The effect of smoking on caffeine elimination: implications for its use as a semiquantitative test of liver function

Pharmacokinetics of Caffeine: A Systematic Analysis of Reported Data for Application in Metabolic Phenotyping and Liver Function Testing

Caffeine is by far the most ubiquitous psychostimulant worldwide found in tea, coffee, cocoa, energy drinks, and many other beverages and food. Caffeine is almost exclusively metabolized in the liver by the cytochrome P-450 enzyme system to the main product paraxanthine and the additional products theobromine and theophylline. Besides its stimulating properties, two important applications of caffeine are metabolic phenotyping of cytochrome P450 1A2 (CYP1A2) and liver function testing. An open challenge in this context is to identify underlying causes of the large inter-individual variability in caffeine pharmacokinetics. Data is urgently needed to understand and quantify confounding factors such as lifestyle (e.g., smoking), the effects of drug-caffeine interactions (e.g., medication metabolized via CYP1A2), and the effect of disease. Here we report the first integrative and systematic analysis of data on caffeine pharmacokinetics from 141 publications and provide a comprehensive high-quality data set on the pharmacokinetics of caffeine, caffeine metabolites, and their metabolic ratios in human adults. The data set is enriched by meta-data on the characteristics of studied patient cohorts and subjects (e.g., age, body weight, smoking status, health status), the applied interventions (e.g., dosing, substance, route of application), measured pharmacokinetic time-courses, and pharmacokinetic parameters (e.g., clearance, half-life, area under the curve). We demonstrate via multiple applications how the data set can be used to solidify existing knowledge and gain new insights relevant for metabolic phenotyping and liver function testing based on caffeine. Specifically, we analyzed 1) the alteration of caffeine pharmacokinetics with smoking and use of oral contraceptives; 2) drug-drug interactions with caffeine as possible confounding factors of caffeine pharmacokinetics or source of adverse effects; 3) alteration of caffeine pharmacokinetics in disease; and 4) the applicability of caffeine as a salivary test substance by comparison of plasma and saliva data. In conclusion, our data set and analyses provide important resources which could enable more accurate caffeine-based metabolic phenotyping and liver function testing.

The Energy to Smoke: Examining the Longitudinal Association between Beverage Consumption and Smoking and Vaping Behaviours among Youth in the COMPASS Study

This study examined the longitudinal association between changes in sugar-sweetened and/or caffeinated beverage consumption and smoking/vaping behaviour among Canadian adolescents. Using longitudinal data from the COMPASS study (2015/16 to 2017/18), four models were developed to investigate whether beverage consumption explained variability in smoking and vaping behaviour in adolescence: (1) smoking initiation, (2) vaping initiation, (3) current smoking status, and (4) current vaping status. Models were adjusted for demographic factors. Multinomial logit models were used for model 1, 2, and 3. A binary logistic regression model was used for model 4. An association between change in frequency of beverage consumption and smoking/vaping behaviour was identified in all models. A one-day increase in beverage consumption was associated with smoking initiation (OR = 1.38, 95% CI: 1.25, 1.51), vaping initiation (OR = 1.23, 95% CI: 1.14, 1.32), identifying as a current smoker (OR = 1.17, 95% CI: 1.01, 1.35), and currently vaping (OR = 1.08, 95% CI: 1.04, 1.11). Change in high-energy drink consumption was the best predictor of smoking behaviours and vaping initiation but not current vaping status. Given the health consequences of smoking and vaping and their association with high-energy drink and coffee consumption, policy initiatives to prevent smoking/vaping initiation, and to limit youth access to these beverages, warrant consideration.

Caffeine Consumption through Coffee: Content in the Beverage, Metabolism, Health Benefits and Risks

Caffeine (1,3,7-trimethylxanthine) is the most consumed psychoactive substance in the world, acting by means of antagonism to adenosine receptors, mainly A1 and A2A. Coffee is the main natural source of the alkaloid which is quite soluble and well extracted during the brew’s preparation. After consumption, caffeine is almost completely absorbed and extensively metabolized in the liver by phase I (cytochrome P450) enzymes, mainly CYP1A2, which appears to be polymorphically distributed in human populations. Paraxanthine is the major caffeine metabolite in plasma, while methylated xanthines and methyluric acids are the main metabolites excreted in urine. In addition to stimulating the central nervous system, caffeine exerts positive effects in the body, often in association with other substances, contributing to prevention of several chronic diseases. The potential adverse effects of caffeine have also been extensively studied in animal species and in humans. These aspects will be approached in the present review.

Tobacco smoking and its drug interactions with comedications involving CYP and UGT enzymes and nicotine

Tobacco smoking is a global public health threat causing several illnesses including cardiovascular disease (Myocardial infarction), cerebrovascular disease (Stroke), peripheral vascular disease (Claudication), chronic obstructive pulmonary disease, asthma, reduced female infertility, sexual dysfunction in men, different types of cancer and many other diseases. It has been estimated in 2015 that approximately 1.3 billion people smoke, around the globe. Use of medications among smokers is more common, nowadays. This review is aimed to identify the medications affected by smoking, involving Cytochrome P450 (CYP) and uridine diphosphate-glucuronosyltransferases (UGTs) enzymes and Nicotine. Polycyclic aromatic hydrocarbons (PAHs) of tobacco smoke have been associated with the induction of CYP enzymes such as CYP1A1, CYP1A2 and possibly CYP2E1 and UGT enzymes. The drugs metabolized by CYP1A1, CYP1A2, CYP2E1 and UGT enzymes might be affected by tobacco smoking and the smokers taking medications metabolized by those enzymes, may need higher doses due to decreased plasma concentrations through enhanced induction by PAHs of tobacco smoke. The prescribers and the pharmacists are required to be aware of medications affected by tobacco smoking to prevent the toxicity-associated complications during smoking cessation.

Coffee, caffeine, and risk of completed suicide: results from three prospective cohorts of American adults

OBJECTIVE

To evaluate the association between coffee and caffeine consumption and suicide risk in three large-scale cohorts of US men and women.

METHODS

We accessed data of 43,599 men enrolled in the Health Professionals Follow-up Study (HPFS, 1988-2008), 73,820 women in the Nurses' Health Study (NHS, 1992-2008), and 91,005 women in the NHS II (1993-2007). Consumption of caffeine, coffee, and decaffeinated coffee, was assessed every 4 years by validated food-frequency questionnaires. Deaths from suicide were determined by physician review of death certificates. Multivariate adjusted relative risks (RRs) were estimated with Cox proportional hazard models. Cohort specific RRs were pooled using random-effect models.

RESULTS

We documented 277 deaths from suicide. Compared to those consuming ≤ 1 cup/week of caffeinated coffee (< 8 oz/237 ml), the pooled multivariate RR (95% confidence interval [CI]) of suicide was 0.55 (0.38-0.78) for those consuming 2-3 cups/day and 0.47 (0.27-0.81) for those consuming ≥ 4 cups/day (P trend < 0.001). The pooled multivariate RR (95% CI) for suicide was 0.75 (0.63-0.90) for each increment of 2 cups/day of caffeinated coffee and 0.77 (0.63-0.93) for each increment of 300 mg/day of caffeine.

CONCLUSIONS

These results from three large cohorts support an association between caffeine consumption and lower risk of suicide.

Modeling caffeine concentrations with the Stanford Caffeine Questionnaire: preliminary evidence for an interaction of chronotype with the effects of caffeine on sleep

OBJECTIVE

To examine the validity of a novel caffeine intake questionnaire and to examine the effects of caffeine on sleep in college students.

METHODS

One-week, ad libitum behavior of 50 university students (28 female, 22 male; aged 20.9 ± 1.78 years) was examined with sleep logs, wrist actigraphy, and a novel daily questionnaire assessing caffeine intake at different times of day. Saliva samples were collected for caffeine assessment (questionnaire validation) and DNA extraction, and for analysis of a single nucleotide polymorphism in the adenosine receptor 2A (ADORA2A) gene.

RESULTS

The caffeine questionnaire was able to accurately predict salivary concentrations of caffeine (R(2) = 0.41, P<0.001). Estimations of integrated salivary caffeine concentration during sleep were correlated with wake after sleep onset (WASO) most strongly in morning-type individuals (R(2) = 0.49; P<0.001, ANOVA), less so in intermediate chronotypes (R(2) = 0.16; P<0.001, ANOVA), and not significantly in evening-types (R(2) = 0.00098; P = 0.13, ANOVA). Using multivariate modeling methods we found that the ADORA2A genotype did not moderate the effects of caffeine on WASO, but did independently alter WASO such that those with the CC genotype had nearly three-times as much WASO as those with CT or TT.

CONCLUSIONS

Our questionnaire was able to accurately predict salivary caffeine concentrations and helped to describe a novel relationship between the effects of caffeine on sleep and genotype and chronotype.

Pharmacokinetics and metabolism of natural methylxanthines in animal and man

Caffeine, theophylline, theobromine, and paraxanthine administered to animals and humans distribute in all body fluids and cross all biological membranes. They do not accumulate in organs or tissues and are extensively metabolized by the liver, with less than 2% of caffeine administered excreted unchanged in human urine. Dose-independent and dose-dependent pharmacokinetics of caffeine and other dimethylxanthines may be observed and explained by saturation of metabolic pathways and impaired elimination due to the immaturity of hepatic enzyme and liver diseases. While gender and menstrual cycle have little effect on their elimination, decreased clearance is seen in women using oral contraceptives and during pregnancy. Obesity, physical exercise, diseases, and particularly smoking and the interactions of drugs affect their elimination owing to either stimulation or inhibition of CYP1A2. Their metabolic pathways exhibit important quantitative and qualitative differences in animal species and man. Chronic ingestion or restriction of caffeine intake in man has a small effect on their disposition, but dietary constituents, including broccoli and herbal tea, as well as alcohol were shown to modify their plasma pharmacokinetics. Using molar ratios of metabolites in plasma and/or urine, phenotyping of various enzyme activities, such as cytochrome monooxygenases, N-acetylation, 8-hydroxylation, and xanthine oxidase, has become a valuable tool to identify polymorphisms and to understand individual variations and potential associations with health risks in epidemiological surveys.

Differences in pharmacokinetic and electroencephalographic responses to caffeine in sleep-sensitive and non-sensitive subjects

The present study investigated pharmacokinetic and electroencephalographic responses to caffeine (140 mg) in two groups of healthy volunteers reporting, or not, caffeine-related sleep disturbances. Significant differences in caffeine consumption and smoking habits were observed between the two groups. Plasma samples were taken from each subject before (T0) and after caffeine intake at 0.5, 1, 2, 4, 6 and 24 h. Three pharmacokinetic parameters: half-life (t1/2), maximum time (Tmax) and maximum plasma concentration (Cmax) were calculated from caffeine plasma concentration measurements determined by reversed phase HPLC analysis. Caffeine-sensitive subjects showed significantly greater half-life values when calculated on 24 h after the administration than tolerant subjects (p<0.05). Since the elimination kinetics were similar on the first 6 h after caffeine administration, the increased caffeine clearance observed overnight, when smoking was resumed in the control group, may indicate a short delay for the induction of hepatic cytochrome, reported here for the first time. Electrophysiological responses to caffeine, including vigilance and cortical activity, were assessed by ambulatory electroencephalographic (EEG) recorded during a period of 6 h before and after caffeine consumption. Following caffeine intake, the caffeine-intolerant subjects presented an increase in vigilance levels with faster peak alpha, beta frequency and lower delta and theta power when compared to tolerant subjects. Pharmacokinetic parameters and EEG data showed significant differences between sleep-sensitive and control subjects. These variations may be, in part, explained by cigarette smoking and the higher caffeine intake observed in the subjects of the control groups while caffeine sleep-sensitive subjects have a significantly lower caffeine intake, as already reported in previous studies on patients with sleep disturbances.

Physiologically based pharmacokinetic (PBPK) modeling of caffeine and theophylline in neonates and adults: implications for assessing children's risks from environmental agents

Children's risks can differ from those in adults for numerous reasons, one being differences in the pharmacokinetic handling of chemicals. Immature metabolism and a variety of other factors in neonates can affect chemical disposition and clearance. These factors can be incorporated into physiologically based pharmacokinetic (PBPK) models that simulate the fate of environmental toxicants in both children and adults. PBPK models are most informative when supported by empirical data, but typically pediatric pharmacokinetic data for toxicants are not available. In contrast, pharmacokinetic data in children are readily available for therapeutic drugs. The current analysis utilizes data for caffeine and theophylline, closely related xanthines that are both cytochrome P-450 (CYP) 1A2 substrates, in developing PBPK models for neonates and adults. Model development involved scale-up of in vitro metabolic parameters to whole liver and adjusting metabolic function for the ontological pattern of CYP1A2 and other CYPs. Model runs were able to simulate the large differences in half-life and clearance between neonates and adults. Further, the models were able to reproduce the faster metabolic clearance of theophylline relative to caffeine in neonates. This differential between xanthines was found to be due primarily to an extra metabolic pathway available to theophylline, back-methylation to caffeine, that is not available to caffeine itself. This pathway is not observed in adults exemplifying the importance of secondary or novel routes of metabolism in the immature liver. Greater CYP2E1 metabolism of theophylline relative to caffeine in neonates also occurs. Neonatal PBPK models developed for these drugs may be adapted to other CYP1A2 substrates (e.g., arylamine toxicants). A stepwise approach for modeling environmental toxicants in children is proposed.

Uncertainty factors for chemical risk assessment. human variability in the pharmacokinetics of CYP1A2 probe substrates

A 100-fold uncertainty factor is used to derive acceptable daily intakes for compounds causing thresholded toxicity. The 10-fold factor for human variability can be further subdivided into two factors of 10(0.5) (3.16) to allow for toxicokinetics and toxicodynamics. The validity of the human kinetic subfactor has been analysed in relation to CYP1A2 metabolism using published in vivo pharmacokinetic parameters selected to reflect chronic exposure (metabolic and total clearances and area under the plasma concentration-time curve: CLm, CL and AUC) and acute exposure (the peak plasma concentration, C(max)). The variability in CYP1A2 activity in healthy adults, based on data after oral and intravenous dosage (CLm, CL and AUC), ranged from 34 to 42%. The variability in C(max) was 21%. The default kinetic factor of 3.16 would cover at least 99% of the healthy adult population, assuming that the data were log-normally distributed, but would give lower protection for some subgroups (pregnant women at term, healthy elderly, patients with liver disease), and was inadequate for neonates. This analysis of in vivo kinetic data for CYP1A2 substrates illustrates the importance of quantifying human variability in specific metabolic pathways, and of identifying potentially susceptible subgroups of the human population, in order to determine the scientific validity of uncertainty factors.

Are we dependent upon coffee and caffeine? A review on human and animal data

Caffeine is the most widely used psychoactive substance and has been considered occasionally as a drug of abuse. The present paper reviews available data on caffeine dependence, tolerance, reinforcement and withdrawal. After sudden caffeine cessation, withdrawal symptoms develop in a small portion of the population but are moderate and transient. Tolerance to caffeine-induced stimulation of locomotor activity has been shown in animals. In humans, tolerance to some subjective effects of caffeine seems to occur, but most of the time complete tolerance to many effects of caffeine on the central nervous system does not occur. In animals, caffeine can act as a reinforcer, but only in a more limited range of conditions than with classical drugs of dependence. In humans, the reinforcing stimuli functions of caffeine are limited to low or rather moderate doses while high doses are usually avoided. The classical drugs of abuse lead to quite specific increases in cerebral functional activity and dopamine release in the shell of the nucleus accumbens, the key structure for reward, motivation and addiction. However, caffeine doses that reflect the daily human consumption, do not induce a release of dopamine in the shell of the nucleus accumbens but lead to a release of dopamine in the prefrontal cortex, which is consistent with caffeine reinforcing properties. Moreover, caffeine increases glucose utilization in the shell of the nucleus accumbens only at rather high doses that stimulate most brain structures, non-specifically, and likely reflect the side effects linked to high caffeine ingestion. That dose is also 5-10-fold higher than the one necessary to stimulate the caudate nucleus, which mediates motor activity and the structures regulating the sleep-wake cycle, the two functions the most sensitive to caffeine. In conclusion, it appears that although caffeine fulfils some of the criteria for drug dependence and shares with amphetamines and cocaine a certain specificity of action on the cerebral dopaminergic system, the methylxanthine does not act on the dopaminergic structures related to reward, motivation and addiction.

Impact of long-term ethanol consumption on CYP1A2 activity

Ethanol is a well-known inducer of CYP2E1; whether or not it is an inducer of other cytochromes has not been investigated systematically. The aim of our study was to evaluate the impact of ethanol consumption on the activity of CYP1A2, which has been shown to be influenced by drugs (inhibited or induced). We evaluated CYP1A2 activity by the ratio of the molar urinary concentrations of the three end products of paraxanthine demethylation of caffeine to the molar concentration of a paraxanthine 8-hydroxylation product. This urinary metabolite ratio has previously been shown to correlate with caffeine clearance. The caffeine metabolites were measured in urine collected during the 3 hours after oral administration of 200 mg caffeine. The caffeine test was performed in 12 smokers (> 25 cigarettes/day) and 12 nonsmokers, all of whom were alcoholic inpatients (daily intake > 100 mg absolute ethanol), within the first 3 days of their hospital stay and after 14 days of abstinence from ethanol. In alcoholic patients who were smokers the molar urinary concentration ratio was 3.14 +/- 0.97 before withdrawal and 4.01 +/- 0.92 after 14 days of abstinence from ethanol. In contrast, in alcoholic patients who were nonsmokers it was 2.62 +/- 0.95 and 2.18 +/- 0.96 before and after withdrawal, respectively. In volunteers who were smokers the molar urinary concentration ratio was 5.02 +/- 1.51, whereas in volunteers who were nonsmokers it was 3.22 +/- 1.46. Our results confirm the well-known induction of CYP1A2 activity by tobacco smoking and show that this induction is masked by long-term ethanol consumption.

Xenobiotic-induced hepatotoxicity: mechanisms of liver injury and methods of monitoring hepatic function

Xenobiotic-induced liver injury is a clinically important etiology of hepatic disease that, if not recognized, can lead to hepatic failure. In this article we discuss the mechanisms of xenobiotic-induced liver injury, various factors that can alter the risk and severity of injury, the clinical and laboratory manifestations of injury, and the methods used to detect the presence of injury and (or) functioning liver mass.

Dose-dependent pharmacokinetics and psychomotor effects of caffeine in humans

Twelve healthy volunteers received oral placebo, 250 mg of caffeine, and 500 mg of caffeine in a randomized, double-blind, single-dose crossover study. Caffeine kinetics were nonlinear, with clearance significantly reduced and elimination half-life prolonged at the 500-mg compared to the 250-mg dose. The lower dose of caffeine produced more favorable subjective effects than the higher dose (elation, peacefulness, pleasantness), whereas unpleasant effects (tension, nervousness, anxiety, excitement, irritability, nausea, palpitations, restlessness) following the 500-mg dose exceeded those of the 250-mg dose. The lower dose of caffeine enhanced performance on the digit symbol substitution test and a tapping speed test compared to placebo; high-dose caffeine produced less performance enhancement than the lower dose. The plasma concentration versus response relationship revealed concentration-dependent increases in anxiety and improvements in cognitive and motor performance at low to intermediate concentrations. Both caffeine doses reduced electroencephalographic amplitude over the 4 Hz to 30 Hz spectrum, as well as in the alpha (8-11 Hz) and beta (12-30 Hz) ranges; however, effects were not dose-dependent. While favorable subjective and performance-enhancing stimulant effects occur at low to intermediate caffeine doses, the unfavorable subjective and somatic effects, as well as performance disruption, from high doses of caffeine may intrinsically limit the doses of caffeine used in the general population.

Caffeine and nicotine: a review of their joint use and possible interactive effects in tobacco withdrawal

There is a strong, significant relationship between coffee consumption and smoking. In six epidemiological studies reviewed and analyzed here, 86.4% of smokers consumed coffee versus 77.2% of nonsmokers. Exsmokers use more coffee than nonsmokers but somewhat less than smokers. Seventeen experimental studies suggest that the pharmacologic effect of caffeine in coffee may be partially but not totally responsible for the relationship. Conditioning, a reciprocal interaction (caffeine intake increases anxiety/arousal--nicotine decreases it), or joint effect of a third variable (e.g., stress, alcohol) may account for the relationship. In abstinent smokers, blood caffeine levels increase and remain elevated for as long as 6 months. These higher caffeine plasma levels may be sufficient to produce caffeine toxicity syndrome. A review of 86 studies of nicotine withdrawal, caffeine withdrawal, and caffeine toxicity suggests that the symptoms are similar enough to be confused, and that reported nicotine withdrawal symptoms may be a mixture of nicotine withdrawal and caffeine toxicity.

Effects of tobacco smoking on caloric intake

Reduced body weight due to smoking may be an important factor inhibiting smoking cessation and promoting relapse after cessation in some smokers. It is popularly believed that smoking decreases body weight by suppressing appetite. However, cross-sectional studies show that, despite their lower body weights, smokers do not eat less than non-smokers or ex-smokers and, in fact, tend to eat slightly more. Similarly, laboratory studies show no acute effects of smoking or nicotine intake via other means on caloric intake in smokers, although intake of non-smokers may be reduced after nicotine. In contrast, longitudinal studies show that eating consistently increases in the first weeks after stopping smoking, but may recede to pre-cessation levels with longer-term abstinence, while resumption of smoking after cessation is accompanied by a reduction in eating. A similar pattern of results is seen for self-reported hunger and some, but not all, constituents of diet. Thus, there appear to be no acute or chronic effects of smoking on eating in smokers maintaining regular smoking, but changes in eating are observed concomitant with changes in smoking status (i.e. cessation or relapse). Although tolerance to anorectic effects of nicotine is one potential explanation, these findings may be more parsimoniously explained by viewing changes in eating due to smoking as secondary to an alteration in the set point around which body weight is regulated. According to this notion, cessation is accompanied by increased eating only until a new, higher body weight set point is reached, while relapse (and perhaps initiation of smoking) decreases eating only until a lower set point is reached. Implications of these findings and a set point explanation for them are discussed.

Comparison of in vitro and in vivo biotransformation in patients with liver disease of differing severity

The activity of 7-ethoxycoumarin O-deethylase (ECOD) has been measured in liver biopsy samples from 23 patients (smokers and non-smokers) with different degrees of structural liver damage. The results, which reflect in vitro cytochrome P450-dependent biotransformation, were correlated with various measures of the P450-dependent in vivo elimination of caffeine and metamizol. The relatively non-specific, low affinity component of ECOD activity was significantly correlated with the kinetics of metamizol (mean residence time, apparent clearance, half-life, area under the concentration-time curve, and metabolite excretion in the urine). Thus, metamizol elimination, which is mainly due to P450 IIB, and the low affinity component of ECOD both reflect, at least in part, the activity of the same form of P450. In contrast, caffeine biotransformation, which is via P450 IA, was not correlated with ECOD activity. There was no relation between the kinetics of metamizol and caffeine, perhaps because of the inducing effect that smoking has on caffeine elimination. In patients with liver disease, smoking appears to alter the elimination of caffeine more than the degree of liver disease.

Effects of smoking cessation on consumption of alcohol and sweet, high-fat foods

Smoking cessation may have significant effects on consumption of certain foods and other substances which may influence health. This study of seven young female smokers examined consumption of alcohol, coffee, soda, and sweets (sweet, high-fat foods) across 3 weeks, involving baseline ad lib smoking (week 1), complete cessation (week 2), and resumption of smoking (week 3). TV watching (i.e., nondietary activity) and subjective measures of craving and tension-anxiety also were assessed. Results showed increased intake of sweets and, to a lesser extent, alcohol after cessation which was reversed upon resumption of smoking. There were no significant changes across weeks in other substances. The changes in alcohol and sweet intake did not appear to be related directly to the subjective changes. These findings indicate that smoking cessation, a behavior change which promotes health, may lead to alterations in consumption of other substances, which may have adverse effects on health risk.