Metabolism of drugs. LIX. A new metabolite of antipyrine

Antipyrine as a probe for human oxidative drug metabolism: identification of the cytochrome P450 enzymes catalyzing 4-hydroxyantipyrine, 3-hydroxymethylantipyrine, and norantipyrine formation

BACKGROUND AND OBJECTIVE

Antipyrine has been widely used as a probe drug for human oxidative drug metabolism. To evaluate the role of antipyrine as a model drug, we have identified the cytochrome P450 enzymes involved in 4-hydroxyantipyrine, 3-hydroxymethylantipyrine, and norantipyrine formation.

METHODS

We used the following methods for this study: (1) determination of enzyme kinetics for antipyrine metabolite formation in human liver microsomes, (2) inhibition studies with antibodies and inhibitors, and (3) formation of metabolites by stable expressed human P450 enzymes.

RESULTS

Antipyrine biotransformation could be described by Michaelis-Menten kinetics: norantipyrine: maximum rate of metabolite formation (Vmax), 0.91 +/- 0.04 nmol . mg-1 . min-1; Michaelis-Menten constant (Km), 19.0 +/- 0.8 mmol/L; 4-hydroxyantipyrine: Vmax, 1.54 +/- 0.08 nmol . mg-1 . min-1;Km,39.6 +/- 2.5 mmol/L. Antibodies against CYP3A4 inhibited the formation of 4-hydroxyantipyrine by 25% to 65%. LKM-2 antibodies (anti-CYP2C) caused a 75% to 100% inhibition of norantipyrine and a 58% to 80% inhibition of 3-hydroxymethylantipyrine formation. Sulfaphenazole inhibited the formation of 3-hydroxymethylantipyrine and norantipyrine by about 50%. Furafylline and fluvoxamine inhibited norantipyrine, 4-hydroxyantipyrine, and 3-hydroxymethylantipyrine formation by about 30%, 30%, and 50%, respectively. Ketoconazole reduced formation of norantipyrine, 3-hydroxymethylantipyrine, and 4-hydroxyantipyrine by up to 80%. Formation in stable expressed enzymes indicated involvement of CYP1A2, CYP2B6, CYP2C, and CYP3A4 in metabolite formation.

CONCLUSION

Antipyrine metabolites are formed by at least six hepatic cytochrome P450 enzymes (CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C18, and CYP3A4). 4-Hydroxylation is mainly catalyzed by CYP3A4 and, to a lesser extent, by CYP1A2. The CYP2C subfamily contains the predominant enzymes for norantipyrine formation, and CYP1A2 is also involved. Formation of 3-hydroxymethylantipyrine is mediated by CYP1A2 and CYP2C9. Because several cytochrome P450 enzymes are involved in the formation of each metabolite, antipyrine is not well suited as a probe for distinct human cytochrome P450 enzymes.

Effect of deltamethrin on antipyrine pharmacokinetics and metabolism in rat

The effect of deltamethrin pretreatment on the pharmacokinetics and metabolism of antipyrine was studied in male rats. The total plasma clearance of antipyrine was significantly decreased by deltamethrin pretreatment (20 mg/kg and 40 mg/kg daily for 6 days prior to antipyrine administration), while the elimination half-life at beta phase, the area under the concentration-time curve and the mean residence time of antipyrine were significantly increased. The magnitude of the observed changes was dose dependent. The urinary excretion of norantipyrine, 4-hydroxyantipyrine and 3-hydroxymethylantipyrine was decreased by 39%, 32% and 26%, respectively (p less than 0.001) in the presence of deltamethrin. In addition, the rate constants for formation of each of these metabolites were significantly decreased by an average of approximately 71%. These results suggest that deltamethrin is capable of inhibiting oxidative metabolism, a finding which could be of clinical and toxicological significance.

High-performance liquid chromatographic method for the determination of the three main oxidative and 3-carboxylic antipyrine metabolites in human urine

The oxidative metabolism of xenobiotics is usually explored using the antipyrine test, which consists of determining the production clearances and urinary percentages of three major antipyrine metabolites 4-hydroxyantipyrine, norantipyrine and 3-hydroxymethylantipyrine. The total forms of these compounds are generally determined by high-performance liquid chromatography (HPLC). However, the 3-carboxylic acid metabolite (3-carboxyantipyrine), which is the ultimate oxidation form in the 3-hydroxylation pathway, should also be taken into account, but so far its determination by HPLC has not been reported. A simple and accurate HPLC method has now been developed to determine the three major metabolites plus 3-carboxyantipyrine. In this method, all compounds are extracted in an aprotic non-polar solvent, at pH 3.5 for the major metabolites and unchanged antipyrine, then at pH 0.9 for 3-carboxyantipyrine. Total forms are evaluated after enzymatic hydrolysis. Throughout the procedure, attention is paid to the relative instability of norantipyrine and 4-hydroxyantipyrine. Recovery, accuracy and precision are discussed. The method has been applied to the determination of relative amounts (percentage of the dose administered) excreted in the urine of ten adult subjects 48 h after ingestion of antipyrine (600 mg). The proportion of 3-carboxyantipyrine excreted was 4.5 +/- 0.2%, which is in agreement with published values obtained by gas chromatography. The excretion rates of the major metabolites also were similar to those reported in the literature, thereby confirming that the reported method is valid. 3-Carboxyantipyrine is totally excreted as the free form and norantipyrine almost completely as glucuroconjugate.

Studies on in vitro antipyrine metabolism by 13C,15N double labeled method

The capacities of forms of cytochrome P-450 to oxidize antipyrine were compared. An isotope dilution gas chromatography/mass spectrometry/selected ion monitoring assay was developed to quantify the three main metabolites, norantipyrine, 3-hydroxymethylantipyrine and 4-hydroxyantipyrine. 13C,15N-Double labeled antipyrine was used as a substrate and the metabolites were analyzed as their trimethylsilyl derivatives. Among forms of cytochrome P-450 examined, a male-specific form of P-450, namely P-450-male, showed higher activity to form all the three metabolites. The other forms were responsible only for the formation of norantipyrine and 4-hydroxyantipyrine. The activities of liver microsomes from untreated male and female rats and rats treated with phenobarbital, 3-methylcholanthrene or polychlorinated biphenyl were expressed dependent on the activities of forms of cytochrome P-450 examined.

Antipyrine metabolite kinetics in healthy human volunteers during multiple dosing of phenytoin and carbamazepine

Antipyrine total clearance and the formation clearance of its major metabolites were studied in normal, healthy male volunteers before and after multiple dosing for approximately three weeks with phenytoin (six subjects) and carbamazepine (six subjects). Total antipyrine clearance increased on average by 91% after phenytoin dosing and by 61% after carbamazepine and individual increases correlated well with mean plasma concentrations of the anti-epileptic drug. The increase in total clearance resulted largely from increased formation clearances of the 4-hydroxy and 3-hydroxymethylantipyrine metabolites with minimal effect on the norantipyrine pathway, following treatment with both enzyme-inducing drugs. It is concluded that both phenytoin and carbamazepine have similar effects on antipyrine metabolism and that these effects are mediated by induction of specific forms of cytochrome P450.

Isolation and characterization of the four antipyrine glucuronides and determination of their urinary excretion pattern in man by a reversed-phase h.p.l.c. assay

A large-scale procedure for the isolation of four urinary glucuronides in antipyrine metabolism is described; the isolated compounds are used as standards in a direct h.p.l.c. assay. The four glucuronides were characterized by u.v. and 1H-n.m.r. spectroscopy, and after hydrolysis by a t.l.c. assay of the corresponding aglycones. A reversed-phase h.p.l.c. assay procedure has been developed for the direct quantification of the four antipyrine glucuronides; this separates 3-hydroxymethyl-antipyrine glucuronide, 4,4'-dihydroxy-antipyrine glucuronide, norantipyrine glucuronide and 4-hydroxy-antipyrine glucuronide in a single run. Urinary elimination patterns of these glucuronides have been determined in five female and five male volunteers after antipyrine (1200 mg) administration. The direct assay of urinary glucuronides enables the simultaneous determination of glucuronidation activities and four different phase-I metabolites of antipyrine in vivo.

Metabolism of the amino acid beta-pyrazol-1-ylalanine and its parent base pyrazole

beta-Pyrazol-1-yl-DL-alanine, an uncommon amino acid from plants of the Cucurbitaceae, was fed to mice. Although pyrazole is known to affect the liver enzymes UDP-glucose dehydrogenase, UDP-glucuronyl transferase and UDP-glucuronic acid pyrophosphatase, and also depresses their liver glycogen concentrations, beta-pyrazol-1-ylalanine had no such effects. beta-Pyrazol-1-ylalanine could not be detected in the liver of the experimental animals but was present in the urine. No other change in urinary amino acid content was observed. Studies with [14C]-beta-pyrazol-1-yl-DL-alanine showed the administered amino acid was excreted over a 4-day period, 93% of the compound supplied was recovered. Similar recoveries were obtained with the L-enantiomer from cucumber seed. The metabolic inertness of beta-pyrazol-1-ylalanine was also apparent in experiments involving subcutaneous injection of this compound. Administration of pyrazole confirmed an earlier report of resultant increased activity of liver UDP-glucose dehydrogenase and UDP-glucuronyl transferase, and of the depression of activity of liver UDP-glucuronic acid pyrophosphatase. A concomitant 40% decrease in liver glycogen content was seen. The urine contained a novel metabolite, identified as a peptide conjugate of a pyrazole derivative. Mass spectrometry and p.m.r. spectroscopy indicate that this derivative is 3,4,4-trimethyl-5-pyrazolone. The amino acid constituents are aspartic acid, threonine, serine, glutamic acid, proline, glycine, alanine, valine and leucine. The urine of mice receiving pyrazole contained less free glycine and alanine than controls. From the results, it is concluded that pyrazole is not a catabolite of dietary beta-pyrazol-1-ylalanine but to the contrary, the amino acid is essentially excreted unchanged. Formation of 3,4,4-trimethyl-5-pyrazolone from pyrazole would imply C-methylation, a process that has not been previously observed in a mammalian detoxication context.

Tolbutamide as a model drug for the study of enzyme induction and enzyme inhibition in the rat

The effects of various drugs on the pharmacokinetics of tolbutamide have been examined in the rat. Phenobarbitone pretreatment caused a significant decrease in half life and area under the curve (AUC) and a significant increase in clearance and volume of distribution (Vd). Acute administration of primaquine significantly increased half life and AUC and decreased clearance. In contrast, the related animoquinolone chloroquine, was without effect. Acute administration of cimetidine produced similar changes to primaquine but of lesser magnitude. Formation of the major metabolite hydroxytolbutamide, was markedly enhanced by phenobarbitone and reduced by primaquine and cimetidine. We conclude that due to its single pathway of metabolism, tolbutamide is a good substrate to use when examining pharmacokinetic interactions involving hepatic enzyme induction and inhibition.

Some new urinary metabolites of famprofazone and morazone in man

The human urinary metabolism of two pyrazolone derivatives, morazone and famprofazone, has been investigated. After administration of morazone, the metabolites p-hydroxymorazone and phenazone-4-carboxylic acid were excreted in addition to the known metabolite, phenmetrazine, and unchanged morazone. Metabolism of famprofazone led to the formation of methamphetamine; the pyrazolone moiety was excreted as 3-hydroxymethyl-propyphenazone.

Monogenic control of variations in antipyrine metabolite formation. New polymorphism of hepatic drug oxidation

To investigate mechanisms that control large variations among normal uninduced subjects in the elimination of the model compound antipyrine (AP) and other drugs, AP was administered to 144 subjects (83 unrelated adults and 61 members of 13 families). Thereafter, at regular intervals for 72 h, the urine of each subject was collected and concentrations of AP and its three main metabolites measured. From these urinary concentrations, rate constants for formation of each AP metabolite were calculated. Trimodal curves were observed when values for each AP rate constant were plotted in 83 unrelated subjects; probit plots of these values showed inflections at the two antimodes of each trimodal distribution. All members of our 13 families were assigned one of three phenotypes determined by where their AP metabolite rate constant placed them in the trimodal distributions derived from the 83 unrelated subjects. In each family, pedigree analysis to identify the mode of transmission of these three phenotypes was consistent with their monogenic control. These results provide evidence for a new polymorphism of drug oxidation in man.

Human metabolism of antipyrine labelled with 14C in the pyrazolone ring or in the N-methyl group

After ingestion of [N-14CH3]antipyrine by two healthy male subjects, the urinary recoveries of radioactivity plus norantipyrine (non-radioactive) were 63 and 73%. After ingestion of [3-14C]antipyrine in the same two subjects, the urinary recoveries of radioactivity were 84 and 99%. Therefore, N-demethylated metabolites which have not been identified before, besides norantipyrine, must account for 21--26% of the dose. Serum half-lives of total 14C were about 50% greater than those of unchanged antipyrine. The difference was less in the saliva. Three major metabolites of antipyrine, norantipyrine, 4-hydroxyantipyrine and 3-hydroxymethylantipyrine, in urine were determined by radio-t.l.c. and g.l.c. These three metabolites and antipyrine accounted for 50--69% of the administered dose. The urinary excretion half-lives of these three metabolites were similar to each other and to the serum half-life of antipyrine. 3-Hydroxymethylantipyrine in one subject was excreted more slowly than the other metabolites. The radioactive metabolite not extracted from urine by organic solvents was very polar, as judged by t.l.c.

Studies of the different metabolic pathways of antipyrine in man. Oral versus i.v. administration and the influence of urinary collection time

The pharmacokinetics of antipyrine in plasma and saliva, and urinary excretion of its major metabolites, were studied following i.v. and oral administration of antipyrine 500 mg to 6 healthy volunteers. Data from both plasma and saliva showed that the oral bioavailability of antipyrine given as an aqueous solution was complete. The saliva/plasma concentration ratio was constant with time from about 3 h onwards, with a mean value of 0.87 after oral and 0.91 after i.v. administration. It is concluded that the pharmacokinetic parameters of antipyrine can be satisfactorily established on the basis of salivary data, although the volume of distribution and clearance values are then slightly too high. After i.v. administration, 3.8 +/- 1.9% of the dose was excreted in urine as unchanged antipyrine in 48 h, 24.9 +/- 6.3% as 4-hydroxyantipyrine, 16.5 +/- 3.2% as norantipyrine, 13.0 +/- 2.2% as 3-hydroxymethyl-antipyrine and 5.8 +/- 1.0% as 3-carboxy-antipyrine. No significant differences were observed following oral administration. The half-lives calculated from the linear part of the urinary excretion rate curves of the metabolites were about the same for oral and i.v. administration, and were of the same order of magnitude as the elimination half-life of parent drug in plasma and saliva. It is important for determination of the ultimate metabolite ratio that urine is collected for at least 36 h, because there is a delay in the excretion of 3-hydroxymethyl-antipyrine in urine.

Differential effects of 3-methylcholanthrene and phenobarbitone treatment on the oxidative metabolism of antipyrine in vitro by microsomal fractions of rat liver

1. The effects of treating rats with the inducers phenobarbitone (4 X 80 mg/kg per day) and 3-methylcholanthrene (80 mg/kg) on the kinetics of the formation of the three major oxidative metabolites of antipyrine in vitro by hepatic microsomal fractions have been investigated. 2. Phenobarbitone treatment significantly increased the Vmax of 4-hydroxyantipyrine formation (by 2.3 fold) and of norphenazone formation (by 2.3-fold). 3-Methylcholanthrene treated caused a slight, but significant (P less than 0.05), reduction in Vmax for 3-hydroxymethylantipyrine formation. 3. Phenobarbitone markedly reduced the Km for 3-hydroxymethylantipyrine formation (from 2.2 +/- 0.5 mM to 0.65 +/- 0.09 mM), whereas 3-methylcholanthrene treatment resulted in an increase (P less than 0.05) in the Km (to 4.9 +/- 1.1 mM). The only other significant change in Km was a slight decrease in that of 4-hydroxyantipyrine formation following phenobarbitone treatment (from 4.6 +/- 1.1 mM to 2.2 +/- 0.3 mM, P less than 0.05). 4. Calculation of the ratio Vmax/Km permitted an estimate of the clearance in vivo to the metabolites. There was good agreement between predicted values and those for total body clearance of antipyrine, and four changes in clearance to individual metabolites in vivo following treatment with enzyme inducers. 5. Kinetics analysis of formation of antipyrine metabolites in vitro has enabled the enzymic basis for changes observed in their excretion to be established. Further evidence was also obtained for the involvement of multiple forms of cytochrome P-450 in antipyrine oxidation.

Differential effects of enzyme induction on antipyrine metabolite formation

1 The influence of enzyme induction with antipyrine and pentobarbitone was studied on the rates of formation of the major metabolites of antipyrine: 4-hydroxyantipyrine, norantipyrine and 3-hydroxymethyl-antipyrine + 3-carboxy-antipyrine. The inducing drugs were given to panels of healthy volunteers for 8 days and prior to and after this period antipyrine total elimination clearance was determined in plasma, whereas the partial clearances for production of the individual metabolites were assessed on the basis of urinary excretion data. 2 Antipyrine total clearance had significantly increased by 16% following treatment with antipyrine, which could almost entirely be attributed to a selective increase in the rate of production of norantipyrine. 3 With pentobarbitone total clearance of antipyrine had increased by 60%, which was associated with a significant increase in the clearance of production of all three metabolites. However, the increase in norantipyrine formation was significantly higher than the increase in 4-hydroxyantipyrine and 3-hydroxymethyl-antipyrine formation. 4 The most likely explanation for these differences in the degree of induction of the different metabolic routes of antipyrine, is that different enzymes are involved in the different routes. Apparently the enzyme involved in norantipyrine formation is most sensitive to induction by antipyrine and pentobarbitone. By measuring rates of antipyrine metabolite formation it may be possible to study the degree of selectivity of enzyme inducers on oxidative drug metabolism.

Quantitation and urinary pattern of 4,4'-dihydroxy-antipyrine, 4-hydroxy-antipyrine and 3-hydroxymethyl-antipyrine, as main metabolites of antipyrine in man and rat

A t.l.c.-assay has been developed for the simultaneous determination from the urine of man and animals of three major hydroxylated metabolites of antipyrine (4,4'-dihydroxy-antipyrine, 4-hydroxy-antipyrine, 3-hydroxymethyl-antipyrine). The methodology is also applicable to bile fluid, liver perfusate and liver homogenate. Genuine conjugates are cleaved by acid hydrolysis and free, acid stable metabolites are extracted. Extracts are subjected to t.l.c. and the chromatograms analysed quantitatively by u.v.-reflectance measurements using authentic materials as standards. Calibration curves are linear with a correlation coefficient r greater than 0.990. Recovery for each metabolite is greater than 95%. Reproducibility of the method is good, with variation coefficients in the range of 3-7%, depending on concentration. The sensitivity of the method is sufficient for practical needs. The specificity of the procedure was confirmed using radio-labelled antipyrine. In man, 4-hydroxy-antipyrine is the principal hydroxylation product in this series, accounting for about 35-40% of the dose. 3-Hydroxymethyl-antipyrine makes up for about 13-17% and 4,4'-dihydroxy-antipyrine represents 3-6% of the dose of antipyrine. In the rat, 4-hydroxy-antipyrine accounts for about 15-31%, 3-hydroxymethyl-antipyrine for 22-28% and 4,4'-dihydroxy-antipyrine for up to 11-18% of the dose. Variation of this pattern in different strains is moderate. In both species, the major portion of phase-I metabolites is excreted as conjugates. Part of them appears in a free form.

Genetic variation in rates of antipyrine metabolite formation: a study in uninduced twins

Adult, male, unmedicated twins received antipyrine orally under carefully controlled environmental conditions. Relative contributions of genetic and environmental factors to 2-fold interindividual variations in rate constants for formation of the three main antipyrine metabolites were compared. Heritabilities for rate constants for formation of 4-hydroxyantipyrine, N-demethylantipyrine, and 3-hydroxymethylantipyrine were 0.88, 0.85, and 0.70, respectively. These results suggest that each molecular form of cytochrome P-450 that converts antipyrine to a different metabolite exhibits genetically controlled interindividual variations in activity. Unrelated adult male subjects whose environments were also carefully controlled exhibited highly reproducible rate constants for formation of antipyrine metabolites. Because the rate constant for metabolite formation sensitively detects certain variations in the gene product, it should be used in future pharmacogenetic studies on rates of production of multiple metabolites from a single parent drug.

Drug distribution as a function of binding competition. Experiments with the distribution dialysis technique

In the distribution dialysis technique each of the two dialysis chambers contains a binding system, and a drug is allowed to distribute between them. This technique was tested by using various intracellular and extracellular binder preparations over wide concentration ranges, and model drugs selected for their known binding properties. The drugs were then tested at therapeutic concentrations in standardized systems of liver homogenate (0.5 g ml-1) and whole blood (0.02 ml ml-1). The resulting intracellular/extracellular concentrations ratios were characteristic for the binding properties of the various drugs. Thus, for imipramine, a drug with strong tissue and weaker plasma binding properties, the concentration ratios were 25 for the system homogenate/buffer, 0.8 for buffer/blood, and 15 for the competitive system homogenate/blood. In experiments with homogenates from various tissues (liver, lung, kidney, intestine, brain) and blood in the standard system, the following approximate ratios were obtained: 1 for antipyrine, 2 for phenylbutazone, 14 for imipramine (but only 8 with muscle, skin and adipose tissue). These results reflect both the individual binding to intracellular and extracellular components and the tissue/blood concentration ratios in vivo. It is suggested that distribution dialysis is an in vitro method for characterizing the distribution of the drugs. It is also concluded that drug distribution is largely determined by a binding competition between tissue and blood sites.

Antipyrine metabolite formation in children in the acute phase of malnutrition and after recovery

1. The plasma elimination rate of antipyrine and the urinary excretion of antipyrine and its primary metabolites 4-hydroxy-antipyrine, norantipyrine, 3-hydroxymethyl-antipyrine and 3-carboxyantipyrine were measured in five children in the acute phase of malnutrition and after recovery. The results were compared with those obtained in 3 normal children. 2. Upon nutritional rehabilitation antipyrine clearance increased from 0.65 +/- 0.14 ml min-1 kg-1 to 1.07 +/- 0.20 ml min-1 kg-1. 3. The urinary excretion of 4-hydroxy-antipyrine increased from 6.1 +/- 4.5 to 14.7 +/- 5.9%, norantipyrine from 8.8 +/- 5.7 to 14.3 +/- 5.4 and 3-hydroxy-methyl-antipyrine from 11.8 +/- 8.3 to 20.5 +/- 5.6% (% of dose/24h urine). Excretion of unchanged antipyrine decreased from 5.2 +/- 3.7 to 2.7 +/- 0.9% dose. The metabolite profile (ratio between the amounts of the various metabolites excreted) was not significantly different. 4. It is concluded that malnutrition decreases the rate of antipyrine metabolism, but it does not affect the three oxidative pathways differently.

Interspecies variation in liver weight, hepatic blood flow, and antipyrine intrinsic clearance: extrapolation of data to benzodiazepines and phenytoin

The literature was reviewed to obtain data from 11 mammalian species on liver weight, hepatic blood flow, and antipyrine intrinsic clearance. It was demonstrated that liver weight and hepatic blood flow in all species could be readily related to body weight by a simple equation. Additionally, hepatic blood flow in all species was directly proportional to liver weight. With the exception of man, antipyrine intrinsic clearance was also directly proportional to liver weight. Man's intrinsic clearance was approximately one-seventh of that which would be predicted from other species. Data on benzodiazepines and phenytoin showed a similar pattern.

Studies on the different metabolic pathways of antipyrine in man. I. Oral administration of 250, 500 and 1000 mg to healthy volunteers

1 The plasma elimination rate of antipyrine, as measured by the salivary concentration decay, and the urinary excretion of antipyrine and its primary metabolites 4-hydroxy-antipyrine, norantipyrine, 3-hydroxymethyl-antipyrine and 3-carboxy-antipyrine was studied in five healthy volunteers, who received 250, 500 and 1000 mg antipyrine orally in a cross-over design.

2 The mean antipyrine half-life and metabolic clearance were 11.5 ± 2.5 h (range 10.2-16.9 h) and 3.4 ± 0.9 l/h (range 1.7-4.2 l/h) respectively after 500 mg. These values were not significantly different after 250 or 1000 mg (P > 0.1; paired t-test).

3 In 52 h urine 3.3 ± 1.2% of the dose of 500 mg antipyrine was excreted unchanged as antipyrine, 28.5 ± 2.2% as 4-hydroxy-antipyrine, 16.5 ± 6.0% as norantipyrine, 35.1 ± 7.2% as 3-hydroxymethyl-antipyrine and 3.3 ± 0.8% as 3-carboxy-antipyrine. The values obtained at the other dose levels were not significantly different (P > 0.1; paired t-test).

4 At all dose levels 4-hydroxy-antipyrine and norantipyrine were excreted in urine entirely as glucuronides. After 500 mg antipyrine, 3-hydroxymethyl-antipyrine was excreted as glucuronide to the extent of 58 ± 9% of the total excreted amount. This percentage was not significantly different at the other dose levels. 3-Carboxy-antipyrine was excreted in the free form at all three dose levels.

5 From 12 h of drug intake onwards, the urinary excretion rate curves of antipyrine and all its metabolites declined mono-exponentially with about the same half-life as the parent compound in saliva. The half-lives calculated from the excretion rate curves of 4-hydroxy-antipyrine, norantipyrine and 3-hydroxymethyl-antipyrine correlated significantly with the half-life of antipyrine in plasma. At all dose levels a relative delay in urinary excretion of 3-hydroxymethyl-antipyrine was observed compared to the urinary excretion of antipyrine and the other metabolites.

6 The ratios of the cumulative amounts of metabolites excreted in 24 h, were essentially the same as those measured in the 52 h samples.

Studies on the different metabolic pathways of antipyrine in rats: influence of phenobarbital and 3-methylcholanthrene treatment

1. The amounts of antipyrine and its metabolites excreted in 24 h urine after i.v. injection of 10 mg antipyrine into male Wistar rats were quantified after enzymic hydrolysis with beta-glucuronidase/aryl sulphatase. In 24 h 2.7% of the administered dose was excreted as unchanged antipyrine, 13.3% as 4-hydroxyantipyrine, 7.4% as norantipyrine, 28.9% as 3-hydroxymethylantipyrine and 1.1% as 3-carboxyantipyrine. 2. Treatment with phenobarbital decreased the antipyrine half-life from 65 to 30 min, but did not significantly change the urinary metabolite profile. Only the amount of 3-carboxyantipyrine was significantly different and increased from 1.1 to 2.6% dose. 3. 3-Methylcholanthrene treatment resulted in a decrease of antipyrine half-life from 72 to 37 min. After treatment 4-hydroxyantipyrine was increased from 13.4% to 25.6% dose, whereas 3-hydroxymethylantipyrine was decreased from 26.8% to 8.5% and 3-carboxyantipyrine from 1.3% to 0.2% of the dose respectively; norantipyrine was unchanged. 4. It is concluded that different types of hepatic cytochrome P-450 may be involved in the formation of 4-hydroxyantipyrine on one hand and the formation of 6-hydroxymethylantipyrine on the other. Another possibility is that in methylcholanthrene-treated animals another haemoprotein is formed that results in the formation of more 4-hydroxyantipyrine and less 3-hydroxymethylantipyrine. In any case, the urinary metabolite profile of antipyrine can be used to study changes in the activity of different cytochromes in drug metabolism studies.

4,4'-Dihydroxyphenazone as an urinary metabolite of phenazone in different species including man

4,4'-Dihydroxyphenazone (4-hydroxy-1-4'-hydroxyphenyl)-2,3-dimethyl-3-pyrazoline-5-one) was isolated as a metabolite of phenazone (antipyrine) after acid hydrolysis of rat urine. This was characterized and identified by NMR, MS, IR, UV, Fp and epsilon270. After dosage with phenazone, 4,4'-dihydroxyphenazone is excreted in conjugated form by man, rabbit, guinea pig and rat. In the rat it is further shown, that conjugates of 4,4'-dihydroxyphenazone are formed by glucuronidation as well as by sulfatation. The metabolite seems to add substantially to the overall metabolic pattern of phenazone in these species.

Michaëlis--Menten kinetics of phenazone elimination in the perfused pig liver

The purpose of the present study was to define the elimination kinetics of phenazone (NFN) in the isolated perfused pig liver. In five experiments phenazone was administered as constant infusion to obtain steady-state periods over a wide range of concentrations. The elimination of phenazone followed saturation kinetics (concentrations 0.1-12 mmol x 1(-1) and the maximal elimination rate (Vmax) was on average 102 mumol x min-1 x kg-1 liver and the Michaëlis-constant (Km) of 2.6 mmol x 1(-1). Estimates of Vmax and Km for the microsomal phenazone hydroxylase activity measured in liver biopsies found to be considerably lower than in the perfused liver. The hepatic elimination of phenazone during perfusion of pig liver at phenazone concentrations corresponding to human therapeutic doses follows first-order kinetics.

The plasma half-life of antipyrine in chromic uraemic and normal subjects

1. Antipyrine was given intravenously in a dose of 18 mg/kg body weight to twelve patients with chronic renal failure (plasma creatinine greater than 4.9 mg/100 ml) who were not taking drugs and twenty normal subjects. 2. Plasma antipyrine levels were measured by a specific method, the plasma half-life of the drug was determined and used as an index of drug oxidation. 3. The mean (+/- s.d) plasma antipyrine half-life in patients with chronic renal failure (7.3 +/- 2.0 h) was significantly shorter than in normal subjects (13.2 +/- 4.3 h: P less than 0.002). There was no difference in the apparent volume of distribution of antipyrine between the two groups (P greater than 0.6). 4. Pretreatment of five patients with chronic renal failure and seven normal subjects with antipyrine or phenobarbitone for weeks significantly shortened the mean plasma antipyrine half-life from 7.4 +/- 2.5 h to 5.0 +/- 1.5 h in uraemics (P less than 0.005) and from 13.2 +/- 4.5 h to 6.9 +/- 1.5 h in normal subjects (P less than 0.0025).5. These results suggest that oxidation of antipyrine by hepatic microsomal enzymes is increased in patients with chronic renal failure, but a state of maximal induction of these enzymes was not observed. The clinical implication of this finding with regard to the association between liver microsomal enzyme induction and vitamin D resistant osteomalacia is discussed.