Plasma Valproate Concentrations Research Articles

Pharmacokinetics of Valproate After Multiple‐Dose Oral and Intravenous Infusion Administration: Gastrointestinal‐Related Diurnal Variation

A randomized, crossover study was conducted in healthy male volunteers to assess the diurnal variation in the steady-state pharmacokinetics of valproate after multiple 250-mg oral and intravenous infusion doses after an intravenous 750-mg loading dose. Multiple blood samples were collected throughout each 168-hour study period, and plasma valproate concentrations were quantitated using a gas chromatographic technique. Within-regimen comparisons indicated statistically significant differences for mean steady-state peak (Cmax) and trough (Cmin) plasma concentrations and area under the plasma concentration-versus-time curve (AUC) between the second and third doses on day 4 after oral dosing, indicating a diurnal variation in the rate of valproate absorption from the delayed-release tablet preparation. Between-regimen steady-state comparisons of pharmacokinetic parameters revealed some significant differences in mean time to Cmax (Tmax), Cmax, Cmin, AUC, and total body clearance for respective day-4 dosing intervals, but not for the entire day 4, except for Cmin and Tmax. Mean elimination rate constant and half-life did not significantly differ between the regimens. The regimens were bio-equivalent at steady state, as assessed by 90% confidence intervals (two one-sided test procedures) for Cmax, Cmin, and AUC, with similarity in degrees of fluctuation ((Cmax-Cmin)/Caverage). Despite the presence of diurnal variation in valproate absorption after oral dose administration, the steady-state plasma concentration-versus-time profile was well maintained by both regimens within the accepted therapeutic range of 50 to 100 micrograms/mL.

Pharmacokinetics of Multiple Oral Dose Divalproex Sodium after Intravenous Loading Dose Administration in Healthy Volunteers

A randomised study was conducted in healthy male volunteers to assess the pharmacokinetics of valproate after multiple 500mg (every 8 hours) or 250mg (every 6 hours) doses commencing 1 to 3 hours following an intravenous 1000mg loading dose administered over 10 minutes. Multiple blood samples were collected throughout the 73- to 75-hour study period and plasma valproate concentrations were quantified using a gas Chromatographic technique. All 3 regimens produced near steady-state trough plasma concentrations generally above 50 mg/L throughout the first study day. By the morning of day 2, pseudo-steady-state conditions had been established. Overall, an intravenous 1000mg loading dose of valproic acid followed within 3 hours by oral regimens of divalproex sodium 500mg every 8 hours or 250mg every 6 hours appears to be an acceptable approach to rapidly achieve therapeutic concentrations with steady-state trough plasma concentrations above 50 mg/L.

Valproate Metabolism During Hepatotoxicity Associated with the Drug

Plasma concentrations of valproate and certain of its metabolites and their patterns of excretion in urine are described in three adults who developed hepatotoxicity during treatment of epilepsy with sodium valproate. One patient also developed a degree of reversible renal insufficiency, whilst another may have had associated infectious mononucleosis. All three cases showed evidence of impaired mitochondrial beta-oxidation of valproate. In one the impairment was at the stage catalysed by fatty acyl-CoA dehydrogenase, in another at the stage catalysed by 3-hydroxyacyl-CoA dehydrogenase and in the third at the stage catalysed by enoyl-CoA hydratase and possibly also at the next stage catalysed by 3-hydroxyacyl-CoA dehydrogenase. The impaired beta-oxidation meant that valproate metabolism was diverted into various alternative pathways. Plasma concentrations of the suspected hepatotoxic metabolite 4-en-valproate were normal for the valproate-treated population in all cases. By analogy with certain spontaneous and acquired human disorders of branched chain amino acid metabolism, it is suggested that valproate-associated hepatotoxicity may represent the consequences of a valproate overload on a limited mitochondrial beta-oxidation capacity, causing accumulation of a toxic product of endogenous branched chain amino acid metabolism.

An influencing factor to predict plasma valproate concentrations: Circadian stage-dependent kinetics.

This study was designed to examine how the dosing time of the day influenced the accuracy in predicting plasma valproate (VPA) concentrations at steady state. Eight healthy male volunteers took 400-mg doses of VPA for 9 days on a twice-daily basis (08: 30 and 20: 30). The circadian changes in VPA kinetics occurred at the absorption phase. The prediction of plasma VPA concentrations at 2hr (around Cmax) and at 12hr after both the morning and the evening dose on the ninth day was performed using the individual subject's pharmacokinetic parameters and population pharmacokinetic parameters (Bayesian method) obtained from the morning trial on the eighth day. The predictive accuracy for VPA concentration around Cmax after the morning dose was better, but that for VPA concentration after the evening dose was significantly biased toward overestimation. The individual pharmacokinetic parameters obtained from the morning trial became better sources for the prediction of VPA concentrations around Cmax after the morning dose, but worse sources for that after the evening dose. The predictive performance based on the Bayesian approach also showed the similar finding to that based on individual pharmacokinetic parameters. The overestimation of the target points around Cmax after the evening dose is closely related to that of Ka value produced by using population parameters obtained from morning trial. Therefore, these results suggest that the time in the circadian stage at which VPA is administered is important for evaluating exactly the predictive accuracy.

β‐Oxidation of Valproic Acid.

The effect of fasting and glucose-infusion on valproate (VPA) disposition was investigated to determine the involvement of endogenous fatty acid (FA) beta-oxidation in the metabolism of VPA. Fifteen healthy volunteers received multiple oral doses of VPA to achieve steady state under both conditions. Depressed plasma FA concentrations in the glucose-infused state (median 51%, p less than 0.0001) were associated with lower unbound plasma VPA fractions (median 17%, p less than 0.0001). Unbound plasma VPA concentrations were notably lower in the glucose-infused state due to significantly higher (median 41%, p less than 0.0001) metabolic clearance, beta-oxidative metabolite formation clearance, representing the largest urinary dose fraction recovered, was significantly higher (median 60%, p less than 0.004) in the glucose-infused state. This finding is consistent with competition between endogenous FA and VPA for the enzymes of beta-oxidation modulated by conditions which affect FA mobilization to the site of catabolism.

Chronotoxicity of Sodium Valproate and Its Mechanisms in Mice: Dose-Concentration-Response Relationship

A significant circadian rhythm of acute toxicity was demonstrated in mice with intraperitoneal (i.p.) injection of sodium valproate (VPA). The role of pharmacokinetics on the rhythms of the toxicity and electroshock seizure (ES) threshold was investigated. ICR male mice, housed under a light-dark (12:12) cycle, were injected intraperitoneally 1200 mg/kg for the acute toxicity study and 300 mg/kg for the anticonvulsant effect study. In the acute toxicity, the highest mortality was found when VPA was injected at 1700 and the lowest at 0900 or 0100. The time course of mean plasma and brain VPA concentrations after an injection of VPA was not different between mice injected at 1700 and mice injected at 0100. In the anticonvulsant effect, no significant circadian rhythm was demonstrated for both the ES threshold and the plasma VPA concentrations after i.p. injection, although a significant rhythm has been reported for them after oral administration. The results suggest that the circadian rhythm in the mortality after an i.p. injection of VPA may be due to the rhythm in the sensitivity of the central nervous system to the drug and that the mechanism underlying the rhythm of VPA acute toxicity is different from that of the anticonvulsant action of VPA. The route and the time of drug administration are essentially important to study the anticonvulsant effect and acute toxicity of VPA in mice.

Valproate in the treatment of absence epilepsy in children: a study of dose-response relationships.

Seven children with absence epilepsy were treated with valproate (VPA). All but one child became free of absence seizures during VPA monotherapy. EEG was recorded for 24 h before start of VPA treatment and repeatedly during treatment. Correlation between plasma VPA concentration and reduction of the number of epileptic discharges was significant. Plasma concentration of 440-660 microM VPA was needed to achieve at least 50% reduction of seizure activity.

Chronopharmacological Study of Sodium Valproate in Mice: Dose-Concentration-Response Relationship

Effects of the time-of-day of drug administration on the pharmacokinetics electroshock seizure (ES) threshold and acute toxicity were investigated in mice with sodium valproate (VPA). ICR male mice, housed under a light-dark (12:12) cycle, were orally administered 600 mg/kg VPA for anticonvulsant effect studies and administered 1500 mg/kg VPA for acute toxicity studies. A significant circadian rhythm was demonstrated for the ES threshold at 30 min after VPA administration, with the highest value in the light phase and the lowest in the dark phase, although no rhythm was shown in the nondrugged state. A significant circadian rhythm was also shown for plasma and brain VPA concentrations. This finding nicely corresponded to the circadian rhythm in the ES threshold. The positive relationship between the brain VPA concentration and the ES threshold was not different between the light phase and the dark phase. There was also a significant circadian rhythm in the acute toxicity induced by VPA, with the highest mortality in the light phase and the lowest in the dark phase. The results suggest the importance of time in the circadian stage at which VPA is administered in the experimental studies in mice and the significance of circadian rhythm in VPA kinetics in relation to the rhythm of ES threshold.

Chronopharmacological study of sodium valproate in mice: dose-concentration-response relationship.

Effects of the time-of-day of drug administration on the pharmacokinetics electroshock seizure (ES) threshold and acute toxicity were investigated in mice with sodium valproate (VPA). ICR male mice, housed under a light-dark (12:12) cycle, were orally administered 600 mg/kg VPA for anticonvulsant effect studies and administered 1500 mg/kg VPA for acute toxicity studies. A significant circadian rhythm was demonstrated for the ES threshold at 30 min after VPA administration, with the highest value in the light phase and the lowest in the dark phase, although no rhythm was shown in the nondrugged state. A significant circadian rhythm was also shown for plasma and brain VPA concentrations. This finding nicely corresponded to the circadian rhythm in the ES threshold. The positive relationship between the brain VPA concentration and the ES threshold was not different between the light phase and the dark phase. There was also a significant circadian rhythm in the acute toxicity induced by VPA, with the highest mortality in the light phase and the lowest in the dark phase. The results suggest the importance of time in the circadian stage at which VPA is administered in the experimental studies in mice and the significance of circadian rhythm in VPA kinetics in relation to the rhythm of ES threshold.

PLASMA CONCENTRATIONS OF UNBOUND VALPROATE AND THE MANAGEMENT OF EPILEPSY

The range of protein binding of valproate and the use of unbound and total plasma concentrations of the drug were studied in an outpatient population of 70 epileptics. The unbound fraction of plasma valproate ranged from 4.2% to 11.7% with a median of 7.1%. A non-linear relationship was found between unbound and total plasma valproate concentrations and was best described by a cubic regression (r2 = 0.88). This concentration dependent protein binding was also demonstrated by a linear relationship between total plasma valproate concentration and unbound fraction (r = 0.46). As expected, there was no correlation across the patient population between plasma concentrations of valproate and seizure frequency. In an individual patient, however, plasma valproate levels usually correlated with change in clinical status, although this correlation was no better for unbound levels than total levels. There were only three patients in whom unbound valproate levels correlated better with clinical effect than total levels, whereas there were six patients in whom total levels correlated better than unbound levels. It is therefore concluded that monitoring sodium valproate therapy with unbound concentrations is rarely helpful and the routine use of unbound valproate levels cannot be advocated.

Chronic sodium valproate treatment in the rat: toxicity versus protection against seizures induced by indoklon

A model for chronic treatment of rats with sodium valproate has been developed balancing efficacy, toxicity and dose control. The dose (300 mg/kg) and frequency (every 8 h) selected were somewhat toxic as measured by weight gain and failed to provide continuous protection against Indoklon induced seizures but yielded plasma valproate levels near the range of therapeutic human levels. Chronic treatment of rats at this dose and frequency yielded a significant negative correlation between weight gain and length of treatment as well as a significant negative correlation between plasma valproate concentration and length of treatment. It was concluded that due to the short half-life of valproate in rats it is impossible to maintain continuously protective, nontoxic levels of valproate with a reasonable frequency (every 8 h) of controlled dose administration.

Diurnal oscillations of CSF valproate in monkey

Valproate (VPA) was administered to four rhesus monkeys by constant-rate intravenous infusion for two weeks under controlled conditions. Plasma and CSF samples were collected for a period of 27 hours at 3-hour intervals during steady-state and post-infusion periods. The mean correlation coefficient between total plasma and CSF VPA concentrations was found to be 0.78 ± 0.09. The CSF VPA levels reflected the changes in free VPA in plasma but the two were not equivalent. Diurnal fluctuations in CSF VPA concentration were similar to those found in plasma but the inter-animal variation was greater in CSF than in plasma.

Distribution of sodium valproate in normal whole blood and in blood from patients with renal or hepatic disease.

Sodium valproate at a concentration of 300 mumol/l in whole blood, partitioned between the red blood cell and plasma to produce a red blood cell/plasma partition ratio of 0.20. Red blood cell uptake was proportional to percent free drug in plasma and uptake was maximal when plasma was replaced by buffer, producing a red blood cell/buffer ratio of 0.87. Reduction of plasma protein binding by plasma dilution, by increasing the total sodium valproate plasma concentration, or by renal or hepatic disease in 24 patients, caused a predictable rise in red blood cell uptake of drug. The red blood cell represented a relatively small compartment for free sodium valproate in blood, however uptake of the drug into this compartment increased considerably in states of reduced plasma protein binding. Because the concentration of drug in the red blood cell reflects free drug concentration in plasma, the red blood cell may serve as an indicator of free drug changes in blood.

Le valproate de sodium: Une drogue hyperammonemiante: Étude chez l'épileptique et chez le volontaire sain

Sodium valproate: a hyperammonemic drug — Study in epileptics and normal subjects Sodium valproate (VPA) consistently induces an arterial hyperammonemia in epileptics tolerant of this drug and in normal subjects. The hyperammonemia appears with the first oral or intravenous dose of the drug, 15–25 mg/kg, and is established within minutes following drug absorption. In 20 epileptics treated with VPA alone for 4 days, the mean arterial ammonemia measured 2–3 h after breakfast and the day's first VPA dose was 27 ± 9 μmol/l in non-alcoholics, and 77 ± 7 μmol/l in alcoholics. Hyperammonemia persisted during chronic treatment; in 10 epileptics who had had received only VPA for over a month, the mean hyperammonemia was 87 ± 6 μmol/l (normal value X ̄ ± 2 SD = 28 ± 12 μ mol/l ). The ammonemia varied in the course of the day; sharp peaks 7 or more times the base value were observed. These variations, differing among subjects, depended on the VPA plasma concentration, and above all on the meal composition and the relative timing of the meal and the drug administration. No secondary effects were seen; in particular, hepatic and pancreatic tests were normal. The hyperammonemia would seem to be due to physiopathological mechanisms other than those giving rise to the hepatic complications occasionally observed with VPA. The permanence and the extent of the hyperammonemia raise questions as to its origin, its relation to the stuporous states induced by VPA, and its eventual repercussions on the functioning of neurons.

Plasma concentrations of valproate during maintenance therapy in epileptic children.

A total of 20 children with various types of epilepsy were treated with valproate, 11 with monotherapy and 9 with valproate in combination with phenobarbitone, phenytoin, or carbamazepine. Valproate was given either every 8 or 12 h. At least two different dose levels were tried in each patient. The pharmacokinetics of valproate during the interval between doses was determined using a gas chromatographic technique. The clinical effect of the treatment was assessed by interviewing the parents. The plasma concentrations showed considerable fluctuation during the intervals between doses. The mean increase from pre-administration to peak level was 82% when the dose interval was 12 h, and 62% when it was 8 h. The mean plasma half-life of valproate, using a one-compartment model, was 10.9 +/- 1.3 h (mean +/- SD). The plasma half-life of valproate was decreased when the drug was combined with the other anti-epileptics. The calculated area under the concentration versus time curve was linearly related to dose, both in a single patient on four dose levels and when different patients were compared. The clinical effect of valproate monotherapy was best in patients with absences, usually good in myoclonus and less favourable in other types of epilepsy. For children with absences, the optimal dose range of valproate was between 20 and 40 mg/kg/24 h. In comparison, the myoclonic types of epilepsy needed a slightly higher dose level, between 30 and 60 mg/kg/24 h. In the latter group a "therapeutic window" seems to exist, since patients below and above the suggested dose levels were not well-controlled. Therapeutic monitoring of valproate does not appear meaningful when the drug is used as monotherapy. However, in combination therapy, determination of the plasma levels of all anti-convulsants used may be helpful. The large fluctuations of valproate during a dose interval must be taken into consideration when the clinical effects are analysed.

Intra-dose variation in plasma protein binding of sodium valproate in epileptic patients.

1 The fluctuations in protein binding of sodium valproate during one dosing interval were studied in five patients stabilized on valproate and taking concurrent anticonvulsant therapy. 2 The patients took their usual morning dose of valproate (400-800 mg) and serial blood samples were collected by venepuncture at 0, 1, 2, 3, 4, and 6 h post-dose. 3 Free valproate was separated from protein bound drug by plasma ultrafiltration and the ultrafiltrate and total plasma valproate concentrations were measured by a gas chromatographic method. 4 The maximum and minimum concentrations in the ultrafiltrates occurred at the same times as in the plasma. However, the percentage fluctuation was always greater in the ultrafiltrates (range 192-412%) compared with the plasma (range 153-374%) due to the concentration-dependent nature of valproate protein binding. 5 If free valproate levels are to be monitored, knowledge of sampling time and dosage history is important for interpretation of the results.

Phenytoin-valproate interaction: importance of saliva monitoring in epilepsy.

Sodium valproate is often used with phenytoin when epilepsy cannot be controlled by a single drug. Sodium valproate depresses phenytoin protein binding and so invalidates plasma phenytoin monitoring as a means of determining precise phenytoin dosage requirements. Plasma and saliva phenytoin and plasma valproate concentrations were measured in 42 patients with epilepsy receiving both drugs. Phenytoin protein binding was also measured by ultrafiltration in 19 of these patients and 19 patients taking phenytoin alone. Saliva phenytoin concentration bore the same close correlation to unbound (therapeutically active) phenytoin in patients receiving both drugs as it did in patients receiving phenytoin alone, whereas plasma total phenytoin did not. The same therapeutic range for saliva phenytoin (4-9 mumol/1; 1-2 microgram/ml) was therefore valid in both groups. The depression of phenytoin binding was directly related to the plasma concentration of valproate both in random samples taken from the 42 patients and in samples taken throughout the day in two of these patients. This was confirmed in vitro. Even when the concentration of valproate is known the degree of binding cannot be predicted. Saliva rather than plasma monitoring of phenytoin treatment is therefore valuable in the presence of valproate and with reduced phenytoin binding from any cause.

On-column propylation method for measuring plasma valproate concentration by gas chromatography.

A gas-chromatographic method is described for measuring plasma valproate concentrations. This method incorporates double-solvent extraction of the plasma sample and on-column propylation of valproic acid and 2-propylhexanoic acid, used as the internal standard. The identity of the propyl ester derivatives of these compounds was confirmed by gas chromatography-mass spectrometry. The accuracy and simplicity of the method make it suitable for routine laboratory use. The assay is sufficiently rapid that a plasma sample can be analyzed in duplicate within 30 min.

Therapeutic Monitoring of Valproic Acid

Valproic acid, a new type of antiepileptic drug, has developed into one of the drugs of first choice in all types of epilepsy with the exception of infantile spasms and partial seizures. Due to its short biological half-life, a steady-state plasma concentration will be reached rapidly after initiation of therapy. However, no direct relationship between this plasma concentration and the clinical effect can be found. In autoradiography experiments, brain distribution appears to change with time, suggesting a gradual accumulation of the drug or its metabolites in certain brain areas. Two metabolites are detected in monkey and human blood plasma. Valprolactone is also found in brain tissue, but the substance is rapidly eliminated. beta, gamma-Unsaturated valproic acid has the longest half-life and is found in concentrations that can be quantitated by routine gas chromatographic analysis. The therapeutic effect appears to correlate better with dosage per kilogram body weight than with the actual plasma concentrations. Routine monitoring of valproate plasma concentrations in epileptic patients seems to be of limited value.

Linear Relationship between Plasma Concentration and Dosage of Sodium Valproate

Plasma valproate concentration was studied in 7 hospitalized nonepileptic patients who received sodium valproate at daily doses of up to 60 mg/kg. A linear relationship between dose and plasma concentration of valproate (r = 0.81--0.99) was found in each patient, although the slopes of the regression lines (reflecting clearance rates) varied twofold. This suggests that if the valproate plasma concentrations at two different doses are known and the clearance of valproate is not altered by concomitant administration of other drugs, it should be possible to predict the plasma valproate concentration that will result from further dose increments.