Enantioselective pharmacokinetics of the enantiomers of clevidipine following intravenous infusion of the racemate in essential hypertensive patients

The aim of the study was to characterize the individual pharmacokinetics of (-)-R- and (+)-S-clevidipine following intravenous constant rate infusion of rac-clevidipine to essential hypertensive patients. Twenty patients received three out of five randomized treatments with clevidipine. The pharmacokinetics of the separate enantiomers were evaluated by compartmental analysis of blood concentrations vs. time curves using the population approach. The derived pharmacokinetic parameters were used to simulate the time for 50 and 90% postinfusion decline following various infusion times of rac-clevidipine. A two-compartment model was used to describe the dispositions of the enantiomers; there were only minor differences between the estimated pharmacokinetic parameters of the separate enantiomers. The mean blood clearance values of (-)-R- and (+)-S-clevidipine were 0.103 and 0.096 l/min/kg, and the corresponding volumes of distribution at steady state were 0.39 and 0.54 l/kg, respectively. The context-sensitive half-time was approximately 2 min regardless of stereochemical configuration, and a 90% decline in concentration was achieved approximately 8 min postinfusion for (-)-R-clevidipine and 11 min for (+)-S-clevidipine, following clinically relevant infusion times with clevidipine. In conclusion, both enantiomers are high-clearance compounds with similar blood clearance values. The volume of distribution for the enantiomers is slightly different, presumably due to differences in the protein binding. From a pharmacokinetic point of view, the use of a single enantiomer as an alternative to the racemic clevidipine will not offer any clinical advantages.

Affinities at the verapamil binding site of MDR1-encoded P-glycoprotein: drugs and analogs, stereoisomers and metabolites

OBJECTIVES

The multidrug transporter P-glycoprotein (P-gp) appears to play a significant role in drug absorption and disposition. Hence, it is of interest to evaluate structure-affinity relationships for the purpose of making predictions.

METHODS

The affinity to P-gp of related molecular structures from various groups of drugs was determined using competitive radioligand-binding assays. Structural analogs, stereoisomers and metabolites of verapamil-type calcium antagonists, beta-adrenoceptor antagonists as well as omeprazole, omeprazole enantiomers and the sulfone metabolite of omeprazole were investigated.

RESULTS

Whereas some stereoselectivity was detected for the high-affinity P-gp substrates verapamil and carvedilol, little or no differences were observed in the case of other beta-blockers. One of the 4 labetalol stereoisomers, the R,R-isomer dilevalol, had an IC50 value half that of labetalol.

CONCLUSIONS

Metabolites of verapamil, gallopamil, carvedilol and omeprazole are characterized by having higher IC50 values (lower P-gp affinity) than the respective parent compounds. Only the acebutolol metabolite, diacetolol, had a P-gp affinity comparable to that of the parent compound.

Pharmacokinetics and arteriovenous differences in clevidipine concentration following a short- and a long-term intravenous infusion in healthy volunteers

BACKGROUND

Clevidipine is an ultra-short-acting calcium antagonist developed for reduction and control of blood pressure during cardiac surgery. The objectives of the current study were to determine the pharmacokinetics of clevidipine after 20-min and 24-h intravenous infusions, and to determine the relation between the arterial and venous concentrations and the hemodynamic responses to clevidipine in healthy volunteers.

METHODS

Four volunteers received clevidipine for 20 min, and eight subjects were administered clevidipine intravenously for 24 h at two different dose rates. Arterial and venous blood samples were drawn for pharmacokinetic evaluation, and blood pressure and heart rate were recorded.

RESULTS

A triexponential disposition model described the pharmacokinetics of clevidipine. The mean arterial blood clearance of clevidipine was 0.069l/kg-1/min-1 and the mean volume of distribution at steady state was 0.19 l/kg. The duration of the infusion had negligible effect on the pharmacokinetic parameters, and the context-sensitive half-time for clevidipine, simulated from the mean pharmacokinetic parameters derived after 24 h infusion at the highest dose, was less than 1 min. The arterial blood levels reached steady state within 2 min of the start of infusion and were about twice as high as those in the venous blood at steady state. The peak response preceded the peak venous concentration and was slightly delayed from the peak arterial blood concentration.

CONCLUSION

Clevidipine is a high clearance drug with a small volume of distribution, resulting in extremely short half-lives in healthy subjects. The initial rapid increase in the arterial blood concentrations and the short equilibrium time between the blood and the biophase suggest that clevidipine can be rapidly titrated to the desired effect.

Caco-2 versus Caco-2/HT29-MTX co-cultured cell lines: permeabilities via diffusion, inside- and outside-directed carrier-mediated transport

PURPOSE

The objective of this study was a systematic characterization and evaluation of cell culture models based on mixtures of Caco-2/HT29-MTX co-cultures for their use in screening for drug absorption and intestinal permeability in comparison to the properties of the respective mono-cultures.

METHODS

Co-cultures of Caco-2 cells (absorptive-type) and HT29-MTX cells (goblet-type) were set up. Three different co-cultures (initial seeding ratios Caco-2/HT29-MTX: 90/10, 70/30, and 50/50) were grown on permeable filter supports, and monolayers were used for permeability studies with model compounds for paracellular absorption (atenolol, furosemide, H334/75, mannitol, terbutaline), transcellular absorption (antipyrine, ketoprofen, metoprolol, piroxicam), carrier-mediated absorption (D-glucose, Gly-Pro, and L-phenylalanine) as well as substrates for carrier-mediated secretion via P-glycoprotein (cimetidine and talinolol). Electrophysiological and microscopic controls were performed to characterize the cell cultures.

RESULTS

For compounds undergoing passive intestinal absorption permeabilities were generally higher in co-cultures than in Caco-2 monolayers, yielding highest values in pure HT29-MTX monolayers. This difference was most obvious for compounds transported via the paracellular pathway, where HT29-MTX cells may be up to 30 times more permeable than Caco-2 cells, whereas for lipophilic and highly permeable compounds, the difference in permeability values was less obvious. For drugs undergoing intestinal secretion mediated by P-glycoprotein, co-cultivation of Caco-2 cells with HT29-MTX cells led to increased apical to basolateral permeability which was decreased in the opposite direction, consistent with the fact that HT29-MTX cells do not express P-glycoprotein. When a carrier-mediated absorption mechanism is involved, the permeabilities observed were lower than the values reported for human small intestine and co-cultivation of HT29-MTX cells with Caco-2 cells resulted in even lower values as compared to the plain Caco-2 cultures.

CONCLUSIONS

Co-cultures of HT29-MTX and Caco-2 cells offer the opportunity of modifying the permeability barrier of the cell monolayers both with respect to paracellular resistance and secretory transport via P-gp. Thus, in special cases, they allow more flexibility in adapting the in vitro system to the in vivo situation as compared to the monocultures. Another advantage is the obvious robustness of the method with respect to the reproducibility of the results. A problem remaining, however, is the quantitative expression of carriers involved in intestinal uptake of many nutrients and drugs.

Role of P-glycoprotein-mediated secretion in absorptive drug permeability: An approach using passive membrane permeability and affinity to P-glycoprotein

It has been shown in vivo and in vitro that P-glycoprotein (P-gp) may be able to influence the permeability of its substrates across biological membranes. However, the quantitative contribution of the secretion process mediated by P-gp on the overall permeability of membranes has not been determined yet. In particular, observations need to be clarified in which substrates showing high affinity to P-glycoprotein, e.g., verapamil, apparently do not seem to be greatly influenced by P-gp in their permeability and consequently also with respect to their extent of GI-absorption after oral administration, whereas weaker substrates of P-gp, e.g., talinolol, have clearly shown P-gp-related absorption phenomena such as nonlinear intestinal permeability and bioavailability. Experiments with Caco-2 cell monolayers and mathematical simulations based on a mechanistic permeation model should aid in clarifying the underlying mechanism for these observations and quantifying the influence of passive membrane permeability and affinity to P-gp to the overall transmembrane drug flux. In addition, the concentration range of drug at which P-glycoprotein-mediated transport across the biological membrane is relevant should be examined. The permeability of various drugs in Caco-2 monolayers was determined experimentally and modeled using a combination of passive absorptive membrane permeability and a Michaelis-Menten-type transport process in the secretory direction. The passive permeabilities were experimentally obtained for the apical and basolateral membrane by efflux experiments using Caco-2 monolayers in the presence of a P-gp inhibitor. The Michaelis-Menten parameters were determined by a newly developed radioligand-binding assay for the quantification of drug affinity to P-gp. The model was able to accurately simulate the permeability of P-glycoprotein substrates, with differing passive membrane permeabilities and P-glycoprotein affinities. Using the outlined approach, permeability vs donor-concentration profiles were calculated, and the relative contribution of passive and active transport processes to the overall membrane permeability was evaluated. A model is presented to quantitatively describe and predict direction-dependent drug fluxes in Caco-2 monolayers by knowing the affinity of a compound to the exsorptive transporter P-gp and its passive membrane permeability. It was shown that a combination of high P-gp affinity with good passive membrane permeability, e.g., in the case of verapamil, will readily compensate for the P-gp-mediated reduction of intestinal permeability, resulting in a narrow range in which the permeability depends on the apical drug concentration. On the other hand, the permeability of compounds with low passive membrane permeability (e. g., talinolol) might be affected over a wide concentration range despite low affinity to P-gp.

Pharmacokinetics of new calcium channel antagonist clevidipine in the rat, rabbit, and dog and pharmacokinetic/pharmacodynamic relationship in anesthetized dogs

Clevidipine is a new vascular selective calcium channel antagonist of the dihydropyridine type, structurally related to felodipine. Clinical trials have shown that the drug can be used to effectively control the blood pressure in connection with cardiac surgical procedures. The compound is tailored to be a short-acting drug and, due to incorporation of an ester linkage into the drug molecule, clevidipine is rapidly metabolized by ester hydrolysis. The pharmacokinetics of clevidipine and its primary metabolite, H 152/81, were studied in rats, rabbits, and dogs. In addition, the influence of the pharmacokinetics on the effect on mean arterial blood pressure was evaluated in anesthetized dogs. Compartmental nonlinear mixed effect regression analysis was used to calculate the population mean and individual pharmacokinetics of clevidipine, whereas nonlinear regression analysis of individual data was used to determine the pharmacokinetics of the primary metabolite. A linked Emax model was fitted to the individual pharmacodynamic/pharmacokinetic data in dogs. According to the results, clevidipine is a high-clearance drug with a relatively small volume of distribution, resulting in an extremely short half-life in all species studied. The median initial half-life of the individual value (Bayesian estimates) is 12, 20, and 22 s in the rabbit, rat, and dog, respectively. The primary metabolite is a high-clearance compound in the dog, whereas it is a low-clearance compound in the rat. A significant gender difference in the clearance of the metabolite was observed in the rat. The mean maximum reduction in arterial blood pressure is 38 +/- 12% (Emax) and is achieved at 85 +/- 46 nM (EC50). The half-life for reaching equilibrium between the central and the effect compartment (T1/2ke0) is 47 +/- 49 s.

Clinical and pharmacokinetic results with a new ultrashort-acting calcium antagonist, clevidipine, following gradually increasing intravenous doses to healthy volunteers

AIMS

To investigate the tolerability and safety of clevidipine in healthy male volunteers during intravenous infusion at gradually increasing dose rates and to obtain preliminary information on the pharmacokinetics and pharmacodynamic effects of the drug.

METHODS

Twenty-five subjects were enrolled in the study and twenty-one of them were included twice, resulting in a total of forty-six study entries encompassing 20 min infusions of clevidipine at target dose rates ranging from 0.12 to 48 nmol min-1 kg-1. Haemodynamic variables and adverse events were recorded throughout the study. Concentrations of clevidipine and its primary metabolite, H 152/81, were followed in whole blood, and the pharmacokinetics were evaluated by non-compartmental and compartmental analysis. An Emax model was fitted to the effect on mean arterial pressure (MAP) over heart rate (HR) and the corresponding blood concentrations of clevidipine.

RESULTS

Clevidipine was administered up to a target dose rate of 48 nmol min-1 kg-1, where a pre-determined escape criterion was reached (HR>120 beats min-1 ) and the study was stopped. The most common adverse events were flush and headache, which can be directly related to the mechanism of action of clevidipine. There was a linear relationship between blood concentration and dose rate in the range studied. The median clearance value determined by non-compartmental analysis was 0.125 l min-1 kg-1. Applying the population approach to the sparse data on clevidipine concentrations, an open two compartment pharmacokinetic model was found to be the best model in describing the disposition of the drug. The population mean clearance value determined by this method was 0.121 l min-1 kg-1, and the volume of distribution at steady state was 0.56 l kg-1. The initial half-life, contributing by more than 80% to the total area under the blood concentration-time curve following i.v. bolus administration, was 1.8 min, and the terminal half-life was 9.5 min. At the highest dose rates, MAP was reduced by approximately 10%, and the HR reached the pre-determined escape criterion for this study (>120 beats min-1 ).

CONCLUSIONS

Clevidipine is well tolerated and safe in healthy volunteers at dose rates up to at least 48 nmol min-1 kg-1. The pharmacokinetics are linear over a wide dose range. Clevidipine is a high clearance drug with extremely short half-lives. The effect of clevidipine on the blood pressure was marginal, probably due to a compensatory baroreflex activation in this population of healthy volunteers. A simple Emax model adequately describes the relationship between the pharmacodynamic response (MAP/HR) and the blood concentrations of clevidipine.

In vitro hydrolysis rate and protein binding of clevidipine, a new ultrashort-acting calcium antagonist metabolised by esterases, in different animal species and man

The objectives of this study were to investigate the protein binding and the in vitro hydrolysis rate of clevidipine and its enantiomers in the rat, dog and man in different biological matrices including blood and plasma from volunteers with deficient pseudocholinesterase activity. The in vitro half-life in blood was 0.6 min (rat), 15.7 min (dog) and 5.8 min in man with normal pseudocholinesterase activity, while the half-life was approximately 9 min in blood from pseudocholinesterase deficient volunteers. The half-life in pseudocholinesterase deficient volunteers was prolonged, although the hydrolysis rates in blood and red blood cells (RBC) were much higher than in plasma, suggesting that esterases located in the RBC are most important in the blood metabolism of clevidipine. A decrease in temperature increased the half-life of clevidipine in blood, whereas dilution of the blood did not affect the in vitro half-life of clevidipine. The albumin concentration affected the hydrolysis rate of clevidipine in RBC suspended with saline. The protein binding of clevidipine and its enantiomers was >99.5% in plasma from all species studied. There was a difference between the free fractions of S- and R-clevidipine in man, 0.43 and 0.32%, respectively, and this stereoselective binding might be the reason for the 10% difference between the in vitro hydrolysis rates of the enantiomers in human blood.

Pharmacokinetics and pharmacodynamics of clevidipine in healthy volunteers after intravenous infusion

OBJECTIVE

To determine the pharmacokinetics and pharmacodynamics of clevidipine, a new ultrashort-acting calcium antagonist, in healthy male volunteers following a constant rate infusion.

METHODS

Eight healthy male volunteers received 1030 nmol x min(-1) of clevidipine together with a tracer dose of 3[H]-clevidipine for 1 h as an i.v. infusion. Frequent venous blood samples and effect recordings were obtained during ongoing infusion and up to 32 h following termination of the infusion. The excretion of radioactivity in urine and faeces was followed for 7 days.

RESULTS

A two-compartment model gave the best fit to the individual clevidipine blood levels, resulting in a mean blood clearance of 0.14 (0.03) l x min(-1) x kg(-1) and a mean volume of distribution at steady state of 0.6 (0.1) l x kg(-1). The initial half-life was 1.6 (0.3) min, and the terminal half-life was 15 (5) min. The maximum concentration of the metabolite H 152/81 was reached 2.2 (1.3) min following termination of the infusion. The mean terminal half-life of the inactive primary metabolite was 9.5 (0.8) h and the mean recovery of the radioactive dose reached 83 (3)%. Following termination of the 1 h infusion, the effect on blood pressure (BP) and heart rate was back to pre-dose values within 15 min.

CONCLUSION

Clevidipine is a high clearance drug, which is rapidly metabolized to the corresponding inactive acid. The tmax value of the primary metabolite, and a virtually identical value of the initial half-life and the half-life for elimination from the central compartment, indicate that the initial rapid decline of the post-infusion blood levels is mainly due to elimination rather than distribution. The duration of action of clevidipine is short.

Characterization of binding properties to human P-glycoprotein: development of a [3H]verapamil radioligand-binding assay

Interaction with the exsorptive transporter P-glycoprotein (P-gp) is a possible source of peculiarities in drug pharmacokinetics, including dose-dependent absorption, drug-drug interactions, intestinal secretion, and limited permeability of the blood-brain barrier. Among the established in vitro methods of the analysis of drug interactions with P-gp, none directly quantifies the affinity of ligands with P-gp. Instead, they measure the result of a membrane permeation and a receptor-binding process; this may lead to difficulties in the interpretation of results. An assay for quantification of drug affinity to the transporter is presented on the basis of the radioligand-binding assay principle. This has the advantage of directly quantifying the interaction between drugs and P-gp. Because of the reversible and competitive interaction of numerous substrates with P-gp, a radioligand-binding assay was developed by taking [3H]verapamil and [3H]vinblastine as radioligands and the human intestinal Caco-2 cells, overexpressed with P-gp by culturing in the presence of vinblastine or transfecting with multidrug resistance gene MDR-1 as receptor preparation. The assay was performed in 96-well plates and has the potential to be used as a high-throughput method. A clear induction of the expression of P-gp was demonstrated in the Caco-2 cells grown in the presence of vinblastine, as well as in the transfected cells, although to a lesser extent. Both radioligands were shown to bind to P-gp. Verapamil was the radioligand of choice for further investigations due to its lower nonspecific binding to the transporter preparation. Kinetics as well as specificity of the binding of verapamil to the P-gp preparation were demonstrated. A two-affinity model was found to adequately describe the data derived from saturation as well as from competition experiments, in accordance with previous findings on two exsorption sites for P-gp. The binding properties of [3H]verapamil and [3H]vinblastine to a P-gp preparation derived from induced Caco-2 cells are described. The concentration-dependent displacement of the radioligand by nonlabeled substrates for P-gp should be a suitable principle for the determination of drug affinity to the respective binding sites at the human intestinal multidrug transporter P-gp.

Radioligand-binding assay employing P-glycoprotein-overexpressing cells: testing drug affinities to the secretory intestinal multidrug transporter

PURPOSE

To develop a rapid and reliable system for affinity determination of conventional as well as newly synthesized compounds to P-gp.

METHODS

The principles of radioligand-binding assay were adapted to the human intestinal P-gp. Acceptor protein was obtained from the human carcinoma cell line Caco-2, where overexpression of P-gp was induced by growing cells in the presence of the cytostatic drug vinblastine. 3H-Verapamil was chosen as radioligand.

RESULTS

The saturability and specificity of 3H-verapamil as the radioligand for the binding to P-gp was demonstrated. From concentration dependence of displacement of the radioligand by various non-labeled ligands for P-gp, affinity constants to P-gp binding sites were calculated. The binding results obtained were in agreement with those published earlier where influx and efflux experiments with cell monolayers had been conducted in order to functionally characterize the P-gp -drug interaction.

CONCLUSIONS

A radioligand-binding assay on the basis of P-gp overexpressing Caco-2 cells has been developed. The method might be suitable for high-throughput screening of drug interaction with human P-gp. It will allow modeling of the interaction of drugs with the human multidrug transporter and has also the potential to serve as a high-throughput screening tool to detect compounds prone to P-gp mediated intestinal secretion and potential P-gp related drug/drug interactions in drug discovery and early development.

The interaction effect of grapefruit juice is maximal after the first glass

OBJECTIVE

To compare the acute effect of grapefruit juice intake on the pharmacokinetics and haemodynamic effects of felodipine ER tablets with the interaction after 14 days intake of drug with juice.

METHODS

Twelve healthy male volunteers were included in this cross-over trial and randomly allocated to a daily intake of a 10-mg felodipine extended release tablet with water or grapefruit juice for 14 days. The two study periods were separated by at least 14 days. The pharmacokinetics of felodipine and dehydrofelodipine, as well as the haemodynamic effects of the drug, were studied during day 1 and 14 in each period.

RESULTS

Similarly to previous single-dose studies, the treatment during the first day with grapefruit juice increased the AUC (+73%) and Cmax (+138%) of felodipine when compared with the control treatment. On day 14 a similar effect of grapefruit juice was observed, with an increased AUC24 (+57%) and Cmax (+114%) of felodipine compared with the control experiment. A significant accumulation of felodipine occurred during both the control (+37%) and grapefruit juice (+25%) period. The extent of accumulation was not significantly different in the two treatment periods. The pharmacokinetics of the metabolite dehydrofelodipine were affected to a similar extent by the juice on day 1 and day 14. The first dose of felodipine together with grapefruit juice was associated with a significant additional increase in heart rate when compared with the control therapy, whereas there was no additional effect on blood pressure when therapy included grapefruit juice. On day 14 the intake of drug with juice resulted in an additional increase in heart rate and reduction in diastolic blood pressure in comparison with the control experiment. Furthermore, the vascularly related adverse events were more frequent in the period including grapefruit juice.

CONCLUSION

The interaction between grapefruit juice and felodipine appears to be already fully developed after the first glass of grapefruit juice, as the change in pharmacokinetics in comparison with the control experiment is similar on day 1 and on day 14. Concomitant intake of 10 mg felodipine ER and the juice is associated with increased haemodynamic effects in healthy subjects both after a single dose and following 14 days of concomitant intake.

Animal pharmacokinetics of inogatran, a low-molecular-weight thrombin inhibitor with potential use as an antithrombotic drug

Pharmacokinetics, excretion, and metabolism of inogatran, a low-molecular-weight thrombin inhibitor, were studied in the rat, dog, and cynomolgus monkey. After intravenous administration the half-life was short in all three animal species, due to a small volume of distribution and a relatively high clearance. At doses of 0.1-5 mumol kg-1, the mean residence time was about 10 min in the rat, 35 min in the dog, and 20 min in the cynomolgus monkey. The oral bioavailability of inogatran was incomplete, presumably due to a low membrane permeability, and dose dependent. The bioavailability was 4.8% at 20 mumol kg-1 and 32-51% at 500 mumol kg-1 in rats, 14% at 10 mumol kg-1 and 34-44% at 150 mumol kg-1 in dogs, and 2.1% at 1 mumol kg-1 in cynomolgus monkeys. The radioactivity excreted in urine and faeces was predominantly unchanged inogatran. After intravenous administration the percentage of the radioactivity recovered in faeces was about equal to or higher than the urinary recovery, which indicates biliary excretion of inogatran. After oral dosing, most of the dose was excreted in faeces, as expected from the estimates of oral bioavailability. The plasma protein binding of inogatran in rat, dog, and human plasma, was 20-28%. The blood-plasma concentration ratio was 0.39-0.56, indicating limited distribution into red blood cells.

Effects of grapefruit juice ingestion--pharmacokinetics and haemodynamics of intravenously and orally administered felodipine in healthy men

OBJECTIVE

To examine the effect of grapefruit juice on the metabolism of felodipine following intravenous and oral administration.

METHODS

The study had a randomised, four-way, crossover design in 12 healthy males. Single doses of felodipine were given as an intravenous infusion for 1 h (1.5 mg) or as an oral extended release (ER) tablet (10 mg). Grapefruit juice (150 ml) or water was ingested 15 min prior to drug intake.

RESULTS

Intake of grapefruit juice did not significantly alter the intravenous pharmacokinetics of felodipine compared to control treatment, whereas after oral drug administration it did lead to an increase in the mean AUC and Cmax by 72% and 173%, respectively, and the mean absolute bioavailability was increased by 112%. The fraction of the oral felodipine dose reaching the portal system was increased from 45% to 80% when intake of drug was preceded by grapefruit juice ingestion. The pharmacokinetics of the primary metabolite, dehydrofelodipine, was affected by the intake of juice, resulting in a 46% increase in Cmax. Juice intake immediately before oral felodipine resulted in more pronounced haemodynamic effects of the drug as measured by diastolic blood pressure and heart rate. However, the haemodynamic effects of the intravenous administration were not altered by juice intake. Vascular-related adverse events were reported more frequently when oral drug administration was preceded by juice intake compared with control treatment. Taking grapefruit juice immediately prior to intravenous felodipine administration did not cause any alteration in the adverse event pattern.

CONCLUSION

The main acute effect of the grapefruit juice on the plasma concentrations of felodipine is mediated by inhibition of gut wall metabolism.

Relationship between time of intake of grapefruit juice and its effect on pharmacokinetics and pharmacodynamics of felodipine in healthy subjects

In this randomised, cross-over study, in nine healthy males given felodipine ER 10 mg PO 200 ml grapefruit juice was found to increase the plasma levels of felodipine even when the juice was taken 24 hours before the drug. Grapefruit juice drunk simultaneously with and 1, 4, 10 or 24 hours before the drug administration resulted in a 32-99% increase in mean Cmax values of felodipine, relative to concomitant water and felodipine intake. The effect on AUC was also significant when juice was taken up to 10 h before the drug. The effect of the interaction decreased with increasing time between juice and drug intake. All treatments produced a significant decrease in diastolic blood pressure and an increase in heart rate in comparison with morning basal values. The change in haemodynamic variables was approximately the same after all treatment combinations. Headache was reported more frequently after treatments including grapefruit juice.

Interethnic differences in omeprazole metabolism in the two S-mephenytoin hydroxylation phenotypes studied in Caucasians and Orientals

Two independent studies showed that the 5-hydroxylation, but not the sulfoxidation, of omeprazole is more rapid in extensive metabolizers (EMs) than in poor metabolizers (PMs) of S-mephenytoin. In Caucasian, Chinese, and Korean PMs, the mean oral clearances were similar and not significantly different (85, 73, and 59 ml h-1 kg-1, respectively). However, the geometric mean clearance in Caucasian EMs (950 ml h-1 kg-1) was higher than in both Chinese EMs (426 ml h-1 kg-1, p < 0.05) and Korean EMs (446 ml h-1 kg-1, p < 0.01). The incidence of PMs of S-mephenytoin is higher in Chinese (14.6%) and Koreans (12.6%) than in Swedish Caucasians (3.3%). Therefore, the proportion of heterozygous compared to homozygous EMs is higher in Orientals than in Caucasians. This might explain the higher clearance of omeprazole in Caucasian EMs compared to the clearance of omeprazole in Chinese and Korean EMs of S-mephenytoin.

Central nervous and systemic kinetics of ramipril and ramiprilat in the conscious dog

The central nervous and systemic kinetics of ramipril, an angiotensin-converting enzyme inhibitor prodrug, and its active metabolite ramiprilat were studied in conscious beagle dogs after sampling of cerebrospinal fluid (CSF) and blood during chronic drug administration. The cardiovascular effect of angiotensin-converting enzyme inhibitors have been suggested to be mediated partly by central action. Ramiprilat and ramipril were determined in CSF and plasma by a gas chromatographic-mass spectrometric assay method. After 10 mg/kg of ramipril, given orally to four dogs once daily for 7 days, significant concentrations of ramiprilat were measured in CSF over the 24-h period after both the 1st and 7th day of treatment. The CSF/plasma (unbound) ratios of ramiprilat on day 7 were (mean +/- S.D.): 0.01 +/- 0.003 (2 h after dose), 0.18 +/- 0.05 (12 h after dose) and 0.32 +/- 0.11 (24 h after dose). Measurable concentrations of ramipril were recorded in plasma after oral dosing (bioavailability approximately 45%), whereas in CSF the prodrug concentration was below the minimal determinable levels in most cases. In a second set of experiments, ramiprilat (3 mg/kg) or ramipril (3 mg/kg) were given i.v. to three dogs once daily for 7 days. Ramipril was rapidly cleared from the plasma, clearance being approximately 140 ml/min/kg and half-life about 0.5 h on day 7. The corresponding values for ramiprilat were 8 ml/min/kg and 0.75 h. The CSF/plasma ratios for ramiprilat were essentially the same after i.v. administration of ramiprilat and ramipril and, furthermore, the ratios did not differ significantly from the ratios observed after oral administration of the prodrug.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for an interaction between the beta-blocker pafenolol and bile salts in the intestinal lumen of the rat leading to dose-dependent oral absorption and double peaks in the plasma concentration-time profile

Pafenolol is a beta-blocker with unusual oral absorption properties. The blood concentration-time profile exhibits two peaks, and the bioavailability is low and dose dependent because of incomplete and nonlinear intestinal uptake. We addressed the question whether the intestinal absorption of pafenolol was affected by bile depletion in the gut lumen of rats. Further, the hypothesis that variable gastric emptying accounts for double peaks in blood was tested by duodenal administration of pafenolol. Following intraduodenal administration to rats with intact bile secretion, double peaks were observed in the blood concentration-time curve. The bioavailability was 6.8 +/- 0.7% for the low dose (1 mumol/kg) and increased significantly to 28 +/- 10% following the high duodenal dose (25 mumol/kg). These blood concentration-time profiles exclude interrupted gastric emptying as cause of the twin peaks. In bile duct-cannulated rats the intestinal absorption of the low dose (1 mumol/kg) was still poor (F = 10.7 +/- 5.5%) and the blood concentration-time profile contained two peaks. Following administration of a high duodenal dose (25 mumol/kg) to rats with an almost bile-free small intestine, the absorption rate increased and the double-peak phenomenon disappeared in five of seven rats, while the bioavailability increased significantly, to 62 +/- 27%. These results suggest that the low bioavailability of pafenolol is due to a complexation between bile and pafenolol in the gut lumen, preventing intestinal uptake in the major part of the small intestine. Further, such complex formation in the intestinal lumen may be the underlying mechanism of the double peaks observed in the blood concentration-time profile.

Dose-dependent intestinal absorption and significant intestinal excretion (exsorption) of the beta-blocker pafenolol in the rat

The elimination of [3H]pafenolol and metabolites was investigated in fasted and fed rats. Separate groups received intravenous doses (0.3 and 3.0 mumol/kg) and oral doses (1 and 25 mumol/kg). After iv administration of pafenolol, the excretion of unchanged drug into urine and feces was about 50 and 25-30% of the given dose, respectively. The predominating mechanism for the excretion of pafenolol into feces was intestinal excretion (exsorption) directly from blood into gut lumen, since only about 3% of a given iv dose was recovered as pafenolol in the bile. When the oral dose was raised from 1 to 25 mumol/kg, the mean (+/- SD) bioavailability, calculated from urine data, increased from 14 +/- 9 to 30 +/- 11% (P < 0.05) in the starved rats and from 14 +/- 3 to 16 +/- 3% in the fed animals. In parallel, the fraction absorbed from the gut (fa) increased from 19 +/- 9 to 31 +/- 10% in the starved rats and from 16 +/- 4 to 19 +/- 5% in the fed animals, respectively. This indicates that the low bioavailability is due primarily to poor intestinal uptake.

Presystemic elimination of the beta-blocker pafenolol in the rat after oral and intraperitoneal administration and identification of a main metabolite in both rats and humans

Pafenolol is a beta 1-adrenoreceptor antagonist exhibiting some interesting oral absorption properties in both rat and humans. The blood concentration-time profile exhibits two peaks, and the bioavailability is low and dose-dependent due to an incomplete and nonlinear intestinal uptake. The origin of the presystemic metabolism was studied in rats after oral and intraperitoneal administration of tritium-labeled pafenolol with reference to the intravenous route by means of urinary excretion data of pafenolol and metabolites specifically assayed by HPLC and radioisotope detection. The oral-bioavailability increased from 15.8 +/- 4.1% (1.0 mumol/kg) to 33.3 +/- 5.8% (25 mumol/kg, p < 0.001). This was primarily due to a change in the fraction of the absorbed dose (fa) from 21.9 +/- 4.6 to 39.5 +/- 7.9% (p < 0.01). The bioavailability following an intraperitoneal dose was almost complete indicating that the presystemic metabolism was due to gut wall metabolism. Saturation of the presystemic metabolism contributed only by approximately 15-20% to the 2-fold increase of bioavailability. This clearly indicates that the underlying mechanism for the low and dose-dependent bioavailability was an incomplete and nonlinear intestinal uptake. The metabolic pattern showed that at least eight metabolites are formed in the rat. One of these is an alpha-OH pafenolol, identified as the main metabolite in human urine by mass spectrometry.

Pharmacokinetics of [14C]omeprazole in patients with liver cirrhosis

The pharmacokinetics of omeprazole and its metabolites following single doses were studied in 8 patients with liver cirrhosis. Each patient participated in 2 experiments in which [14C]omeprazole was administered either intravenously (20mg) or in an oral solution (40mg) in a randomised crossover design. Plasma concentrations of omeprazole and 2 of its identified metabolites, as well as total radioactivity were followed for 24h; urinary excretion was followed for 96h. The mean elimination half-life of omeprazole in the patients with cirrhosis was 2.8h and the mean total plasma clearance was 67 ml/min (4.02 L/h); corresponding values from separate studies in young healthy volunteers were 0.7h and 594 ml/min (35.64 L/h). The mean systemic availability was nearly 100% in the patients with cirrhosis whereas the previously reported value in young volunteers was only 56%. Almost 80% of a given dose was excreted as urinary metabolites in both patients and young volunteers. It is concluded that, as the hepatic clearance of omeprazole was substantially reduced in these patients, the dose of omeprazole needed for a certain degree of acid suppression is lower in patients with liver cirrhosis.

Regional gastrointestinal absorption of the beta-blocker pafenolol in the rat and intestinal transit rate determined by movement of 14C-polyethylene glycol (PEG) 4000

The gastrointestinal absorption characteristics of pafenolol following oral administration as a solution in man and rat has previously been found to be a double-peak phenomenon and exhibited dose-dependent bioavailability, despite negligible presystemic metabolism. In both man and rat the first peak appeared approximately 0.5-1 hr postdose and the second, more pronounced peak 3-4 hr postdose. In rat more than 90% of the available dose was absorbed during the second peak. In the present study we investigated the absorption of a solution of pafenolol in rats after intrajejunal and intraileal administration. The resulting blood concentration-time profile of pafenolol exhibited one peak only; the extent of absorption was similar to that observed when the same dose was given orally. The small intestinal transit time of the 14C-PEG 4000 solution was found to be more than 3 hr. The transit rate was higher in the proximal part of the small intestine compared to the more distal part, where the transit of the solution was staggered. In conclusion, the results of the intestinal transit time investigation and the administrations of pafenolol at different levels of the alimentary tract indicate that pafenolol is a drug with a specific absorption site located in the ileocolonic region.

Pharmacokinetic study of omeprazole in elderly healthy volunteers

The pharmacokinetics of omeprazole and its metabolites were studied in 8 healthy elderly volunteers using [14C]omeprazole. In another 6 healthy elderly volunteers, the pharmacokinetics of omeprazole were studied using unlabelled drug. Each volunteer received single doses of omeprazole intravenously (20mg) and orally (40mg) as solutions in a randomized crossover design. The plasma concentrations and urinary excretion of omeprazole and metabolites were followed for 24 and 96h, respectively. The results indicate that the average metabolic capacity of omeprazole is decreased in the elderly compared with that found in earlier studies of healthy young individuals. This was reflected in an increase in bioavailability from 56 to 76%, a reduction in mean systemic clearance by approximately 50% (0.25 L/min) and a prolongation of the mean elimination half-life from 0.7 to 1.0h compared with the young. Despite these findings, the considerable overlap in these parameters between young and old volunteers, together with data from previous pharmacodynamic studies and the wide therapeutic range of omeprazole, indicate that dosage reductions are not needed in the elderly.

The effect of felodipine on bile flow in pentobarbital anaesthetized rats and conscious rats receiving bile salt supplementation

Felodipine, a vasoselective dihydropyridine calcium antagonist, has been given i.v. (0.2 mumol/kg) to anaesthetized and conscious male rats. There was no effect of pentobarbital anaesthesia on bile flow over a 6 h observation period. Felodipine increased the 6 h recovery of bile by approximately 25% in the conscious rat but in the anaesthetized rat there was a 20% decrease in bile flow following i.v. felodipine. A positive effect of Na-taurocholate infusion (1 mumol/min/100 g body weight) on bile flow in conscious rats was reinforced by concomitant felodipine dosing. Accumulated 6 h recoveries were 2.32 +/- 0.80 g/100 g body weight (control), 3.09 +/- 0.91 g/100 g body weight (taurocholate) and 5.00 +/- 0.80 g/100 g body weight. (taurocholate plus felodipine). The excretion of felodipine in the bile was significantly reduced during anaesthesia and during infusion of 2% bovine serum albumin (0.01 ml/min/100 g body weight) to conscious rats.

Inhibition of dihydropyridine metabolism in rat and human liver microsomes by flavonoids found in grapefruit juice

The effects of naringenin, quercetin and kaempferol, flavonoids found in grapefruit as glycosides, on the metabolism of nifedipine and the enantiomers of felodipine were studied in microsomes from rat and human liver. Flavonoid concentrations of 10, 50 and 100 mumol/l were added to rat liver microsomes. The metabolism of nifedipine, (R)- and (S)-felodipine was inhibited to a similar extent, and the inhibition was dependent on the chemical structure and the concentration of flavonoid. Naringenin had lower inhibitory potency than quercetin and kaempferol. These flavonoids exhibited the same order of inhibitory potency in human liver microsomes. No inhibition of naringenin was found, however, until higher concentrations, 300 and 500 mumol/l, were added. A likely mechanism is inhibition of cytochrome P-450 IIIA4, the isoenzyme that catalyzes the oxidation of the dihydropyridine ring to form the corresponding pharmacologically inactive pyridine metabolite. This is a predominant metabolic step that determines the extent of first-pass extraction of dihydropyridines. Grapefruit juice has been shown recently to increase the p.o. bioavailability of the dihydropyridine calcium antagonists nifedipine and felodipine. The interaction may be explained by an inhibition of the first-pass metabolism by flavonoids in grapefruit juice. Furthermore, the results indicate that the rat may be used for in vivo studies of interactions between flavonoids and dihydropyridines or other drugs that are metabolized by cytochrome P-450 IIIA4.

Acute effects of drinking grapefruit juice on the pharmacokinetics and dynamics of felodipine--and its potential clinical relevance

The effect of drinking grapefruit juice on the acute pharmacokinetic and haemodynamic actions of the dihydropyridine calcium antagonist felodipine given as a 5 mg plain tablet has been studied in nine, healthy, middle-aged males. Compared to water, grapefruit juice caused an increase in Cmax from mean 6 to 16 nmol.l-1, and in the AUC from 23 to 65 nmol.h.l-1. The change in AUC corresponded to an increase in the systemic availability of felodipine from 15 to 45%, assuming no change in its clearance. This change was probably caused by inhibition of the oxidation of felodipine to the inactive dehydrofelodipine by flavonoids in grapefruit juice. The interaction with grapefruit juice is believed to be a class effect for the dihydropyridines, as oxidation of the dihydropyridine ring to the corresponding pyridine derivative is a major metabolic route for all these drugs. The higher plasma concentrations of felodipine taken with grapefruit juice resulted in a greater change in blood pressure measured in the morning 3 h after dosing (-9%) than did water (0%).

Polymorphic hydroxylation of S-mephenytoin and omeprazole metabolism in Caucasian and Chinese subjects

Twelve Caucasian healthy men and women, of whom six were poor metabolizers (PMs) and six were extensive metabolizers (EMs) of S-mephenytoin, together with 13 Chinese healthy men and women (five PMs and eight EMs), received a single oral 20 mg dose of omeprazole. Plasma levels of omeprazole and its two main metabolites, omeprazole sulphone and hydroxyomeprazole, were determined by HPLC. The mean (+/- SD) area under the plasma concentration-time curve (AUC) for omeprazole was 11.1 +/- 2.6 and 0.9 +/- 0.4 microM h in the Caucasian PMs and EMs, respectively. Corresponding values for elimination half-life were 2.3 +/- 0.4 and 0.7 +/- 0.4 h. In the Chinese PMs and EMs the AUC of omeprazole as 13.3 +/- 5.6 and 2.6 +/- 1.8 microM h. The AUCs of omeprazole were significantly higher in the Chinese EMs than in the Caucasian EMs possibly due to the higher proportion of heterozygotes in the former than in the latter group. The elimination half-life in the Chinese PMs and EMs was 2.4 +/- 0.2 and 0.8 +/- 0.2 h--similar to the observations in the Caucasian subjects. The maximum plasma concentration of hydroxyomeprazole was five-fold and four-fold higher in EMs compared to PMs among Caucasians and Chinese, respectively. The elimination half-life of the hydroxy metabolite was also longer in PMs than in EMs in both populations. The ratio between AUC for omeprazole and AUC for hydroxyomeprazole was 11.9 +/- 2.1 and 0.6 +/- 0.1 in Caucasian PMs and EMs, respectively. Corresponding values in the Chinese were 13.1 +/- 2.9 and 1.6 +/- 0.5.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of four carboxylic acid metabolites of felodipine in plasma by high-performance liquid chromatography

A reversed-phase high-performance liquid chromatography method with ultraviolet detection at 220 nm was developed to determine four carboxylic acid metabolites in plasma following therapeutic doses of the calcium antagonist felodipine. After the addition of an internal standard the analytes were isolated by liquid-liquid and solid-phase extraction. The metabolites were applied to a C2 cartridge in their free acid form, but they were transformed and retained as ion pairs with tetrabutylammonium during a wash with phosphate buffer (pH 7), prior to automated elution and injection by the Varian AASP system onto the analytical C18 column. Using a sample volume of 1 ml of plasma, the lower limit of determination for the metabolites was about 20 nmol/l. The influence of the pH of the mobile phase on the retention time of the metabolites and the structural requirements for the internal standard were studied. The method was applied to plasma samples from four dogs collected after an oral dose of felodipine. The plasma concentration-time profiles of the metabolites gave useful information about the mechanisms by which they were formed and eliminated.

Stereoselective metabolism of felodipine in liver microsomes from rat, dog, and human

Felodipine, a 1,4-dihydropyridine derivative, is a potent, vasoselective calcium antagonist that is used clinically as a racemic mixture of two stereoisomers. In the rat, dog, and human, the bioavailability of an oral dose is about 15% because of high first-pass metabolism. Oxidation of the dihydropyridine ring to the corresponding achiral, pharmacologically inactive pyridine metabolite is the predominant metabolic step. In the liver, this metabolism is catalyzed by cytochrome P-450. In the present study, the metabolism of (R)- and (S)- felodipine was compared in vitro in liver microsomes from rats, dogs, and humans. Slightly higher rates of metabolism were found for the (S)-enantiomer in rat and dog liver microsomes. However, no significant differences were observed in the Michaelis-Menten parameters, Vmax or KM. In human liver microsomes, the (R)-enantiomer was metabolized more readily than (S)-felodipine. The mean value of KM was lower for (R)-felodipine, while the Vmax values of the enantiomers were similar. The intrinsic clearance, defined as the ratio of Vmax and KM, was about two times higher for (R)-felodipine. Assuming complete absorption and that the bioavailability is determined by the first-pass metabolism in the liver, these in vitro results suggest that the bioavailability of (S)-felodipine in vivo is about two times higher than that of (R)-felodipine.

Pharmacokinetics of felodipine in chronic hemodialysis patients

Five chronic hemodialysis patients (1 woman and 4 men, aged, 46-68 yr) were given an oral dose of 10 mg felodipine followed by 0.057 mg [3H] felodipine IV. After 5 hours, a hemodialysis treatment lasting 4 hours was performed. Blood and dialysate flows were 200 mL/min and 500 mL/min, respectively. Capillary dialyzers with 1.3 m2 cellulose acetate membrane were used. The pharmacokinetic characteristics and reduction in diastolic BP were similar to those in hypertensive patients with normal renal function and in uremic patients who were not treated with dialysis. There was no measurable removal of felodipine by hemodialysis. Dialyzer clearance of radioactive metabolites was about 10 mL/min, and only 8.9% of the dose was eliminated by the treatment. The half-life of radioactive metabolites was 10 days (6-14 days) in three patients dialyzed thrice weekly. Since the metabolites are biologically inactive, no adjustment of dose is required in hemodialysis patients.

Pharmacokinetics of the enantiomers of felodipine in the dog after oral and intravenous administration of a pseudoracemic mixture

1. A pseudoracemic mixture of deuterated (S)-felodipine and unlabelled (R)-felodipine was administered as single i.v. or oral doses to four dogs. Plasma concentrations of the enantiomers and their corresponding pyridine metabolites were determined by g.l.c.-mass spectrometry. 2. No isotope effects were observed after oral administration of equimolar amounts of deuterated and unlabelled (S)-felodipine. 3. The pharmacokinetic parameters of the enantiomers were similar after i.v. administration, indicating that the disposition of felodipine was not stereoselective. 4. After oral administration the bioavailability of (R)-felodipine was slightly higher than that of (S)-felodipine in two of the dogs, presumably due to a lower first-pass extraction of the (R)-enantiomer, while no difference was observed in the other two dogs. 5. No substantial differences in Cmax or AUC were observed between the deuterated and unlabelled pyridine metabolites, indicating that the oxidative clearances of the felodipine enantiomers were similar.

Lack of effect of omeprazole treatment on steady-state plasma levels of metoprolol

In a randomised double-blind crossover study, seven healthy males were concomitantly given metoprolol 100 mg o.d. as a controlled release formulation, and omeprazole 40 mg o.d. or placebo, for 8 days. Plasma levels of the R- and S-enantiomers of metoprolol were determined on the 8th day of each treatment. The subjects were also characterised by their metabolic capacity to hydroxylate debrisoquine. Concomitant omeprazole treatment had no significant influence on the steady-state plasma levels of the two enantiomers of metoprolol. All subjects were characterised by extensive debrisoquine hydroxylation, i.e. extensive metoprolol metabolism. As metoprolol is metabolised to a great extent by debrisoquine hydroxylase (IID6), it is concluded that concomitant omeprazole treatment will probably have a negligible influence on the metabolism of the relatively large number of drugs mainly metabolised by this isoenzyme of the cytochrome P450 family.

Pharmacokinetics of a single intravenous and oral dose of pafenolol--a beta 1-adrenoceptor antagonist with atypical absorption and disposition properties--in man

The pharmacokinetics of pafenolol were studied in eight young healthy individuals. The doses were 10 mg iv and 40 mg orally. Each dose was labeled with 100 microCi [3H]pafenolol. The plasma concentration-time curve of the oral dose exhibited dual maxima. The second peak was about four times higher than the first one. Maximum concentrations were attained after 0.9 +/- 0.2 and 3.7 +/- 0.6 hr. The mean bioavailability (F) of the oral dose was 27.5 +/- 15.5%. The reduction in F was due mainly to incomplete gastrointestinal absorption. The drug was rapidly distributed to extravascular sites; t1/2 lambda 1 was 6.6 +/- 1.8 min. The volumes of distribution were Vc = 0.22 +/- 0.08 liter/kg, Vss = 0.94 +/- 0.17 liter/kg, and Vz = 1.1 +/- 0.16 liters/kg. The iv dose of pafenolol was excreted in unchanged form in the urine to 55.6 +/- 5.1% of the given dose and in the feces to 23.8 +/- 5.7% within 72 hr. The corresponding recoveries of the oral dose were 15.8 +/- 5.9 and 67.0 +/- 10.2%, respectively. About 10% of both doses was recovered as metabolites in the excreta. Approximately 6% of the oral dose was metabolized to nonabsorbable compounds in the intestine. The mean total plasma clearance was 294 +/- 57 ml/min, of which renal clearance, metabolic clearance, and gastrointestinal and/or biliary clearance were responsible for 165 +/- 31, 31 +/- 15, and 95 +/- 32 ml/min, respectively. The half-life of the terminal phase determined from plasma levels up to 24 hr after dosing was 3.1 +/- 0.3 hr for the iv dose and 6.7 +/- 0.7 hr for the oral dose.

Acute and steady-state pharmacokinetics and antihypertensive effects of felodipine in patients with normal and impaired renal function

Eighteen patients (14 men and 4 women, aged 36-74 years) treated with metoprolol for a month were included in the study. Twelve had impaired renal function (IRF) with a glomerular filtration rate (GFR) of 7.5-77.1 mL/min and six having normal renal function (NRF) with a GFR of 84.9-113.0 mL/min. Plasma and urine concentrations of felodipine and metabolites, heart rate, and blood pressure were recorded over 24 hours on day 1 after an oral dose of 10 mg felodipine and 0.04 mg 3H-felodipine IV and repeated on day 29 during continuous treatment with felodipine, 10 mg bid. The bioavailability of the oral dose on day 1 and day 29 was 13% and 12.5%, respectively. The terminal plasma half-life (t1/2) on day 29 was 22 hours and systemic clearance was 490 mL/min on day 1 and 434 mL/min on day 29 (NS). There were no differences in these parameters between NRF and IRF. The protein binding determined by equilibrium dialysis in the six patients with the lowest GFR was 99.74% on day 1 and 99.73% on day 29 and did not differ significantly from previously reported values in healthy subjects. The mean supine blood pressure before the acute dose of felodipine was 164/96 mm Hg in the IRF patients and 145/95 mm Hg in the NRF patients. A maximum decrease of 37/22 mm Hg and 32/19 mm Hg, respectively, was seen within 1.5 hours after dose and at 12 hours the reduction was 12/9 and 15/10 mm Hg, respectively, compared to baseline values. At steady state the morning blood pressure before dose was 152/87 mm Hg in the IRF patients and 129/86 mm Hg in the NRF patients. Similar maximum decreases and effects at 12 hours were seen after dose on day 29 as on day 1. Data on the effect on diastolic blood pressure and plasma felodipine concentrations were well fitted to the Emax model. The maximum reduction in diastolic blood pressure using this model was 27% and the plasma concentration leading to 50% of the maximum effect was 6.2 nmol/L. In conclusion, renal disease does not affect the pharmacokinetics of felodipine. The pharmacokinetic and pharmacodynamic effects of felodipine are not altered during steady state. The renal excretion of inactive metabolites is reduced in IRF. However, the accumulation of metabolites in the blood does not affect the protein binding or the clearance of felodipine. No dosage adjustment of felodipine seems to be necessary in patients with hypertension and renal impairment.

Pharmacokinetics of pafenolol in the rat: a suitable model for studying absorption mechanisms of a drug exhibiting unusual absorption properties in man

The pharmacokinetics of pafenolol, a highly selective beta 1-adrenoceptor antagonist, have been studied in starved and unstarved rats. Separate groups received intravenous doses (0.3 and 3.0 mumol kg-1) and oral doses (1 and 25 mumol kg-1). The systemic clearance of pafenolol was constant in the dose range investigated where the absolute oral bioavailability increased from 15 to 27 per cent in the starved and from 9.1 to 21 per cent in the unstarved rats, when the oral dose was raised from 1 to 25 mumol kg-1. The blood concentration profile after an oral solution of pafenolol exhibited two peaks in the majority of the rats. The major part of the absorption was associated with the second peak which appeared about 4 h after dosing in both starved and unstarved rats. Food lowered the degree of bioavailability and shifted the tmax1 to about 1 hour compared to half an hour in starved rats. The low bioavailability was primarily due to incomplete uptake from the gastrointestinal tract. Our study shows that pafenolol is absorbed in a similar way to that in man. The mechanisms behind the dose-dependent bioavailability and the two peaks in the absorption profile after an oral solution will be further explained in the rat.

Pharmacokinetics of various single intravenous and oral doses of omeprazole

The influence of dose on the kinetics of omeprazole and two of its metabolites, hydroxyomeprazole and the sulphone, has been studied. Ten healthy subjects were given omeprazole 10 and 40 mg iv and 10, 40 and 90 mg orally. No significant dose-related difference in any parameter calculated from the iv experiments was detected. Following the oral solutions, however, there was a dose-dependent increase in systemic availability, probably due to saturable first-pass elimination. The AUC of the sulphone also seemed to increase non-linearly with increasing dose, and that of the hydroxyomeprazole increased in proportion to dose. The slight dose-dependency of the bioavailability of the solution is considered to be of no or limited clinical relevance. Furthermore, since omeprazole is given orally as slowly absorbed enteric coated granules in the dose of 20 mg o.d., the potential for dose-dependent kinetics in clinical practice would be much less than in the present study.

Biliary secretion of felodipine metabolites in man after intravenous [14C]felodipine

The biliary secretion of [14C]felodipine in 4 healthy human subjects was studied by use of the multiple marker dilution principle with double lumen tubes placed in the stomach and intestine. Insignificant amounts of 14C activity were recovered from gastric aspirates. The individual recovery from intestinal aspirates varied from 2.9 to 8.5% of the dose of radioactivity over the period of 4.5 h after dosing. Less than 0.1% was identified as unchanged felodipine. The results show that biliary secretion is a minor route of elimination of felodipine or its metabolites. Bile collection for 4.5 h had no significant effect on the pharmacokinetics of felodipine, although the 72 h urinary recovery of radioactivity tended to be lower when bile was collected (59%) than in the control experiment (66%).

The pharmacokinetics of omeprazole in humans--a study of single intravenous and oral doses

The pharmacokinetics of omeprazole, hydroxyomeprazole, omeprazolesulfone, and "remaining metabolites" have been studied in eight young healthy subjects following an acute i.v. and oral dose of 10 and 20 mg of 14C-labeled drug, respectively. The oral dose was given as a buffered solution. Two subjects exhibited essentially higher and more sustained plasma levels of omeprazole than the others. This was due to a higher bioavailability, lower clearance, and longer t1/2 of omeprazole in these two subjects. Maximum concentration (0.7-4.6 mumol/L) was reached between 10 and 25 min after oral dosing. The median bioavailability was 39% (25-117%) and the median systemic plasma clearance was 624 ml/min (range of 59-828 ml/min). The corresponding t1/2 for the i.v. dose was 35 min (16-150 min) and 39 min (14-186 min) after oral administration. The drug was rapidly distributed to extravascular sites (mean t1/2 lambda 1 = 3.0 +/- 0.8 min). Mean Vss was 0.23 +/- 0.04 L/kg. Low systemic clearance of omeprazole was associated with a decreased formation rate of hydroxyomepraxole and "remaining metabolites" while omeprazolesulfone formation seemed to be less affected. However, there was a clear-cut correlation between the t1/2 of omeprazole and of its omeprazolesulfone metabolite, indicating that the elimination of these two compounds is mediated by the same isoenzyme. The mean urinary recovery of the radioactive dose during 96 h was 78.3 +/- 2.3 and 75.7 +/- 2.6% for the i.v. and oral dose, respectively. Insignificant amounts were due to unchanged drug and omeprazolesulfone. The excretion of hydroxyomeprazole during the first 12 h varied between 4.6 to 15.5% of a given dose. The mean recovery of radioactivity in the feces was 19.3 +/- 3.1% of a given i.v. dose and 18.2 +/- 2.3% when given orally. It is concluded that omeprazole is mainly eliminated metabolically and that there is a substantial interindividual variation in the rate of formation of primary and secondary metabolites. This variation in omeprazole disposition is probably of limited clinical importance. The half-life, with a maximum of approximately 3 h, is too short to cause accumulation when the drug is administered in a once-daily regimen.

Pharmacokinetic and biopharmaceutic aspects of once daily treatment with metoprolol CR/ZOK: a review article

In the development of a new controlled release preparation and its subsequent assessment there are a number of factors that need to be considered both related to the drug itself and to the pharmaceutical preparation. This review describes the biopharmaceutical and pharmacokinetic properties of metoprolol CR/ZOK, a recently introduced formulation of a widely used beta 1-selective adrenoceptor antagonist intended for once daily usage. The metoprolol CR/ZOK preparation provides reproducible dissolution and absorption properties resulting in stable plasma concentrations with minimum fluctuations over a 24-hour dosage interval. This has been shown in extensive studies comprising over 200 healthy volunteers. Considerations for a new drug preparation such as bioavailability and variability in relation to a standard treatment and the clinical significance of taking the drug with food and in increasing doses, are potential concerns that do not seem to be a problem for the therapeutic use of metoprolol CR/ZOK. Furthermore, the pharmacokinetic data achieved in young healthy subjects appear to be relevant for the treatment of patients, as shown by the consistent plasma concentration profiles obtained in elderly and in hypertensive patients.

The relationship between metoprolol plasma concentration and beta 1-blockade in healthy subjects: a study on conventional metoprolol and metoprolol CR/ZOK formulations

Four studies using different daily doses (100 mg, 200 mg, 300 mg and 400 mg) have examined the bioequivalence of the once daily formulation metoprolol CR/ZOK and conventional metoprolol tablets (CT). These studies showed metoprolol CR/ZOK to have a similar beta 1-blocking activity to metoprolol CT but that the bioavailability of the new formulation was lower. This paper presents the analysis of data from all four studies, looking at the relationship between plasma concentrations and beta 1-blockade measured as reduction of exercise induced tachycardia. The log linear pharmacodynamic model was used for each of the four doses to compare the two formulations. Mean slopes and intercepts in the linear regression analysis of log plasma concentration of metoprolol versus beta 1-blockade did not differ significantly between CR/ZOK and CT in any of the four studies. After having shown lack of influence of the absorption rate of the plasma concentration-effect relationship, data for CR/ZOK and CT formulations from the 200 mg study were pooled for each individual subject and fitted to an Emax model. The maximal beta 1-blocking effect (Emax) was 28% (95% confidence interval: 25-31%) and the plasma concentration for obtaining 50% of Emax (C50) was 105 nmol/L (95% confidence interval: 74-135 nmol/L). The plasma concentration-effect data from the 100 mg, 300 mg and 400 mg studies were reasonably well within the 95% prediction interval based on the 200 mg study, which showed the validity of the obtained relationship.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic and pharmacodynamic properties of atenolol in acute myocardial infarction

A commonly used combined intravenous (i.v.) and oral dose regimen of atenolol was evaluated in 21 patients with acute myocardial infarction (MI). Atenolol 5-10 mg was administered i.v. and was followed by 50 mg twice daily during the first 24 h. Starting on the second day, 100 mg was administered as a single dose. Dose adjustments were made as necessary. The i.v. dose resulted in a rapid plasma peak concentration, but absorption of the first oral dose appeared to be delayed. The half-life (t1/2) was significantly prolonged on day 2 as compared with day 6, but the tmax, Cmax, and area under the curve (AUC) were all comparable to what has been reported in patients without MI. The change in heart rate (HR) or blood pressure (BP) was not related to the initial plasma level of atenolol. The interindividual variation was large, but most patients obtained plasma levels of atenolol which in studies of other diseases have been shown to be effective.

The rhesus monkey as a model for studies of pregnancy induced changes in metoprolol metabolism

Five pregnant Rhesus monkeys were catheterized in the hepatic and femoral veins. They were simultaneously given 168 or 176 micrograms of 3H-metoprolol intravenously, and 9 mg of metoprolol per kg body weight orally. The same procedure was repeated a few months after delivery. Analyses of the unlabelled drug in blood were made by gas-chromatography and of the 3H-labelled metoprolol, by liquid scintillation. The apparent volume of distribution as well as the terminal half-lives of metoprolol were in the same range during pregnancy and in non-pregnancy. The oral bioavailability of metoprolol was lower (6-22%) during pregnancy than in non-pregnancy (9-49%). The apparent oral clearance and the intrinsic hepatic clearance were in a similar range although there was a greater variation in the intrinsic clearance values. The former clearance estimate was lower in the non-pregnant state only for three of the five animals. The systemic clearance varied very little and was in the same range during pregnancy and in non-pregnancy. The changes in apparent oral clearance and in oral bioavailability of metoprolol between the pregnant and non-pregnant Rhesus monkey are similar to the changes observed in pregnant women, although the absolute values are different.

Contribution of the intestine to the first-pass metabolism of felodipine in the rat

The systemic availability of intraduodenally (i.d.) administered felodipine in the rat is about 10%. The purpose of this study was to determine to what extent intestinal metabolism contributes to the first-pass elimination of felodipine in the rat. Four different types of experiments were performed. 1) [3H]Felodipine was given i.v. and i.p.; 2) the uptake of i.p. administered [3H]felodipine by the lymph was studied for 3 hr after dosing; 3) portal blood was collected quantitatively for 40 min after i.d. administration of [3H]felodipine; and 4) the in vitro metabolism of felodipine was studied in intestinal cell suspensions. The mean bioavailability of the i.p. dose was approximately 48%. The uptake via the lymph was negligible as an insignificant amount of the radioactive i.p. dose was recovered in lymph from a main lymph vessel in the peritoneal cavity. An average of 21 +/- 12% of given radioactive dose was recovered in portal blood during the first 40 min after i.d. dosing. The recovered radioactivity was to 40 to 70% due to felodipine and 9 to 16% was due to dehydro-felodipine. These results indicate that substantial first-pass elimination occurs in the intestine of the rat. Further support for gastrointestinal metabolism of felodipine in the rat was obtained from incubations with intestinal cells.

Central nervous system kinetics of atenolol and metoprolol in the dog during long term treatment

Beagle dogs with catheters chronically implanted into the lateral cerebral ventricle were used to study the distribution of atenolol and metoprolol between the cerebrospinal fluid (CSF) and blood plasma over a 24-hr period during long term treatment. The concentration of atenolol declined more slowly in CSF than in blood plasma and the CSF/plasma ratio of atenolol (after iv administration for 7 days) increased from 0.08 +/- 0.02 (2 hr after dose) to 0.83 +/- 0.14 (24 hr after dose) (mean +/- SD). Furthermore, the CSF concentration of atenolol, relative to the plasma concentration, increased during repeated drug administration. The CSF/plasma ratio 24 hr after an iv dose was 0.48 +/- 0.12 on day 1 and 0.83 +/- 0.14 on day 7. The CSF concentration of the more lipophilic beta 1-adrenoceptor antagonist metoprolol was almost the same as the concentration of the drug in blood plasma. After 7 days of oral treatment, the CSF/plasma ratio of metoprolol 24 hr after dosing was 0.81 +/- 0.10. The regional CSF concentration of atenolol along the neuraxis was determined in anaesthetized dogs after acute iv administration of the drug. The atenolol concentration in CSF from the lateral cerebral ventricle was similar to that in the cisterna magna but lower than the concentration in CSF sampled from the lumbar region. It is concluded that the CSF concentration of the moderately lipophilic beta 1-adrenoceptor antagonist metoprolol equilibrates with the plasma concentration of the drug more rapidly compared with the hydrophilic drug atenolol.

Pharmacokinetics of felodipine in patients with liver disease

Nine patients (6 males, 3 females) with biopsy-proven liver cirrhosis participated in an open, cross-over, three centre study of the effect of impaired liver function on the pharmacokinetics of felodipine. Two of the nine patients had undergone porto-caval anastomosis. Each patient was given 0.75 mg i.v. and 10 mg p.o. on separate occasions. The results of this study have been compared with published data from younger subjects and elderly hypertensive patients. The mean peak plasma concentration normalized to a dose of 10 mg (Cmax 46 nmol/l) was twice as high in the cirrhotic patients as in the healthy subjects, but the bioavailability, f, (17.0%) was comparable. Subjects with a porto-caval shunt did not have higher f than the mean for the group. The volume of distribution at steady-state, Vss, was significantly lower than in the healthy subjects. Protein binding was significantly lower in the patients with cirrhosis: 99.46% compared to 99.64% in the healthy subjects. The weight-corrected clearance was 1/3 of the value in healthy subjects. No correlation between systemic availability and oral clearance was found, so it is proposed that felodipine is metabolized both in the liver and also in the gut wall. The results suggest that at least the starting dose should be reduced in patients with severe liver disease.