Clinical pharmacokinetics in patients with liver disease

Alcohol exposure and paracetamol-induced hepatotoxicity

Most instances of hepatotoxicity due to paracetamol in the United Kingdom and Australia are the result of large overdoses of the drug taken with suicidal or parasuicidal intent. In contrast, serious hepatotoxicity at recommended or near-recommended doses for therapeutic purposes has been reported, mainly from the United States and in association with chronic alcohol use, leading to the widely held belief that chronic alcoholics are predisposed to paracetamol-related toxicity at relatively low doses. Yet the effects of alcohol on paracetamol metabolism are complex. Studies performed in both experimental animals and humans indicate that chronic alcohol use leads to a short-term, two- to threefold increase in hepatic content of cytochrome P4502E1, the major isoform responsible for the generation of the toxic metabolite from paracetamol, although increased oxidative metabolism of paracetamol at recommended doses has not been demonstrated clinically. A reduced hepatic content of glutathione, required to detoxify the reactive metabolite, has been documented in chronic alcoholics, due probably to associated fasting and malnutrition, providing a metabolic basis for any possible predisposition of this group to hepatotoxicity at relatively low paracetamol doses. Simultaneous alcohol and paracetamol ingestion reduces oxidative metabolism of paracetamol in both rodents and humans, predominantly as a consequence of depletion in cytosol of free NADPH. The possibilities that chronic alcohol use may predispose to paracetamol-related hepatotoxicity and that alcohol taken with paracetamol may protect against it, based on these metabolic observations, are examined in this review.

Comparative tacrolimus pharmacokinetics: normal versus mildly hepatically impaired subjects

Tacrolimus (FK506, Prograf), marketed for the prophylaxis of organ rejection following allogenic liver or kidney transplantation, is virtually completely metabolized. The major metabolic pathways are P450 3A4-mediated hydroxylation and demethylation. Since P450 hepatic drug-metabolizing enzymes may be impaired in hepatic dysfunction, a study was conducted to characterize oral and intravenous tacrolimus pharmacokinetics in 6 patients with mild hepatic dysfunction and compared with parameters to those from normal subjects obtained in a separate study. Patients received two treatments: a single 0.020 mg/kg ideal body weight (IBW) i.v. dose infused over 4 hours and approximately 0.12 mg/kg IBW orally; normal subjects were dosed at 0.02 mg/kg 4-hour i.v. and 5 mg (0.065 mg/kg) p.o. Mean blood pharmacokinetic parameters with mild hepatic dysfunction were as follows: clearance = 0.035 L/h/kg, terminal exponential volume of distribution = 2.59 L/kg, terminal exponential half-life = 60.6 hours (i.v.), p.o. maximum blood concentration = 48.2 ng/mL, time of p.o. maximum blood concentration = 1.5 hours, and absolute bioavailability = 22.3%. The respective parameters in normal subjects were as follows: 0.040 L/h/kg, 1.91 L/kg, 34.2 hours (i.v.), 29.7 ng/mL, 1.6 hours, and 17.8%. Inasmuch as clearance and bioavailability were not substantially different from that in normal subjects, patients with mild hepatic impairment may initially be treated with conventional tacrolimus doses, with subsequent dosage adjustments based on response, toxicity, and therapeutic drug monitoring.

Carbon monoxide wash-in method to determine gas transfer in vascular beds: application to rat hindlimb

The vascular barrier to gas transfer is an important physiological parameter; however, no readily applicable technique exists to quantitate the process. A simple technique to measure the permeability-surface area (PS) product for gas transfer in vascular beds is proposed using wash in of carbon monoxide (CO) and Crone-Renkin analysis. Wash-in experiments were performed on the perfused hindlimbs of male Wistar rats (n = 15) by using CO as a surrogate marker for oxygen and technetium-99m-labeled albumin as the vascular marker. The use of CO and erythrocyte-free perfusate and the collection of outflow samples into tubes preloaded with erythrocytes obviated the need for an anaerobic collection device or consideration of Hb binding in the analysis. The PS product for CO was determined from the early extraction as 0.013 +/- 0.006 ml. s(-1). g(-1). Compartmental analysis revealed that the fractional recovery of CO was 0.45 +/- 0.14 and the volume of distribution was 2.31 +/- 0.76 ml/g. This technique detected a small measurable barrier to the transfer of CO across the hindlimb vasculature and is potentially applicable to other vascular beds in health and disease.

Surgery and intensive care procedures affect the target site distribution of piperacillin

OBJECTIVE

Therapeutic failure of antibiotic therapy has been ascribed to pharmacokinetic alterations in compromised patient populations. The present study, therefore, aimed at examining the influences of cardiac surgery and intensive care procedures on the postoperative target site distribution of piperacillin. For this purpose, the penetration of piperacillin to the interstitial space fluid, the relevant target site for most bacterial infections, was compared between patients after aortic valve replacement and healthy volunteers.

DESIGN

Comparative study in two study populations.

SETTING

The intensive care unit and research ward of a university hospital.

PATIENTS

The study population included six otherwise healthy patients scheduled to undergo aortic valve replacement and a control group of six healthy male volunteers.

INTERVENTIONS

After the administration of a single i.v. infusion of 4.0 g piperacillin, free piperacillin concentrations were measured in the interstitium of skeletal muscle and subcutaneous tissue by in vivo microdialysis and in venous serum. Piperacillin concentrations were assayed with reversed phase high-performance liquid chromatography.

MEASUREMENTS AND MAIN RESULTS

Interstitial piperacillin concentrations in muscle and subcutaneous adipose tissue were significantly lower in patients compared with volunteers with the area under the curve for the interstitium/area under the curve for serum concentration ratios ranging from 0.25 to 0.27 and from 0.43 to 1.22 in patients and volunteers, respectively (p < .05 between groups). The terminal elimination half-life was markedly prolonged in patients, leading to a concomitant increase in t > minimal inhibitory concentration (MIC) values, the relevant surrogate for therapeutic success of therapy with beta-lactam antibiotics, for strains with MIC50 <4 microg/mL. For strains with MIC50 >20 microl/mL, however, inadequate target site concentrations were attained in the patient population.

CONCLUSIONS

During the postoperative and intensive care periods, target site concentrations of piperacillin are markedly altered and decreased. This may also be true for other antibiotic agents and may have clinical implications in that current dosing guidelines may result in inadequate target site concentrations for high-MIC strains. Conceivably, this could lead to therapeutic failure in some patients.

The single dose pharmacokinetics of ribavirin in subjects with chronic liver disease

AIMS

The primary objective of this study was to describe the single dose pharmacokinetics of ribavirin in subjects with normal liver function and those with various degrees of stable chronic liver disease. Additionally this study assessed the safety and tolerability of ribavirin in this population.

METHODS

Single oral 600 mg doses of ribavirin were administered to healthy male and female volunteers (n = 6) and patients with stable chronic liver disease (n = 17), in a parallel group study. Pharmacokinetic sampling and tolerability assessments were performed up to 168 h post dose.

RESULTS

Single oral doses of 600 mg ribavirin were well tolerated by healthy volunteers and patients with varying degrees of hepatic dysfunction. Although mean Cmax increased with the severity of hepatic dysfunction, there was no change in extent of absorption or renal clearance of ribavirin.

CONCLUSIONS

There are no pharmacokinetic reasons for initial dose adjustment of ribavirin in patients with hepatic dysfunction.

Oxidative injury reproduces age-related impairment of oxygen-dependent drug metabolism

The Oxygen Diffusion Barrier Hypothesis states that aging in the liver is associated with restricted oxygen uptake that explains the age-related impairment of phase I drug clearance observed in vivo with preservation of in vitro phase I enzyme activity and in vivo phase II drug clearance. Aging in the liver may be secondary to oxidative stress. Therefore we examined the effects of oxidative injury on oxygen uptake, and phase I and phase II drug metabolism in the liver. Oxidative stress was induced in the perfused rat liver with hydrogen peroxide. The intrinsic clearances of propranolol and morphine were used as markers of phase I and phase II activity, respectively. Oxidative injury was associated with a 14+/-99% (P=0.03) reduction in oxygen uptake. The decrease in the intrinsic clearance of propranolol was greater than that of morphine (57+/-14% vs 34+/-7% P<0.005). This result supports the concept of a restriction of oxygen supply constraining hepatic drug metabolism following oxidative stress. This has implications for aging and hepatic drug metabolism.

Effects of liver disease on pharmacokinetics. An update

Liver disease can modify the kinetics of drugs biotransformed by the liver. This review updates recent developments in this field, with particular emphasis on cytochrome P450 (CYP). CYP is a rapidly expanding area in clinical pharmacology. The information currently available on specific isoforms involved in drug metabolism has increased tremendously over the latest years, but knowledge remains incomplete. Studies on the effects of liver disease on specific isoenzymes of CYP have shown that some isoforms are more susceptible than others to liver disease. A detailed knowledge of the particular isoenzyme involved in the metabolism of a drug and the impact of liver disease on that enzyme can provide a rational basis for dosage adjustment in patients with hepatic impairment. The capacity of the liver to metabolise drugs depends on hepatic blood flow and liver enzyme activity, both of which can be affected by liver disease. In addition, liver failure can influence the binding of a drug to plasma proteins. These changes can occur alone or in combination; when they coexist their effect on drug kinetics is synergistic, not simply additive. The kinetics of drugs with a low hepatic extraction are sensitive to hepatic failure rather than to liver blood flow changes, but drugs having a significant first-pass effect are sensitive to alterations in hepatic blood flow. The drugs examined in this review are: cardiovascular agents (angiotensin converting enzyme inhibitors, angiotensin II receptor antagonists, calcium antagonists, ketanserin, antiarrhythmics and hypolipidaemics), diuretics (torasemide), psychoactive and anticonvulsant agents (benzodiazepines, flumazenil, antidepressants and tiagabine), antiemetics (metoclopramide and serotonin antagonists), antiulcers (acid pump inhibitors), anti-infectives and antiretroviral agents (grepafloxacin, ornidazole, pefloxacin, stavudine and zidovudine), immunosuppressants (cyclosporin and tacrolimus), naltrexone, tolcapone and toremifene. According to the available data, the kinetics of many drugs are altered by liver disease to an extent that requires dosage adjustment; the problem is to quantify the required changes. Obviously, this requires the evaluation of the degree of hepatic impairment. At present there is no satisfactory test that gives a quantitative measure of liver function and its impairment. A critical evaluation of these methods is provided. Guidelines providing a rational basis for dosage adjustment are illustrated. Finally, it is important to consider that liver disease not only affects pharmacokinetics but also pharmacodynamics. This review also examines drugs with altered pharmacodynamics.

31P and 1H NMR spectroscopic studies of liver extracts of carbon tetrachloride-treated rats

NMR spectroscopy was used to examine hepatic metabolism in cirrhosis with a particular focus on markers of functional cellular hypoxia. (31)P and (1)H NMR spectra were obtained from liver extracts from control rats and from rats with carbon tetrachloride-induced cirrhosis. A decrease of 34% in total phosphorus content was observed in cirrhotic rats, parallelling a reduction of 40% in hepatocyte mass as determined by morphometric analysis. Hypoxia appeared to be present in cirrhotic rats, as evidenced by increased inorganic phosphate levels, decreased ATP levels, decreased ATP:ADP ratios (1.72 +/- 0.40 vs 2.48 +/- 0.50, p < 0.01), and increased inorganic phosphate:ATP ratios (2.77 +/- 0.48 vs 1.62 +/- 0.24, p < 0.00001). When expressed as a percentage of the total phosphorus content, higher levels of phosphoethanolamine and lower levels of NAD and glycerophosphoethanolamine were detected in cirrhotic rats. Cirrhotic rats also had increased phosphomonoester:phosphodiester ratios (5.73 +/- 2.88 vs 2.53 +/- 0.52, p < 0.01). These findings are indicative of extensive changes in cellular metabolism in the cirrhotic liver, with many findings attributable to the presence of intracellular hypoxia.

Carbon monoxide disposition in the perfused rat liver

A simple method for determining carbon monoxide (CO) disposition in the rat liver perfused with erythrocyte-free buffer was developed. Wash-in experiments were performed with buffer containing tracer quantities of [14C]sucrose and 3H2O and equilibrated with CO. Outflow samples were collected into tubes containing human erythrocytes, which avidly bind CO. Outflow curves were analyzed using compartmental models. Fractional recovery of CO was 1.07 +/- 0. 17, and the apparent volume of distribution was 1.37 +/- 0.30 ml/g of liver (n = 8). A flow-limited model fitted the data most effectively, although estimates of the permeability-to-surface area product were attempted using a barrier-limited model. This technique will facilitate investigation of the effects of disease on gaseous substrate disposition in perfused organs.

Antipyrine, oxazepam, and indocyanine green clearance in patients with chronic pancreatitis and healthy subjects

BACKGROUND

Hepatic drug metabolism was examined in patients with chronic pancreatitis and healthy controls by using a cocktail design with three different model compounds: antipyrine to express phase-I oxidation, oxazepam to express phase-II conjugation, and indocyanine green (ICG), a high-clearance compound.

METHODS

Eight patients with chronic pancreatitis and seven healthy controls participated. Patients were diagnosed by the presence of typical morphologic changes of the pancreas on imaging and had a moderately but significantly reduced exocrine function and no or only slight impairment of the glucose tolerance. No one had a history or clinical signs of liver disease. Clearance of the three model compounds was estimated after the administration of 1 g antipyrine and 15 mg oxazepam orally and a bolus of indocyanine green, 0.5 mg/kg body weight, intravenously.

RESULTS

The antipyrine clearance and ICG clearance were significantly decreased in the patients compared with the controls (mean, 27.2 ml/min; 95% confidence interval (CI), 19.4-35; versus 46.2 ml/min; 34.7-58.7, and 501 ml/min; 4014601, versus 771 mU/min; 677-865 (P < 0.05), respectively). The oxazepam clearance did not differ significantly between the two groups (181 ml/min (145-217) versus 178 ml/min (152-204)). The model drug clearance ratios between the patient and control clearances showed decreased values for antipyrine and ICG compared with the oxazepam data (0.59 and 0.65 versus 1.02, respectively). Patients and controls were characterized by a body weight of 58.2 kg (53.1-63.3) and 83.4 kg (72.7-94.1), respectively, and a body mass index (BMI) of 19.6 kg/m2 (17.9-21.3) versus 25.9 kg/m2 (23.4-28.4) (P < 0.05 for both).

CONCLUSIONS

Patients with chronic pancreatitis characterized by a moderately reduced exocrine function and absence of diabetes mellitus and overt liver disease had a decreased antipyrine oxidation and ICG clearance, whereas no difference was seen in oxazepam conjugation when compared with healthy volunteers. In chronic pancreatitis the hepatic phase-I oxidation is reduced compared with the phase-II conjugation, as shown by the model drug clearance ratios. The clearance of ICG was also affected, pointing at a reduced hepatic plasma flow, provided that the hepatic extraction fraction is normal for these patients.

Comparative effects of oxygen supplementation on theophylline and acetaminophen clearance in human cirrhosis

BACKGROUND & AIMS

Sinusoidal capillarization in cirrhosis may impair the transfer of oxygen into hepatocytes; this may contribute to impaired oxidative drug metabolism. The aim of this study was to test this hypothesis by comparing the effects of oxygen supplementation in cirrhotic patients on the clearance of theophylline, which is dependent on hepatic oxidative metabolism, with its effect on the clearance of acetaminophen, which is reliant on hepatic conjugation reactions.

METHODS

Ten cirrhotic patients awaiting liver transplant and 5 control subjects were studied. Oral acetaminophen (1000 mg) and intravenous theophylline (3 mg/kg) were administered simultaneously on two separate occasions, 7 days apart. Subjects were randomized to breathe either room air or oxygen via face mask at 12 L/min for 9 hours of blood sampling.

RESULTS

Theophylline and acetaminophen clearances were significantly reduced by a mean of 54% and 50%, respectively, in cirrhotic patients compared with controls. Oxygen supplementation improved plasma theophylline clearance in cirrhotic patients by a mean of 34% (P = 0. 001), whereas acetaminophen clearance remained unchanged.

CONCLUSIONS

These findings indicate that, in cirrhosis, impaired hepatocyte oxygenation contributes to reduced oxidative drug metabolism and that oxidative drug metabolism can be improved by oxygen supplementation.

Toxicokinetics of organic solvents: a review of modifying factors

This article reviews, with an emphasis on human experimental data, factors known or suspected to cause changes in the toxicokinetics of organic solvents. Such changes in the toxicokinetic pattern alters the relation between external exposure and target dose and thus may explain some of the observed individual variability in susceptibility to toxic effects. Factors shown to modify the uptake, distribution, biotransformation, or excretion of solvent include physical activity (work load), body composition, age, sex, genetic polymorphism of the biotransformation, ethnicity, diet, smoking, drug treatment, and coexposure to ethanol and other solvents. A better understanding of modifying factors is needed for several reasons. First, it may help in identifying important potential confounders and eliminating negligible ones. Second, the risk assessment process may be improved if different sources of variability between external exposures and target doses can be quantitatively assessed. Third, biological exposure monitoring may be also improved for the same reason.

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.

Oral bioavailability and disposition characteristics of irbesartan, an angiotensin antagonist, in healthy volunteers

Absolute oral bioavailability and disposition characteristics of irbesartan, an angiotensin II receptor antagonist, were investigated in 18 healthy young male volunteers. Subjects received [14C] irbesartan as a 30-minute intravenous infusion (50 mg), [14C] irbesartan orally as a solution (50 mg or 150 mg), or irbesartan capsule (50 mg). Irbesartan was rapidly and almost completely absorbed after oral administration, and exhibited a mean absolute oral bioavailability of 60% to 80%. Mean total body clearance was approximately 157 mL/min, and renal clearance was 3.0 mL/min. Volume of distribution at steady state was 53 L to 93 L, and terminal elimination half-life was approximately 13 to 16 hours. Hepatic extraction ratio was low (0.2). There were no major circulating metabolites, and approximately 80% of total plasma radioactivity was attributable to unchanged irbesartan. Regardless of route of administration, approximately 20% of dose was recovered in urine and the remainder in feces.

Selective effect of liver disease on the activities of specific metabolizing enzymes: investigation of cytochromes P450 2C19 and 2D6

BACKGROUND AND OBJECTIVES

Drug metabolism is influenced by liver disease because of the central role that the liver plays in metabolic activities in the body. However, it is still unclear how activities of specific drug-metabolizing enzymes are influenced by the presence and severity of liver disease. As a consequence, alteration in metabolism of specific drugs cannot be easily predicted or appropriate dosage adjustment recommendations made.

METHODS

The activities of cytochromes P450 (CYP) 2C19 and 2D6 were investigated in a group of patients with mild or moderate liver disease (n = 18) and a group of healthy control subjects (n = 10). The disposition of racemic mephenytoin for CYP2C19 and debrisoquin for CYP2D6 were characterized in plasma and urine samples collected over 192 hours.

RESULTS

The elimination of S-mephenytoin was severely reduced among patients with liver disease, resulting in a 79% decrease in plasma clearance for all patients combined. This reduction was related to the severity of disease, patients with moderate disease being affected more severely than patients with mild disease. Similar differences were observed in the urinary excretion of 4'-hydroxymephenytoin metabolite. By contrast, there was no effect on the disposition of R-mephenytoin or debrisoquin.

CONCLUSION

These results show selectivity in the effect of liver disease on activities of specific metabolizing enzymes, CYP2C19 being more sensitive than CYP2D6. They suggest that recommendations for modification in drug dosage in the presence of liver disease should be based on knowledge of the particular enzyme involved in metabolism of the drug. The results emphasize the need for further studies of each specific drug-metabolizing enzyme in the presence of liver disease.

Effect of liver impairment on the pharmacokinetics of tolcapone and its metabolites

OBJECTIVE

To assess the effect of liver impairment on the pharmacokinetics of tolcapone and to derive appropriate dose recommendations for patients with this disease who are undergoing treatment for Parkinson's disease.

STUDY DESIGN

In an open, two-way crossover study, 16 patients with moderate liver disease (eight with cirrhotic and eight with noncirrhotic liver disease) and eight healthy subjects received an oral dose of 200 mg tolcapone and an intravenous dose of 50 mg tolcapone on separate occasions. The concentrations of total and unbound tolcapone and its three major metabolites (tolcapone glucuronide, carboxylic acid, and 3-O-methyl metabolite) were assessed in plasma and urine.

RESULTS

On the basis of total drug concentration, the differences in tolcapone pharmacokinetics between the groups were small. However, lower clearance and volume of distribution of unbound drug were found among patients with cirrhosis than among control subjects. Plasma concentration of the pharmacologically inactive glucuronide metabolite was increased among patients with cirrhosis.

CONCLUSIONS

Half of the recommended dosage of tolcapone should be administered to patients with cirrhosis of the liver to maintain the target steady-state concentration of unbound drug and to avoid accumulation of tolcapone glucuronide. Our data did not indicate a requirement for dosage adjustment in the presence of moderate chronic hepatitis.

Clinical importance of non-genetic and genetic cytochrome P450 function tests in liver disease

Liver disease is associated with reduced metabolic capacity for drugs that are metabolized by oxidative biotransformation. Three cytochrome P450 (P450 or CYP) gene families in liver microsomes (CYP 1, CYP2 and CYP3) appear to be responsible for much of the drug metabolism that takes place. The genetic polymorphism of the CYPs responsible for debrisoquine/ sparteine (CYP2D6) metabolism and S-mephenytoin (CYP2C19) metabolism has been well documented, but information on the impairment of each isoform in liver disease is still limited. There are two types of hepatic P450 function tests. One type consists of non-genetic P450 function tests (CYP1A2, 2A6, 2C9/10, 2E1 and 3A3/4), and probe drugs include caffeine, catalysed by CYP1A2, coumarin by CYP2A6, phenytoin by CYP2C6, chlorzoxazone by CYP2E1, and nifedipine, erythromycin and lidocaine by CYP3A3/4. The second type of genetic P450 function tests (CYP2C19 and CYP2D6) involves probe drugs such as S-mephenytoin, catalysed by CYP2C19, and debrisoquine and sparteine, catalysed by CYP2D6. The metabolism of the probe drugs used in non-genetic P450 function tests in patients with liver disease falls into two categories: reduced (CYP1A2, CYP2C, 2E1 and 3A) and unchanged (CYP2C). In genetic P450 function tests there seems to be a lesser degree of inhibition in poor metabolizers (PMs) than extensive metabolizers (EMs) among patients with liver disease. There have been very few reports on changes in metabolism of the probe drugs used in genetic P450 function tests in liver disease. In this paper the subject is reviewed.

Pharmacokinetics of rifapentine in patients with varying degrees of hepatic dysfunction

In this open-label investigation, the pharmacokinetics of rifapentine and its active metabolite, 25-desacetyl-rifapentine, were characterized in patients with varying degrees of hepatic dysfunction. Eight patients with mild-to-moderate chronic, stable hepatic dysfunction and seven patients with moderate-to-severe hepatic dysfunction received single oral 600-mg doses of rifapentine. Maximum plasma concentration of rifapentine was lower, time to maximum plasma concentration (tmax) was greater, and elimination half-life (t 1/2) was longer in the patients with moderate-to-severe hepatic dysfunction than in those with mild-to-moderate dysfunction. However, mean area under the concentration-time curve extrapolated to infinity (AUC0-infinity) for the two groups was similar. AUC0-infinity values in patients with hepatic dysfunction were 19% to 25% higher than values previously reported for healthy volunteers. The 25-desacetyl metabolite appeared in plasma slowly after the single oral dose of rifapentine. Similar to findings for the parent drug, comparable plasma exposures of 25-desacetyl-rifapentine based on AUC0-infinity were found in the two groups of patients with mild-to-moderate and moderate-to-severe hepatic dysfunction. Rifapentine was well tolerated in this patient population, irrespective of the etiology or severity of hepatic dysfunction. These safety and pharmacokinetic results suggest that no dosage adjustments for rifapentine are needed in patients with hepatic impairment.

The aging liver. Drug clearance and an oxygen diffusion barrier hypothesis

A change in drug clearance with age is considered an important factor in determining the high prevalence of adverse drug reactions associated with prescribing medications for the elderly. Despite this, no general principles have been available to guide drug administration in the elderly, although a substantial body of clearance and metabolism data has been generated in humans and experimental animals. A review of age-related change in drug clearances established that patterns of change are not simply explained in terms of hepatic blood flow, hepatic mass and protein binding changes. In particular, the maintained clearance of drugs subject to conjugation processes while oxygen-dependent metabolism declines, and all in vitro tests of enzyme function have been normal, requires new explanations. Reduction in hepatic oxygen diffusion as part of a general change in hepatocyte surface membrane permeability and conformation does provide one explanation for the paradoxical patterns of drug metabolism, and increased hepatocyte volume would also modify oxygen diffusion path lengths (the 'oxygen diffusion barrier' hypothesis). The reduction in clearances of high extraction drugs does correlate with observed reduction in hepatic perfusion. Dosage guidelines emerge from these considerations. The dosage of high clearance drugs should be reduced by approximately 40% in the elderly while the dosage of low clearance drugs should be reduced by approximately 30%, unless the compound is principally subject to conjugation mechanisms. If the hepatocyte diffusion barrier hypothesis is substantiated, this concept may lead to therapeutic (preventative and/or restorative) approaches to increased hepatocyte oxygenation in the elderly. This may lead to approaches for modification of the aging process in the liver.

Pharmacokinetics of milnacipran in liver impairment

The pharmacokinetics of single 50 mg oral and intravenous doses of milnacipran, a new non tricyclic antidepressant drug, were compared in 11 chronic liver impaired (LI) subjects and in 6 control subjects. Hepatic impairments, classified according to the PUGH scale were moderate (1 grade A), intermediate (6 grade B) and severe (4 grade C). Concentrations of unchanged drug and its conjugated form (its main metabolite) were measured in plasma and urines. In control subjects, milnacipran present high absolute bioavailability (mean value of 90%). Around 50% of the dose are excreted in urines as unchanged, while around 14% are excreted as glucuroconjugate. The remaining is composed of free and conjugated phase I inactive metabolites. Administration of milnacipran in LI subjects results in non significant changes in its pharmacokinetics, although its variability is increased. Unchanged drug exposure is not modified in LI subjects, while plasma levels of the conjugate are slightly decreased compared to the control group. This could either be due to a slight reduction in the conjugation process, or to a change in the distribution of the drug as urine excretion of both unchanged and conjugated forms are not modified compared to the control group. A few LI subjects present supra-bioavailability resulting in higher drug exposure after oral administration than after intravenous infusion. These modifications are not clinically relevant as drug exposure of the parent drug is not modified. As the unchanged drug is the only compound responsible for the activity of milnacipran, no dosage adjustment is needed in patients presenting liver impairment.

Drug acetylation in liver disease

N-Acetylation is a phase II conjugation reaction mediated in humans by the polymorphic N-acetyltransferase 2 (NAT2) and N-acetyltransferase 1 (NAT1). Acetylation of some drugs may be modestly decreased in patients with chronic liver disease, whereas acute liver injury has no effect on drug acetylation. For NAT2 substrates, the impairment in acetylation capacity seems to be phenotype-specific, with a more prominent effect being exerted in rapid than slow acetylators. Thus, in the presence of significant hepatic dysfunction, the activity of NAT2 may not exhibit its usual bimodal distribution, and hence phenotypic assignment may not be reliable. Furthermore, it remains to be evaluated whether the precautions advised for slow acetylators when treated with drugs metabolised by NAT2 apply to all patients (regardless of phenotype) with liver cirrhosis.

Liver damage induced by bile duct ligation affects CYP isoenzymes differently in rats

We examined the influence of liver damage induced by bile duct ligation on the activity and the expression of hepatic cytochrome P450 (CYP) 1A, 2B, 2C6, 2C11, 2E1 and 3A2 in male Sprague-Dawley rats. In the ligation group, testosterone 2 alpha-, 16 alpha-, and 6 beta-hydroxylase activities were severely decreased, whereas ethoxyresorufin O-deethylase and progesterone 21-hydroxylase activities relatively remained. Pentoxyresorufin O-deethylase and chlorzoxazone 6-hydroxylase activities were reduced to approximately one thirds those of control. The protein contents of these isoenzymes expressed in hepatic microsomes of the ligation group were decreased to 45%, 32%, 79%, 13%, 58%, and 23% of control for CYP1A, 2B, 2C6, 2C11, 2E1 and 3A2, respectively. The rank order of magnitude of the influence of bile duct ligation on CYP isoenzymes, assessed by the reduction in the enzyme activity and the protein content, corresponded with each other except CYP1A. The reduction of the enzyme activities significantly correlated with the reduction in the protein contents of different isoenzymes. These results suggested that bile duct ligation affected CYP isoenzyme activities and contents with different extent.

Impairment of the metabolism of dipyrone in asymptomatic carriers of the hepatitis B virus

BACKGROUND

The pharmacokinetics of a number of drugs has been shown to be impaired in patients with acute or chronic viral liver disease.

OBJECTIVE

To examine the effect of the asymptomatic hepatitis B virus carrier state on the metabolism of dipyrone (INN, metamizole) as a model drug.

METHODS

The pharmacokinetics of the metabolites of dipyrone-4-methylaminoantipyrine, 4-aminoantipyrine, 4-formylaminoantipyrine, and 4-acetylaminoantipyrine-after a 1.0 gm oral dose of dipyrone were evaluated in nine asymptomatic carriers of hepatitis B virus with normal liver function tests and nine healthy subjects. All subjects displayed the slow acetylator phenotype.

RESULTS

The nonrenal (metabolic) clearance of 4-methylaminoantipyrine was significantly reduced (mean +/- SEM) (123.3 +/- 15.8 versus 182.9 +/- 15.1 ml.min-1, respectively; p < 0.02) in the carriers of hepatitis B virus compared with the healthy subjects, and the elimination half-life of this metabolite was significantly longer (3.69 +/- 0.35 versus 2.64 +/- 0.28 hours, respectively; p < 0.03). The formation clearances of 4-aminoantipyrine and 4-formylaminoantipyrine were significantly smaller in the carriers of hepatitis B virus compared with healthy subjects (33.8 +/- 6.2 versus 55.2 +/- 6.4 ml.min-1; p < 0.03, and 16.7 +/- 2.2 versus 34.2 +/- 4.2 ml.min-1; p < 0.002; respectively). However, the elimination half-life of 4-formylaminoantipyrine was found to be slightly shorter in the carriers of hepatitis B virus. No significant differences were noted between the groups in the pharmacokinetics of 4-acetylaminoantipyrine.

CONCLUSION

The metabolism of dipyrone is impaired in asymptomatic carriers of hepatitis B virus. Clinically latent infection with hepatitis B virus seems to exert a differential effect on metabolism of the drug. Oxidative pathways to produce 4-aminoantipyrine and 4-formylaminoantipyrine were significantly affected, whereas acetylation remained intact. This study provided an additional example of the effect of a virus on the disposition of a drug.

Role of viral infections in the induction of adverse drug reactions

A spectrum of adverse drug reactions that are caused by the combined action of drugs and viruses has been described: ampicillin rash in acute infectious mononucleosis; Reye's syndrome; hypersensitivity reactions to sulphonamides in patients with HIV infection; drug-induced agranulocytosis; paracetamol (acetaminophen) hepatotoxicity; aspirin (acetylsalicyclic acid)-induced asthma; Epstein-Barr virus-associated lymphoma and methotrexate; and AIDS-related Kaposi's sarcoma and nitrite use. Changes in pharmacokinetics have been reported for: caffeine, sulfamethoxazole and fluconazole in patients with HIV infection; theophylline, following influenza and influenza vaccination; and recently, dipyrone metabolites in carriers of the hepatitis B virus. In addition increased drug- and drug metabolite-related toxicity has been observed in virally infected cells. Pathogenetic mechanisms for the interaction between drugs and viruses are varied, and include biological mechanisms (often immunological) and changes in drug metabolism. The combined effects of chemical and biological exposure provide a unique model for the study of disease induction.

Effects of liver disease on the disposition of the opioid antagonist nalmefene

OBJECTIVES

The pharmacokinetics of nalmefene and its glucuronide metabolite were investigated in 12 patients with liver disease (four patients with mild, five patients with moderate, and three patients with severe liver disease) and 12 age-, weight-, and gender-matched control subjects.

METHODS

Subjects received a single intravenous bolus 2.0 mg dose of nalmefene. Multiple blood and urine samples were collected for 48 hours. Within 1 week of nalmefene administration, antipyrine and galactose clearances were determined as general markers of hepatic metabolism and effective liver plasma flow, respectively. Plasma concentrations of nalmefene were determined by radioimmunoassay.

RESULTS

The antipyrine and galactose clearance values were 56% and 33% lower, respectively, in the patients with liver disease compared with the normal healthy control subjects. The systemic clearance of nalmefene was reduced by 32% (0.61 +/- 0.21 versus 0.90 +/- 0.27 L/hr/kg [mean +/- SD]) and the terminal elimination half-life was increased by 31% (10.5 +/- 1.9 versus 8.0 +/- 2.2 hours) in the patients with liver disease. This was primarily the result of a 31% reduction (0.181 +/- 0.067 versus 0.263 +/- 0.072 L/hr/kg) in nalmefene glucuronide formation clearance. There were no significant differences in nalmefene volumes of distribution or protein binding. There was a significant inverse relationship between nalmefene clearance and Pugh score (r = -0.57; p = 0.004), indicating decreasing nalmefene clearance with increasing severity of liver disease.

CONCLUSIONS

The clearance of nalmefene was significantly reduced in the presence of liver disease. However, because nalmefene will be primarily used in the acute care setting for reversal of opioid-induced effects, it is not likely that these alterations will necessitate a dosage modification.

Trimethadione as a probe drug to estimate hepatic oxidizing capacity in humans

Trimethadione (TMO) has the properties required of probe drugs for the evaluation of hepatic drug-oxidizing capacity in humans in vivo. TMO is demethylated to dimethadione (DMO), its only metabolite, in the liver after oral administration. Involvement of two cytochrome P450's--CYP2C9 and 3A4--in TMO metabolism has been seen in humans, but involvement of 1A2 is not clearly established. In humans with various types of liver disease and hepatectomy, the serum DMO/TMO ratios, which were measured on blood samples obtained by a single collection 4 hr after oral administration of TMO, correlated well with the degree of hepatic damage. This finding suggests that TMO may be used as a probe drug in the rapid determination of the functional reserve mass of the liver as well as hepatic drug-oxidizing capacity in humans in vivo.

Pharmacokinetics of trapidil in patients with chronic liver disease

The pharmacokinetics of trapidil were studied in 15 patients with chronic liver disease (12 with hepatic cirrhosis, 2 with alcoholic fatty liver, 1 with liver fibrosis). Trapidil was administered intravenously as a 100-mg bolus. Serum samples were analyzed for trapidil by means of high-performance liquid chromatography. Mean pharmacokinetic parameters were compared with those found in a previous study of 12 healthy volunteers. Total plasma clearance was decreased significantly in patients with hepatic cirrhosis (96 mL/ min versus 258 mL/min in healthy individuals and 252 mL/min in patients with noncirrhotic liver disease). No difference in clearance was observed between patients with compensated or decompensated cirrhosis, and portal hypertension did not affect this clearance of trapidil. It can be concluded that trapidil clearance is a parameter that is very sensitive to alterations in hepatic clearance caused by liver cirrhosis, and that the dosage of trapidil should be adjusted accordingly in such patients.

Drug metabolism in liver disease: implications for therapeutic drug monitoring

Over the last decades, a bulk of evidence has accumulated on the effect of liver disease on drug metabolism. It has convincingly been demonstrated that liver disease is associated with a reduced metabolic capacity with respect to drugs undergoing oxidative biotransformation, whereas conjugation reactions, especially glucuronidation, seem less affected. Nevertheless, many data have been conflicting, and it has become increasingly clear that differences in patient selection and severity of disease can account for these. Further, more recent communications suggest that liver disease led to a differential alteration of the cytochrome P-450s with regard to protein content and activity. From a clinical point of view, these findings may have important implications. However, when treating liver patients, we still have no generally accepted model for dose predictions; the best approach should be empiric and based on the clinical response. In selected cases, monitoring of plasma drug concentrations and liver function is recommended.

Changes in the metabolism of three model substrates catalysed by different P450 isozymes when administered as a cocktail to the carbon tetrachloride-intoxicated rat

1. Caffeine (CA) metabolism in animal and man is mainly catalysed by P4501A2. Lidocaine (LID) and phenytoin (PHT) are metabolized by P4503A2 and 2B1/2 in animals and 3A4 and 2C9 in man, respectively. 2. We investigated the possibility of predicting liver damage from changes in blood concentrations after simultaneous i.v. administration (cocktail study) of the three probe drugs CA (10 mg/kg), LID (4 mg/kg) and PHT (4 mg/kg), and their main metabolites, paraxanthine (PX), monoethylglycinexylide (MEGX) and 5-(p-hydroxyphenyl)-5-phenylhydantoin (p-HPPH) respectively. 3. The metabolism of CA, LID and PHT and production of their metabolites (PX, MEGX and p-HPPH) in the carbon tetracholride (CCl4 0.25 and 0.5 ml/kg)-treated rat were reduced in comparison with the control group. 4. The ratios (PX/CA and p-HPPH/PHT) of CA and PHT to the serum levels of the their metabolites 2 h after i.v. administration of three drugs to the CCl4-treated rat were significantly reduced. 5. The correlation coefficients among CLs of CA, LID and PHT, and the PX/CA and p-HPPH/PHT ratios in rat pretreated with different doses of CCL4 are very high. 6. These results suggest that different estimates of hepatic oxidizing capacity, catalysed mainly by P4501A2, 2C and 3A, may be related to the extent of liver disease in the CCl4-intoxicated rat. Therefore a 'cocktail' is not in fact needed to assess hepatic damage in the CCl4-intoxicated rat.

Plasma hydroxy-metronidazole/metronidazole ratio in patients with liver disease and in healthy volunteers

Metronidazole pharmacokinetics were studied in patients with different degrees of liver cirrhosis, classified according to the Child-Pugh algorithm (A, B or C, as liver disease severity increases) and in schistosomic patients. Metronidazole (500 mg) was administered i.v. as a slow infusion over 20 min, and blood samples were collected at set intervals after the end of the infusion. The plasma concentrations of metronidazole and its main metabolite hydroxy-metronidazole were quantified by reversed-phase h.p.l.c. with u.v. detection. The metronidazole and hydroxy-metronidazole areas under the curve from 0 to 24 h (AUC0,24h), the metronidazole terminal elimination half-life (t1/2), the total clearance (CL), the metronidazole volume of distribution (V) values and the hydroxy-metronidazole/metronidazole concentration ratios as a function of time were calculated for each group. Comparison of the metronidazole AUC0,24h, t1/2 and CL values revealed that metronidazole metabolism is progressively impaired as the severity of liver disease increases. There were no variations in these parameters between the schistosomic and Child-Pugh A groups. In addition, there were no differences in the V and hydroxy-metronidazole AUC0,24h among the various groups studied. However, metronidazole metabolism was delayed in patients with hepatic disease, as illustrated by the hydroxy-metronidazole/metronidazole ratio 10 min after the end of metronidazole infusion. These results indicate that the clinical assessment of liver disease is paralleled by an impairment of metronidazole metabolism. Of the studied variables, we propose the hydroxy-metronidazole/metronidazole ratio 10 min after metronidazole infusion as a suitable and practical index for liver function evaluation.

Dipyrone metabolism in liver disease

BACKGROUND AND OBJECTIVES

Dipyrone is an analgesic, antipyretic, and anti-inflammatory drug. After oral administration it is hydrolyzed to 4-methylaminoantipyrine and further metabolized to 4-aminoantipyrine, 4-formylaminoantipyrine, and 4-acetylaminoantipyrine. This study investigated the disposition of dipyrone metabolites in 12 hospitalized patients with cirrhosis (age, 25 to 65 years) and 27 healthy subjects of two age groups (young, 21 to 40 years; elderly, 73 to 90 years).

METHODS

Subjects received 1 gm dipyrone orally, and blood samples were drawn and urine collected over 72 hours. Plasma and urine concentrations of the four metabolites were determined by HPLC.

RESULTS

4-Methylaminoantipyrine terminal elimination half-life (t1/2 beta) in patients with cirrhosis was prolonged compared with young and elderly subjects (mean +/- SEM, 10.6 +/- 0.6 versus 3.1 +/- 0.2 and 4.9 +/- 0.6 hours, p < 0.001), and the nonrenal clearance was reduced compared with the young subjects (1.069 +/- 0.243 versus 2.165 +/- 0.154 ml/min/kg, p < 0.005). 4-Formylaminoantipyrine was undetectable in two patients and in the remaining 10 patients, t1/2 was longer than in the young subjects (26.4 +/- 4.3 versus 10.8 +/- 0.7 hour, p < 0.01), whereas the elderly had intermediate values (18.1 +/- 2.8 hours). Clearance for production of 4-formylaminoantipyrine was reduced in the patients with cirrhosis than in the young and elderly subjects (0.109 +/- 0.024 versus 0.363 +/- 0.031 and 0.340 +/- 0.053 ml/min/kg, p < 0.001). The acetylation phenotype was determined to evaluate the pharmacokinetic parameters of 4-aminoantipyrine and 4-acetylaminoantipyrine. Prolongation of the 4-aminoantipyrine t1/2 and decrease in its clearance for production was found for the patients with cirrhosis, both slow and rapid acetylators, compared with the young and elderly subjects (p < 0.01). 4-Acetylaminoantipyrine t1/2 was also prolonged for patients with cirrhosis, slow and rapid acetylators, compared with the young subjects (p < 0.005). In the slow acetylators, clearance for production of 4-acetylaminoantipyrine did not differ between the patients with cirrhosis and the young subjects (p < 0.5); however, a difference was found for the rapid acetylators (p < 0.001).

CONCLUSION

Our results show that the disposition of 4-methylaminoantipyrine, 4-aminoantipyrine, 4-formylaminoantipyrine, and 4-acetylaminoantipyrine is reduced by chronic liver disease after a single oral dose of dipyrone.

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.

Oxygen supplementation restores theophylline clearance to normal in cirrhotic rats

BACKGROUND/AIMS

Capillarization associated with hepatic fibrosis may present a functional barrier to oxygen diffusion into the hepatocyte, and restriction on cellular oxygen supply may represent the rate-limiting constraint on hepatic oxidative drug metabolism. The aim of this study was to test this hypothesis by examining the effect of oxygen supplementation on plasma theophylline clearance in 10 control and 10 cirrhotic rats.

METHODS

Theophylline (3 mg/kg) was administered intravenously on two separate occasions, 24 hours apart, during which time the rats breathed either room air or oxygen (95%) from 1 hour before dosing until the end of plasma sampling with a randomized order of gas exposure.

RESULTS

Theophylline clearance was significantly reduced by a mean of 37% (n = 10; P = 0.003) in cirrhotic rats compared with controls. Oxygen supplementation significantly improved plasma theophylline clearance in cirrhotic rats by a mean of 40% (n = 10; P = 0.007), whereas clearance remained unchanged in healthy rats. Clearance in oxygen-supplemented cirrhotic rats was not significantly different from that in controls (P > 0.05).

CONCLUSIONS

These novel findings indicate an important role for hepatic oxygenation in improving drug disposition in cirrhosis, which may have potentially important clinical implications for the management of this disease.

The use of in vitro metabolism techniques in the planning and interpretation of drug safety studies

An important issue in toxicology is the suitability of the data obtained with experimental animals for human risk assessment. Because it is not possible to use humans in long-term toxicological studies, the use of animals will continue. However, the data obtained in animal studies can be better extrapolated to the patient by utilizing bridging studies with in vitro models of human drug metabolism. There are 2 basic categories of in vitro methods for the examination of human liver drug metabolism. The first group of in vitro methods consists of the cellular models, which include primary hepatocytes, liver slices, and cell lines. The second group is the use of preparations of the drug-metabolizing enzymes such as tissue homogenates, subcellular fractions, and isolated enzymes. Studies modeling both the human and experimental animal metabolism of a drug are useful in the design of toxicological studies. In vitro studies can identify metabolites, species-specific metabolic routes, and the experimental animal model that best reflects the potential human exposure to the drug and its metabolites. This information can also be useful in the design of the clinical studies by identifying human metabolites, the enzymes responsible for the metabolic clearance of the drug, the effects of genetics and other host factors on the metabolism of the drug, and potential drug-drug interactions. An example of how such information is generated and interpreted is presented.

The role of cytochrome P450 enzymes in hepatic and extrahepatic human drug toxicity

The human cytochrome P450 enzyme system metabolises a wide array of xenobiotics to pharmacologically inactive metabolites, and occasionally, to toxicologically active metabolites. Impairment of cytochrome P450 activity, which may be either genetic or environmental, may lead to toxicity caused by the parent compound itself. In practise, this usually only applies to drugs that have a narrow therapeutic index and when their clearance is critically dependent upon the fraction normally metabolised by that pathway. P450 enzymes may also convert the drug to a chemically reactive metabolite, which, if not detoxified, may lead to various forms of hepatic and extrahepatic toxicity, including cellular necrosis, hypersensitivity, teratogenicity, and carcinogenicity, depending on the site of formation and the relative stability of the metabolite, and the cellular macromolecule with which it reacts. Variation in the regulation and expression of the drug metabolising enzymes may play a key role in both interindividual variation in sensitivity to drug toxicity and tissue-specific damage. Avoidance of toxicity may be possible in rare instances by prediction of individual susceptibility or by designing new chemical entities that are metabolised by a range of enzymes (both cytochromes P450 and others) and do not undergo bioactivation.

Pharmacokinetics of famciclovir in subjects with chronic hepatic disease

The pharmacokinetic profile of penciclovir was determined after a single 500-mg dose of its oral precursor, famciclovir, in 9 healthy volunteers and in 14 patients with chronic hepatic disease. Plasma and urine samples were analyzed for concentrations of penciclovir and 6-deoxy-penciclovir using a reverse-phase high-performance liquid chromatography (HPLC) method. Famciclovir was not quantifiable in patients with hepatic disease, and 6-deoxy-penciclovir was quantifiable in only a limited number of specimens. The extent of systemic availability of penciclovir, as measured by AUC0-infinity, was similar in patients with hepatic disease and in healthy subjects. In contrast, Cmax was significantly lower (average decrease of 43%) in subjects with hepatic disease relative to healthy normal subjects. Median Tmax for subjects with hepatic disease was significantly increased (by 0.75 hours) compared with subjects with normal liver function. These data suggest a decrease in the rate, but not the extent, of systemic availability of penciclovir in patients with hepatic disease. It should be unnecessary to modify the dose of famciclovir for subjects with compensated hepatic disease and normal renal function.

Effect of renal or hepatic dysfunction on neurotoxic convulsion induced by ranitidine in mice

We investigated the effect of acute renal and hepatic dysfunction on the neurotoxicity of ranitidine, a histamine H2 receptor antagonist. Experimental acute hepatic and renal dysfunction in mice were produced by i.p. injection of uranyl nitrate (UN) and carbon tetrachloride (CT), respectively. Ranitidine was then constantly infused into the tail vein until the onset of clonic convulsion. When compared to control mice, UN treated mice had a significantly shorter onset time to clonic convulsion, lower total dose and higher plasma concentration at initiation of clonic convulsion. In contrast, the convulsive threshold concentration in the brain of UN treated mice was not significantly different from that of control mice. In CT treated mice, all pharmacokinetic and pharmacodynamic data described above were not significantly different from those of the control mice. No significant difference in the brain/plasma concentration ratio was observed between both disease models and the corresponding control mice. Finally, the effect of UN and CT treatment on the convulsive potency after intracerebral (i.c.) administration of ranitidine was investigated in mice. Potentiation of the intrinsic neurotoxic sensitivity to ranitidine could not be demonstrated for mice with renal or hepatic dysfunction. From these findings, we conclude that renal dysfunction is a risk factor for ranitidine neurotoxicity, and this increased risk results from increase in the drug concentration in plasma and brain as a result of impaired renal excretion. No apparent effect of acute hepatic dysfunction was observed on both the pharmacokinetic and pharmacodynamic behavior of the drug.

Assessment of liver metabolic function. Clinical implications

Inter- and intraindividual variability in pharmacokinetics of most drugs is largely determined by variable liver function as described by parameters of hepatic blood flow and metabolic capacity. These parameters may be altered as a result of disease affecting the liver, genetic differences in metabolising enzymes, and various types of drug interactions, including enzyme induction, enzyme inhibition or down-regulation. With the now known large number of drug metabolising enzymes, their differential substrate specificity, and their differential induction or inhibition, each test substance of liver function should be used as a probe for its specific metabolising enzyme. Thus, the concept of model test-substances providing general information about liver function has severe limitations. To test the metabolic activity of several enzymes, either several test substances may be given (cocktail approach) or several metabolites of a single test substance may be analysed (metabolic fingerprint approach). The enzyme-specific analysis of liver function results in a preference for analysis of the metabolites rather than analysis of the clearance of the parent test substance. There are specific methods to quantify the activity of cytochrome P450 enzymes such as CYP1A2, CYP2C9, CYP2C19MEPH, CYP2D6, CYP2E1, and CYP3A, and phase II enzymes, such as glutathione S-transferases, glucuronyl-transferases or N-acetyltransferases, in vivo. Interactions based on competitive or noncompetitive inhibition should be analysed specifically for the cytochrome P450 enzyme involved. At least 5 different types of cytochrome P450 enzyme induction may result in major variability of hepatic function; this may be quantified by biochemical parameters, clearance methods, or highly enzyme-specific methods such as Western blot analysis or molecular biological techniques such as mRNA quantification in blood and tissues. Therapeutic drug monitoring is already implicitly used for quantification of the enzyme activities relevant for a specific drug. Selective impairment of hepatic enzymes due to gene mutations may have an effect on the pharmacokinetics of certain drugs similar to that caused by cirrhosis. Assessment of this heritable source of variability in liver function is possible by in vivo or ex vivo enzymological methods. For genetically polymorphic enzymes and carrier proteins involved in drug disposition, molecular genetic methods using a patient's blood sample may be used for classification of the individual into: (i) the impaired or poor metaboliser (homozygous deficient); (ii) the extensive (homozygous active) metaboliser group; and (iii) the moderately extensive metaboliser (heterozygous) group. For hepatic blood flow determinations, galactose or sorbitol given at relatively low doses may be much better indicators than the indocyanine green.(ABSTRACT TRUNCATED AT 400 WORDS)

Individual variation in first-pass metabolism

Individual variation in pharmacokinetics has long been recognised. This variability is extremely pronounced in drugs that undergo extensive first-pass metabolism. Drug concentrations obtained from individuals given the same dose could range several-fold, even in young healthy volunteers. In addition to the liver, which is the major organ for drug and xenobiotic metabolism, the gut and the lung can contribute significantly to variability in first-pass metabolism. Unfortunately, the contributions of the latter 2 organs are difficult to quantify because conventional in vivo methods for quantifying first-pass metabolism are not sufficiently specific. Drugs that are mainly eliminated by phase II metabolism (e.g. estrogens and progestogens, morphine, etc.) undergo significant first-pass gut metabolism. This is because the gut is rich in conjugating enzymes. The role of the lung in first-pass metabolism is not clear, although it is quite avid in binding basic drugs such as lidocaine (lignocaine), propranolol, etc. Factors such as age, gender, disease states, enzyme induction and inhibition, genetic polymorphism and food effects have been implicated in causing variability in pharmacokinetics of drugs that undergo extensive first-pass metabolism. Of various factors considered, age and gender make the least evident contributions, whereas genetic polymorphism, enzymatic changes due to induction or inhibition, and the effects of food are major contributors to the variability in first-pass metabolism. These factors can easily cause several-fold variations. Polymorphic disposition of imipramine and propafenone, an increase in verapamil first-pass metabolism by rifampicin (rifampin), and the effects of food on propranolol, metoprolol and propafenone, are typical examples. Unfortunately, the contributions of these factors towards variability are unpredictable and tend to be drug-dependent. A change in steady-state clearance of a drug can sometimes be exacerbated when first-pass metabolism and systemic clearance of a drug are simultaneously altered. Therefore, an understanding of the source of variability is the key to the optimisation of therapy.

Differential expression of drug metabolizing enzymes in primary and secondary liver neoplasm: immunohistochemical characterization of cytochrome P4503A and glutathione-S-transferase

The question whether expression of drug metabolizing enzymes in human liver is altered by liver neoplasm remains controversial; however, the ability or unability of tumour cells to metabolize certain drugs may be important for developing therapeutic strategies. We therefore investigated the abundance and localization of two classes of drug metabolizing enzymes [cytochrome P4503A (CYP3A) and pi-type glutathione-S-transferase] by means of immunohistochemistry (standard ABC technique) in patients with hepatocellular carcinoma (HCC, n = 16) and with liver metastasis from adenocarcinoma (n = 53) in comparison to normal controls (n = 5). The distribution of CYP3A in normal liver samples showed a characteristic pattern of four to five layers of stained hepatocytes surrounding the central vein. Eleven out of 16 cases of HCC showed expression of CYP3A; staining was less intense than in normal liver and zonation was completely lost. In contrast, only 5 out of 53 samples of metastasis stained positively for CYP3A. The difference between primary and secondary neoplasm was statistically significant (chi-square, P < 0.0001). Pi-type glutathione-S-transferase (GST) stained positively in 9 out of 16 HCC and in 48 out of 53 cases of liver metastasis (chi-square, P < 0.01) indicating a higher percentage of immunostaining in liver metastasis. In summary, we observed differences in the abundance and distribution pattern of CYP3A and GST between primary and secondary neoplasma of human liver and in comparison to normal controls. In combination with established methods these data may contribute to the establishment of reliable test systems for distinguishing primary from secondary liver tumours.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of acute experimental liver dysfunction on the pharmacodynamics of heptabarbital in rats

The present study aimed to investigate whether the pharmacodynamics of a lipophilic barbiturate, heptabarbital, is altered by liver dysfunction. Heptabarbital was slowly infused i.v., to rats with liver necrosis induced by carbon-tetrachloride, until the rats lost their righting reflex. The concentrations of the drug in serum, brain and CSF were determined in both the diseased animals and solvent-treated controls. Although the concentrations of heptabarbital in the CSF, serum and brain were not significantly different between controls and diseased rats, the total heptabarbital dose required to induce sleep was markedly lower in the diseased animals. Accordingly, the pharmacodynamics of heptabarbital is unaffected in experimental liver dysfunction in rats.

Both phenolic and acyl glucuronidation pathways of diflunisal are impaired in liver cirrhosis

The pharmacokinetics of diflunisal, a salicylate derivative that undergoes phenolic and acyl glucuronidation as well as sulphate conjugation, has been studied after a single oral dose (250 mg) in patients with cirrhosis (n = 5) and in healthy controls (n = 5). The plasma clearance of total (bound + unbound) diflunisal was 10.2 ml.min-1 in the control subjects and it was not affected by cirrhosis (10.9 ml.min-1). The plasma protein binding of diflunisal was significantly reduced in cirrhosis; the percentage of unbound diflunisal in plasma was 0.089 in the controls and 0.147 in the patients with cirrhosis. Plasma clearance of unbound diflunisal was significantly impaired in cirrhosis: 11.5 l.min-1 in control subjects vs 7.41.min-1 in cirrhotics. In cirrhotic patients, the unbound partial clearances to the phenolic and acyl glucuronides were both significantly reduced, by approximately 38%. The unbound partial clearance to the sulphate conjugate was not significantly affected by cirrhosis. The results show that both the phenolic and acyl glucuronidation pathways of diflunisal are equally susceptible to the effects of liver cirrhosis.

Malaria infection impairs glucuronidation and biliary excretion by the isolated perfused rat liver

1. The effect of the erythrocyte stage of malaria infection on hepatic glucuronidation, biliary excretion and oxidation processes was investigated using harmol, salbutamol, taurocholate and propranolol. Livers from rats infected with the rodent malaria parasite P. berghei were isolated and perfused in a single-pass (harmol, taurocholate, propranolol) or recirculating (harmol, salbutamol) design. The degree of erythrocytic parasitaemia ranged from 16% to 63%. 2. The hepatic clearance (Cl) of harmol decreased from 7.8 +/- 0.4 ml/min in controls to 5.7 +/- 1.1 ml/min in the malaria-infected group in single-pass studies. This corresponded to a 40-60% reduction in hepatic intrinsic clearance (Clint). Similar changes were observed using the recirculating design when glucuronidation accounted for greater than 90% of harmol metabolism. 3. The Cl of salbutamol, metabolized exclusively by glucuronidation under the conditions used, also decreased significantly from 8.5 +/- 0.8 in controls to 6.6 +/- 1.4 ml/min in the malaria-infected group. This corresponded to a 40-70% reduction in Clint. 4. The Cl of taurocholate, excreted unchanged in bile, decreased slightly but significantly from 9.6 +/- 0.3 ml/min in controls to 8.3 +/- 0.9 ml/min in the malaria-infected group. In the same livers, there was also a slight but significant decrease in propranolol Cl (10.0 +/- 0.1 ml/min and 9.9 +/- 0.1 ml/min, respectively). Both these compounds undergo flow-limited hepatic clearance; the decreases in Clint of taurocholate and propranolol were 87% and 35%, respectively. 5. Cl and Clint of each of the compounds studied were found to correlate significantly with the degree of erythrocytic parasitaemia. This study shows that glucuronidation, biliary excretion and oxidation by liver are impaired in malaria infection in rats, with biliary excretion being the most affected. The data indicate that there is a general decrease in hepatic elimination processes during the erythrocytic phase of malaria infection.