In-vitro characterization of the cytochrome P450 isoenzymes involved in the back oxidation and N-dealkylation of reduced haloperidol.

In-vitro studies were performed using human liver microsomes and c-DNA-expressed human P450 isoforms to identify the cytochrome P450 isoenzyme(s) involved in the back oxidation and N-dealkylation of reduced haloperidol. Back oxidation and N-dealkylation of reduced haloperidol were assessed by measuring the formation of haloperidol and 4-(4-chlorophenyl)-4-hydroxypiperidine (CPHP), respectively. The haloperidol and CPHP formation rates as a function of substrate concentration, measured in three livers, followed monophasic enzyme kinetics. For haloperidol formation Km values ranged from 51-59 microM, and Vmax values from 190-334 pmol mg(-1) min(-1); for CPHP formation Km values were 44-49 microM, and Vmax values 74-110 pmol mg(-1) min(-1). Haloperidol and CPHP formation rates in the nine liver preparations were significantly correlated with dextromethorphan N-demethylase activity (a marker of CYP3A4 activity), but not with the CYP2D6, CYP1A2 and CYP2C9 activity. Ketoconazole and troleandomycin, inhibitors of CYP3A4, inhibited competitively both haloperidol and CPHP formation, with a Ki value lower than 0.2 microM for ketoconazole and lower than 0.3 microM for troleandomycin. Sulphaphenazole (CYP2C9), furafylline (CYP1A2) and quinidine and paroxetine (CYP2D6) gave only little inhibition (IC50 > 60 microM). CPHP and haloperidol formation were, moreover, enhanced by alpha-naphthoflavone, an effect known for CYP3A4 mediated reactions. Anti-CYP3A4 antibodies strongly inhibited haloperidol and CPHP formation, whereas CYP2D6 antibodies did not. Among the recombinant human CYP isoforms tested, CYP3A4 exhibited the highest activity with respect to haloperidol and CPHP formation rates, with no detectable effect of CYP1A2, CYP2D6 and CYP2C9. These results strongly suggest that back oxidation and N-dealkylation of reduced haloperidol in human liver microsomal preparations are mediated by CYP3A4.

Paroxetine affects metoprolol pharmacokinetics and pharmacodynamics in healthy volunteers.

OBJECTIVE: To investigate the effect of multiple-dose paroxetine intake on the stereoselective pharmacokinetics and the pharmacodynamics of metoprolol. METHODS: We conducted an open trial with two sessions in eight healthy male volunteers. Racemic metoprolol (100 mg single oral dose) was administered before and after paroxetine treatment (20 mg/day for 6 days). The (R)- and (S)-metoprolol pharmacokinetics, metoprolol metabolic ratio (MR), exercise heart rate and blood pressure were assessed for 12 (pharmacodynamic data) to 24 (pharmacokinetic data) hours after each metoprolol intake. RESULTS: Paroxetine treatment increased the mean area under the plasma concentration-time curve extrapolated to infinity (AUC) of (R)- and (S)-metoprolol significantly (169 to 1,340 ng x h/mL [P < .001] and 279 to 1,418 ng x h/mL [P < .001], respectively), with an approximately twofold increase in both maximum plasma concentration and terminal elimination half-life. Furthermore, the (S)/(R) AUC ratio was significantly decreased, from 1.72 to 1.07 (P < .001). The mean metoprolol MR was significantly increased, from 0.17 to 5.69 (P < .05). The AUC of the metoprolol-induced decrease in exercise heart rate versus time curve was increased, with 46% (P < .01) after multiple-dose paroxetine intake, reaching significance from 6 hours after metoprolol intake, illustrating a more sustained beta-blockade. Similar results were obtained for the effect on exercise systolic blood pressure. Multiple-dose metoprolol administration combined with paroxetine can lead to an accumulation of the beta-blocking (S)-enantiomer of metoprolol, possibly resulting in unacceptable bradycardia, loss of cardioselectivity, or both. CONCLUSION: Multiple-dose paroxetine intake affects both metoprolol pharmacokinetics and pharmacodynamics and suggests that when paroxetine is added to an ongoing metoprolol therapy, caution is warranted and a reduction of the metoprolol dose may be required to prevent undesired adverse effects.

Stereoselective pharmacokinetics of oxprenolol and its glucuronides in humans.

OBJECTIVE: To study the pharmacokinetics of R(+)- and S(-)-oxprenolol and their corresponding glucuronide conjugates in healthy subjects. METHODS: An oral dose of 80 mg racemic oxprenolol was given to eight male volunteers. Venous blood samples and urine were collected as a function of time. Oxprenolol enantiomers in plasma and urine were determined by an enantiospecific HPLC method. Oxyprenolol glucuronides in plasma and urine were measured as oxprenolol equivalents after enzymatic hydrolysis. RESULTS: For R-oxprenolol the area under the plasma concentration-time curve was slightly higher (R/S ratio, 1.19) and the oral clearance slightly lower (R/S ratio, 0.84) than those parameters for S-oxprenolol. The free fraction of R-oxprenolol in plasma was 4% higher than that of S-oxprenolol. The intrinsic clearance of S-oxprenolol was 1.5 times larger than that of R-oxprenolol, and a maximum of 3% of the dose was excreted as unchanged enantiomers in the urine. The plasma concentrations of S-oxprenolol glucuronide were more than three times higher than those of R-oxprenolol glucuronide. Twenty-five percent of the dose of the R-enantiomer was excreted in the urine as R-oxprenolol glucuronide; 29% of the S-enantiomer dose was excreted as S-oxprenolol glucuronide. The renal clearance of R-oxprenolol glucuronide was, on average, 172 ml/min, suggesting active tubular secretion. In contrast, the renal clearance of S-oxprenolol glucuronide was only 49 ml/min, which can be explained by the plasma binding of the compound. CONCLUSIONS: Our results show small differences in disposition between R- and S-oxprenolol but a marked difference in disposition between the glucuronides. The difference in plasma concentrations between the oxprenolol glucuronides is mainly attributable to the stereoselectivity of the renal excretion.

Stereoselective pharmacokinetics of atenolol in the rat: influence of aging and of renal failure.

The pharmacokinetics of R- and S-atenolol after intravenous administration of racemic atenolol were studied in 3-, 12- and 24-month-old rats and in 3-month-old rats with renal failure induced by uranyl nitrate. In all age groups, the area under the plasma concentration-time curves is higher for R- than for S-atenolol; volume of distribution, total clearance and renal clearance are lower for R-atenolol than for S-atenolol, but the differences are small. In function of age there is for both enantiomers a significant increase in AUC, due, at least in part, to a decreased renal clearance; the effect of aging is not stereoselective. In rats with renal failure, the AUC of both enantiomers increases, due mainly to a decrease in renal clearance, but to a lesser degree also to a decrease in nonrenal clearance. For both enantiomers, the volume of distribution decreases and the half-life increases in the uraemic rats. The total amount of both enantiomers excreted in the urine is decreased in the rats with renal failure. There are no stereoselective effects of treatment of the rats with uranyl nitrate.

Influence of age on stereoselective pharmacokinetics and metabolism of hexobarbital in the rat.

The influence of age on stereoselective pharmacokinetics and in vitro metabolism of R- and S-hexobarbital was studied in the rat. After intravenous administration of the racemate, the plasma concentrations of S-hexobarbital are markedly lower than those of R-hexobarbital. For S-hexobarbital the half-life is somewhat shorter and the volume of distribution and plasma clearance is higher than for its antipode. For both enantiomers an increase in AUC and half-life, and a decrease in clearance are observed with aging. These changes occur mainly between the 3rd and the 12th month and are slightly more pronounced for R- than for S-hexobarbital, as appears from the S/R ratios. The volume of distribution shows no changes with aging. In vitro disappearance rate in 3-month-old rats is significantly higher for S- than for R-hexobarbital. There is for both enantiomers an increase in disappearance rate in 12-month-old rats as compared to younger or older rats, but this is significant only for the R-enantiomer. There are pronounced differences in the kinetics and metabolism of both hexobarbital enantiomers; changes with aging occur, but are only slightly and not always significantly more important for R- than for S-hexobarbital.

Binding to serum alpha 1-acid glycoprotein and effect of beta-adrenoceptor antagonists in rats with inflammation.

The beta-blocking effect of 4 beta-adrenoceptor antagonists with different pharmacokinetic properties was studied after intravenous and intraportal administration to control rats and to rats with experimental inflammation. In rats with inflammation the effects of propranolol and oxprenolol, which are mainly bound to alpha 1-acid glycoprotein (alpha 1-AGP), were significantly less after intravenous administration, but not after intraportal administration. In contrast, for metoprolol and atenolol, which are only negligibly serum bound, no difference was observed between control rats and rats with inflammation for either route of administration. Total and unbound serum concentrations of propranolol were measured 20 min after intravenous and intraportal administration. After intravenous administration, in the rats with inflammation total concentrations of propranolol were more than twice, and unbound concentrations less than half those of control rats. After intraportal administration the total concentrations were 8 times, and the unbound concentrations 3 times higher in the rats with inflammation. There was a significant correlation between the beta-blocking effect and the unbound concentrations of propranolol after intravenous administration, but not after intraportal administration. The latter finding is probably because the unbound concentrations were supramaximal.

Binding of oxprenolol and propranolol to serum, albumin and alpha 1-acid glycoprotein in man and other species.

Species differences in binding of basic drugs have only occasionally been studied and we have therefore measured the binding of the beta-adrenergic blockers oxprenolol and propranolol in (1) serum of healthy humans, dogs, rats and rabbits and of rabbits with experimental arthritis, (2) a solution of albumin of these species and (3) a solution of human alpha 1-AGP. In humans, dogs, rats and arthritic rabbits, binding of oxprenolol and propranolol was much higher in serum than in albumin solution; in healthy rabbits serum binding was very low and not different from albumin binding. For both drugs, concentration-dependency was seen in serum of dogs, humans and rats and of arthritic rabbits; a similar concentration-dependency was found for human alpha 1-AGP solution, but not for human albumin and for serum of healthy rabbits. Tris (2-butoxyethyl)-phosphate (TBEP), a known displacer of drugs from alpha 1-AGP in humans, decreased binding in serum of all species except the rabbit. For both beta-blockers, species differences in capacity constants were found; species differences in affinity constants were present only for propranolol. These results suggest that in humans, dog and rat, but much less in rabbits, oxprenolol and propranolol bind mainly to alpha 1-AGP and that binding to alpha 1-AGP is more important for oxprenolol than for propranolol.

Influence of human recombinant interleukin-1 beta on the enantioselective disposition of propranolol in rats.

The influence of i.v. administration of 10 micrograms/kg recombinant human interleukin-1 beta (rhIL-1 beta), a putative mediator of inflammation, on the pharmacokinetics and metabolism of the propranolol enantiomers was studied in rats aged 3, 12 and 24 months. After oral administration of rac-propranolol to control rats of the three age groups, the plasma concentrations of (R)-propranolol were higher than those of (S)-propranolol. Administration of IL-1 beta increased the plasma concentrations of the (R)-enantiomer markedly and significantly, those of the (S)-enantiomer only to a lesser degree. For both enantiomers an important increase in plasma binding was found in the IL-1 beta-treated rats, which was linked to the increase in alpha 1-acid glycoprotein levels. The in vitro clearance, measured in 3-month-old rats using the 9000 g liver fraction, was for neither of the propranolol enantiomers influenced by IL-1 beta treatment, which is in keeping with the unchanged cytochrome P450 content. The enantioselective influence of IL-1 beta treatment on the pharmacokinetics of propranolol was also present in 12- and 24-month-old rats, although somewhat less pronounced in the latter group. Our results show an enantioselective influence of IL-1 beta treatment on the pharmacokinetics of propranolol in the rat, favouring the (R)-enantiomer.

Binding of diltiazem to albumin, alpha 1-acid glycoprotein and to serum in man.

Binding of drugs can vary considerably. Therefore, binding of the calcium antagonist diltiazem was studied in protein solutions and in serum of healthy persons, patients with renal failure, patients with cirrhosis, patients with rheumatoid arthritis, patients with myocardial infarction, and intensive care patients. The effect of in vitro addition of some cardiovascular drugs (lidocaine, disopyramide, quinidine, and bupivacaine) on the binding of diltiazem in serum of healthy volunteers was also investigated. Diltiazem is bound as well to alpha 1-acid glycoprotein (AAG) as to albumin. In patients with renal failure, myocardial infarction, and rheumatoid arthritis and in intensive care patients, AAG concentrations are increased, and in the patients with myocardial infarction an increased binding of diltiazem is found. In patients with cirrhosis, AAG concentrations and diltiazem binding are decreased. In vitro addition of lidocaine, disopyramide, bupivacaine, or quinidine in concentrations between 5 and 100 micrograms/mL, before dialysis, decreases the binding of diltiazem; the displacing effect is most pronounced with bupivacaine.

Meptazinol (Wy 22811), a new analgesic: preliminary pharmacokinetic data.

The authors report on the pharmacokinetic profile of a new analgesic, meptazinol, from studies of plasma and urine levels after intravenous injection in 4 healthy volunteers. Conjugated product is formed very soon after injection and its concentration changes little over the period studied compared with the unchanged product where a fast, then a relatively slow phase of decline can easily be distinguished, suggesting a two compartment open system model. Meptazinol is predominantly eliminated in this conjugated form by urinary excretion of the metabolites. Excretion is very rapid in the first few hours after dosing and elimination is almost completed after 24 hours.

Glycan microheterogeneity of alpha 1-acid glycoprotein in sera and dialysates of patients on continuous ambulatory peritoneal dialysis.

Total concentration and concanavalin (Con A) dependent microheterogeneity of alpha 1-acid glycoprotein (AGP) were studied in sera of eight chronic renal failure patients before the start of continuous ambulatory peritoneal dialysis (CAPD), and in sera and dialysate fluids for up to 6 months of CAPD. The glycan heterogeneity of AGP in the samples was expressed as a reactivity coefficient, i.e. the ratio of the Con A-reactive AGP components to the Con A non-reactive AGP component. Concentrations of AGP in serum and reactivity coefficients were markedly elevated in non-dialysed uraemic patients. AGP concentrations in serum increased further during the first week on CAPD, and then gradually decreased to pre-dialysis values, which were reached after 1 to 6 mth. The reactivity coefficients did not change significantly during the CAPD treatment. Dialysate AGP concentrations were low in comparison to those in serum, and there was a good correlation between the reactivity coefficients in the dialysate fluids and those in the corresponding sera. The effect of peritonitis was evaluated in a separate group of eight CAPD patients. Serum and dialysate AGP concentrations were significantly higher during than after peritonitis while the corresponding reactivity coefficients were only slightly elevated.

The effect of different calcium antagonists and a calcium agonist on the metabolism of propranolol by isolated rat hepatocytes.

The influence of the dihydropyridine calcium entry blockers nicardipine, amlodipine, nifedipine, isradipine and of the dihydropyridine calcium entry promotor BAY K 8644 on the disappearance rate of propranolol by isolated rat hepatocytes was compared to the effect of diltiazem and verapamil, two non-dihydropyridine calcium channel blockers and known inhibitors of hepatic cytochrome P450 mixed function oxidases. All compounds dose-dependently inhibited the disappearance rate of propranolol. Nicardipine and isradipine were more potent in inhibiting the disappearance rate of propranolol than the other dihydropyridines and than diltiazem and verapamil. The inhibitory effect of nicardipine on the disappearance rate of propranolol was not stereoselective and was not influenced by age.

Effect of turpentine-induced inflammation on the disposition kinetics of propranolol, metoprolol, and antipyrine in the rat.

Plasma concentrations after oral administration of the high extraction drug propranolol are increased in patients and animals with inflammation. This could be due to increased serum propranolol binding, but also to decreased first-pass metabolism. We studied the pharmacokinetics of 3 drugs in control rats and in rats with turpentine-induced inflammation: propranolol, which is bound extensively to alpha 1-acid glycoprotein (alpha 1-AGP); metoprolol, another high extraction drug, but which is negligibly bound to alpha 1-AGP; and antipyrine, a low extraction drug, not bound to serum proteins. After IV administration of propranolol in rats with inflammation, systemic clearance, volume of distribution, and free fraction decreased, and the area under the curve (AUC) increased, whereas the half-life did not change. As the systemic clearance of a high extraction drug such as propranolol depends on hepatic blood flow only, a fall in hepatic blood flow or transition to a low extraction situation should be postulated. After oral administration of propranolol, the AUC was increased 20-fold in rats with inflammation; as the decrease in free fraction was only 4-fold, it can be concluded that a considerable decrease in hepatic intrinsic clearance was present. For metoprolol, in contrast to propranolol, after IV administration, no changes in pharmacokinetic parameters as a result of inflammation were observed. After oral administration, the AUC was increased about 4 times in rats with inflammation; as metoprolol is only negligibly bound to serum proteins, the increase in AUC can be attributed to a decrease in hepatic intrinsic clearance.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of hemorrhagic shock on the pharmacokinetics and the analgesic effect of morphine in the rat.

The influence of hemorrhagic shock (removal of 30% of the blood volume) on the pharmacokinetics and the analgesic effect of morphine was investigated in conscious rats. Plasma concentrations of morphine after a bolus injection (5 mg/kg) are higher in the shock animals, which is attributed to a small decrease in clearance (-22%; P > 0.05) and a significant decrease in distribution volume (-33%; P < 0.05) of the drug. The areas under the plasma concentration-time curve of the metabolite morphine-3-glucuronide (M3G) are significantly higher (+237%; P < 0.01) in the shock rats, which is probably explained by a decreased distribution and renal excretion. The analgesic effect of morphine was evaluated using the tail-flick test during a continuous infusion (10 mg/kg/h) with measurement of the plasma concentrations of morphine and M3G. Data from these experiments show higher plasma concentrations of morphine (+33%; P < 0.05) and M3G (+66%; P > 0.05) during shock, and a significantly increased analgesic effect (+43%; P < 0.05). Our data suggest that the increased analgesic effect of morphine during hemorrhagic shock can most likely be explained by pharmacokinetic changes resulting in higher morphine concentrations.

High-performance liquid chromatographic determination of metoprolol in plasma.

A high-performance liquid chromatographic method is presented for the determination of metoprolol in human plasma. Metoprolol was extracted from plasma by a single extraction procedure with 4-methylpropranolol as the internal standard. Chromatography was done on a reversed-phase column with fluorescence detection. The minimum detectable concentration was 5.0 ng/ml of plasma. The standard curve was linear in the range evaluated, 10-300 ng/ml. The within-run coefficient of variation was 2.3-6.0%, and the day-to-day variation was 6.8%. The method is free from interference by major metoprolol metabolites.

Oral diazoxide contraindicated in severe hypertension with renal failure.

Diazoxide was given orally to nine hypertensive patients with renal failure and its effect on blood pressure and on glucose metabolism was studied. There was no long-term antihypertensive effect. During treatment insulin release and glucose assimilation after an intravenous glucose load were frankly impaired, but this impairment was reversible after stopping the treatment. Two major complications (diabetic ketoacidosis and pancreatitis) were observed. In view of these observations, the authors are of the opinion that oral diazoxide is contraindicated in the treatment of hypertension in patients with renal failure.

High performance liquid chromatography determination of dextromethorphan and its metabolites in urine using solid-phase extraction.

A high performance liquid chromatography assay coupled with fluorescence detection was developed for the determination of dextromethorphan and its metabolites in urine. The products and the internal standard, pholcodine, were separated on an Alltima C18, 5 microns column (250 x 4.6 mm), using a mobile phase containing sodium dodecyl sulphate (1 mM) in a mixture of acetonitrile-sodium dihydrogen phosphate (0.01 M) 40.5:59.5, v/v) (pH* = 2.5). A novel solid-phase extraction procedure with strong cation exchange, non end-capped, Isolute SCX cartridges allows good recovery of the products (mean 85% or more). For all analytes, the assay is sensitive (LOQ 25 ng ml-1, using 200 microliters urine), reproducible (RSD < 15%) and accurate (< 15% deviation of the nominal value) over the range evaluated. This method can be used to measure dextromethorphan and its metabolites to phenotype individuals as poor or extensive metabolizers of drugs metabolized by CYP2D6.

Effect of inflammation and proadifen on the disposition of antipyrine, lignocaine and propranolol in rat isolated perfused liver.

The effect of inflammation, induced in rats by injection of turpentine oil, on drug disposition has been evaluated in rat isolated perfused livers. The drugs studied were a low extraction drug, antipyrine, and two high extraction drugs, lignocaine and propranolol. Turpentine significantly increased the half-life of antipyrine and of propranolol, but not that of lignocaine. Proadifen (SKF 525A) significantly increased the half-life of all three drugs. Turpentine decreased the clearance of antipyrine significantly by about 50% and that of propranolol non-significantly by about 20%, but did not affect the clearance of lignocaine. Proadifen significantly decreased the clearance of all three drugs, but this was most pronounced for antipyrine. In both turpentine- and proadifen-treated rats a significant increase in volume of distribution of propranolol was observed. The results show that, as with proadifen, turpentine-induced inflammation affects the hepatic clearance of antipyrine in the rat isolated perfused liver. With both high extraction drugs, the effect of inflammation on their clearance was low or absent, in contrast to the effect of proadifen. This suggests that a possible effect of inflammation on intrinsic clearance is not large enough to influence the hepatic clearance of the high extraction drugs.

In-vitro biotransformation of antipyrine, lignocaine and propranolol in the liver of rats with turpentine-induced inflammation.

In rats with inflammation induced by turpentine injection, changes in drug disposition occur in-vivo and in the perfused isolated liver. Therefore the biotransformation of a low extraction drug, antipyrine, and of two high extraction drugs, lignocaine and propranolol, has been evaluated in the 9000g supernatant fraction of the liver of turpentine-treated rats. Aminopyrine N-demethylase activity and cytochrome P450 content were also measured. Turpentine treatment significantly reduced the in-vitro breakdown of the three drugs; aminopyrine N-demethylase activity and cytochrome P450 content were also decreased. Similar results were found in the proadifen-treated rats, except that in those, the cytochrome P450 content was slightly increased. The changes in drug disposition seen after turpentine-induced inflammation, could therefore be due in part to a change in hepatic enzymatic activity.

Influence of lignocaine on plasma protein binding and pharmacokinetics of verapamil in dogs.

The effect of lignocaine (lidocaine) on the plasma protein binding of verapamil has been studied in-vitro and in-vivo in dogs. The binding of verapamil was ca 85%. In-vitro addition of lignocaine at therapeutic concentrations displaced verapamil from its plasma binding sites. Lignocaine in this regard was equipotent with tris(2-butoxyethyl)phosphate, suggesting an interaction at the level of alpha 1-acid glycoprotein binding sites. On in-vivo administration of 4 mg kg-1 in a bolus to dogs in which steady state concentrations of verapamil were present, the free fraction of verapamil increased transiently. During the lignocaine maintenance infusion, it then decreased to a level higher than that before administration of the local anaesthetic. The free verapamil concentrations increased suddenly upon the administration of the lignocaine loading dose, and then returned to values slightly higher than those before lignocaine. After a bolus injection of verapamil during a lignocaine infusion, the verapamil total plasma concentrations were lower than during a saline infusion, but the free concentrations were not different. The volume of distribution of verapamil was increased, whereas the blood clearance had not changed; the lignocaine infusion did not change the hepatic blood flow, as measured by indocyanine green clearance. These results show that lignocaine displaces verapamil in-vitro and in-vivo from its plasma protein binding sites, but the ensuing pharmacokinetic changes do not lead to significant changes in free verapamil concentrations.