Inter-individual and inter-organ variability in the bioactivation of paracetamol by human liver and kidney tissues

Toxin-Induced Liver Injury and Extracorporeal Treatment of Liver Failure

Poisoning with a large variety of drugs and naturally occurring toxins may result in acute liver injury and failure. Drug-induced liver injury is a major cause of liver failure nationwide, and it is likely that nephrologists will be involved in treating patients with these conditions. A number of xenobiotics resulting in liver toxicity may cause acute kidney injury or other organ injury as well. Most agents causing drug- or toxin-induced liver failure lack specific therapies, although a few xenobiotics such as acetaminophen have effective antidotal therapies if administered prior to development of hepatotoxicity. The nephrologist should be aware that extracorporeal treatment of liver failure associated with drugs and toxins may be indicated, including therapies conventionally performed by nephrologists (hemodialysis, continuous kidney replacement therapy), therapies occasionally performed by nephrologists and other specialists (plasma exchange, albumin dialysis, hemadsorption), and therapies performed by other specialists (extracorporeal membrane oxygenation). An overview of the role of these therapies in liver failure is provided, as well as a review of their limitations and potential complications.

Role of Oxidative Stress and Reactive Metabolites in Cytotoxicity & Mitotoxicity of Clozapine, Diclofenac and Nifedipine in CHO-K1 Cells In Vitro

BACKGROUND

CHO-K1 cells were used as in vitro model to explore mechanisms of cytotoxicity of the test drugs.

AIM

To provide in vitro data on toxicity mechanisms of clozapine, diclofenac and nifedipine.

OBJECTIVE

Cytotoxic mechanisms of clozapine (CLZ), diclofenac (DIC) and nifedipine (NIF) were studied in CHO-K1 cells in vitro. All three drugs induce adverse reactions in some patients with partially unknown mechanisms.

METHODS

Following the determination of time- and dose-dependency of cytotoxicity by the MTT test, cytoplasmic membrane integrity was explored by the LDH leakage test. Both end-points were further examined in the presence of soft and hard nucleophilic agents, glutathione (GSH) and potassium cyanide (KCN), respectively, and either individual or general cytochrome P450 (CYP) inhibitors, whether CYPcatalysed formation of electrophilic metabolites play a role in the observed cytotoxicity and membrane damage. The generation of reactive metabolites during the incubations was also explored. Formation of malondialdehyde (MDA) and oxidation of dihydrofluorescein (DCFH) were monitored whether peroxidative membrane damage and oxidative stress take place in cytotoxicity. Incubations were also conducted in the presence of chelating agents of EDTA or DTPA to explore any possible role of metals in cytotoxicity by facilitating electron transfer in redox reactions. Finally, mitochondrial membrane oxidative degradation and permeability transition pore (mPTP) induction by the drugs were tested as markers of mitochondrial damage.

RESULTS

The presence of an individual or combined nucleophilic agents significantly diminished CLZand NIF-induced cytotoxicities, while the presence of both agents paradoxically increased DIC-induced cytotoxicity by a factor of three with the reason remaining unknown. The presence of GSH significantly increased DIC-induced membrane damage too. Prevention of membrane damage by the hard nucleophile KCN suggests the generation of a hard electrophile upon DIC and GSH interaction. The presence of CYP2C9 inhibitor sulfaphenazole significantly diminished DIC-induced cytotoxicity, probably by preventing the formation of 4-hydroxylated metabolite of DIC, which further converts to an electrophilic reactive intermediate. Among the chelating agents, EDTA caused a marginal decrease in CLZ-induced cytotoxicity, while DIC-induced cytotoxicity was amplified by a factor of five. Both reactive and stable metabolites of CLZ could be detected in the incubation medium of CLZ with CHO-K1 cells, which are known to have low metabolic capacity. All three drugs caused a significant increase in cytoplasmic oxidative stress by means of DCFH oxidation, which was confirmed by increased MDA from cytoplasmic as well as mitochondrial membranes. The addition of GSH paradoxically and significantly increased DICinduced MDA formation, in parallel with the increase in membrane damage when DIC and GSH combined.

CONCLUSION

Our results suggested that the soft electrophilic nitrenium ion of CLZ is not responsible for the observed in vitro toxicities, and this may originate from a relatively low amount of the metabolite due to the low metabolic capacity of CHO-K1. A hard electrophilic intermediate may contribute to cellular membrane damage incubated with DIC, while a soft electrophilic intermediate seems to exacerbate cell death by a mechanism other than membrane damage. A significant decrease in cytotoxicity of NIF by GSH and KCN suggested that both soft and hard electrophiles contribute to NIF-induced cytotoxicity. All three drugs induced peroxidative cytoplasmic membrane damage, while only DIC and NIF induced peroxidative mitochondrial membrane damage, which suggested mitochondrial processes may contribute to adverse effects of these drugs in vivo.

Using liver models generated from human-induced pluripotent stem cells (iPSCs) for evaluating chemical-induced modifications and disease across liver developmental stages

The liver is a pivotal organ regulating critical developmental stages of fetal metabolism and detoxification. Though numerous studies have evaluated links between prenatal/perinatal exposures and adverse health outcomes in the developing fetus, the central role of liver to health disruptions resulting from these exposures remains understudied, especially concerning early development and later-in-life health outcomes. While numerous in vitro methods for evaluating liver toxicity have been established, the use of iPSC-derived hepatocytes appears to be particularly well suited to contribute to this critical research gap due to their potential to model a diverse range of disease phenotypes and different stages of liver development. The following key aspects are reviewed: (1) an introduction to developmental liver toxicity; (2) an introduction to embryonic and induced pluripotent stem cell models; (3) methods and challenges for deriving liver cells from stem cells; and (4) applications for iPSC-derived hepatocytes to evaluate liver developmental stages and their associated responses to insults. We conclude that iPSC-derived hepatocytes have great potential for informing liver toxicity and underlying disease mechanisms via the generation of patient-specific iPSCs; implementing large-scale drug and chemical screening; evaluating general biological responses as a potential surrogate target cell; and evaluating inter-individual disease susceptibility and response variability.

Mitochondrial versus microsomal bioactivation of paracetamol by human liver and kidney tissues

Mitochondria appeared to be a major target for paracetamol (PAR)-induced hepatotoxicity. Studies suggested that microsomal CYPs catalyse bioactivation of PAR to N-acetyl-p-benzoquinone imine (NAPQI), which alkylates mitochondrial proteins, and causes transmission of death signal from mitochondria to nucleus. We hypothesised that local formation of NAPQI within mitochondria seems more likely compared to the translocation of NAPQI. We therefore tested whether the formation of NAPQI may be catalysed by mitochondrial CYPs. Cellular fractions were isolated from human liver and kidney to compare the metabolic capacities. Liver and kidney mitochondria are capable to generate NAPQI. Mitochondrial CYP2E1 and CYP3A4 activities were comparable to the microsomal counterparts in both organs. Previously reported higher kidney microsomal CYP2E1 activity in men compared women were observed in mitochondrial CYP2E1 as well in the present study. On the other hand, no correlation between kidney CYP2E1 activity and quantity of NAPQI formation, as well as no induction on mitochondrial permeability transition pore (mPTP) opening by PAR in kidney mitochondria strongly suggested a different toxicity mechanism in this organ.

Comparing N-acetylcysteine and 4-methylpyrazole as antidotes for acetaminophen overdose

Acetaminophen (APAP) overdose can cause hepatotoxicity and even liver failure. N-acetylcysteine (NAC) is still the only FDA-approved antidote against APAP overdose 40 years after its introduction. The standard oral or intravenous dosing regimen of NAC is highly effective for patients with moderate overdoses who present within 8 h of APAP ingestion. However, for late-presenting patients or after ingestion of very large overdoses, the efficacy of NAC is diminished. Thus, additional antidotes with an extended therapeutic window may be needed for these patients. Fomepizole (4-methylpyrazole), a clinically approved antidote against methanol and ethylene glycol poisoning, recently emerged as a promising candidate. In animal studies, fomepizole effectively prevented APAP-induced liver injury by inhibiting Cyp2E1 when treated early, and by inhibiting c-jun N-terminal kinase (JNK) and oxidant stress when treated after the metabolism phase. In addition, fomepizole treatment, unlike NAC, prevented APAP-induced kidney damage and promoted hepatic regeneration in mice. These mechanisms of protection (inhibition of Cyp2E1 and JNK) and an extended efficacy compared to NAC could be verified in primary human hepatocytes. Furthermore, the formation of oxidative metabolites was eliminated in healthy volunteers using the established treatment protocol for fomepizole in toxic alcohol and ethylene glycol poisoning. These mechanistic findings, together with the excellent safety profile after methanol and ethylene glycol poisoning and after an APAP overdose, suggest that fomepizole may be a promising antidote against APAP overdose that could be useful as adjunct treatment to NAC. Clinical trials to support this hypothesis are warranted.

Bioactivation of clozapine by mitochondria of the murine heart: Possible cause of cardiotoxicity

The mechanism of clozapine-associated cardiotoxicity has not been elucidated. The formation of a reactive nitrenium ion from the drug has been suggested as the cause, however, the reason why the heart is a target remains unknown. The heart is one of the most perfused organs; therefore, it contains a large number of mitochondria per cell; these organelles are responsible for both oxygen metabolism and energy production due to high energy expenditure. Given that mitochondria play critical roles in cellular homeostasis and maintenance, this study tested the hypothesis that cardiac mitochondria are both a target and initiator of clozapine-induced cardiotoxicity through activating the drug. We investigated whether murine heart receives a relatively high amount of systemically administered drug (20 mg/kg, i.p., Wistar albino rats) and whether cardiac mice (Swiss albino) and rat (Wistar albino) mitochondria locally activate clozapine (100 μM) to a reactive metabolite. We observed a relatively large distribution of clozapine to heart tissue as well as the formation of reactive metabolites by cardiac mitochondria in situ. Mitochondrial cytochrome P450 enzymes (CYP) in cardiac tissue responsible for biotransformation of clozapine were also characterized. CYP3A4 has been found to be the major enzyme catalyzes CLZ bioactivation, while CYP1A largely and CYP3A4 partially catalyzes the formation of stable metabolites of CLZ. At 100 μM concentration, clozapine caused a significant decline in mitochondrial oxygen consumption rate in vitro as much as positive control (antimycin A), while it did not induce mitochondrial permeability transition pore opening. These data provide an explanation as to why the heart is a target for clozapine adverse effects.

4-methylpyrazole protects against acetaminophen-induced acute kidney injury

Acetaminophen (APAP) hepatotoxicity is the most common cause of acute liver failure in the United States, and while a significant percentage of APAP overdose patients develop kidney injury, molecular mechanisms involved in APAP-induced nephrotoxicity are relatively unknown. We have shown that 4-methylpyrazole (4MP, Fomepizole) protects against APAP-induced liver injury by inhibiting reactive metabolite formation through Cyp2E1, and analysis of data from APAP overdose patients indicated that kidney dysfunction strongly correlated with severe liver injury. Since Cyp2E1 is also expressed in the kidney, this study explored protection by 4MP against APAP-induced nephrotoxicity. Male C57BL/6 J mice were treated with either 300 or 600 mg/kg APAP with or without 4MP for 2, 6 or 24 h, followed by measurement of APAP metabolism and tissue injury. Interestingly, levels of APAP and its non-oxidative metabolites were significantly higher in kidneys when compared to the liver. APAP-protein adducts were present in both tissues within 2 h, but were absent in kidney mitochondria, unlike in the liver. While GSH depletion was seen in both tissues, activation of c-jun N-terminal kinase and its translocation to the mitochondria, which is a critical feature of APAP-induced liver injury, was not detected in the kidney. Treatment with 4MP attenuated APAP oxidative metabolite generation, GSH depletion as well as kidney injury indicating its potential use in protection against APAP-induced nephrotoxicity. In conclusion, since reactive metabolite formation seems to be common in both liver and kidney, 4MP mediated inhibition of Cyp2E1 protects against APAP-induced nephrotoxicity. However, downstream mechanisms of APAP-induced nephrotoxicity seem distinct from the liver.

Mechanisms of titanium dioxide nanoparticle-induced oxidative stress and modulation of plasma glucose in mice

Titanium dioxide nanoparticles (TiO₂ NPs) are reported to increase plasma glucose levels in mice at specific doses. The production and accumulation of reactive oxygen species (ROS) is potentially the most important factor underlying the biological toxicity of TiO₂ NPs but the underlying mechanisms are unclear at present. Data from genome-wide analyses showed that TiO₂ NPs induce endoplasmic reticulum (ER) stress and ROS generation, leading to the inference that TiO₂ NP-induced ER stress contributes to enhancement of ROS in mice. Resveratrol (Res) effectively relieved TiO₂ NP-induced ER stress and ROS generation by ameliorating expression of a common set of activated genes for both processes, signifying that ER stress and ROS are closely related. TiO₂ NP-induced ER stress occurred earlier than ROS generation. Upon treatment with 4-phenylbutyric acid to relieve ER stress, plasma glucose levels tended toward normal and TiO₂ NP increased ROS production was inhibited. These results suggest that TiO2 NP-induced ER stress promotes the generation of ROS, in turn, triggering increased plasma glucose levels in mice. In addition, Res that displays the ability to reduce ER stress presents a dietary polyphenol antioxidant that can effectively prevent the toxicological effects of TiO₂ NPs on plasma glucose metabolism.