a reduction of free glutathione (GSH). Humans that have taken an overdose of paracetamol are often treated with N-acetyl cysteine (NAC). MethodXenopus laevis embryos were treated with up to 5 mM paracetamol from stage 38 to stage 45 during development, when the liver is functional. The presence of paracetamol-induced liver injury was assessed by: (1) microRNA-122 (miR-122) expression (a hepatic marker), (2) free GSH concentration (a marker of oxidative stress) and (3) NAC antioxidant intervention. ResultsThe amount of free GSH decreased significantly in embryos exposed to increasing paracetamol concentration. In embryos exposed to 5 mM paracetamol, 22.57 ± 4.25 nmol/mg GSH was detected compared to 47.11 ± 7.31 nmol/mg untreated embryos (mean ± SEM). In tail tissue, miRNA-122 expression increased 6.3-fold with 3 mM paracetamol concentration treatment compared to untreated embryos. NAC treatment altered the free GSH decline for embryos treated with up to 5 mM. Embryos exposed to 1 mM paracetamol and then exposed to 0.5 mM NAC 24 h prior to harvest, had a significantly higher amount of GSH compared to embryos that were only exposed to 1 mM paracetamol (mean ± SEM; 97.1 ± 9.6 nmol/mg and 54.5 ± 6.6 nmol/mg respectively). ConclusionXenopus laevis embryos exhibit similar characteristics of paracetamol-induced liver injury observed in mammalian models. These data indicate that the Xenopus embryo could be a useful in vivo model to assess DILI and aid lead compound prioritisation during the early phase of drug development, in combination with pre-clinical in vitro studies. Consequently, the Xenopus embryo could contribute to the reduction principle as defined by the National Centre for the Replacement, Refinement and Reduction of Animals in Research.