Measurement of hepatic drug-metabolizing enzyme activity in man

Summary Three parameters of hepatic drug metabolism, cytochrome P-450 (P-450) content, antipyrine metabolism and urinary excretion of glucaric acid (GA) were investigated in 161 patients who underwent diagnostic liver needle biopsy. P-450 and antipyrine metabolism, but not GA were related to histological changes in liver. All the parameters were significantly increased in subjects treated with enzyme-inducing drugs, the extent of induction being related to alterations in liver histology. The largest responses were seen in subjects with an intact liver and the smallest in those with hepatitis or cirrhosis. Therapy with inducers partly compensated for the impairement in drug metabolism caused by the disease process; thus, some patients with altered liver had normal values in the tests if they had been treated with inducers. There were significant correlations between in vivo and in vitro parameters of drug metabolism, but in the interpretation of data selection of the material and the parameters influenced the results. Thus, the antipyrine plasma clearance rate was directly related to P-450 and GA values, whereas the correlation between the latter and the drug half-life was non-linear. Also, comparison of selected groups of patients resulted in better correlations between the indices of drug metabolism than in the entire series. The results demonstrate that the overall picture of hepatic drug metabolism in man is largely determined by histological changes in the liver and by exposure to drugs, which are reflected differently in various assays of hepatic drug metabolism. Quantitative evaluation of these factors, and selection of the appropriate assay method, seem to be of importance in investigating hepatic drug metabolism in man.     

Serum low-density lipoprotein and high-density lipoprotein cholesterol,and liver size in subjects on drugs inducing hepatic microsomal enzymes

Summary Serum low-density lipoprotein cholesterol and high-density lipoprotein cholesterol concentration and the ratio between them, major risk factors of coronary heart disease, and liver size were investigated in 18 subjects who were on enzyme-inducing anti-convulsants, phenytoin alone or in combination with phenobarbital and/or carbamazepine. The subjects with a high liver cytochrome P-450, indicating hepatic microsomal enzyme induction, who showed an increase in liver size, had an elevated high-density lipoprotein concentration and high-density lipoprotein cholesterol/total cholesterol ratio, and a reduced low/high-density lipoprotein cholesterol ratio. The high-density lipoprotein cholesterol concentration and its ratio to total cholesterol were directly and related to the ratio between low and high-density lipoprotein cholesterol were inversely related to the extent of liver enlargement. The serum cholesterol distribution profile associated with an increase in liver size was typical of subjects with a low risk of coronary heart disease. The results suggest that enzyme-inducers, such as phenytoin and phenobarbital, induce structural and functional changes in hepatocellular membranes associated with liver enlargement and cholesterol distribution characteristic of low susceptibility to atherosclerotic vascular disease.     

One year study of effects of an oestrogen-dominant oral contraceptive on serum high-density lipoprotein cholesterol,apolipoproteins A-I and A-II and hepatic microsomal function

Summary The effects over 1 year of an oestrogendominant oral contraceptive containing ethinylestradiol and levonorgestrel on serum high-density lipoprotein cholesterol, apolipoproteins A-I and A-II and liver microsomal enzyme activity assessed by antipyrine kinetics, were investigated in 21 healthy, young women. HDL cholesterol and apolipoprotein concentrations rose and hepatic microsomal enzyme activity fell during the first month of the treatment, and remained affected throughout the year. After discontinuation of treatment, the lipid and apolipoprotein concentrations and the microsomal enzyme activity returned to their pretreatment levels within 1 month. The drug, by reducing hepatic enzyme activity, may have influenced both the antipyrine elimination rate and the high-density lipoprotein concentration.     

Changes in circulating thyroid hormones during short-term hepatic enzyme induction with carbamazepine

Summary The effect of short-term hepatic enzyme induction with carbamazepine (CBZ) on circulating thyroid hormone concentrations was studied in 10 healthy male subjects. CBZ 400 mg per day was given for 21 days in 6 subjects and for 14 days in a further 4. In the former group the effect of therapy on the pituitary/thyroid axis was also assessed by measuring thyroid stimulating hormone (TSH) response to thyrotrophin-releasing hormone. CBZ therapy resulted in induction of hepatic monooxygenase activity, evidenced by a fall in antipyrine half-life (11.1±0.7 to 7.6±0.7 h; p<0.001), and a rise in antipyrine clearance (0.72±0.06 to 0.98±0.1 ml min^–1 kg^–1; p<0.001). A significant fall in total serum thyroxine (T4) (81.9±2.9 to 75.1±2.9 nmol l^–1), and triiodothyronine (T3); (1.59±0.07 to 1.37±0.05 nmol l^–1) and free T4 (16.03±0.82 to 14.2±0.8 pmol l^–1) was seen after CBZ therapy. (all p<0.05). No significant change in reverse T3 or thyroid binding globulin occurred. In the 6 subjects studied for 21 days, maximal changes were found following 14 days' treatment. Basal and stimulated TSH remained unaltered. These effects on circulating thyroid hormone concentrations are likely to be secondary to hepatic enzyme induction leading to accelerated nondeiodinative hepatic hormone disposal. The reason for the failure of pituitary TSH secretion to rise in response to the fall in circulating T4 and T3 is unclear but may have implications for chronic treatment with CBZ in epileptic patients.     

Effects of enzyme induction and inhibition on microsomal oxidation of 1,4-benzodiazepines

Summary Metabolic oxidative profiles of diazepam (I) were obtained by aromatic C-4-hydroxylation, N-1-demethylation, and 3-hydroxylation using a supernatant of rat liver. Incubations of 3-methyldiazepam (VI), which suppressed 3-hydroxylation, and N-1-nor-3-methyldiazepam (VII), were used to separately investigate these three oxidative pathways. Treatment of animals with phenobarbital enhanced N-1-demethylation and 3-hydroxylation, and to a variable extent C-4-hydroxylation. Application of metyrapone reduced metabolite formation by 3-hydroxylation and N-1-demethylation, but had no effect on C-4-hydroxylation. Metyrapone inhibition was more pronounced following than prior to phenobarbital treatment. C-2-hydroxylation was studied using medazepam (XX) incubations. This pathway was increased by phenobarbital pretreatment and reduced by metyrapone inhibition which was again more pronounced following than prior to phenobarbital pretreatment.These results support earlier conclusions on the heterogeneity of liver microsomes and suggest the presence of different species of hepatic microsomal terminal oxidases. Phenobarbital treatment and metyrapone change the metabolic profile via induction and inhibition, respectively, and, thus, in the case of 1,4-benzodiazepines, the formation of metabolites with varying pharmacological activity. This could become important in clinical situations as a diagnostic mean to determine induction under various treatment or, possibly, during cumulation of metabolites with a long half-life.     

Induction of hepatic microsomal drug-metabolizing enzymes by inhibitors of 5-lipoxygenase (5-LO): studies in rats and 5-LO knockout mice.

The effect of 5-lipoxygenase (5-LO) inhibitors on the hepatic microsomal mixed-function oxidase (MFO) system of rodents was investigated. After establishing the relative in vitro and in vivo potencies of the 3 test compounds, male Crl:CD (SD) BR rats received CJ-11,802 (0, 10, 50, or 200 mg/kg/day), zileuton (0, 10, 60, or 300 mg/kg/day) or ZD2138 (0 or 200 mg/kg/day) once daily by oral gavage for 14 (zileuton and ZD2138) or 30 (CJ-11,802) consecutive days. Controls were given an equivalent volume of 0.5% methylcellulose vehicle. At necropsy, all livers were weighed, and sections from representative animals (control and highest dose for each compound) were utilized to prepare hepatic microsomal fractions, which were assayed for cytochrome P-450 (CYP) content and the activities of cytochrome c reductase (CRed), para-nitroanisole O-demethylase (p-NOD), ethoxyresorufin O-deethylase (EROD), and pentoxyresorufin O-dealkylase (PROD). A dose-related increase in liver weight occurred in rats given CJ-11,802 and zileuton, while animals administered ZD2138 were unaffected. Rats given CJ-11,802 (200 mg/kg/day) and zileuton (300 mg/kg/day) had increases in CYP, EROD, PROD, CRed and p-NOD compared to corresponding controls, while only the latter two activities were elevated in animals administered ZD2138. To determine if induction of the hepatic microsomal MFO system was related to 5-LO inhibition, male DBA wild-type and 5-LO knockout mice were administered either CJ-11,802 (200 mg/kg/day) or vehicle by oral gavage for 14 consecutive days. At necropsy, liver weight, CYP content, and CRed activity were measured and all were increased similarly in the treated wild-type and knockout mice compared to corresponding controls, indicating that induction was not related to inhibiting 5-LO.     

Dose dependent enzyme induction by oxcarbazepine?

Summary Antipyrine half life and clearance was compared in four patients with classical idiopathic trigeminal neuralgia during carbamazepine (CBZ) or CBZ/phenytoin (PHT) and after substitution with oxcarbazepine (OXC) monotherapy.OXC is observed to be less of a hepatic enzyme inducer than CBZ or CBZ/PHT in combination, however induction by OXC may be dose related.     

Induction of drug metabolizing enzyme system in the liver

Summary 1. Accelerated drug metabolism in liver cells was discovered during studies of the cause of barbiturate tolerance and of the way in which polycyclic hydrocarbons protect against the actions of several carcinogenic compounds. — 2. It is elicited by many lipid soluble drugs and is accompanied by increased synthesis of haem and enzyme proteins. It is due to increased activity of the drug metabolizing microsomal enzyme-complexes in the liver which consist of non specific mixed function oxidases and various transferases. — 3. Acceleration of drug metabolism is accompanied by hypertrophy of smooth intracellular membranes and enlargement of the liver. It is dependent on the concentration of the inducer in the endoplasmic reticulum and seems to be influenced by the stability of a complex formed between the drug and cytochrome P-450, as well as by certain, as yet unidentified properties possessed by certain types of inducing agents. — 4. In man it has been observed in patients during and after treatment with a few drugs which have a high inducing capacity and are prescribed in large doses. — 5. Accelerated drug metabolism might be highly advantageous and beneficial in protecting an organism against an overload of endogenous or exogenous toxic compounds, but it can also be harmful if it results in increased production of toxic metabolites. — 6. It can be regarded as a hitherto unknown pharmacological effect on the regulation of the synthesis-rate of enzymes involved in drug metabolism.Presented as Feldberg Foundation Lecture 1972 in St. Mary's Hospital Medical School on March 8th, 1972     

大鼠肝睾酮羟化酶体外诱导模型的建立

目的建立大鼠原代培养肝细胞微粒体睾酮羟化酶系列同工酶(CYP2A1,CYP2B1/2,CYP2C11,CYP3A1/2)的体外诱导模型。方法应用胶原酶原位灌流法分离大鼠肝细胞进行原代培养;用苯巴比妥钠(PB,1mmol·L-1)、地塞米松(Dex,10μmol·L-1)和β-萘酚黄酮(β-NF,50μmol·L-1)诱导培养肝细胞72 h,提取肝细胞微粒体,进行其肝CYP总量、肝微粒体蛋白含量和睾酮羟化酶比活性的测定。另外采用体内诱导方法,SD大鼠分别给予PB 80mg·kg-1,Dex50mg·kg-1和β-NF80mg·kg-1,ip,每天1次,连续5d,停药24h后制备肝微粒体进行上述指标的测定。结果在体外和体内实验中,PB,Dex和β-NF对肝微粒体蛋白含量、CYP总量和睾酮羟化酶同工酶活性均具有较高的诱导效应。PB对肝CYP总量在体外的诱导效应高于体内,对睾酮不同位置羟化作用的诱导效应体内外无显著性差异;Dex和β-NF对肝CYP总量及睾酮不同位置羟化作用的诱导效应体内外无显著性差异。结论大鼠肝睾酮羟化酶体外诱导模型可替代体内实验,用于药物代谢、新药安全性评价及其他外源性化合物代谢和毒性研究。     

Differential induction of antipyrine metabolism by rifampicin

Summary Antipyrine is oxidised to three main metabolites in man. There is evidence that the different metabolites are products of different forms of cytochrome P-450. The effect of rifampicin administration for two weeks on the rates of formation of these metabolites was investigated in healthy volunteers. Rifampicin increased antipyrine clearance and shortened its half-life. Two weeks after stopping rifampicin the induction had largely been reversed. Clearance to all three metabolites was increased by rifampicin. Clearance to 3-hydroxymethylantipyrine was increased from 7.8±0.9 ml/min to 13.3±1.3 ml/min, to norphenazone from 5.8±0.6 ml/min to 19.3±2.1 ml/min and to 4-hydroxyantipyrine from 14.3±2.2 ml/min to 21.9±3.9 ml/min. Thus clearance to norphenazone was increased to a much greater extent than to either of the other two metabolites. It is concluded that this provides evidence for the involvement of at least two different forms of cytochrome P-450 in antipyrine metabolism in man.     

High-density lipoprotein subfractions,apolipoproteins and antipyrine clearance in normal subjects

Summary Serum high density lipoprotein (HDL) subfractions HDL₂ and HDL₃, apolipoproteins, and plasma antipyrine clearance (AP-CL) rate, an index of liver microsomal enzyme activity, were determined in 21 healthy subjects. High HDL cholesterol and HDL₂ cholesterol concentrations and HDL cholesterol/cholesterol and HDL₂/HDL₃ cholesterol ratios were associated with high AP-CL. Phenobarbital enhanced antipyrine elimination and increased the apolipoprotein A-I/A-II ratio. Subjects who had high AP-CL had a more antiatherogenic HDL subfraction and apolipoprotein profile than those with low AP-CL.     

A simple method to evaluate liver microsomal enzyme activity for phenol with a high-performance liquid chromatograph

A method to evaluate liver microsomal cytochrome P-450-related enzyme activity for phenol is described. Phenol of a known quantity q was incubated with enzyme and cofactor for a period of time t. At the end of the incubation, phenol remaining unchanged in the reaction mixture was analyzed with a high-performance liquid chromatograph (HPLC). No direct determination of phenol concentration was necessary: only the ratio of chromatographic peak area a1 obtained from a sample vessel (cofactor solution contained NADP) to the area a from a reference vessel (cofactor solution devoid of NADP) was needed to calculate the rate of enzymatic reaction V, i.e., V = q(1 - a'/a)/t. The time-activity and enzyme-activity studies proved the method to be useful for the evaluation of enzyme activity. This method is expected to have wide application in the field of drug metabolism study.     

Relationship between lipid composition and drug metabolizing capacity of human liver

Summary The relationship between hepatic glycerolipids and microsomal drug-metabolizing enzymes was studied in liver biopsies from 41 subjects. The series included obese, diabetic, epileptic and chronic alcoholic patients, all of whom were hospitalized for suspected hepatic ailments (fatty liver, hepatitis or cirrhosis). Therapy with enzyme-inducing anticonvulsants was associated with high phospholipid and cytochrome P-450 and low triacylglycerol concentration in the liver. In patients with fatty liver or cirrhosis low phospholipid and cytochrome P-450 and high triacylglycerol concentrations were observed. There was a significant correlation (r (Pearsons's product moment correlation coefficient) =0.91) between the hepatic phospholipid and cytochrome P-450 concentration. The cytochrome P-450 concentration was inversely related (r=–0.74) to the triacylglycerol concentration. The positive correlation between hepatic phospholipids and drug-metabolizing enzymes could be interpreted as indicating that in human liver phospholipid and cytochrome P-450 synthesis share common regulators, or that phospholipids are necessary for the maximum rate of cytochrome P-450 synthesis.     

Effect of ampicillin on hepatic microsomal mixed-function oxidase system in male mice

The effect of ampicillin [(D)-alpha-aminobenzyl penicillin] administration on the hepatic mixed-function oxidase (MFO) system was studied in male mice. Ampicillin (100 mg/kg, i.p., 3 days) decreased the levels of cytochrome P-450, aminopyrine N-demethylase, acetanilide hydroxylase and cytochrome c-reductase activity significantly. In carbon tetrachloride (CCl4)-pretreated mice, ampicillin increased acetanilide hydroxylation compared with CCl4 treatment alone; however, all other parameters of the MFO system remained unchanged. Ampicillin exhibited type II binding with microsomes (trough at 388 nm, peak at 430 nm). Thus ampicillin acts as an inhibitor of the MFO system.     

细胞色素氧化酶P450家族在中药毒性研究中的应用进展

细胞色素氧化酶P450(CYP)是最重要的药物代谢酶,多种中药能抑制或诱导CYP3A, CYP2C等亚型活性,而影响中药代谢,其中抑制作用是药物不良反应的主要原因;CYP1A2和CYP2E1等参与了中药前毒物的活化引起毒性反应。CYP参与中药代谢性相互作用,故可通过对CYP抑制或诱导作用的分析,研究中药配伍减毒作用或配伍禁忌的毒性(增毒)作用;应重视研究中西药合用引起不良反应的CYP作用。本文综述了CYP在中药毒性研究中的应用概况,进一步展望CYP的应用前景,以期对中药毒性研究提供新思路。     

Hepatic blood flow and drug metabolism in patients on enzyme-inducing anticonvulsants

Summary Liver blood flow and indices of hepatic drug metabolism (antipyrine elimination rate and cytochrome P-450 concentration in liver biopsy specimens) were studied in 19 epileptics on long-term anticonvulsant treatment, and in 18 controls. The size of the liver and the total estimated liver blood flow were greater in the epileptics than in the controls, whereas the relative liver blood flow (per unit weight of the liver) was not significantly different. The epileptics had higher cytochrome P-450 levels and they eliminated antipyrine faster than the controls. It was concluded that long-term ingestion of enzyme-inducing anticonvulsants is associated with an increase in the total hepatic blood flow in parallel with the increase in liver size, and not as an independent phenomenon. Since the relative perfusion rate of the hepatocytes was unchanged, the enhanced activity of drug metabolizing enzymes is presumed to be mainly responsible for the increased drug clearance observed in epileptic subjects.     

黄酮类化合物对细胞色素P450 CYP1，2E1，3A4和19的影响

黄酮类化合物广泛存在于蔬菜、坚果、水果和饮料及中草药中，可诱导或抑制多种细胞色素P450的活性。本篇综述主要集中回顾黄酮类化合物对于细胞色素P450 CYP1，2E1，3A4和19的影响。归纳总结了该类物质抑制和诱导细胞色素P450的多种机制，如刺激特定的受体、稳定相关mRNA等。并总结了该类物质对细胞色素P450的影响体内和体外水平的研究结果并非总是一致的原因，如体内外的浓度的差异、基因和其他环境因素的影响。由于黄酮类化合物可通过影响细胞色素P450的活性影响药物代谢从而导致药物不良反应和药物相互作用，因此在将该类物质与其他药物合用时应高度重视。     

Age- and Sex-related Differences in Rat Liver Microsomal Enzymes and their Inducibility by Toluene

Abstract: In the liver microsomes of toluene-treated and control Sprague-Dawley rats (n=148, males and females aged 13–93 days), the contents of cytochrome P-450 and b₅ and the activities of NADPH – cytochrome c reductase and four monooxygenases were studied. In male control rats, cytochrome contents and NADPH – cytochrome c reductase, aminopyrine N-demethylase and aniline hydroxylase activities increased to the age of one month, and after a slight decrease in cytochrome concentrations, the average adult level was reached by the age of two months. Aniline hydroxylase and 7-ethoxycoumarine O-deethylase activities decreased to about half at the same age period. In control female rats, the activities of aminopyrine N-demethylase and aniline hydroxylase decreased after the age of one month, and they remained at a considerably lower level in adult females than in males. The sex-dependence of other enzymes was negligible. Toluene induction was already well developed in the youngest age group of both sexes; in most cases the induced enzyme levels in young rats were as high or higher than in adults. In adult female rats, toluene induction of all enzymes was weaker than in males. In male rats, the toluene-induced level of aniline hydroxylase and 7-ethoxycoumarine O-deethylase showed deep minima at the age of 43–53 days, at the puberty of rats.     

Induction of tirilazad clearance by phenytoin

Tirilazad is a membrane lipid peroxidation inhibitor being studied for the management of subarachnoid hemorrhage; phenytoin is used for seizure prophylaxis in the same disorder. The induction of tirilazad clearance by phenytoin was assessed in 12 volunteers (6 male, 6 female). Subjects received phenytoin orally every 8 h for 7 days (200 mg for nine doses and 100 mg for 13 doses) in one phase of a crossover study. In both study phases, 1.5 mg kg⁻¹ tirilazad mesylate was administered by IV infusion every 6 h for 29 doses. Tirilazad mesylate and U-89678 (an active metabolite) in plasma were quantified by HPLC. After the final dose, tirilazad clearance was increased by 91.8% in subjects receiving phenytoin+tirilazad versus tirilazad alone. AUC_0–6 for U-89678 after the last tirilazad dose was reduced by 93.1% by concomitant phenytoin. These effects were statistically significant. The time course of induction was consistent with that of phenytoin's effect on the ratio of urinary 6β -hydroxycortisol to cortisol, a measure of hepatic CYP3A activity. The results show that phenytoin induces metabolism of tirilazad and U-89678 in healthy subjects and that, under these conditions, tirilazad clearance approaches liver blood flow. © 1998 John Wiley & Sons, Ltd.     

Hepatic injury and drug metabolism in patients with alpha-methyldopa-induced liver damage

Summary The clinical picture and drug metabolism in 36 consecutive patients with alpha-methyldopa — induced hepatic injury were investigated. The diagnosis was based on case history and biochemical, histological and follow-up studies after withdrawal the drug. Alpha-methyldopa-induced liver damage was found to occur in two phases, acutely within months and chronically within years after beginning treatment. Differences were also found in clinical symptoms and the results of liver tests on the patients if they were divided on the basis of the time factor. Drug metabolism was impaired in patients with alpha-methyldopa-induced liver damage, as indicated by low cytochrome P-450 level in liver biopsies and prolonged antipyrine elimination rate from plasma. Disappearance of the symptoms and normalisation of the liver tests after drug withdrawal occurred faster in patients with an acute type of hepatotoxicity than in subjects with delayed onset of the symptoms. The occurrence of hepatotoxicity in four members of a family suggests a genetic disposition to alpha-methyldopa-induced hepatic injury. The occurrence of two phases of liver damage suggests that a possible mechanism for acute hepatotoxicity might be an allergic reaction to metabolic intermediates produced during breakdown of alpha-methyldopa in the liver. For cases of delayed onset the cause might be increasing damage to microsomal liver protein due to covalent binding during long-term exposure to the drug.