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

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.

Studies on regional blood flow of the mouse using whole-body autoradiography of 14C-iodoantipyrine

In order to visualize regional blood flow in various tissues of the mouse at the same time, the distribution of radioactive carbon from 14C-iodoantipyrine was studied by whole-body autoradiography. The mice were frozen with Dry-Ice-hexane at 1, 10, 30 min, and 1 h and 3 h after intravenous injection of 14C-iodoantipyrine. Whole-sagittal sections of the frozen mouse, obtained by using a cryostat microtome, were dried in a cryostat and subjected to autoradiograhy. The resulting dry autoradiographs are called untreated autoradiographs in the present work. The sections were then fixed in cold 6% (w/v) HClO4, dried at room temperature and again subjected to autoradiography. Autoradiographs thus obtained are referred to as treated autoradiographs. It was found that the method could be suitable for the estimation of regional blood flow of the renal cortex, spleen, lung, skeletal muscle, bone marrow, thymus, testes, and brain.

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.

Comparison of the in vivo and in vitro rates of formation of the three main oxidative metabolites of antipyrine in man

1 The metabolism of antipyrine to its three main oxidative metabolites, 4-hydroxyantipyrine, 3-hydroxymethylantipyrine and norphenazone was investigated in vivo and in vitro in separate groups of subjects with normal hepatic function and in the same group of patients with suspected liver disease. 2 The rank order for the rate of formation of the three metabolites of antipyrine was similar in vivo and in vitro. 3 There was no significant correlation between the rates of formation of any pair of antipyrine metabolites either in vivo or in vitro. 4 Despite this there was a significant correlation between the in vivo and in vitro rates for formation of each of the three metabolites in the same group of patients. 5 It is concluded that determination of rates of formation of antipyrine metabolites from their excretion in urine provides an indication of the activity of the enzymes involved in their formation.

Inhibition of antipyrine metabolism by beta-adrenoceptor antagonists

1 The effects of two beta-adrenoceptor antagonists (propranolol and metoprolol), and of the beta-adrenoceptor agonist, terbutaline, on the plasma kinetics of antipyrine were studied in five normal subjects. In addition, the influence of propranolol on the clearance of antipyrine to three of its major metabolites was investigated. 2 At the same level of beta-adrenoceptor blockade, assessed by lowering of exercise tachycardia, propranolol decreased antipyrine clearance by 37.3 +/- 9.9 s.d. % (P less than 0.001) and metoprolol decreased it by 18.0 +/- 4.7 s.d. % (P less than 0.01). Terbutaline had no effect on antipyrine clearance. The volume of distribution of antipyrine was unchanged following treatment with all three drugs. 3 Only the metabolic clearance of antipyrine to its 3-hydroxymethyl product was impaired to a statistically significant degree by propranolol. However, four of the five subjects also showed impaired clearance to 4-hydroxyantipyrine and three of the five to norantipyrine after propranolol treatment. In four of the five subjects propranolol lowered the renal clearance of antipyrine. 4 Inhibition of the metabolism of antipyrine by beta-adrenoceptor antagonists may be related to their lipid-solubility and extent of metabolism and is independent of their effect on beta-adrenoreceptors.

Clinical implications of enzyme induction and enzyme inhibition

The pharmacological effect of a drug is partly dependent upon its concentration at its site of action, which in turn is partly dependent upon its rate of elimination. The rate of elimination of many lipophilic drugs is governed by the activity of the hepatic microsomal mixed-function oxidases. Consequently any alteration in the activity of these enzymes may result in a modification of drug action. A wide range of chemically unrelated substances may stimulate the activity of the mixed-function oxidases by enzyme induction. The drugs most frequently encountered as enzyme-inducing agents in man are barbiturates, rifampicin and phenytoin. Enhancement of drug metabolism by ethanol, tobacco smoking and diet may also involve enzyme induction. Enzyme induction is normally associated with a reduction in the drug efficacy but may also alter the toxicity of certain substances. Enzyme induction has been assessed in man by measuring changes in the pharmacokinetics of a marker drug, or changes in the disposition of endogenous compounds such as gamma-glutamyltranspeptidase, D-glucaric acid and 6beta-hydroxycortisol. The therapeutic problems associated with enzyme inhibition have received much less attention than those associated with enzyme induction. The effect on the rate of elimination of a particular drug will depend upon the fraction of the dose that is normally metabolised by the inhibited enzyme and on the affinity of the enzyme for the drug and the inhibitor. An alteration in the dosage schedule is usually only necessary for drugs with a small therapeutic ratio.

Differential induction of antipyrine metabolism by rifampicin

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.

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.