Caffeine: a model compound for measuring liver function

Liver Cirrhosis Affects the Pharmacokinetics of the Six Substrates of the Basel Phenotyping Cocktail Differently

BACKGROUND

Activities of hepatic cytochrome P450 enzymes (CYPs) are relevant for hepatic clearance of drugs and known to be decreased in patients with liver cirrhosis. Several studies have reported the effect of liver cirrhosis on CYP activity, but the results are partially conflicting and for some CYPs lacking.

OBJECTIVE

In this study, we aimed to investigate the CYP activity in patients with liver cirrhosis with different Child stages (A-C) using the Basel phenotyping cocktail approach.

METHODS

We assessed the pharmacokinetics of the six compounds and their CYP-specific metabolites of the Basel phenotyping cocktail (CYP1A2: caffeine, CYP2B6: efavirenz, CYP2C9: flurbiprofen, CYP2C19: omeprazole, CYP2D6: metoprolol, CYP3A: midazolam) in patients with liver cirrhosis (n = 16 Child A cirrhosis, n = 15 Child B cirrhosis, n = 5 Child C cirrhosis) and matched control subjects (n = 12).

RESULTS

While liver cirrhosis only marginally affected the pharmacokinetics of the low to moderate extraction drugs efavirenz and flurbiprofen, the elimination rate of caffeine was reduced by 51% in patients with Child C cirrhosis. For the moderate to high extraction drugs omeprazole, metoprolol, and midazolam, liver cirrhosis decreased the elimination rate by 75%, 37%, and 60%, respectively, increased exposure, and decreased the apparent systemic clearance (clearance/bioavailability). In patients with Child C cirrhosis, the metabolic ratio (ratio of the area under the plasma concentration-time curve from 0 to 24 h of the metabolite to the parent compound), a marker for CYP activity, decreased by 66%, 47%, 92%, 73%, and 43% for paraxanthine/caffeine (CYP1A2), 8-hydroxyefavirenz/efavirenz (CYP2B6), 5-hydroxyomeprazole/omeprazole (CYP2C19), α-hydroxymetoprolol/metoprolol (CYP2D6), and 1'-hydroxymidazolam/midazolam (CYP3A), respectively. In comparison, the metabolic ratio 4-hydroxyflurbiprofen/flurbiprofen (CYP2C9) remained unchanged.

CONCLUSIONS

Liver cirrhosis affects the activity of CYP isoforms differently. This variability must be considered for dose adjustment of drugs in patients with liver cirrhosis.

CLINICAL TRIAL REGISTRATION

NCT03337945.

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

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

Prediction of Survival After Partial Hepatectomy Using a Physiologically Based Pharmacokinetic Model of Indocyanine Green Liver Function Tests

The evaluation of hepatic function and functional capacity of the liver are essential tasks in hepatology as well as in hepatobiliary surgery. Indocyanine green (ICG) is a widely applied test compound that is used in clinical routine to evaluate hepatic function. Important questions for the functional evaluation with ICG in the context of hepatectomy are how liver disease such as cirrhosis alters ICG elimination, and if postoperative survival can be predicted from preoperative ICG measurements. Within this work a physiologically based pharmacokinetic (PBPK) model of ICG was developed and applied to the prediction of the effects of a liver resection under various degrees of cirrhosis. For the parametrization of the computational model and validation of model predictions a database of ICG pharmacokinetic data was established. The model was applied (i) to study the effect of liver cirrhosis and liver resection on ICG pharmacokinetics; and (ii) to evaluate the model-based prediction of postoperative ICG-R15 (retention ratio 15 min after administration) as a measure for postoperative outcome. Key results are the accurate prediction of changes in ICG pharmacokinetics caused by liver cirrhosis and postoperative changes of ICG-elimination after liver resection, as validated with a wide range of data sets. Based on the PBPK model, individual survival after liver resection could be classified, demonstrating its potential value as a clinical tool.

Finger sweat analysis enables short interval metabolic biomonitoring in humans

Metabolic biomonitoring in humans is typically based on the sampling of blood, plasma or urine. Although established in the clinical routine, these sampling procedures are often associated with a variety of compliance issues, which are impeding time-course studies. Here, we show that the metabolic profiling of the minute amounts of sweat sampled from fingertips addresses this challenge. Sweat sampling from fingertips is non-invasive, robust and can be accomplished repeatedly by untrained personnel. The sweat matrix represents a rich source for metabolic phenotyping. We confirm the feasibility of short interval sampling of sweat from the fingertips in time-course studies involving the consumption of coffee or the ingestion of a caffeine capsule after a fasting interval, in which we successfully monitor all known caffeine metabolites as well as endogenous metabolic responses. Fluctuations in the rate of sweat production are accounted for by mathematical modelling to reveal individual rates of caffeine uptake, metabolism and clearance. To conclude, metabotyping using sweat from fingertips combined with mathematical network modelling shows promise for broad applications in precision medicine by enabling the assessment of dynamic metabolic patterns, which may overcome the limitations of purely compositional biomarkers.

Shared Metabolic Profile of Caffeine in Parkinsonian Disorders

Objective

The objective of this study was to determine comprehensive metabolic changes of caffeine in the serum of patients with parkinsonian disorders including Parkinson's disease (PD), progressive supranuclear palsy (PSP), and multiple system atrophy (MSA) and to compare this with healthy control serum.

Methods

Serum levels of caffeine and its 11 downstream metabolites from independent double cohorts consisting of PD (n = 111, 160), PSP (n = 30, 19), MSA (n = 23, 17), and healthy controls (n = 43, 31) were examined by liquid chromatography–mass spectrometry. The association of each metabolite with clinical parameters and medication was investigated. Mutations in caffeine‐associated genes were investigated by direct sequencing.

Results

A total of 9 metabolites detected in more than 50% of participants in both cohorts were decreased in 3 parkinsonian disorders compared with healthy controls without any significant association with age at sampling, sex, or disease severity (Hoehn and Yahr stage and Unified Parkinson's Disease Rating Scale motor section) in PD, and levodopa dose or levodopa equivalent dose in PSP and MSA. Of the 9 detected metabolites, 8 in PD, 5 in PSP, and 3 in MSA were significantly decreased in both cohorts even after normalizing to daily caffeine consumption. No significant genetic variations in CYP1A2 or CYP2E1 were detected when compared with controls.

Conclusion

Serum caffeine metabolic profiles in 3 parkinsonian diseases show a high level of overlap, indicative of a common potential mechanism such as caffeine malabsorption from the small intestine, hypermetabolism, increased clearance of caffeine, and/or reduced caffeine consumption. © 2020 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Pharmacokinetics in Morbid Obesity: Influence of Two Bariatric Surgery Techniques on Paracetamol and Caffeine Metabolism

PURPOSE

The purpose of the study was to study the impact of the two most common bariatric surgery techniques on paracetamol pharmacokinetics (a marker of gastric emptying) and caffeine metabolism (a marker of liver function).

MATERIALS AND METHODS

In the present prospective study, we studied 24 morbid obese patients before, at 4 weeks, and 6 months after having undergone sleeve gastrectomy (n = 10) or Roux-en-Y gastric bypass (n = 14). For comparative purposes, 28 healthy controls (14 normal weights and 14 overweights) were also included in the study.

RESULTS

Paracetamol pharmacokinetics was altered in the obese participants leading to lower bioavailability. Bariatric surgery resulted in faster absorption and normalized pharmacokinetic parameters, prompting an increase in paracetamol bioavailability. No differences were found between surgical procedures. In the case of caffeine, the ratio paraxanthine/caffeine did not differ between morbid obese and healthy individuals. This ratio remained unmodified after surgery, indicating that the liver function (assessed by cytochrome P450 1A2 activity) was unaffected by obesity or bariatric surgery.

CONCLUSIONS

Paracetamol pharmacokinetics and caffeine plasma levels are altered in severely obese patients. The two studied bariatric surgical techniques normalize paracetamol oral bioavailability without impairing the liver function (measured by cytochrome P450 1A2 activity).

Clinical Pathology Approaches to Hepatic Injury

Testing the blood for evidence of hepatic damage and dysfunction frequently involves measuring several blood constituents simultaneously to screen for disease. While useful, this approach occasionally leads to apparent disparities between the blood test results, and the results of other diagnostic tests such as histology. In part, these perceived discrepancies may stem from a lack of appreciation for tissue, cellular, and molecular factors that affect the appearance of hepatic disease biomarkers in the blood. Further confusing the matter is that in some instances the mechanisms responsible for the appearance of diagnostic compounds in blood are only partially understood. Many of the known factors that affect hepatic biomarkers are similar to those affecting other tissue markers, while others are unique to the liver, such as those involved with cholestasis. Disease conditions can also cause misleading results by affecting tissue concentrations of test compounds, hepatic mass, and the clearance rate of compounds from the blood. Knowledge of the factors affecting the blood concentrations of biomarkers, as well as investigations into the mechanisms behind changes to hepatic biomarker concentrations, may allow for a better interpretation of blood test results and fewer inconsistencies between diagnostic results.

A pharmacometric approach to investigate the impact of methylxanthine abstinence and caffeine consumption on CYP1A2 activity

This study aimed to investigate the impact of methylxanthine abstinence (MA) periods on CYP1A2 activity in individuals with varying levels of caffeine consumption through development of a population pharmacokinetic model of caffeine and its major metabolite paraxanthine. This study developed and evaluated a mixed-effects pharmacokinetic model for caffeine and paraxanthine concentration-time data derived from a sequential single-dose cross-over study in healthy male volunteers (n = 30) who received oral 100 mg caffeine doses. Participants received caffeine with and without a MA period. Participants were classified as low (0-100 mg/d), medium (100-200 mg/d), or high (>200 mg/d) caffeine consumers (LCCs, MCCs, or HCCs, respectively). All caffeine and paraxanthine concentration-time data were simultaneously modeled. Caffeine pharmacokinetics was described by a two-compartment model with first-order absorption and two first-order elimination pathways. Paraxanthine was described by a one-compartment model with first-order absorption and elimination. Among LCCs (n = 16) and MCCs (n = 9), there was no difference in the mean (95% confidence interval) total apparent caffeine clearance (CL) between the MA period [LCCs: 6.88 (5.61-8.16 l/h); MCCs: 10.09 (7.57-12.60 l/h)] versus the no MA period [LCCs: 6.22 (4.97-7.46 l/h); MCCs: 9.68 (7.12-12.24 l/h)]. The mean CL among HCCs (n = 5) was considerably higher in the MA period [10.48 (5.62-15.33 l/h)] compared with the no MA period [6.30 (3.40-9.20 l/h)] (P < 0.05). The decrease in CL in the no MA period among HCC appears to be due to alternative caffeine elimination pathways, rather than CYP1A2.

Serum Metabolite/Caffeine Ratios as a Test for Liver Function

The aim of this study was to test the usefulness of the metabolite/caffeine ratio for the evaluation of hepatic dysfunction. Subjects with liver cirrhosis and chronic hepatitis, as well as healthy volunteers, were given the oral dose of 300 mg caffeine. Blood samples were collected after 4, 8, and 12 hours. Concentrations of caffeine (CA) and its three metabolites-paraxanthine (PX), theobromine (TB), and theophylline (TP)-were determined by high-performance liquid chromatography. Pharmacokinetic parameters of caffeine and PX/CA, TB/CA, and TP/CA ratios were calculated. Elimination of caffeine was decreased in cirrhotics in comparison with healthy volunteers, as proved by the values of clearance (0.035 vs. 0.094 L/h/kg), elimination coefficient (0.061 vs. 0.153 h(-1)), and half-life (11.4 vs. 4.3 h). Serum metabolite/caffeine ratios were significantly reduced in cirrhotic patients: PX/CA by more than 80%, TB/CA by 50% to 70%, and TP/CA by 40% to 70%. The reduction of the ratios in chronic hepatitis patients was lower and did not occur at all time points. A high correlation was found between caffeine clearance and metabolite/caffeine ratios. Metabolite/caffeine ratios calculated in a single blood sample collected 8 or 12 hours after caffeine administration could provide a practical assessment of hepatic function in cirrhotic patients. The value of the test for the chronic hepatitis patients is limited.

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.

Quantitative rat liver function test by galactose single point method

Summary

The purpose of this study was to investigate the galactose single point (GSP) method, a residual liver function test recently recommended by the US Food and Drug Administration, which can be a useful tool for rat liver function measurement. Rats were treated either with carbon tetrachloride (CCl4) alone (1 mL/kg, intraperitoneally [i.p.]) for one day or with isoniazid (INH) alone (150 mg/kg, i.p.) or (in order to ameliorate the effects of INH) with a combination of INH and bis- p-nitrophenyl phosphate (BNPP) (25 mg/kg, i.p.) for 21 days. Hepatotoxicity was assayed by plasma aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities and scores of histological activity index-necroinflammation (HAI-NI) of the respective liver specimens. The GSP method in rats was defined by the galactose blood level after 60 min. Significant differences in GSP values were observed between controls and the CCl4-treated rats. After 21 days of treatment, no significant changes in AST and ALT values were observed among the control, INH and INH-BNPP groups. There were significant differences in average GSP values for controls ( P < 0.001) and INH-BNPP ( P < 0.001) compared with INH alone. Highly significant correlations ( P < 0.001) were obtained between GSP and scores of HAI-NI for all the groups. GSP was concluded to be a more sensitive biomarker of INH-induced hepatotoxicity than AST or ALT in the rats. The GSP method has been proved to be a simple and useful tool for the quantitative determination of liver function in rats, which can possibly be extended to other animals.

Pharmacokinetics and dosage adjustment in patients with hepatic dysfunction

The liver plays a central role in the pharmacokinetics of the majority of drugs. Liver dysfunction may not only reduce the blood/plasma clearance of drugs eliminated by hepatic metabolism or biliary excretion, it can also affect plasma protein binding, which in turn could influence the processes of distribution and elimination. Portal-systemic shunting, which is common in advanced liver cirrhosis, may substantially decrease the presystemic elimination (i.e., first-pass effect) of high extraction drugs following their oral administration, thus leading to a significant increase in the extent of absorption. Chronic liver diseases are associated with variable and non-uniform reductions in drug-metabolizing activities. For example, the activity of the various CYP450 enzymes seems to be differentially affected in patients with cirrhosis. Glucuronidation is often considered to be affected to a lesser extent than CYP450-mediated reactions in mild to moderate cirrhosis but can also be substantially impaired in patients with advanced cirrhosis. Patients with advanced cirrhosis often have impaired renal function and dose adjustment may, therefore, also be necessary for drugs eliminated by renal exctretion. In addition, patients with liver cirrhosis are more sensitive to the central adverse effects of opioid analgesics and the renal adverse effects of NSAIDs. In contrast, a decreased therapeutic effect has been noted in cirrhotic patients with beta-adrenoceptor antagonists and certain diuretics. Unfortunately, there is no simple endogenous marker to predict hepatic function with respect to the elimination capacity of specific drugs. Several quantitative liver tests that measure the elimination of marker substrates such as galactose, sorbitol, antipyrine, caffeine, erythromycin, and midazolam, have been developed and evaluated, but no single test has gained widespread clinical use to adjust dosage regimens for drugs in patients with hepatic dysfunction. The semi-quantitative Child-Pugh score is frequently used to assess the severity of liver function impairment, but only offers the clinician rough guidance for dosage adjustment because it lacks the sensitivity to quantitate the specific ability of the liver to metabolize individual drugs. The recommendations of the Food and Drug Administration (FDA) and the European Medicines Evaluation Agency (EMEA) to study the effect of liver disease on the pharmacokinetics of drugs under development is clearly aimed at generating, if possible, specific dosage recommendations for patients with hepatic dysfunction. However, the limitations of the Child-Pugh score are acknowledged, and further research is needed to develop more sensitive liver function tests to guide drug dosage adjustment in patients with hepatic dysfunction.

Pharmacokinetics and dosage adjustment in patients with hepatic dysfunction

The liver plays a central role in the pharmacokinetics of the majority of drugs. Liver dysfunction may not only reduce the blood/plasma clearance of drugs eliminated by hepatic metabolism or biliary excretion, it can also affect plasma protein binding, which in turn could influence the processes of distribution and elimination. Portal-systemic shunting, which is common in advanced liver cirrhosis, may substantially decrease the presystemic elimination (i.e., first-pass effect) of high extraction drugs following their oral administration, thus leading to a significant increase in the extent of absorption. Chronic liver diseases are associated with variable and non-uniform reductions in drug-metabolizing activities. For example, the activity of the various CYP450 enzymes seems to be differentially affected in patients with cirrhosis. Glucuronidation is often considered to be affected to a lesser extent than CYP450-mediated reactions in mild to moderate cirrhosis but can also be substantially impaired in patients with advanced cirrhosis. Patients with advanced cirrhosis often have impaired renal function and dose adjustment may, therefore, also be necessary for drugs eliminated by renal exctretion. In addition, patients with liver cirrhosis are more sensitive to the central adverse effects of opioid analgesics and the renal adverse effects of NSAIDs. In contrast, a decreased therapeutic effect has been noted in cirrhotic patients with beta-adrenoceptor antagonists and certain diuretics. Unfortunately, there is no simple endogenous marker to predict hepatic function with respect to the elimination capacity of specific drugs. Several quantitative liver tests that measure the elimination of marker substrates such as galactose, sorbitol, antipyrine, caffeine, erythromycin, and midazolam, have been developed and evaluated, but no single test has gained widespread clinical use to adjust dosage regimens for drugs in patients with hepatic dysfunction. The semi-quantitative Child-Pugh score is frequently used to assess the severity of liver function impairment, but only offers the clinician rough guidance for dosage adjustment because it lacks the sensitivity to quantitate the specific ability of the liver to metabolize individual drugs. The recommendations of the Food and Drug Administration (FDA) and the European Medicines Evaluation Agency (EMEA) to study the effect of liver disease on the pharmacokinetics of drugs under development is clearly aimed at generating, if possible, specific dosage recommendations for patients with hepatic dysfunction. However, the limitations of the Child-Pugh score are acknowledged, and further research is needed to develop more sensitive liver function tests to guide drug dosage adjustment in patients with hepatic dysfunction.

Assessing liver function

PURPOSE OF REVIEW

This is a review on the techniques for assessing liver function in critically ill patients.

RECENT FINDINGS

Actually, there is no ideal real-time and bedside technique for assessing liver function in critically ill patients. Though not allowing to differentiate between liver blood flow and cell function, dynamic tests, that is indocyanine green plasma disappearance rate and lidocaine metabolism (monoethylglycinxylidide test), are superior, however, to static tests. Recently, the indocyanine green plasma disappearance rate, which nowadays can be measured reliably by a transcutaneous system in critically ill patients, was confirmed to correlate well with indocyanine green clearance. In general, the indocyanine green plasma disappearance rate is superior to bilirubin, which is still used as a marker of liver function, and comparable or even superior to complex intensive care scoring systems in terms of outcome prediction. Furthermore, indocyanine green plasma disappearance rate is more sensitive than serum enzyme tests for assessing liver dysfunction and early improvement in the indocyanine green plasma disappearance rate after onset of septic shock is associated with better outcome.

SUMMARY

Since no ideal tool is currently available, dynamic tests such as indocyanine green plasma disappearance rate and monoethylglycinxylidide test may be recommended for assessing liver function in critically ill patients. The indocyanine green plasma disappearance rate has the advantage, however, of being measurable noninvasively at the bedside and providing results within a few minutes.

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.

Dose adjustment in patients with liver disease

Unfortunately, there is no endogenous marker for hepatic clearance that can be used as a guide for drug dosing. In order to predict the kinetic behaviour of drugs in cirrhotic patients, agents can be grouped according to their extent of hepatic extraction. For drugs with a high hepatic extraction (low bioavailability in healthy subjects), bioavailability increases and hepatic clearance decreases in cirrhotic patients. If such drugs are administered orally to cirrhotic patients, their initial dose has to be reduced according to hepatic extraction. Furthermore, their maintenance dose has to be adapted irrespective of the route of administration, if possible, according to kinetic studies in cirrhotic patients. For drugs with a low hepatic extraction, bioavailability is not affected by liver disease, but hepatic clearance may be affected. For such drugs, only the maintenance dose has to be reduced, according to the estimated decrease in hepatic drug metabolism. For drugs with an intermediate hepatic extraction, initial oral doses should be chosen in the low range of normal in cirrhotic patients and maintenance doses should be reduced as for high extraction drugs. In cholestatic patients, the clearance of drugs with predominant biliary elimination may be impaired. Guidelines for dose reduction in cholestasis exist for many antineoplastic drugs, but are mostly lacking for other drugs with biliary elimination. Dose adaptation of such drugs in cholestatic patients is, therefore, difficult and has to be performed according to pharmacological effect and/or toxicity. Importantly, the dose of drugs with predominant renal elimination may also have to be adapted in patients with liver disease. Cirrhotic patients often have impaired renal function, despite a normal serum creatinine level. In cirrhotic patients, creatinine clearance should, therefore, be measured or estimated to gain a guideline for the dosing of drugs with predominant renal elimination. Since the creatinine clearance tends to overestimate glomerular filtration in cirrhotic patients, the dose of a given drug may still be too high after adaptation to creatinine clearance. Therefore, the clinical monitoring of pharmacological effects and toxicity of such drugs is important. Besides the mentioned kinetic changes, the dynamics of some drugs is also altered in cirrhotic patients. Examples include opiates, benzodiazepines, NSAIDs and diuretics. Such drugs may exhibit unusual adverse effects that clinicians should be aware of for their safe use. However, it is important to realise that the recommendations for dose adaptation remain general and cannot replace accurate clinical monitoring of patients with liver disease treated with critical drugs.

Validity of 13C-phenylalanine breath test to evaluate functional capacity of hepatocyte in patients with liver cirrhosis and acute hepatitis

BACKGROUND

No definitive method for quantitative evaluation of hepatic function has as yet been established.

AIM

To investigate whether the (13)C-phenylalanine breath test would be useful for the evaluation of hepatic function in patients with liver cirrhosis and acute hepatitis.

METHODS

L-[1-(13)C]-phenylalanine was administered orally in a dose of 100 mg to 25 patients with liver cirrhosis, 22 patients with acute hepatitis and 10 healthy subjects. The relationships of the cumulative excretion with the (13)C-%dose/h, blood biochemical parameters and asialoscintigraphy were investigated.

RESULTS

In liver cirrhosis patients, the cumulative excretion showed correlations with hepatic function tests, asialoscintigraphy, clinical stage and portal hypertension. In acute hepatitis patients, the cumulative excretion showed correlations with hepatic function tests. There were positive correlations between the cumulative excretion and the (13)C-%dose/h at 20 min (Phe20) and (13)C-%dose/h at 30 min (Phe30) in liver cirrhosis and acute hepatitis patients. Multiple regression analysis demonstrated that total bilirubin, total cholesterol and absence of varices were independent determinants of cumulative excretion in liver cirrhosis patients and prothrombin time in acute hepatitis patients.

CONCLUSION

The (13)C-phenylalanine breath test may allow hepatic function to be evaluated non-invasively in liver cirrhosis and acute hepatitis patients, and the Phe20 and Phe30 may be useful for determination of function at a single time-point.

Population Pharmacokinetics of Caffeine and its Metabolites Theobromine, Paraxanthine and Theophylline after Inhalation in Combination with Diacetylmorphine

The stimulant effect of caffeine, as an additive in diacetylmorphine preparations for study purposes, may interfere with the pharmacodynamic effects of diacetylmorphine. In order to obtain insight into the pharmacology of caffeine after inhalation in heroin users, the pharmacokinetics of caffeine and its dimethylxanthine metabolites were studied. The objectives were to establish the population pharmacokinetics under these exceptional circumstances and to compare the results to published data regarding intravenous and oral administration in healthy volunteers. Diacetylmorphine preparations containing 100 mg of caffeine were used by 10 persons by inhalation. Plasma concentrations of caffeine, theobromine, paraxanthine and theophylline were measured by high performance liquid chromatography. Non-linear mixed effects modelling was used to estimate population pharmacokinetic parameters. The model was evaluated by the jack-knife procedure. Caffeine was rapidly and effectively absorbed after inhalation. Population pharmacokinetics of caffeine and its dimethylxanthine metabolites could adequately and simultaneously be described by a linear multi-compartment model. The volume of distribution for the central compartment was estimated to be 45.7 l and the apparent elimination rate constant of caffeine at 8 hr after inhalation was 0.150 hr(-1) for a typical individual. The bioavailability was approximately 60%. The presented model adequately describes the population pharmacokinetics of caffeine and its dimethylxanthine metabolites after inhalation of the caffeine sublimate of a 100 mg tablet. Validation proved the stability of the model. Pharmacokinetics of caffeine after inhalation and intravenous administration are to a large extent similar. The bioavailability of inhaled caffeine is approximately 60% in experienced smokers.

Validity of the 13C-caffeine breath test as a noninvasive, quantitative test of liver function

The properties of caffeine render it an ideal substrate for a quantitative test of liver function. The aim of this study was to determine whether the caffeine breath test (CBT) using orally administered 13C-caffeine correlates reliably with plasma caffeine clearance and reflects varying degrees of liver dysfunction. The CBT was performed in 25 healthy controls; 20 subjects with noncirrhotic, chronic hepatitis B or C; and 20 subjects with cirrhosis. Plasma caffeine clearance was assayed simultaneously with the CBT in a cohort of these subjects. Over a broad range of caffeine clearances, the CBT exhibited a highly significant correlation with plasma clearance (r = 0.85, P <.001). Cirrhotic patients were characterized by significantly reduced CBT values (1.15 +/- 0.75 delta per thousand mg(-1)) compared with controls (2.23 +/- 0.76; P =.001) and hepatitic patients (1.83 +/- 1.05; P =.04). There was a significant inverse relationship between the CBT and Child-Pugh score (r = -.74, P =.002). The intraclass correlation coefficient between repeated CBTs in 20 subjects with normal and cirrhotic livers was 0.89. Although smoking was associated with an 86% to 141% increase in CBT in all groups, the CBT was able to distinguish control, hepatitic, and cirrhotic smokers (5.36 +/- 0.82, 3.63 +/- 1.21, and 2.14 +/- 1.14, respectively, P =.001). Multivariate analysis revealed that only smoking (P <.001) and disease state (P =.001) were significant predictors of the CBT. In conclusion, the 13C-CBT represents a valid indicator of plasma caffeine clearance and correlates reproducibly with hepatic dysfunction.

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.

Recovery of liver function in two-third partial hepatectomized rats evaluated by L-[1-13C]phenylalanine breath test

BACKGROUND

We have previously reported that by means of a breath test with intravenously administered L-[1-13C] phenylalanine (13Cphe), hepatopathy could be quantitatively evaluated by measuring expiratory 13CO2 levels in a short period. It is known that phenylalanine hydroxylase activity (PAHA) plays an important role in phenylalanine metabolism. We examined the relationship between changes in PAHA and the results of the 13Cphe breath test during hepatic regeneration in a rat model of 70% hepatectomy, to assess their usefulness for evaluating hepatic regeneration.

METHODS

Male Wistar rats (Shizvoka Laboratory Animal Center, Hamamatsu, Japan) weighing 230 to 290 g were subjected to 70% hepatectomy under anesthesia with sodium pentobarbital. One, 2, 3, 5, 7, and 14 days postoperatively, 30 mg/kg 13Cphe was intravenously injected into the femoral vein, and the increase in exhaled 13CO2 (Delta 13CO2) was measured for 15 minutes. Simple laparotomy was performed in control rats. After the breath test, the regenerated liver was removed and weighed. The amount of DNA, amount of hepatic tissue total protein (TP), and PAHA were determined.

RESULTS

The r between liver weight/body weight and PAHA, between DNA and PAHA, and between TP and PAHA were 0.832, 0.720, and 0.758, respectively. Breath test graphs revealed that liver weight/body weight, DNA, and TP showed the best correlations with the peak value of Delta 13CO2 (liver weight/body weight percentage, r = 0.801; DNA, r = 0.660; TP, r = 0.706), and r between PAHA and peak value was 0.638.

CONCLUSIONS

These results suggest that measurement of PAHA in regenerated liver is an effective method for following up liver function after hepatic resection. Moreover, the 13Cphe breath test may also be useful to evaluate liver function after partial hepatectomy.

Differential alteration of cytochrome P450 isoenzymes in two experimental models of cirrhosis

Liver diseases are associated with a decrease in hepatic drug elimination, but there is evidence that cirrhosis does not result in uniform changes of cytochrome P450 (CYP) isoenzymes. The objective of this study was to determine the content and activity of four CYP isoenzymes in the bile duct ligation and carbon tetrachloride (CCl4)-induced models of cirrhosis. The hepatic content of CYP1A, CYP2C, CYP2E1, and CYP3A was measured by Western blot analysis. CYP activity in vivo was evaluated with breath tests using substrates specific for different isoenzymes: caffeine (CYP1A2), aminopyrine (CYP2C11), nitrosodimethylamine (CYP2E1), and erythromycin (CYP3A). Bile duct ligation resulted in biliary cirrhosis; CYP1A, CYP2C and CYP3A content was decreased and the caffeine, aminopyrine, and erythromycin breath tests were reduced whereas CYP2E1 content and the nitrosodimethylamine breath test were unchanged compared with controls. CCl4 treatment resulted in cirrhosis of varying severity as assessed from the decrease in liver weight and serum albumin. In rats with mild cirrhosis, CYP content was comparable with controls except for a decrease in CYP2C. The activity of CYPs was also unchanged except for an increase in CYP2E1 activity. In rats with more severe cirrhosis, the content of all four CYP isoenzymes and the caffeine, aminopyrine, and erythromycin breath tests were reduced whereas the nitrosodimethylamine breath test was unchanged. In both models of cirrhosis, there was a significant correlation between the breath tests results and the severity of cirrhosis as assessed from serum albumin levels. These results indicate that content and the catalytic activity of individual CYP enzymes are differentially altered by cirrhosis in the rat and also suggest that drug probes could be useful to assess hepatic functional reserve.

The MEGX test: a tool for the real-time assessment of hepatic function

The dynamic liver function test based on the hepatic conversion of lidocaine to monoethylglycinexylidide (MEGX) can complement established static liver function tests if prognostic information is of particular interest. Because of its ease of use and rapid turnaround, the MEGX test has found widespread application for realtime assessment of hepatic function in transplantation, critical care medicine, and various experimental models. Lidocaine is metabolized primarily by the liver cytochrome P450 system through sequential oxidative N-dealkylation, the major initial metabolite in humans being MEGX. Because of the relatively high extraction ratio of lidocaine, this liver function test depends not only on hepatic metabolic capacity but also on hepatic blood flow. For the determination of MEGX in serum, an immunoassay based on the fluorescence polarization immunoassay technique high-performance liquid chromatography and gas liquid chromatography methods have been described. Whereas high-performance liquid chromatography and gas liquid chromatography are specific for MEGX, the fluorescence polarization immunoassay also cross-reacts with 3-OH-MEGX. Although this is not a problem in humans, some species, such as the rat, produce significant amounts of this metabolite. The findings of most studies published so far suggest that the MEGX test is a useful tool that can improve our decision-making process with respect to the selection of transplant candidates. Patients with a MEGX 15- or 30-minute test value <10 microg/L have a particularly poor 1-year survival rate. Serial monitoring of liver graft recipients early after transplantation with the MEGX test may initially alert the clinician to a major change in liver function; if used with other tests, such as serum hyaluronic acid concentrations, it may become more discriminatory. In critically ill patients, several studies have shown that an initially rapid decrease in MEGX test values is associated with an enhanced risk for the development of multiple organ dysfunction syndrome and a poor outcome. Further, this decrease appears to be associated with an enhanced systemic inflammatory response. The MEGX test has potential for investigating the pathogenesis of multiple organ dysfunction syndrome with regard to early hepatic functional impairment in critically ill patients after polytrauma or sepsis.

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.

Plasma hydroxy-metronidazole/metronidazole ratio can detect early changes in hepatic function in ethanol-induced liver injury

AIMS

To evaluate the usefulness of plasma hydroxy-metronidazole/metronidazole (OH-MET/MET) ratios as a dynamic liver function test in ethanol abusers with or without liver cirrhosis.

METHODS

Metronidazole was administered intravenously for 20 min to healthy volunteers, and to patients with alcohol-induced, non-cirrhotic hepatopathy and liver cirrhosis. Plasma concentrations of metronidazole and hydroxy-metronidazole were measured by high performance liquid chromatography in samples collected 5, 10, 20 and 30 min after the metronidazole infusion.

RESULTS

Patients with non-cirrhotic alcoholic hepatopathy had significantly elevated aminotransferase levels compared to healthy volunteers and Child A patients. Child-Pugh C patients had significantly prolonged prothrombin times when compared to healthy volunteers and patients with non-cirrhotic hepatopathy. Metronidazole metabolism, as measured by the OH-MET/MET ratio following the intravenous administration of 500 mg of the drug, was significantly impaired in all ethanol-abusing individuals, including patients with non-cirrhotic alcoholic hepatopathy.

CONCLUSIONS

Metronidazole metabolism was impaired in ethanol abusers, even in the absence of liver cirrhosis, indicating that ethanol was capable of affecting liver function in the early stages of alcohol-induced liver disease.

Salivary clearance and urinary metabolic pattern of caffeine in healthy children and in pediatric patients with hepatocellular diseases

Measurement of salivary clearance and urinary metabolites of caffeine is an excellent noninvasive tool for assessing liver function, particularly the activity of cytochrome P4501A2 (CYP1A2), N-acetyltransferase (NAT), and xanthine oxidase (XO). This study was undertaken to measure the clearance of caffeine using saliva as a biological fluid and to assess the activities of the above-mentioned enzymes in healthy children and pediatric patients with liver diseases using urinary molar ratios of different caffeine metabolites. The well-established two-sample saliva approach was used to measure the clearance of caffeine in nine pediatric patients with liver diseases (LD) and in nine healthy children. The caffeine metabolites were also measured in the urine of these subjects by high-performance liquid chromatography, and urinary molar ratios of 5-acetylamino-6-formylamino-3-methyluracil (AFMU), 1-methylxanthine (1X), 1-methyluric acid (1U), and 1,7-dimethyluric acid (17U) were employed to estimate the activities of CYP1A2, NAT, and XO. The caffeine salivary clearance and the percentage of the dose excreted in the form of various metabolites were significantly (p < 0.035) smaller in the LD patients than those in healthy children. The urinary molar ratio of [AFMU + 1U + 1X]/17U, which reflects the activity of CYP1A2, was also significantly (p < 0.0005) reduced in these patients. However, there were no significant differences between the two groups in the ratios of AFMU/1X and 1U/1X, which estimate the activities of NAT and XO, respectively. In conclusion, the data obtained suggest that liver disease in pediatric subjects significantly reduces the salivary clearance of caffeine and the activity of cytochrome P4501A2, but it has no impact on the activities of NAT and XO.

Effect of hepatic insufficiency on pharmacokinetics and drug dosing

The liver plays a central role in the pharmacokinetics of many drugs. Liver dysfunction may not only reduce the plasma clearance of a number of drugs eliminated by biotransformation and/or biliary excretion, but it can also affect plasma protein binding which in turn could influence the processes of distribution and elimination. In addition, reduced liver blood flow in patients with chronic liver disease will decrease the systemic clearance of flow limited (high extraction) drugs and portal-systemic shunting may substantially reduce their presystemic elimination (first-pass effect) following oral administration. When selecting a drug and its dosage regimen for a patient with liver disease additional considerations such as altered pharmacodynamics and impaired renal excretion (hepatorenal syndrome) of drugs and metabolites should also be taken into account. Consequently, dosage reduction is necessary for many drugs administered to patients with chronic liver disease such as liver cirrhosis.

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.

Evaluation of pharmacokinetic methods used to estimate caffeine clearance and comparison with a Bayesian forecasting method

Simplified pharmacokinetic methods have been used to estimate caffeine clearance in subjects with liver disease. There is a need to have a reliable, easy to implement method for research and possible clinical use. This study evaluates the use of Bayesian pharmacokinetic forecasting techniques to estimate caffeine clearance and compares its performance to other published methods. Commonly used published methods include the two-concentration overnight salivary clearance method (Jost method) and a method that samples caffeine concentrations over a 4-hour time period (Nagel method). Both have been used in studies incorporating serial measurements of caffeine clearance to predict clinical outcomes in subjects with liver disease, but these approaches have not been proven useful. However, neither method has been formally evaluated for accuracy in estimating caffeine clearance in subjects with cirrhosis. The performance of the Jost, Nagel, and Bayesian methods was compared to a Gold Standard method that accurately measured caffeine clearance in healthy subjects and subjects with cirrhosis using an intravenous infusion of stable isotope-labeled caffeine. The Bayesian method, even when only one measured concentration of caffeine was used, was more accurate, better correlated to the Gold Standard method, and had less intraindividual variation than the two previously published methods. Before the idea of using serial measurements of caffeine clearance for clinical usefulness is rejected, a reevaluation using methods of estimating caffeine clearance that are more accurate than previous paradigms is needed.

Caffeine in saliva after peroral intake: early sample collection as a possible source of error

The influence of collection time on the correlation of caffeine concentrations in saliva and serum was examined in six healthy adults after peroral administration of 5 mg/kg caffeine citrate. Saliva was obtained from three different salivary glands (sublingual, right parotid, and left parotid) and evaluated separately. Caffeine concentrations in saliva and serum samples were determined by high-performance liquid chromatography. There were no differences in the caffeine concentrations in saliva from the three investigated glands (alpha = 0.05). Saliva samples collected earlier than 2 hours after caffeine intake showed higher caffeine concentrations than could be expected from the corresponding serum samples. Gingiva contamination was shown to be responsible for the higher caffeine concentrations in saliva, and it was concluded that saliva is a feasible matrix for therapeutic drug monitoring of caffeine. If caffeine is administered orally, saliva samples should be taken at least 2 hours after caffeine intake. If caffeine-containing beverages are used as the source of caffeine or if subjects do not cooperate by rinsing the mouth of caffeine contamination, an additional 60 minutes should be added before saliva sampling.

Effect of liver cirrhosis on the systemic availability of naltrexone in humans

BACKGROUND/AIMS

Naltrexone is a competitive opiate antagonist with high hepatic extraction. It is used for detoxification treatment for heroin addicts and has been proposed as a possible treatment of pruritus in cholestasis. Such patients are likely to have impaired liver function, underscoring the need to understand the pharmacokinetic behavior of naltrexone in liver disease. These studies were undertaken to evaluate the effect of liver cirrhosis on the plasma time-course of naltrexone.

METHODS

A total of 18 patients were investigated: seven migraine patients with normal liver function regarded as controls and 11 patients with liver cirrhosis (six with decompensated disease and five with preserved liver function). A bolus of 100 mg of naltrexone was administered orally in the morning, after an overnight fast. Blood samples were taken in basal conditions and at fixed intervals, up to 24 h after administration. Serum levels of naltrexone and of its major active metabolite, 6 beta-naltrexol, were assayed by reversed-phase HPLC analysis.

RESULTS

In control subjects, circulating concentrations of naltrexone were always much lower than those of 6 beta-naltrexol (area under the curve: naltrexone, 200 +/- 97 ng/ml x 24 h; 6 beta-naltrexol, 2467 +/- 730 ng/ml x 24 h, p < 0.01). In severe cirrhosis serum levels of 6 beta-naltrexol increased more slowly, so that circulating levels of naltrexone during the first 2-4 h after drug intake were higher than those of 6 beta-naltrexol (6 beta-naltrexol/naltrexone ratio at 2 h: controls, 10.91 +/- 4.80; cirrhosis, 0.39 +/- 0.18, p < 0.01). The area under the curve for naltrexone (1610 +/- 629 ng/ml x 24 h) was significantly greater than in controls, whereas that for 6 beta-naltrexol (2021 +/- 955 ng/ml x 24 h) was not significantly different. Patients with compensated cirrhosis showed an intermediate pattern. No differences in elimination half-life of the two drugs were detected among the groups.

CONCLUSIONS

Our data suggest the occurrence of important changes in the systemic availability of naltrexone and 6 beta-naltrexol in liver cirrhosis; such alterations are consistent with lesser reduction of naltrexone to 6 beta-naltrexol and appear to be related to the severity of liver disease. This must be considered when administering naltrexone in conditions of liver insufficiency.

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

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

Assessment of liver function: pre- and peritransplant evaluation

Liver transplantation has been demonstrated to be a successful therapeutic modality for patients with end-stage liver disease. The high rate of survival for an otherwise terminal condition has resulted in significant expansion of the indications and diseases treated by this procedure, and is hampered only by the limited numbers of organs available for transplantation. Efforts in clinical and laboratory medicine should be directed to identify candidates who would benefit most from this procedure, to provide better means for accurate assessment of liver reserve and the appropriate timing for transplantation, to identify quality liver grafts that would have the potential to tolerate cold preservation and reperfusion injury, and to assist in accurate monitoring of graft function immediately after transplantation. The aim of this manuscript is to describe the existing pathways for clinical and laboratory assessment of pretransplant residual liver function, the donor liver graft, and immediate posttransplantation function.

L-[1-(13)C] Phenylalanine oxidation as a measure of hepatocyte functional capacity in end-stage liver disease

BACKGROUND

Liver disease is associated with impaired metabolism of these amino acids phenylalanine and tyrosine. Decreased metabolism of these amino acids leads to abnormal plasma elevations and impaired clearance rates. We have developed a noninvasive breath test that measures hepatic cytosolic enzyme activity.

METHODS

The rate of hepatic phenylalanine metabolism was quantitatively calculated from the appearance of 13CO2 in the breath using the nonradioactive tracer L-[1-(13)C]phenylalanine.

RESULTS

Normal controls (n = 47) oxidized phenylalanine more than twice that of end-stage liver disease patients (n = 117). Significant differences in the percent of phenylalanine oxidized per hour (mean +/- SEM) were found between controls (7.08% +/- 0.33%, 95% CI: 6.42%-7.74%) and Child Pugh classification patients, class A (4.96% +/- 0.69%, 95% CI: 3.50%-6.42%), class B (2.88% +/- 0.13, 95% CI: 2.39%-3.38%) and class C (1.75% +/- 0.13, 95% CI: 1.50%-2.01%). The phenylalanine breath test score significantly correlated with albumin levels, prothrombin time and total bilirubin.

CONCLUSION

We have demonstrated that phenylalanine oxidation is significantly decreased with end-stage liver disease and is correlated with the best clinical measures of liver disease.

Use of salivary caffeine tests to assess the inducer effect of a drug on hepatic metabolism

OBJECTIVE

To validate the use of successive salivary caffeine tests in evaluating how long inducer drugs affect hepatic metabolism. The time course of the inducer effect of rifampin found in other studies using different methodologies was chosen as the time course of reference.

DESIGN

Open-label, prospective, longitudinal study.

SETTING

A university hospital.

PARTICIPANTS

Five healthy volunteers.

MAIN OUTCOME MEASURES

Rifampin 600 mg/d was administered for 21 days. Anhydrous caffeine 300 mg was concurrently administered on each study day. Salivary caffeine tests were carried out on the following days: predose (baseline), and days 1, 5, 9, 13, and 17. Salivary tests were performed for up to 13 days after the last dose of rifampin (study days 21, 25, 29, and 33).

RESULTS

The mean systemic caffeine clearance was increased for up to 17 days after the intake of rifampin, reaching the maximum inducer effect between days 5 and 9, and returning to previous values progressively during several days after rifampin was discontinued.

CONCLUSIONS

Our results suggest that successive salivary caffeine measurements could be a safe, reliable, noninvasive, and suitable test for exploring the time course of the inducer effect of drugs on hepatic metabolism activity.

The effect of liver disease on urine caffeine metabolite ratios

OBJECTIVES

A number of caffeine metabolite ratios (CMRs) have been proposed to measure CYP1A2 activity in vivo. The effect of liver disease on these ratios is not clear. The objective of this study was to determine the influence of liver disease on caffeine metabolite ratios.

STUDY DESIGN

Two studies were performed. First, in healthy control subjects and in subjects with cirrhosis, caffeine clearance was measured by intravenous infusion of stable isotope-labeled caffeine while subjects consumed oral caffeine. Second, spot urine samples were collected from control subjects and from subjects with either chronic hepatitis or cirrhosis while they consumed caffeine.

RESULTS

In study 1, caffeine clearance was decreased in subjects with cirrhosis (control mean, 0.14 L/hr/kg; cirrhosis mean, 0.04 L/hr/kg; p = 0.003). CMRs were affected by liver disease (e.g., ratio characterizing paraxanthine demethylation [AAMU + 1X + 1U/17U], control median, 8.3; cirrhosis median, 6.2; p = 0.005). AAMU + 1X + 1U/17U correlated significantly with caffeine clearance in the control group (r2 = 0.68; p = 0.0001) and in subjects with cirrhosis (r2 = 0.41; p = 0.05). In study 2, there was a significant difference between control subjects and subjects with cirrhosis for AAMU + 1X + 1U/17U (median for control subjects, 6.2; median for subjects with cirrhosis, 4.3; p = 0.001) but not between control subjects and patients with chronic hepatitis.

CONCLUSIONS

We conclude that AAMU + 1X + 1U/17U is affected by liver disease and reflects the decrease in CYP1A2 activity in subjects with cirrhosis. AAMU + 1X + 1U/17U measured from a spot urine sample reflects caffeine clearance in subjects with cirrhosis and may be useful as a hepatic function test. Despite the large interindividual variation observed, the test can be repeated easily in the same patient and an individual patient's decline in CYP1A2 activity, such as in patients with progressively deteriorating liver function, can be monitored.

The measurement of caffeine concentration in scalp hair as an indicator of liver function

Caffeine concentration in plasma and scalp hair has been determined for subjects consuming normal daily amounts of caffeine and the results used as an indicator of individual hepatic metabolic capacity. Daily exposure to caffeine was assessed in six healthy Japanese volunteers by direct HPLC measurement of the concentrations of caffeine in aliquots of all caffeine-containing beverages consumed by the subjects. The measurements were repeated on three different occasions for each subject and caffeine consumption (mean +/- s.d.) was calculated as 178.0 +/- 84.3 mg day-1 with an intra-individual variability of 23.8 +/- 6.3% as coefficient of variation. A survey of daily caffeine consumption in 121 adult Japanese by means of a questionnaire revealed a similar value (231.8 +/- 177.8 mg day-1). Caffeine concentration in the plasma sampled during an overnight caffeine-free interval was measured by HPLC and a comparison made between healthy subjects and patients with liver disease (0.71 +/- 0.32, 0.77 +/- 0.45 and 3.92 +/- 1.91 micrograms mL-1 for healthy volunteers (n = 6), patients with hepatitis (n = 11) and those with liver cirrhosis (n = 4), respectively). Strands of scalp hair were collected from six healthy subjects and six patients with liver cirrhosis. Caffeine in hair was identified and measured by gas chromatography-mass spectrometry after digestion of the hair matrix with protease and extraction of the caffeine with chloroform. Caffeine concentration in hair collected from patients with liver cirrhosis (26.5 +/- 5.04 ng mg-1 hair) was significantly higher than that in hair sampled from healthy subjects (7.21 +/- 3.11 ng mg-1). These findings suggest that the determination of caffeine concentration in the plasma and hair of subjects consuming normal daily amounts of caffeine-containing beverages provides a practical assessment of individual liver metabolic capacity.

Nondispersive isotope-selective infrared spectroscopy: a new analytical method for 13C-urea breath tests

BACKGROUND

Currently, stable isotope techniques in breath tests using 13C-labeled substrates are limited to a few centers equipped with expensive and complex isotope ratio mass spectrometry (IRMS). Although breath samples can be mailed to these centers, widespread application of 13C breath tests would be more feasible with a cheaper and more practicable analysis system at hand.

METHODS

We therefore tested the newly developed nondispersive isotope-selective infrared spectrometer (NDIRS) with reference to IRMS in a clinical setting comparing the results of both techniques in 538 consecutive 13C-urea breath tests performed for the detection of Helicobacter pylori infection.

RESULTS

With NDIRS five false-positive and three false-negative results were observed; that is, the sensitivity of NDIRS was 98.3%, and the specificity was 98.6%. The delta over base-line values of both devices correlated linearly (Y = 0.87 +/- 0.01 X + 0.29 +/- 1.5; r = 0.95; p < 0.0001; n = 538).

CONCLUSIONS

When running this large number of breath tests in 3 days, the NDIRS proved to be a reliable, stable, and easy-to-operate analytical tool, which is well qualified for gastroenterologic application in the diagnostic routine. Both the price and the easy handling of NDIRS will facilitate the widespread use of the noninvasive stable isotope technique for 13C breath tests.

Liver function tests

For optimal timing of liver transplantation and for the evaluation of new pharmacotherapeutic options, objective modalities for estimating the liver's functional reserve and prognosis in an individual patient are highly desirable. In the past a number of tests and several scoring systems have been proposed and validated to varying degrees for this purpose. The issues still to be clarified include: (1) any observed prognostic value of individual quantitative function tests and of scoring systems must be validated in independent, large enough and well defined patient populations; (2) it must be prospectively defined which (serially performed) quantitative test(s) add(s) prognostic information for the individual patient to the survival estimates defined by the more universally available scores and in which disease state(s); and (3) existing scoring systems must be validated, or new ones developed, that allow follow-up data to be used in order to adapt the original prognosis estimate to the evolution of the disease, e.g. during therapy.

Clinical significance of the trimethadione tolerance test in chronic hepatitis: a useful indicator of hepatic drug metabolizing capacity

Trimethadione (TMO) was chosen as an indicator of quantitative hepatic microsomal function, and its pharmacokinetics were studied in 52 patients with chronic hepatitis. Findings in these patients were compared with those for 26 healthy subjects and 13 patients with renal failure. Patients with chronic hepatitis, but not those with renal failure, showed significant reduction in clearance (CL) and prolongation of half-life (t1/2), and the extent of abnormalities was found to reflect the severity of histologic changes in liver tissue. The serum dimethadione (DMO)/TMO ratio 4 h after the administration of TMO altered in parallel with the CL and t1/2 of TMO, and abnormalities in this simple ratio were also related to the histologic severity of changes in the liver tissue. A low DMO/TMO ratio (< 0.4) was associated with advanced histologic changes (chronic active hepatitis with bridging or chronic active hepatitis with cirrhosis), whereas a high DMO/TMO ratio (> 0.4) was associated with mild histologic changes (chronic persistent hepatitis or chronic active hepatitis) (sensitivity, 0.81; specificity, 0.86). These results indicate that the DMO/TMO ratio, which can be obtained from a single blood sampling, reflects the histologic severity of changes in tissue liver, and that the TMO tolerance test is a useful indicator of quantitative liver function.

Review article: quantitative tests of liver function

The search continues for a single reliable test of liver function that provides accurate prognostic information in chronic liver disease, in acute liver failure, and about graft function following orthotopic liver transplantation. Although transaminases, the commonly used markers of hepatocellular injury, have a high sensitivity in screening for liver disease, they do not provide any information about prognosis. Rational assessment of liver function using bilirubin, serum albumin and prothrombin-time is limited by the relative lack of sensitivity of these measurements and their inability to identify the functional reserve of the liver. Dynamic liver function tests are an improvement on the static tests but are generally cumbersome. The ideal liver function test would be cheap, easy to perform and analyse, safe, have a simple pharmacokinetic profile with minimal drug interactions, have a high predictive value and provide quick results. Numerous quantitative liver function tests have been developed and have shown promise in some studies. The aim of this review is to assess the place of these tests in the practical management of liver disease.