Free fraction of valproic acid: in vitro time-dependent increase and correlation with free fatty acid concentration in human plasma and serum

Effects of intraoperative fluid infusions, sample storage time, and sample handling on unbound propofol assay in human blood plasma

Free drug assay in physiological fluids is getting more and more attention nowadays. The principal reason is the fact that the unbound drug form is responsible for the therapeutic or toxic effects of its application. Unbound drug concentration significantly depends on the extent of its binding by plasma. This article describes the influence of different factors on unbound propofol concentration. These factors are presence of infusion fluids in blood, type of anticoagulant, sample storage time and plasma freezing. The following conclusions result from the experiments carried out: 1. The lowest free drug fractions are observed in samples containing carbohydrate infusion fluids. The free drug percentage is virtually independent of its total concentration in the range of clinically relevant concentrations. 2. There is no evident anticoagulant influence (heparin, citrate, EDTA and oxalate) on free propofol level in plasma samples. 3. Longer storage of plasma at 4 degrees C causes a slight rise of free propofol concentration in heparinised plasma and no evident changes in plasma containing citrate. 4. Plasma freezing induces the increase of free drug concentration both for citrate and heparin. These findings are valuable both for clinicists and pharmacologists, and important for chemical analysts.

Pharmacokinetic considerations in the treatment of childhood epilepsy

Organogenesis throughout childhood affects almost every aspect of pediatric pharmacotherapy. The antiepileptic drugs (AEDs) are particularly impacted since most elimination rates are diminished for the first 6 months of infancy, but quickly attain and supersede adult values. When children enter a hypermetabolic stage, large doses of AEDs may be necessary to maintain effective serum concentrations. Medication noncompliance is frequently confused as hypermetabolism, since both present with low serum drug concentrations. Amazingly, noncompliance among children with chronic illness approaches a similar incidence to that reported in the adult population. It is obviously important to include this in the differential diagnosis of the etiology of subtherapeutic serum AED concentrations. Maturational differences also affect gastrointestinal drug absorption. Intestinal transit time and absorptive surface area are both diminished in young children. Drug delivery systems suitable in adults may not deliver the total dosage in children. Differences in the composition of body compartments and protein binding can alter the volume of drug distribution and, consequently, serum concentrations. In addition to pathophysiologic changes, there is evidence to suggest differences between a mature and immature brain. These differences include quantitative and qualitative responses to neurotransmitters. Hence, it is understandable why seizure semiology is different in children compared with adults. This constellation of factors contributes to the challenges of caring for children with epilepsy.

Unbound valproate fraction in plasma and subcutaneous microdialysate in steady state and after a single dose in humans

The aim of the current study was to characterize the observed discrepancy between unbound plasma valproate (VPA) in single dose and steady state in humans. Unbound and total plasma VPA and subcutaneous microdialysate VPA concentrations were estimated in single dose (6 subjects, n = 33) and steady state (11 subjects, n = 110). Trough plasma samples from 14 patients with total VPA concentrations of 300 micromol/L and 14 patients with VPA concentrations ranging from 600 to 700 micromol/L were analyzed for the unbound VPA fraction and compared with the unbound VPA fraction in spiked plasma samples from healthy subjects containing similar total VPA concentrations. The unbound plasma VPA fraction was significantly higher (P < 0.001) in the steady-state group compared with the single-dose group. The unbound VPA fraction was significantly higher in steady state compared with spiked plasma samples at high and low total VPA concentrations (P < 0.001). The difference between microdialysate and unbound plasma VPA concentrations was significant in the steady-state group (P < 0.001), while no difference was observed in the single-dose group. The mean (+/- SD) subcutaneous microdialysate-to-unbound plasma ratio in the single-dose and steady-state groups was 1.08 (+/- 0.401) and 0.74 (+/- 0.123), respectively. The ratio difference between the groups was significant (P < 0.001). The results of the current study show that unbound plasma fractions of VPA are consistently higher in steady state compared with single dose. Together with the finding of higher unbound VPA fraction in steady state compared with spiked plasma samples, these results provide indirect evidence of displacement of VPA from plasma proteins by product(s) of VPA biotransformation. In addition, subcutaneous microdialysate VPA levels were consistently lower than unbound plasma levels in steady state but not after single dose. The mechanisms underlying this observation need to be studied further.

Comparison of the anticonvulsant efficacies and neurotoxic effects of valproic acid, phenytoin, and the ketogenic diet

PURPOSE

The purpose of this study was to measure quantitatively the effectiveness of the ketogenic diet (KD) in comparison to two clinically important anticonvulsant drugs (AEDs), valproic acid (VPA) and phenytoin (PHT), and to evaluate possible associated neurotoxicity.

METHODS

Rats were maintained on either a calorie-restricted, KD or calorie-restricted, rodent-chow diet for 3-5 weeks, after which neurobehavioral and seizure testing was completed. AEDs (either VPA or PHT) were injected acutely at the time to peak effect before neurotoxic and seizure assessment. Seizures were induced by timed infusion of pentylenetetrazole (PTZ) and maximal electroshock (MES).

RESULTS

VPA protected from both MES- and PTZ-induced seizures, whereas the KD only elevated PTZ seizure threshold; PHT only attenuated MES-induced seizures. The KD was as effective as a high dose of VPA (i.e., 300 mg/kg) and combined treatment (i.e., KD + VPA) showed an additive increase in PTZ seizure threshold. No observed neurobehavioral deficits were associated with either diet treatment; however, drug-related side effects were noted with high doses of either VPA or PHT.

CONCLUSIONS

These data suggest that the KD ranks among VPA and PHT as an effective treatment for seizures, without observed drug-associated neurobehavioral contraindications. In combination with AEDs, our results indicate that the KD plus VPA work synergistically to increase seizure threshold, whereas the KD plus PHT may be complementary, elevating seizure threshold (KD) and reducing seizure severity (PHT). These findings may provide insights into future directions for rational polytherapy; however, it is important to be aware that the KD has been shown to elevate VPA-induced hepatotoxicity.

Elevated free fatty acid concentrations in lipemic sera reduce protein binding of valproic acid significantly more than phenytoin

Higher concentrations of free valproic acid and phenytoin have been reported in patients with uremia and liver disease. Free fatty acids also displace valproic acid and phenytoin. This is a study of the magnitude of displacement of valproic acid and phenytoin from protein binding by free fatty acid in lipemic sera. Higher concentrations of free fatty acids in lipemic sera affected protein binding of valproic acid significantly more than that of phenytoin. Supplementing normal sera with free fatty acids also increased the free concentrations of both valproic acid and phenytoin as expected, but the observed effect was several times higher in magnitude with valproic acid. There was an increased free fraction of valproic acid in patients who received valproic acid and had hypertriglyceridemia. In a patient with uremia, there was also a significant increase in free valproic acid concentration after routine hemodialysis caused by an increase in free fatty acid concentration secondary to hemodialysis. Increased protein binding of valproic acid in sera was observed after treatment with activated charcoal because charcoal can remove free fatty acid. Because higher free fatty acid concentration significantly affects protein binding of valproic acid, careful monitoring of free valproic acid in patients with lipid disorder may be beneficial.

Age-related changes in valproic acid binding to rat serum proteins in vitro

The effect of age on the in vitro binding of valproic acid (VPA) to serum proteins was investigated in rats ranging in age from 14 days (preweaning) to 24 months (senescent). The influence of free fatty acid (FFA) and total protein (TP) concentrations on age-related changes in binding was examined. The protein binding of VPA was altered during development and aging. The VPA fraction unbound (fu) at low VPA concentrations was significantly higher in older age groups (12 and 24 months old; fu = 0.26-0.30) than in younger animals (14, 20, and 40 days old; fu = 0.16-0.18). Binding was best described by a model incorporating a saturable and a nonsaturable binding site. Binding affinity at the saturable binding site was lowest at the extremes of age. Changes in binding at either the saturable or the nonsaturable site were not predicted by changes in TP or FFA with age. Changes in nonsaturable binding were marginally associated with age (p = 0.0952). A 3-fold increase in FFA concentrations was necessary to produce a 1.5-fold increase in VPA fu. There was less than a 2-fold difference in FFA concentrations between the age groups (range 0.219-0.379 mmol/L). Thus, the difference in FFA concentrations between the age groups may not have been large enough to cause measurable differences in displacement of VPA from binding sites. Changes in protein binding may contribute to age-related changes in disposition of VPA observed in the rat. Changes in the serum concentrations of specific FFA or proteins may play a role in the altered VPA binding with age, but changes in total FFA or protein concentrations do not account for the age-related differences observed. Further investigation is required to identify the mechanism(s) responsible for age-related changes in binding of VPA to serum proteins measured in vitro.

SPECT in the localisation of extratemporal and temporal seizure foci

The yield of ictal, postictal, and interictal SPECT was compared in the localisation of seizure foci in 177 patients with partial epilepsy. In 119 patients with known unilateral temporal lobe epilepsy ictal SPECT (97% correct localisation) was superior to postictal SPECT (71% correct), which was better than interictal studies (48% correct). Similarly, in cases of known or suspected extratemporal epilepsy the yield of ictal SPECT studies was high (92%). By contrast, the yield of postictal studies was much lower (46%) and usually only very early postictal studies were diagnostic. Interictal SPECT was of little value. The accuracy of ictal SPECT in localising temporal lobe seizures is now well established. Extratemporal seizures are often brief and difficult to localise. This report shows that ictal SPECT also has a high diagnostic yield in a wide range of extratemporal epilepsies. The brevity of many extratemporal seizures means that true ictal SPECT examinations can be difficult to achieve, but the high diagnostic yield justifies the special organisational effort needed to obtain such studies.

Pharmacokinetics and pharmacodynamics of valproate analogues in rats. IV. Anticonvulsant action and neurotoxicity of octanoic acid, cyclohexanecarboxylic acid, and 1-methyl-1-cyclohexanecarboxylic acid

We examined the pharmacodynamics of valproate (VPA) and three structural analogues, octanoic acid (OA), cyclohexanecarboxylic acid (CCA), and 1-methyl-1-cyclohexanecarboxylic acid (MCCA) in rats. A pentylenetetrazol (PTZ) infusion seizure model was used to determine threshold convulsive doses of PTZ; the increase in PTZ threshold dose after administration of test compound was taken as an index of anticonvulsant activity. Each of the compounds investigated antagonized PTZ-induced seizures, with MCCA evidencing the highest potency. Both CCA and MCCA appeared to have an approximate twofold advantage relative to VPA in protective index (i.e., the ratio of concentrations that produce toxicity to concentrations that produce anticonvulsant effect), based on a rotorod assay of neurotoxicity. Examination of the time course of PTZ antagonism indicated that there was significant dissociation between pharmacokinetics and pharmacodynamics of VPA, with a marked delay in production of maximal anticonvulsant activity. In contrast, only a slight delay in production of maximal protection against PTZ-induced seizures was observed for MCCA, and no delay was evident for CCA. The data indicate that the dynamics of anticonvulsant action differ between these low-molecular-weight carboxylic acids despite their similar chemical structures.

Protein binding of four antiepileptic drugs in maternal and umbilical cord serum

The total and protein free levels of 4 antiepileptic drugs (AEDs) in serum from 35 maternity patients who had been treated with AED monotherapy throughout pregnancy were studied. Results were compared with those in the umbilical cord serum at the time of delivery, and the placental transfer of AEDs was evaluated from the viewpoint of the protein binding capacity of the drug. The materials consisted of 35 samples of maternal and umbilical cord serum in total and included 13 patients on phenobarbital (PB), 7 on phenytoin (PHT), 7 on carbamazepine (CBZ) and 8 on valproic acid (VPA). The mean fetal/maternal total concentration ratios were 0.86, 0.91, 0.73 and 1.59 for PB, PHT, CBZ and VPA, respectively, only the VPA ratio being above 1. On the other hand, the mean fetal/maternal free fraction ratios were 1.13, 1.10, 1.42 and 0.50 for PB, PHT, CBZ and VPA, respectively, only the VPA ratio being less than 1. Correlation of the 2 ratios showed a reciprocal proportion with a correlation coefficient of -0.90 (P < 0.005). It was considered that the fetal/maternal total concentration ratio of 4 AEDs was regulated by the fetal/maternal free fraction ratio of the corresponding AEDs and that the difference in fetal/maternal free fraction ratio depended on the type of drug being administered.

Developmental toxicity of valproic acid

Valproic acid is a very effective anticonvulsant agent widely used in the management of various forms of epilepsy. Administration of the drug during pregnancy results in increased incidence of congenital abnormalities in both humans and experimental animals. In recent years, a significant number of research efforts have attempted to define the contributory role of valproic acid to the impairment of normal prenatal growth and development. The present report summarizes current knowledge that has emerged from clinical and research studies. The specific topics include: the placental transfer of valproic acid; the teratogenic potential; structure-teratogenicity and dose-response relationships; species and strain differences; biochemical changes evoked by the drug in the fetus.

Nonlinear binding of valproic acid (VPA) and E-delta 2-valproic acid to rat plasma proteins

The binding characteristics of valproic acid (VPA) and its pharmacologically active monounsaturated metabolite, E-delta 2-VPA, to rat plasma proteins were compared. The plasma free fraction was determined by a rapid equilibrium procedure, which minimizes the interfering effects of nonesterified fatty acids liberated by in vitro lipolysis. Nonlinear binding behavior was observed with both compounds over their respective pharmacologic concentration range. Multiple binding-site models were invoked to explain the binding isotherm. The 2-unsaturated compound has a much higher affinity for the rat plasma proteins (mainly albumin) than its saturated precursor. The equilibrium association constants for the high- and intermediate-affinity sites were more than an order of magnitude higher with E-delta 2-VPA than with VPA (10(4)-10(6) versus 10(3) M-1). This difference in binding affinity was also reflected by a lower plasma free fraction for E-delta 2-VPA compared with VPA (much less than 10 versus greater than 20% at total concentrations of less than 100 micrograms/ml). A more pronounced dose- and concentration-dependent variation in the distribution and clearance kinetics is predicted for the 2-unsaturated analogue compared to VPA. Also, the structural dependency in plasma protein binding observed with these branched-chain fatty acids may provide insights into the mechanism of interaction between fatty acyl molecules and albumin.

Clinical pharmacokinetics of valproic acid--1988

Sodium valproate (valproic acid) has been widely used in the last decade and is now considered a relatively safe and effective anticonvulsant agent. Recently, several investigators have proposed its use in the treatment of anxiety, alcoholism and mood disorders, although these indications require further clinical studies. Valproic acid is available in different oral formulations such as solutions, tablets, enteric-coated capsules and slow-release preparations. For most of these formulations bio-availability approaches 100%, while the absorption half-life varies from less than 30 minutes to 3 or 4 hours depending on the type of preparation used. Once absorbed, valproic acid is largely bound to plasma proteins and has a relatively small volume of distribution (0.1 to 0.4 L/kg). Its concentration in CSF is approximately one-tenth that in plasma and is directly correlated with the concentration found in tears. At therapeutic doses, valproic acid half-life varies from 10 to 20 hours in adults, while it is significantly shorter (6 to 9 hours) in children. Valproic acid undergoes extensive liver metabolism. Numerous metabolites have been positively identified and there is reasonable evidence that several of them contribute to its pharmacological and toxic actions. In fact, several valproic acid metabolites have anti-convulsant properties, while many of the side effects it may cause (e.g. those related to hyperammonaemia or liver damage) are most often observed in patients previously treated with phenobarbitone. This could indicate that induction of liver enzymes is responsible for the formation of toxic valproic acid metabolites.

Free drug concentration monitoring in clinical practice. Rationale and current status

Recent advances in techniques to determine free drug concentrations have lead to a substantial increase in the monitoring of this parameter in clinical practice. The majority of drug binding to macromolecules in serum can be accounted for by association with albumin and alpha 1-acid glycoprotein. Albumin is the primary binding protein for acidic drugs, while binding to alpha 1-acid glycoprotein is more commonly observed with basic lipophilic agents. Alterations in the concentrations of either of these macromolecules can result in significant changes in free fraction. Diseases such as cirrhosis, nephrotic syndrome and malnourishment can result in hypoalbuminaemia. Burn injury, cancer, chronic pain syndrome, myocardial infarction, inflammatory diseases and trauma are all associated with elevations in the concentration of alpha 1-acid glycoprotein. Treatment with a number of drugs has also been shown to increase alpha 1-acid glycoprotein serum concentrations. A wide variety of biological fluids have been examined for their ability to provide an estimation of free drug concentration at receptor sites. The most useful fluid for estimating free drug concentrations appears to be plasma or serum, with subsequent treatment of the sample to separate free and bound drug by an appropriate technique. The two most widely used methods are equilibrium dialysis and ultrafiltration. Of these two, ultrafiltration has the greatest utility clinically because it is rapid and relatively simple. The major difficulty associated with this method involves the binding of drug to the ultrafilters, but significant progress has been made in solving this problem. Several authors have endorsed the routine use of free drug concentration monitoring. Data examining the clinical usefulness of free drug concentration monitoring for phenytoin, carbamazepine, valproic acid, disopyramide and lignocaine (lidocaine) are reviewed. While available evidence suggests that free concentrations may correlate with clinical effects better than total drug concentrations, there are insufficient data to justify the recommendation of the routine use of free drug concentration monitoring for any of these agents at present.

An update on sodium valproate

Sodium valproate has been in clinical use for the treatment of epilepsy in Great Britain since 1973 and in the United States since 1978. It is chemically quite different from the existing antiepileptic drugs. Although most authorities concentrate on its modification of GABAergic inhibitory transmission in the central nervous system, its mechanism of action remains obscure. It has been shown to be an effective antiepileptic drug in a wide variety of seizure types, but clinically, its major use to date has been in generalized seizures. It is particularly effective in photosensitive epilepsy and myoclonus. Most adverse reactions to sodium valproate are mild and reversible, but with increasing experience, the drug's rare, idiosyncratic, adverse effects are becoming apparent, particularly hepatotoxicity and teratogenicity. The role of therapeutic drug monitoring in the management of patients taking sodium valproate is controversial.

Pharmacokinetics of valproic acid in volunteers after a single dose study

The pharmacokinetics of valproic acid (VPA) was investigated in six healthy volunteers. This was done by monitoring total and free (unbound) valproic acid levels in the serum, and the amount of one of its metabolites, VPA glucuronide, in the urine as a function of time, after a single dose administration of the parent drug. VPA half-life calculated from the urine data of the metabolite was shorter than the half-life calculated from the blood data. About 15 to 20 per cent of the administered oral dose of VPA was excreted in the urine as VPA glucuronide. The average free fraction of VPA obtained in this study, by using the EMIT technique, ranged from 1.5 to 11.5 per cent with a mean value of 4.9 per cent.

In vivo plasma protein binding interaction between valproic acid and naproxen

The effect of naproxen on the kinetics of free and total plasma valproic acid (VPA) was investigated in 6 normal volunteers by using a recently developed simple ultrafiltration technique associated with an immuno-enzymatic assay (Free Level System I, Syva). Each subject received a single oral dose of sodium valproate on two occasions: a) on a control day and b) during concurrent treatment with naproxen (500 mg b.i.d. for 5 consecutive days). Naproxen caused a slight but significant decrease in total plasma VPA levels but left free VPA levels essentially unchanged. The free VPA fraction increased with increasing total VPA concentrations: at equivalent values of total VPA, however, the free fraction was higher in the presence of naproxen. It is concluded that naproxen exerts a moderate displacing effect on protein bound VPA, thereby increasing the clearance of total drug but leaving essentially unchanged the clearance of free drug.

Pharmacokinetics of valproic acid in the elderly

The kinetics of a single oral dose of sodium valproate was studied in six healthy elderly patients (age 68-89 years) and six young control subjects (age 24-26 years). The profiles of total plasma valproic acid (VPA) concentrations were very similar in the elderly and in the young. Half-lives (15.3 +/- 0.7 s.e. mean in the elderly vs 13.0 +/- 1.0 h in the young), volumes of distribution (0.16 +/- 0.01 l/kg in the elderly vs 0.14 +/- 0.01 l/kg in the young) and clearance (7.5 +/- 0.9 ml h-1 kg-1 in the elderly vs 7.7 +/- 0.6 ml h-1 kg-1 in the young) did not differ significantly between the two groups. Free VPA concentrations were significantly increased in the elderly. The clearance of the free drug (intrinsic clearance) was reduced from 127.0 +/- 12 ml h-1 kg-1 (control value in the young) to 77.7 +/- 5.5 ml h-1 kg-1 (P less than 0.02). Free VPA fraction was 9.5 +/- 0.6% in the elderly and 6.6 +/- 0.5% in the young (P less than 0.02). These findings suggest that the pharmacokinetic alterations of VPA in old age are complex and include at least two separate mechanisms: a decrease in plasma protein binding and a reduction of drug metabolizing capacity resulting in decreased clearance of free drug by the liver.

Differential transplacental binding of valproic acid: influence of free fatty acids

The unbound fraction of valproic acid (VPA) was found to be significantly lower in cord serum (6.0 +/- 0.8%) than in maternal serum collected before oxytocin (12.2 +/- 2.7%) or after delivery (9.9 +/- 2.3%). The difference was probably due to the concentration of free fatty acids (acting as displacing agents) being higher in maternal serum. The transplacental binding gradient explains the clinical observation that total VPA levels at delivery are higher in the newborn than in the mother.

Protein binding of valproic acid in maternal and umbilical cord serum

The serum valproic acid levels of 18 maternity patients at the time of delivery were compared with the valproic acid levels in the umbilical cord serum. The levels in the umbilical cord serum were 1.1-4.6 times higher than those in the maternal serum, with a mean value of 1.38. One explanation for this difference apparently is an increased protein binding of valproic acid in the infant's serum. Protein binding was determined in nine patients, in six by equilibrium dialysis and in three by ultrafiltration. The median value of the free fraction of valproic acid was 9.1% (range, 5.8-16.4%) in the umbilical cord serum (equilibrium dialysis) and 15% (range, 12.7-35.3%) in the maternal serum. The difference between the infant and the maternal serum is significant.

Valproic acid in the perinatal period: decreased maternal serum protein binding results in fetal accumulation and neonatal displacement of the drug and some metabolites

The total concentrations of valproic acid were higher in cord serum than in the serum of epileptic mothers given this drug (fetal/maternal total concentration ratios 1.7 +/- 0.5). The maternal free fractions of VPA correlated with the fetal accumulation of the drug. The fetal/maternal free fraction ratios (0.47 +/- 0.24) correlated inversely with the fetal/maternal total concentration ratios. The free concentrations of VPA in fetal blood were similar to those in maternal blood. These results obtained in vivo were confirmed by an in vitro study in which the drug had been added to drug-free serum samples. The free fractions (x100) of VPA in the maternal serum at birth (27.3 +/- 6.3) were significantly higher than in the serum of adult controls (8.0 +/- 2.4) and in cord serum (11.8 +/- 1.3). The pattern of VPA metabolite binding in the three groups was similar to that of VPA, although an unsaturated metabolite (2-en) was bound to a much higher degree than VPA (greater than 98%). The high total drug load in the fetus was partially displaced from binding sites during the first few postnatal days. The free fractions of the drug and metabolites in the neonates were almost twice as high as those in the fetus at birth. The decreased protein binding of VPA in the mothers at birth and in the neonates during the first postnatal week was related to increased free fatty acid levels. VPA concentrations in mother's milk were much lower than the free concentrations in plasma milk/plasma ratios 0.025 +/- 0.01). In neonates, half-lives for VPA were prolonged (43 +/- 14 hours). Our results indicate that increased free fatty acid concentrations in the maternal blood at the time of birth result in partial displacement of VPA from maternal binding sites, additional placental transfer, and thus fetal accumulation of the drug. The high drug load in the fetus is subsequently partially displaced after birth, resulting in increased free fractions and free concentrations of VPA in the neonate.

Rationale for monitoring free drug levels

Recent developments in our knowledge of drug binding prompt the questions, in what circumstances are measurements of free levels necessary, desirable or not warranted? Free level monitoring should be considered for drugs for which the usefulness of plasma level monitoring has been established, drugs which are highly bound to plasma proteins, and which exhibit a variable free fraction. In terms of drug distribution, free drug concentration is independent of free fraction, while total concentration is a resultant of free concentration and free fraction. For drugs with a low extraction ratio, the total concentration at steady-state depends upon free fraction, but the free concentration is independent of free fraction. With high extraction ratio drugs, the free concentration is dependent upon free fraction but only after parenteral administration. There are numerous examples of drugs which are highly bound and exhibit large variations in free fraction. Free fraction may increase with drug concentration. Pathophysiological changes in the binding ligands, albumin and alpha 1-acid glycoprotein, as well as changes in the concentration of endogenous compounds which compete for binding sites, may also cause changes in free drug fraction. Binding interactions resulting from polytherapy may also result in a variable free fraction. Ultrafiltration devices now enable the convenient and accurate determination of free drug levels. Areas in which this should prove useful are investigations of concentration-effect relationships and clinical situations requiring intensive monitoring where binding changes are suspected.

Plasma protein-binding and CSF concentrations of valproic acid in man following acute oral dosing

Simultaneous cerebrospinal fluid (CSF), total and free plasma valproic acid (VPA) concentrations were measured in 17 patients receiving two weight-adjusted VPA doses as seizure prophylaxis prior to diagnostic myelography or cisternography. Free drug concentrations were similar when measured by equilibrium dialysis (ED) at 37 degrees C for 24 h (Dianorm) or by a novel ultrafiltration (UF) method (EMIT freelevel system 1, SYVA) (ED:2.3-35.5 mg-1; UF:1.3-33.6 mg-1; r = 0.78, P less than 0.002). There was wide variation in total VPA concentration (39-154 mg-1) and in free fraction (ED: 3.3-25.6%; UF: 5.9-24%). Concentration dependent protein binding was not demonstrated. CSF VPA varied between 4.2 and 25.6 mg-1 and was accurately reflected by free plasma VPA concentrations (ED: r = 0.75, P less than 0.005: UF: r = 0.93, P less than 0.001). CSF concentration also correlated with the total plasma VPA (r = 0.76, P less than 0.005). The Emit freelevel system 1 provides a rapid measure of unbound VPA in the plasma which may be suitable for routine clinical use.