Effect of Amiodarone and Desethylamiodarone on the Pharmacokinetics of Antipyrine in the Rat

The effect of amiodarone on hepatic drug metabolism in vivo was examined in the rat using antipyrine as a model substrate. Pretreatment with oral amiodarone hydrochloride, 100 mg/kg/day, for 5 days resulted in a 19% reduction in antipyrine clearance and a 22% increase in half-life. The administration of single oral doses of amiodarone hydrochloride, 100 mg/kg, 1 or 5 hr prior to antipyrine administration had no significant effect on antipyrine pharmacokinetics. The administration of a single intravenous dose of amiodarone hydrochloride, 50 mg/kg, reduced antipyrine clearance by 32% and increased the half-life by 46%. The desethyl metabolite of amiodarone was also found to reduce antipyrine clearance (21%) after a single oral dose of 100 mg/kg.     

Dextromethorphan Pretreatment Induces Antipyrine Clearance in the Rat

Numerous agents that undergo extensive first-pass metabolism have been shown to inhibit oxidative drug metabolism. To examine whether this effect is related to the chemical structure or pharmacokinetic characteristics of the inhibiting agent, we determined the effect of dextromethorphan (a compound which exhibits pharmacokinetic similarities to, but is chemically dissimilar from, previously studied agents) on the disposition of antipyrine. A single oral dose of dextromethorphan hydrobromide, 100 mg/kg, 1 hr prior to antipyrine administration had no significant effect on the pharmacokinetics of this model substrate. The administration of dextromethorphan at the same dose twice daily for 3 days and an additional dose 1 hr prior to antipyrine administration resulted in a 33% increase in the clearance of antipyrine. These data indicate that dextromethorphan is capable of inducing hepatic microsomal enzymes. Studies are needed to determine if this effect also occurs upon chronic administration in humans. These data suggest that the pharmacokinetic characteristic of extensive first-pass metabolism is not necessarily associated with inhibition of drug metabolism.     

Hydralazine Pharmacokinetics and Interaction with Food: An Evaluation of the Dog as an Animal Model

The intravenous and oral dose-dependent pharmacokinetics of hydralazine and the effect of concurrent administration of food with hydralazine in dogs were evaluated for comparison with published human data. Four dogs were given intravenous and oral doses of hydralazine at 0.25, 1.0, 2.5, and 4.0 mg/kg. In addition, the oral 2.5 mg/kg dose was given with a meal. Blood samples were collected at appropriate intervals and analyzed for hydralazine. Pharmacokinetic analysis showed that AUC_oral/ dose (5552 to 13218 mg-min/ml) and F (0.36 to 0.77) increased significantly with dose, indicating saturation of first-pass metabolism, as is seen in humans. Total-body clearance (70 ml/min/kg) and steady-state volume of distribution (9 L/kg) were similar to human values. The bioavailability of hydralazine in the dog was decreased by 63% when the dose was given with a meal, which is comparable to some human data. It was concluded that the dog may be a useful model in which to study mechanisms of the hydralazine-food interaction.     

The effect of liver cirrhosis on the pharmacokinetics of phenprocoumon

Summary Phenprocoumon was given orally to 9 patients with biopsy proven liver cirrhosis (dose range 0.12–0.25 mg/kg) and to 7 healthy volunteers (0.23 mg/kg). Concentrations of phenprocoumon were determined using HPLC in plasma and urine samples obtained for 6–7 days after drug administration. The binding of [³H]-phenprocoumon in plasma from all subjects was determined by equilibrium dialysis. Antipyrine plasma concentrations were determined spectrophotometrically following oral administration of antipyrine (1200 mg). The total body clearance of phenprocoumon was higher in the cirrhotic patients (1.64±0.16 ml/h/kg mean ± SEM) than in the healthy volunteers (0.90±0.07 ml/h/kg), however the free drug clearance was not significantly different in the patients (144±14 ml/h/kg) compared with normal (113±11 ml/h/kg). In contrast the clearance of antipyrine was much reduced in the cirrhotic group (17.5±2.9 ml/h/kg) compared with normal (35.6±3.9 ml/h/kg). The metabolic clearance of phenprocoumon via glucuronidation, is relatively unaffected during cirrhosis compared with antipyrine clearance via oxidation.     

Cimetidine-phenytoin interaction: Effect on serum phenytoin concentration and antipyrine test

Summary In a prospective study in nine patients the effects of phenytoin and of cimetidine (1000mg/day) + phenytoin on the antipyrine test and serum phenytoin concentrations were studied. Serum phenytoin increased from the steady state level of 5.7±1.3 mg/l to 9.1±1.4mg/l after three weeks on cimetidine (p<0.01), and fell to 5.8±1.2 mg/l within two weeks after withdrawal of cimetidine. The protein binding of phenytoin was not changed by cimetidine. After use of phenytoin for 2–4 months, antipyrine clearance increased from 0.67±0.06ml/min/kg to 1.61±0.22 ml/ min/kg, and antipyrine half-live fell from 10.9±1.3h to 4.5±0.6h as compared to the values before phenytoin treatment (p<0.01). After three weeks combined use of cimetidine and phenytoin, antipyrine clearance was decreased to 1.01±0.07 ml/min/kg and antipyrine half-life was prolonged to 6.1±0.5h, (p<0.01) compared to the values on phenytoin alone. The distribution volume of antipyrine was not affected by phenytoin nor by cimetidine + phenytoin. The half-life of cimetidine was 2.8±0.3h in the patients on longterm phenytoin treatment. There was a significant positive correlation (p<0.001) between the increase in serum phenytoin concentration and the prolongation of antipyrine half-life caused by cimetidine. Thus, cimetidine increases serum phenytoin concentration, very probably by inhibiting its metabolism. Care should be taken in the concomitant use of cimetidine and phenytoin, and the dose of phenytoin should be modified according to the clinical symptoms and serum phenytoin concentrations.Part of this work was presented at the Joint Meeting of the Scandinavian and German Pharmacological Societies, September 1980 [14]     

Influence of allylisopropylacetamide and phenobarbital treatment on in vivo antipyrine metabolite formation in rats

1. The influence of pretreatment with allylisopropylacetamide (AIA) and phenobarbital (PB) on the pharmacokinetics and metabolite profile of antipyrine was studied in rats in vivo. Antipyrine concentrations were measured in blood and urine, and four metabolites (4-hydroxyantipyrine, norantipyrine, 3-hydroxymethylantipyrine and 4,4′-dihydroxyantipy-rine) were determined in urine.

2. Treatment with PB increased antipyrine blood clearance from 11.1 to 59.1 ml/min per kg. The clearances for production of metabolites all increased between four- and five-fold, indicating non-selective induction.

3. Treatment with AIA resulted in a reduction of antipyrine clearance to 5.6 ml/min per kg. The clearances to all four metabolites were decreased to about the same extent (52–65% of control values) indicating non-selective inhibition.

4. Treatment with AIA after PB treatment strongly inhibited drug-metabolizing enzyme activity. Blood clearance of antipyrine was reduced from 59.1 to 12.3 ml/min per kg. Clearances to the metabolites were again inhibited non-selectively (to 20–28% of PB-induced values).

5. In contrast to previous reports, AIA in this study inhibited non-induced oxidative microsomal enzyme activity. This inhibition closely resembled AIA inhibition of PB-induced cytochromes. Therefore it is concluded that in untreated rats antipyrine is predominantly metabolized by PB-types of cytochrome P-450.     

Inhibition of drug metabolism by the antimalarial drugs chloroquine and primaquine in the rat

The effect of the antimalarial drugs chloroquine (CQ) and primaquine (PQ) on rat liver microsomal drug metabolism has been studied in vitro and in vivo. After acute administration, PQ increased pentobarbitone sleeping time in a dose-related manner [control, 94.0 ± 9.4 min; 10mg/kg, 137.0 ± 2.4 min; 20mg/kg, 197.0 ± 7.5 min; 50 mg/kg, 269.0 ± 2.9 min (means ± S.E.M.)], prolonged zoxazolamine paralysis time (control, 140.0 ± 10.0 min; 50 mg/kg, 341.5 ± 25.6 min) and decreased antipyrine blood clearance from 2.17 ± 0.19 to 0.86 ± 0.12 ml/min. CQ showed no effect on pentobarbitone sleeping time or zoxazolamine paralysis time, but decreased antipyrine clearance from 2.17 ± 0.19 to 1.11 ± 0.18 ml/min. Both drugs inhibited aminopyrine N-demethylase activity, although the concentration required to produce 50% inhibition was much greater for CQ (10 mM) than for PQ (approximately 0.1 mM). Lineweaver-Burk plots showed that CQ inhibited competitively whereas PQ inhibition was apparently non-competitive. Ethoxyresorufin O-deethylase activity decreased by about 40 and 50% in the presence of CQ and PQ respectively (250 nM, equimolar with substrate). There was no evidence of induction following chronic administration of CQ and PQ (50 mg/kg/day for 4 days). There was an apparent decrease in cytochrome P-450 content and aminopyrine N-demethylase activity was decreased. These results demonstrate that PQ and CQ inhibit hepatic drug metabolism both in vitro and in vivo and that PQ appears to be the more potent inhibitor.     

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 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<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.     

Penbutolol and propranolol: a comparison of their effects on antipyrine clearance in man.

The effects of two beta-adrenoceptor antagonists, penbutolol (administered on separate occasions as (+/-)- and (-)-forms) and propranolol, on the kinetics of antipyrine were studied in eight normal subjects. At the same degree of beta-adrenoceptor blockade, as assessed by the lowering of exercise tachycardia, propranolol decreased antipyrine clearance by 31 +/- 11 s.d.% (P less than 0.001) whereas neither of the two penbutolol formulations had a significant effect. The volume of distribution of antipyrine was unchanged following any of the beta-adrenoceptor antagonist treatments. The lack of effect of penbutolol on oxidative drug metabolism is not consistent with in vitro data suggesting a relationship between the lipid solubility of beta-adrenoceptor antagonists and inhibition of metabolism.     

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.     

Effect of single and multiple doses of sulphinpyrazone on antipyrine metabolism and urinary excretion of 6-beta-hydroxycortisol

Summary Sulphinpyrazone decreases the plasma clearance of tolbutamide and S-warfarin and increases the clearance of R-warfarin, theophylline and antipyrine. In order to determine whether sulphinpyrazone is an inducer or inhibitor or both of oxidative drug metabolism, antipyrine and its metabolites as well as 6-beta-hydroxycortisol were measured in urine before, 24 h and after 23 days of chronic administration of sulphinpyrazone (4×200 mg/day). During chronic treatment sulphinpyrazone increased the ratio of 6-beta-hydroxycortisol to the 17-hydroxycorticosteroids by 70% (p<0.02). The renal clearance of the main oxidative metabolites of antipyrine (4-hydroxyantipyrine, 3-hydroxymethylantipyrine and norantipyrine) were increased after sulphinpyrazone (p<0.02). Except for norantipyrine, no change in total excretion of antipyrine and its metabolites occurred after 24 h or after 23 days. It is concluded that sulphinpyrazone induces the enzymes which metabolize antipyrine and cortisol.     

Stability and pharmacokinetics of flumazenil in the rat

The pharmacokinetics of flumazenil in the rat were determined after 2.5 mg/kg intravenous and 25 mg/kg oral administration. Following intravenous administration flumazenil was rapidly eliminated with an extremely short terminal half-life (mean±SE,n=8) of 8.3±0.3 min due to a large total blood clearance of 147±7 ml/kg/min combined with a relatively small volume of distribution at steady-state of 1.33±0.07 l/kg. After oral administration flumazenil was rapidly absorbed; however, the bioavailability was low (28±4%) and variable. Flumazenil was found to be unstable in rat blood in vitro and disappeared with a half-life (mean±SE,n=5) of 8.3±1 min and 31±4 min at body and room temperature, respectively. The blood samples were stabilized by addition of sodium fluoride (NaF) and cooling to 0°C. The samples had to be stored at –35°C when analyzed at later times. Presumably esterases in rat blood are responsible for the observed instability. A sensitive HPLC assay to measure flumazenil concentrations in small blood samples is also described.     

Effect of glutathione depletion on the in vivo inhibition of drug metabolism by agents forming an inactive cytochrome P-450 Fe(II):metabolite complex. Studies with amiodarone and troleandomycin

The relative contribution of competitive inhibition versus formation of a P-450:metabolite complex to the in vivo inhibition of drug metabolism for several agents is unclear. The present investigation examined the contribution of these two mechanisms to the in vivo inhibition of drug metabolism by amiodarone through manipulation of glutathione turnover. In vivo P-450-dependent metabolism in rats was assessed by determining antipyrine clearance. Pretreatment with amiodarone (50 mg/kg, iv) decreased antipyrine clearance with or without prior glutathione depletion. Depletion of glutathione by buthionine sulfoximine (1.6 g/kg, ip) did not enhance the magnitude of inhibition of antipyrine clearance by amiodarone. Moreover, administration of a normally subinhibitory dose of amiodarone after buthionine sulfoximine pretreatment did not influence antipyrine clearance. Similarly, depletion of glutathione via buthionine sulfoximine or diethylmaleate (1 mL/kg, po) did not influence the magnitude of inhibition caused by a single po dose of troleandomycin (500 or 350 mg/kg, respectively). These data indicate that glutathione content may not be a critical determinant for the in vivo inhibition of drug metabolism by agents which form a P-450:metabolite complex.     

Effect of rifampicin treatment on the kinetics of mexiletine

Summary To study the effects of enzyme induction on its pharmacokinetics, a single oral dose of the new antiarrhythmic agent mexiletine hydrochloride 400 mg was administered to 8 healthy volunteers before and after treatment with rifampicin 300 mg b.i.d. for ten days. The absorption and distribution of mexiletine were not changed after rifampicin, but its elimination half-life fell from 8.5±0.8 h (mean±SE) to 5.0±0.4 h (p<0.01), and its nonrenal clearance increased from 435±68 ml/min to 711±101 ml/min (p<0.01). The mean renal clearance of mexiletine did not change, but it showed an exponential correlation with urinary pH. The amount of unchanged mexiletine excreted in urine over two days decreased from 32±7 to 18±3 mg (p<0.01). The half-life of antipyrine fell from 11.8±0.4 to 5.5±0.3 h and its clearance increased from 40±3 ml to 74±3 ml/min (p<0.01). There was a significant (p<0.05) positive linear correlation between both the half-lives and the clearances of antipyrine and mexiletine. The clearances were positively correlated with serum -glutamyl transpeptidase. The results suggest that the dosage of mexiletine should be adjusted when enzyme inducing drugs are started or stopped during therapy with it.     

Ethanol-induced inhibition of hepatic uptake of propranolol in perfused rat liver and in man

Summary Studies were conducted to determine the mechanism whereby ethanol alters the hepatic disposition of propranolol. In eight isolated perfused rat livers, ethanol ( =40.1 mmol/l diminished the clearance of dl-propranolol (1.93±0.43 to 1.24±0.22 ml/min/g liver, p<0.05); increased its t_1/2 (12.8±1.5 to 20.7±3.25 min, p<0.01); and decreased the proportion metabolized (68.7±4.7% to 34.3±10.3%, p<0.01). These results suggest that ethanol could substantially increase the oral bioavailability of propranolol in humans. However, in normal human volunteers administered 80 mg of propranolol orally, alone, or preceded and followed by ethanol to maintain breath ethanol concentrations of 800–1000 mg/l, increases in propranolol AUC were smaller than anticipated. Seven subjects had increases in free propranolol AUC_0–8h (32%, range: 12–61%) (p<0.05), while total propranolol AUC_0–8h increased by a mean 22% (range: –4–+49%). Propranolol free fraction varied with time and was higher after ethanol ( =0.090 vs 0.084) (p<0.077). The extent of the propranolol-induced slowing of heart rate was not influenced by ethanol (mean decrease from baseline of 13 bpm at peak propranolol effect vs 9 bpm without ethanol); mean heart rates following propranolol with ethanol were higher at all times (mean of 7.5 bpm) (p<0.001) than after propranolol alone. Ethanol inhibits the hepatic oxidative metabolism of propranolol in vitro; however, any effect on heart rate of higher concentrations of propranolol induced by ethanol in humans is off-set by the cardio-acceleratory effect of ethanol.     

Influence of prednisolone on antipyrine elimination in patients with obstructive lung disease

Summary The influence of prednisolone on the elimination of antipyrine has been investigated. The one-sample antipyrine clearance was estimated in 23 outpatients with obstructive lung disease before and after treatment with prednisolone 30 or 50 mg/day for 7 days. During prednisolone administration antipyrine clearance decreased from 54.9±14.8 to 51.7±14.6 ml/min (mean±SD; p<0.05). The results indicate that prednisolone decreases the rate of antipyrine elimination, but not to an extent suggesting a clinically important change in hepatic drug metabolism.     

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.     

Pharmacokinetics of haloperidol in psychotic patients

Nine psychotic patients under continuous oral treatment with haloperidol were randomly given a test dose of 1.5–5 mg haloperidol orally and/or intravenously. Serum levels of haloperidol were determined by high performance liquid chromatography and serum concentration data obtained were submitted to pharmacokinetic analysis. The steady state concentration ratio between blood and plasma was determined and found to be 0.79±0.03. The blood clearance was then calculated to be 550±133 ml/min. The mean hepatic extraction ratio was intermediate (0.37). Consequently, for a drug mainly eliminated by hepatic metabolism like haloperidol, the total blood clearance and the extent of oral bioavailability can be affected by changes in hepatic blood flow, hepatic enzyme activities and drug binding. During continuous oral treatment with haloperidol, however, it can be shown that changes in the total metabolic capacity of the liver due to hepatic enzyme induction or inhibition should be important for the therapeutic effects of haloperidol. The volume of distribution at steady state (Vdss) was large (7.9±2.5 l/kg). The terminal half-life was 18.8 h after intravenous and 18.1 h after oral administration. The oral bioavailability (0.60±0.18) were in accordance with previous results in healthy subjects. A mean lag time after oral dose was 1.3±1.1 h and a longer absorption half-life (1.9±1.4 h) was found in the patients compared with healthy volunteers.     

Effects of chlorpromazine on the disposition and beta-adrenergic blocking activity of propranolol in the dog

The effects of chlorpromazine (100mg p.o., 2hr before propranolol) on the disposition and beta-adrenergic blocking actions of both intravenous (6 mg) and oral (40 mg) propranolol were studied in the dog. Chlorpromazine pretreatment significantly reduced (69%) the oral clearance of propranolol, resulting in significant increases in propranolol bioavailability (159%), and in the total beta-adrenergic blocking activity (111%) after the oral dose. The increase in the total beta-adrenergic blocking activity of oral propranolol after chlorpromazine pretreatment was mostly due to an increased contribution from the parent compound; the apparent activity from active propranolol metabolites was not affected by chlorpromazine. Chlorpromazine pretreatment had no significant influence on the systemic clearance, elimination half-life, apparent volume of distribution, and plasma binding of propranolol, or on the apparent hepatic blood flow. After intravenous propranolol, chlorpromazine pretreatment had no effect on either the total amount of beta-adrenergic blocking activity or the amount of activity attributable to active metabolites. The decreased oral propranolol clearance by chlorpromazine was seen as a shift to the left in the propranolol dose vs. AUCrelationship, eliminating the apparent nonlinear kinetic behavior of oral propranolol, and reducing the apparent oral threshold dose. Chlorpromazine's major, if not only, effect on propranolol disposition was to reduce the presystemic elimination of propranolol, possibly through inhibition of its metabolism via a pathway other than ring oxidation.This work was supported by a US Public Health Grant No. HL-22718.     

The effect of induction with phenobarbital on the kinetics and bioavailability of antipyrine in the dog

In dogs, the chronic administration of 400 mg/d of phenobarbital produced peak plasma concentrations of 78 micrograms/ml which so markedly induced the metabolism of antipyrine that the conventional schemes by which antipyrine kinetics assess induction were rendered invalid. The bioavailability of antipyrine was only 20% due to first-pass metabolism. Induction raised the observed antipyrine clearance to 31 ml/min/kg, a value approaching hepatic blood flow. If one assumes antipyrine was an ideal test compound, one would calculate that metabolism was induced by a factor of 3.8. Acknowledging that antipyrine is nonideal, one can calculate a correct induction factor of 13.6. Oral, rather than intravenous, antipyrine is the metabolism test of choice under these conditions.