Biotransformation of caffeine by microsomes from human liver. Kinetics and inhibition studies

A double-blind, randomized, two-part, two-period crossover study to evaluate the pharmacokinetics of caffeine versus d9-caffeine in healthy subjects

The deuterium kinetic isotope effect has been used to affect the cytochrome P450 metabolism of the deuterated versions of substances. This study compares the pharmacokinetics of caffeine, a Generally Recognized As Safe food and beverage ingredient, versus d9-caffeine, a potential caffeine alternative, and their respective metabolites at two dose levels in 20 healthy adults. A single dose of 50 mg or 250 mg of caffeine, or a molar-equivalent dose of d9-caffeine, were orally administered in solution with blood samples collected for up to 48 h post-dose. Plasma concentrations of parent and metabolites were analyzed using validated LC-MS/MS methods. Both d9-caffeine and caffeine were rapidly absorbed; however, d9-caffeine exhibited a higher (ca. 29%-43%) C_max and 4-5-fold higher AUC_last than caffeine, and lower C_max, lower AUC_last, and a 5-10-fold reduction in the relative exposure to the active metabolites of caffeine. Results were consistent in normal and rapid metabolizers, and both substances were well tolerated.

Employing in vitro metabolism to guide design of F-labelled PET probes of novel α-synuclein binding bifunctional compounds

A challenge in the development of novel ¹⁸F-labelled positron emission tomography (PET) imaging probes is identification of metabolically stable sites to incorporate the ¹⁸F radioisotope. Metabolic loss of ¹⁸F from PET probes in vivo can lead to misleading biodistribution data as displaced ¹⁸F can accumulate in various tissues.In this study we report on in vitro hepatic microsomal metabolism of novel caffeine containing bifunctional compounds (C₈-6-I, C₈-6-N, C₈-6-C₈) that can prevent in vitro aggregation of α-synuclein, which is associated with the pathophysiology of Parkinson's disease. The metabolic profile obtained guided us to synthesize stable isotope ¹⁹F-labelled analogues in which the fluorine was introduced at the metabolically stable N7 of the caffeine moiety.An in vitro hepatic microsomal metabolism study of the ¹⁹F-labelled analogues resulted in similar metabolites to the unlabelled compounds and demonstrated that the fluorine was metabolically stable, suggesting that these analogues are appropriate PET imaging probes. This straightforward in vitro strategy is valuable for avoiding costly stability failures when designing radiolabelled compounds for PET imaging.

The importance of early use of beta blockers and gastric decontamination in caffeine overdose: A case report

Caffeine is a common stimulant consumed daily worldwide and available in a wide variety of over-the-counter formulations. It is a mild central nervous system stimulant when used in recommended doses. However, it can be fatal if taken as an intentional or accidental overdose. We report a case of a 48-year-old lady with depression and post-traumatic stress disorder who consumed a significant overdose of caffeine, triggered by the stress that she had contracted coronavirus disease 19. This led to significant cardiovascular and central nervous system toxicity. The condition was identified early and managed appropriately with early β-blockers and gastric decontamination, which saved her life. There are few studies with regard to such modalities on treatment for caffeine overdose; our patient responded rapidly and favourably to the treatment. Why should an emergency physician be aware of this? Caffeine overdose is uncommon but one that clinicians should be aware of. Early identification and intervention with β-adrenergic antagonists and activated charcoal is paramount in caffeine toxicity.

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.

Physiologically Based Pharmacokinetic/Pharmacodynamic Model for Caffeine Disposition in Pregnancy

Caffeine is the most consumed active stimulant. About 80% of pregnant women consume caffeine orally on a daily basis. Many reports indicated consumption of >200 mg caffeine during pregnancy could increase the likelihood of miscarriage. In this article, we developed a pregnancy physiological-based pharmacokinetic/pharmacodynamic (PBPK/PD) model for caffeine to examine association between maternal caffeine consumption during pregnancy and caffeine plasma levels at doses lower and higher than 200 mg to predict changes in caffeine concentrations across the 3 trimesters, and to predict associated changes in caffeine PD parameters. Two models were successfully developed using GastroPlus software, a nonpregnant model for validation purposes and a pregnant model for validation and prediction of maternal caffeine plasma concentrations following single and multiple dosing. Using observed and predicted data, we were able to validate and simulate PK changes of caffeine in nonpregnant women and the PD effect of caffeine on certain enzymes and catecholamines associated with caffeine intake. Furthermore, the pregnancy PBPK model successfully predicted changes in caffeine PK across the three trimesters. Caffeine increased exposure during pregnancy was related to reduced activity of caffeine metabolizing enzyme CYP1A2. The model also predicted increased levels of caffeine in the fetoplacental compartment (FPC) due to increased maternal caffeine plasma concentrations. Increased caffeine levels in maternal blood was accompanied by greater inhibition of the phosphodiesterase enzyme, higher cyclic adenosine monophosphate, and greater increase of epinephrine levels, which could increase the risk of pregnancy loss. The application of the developed PBPK model to predict the PD effect could provide a useful tool to help define potential cut-offs for caffeine intake in various stages of pregnancy.

Phenotyping of CYP 4501A2 Activity by Total Overnight Salivary Caffeine Assessment (TOSCA) in Patients on Warfarin Treatment: A Cross-Sectional Study

Warfarin is an oral anticoagulant, commonly used for primary and secondary prevention of venous and arterial thromboembolic events. The drug is characterized by narrow therapeutic index, widespread individual variability in clinical response, and high rates of adverse events, particularly bleeding complications. For these reasons, a close monitoring of the dosage, using the frequent assessment of coagulation status by means of International Normalized Ratio value, is mandatory. Warfarin is metabolized by hepatic cytochrome P-450. High CYP 450 activity may lead to low drug concentration and requires high warfarin doses to reach efficacy; conversely, low CYP 450 activity is responsible for high drug concentration and needs for low doses to avoid potential toxicity risks. The major isoforms of CYP involved in the metabolism of warfarin sodium are CYP1A2 (for the R-warfarin) and CYP2C9 (for the S-warfarin). The probes for testing CYP1A2 are phenacetin and caffeine while for CYP2C9 tolbutamide. Although S-warfarin has major activity, it was decided to exclude its phenotyping for ethical issues, being mandatory to use a drug (tolbutamide). Instead, it was chosen to test the 1A2 isoform, as the activity of the latter isoform could be investigated by using caffeine contained in the caffeinated beverages. Specifically, a single-point concentration of salivary caffeine (total overnight salivary caffeine assessment [TOSCA]) after an overnight period of the caffeinated beverages abstinence was utilized. In the present study, 75 nonsmoker patients regularly receiving warfarin sodium were enrolled. The results have showed a significant association of the warfarin dose with TOSCA values (coefficient = -0.15, standard error = 0.04, 95% confidence interval = -0.24 to -0.06, t = -3.23, P = .002). In conclusion, the phenotyping of CYP1A2 by TOSCA could be useful, if further proven, to help manage patients on warfarin in order to lessen severe adverse events.

Caffeine in tea Camellia sinensis--content, absorption, benefits and risks of consumption

Therapeutic properties of tea Camellia sinensis are of particular interest since it has been consumed for ages and was always regarded as safe beverage. Tea is most popular beverage in the world because of its attractive aroma, exceptional taste, health promoting and pharmaceutical potential. Current results showed that antioxidative, antibacterial and other health effects are attributed to its caffeine content and caffeine - polyphenols interactions. An overview is given on caffeine content in different tea leaves beverage. Special attention is drawn to caffeine physiological effect on human organism. Controversies concerning the possible caffeine influence on human physical and psychological health are briefly summarized and presented.

Monkey liver cytochrome P450 2C9 is involved in caffeine 7-N-demethylation to form theophylline

Caffeine (1,3,7-trimethylxanthine) is a phenotyping substrate for human cytochrome P450 1A2. 3-N-Demethylation of caffeine is the main human metabolic pathway, whereas monkeys extensively mediate the 7-N-demethylation of caffeine to form pharmacological active theophylline. Roles of monkey P450 enzymes in theophylline formation from caffeine were investigated using individual monkey liver microsomes and 14 recombinantly expressed monkey P450 enzymes, and the results were compared with those for human P450 enzymes. Caffeine 7-N-demethylation activity in microsomes from 20 monkey livers was not strongly inhibited by α-naphthoflavone, quinidine or ketoconazole, and was roughly correlated with diclofenac 4'-hydroxylation activities. Monkey P450 2C9 had the highest activity for caffeine 7-N-demethylation. Kinetic analysis revealed that monkey P450 2C9 had a high Vmax/Km value for caffeine 7-N-demethylation, comparable to low Km value for monkey liver microsomes. Caffeine could dock favorably with monkey P450 2C9 modeled for 7-N-demethylation and with human P450 1A2 for 3-N-demethylation. The primary metabolite theophylline was oxidized to 8-hydroxytheophylline in similar ways by liver microsomes and by recombinant P450s in both humans and monkeys. These results collectively suggest a high activity for monkey liver P450 2C9 toward caffeine 7-N-demethylation, whereas, in humans, P450 1A2-mediated caffeine 3-N-demethylation is dominant.

In vivo evaluation of CYP2A6 and xanthine oxidase enzyme activities in the Serbian population

PURPOSE

The main aim of the study was to investigate the distribution of cytochrome P450 2A6 (CYP2A6) and xanthine oxidase (XO) enzyme activities in the Serbian population. Secondly, we tested the influence of genetics (CYP2A6 polymorphism), sex, and cigarette smoking on both enzymes.

METHODS

One hundred forty healthy Serbian volunteers were genotyped for common CYP2A6 alleles. In 100 of them, CYP2A6 and XO activities were determined by the urinary 17U/17X and 1U/(1U + 1X) ratios, respectively, after oral administration of 100 mg caffeine as a probe.

RESULTS

A 21-fold variation in the 17U/17X ratio was observed (range: 0.49-10.28, mean = 1.65, 95% CI: 1.49-1.83). The urinary 1U/(1U + 1X) ratios displayed four-fold variation, ranging from 0.17 to 0.71 (mean = 0.43, 95% CI: 0.41-0.45). CYP2A6 alleles *1A, *1B1, *9, *4 and *1B1x2 were found with frequencies of 0.579, 0.307, 0.082, 0.029, and 0.004 respectively. CYP2A6*5 was not detected. CYP2A6 genotype influenced interindividual variability in CYP2A6 enzyme activity (P = 0.04). Cigarette smoking inhibited CYP2A6 enzyme activity (P = 0.02), but had no effect on activity of XO (P = 0.16).There was no significant difference between men and women in terms of CYP2A6 or XO activity.

CONCLUSIONS

Serbs displayed interindividual variations in CYP2A6 activity. CYP2A6 genotype and cigarette smoking, but not sex, influenced CYP2A6 enzyme activity. Unimodal distribution of XO enzyme activity in Serbs implies the absence of subjects with low enzyme activity in this population. XO activity is not influenced by sex or cigarette smoking.

The relative contribution of human cytochrome P450 isoforms to the four caffeine oxidation pathways: an in vitro comparative study with cDNA-expressed P450s including CYP2C isoforms

The aim of the present study was to estimate the relative contribution of cytochrome P450 isoforms (P450s), including P450s of the CYP2C subfamily, to the metabolism of caffeine in human liver. The experiments were carried out in vitro using cDNA-expressed P450s, liver microsomes and specific P450 inhibitors. The obtained results show that (1) apart from the 3-N-demethylation of caffeine - a CYP1A2 marker reaction and the main oxidation pathway of caffeine in man - 1-N-demethylation is also specifically catalyzed by CYP1A2 (not reported previously); (2) 7-N-demethylation is catalyzed non-specifically, mainly by CYP1A2 and, to a smaller extent, by CYP2C8/9 and CYP3A4 (and not by CYP2E1, as suggested previously); (3) C-8-hydroxylation preferentially involves CYP1A2 and CYP3A4 and, to a smaller degree, CYP2C8/9 and CYP2E1 (and not only CYP3A, as suggested previously) at a concentration of 100 microM corresponding to the maximum therapeutic concentration in humans. At a higher caffeine concentration, the contribution of CYP1A2 to this reaction decreases in favour of CYP2C8/9. The obtained data show for the first time the contribution of CYP2C isoforms to the metabolism of caffeine in human liver and suggest that apart from 3-N-demethylation, 1-N-demethylation may also be used for testing CYP1A2 activity. Moreover, they indicate that the C-8-hydroxylation is not exclusively catalyzed by CYP3A4.

Inhibitory Effects of Propafenone on the Pharmacokinetics of Caffeine in Humans

CYP1A2 is involved in the metabolism of both caffeine and propafenone, a class Ic antiarrhythmic agent. Despite the widespread consumption of caffeine, drug-drug interactions with this agent are often overlooked. This study investigated effects of propafenone on the pharmacokinetics of caffeine. Eight healthy volunteers were included in our study. A total of 300 mg of caffeine was given on 2 occasions, once alone and once during the coadministration of 300 mg propafenone. Serial blood samples were collected and pharmacokinetic parameters were estimated using a population pharmacokinetic approach. A one-compartment PK model with first-order absorption and elimination described plasma concentration profiles. Concomitant administration of propafenone decreased caffeine oral clearance from 8.3 +/- 0.9 L/h to 5.4 +/- 0.7 L/h (P < 0.05). Elimination half-life of caffeine was also increased 54% by propafenone. One of our volunteers was a poor metabolizer of CYP2D6. Concomitant administration of propafenone to this volunteer caused the greatest increase in caffeine plasma concentrations. These results support the concept of competitive inhibition between propafenone and caffeine. Our results suggest that propafenone causes significant inhibition of CYP1A2 activity leading to a decrease in the clearance of caffeine. Caffeine has intrinsic proarrhythmic effects; thus, its coadministration with an antiarrhythmic agent such as propafenone should be used with caution, especially in patients with poor CYP2D6 activity.

Probing the CYP3A4 active site by cysteine scanning mutagenesis and photoaffinity labeling

The mechanism of CYP3A4-substrate interactions has been investigated using a battery of techniques including cysteine scanning mutagenesis, photoaffinity labeling, and structural modeling. In this study, cysteine scanning mutagenesis was performed at seven sites within CYP3A4 proposed to be involved in substrate interaction and/or cooperativity. Photolabeled CYP3A4 peptide adducts were further characterized by mass spectrometric analysis for each mutant after proteolytic digestion and isolation of fluorescent photolabeled peptides. Among the tryptic peptides of seven tested mutants, three photolabeled peptides of the F108C mutant, ECYSVFTNR (positions 97-105), VLQNFSFKPCK (positions 459-469), and RPCGPVGFMK (positions 106-115) were identified by MALDI-TOF-MS and nano-LC/ESI QTOF MS. The site of modification was further localized to the substituted Cys-108 residue in the mutant peptide adduct RPCGPVGFMK (positions 106-115) by nano-LC/ESI QTOF MS/MS. In summary, we described a potentially useful method to study P450 active sites using a combination of cysteine scanning mutagenesis and photoaffinity labeling.

Caffeine Use in Sports, Pharmacokinetics in Man, and Cellular Mechanisms of Action

Caffeine is the most widely consumed psychoactive 'drug' in the world and probably one of the most commonly used stimulants in sports. This is not surprising, since it is one of the few ergogenic aids with documented efficiency and minimal side effects. Caffeine is rapidly and completely absorbed by the gastrointestinal tract and is readily distributed throughout all tissues of the body. Peak plasma concentrations after normal consumption are usually around 50 microM, and half-lives for elimination range between 2.5-10 h. The parent compound is extensively metabolized in the liver microsomes to more than 25 derivatives, while considerably less than 5% of the ingested dose is excreted unchanged in the urine. There is, however, considerable inter-individual variability in the handling of caffeine by the body, due to both environmental and genetic factors. Evidence from in vitro studies provides a wealth of different cellular actions that could potentially contribute to the observed effects of caffeine in humans in vivo. These include potentiation of muscle contractility via induction of sarcoplasmic reticulum calcium release, inhibition of phosphodiesterase isoenzymes and concomitant cyclic monophosphate accumulation, inhibition of glycogen phosphorylase enzymes in liver and muscle, non-selective adenosine receptor antagonism, stimulation of the cellular membrane sodium/potassium pump, impairment of phosphoinositide metabolism, as well as other, less thoroughly characterized actions. Not all, however, seem to account for the observed effects in vivo, although a variable degree of contribution cannot be readily discounted on the basis of experimental data. The most physiologically relevant mechanism of action is probably the blockade of adenosine receptors, but evidence suggests that, at least under certain conditions, other biochemical mechanisms may also be operational.

Evaluation of cytochrome P450 probe substrates commonly used by the pharmaceutical industry to study in vitro drug interactions

Pharmaceutical industry investigators routinely evaluate the potential for a new drug to modify cytochrome p450 (p450) activities by determining the effect of the drug on in vitro probe reactions that represent activity of specific p450 enzymes. The in vitro findings obtained with one probe substrate are usually extrapolated to the compound's potential to affect all substrates of the same enzyme. Due to this practice, it is important to use the right probe substrate and to conduct the experiment under optimal conditions. Surveys conducted by reviewers in CDER indicated that the most common in vitro probe reactions used by industry investigators include the following: phenacetin O-deethylation for CYP1A2, coumarin 7-hydroxylation for CYP2A6, 7-ethoxy-4-trifluoromethyl coumarin O-dealkylation for CYP2B6, tolbutamide 4'-hydroxylation for CYP2C9, S-mephenytoin 4-hydroxylation for CYP2C19, bufuralol 1'-hydroxylation for CYP2D6, chlorzoxazone 6-hydroxylation for CYP2E1, and testosterone 6 beta-hydroxylation for CYP3A4. We reviewed the validation information in the literature on these reactions and other frequently used reactions, including caffeine N3-demethylation for CYP1A2, S-mephenytoin N-demethylation for CYP2B6, S-warfarin 7'-hydroxylation for CYP2C9, dextromethorphan O-demethylation for CYP2D6, and midazolam 1'-hydroxylation for CYP3A4. The available information indicates that we need to continue the search for better probe substrates for some enzymes. For CYP3A4-based drug interactions it may be necessary to evaluate two or more probe substrates. In many cases, the probe reaction represents a particular enzyme activity only under specific experimental conditions. Investigators must consider appropriateness of probe substrates and experimental conditions when conducting in vitro drug interaction studies and when extrapolating the results to in vivo situations.

Application of isotopic ratio mass spectrometry for the in vitro determination of demethylation activity in human liver microsomes using N-methyl-13C-labeled substrates

The reaction of demethylation mediated by cytochrome P450 (CYP) leads to the equimolar production of demethylated metabolite and formaldehyde. From a 13C-substrate labeled on a carbon of the methyl moiety, [13C]formaldehyde (H13CHO) is liberated. A highly sensitive and specific assay involving the oxidation of H13CHO to 13CO(2) by a double-enzymatic-step reaction is reported. The 13CO(2) was quantified by the method of reverse isotopic dilution based on gas chromatography-isotope ratio mass spectrometry analysis. The method first involves the limiting step of the CYP-dependent reaction, which is stopped with a mixture of zinc sulfate 5 mM and trichloroacetic acid 100 mM. Then, the transformation of H13CHO to 13CO(2) is performed with the formaldehyde (0.2 unit) and the formate (0.2 unit) dehydrogenase NAD-dependent enzymes. The recovery of 13CO(2) from the incubation mixture was equal to 91.4 +/- 3.0%. The accuracy and the precision of the present method were within 12 and 10%, respectively. The limit of quantification was set to 25 pmol. The performance of the assay was validated on human liver microsomes with five probes: [13C]erythromycin, [1-13C]caffeine, [3-13C]caffeine, [7-13C]caffeine, and [13C(2)]aminopyrine. This method is useful for the rapid determination of N-demethylase activity of human liver microsomes from methyl-13C-substrates.

Differences in caffeine and paraxanthine metabolism between human and murine CYP1A2

For the characterisation of murine models of CYP1A2 mediated metabolism in humans we compared the metabolism of caffeine and paraxanthine in human liver microsomes (LM) (two samples) and in LM from CYP1A2-null and wild-type mice. Inhibition experiments were carried out with the quinolones norfloxacin and pefloxacin and the substrate, caffeine. Additionally, in vivo pharmacokinetics of paraxanthine was determined in CYP1A2-null and wild-type mice. All LM produced the primary metabolites of caffeine and paraxanthine. In human LM, the main metabolite of caffeine was paraxanthine (K(M) 0.4 and 0.5 mmol L(-1)). In wild-type and CYP1A2-null mice LM, the main caffeine metabolite was 1,3,7-trimethylurate, but formation was not saturable. Apparent K(M) for paraxanthine formation from caffeine in wild-type and CYP1A2-null murine LM were 0.2 and 4.9 mmol L(-1), respectively. The main metabolite of paraxanthine was 1-methylxanthine in human (K(M) 0.13 and 0.2 mmol L(-1)) and in wild-type mice LM (K(M) 0.53 mmol L(-1)). In CYP1A2-null murine LM, the main paraxanthine metabolite was 7-methylxanthine. The quinolones competitively inhibited caffeine metabolism in human but not in wild-type or CYP1A2-null murine LM. No obvious differences were seen for blood pharmacokinetics and urinary metabolite excretion of paraxanthine between CYP1A2-null and wild-type mice. Thus, for paraxanthine, norfloxacin and pefloxacin interaction with CYP1A2 there were clear differences between mice and man. Our results suggest that an interspecies comparison is required for the metabolism of individual xenobiotics interacting with CYP1A2 prior to the use of mice models to predict its toxicity and/or pharmacological activity in man.

Fluvoxamine is a potent inhibitor of the metabolism of caffeine in vitro

The selective serotonin re-uptake inhibitor, fluvoxamine, is a very potent inhibitor of CYP1A2, and accordingly causes pharmacokinetic interactions with drugs metabolised by CYP1A2, such as caffeine, theophylline, imipramine, tacrine and clozapine. Interaction between caffeine and fluvoxamine has been described in vivo, leading to lowering of total clearance of caffeine by 80% during fluvoxamine intake. The main purpose of the present study was to evaluate this interaction in vitro in human liver microsomes. A high-performance liquid chromatography method was developed in order to assay 1,3-dimethylxanthine, 1,7-dimethylxanthine, 3,7-dimethylxanthine and 1,3,7-trimethyluric acid formed from caffeine by human liver microsomes. The limit of detection was 0.06 nmol.mg protein-1.hr-1. As expected, fluvoxamine was a very potent inhibitor of the formation of the N-demethylated caffeine metabolites, displaying Ki values of 0.08-0.28 microM. The formation of 1,7-dimethylxanthine was virtually abolished by 10 microM of fluvoxamine, indicating that the N3-demethylation of caffeine is almost exclusively catalysed by CYP1A2. The CYP3A4 inhibitors, ketoconazole and bromocriptine, inhibited 1,3,7-trimethyluric acid formation with Kis of 0.75 microM and 5 microM, respectively, thus further supporting the involvement of CYP3A4 in the 8-hydroxylation of caffeine. The study shows that fluvoxamine, as expected, is a potent inhibitor of the metabolism of caffeine in vitro.

Stereoselective metabolism of bisoprolol enantiomers in dogs and humans

To clarify the mechanism of the species difference in the metabolism of bisoprolol enantiomers, in vitro metabolic studies were performed using dog liver microsomes and human cytochrome P450 (CYP) isoforms. The O-deisopropylation of bisoprolol enantiomers showed biphasic kinetics in dog liver microsomes. The intrinsic clearance (Vmax/Km) for O-deisopropylation of R(+)-bisoprolol was higher than S(-)-isomer in both high-affinity and low-affinity components. The R/S ratio of the intrinsic clearance in high- and low-affinity components was 1.34 and 1.65, respectively. The inhibition studies in dog liver microsomes using CYP isoform-selective inhibitors indicated that the O-deisopropylation of both bisoprolol enantiomers was mediated via the CYP2D and CYP3A subfamily, and suggested that high-affinity oxidation was dependent on CYP2D. The kinds of CYP subfamilies in dogs, which contribute to the metabolism of bisoprolol enantiomers, were the same as those in humans. The intrinsic clearance for O-deisopropylation of R(+)bisoprolol by human recombinant CYP2D6 was also different from that of S(-)-enantiomers (R/S:1.50). However, unlike the dog microsomes, the intrinsic clearance by the human recombinant CYP3A4 did not show a stereoselective difference. Therefore, the species difference in the R/S ratio of metabolic clearance for the oxidation of bisoprolol enantiomers (dog > human) is mainly due to the species difference in the stereoselectivity of one of the cytochrome P450 subfamilies (CYP3A).

Changes in the enzymatic activities of beagle liver during maturation as assessed both in vitro and in vivo

1. We have examined changes in caffeine and trimethadione (TMO) metabolism in vivo, agents which are used as probe drugs. In this study the total body clearance (Cl) of caffeine and TMO was low 1 week after birth (week 1), increased rapidly from week 3, peaked and then decreased gradually until reaching the level for the mature, adult dog. The elimination half-life (t1/2) of caffeine and TMO was prolonged during week 1; however, it then gradually became shorter. Gradually it became longer and reached the level for the adult dog. The apparent volume of distribution (Vd) of caffeine did not change throughout the study. However, the Vd of TMO was only high during week 1. 2. The in vitro changes in a variety of typical substrates for seven different cytochrome P450 (CYP) isozymes were examined. In this study three different patterns of metabolism can be identified: (1) activity is low immediately after birth, increases, peaks and then decreases to the adult dog level (p-nitroanisole; CYP1A1, caffeine; CYP1A2, benzphetamine; CYP3A/2B(?), aniline; 2E1 and TMO; CYP2C9/2E1/3A4); (2) activity generally increases rapidly soon after birth, continues to increase, peaks and then gradually decreases to the adult level (phenytoin; CYP2C9); and (3) activity is high (about the same level as the adult) immediately after birth, decreases and then gradually increases to the adult level (erythromycin; CYP3A4/5). 3. The results of these in vivo and in vitro studies suggest that changes in enzyme activity are due to differences in P450 isoenzymes during development.

Interlaboratory comparison of the assessment of P450 activities in human hepatic microsomal samples

1. Although the importance of in vitro technology in supporting drug development is widely accepted, there is no real consensus about which approaches should be taken, which substrates should be used, or on the reliability and application of in vitro data. Consequently, as part of a collaborative project to characterize human liver with respect to the major forms of cytochrome P450, an interlaboratory comparison of the analysis of samples for form-specific activities was undertaken. 2. Microsomal fractions were isolated from five different human liver samples taken from the liver bank maintained at the Royal Postgraduate Medical School (RPMS). Aliquots from the five samples were sent to the 11 collaborating laboratories for characterization using their in-house, form-specific assays for cytochrome P450 activities. Although each laboratory assayed protein concentration, total cytochrome P450 content and enzyme activities were calculated using the protein estimation generated by RPMS to eliminate this possible source of variability. 3. With the exception of one laboratory, all estimates of protein concentration were similar (coefficient of variation, CoV, 9-13%) and the rank-order of the five samples was consistent across the laboratories. There was greater variability in the estimates of total cytochrome P450 content (CoV 28-43%), although again rank order of the samples across laboratories was fairly consistent. 4. The various laboratories used a number of different probe substrates, together with a range of conditions (substrate concentration, time of incubation, amount of protein), to assay for activity of CYP1A2, CYP2C9, CYP2D6, CYP2E1 and CYP3A4. However, apart from the occasional outlier, the five samples were ranked for activity of all these forms of cytochrome P450 with a high degree of consistency by the various laboratories and the choice of substrate had no appreciable effect on the ranking of the samples. 5. While this interlaboratory comparison has shown that greater consistency in the approach to in vivo determination of drug-metabolizing activity is desirable, there was little indication that any particular approach or substrate was superior to the others.

Strain differences in CYP3A-mediated C-8 hydroxylation (1,3,7-trimethyluric acid formation) of caffeine in Wistar and Dark Agouti rats. Rapid metabolism of caffeine in debrisoquine poor metabolizer model rats

We observed significant strain differences [Dark Agouti (DA) > Wistar] in 1,3,7-trimethyluric acid formation (C-8 hydroxylation) during caffeine metabolism, though not in N-demethylations, in adult male DA and Wistar rats. In contrast, adult female and immature male rats of both DA and Wistar strains did not show significant differences in activity levels of C-8 hydroxylation. Kinetic studies using liver microsomes revealed that adult male DA rats have a larger Vmax for C-8 hydroxylation than do Wistar rats. Troleandomycin (TAO), known as a cytochrome P450 (CYP) 3A inhibitor, and an anti-rat CYP3A2 polyclonal antibody effectively reduced C-8 hydroxylation by rat liver microsomes in a concentration-dependent manner, suggesting that C-8 hydroxylation in rats is mediated largely by an isoform(s) of the CYP3A subfamily. Troleandomycin and the antibody did not inhibit the N-demethylations of caffeine by rat liver microsomes. Treatment of rats with CYP3A inducers caused a marked increase in C-8 hydroxylase activity. These results indicate that the rat CYP3A subfamily is capable of catalyzing C-8 hydroxylation of caffeine as is the case for human CYP3A4. The results of western blotting analysis using anti CYP3A antiserum showed that the staining intensity of the protein band in DA rat liver microsomes was higher than that in Wistar rat liver microsomes. We concluded that marked sex-dependent strain differences in C-8 hydroxylation of caffeine between Wistar and DA rats are due to the differences in the levels of expression of CYP3A in these strains of rats.

Structure-related inhibition of human hepatic caffeine N3-demethylation by naturally occurring flavonoids

The effects of flavonoids on caffeine N3-demethylation, a marker activity of CYP1A2, in human liver microsomes were investigated to elucidate the inhibition mechanism and the structure-activity relationship. Caffeine N3-demethylase activity was inhibited by the presence of various flavonoids, whose structures seem to be closely related to the degree of inhibition. Among twenty-one compounds tested, the most active was chrysin with an IC50 value of 0.2 microM. Others had IC50 values ranging from 1 to more than 500 microM. Kinetic analysis revealed that the mechanism of inhibition varied among the flavonoids. The inhibitory effect was postulated to be governed by factors such as the number of hydroxyl groups and glycosylation of these free hydroxyl groups. An increase in the number of free hydroxyl groups reduced the inhibitory effect on P450 activity. Analysis of the quantitative structure-activity relationship (QSAR) showed that the volume to surface area ratio was the most effective factor on the inhibition of caffeine N3-demethylation, and the electron densities on the C3 and C4' atoms exercised significant influence on the inhibitory effect. The calculated inhibitory effect of flavonoids on CYP1A2 activity was highly correlated with the antimutagenicity of flavonoids in 2-amino-3,4-dimethylimidazo[4,5-flquinoline (MelQ)-induced umu response.

Three-dimensional modelling of human cytochrome P450 1A2 and its interaction with caffeine and MeIQ

The three-dimensional modelling of proteins is a useful tool to fill the gap between the number of sequenced proteins and the number of experimentally known 3D structures. However, when the degree of homology between the protein and the available 3D templates is low, model building becomes a difficult task and the reliability of the results depends critically on the correctness of the sequence alignment. For this reason, we have undertaken the modelling of human cytochrome P450 1A2 starting by a careful analysis of several sequence alignment strategies (multiple sequence alignments and the TOPITS threading technique). The best results were obtained using TOPITS followed by a manual refinement to avoid unlikely gaps. Because TOPITS uses secondary structure predictions, several methods that are available for this purpose (Levin, Gibrat, DPM, NnPredict, PHD, SOPM and NNSP) have also been evaluated on cytochromes P450 with known 3D structures. More reliable predictions on alpha-helices have been obtained with PHD, which is the method implemented in TOPITS. Thus, a 3D model for human cytochrome P450 1A2 has been built using the known crystal coordinates of P450 BM3 as the template. The model was refined using molecular mechanics computations. The model obtained shows a consistent location of the substrate recognition segments previously postulated for the CYP2 family members. The interaction of caffeine and a carcinogenic aromatic amine (MeIQ), which are characteristic P450 1A2 substrates, has been investigated. The substrates were solvated taking into account their molecular electrostatic potential distributions. The docking of the solvated substrates in the active site of the model was explored with the AUTODOCK programme, followed by molecular mechanics optimisation of the most interesting complexes. Stable complexes were obtained that could explain the oxidation of the considered substrates by cytochrome P450 1A2 and could offer an insight into the role played by water molecules.

Drug interactions with proton pump inhibitors

Omeprazole, lansoprazole and pantoprazole are all mainly metabolised by the polymorphically expressed cytochrome P450 (CYP) isoform CYP2C19 (S-mephenytoin hydroxylase). All 3 proton pump inhibitors have a very limited potential for drug interactions at the CYP level. Small effects on CYP reported for these compounds are usually of no clinical relevance. No dose related adverse effects have been identified, suggesting that the small proportion of slow metabolisers is at no additional risk for clinically important drug interactions. The absorption of some compounds, e.g. benzylpenicillin (penicillin G), are altered during treatment with proton pump inhibitors as a result of the increased intragastric pH. A synergy has been confirmed between omeprazole and amoxicillin or clarithromycin in the antibacterial effect against Helicobacter pylori.

Pharmacokinetics, metabolism and interactions of acid pump inhibitors. Focus on omeprazole, lansoprazole and pantoprazole

This review updates and evaluates the currently available information regarding the pharmacokinetics, metabolism and interactions of the acid pump inhibitors omeprazole, lansoprazole and pantoprazole. Differences and similarities between the compounds are discussed. Omeprazole, lansoprazole and pantoprazole are all mainly metabolished by the polymorphically expressed cytochrome P450 (CYP) isoform S-mephenytoin hydroxylase (CYP2C19), which means that within a population a few individuals (3% of Caucasians) metabolise the compounds slowly compared with the majority of the population. For all 3 compounds, the area under the plasma concentration-versus-time curve (AUC) for a slow metaboliser is, in general, approximately 5 times higher than that in an average patient. Since all 3 compounds are considered safe and well tolerated, and no dosage-related adverse drug reactions have been identified, this finding seems to be of no clinical relevance. The acid pump inhibitors seem to be similarly handled in the elderly, where a somewhat slower elimination can be demonstrated compared with young individuals. In patients with renal insufficiency, omeprazole is eliminated as in healthy individuals, whereas the data on lansoprazole and pantoprazole are unresolved. In patients with hepatic insufficiency, as expected, the elimination rates of all 3 compounds are substantially decreased. No clinically relevant effects on specific endogenous glandular functions, such as the adrenal (cortisol), the gonads or the thyroid, were demonstrated for omeprazole and pantoprazole, whereas a few minor concerns have been raised regarding lansoprazole. The absorption of some compounds, e.g. digoxin, might be altered as a result of the increased gastric pH obtained during treatment with acid pump inhibitors, and, accordingly, similar effects are expected irrespective of which acid pump inhibitor is given. The effect of the acid pump inhibitors on enzymes in the liver has been intensely debated, and some authors have claimed that lansoprazole and pantoprazole have less potential than omeprazole to interact with other drugs metabolised by CYP. However, after assessment of available data in this area, the conclusion is that all 3 acid pump inhibitors have a very limited potential for drug interactions at the CYP level. In addition, the small effects on CYP reported for these compounds are rarely of any clinical relevance, considering the normal intra- (and inter-)individual variations in metabolism observed for most drugs. In conclusion, omeprazole, lansoprazole and pantoprazole are structurally very similar, and an evaluation of available data indicates that also with respect to pharmacokinetics, metabolism and interactions in general they demonstrate very similar properties, even though omeprazole has been more thoroughly studied with regard to different effects.

Specificity of substrate and inhibitor probes for cytochrome P450s: evaluation of in vitro metabolism using cDNA-expressed human P450s and human liver microsomes

1. We evaluated the specificity of 15 substrates and 14 inhibitors of the cytochrome P450s using nine human P450 forms expressed in HepG2 cells using a recombinant vaccinia virus and also in human liver microsomes. 2. Coumarin, 7-ethoxyresorufin, 7-benzyloxyresorufin, tolbutamide, aniline and diazepam were form-selective substrates towards CYP2A6, the CYP1A subfamily, CYP2B6, the CYP2C subfamily, CYP2E1 and the CYP3A subfamily respectively. However, a selective substrate for CYP2D6 was not found among the chemicals tested. 3. SKF-525A inhibited > 40% of the metabolic activity of all substrates tested, and the inhibitory effects differed among P450 forms. Sulphaphenazole, 7,8-benzoflavone, quinidine and troleandomycin were selective inhibitors of the CYP2C subfamily (except CYP2C19), the CYP1A subfamily, CYP2D6 and the CYP3A subfamily respectively. Methoxsalen (CYP2A6 inhibitor) inhibited the metabolic activity of CYP1A2 as well as that of CYP2A6. Diethyldithiocarbamate (CYP2E1 inhibitor) inhibited the metabolic activities of CYP2A6 and CYP2C19 in addition to that of CYP2E1. 4. Our results indicated that substrates and inhibitors reported as P450 selective probes are not necessarily specific for individual human P450 forms. These results may provide useful information regarding human P450 substrates and inhibitors in vitro using human liver microsomal samples.

Biotransformation of caffeine by cDNA-expressed human cytochromes P-450

OBJECTIVES

The biotransformation of caffeine has been studied in vitro using human cytochrome P-450 isoenzymes (CYPs) expressed in human B-lymphoblastoid cell lines, namely CYP1A1, 1A2, 2A6, 2B6, 2D6-Val, 2E1 and 3A4, and microsomal epoxide hydroxylase (EH). In addition, CYP2D6-Met was also studied, in which a valine in the wild type (CYP2D6-Val) has been replaced by a methionine due to a G to A mutation in position 112.

RESULTS

At caffeine 3 mmol center dot l-1, five CYPs (1A1, 1A2, 2D6-Met, 2E1 and 3A4) catalysed the biotransformation of caffeine. Among the enzymes studied, CYP1A2, which predominantly catalysed paraxanthine formation, had the highest intrinsic clearance (160 l center dot h-1 center dot mmol-1 CYP). Together with its high abundance in liver, it should be considered, therefore, to be the most important isoenzyme in caffeine metabolism. The affinity of caffeine for CYP1A1 was comparable to that of its homologue 1A2. CYP2D6-Met, which catalysed caffeine metabolism by demethylation and 8-hydroxylation, also had a relatively high intrinsic clearance (3.0 l center dot h-1 mmol-1 CYP), in particular for theophylline and paraxanthine formation, with kM values between 9-16 mmol center dot l-1. In contrast, the wild type, CYP2D6-Val, had no detectable activity. In comparison, CYP2E1 played a less important role in in vitro caffeine metabolism. CYP3A4 predominantly catalysed 8-hydroxylation with a kM value of 46 mmol center dot l-1 and an intrinsic clearance of 0.60 l center dot h-1 center dot mmol-1 CYP. Due to its high abundance in human liver, the latter CYP may contribute significantly to the in vivo formation of TMU.

CONCLUSION

The findings of this study indicate that i) microsomes from transfected human B-lymphoblastoid cell lines give results close to those obtained with microsomes isolated from human liver, ii) at least four CYP isoforms are involved in caffeine metabolism, iii) at a substrate concentration <0.1 mmol center dot l-1, CYP1A2 and 1A1 are the most important isoenzymes, iv) at higher concentrations the participation of other isoenzymes, in particular CYP3A4, 2E1 and possibly also CYP2D6-Met, are important in caffeine metabolism, and v) the nucleotide composition at position 1120 of CYP2D6 determines the activity of this isoenzyme in caffeine metabolism.

Determination of P4501A2 activity in human liver microsomes using [3-14C-methyl]caffeine

1. Caffeine N3-demethylation, the major pathway of caffeine metabolism in man, is mediated by P4501A2. The carbon of the methyl group lost during N3-demethylation is eliminated as carbon dioxide in vivo, or as formaldehyde and formic acid in vitro. 2. A simple and sensitive assay was developed to quantify the [14C]formaldehyde/[14C]formic acid produced following incubation of human microsomes with [3-14C-methyl]caffeine. This assay, using solid-phase extraction, enables quantitation of [14C]formaldehyde/[14C]formic acid with acceptable precision (within 5%) and accuracy (within 10%). 3. Typical Km and Vmax for the N3-demethylation of caffeine were determined by this assay to be 500 (range 220-1200) microM, and 250 (range 115-450) pmol.mg protein-1.min-1 respectively. 4. The N3-demethylation activity determined in microsomes from a range of human livers correlated significantly with other P4501A2 activities (p < 0.001) and was inhibited (> 95%) by furafylline. In addition, caffeine N3-demethylation was catalysed by microsomes from cell lines transfected with human P4501A2 cDNA. 5. This assay, for quantitation of [14C]formaldehyde/[14C]formic acid in human liver microsomes, is suitable for use in in vitro drug interaction studies as a probe for P4501A2 activity.

Effects of capsaicinoids on oxidative metabolism of caffeine in isolated rat hepatocytes

The metabolism of caffeine was studied in isolated rat hepatocytes, in the absence and presence of capsaicinoids. Caffeine and four primary metabolite fractions were identified by high performance liquid chromatography: 1,7-dimethylxanthine, 3,7-dimethylxanthine, 1,3-dimethylxanthine and 1,3,7-trimethyluric acid. The incubation with the lowest concentrations (0.1 and 1 microM) of capsaicinoids (natural extract, capsaicin, dihydrocapsaicin) showed a stimulatory effect on caffeine metabolism, which was further enhanced with capsaicin. At 10 microM, capsaicin stimulated the two pathways of metabolism of caffeine (N-demethylation and C-8 oxidation). In contrast, dihydrocapsaicin and the natural extract seem to inhibit the N-demethylation pathways without affecting the C-8 oxidation route. The inhibitory activity on the N-demethylation pathways and especially the N-7 demethylation pathway was pronounced at the first 30 min of incubation. These results suggest that the two pathways (N-demethylation and C-8 oxidation) are mediated by different isozymes of cytochromes P-450. This is in agreement with recent findings.

Effects of methylxanthine derivatives on adriamycin concentration and antitumor activity

We studied the mechanism whereby caffeine acts as a biochemical modulator of adriamycin, and examined various methylxanthine derivitives to determine whether they would be of value as biochemical modulators. In an in vitro study of adriamycin efflux in Ehrlich ascites carcinoma cells, theophylline, pentoxifylline, and theobromine inhibited this efflux, while caffeine metabolites did not. The effects of several methylxanthine derivatives on the antitumor activity of adriamycin and on adriamycin concentration in tissue were also examined in CDF1 tumor-bearing mice. Theobromine, which inhibited adriamycin efflux in vitro, increased the antitumor activity of adriamycin and the concentration of adriamycin in tumors. The caffeine metabolites, which had no effect on the adriamycin efflux, did not increase antitumor activity. These results suggest that the metabolism of caffeine may weaken its effect as a biochemical modulator, and that pentoxifylline and theobromine would be of value as biochemical modulators of adriamycin.

Caffeine metabolism in patients with chronic liver disease

An oral load of 200 mg (1030 mumol) caffeine (CA) was given to 13 patients with chronic liver diseases and to 11 healthy controls. The metabolism of CA was determined by following plasma concentrations and urinary excretion of CA and its metabolites. In addition, [2-14C]-caffeine was given orally to six patients to confirm the excretion through the different pathways. CA and its 14 main metabolites were separated and quantified by high performance liquid chromatography and capillary electrophoresis. Median (interquartile range) half-lives of CA were 19 (6.3-32) h in the patients and 3.8 (3.4-4.7) h in the controls. The wide range in the patients indicated varying degrees of liver dysfunction. Only 3 (2-4)% of administered CA was excreted unmetabolized in urine in the controls and the main degradation was through the paraxanthine (PX) pathway 82 (75-83)%. The combined theobromine (TB) and theophylline (TP) pathways (TB + TP) accounted for 15 (13-21)% of CA metabolism. Although the excretion of unmetabolized CA in the patients 6 (3-8)%, was similar to that in the controls, the metabolism through the PX pathway, 62 (44-65)%, decreased (p < 0.01 vs. controls), whereas the metabolism through the TB + TP pathways increased to 33 (30-47)%, p < 0.01. In controls, N3-, N7- and N1-demethylations were observed in 86 (83-89)%, 66 (62-70)% and 13 (9-18)%, respectively, of excreted metabolites. In patients the N3-demethylations, 71 (66-77)%, and N7-demethylations, 54 (48-59)%, decreased (p < 0.01 vs. controls), whereas N1-demethylation increased 30 (21-46)%, p < 0.01. The major C8-oxidation reaction, the oxidation of 1-methylxanthine, increased in patients (p < 0.01). We conclude that the slowed metabolism of CA in chronic liver disease is due to reduced N3- and N7-demethylations affecting biotransformation through the PX pathway.

Caffeine metabolism by human hepatic cytochromes P450: contributions of 1A2, 2E1 and 3A isoforms

Caffeine (CA) N1-, N3- and N7-demethylase, CA 8-hydroxylase and phenacetin O-deethylase activities were measured in microsomes from 18 separate human livers which had been characterized previously for a range of cytochrome P450 (CYP) isoform-specific activities and immunoreactive CYP protein contents. Correlations between the high affinity components of the three separate CA N-demethylations were highly significant (r = 0.77-0.91, P < 0.001) and each of the three high affinity CA N-demethylations correlated significantly (r = 0.64-0.93, P < 0.05-0.001) with the high affinity phenacetin O-deethylase, 2-acetylaminofluorene N-hydroxylation and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) mutagenicity (all predominantly CYP1A2-mediated reactions). Consistent with these observations, cDNA-expressed human CYP1A2 catalyzed the N1-, N3- and N7-demethylation of CA and apparent Km values were similar (0.24-0.28 mM) for all three reactions and comparable to those observed previously with human liver microsomes. The low affinity components of CA N1- and N7-demethylation correlated significantly (r = 0.55-0.85, P < 0.05-0.001) with immunoreactive CYP2E1 content and the CYP2E1-specific activities 4-nitrophenol and chlorzoxazone hydroxylation. Diethyldithiocarbamate, a selective inhibitor of CYP2E1, inhibited the low affinity CA N1- and N7-demethylation, with IC50 values of 23 microM and 11 microM, respectively. The apparent Km values for CA N1- and N7-demethylation by cDNA-expressed CYP2E1 (namely 28 and 43 mM, respectively) were of a similar order to those calculated for the low affinity microsomal activities. Significant correlations (r = 0.87-0.97, P < 0.001) were observed between CA 8-hydroxylation and immunoreactive CYP3A content and the CYP3A-mediated reactions benzo(a)pyrene hydroxylation, omeprazole sulfoxidation and aflatoxin B1 mutagenesis. Effects of alpha-naphthoflavone, erythromycin, troleandomycin and nifedipine on microsomal CA 8-hydroxylation were generally consistent with CYP3A involvement. Taken together with previous data, the results indicate a major involvement of CYP1A2 in the high affinity component of all three human hepatic CA N-demethylations. In contrast, CYP2E1 appears to be the main enzyme involved in the low affinity components of CA N1- and N7-demethylation while CA 8-hydroxylation is catalysed predominantly by a CYP3A isoform(s).

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)

Saccharomyces cerevisiae: an alternative source for human microsomal liver enzymes and its use in drug interaction studies

Heterologous expression of human cDNAs in the yeast Saccharomyces cerevisiae represents an attractive alternative source of human enzymes and allows metabolic studies to be performed without the need of human tissue. Here we report on the functional expression of human microsomal epoxide hydrolase (hmEH) and cytochrome P450 1A1 and 1A2 in yeast. Microsomal fractions of corresponding yeast strains exhibited enzyme specific activities which allowed the characterization of the heterologous enzymes. The use of these microsomes enabled us to study drug interactions on the respective enzymes with pharmacologically relevant drugs such as carbamazepine epoxide, valpromide and ketoconazole.

Biotransformation of methylxanthines in mammalian cell lines genetically engineered for expression of single cytochrome P450 isoforms. Allocation of metabolic pathways to isoforms and inhibitory effects of quinolones

V79 Chinese hamster cells genetically engineered for stable expression of single forms of rat cytochromes P450IA1, P450IA2, P450IIB1, human P450IA2, and rat liver epithelial cells expressing murine P450IA2 were used to allocate metabolic pathways of methylxanthines to specific isoforms and to test the suitability of such cell lines for investigations on drug interactions occurring at the cytochrome expressed. The cell lines were exposed to caffeine and/or theophylline and concentrations of metabolites formed in the medium were determined by HPLC. Caffeine was metabolized by human, rat and murine P450IA2, resulting in the formation of four primary demethylated and hydroxylated metabolites. However, there were differences in the relative amounts of the metabolites. The human and the mouse P450IA2 isoforms predominantly mediated 3-demethylation of caffeine. The rat cytochrome P450IA2 mediated both 3-demethylation and 1-demethylation of caffeine to a similar extent. The results support the hypothesis that caffeine plasma clearance is a specific in vivo probe for determining human P450IA2 activity. Addition of the quinolone antibiotic agents pipemidic acid or pefloxacin, both known to inhibit caffeine metabolism in vivo and in human liver microsomes, reduced formation rates of all metabolites of caffeine in cells expressing rat and human P450IA2. Theophylline was mainly metabolized via 8-hydroxylation. All cell lines tested were able to carry out this reaction, with highest activities in cell lines expressing rat or human P450IA2, or rat P450IA1.

Effects of ciprofloxacin and enrofloxacin on zoxazolamine kinetics, plasma concentration and sleeping times in mice

The treatment of CD1 male mice with either ciprofloxacin (CP) or enrofloxacin (EF) prior to zoxazolamine (ZX) administration increased the mean ZX sleeping times to, respectively, 162 and 156% of the control (ZX alone). At the end of the sleeping time, the mean ZX plasma concentration in controls was 27.2 micrograms/ml and was not different in EF- or CP-treated groups (87% and 95% of controls, respectively). The animals coadministered with CP or EF and ZX eliminated the latter more slowly than the controls. The estimated zero-time drug concentration of the disposition curves of both the CP- and EF-treated groups as well as the apparent half-life of elimination and apparent overall rate of elimination of the CP-treated group were different from the control values.

Effect of pregnandiol on caffeine metabolism in female rats

Three groups of six 5-week-old Sprague Dawley female rats received i.p. injections of pregnandiol, 1.25, 2.50 or 5 mg/kg, respectively, in triolein daily for 7 days. Caffeine metabolism was studied in liver slices on day 8 by HPLC. Only primary metabolites were formed. N-1 demethylation was the most important pathway (theobromine represented 51% of total dimethylxanthines). Unlike in human in vitro or in vivo, 1,3,7-DAU (6-amino-5-(N-formylmethylamino)-1,3-dimethyluracil) was an important metabolite (9.7% of total caffeine metabolites). Pregnandiol inhibited N-1, N-3 and N-7 demethylation in vitro (-33%, -33% and -28%, respectively, at 5 mg/kg/day), but it had no effect on N-1 demethylation at 1.25 or 2.50 mg/kg/day. Pregnandiol at all doses had no effect on 1,3,7-trimethyluric acid and 1,3,7-DAU formation. These results are consistent with the hypothesis that C-8 hydroxylation and demethylation of caffeine are mediated by different isoenzymes. They indicate that pregnandiol is a potent inhibitor of microsomal drug metabolism, specifically of cytochrome P450 IA, which could explain the immaturity of some metabolic pathways of caffeine in neonates.

Interspecies variations in caffeine metabolism related to cytochrome P4501A enzymes

1. Interspecies (including man, monkey, rabbit, rat and mouse) variations in caffeine metabolism by liver microsomes were studied. While N-3 demethylation was the major pathway in man (81% of total dimethylxanthines), N-7 demethylation was predominant in monkey (89%), and the three demethylation pathways were about equal in mouse, rabbit and rat. 2. Three monooxygenase activities (methoxyresorufin O-demethylase, phenacetin O-deethylase and acetanilide 4-hydroxylase) correlated significantly with the rate of metabolism of caffeine. 3. P4501A1 and 1A2 enzymes were immunodetected in different species. P4501A2 was the only isoform detected in liver of man, rat and mouse, while no polypeptide immunorelated to P4501A was detected in monkey and only a minor band of P4501A1 was detected in rat and rabbit. 4. All in vitro data indicate that paraxanthine formation is mediated mainly by P4501A2 in mammals while theophylline formation is mediated mainly by cytochromes P-450 other than those of the 1A family.

Inhibitory potency of quinolone antibacterial agents against cytochrome P450IA2 activity in vivo and in vitro

Inhibition of cytochrome P450IA2 activity is an important adverse effect of quinolone antibacterial agents. It results in a prolonged half-life for some drugs that are coadministered with quinolones, such as theophylline. The objective of the study described here was to define the parameters for quantifying the inhibitory potencies of quinolones against cytochrome P450IA2 in vivo and in vitro and to investigate the relationship between the results of both approaches. Cytochrome P450IA2 activity in vitro was measured by using the 3-demethylation rate of caffeine (500 microM) in human liver microsomes. The inhibitory potency of a quinolone in vitro was determined by calculating the decrease in the activity of cytochrome P450IA2 caused by addition of the quinolone (500 microM) into the incubation medium. The mean values (percent reduction of activity without quinolone) were as follows: enoxacin, 74.9%; ciprofloxacin, 70.4%; nalidixic acid, 66.6%; pipemidic acid, 59.3%; norfloxacin, 55.7%; lomefloxacin, 23.4%; pefloxacin, 22.0%; amifloxacin, 21.4%; difloxacin, 21.3%; ofloxacin, 11.8%; temafloxacin, 10.0%; fleroxacin, no effect. The inhibitory potency of a quinolone in vivo was defined by a dose- and bioavailability-normalized parameter calculated from changes of the elimination half-life of theophylline and/or caffeine reported in previously published studies. Taking the pharmacokinetics of the quinolones into account, it was possible to differentiate between substances with and without clinically relevant inhibitory effects by using results of in vitro investigations. The in vitro test described here may help to qualitatively predict the relevant drug interactions between quinolones and methylxanthines that occur during therapy.

Biotransformation of caffeine and theophylline in mammalian cell lines genetically engineered for expression of single cytochrome P450 isoforms

Primary steps in the metabolism of caffeine and theophylline are cleavage of methyl groups and/or hydroxylation at position 8, mediated by cytochromes P450. V79 Chinese hamster cells genetically engineered for stable expression of single forms of rat cytochromes P450IA1, P450IA2 and P450IIBI and human P450IA2 and rat liver epithelial cells expressing murine P450IA2 were used to overcome problems arising in the proper allocation of metabolic pathways to specific isoforms by conventional techniques. These cell lines were exposed to caffeine and/or theophylline, and concentrations of metabolites formed in the medium were determined by HPLC. Caffeine was metabolized by human, rat and murine P450IA2, resulting in the formation of four primary demethylated and hydroxylated metabolites. However, there were differences in the relative amounts of the metabolites. The human and the mouse P450IA2 isoforms predominantly mediated 3-demethylation of caffeine. The rat cytochrome P450IA2 mediated both 3-demethylation and 1-demethylation of caffeine to a similar extent. Theophylline was metabolized mainly via 8-hydroxylation. All cell lines tested were able to carry out this reaction, with highest activities in cell lines expressing rat or human P450IA2, or rat P450IA1. These results support the hypothesis that caffeine plasma clearance is a specific in vivo probe for determining human P450IA2 activity.

Omeprazole drug interaction studies

This review examines the literature on drug interactions with omeprazole. Different mechanisms have been proposed as potential causes for such interactions. First, the absorption of some drugs might be altered due to the decreased intragastric acidity resulting from omeprazole treatment. There was no effect of omeprazole on the absorption of amoxycillin, bacampicillin and alcohol, while the amount of digoxin and nifedipine absorbed was increased by 10 and 21%, respectively, both increases probably being of no clinical significance. Secondly, the metabolism of high clearance drugs might be altered by changes in liver blood flow, although that is not affected by omeprazole, as indicated by the unchanged elimination of indocyanine green. In addition, the clearance of intravenously administered lidocaine (lignocaine) [a high clearance drug] was unaffected by omeprazole, further indicating that the latter does not alter liver blood flow. Thirdly, since omeprazole is a substituted benzimidazole, it might have the potential to interfere with the metabolism of other drugs by altering the activity of drug metabolising enzymes in the cytochrome P450 system, through either induction or inhibition. There is no indication of induction of this enzyme system in any interaction study with omeprazole. As regards inhibition, on the other hand, there is now considerable information available which indicates that omeprazole has the potential to partly inhibit the metabolism of drugs metabolised to a great extent by the cytochrome P450 enzyme subfamily IIC (diazepam, phenytoin), but not of those metabolised by subfamilies IA (caffeine, theophylline), IID (metoprolol, propranolol) and IIIA (cyclosporin, lidocaine, quinidine). Since relatively few drugs are metabolised mainly by IIC compared with IID and IIIA, the potential for omeprazole to interfere with the metabolism of other drugs appears to be limited.

Characterization of metronidazole metabolism by human liver microsomes

The metabolism of metronidazole was studied in microsomes isolated from livers of human kidney donors. The formation of the major in vivo metabolite, hydroxymetronidazole, proceeded according to biphasic kinetics, suggesting the involvement of at least two enzymatic sites. The affinity constant (Km) of the high affinity site ranged from 140 to 320 microM and metabolism at this site contributed more than 75% of the intrinsic clearance. Thus, at therapeutic doses of metronidazole most of the hydroxylation in vivo should be associated with this site. Antipyrine, cimetidine, alpha-naphthoflavone, caffeine, theophylline, mephenytoin, tolbutamide, quinidine, acetone and nifedipine were poor inhibitors of the formation of hydroxymetronidazole by human liver microsomes. Propranolol (500 microM) inhibited the hydroxylation rate by 70%. Phenacetin inhibited metronidazole hydroxylation with a competitive inhibition constant (Ki) of 4-5 microM. However, metronidazole did not inhibit the O-deethylation of phenacetin. It is concluded that cytochromes P450 IA2, IIC9, IIC10, IID6, IIE1 and IIIA3 do not contribute significantly to the high affinity hydroxylation of metronidazole in man.

Selective in-vitro inhibition of hepatic oxidative metabolism by quinolones: 7-ethoxyresorufin and caffeine as model substrates

The in-vitro inhibition of several metabolic pathways has been studied in 3-methylcholanthrene-treated rats. The specificity of the 7-ethoxyresorufin O-de-ethylase reaction has been determined in the presence and absence of ciprofloxacin, enoxacin, norfloxacin, ofloxacin, nalidixic acid, oxolinic acid and pipemidic acid. For the caffeine N3-demethylation reaction, enoxacin and pipemidic acid were used. Enoxacin (IC50 = 105 microM, Ki = 65 microM) and pipemidic acid (IC50 = 115 microM, Ki = 160 microM) significantly inhibited 7-ethoxyresorufin O-de-ethylase reaction and caffeine N3-demethylation (IC50 = 60 microM for enoxacin and IC50 = 185 microM for pipemidic acid) by a competitive mechanism. Other quinolones had lower or no (ofloxacin) inhibitory capacity. The order of inhibitory activity observed is in agreement with results obtained previously from in-vivo studies in man. No activity was detected towards ethylmorphine N-demethylation.

Constitutive and inducible expression of human cytochrome P450IA1 in yeast Saccharomyces cerevisiae: an alternative enzyme source for in vitro studies

A cDNA of human cytochrome P450IA1 was expressed in yeast Saccharomyces cerevisiae on a multicopy plasmid under the control of the constitutive GAPFL or the inducible PHO5 promoter. Microsomes of transformed yeast contained substantial amounts of the heterologous enzyme as determined by reduced CO-difference spectra (156-68 pmol/mg). Enzyme kinetics with 7-ethoxyresorufin as substrate resulted in a Km value of 92 nM and a Vmax value of 223 pmol/mg/min, which is comparable to data obtained with human liver microsomes. The antimycotic drug ketoconazole (Ki = 22nM) as well as the isozyme specific P450 inhibitor alpha-naphthoflavone (Ki = 1.2 nM) were shown to be strong inhibitors of human P450IA1. Taken together, these data show that heterologous P450 gene expression in yeast is a potent instrument for the study of enzyme specific parameters and might be used to answer further questions with regard to substrate specificity as well as drug interaction in a background with no interfering activities.

Demethylation pathways in caffeine metabolism as indicators of variability in 1,1,1-trichloroethane oxidation in man

Twenty volunteers were exposed to 191 +/- 7 p.p.m. 1,1,1-trichloroethane or 50 +/- 2 p.p.m. perchloroethylene vapour for 6 hr. They were then evaluated for their rate of caffeine metabolism, mephenytoin hydroxylation and debrisoquine hydroxylation. Seven subjects were identified as 'fast acetylators' of caffeine and one person as a slow metabolizer of debrisoquine. The "slow acetylators" exposed to perchloroethylene excreted an average of 3.83 +/- 0.35 mg trichloroacetic acid within 24 hr (N = 13, +/- S.E.) and the 'fast acetylators' 3.58 +/- 0.48 mg (N = 7, +/- S.E.). The excretion of trichloroethanol by the same persons after 1,1,1-trichloroethane exposure was 14.1 +/- 1.12 mg and 16.7 +/- 1.48 mg, respectively. The excretion of trichloroacetic acid in the latter exposure varied significantly between the seven 'fast' and 13 'slow acetylators' (0.43 +/- 0.08 mg versus 1.03 +/- 0.19 mg; +/- S.E.; P = 0.037). A multiple linear regression analysis confirmed this association when other factors, such as body weight, creatinine clearance, smoking habit and alcohol consumption, were taken into account. One volunteer proved to be a poor hydroxylator of debrisoquine and excreted half the amount of trichloroacetic acid in the perchloroethylene exposure and half the amount of trichloroethanol in the 1,1,1-trichloroethane exposure compared to the others. A reduction in the solvent metabolism could thus be predicted by the debrisoquine test. On the other hand, the caffeine test predicted faster oxidation of trichloroethanol which could be of toxicological and pharmacological importance e.g. in the clinical use of chloral.

Furafylline is a potent and selective inhibitor of cytochrome P450IA2 in man

1. Furafylline (1,8-dimethyl-3-(2'-furfuryl)methylxanthine) is a methylxanthine derivative that was introduced as a long-acting replacement for theophylline in the treatment of asthma. Administration of furafylline was associated with an elevation in plasma levels of caffeine, due to inhibition of caffeine oxidation, a reaction catalysed by one or more hydrocarbon-inducible isoenzymes of P450. We have now investigated the selectivity of inhibition of human monooxygenase activities by furafylline. 2. Furafylline was a potent, non-competitive inhibitor of high affinity phenacetin O-deethylase activity of microsomal fractions of human liver, a reaction catalysed by P450IA2, with an IC50 value of 0.07 microM. 3. Furafylline had either very little or no effect on human monooxygenase activities catalysed by other isoenzymes of P450, including P450IID1, P450IIC, P450IIA. Of particular interest, furafylline did not inhibit P450IA1, assessed from aryl hydrocarbon hydroxylase activity of placental samples from women who smoked cigarettes. 4. It is concluded that furafylline is a highly selective inhibitor of P450IA2 in man. 5. Furafylline was a potent inhibitor of the N3-demethylation of caffeine and of a component of the N1- and N7-demethylation. This confirms earlier suggestions that caffeine is a selective substrate of a hydrocarbon-inducible isoenzyme of P450 in man, and identifies this as P450IA2. Thus, caffeine N3-demethylation should provide a good measure of the activity of P450IA in vivo in man. 6. Although furafylline selectively inhibited P450IA2, relative to P450IA1, in the rat, this was at 1000-times the concentration required to inhibit the human isoenzyme, suggesting a major difference in the active site geometry between the human and the rat orthologues of P50IA2.