Genotoxicity studies with paracetamol

The Role of Mechanistic Biomarkers in Understanding Acetaminophen Hepatotoxicity in Humans

Our understanding of the fundamental molecular mechanisms of acetaminophen (APAP) hepatotoxicity began in 1973 to 1974, when investigators at the US National Institutes of Health published seminal studies demonstrating conversion of APAP to a reactive metabolite that depletes glutathione and binds to proteins in the liver in mice after overdose. Since then, additional groundbreaking experiments have demonstrated critical roles for mitochondrial damage, oxidative stress, nuclear DNA fragmentation, and necrotic cell death as well. Over the years, some investigators have also attempted to translate these mechanisms to humans using human specimens from APAP overdose patients. This review presents those studies and summarizes what we have learned about APAP hepatotoxicity in humans so far. Overall, the mechanisms of APAP hepatotoxicity in humans strongly resemble those discovered in experimental mouse and cultured hepatocyte models, and emerging biomarkers also suggest similarities in liver repair. The data not only validate the first mechanistic studies of APAP-induced liver injury performed 50 years ago but also demonstrate the human relevance of numerous studies conducted since then. SIGNIFICANCE STATEMENT: Human studies using novel translational, mechanistic biomarkers have confirmed that the fundamental mechanisms of acetaminophen (APAP) hepatotoxicity discovered in rodent models since 1973 are the same in humans. Importantly, these findings have guided the development and understanding of treatments such as N-acetyl-l-cysteine and 4-methylpyrazole over the years. Additional research may improve not only our understanding of APAP overdose pathophysiology in humans but also our ability to predict and treat serious liver injury in patients.

DNA-binding activities of compounds acting as enzyme inhibitors, ion channel blockers and receptor binders

The DNA-binding activities of compounds used as remedies can display DNA-protection, but also damaging effects in biological systems. The current review compiles literature data on DNA-binding activities of drugs widely used as remedies with different therapeutic indications. The compounds are classified according their mechanism of action: enzyme inhibitors, ion channel inhibitors, inhibitors of viral RNA replication and HIV protease and receptor agonists. DNA binding was reported for such widely used drugs as paracetamol, aspirin, metformin, statins and many others. The capability of the drug to bind DNA is sometimes coupled to genotoxic effects, but in some cases - to genome protection. Data on atoms and chemical groups involved in the drug-DNA interactions are also presented. In many cases the same atoms are involved in both interactions of the compounds with proteins and DNA.

Protective effects of resveratrol and avocado oil against paracetamol-induced hepatotoxicity in rats

This study sought to assess the protective effects of resveratrol and avocado oil in relation to paracetamol-induced hepatotoxicity in rats. The rats were divided into five groups, namely the control, paracetamol (600 mg/kg), resveratrol (RES; 10 mg/kg) + paracetamol, avocado oil (AVO; 200 mg/kg) + paracetamol, and RES + AVO + paracetamol groups. The hepatoprotective activity was evaluated by measuring biochemical parameters such as the total antioxidant status (TAS) and the total oxidant status (TOS) in each rat's liver homogenates. Any DNA damage was assessed by means of a comet assay. The results showed that the TOS levels were significantly increased in the paracetamol group when compared with the control group. The TOS levels were found to be significantly lower in the paracetamol groups, in comparison with the RES, AVO, and RES + AVO groups. Moreover, the TAS levels significantly increased in the RES and RES + AVO groups when compared with the paracetamol group. The histopathological examination revealed necrotic areas in the rats' livers. Pretreatment with both RES and RES + AVO was found to reverse the oxidative stress parameters, DNA damage, and necrosis induced by paracetamol. These results suggest that a combination of REV and AVO may protect against paracetamol-induced hepatotoxicity due to their antioxidant properties.

A comprehensive weight of evidence assessment of published acetaminophen genotoxicity data: Implications for its carcinogenic hazard potential

In 2019, the California Office of Environmental Health Hazard Assessment initiated a review of the carcinogenic hazard potential of acetaminophen, including an assessment of its genotoxicity. The objective of this analysis was to inform this review process with a weight-of-evidence assessment of more than 65 acetaminophen genetic toxicology studies that are of widely varying quality and conformance to accepted standards and relevance to humans. In these studies, acetaminophen showed no evidence of induction of point or gene mutations in bacterial and mammalian cell systems or in in vivo studies. In reliable, well-controlled test systems, clastogenic effects were only observed in unstable, p53-deficient cell systems or at toxic and/or excessively high concentrations that adversely affect cellular processes (e.g., mitochondrial respiration) and cause cytotoxicity. Across the studies, there was no clear evidence that acetaminophen causes DNA damage in the absence of toxicity. In well-controlled clinical studies, there was no meaningful evidence of chromosomal damage. Based on this weight-of-evidence assessment, acetaminophen overwhelmingly produces negative results (i.e., is not a genotoxic hazard) in reliable, robust high-weight studies. Its mode of action produces cytotoxic effects before it can induce the stable, genetic damage that would be indicative of a genotoxic or carcinogenic hazard.

Assessment of the biochemical pathways for acetaminophen toxicity: Implications for its carcinogenic hazard potential

In 2019 California's Office of Environmental Health Hazard Assessment (OEHHA) initiated a review of the carcinogenic hazard potential of acetaminophen. In parallel with this review, herein we evaluated the mechanistic data related to the steps and timing of cellular events following therapeutic recommended (≤4 g/day) and higher doses of acetaminophen that may cause hepatotoxicity to evaluate whether these changes indicate that acetaminophen is a carcinogenic hazard. At therapeutic recommended doses, acetaminophen forms limited amounts of N-acetyl-p-benzoquinone-imine (NAPQI) without adverse cellular effects. Following overdoses of acetaminophen, there is potential for more extensive formation of NAPQI and depletion of glutathione, which may result in mitochondrial dysfunction and DNA damage, but only at doses that result in cell death - thus making it implausible for acetaminophen to induce the kind of stable, genetic damage in the nucleus indicative of a genotoxic or carcinogenic hazard in humans. The collective data demonstrate a lack of a plausible mechanism related to carcinogenicity and are consistent with rodent cancer bioassays, epidemiological results reviewed in companion manuscripts in this issue, as well as conclusions of multiple international health authorities.

Glutathione S-Transferase P1 Protects Against Amodiaquine Quinoneimines-Induced Cytotoxicity but Does Not Prevent Activation of Endoplasmic Reticulum Stress in HepG2 Cells

Formation of the reactive amodiaquine quinoneimine (AQ-QI) and N-desethylamodiaquine quinoneimine (DEAQ-QI) plays an important role in the toxicity of the anti-malaria drug amodiaquine (AQ). Glutathione conjugation protects against AQ-induced toxicity and GSTP1 is able to conjugate its quinoneimine metabolites AQ-QI and DEA-QI with glutathione. In this study, HepG2 cells transiently transfected with the human GSTP1 construct were utilized to investigate the protective effect of GSTP1 in a cellular context. HepG2 cells were exposed to synthesized QIs, which bypasses the need for intracellular bioactivation of AQ or DEAQ. Exposure was accompanied by decreased cell viability, increased caspase 3 activity, and decreased intracellular GSH levels. Using high-content imaging-based BAC-GFP reporters, it was shown that AQ-QI and DEAQ-QI specifically activated the endoplasmic reticulum (ER) stress response. In contrast, oxidative stress, DNA damage, or inflammatory stress responses were not activated. Overexpression of GSTP1 resulted in a two-fold increase in GSH-conjugation of the QIs, attenuated QI-induced cytotoxicity especially under GSH-depletion condition, abolished QIs-induced apoptosis but did not significantly inhibit the activation of the ER stress response. In conclusion, these results indicate a protective role of GSTP1 by increasing enzymatic detoxification of AQ-QI and DEAQ-QI and suggest a second protective mechanism by interfering with ER stress induced apoptosis.

Hypersensitivity of mouse embryonic fibroblast cells defective for DNA polymerases η, ι and κ to various genotoxic compounds: Its potential for application in chemical genotoxic screening

Genotoxic agents cause modifications of genomic DNA, such as alkylation, oxidation, bulky adduct formation, and strand breaks, which potentially induce mutations and changes to the structure or number of genes. Majority of point mutations are generated during error-prone bypass of modified nucleotides (translesion DNA synthesis, TLS); however, when TLS fails, replication forks stalled at lesions eventually result in more lethal effects, formation of double-stranded breaks (DSBs). Here we compared sensitivities to various compounds among mouse embryonic fibroblasts derived from wild-type and knock-out mice lacking one of the three Y-family TLS DNA polymerases (Polη, Polι, and Polκ) or all of them (TKO). The compounds tested in this study include genotoxins such as methyl methanesulfonate (MMS) and nongenotoxins such as ammonium chloride. We found that TKO cells exhibited the highest sensitivities to most of the tested genotoxins, but not to the non-genotoxins. In order to quantitatively evaluate the hypersensitivity of TKO cells to different chemicals, we calculated ratios of half-maximal inhibitory concentration for WT and TKO cells. The ratios for 9 out of 10 genotoxins ranged from 2.29 to 5.73, while those for 5 nongenotoxins ranged from 0.81 to 1.63. Additionally, the two markers for DNA damage, ubiquitylated proliferating cell nuclear antigen and γ-H2AX after MMS treatment, were accumulated in TKO cells more greatly than in WT cells. Furthermore, following MMS treatment, TKO cells exhibited increased frequency of sister chromatid exchange compared with WT cells. These results indicated that the hypersensitivity of TKO cells to genotoxins resulted from replication fork stalling and subsequent DNA double-strand breaks, thus demonstrating that TKO cells should be useful for evaluating chemical genotoxicity.

2 Final Report on the Safety Assessment of p-Aminophenol, m-Aminophenol, and o-Aminophenol

p-Aminophenol (PAP), m-Aminophenol (MAP), and o-Aminophenol (OAP) are used in permanent (oxidative) hair dyes at concentrations from 0.1 to 5%. In vivo and in vitro skin absorption studies indicated that 11% of the dermally applied 14C-PAP was detected in the excreta, viscera, and skin of the test animals. The oral LD50s of PAP, MAP, and OAP in rats ranged from 600 to 1300 mg/kg. Topical application of PAP at concentrations up to 8.00 g/kg to the skin of New Zealand white (NZW) rabbits produced no skin irritation and no mortality. PAP, MAP, and OAP were irritating to eyes of NZW rabbits at a concentration of 2.5%. MAP at 3% was nonsensitizing in guinea pigs; PAP at 2% sensitized 9 of 10 guinea pigs. Neither PAP nor MAP produced photosensitization in guinea pigs. No treatment-related toxicity was found in three separate four-generation chronic dermal toxicity and reproduction studies of hair dye formulations containing the three Aminophenols. Additional studies on the pure ingredients were also nonteratogenic; embryotoxicity was reported. A range of results was obtained from studies assessing the mutagenic activity of the Aminophenols. PAP tested positive in six of eight mutagenicity tests. MAP and OAP gave positive results in two of eight and five of seven mutagenicity tests, respectively. Oxidative hair dye formulations containing PAP, MAP, and OAP did not produce gross or microscopic alterations or have carcinogenic effects after chronic topical application to mice. Feeding of OAP-HCl and PAP to rats at a dose of 8 mmol/kg produced neither hepatic cirrhosis nor neoplastic lesions. A 3% solution of MAP in an aqueous vehicle was neither a significant irritant nor sensitizer in two clinical studies. A variety of epidemiological studies have not indicated that occupational exposure to, and personal use of, hair dyes containing the Aminophenols presented a carcinogenic risk. A discussion of the significance of the mutagenic data in the safety assessment and the potential for human effects is presented. On the basis of the available animal and clinical data presented in this report it is concluded that p-, m-, and o-Aminophenols are safe as cosmetic ingredients in the present practices of use and concentrations.

Interindividual variation in gene expression responses and metabolite formation in acetaminophen-exposed primary human hepatocytes

Acetaminophen (APAP) is a readily available over-the-counter drug and is one of the most commonly used analgesics/antipyretics worldwide. Large interindividual variation in susceptibility toward APAP-induced liver failure has been reported. However, the exact underlying factors causing this variability in susceptibility are still largely unknown. The aim of this study was to better understand this variability in response to APAP by evaluating interindividual differences in gene expression changes and APAP metabolite formation in primary human hepatocytes (PHH) from several donors (n = 5) exposed in vitro to a non-toxic to toxic APAP dose range. To evaluate interindividual variation, gene expression data/levels of metabolites were plotted against APAP dose/donor. The correlation in APAP dose response between donors was calculated by comparing data points from one donor to the data points of all other donors using a Pearson-based correlation analysis. From that, a correlation score/donor for each gene/metabolite was defined, representing the similarity of the omics response to APAP in PHH of a particular donor to all other donors. The top 1 % highest variable genes were selected for further evaluation using gene set overrepresentation analysis. The biological processes in which the genes with high interindividual variation in expression were involved include liver regeneration, inflammatory responses, mitochondrial stress responses, hepatocarcinogenesis, cell cycle, and drug efficacy. Additionally, the interindividual variation in the expression of these genes could be associated with the variability in expression levels of hydroxyl/methoxy-APAP and C8H13O5N-APAP-glucuronide. The before-mentioned metabolites or their derivatives have also been reported in blood of humans exposed to therapeutic APAP doses. Possibly these findings can contribute to elucidating the causative factors of interindividual susceptibility toward APAP.

Protective effect of artichoke leaf extract against paracetamol-induced hepatotoxicity in rats

CONTEXT

Paracetamol overdose is a predominant cause of hepatotoxicity in both humans and experimental animals.

OBJECTIVE

In this study, we investigated the protective effect of aqueous artichoke leaf extract (ALE) against paracetamol-induced liver injury in rats using N-acetylcysteine (NAC) as a reference drug.

MATERIALS AND METHODS

Rats were divided into five groups: negative control, paracetamol (2 g/kg, single oral dose), ALE (1.5 g/kg, orally for 14 d), ALE + paracetamol, and NAC (100 mg/kg) + paracetamol. Indices of liver damage (serum alanine aminotransferase and aspartate aminotransferase) were measured. Liver homogenates were analyzed for oxidative stress biomarkers (MDA, malondialdehyde; SOD activity, superoxide dismutase activity; NO, nitric oxide; GSH content, reduced glutathione), glutathione cycling (GR, glutathione reductase), and utilization (GST, glutathione-S-transferase). Apoptosis was assessed using the comet assay.

RESULTS

Paracetamol caused marked liver damage as noted by significant increased activities of serum aminotransferases (p < 0.05) as well as a significant increase in hepatic MDA and NO levels (p < 0.001) compared with the negative control group. GSH content, GR, GST, and SOD activities were decreased significantly (p < 0.001). Comet assay parameters (tail length, percentage of tailed cells, percentage of migrated DNA, and tail moment) were increased (p < 0.05), indicating apoptosis. Histopathological examination showed necrotic areas. Pre-treatment with ALE replenished hepatic GSH, reversed oxidative stress parameters, DNA damage, and necrosis induced by paracetamol.

DISCUSSION AND CONCLUSION

These results suggest that ALE may protect from paracetamol-induced liver toxicity via its antioxidant and anti-apoptotic properties.

Plasmid DNA linearization in the antibacterial action of a new fluorescent Ag nanoparticle-paracetamol dimer composite

Herein, we report the generation of a composite comprised of p-hydroxyacetanilide dimer and Ag nanoparticles (NPs) by reaction of AgNO(3) and p-hydroxyacetanilide. The formation of the composite was established by UV-vis, FTIR and NMR spectroscopy, transmission electron microscopy and X-ray diffraction along with substantiation by mass spectrometry. Interestingly, the composite exhibited an emission spectrum with a peak at 435 nm when excited by light of wavelength 320 nm. The composite showed superior antimicrobial activity with respect to its individual components against a wide range of Gram positive and Gram negative bacteria at relatively low concentrations of Ag NPs and at which there was no apparent cytotoxicity against mammalian cells. Our results suggest that the composite strongly interacted with the bacterial cell walls leading to cell bursting. Interestingly, enhancement in the reactive oxygen species (ROS) generation in bacteria was observed in the presence of the composite. It is proposed that the ROS generation led to oxidation of the dimer to N-acetyl-p-benzoquinone imine (NAPQI). The generated NAPQI acted as a DNA gyrase inhibitor causing cell death following linearization of DNA.

Animal products, diseases and drugs: a plea for better integration between agricultural sciences, human nutrition and human pharmacology

Eicosanoids are major players in the pathogenesis of several common diseases, with either overproduction or imbalance (e.g. between thromboxanes and prostacyclins) often leading to worsening of disease symptoms. Both the total rate of eicosanoid production and the balance between eicosanoids with opposite effects are strongly dependent on dietary factors, such as the daily intakes of various eicosanoid precursor fatty acids, and also on the intakes of several antioxidant nutrients including selenium and sulphur amino acids. Even though the underlying biochemical mechanisms have been thoroughly studied for more than 30 years, neither the agricultural sector nor medical practitioners have shown much interest in making practical use of the abundant high-quality research data now available. In this article, we discuss some specific examples of the interactions between diet and drugs in the pathogenesis and therapy of various common diseases. We also discuss, using common pain conditions and cancer as specific examples, how a better integration between agricultural science, nutrition and pharmacology could lead to improved treatment for important diseases (with improved overall therapeutic effect at the same time as negative side effects and therapy costs can be strongly reduced). It is shown how an unnaturally high omega-6/omega-3 fatty acid concentration ratio in meat, offal and eggs (because the omega-6/omega-3 ratio of the animal diet is unnaturally high) directly leads to exacerbation of pain conditions, cardiovascular disease and probably most cancers. It should be technologically easy and fairly inexpensive to produce poultry and pork meat with much more long-chain omega-3 fatty acids and less arachidonic acid than now, at the same time as they could also have a similar selenium concentration as is common in marine fish. The health economic benefits of such products for society as a whole must be expected vastly to outweigh the direct costs for the farming sector.

Genotoxic Activities of Aniline and its Metabolites and Their Relationship to the Carcinogenicity of Aniline in the Spleen of Rats

Aniline (in the form of its hydrochloride) has been shown to induce a rather rare spectrum of tumors in the spleen of Fischer 344 rats. The dose levels necessary for this carcinogenic activity were in a range where also massive effects on the blood and non-neoplastic splenotoxicity as a consequence of methemoglobinemia were to be observed. This review aimed at clarifying if aniline itself or one of its metabolites has a genotoxic potential which would explain the occurrence of the spleen tumors in rats as a result of a primary genetic activity. The database for aniline and its metabolites is extremely heterogeneous. With validated assays it ranges from a few limited Ames tests (o- and m-hydroxyacetanilide, phenylhydroxylamine, nitrosobenzene) to a broad range of studies covering all genetic endpoints partly with several studies of the same or different test systems (aniline, p-aminophenol, p-hydroxyacetanilide). This makes a direct comparison rather difficult. In addition, a varying number of results with as yet not validated systems are available for aniline and its metabolites. Most results, especially those with validated and well performed/documented studies, did not indicate a potential of aniline to induce gene mutations. In five different mouse lymphoma tests, where colony sizing was performed only in one test, aniline was positive. If this indicates a peculiar feature of a point mutagenic potential or does represent a part of the clastogenic activity for which there is evidence in vitro as well as in vivo remains to be investigated. There is little evidence for a DNA damaging potential of aniline. The clastogenic activity in vivo is confined to dose levels, which are close to lethality essentially due to hematotoxic effects. The quantitatively most important metabolites for experimental animals as well as for humans (p-aminophenol, p-hydroxyacetanilide) seem to have a potential for inducing chromosomal damage in vitro and, at relatively high dose levels, also in vivo. This could be the explanation for the clastogenic effects that have been observed after high doses/concentrations with aniline. They do not induce gene mutations and there is little evidence for a DNA damaging potential. None of these metabolites revealed a splenotoxic potential comparable to that of aniline in studies with repeated or long-term administration to rats. The genotoxicity database on those metabolites with a demonstrated and marked splenotoxic potential, i.e. phenylhydroxylamine, nitrosobenzene, is unfortunately very limited and does not allow to exclude with certainty primary genotoxic events in the development of spleen tumors. But quite a number of considerations by analogy from other investigations support the conclusion that the effects in the spleen do not develop on a primary genotoxic basis. The weight of evidences suggests that the carcinogenic effects in the spleen of rats are the endstage of a chronic high-dose damage of the blood leading to a massive overload of the spleen with iron, which causes chronic oxidative stress. This conclusion, based essentially on pathomorphological observations, and analogy considerations thereof by previous authors, is herewith reconfirmed under consideration of the more recently reported studies on the genotoxicity of aniline and its metabolites, on biochemical measurements indicating oxidative stress, and on the metabolism of aniline. It is concluded that there is no relationship between the damage to the chromosomes at high, toxic doses of aniline and its major metabolites p-aminophenol/p-hydroxyacetanilide and the aniline-induced spleen tumors in the rat.

Effect of crude sulphated polysaccharide from brown algae against acetaminophen-induced toxicity in rats

This study was conducted to examine the protective role of crude polysaccharide from brown seaweed Sargassum polycystum against acetaminophen-induced abnormality in blood glucose, serum albumin/globulin ratio, and liver glycogen, lactate, and pyruvate. Liver and renal tissue histology was performed to confirm the efficacy of Sargassum polysaccharide. A toxic dose of acetaminophen (800 mg/kg body weight intraperitoneally) induced severe abnormality in all basic parameters with apparent toxicity in liver (enlargement of hepatocytes, loss of cytoplasmic content with disruption in the hepatic plates and sinusoidal dilation) and renal tissue (glomerular damage with congestion of tubules). The isolated liver cells were stained with acridine orange and examined under fluorescence microscope, which revealed that the acetaminophen induced significant damage. In contrast, the rats pretreated with Sargassum polysaccharide (200 mg/kg body weight) daily for 3 weeks did not show liver and renal tissue with these severe aberrations induced by acetaminophen. Histology results were also consistent with analyzed basic biochemical parameters, which confirmed the effectiveness of the crude polysaccharide against acetaminophen-induced abnormality in rats.

Hepatoprotective and antioxidant effects of Cuscuta chinensis against acetaminophen-induced hepatotoxicity in rats

Tu-Si-Zi, the seeds of Cuscuta chinensis Lam. (Convolvulaceae), is a traditional Chinese medicine that is commonly used to nourish and improve the liver and kidney conditions in China and other Asian countries. As oxidative stress promotes the development of acetaminophen (APAP)-induced hepatotoxicity, the aim of the present study was to evaluate and compare the hepatoprotective effect and antioxidant activities of the aqueous and ethanolic extracts of C chinensis on APAP-induced hepatotoxicity in rats. The C chinensis ethanolic extract at an oral dose of both 125 and 250mg/kg showed a significant hepatoprotective effect relatively to the same extent (P<0.05) by reducing levels of glutamate oxaloacetate transaminase (GOT), glutamate pyruvate transaminase (GPT), and alkaline phosphatase (ALP). In addition, the same ethanolic extract prevented the hepatotoxicity induced by APAP-intoxicated treatment as observed when assessing the liver histopathology. Regarding the antioxidant activity, C chinensis ethanolic extract exhibited a significant effect (P<0.05) by increasing levels of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx), and by reducing malondialdehyde (MDA) levels. In contrast, the same doses of the aqueous extract of C chinensis did not present any hepatoprotective effect as seen in the ethanolic extract, and resulted in further liver deterioration. In conclusion, these data suggest that the ethanolic extract of Cuscuta chinensis can prevent hepatic injuries from APAP-induced hepatotoxicity in rats and this is likely mediated through its antioxidant activities.

Protective effect of Moutan Cortex extract on acetaminophen-induced hepatotoxicity in mice

Previously, we demonstrated that Moutan Cortex prevents acetaminophen (AAP)-induced cytotoxicity in vitro. The present study examined the protective effect of Moutan Cortex on AAP induced hepatotoxicity and the possible mechanisms underlying this effect in mice. When Montan Cortex was administered to ICR mice, followed by hepatotoxic dose of AAP (400 mg/kg, i.p.), Moutan Cortex pre-exposure prevented liver injury as indicated by the decrease of serum alanine aminotransferase level. Moutan Cortex also protected AAP-induced hepatic glutathione depletion. Cytochrome P450 2E1-dependent aniline and p-nitrophenol hydroxylases activities in microsomal incubations were significantly inhibited by Moutan Cortex. Abrogation of toxicity was also mirrored in DNA fragmentation. These observations demonstrate that Moutan Cortex pre-exposure may attenuate AAP-induced GSH depletion, cytochrome P450 2E1 activity, and hepatic DNA damage in vivo.

Antioxidants protect primary rat hepatocyte cultures against acetaminophen-induced DNA strand breaks but not against acetaminophen-induced cytotoxicity

Acetaminophen, a safe analgesic when dosed properly but hepatotoxic at overdoses, has been reported to induce DNA strand breaks but it is unclear whether this event preceeds hepatocyte toxicity or is only obvious in case of overt cytotoxicity. Moreover, it is not known whether the formation of reactive oxygen species (ROS) is involved in the formation of the DNA strand breaks. In the present study, the dose-response curves for cytotoxicity and DNA strand breaks and the response to antioxidant protection have been compared. In primary hepatocytes from untreated male rats, cytotoxicity as measured by the MTT test and by Neutral Red accumulation was obvious at 10 mM acetaminophen but DNA strand breaks as measured by the comet assay were only found at 25-30 mM acetaminophen. Non-cytotoxic concentrations of three compounds with antioxidant activity, the glutathione precursor N-acetylcysteine (100 micro M), the plant polyphenol silibin (25 micro M) and the antioxidant vitamin alpha-tocopherol (50 micro M), were not able to inhibit acetaminophen toxicity at any acetaminophen concentration, while they completely prevented the formation of DNA strand breaks at 25-30 mM acetaminophen. The occurrence of oxidative stress in our experiments was indicated by a slight increase of malondialdehyde formation at 40 mM acetaminophen and by an adaptive increase in catalase mRNA concentration. We conclude that in acetaminophen-treated hepatocytes ROS-independent cell death and ROS-dependent DNA strand breaks occur which appear not to be causally related as judged from their dose dependency and their response to antioxidants.

Rat testicular structure and ultrastructure after paracetamol treatment

The morphological modifications of Wistar rat testicles were investigated for animals treated with paracetamol. The rats (n = 6) received a single dose of 4.4 mmol/kg paracetamol, while controls (n = 6) received the same volume of physiological solution. After 5, 10 and 50 days, the rats were perfused with Karnovsky's fixative and processed by routine methods for light and electron microscopy. Some altered seminiferous tubules were found, as well as a few degenerating tubules. Within the tubules, Sertoli cells appeared fragmented while the spermatids showed unusually well-developed rough endoplasmic reticulum and Golgi complexes and had irregularly compacted chromatin. Morphologically altered late spermatids were found retaining a larger volume of residual cytoplasm than spermatids of controls. An indirect cause of alterations was also suggested based on the occurrence of dilated blood vessels and edema.

Protective effect of moutan cortex extract on acetaminophen-induced cytotoxicity in human Chang liver cells

The aim of this study was to investigate the effect of Moutan Cortex on acetaminophen (AAP)-induced toxicity in human Chang liver cells. Cells were incubated with AAP (0-30 mM) to evaluate the drug's ability to reduce cytoviability. For the cells treated with 10, 20 and 30 mM AAP, LDH leakage was 39.8%, 49.0% and 57.6%, respectively. Administration of Moutan Cortex reduced cytotoxicity in a dose-dependent manner. Glutathione (GSH) concentration in human liver cells decreased significantly after exposure to 20 (p<0.05) and 30 mM (p<0.01) AAP, and increased (p<0.05) if incubated with AAP and Moutan Cortex. The ability of AAP to inhibit mitochondrial function and its counteraction by Moutan Cortex was also evaluated. Moutan Cortex showed dose-dependent increases in MTT metabolism and ATP levels in AAP-treated cells. The DNA content of AAP-treated cells increased with the treatment of Moutan Cortex. These observations demonstrate that Moutan Cortex may significantly attenuate AAP-induced toxicity. It can be considered a cytoprotective agent in this in vitro model of drug toxicity.

Sulphur mustard induced DNA damage in mice after dermal and inhalation exposure

Sulphur mustard (SM) is a chemical warfare agent of the blistering agent category for which there is still no effective therapy. SM, being a strong electrophile, readily reacts with a wide range of cellular macromolecules including DNA, RNA and protein. Since the main intoxication routes for SM are inhalation and dermal penetration, in the present study we have exposed female mice to different concentrations of SM by dermal and inhalation exposures and estimated the DNA damage in different organs viz., liver, lung, spleen and thymus. SM was applied at 38.7, 77.4, 154.7 mg/kg body weight, on the hair-clipped skin (dermal exposure) equivalent to 0.25, 0.5 and 1.0 of the LD50. Inhalation exposure was carried out at 10.6, 21.2 and 42.3 mg/m3 for 1 h duration equivalent to 0.25, 0.5 and 1.0 LC50. SM induced a dose-dependent DNA damage in all the organs except the lung in dermal exposure. Similarly the inhalation exposure resulted in dose- and time-dependent effect in all the organs including lung. By both routes of exposure liver was the most affected organ followed by spleen, thymus and lung in decreasing order. The quantitative data were corroborated by qualitative analysis of DNA on agarose gel electrophoresis. The genomic DNA analysis of the organs had revealed random nuclear DNA fragmentation resulting in a 'smear' typical of necrotic form of cell death. Since DNA damage is not reversible especially in liver, this can be used as a marker for SM exposure through either the dermal or inhalation route.

Acetaminophen binds to mouse hepatic and renal DNA at human therapeutic doses

Alkylation of DNA by acetaminophen metabolites has been reported previously, but has received little attention, and the biological impact of this alkylation is essentially unknown. In the present study, apparent covalent binding of acetaminophen metabolites to DNA in male ICR mice was observed at levels of 2.0 +/- 0.4 to 18.5 +/- 5.5 pmol of acetaminophen/mg of DNA in liver and 0.6 +/- 0.1 to 26.9 +/- 2.6 pmol of acetaminophen/mg of DNA in kidney with doses ranging from 10 to 400 mg/kg. Investigations of the reaction of [3H]-N-acetyl-p-benzoquinone imine (NAPQI) or [ring-14C]NAPQI with DNA in vitro yielded low levels of DNA alkylation. Greater apparent binding of [3H]NAPQI to DNA occurred in reactions containing nuclear proteins, such as by using chromatin or whole nuclei. The binding of NAPQI to purified DNA also was enhanced by the presence of 0.1 mM cysteine, but not by 1.0 mM cysteine. Increased binding of NAPQI to DNA in the presence of cysteine or nuclear protein is in contrast to the effects of alternate sulfhydryls on the binding of NAPQI to proteins, which implies that the mechanisms responsible for binding to DNA may be different than the mechanisms that mediate alkylation of protein. The alkylation of DNA by [ring-14C]NAPQI was enhanced markedly at buffer pH < 4.0, suggesting participation of a protonated form of NAPQI in binding to DNA under these conditions. Acetaminophen binding to DNA also was assessed in metabolic activation systems, including microsomes with cumene hydroperoxide or NADPH, and with horseradish peroxidase (HRP) and H2O2. Measurable binding was obtained in all systems, but HRP and H2O2 produced binding levels 200-fold greater than was observed with the microsomal systems. The 32P-postlabeling of DNA from acetaminophen-treated mice, and of DNA reacted with acetaminophen, HRP, and H2O2, produced unique spots that were not identical. The present data further support the hypothesis that acetaminophen metabolites bind covalently to DNA and demonstrate that this apparent binding is observed in experimental animals in vivo at doses that mimic therapeutic doses in humans.

Inhibition of DNA synthesis by paracetamol in different tissues of the rat in vivo

DNA synthesis in the spleen, testis, thymus, stomach, small intestine and bone marrow was inhibited by 70-90% at 1 h following an oral dose of paracetamol (1 g/kg). This inhibitory effect was still apparent using a lower dose of 125 mg/kg paracetamol, but not when the dose was reduced to 60 mg/kg. In contrast, the liver was resistant to the inhibitory action of paracetamol on DNA synthesis, there being no significant inhibition of DNA synthesis at 500 mg/kg or 1 g/kg paracetamol. These doses and the associated plasma levels are in the range found in human overdose. Tissue levels of paracetamol in the liver, spleen, thymus, kidney and testis were essentially the same as the plasma level. However the apparent paracetamol tissue levels in the stomach wall and duodenum were orders of a magnitude higher than the plasma level. The tissue levels of paracetamol did not explain the differences between tissues in the degree of inhibition of DNA synthesis, in particular the high levels of paracetamol in the tissue of the stomach and duodenum did not result in higher levels of inhibition in these tissues. This study also shows that the inhibitory effect of paracetamol on DNA synthesis is transient. All the tissues, except the spleen, no longer showed inhibition of DNA synthesis by 4 h post paracetamol dosing.

Acetaminophen-arylated proteins are detected in hepatic subcellular fractions and numerous extra-hepatic tissues in CD-1 and C57B1/6J mice

To identify acetaminophen (APAP)-bound proteins in addition to the major 44 and 58 kDa APAP-binding proteins (Bartolone et al., 1992, Toxicol. Appl. Pharmacol. 113. 19-9; Pumford et al., 1992, Biochem. Biophys. Res. Commun. 182, 1348-1355; Bulera et al., 1995, Toxicol, Appl. Pharmacol. 134, 313-320), we investigated subcellular localization of liver proteins and tissue distribution of proteins arylated by a hepatotoxic dose of APAP in CD-1 and C57B1/6J mice. Western blot analysis with affinity-purified, anti-APAP antibodies allowed the detection of covalently bound proteins in liver mitochondria, nuclei, membrane, cytosol, and microsomes. Enzyme market assays revealed that subcellular fractions were 90-98% pure. The lack of contamination from other isolated subcellular fractions indicates that covalently bound proteins were specific to the particular subcellular fraction. APAP-arylated proteins with molecular weights similar to those detected in the liver were found in cytosolic fractions from kidney, lung, pancreas, heart, skeletal muscle, and stomach. The presence of arylated proteins in extra-hepatic organs suggests that other organs may be susceptible to APAP toxicity and may contain critical protein targets that are important in APAP toxicity. In contrast, covalently bound proteins were not detected in cytosols isolated from spleen, small intestine, brain, and testis. The characterization of the APAP-arylated proteins identified in this study will aid in elucidating the mechanism of APAP-induced toxicity.

Paracetamol-induced spindle disturbances in V79 cells with and without expression of human CYP1A2

Spindle disturbing effects in terms of c-mitosis and cytotoxicity of paracetamol were investigated in two Chinese hamster V79 cell lines, one of which (V79MZh1A2) was transfected with human CYP1A2. This enzyme catalyses the oxidative formation of the reactive paracetamol metabolite, NAPQI, believed to initiate hepatoxicity by covalent binding to proteins after overdose. In the native V79 cell line paracetamol increased c-mitosis frequency in a concentration dependent manner from 8.7 + or - 3.5% (control) to 66 + or - 18% at 20 mM. A significant increase to 13.3 + or - 3.5% was first seen at 2.5 mM in the native cell line (P<0.05). In the V79MZh1A2 cells the concentration-effect curve was slightly shifted to the left (P<0.05) with c-mitosis frequency increased to 12.1 + or - 2.6% (P<0.05) at 1 mM paracetamol. At 5 mM paracetamol the c-mitosis frequency was 14.4 + or - 5.0% and 19.0 + or - 3.8% in the native and CYP1A2 expressing cell lines, respectively (P<0.05). At 20 mM paracetamol the c-mitosis frequency was 61 + or - 10% in the V79MZh1A2 cells. Cell survival was reduced to approximately 50% at 5-10 mM paracetamol in both cell lines. At 20 mM paracetamol survival was further decreased to 39 + or - 9% in V79MZh1A2 cells only (P<0.05). The present study demonstrated that paracetamol may disturb the spindle of dividing cells conveying a risk of aneuploidy. The spindle disturbing effect was only slightly enhanced by expression of CYP1A2, suggesting that metabolic activation plays only a minor role in this genotoxic effect. The reduction of survival mirrored the increase in c-mitosis frequency.

Series: current issues in mutagenesis and carcinogenesis, No. 65. The genotoxicity and carcinogenicity of paracetamol: a regulatory (re)view

The publication of several studies reporting genotoxic effects of paracetamol, one of the world's most popular over-the-counter drugs, has raised the question of regulatory action. Paracetamol does not cause gene mutations, either in bacteria or in mammalian cells. There are, however, published data giving clear evidence that paracetamol causes chromosomal damage in vitro in mammalian cells at high concentrations and indicating that similar effects occur in vivo at high dosages. Available data point to three possible mechanisms of paracetamol-induced genotoxicity: (1) inhibition of ribonucleotide reductase; (2) increase in cytosolic and intranuclear Ca2+ levels; (3) DNA damage caused by NAPQI after glutathione depletion. All mechanisms involve dose thresholds. Studies of the relationship between genotoxicity and toxic effects in the rat (induction of micronuclei in rat bone marrow including dose-response relationship, biotransformation of paracetamol at different dosages, concomitant toxicity and biochemical markers) have recently been completed. These studies, which employed doses ranging from the dose resulting in human therapeutic peak plasma levels to highly toxic doses, give convincing evidence that genotoxic effects of paracetamol appear only at dosages inducing pronounced liver and bone marrow toxicity and that the threshold level for genotoxicity is not reached at therapeutic dosage. Reliable studies on the ability of paracetamol to affect germ cell DNA are not available. However, based on the amount of drug likely to reach germ cells and the evidence of thresholds, paracetamol is not expected to cause heritable damage in man. Various old and poorly designed long-term studies of paracetamol in the mouse and rat have given equivocal results. A few of these studies showed increased incidence of liver and bladder tumours at hepatotoxic doses. National Toxicology Program (U.S.A.) feeding studies have shown that paracetamol is non-carcinogenic when given at non-hepatotoxic doses up to 300 mg/kg/d to the rat and up to 1000 mg/kg/d to the mouse. Taking into account the knowledge of the hepatotoxicity and metabolism of paracetamol and the existence of thresholds for its genotoxicity, the animal studies do not indicate a carcinogenic potential at non-hepatotoxic dose levels. Based on this updated assessment of the genotoxicity and carcinogenicity of paracetamol, it is concluded that there is no need for regulatory action.

Inhibitory effects of paracetamol on DNA repair in mammalian cells

Paracetamol blocks DNA replication by inhibiting deoxyribonucleotide (dNTP) synthesis and may therefore also interfere with DNA repair. In the present work various mammalian cell types were treated with genotoxic agents and allowed to repair in the presence or absence of paracetamol. Alkaline elution was used to assay DNA single-strand breaks plus alkali-labile sites (= SSBs). Resting human mononuclear blood cells (MNC) exposed to 4-nitroquinoline N-oxide (NQO, 3 microM) plus 0.3 mM paracetamol contained twice as many DNA SSBs compared to MNC exposed to NQO alone, and the level of SSBs decreased more slowly during repair in the presence of paracetamol. Deoxyribonucleosides reversed the effects of paracetamol. SSBs induced by MMS or X-rays (2.6 Gy) were not increased by paracetamol. Resting and growth-stimulated MNC, HL-60 cells, rat hepatocytes and human fibroblasts exposed to UV-C (3-12 J/m2) showed varying levels of transient SSBs formed during repair but these were consistently higher in the presence of paracetamol (0.3-1 mM). In rat testicular cells SSBs were induced by NQO and the levels were further increased in the presence of paracetamol, whereas after UV almost no SSBs were detected during repair. The cell-type specific levels of transient SSBs after UV did not correlate with the rate of incision of DNA lesions, measured as the rate of SSB accumulation in the presence of repair inhibitors Ara C plus hydroxyurea. Transient SSBs were present in resting MNC for at least 24 h after UV and paracetamol increased these breaks 4-fold however the overall rate of removal of excisable photodamage during repair did not appear to be reduced by the presence of paracetamol. The present data indicate that paracetamol interferes with nucleotide excision repair in several mammalian cell types. This constitutes a mechanism by which paracetamol may contribute to genotoxicity in humans.

International Commission for Protection against Environmental Mutagens and Carcinogens. An evaluation of the genetic toxicity of paracetamol

During the last years, several reports have indicated genotoxic effects of paracetamol, a widely used non-prescription analgesic and antipyretic drug. Thus, a careful evaluation of a possible genotoxic effect related to paracetamol use is warranted. Studies in vitro and in vivo indicate that the reactive metabolite of paracetamol can bind irreversibly to DNA and cause DNA strand breaks. Paracetamol inhibits both replicative DNA synthesis and DNA repair synthesis in vitro and in experimental animals. Paracetamol does not cause gene mutations, either in bacteria or in mammalian cells. On the other hand, a co-mutagenic effect of paracetamol has been reported. Furthermore, paracetamol increases the frequency of chromosomal damage in mammalian cell lines, isolated human lymphocytes and experimental animals. Two independent studies have shown an increase in chromosomal damage in lymphocytes of human volunteers after intake of therapeutic doses of paracetamol, whereas a third study was negative. Paracetamol-induced chromosomal damage appears to be caused by an inhibition of ribonucleotide reductase. This indicates that a threshold level for the paracetamol-induced chromosomal damage may exist. Genotoxic effects of paracetamol have, however, been demonstrated both in vitro and in vivo at or near therapeutic concentrations. The data indicate that the use of paracetamol may contribute to an increase in the total burden of genotoxic damage in man. Thus, there may be a need to evaluate the therapeutic benefit of paracetamol, taking into consideration not only its potential to induce acute and chronic organ damage, but also genotoxic effects.

Modification by analgesics of lesion development in the urinary tract and various other organs of rats pretreated with dihydroxy-di-N-propylnitrosamine and uracil

Effects of the analgesics phenacetin, acetaminophen and antipyrine on lesion development in the urinary tract and other organs in male F344 rats were investigated. Animals were concurrently administered with 0.1% dihydroxy-di-N-propylnitrosamine (DHPN) in drinking water and 3.0% uracil in the diet for 4 weeks and then, starting 1 week after the cessation of this treatment, received basal diet or diet containing phenacetin, acetaminophen or antipyrine for 35 weeks. The occurrences of renal cell tumors were increased in the groups given phenacetin or antipyrine, as compared with the DHPN + uracil alone controls. Antipyrine, but not the two other compounds, also enhanced development of hyperplastic lesions in the renal pelvis and ureter. In the urinary bladder, phenacetin and antipyrine treatments were both associated with increased incidence of preneoplastic or neoplastic lesions. Furthermore, phenacetin alone, without the initiating agent pretreatments, induced simple hyperplasias of the urinary bladder at high incidence. Antipyrine enhanced induction of hyperplastic lesions in the ureter and was also found to increase the incidences of preneoplastic and neoplastic lesions in the liver. Although decreased incidences of tumor development of lung and thyroid were observed for the group given phenacetin, this might have been linked to the decreased weight gain. The results confirmed that combination treatment with DHPN + uracil is effective for wide-spectrum initiation of carcinogenesis in the urological tract and demonstrated significant modification potential for both phenacetin and antipyrine.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Nitrosamine formation via non-prescription drugs?

Both carcinogenic and non-carcinogenic nitrosamines can be formed under physiological conditions in the human body by a reaction between nitrite and secondary or tertiary amines. A large number of people are exposed daily through drinking water to high levels of nitrate, which can be reduced to nitrite. Moreover, nitrate and nitrite are present in vegetables and nitrite is used in food preservation. Dietary exposure to amines is normally below 100 mg per day, whereas paracetamol and antazolin, both secondary amines, are used therapeutically at much higher doses. Knowledge about the possible interactions between these widely used drugs and the background exposure to nitrite is presently not available. Therefore, an evaluation of the carcinogenic hazard related to this combination is needed.

The genetic toxicology of paracetamol and aspirin: a review

Paracetamol and aspirin are the two most widely used analgesics available for human use without prescription in several parts of the world. Paracetamol has an antipyretic activity, and aspirin has both antipyretic and anti-inflammatory activities. Characterization of the mutagenicity and clastogenicity of these drugs is essential for their overall safety assessment. In the present review, an attempt is made to evaluate the genotoxic effects of these two widely used analgesics based on available literature.

Investigation of possible mechanisms of hepatic swelling and necrosis caused by acetaminophen in mice

Vascular congestion and liver swelling have long been recognized as features of the hepatotoxic effects of acetaminophen (AAP) in mice and rats and have been proposed as contributing factors to the eventual extent of necrosis produced. Neutrophil accumulation in the hepatic microcirculation has been proposed as being responsible for the blockage of hepatic blood flow and thereby the expansion of the region of damage. We therefore determined in mice the effects of hepatotoxic doses of AAP on the messenger RNA for intercellular adhesion molecule-1 (ICAM-1), which is a critical determinant of neutrophil adhesion, activation and ultimately of neutrophil-mediated tissue injury. Hepatotoxic doses of AAP did not upregulate ICAM-1 messenger RNA. However, doses of bacterial lipopolysaccharide (LPS) did cause a rapid and dramatic increase in ICAM-1 message, which was accompanied by a much greater hepatic accumulation of neutrophils, but which led to only scattered single cell necrosis. In addition, we investigated the effects of pentoxifylline (PTX) on AAP-induced vascular congestion and on hepatic necrosis as evaluated histologically and by measurement of plasma transaminase activities. Although PTX has been shown to increase blood cell deformability and improve vascular perfusion in a number of animal models of restricted blood flow, and is used in humans for the treatment of intermittent claudication, we found no decrease in AAP-induced hepatic swelling or in AAP-induced necrosis in response to PTX. With some dosing regimens, PTX-treated animals proved to be slightly more susceptible to AAP, which may be related to the reported potentiation of the cytotoxicities of a number of alkylating anti-cancer drugs by PTX and other methylxanthines. We conclude from these studies that upregulation of ICAM-1 and subsequent adhesion and vascular plugging by neutrophils are not significant determinants of AAP-induced liver swelling and necrosis and that whatever hemorheological advantages PTX might offer in AAP-induced hepatic damage appear to be overshadowed by effects that potentiate the toxic responses.

Overview of genotoxic carcinogens and non-genotoxic carcinogens

It is known that carcinogens designated on the basis of longterm animal test results are extremely diverse in character, both in terms of potencies and the mechanism of action, which leads to complexity in their assessment for cancer risk to humans. The classification of carcin0ogens into two categories, namely, genotoxic and non-genotoxic varieties has been proposed to give a longical foundation on which cancer risk assessment can be reasonably based. The term "genotoxic carcinogen" indicates a chemical capable of producing cancer by directly altering the genetic material of target cells, while "non-genotoxic carcinogen" represents a chemical capable of producing cancer by some secondary mechanism not related to direct gene damage. This classification has contributed to the exclusion of various rodent-specific carcinogens from the group of chemicals with potential cancer risk to humans. However, the term, "nongenotoxic carcinogen" tends to give the mistaken impression that carcinogens shown to be negative for mutagenicity in a series of test systems might be harmless to humans. It should be realized that clear-cut criteria for this classification have not been established because of insufficiencies in the available information concerning mechanisms of action of non-genotoxic carcinogens. Future scientific advances leading to elucidation of the subcellular mechanisms of carcinogenesis are necessary for establishment of the unified, more realistic and mechanism-based approach to cancer risk estimation form exposure to chemicals.

Normal chromosomal aberration frequencies in peripheral lymphocytes of healthy human volunteers exposed to a maximum daily dose of paracetamol in a double blind trial

Paracetamol (acetaminophen) has been examined for mutagenic potential in numerous studies: gene mutation tests consistently gave negative results while in vitro chromosomal aberration tests showed equally consistently positive effects. In vivo studies for chromosome breaking activity gave clearly negative, equivocal or weakly positive results. In particular two reports have indicated that human volunteers taking a maximum daily dose of paracetamol (3 x 1000 mg over 8 h) exhibited significantly elevated frequencies of chromatid breaks in their peripheral lymphocytes 24 h later. In the one study evaluating the time course, levels returned to normal between 3 and 7 days later. We performed a carefully controlled double-blind study in which volunteers were pre-screened for normal liver function, they all were non-smoking and their diet and environmental exposures were controlled during the study. Cell-cycle kinetics were monitored and paralleled and a placebo group was included. Although a larger number of cells than in the other studies was analysed we were unable to reproduce their findings. No significant increases in structural chromosome aberrations (CA) were found either when the paracetamol group (male, female or both) post-dosing values were compared with pre-dosing values, or when treated groups at any sampling time were compared with the placebo groups. There was not even any evidence that individuals responded to the clastogenic potential of paracetamol or that a group response may have been masked by non-responders. In conjunction with the recently published results of the NTP bioassay, showing no carcinogenic activity in mice and no carcinogenic activity in rats except an increase of mononuclear cell leukaemia in female rats which is of doubtful relevance, the study presented here argues that paracetamol does not pose an unacceptable (if any) genotoxic/carcinogenic risk to man.

The role of the nucleus and other compartments in toxic cell death produced by alkylating hepatotoxicants

Hepatocellular necrosis occurs under a wide range of pathological conditions. In most cases, toxic cell death takes place over a finite span of time, delayed from the point of initial injury and accompanied by homeostatic counterresponses that are varied and complex. The present strategies for discovering critical steps in cell death recognize that (1) different toxins produce similar morphologic changes that precede killing in widely varied cell types, and that (2) lethal events are likely to involve one or more compartmentalized functions that are common to most cells. Investigations of the plasma membrane, endoplasmic reticulum, cytoplasm, mitochondrion, and nucleus have greatly advanced our understanding of acute hepatocellular necrosis. This report examines each compartment but emphasizes molecular changes in the nucleus which may explain cell death caused by alkylating hepatotoxicants. Accumulating knowledge about two distinct modes of cell death, necrosis and apoptosis, indicates that loss of Ca2+ regulation and subsequent damage to DNA may be critical steps in lethal damage to liver cells by toxic chemicals.

Sister-chromatid exchange and chromosome aberrations induced by paracetamol in vivo in bone-marrow cells of mice

Sister-chromatid exchange (SCE) and chromosome aberrations (CA) induced by paracetamol (PC), a common analgesic, were studied in vivo on bone-marrow cells of mice. The trend tests for the evidence of dose-response effects for both SCE and CA were significant. The significant increase in SCE as well as CA induced by PC may be attributed to the fact that PC can induce genotoxicity through DNA damage. Thus, the present study indicates that PC was genotoxic in vivo in bone-marrow cells of mice.

The covalent binding of acetaminophen to cellular nucleic acids as the result of the respiratory burst of neutrophils derived from the HL-60 cell line

After being induced to differentiate into a neutrophilic type, cultures of the leukemic cell line HL-60 were able to cause the bioactivation and nucleic acid binding of acetaminophen upon stimulation of the respiratory burst. This phenomenon was found to simulate the same process as that previously shown with normal human granulocytes. Binding to both DNA and RNA of the cells was determined quantitatively by use of 14C-labeled acetaminophen congeners. Protein binding occurred to about the same extent as did RNA binding. Simultaneous labeling experiments with [ring-14C]- and [14C = O]acetaminophen further showed that the acetaminophen molecule was bound to DNA in an intact manner, while binding to RNA showed about a 50% excess binding of the acetaminophen ring relative to the carbonyl group. Experiments with certain inhibitors showed that catalase and azide ion strongly inhibited DNA binding, while superoxide dismutase had a slight stimulatory effect on binding. These results suggest a significant role for myeloperoxidase in the bioactivation process, which contrasts with the proposed bioactivation mechanism of certain arylamine compounds. A mechanism was proposed for acetaminophen binding to nucleic acids that requires the 1 e- oxidation of this substrate to its phenoxyl radical, although the production of the N-acetyl-p-benzoquinoneimine metabolite, which has been proposed to account for the extensive protein binding known to occur for acetaminophen, might also contribute to such binding. The potential genotoxicity of acetaminophen was considered in view of what might be a unique pathway which can metabolize this chemical to a nucleic acid-binding species.

Acetaminophen-induced cytotoxicity in cultured mouse hepatocytes: effects of Ca(2+)-endonuclease, DNA repair, and glutathione depletion inhibitors on DNA fragmentation and cell death

Hepatotoxic alkylation of mouse liver cells by acetaminophen is characterized by an early loss of ion regulation, accumulation of Ca2+ in the nucleus, and fragmentation of DNA in vitro and in vivo. Acetaminophen-induced DNA cleavage is accompanied by the formation of a "ladder" of DNA fragments characteristic of Ca(2+)-mediated endonuclease activation. These events unfold well in advance of cytotoxicity and the development of necrosis. The present study utilized cultured mouse hepatocytes and mechanistic probes to test whether DNA fragmentation and cell death might be related in a "cause-and-effect" manner. Cells were isolated by collagenase perfusion, cultured in Williams' E medium for 22-26 hr, and exposed to acetaminophen. Aurintricarboxylic acid, a general Ca(2+)-endonuclease inhibitor, and EGTA, a chelator of Ca2+ required for endonuclease activation, significantly decreased DNA fragmentation at 6 and 12 hr and virtually abolished cytotoxicity. N-Acetylcysteine also eliminated DNA fragmentation and cytotoxicity. 3-Aminobenzamide, an inhibitor of poly(ADP-ribose) polymerase-stimulated DNA repair, failed to alter the amount of DNA fragmentation at 6 hr but substantially increased acetaminophen cytotoxicity in hepatocytes at 12 hr. With the exception of when DNA repair was inhibited by 3-aminobenzamide, Ca2+ accumulation in the nucleus, DNA fragmentation, and hepatocyte death varied consistently and predictably with one another. Collectively, these findings suggest that unrepaired damage to DNA contributes to acetaminophen-induced cell death in vivo and may play a role in necrosis in vivo.

Increased frequency of sister-chromatid exchange and chromatid breaks in lymphocytes after treatment of human volunteers with therapeutic doses of paracetamol

Paracetamol was given to 10 healthy human volunteers in 3 doses of 1 g each during a period of 8 h. Blood samples for lymphocyte cultures were taken before and 24 h after paracetamol administration. A small but significant increase was found in the frequency of sister-chromatid exchanges (SCE) after intake of paracetamol (0.187 +/- 0.030 per chromosome before and 0.208 +/- 0.024 per chromosome after). After exposure the mean frequency of chromatid breaks per 100 cells was significantly increased (2.16 +/- 1.33 versus 0.33 +/- 0.50 before exposure). Exposure of human lymphocytes in vitro showed that concentrations of paracetamol above 0.1 mM induced inhibition of replicative DNA synthesis. Increased SCE was found in lymphocytes exposed to 1-10 mM paracetamol for 2 h. Furthermore, 0.75-1.5 mM paracetamol exposure for 24 h increased the frequency of chromatid and chromosome breaks in the lymphocytes. The paracetamol-induced SCE and chromosome aberrations may be secondary effects of paracetamol-induced inhibition of DNA synthesis or due to covalent binding of paracetamol metabolite(s) to DNA.

Comparative cytotoxic effects of acetaminophen (N-acetyl-p-aminophenol), a non-hepatotoxic regioisomer acetyl-m-aminophenol and their postulated reactive hydroquinone and quinone metabolites in monolayer cultures of mouse hepatocytes

Toxic effects of acetaminophen (paracetamol, N-acetyl-p-aminophenol, APAP) in monolayer cultures of mouse hepatocytes developed over a period of 18 hr. N-Acetyl-m-aminophenol (AMAP) was approximately 10-fold less toxic than APAP, despite the fact that it bound covalently to a greater extent to hepatocyte macromolecules. AMAP did not deplete glutathione to as great an extent as APAP, indicating that their reactive metabolites may bind to different proteins or that oxidative damage in addition to arylation of proteins may be involved in the development of cell death. The toxicity of 3-methoxy-acetyl-p-aminophenol was similar to that of APAP, whereas the other hydroquinone and quinone metabolites were 8-10 times more cytotoxic than APAP. The potencies of these analogs were in the order: acetyl-m-aminophenol-p-benzoquinoneimine greater than or equal to 2,5-dihydroxyacetanilide greater than or equal to 3-methoxy-p-benzoquinone greater than or equal to N-acetyl-p-benzoquinone imine (NAPQI) greater than or equal to acetyl-m-aminophenol-o-benzoquinone greater than or equal to 3-hydroxy-acetyl-p-aminophenol. The relative toxic potencies of the hydroquinone and quinone metabolites of AMAP were comparable to that of NAPQI, and do not readily explain the marked difference between the cytotoxic effects of AMAP and APAP.

Further investigations on the clastogenicity of paracetamol and acetylsalicylic acid in vitro

Paracetamol (PCM) and acetylsalicylic acid (ASA), both widely used analgesics, were tested for their clastogenicity in V79 cells in vitro. Rat liver S9 mix and primary rat hepatocytes (PRH) were used as external activation systems. ASA was found to be negative with and without activation system in concentrations up to 10(-2) M. In contrast PCM induced concentration-dependent chromosomal aberrations with and without activation system within the range of 3 x 10(-3) and 10(-2) M. The greatest effects were observed following continuous treatment with PRH activation and without external metabolization. Pulse treatments without external metabolization, with S9 mix and PRH were less effective. The clastogenic potency of PCM seems to be partly independent of metabolic activation. Although clastogenic effects in vitro were observed only in very high concentrations pharmacokinetic data and other published mutagenicity data indicate that there might be a risk for human use. Peak plasma levels of more than 10(-4) M have been reported (Forrest et al., 1982) and 2 groups of investigators (Kocisova et al., 1988; Hongslo et al., 1990) found PCM to be weakly clastogenic in human lymphocytes in vivo in the maximum human therapeutic dose range.

The effect of paracetamol on oxidative damage in human peripheral lymphocytes

Unscheduled DNA synthesis (UDSox) and lipid peroxidation (LPO) induced by non-enzymatic activation of molecular oxygen (Fe2+ +H2O2) were measured in human peripheral lymphocytes from healthy volunteers. The effect of paracetamol (PC) in a final concentration range of 0.05-10 mmole/l on these oxidative processes and on DNA repair induced by MNNG (UDSmut) was investigated. The level of induced LPO was measured by the thiobarbituric acid assay, UDSox and UDSmut were determined by scintillometric measurement of incorporated [methyl-3H]thymidine into damaged DNA. PC at concentrations lower than 1 mmole/l significantly potentiates the non-enzymatically induced LPO and UDSox with the maximum of the activation being around 0.1 mmole/l. In contrast, PC at concentrations higher than 1 mmole/l exhibits an inhibitory effect on both LPO and UDSox. On the other hand, concentrations higher than 1 mmole/l significantly suppressed DNA-repair synthesis induced by MNNG.