Maturation of caffeine N-demethylation in infancy: a study using the 13CO2 breath test

Caffeine: cardiorespiratory effects and tissue protection in animal models

The aim of this review is to analyze the cardiorespiratory and tissue-protective effects of caffeine in animal models. Peer-reviewed literature published between 1975 and 2021 was retrieved from CAB Abstracts, PubMed, ISI Web of Knowledge, and Scopus. Extracted data were analyzed to address the mechanism of action of caffeine on cardiorespiratory parameters (heart rate and rhythm), vasopressor effects, and some indices of respiratory function; we close this review by discussing the current debate on the research carried out on the effects of caffeine on tissue protection. Adenosine acts through specific receptors and is a negative inotropic and chronotropic agent. Blockage of its cardiac receptors can cause tachycardia (with arrhythmogenic potential) due to the intense activity of β1 receptors. In terms of tissue protection, caffeine inhibits hyperoxia-induced pulmonary inflammation by decreasing proinflammatory cytokine expression in animal models. The protection that caffeine provides to tissues is not limited to the CNS, as studies have demonstrated that it generates attenuation of inflammatory effects in pulmonary tissue. It inhibits the effects of some pro-inflammatory cytokines and prevents functional and structural changes.

Proof of Concept: First Pediatric [14 C]microtracer Study to Create Metabolite Profiles of Midazolam

Growth and development affect drug-metabolizing enzyme activity thus could alter the metabolic profile of a drug. Traditional studies to create metabolite profiles and study the routes of excretion are unethical in children due to the high radioactive burden. To overcome this challenge, we aimed to show the feasibility of an absorption, distribution, metabolism, and excretion (ADME) study using a [14 C]midazolam microtracer as proof of concept in children. Twelve stable, critically ill children received an oral [14 C]midazolam microtracer (20 ng/kg; 60 Bq/kg) while receiving intravenous therapeutic midazolam. Blood was sampled up to 24 hours after dosing. A time-averaged plasma pool per patient was prepared reflecting the mean area under the curve plasma level, and subsequently one pool for each age group (0-1 month, 1-6 months, 0.5-2 years, and 2-6 years). For each pool [14 C]levels were quantified by accelerator mass spectrometry, and metabolites identified by high resolution mass spectrometry. Urine and feces (n = 4) were collected up to 72 hours. The approach resulted in sufficient sensitivity to quantify individual metabolites in chromatograms. [14 C]1-OH-midazolam-glucuronide was most abundant in all but one age group, followed by unchanged [14 C]midazolam and [14 C]1-OH-midazolam. The small proportion of unspecified metabolites most probably includes [14 C]midazolam-glucuronide and [14 C]4-OH-midazolam. Excretion was mainly in urine; the total recovery in urine and feces was 77-94%. This first pediatric pilot study makes clear that using a [14 C]midazolam microtracer is feasible and safe to generate metabolite profiles and study recovery in children. This approach is promising for first-in-child studies to delineate age-related variation in drug metabolite profiles.

13C Breath Tests in Infections and Beyond

Stable isotope labeled compounds are widely used as diagnostic probes in medicine. These diagnostic stable isotope probes are now being expanded in their scope, to provide precise indications of the presence or absence of etiologically significant change in metabolism due to a specific disease. This concept exploits a labeled tracer probe that is a specifically designed substrate of a “gateway” enzyme in a discrete metabolic pathway, whose turnover can be measured by monitoring unidirectional precursor product mass flow. An example of such a probe is the 13C-urea breath test, where labeled urea is given to patients with H. pylori infection. Another example of this kind of probe is used to study the tripeptide glutathione (glu-cys-gly, GSH), which is the most abundant cellular thiol, and protects cells from the toxic effects of reactive oxygen species. Within the gamma glutamyl cycle, 5-oxoproline (L-pyroglutamic acid) is a metabolite generated during GSH catabolism, and is metabolized to glutamic acid by 5-oxoprolinase. This enzyme can also utilize the substrate L-2-oxothiazolidone-4-carboxylate (OTC), to generate intracellular cysteine, which is beneficial to the cell. Thus, labeled (13C) OTC would, under enzymatic attack yield cysteine and 13CO2, and can thus track the state and capacity of glutathione metabolism. Similarly, stable isotope labeled probes can be used to track the activity of the rate of homocysteine clearance, lymphocyte CD26, and liver CYP (cytochrome P450) enzyme activity. In the future, these applications should be able to titrate, in vivo, the characteristics of various specific enzyme systems in the body and their response to stress or infection as well as to treatment regimes.

Interindividual Differences in Caffeine Metabolism and Factors Driving Caffeine Consumption

Most individuals adjust their caffeine intake according to the objective and subjective effects induced by the methylxanthine. However, to reach the desired effects, the quantity of caffeine consumed varies largely among individuals. It has been known for decades that the metabolism, clearance, and pharmacokinetics of caffeine is affected by many factors such as age, sex and hormones, liver disease, obesity, smoking, and diet. Caffeine also interacts with many medications. All these factors will be reviewed in the present document and discussed in light of the most recent data concerning the genetic variability affecting caffeine levels and effects at the pharmacokinetic and pharmacodynamic levels that both critically drive the level of caffeine consumption. The pharmacokinetics of caffeine are highly variable among individuals due to a polymorphism at the level of the CYP1A2 isoform of cytochrome P450, which metabolizes 95% of the caffeine ingested. Moreover there is a polymorphism at the level of another critical enzyme, N-acetyltransferase 2. At the pharmacodynamic level, there are several polymorphisms at the main brain target of caffeine, the adenosine A2A receptor or ADORA2. Genetic studies, including genome-wide association studies, identified several loci critically involved in caffeine consumption and its consequences on sleep, anxiety, and potentially in neurodegenerative and psychiatric diseases. We start reaching a better picture on how a multiplicity of biologic mechanisms seems to drive the levels of caffeine consumption, although much more knowledge is still required to understand caffeine consumption and effects on body functions.

Mechanisms of modulation of cytokine release by human cord blood monocytes exposed to high concentrations of caffeine

BACKGROUND

Serum caffeine concentrations >20 μg/ml (100 μmol/l) in infants treated for apnea of prematurity increases TNF-α and decreases IL-10, changes that perhaps are linked to comorbidities. We hypothesize that this proinflammatory cytokine profile may be linked to differential binding of caffeine to adenosine receptor subtypes (AR), inhibition of phosphodiesterases (PDEs), and modulation of toll-like receptors (TLR).

METHODS

Lipopolysaccharide-activated cord blood monocytes (CBM) from 19 infants were exposed to caffeine (0-200 μmol/l) with or without previous exposure to A1R, A3R, or PDE IV antagonists to determine changes in dose-response curves. Cytokines levels (enzyme-linked immunosorbent assay (ELISA)), intracellular cyclic adenosine monophosphate (cAMP) accumulation (enzyme immunoassay (EIA)), and TLR gene expression (real time qRT PCR) were measured.

RESULTS

Caffeine at ≤100 μmol/l decreased TNF-α levels (~25%, P = 0.01) and cAMP. All caffeine concentrations decreased IL-10 levels (17-35%, P < 0.01). A1R, A3R, and PDE blockades decreased TNF-α (31, 21, and 88%, P ≤ 0.01), but not IL-10. Caffeine further decreased TNF-α following A3R and PDE blockades. Caffeine concentrations directly correlated to TLR4 gene expression (r = 0.84; P < 0.001).

CONCLUSION

Neither A3R, nor PDE blockades are involved in caffeine's modulation of cytokine release by CBM at any concentration. Besides A1R blockade, caffeine's upregulation of TLR4 may promote inflammation at high concentrations.

Decreased metabolism of 13C-caffeine via hepatic CYP1A2 in marasmus and kwashiorkor based on breath test

BACKGROUND

Hepatic metabolism of drugs has been rarely studied in children with malnutrition. Caffeine breath test (CBT) has been used to determine the activity of cytochrome P450 1A2 (CYP1A2) enzymes in children. We used the CBT to assess how marasmus and kwashiorkor differentially affect 13C-caffeine metabolism.

METHODS

A total of 45 children composed of 15 children per group of malnutrition (marasmus, marasmic-kwashiorkor, and kwashiorkor) were studied during malnutrition and after nutritional recovery. After an overnight fast, patients ingested 3 mg/kg 13C-caffeine at 0900 h. Duplicate breath samples were collected into an Exetainer bottle at -20, -10, and -1 min and at 15-min intervals for 2 h. The mean cumulative percent dose recovered (CPDR) of 13C-caffeine in the expired 13CO2 was determined over the study period.

RESULTS

The CPDR in the expired 13CO2 in 2 h significantly increased after nutritional recovery in children with marasmus (from 6.80%±3.00% to 7.67%±2.81%; Student's t-test, p=0.001), marasmic-kwashiorkor (from 6.61%±2.26% to 7.56%±2.46%, p=0.041), and kwashiorkor (from 6.29%±1.06% to 7.20%±1.80%, p=0.002). When the three groups of malnutrition were compared, there was no significant difference in their mean CPDR in 2 h during malnutrition [p=0.820, analysis of variance (ANOVA)] and after nutritional recovery (p=0.810, ANOVA).

CONCLUSIONS

Hepatic metabolism of caffeine significantly decreased in children with marasmus, marasmic-kwashiorkor, and kwashiorkor compared to after they had recovered nutritionally. This suggests a decreased CYP1A2 activity in all categories of malnutrition.

Effect of Casilan(®) on (13)C-caffeine metabolism in overnight-fasted healthy Nigerian children

Objective:

To determine the effect of Casilan^® on ¹³C-caffeine metabolism in healthy Nigerian children.

Materials and Methods:

Twelve healthy Nigerian children (male: six, female: six) aged 3–8 years were studied on three occasions. After an overnight fast, the children were studied after ingesting Casilan^® only (Week 1). They were restudied after ingesting 3 mg/kg of labeled caffeine only (Week 2), and further re-studied after ingesting both Casilan^® and labeled caffeine (Week 3). Breath samples were collected by blowing via a straw into an exentainer bottle. The cumulative percentage of ¹³C-caffeine exhaled as ¹³CO₂ was measured over 2 h.

Results:

The time courses of ¹³C-enrichments in exhaled CO₂ for all the children, after they had ingested labeled caffeine only and after they had ingested both Casilan^® and labeled caffeine, were identical. There was a gradual rise and peak of the enrichments at about 60–75 min, followed by a gradual fall (II) or a plateau (III). Contrarily, the time course of ¹³C-enrichments for all the children was consistently low and stable after they had ingested Casilan^® only (I). The mean values of cumulative percent ¹³C-doses recovered in the CO₂ exhaled over a 2-h period, after ingesting labeled caffeine only (8.59 ± 1.10 δ%/mg) and after ingesting both Casilan^® and labeled caffeine (8.58 ± 1.33 δ%/mg), were identical, with no statistically significant difference (P = 0.972). This suggests that Casilan^® did not affect the CYP1A2 metabolic pathway.

Conclusions:

Casilan^® is a safe, reliable and quantitative food supplement for overnight-fasted children undergoing caffeine breath test.

Profound changes in drug metabolism enzymes and possible effects on drug therapy in neonates and children

INTRODUCTION

There are profound changes that take place in drug metabolism enzymes during fetal and postnatal development. These changes may significantly impact drug therapy in children.

AREAS COVERED

A combination of focused and comprehensive literature searches using PubMed and reference lists (from inception to 7 November 2009) is undertaken to identify reports on in vitro and in vivo development of drug metabolism enzymes as well disposition of selected drugs and their effect in children. The article provides an update on development of drug metabolism enzymes and their impact on drug substrate disposition and disease, which may aid to improve clinical practice and optimally design clinical trials in children.

EXPERT OPINION

Drug metabolism enzyme activity changes profoundly throughout the continuum of postnatal development and often results in different disposition pathways than in adults. Genetics and co-morbidity interact significantly with these developmental changes. Translation of existing knowledge into age-adjusted dosing guidelines and clinical trial design is highly needed for there to be an improvement in drug therapy in children.

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.

Sample collection, biobanking, and analysis

Pediatric pharmacokinetic studies require sampling of biofluids from neonates and children. Limitations on sampling frequency and sample volume complicate the design of these studies. In addition, strict guidelines, designed to guarantee patient safety, are in place. This chapter describes the practical implications of sample collection and their storage, with special focus on the selection of the appropriate type of biofluid and withdrawal technique. In addition, we describe appropriate measures for storage of these specimens, for example, in the context of biobanking, and the requirements on drug assay methods that they pose. Pharmacokinetic studies in children are possible, but they require careful selection of an appropriate sampling method, specimen volume, and assay method. The checklist provided could help prospective researchers with the design of an appropriate study protocol and infrastructure.

A systematic review of pharmacokinetics studies in children with protein-energy malnutrition

OBJECTIVE

protein energy malnutrition (PEM) is a nutritional problem affecting many children world-wide. Its association with a wide spectrum of infections necessitates multiple drug therapies. A systematic review was performed to determine the effects of PEM on drug pharmacokinetics.

METHODS

literature searches in the MEDLINE and EMBASE databases (January 1960 to December 2009) were performed. Malnutrition, undernutrition, underweight, protein-energy malnutrition, protein-calorie malnutrition, marasmus, marasmic-kwashiorkor or kwashiorkor was the medical subject heading (MeSH) descriptor used. Inclusion criteria were abstracts that assessed or discussed absorption, distribution, metabolism, elimination, clearance, pharmacokinetics or pharmacodynamics of drugs, except micronutrients and appetite-stimulating drugs.

RESULTS

altogether, 41 publications were identified. A total of 34 drugs were studied. The absorption of 18 drugs was studied; the extent of absorption (AUC) was unaffected for 10 drugs. The plasma protein binding of 20 drugs was evaluated; it was significantly reduced for 12 drugs. The volume of distribution (Vd) of 13 drugs was evaluated; it was, however, unaffected for most of the drugs. The effect of PEM on total clearance and the half-life of drugs primarily metabolised by the liver was studied for 8 drugs. There was decreased total clearance and an associated increased half-life of 5 drugs. For 2 drugs (chloramphenicol and quinine), different degrees of PEM affected total clearance differently. The total clearance of six drugs primarily eliminated by the kidneys was studied; it was unaffected for four drugs, but significantly decreased for two drugs (cefoxitin and penicillin).

CONCLUSIONS

considering the proportion of children affected with PEM world-wide, there have been relatively few pharmacokinetic studies of drugs frequently used for their treatment. More studies are therefore required to establish the appropriate dose and safety of these drugs for PEM children. The studies need to recognise that PEM is a disease spectrum and should further look at the differential effects of kwashiorkor and marasmus on drug pharmacokinetics in children.

Challenges for drug studies in children: CYP3A phenotyping as example

A paucity of data exists on the disposition and effect of drugs in young children. This information gap can be reduced by elucidating developmental principles of absorption, distribution, metabolism and excretion (ADME) in vivo. Such knowledge might enable the prediction of the disposition of individual drugs in children over the whole pediatric age range. CYP3A, the most abundant human drug metabolizing enzyme, is involved in the metabolism of more than 50% of all marketed drugs. Hence, elucidating the developmental pattern of CYP3A in relation to genetic background, disease and comedications might greatly enhance our knowledge on drug disposition in children. Several methods have been used to determine in vivo CYP3A activity in human adults, while similar studies in children face several ethical, practical and scientific challenges. The aim of this review is to identify these challenges and offer feasible solutions for studying drugs in young children, with an emphasis on CYP3A phenotyping as an example.

13C-Erythromycin Breath Test as a Noninvasive Measure of CYP3A Activity in Newborn Infants: A Pilot Study

The intravenous C-erythromycin breath test (EBMT) has been largely used in adults as a validated probe to measure hepatic cytochrome P450 3A4 and 3A5 (CYP3A4/5) activity in vivo. Additionally, the oral EBMT has been suggested to measure combined hepatic and intestinal CYP3A4/5 activity. Both hepatic and intestinal CYP3A4/5 activities are low in neonates, but the exact developmental pattern is not known. Also, a knowledge gap exists on the impact of comedication or disease state on CYP3A4/5 activity in this population. However, to use the radioactive test in newborns is not feasible, for obvious ethical reasons. Hence, the aim of this pilot study was to determine if stable isotope-labeled C-erythromycin could be used alternatively. Preterm infants who needed treatment with erythromycin for ureaplasma infection were given an oral 10 to 15 mg/kg C-(N-dimethyl)-erythromycin dose. Pharmacy regulations did not permit intravenous administration. Exhaled air samples were collected predose and up to 24 hours post-dose and analyzed for CO2 and CO2 with gas chromatography-mass spectrometry. Three patients received oral C-erythromycin. CO2 did not change significantly from baseline, showed a maximum blood concentration at 20 hours (+12%), and decreased over 24 hours (-16%) in these patients, respectively. Because none of these patients showed a consistent peak in C enrichment, in accordance with maximum blood concentration of oral erythromycin in preterms, we stopped this pilot trial after 3 patients. In conclusion, the lack of a consistent change in exhaled CO2 after oral C-erythromycin in this pilot study precludes the routine use of oral 13C-EBMT in preterm infants as a noninvasive probe of CYP3A4/5 activity. We speculate that this lack of change is due to developmentally low intestinal and hepatic CYP3A activity.

Differences in absorption, distribution, metabolism and excretion of xenobiotics between the paediatric and adult populations

In children, the therapeutic benefits and potential risks associated with drug treatment may be different from those in adults and will depend on the exposure, receptor sensitivity and relationship between effect and exposure. In this paper, key factors undergoing maturational changes accounting for differences in drug metabolism and disposition in the paediatric population compared with adults are reviewed. Gastric and duodenal pH, gastric emptying time, intestinal transit time, secretion and activity of bile and pancreatic fluid, bacterial colonisation and transporters, such as P-glycoprotein (P-gp), are important factors for drug absorption, whereas key factors explaining differences in drug distribution between the paediatric population and adults are organ size, membrane permeability, plasma protein concentration and characteristics, endogenous substances in plasma, total body and extracellular water, fat content, regional blood flow and transporters such as P-gp, which is present not only in the gut, but also in liver, kidney, brain and other tissues. As far as drug metabolism is concerned, important differences have been found in the paediatric population compared with adults both for phase I enzymes (oxidative [e.g., cytochrome P450 (CYP)1A2, and CYP3A7 versus -3A4], reductive and hydrolytic enzymes) and phase II enzymes (e.g., N-methyltransferases and glucuronosyltransferases). Generally, the major enzyme differences observed in comparison with the adult age are in newborn infants, although for some enzymes (e.g., glucuronosyltransferases and other phase II enzymes) important differences still exist between infants and toddlers and adults. Finally, key factors undergoing maturational changes accounting for differences in renal excretion in the paediatric population compared with adults are glomerular filtration and tubular secretion. The ranking of the key factors varies according to the chemical structure and physicochemical properties of the drug examined, as well as to the characteristics of its formulation. It would be important to generate additional information on the developmental aspects of renal P-gp and of other renal transporters, as has been done and is still being done with the different -isozymes involved in drug metabolism.

A Mechanistic Approach for the Scaling of Clearance in Children

BACKGROUND AND OBJECTIVE

Clearance is an important pharmacokinetic concept for scaling dosage, understanding the risks of drug-drug interactions and environmental risk assessment in children. Accurate clearance scaling to children requires prior knowledge of adult clearance mechanisms and the age-dependence of physiological and enzymatic development. The objective of this research was to develop and evaluate ontogeny models that would provide an assessment of the age-dependence of clearance.

METHODS

Using in vitro data and/or in vivo clearance values for children for eight compounds that are eliminated primarily by one process, models for the ontogeny of renal clearance, cytochrome P450 (CYP) 3A4, CYP2E1, CYP1A2, uridine diphosphate glucuronosyltransferase (UGT) 2B7, UGT1A6, sulfonation and biliary clearance were developed. Resulting ontogeny models were evaluated using six compounds that demonstrated elimination via multiple pathways. The proportion of total clearance attributed to each clearance pathway in adults was delineated. Each pathway was individually scaled to the desired age, inclusive of protein-binding prediction, and summed to generate a total plasma clearance for the child under investigation. The paediatric age range included in the study was premature neonates to sub-adults.

RESULTS

There was excellent correlation between observed and predicted clearances for the model development (R2 = 0.979) and test sets (Q2 = 0.927). Clearance in premature neonates could also be well predicted (development R2 = 0.951; test Q2 = 0.899).

CONCLUSION

Paediatric clinical trial development could greatly benefit from clearance scaling, particularly in guiding dosing regimens. Furthermore, since the proportion of clearance via different elimination pathways is age-dependent, information could be gained on the developmental extent of drug-drug interactions.

Human variability in xenobiotic metabolism and pathway-related uncertainty factors for chemical risk assessment: a review

This review provides an account of recent developments arising from a database that defined human variability in phase I metabolism (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, hydrolysis, alcohol dehydrogenase), phase II metabolism (N-acetyltransferases, glucuronidation, glycine conjugation, sulphation) and renal excretion. This database was used to derive pathway-related uncertainty factors for chemical risk assessment that allow for human variability in toxicokinetics. Probe substrates for each pathway of elimination were selected on the basis that oral absorption was >95% and that the metabolic route was the primary route of elimination of the compound (60-100% of a dose). Intravenous data were used for compounds for which absorption was variable. Human variability in kinetics was quantified for each compound from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups of the population using parameters relating to chronic exposure (metabolic and total clearances, area under the plasma concentration-time curve (AUC)) and acute exposure (Cmax) (data not presented here). The pathway-related uncertainty factors were calculated to cover 95%, 97.5% and 99% of the population of healthy adults and of each subgroup. Pathway-related uncertainty factors allow metabolism data to be incorporated into the derivation of health-based guidance values. They constitute an intermediate approach between the general kinetic default factors (3.16) and a chemical-specific adjustment factor. Applications of pathway-related uncertainty factors for chemical risk assessment and future refinements of the approach are discussed. A knowledge-based framework to predict human variability in kinetics for xenobiotics showing a threshold dose below which toxic effects are not observed, is proposed to move away from default assumptions.

Impact of inter-individual differences in drug metabolism and pharmacokinetics on safety evaluation

Safety evaluation aims to assess the dose-response relationship to determine a dose/level of exposure for food contaminants below which no deleterious effect is measurable that is 'without appreciable health risk' when consumed daily over a lifetime. These safe levels, such as the acceptable daily intake (ADI) have been derived from animal studies using surrogates for the threshold such as the no-observed-adverse-effect-level (NOAEL). The extrapolation from the NOAEL to the human safe intake uses a 100-fold uncertainty factor, defined as the product of two 10-fold factors allowing for human variability and interspecies differences. The 10-fold factor for human variability has been further subdivided into two factors of 10(0.5) (3.16) to cover toxicokinetics and toxicodynamics and this subdivsion allows for the replacement of an uncertainty factor with a chemical-specific adjustment factor (CSAF) when compound-specific data are available. Recently, an analysis of human variability in pharmacokinetics for phase I metabolism (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, hydrolysis, alcohol dehydrogenase), phase II metabolism (N-acetyltransferase, glucuronidation, glycine conjugation, sulphation) and renal excretion was used to derive pathway-related uncertainty factors in subgroups of the human population (healthy adults, effects of ethnicity and age). Overall, the pathway-related uncertainty factors (99th centile) were above the toxicokinetic uncertainty factor for healthy adults exposed to xenobiotics handled by polymorphic metabolic pathways (and assuming the parent compound was the proximate toxicant) such as CYP2D6 poor metabolizers (26), CYP2C19 poor metabolizers (52) and NAT-2 slow acetylators (5.2). Neonates were the most susceptible subgroup of the population for pathways with available data [CYP1A2 and glucuronidation (12), CYP3A4 (14), glycine conjugation (28)]. Data for polymorphic pathways were not available in neonates but uncertainty factors of up to 45 and 9 would allow for the variability observed in children for CYP2D6 and CYP2C19 metabolism, respectively. This review presents an overview on the history of uncertainty factors, the main conclusions drawn from the analysis of inter-individual differences in metabolism and pharmacokinetics, the development of pathway-related uncertainty factors and their use in chemical risk assessment.

Drug metabolism and disposition in children

Key factors undergoing maturational changes accounting for differences in drug metabolism and disposition in the pediatric population compared with adults are reviewed. Gastric and duodenal pH, gastric emptying time, intestinal transit time, bacterial colonization and probably P-glycoprotein are important factors for drug absorption, whereas key factors explaining differences in drug distribution between the pediatric population and adults are membrane permeability, plasma protein concentration and plasma protein characteristics, endogenous substances in plasma, total body and extracellular water, fat content, regional blood flow and probably P-glycoprotein, mainly that present in the gut, liver and brain. As far as drug metabolism is concerned, important differences have been found in the pediatric population compared with adults both for phase I enzymes [oxidative (e.g. cytochrome CYP3A7 vs. CYP3A4 and CYP1A2), reductive and hydrolytic enzymes] and phase II enzymes (e.g. N-methyltransferases and glucuronosyltransferases). Finally, key factors undergoing maturational changes accounting for differences in renal excretion in the pediatric population compared with adults are glomerular filtration and tubular secretion. It would be important to generate information on the developmental aspects of renal P-glycoprotein and of other renal transporters as done and still being done with the different isozymes involved in drug metabolism.

Review article: breath testing for human liver function assessment

Carbon-labelled breath tests were proposed as tools for the evaluation of human liver function 30 years ago, but have never become part of clinical routine. One reason for this is the complex role of the liver in metabolic regulation, making it difficult to provide essential information for the management of patients with liver disease with a single test and to satisfy the hepatology community. As a result, a battery of breath tests have been developed. Depending on the test compound administered, different metabolic pathways (microsomal, cytosolic, mitochondrial) can be examined. Most available data come from microsomal function tests, whilst information about cytosolic and mitochondrial liver function is more limited. However, breath tests have shown promise in some studies, in particular to predict the outcome of patients with chronic liver disease or to monitor hepatic function after treatment. Whilst we await new substrates that can be used to measure liver function in a more valid manner, and large prospective studies to assess the usefulness of available test compounds, the aim of this review is to describe how far we have come in this controversial and unresolved issue.

Potential benefit of Bayesian forecasting for therapeutic drug monitoring in neonates

Therapeutic drug monitoring in neonate has been hampered by invasiveness of blood samplings raising ethical problems. A methodologic approach has been developped in adults and in children that is still unsufficiently developped in neonates, the Bayesian forecasting of drug plasma concentration. This method is particularly attractive in neonates using a few blood samples from an individual patient and more informations from a prior patient sample representative of the population the individual patient belongs to. The present article aims at reviewing the different procedures and methods to minimize invasiveness during therapeutic drug monitoring in neonate and at reviewing the methods for improving the quality of different dose adjustments using a Bayesian approach.

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.

Caffeine metabolism in premature infants

Caffeine has been used frequently in the treatment and prevention of apnea of prematurity. The metabolism of caffeine depends on the activities of the hepatic enzymes that vary from one infant to another. The objective of this study was to determine the influence of postnatal age (PNA), birth weight (BW), study weight (SW), gestational age (GA), postconceptual age (PCA), and gender on the maturation of caffeine metabolism in premature infants. The caffeine base was administered orally as a loading dose of 10 mg/kg, followed by a maintenance dose of 2 mg/kg every 24 hours. The steady-state concentration of caffeine and metabolites was measured in plasma taken on the 5th-day postloading dose. The molar concentration ratios for the N3 (N3-), N7 (N7-), N1 (N1-), and all methyl (Nall-) demethylation processes; clearance (CL); and the percentage of molar concentration of caffeine found in plasma to that of the total caffeine and metabolites (%CAF) were calculated from samples collected from 80 neonatal infants. The 48 male and 32 female premature infants had median (range) BW (g), GA (weeks), SW (g), PCA (weeks), and PNA (days) of 1300 (650-2260), 30 (24-34), 1630 (980-2670), 34 (29-40), and 28 (5-60), respectively. The median (range) of the ratios for the %CAF, CL, and the N3-, N7-, N1-, and Nall- were 86.9 (52.9-99.0), 0.127 (0.046-0.503) ml.kg-1.min-1, 0.032 (0-0.438), 0.070 (0.007-0.471), 0.026 (0-0.283), and 0.0463 (0.003-0.303), respectively. When the patients were stratified into four PNA age groups, each older group showed a consistently higher level of caffeine metabolic activity for the N3-, N7-, and Nall- pathways with a corresponding decrease in the %CAF, whereas no significant differences were seen for the N1-pathway or for CL. No pattern of significant differences between the demethylation process ratios, %CAF, or CL was seen between groups of infants when they were stratified according to BW, SW, PCA, or GA. The female infants were found to have significantly higher rates of caffeine metabolism as shown by %CAF, N1-, N3-, and Nall- processes but not the N7-. Multivariate linear regression analysis by two methods demonstrated that PNA is significantly related to %CAF and Nall-, whereas the female patients had higher levels of metabolic activity for the %CAF and N1- process. The authors conclude that the N7-demethy-lation process is the predominate caffeine metabolic process in premature infants. Furthermore, the maturation of the caffeine metabolism in premature infants with a PNA of less than 60 days increases with postnatal age, regardless of birth weight, gestational age, postconceptual age, and study weight. The female neonatal patients demonstrated a higher rate of caffeine metabolism than the males.

13C breath tests: visions and realities

Breath tests have been used in research laboratories for over 25 y. Originally, the tests were based on the use of (14)C, rather than on the nonradioactive isotope, (13)C. When (13)C became widely available at a reasonable cost, research groups in the United States and Europe developed methodologies to measure (13)C abundance in samples of CO(2). The tests used a variety of substrates and measured pancreatic function, fat absorption, bacterial overgrowth and P(450) mixed-function oxidase. Thus far, the only test to be approved by the Food and Drug Administration is the (13)C-urea breath test. This manuscript describes the process by which approval is gained, and indicates the steps necessary for other tests to receive Food and Drug Administration approval.

Phenotyping of drug-metabolizing enzymes in adults: a review of in-vivo cytochrome P450 phenotyping probes

Cytochrome P450 phenotyping provides valuable information about real-time activity of these important drug-metabolizing enzymes through the use of specific probe drugs. Despite more than 20 years of research, few conclusions regarding optimal phenotyping methods have been reached. Caffeine offers many advantages for CYP1A2 phenotyping, but the widely used caffeine urinary metabolic ratios may not be the optimal method of measuring CYP1A2 activity. Several probes of CYP2C9 activity have been suggested, but little information exists regarding their use, largely due to the narrow therapeutic index of most CYP2C9 probes. Mephenytoin has long been considered the standard CYP2C19 phenotyping probe, but problems such as sample stability and adverse effects have prompted the investigation of potential alternatives, such as omeprazole. Several well-validated CYP2D6 probes are available, including dextromethorphan, debrisoquin and sparteine, but, in most cases, dextromethorphan may be preferred due to its wide safety margin and availability. Chlorzoxazone remains the only CYP2E1 probe that has received much study. However, questions concerning phenotyping method and involvement of other enzymes have impaired its acceptance as a suitable CYP2E1 phenotyping probe. CYP3A phenotyping has been the subject of numerous investigations, reviews and commentaries. Nevertheless, much controversy regarding the selection of an ideal CYP3A probe remains. Of all the proposed methods, midazolam plasma clearance and the erythromycin breath test have been the most rigorously studied and appear to be the most reliable of the available methods. Despite the limitations of many currently available probes, with continued research, phenotyping will become an even more valuable research and clinical resource.

Caffeine based measures of CYP1A2 activity correlate with oral clearance of tacrine in patients with Alzheimer's disease

AIMS

To study the potential utility of caffeine based probes of CYP1A2 enzyme activity in predicting the pharmokinetics of tacrine in patients with Alzheimer's disease.

METHODS

The pharmokinetics of a single 40 mg oral dose of tacrine were measured in 19 patients with Alzheimer's disease. Each patient also received 2 mg kg(-1) [13C-3-methyl] caffeine orally and had breath and urine samples collected.

RESULTS

Tacrine oral clearance (CL F(-1) kg(-1)), which varied 15-fold among the patients, correlated significantly with the 2 h total production of 13CO2 in breath (r=0.56, P=0.01), and with each of two commonly used urinary caffeine metabolite ratios: the 'paraxanthine/caffeine ratio' (1,7X + 1, 7U)/1,3,7X) (r=0.76, P=0.0002) and the 'caffeine metabolic ratio' (AFMU + 1X + 1U)/1, 7U)(r=0.76, P=0.0001).

CONCLUSIONS

These observations support a central role for CYP1A2 in the in vivo disposition of tacrine and the potential for drug interactions when tacrine treated patients receive known inducers or inhibitors of this enzyme. The magnitude of the correlations we observed, however, are probably not sufficient to be clinically useful in individualizing tacrine therapy.

Developmental expression of cytochrome P450 enzymes in human liver

Drug-metabolizing cytochrome P450 enzymes, the major phase I enzymes, are active in human liver already at very early stages of intrauterine development, although presumably at fairly low concentrations and in low numbers. During maturation, these enzymes go through various developmental programmes towards adulthood. The major increase both in abundance as well as in number of different enzymes takes place after birth, probably during the first year of life. Detailed information concerning these developmental changes is still limited. The major drug-metabolizing P450 enzymes appear to be primarily members of the CYP3A subfamily in all stages of development. The balance between different members of this subfamily, however, undergoes significant switches from the foetal predominant CYP3A7 to the major adult form CYP3A4. The ontogeny of the other cytochrome P450 enzymes is less well characterized, but the major switch-on appears to occur mainly after birth. Developmental expression of P450 enzymes is one of the key factors determining the pharmacokinetic status of developing individuals both pre- and postnatally.

Inhibition of caffeine metabolism by ciprofloxacin in children with cystic fibrosis as measured by the caffeine breath test

The caffeine breath test was carried out in six children with cystic fibrosis, before and during a course of ciprofloxacin. There was a significant decrease in the 2 h cumulative labelled CO2 exhaled during ciprofloxacin treatment, mean difference (s.d.) -5.2(3.3)%, P < 0.02. The results suggest an inhibition of 3-N-demethylation of caffeine (CYP1A2 enzyme activity) by ciprofloxacin. Ciprofloxacin may cause significant drug interactions in children with cystic fibrosis. The caffeine breath test can be used to study drug interactions involving CYP1A2 in children.

Biotransformation of caffeine in human liver microsomes from foetuses, neonates, infants and adults

1. Caffeine metabolism was studied in human liver microsomes from foetuses (n = 10), neonates (n = 10), infants (n = 9) and adults (n = 5). Caffeine and its metabolites, 1-3-7-trimethyluric acid, paraxanthine, theophylline and theobromine, were assayed by h.p.l.c. Methoxyresorufin-O-demethylase activity (MEROD) was determined and immunoquantifiable levels of CYP1A2 were measured. 2. The formation of the dimethylxanthines by N-3, N-7 or N-1-demethylation was significantly less in foetuses, neonates and infants than in adults, as shown previously in vivo. The formation of 1-3-7-trimethyluric acid (C-8-hydroxylation) was not significantly different between age groups. The production of total dimethylxanthines, paraxanthine and theophylline increased significantly with age within the neonate-infant group over at least the 0-300 day range (rs = 0.739, 0.667, 0.682, respectively). These data differ from those reported in vivo which suggested that N-3 and N-7-demethylations matured at about 120 days. The difference in maturational profiles of each metabolic pathway suggests that the reactions depend on different isoenzymes. The delay in the maturation of N-1 compared with N-3 and N-7-demethylation is in agreement with previous in vivo data. 3. In the neonate-infant group, only N-3-demethylation correlated with both MEROD activity (rs = 0.681; P < 0.05) and CYP1A2 microsomal concentration (rs = 0.454; P approximately 0.05), suggesting that, as in adults, this reaction depends on CYP1A2. 4. In the foetal samples, the production of total dimethylxanthines, paraxanthine and theobromine decreased significantly (rs = -0.879, -0.767, -0.708, respectively) with increasing gestational age.(ABSTRACT TRUNCATED AT 250 WORDS)