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

In vitro approaches to develop weight of evidence (WoE) and mode of action (MoA) discussions with positive in vitro genotoxicity results

A recent analysis by Kirkland et al. [Kirkland, D., Aardema, M., Henderson, L. and Müller, L. (2005) Evaluation of the ability of a battery of 3 in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens. I. Sensitivity, specificity and relative predictivity. Mutat. Res. 584, 1-256] demonstrated an extremely high false positive rate for in vitro genotoxicity tests when compared with carcinogenicity in rodents. In many industries, decisions have to be made on the safety of new substances, and health risk to humans, without rodent carcinogenicity data being available. In such cases, the usual way to determine whether a positive in vitro genotoxicity result is relevant (i.e. indicates a hazard) for humans is to develop weight of evidence (WoE) or mode of action (MoA) arguments. These are based partly on further in vitro investigations, but usually rely heavily on tests for genotoxicity in one or more in vivo assays. However, for certain product types in the European Union, the use of animals for genotoxicity testing (as well as for other endpoints) will be prohibited within the next few years. Many different examples have been described that indicate DNA damage and genotoxic responses in vitro can arise through non-relevant in vitro events that are a result of the test systems and conditions used. The majority of these non-relevant in vitro events can be grouped under a category of 'overload of normal physiology' that would not be expected to occur in exposed humans. However, obtaining evidence in support of such MoAs is not easy, particularly for those industries prohibited from carrying out in vivo testing. It will become necessary to focus on in vitro studies to provide evidence of non-DNA, threshold or in vitro-specific processes and to discuss the potential for such genotoxic effects to occur in exposed humans. Toward this end, we surveyed the published literature for in vitro approaches that may be followed to determine whether a genotoxic effect observed in vitro will occur in humans. Unfortunately, many of the approaches we found are based on only a few published examples and validated approaches with consensus recommendations often do not exist. This analysis highlights the urgent need for developing consensus approaches that do not rely on animal studies for dealing with in vitro genotoxins.

How to reduce false positive results when undertaking in vitro genotoxicity testing and thus avoid unnecessary follow-up animal tests: Report of an ECVAM Workshop

Workshop participants agreed that genotoxicity tests in mammalian cells in vitro produce a remarkably high and unacceptable occurrence of irrelevant positive results (e.g. when compared with rodent carcinogenicity). As reported in several recent reviews, the rate of irrelevant positives (i.e. low specificity) for some studies using in vitro methods (when compared to this "gold standard") means that an increased number of test articles are subjected to additional in vivo genotoxicity testing, in many cases before, e.g. the efficacy (in the case of pharmaceuticals) of the compound has been evaluated. If in vitro tests were more predictive for in vivo genotoxicity and carcinogenicity (i.e. fewer false positives) then there would be a significant reduction in the number of animals used. Beyond animal (or human) carcinogenicity as the "gold standard", it is acknowledged that genotoxicity tests provide much information about cellular behaviour, cell division processes and cellular fate to a (geno)toxic insult. Since the disease impact of these effects is seldom known, and a verification of relevant toxicity is normally also the subject of (sub)chronic animal studies, the prediction of in vivo relevant results from in vitro genotoxicity tests is also important for aspects that may not have a direct impact on carcinogenesis as the ultimate endpoint of concern. In order to address the high rate of in vitro false positive results, a 2-day workshop was held at the European Centre for the Validation of Alternative Methods (ECVAM), Ispra, Italy in April 2006. More than 20 genotoxicity experts from academia, government and industry were invited to review data from the currently available cell systems, to discuss whether there exist cells and test systems that have a reduced tendency to false positive results, to review potential modifications to existing protocols and cell systems that might result in improved specificity, and to review the performance of some new test systems that show promise of improved specificity without sacrificing sensitivity. It was concluded that better guidance on the likely mechanisms resulting in positive results that are not biologically relevant for human health, and how to obtain evidence for those mechanisms, is needed both for practitioners and regulatory reviewers. Participants discussed the fact that cell lines commonly used for genotoxicity testing have a number of deficiencies that may contribute to the high false positive rate. These include, amongst others, lack of normal metabolism leading to reliance on exogenous metabolic activation systems (e.g. Aroclor-induced rat S9), impaired p53 function and altered DNA repair capability. The high concentrations of test chemicals (i.e. 10 mM or 5000 microg/ml, unless precluded by solubility or excessive toxicity) and the high levels of cytotoxicity currently required in mammalian cell genotoxicity tests were discussed as further potential sources of false positive results. Even if the goal is to detect carcinogens with short in vitro tests under more or less acute conditions, it does not seem logical to exceed the capabilities of cellular metabolic turnover, activation and defence processes. The concept of "promiscuous activation" was discussed. For numerous mutagens, the decisive in vivo enzymes are missing in vitro. However, if the substrate concentration is increased sufficiently, some other enzymes (that are unimportant in vivo) may take over the activation-leading to the same or a different active metabolite. Since we often do not use the right enzyme systems for positive controls in vitro, we have to rely on their promiscuous activation, i.e. to use excessive concentrations to get an empirical correlation between genotoxicity and carcinogenicity. A thorough review of published and industry data is urgently needed to determine whether the currently required limit concentration of 10mM or 5000 microg/ml, and high levels of cytotoxicity, are necessary for the detection of in vivo genotoxins and DNA-reactive, mutagenic carcinogens. In addition, various measures of cytotoxicity are currently allowable under OECD test guidelines, but there are few comparative data on whether different measures would result in different maximum concentrations for testing. A detailed comparison of cytotoxicity assessment strategies is needed. An assessment of whether test endpoints can be selected that are not intrinsically associated with cytotoxicity, and therefore are less susceptible to artefacts produced by cytotoxicity, should also be undertaken. There was agreement amongst the workshop participants that cell systems which are p53 and DNA-repair proficient, and have defined Phase 1 and Phase 2 metabolism, covering a broad set of enzyme forms, and used within the context of appropriately set limits of concentration and cytotoxicity, offer the best hope for reduced false positives. Whilst there is some evidence that human lymphocytes are less susceptible to false positives than the current rodent cell lines, other cell systems based on HepG2, TK6 and MCL-5 cells, as well as 3D skin models based on primary human keratinocytes also show some promise. Other human cell lines such as HepaRG, and human stem cells (the target for carcinogenicity) have not been used for genotoxicity investigations and should be considered for evaluation. Genetic engineering is also a valuable tool to incorporate missing enzyme systems into target cells. A collaborative research programme is needed to identify, further develop and evaluate new cell systems with appropriate sensitivity but improved specificity. In order to review current data for selection of appropriate top concentrations, measures and levels of cytotoxicity, metabolism, and to be able to improve existing or validate new assay systems, the participants called for the establishment of an expert group to identify the in vivo genotoxins and DNA-reactive, mutagenic carcinogens that we expect our in vitro genotoxicity assays to detect as well as the non-genotoxins and non-carcinogens we expect them not to detect.

Disruption of the Glutathione Transferase Pi Class Genes

Glutathione transferases are a multi-gene family of enzymes responsible for the metabolism of a wide range of both endogenous and exogenous substrates. These polymorphic enzymes, which form part of an adaptive response to chemical and oxidative stress, are widely distributed and ubiquitously expressed and are subject to regulation by a number of structurally unrelated chemicals. One of these enzymes, GST P, has been the focus of much research in recent years in relation to its involvement in the etiology of disease, particularly cancer. As part of our research efforts into GST P, we have developed a mouse line that lacks this enzyme and have used this model to investigate the consequences of the absence of GST P on tumorigenesis, drug metabolism, and toxicity.

Preliminary Evaluation of DNA Damage Related with the Smoking Habit Measured by the Comet Assay in Whole Blood Cells

The alkaline single-cell gel electrophoresis (SCGE) assay, also called the comet assay, is a rapid and simple method for the detection of DNA damage in individual cells. The objective of this study was to establish if the alkaline SCGE assay in whole blood cells gives similar results as the same method in isolated lymphocytes, because whole blood cells are simpler and more economical to use, specifically in human genotoxic biomonitoring. To validate the method, we first used mouse blood cells, because mouse is one of the most commonly used animals in genetic toxicology testing. Groups of seven CF1 male mice were given i.p. injections of relatively low doses of methyl methanesulfonate (25 mg/kg body weight), a direct acting genotoxic agent, or cyclophosphamide (50 mg/kg body weight), which requires metabolic activation. Three, 6, 8, 12, 16, 20, and 65 hours after treatment, 5 μL of blood were collected from each animal and were processed for the alkaline SCGE assay. On the basis of an analysis of tail moment, the results showed that this assay can detect DNA damage induced by both kinds of alkylating mutagens. We then did a preliminary study to assess the status of DNA damage in a young (19 to 23 years old) healthy population of male smokers (n = 6) and nonsmokers (n = 6) using the comet assay in whole blood cells. A significant difference was observed between the two groups, showing that the method is able to detect DNA damage in the smoking group despite the short time that the volunteers had actually been smoking.

Genotoxicity of hormonal steroids

Hormonal steroids have a widespread use in medicine and their side effects are continuously debated. The possible genotoxic activity of steroids has been the subject of many investigations. The natural estrogens estradiol, estrone and estriol are generally negative in the ICH core battery of tests, but several positive results have been obtained when using additional endpoints of genotoxicity. The genotoxic activity of the 4-hydroxy metabolites of estradiol and estrone is well established. The synthetic steroidal estrogens have a comparable profile of negative and positive test results. Cyproterone acetate and some of its analogues have a special position within the group of progestins. Their genotoxic potential has been established. Other progestins are generally negative in the routine tests. Anti-glucocorticoids, anti-progestins, corticosteroids, androgens, anabolics and anti-androgens appear to be devoid of genotoxic activities. The genotoxic potential of estradiol, estrone and cyproterone acetate with its analogues may play no role under normal physiological and therapeutic conditions. The metabolic conditions that are needed for the formation of DNA-reactive metabolites and oxygen radicals may not be present in humans. Epidemiological cancer data seem to support this view. The importance of thresholds in the dose-effect-relationship of genotoxicity data and their use in risk assessment is discussed.

Paracetamol and metabolite pharmacokinetics in infants

BACKGROUND

Data concerning metabolism of paracetamol in infants are scant. Previous studies have examined urinary metabolite recovery rates after a single dose of paracetamol in either neonates (<6 weeks) or children (3-9 years). There are no studies investigating infants.

METHODS

Infants ( n=47) undergoing major craniofacial surgery were given paracetamol 19-45 mg/kg 6-, 8-, or 12-hourly as either elixir or suppository formulation for postoperative analgesia, after a loading dose of 33-59 mg/kg rectally during the operation. Serum was assayed for paracetamol concentration in 40 of these infants at 5, 8, 11, 14, 17 and 20 h postoperatively. Urine samples were collected every 3 h for 24 h in 15 of these infants. The clearances of paracetamol to glucuronide and sulphate metabolites as well as the urinary clearance of unmetabolised paracetamol were estimated using non-linear, mixed-effects models.

RESULTS

Mean (+/-SD) age and weight of the patients were 11.8+/-2.5 months and 9.1+/-1.9 kg. Clearances of paracetamol to paracetamol-glucuronide (%CV) and to paracetamol-sulphate were 6.6 (11.5) l/h and 7.5 (11.5) l/h respectively, standardised to a 70-kg person using allometric "1/4 power" models. Glucuronide formation clearance, but not sulphate formation, was related to age and increased with age from a predicted value in a neonate of 2.73 l/h/70 kg to a mature value of 6.6 l/h/70 kg with a maturation half-life of 8.09 months. Urine clearance of paracetamol-glucuronide, paracetamol-sulphate and unchanged paracetamol (%CV) were, respectively, 2.65, 3.03 and 0.55 (28) l/h/70 kg. The urine clearance of unchanged paracetamol and metabolites was related to urine volume flow rate. Clearance attributable to pathways other than these measured in urine was not identifiable. The glucuronide/sulphate formation clearance ratio was 0.69 at 12 months of age. Sulphate metabolism contributed 50% towards paracetamol clearance.

CONCLUSION

Glucuronide formation clearance increases with age in the infant age range but sulphate formation does not. Renal clearance of paracetamol and its metabolites increases with urine flow rate. This and other studies show that paracetamol metabolism to glucuronide appears to be similar in infants and children, but in adults is increased in comparison with children. Oxidative pathways were undetectable in this infant study and may explain, in part, the reduced incidence of hepatotoxicity in infants.

Regulatory recognition of indirect genotoxicity mechanisms in the European Union

The European Union (EU) system for the regulation of chemicals includes approval systems for pharmaceuticals, pesticides and biocides, requirements for hazard classification and for risk assessment of industrial chemicals. Regulators have traditionally used the commonly accepted categorisation of chemicals into genotoxic (DNA-reactive) or non-genotoxic agents in their decision-making processes, and have generally considered that there is no threshold level for the former group. The recognition that a number of genotoxic agents operate by indirect genotoxicity mechanisms such as induction of aneuploidy, oxidative stress, inhibition of DNA synthesis or cytotoxicity presents new problems for the regulator. The dose-response relationship for a number of such agents is generally accepted to show a threshold, however, the degree of acceptance of the threshold effect differs in different EU regulatory systems. The classification system for mutagens is based primarily on intrinsic hazard rather than risk, and the classification criteria do not allow for a less stringent classification for chemicals operating by a threshold mechanism. In contrast, regulatory approval systems for plant protection products and therapeutic agents are based on a risk assessment approach, in which a demonstrated threshold effect for a genotoxic agent is likely to be an important factor in reaching a decision concerning authorisation of the product.

Paracetamol (acetaminophen)-induced toxicity: molecular and biochemical mechanisms, analogues and protective approaches

An overview is presented on the molecular aspects of toxicity due to paracetamol (acetaminophen) and structural analogues. The emphasis is on four main topics, that is, bioactivation, detoxication, chemoprevention, and chemoprotection. In addition, some pharmacological and clinical aspects are discussed briefly. A general introduction is presented on the biokinetics, biotransformation, and structural modification of paracetamol. Phase II biotransformation in relation to marked species differences and interorgan transport of metabolites are described in detail, as are bioactivation by cytochrome P450 and peroxidases, two important phase I enzyme families. Hepatotoxicity is described in depth, as it is the most frequent clinical observation after paracetamol-intoxication. In this context, covalent protein binding and oxidative stress are two important initial (Stage I) events highlighted. In addition, the more recently reported nuclear effects are discussed as well as secondary events (Stage II) that spread over the whole liver and may be relevant targets for clinical treatment. The second most frequent clinical observation, renal toxicity, is described with respect to the involvement of prostaglandin synthase, N-deacetylase, cytochrome P450 and glutathione S-transferase. Lastly, mechanism-based developments of chemoprotective agents and progress in the development of structural analogues with an improved therapeutic index are outlined.

Strategies and testing methods for identifying mutagenic risks

The evolution of testing strategies and methods for identification of mutagenic agents is discussed, beginning with the concern over potential health and population effects of chemical mutagens in the late 1940s that led to the development of regulatory guidelines for mutagenicity testing in the 1970s and 1980s. Efforts to achieve international harmonization of mutagenicity testing guidelines are summarized, and current issues and needs in the field are discussed, including the need for quantitative methods of mutagenic risk assessment, dose-response thresholds, indirect mechanisms of mutagenicity, and the predictivity of mutagenicity assays for carcinogenicity in vivo. Speculation is offered about the future of mutagenicity testing, including possible near-term changes in standard test batteries and the longer-term roles of expression profiling of damage-response genes, in vivo mutagenicity testing methods, and models that better account for differences in metabolism between humans and laboratory model systems.

Increased resistance to acetaminophen hepatotoxicity in mice lacking glutathione S-transferase Pi

Overdose of acetaminophen, a widely used analgesic drug, can result in severe hepatotoxicity and is often fatal. This toxic reaction is associated with metabolic activation by the P450 system to form a quinoneimine metabolite, N-acetyl-p-benzoquinoneimine (NAPQI), which covalently binds to proteins and other macromolecules to cause cellular damage. At low doses, NAPQI is efficiently detoxified, principally by conjugation with glutathione, a reaction catalyzed in part by the glutathione S-transferases (GST), such as GST Pi. To assess the role of GST in acetaminophen hepatotoxicity, we examined acetaminophen metabolism and liver damage in mice nulled for GstP (GstP1/P2((-/-))). Contrary to our expectations, instead of being more sensitive, GstP null mice were highly resistant to the hepatotoxic effects of this compound. No significant differences between wild-type (GstP1/P2((+/+))) mice and GstP1/P2((-/-)) nulls in either the rate or route of metabolism, particularly to glutathione conjugates, or in the levels of covalent binding of acetaminophen-reactive metabolites to cellular protein were observed. However, although a similar rapid depletion of hepatic reduced glutathione (GSH) was found in both GstP1/P2((+/+)) and GstP1/P2((-/-)) mice, GSH levels only recovered in the GstP1/P2((-/-)) mice. These data demonstrate that GstP does not contribute in vivo to the formation of glutathione conjugates of acetaminophen but plays a novel and unexpected role in the toxicity of this compound. This study identifies new ways in which GST can modulate cellular sensitivity to toxic effects and suggests that the level of GST Pi may be an important and contributing factor in the sensitivity of patients with acetaminophen-induced hepatotoxicity.

Repeated acetaminophen dosing in rats: adaptation of hepatic antioxidant system

Repeated dosing of acetaminophen (paracetamol) to rats is reported to decrease their sensitivity to its hepatotoxic effects, which are associated with oxidative stress and glutathione depletion. We determined if repeated acetaminophen dosing produced adaptive response of key antioxidant system enzymes. Male rats (Sprague-Dawley, 10 weeks) were given 800, 1200, or 1600 mg/kg/day acetaminophen by oral gavage for 4 days. Liver was assayed for oxidative stress and antioxidant markers: malondialdehyde (MDA), thiobarbituric acid reactive substance (TBARS), total antioxidant status (TAS), glutathione (GSH), glutathione reductase (GR), glutathione peroxidase (GPx), glucose-6-phosphate dehydrogenase (G6PD), catalase (CAT), and superoxide dismutase (SOD), and alanine transaminase (ALT) as a marker of hepatocellular injury. Acetaminophen at 1200/1600 mg/kg decreased GSH 26/47%, GPx 21/26%, CAT 35/28%, SOD 21/12%; and TAS 28/18% (correlated with CAT, r=0.91; SOD, r=0.66; GPx, r=0.45). Despite antioxidant deficiencies, and no TBARS change, MDA decreased 26%/33%/37% at 800/1200/1600 mg/kg, which correlated with increased GR (61%/62%/76%, r=0.77) and G6PD (130%/110%/190%, r=0.78). Both MDA (r=0.68) and G6PD (r=0.71) correlated with hepatic ALT, which decreased 27%/43%/48%, respectively. Resistance to acetaminophen hepatotoxicity produced by repeated exposure is partially attributable to upregulation of hepatic G6PD and GR activity as an adaptive and protective response to oxidative stress and glutathione depletion.

Human biological relevance and the use of threshold-arguments in regulatory genotoxicity assessment: experience with pharmaceuticals

Issues of biological relevance and thresholds for genotoxicity are discussed here based upon the background of experience with the submissions for the approval of new pharmaceuticals to the German regulatory authority over the period between 1990 and 1997. This experience shows that out of the genotoxicity test systems which are required according to existing guidelines in the European Union (EU), the in vitro tests for chromosomal aberrations (CA) and the mouse lymphoma tk assays (MLA) yield a rate of positives that is about four-fold higher than that of other genotoxicity tests. A detailed analysis of chemical and pharmacological classes of compounds and their effects in these systems reveals that in addition to direct DNA reactivity several mechanisms of indirect genotoxicity such as nucleoside analogue incorporation into DNA, interaction with microtubule assembly, topoisomerase inhibition and high levels of cytotoxicity are relevant. New pharmaceuticals, for which the latter mechanisms apply, often display threshold-like characteristics in their genotoxic effects in vitro or even in vivo in experimental animals. This casts doubt upon the relevance of positive in vitro test results for such compounds. However, the discussion of examples shows that it may not be easy to demonstrate the exact thresholded mechanism of genotoxicity in a given case. In particular, the demonstration of a coincidence of genotoxicity and high levels of cytotoxicity, which seems to be a major factor for biologically non-relevant in vitro positive new pharmaceuticals, usually requires quite extensive testing. Hence, for new pharmaceuticals it is practice to provide in addition to in vitro results that may be thresholded a wealth of information from in vivo studies on genotoxicity, carcinogenicity, metabolism, pharmacokinetics, etc. the results of which help in assessing the biological relevance of in vitro positives. The regulatory acknowledgement of biologically non-relevant, thresholded mechanisms of (in vitro) genotoxicity in addition to those that are considered relevant for human risk ensures a better understanding of test results and is needed for the credibility of genotoxicity testing practice in general.

Interpretation of the biological relevance of genotoxicity test results: the importance of thresholds

Despite recent improvements in genotoxicity protocols, we have observed an increase in the occurrence of positive results, particularly in chromosomal aberration tests in vitro, yet very few of these are accompanied by positive responses in vivo. Thus, the positive results may not be biologically relevant either for rodents or humans in vivo, but how should we determine "biological relevance"? Chemicals that produce thresholded dose-responses may well not pose a genotoxic risk at low (relevant to human) exposures, but thresholds should not just be "seen"; there must be an explanation and understanding of the underlying mechanism. In addition to extremes of pH, ionic strength and osmolality, as have been identified previously, such mechanisms include indirect genotoxicity resulting from interaction with non-DNA targets, chemicals/metabolites which are inherently genotoxic but which, at low concentrations, are effectively conjugated and unable to form adducts, and production of specific metabolites under in vitro conditions that are not formed in rodents or humans in vivo. If such thresholded mechanisms can be identified at exposures which are well in excess of expected human exposure, then there may be a strong argument that the positive results are not biologically relevant.

Thresholds in genotoxicity responses

It has been commonly accepted that risk assessments of genotoxic chemicals are based on linear extrapolation methods. However, there is substantial evidence that some chemicals may be genotoxic only at high doses by mechanisms that do not occur at low doses, or only under specific conditions in genotoxicity assays, but are inactive at concentrations within the range of human exposure levels. There are a variety of possible mechanisms of thresholded genotoxicity, including disruption of cell division and chromosome segregation, inhibition of DNA synthesis, overloading of oxidative defence mechanisms, metabolism or plasma binding capacity, disturbances of metal homeostasis, cytotoxicity and physiological perturbations in in vivo assays. The degrees of evidence supporting the proposed mechanisms are variable and not all are sufficiently robust to be universally accepted as yet by the scientific community. However, a survey of industrial companies indicated that data have been accepted by some regulatory authorities indicating thresholds contributing to genotoxicity responses.

Toxicologic and carcinogenic effects of the type IV phosphodiesterase inhibitor RP 73401 on the nasal olfactory tissue in rats

RP 73401, a type IV phosphodiesterase inhibitor, caused toxic effects in the nasal olfactory region of Sprague-Dawley rats when administered by either oral or inhalation exposure. A single oral administration of RP 73401 (at a dose of > or = 50 mg/kg) or 5-day inhalation exposure (1 hr/day) at a dose of approximately 1.0 mg/kg per day caused degeneration and sloughing of the olfactory surface epithelium. Degeneration and loss of Bowman's glands were noted in the underlying lamina propria and submucosa. Electron microscopy of these lesions demonstrated that sustentacular cells and the epithelial cells lining Bowman's glands were the primary target cells in the olfactory mucosa. The earliest ultrastructural changes detected in these cells were dilatation and vesiculation of the endoplasmic reticulum, suggesting that metabolic activation is important for the toxic effects. In repeated-dose studies, 13 wk of oral dosing at 2.0 or 6.0 mg/kg per day resulted in subtle disorganization of the olfactory epithelium, whereas basal cell hyperplasia in the olfactory epithelium was identified in a 6-month inhalation study at a dose of 1.0 mg/kg per day. A 2-yr inhalation carcinogenicity study resulted in tumors of the nasal olfactory region in rats treated at 0.5 and 1.0 mg/kg per day. Most tumors were classified as olfactory neuroblastomas, and immunohistochemistry on selected tumors was consistent with their being of neuroectodermal origin. Of the species studied (rat, mouse, and dog), the olfactory toxicity of RP 73401 was confined to the rat, and the toxicity was likely related to metabolic activation by olfactory epithelial cells rather than the phosphodiesterase activity of the compound.

Analgesic agents for the postoperative period. Nonopioids

For many reasons, nonopioid analgesics have proven to be of immense benefit in postoperative pain relief. Consideration of the limitations and side effects of opioids confirms the need for alternative, complementary analgesics. The current understanding of pain pathophysiology recognizes that many tissue and neuronal factors and changes are invoked by tissue damage, producing peripheral and central sensitization, and some of these may be modulated by the use of NSAIDs, NMDA antagonists, and local anesthetic agents. If successful preemptive analgesic techniques are developed, they will likely include the use of NSAIDs and perhaps NMDA antagonists. Nonopioids are of benefit in multimodal analgesia and allow acute rehabilitation of surgical patients. Acetaminophen, NSAIDs, alpha 2-antagonists, and NMDA antagonists are in routine use as components of multimodal analgesia, in combination with opioids or local anesthetic techniques. Tramadol is interesting because it has nonopioid and opioid actions that can be attributed to the two isomers found in the racemic mixture. Spinal neostigmine and the use of adenosine represent completely different mechanisms of nonopioid analgesia being investigated. Nonopioids, including lidocaine, ketamine, the anticonvulsants, and the antidepressants, are necessary for the treatment of patients with the difficult clinical problem of neuropathic pain that can present in the postoperative period.

Chromosome aberration testing in genetic toxicology-past, present and future

In vitro metaphase tests for chromosomal aberrations (CA) have undergone considerable evolutionary changes over the last 20 yr. Treatment and sampling times have been a particular focus of attention as we have tried to develop protocols that detect weak genotoxins. Different approaches evolved in different parts of the world and led to a need to harmonise. At the same time, we have increasingly challenged the conditions in which clastogens produce positive responses, and several situations have been described in which clastogenic responses would be considered not to be biologically relevant. Now there is a strong case to replace the conventional metaphase analysis test with an in vitro micronucleus test. The time is therefore right to carefully consider whether the type of damage scored in CA tests is relevant for human health.

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.