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

Accuracy of Calculated Free Valproate Levels in Adult Patients With Status Epilepticus

OBJECTIVE

To test the accuracy of an equation in adult patients with status epilepticus that calculates the free concentration of serum valproic acid (fVPA) from the total concentration of serum valproic acid (tVPA) and serum albumin.

METHODS

All adult patients with status epilepticus who were treated at a Swiss academic medical center between 2005 and 2018 with concurrent measurements of tVPA, fVPA, and serum albumin were included. fVPA was categorized as subtherapeutic, therapeutic (5-10 mg/L), or supratherapeutic. Agreement was defined as the proportion of measured and calculated fVPA falling within the same category.

RESULTS

Of 676 patients with status epilepticus, 104 had 506 measurements, with a median of 3 (interquartile range [IQR] 1.5-6.5) per patient. The median tVPA was 43.5 mg/L (27.4-63.6), with measured fVPA 9.1 mg/L (4.5-14.7) and calculated fVPA 10.1 mg/L (7.0-13.0), respectively. The median deviation of calculated from measured fVPA was -0.8 mg/L (-3.2 to 2.5) with 336 measurements >1 mg/L. While the association between measured and calculated fVPA was linear (regression coefficient 1.1, 95% confidence interval 0.9-1.2, p < 0.0001), the agreement on effective drug levels did not match in 39.8% of measurements regardless of serum albumin levels, with calculated fVPA overestimating measured fVPA in 30.4%. tVPA and serum albumin independently influenced the accuracy of the calculated fVPA in the multivariable model.

CONCLUSIONS

Calculated fVPA is inaccurate when using the proposed equation in adult patients with status epilepticus, calling for drug monitoring based on measured fVPA in this context.

Determination of unbound valproic acid in plasma using centrifugal ultrafiltration and gas chromatography:Application in TDM

AIM

In order to monitor the free concentration of VPA in plasma, a simple and rapid method needs to be developed.

METHODS

The free fraction of VPA in plasma was obtained by centrifugal ultrafiltration (CF-UF) devices. Cyclohexanecarboxylic acid was used as internal standard. Valproate in plasma was converted to VPA by sulphuric acid acidification, and dichloromethane was used as solvent for extraction. Nitrogen was the carrier gas, the samples were separated by capillary column, and the flame ionization detector was used to detect VPA fragment ions for quantitative analysis.

RESULTS

The assay had good specificity and stability. The linear range of the assay was 0.56-28.11 mg/L. The intra-day and inter-day precision (RSDs) of the assay were all within 15%, and the accuracy (RE) was 2.58%. The recoveries of VPA with three different concentrations were 102.03 ± 1.05, 101.45 ± 2.08 and 102.58 ± 3.38. The results of therapeutic drug monitoring (TDM) in pediatric inpatient group and outpatient group showed significant differences between the two groups (P < 0.001).

CONCLUSION

This assay has low cost and good analytical performance, so it can be developed into a routine TDM method of unbound VPA. We recommend the monitoring of unbound VPA concentration in pediatric inpatients during clinical use of VPA.

Analysis of the Variables Influencing Valproic Acid Concentration in the Serum and Cerebrospinal Fluid of Chinese Patients After Craniotomy

BACKGROUND

Valproic acid (VPA) has been widely used in Chinese patients after craniotomy. Many studies have focused on the influencing factors of VPA serum concentration, but conclusions are sometimes paradoxical. Furthermore, the concentration of VPA in the cerebrospinal fluid (CSF) has been rarely reported. In the present study, VPA CSF concentrations were measured, and the potential factors influencing serum concentration and CSF distribution of VPA were investigated. In addition, the functional relationship between serum and CSF concentration was explored.

METHODS

Subjects were patients who underwent craniotomy and were administrated with VPA and had a lumbar puncture. Serum and CSF VPA concentrations were measured by use of the Abbott i1000 system. CYP2C9 (430 C>T, 1075 A>C, 1076 T>C, 1080 C>G), UGT1A6 (541 A>G, 552 A>C), UGT2B7 (211 G>T, 802 C>T), and ABCB1 (1236 C>T, 2677 G>T/A, 3435 C>T) genotypes were determined by direct sequencing. Information, such as age, gender, and height, was collected, and their effect on serum and CSF VPA concentrations was investigated by univariate analysis and multiple linear regression analysis.

RESULTS

First, the concomitant use of carbapenems (β' = -0.422) and UGT1A6 (552 AA → AC) (β' = -0.249) had a significant negative correlation with the weight-adjusted VPA serum concentration (C:W ratio), whereas CYP2C9 (1075 AA → AC) (β' = 0.186) and gender (female compared with male) (β' = 0.322) showed a positive correlation with VPA serum C:W ratio. The coefficient of determination (R) was only 0.348. Second, the relationship between the serum concentration and the CSF square root of the concentration (R = 0.705) had a better linear fit. Third, serum VPA concentration (β' = 0.810), concomitant use of glycerol fructose (β' = 0.160), and age (≥65 compared with <65) (β' = 0.118) showed a positive correlation (R = 0.748) with the variability of square root of the concentration of the CSF.

CONCLUSIONS

In Chinese patients, after craniotomy, female patients with 1 or more of CYP2C9 (1075 AC) and UGT1A6 (552 AA) genotypes required a lower VPA dosage compared with male patient. There was a better-fitted linear relationship between VPA serum and the square root of CSF concentrations. CSF VPA concentrations were relatively stable, with only age and the use of glycerol fructose having a small influence.

Plasma protein binding: from discovery to development

The importance of plasma protein binding (PPB) in modulating the effective drug concentration at pharmacological target sites has been the topic of significant discussion and debate amongst drug development groups over the past few decades. Free drug theory, which states that in absence of energy-dependent processes, after steady state equilibrium has been attained, free drug concentration in plasma is equal to free drug concentration at the pharmacologic target receptor(s) in tissues, has been used to explain pharmacokinetics/pharmacodynamics relationships in a large number of cases. Any sudden increase in free concentration of a drug could potentially cause toxicity and may need dose adjustment. Free drug concentration is also helpful to estimate the effective concentration of drugs that potentially can precipitate metabolism (or transporter)-related drug-drug interactions. Disease models are extensively validated in animals to progress a compound into development. Unbound drug concentration, and therefore PPB information across species is very informative in establishing safety margins and guiding selection of First in Human (FIH) dose and human efficacious dose. The scope of this review is to give an overview of reported role of PPB in several therapeutic areas, highlight cases where PPB changes are clinically relevant, and provide drug metabolism and pharmacokinetics recommendations in discovery and development settings.

Correlation between plasma ammonia level and serum trough concentration of free valproic acid in patients with epilepsy

Therapeutic drug monitoring of valproic acid (VPA) is essential to prevent toxicity, but the correlation between plasma ammonia level and serum VPA concentration remains unclear. We examined the correlation of plasma ammonia level with VPA dose and serum trough concentrations of total and free VPA in Japanese patients with epilepsy. Thirty-eight data sets from 19 Japanese patients with epilepsy were analyzed. The relations of VPA dose and serum total and free VPA concentrations with plasma ammonia level, and the breakpoints of VPA parameters predicting hyperammonemia (plasma ammonia higher than 60 µmol/L) were analyzed. A significant positive correlation was observed between plasma ammonia level and VPA dose (r(s)=0.56, p=0.00062), serum trough total VPA concentration (r(s)=0.55, p=0.00086) and serum trough free VPA concentration (r(s)=0.58, p=0.00041). The breakpoints predicting hyperammonemia were VPA dose of 30.4 mg/kg, serum trough total VPA concentration of 90.9 µg/mL, and serum trough free VPA concentration of 8.65 µg/mL, with impurity reductions at 1.35, 1.35 and 2.02, respectively. These findings suggest that serum trough concentration of free VPA is the most reliable predictor for hyperammonemia, and that the risk of developing hyperammonemia may increase in patients with serum trough free VPA concentrations higher than 8.65 µg/mL.

No effect of co-administered antiepileptic drugs on in-vivo protein binding parameters of valproic acid in patients with epilepsy

Objectives

The aim of this study was to establish the population protein binding parameters of valproic acid (VPA) in patients with epilepsy receiving VPA monotherapy and those receiving VPA combined with other antiepileptic drugs.

Methods

One hundred and thirty nine data sets from 63 Japanese patients with epilepsy were analysed. These patients were separated into two groups: VPA monotherapy and VPA combined with other binding-sensitive antiepileptic drugs, including phenytoin, clonazepam, clobazam, carbamazepine and phenobarbital (VPA polytherapy). The population protein-binding parameters of VPA were obtained by non-linear least-squares method in each group.

Key findings

The mean (95% confidence interval) dissociation constants were 38.9 µm (33.2–44.6 µm) and 36.9 µm (26.7–47.1 µm), and the numbers of binding sites were 1.36 (1.27–1.44) and 1.33 (1.19–1.47) in the monotherapy and polytherapy groups, respectively. No significant differences in the binding parameters of VPA to serum albumin were observed between the two groups.

Conclusions

The steady-state serum albumin binding of VPA in Japanese patients with epilepsy is not affected by co-administration of other antiepileptic drugs. These findings suggest that serum VPA concentration is stable at the steady state with regard to interaction by protein binding, even when other antiepileptic drugs with moderate-to-high binding properties are co-administered.

Characterization of non-linear relationship between total and unbound serum concentrations of valproic acid in epileptic children

OBJECTIVE

To establish a regression equation to properly estimate the unbound serum concentration of valproic acid (VPA) from its total serum concentration; the relationship between total and unbound serum VPA concentrations was retrospectively characterized.

METHODS

Data were obtained from the clinical examination records that were routinely archived during therapeutic drug monitoring. The screening encompassed 342 records of 108 paediatric patients whose total and unbound VPA concentrations had been determined. The relationship between total and unbound VPA concentrations was characterized according to the Langmuir equation by taking account of inter-individual variability with the nonmem program.

RESULTS

The total VPA concentration (C(t)) in the screened patients ranged from 5.5 to 179.8 microg/mL, and the unbound VPA concentration (C(f)) increased in a non-linear manner as the total VPA concentration increased. Taking account of the effects of antiepileptics concurrently administered, the VPA dissociation constant (K(d)) and maximum binding site concentration (B(m)) were 7.8 +/- 0.7 and 130 +/- 4.5 microg/mL respectively, for the regression equation, C(t) = C(f) + B(m) x C(f)/(K(d) + C(f)). An alteration in the unbound concentration was seen in patients who were treated with the combination of VPA and ethosuximide and in those who received two additional antiepileptics.

CONCLUSIONS

A regression equation for estimation of the unbound VPA concentration, based on total VPA concentration collected during routine therapeutic drug monitoring was established. Use of two additional antiepileptics and ethosuximide treatment was considered as potential factors affecting unbound VPA concentration.

Plasma and cerebrospinal fluid pharmacokinetics of valproic acid after oral administration in non-human primates

PURPOSE

Valproic acid (VPA), a widely used antiepileptic, also inhibits histone deacetylase (HDAC), and is undergoing evaluation as an anti-cancer agent. We studied the pharmacokinetics of VPA in the plasma and cerebrospinal fluid (CSF) in a non-human primate model that is highly predictive of human CSF penetration to determine if levels of VPA required to inhibit HDAC in in vivo models can be attained.

METHODS

Oral VPA, 75 mg/kg, was administered to four non-human primates. Serial samples of blood (n = 4) and CSF (n = 3) were obtained for pharmacokinetic studies of total and free VPA. Plasma and CSF VPA concentrations were measured using the commercially available Abbott AxSYM VPA assay reagent system (Abbott Laboratories, Abbott Park, IL, USA). The resultant plasma and CSF data were evaluated using pharmacokinetic modeling methods.

RESULTS

At a dose of 75 mg/kg, the maximum plasma concentration of VPA was 130.1 +/- 70.6 microg/ml (mean +/- standard deviation) for total drug and 53.3 +/- 44.4 microg/ml for free drug. The mean plasma area under the curve (AUC) for total drug was 680 +/- 233 microg/ml h and for free drug 146 +/- 89 microg/ml hr. The maximum CSF concentration occurred 2-3 h after administration and was 28.2 +/- 18.6 microg/ml. The CSF AUC for VPA was 108 +/- 52 microg/ml h. The CSF penetration of VPA was 12.9 +/- 5.1% for total drug and 57.0 +/- 8.7% for free drug. Disappearance from the plasma followed non-linear kinetics with a V (max) of 321.2 +/- 65.6 microg/kg/min and a K (m) of 17.2 +/- 13.7 mg/l.

CONCLUSION

Valproic acid deserves further study for the treatment of CNS tumors given its high CSF penetration after oral dosing coupled with the anti-tumor activity observed in preclinical studies.

Gender- or age-related binding characteristics of valproic acid to serum proteins in adult patients with epilepsy

The aim of the present study was to determine the gender- or age-related binding characteristics of valproic acid (VPA) to serum proteins in the adult population. Serum samples examined in the study were obtained from 70 adult patients (36 males, 34 females) with epilepsy on VPA monotherapy. Their age ranged from 16 to 68 years (mean age with (SD), 37.7 (15.7) years; <45 years, n=44; >/=45 years, n=26). The in vivo population binding parameters of VPA to serum proteins and theoretical minimal unbound serum VPA fraction (Fu) were determined using an equation derived from the Scatchard equation in: (1), all; (2), male and female subgroups; and (3), younger (<45 years) and older (>/=45 years) subgroups. There was a significant difference in serum concentration of unbound VPA between male and female patients. The mean association constant (K) was 0.010 microM(-1) in all, male, and female patients. The mean total concentration of binding sites (n(Pt)) was 1453 microM for all patients, and 1561 and 1394 microM for male and female patients, respectively. The Fu was 0.064 for all patients, and 0.060 and 0.067 for male and female patients, respectively. There were no significant differences in the binding characteristics of VPA to serum proteins between the male and female groups. On the other hand, there were significant differences in the serum albumin concentration and molar concentration ratio of free fatty acids to albumin in serum between the younger and older patients. The mean value of K was 0.016 microM(-1) for the younger patients and 0.007 microM (-1) for the older patients. The mean n(Pt) was 1157 microM for the younger patients and 1703 microM for the older patients. The Fu was 0.051 for the younger patients and 0.077 for the older patients. Thus, significant differences were observed in the binding characteristics of VPA to serum proteins between the younger and older groups. Our results show that age, but not gender, has significant influences on the binding characteristics of VPA to serum proteins in our patient population.

Distribution of valproate to subdural cerebrospinal fluid, subcutaneous extracellular fluid, and plasma in humans: a microdialysis study

We sought to study the time course of the distribution of valproate (VPA) to subdural cerebrospinal fluid (CSF) in relation to subcutaneous extracellular fluid (ECF) and plasma after a single oral dose and to study the distribution to these three compartments under steady-state conditions. Microdialysis was used to estimate unbound VPA concentrations in subdural CSF and subcutaneous ECF, and blood samples were drawn for estimation of total and unbound VPA plasma concentrations in four patients with drug-resistant partial epilepsy undergoing presurgical evaluation with subdural EEG monitoring. Three patients were given a single oral dose of VPA, and one patient was receiving regular VPA treatment. VPA was analyzed by gas chromatography with flame ionization detection. The distribution of VPA to subdural CSF was rapid (Tmax, 3.5 h in two patients and 5.5 h in one patient) and subject to a minor delay in all three patients compared with that in the subcutaneous tissue ECF (Tmax, 2.5 h in all three patients), which in turn exhibited no evidence of a distribution delay compared with plasma. Subdural CSF levels of VPA were slightly lower than subcutaneous ECF levels (mean ratio, 0.78) and unbound plasma levels (mean ratio, 0.91). VPA rapidly enters the subdural CSF in unbound concentrations marginally lower than those obtained in subcutaneous ECF and plasma. These findings provide a pharmacokinetic rationale for acute administration of VPA. The good correlation between VPA concentrations in subcutaneous ECF and subdural CSF indicates that estimation of unbound VPA concentrations in subcutaneous tissue using microdialysis sampling has the potential to be useful for monitoring purposes.

Cell motility is inhibited by the antiepileptic compound, valproic acid and its teratogenic analogues

Valproic acid (VPA) is an established human teratogen that causes neural tube defects in 1-2% of human foetuses exposed to the drug during early pregnancy. In this study, individual cell motility was evaluated using short- and long-term time-lapse video-recording and computer assisted image analysis, and it was found that VPA and selected VPA-analogues inhibited individual cell motility of L-cells in a dose-dependent manner. The compounds caused a decrease in the root-mean-square speed, S, and in the rate of diffusion, R, but an increase in the time of persistence in direction, P. Using short-term recordings and measurements of mean-cell speed, the reduction in the motile behaviour was shown to correlate with the teratogenic potency of the tested compounds. The observed effects of VPA on cell motility was independent of the employed L-cell clone, and could be reproduced in cells containing the neuronal marker NCAM and in the neuronal cell line N2a. Furthermore, the observed effect was independent of culture substratum, being observed for L-cells grown on fibronectin as well as on plastic. Immunofluorescence microscopy revealed that VPA-treatment of mouse L-cells caused a redistribution of F-actin and of a series of focal adhesion proteins, indicating that the effect of VPA on cell motility may be causally related to increased cell-substratum interactions or to alterations in the organisation or dynamics of the actin cytoskeleton.

No effect of gender or age on binding characteristics of valproic acid to serum proteins in pediatric patients with epilepsy

The gender- and age-related binding characteristics of valproic acid to serum proteins were determined in the pediatric population. Serum samples examined in the study were obtained from 61 pediatric patients (28 males, 33 females) with epilepsy on valproic acid monotherapy. Their ages ranged from 1 to 15 years (mean age with [SD]: 7.8 [3.9] years; < 10 years, n = 41; > or = 10 years, n = 20). The in vivo population binding parameters of valproic acid to serum proteins and theoretical minimal unbound serum fraction (fu) of valproic acid were determined in (1) all, (2) male and female subgroups, and (3) prepubescent (< 10 years) and pubescent (> or = 10 years) subgroups. The association constant (K) was approximately 1.4 times higher in male (0.018 L/mumol) than in female (0.013 L/mumol) patients, while the total concentration of binding sites (n(Pt)) was 1.2 times greater in female (1235 mumol/L) than in male (997 mumol/L) patients. The fu was 0.053 and 0.059 for male and female patients, respectively. The value of K was approximately 1.6 times higher in the pubescent (0.019 L/mumol) than in the prepubescent (0.012 L/mumol) patients, while the n(Pt) was 1.2 times higher in the prepubescent (1244 mumol/L) than in the pubescent (1057 mumol/L) patients. The fu was 0.063 for the prepubescent and 0.047 for the pubescent patients. No significant differences were observed in binding characteristics of valproic acid to serum proteins between male and female or younger and older patients. However, the differences in valproic acid binding to serum proteins appear to be relatively larger in binding affinity than in binding capacity between the two groups. Because no significant differences were observed in serum concentrations of total and unbound valproic acid, albumin, or free fatty acids between any subgroups (male and female, younger and older), the results suggest that gender or age may not be factors for the determination of the binding characteristics of valproic acid to serum proteins in pediatric patients.

Management of Seizures in the Elderly: A Survey of UK Geriatricians

The incidence and prevalence of epilepsy increase substantially with old age. Despite this, the investigation and management of this patient population remains a grey area. Four hundred and eleven (53%) consultant geriatricians responded to a questionnaire exploring their approach to seizures in the elderly in order to establish an overview of current clinical practice. Between one and five patients presenting with seizures, predominantly aged between 75–85 years, were reviewed monthly. Seventy per cent of geriatricians undertook to investigate the patients themselves with biochemical and haematological profiles performed by most. Electroencephalography and computerized tomographic scanning were routinely requested by a quarter of responders. Only 58% would themselves initiate therapy with antiepileptic drugs, with 16% of consultants starting treatment following the first seizure, 59% after a second and 5% after a third. Phenytoin was first choice for generalized tonic-clonic seizures with carbamazepine preferred for partial seizures. If good control was not obtained, 67% would substitute another first line drug, while 27% would add in a second. Less than 3% would use the new anticonvulsants lamotrigine or vigabatrin. Sixty per cent monitored anticonvulsant concentrations in patients with poor control or suspected toxicity. A wide variability was seen in the current approach to seizures in the elderly, which reflects a lack of established practice. Epilepsy clinics for the elderly would encourage structured research into the many unanswered questions affecting the care of older people with seizures.

Simultaneous and continuous measurement of free concentration of valproate in blood and extracellular space of rat cerebral cortex

Free concentration of valproate (VPA) was measured simultaneously and continuously in blood and in the extracellular space of cerebral cortex of rats by VPA-selective microelectrodes. Constant amounts of VPA were injected into the femoral vein with differing duration of injection. Immediately after drug application, the concentration of free VPA in blood and brain increased to a peak value, the degree of which increased with the speed of injection. Ten to 15 min after VPA injection, a plateau value was reached. This plateau value was equal in the extracellular space of cortex and in blood. The data indicate that VPA can "freely" cross the blood-brain barrier (BBB).

Variability and clinical relevance of the interaction between sodium valproate and carbamazepine in epileptic patients

Twenty-four epileptic patients (16 females, 8 males; aged 13-62 years) were studied before and after the addition of sodium valproate (VPA) 500 mg twice daily for 5 days. All had been established previously on carbamazepine (CBZ) as monotherapy (300-1600 mg daily in divided doses). Sixteen of these patients undertook a battery of cognitive function tests before and after VPA introduction. VPA had no effect on total or free CBZ concentrations. However, median concentrations of the active metabolite, CBZ 10,11 epoxide (CBZ-E), were significantly increased (CBZ-E before VPA 1.3 mg/l, after VPA 2.1 mg/l, P less than 0.01). The median rise was 25%, although the extent of the interaction ranged from a 25% decrease to an increase of 123% in CBZ-E concentrations. This was related to the marked inter-individual variation in circulating VPA (mean 25-69 mg/l), as CBZ-E concentrations correlated significantly with total (r = 0.5, P less than 0.05, 95% CI 0 to +0.08) and free (r = 0.7, P less than 0.001, 95% CI +0.09 to +0.25) VPA levels in individual patients. Although uncontrolled, no deterioration in performance of any of the cognitive function tests was observed following the addition of VPA. This study does not support immediate clinical relevance for this drug interaction between VPA and CBZ.

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.

Penetration of valproate and its active metabolites into cerebrospinal fluid of children with epilepsy

Levels of the antiepileptic drug valproate (VPA) and five of its active metabolites (2-en-VPA, 3-keto-VPA, and 3-,4-, and 5-hydroxy-VPA) were determined by gas chromatography-mass spectrometry in cerebrospinal fluid (CSF) of 15 epileptic children undergoing chronic treatment with VPA. In eight of these children, total and free drug and metabolite concentrations in plasma were also measured. All VPA metabolites present in plasma could also be detected in CSF, although concentrations were substantially lower than those of the parent compound. CSF concentrations of VPA and most of its metabolites were positively correlated with total and free concentrations in plasma. However, concentrations in CSF were always significantly lower than free plasma concentrations, which may be explained by asymmetric transport at the blood-CSF barrier. The data on low CSF levels of VPA metabolites do not exclude the possibility that accumulation of active metabolite(s) may occur in certain brain areas during chronic treatment of epileptic patients with VPA.

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.

The prophylactic use of phenytoin during iopamidol contrast studies of the subarachnoid space

Contrast examinations of the subarachnoid space are associated with side effects including convulsions. Attention has been given to the prophylactic use of anticonvulsants. We describe a simple oral regimen using the established anticonvulsant phenytoin that can be administered to short-stay patients and that achieves effective serum and CSF concentrations. A preliminary account of this work was presented to the British Pharmacological Society in January 1984 in London.

Therapeutic levels of sodium valproate inhibit mitotic indices in cells of neural origin

Sodium valproate (VPA) inhibited the mitotic index of neuroblastoma (Neuro-2A) and glioma (C6) cells with an IC50 of 0.5 and 1.0 mM, respectively. Continued exposure of these cell lines to 1 mM VPA induced differentiation and increased adhesiveness. These observations are characteristic of putative teratogens and this implication for VPA is discussed.

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.

Protein binding and CSF penetration of phenytoin following acute oral dosing in man

Prophylactic phenytoin (DPH) has been evaluated in 20 patients undergoing diagnostic myelography. DPH (0.75 g) was ingested at 20.00 h the night before and 0.5 g at 08.00 h on the morning of the procedure. Total DPH concentrations at myelography (mean +/- s.d.: 12.7 +/- 4.3 mg l-1; range 6.3-21.5 mg l-1) correlated with CSF values (1.3 +/- 0.46 mg l-1; range 0.7-2.2 mg l-1; r = 0.83, P less than 0.001). DPH protein binding at that time varied two-fold (9.2-18.5%) and free drug levels (1.7 +/- 0.6 mg l-1) correlated with CSF (r = 0.83, P less than 0.001) and total (r = 0.89, P less than 0.001) plasma DPH concentrations. There were significant negative correlations between patient weight (n = 17) and total (r = 0.57, P less than 0.05) and CSF (r = -0.55, P less than 0.05) DPH concentrations at myelography. Total plasma DPH levels 8 h (14.5 +/- 3.9 mg l-1; range 7.3-20.6 mg l-1) and 24 h (12.3 +/- 3.8 mg l-1; range 5.0-19.8 mg l-1) after myelography were largely within the 'therapeutic range' of 10-20 mg l-1 for the drug. No patient suffered a seizure although, in two, spike discharges were seen on a post-myelography electroencephalogram. A simple regime involving two doses of DPH would provide acceptable plasma CSF concentrations as a basis for controlled studies in seizure prophylaxis following neuroradiological investigations involving intrathecal contrast.

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.