Prediction of xenobiotic metabolism by non-invasive methods.
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Acetylation↗
Biomedical subjects
S Loft
Publications and source records attributed to S Loft.
Metronidazole and antipyrine as probes for the study of foreign compound metabolism.
The aim of the present work was to develop a tool for the study of the enzyme activities relevant for the biotransformation of foreign compounds, their elimination and/or activation to toxic substances. The activity of an enzyme may be assessed by the rate of metabolism of a preferably specific probe or model compound. The cytochrome P450'ies, the key enzymes for the elimination and/or activation of most foreign compounds, exist in multiple forms with variable substrate specificity and regulation. Some cytochrome P450'ies are under genetic control, whereas the activity of others is mainly regulated by the influence from factors in the environment. Only some of the cytochrome P450'ies are relevant for the formation of harmful metabolites. Thus, the activity of as many cytochrome P450 forms as possible should be assessable, preferably simultaneously. The present work evaluated metronidazole in a cocktail with antipyrine as a tool for the study of the regulation of foreign compound metabolism in the liver. The cytochrome P450 catalyzed metabolism of metronidazole and antipyrine was studied in humans and in isolated rat hepatocytes. In humans the influence of dose, route of administration, enzyme induction and inhibition and liver disease was investigated. Rats of either sex were studied with and without pretreatment with specific enzyme inducers and incubations included specific enzyme inhibitors. Evidence was provided that the oxidative formation of the five major metabolites, two from metronidazole and three from antipyrine, depends on different cytochrome P450'ies. In humans it was demonstrated that the clearance of metronidazole and antipyrine could be determined from the same saliva sample collected 16-24 hours after their oral administration and so could the clearance for formation of each metabolite if urine was collected for 48 hours. Thus, with the cocktail of metronidazole and antipyrine and simple non-invasive sampling the activity of five different cytochrome P450'ies can be assessed in vivo. In addition, metronidazole may also be used for assessment of the glucuronidation capacity although this is a minor pathway in man. Because the variation within subjects is much less than between them, the cocktail test is particularly suited for paired designs with measurements before and after an environmental change and the subjects serving as their own control. The metronidazole/antipyrine cocktail may have many applications in the study of the regulation of foreign compound metabolism in man and in animals, in vivo and in vitro.
Animals↗
Metronidazole elimination is preserved in the elderly.
1 The disposition of metronidazole and its major metabolites was compared in 11 subjects aged 86 +/- 6 years and 8 aged 30 +/- 6 years. 2 The plasma clearance of metronidazole was 1.20 +/- 0.53 and 1.25 +/- 0.22 ml min-1 kg-1, the volume of distribution 0.77 +/- 0.27 and 0.77 +/- 0.09 1 kg-1 and the half-life 7.8 +/- 1.9 and 7.2 +/- 0.9 h in elderly and young subjects, respectively (P less than 0.05). 3 The area under the plasma concentration-time curve of the hydroxy metabolite was 32 +/- 14 and 21 +/- 3 mM min-1 (P less than 0.05) whereas its half-life was 21 +/- 14 and 12 +/- 2 h (P less than 0.05) in the elderly and young subjects, respectively. 4 The recovery in the urine of metronidazole and its metabolites was 42 +/- 21% and 87 +/- 6% of dose in elderly and young subjects, respectively (P less than 0.05). With this reservation the only elimination pathways of metronidazole affected by old age were the renal excretion of unchanged compound and the hydroxy metabolite. 5 It is concluded that the ability to eliminate metronidazole is preserved in old age and that age-related dose adjustments are not necessary.
Adult↗
Metabolism of metronidazole and antipyrine in hepatocytes isolated from mouse and rat.
1. In order to study species-related differences and select a model for the human metabolism of metronidazole and antipyrine, the Michaëlis-Menten kinetics of metabolite formation from the two compounds were investigated in freshly isolated mouse and rat hepatocytes. 2. The average Km values for the formation of the major metronidazole metabolites ranged from 0.6 to 3 mM. The intrinsic clearance values (Vmax/Km) of metronidazole to the acetic acid, hydroxy and glucuronide metabolites were 58 (36-125) and 21 (12-28; P less than 0.05), 156 (63-263) and 36 (19-56; P less than 0.05), and 269 (102-452) and 500 (389-1616; P less than 0.05) nl/min per 10(6) hepatocytes, for mouse and rat, respectively (median with range, n = 6). 3. The average Km values for the formation of antipyrine metabolites ranged from 2 to 10 mM. The intrinsic clearance values for production of 3-hydroxymethyl-, nor- and 4-hydroxyantipyrine were 232 (43-519) and 487 (296-793; P less than 0.05), 594 (168-813) and 93 (55-180; P less than 0.05), and 118 (23-505) and 239 (134-501; P greater than 0.05) nl/min per 10(6) hepatocytes, for mouse and rat, respectively (median with range, n = 6). 4. The results demonstrate that metronidazole and antipyrine are metabolized with quantitative, but not qualitative, differences in isolated hepatocytes from mice and rats. Neither species provided an ideal model for the human metabolism of the two compounds.
Acetates↗
Metabolism of metronidazole and antipyrine in isolated rat hepatocytes. Influence of sex and enzyme induction and inhibition.
The metabolism of metronidazole and antipyrine was investigated in freshly isolated hepatocytes from 7 male and 6 female control Wistar rats, 8 males and 5 females pretreated with phenobarbital (PB) and 3 males pretreated with 3-methylcholanthrene (MC). Pretreatment with PB increased the intrinsic clearance (CLi = Vmax/Km) of metronidazole to its acetic acid (MAA) and hydroxy metabolite (HM) 7- and 2.8-fold in the males and 3.2- and 3.0-fold in the females, whereas MC treatment increased the values 9- and 10-fold, respectively (P less than 0.05). The CLi of metronidazole to HM and its glucuronide conjugate was higher in the control and PB treated male than in the corresponding female groups, whereas the rank order was reversed for sulphate formation (P less than 0.05). SKF 525A was a more potent inhibitor of MAA formation than of HM formation, except in the PB treated male group. Pretreatment with MC increased the inhibitory potency of alpha-naphthoflavone and antipyrine toward MAA and HM formation. In male rats PB treatment increased the CLi of antipyrine to 3-hydroxymethyl-(HMAP), nor-(NORAP) and 4-hydroxyantipyrine (OHAP) 2.5-, 2.1- and 4.5-fold, respectively (P less than 0.05). Pretreatment with MC in male and with PB in female rats had no significant effect on antipyrine metabolism. SKF 525A was a more potent inhibitor of HMAP and OHAP formation than of NORAP formation. Treatment with MC increased the inhibitory potency of alpha-naphthoflavone toward the formation of all antipyrine metabolites. Metronidazole increased the formation rate of HMAP, but inhibited the formation of NORAP and OHAP, particularly the latter. The results suggest that the formation of MAA, HM, HMAP, NORAP and OHAP from metronidazole and antipyrine is catalyzed by different cytochrome P-450 isozymes, which may be supplemented or substituted by PB or MC induced species. The involved P-450 isozymes have more or less overlapping substrate and product specificity. Metronidazole appears to be a sensitive probe for detection and identification of PB and MC type induction.
Acetates↗
Methyl parathion toxicity in rats is changed by pretreatment with the pesticides chlordecone, mirex and linuron.
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Acetylcholinesterase↗
Influence of a very low calorie diet on the clearance of oxazepam and antipyrine in man.
A very low calorie diet (Prodi) was administered to eleven otherwise healthy obese subjects for fourteen days. The daily intake of protein was 52.7 g and carbohydrate 25.7 g, corresponding to 360 kcal. The clearance of oxazepam and antipyrine was investigated before and after the diet period. Total oxazepam clearance was 1.04 ml.min-1.kg-1 and it decreased 0.88-fold after the diet. The mean clearance of unbound oxazepam was correspondingly reduced 0.88-fold. The elimination half-life increased to 1.22-times the control value, 7.9 h. No significant change was found in the volume of distribution or protein binding of oxazepam. Antipyrine clearance, estimated by the one-sample technique, was 52.4 and 51.8 ml.min-1, before and after the diet, respectively. It appears that a very low calorie diet with a sufficient protein and a very low carbohydrate content decreases the metabolism of oxazepam by glucuro-conjugation, whereas no effect was seen on the oxidative metabolism of antipyrine.
Adult↗
[Metabolism of drugs in the liver and disease].
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Humans↗
Antipyrine and metronidazole metabolism during occupational exposure to gasoline.
Antipyrine and metronidazole clearance was measured in 18 fuel-filling attendants by the single-sample method while the attendants were being exposed occupationally to gasoline; the measurements were repeated after 2-4 weeks with no exposure. Eighteen office workers were investigated simultaneously. The median concentration of gasoline in the breathing zone of the fuel-filling attendants during filling and cleaning operations was 270 mgm-3 (range 18-1758 mgm-3). Antipyrine clearance was 18% higher during exposure to gasoline than after 2-4 weeks of vacation (P less than 0.01), while antipyrine clearance was unchanged in the office workers. No change was found in metronidazole clearance in either group. Antipyrine clearance was on average 26% higher in the smokers than in the nonsmokers (P less than 0.05), while metronidazole clearance was similar in smokers and nonsmokers. We conclude that gasoline is an inducer of antipyrine elimination, with no impact on metronidazole elimination. This indicates that gasoline has a differential inducing effect on the hepatic drug metabolizing enzymes of man.
Adult↗
Lack of effect of cimetidine on the pharmacokinetics and metabolism of a single oral dose of metronidazole.
The time course of the effect of cimetidine on the pharmacokinetics of metronidazole was investigated in 6 healthy volunteers. Cimetidine 1.0 g/day was administered for 9-days and metronidazole 500 mg was administered orally on the second and eighth days, and in a control experiment. During cimetidine treatment the plasma kinetics of metronidazole and its partial clearance by renal excretion of the unchanged compound, glucuronidation, hydroxylation and oxidation to its acetic acid metabolite were not significantly different from the control values. The results indicate that cimetidine does not influence the pharmacokinetics or metabolism of a single oral dose of metronidazole.
Administration, Oral↗
Therapeutic doses of codeine have no effect on acetaminophen clearance or metabolism.
In nine healthy volunteers, the clearance and metabolism of acetaminophen 1000 mg i.v. was evaluated with and without two concomitant oral doses of codeine in order to investigate a possible interaction. Plasma acetaminophen was followed for 720 min and urine was collected for 24 h after each dose for determination of metabolites. When codeine was coadministered, the average total clearance of acetaminophen and its clearance by glucuronidation, sulphation and mercapturate formation were 0.58 to 1.12-times the control values. It is concluded that therapeutic doses of codeine do not influence the clearance or metabolism of acetaminophen.
Acetaminophen↗
Bioavailability and pharmacokinetics of oxazepam.
Six healthy volunteers received oxazepam 15 mg i.v. and orally at an interval of at least one week. The kinetic variables of i.v. oxazepam were: elimination half-life (t1/2 beta) 6.7 h, total clearance (CL) 1.07 ml.min-1.kg-1, volume of distribution (Vc) 0.27 l.kg-1 (0.21-0.49) and volume of distribution at steady-state (Vss) 0.59 l.kg-1. The intravenous disposition of unbound oxazepam was characterized by a clearance of 22.5 ml.min-1.kg-1 and a distribution volume of 12.3 l.kg-1. After oral oxazepam the peak plasma level was reached in 1.7 to 2.8 h. The plasma t1/2 beta at 5.8 h was not significantly different from the i.v. value. Absorption was almost complete, with a bioavailability of 92.8%. Urinary recovery was 80.0 and 71.4% of the dose after intravenous and oral administration, respectively. Renal clearance (CLR) of the glucuronide metabolite was 1.10 ml.min-1.kg-1 (0.98-1.52). Oxazepam was extensively bound to plasma protein with a free fraction of 4.5%.
Administration, Oral↗
Antipyrine as a model drug to study hepatic drug-metabolizing capacity.
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Algorithms↗
Metronidazole clearance: a one-sample method and influencing factors.
After 96 administrations of metronidazole to 36 subjects, it was found that the clearance could be determined from one plasma sample, the dose, and a volume of distribution estimated from sex, age, body weight, and height, without loss of precision and accuracy compared with conventional clearance determinations (r greater than 0.97). In 230 sample pairs the plasma and saliva concentrations of metronidazole were identical (r = 0.99). In 119 subjects the one-sample clearance of metronidazole was unimodally distributed. Body weight (r = 0.28) and the alcohol consumption (r = 0.23) correlated with the metronidazole clearance. In the same subjects the consumption of tobacco (r = 0.28), alcohol (r = -0.19), coffee/tea (r = 0.27), age (r = -0.24), and sex (r = 0.28) correlated with the antipyrine clearance. The clearances of metronidazole and antipyrine were correlated (r = 0.34). The differential influence of the environmental factors on the elimination rates supports differential metabolism of metronidazole and antipyrine.
Administration, Oral↗
Fluorescein and fluorescein glucuronide in plasma after intravenous injection of fluorescein.
After intravenous injection of fluorescein the time-course of the plasma concentrations of fluorescein (F) and fluorescein glucuronide (FG) was studied in 18 insulin-dependent diabetics with various degrees of nephropathy, and in two non-diabetic subjects. Fifteen minutes after injection the molar concentrations of free (non-protein bound) F and FG were almost identical. After one hour the concentration curve integrals of the two substances were of the same magnitude. There was, however, considerable interindividual variation. In diabetic patients with renal insufficiency an increase was found in both integrals, the F integral being less increased (27%) than the FG integral (44%). It appears from the variation in absolute and relative concentrations of F and FG that a separate determination of the two fluorophores in plasma is desirable, when F is used by intravenous administration as an indicator of blood-ocular barrier function. Previous observations of an increase with time after injection of the free fraction of plasma fluorescence are explained by the finding of a higher free fraction of FG (34%) than of F (11%) and a change in relative concentrations of the two fluorophores.
Adult↗
Influence of age and consumption of tobacco, alcohol and caffeine on antipyrine clearance.
1. Antipyrine clearance and average daily consumption of tobacco, alcohol and coffee/tea were determined in 303 healthy men. 2. The antipyrine clearance was positively correlated with the consumption of tobacco (r = 0.24; P less than 0.0001) and coffee/tea (r = 0.18; P less than 0.001), and negatively with age (r = -0.14; P less than 0.05) and the alcohol consumption (r = -0.13; P less than 0.05). 3. The multiple regression coefficients suggested an increase in antipyrine clearance of 0.8% per daily cigarette or cup of tea and 1.4% per daily cup of coffee; the decrease per daily drink or year of age was 2.8% or 0.4%, respectively.
Adult↗
[The influence of environmental factors, age, sex and menstrual cycle on the metabolism of drugs in the liver].
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Adult↗
Inhibition and induction of metronidazole and antipyrine metabolism.
The effect of cimetidine, antipyrine and phenobarbitone on the pharmacokinetics of intravenous metronidazole and oral antipyrine has been examined in 7 healthy volunteers. The administration of cimetidine for 24 h before and throughout the sampling period failed to alter the total clearance of metronidazole or the rate of formation of the hydroxy metabolite, whereas the total and partial clearances of antipyrine were decreased 0.74 and 0.6-0.7-fold, respectively. Seven days of phenobarbitone or antipyrine administration increased the total clearance of metronidazole 1.51- and 1.86-fold, respectively, and the total antipyrine clearance was 1.22 or 1.46-fold increased, respectively. The rate of metronidazole hydroxylation was significantly enhanced by both enzyme inducers. The partial clearance of antipyrine to the normetabolite was significantly increased by both inducers, whereas the rate of 4-hydroxylation was significantly increased only by prior antipyrine administration. The results indicate that the hydroxylation of metronidazole is not inhibited by cimetidine, but that it is inducible by phenobarbitone or antipyrine. It is suggested that metronidazole and antipyrine are metabolized by different enzymatic pathways.
Adult↗