Mechanisms of ethanol-drug-nutrition interactions

Engineered Animal Models Designed for Investigating Ethanol Metabolism, Toxicity and Cancer

Excessive consumption of alcohol is a leading cause of lifestyle-induced morbidity and mortality worldwide. Although long-term alcohol abuse has been shown to be detrimental to the liver, brain and many other organs, our understanding of the exact molecular mechanisms by which this occurs is still limited. In tissues, ethanol is metabolized to acetaldehyde (mainly by alcohol dehydrogenase and cytochrome p450 2E1) and subsequently to acetic acid by aldehyde dehydrogenases. Intracellular generation of free radicals and depletion of the antioxidant glutathione (GSH) are believed to be key steps involved in the cellular pathogenic events caused by ethanol. With continued excessive alcohol consumption, further tissue damage can result from the production of cellular protein and DNA adducts caused by accumulating ethanol-derived aldehydes. Much of our understanding about the pathophysiological consequences of ethanol metabolism comes from genetically-engineered mouse models of ethanol-induced tissue injury. In this review, we provide an update on the current understanding of important mouse models in which ethanol-metabolizing and GSH-synthesizing enzymes have been manipulated to investigate alcohol-induced disease.

Lifetime alcohol intake is associated with an increased risk of KRAS+ and BRAF-/KRAS- but not BRAF+ colorectal cancer

Ethanol in alcoholic beverages is a causative agent for colorectal cancer. Colorectal cancer is a biologically heterogeneous disease, and molecular subtypes defined by the presence of somatic mutations in BRAF and KRAS are known to exist. We examined associations between lifetime alcohol intake and molecular and anatomic subtypes of colorectal cancer. We calculated usual alcohol intake for 10-year periods from age 20 using recalled frequency and quantity of beverage-specific consumption for 38,149 participants aged 40-69 years from the Melbourne Collaborative Cohort Study. Cox regression was performed to derive hazard ratios (HRs) and 95% confidence intervals (CIs) for the association between lifetime alcohol intake and colorectal cancer risk. Heterogeneity in the HRs across subtypes of colorectal cancer was assessed. A positive dose-dependent association between lifetime alcohol intake and overall colorectal cancer risk (mean follow-up = 14.6 years; n = 596 colon and n = 326 rectal cancer) was observed (HR = 1.08, 95% CI: 1.04-1.12 per 10 g/day increment). The risk was greater for rectal than colon cancer (p_homogeneity  = 0.02). Alcohol intake was associated with increased risks of KRAS+ (HR = 1.07, 95% CI: 1.00-1.15) and BRAF-/KRAS- (HR = 1.05, 95% CI: 1.00-1.11) but not BRAF+ tumors (HR = 0.89, 95% CI: 0.78-1.01; p_homogeneity  = 0.01). Alcohol intake is associated with an increased risk of KRAS+ and BRAF-/KRAS- tumors originating via specific molecular pathways including the traditional adenoma-carcinoma pathway but not with BRAF+ tumors originating via the serrated pathway. Therefore, limiting alcohol intake from a young age might reduce colorectal cancer originating via the traditional adenoma-carcinoma pathway.

Alcohol drinking as an unfavorable prognostic factor for male patients with nasopharyngeal carcinoma

The relationship between alcohol drinking and the prognosis of nasopharyngeal carcinoma (NPC) is unknown. To investigate the prognostic value of alcohol drinking on NPC, this retrospective study was conducted on 1923 male NPC patients. Patients were classified as current, former and non-drinkers according to their drinking status. Furthermore, they were categorized as heavy drinkers and mild/none drinkers based on the intensity and duration of alcohol drinking. Survival outcomes were compared using Kaplan–Meier analysis and Cox proportional hazards model. We found that current drinkers had significantly lower overall survival (OS) rate (5-year OS: 70.2% vs. 76.4%, P < 0.001) and locoregional recurrence-free survival (LRFS) rate (5-year LRFS: 69.3% vs. 77.5%, P < 0.001) compared with non-drinkers. Drinking ≥14 drinks/week, and drinking ≥20 years were both independent unfavorable prognostic factors for OS (hazard ratio [HR] = 1.38, 95% confidence interval [CI] 1.05–1.81, P = 0.022; HR = 1.38, 95% CI 1.09–1.75, P = 0.007). Stratified analyses further revealed that the negative impacts of alcohol were manifested mainly among older patients and among smokers. In conclusion, alcohol drinking is a useful predictor of prognosis in male NPC patients; drinkers, especially heavy drinkers have poorer prognosis.

The predicted impact of reducing the nicotine content in cigarettes on alcohol use

INTRODUCTION

Product standards reducing the level of nicotine in cigarettes could significantly improve public health by reducing smoking behavior and toxicant exposure. However, relatively little is known about how the regulatory strategy could impact alcohol use, a closely related health behavior that is also a major contributor to morbidity and mortality. The primary objective of this paper is to predict the effect of nicotine reduction on alcohol use, identify priorities for future research, and highlight areas for mitigating any adverse outcomes.

METHODS

We critically reviewed and integrated literatures examining the effects of very low nicotine content (VLNC) cigarettes on smoking-related outcomes (nicotine exposure, nicotine withdrawal, and smoking as a cue to drink) and, in turn, the effects of those outcomes on alcohol use.

RESULTS

Current evidence suggests reducing the nicotine content of cigarettes may benefit public health by reducing alcohol use and problematic drinking over time as a consequence of reduced exposure to nicotine and the smoking cues associated with drinking. Nicotine withdrawal could increase risk of drinking, although these effects should be short-lived and could be mitigated by other sources of nicotine. Gender, hazardous drinking, and psychiatric comorbidities are likely to be important moderators of the effects of VLNC cigarettes.

CONCLUSIONS

It is imperative to broadly assess the public health impact of potential tobacco product regulations by including measures of closely related health behaviors that could be impacted by these interventions. Nicotine reduction in cigarettes may contribute to improved public health through reductions in alcohol use.

Association between in vivo alcohol metabolism and genetic variation in pathways that metabolize the carbon skeleton of ethanol and NADH reoxidation in the alcohol challenge twin study

BACKGROUND

Variation in alcohol metabolism affects the duration of intoxication and alcohol use. While the majority of genetic association studies investigating variation in alcohol metabolism have focused on polymorphisms in alcohol or aldehyde dehydrogenases, we have now tested for association with genes in alternative metabolic pathways that catalyze the carbon skeleton of ethanol (EtOH) and NADH reoxidation.

METHODS

Nine hundred fifty single nucleotide polymorphisms (SNPs) spanning 14 genes (ACN9, ACSS1, ACSS2, ALDH1A1, CAT, CYP2E1, GOT1, GOT2, MDH1, MDH2, SLC25A10, SLC25A11, SLC25A12, SLC25A13) were genotyped in 352 young adults who participated in an alcohol challenge study. Traits tested were blood alcohol concentration (BAC), breath alcohol concentration (BrAC), peak alcohol concentration, and rates of alcohol absorption and elimination. Allelic association was tested using quantitative univariate and multivariate methods.

RESULTS

A CYP2E1 promoter SNP (rs4838767, minor allele frequency 0.008) exceeded the threshold for study-wide significance (4.01 × 10(-5) ) for 2 early BAC, 8 BrAC measures, and the peak BrAC. For each phenotype, the minor C allele was related to a lower alcohol concentration, most strongly for the fourth BrAC (p = 2.07 × 10(-7) ) explaining ~8% of the phenotypic variance. We also observed suggestive patterns of association with variants in ALDH1A1 and on chromosome 17 near SLC25A11 for aspects of blood and breath alcohol metabolism. An SNP upstream of GOT1 (rs2490286) reached study-wide significance for multivariate BAC metabolism (p = 0.000040).

CONCLUSIONS

Overall, we did not find strong evidence that variation in genes coding for proteins that further metabolize the carbon backbone of acetaldehyde, or contribute to mechanisms for regenerating NAD from NADH, affects alcohol metabolism in our European-descent subjects. However, based on the breath alcohol data, variation in the promoter of CYP2E1 may play a role in preabsorptive or early hepatic alcohol metabolism, but more samples are required to validate this finding.

Pharmacogenetics of drug dependence: role of gene variations in susceptibility and treatment

Drug dependency is a highly prevalent mental health disorder that imposes a significant burden on those directly affected, health care systems, and society in general. There is substantial heritability in the susceptibility to drug addiction, which indicates that there are genetic risk factors. Variation in the human genome is abundant and can directly affect drug dependency phenotypes, for example, by altering the function of a gene product or by altering gene expression. Pharmacogenetic studies can assess the effects of genetic variation on the risk for a particular phenotype (e.g., being an alcoholic). In addition, pharmacogenetic variability in treatment efficacy and adverse reactions can be investigated to identify particular genetic variants associated with altered responses. This review highlights examples of genetic variations that are important in the development and maintenance of specific drug dependencies as well as those that affect the response to treatment.

Differential induction of ethanol-metabolizing CYP2E1 and nicotine-metabolizing CYP2B1/2 in rat liver by chronic nicotine treatment and voluntary ethanol intake

Alcohol and nicotine are frequently co-used and co-abused, and use of both drugs alone can affect hepatic drug metabolism. We investigated the influences of chronic nicotine treatment and voluntary ethanol intake on the induction of rat hepatic cytochrome P450 (CYP) enzymes that metabolize ethanol and nicotine. Rats were trained to voluntarily drink ethanol (6% v/v, 1 h) with nicotine pretreatment for 10 days. Another group of rats were treated with the same nicotine doses alone. Hepatic CYP2E1, CYP2B1/2 and CYP2D1 proteins were assessed by immunoblotting. Nicotine pretreatment (0.4, 0.8 and 1.2 mg/kg) increased voluntary ethanol intake on day 10 by 1.8, 2.0, and 1.4 fold respectively compared to saline pretreatment (P<0.01-0.3). CYP2E1 was increased 1.7, 1.8, and 1.4 fold by the three doses of nicotine alone (P<0.02-0.21); CYP2E1 levels were increased by voluntary ethanol intake alone and a further 2.4, 2.2, and 1.8 fold by 0.4, 0.8, and 1.2 mg/kg nicotine respectively versus saline pretreatment (P<0.002-0.06). CYP2B1/2 proteins were not induced by nicotine alone, but were increased by 2.2-2.5 fold by ethanol drinking (P<0.05). CYP2E1 (r=0.67, P<0.001) and CYP2B1/2 levels (r=0.49, P=0.007) correlated with alcohol consumption on day 10. There was no change in CYP2D1. Chronic nicotine increased voluntary ethanol intake thereby enhancing CYP2E1 and CYP2B1/2 levels. Thus CYPs are regulated not only directly by nicotine and ethanol, but also indirectly via an increase in the ethanol consumption in the presence of nicotine pretreatment. Together this may contribute to the co-abuse of these drugs and alter the metabolism of clinical drugs and endogenous substrates.

Alcohol dehydrogenase-catalyzed in vitro oxidation of anandamide to N-arachidonoyl glycine, a lipid mediator: synthesis of N-acyl glycinals

N-Arachidonoyl ethanolamide or anandamide is an endocannabinoid found in most tissues where it acts as an important signaling mediator in a number of physiological and pathophysiological processes. Consequently, intense effort has been focused on understanding all its biosynthetic and metabolic pathways. Herein we report human alcohol dehydrogenase-catalyzed sequential oxidation of anandamide to N-arachidonoyl glycine, a prototypical member of the class of long chain fatty acyl glycines, a new group of lipid mediators with a wide array of physiological effects. We also present a straightforward synthesis for a series of N-acyl glycinals including N-arachidonoyl glycinal, an intermediate in the alcohol dehydrogenase-catalyzed oxidation of anandamide.

Role of cytochrome P450 in drug interactions

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues. Many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

A haplotype analysis of CYP2E1 polymorphisms in relation to alcoholic phenotypes in Mexican Americans

BACKGROUND

Studies regarding the association between the 4 polymorphisms of CYP2E1 (CYP2E1*1D, *5B, *6, and *1B) and alcoholism are inconsistent and inconclusive. The purpose of the present study was to clarify previously discordant studies by haplotype analysis in the Mexican American population.

METHODS

The 4 polymorphisms of CYP2E1 were studied in 334 alcoholics and 365 controls. Genotype, allele, and haplotype frequency comparisons between alcoholics and controls were assessed. Patterns of linkage disequilibrium (LD) at CYP2E1 were determined. Reconstructed haplotypes were tested for associations with clinical phenotypes (age onset of drinking, Maxdrinks, and smoking status).

RESULTS

No significant associations between the 4 polymorphisms of CYP2E1 and alcoholism were revealed by single allele tests. High LD was found between the CYP2E1 c2 and C alleles in Mexican Americans. Eleven haplotypes were present in the 699 participants. The 6 main haplotypes with frequencies higher than 1% made up 97% of the total halpotypes. The frequency of subjects carrying H6 (1C-c2-C-A2) was significantly higher in alcoholics than in controls (p = 0.0001). In contrast, the frequencies of H7 (1C-c2-C-A1) and H10 (1C-c2-D-A1) were significantly lower in alcoholics than in controls (p = 0.0072 for H7 and p = 0.0407 for H10). The frequency of H6 was significantly higher in alcoholics who had late onset of drinking than in nonalcoholic controls. Furthermore, the frequencies of H6 haplotype were also consistently higher in groups who had high number of maximum drinks (9 to 32 drinks) than in controls. When smokers are excluded, the frequencies of H6, H7, and H9 (1C-c2-D-A2) showed statistically significant differences between alcoholics and controls (p < 0.05). Moreover, the association between H6 and alcoholism become more robust when smokers are excluded. Furthermore, the frequency of H1 (1C-c1-D-A2) in alcoholic-smokers was much higher than in alcoholic-nonsmokers (p = 0.0028). In contrast, alcoholic-smokers carried less H2 (1C-c1-D-A1) in comparison with alcoholic-nonsmokers (p = 0.0417). The H3 (1D-c2-C-A2) frequency in alcoholic-smokers was much lower than in alcoholic-nonsmokers (p = 0.0042) and control-smokers (p = 0.0363).

CONCLUSIONS

Our data demonstrate that carrying haplotype H6 might enhance susceptibility to developing alcoholism, but possessing the H7 or H10 haplotype appears to decrease this susceptibility. The H6, H7, and H9 haplotypes may play certain roles in different clinical phenotypes in Mexican American alcoholics. In addition, our data suggest that the H1, H2, and H3 haplotypes are associated with alcohol drinking and smoking. These results support that haplotype analysis is much more informative than single allele analysis. Our findings clearly indicate the importance of H6 haplotype in alcohol drinking in Mexican Americans.

Metabolic interaction between ethanol, high-dose alprazolam and its two main metabolites using human liver microsomes in vitro

Alprazolam is widely used as a short-acting antidepressant and anxiolytic agent and its effect appears at very low doses while ethanol is used as a social drug worldwide. Sometimes, toxic interactions occur following combined administration of these two drugs. In this study we have investigated the interaction between ethanol and high-dose alprazolam using human liver microsomes in vitro. The interaction effects between ethanol and alprazolam were examined by a mixed-function oxidation reaction using a human liver microsomal preparation. Alprazolam and its two main metabolites (alpha-hydroxyalprazolam: alpha-OH alprazolam, 4-hydroxyalprazolam: 4-OH alprazolam) were measured by HPLC/UV. The production of 4-OH alprazolam, one main metabolite of alprazolam, was weakly inhibited by higher dose of ethanol, but not alpha-OH alprazolam. These results using a human liver microsomal preparation show that the production of 4-OH alprazolam is weakly inhibited by ethanol but not alpha-OH alprazolam. Toxic levels may be reached by simultaneous administration of ethanol and high-dose alprazolam.

Lifetime and baseline alcohol intake and risk of colon and rectal cancers in the European prospective investigation into cancer and nutrition (EPIC)

Alcohol consumption may be associated with risk of colorectal cancer (CRC), but the epidemiological evidence for an association with specific anatomical subsites, types of alcoholic beverages and current vs. lifetime alcohol intake is inconsistent. Within the European Prospective Investigation into Cancer and Nutrition (EPIC), 478,732 study subjects free of cancer at enrolment between 1992 and 2000 were followed up for an average of 6.2 years, during which 1,833 CRC cases were observed. Detailed information on consumption of alcoholic beverages at baseline (all cases) and during lifetime (1,447 CRC cases, 69% of the cohort) was collected from questionnaires. Cox proportional hazard models were used to examine the alcohol-CRC association. After adjustment for potential confounding factors, lifetime alcohol intake was significantly positively associated to CRC risk (hazard ratio, HR=1.08, 95%CI=1.04-1.12 for 15 g/day increase), with higher cancer risks observed in the rectum (HR=1.12, 95%CI=1.06-1.18) than distal colon (HR=1.08, 95%CI=1.01-1.16), and proximal colon (HR=1.02, 95%CI=0.92-1.12). Similar results were observed for baseline alcohol intake. When assessed by alcoholic beverages at baseline, the CRC risk for beer (HR=1.38, 95%CI=1.08-1.77 for 20-39.9 vs. 0.1-2.9 g/day) was higher than wine (HR=1.21, 95%CI=1.02-1.44), although the two risk estimates were not significantly different from each other. Higher HRs for baseline alcohol were observed for low levels of folate intake (1.13, 95%CI=1.06-1.20 for 15 g/day increase) compared to high folate intake (1.03, 95%CI=0.98-1.09). In this large European cohort, both lifetime and baseline alcohol consumption increase colon and rectum cancer risk, with more apparent risk increases for alcohol intakes greater than 30 g/day.

Geriatric alcoholism

BACKGROUND

The authors reviewed the clinical features, epidemiology, diagnosis, medical treatment, orofacial findings and dental treatment of geriatric patients with alcoholism.

TYPES OF STUDIES REVIEWED

The authors conducted MEDLINE searches for the period 1995 through 2004 using the terms "alcoholism," "geriatric," "pathophysiology," "treatment" and "dentistry." They selected reports published in English in peer-reviewed journals for further review.

RESULTS

Physiological changes associated with aging permit the harmful effects of drinking alcohol to arise at lower levels of consumption than in younger people. Excessive use of alcohol exacerbates the medical and emotional problems associated with aging and predisposes the person to adverse drug reactions with medications controlling these illnesses.

CLINICAL IMPLICATIONS

The incidence of dental disease in this population is extensive because of diminished salivary flow and a disinterest in performing appropriate oral hygiene techniques. Concurrent abuse of tobacco products worsens dental disease and heightens the risk of developing oral cancer. Identification of patients who abuse alcohol, a cancer-screening examination, preventive dental education, and use of saliva substitutes and anticaries agents are indicated. Clinicians must take precautions when performing surgery and when prescribing or administering analgesics, antibiotics or sedative agents that are likely to have an adverse interaction with alcohol.

A study of the in vitro interaction between ethanol, and triazolam and its two metabolites using human liver microsomes

AIM

Triazolam is widely used as an ultrashort-acting anxiolytic drug and hypnosedatives and its effect appears at very low doses. Ethanol is used as a social drug worldwide. Sometimes, toxic interactions occur following combined administration of these two drugs. In this study, we have investigated the interaction between alcohol and triazolam in vitro.

METHODS

The interaction effects between alcohol and triazolam were examined by a mixed-function oxidation reaction using a human liver microsomal preparation. Triazolam and its two metabolites (alpha-hydroxytriazolam: alpha-OH triazolam, 4-hydroxytriazolam: 4-OH triazolam) were measured by HPLC/UV.

RESULTS

The production of alpha-OH triazolam and 4-OH triazolam was shown to be weakly inhibited by 13-29% (p < 0.05) and 8-14%,respectively, by ethanol (20-80 mM).

CONCLUSIONS

These results using a human liver microsomal preparation show that the formation of both metabolites of triazolam is weakly inhibited by ethanol. Toxic levels may be reached by simultaneous administration of ethanol and triazolam.

Alcohol and cancer: genetic and nutritional aspects

Chronic alcohol consumption is a major risk factor for cancer of upper aero-digestive tract (oro-pharynx, hypopharynx, larynx and oesophagus), the liver, the colo-rectum and the breast. Evidence has accumulated that acetaldehyde is predominantly responsible for alcohol-associated carcinogenesis. Acetaldehyde is carcinogenic and mutagenic, binds to DNA and protein, destroys the folate molecule and results in secondary cellular hyper-regeneration. Acetaldehyde is produced by mucosal and cellular alcohol dehydrogenase, cytochrome P450 2E1 and through bacterial oxidation. Its generation and/or its metabolism is modulated as a result of polymorphisms or mutations of the genes responsible for these enzymes. Acetaldehyde can also be produced by oral bacteria. Smoking, which changes the oral bacterial flora, also increases salivary acetaldehyde. Cigarette smoke and some alcoholic beverages, such as Calvados, contain acetaldehyde. In addition, chronic alcohol consumption induces cytochrome P450 2E1 enxyme activity in mucosal cells, resulting in an increased generation of reactive oxygen species and in an increased activation of various dietary and environmental carcinogens. Deficiencies of riboflavin, Zn, folate and possibly retinoic acid may further enhance alcohol-associated carcinogenesis. Finally, methyl deficiency as a result of multiple alcohol-induced changes leads to DNA hypomethylation. A depletion of lipotropes, including methionine, choline, betaine and S-adenosylmethionine, as well as folate, results in the hypomethylation of oncogenes and may lead to DNA strand breaks, all of which are associated with increased carcinogenesis.

Posttraumatic stress disorder: psychopathology, medical management, and dental implications

PTSD is a chronic mental illness that may arise after an individual experiences or witnesses a life-threatening event. Symptoms consist of persistent reexperiencing of the event, avoidance of reminders of the event, a numbing of positive emotions, and social withdrawal. A depressed mood and excessive use of alcohol and tobacco may accompany the disorder. PTSD afflicts approximately 5% of men and 11% of women. Dental disease may be extensive because of neglect of oral hygiene compounded by cigarette smoking. Dental treatment includes preventive education, oral cancer screening, and prescribing saliva substitutes or stimulants and anticaries agents to combat medication-induced xerostomia. Precautions must be taken when prescribing or administering certain analgesics, antibiotics, or sedative agents that may adversely interact with the psychiatric medications or when performing surgery because of the long-term effects of alcohol abuse.

Genetic evidence that retinaldehyde dehydrogenase Raldh1 (Aldh1a1) functions downstream of alcohol dehydrogenase Adh1 in metabolism of retinol to retinoic acid

Vitamin A (retinol) is a nutrient that is essential for developmental regulation but toxic in large amounts. Previous genetic studies have revealed that alcohol dehydrogenase Adh1 is required for efficient clearance of excess retinol to prevent toxicity, thus demonstrating that the mechanism involves oxidation of excess retinol to retinoic acid (RA). Whereas Adh1 plays a dominant role in the first step of the clearance pathway (oxidation of retinol to retinaldehyde), it is unknown what controls the second step (oxidation of retinaldehyde to RA). We now present genetic evidence that aldehyde dehydrogenase Aldh1a1, also known as retinaldehyde dehydrogenase Raldh1, plays a dominant role in the second step of retinol clearance in adult mice. Serum RA levels following a 50 mg/kg dose of retinol were reduced 72% in Raldh1-/- mice and 82% in Adh1-/- mice. This represented reductions in RA synthesis of 77-78% for each mutant after corrections for altered RA degradation in each. After retinol dosing, serum retinaldehyde was increased 2.5-fold in Raldh1-/- mice (indicating defective retinaldehyde clearance) and decreased 3-fold in Adh1-/- mice (indicating defective retinaldehyde synthesis). Serum retinol clearance following retinol administration was decreased 7% in Raldh1-/- mice and 69% in Adh1-/- mice. LD50 studies indicated a small increase in retinol toxicity in Raldh1-/- mice and a large increase in Adh1-/- mice. These observations demonstrate that Raldh1 functions downstream of Adh1 in the oxidative metabolism of excess retinol and that toxicity correlates primarily with accumulating retinol rather than retinaldehyde.

Alcohol abuse and dependence: psychopathology, medical management and dental implications

BACKGROUND

The authors review the clinical features, epidemiology, pathophysiology, medical management, dental findings and dental treatment of patients with alcoholism.

LITERATURE REVIEWED

The authors conducted a MEDLINE search for 1995 through 2001 using the key terms of alcoholism, epidemiology, pathophysiology, treatment and dentistry. Reports selected for further review included those published in English in peer-reviewed journals. The authors gave preference to articles reporting randomized, controlled trials.

CONCLUSIONS

Alcoholism is a chronic and progressive psychiatric illness that afflicts more than 14 million Americans. It is characterized by a loss of control over the use of alcohol, resulting in impaired social functioning, and the consequent development of medical illnesses. The disease arises in genetically vulnerable people when they are overwhelmed by their cravings for the alcohol-associated euphoria that results from the actions of several neurotransmitter systems in the brain's pleasure center. New medications to counteract alcohol-induced neurotransmission imbalance may assist patients in reducing their craving.

CLINICAL IMPLICATIONS

The prevalence of dental disease usually is extensive because of a disinterest in performing appropriate oral hygiene techniques and diminished salivary flow. Concurrent abuse of tobacco products worsens dental disease and heightens the risk of developing oral cancer. Identification of the alcohol-abusing patient, a cancer-screening examination, preventive dental education, and use of saliva substitutes and anticaries agents are indicated. Special precautions must be taken when performing surgery and when prescribing or administering analgesics, antibiotics or sedative agents that are likely to have an adverse interaction with alcohol or psychiatric medications.

Induction of nicotine-metabolizing CYP2B1 by ethanol and ethanol-metabolizing CYP2E1 by nicotine: summary and implications

Alcohol and tobacco are frequently co-abused. Increased alcohol use and alcoholism are associated with smoking, and vice versa. Functional and/or metabolic cross-tolerance may contribute to this occurrence. This review summarizes recent studies published from our laboratory focusing on metabolic aspects of tolerance, which demonstrate that in rat, subchronic, behaviourally relevant doses of ethanol induce hepatic nicotine-metabolizing cytochrome P450 (CYP) 2B1, and that subchronically administered nicotine, at behaviourally relevant doses, induces hepatic ethanol-metabolizing CYP2E1. Increased CYP2B1 protein, mRNA and CYP2B1-mediated nicotine metabolism was observed following ethanol treatments. CYP2E1 protein and activity were induced by nicotine, but no changes were seen in levels of CYP2E1 mRNA. These data indicate that metabolic cross-tolerance may occur between nicotine and ethanol, suggesting that nicotine use may increase the elimination of ethanol, and ethanol use may increase the elimination of nicotine. Other implications, such as altered pharmacology and toxicology of drugs metabolized by these enzymes, as well as changes in pro-carcinogen and pro-toxin activation are also discussed.

Characterisation of a novel mouse liver aldo-keto reductase AKR7A5

We have characterised a novel aldo-keto reductase (AKR7A5) from mouse liver that is 78% identical to rat aflatoxin dialdehyde reductase AKR7A1 and 89% identical to human succinic semialdehyde (SSA) reductase AKR7A2. AKR7A5 can reduce 2-carboxybenzaldehyde (2-CBA) and SSA as well as a range of aldehyde and diketone substrates. Western blots show that it is expressed in liver, kidney, testis and brain, and at lower levels in skeletal muscle, spleen heart and lung. The protein is not inducible in the liver by dietary ethoxyquin. Immunodepletion of AKR7A5 from liver extracts shows that it is one of the major liver 2-CBA reductases but that it is not the main SSA reductase in this tissue.

Toxicological interactions between alcohol and benzodiazepines

BACKGROUND

We review recentfindings on the toxicological interactions between alcohol (ethanol) and benzodiazepines, and the combined use of benzodiazepines and alcohol in fatal poisoning. Acute ingestion of alcohol combined with benzodiazepines is responsible for several toxicological interactions that can have significant clinical implications. In general, metabolism of these drugs is delayed when combined with acute alcohol ingestion although some reports suggest otherwise. Alternately, the drugs metabolized during chronic alcohol ingestion have an increased clearance. The net effect may also be influenced by internal (e.g., disease, age) and external (e.g., environment, diet) factors. Fatal poisoning involving coadministration of alcohol and benzodiazepine, especially triazolam, continues to be a serious problem.

Excessive vitamin A toxicity in mice genetically deficient in either alcohol dehydrogenase Adh1 or Adh3

Alcohol dehydrogenase (ADH) deficiency results in decreased retinol utilization, but it is unclear what physiological roles the several known ADHs play in retinoid signaling. Here, Adh1, Adh3, and Adh4 null mutant mice have been examined following acute and chronic vitamin A excess. Following an acute dose of retinol (50 mg.kg(-1)), metabolism of retinol to retinoic acid in liver was reduced 10-fold in Adh1 mutants and 3.8-fold in Adh3 mutants, but was not significantly reduced in Adh4 mutants. Acute retinol toxicity, assessed by determination of the LD(50) value, was greatly increased in Adh1 mutants and moderately increased in Adh3 mutants, but only a minor effect was observed in Adh4 mutants. When mice were propagated for one generation on a retinol-supplemented diet containing 10-fold higher vitamin A than normal, Adh3 and Adh4 mutants had essentially the same postnatal survival to adulthood as wild-type (92-95%), but only 36% of Adh1 mutants survived to adulthood with the remainder dying by postnatal day 3. Adh1 mutants surviving to adulthood on the retinol- supplemented diet had elevated serum retinol signifying a clearance defect and elevated aspartate aminotransferase indicative of increased liver damage. These findings indicate that ADH1 functions as the primary enzyme responsible for efficient oxidative clearance of excess retinol, thus providing protection and increased survival during vitamin A toxicity. ADH3 plays a secondary role. Our results also show that retinoic acid is not the toxic moiety during vitamin A excess, as Adh1 mutants have less retinoic acid production while experiencing increased toxicity.

Distinct retinoid metabolic functions for alcohol dehydrogenase genes Adh1 and Adh4 in protection against vitamin A toxicity or deficiency revealed in double null mutant mice

The ability of class I alcohol dehydrogenase (ADH1) and class IV alcohol dehydrogenase (ADH4) to metabolize retinol to retinoic acid is supported by genetic studies in mice carrying Adh1 or Adh4 gene disruptions. To differentiate the physiological roles of ADH1 and ADH4 in retinoid metabolism we report here the generation of an Adh1/4 double null mutant mouse and its comparison to single null mutants. We demonstrate that loss of both ADH1 and ADH4 does not have additive effects, either for production of retinoic acid needed for development or for retinol turnover to minimize toxicity. During gestational vitamin A deficiency Adh4 and Adh1/4 mutants exhibit completely penetrant postnatal lethality by day 15 and day 24, respectively, while 60% of Adh1 mutants survive to adulthood similar to wild-type. Following administration of a 50-mg/kg dose of retinol to examine retinol turnover, Adh1 and Adh1/4 mutants exhibit similar 10-fold decreases in retinoic acid production, whereas Adh4 mutants have only a slight decrease. LD(50) studies indicate a large increase in acute retinol toxicity for Adh1 mutants, a small increase for Adh4 mutants, and an intermediate increase for Adh1/4 mutants. Chronic retinol supplementation during gestation resulted in 65% postnatal lethality in Adh1 mutants, whereas only approximately 5% for Adh1/4 and Adh4 mutants. These studies indicate that ADH1 provides considerable protection against vitamin A toxicity, whereas ADH4 promotes survival during vitamin A deficiency, thus demonstrating largely non-overlapping functions for these enzymes in retinoid metabolism.

The role of pharmacogenetically-variable cytochrome P450 enzymes in drug abuse and dependence

The risk of drug dependence is determined by the interaction of drug, individual and environment. 'Pharmacogenetics' is the study of the influence of heredity on the response to drugs and their fate in the body; these studies aim to improve the understanding of inter-individual variability in drug response. The authors have applied this research approach to the study of drug metabolism and dependence. Specifically the interaction of genetically variable hepatic cytochrome P450 (CYP) enzymes and their effect on self-administration of drugs has been examined. Many drugs of abuse are substrates (e.g., amphetamines, codeine, nicotine) or inhibitors (e.g., (-)-cocaine) of polymorphic CYPs. Drug metabolism by genetically polymorphic enzymes can have significant clinical implications relating to drug toxicity, therapeutic failure, drug-drug interactions, disease susceptibility and abuse liability. There is good evidence that drug metabolism by genetically variable CYPs can influence the risk of drug dependence, the amount of drug consumed by dependent individuals and some of the toxicities associated with drug-taking behavior. It is anticipated that pharmacogenetics will be used to identify individuals at a greater risk for specific drug dependencies, provide information that can lead to novel treatment and prevention approaches as well as provide guidance for individualization of treatment choice.

Cytochrome P450 2E1: its clinical and toxicological role

Cytochrome (CYP) P450 2E1 is clinically and toxicologically important and it is constitutively expressed in the liver and many other tissues. In contrast to many other CYP isoenzymes, indisputable evidence for a functionally important polymorphism of CYP2E1 in the human population is lacking. CYP2E1 metabolizes a wide variety of chemicals with different structures, in particular small and hydrophobic compounds, including potential cytotoxic and carcinogenic agents. In addition, chlorzoxazone and trimethadione metabolism are good CYP2E1 probes for liver disease in vivo and in vitro. In the future, methods for fully analysing the function of CYP2E1 using knockout mice will be established. This article reviews recent advances in our understanding of the role of human CYP2E1 in drug metabolism.

Pharmacokinetic interaction of abacavir (1592U89) and ethanol in human immunodeficiency virus-infected adults

While in vitro results at clinically relevant concentrations do not predict abacavir (1592U89) interactions with drugs highly metabolized by cytochrome P450, the potential does exist for a pharmacokinetic interaction between abacavir and ethanol, as both are metabolized by alcohol dehydrogenase. Twenty-five subjects were enrolled in an open-label, randomized, three-way-crossover, phase I study of human immunodeficiency virus-infected male subjects. The three treatments were administration of (i) 600 mg of abacavir, (ii) 0.7 g of ethanol per kg of body weight, and (iii) 600 mg of abacavir and 0.7 g of ethanol per kg. Twenty-four subjects completed the study with no unexpected adverse events reported. Ethanol pharmacokinetic parameters were unchanged with abacavir coadministration. The geometric least squares mean area under the concentration curve extrapolated to infinite time for abacavir increased 41% (from 11.07 to 15.62 microg. h/ml), and the half-life increased 26% (from 1.42 to 1.79 h) in the presence of ethanol (mean ethanol maximum concentration in plasma of 498 microg/ml). The percentages of abacavir dose recovered in urine as abacavir and its two major metabolites were each altered in the presence of ethanol, but there was no change in the total percentage ( approximately 50%) of administered dose recovered in the 12-h collection interval. In conclusion, while a single 600-mg dose of abacavir does not alter blood ethanol concentration, ethanol does increase plasma abacavir concentrations.

Clinically significant pharmacokinetic drug interactions with benzodiazepines

The reports of interactions between benzodiazepines (BZPs) and other drugs (e.g., antidepressants, selective serotonin reuptake inhibitors, antiulcer drugs, antiepileptic drugs, macrolide antibiotics) during their combined use are reviewed. In general, metabolism of BZPs is delayed when combined with a number of other drugs but some reports have suggested otherwise. In recent years, the cytochrome P450 (P450 or CYP) isoenzyme that catalyses the metabolism of BZPs has also been identified. BZPs are mainly catalysed by CYP3A4. When published reports are studied, it appears necessary to be exceptionally careful about interactions mainly between BZPs and selective serotonin reuptake inhibitors, cimetidine, antiepileptic drugs, macrolide antibiotics and antimycotics. More information is necessary to identify individuals at greatest risk of drug interactions and adverse events.

Expression and localization of human alcohol and aldehyde dehydrogenase enzymes in skin

Alcohol dehydrogenase (ADH; EC 1.1.1.1) and aldehyde dehydrogenase (ALDH; EC 1.2.1.3.) are important enzymes involved in the biotransformation of both alcohols and aldehydes. Today, six classes of ADH and twelve classes of ALDH have been defined in mammals. Here we report the detection and localisation of three classes of ADH and two classes of ALDH in human skin, using Western blot analysis and immunohistochemistry with class-specific antisera. Western blot analysis of human skin cytosol revealed that class I-III ADH and class 1 and class 3 ALDH enzymes are expressed, constitutively, in three different anatomical regions of human skin (foreskin, breast, abdomen). Densitometric analysis of the immunoreactive bands revealed differential constitutive expression of these enzymes in foreskin, breast, and abdomen skin. Immunohistochemistry showed the presence of class I ADH and class III ADH enzymes, predominantly in the epidermis with some localised expression in the dermal appendages of human skin. In comparison, staining for class II ADH was more faint in the epidermis with very little dermal expression. Class 1 ALDH and class 3 ALDH were predominantly localised to the epidermis with minimal, highly localised dermal appendageal expression. These cutaneous ADH and ALDH enzymes may play significant roles in the metabolism of endogenous or xenobiotic alcohols and aldehydes.

Beneficial effects of S-adenosyl-L-methionine on aminolevulinic acid dehydratase, glutathione, and lipid peroxidation during acute lead-ethanol administration in mice

Beneficial effects of S-adenosyl-L-methionine (SAM) in preventing inhibition of blood delta-aminolevulinic acid dehydratase (ALAD), alterations in blood and hepatic glutathione (GSH), hepatic and brain malondialdehyde (MDA) formation, and uptake of lead following acute lead plus ethanol coexposure were investigated in mice. Whereas exposure to both lead or ethanol individually produced a significant inhibition of blood delta-aminolevulinic acid dehydratase (ALAD) activity, ethanol administration alone produced only a marginal depletion of hepatic glutathione (GSH). A significant elevation of hepatic MDA concentration was observed following lead or ethanol ingestion. An appreciable increase in brain GSH following ethanol administration whereas a moderate elevation in MDA level following lead plus ethanol administration was observed. Combined lead plus ethanol exposure produced a more pronounced depletion of blood ALAD activity and an increase in hepatic MDA level compared to lead- or ethanol-alone administration. Brain GSH concentration showed an increase compared to untreated control animals or lead-alone-exposed mice. Concomitant administration of SAM partially reversed the inhibition of blood ALAD activity in all three exposed groups (i.e., lead, ethanol, or lead plus ethanol). Lead concentration in blood, liver, and brain was significantly reduced by SAM in lead-alone or lead plus ethanol coexposed groups. The results suggest that supplementation of SAM may have beneficial effects in preventing alterations in some biochemical variables and accumulation of lead in blood, liver, and brain during acute lead plus ethanol exposure in animals.

Drug, meal and formulation interactions influencing drug absorption after oral administration. Clinical implications

Drug-drug, drug-formulation and drug-meal interactions are of clinical concern for orally administered drugs that possess a narrow therapeutic index. This review presents the current status of information regarding interactions which may influence the gastrointestinal (GI) absorption of orally administered drugs. Absorption interactions have been classified on the basis of rate-limiting processes. These processes are put in the context of drug and formulation physicochemical properties and oral input influences on variable GI physiology. Interaction categorisation makes use of a biopharmaceutical classification system based on drug aqueous solubility and membrane permeability and their contributions towards absorption variability. Overlaying this classification it is important to be aware of the effect that the magnitudes of drug dosage and volume of fluid administration can have on interactions involving a solubility rate limits. GI regional differences in membrane permeability are fundamental to the rational development of extended release dosage forms as well as to predicting interaction effects on absorption from immediate release dosage forms. The effect of meals on the regional-dependent intestinal elimination of drugs and their involvement in drug absorption interactions is also discussed. Although the clinical significance of such interactions is certainly dependent on the narrowness of the drug therapeutic index, clinical aspects of absorption delays and therapeutic failures resulting from various interactions are also important.

Clinically important pharmacokinetic drug-drug interactions: role of cytochrome P450 enzymes

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues and many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In the future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

Pharmacokinetic interactions between acute alcohol ingestion and single doses of benzodiazepines, and tricyclic and tetracyclic antidepressants -- an update

Recent reports of interactions between alcohol and benzodiazepines, tricyclic and tetracyclic antidepressants during their acute concomitant use are reviewed. Acute ingestion of alcohol (ethanol) with tranquilizers or hypnotics is responsible for several pharmacokinetic interactions that can have significant clinical implications. In general, metabolism of these drugs is delayed when combined with alcohol but some reports have suggested otherwise. The amount of alcohol consumed, the presence or absence of liver disease, and differences in the dosage and administration of these drugs may account for the observed discrepancies. In recent years, the cytochrome P450 (P450 or CYP) isoenzyme that catalyses the metabolism of these drugs has also been identified. However, since changes in the pharmacogenetic metabolism of benzodiazepines and tricyclic and tetracyclic antidepressants are mainly governed by CYP2C19 and CYP2D6, caution is needed when used together with alcohol.

Molecular cloning, expression and catalytic activity of a human AKR7 member of the aldo-keto reductase superfamily: evidence that the major 2-carboxybenzaldehyde reductase from human liver is a homologue of rat aflatoxin B1-aldehyde reductase

The masking of charged amino or carboxy groups by N-phthalidylation and O-phthalidylation has been used to improve the absorption of many drugs, including ampicillin and 5-fluorouracil. Following absorption of such prodrugs, the phthalidyl group is hydrolysed to release 2-carboxybenzaldehyde (2-CBA) and the pharmaceutically active compound; in humans, 2-CBA is further metabolized to 2-hydroxymethylbenzoic acid by reduction of the aldehyde group. In the present work, the enzyme responsible for the reduction of 2-CBA in humans is identified as a homologue of rat aflatoxin B1-aldehyde reductase (rAFAR). This novel human aldo-keto reductase (AKR) has been cloned from a liver cDNA library, and together with the rat protein, establishes the AKR7 family of the AKR superfamily. Unlike its rat homologue, human AFAR (hAFAR) appears to be constitutively expressed in human liver, and is widely expressed in extrahepatic tissues. The deduced human and rat protein sequences share 78% identity and 87% similarity. Although the two AKR7 proteins are predicted to possess distinct secondary structural features which distinguish them from the prototypic AKR1 family of AKRs, the catalytic- and NADPH-binding residues appear to be conserved in both families. Certain of the predicted structural features of the AKR7 family members are shared with the AKR6 beta-subunits of voltage-gated K+-channels. In addition to reducing the dialdehydic form of aflatoxin B1-8,9-dihydrodiol, hAFAR shows high affinity for the gamma-aminobutyric acid metabolite succinic semialdehyde (SSA) which is structurally related to 2-CBA, suggesting that hAFAR could function as both a SSA reductase and a 2-CBA reductase in vivo. This hypothesis is supported in part by the finding that the major peak of 2-CBA reductase activity in human liver co-purifies with hAFAR protein.

Pharmacokinetic interactions between alcohol and other drugs

The frequent use of alcohol (ethanol) together with prescription drugs gives any described pharmacokinetic interaction significant clinical implications. The issue is both the effect of alcohol on the pharmacokinetics of various drugs and also the effect of those drugs on the pharmacokinetics of alcohol. This review discusses these pharmacokinetic interactions but also briefly describes some other effects of alcohol that are clinically relevant to drug prescribing. The use of several different study designs may be required before we can confidently state the presence or absence of any alcohol-drug interaction. Short term administration of alcohol in volunteers is the most common study design but studies of social drinking and prolonged moderate alcohol intake can be important in some situations. Community-based studies may illustrate the clinical relevance of any interaction. Alcohol can affect the pharmacokinetics of drugs by altering gastric emptying or liver metabolism (by inducing cytochrome P450 2E1). Drugs may affect the pharmacokinetics of alcohol by altering gastric emptying and inhibiting gastric alcohol dehydrogenase. The role of gastric alcohol dehydrogenase in the first-pass metabolism of alcohol is reviewed in this article and the arguments for and against any potential interaction between alcohol and H2 receptor antagonists are also discussed. The inhibition of the metabolism of acetaldehyde may cause disulfiram-like reactions. Pharmacodynamic interactions between alcohol and prescription drugs are common, particularly the additive sedative effects with benzodiazepines and also with some of the antihistamine drugs; other interactions may occur with tricyclic antidepressants. Alcohol intake may be a contributing factor to the disease state which is being treated and may complicate treatment because of various pathophysiological effects (e.g. impairment of gluconeogenesis and the risk of hypoglycaemia with oral hypoglycaemic agents). The combination of nonsteroidal anti-inflammatory drugs and alcohol intake increases the risk of gastrointestinal haemorrhage.

Alcoholic beverages and lipid peroxidation: relevance to cardiovascular disease

Overall there is good evidence that alcohol consumption induces oxidative stress, and leads to lipid peroxidation, effects which have been linked to alcohol-related toxicity and disease and may be relevant to alcoholatherosclerosis interrelationships. On the other hand, a protective effect of light to moderate alcohol consumption against cardiovascular disease is well recognized, with the further hypothesis that red wine offers extra cardiovascular protection due to its rich content of antioxidant phenolic compounds. Although this hypothesis is given some credence from in vitro data, controlled studies in humans have produced conflicting results. Clearly, the equally well described pro-oxidant effects of alcohol and its metabolism have been insufficiently considered in the pursuit of what to many is an intuitively attractive hypothesis. Further studies are required to determine if red wine phenolics are actually absorbed from the gut and whether they offer any overall antioxidant protection in vivo. The hypothesis that red wine offers extra cardiovascular protection compared to other alcoholic beverages is not proven and must await the outcome of studies in which the full spectrum of the pro-oxidant and antioxidant effects of alcoholic beverages are duly considered. In the absence of such studies, there are no grounds at present for the promotion of the consumption of alcoholic beverages on the basis of their putative "antioxidant" properties.

Actions of acute ethanol intoxication on cardiopulmonary function after an endotoxin challenge

BACKGROUND

This study examined whether acute ethanol (EtOH) intoxication could alter the systemic inflammatory response evoked by endotoxin (lipopolysaccharide, LPS).

METHODS

Anesthetized (fentanyl) and mechanically ventilated mongrel pigs were administered 20% EtOH (3 gm/kg) or its vehicle (VEH) by means of gastric lavage. After 60 minutes of equilibration, blood levels were 110 to 130 mg/dl, and LPS (1 microgram/kg per 30 minutes) was infused intravenously to mimic the type of sepsis that might be encountered after a penetrating abdominal injury.

RESULTS

LPS caused initial pulmonary vasoconstriction followed by cardiovascular collapse in 7 of 14 pigs with EtOH versus 0 of 14 pigs with VEH (p = 0.0058); survival time averaged 2.4 +/- 0.5 hours for EtOH versus 4.5 +/- 0.3 hours for VEH (p = 0.002). At 3 to 5 hours after LPS infusion the survivors were acidotic (base excess, -5.1 +/- 1.5 versus 0.4 +/- 1.1 mEq/L; p = 0.007) and vasodilated (systemic vascular resistance, 54% +/- 9% versus 111% +/- 9% baseline; p = 0.005). Systemic arterial pressure and cardiac filling pressures were maintained with fluid resuscitation, but more was required for EtOH versus VEH (80 +/- 11 versus 42 +/- 5 ml/kg/hr; p = 0.0034). A diffuse capillary leak was detected with gamma scintigraphy and regional uptake of technetium 99m albumin. With EtOH versus VEH microvascular permeability was higher in abdomen (p = 0.01) and liver (p = 0.06) but not in lung (p = 0.29). These changes were probably mediated in part by leukosequestration: neutrophil counts were initially reduced more than 80% in both groups, but they then rebounded with VEH (p < 0.05) but not EtOH. The early versus late deaths within the EtOH group were distinguished by higher baseline levels of cortisol (1.4 +/- 0.3 versus 0.9 +/- 0.2 micrograms/dl; p = 0.075) and by a 50% decrease evoked by EtOH (p = 0.0042) versus no change in the late death subgroup. After LPS infusion cortisol peaked sooner and then recovered with VEH (p < 0.05), whereas the peak occurred later and there was no decay with EtOH. In addition, at 60 and 90 minutes after LPS infusion tumor necrosis factor was 119 +/- 27 and 240 +/- 40 pg/ml with VEH versus 62 +/- 15 and 95 +/- 23 pg/ml with EtOH (p = 0.041 and p = 0.0030).

CONCLUSIONS

By means of a leukocyte-mediated mechanism in the splanchnic circulation, acute EtOH impaired host defense, which exacerbated LPS-evoked systemic inflammatory response. In context with our earlier experimental study and two large clinical trials, it appears that acute EtOH can have opposite effects on the vulnerability to posttrauma sepsis, depending on the timing of the septic insult and on the immune status at the time of septic challenge.