Oxidation of acetaminophen to N-acetyl-p-aminobenzoquinone imine by human CYP3A4

Cataract formation by a semiquinone metabolite of acetaminophen in mice: possible involvement of Ca(2+)and calpain activation

Acetaminophen, an analgesic/antipyretic, is metabolized by hepatic cytochrome P450 to N -acetyl- p -benzoquinone imine (NAPQI), which is transported by blood circulation to the eye and induces anterior cortical cataract in mice. In this study we injected NAPQI into the anterior chamber of mouse eye and investigated time-dependent cellular responses in the lens. After a lag period of about 2 hr following NAPQI injection, lens opacification as determined by measurement of light scattering by the lens became evident and progressively increased thereafter. There was no difference in the profile of opacity development between a P450-inducer responsive mouse strain and a non-responsive strain. During the lag period, a marked increase in free intracellular Ca(2+)in the lens epithelium was observed at 1 hr by confocal fluorescence microscopy with a Ca(2+)probe. Concurrent with the free Ca(2+)increase, there was a 300% rise in the activity of the non-lysosomal neutral protease calpain in the lens at 1 hr after NAPQI injection. Evidence indicated degradation of vimentin in the lens in which calpain activity was enhanced. Co-injection of calpain inhibitors (N-Ac-Leu-Leu-norleucinol and N-Ac-Leu-Leu-methioninal) with NAPQI protected animals completely from cataract development, although a rise in free intracellular Ca(2+)in the lens epithelium was still observed. Lenses from the protected mice did not exhibit enhanced calpain activity. These results suggest the following sequence of events as a possible mechanism of NAPQI-induced cataract. NAPQI introduced in the anterior chamber of the eye enters the lens epithelial cells and disturbs Ca(2+)homeostasis with a resultant rise in free intracellular Ca(2+)which in turn activates calpain in the epithelium. The neutral protease then degrades cellular proteins (e.g. cytoskeletal proteins) and initiates anterior cortical cataract formation.

Using cytochrome P-450 gene knock-out mice to study chemical metabolism, toxicity, and carcinogenicity

Cytochrome P-450 (CYP) enzymes are heme-containing proteins that carry out oxidative metabolism of a wide range of structurally diverse exogenous chemicals and therapeutic agents as well as endogenous compounds. For some of these xenobiotics, oxidative metabolism results in the formation of toxic, mutagenic, or carcinogenic metabolites. In the past, the role of CYP enzymes in metabolism and chemical-induced toxicity was studied indirectly through use of specific antibodies or inducers and inhibitors of these enzymes. Progress in molecular biology and the ability to bioengineer animal models that do not express CYP1A2, CYP1A1, CYP1B1, CYP2E1, or both CYP1A2 and CYP2E1 isozymes has allowed for direct investigations of the in vivo role of these enzymes in the metabolism, toxicity, and carcinogenicity of xenobiotics. This article reviews research conducted to date that utilizes these genetically bioengineered mice in metabolism, toxicity, or carcinogenicity studies of chemicals. Some studies showed a positive correlation between in vivo results and in vitro predictions for the role of a specific CYP in chemical-induced effects, whereas other studies did not support in vitro predictions. Work reviewed herein demonstrates the importance of using animal models for investigating the role of specific CYP enzymes in metabolism and chemical-induced toxicity or carcinogenicity rather than relying solely on in vitro techniques. Eventually, studies of this nature will facilitate a more accurate assessment of human risks with regard to chemicals by helping us to understand the relationships between chemical metabolism, carcinogenicity, and polymorphisms in CYP enzymes.

Short-term treatment with alcohols causes hepatic steatosis and enhances acetaminophen hepatotoxicity in Cyp2e1(-/-) mice

CYP2E1 has been reported to have an essential role in alcohol-mediated increases in hepatic steatosis and acetaminophen hepatotoxicity. We found that pretreatment of Cyp2e1(-/-) mice with ethanol plus isopentanol, the predominant alcohols in alcoholic beverages, for 7 days resulted in micro- and macrovesicular steatosis in the livers of all mice, as well as a dramatic increase in acetaminophen hepatotoxicity. In Cyp2e1(-/-) mice administered up to 600 mg acetaminophen/kg alone and euthanized 7 h later, there was no increase in serum levels of ALT. In Cyp2e1(-/-) mice pretreated with ethanol and isopentanol, subsequent exposure to 400 or 600 mg acetaminophen/kg resulted in centrilobular necrosis in all mice with maximal elevation in serum levels of ALT. Acetaminophen-mediated liver damage was similar in males and females. Hepatic microsomal levels of APAP activation in untreated females were similar to those in males treated with the alcohols. However, the females, like the males, required pretreatment with the alcohols in order to increase APAP hepatotoxicity. These findings suggest that, in the Cyp2e1(-/-) mice, the alcohol-mediated increase in acetaminophen hepatotoxicity involves the contribution of other factors, in addition to induction of CYP(s) that activate acetaminophen. Alternatively, CYP-mediated activation of acetaminophen measured in vitro may not reflect the actual activity in vivo. Our findings that a 7-day treatment with ethanol and isopentanol causes extensive hepatic steatosis and increases acetaminophen hepatotoxicity in Cyp2e(-/-) mice indicate that CYP2E1 is not essential for either response.

The contributions of various chloroquine salts to the biliary and urinary execretion of hepatic paracetamol conjugation metabolites in the rat

As an approach to explain the possible in vivo interaction of paracetamol (acetaminophen) with various chloroquine salts that are often administered during malaria tropica, the effects of these salts (chloroquine sulphate, chloroquine phosphate, chloroquine hydrochloride and ferrous sulphate) were examined in male rats. The coadministration of chloroquine salts with paracetamol for 7 days showed varied effects on urinary and biliary excretion of paracetamol sulphate and paracetamol glucuronide conjugates--the major metabolites of paracetamol metabolism. These findings suggest that chloroquine sulphate and ferrous sulphate may enhance the sulphation pathway in paracetamol metabolism and influence detoxification of paracetamol in the liver and thus protect the liver. Chloroquine sulphate is therefore a better choice compared to other chloroquine salts in the treatment of malaria with paracetamol as an antipyretic and analgesic.

Acetaminophen produces cataract in DBA2 mice by Ah receptor-independent induction of CYP1A2

The metabolic transformation of acetaminophen to N-acetyl-p-benzoquinone imine by cytochrome P450 enzymes (e.g., cytochrome P450 1A2) is a prerequisite for acetaminophen-induced cataract formation in mice. Aromatic hydrocarbons, such as beta-naphthoflavone, induce cytochrome P450 1A2 in C57BL6 mice via the mediation of the aromatic hydrocarbon receptor and render the animals susceptible to cataract formation by acetaminophen administration but not in DBA2 mice which do not respond to cytochrome P450 1A2 induction by these compounds. Polycyclic hydrocarbons, such as acenaphthylene, were recently found to induce cytochrome P450 1A2 gene expression in young DBA2 mice by aromatic hydrocarbon receptor-independent pathways. In this work, we investigated whether enhanced metabolism of acetaminophen to N-acetyl-p-benzoquinone by cytochrome P450 1A2 induction by acenaphthylene could produce cataract in young DBA2 mice. Fifteen-day-old DBA2 mice were pretreated with two intraperitoneal injections of acenaphthylene and, 24 hr later, with one injection of acetaminophen. In most mice, cataract developed 18-24 hr after acenaphthylene injection. Acenaphthylene treatment of young DBA2 mice resulted in a 2-fold increase in cytochrome P450 1A2-dependent methoxyresorufin O-demethylase activity in the liver. These results support the hypothesis that the aromatic hydrocarbon receptor-independent induction of cytochrome P450 1A2 enzyme leads to accumulation of sufficient N-acetyl-p-benzoquinone in the liver and cataract development in the eye.

Effects of rifampin on the glutathione depletion and cytochrome c reduction by acetaminophen reactive metabolites in an in vitro P450 enzyme system

The present study examined whether rifampin attenuated glutathione (GSH) depletion by acetaminophen reactive metabolites generated in the in vitro P450 enzyme system prepared from mouse liver and the possible mechanism involved in this effect. The results showed that GSH concentration was decreased concentration-dependently by acetaminophen in the in vitro P450 enzyme system. Rifampin significantly attenuated acetaminophen-mediated GSH depletion in a concentration-dependent manner. The concentration-response curve for GSH depletion of acetaminophen was shifted to the right in a parallel fashion in the presence of rifampin at the concentration of 3.2 x 10(-5) M, which appeared to result from the competitive binding of rifampin to acetaminophen metabolites. Cytochrome c was markedly reduced by acetaminophen metabolites in this enzyme system, and GSH concentration-dependently increased the cytochrome c reduction by acetaminophen metabolites. These findings suggested that cytochrome c was reduced by the GSH conjugate of acetaminophen metabolites rather than by acetaminophen-derived superoxide anion (O2*-) and other unbound free radicals. Rifampin was shown to possess an effect similar to that of GSH. It is concluded that the decrease in GSH depletion by rifampin is most likely attributable to the binding of rifampin to the acetaminophen toxic species, and the increase in cytochrome c reduction by rifampin is attributable to the conjugate formed between rifampin and acetaminophen metabolites.

Paracetamol (acetaminophen) cytotoxicity in rat type II pneumocytes and alveolar macrophages in vitro

Paracetamol (acetaminophen, APAP) liver and kidney cytotoxicity is associated with bioactivation by P450 and/or prostaglandin H synthetase (PGHS) to a reactive metabolite, which depletes GSH, covalently binds to proteins, and leads to oxidative stress. Although APAP may also damage the lung, little is known about the mechanism by which this occurs. We studied the in vitro 24-hr-old type II pneumocytes. A time- and concentration-dependent decrease in intracellular GSH occurred in freshly isolated type II pneumocytes and alveolar macrophages exposed to subtoxic (</= 1 mM) APAP concentrations. In 24-hr-old type II pneumocytes, there were no changes in intracellular GSH concentration after APAP exposure. Potassium ethyl xanthate (a P450 inhibitor) and indomethacin (a PGHS inhibitor) significantly decreased APAP-induced GSH depletion in freshly isolated type II pneumocytes and alveolar macrophages, suggesting that P450 and/or PGHS are involved in APAP bioactivation in these cells.

The effect of paracetamol (acetaminophen) on fentanyl metabolism in vitro

BACKGROUND

Fentanyl is a synthetic opioid widely used in anesthesia and analgesia. In experimental animals and in humans it has been shown that most fentanyl biotransformation occurs in the liver, where this opioid is converted primarily to its N-dealkylated derivative, norfentanyl. Recent studies have shown that fentanyl is metabolized to norfentanyl via cytochrome P-450 3A4 (CYP3A4). CYP3A4 is responsible for the metabolism of numerous other therapeutic agents, including those administered concurrently with fentanyl (e.g., nifedipine, lidocaine, erythromycin and cyclosporine). Paracetamol is also metabolized by the CYP3A family, with a Km that is nearly equal to therapeutic blood concentrations. Since paracetamol is widely used, its potential interaction with fentanyl metabolism would be of great interest.

METHODS

In the present study, rat and human liver microsomes were used to assess the ability of paracetamol to inhibit fentanyl metabolism.

RESULTS

In both sets of microsomes, paracetamol produced a concentration-dependent inhibition of fentanyl oxidation to norfentanyl. Kinetic analysis of the data showed that 0.5-5 mM paracetamol inhibited fentanyl metabolism in a noncompetitive fashion. A Dixon plot revealed that the Ki for paracetamol inhibition of fentanyl metabolism is approximately 3.2 mM and 2.8 mM for human and rat liver microsomes, respectively.

CONCLUSIONS

Since these concentrations of paracetamol are approximately one order of magnitude greater than therapeutic concentrations, it would appear that potentially important and possibly harmful fentanyl-paracetamol drug interactions do not occur with therapeutic concentrations of paracetamol.

Paracetamol, alcohol and the liver

It is claimed that chronic alcoholics are at increased risk of paracetamol (acetaminophen) hepatotoxicity not only following overdosage but also with its therapeutic use. Increased susceptibility is supposed to be due to induction of liver microsomal enzymes by ethanol with increased formation of the toxic metabolite of paracetamol. However, the clinical evidence in support of these claims is anecdotal and the same liver damage after overdosage occurs in patients who are not chronic alcoholics. Many alcoholic patients reported to have liver damage after taking paracetamol with 'therapeutic intent' had clearly taken substantial overdoses. No proper clinical studies have been carried out to investigate the alleged paracetamol-alcohol interaction and acute liver damage has never been produced by therapeutic doses of paracetamol given as a challenge to a chronic alcoholic. The paracetamol-alcohol interaction is complex; acute and chronic ethanol have opposite effects. In animals, chronic ethanol causes induction of hepatic microsomal enzymes and increases paracetamol hepatotoxicity as expected (ethanol primarily induces CYP2E1 and this isoform is important in the oxidative metabolism of paracetamol). However, in man, chronic alcohol ingestion causes only modest (about twofold) and short-lived induction of CYP2E1, and there is no corresponding increase (as claimed) in the toxic metabolic activation of paracetamol. The paracetamol-ethanol interaction is not specific for any one isoform of cytochrome P450, and it seems that isoenzymes other than CYP2E1 are primarily responsible for the oxidative metabolism of paracetamol in man. Acute ethanol inhibits the microsomal oxidation of paracetamol both in animals and man. This protects against liver damage in animals and there is evidence that it also does so in man. The protective effect disappears when ethanol is eliminated and the relative timing of ethanol and paracetamol intake is critical. In many of the reports where it is alleged that paracetamol hepatotoxicity was enhanced in chronic alcoholics, the reverse should have been the case because alcohol was actually taken at the same time as the paracetamol. Chronic alcoholics are likely to be most vulnerable to the toxic effects of paracetamol during the first few days of withdrawal but maximum therapeutic doses given at this time have no adverse effect on liver function tests. Although the possibility remains that chronic consumption of alcohol does increase the risk of paracetamol hepatotoxicity in man (perhaps by impairing glutathione synthesis), there is insufficient evidence to support the alleged major toxic interaction. It is astonishing that clinicians and others have unquestion-ingly accepted this supposed interaction in man for so long with such scant regard for scientific objectivity.

Acetaminophen intoxication during treatment: what you don't know can hurt you

For over two decades, pediatricians have been made aware of the potential risk associated with the acute ingestion of large single and/or multiple doses of acetaminophen (APAP). Clearly, APAP-induced hepatotoxicity remains as a recognized medical emergency which, when treated promptly with appropriate gastrointestinal decontamination and when indicated, with the antidote N-acetylcysteine, has a uniformly good clinical outcome. Recently, the hepatotoxic potential associated with "therapeutic" APAP administration has been brought to the attention of the pediatric community. This review explores the issue of APAP toxicity with therapeutic intent by examining both the clinical literature and also, relevant information concerning the basic pharmacology and toxicology of this old and widely used nonprescription drug. A "risk profile" is developed with regard to factors that may predispose infants and children to this iatrogenic form of toxicity so that the awareness of physicians and other caregivers (including parents) can be heightened and preventative education administered. As is true for most all potentially beneficial medicines used in pediatrics, awareness of the actual amount of drug received from all sources and caution to not exceed the age-appropriate dosing guidelines (i.e., both amount and duration) contained in the approved labeling for all products containing APAP will insure safe and effective therapy.

Acetaminophen hepatotoxicity precipitated by short-term treatment of rats with ethanol and isopentanol: protection by triacetyloleandomycin

Ethanol and isopentanol are the predominant alcohols in alcoholic beverages. We have reported previously that pretreatment of rats with a liquid diet containing 6.3% ethanol plus 0.5% isopentanol for 7 days results in a synergistic increase in acetaminophen hepatotoxicity, compared with rats treated with either alcohol alone. Here, we investigated the role of CYP3A in acetaminophen hepatotoxicity associated with the combined alcohol treatment. Triacetyloleandomycin, a specific inhibitor of CYP3A, protected rats pretreated with ethanol along with isopentanol from acetaminophen hepatotoxicity. At both 0.25 and 0.5 g acetaminophen/kg, triacetyloleandomycin partially prevented elevations in serum levels of alanine aminotransferase. At 0.25 g acetaminophen/kg, triacetyloleandomycin completely protected 6 of 8 rats from histologically observed liver damage, and partially protected the remaining 2 rats. At 0.5 g acetaminophen/kg, triacetyloleandomycin decreased histologically observed liver damage in 7 of 15 rats. In rats pretreated with ethanol plus isopentanol, CYP3A, measured immunohistochemically, was decreased by acetaminophen treatment. This effect was prevented by triacetyloleandomycin. These results suggest that CYP3A has a major role in acetaminophen hepatotoxicity in animals administered the combined alcohol treatment. We also found that exposure to ethanol along with 0.1% isopentanol for only 3 days resulted in maximal increases in acetaminophen hepatotoxicity by the combined alcohol treatment, suggesting that short-term consumption of alcoholic beverages rich in isopentanol may be a risk for developing liver damage from acetaminophen.

Utility of acetylcysteine in treating poisonings and adverse drug reactions

As recognition of the role of free radicals and reactive toxins in the pathogenesis of disease, poisoning, and adverse drug reactions has evolved, interest in the use of acetylcysteine as a modulator of these effects has steadily increased in recent years. Acetylcysteine is commonly thought to serve as a glutathione precursor and consequently can increase or sustain intracellular glutathione which scavenges reactive oxygen species caused by toxins or subsequent tissue injury. At least 10 additional mechanisms of action for acetylcysteine have been demonstrated in various laboratory models, but a unifying framework of its actions is still to be proposed. This paper reviews the current experimental and therapeutic status of acetylcysteine for the treatment of poisonings and adverse drug reactions. Of the 45 potential uses of acetylcysteine that were identified for the treatment of poisonings or adverse drug reactions, 14 of the toxic effects have little support for its use while promising results have been demonstrated for 27 toxicities. Currently, treatment of acute paracetamol (acetaminophen) poisoning is the only widely accepted clinical indication for acetylcysteine as a treatment for poisoning or adverse drug reactions. In many clinical situations acetylcysteine is used empirically utilising modifications of dosage regimens employed for paracetamol poisoning. Often it is difficult to determine the benefit of therapy with acetylcysteine owing to the nature of the toxicity being treated, the use of other therapies, the presence of comorbid conditions, and the small number of patients studied. The diverse and positive nature of the investigations suggest that there is considerable promise in acetylcysteine as a research tool and pharmacological agent.

Phenytoin as a possible cause of acetaminophen hepatotoxicity: case report and review of the literature

A 55-year-old woman was hospitalized for treatment of community-acquired pneumonia. Unexplained, moderate elevations in hepatic transaminase and enzyme levels prompted review of her drug regimen. She had taken acetaminophen 1,300-6,200 mg/day during the hospitalization. She also received phenytoin for posttraumatic seizures. Acetaminophen was discontinued, and the patient's liver chemistries returned to normal within 2 weeks of discharge. Acetaminophen is metabolized in part by cytochrome P450 (CYP) 2E1, and inducers of CYP2E1 are known to predispose patients to acetaminophen-related hepatotoxicity. Phenytoin induces CYP2C and CYP3A4 isoforms, but not CYP2E1. The literature suggests, however, that CYP3A4 may participate in acetaminophen metabolism to a greater extent than previously realized, and induction of this isoform may predispose patients to acetaminophen-induced hepatotoxicity.

Warfarin-acetaminophen drug interaction revisited

Physicians and pharmacists routinely advise patients receiving warfarin to take acetaminophen for pain or fever because of its relative safety; however, a recent study questioned the safety of such practice. A comprehensive search of MEDLINE and IPA for human studies and case reports from 1966-1999 revealed evidence that acetaminophen may potentiate the effect of warfarin by a mechanism that has yet to be elucidated. Due to lack of a safer alternative, acetaminophen still should be the analgesic and antipyretic of choice in patients taking warfarin, as long as excessive amounts and prolonged administration (> 1.3 g acetaminophen/day for > 2 wks) are avoided. With the high degree of interpatient variability and the unpredictability of various drug-drug interactions with warfarin, close and frequent monitoring of international normalized ratios is the key for safe oral anticoagulation therapy.

In vitro metabolism of alachlor by human liver microsomes and human cytochrome P450 isoforms

Alachlor (2-chloro-N-methoxymethyl-N-(2,6-diethylphenyl)acetamide) is a widely used pre-emergent chloroacetanilide herbicide which has been classified by the USEPA as a probable human carcinogen. The putative carcinogenic metabolite, 2,6-diethylbenzoquinone imine (DEBQI), is formed through a complex series of oxidative and non-oxidative steps which have been characterized in rats, mice, and monkeys but not in humans. A key metabolite leading to the formation of DEBQI is 2-chloro-N-(2,6-diethylphenyl)acetamide (CDEPA). This study demonstrates that male human liver microsomes are able to metabolize alachlor to CDEPA. The rate of CDEPA formation for human liver microsomes (0.0031 +/- 0.0007 nmol/min per mg) is significantly less than the rates of CDEPA formation for rat liver microsomes (0.0353+/-0.0036 nmol/min per mg) or mouse liver microsomes (0.0106 +/- 0.0007). Further, we have screened human cytochrome P450 isoforms 1A1, 1A2, 2B6, 2C8, 2C9, 2C18, 2C19, 2D6, 2E1, and 3A4 and determined that human CYP 3A4 is responsible for metabolism of alachlor to CDEPA. Further work is necessary to determine the extent to which humans are able to metabolize CDEPA through subsequent metabolic steps leading to the formation of DEBQI.

Protective effect of chlormethiazole, a sedative, against acetaminophen-induced liver injury in mice

OBJECTIVES

The hepatotoxicity of acetaminophen is not a result of the parent compound but is mediated by its reactive metabolite N-acetyl-p-benzoquinone imine. Cytochrome P4502E1 (CYP2E1) is the principal enzyme of this biotransformation, which accounts for approximately 52% of the bioactivation in human microsomes. Recently, chlormethiazole a sedative drug, is reported to be an efficient inhibitor of CYP2E1 activity in human beings. In this study we wished to evaluate whether chlormethiazole, an inhibitor of CYP2E1, could prevent acetaminophen-induced liver injury in mice.

METHODS

Acetaminophen, at doses ranging from 200 to 600 mg/kg, was injected into the peritoneum of female C57BL/6 inbred mice fasted for four hours. Chlormethiazole (60 mg/kg) or 5% dextrose water was given 30 min before or 2 h after acetaminophen. Serum aminotransferase activities, histologic index score, survival rate and hepatic malondialdehyde levels were compared.

RESULTS

Pretreatment with chlormethiazole 30 min before 400 mg/kg of acetaminophen completely inhibited acetaminophen-induced liver injury (median 118.5 U/L, range 75 to 142 vs. 14,070 U/L, range 5980 to 27,680 for AST; 49 U/L, range 41 to 64 vs. 15,330 U/L, range 13,920 to 15,940 for ALT). In mice receiving chlormethiazole 2 h after acetaminophen, the mean AST and ALT levels were also less elevated, reaching only 20% of the value of acetaminophen-only group. These protective effects were confirmed histologically. Whereas more than 50% of mice died at 500 mg/kg of acetaminophen, all the mice pretreated with chlormethiazole survived at the same dose.

CONCLUSION

Chlormethiazole effectively reduces acetaminophen-induced liver injury in mice. Further studies are needed to assess its role in humans.

Drug interactions with tobacco smoking. An update

Cigarette smoking remains highly prevalent in most countries. It can affect drug therapy by both pharmacokinetic and pharmacodynamic mechanisms. Enzymes induced by tobacco smoking may also increase the risk of cancer by enhancing the metabolic activation of carcinogens. Polycyclic aromatic hydrocarbons in tobacco smoke are believed to be responsible for the induction of cytochrome P450 (CYP) 1A1, CYP1A2 and possibly CYP2E1, CYP1A1 is primarily an extrahepatic enzyme found in lung and placenta. There are genetic polymorphisms in the inducibility of CYP1A1, with some evidence that high inducibility is more common in patients with lung cancer. CYP1A2 is a hepatic enzyme responsible for the metabolism of a number of drugs and activation of some procarcinogens. Caffeine demethylation, using blood clearance or urine metabolite data, has been used as an in vivo marker of CYP1A2 activity, clearly demonstrating an effect of cigarette smoking, CYP2E1 metabolises a number of drugs as well as activating some carcinogens. Our laboratory has found in an intraindividual study that cigarette smoking significantly enhances CYP2E1 activity as measured by the clearance of chlorzoxazone. In animal studies, nicotine induces the activity of several enzymes, including CYP2E1, CYP2A1/2A2 and CYP2B1/2B2, in the brain, but whether this effect is clinically significant is unknown. Similarly, although inhibitory effects of the smoke constituents carbon monoxide and cadmium on CYP enzymes have been observed in vitro and in animal studies, the relevance of this inhibition to humans has not yet been established. The mechanism involved in most interactions between cigarette smoking and drugs involves the induction of metabolism. Drugs for which induced metabolism because of cigarette smoking may have clinical consequence include theophylline, caffeine, tacrine, imipramine, haloperidol, pentazocine, propranolol, flecainide and estradiol. Cigarette smoking results in faster clearance of heparin, possibly related to smoking-related activation of thrombosis with enhanced heparin binding to antithrombin III. Cutaneous vasoconstriction by nicotine may slow the rate of insulin absorption after subcutaneous administration. Pharmacodynamic interactions have also been described. Cigarette smoking is associated with a lesser magnitude of blood pressure and heart rate lowering during treatment with beta-blockers, less sedation from benzodiazepines and less analgesia from some opioids, most likely reflecting the effects of the stimulant actions of nicotine. The impact of cigarette smoking needs to be considered in planning and assessing responses to drug therapy. Cigarette smoking should be specifically studied in clinical trials of new drugs.

Acetaminophen hepatotoxicity: An update

Acetaminophen is a widely used nonprescription analgesic and antipyretic agent. It is also a dose-related hepatotoxin that can cause fulminant liver failure when taken in massive overdoses or, much less commonly, at therapeutic doses in susceptible individuals. Persons who regularly consume alcohol or persons who have been fasting may be more susceptible to this hepatotoxicity. This liver injury is due not to the drug itself but to the formation of the toxic metabolite N-acetyl-p-benzoquinine imine generated through the cytochrome P-450 drug-metabolizing system. Normally, hepatic stores of glutathione combine with the toxic metabolite and prevent liver cell injury. When glutathione stores are depleted by overproduction of this metabolite, however, the reactive metabolite binds to liver cell proteins and causes hepatic necrosis. P-450 2E1 is induced by alcohol consumption and possibly starvation, and glutathione depletion can occur due to the inadequate nutrition occurring in chronic alcohol use or in starvation. Recent studies have shown that activated Kupffer cells and their secreted toxic agents such as cytokines may also play a role in this liver injury. This liver injury is characterized by extremely high levels of serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) (> 1000), and bad prognostic signs include severe prolongation of the prothrombin time, renal dysfunction, and, most importantly, acidosis. N-acetylcysteine is a highly effective antidote when given early (within 15 hours) of overdose. Some patients may develop such fulminant liver injury that they require transplantation. Unfortunately, many such patients have a course so rapid that a donor liver may not become available in time. Thus, both the medical community and the general public require a heightened understanding of this clinical problem in order to initiate prevention measures and to implement early therapeutic measures if an overdose situation occurs.

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

In studies designed to simulate a clinical observation in which an individual became tolerant to normally lethal doses of acetaminophen (APAP), mice were pretreated with increasing doses of APAP for 8 days and challenged on day 9 with normally supralethal doses of APAP. These animals developed minimal hepatotoxicity after a challenge dose with a fourfold increase in LD50 to 1,350 mg/kg. The pretreatment regimen resulted in hepatic changes including: centrilobular localization of 3-(cysteine-S-yl)APAP protein adducts, selective down-regulation of cytochrome P4502E1 (CYP2E1) and CYP1A2 that produced the toxic metabolite, N-acetyl-p-benzoquinone imine, higher levels of reduced glutathione (GSH), centrilobular inflammation, and a fourfold increase in hepatocellular proliferation. The protection against the lethal APAP doses afforded by pretreatment is secondary to these changes and to the associated regional shift in the bioactivation of the APAP challenge dose from centrilobular to periportal regions where CYP2E1 is not found, protective GSH is more abundant, and where cell-proliferative responses are better able to sustain repair. This shift in APAP bioactivation results in less-intense covalent binding that is more diffuse and spread uniformly throughout the hepatic lobe, most likely contributing to protection by delaying the early onset of liver injury that has been generally associated with centrilobular localization of the adducts. Intervention of APAP pretreatment-induced cell division in mice with colchicine left them resistant to a 500-mg/kg (normally lethal) dose of APAP, but unable to survive a 1,000-mg/kg APAP challenge dose. The data demonstrate multiple mechanistic components to the protection afforded by APAP pretreatment. Whereas metabolic and physiological changes not dependent on cell proliferation are adequate to protect against 500 mg/kg APAP, these changes plus a potentiated cell-proliferative response are necessary for protection against the supralethal 1,000-mg/kg APAP dose. Furthermore, the data document an uncoupling of the traditional association between covalent binding and toxicity, and suggest that the assessment of toxicity following repeated or chronic APAP exposure must consider altered drug interactions and parameters besides those historically used to assess acute APAP overdose.

Pharmacogenomics - it's not just pharmacogenetics

New technologies in both combinatorial chemistry and combinatorial biology promise to unlock new opportunities for drug discovery and lead optimisation. Using such genome-based technologies to measure the dynamic properties of pharmacological systems, pharmacogenomics can now provide an objective measure of a drug's biological efficacy, including its potential adverse effects.

Protection against acetaminophen toxicity in CYP1A2 and CYP2E1 double-null mice

Acetaminophen (APAP) hepatotoxicity is due to its biotransformation to a reactive metabolite, N-acetyl-p-benzoquinone imine (NAPQI), that is capable of binding to cellular macromolecules. At least two forms of cytochrome P450, CYP2E1 and CYP1A2, have been implicated in this reaction in mice. To test the combined roles of CYP1A2 and CYP2E1 in an intact animal model, a double-null mouse line lacking functional expression of CYP1A2 and CYP2E1 was produced by cross-breeding Cyp1a2-/- mice with Cyp2e1-/- mice. Animals deficient in the expression of both P450s developed normally and exhibited no obvious phenotypic abnormalities. Comparison of the dose-response to APAP (200-1200 mg/kg) indicated that double-null animals were highly resistant to APAP-induced toxicity whereas the wild-type animals were sensitive. Administration of 600 to 800 mg/kg of this drug to male wild-type animals resulted in increased plasma concentrations of liver enzymes (alanine aminotransferase, sorbitol dehydrogenase), lipidosis, hepatic necrosis, and renal tubular necrosis. In contrast, when APAP of equivalent or higher dose was administered to the double-null mice, plasma levels of liver enzymes and liver histopathology were normal. However, administration of 1200 mg of APAP/kg to the double-null mice resulted in infrequent liver lipidosis and mild kidney lesions. Consistent with the protection from hepatotoxicity, the expected depletion of hepatic glutathione (GSH) content was significantly retarded and APAP covalent binding to hepatic cytosolic proteins was not detectable in the double-null mice. Likewise, in vitro activation of APAP by liver microsomes from the double-null mice was approximately one tenth of that in microsomes from wild-type mice. Thus, the protection against APAP toxicity afforded by deletion of both CYP2E1 and CYP1A2 likely reflects greatly diminished production of the toxic electrophile, NAPQI.

Prevention of acetaminophen-induced cataract by a combination of diallyl disulfide and N-acetylcysteine

Injection of acetaminophen (APAP) (350 mg/kg body weight) into C57BL/6 mice in which cytochrome P450 (CYP) 1A1/1A2 had been induced produced acute cataract and other ocular tissue damage. Treatment of APAP-injected mice with one of the major organosulfides in garlic oil, diallyl disulfide (DADS) (200 mg/kg body weight), prevented cataract development and prolonged survival time. N-acetyl L-cysteine (NAC) (500 mg/kg body weight), a prodrug that stimulates glutathione synthesis, also prolonged survival time but was effective only weakly to prevent cataract formation. A combination of DADS and NAC completely prevented cataractogenesis, and all of the treated animals survived APAP toxicity. Neither DADS nor NAC inhibited CYP 1A1/1A2 induction as determined by their effect on the induction of hepatic microsomal ethoxyresorufin O-dealkylase (ERD) activity. However, in the in vitro enzyme assay, DADS, but not NAC, was a potent inhibitor of ERD activity (IC50 = 3.5 mM). Treatment with DADS or NAC slowed but did not stop the decrease of hepatic glutathione (GSH) content. At 4 hours after APAP injection, hepatic GSH began to increase only when DADS and NAC were administered together. These results suggest that the protective effect of DADS is due to its inhibition of biotransformation of APAP to the reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI) by CYP 1A1/1A2 enzymes and that NAC provides protection by increasing cellular cysteine level and GSH synthesis, thus facilitating detoxification of NAPQI by glutathione conjugation. Assay of plasma glutamate-pyruvate transaminase activity, an indicator of liver necrosis, showed that treatment with DADS and NAC together effectively protected the liver. Therefore, the decrease of GSH as much as 30% of normal concentration, by itself, is not responsible for liver damage. The primary cause of hepatic necrosis is rapid accumulation of NAPQI.

Inhibition of chlorzoxazone metabolism, a clinical probe for CYP2E1, by a single ingestion of watercress

To investigate the effect of watercress on the metabolism of chlorzoxazone, an in vivo probe for CYP2E1, the oral pharmacokinetics of chlorzoxazone was studied in 10 healthy volunteers before and after a single ingestion of a watercress homogenate (50 gm). A third chlorzoxazone pharmacokinetic study was performed after a 1-week treatment with isoniazid (300 mg/day), a well-known CYP2E1 inhibitor. Ingestion of watercress or isoniazid did not affect the oral absorption of chlorzoxazone. The area under the chlorzoxazone plasma concentration-time curve was significantly increased by 56% (p < 0.05) after watercress ingestion and by 135% (p < 0.001) with isoniazid treatment. Similarly, chlorzoxazone elimination half-life was prolonged after watercress (53%; p < 0.05) and isoniazid (104%; p < 0.01) administration. These results show that a single ingestion of watercress inhibits the hydroxylation of chlorzoxazone, an in vivo probe for CYP2E1.

Induction of cytochrome P4503A by taxol in primary cultures of human hepatocytes

In primary cultures of human hepatocytes, paclitaxel (Taxol), at pharmacological concentrations, was demonstrated to induce immunoreactive cytochrome P4503A (CYP3A). The magnitude of the inductive response of the hepatocytes to Taxol varied in five separate cultures. In general, exposure to increasing concentrations of Taxol (0.2 to 10 microM) resulted in increases in immunoreactive CYP3A. In four of the cultures, treatment of hepatocytes with the lowest concentration of Taxol tested (0.2 microM) resulted in approximately two-fold increases in CYP3A. In the other culture, however, a six-fold increase in CYP3A was observed at 0.2 microM. Taxol was almost as effective as rifampicin in inducing CYP3A in two of the cultures, but less effective than rifampicin in two other cultures. CYP3A4 mRNA was increased by Taxol. Increases in CYP3A4 mRNA correlated with increases in the levels of immunoreactive CYP3A. These results demonstrate that Taxol is a potent inducer of CYP3A in human hepatocytes. The clinical significance of these findings is discussed.

Temporal variation in hepatotoxicity and metabolism of acetaminophen in mice

Temporal variation in metabolism and hepatotoxicity of acetaminophen (APAP) was examined using male ICR mice. Animals were injected with a single dose of APAP (400 mg/kg, i.p.) at 08:00, 14:00 or 20:00 h. APAP at this dose was markedly hepatotoxic to mice when administered at 20:00 h as determined by increases in serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities, and by decreases in hepatic glucose-6-phosphatase (G-6-Pase) activity. However, mice appeared to be entirely insensitive to an identical dose of APAP given either at 08:00 or 14:00 h. Hepatic glutathione (GSH) level was significantly higher at 08:00, but no difference in GSH levels between 14:00 and 20:00 h was observed in normal mice. APAP and its metabolites in blood were monitored using HPLC for 3 h following the treatment. There were no significant differences in the plasma concentrations of APAP, APAP-glucuronide, APAP-sulfate, or APAP-mercapturate among the mice treated with this drug at 08:00, 14:00 or 20:00 h. However, the APAP-cysteine and APAP-GSH levels measured at 1 h following the APAP treatment were significantly lower in mice treated with this analgesic either at 14:00 or 20:00 h. In vitro hepatic microsomal p-nitrophenol hydroxylase activities were not different between 08:00, 14:00 and 20:00 h. But ethoxyresorufin O-deethylase and aminopyrine N-demethylase activities measured at 14:00 h were significantly lower than those of 08:00 or 20:00 h. Thus, the greater hepatotoxicity of APAP administered at 20:00 h appears to be related to the marked decrease in hepatic GSH at this time period, whereas the simultaneous reduction in APAP activation may be responsible for the lack of hepatotoxicity in mice treated with this analgesic at 14:00 h. These results suggest that the temporal variation in hepatotoxicity and metabolism of APAP is determined by interactions of multiple factors including the hepatic GSH level and drug metabolizing activities.

Use of transgenic animals in understanding molecular mechanisms of toxicity

Understanding molecular mechanisms of chemical toxicity and the potential risks of drugs to man is a pivotal part of the drug development process. With the dramatic increase in the number of new chemical entities arising from high throughput screening, there is an urgent need to develop systems for the rapid evaluation of potential drugs so that those agents which are most likely to be free of adverse effects can be identified at the earliest possible stage in drug development. The complex mechanisms of action of chemical toxins has made it extremely difficult to evaluate the precise toxic mechanism and also the relative role of specific genes in either potentiating or ameliorating the toxic effect. This problem can be addressed by the application of genetic strategies. Such strategies can exploit strain differences in susceptibility to specific toxic agents or, with the rapidly developing technologies, can exploit the use of transgenic animals where specific genes can be manipulated and subsequent effects on chemical toxicity evaluated. Transgenic animals can be exploited in a variety of ways to understand mechanisms of chemical toxicity. For example, a human gene encoding a drug metabolizing enzyme can be directly introduced and the effects on toxic response evaluated. Alternatively, specific genes can be deleted from the mouse genome and the consequences on toxicological response determined. Many toxic chemical agents modulate patterns of gene expression within target cells. This can be used to screen for responses to different types of toxic insult. In such experiments the promotor of a stress-regulated gene can be ligated to a suitable reporter gene, such as lacZ, or green fluorescent protein, and inserted into the genome of an appropriate test species. On administration of a chemical agent, cells which are sensitive to the toxic effects of that chemical will express the reporter, which can then be identified using an appropriate assay system. This latter strategy provides the potential for screening a large number of compounds rapidly for their potential toxic effects and also provides information on tissue and cellular specificity. Experiments using transgenic animals can be complex, and care must be taken to ensure that the results are not affected by background activities within the species being used. For example, the introduction of a specific human cytochrome P450 gene may have no effect on the metabolic disposition of a drug or toxin because of the background activity within the mouse. As the toxicity of a chemical agent is determined by a wide range of different factors including drug uptake, metabolism, detoxification and repair, differences between man and the species being used could potentially generate a toxic response in the animal model whereas no toxicity may be observed in man. In spite of these confounding factors, the application of transgenic animals to toxicological issues has enormous potential for speeding up the drug discovery process and will undoubtedly become part of this process in the future.

Role of CYP2A5 and 2G1 in acetaminophen metabolism and toxicity in the olfactory mucosa of the Cyp1a2(-/-) mouse

Acetaminophen (AP) is a widely-used analgesic agent that has been linked to human liver and kidney disease with prolonged or high-dose usage. In rodents, the target organs that are affected include liver, kidney, and the olfactory mucosa. AP toxicity requires cytochrome P450(CYP)-mediated metabolic activation, and the isozymes CYP1A2, 2E1, and 3A are known to activate AP in the human. In the present study, we determined that olfactory mucosal toxicity of AP was not different between the Cyp1a2(+/+) wild-type and the Cyp1a2(-/-) knockout mouse, whereas the hepatic toxicity of AP was significantly diminished in Cyp1a2(-/-) mice. Western blots of olfactory mucosa revealed that CYP2E1 and CYP3A levels are similar between untreated Cyp1a2(+/+) and Cyp1a2(-/-) mice. Diallyl sulfide (DAS), a known inhibitor of CYP2E1 and of CYP2A10/2A11 (the rabbit orthologue of mouse CYP2A5), completely eliminated olfactory toxicity of AP in both the Cyp1a2(-/-) and wild-type mouse olfactory mucosa. We found that heterologously expressed mouse CYP2A5 and CYP2G1 enzymes (known to be present in olfactory mucosa) form 3-hydroxyacetaminophen (3-OH-AP) and 3-(glutathion-S-yl)acetaminophen (GS-AP); CYP2A5 is considerably more active than 2G1. Addition of GSH caused increases in GS-AP proportional to decreases in 3-OH-AP, suggesting that these two metabolites arise from a common precursor or are formed by way of competing pathways. We also found that both CYP2A5 and CYP2G1 are inhibitable by DAS in vitro. These studies provide strong evidence that, in addition to CYP2E1, CYP2A5 and 2G1 are important in AP bioactivation in the mouse olfactory mucosa and that CYP1A2 appears to be of minor importance for AP olfactory toxicity.

CYP3A4-mediated oxidation of lisofylline to lisofylline 4,5-diol in human liver microsomes

The cytochrome P450s responsible for the conversion of lisofylline, a drug being developed to prevent the complications of high-dose chemotherapy, to lisofylline 4,5-diol, one of two principal metabolites in human liver microsomes, were evaluated. Lisofylline diol formation in microsomes prepared from five adult human livers was biphasic, with respective Km values of 0.0230+/-0.015 and 4.23+/-2.8 mM (mean +/- SD) and respective Vmax values of 0.0565+/-0.052 and 0.429+/-0.15 nmol/min/mg of protein. Through studies with isoform selective chemical inhibitors, CYP3A4 was implicated as the low Km enzyme from 89.0+/-11.2% inhibition of lisofylline 4,5-diol formation by troleandomycin at 50 microM substrate and CYP2A6 was implicated as the high Km enzyme. The formation of lisofylline 4,5-diol by these enzymes was confirmed with cDNA-expressed human CYP3A4 and CYP2A6.

Clinical and toxicological findings in two young siblings and autopsy findings in one sibling with multiple hospital admissions resulting in death. Evidence suggesting Munchausen syndrome by proxy

A 15-month-old girl underwent several emergency department (ED) visits and two admissions for parent-reported histories of ingestions, apnea, and seizures. She was initially admitted following reports of several unusual episodes of syncope accompanied by convulsive movements and was discharged on mephobarbital with a diagnosis of atypical seizure disorder. The day after discharge, she was brought to the ED in cardiopulmonary arrest and was resuscitated after a prolonged period. She was declared brain dead 2 days later. Ante- and postmortem toxicology produced several inconclusive findings, none of which explained death. Autopsy findings, including neuropathology, failed to demonstrate any significant disease processes. Approximately 3 months later, a 4-month-old female sibling was brought to the ED with a parent-reported history of apnea and seizures similar to the deceased child. A stool specimen obtained 2 days after admission contained numerous tiny seeds, which were found by gas chromatography-mass spectrometry analysis to contain lorazepam and temazepam. The role of these benzodiazepines in the apnea episodes in this infant was unknown, but the presence of the seeds in such a young infant coupled with the parent's aberrant behavior, led to the tentative diagnosis of Munchausen syndrome by proxy. This diagnosis was strengthened when results from these studies persuaded legal authorities to remove the surviving sibling from the parents, resulting in an asymptomatic recovery.

The effect of omeprazole pretreatment on acetaminophen metabolism in rapid and slow metabolizers of S-mephenytoin

Omeprazole, a widely used and potent gastric proton pump inhibitor, induces cytochrome P450 (CYP) 1A2 in humans. Induction is most pronounced in slow metabolizers of S-mephenytoin because CYP2C19 (S-mephenytoin hydroxylase) is responsible for the elimination of omeprazole. Acetaminophen (INN, paracetamol), a widely used and effective analgesic and antipyretic agent, causes serious hepatic and renal toxicity at high doses by conversion of acetaminophen to the toxic intermediate N-acetyl-p-benzoquinone imine (NAPQI) through CYP1A2, CYP2E1, and CYP3A4. This study evaluated whether omeprazole pretreatment in five rapid and five slow metabolizers of S-mephenytoin could increase thioether (an estimate of NAPQI production) metabolite formation from acetaminophen. The results of this study show that, despite induction of CYP1A2 activity in slow metabolizers (a 75% increase in plasma clearance of caffeine), the formation of NAPQI from acetaminophen was not increased after 7 days of omeprazole administration (40 mg/day). This suggests that induction of CYP1A2 activity by omeprazole is unlikely to increase the risk of acetaminophen hepatotoxicity.

Delayed sequelae after acute overdoses or poisonings: cranial neuropathy related to ethylene glycol ingestion

A 31-year-old woman came to the hospital with breathlessness, confusion, and a refractory anion gap metabolic acidosis; acute renal failure subsequently developed. Her blood ethylene glycol concentration was 390 mg/L, and she was treated with an intravenous ethanol infusion and hemodialysis. During the tenth and eleventh day after admission bilateral seventh cranial nerve paralysis developed, as well as bilateral dysfunction of cranial nerves II, V, VIII, IX, X and XII. Magnetic resonance imaging of her head showed gadolinium enhancement of the fifth cranial nerve bilaterally and a communicating hydrocephalus. Over the subsequent 11 months she recovered full function of her cranial nerves V, VII, IX, X, and XII, and she had subjective clinical improvement to baseline function in cranial nerves II and VIII. This case serves to introduce a discussion of agents that cause delayed complications after their acute toxic ingestion.

Acetaminophen cytotoxicity in mouse eye: mitochondria in anterior tissues are the primary target

Acetaminophen (APAP) injected into C57BL/6 mice (cytochrome P450 inducer-responsive strain) that had been pretreated with b-naphthoflavone (BNF) produced ocular tissue damage, including cataract. Our previous histocytochemical studies showed that tissue damage spread in association with the flow of the aqueous humor and appeared first in the ciliary epithelium, followed by the iris and corneal endothelium and, finally, the lens. The neural retina, retinal pigmented epithelium and choroid remained unaffected. A close examination of the affected tissues indicated that mitochondria are the primary target of APAP cytotoxicity. In order to investigate whether the respiratory capacity of mitochondria is more sensitive to APAP cytotoxicity than mitochondrial morphology, we determined in this work the oxygen uptake by eye tissues dissected from BNF-pretreated and APAP-injected C57BL/6 mice. Oxygen uptake by the ciliary body/iris decreased about 60% at 90 min and 85% at 120 min after APAP administration. The oxygen uptake was inhibited about 50% by 10 microM rotenone. Since the earliest sign of mitochondrial damage was noted at 120 min, the result indicates that mitochondrial energy dysfunction precedes morphological alterations. It was also observed that oxygen uptake by the retina remained unaffected at least for 120 min after APAP administration; therefore, it is evident that the retina and, possibly, other posterior tissues as well are resistant to APAP cytotoxicity, not only in their morphology but, also, in their capacity of mitochondrial energy metabolism.

Expression of enzymatically active CYP3A4 by Caco-2 cells grown on extracellular matrix-coated permeable supports in the presence of 1alpha,25-dihydroxyvitamin D3

The human colon carcinoma cell line, Caco-2, is widely used as a model for oral absorption of xenobiotics. The usefulness of Caco-2 cells has been limited, however, because they do not express appreciable quantities of CYP3A4, the principle cytochrome P450 present in human small bowel epithelial cells. We report that treatment of Caco-2 cells with 1 alpha,25-dihydroxyvitamin D3, beginning at confluence, results in a dose- and duration-dependent increase in CYP3A4 mRNA and protein, with little apparent effect on the expression of CYP3A5 or CYP3A7. This treatment also results in increases in NADPH cytochrome P450 reductase and P-glycoprotein (the MDR1 gene product) but has no detectable effect on expression of CYP1A1, CYP2D6, cytochrome b5, liver or intestinal fatty acid binding proteins, or villin. Maximal expression of CYP3A4 requires an extracellular matrix on a permeable support and the presence of serum. In the treated cells, the intrinsic formation clearance of 1'-hydroxymidazolam (a reaction characteristically catalyzed by CYP3A enzymes) was estimated to be somewhat lower than that of human jejunal mucosa (1.14 and 3.67 ml/min/g of cells, respectively). The 1'-OH-midazolam/4-OH-midazolam product ratio produced by the cells (approximately 5.3) is comparable to, but somewhat lower than, that observed in human jejunal microsomes (7.4-15.4), which may reflect the presence of CYP3A7 in the Caco-2 cells. 25-Hydroxyvitamin D3 is less efficacious but reproduces the effects of the dihydroxy compound, whereas unhydroxylated vitamin D is without appreciable effect. These observations, together with the time course of response, suggest that the vitamin D receptor may be involved in CYP3A4 regulation. The culture model we describe should prove useful in defining the role of CYP3A4 in limiting the oral bioavailability of many xenobiotics.

Role of CYP3A in ethanol-mediated increases in acetaminophen hepatotoxicity

CYP2E is considered the only form of cytochrome P450 responsible for ethanol-mediated increases in acetaminophen hepatotoxicity. However, in experimental systems used for investigating ethanol-mediated increases in acetaminophen hepatotoxicity, animals are withdrawn from ethanol for 16 to 24 hr before the administration of acetaminophen to ensure the clearance of ethanol from the circulation. In rats, CYP2E has been shown to decrease to control levels after this time period of withdrawal from ethanol. We have previously shown in cultured human and rat hepatocytes, and in intact rats, that ethanol induces CYP3A in addition to CYP2E. To determine if there might be a role for CYP3A in ethanol-mediated APAP hepatotoxicity in addition to the recognized role for CYP2E, we investigated the effect of triacetyloleandomycin (TAO) on acetaminophen hepatotoxicity in ethanol-pretreated rats, as well as the effect of 11 hr withdrawal from ethanol on hepatic levels of CYP3A and CYP2E. TAO was dissolved in saline instead of dimethylsulfoxide, the solvent most usually employed, since dimethylsulfoxide inhibits CYP2E. Rats were administered 6.3% ethanol as part of the Lieber-DeCarli diet for 7 days, followed by replacement of the liquid diet with water for 11 hr. This 11-hr withdrawal from ethanol resulted in a decrease in hepatic levels of ethanol-induced CYP2E; however, considerable induction was still evident. There was no significant decrease in CYP3A. TAO completely prevented the histologically observed liver damage from acetaminophen in ethanol-pretreated rats, but did not prevent the increase in serum levels of AST. In ethanol-pretreated rats, exposure to APAP in the absence of TAO was associated with a 75% decrease in CYP3A, compared to animals exposed to APAP in the presence of TAO. These results suggest that CYP3A may have been suicidally inactivated by acetaminophen in the absence of TAO. Our findings suggest that CYP3A has a major role in ethanol-mediated increases in acetaminophen hepatotoxicity.

Catalytic characteristics of CYP3A4: requirement for a phenolic function in ortho hydroxylation of estradiol and mono-O-demethylated methoxychlor

CYP3A4 is the major human cytochrome P-450 in a superfamily of heme-thiolate proteins that catalyze the oxidation of numerous lipophilic compounds. In this investigation, we report that CYP3A4 requires a phenolic function for ortho hydroxylation of estradiol and mono-O-demethylated methoxychlor and that CYP3A4 aromatic hydroxylation in general may be dependent on the presence of a free phenolic group. Indeed, when methoxyls were present instead of phenolic hydroxyls, CYP3A4 essentially failed to catalyze ortho hydroxylation. By contrast, of eight additional cDNA-expressed P-450s (CYP1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2D6, and 2E1) examined, only CYP1A2 and CYP2B6 could catalyze ortho hydroxylation of [o-3H]methoxychlor (7.2 and 14.6 pmol/90 min/pmol P-450, respectively), indicating that these isoforms do not require a phenolic hydroxyl for aromatic hydroxylation and that methoxyls do not sterically hinder catalysis by these CYPs. However, with [o-3H]mono-O-demethylated methoxychlor, containing a phenolic group, five isoforms (CYP1A2, 2B6, 2D6, 2E1, and 3A4) supported ortho hydroxylation. Of these, CYP3A4 exhibited by far the highest rate of hydroxylation at 87.8 pmol/90 min/pmol P-450. Further studies with [2-(3)H]estradiol 3-methyl ether and with [2-(3)H]estradiol revealed a similar and dramatic augmentation of CYP3A4-mediated C2 hydroxylase activity of approximately 75-fold by the presence of the phenolic group in the 3-position. The mechanism of augmentation by the phenolic hydroxyl does not appear to involve the acidic proton of estradiol, since CYP3A4-catalyzed estradiol 2-hydroxylation and testosterone 6-beta-hydroxylation were diminished to an equal extent when incubations were performed at increasing buffer pH values from 7 to 9. Both estradiol and its 3-methoxy derivative bound with similar affinity to cDNA-expressed, microsomal CYP3A4: spectral dissociation constants were 270 and 370 microM, respectively, and both compounds exhibited type I spectra. Thus, the disparities in aromatic hydroxylation rates between compounds containing phenolic hydroxyls and those with methoxyls cannot be explained by differences in their binding affinities. To explain the mode via which the phenolic hydroxyl facilitates ortho hydroxylation, a mechanism in which the phenolic moiety attacks the iron-oxo double bond of CYP3A4, resulting in oxygen transfer to the ortho position, is proposed. It is anticipated that these findings will assist in forecasting the CYP-mediated metabolic fate of phenolic compounds.

Selective protein arylation and acetaminophen-induced hepatotoxicity

More than 20 years have passed since the early reports of acute hepatotoxicity with APAP overdose. During that period investigative research to discover the "mechanism" underlying the toxicity has been conducted in many species and strains of intact animals as well as in a variety of in vitro and culture systems. Such work has clarified the primary role of biotransformation and the protective role of GSH. Understanding the former provides explanations for the toxic interactions which may occur with alcohol or other xenobiotics, while understanding of the latter led to the development of antidotes for the treatment of acute poisoning. Acetaminophen (APAP)-induced hepatotoxicity: roles for protein arylation. Initiating events in toxicity require biotransformation of APAP to NAPQI followed by arylation of several important proteins with subsequent alteration of protein structure and function. The immediate consequence of the alterations is detectable in several organelles and these may represent multiple initiating events which are depicted as acting in concert to cause cell injury (large arrowheads). Arylation of cytosolic 58-ABP with subsequent translocation to the nucleus is depicted as a possible signaling mechanism for determining outcome at the cell or organ level (within dotted boundary). For simplicity NAPQI's potentials for oxidizing protein sulfhydryls and direct binding to DNA have been omitted. Significant light has also been shed on the biochemical and cellular events which accompany APAP-induced hepatotoxicity. However, such studies have not identified a unique mechanism of toxicity that is universally accepted. The recent identification of several protein targets which become arylated during toxicity--along with the findings that arylation of some of those target proteins results in loss of protein function--demonstrates that covalent binding does, indeed, have biological consequences and is not merely an indicator of the fleeting presence of reactive electrophiles. These observations further suggest that multiple independent insults to the cell may be involved in toxicity. it is now apparent that the concept of a multistage process that involves both initiation and progression events is appropriate for APAP toxicity, and it is unlikely that a unique initiating event will ever be identified. In light of recent findings it is more likely that a number of such cellular events occur very early after toxic overdosage, and that they collectively set in motion and perpetuate the biochemical, cellular, and molecular processes which will determine outcome. The importance of 58-ABP arylation with early, apparently selective, translocation to the nucleus remains to be elucidated. To date there is nothing to suggest that this represents an initiating event in toxicity. rather it is plausible that the translocation may play a role in signaling electrophile presence and in calling for cellular defense against electrophile insult. This is reflected in the hypothetical model presented in Fig. 3. Critical experimental testing of this model will advance our understanding of the cellular and molecular responses to toxic electrophile insult.

Rifampicin and isoniazid increase acetaminophen and isoniazid cytotoxicity in human HepG2 hepatoma cells

Acetaminophen (APAP) induced a concentration-dependent (0-30 mM) cytotoxic effect in human HepG2 hepatoma cells which was significantly increased when intracellular reduced glutathione (GSH) content was decreased. The cytotoxic effect of APAP (0-30 mM) was significantly lower in a day 3-treated compared to day 1-treated HepG2 cells. A 3-day preincubation of HepG2 cells with 5 microM 3-methylcholanthrene (3MC), 50 microM rifampicin (RFP) or 1 mM isoniazid (INH) significantly increased 15-30 mM APAP cytotoxicity, of about 15-20% for INH and RFP and 35-50% for 3MC. The cytotoxicity of 10 mM APAP was also increased (about 20%) by a 3-day preincubation with INH but was not affected by 3MC and RFP. INH induced a concentration-dependent (0-40 mM) cytotoxic effect in day-1 treated HepG2 cells and not significantly affected by decreases in intracellular GSH concentrations. INH was not cytotoxic in day 3-treated HepG2 cells. A 3-day preincubation of HepG2 cells with 50 mM RFP or 1 mM INH significantly increased 10-40 mM INH cytotoxicity, respectively of about 10% and 10-25%. A 3-day preincubation with 3MC did not modify the cytotoxic effect of INH at these concentrations. This is to our knowledge the first report of increases by INH and RFP of APAP of INH cytotoxicity in vitro in hepatocellular cells of human origin. It is in accordance with clinical observations of severe hepatotoxicity associated with APAP or INH usage in patients receiving multiple drug therapy (INH, RFP) for tuberculosis or in alcoholics.

Protein targets of xenobiotic reactive intermediates

Many xenobiotics are metabolically activated to electrophilic intermediates that form covalent adducts with proteins; the mechanism of toxicity is either intrinsic or idiosyncratic in nature. Many intrinsic toxins covalently modify cellular proteins and somehow initiate a sequence of events that leads to toxicity. Major protein adducts of several intrinsic toxins have been identified and demonstrate significant decreases in enzymatic activity. The reactivity of intermediates and subcellular localization of major targets may be important in the toxicity. Idiosyncratic toxicities are mediated through either a metabolic or immune-mediated mechanism. Xenobiotics that cause hypersensitivity/autoimmunity appear to have a limited number of protein targets, which are localized within the subcellular fraction where the electrophile is produced, are highly substituted, and are accessible to the immune system. Metabolic idiosyncratic toxins appear to have limited targets and are localized within a specific subcellular fraction. Identification of protein targets has given us insights into mechanisms of xenobiotic toxicity.

Influence of polymorphic N-acetyltransferase phenotype on the inhibition and induction of acetaminophen bioactivation with long-term isoniazid

OBJECTIVE

To determine in patients receiving isoniazid prophylaxis whether an increase in the CYP2E1 dependent formation clearance of acetaminophen (paracetamol) to N-acetyl-p-benzoquinone imine (NAPQI) occurs during a normal 24-hour isoniazid dose interval and whether the interaction is dependent on acetylation status.

METHODS

Acetaminophen elimination kinetics were determined on four different occasions. Ten subjects were assigned to receive acetaminophen either simultaneously with the 8 am dose of isoniazid or 12 hours after the isoniazid dose. One week later, on the last day of isoniazid therapy, subjects received acetaminophen at the alternate time of day. The control phase acetaminophen administrations were repeated 1 and 2 weeks later, following the initial randomization. Isoniazid acetylation (NAT2) genotype was determined by analysis of genomic DNA obtained from peripheral blood leukocytes.

RESULTS

The mean NAPQI formation clearance was inhibited 57% when acetaminophen and isoniazid were coadministered but was unchanged compared with time-matched control when acetaminophen was given 12 hours after the isoniazid dose. However, when data from subjects was segregated according to isoniazid (INH) acetylation phenotype, the mean ratio of NAPQI formation clearances (+INH/-INH) with 8 PM acetaminophen was significantly higher for fast acetylators compared with slow acetylators (1.36 versus 0.68; p = 0.006).

CONCLUSIONS

Fast metabolizers of isoniazid appeared to clear the inducer or inhibitor from the active site of CYP2E1 more rapidly, which resulted in an increased formation of NAPQI 12 hours after the isoniazid dose. In contrast, formation of NAPQI for slow isoniazid metabolizers remained inhibited.

Decrease of plasma and urinary oxidative metabolites of acetaminophen after consumption of watercress by human volunteers

To investigate the effect of the consumption of watercress (Nasturtium officinale R. Br.), a cruciferous vegetable, on acetaminophen metabolism, the pharmacokinetics of acetaminophen and its metabolites were studied in a crossover trial of human volunteers. A single oral dose of acetaminophen (1 gm) was given 10 hours after ingestion of watercress homogenates (50 gm). In comparison with acetaminophen only, the ingestion of watercress resulted in a significant reduction in the area under the plasma cysteine acetaminophen (Cys-acetaminophen) concentration-time curve and in the peak plasma Cys-acetaminophen concentration by 28% +/- 3% and by 21% +/- 4% (mean +/- SE; n = 7; p < 0.005), respectively. Correspondingly, the Cys-acetaminophen formation rate constant and Cys-acetaminophen formation fraction were decreased by 55% +/- 9% and 52% +/- 7% (p < 0.01), respectively. Consistent with the results obtained from the plasma, the total urinary excretion of Cys-acetaminophen in 24 hours was also reduced. A decrease of mercapturate acetaminophen, a Cys-acetaminophen metabolite, was also shown in the plasma and urine samples. However, the plasma pharmacokinetic processes and the urinary excretions of acetaminophen, acetaminophen glucuronide, and acetaminophen sulfate were not altered significantly by the watercress treatment. These results suggest that the consumption of watercress causes a decrease in the levels of oxidative metabolites of acetaminophen, probably due to inhibition of oxidative metabolism of this drug.

Protective effects of diallyl sulfide on acetaminophen-induced toxicities

Diallyl sulfide (DAS), a major flavour component of garlic, is known to modulate drug metabolism and may protect animals from chemically induced toxicity and carcinogenesis. In this study the effects of DAS on the oxidative metabolism and hepatotoxicity induced by acetaminophen (APAP) in rats were investigated. In the hepatotoxicity evaluation of Fischer 344 rats there was a dose-dependent increase in the odds of mortality rate by APAP (P = 0.009); DAS treatment significantly protected rats from APAP-related mortality (P = 0.026). Liver toxicity determined by lactate dehydrogenase activity was significantly increased by APAP treatment (0.75 g/kg). Pretreatment with DAS protected animals from APAP-induced liver toxicity in a time- and dose-dependent fashion. Treatment of DAS (50 mg/kg) 3 hr after APAP dosing significantly (P < 0.05) protected rats from APAP-induced liver toxicity. The metabolism of APAP (50 microM) in vitro was significantly inhibited by DAS (0.3-1 mM) in liver microsomes isolated from F344 rats. As the effect of DAS on APAP-induced hepatotoxicity in vivo was observed only when DAS was administered before or shortly after (< 3 hr) APAP dosing, data suggested that the protective effect of DAS is mainly at the metabolic activation step of APAP. However, the possibility that DAS may also have effects on other drug metabolism systems, such as glutathione (GSH) and glutathione S-transferases, cannot be ruled out.

Acetaminophen toxicity in children: diagnostic confirmation using a specific antigenic biomarker

Chronic acetaminophen (APAP) toxicity poses a difficult diagnostic challenge to the clinician. Signs and symptoms are nonspecific and no currently available laboratory study can confirm APAP as the causative agent of hepatic injury. In this study an antigenic biomarker for APAP toxicity was used to confirm the diagnosis of APAP-induced hepatic failure in two children with chronic APAP toxicity. APAP that has been metabolized to N-acetyl-benzoquinone imine (NAPQI) reacts with cellular proteins to form 3-(cystein-S-yl)-APAP protein adducts (3-Cys-A). Serum from both patients was submitted for quantitation of 3-Cys-A by a competitive inhibition enzyme-linked immunosorbent assay (ELISA). Concentrations of 3-Cys-A in the two patients were 1.97 and 2.77 nmol/mg protein, which are similar to concentrations found in adults with hepatic injury secondary to an overdose of APAP. Individuals with no exposure to APAP have no detectable 3-Cys-A in serum. It was concluded that 3-Cys-A is a useful marker of APAP intoxication after long-term ingestion of APAP when total dose and time course of ingestion are uncertain, and may prove to be a useful clinical and investigative tool in the study of APAP intoxication.

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.

Inhibition of sulfamethoxazole hydroxylamine formation by fluconazole in human liver microsomes and healthy volunteers

Sulfamethoxazole toxicity is putatively initiated by the formation of a hydroxylamine metabolite by cytochromes P450. If this reaction could be inhibited, toxicity may decrease. We have studied--in vitro and in vivo--fluconazole, ketoconazole, and cimetidine as potentially suitable clinical inhibitors of sulfamethoxazole hydroxylamine formation. Both fluconazole and ketoconazole in human liver microsomal incubations competitively inhibited sulfamethoxazole N-hydroxylation, with the inhibitory constant (Ki) values of 3.5 and 6 micromol/L, respectively. Cimetidine exhibited a mixed type of inhibition of sulfamethoxazole hydroxylamine formation in human liver microsomes, with IC 50 values (the concentration required to decrease hydroxylamine formation by 50%) of 80 and 800 micromol/L, the lower value being observed when cimetidine was preincubated with microsomes and reduced nicotinamide adenine dinucleotide phosphate. In an in vivo study in six healthy volunteers the inhibition of the cytochrome P450-mediated generation of the toxic metabolite in the presence of fluconazole was shown by a 94% decrease in the area under the plasma concentration-time curve of sulfamethoxazole hydroxylamine. In contrast, the recovery of hydroxylamine in urine decreased by only 60%. Total clearance of sulfamethoxazole was decreased by 26% by fluconazole, most likely because of the inhibition of unidentified P450 elimination pathways. There was close agreement between the predicted (87%) and observed inhibition (94%) of sulfamethoxazole hydroxylamine formation in vivo. Similarly, there was close agreement between in vivo and in vitro Ki values--1.6 and 3.5 micron/L, respectively.

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.