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

UDP glucuronosyltransferase mRNA levels in human liver disease

The UDP glucuronosyltransferases (UGT) are a family of enzymes in which substrates include drugs, xenobiotics, and products of endogenous catabolism. The main source of most UGT enzymes is the liver, a major organ in the detoxification and inactivation of compounds. Previous studies have indicated that glucuronidation, as measured by pharmacokinetic studies, is relatively spared in liver disease. Because UGT activity toward most substrates is the result of metabolism by different isoforms with overlapping specificities, these studies may not indicate the effect of disease on the levels of individual isoforms. We sought to extend these studies to the measurement of mRNA for individual isoforms in the liver of patients with various forms of liver disease. RNA was extracted from liver tissue samples of patients undergoing clinically necessary percutaneous liver biopsies. UGT mRNA levels for isoforms 1A1, 1A3, 1A4, 1A6, 1A9, 2B4, 2B7, 2B10, 2B11, 2B15, and 2B17 were determined by real-time reverse transcription-polymerase chain reaction. Biopsies were graded using the Metavir system. Results from patients with low fibrosis or inflammatory scores were compared with those with high scores. We found large interindividual variation in the levels of the various isoforms. This was greatest for UGT2B17. A consistent downward trend, reaching statistical significance for UGT1A4, UGT2B4, and UGT2B7, was observed in samples from patients with high inflammation scores. There was no such correlation with the degree of fibrosis. Our results indicate that hepatic UGT mRNA levels are reduced while the tissue is inflamed, but they are not affected in the noninflamed, chronically diseased liver.

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.

Variable cytochrome P450 2D6 expression and metabolism of codeine and other opioid prodrugs: implications for the Australian anaesthetist

Codeine is a popular opioid prodrug dependent on the activity of the specific cytochrome P450 enzyme 2D6 (CYP2D6). This enzyme catalyses the production of the potent analgesic metabolite morphine, but genetic studies have demonstrated that individuals from different ethnic groups exhibit considerable variability in the functional capacities of their expressed CYP2D6 enzymes, and pharmacological studies have shown many commonly prescribed drugs can reduce the action of CYP2D6 enzymes. These findings have significant clinical implications for the rational prescription of effective analgesia, especially in a multicultural country like Australia.

Single and multiple dose pharmacokinetics of nelfinavir and CYP2C19 activity in human immunodeficiency virus-infected patients with chronic liver disease

AIMS

To evaluate the single-dose and multiple-dose pharmacokinetics of nelfinavir and its active M8 metabolite in eight HIV-seropositive patients with liver disease, and to examine the relationship between CYP2C19 activity (genotype and plasma M8/nelfinavir metabolic ratio) and the severity of liver disease in these patients.

METHODS

Nelfinavir was given as a single dose (500 or 750 mg) to patients beginning therapy and twice (500, 750 or 1000 mg) or three times (250 or 750 mg) daily during chronic therapy. Single-dose pharmacokinetic values were used to predict multiple-dose regimens. Peak and total plasma exposures between 2-4 microg ml-1 and 45-75 microg ml-1 h, respectively, and predose levels > 0.7 microg ml-1 were targeted for multidose nelfinavir. Genotype was determined by analysis for CYP2C19*1, CYP2C19*2, and CYP2C19*3. Individuals were grouped according to their genotype, molar M8/nelfinavir AUC ratio (low: < 0.1, intermediate: 0.1-0.3, high > 0.3), and Child-Pugh classification for severity of liver disease.

RESULTS

Nelfinavir pharmacokinetics were characterized by wide interindividual variability, low clearance (181-496 ml min-1 70 kg-1, n = 7), and prolonged half-life (5-20 h, n = 7). M8/nelfinavir AUC ratio increased 58% (n = 4) and alpha 1-acid glycoprotein levels decreased up to 39% (n = 5) from single to multiple dosing. CYP2C19 activity was low (metabolic AUC ratio < 0.1) in four patients with moderate to severe liver disease even though they were genetically extensive CYP2C19 metabolizers (*1/*1 or *1/*2). Three patients required lower daily doses than the standard regimen of 750 mg every 8 h to achieve target concentrations and maintain virologic suppression at < 50 RNA copies ml-1 (up to 20 months).

CONCLUSIONS

Acquired CYP2C19 deficiency from moderate or severe liver disease resulted in decreased M8 formation. Long-term HIV suppression is possible using low nelfinavir doses in patients with liver disease.

Single-dose pharmacokinetics of repaglinide in subjects with chronic liver disease

Repaglinide is a novel insulin secretagogue developed in response to the need for a fast-acting, oral prandial glucose regulator for the treatment of type 2 (non-insulin-dependent) diabetes mellitus. Repaglinide is metabolized mainly in the liver; its pharmacokinetics may therefore be altered by hepatic dysfunction. This open, parallel-group study compared the pharmacokinetics and tolerability of a single 4 mg dose of repaglinide in healthy subjects (n = 12) and patients with chronic liver disease (CLD) (n = 12). Values for AUC and Cmax were significantly higher in CLD patients compared with healthy subjects, and the MRT was prolonged in CLD patients. Values for tmax did not differ between the groups, but t1/2 was significantly prolonged in CLD patients compared with previously determined values in healthy subjects. AUC was inversely correlated with caffeine clearance in CLD patients but not in healthy subjects. Blood glucose profiles were similar in both groups. Adverse events (principally hypoglycemia) were similar in the two groups; none was serious. Repaglinide clearance is significantly reduced in patients with hepatic impairment; the agent should be used with caution in this group.

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.

Pharmacokinetics of opioids in liver disease

The liver is the major site of biotransformation for most opioids. Thus, the disposition of these drugs may be affected in patients with liver insufficiency. The major metabolic pathway for most opioids is oxidation. The exceptions are morphine and buprenorphine, which primarily undergo glucuronidation, and remifentanil, which is cleared by ester hydrolysis. Oxidation of opioids is reduced in patients with hepatic cirrhosis, resulting in decreased drug clearance [for pethidine (meperidine), dextropropoxyphene, pentazocine, tramadol and alfentanil] and/or increased oral bioavailability caused by a reduced first-pass metabolism (for pethidine, dextropropoxyphene, pentazocine and dihydrocodeine). Although glucuronidation is thought to be less affected in liver cirrhosis, and clearance of morphine was found to be decreased and oral bioavailability increased. The consequence of reduced drug metabolism is the risk of accumulation in the body, especially with repeated administration. Lower doses or longer administration intervals should be used to remedy this risk. Special risks are known for pethidine, with the potential for the accumulation of norpethidine, a metabolite that can cause seizures, and for dextropropoxyphene, for which several cases of hepatotoxicity have been reported. On the other hand, the analgesic activity of codeine and tilidine depends on transformation into the active metabolites, morphine and nortilidine, respectively. If metabolism is decreased in patients with chronic liver disease, the analgesic action of these drugs may be compromised. Finally, the disposition of a few opioids, such as fentanyl, sufentanil and remifentanil, appears to be unaffected in liver disease.

Effects of Plasmodium berghei infection on cytochromes P-450 2E1 and 3A2

Metabolism and disposition of most drugs used to treat malaria are substantially altered in malaria infection. Few data are available that specify effects of malaria infection on drug metabolism pathways in humans or animal model systems. In this report, studies were undertaken to determine the effect of Plasmodium berghei infection on cytochrome P-450 (CYP450) 2E1 and 3A2-mediated metabolism and enzyme expression in rat liver microsomes. Malaria infection (MAL) resulted in significant decreases in total cytochrome P-450 content (56%, P < 0.05) and NADPH cytochrome P-450 reductase activity (32%, P < 0.05) as compared to control (CON) rats. Chlorzoxazone 4-hydroxylase activity (CYP2E1-mediated) showed no significant difference between CON and MAL microsomes while testosterone 6-beta-hydroxylase activity (CYP3A2-mediated) was reduced by 41% (P < 0.05) in MAL. Enzyme kinetic studies and immunoblot analysis indicate that the loss of activity for CYP3A2 in malaria infection is due to significantly decreased CYP3A2 protein expression. The altered expression of CYP450s in malaria infection should be taken into account when treating patients with malaria in order to minimize drug-drug interactions or toxicity.