Total Valproate Concentrations Research Articles

Derivation and Validation of a New Equation for Estimating Free Valproate Concentration in Critically Ill Adult Patients.

Protein binding of valproate varies among ICU patients, altering the biologically active free valproate concentration (VPAC). Free VPAC is measured at few laboratories and is often discordant with total VPAC. Existing equations to predict free VPAC are either not validated or are inaccurate in ICU patients. We designed this study to derive and validate a novel equation to predict free VPAC using data from ICU patients and to compare its performance to published equations. Retrospective cohort study. Two academic medical centers. Patients older than 18 years old with concomitant free and total VPACs measured in the ICU were included in the derivation cohort if admitted from 2014 to 2018, and the validation cohort if admitted from 2019 to 2022. Multivariable linear regression was used to derive an equation to predict free VPAC. Modified Bland-Altman plots and the rate of therapeutic concordance between the measured and predicted free VPAC were compared. Demographics, median free and total VPACs, and valproate free fractions were similar among 115 patients in the derivation cohort and 147 patients in the validation cohort. The Bland-Altman plots showed the new equation performed better (bias, 0.3 [95% limits of agreement, -13.6 to 14.2]) than the Nasreddine (-9.2 [-26.5 to 8.2]), Kodama (-9.7 [-30.0 to 10.7]), Conde Giner (-7.9 [-24.9 to 9.1]), and Parent (-9.9 [-30.7 to 11.0]) equations, and similar to Doré (-2.0 [-16.0 to 11.9]). The Doré and new equations had the highest therapeutic concordance rate (73%). For patients at risk of altered protein binding such as ICU patients, existing equations to predict free VPAC are discordant with measured free VPAC. A new equation had low bias but was imprecise. External validation should be performed to improve its precision and generalizability. Until then, monitoring free valproate is recommended during critical illness.

Evaluation of Free Valproate Concentration in Critically Ill Patients.

Retrospective multicenter cohort study. Two academic medical centers. Patients greater than or equal to 18 years of age with concomitant free and total valproate concentrations collected in the ICU. None. Two-hundred fifty-six patients were included in the study, with a median age of 56 years (42-70) and 65% of patients were male. The median total valproate concentration was 53 µg/mL (38-70 µg/mL), the free valproate concentration was 12 µg/mL (7-20 µg/mL), and the free fraction was 23.6% (17.0-33.9%). Therapeutic discordance between the free and total valproate concentration occurred in 70% of patients. On multivariable analysis, increased free valproate concentration was associated with higher total valproate concentration (per 5 µg/mL increase, increase 1.72 µg/mL, 95% CI, 1.48-1.96) and lower serum albumin (per 1 g/dL decrease, increase 4.60 µg/mL, 95% CI, 2.71-6.49). There was no association between free valproate concentration and adverse effects. The valproate total and free concentration was discordant in the majority of patients (70%). Increased valproate free concentration was associated with hypoalbuminemia and total valproate concentration. Clinical decisions based on total valproate concentration may be incorrect for many ICU patients. Prospective, controlled studies are needed to confirm these findings and their clinical relevance.

Monitoring for valproate and phenytoin toxicity in hypoalbuminaemia: A retrospective cohort study.

Equations to calculate albumin-adjusted total concentrations have been validated to correlate with measured free concentrations for both phenytoin and valproate, but there is a lack of data to assess correlation with clinical outcomes. We aimed to assess the association of hypoalbuminaemia and albumin-adjusted total concentrations with concentration-dependent toxicity for phenytoin and valproate and review the impact on management decisions following concentration monitoring in hypoalbuminaemia. Patients undergoing concentration monitoring for phenytoin or valproate between January and December 2018 were included. Patients were identified using a centralised laboratory database with data extracted from medical records. Total phenytoin concentrations were measured for 144 patients, with hypoalbuminaemia (≤30 g L-1 ) recorded in 59 (41%) patients. Albumin-adjusted phenytoin concentration >20 mg L-1 was associated with increased neurological adverse effects (77% vs. 43%, P < .001). On logistic regression, higher albumin-adjusted phenytoin concentration was an independent risk factor for neurotoxicity (OR 1.06, 95% CI 1.01-1.12, P = .011). Total valproate concentrations were measured for 383 patients, with hypoalbuminaemia (≤30 g L-1 ) noted in 53 (14%) patients. For the valproate cohort, hypoalbuminaemia (42% vs. 28%, P = .039) and albumin-adjusted valproate concentration >100 mg L-1 (49% vs. 23%, P < .001) were both associated with increased neurotoxicity. On multiple logistic-regression, valproate daily dose (aOR = 1.01, 95% CI 1.00-1.02, P = .006) and albumin-adjusted valproate concentration (aOR 1.01, 95% CI 1.00-1.02, P = .033) were independent risk factors for neurotoxicity after accounting for confounders. While measuring free drug concentrations in hypoalbuminaemia would be ideal, the adjustment equations can help identify vulnerable patients needing further assessment of potential concentration-dependent toxicity.

Determinants of free serum valproate concentration: A prospective study in patients on divalproex sodium monotherapy

PurposeTo evaluate variables affecting the valproate (VPA) free fraction and develop an equation for computing free VPA concentration from total VPA concentration. MethodsTrough total and free VPA concentrations were collected from patients who participated in a prospective VPA monotherapy trial. All available paired data of trough total and free VPA concentrations were included. Significant variables from the univariate analysis were evaluated in a multivariate model. ResultsA total of 902 concomitant total and free VPA concentrations were available. Multivariate analysis showed that total VPA concentration, age and gender were significantly associated with VPA free fraction. However, the effect size of total VPA concentration was substantially higher than that of gender and age. VPA free fraction remained stable at around 10% for total VPA concentration between 20 and 60 μg/mL with subsequent linear increases for higher concentration. A scatter plot correlating total and free VPA concentrations showed that a quadratic equation best fitted the data, accounting for 88% of the free VPA concentration variance. ConclusionsAn increase in the total VPA concentration results in corresponding linear and non-linear rise in the VPA free fraction and free VPA concentration, respectively. The total daily dose of VPA should be increased in smaller increments whenever a total VPA concentration of 60 μg/mL is reached. When drug monitoring is needed, we recommend measuring the free VPA concentration. If this test is unavailable, and for patients with normal albumin levels, it can be predicted from the total VPA concentration using the generated equation.

Pharmacokinetic variability of valproate during pregnancy – Implications for the use of therapeutic drug monitoring

Background and purposeUse of valproate (VPA) in women of childbearing age is restricted due to dose-dependent risk of teratogenicity. The purpose of this study was to characterise pharmacokinetic variability of VPA in pregnancy, and discuss use of therapeutic drug monitoring (TDM) as guidance to exposure in women. MethodMeasurements of trough total and unbound VPA concentrations before, during and after pregnancy, at assumed steady-state were collected from the TDM-database (2006–2016) at the National Center for Epilepsy in Norway. Additional clinical data were obtained from the Oppland county Perinatal Database (1994–2011). ResultsData from 51 pregnancies in 33 women aged 19–40 years were included. Each woman underwent 1–4 pregnancies, and 1–7 measurements per pregnancy were performed. The variability in total concentration/dose (C/D)-ratios between women was 13-fold, and intra-patient variability extensive. Total C/D-ratios were reduced by 46% from before pregnancy to third trimester (0.48–0.29 μmol/L/mg). Unbound concentrations of VPA were only requested in 10% of the pregnancies. Repeated measurements from two pregnancies in one women revealed increased unbound concentration of VPA during pregnancy. There were 19 with idiopathic generalized epilepsy and two focal based on clinical data from 21 women and 38 pregnancies; 1 major congenital malformation was noted. ConclusionThere is pronounced pharmacokinetic variability of VPA during pregnancy. Unbound concentrations are rarely requested. TDM should be used by measurements of both total and unbound concentrations since total concentrations may be misleading for efficacy and fetal exposure of VPA.

Assessment of the Combined Treatment with Umbelliferone and Four Classical Antiepileptic Drugs Against Maximal Electroshock-Induced Seizures in Mice

Background/Aims: The aim of this study was to determine the effects of umbelliferone (7-hydroxycoumarin; UMB) on the anticonvulsant potency of four classical antiepileptic drugs (carbamazepine (CBZ), phenytoin (PHT), phenobarbital (PB) and valproate (VPA)) in the mouse maximal electroshock-induced seizure (MES) model. Results: UMB administered systemically intraperitoneally (ip) in a dose of 150 mg/kg significantly elevated the threshold for maximal electroconvulsions (p < 0.05) in mice. Moreover, UMB (150 mg/kg) co-administered with PB and VPA significantly enhanced the anticonvulsant potency of these drugs by reducing their median effective doses (ED₅₀ values) from 35.39 to 21.78 mg/kg (p < 0.01) for PB, and from 281.4 to 215.5 mg/kg (p < 0.01) for VPA. In contrast, UMB (150 mg/kg, ip) had no significant effect on the antiseizure activity of CBZ and PHT in the mouse MES model. Neither total brain PB, nor total brain VPA concentrations were altered after ip administration of UMB, indicating a pharmacodynamic nature of interactions between the tested drugs. Conclusions: The selective potentiation of the anticonvulsant potency of PB and VPA by UMB, and lack of any pharmacokinetic interactions between drugs, make the combinations of UMB with PB or VPA worthy of consideration for epileptic patients who are refractory to standard antiepileptic treatment.

Variability of carbamazepine and valproate concentrations in elderly nursing home residents

Measuring antiepileptic drug (AED) concentrations is common practice in nursing homes. Phenytoin (PHT) concentrations fluctuate substantially in many nursing home residents under constant dose conditions; however, the stability of other AED concentrations has not been studied. We investigated the variability of carbamazepine (CBZ) and valproate (VPA) concentrations under constant dose conditions in US nursing home residents. A database of elderly persons (≥65 years) in 119 nursing homes throughout the US was reviewed for residents with at least one measurement of total PHT, CBZ or VPA. Inclusion criteria for this study were three or more serum concentration measurements while on the same dose of CBZ or VPA, a two-month minimum stay, and no interfering co-medications (inducers or inhibitors). Enrollment occurred over a 2-year period. Data were collected on residents for a minimum of 6 months. Of the 593 residents identified, 245 had CBZ or VPA concentrations measured and 44 (18%) met inclusion criteria (22 on CBZ and 22 VPA). Some subjects had little variability in AED concentrations, others had large fluctuations. Total CBZ concentrations within individuals varied as little as 0mg/L to as much as 6.3mg/L and total VPA concentrations as little as 10.0mg/L to as much as 77.6mg/L. The variability of PHT, CBZ, and VPA concentrations in many but not all nursing home residents implies that a re-evaluation of the role of AED concentration measurements in the management of patients is needed. Strategies for use and interpretation of AED concentration measurements need to be reevaluated.

Effect of sildenafil on the anticonvulsant action of classical and second‐generation antiepileptic drugs in maximal electroshock‐induced seizures in mice

The goal of the present study was to evaluate the effects of sildenafil on the threshold for electrically induced seizures in mice. In addition, interactions between sildenafil and classical and second-generation antiepileptic drugs (AEDs), that is, carbamazepine (CBZ), phenobarbital (PB), phenytoin (PHT), valproate (VPA), lamotrigine (LTG), topiramate (TPM), and oxcarbazepine (OXC) were evaluated. Two electroconvulsive tests were used: maximal electroshock seizure threshold (MEST) and maximal electroshock seizure (MES) tests in mice. Acute adverse effects of the studied combinations were investigated in the chimney test, step-through passive avoidance task, and grip-strength test. Total brain and free plasma concentrations of AEDs were also determined. Sildenafil raised the threshold for electroconvulsions in a dose-dependent manner. It also increased the anticonvulsant activity of CBZ, VPA, and TPM in the MES test, whereas the activity of the remaining AEDs was not significantly changed. Sildenafil increased total brain and free (protein unbound) plasma CBZ concentrations and total brain VPA concentration. Neither sildenafil nor its coadministration with the studied AEDs affected motor coordination and long-term memory in mice. Interestingly, sildenafil dose-dependently enhanced the skeletal muscle strength in mice, although combinations of sildenafil with AEDs were ineffective in this respect. Sildenafil significantly raised the threshold for electroconvulsions in mice without any impairment of motor performance and long-term memory, but it enhanced muscle strength. Treatment of patients on CBZ or VPA with sildenafil may not be recommended due to pharmacokinetic interactions. Coadministration of sildenafil with other AEDs, especially with TPM, seems to be a reasonable choice.

Interaction of tiagabine with valproate in the mouse pentylenetetrazole-induced seizure model: an isobolographic analysis for non-parallel dose-response relationship curves

To characterize the interaction between tiagabine (TGB) and valproate (VPA)--two antiepileptic drugs in the mouse pentylenetetrazole (PTZ)-induced clonic seizure model, type I isobolographic analysis for non-parallel dose-response relationship curves (DRRCs) was used. Clonic seizures were evoked in albino Swiss mice by subcutaneous injection of PTZ at its CD97 (100 mg/ kg). To ascertain the nature of interaction between TGB and VPA administered in combination, total brain concentrations of TGB and VPA were estimated by using high-performance liquid chromatography (HPLC) and fluorescence polarization immunoassay (FPIA). TGB and VPA produced clear-cut anticonvulsant effects against PTZ-induced clonic seizures in mice and their DRRCs were not parallel to one another. The type I isobolographic analysis for non-parallel DRRCs revealed that the combination of TGB with VPA at the fixed-ratio of 1:1 exerted additive interaction against PTZ-induced clonic seizures in mice. With FPIA, it was found that TGB did not affect total brain VPA concentrations in experimental animals. Moreover, VPA had no significant impact on total brain concentrations of TGB in mice, as measured with HPLC. The additive interaction between TGB and VPA at the fixed-ratio of 1:1 in the mouse PTZ model was pharmacodynamic in nature.

Isobolographic characterization of interactions of retigabine with carbamazepine, lamotrigine, and valproate in the mouse maximal electroshock-induced seizure model

The aim of this study was to characterize the pharmacodynamic, pharmacokinetic, and adverse-effect profiles of retigabine (RTG) in combination with carbamazepine (CBZ), lamotrigine (LTG), and valproate (VPA). The isobolographic analysis for parallel and nonparallel dose-response effects was used in the mouse maximal electroshock seizure (MES) model for evaluation of pharmacodynamic interaction. Potential adverse-effect profiles of interactions of RTG with CBZ, LTG, and VPA at the fixed ratio of 1:1 in the MES test were evaluated in the chimney (motor performance), passive avoidance (long-term memory), and grip strength (muscular strength) tests. Free plasma and total brain concentrations of CBZ, LTG, and VPA were determined by immunofluorescence and chromatography to assess pharmacokinetic interaction. In the MES model, RTG administered singly had its dose-response relationship curve (DRRC) parallel to that for VPA and nonparallel to that for CBZ and LTG. With isobolography for parallel DRRCs, the combination of RTG with VPA at fixed ratios of 1:3, 1:1, and 3:1 exerted supraadditive (synergistic) interaction. Isobolography for nonparallel DRRCs revealed that the combinations of RTG with CBZ and LTG at the fixed ratio of 1:1 produced additive interaction. In all combinations, neither motor coordination, long-term memory, nor muscular strength were affected. Only the combination of RTG with VPA at the fixed ratio of 3:1 was complicated by a pharmacokinetic increase in both free plasma and total brain VPA concentrations. All remaining combinations of RTG with VPA, CBZ, and LTG were pharmacodynamic in nature. RTG synergistically interacted with VPA and exerted additive interaction with CBZ and LTG in the mouse MES model.

Agmatine enhances the anticonvulsant action of phenobarbital and valproate in the mouse maximal electroshock seizure model

Accumulating evidence indicates that agmatine (AGM--an endogenous neuromodulator/neurotransmitter in the brain) exerts the anticonvulsant action in various in vivo experiments. Therefore, the aim of this study was to assess the influence of AGM on the protective action of numerous conventional and newer antiepileptic drugs [carbamazepine (CBZ), lamotrigine (LTG), oxcarbazepine (OXC), phenobarbital (PB), phenytoin (PHT), topiramate (TPM) and valproate (VPA)] in the mouse maximal electroshock seizure (MES) model. Results indicate that AGM (up to 100 mg/kg, i.p., 45 min before the test) neither altered the threshold for electroconvulsions nor protected the animals against MES-induced seizures in mice. Moreover, AGM (100 mg/kg, i.p.) significantly enhanced the anticonvulsant effects of PB and VPA in the MES test by reducing their ED50 values from 22.54 to 16.82 mg/kg (P < 0.01) for PB, and from 256.1 to 210.6 mg/kg (P < 0.05) for VPA, respectively. In contrast, AGM at 100 mg/kg (i.p.) had no significant effect on the antielectroshock action of the remaining drugs tested (CBZ, LTG, OXC, PHT, and TPM) in mice. Estimation of total brain PB and VPA concentrations revealed that the observed interactions between AGM and PB or VPA in the MES test were pharmacodynamic in nature because neither total brain PB, nor total brain VPA concentrations were altered after i.p. administration of AGM at 100 mg/kg. Moreover, none of the examined combinations of AGM (100 mg/kg) with CBZ, LTG, OXC, PB, PHT, TPM, and VPA (at their ED50 values from the MES test) affected motor coordination in the chimney test, long-term memory in the passive avoidance task, and muscular strength in the grip-strength test in mice, indicating no acute adverse effects in animals. In conclusion, one can ascertain that the selective potentiation of the antielectroshock action of PB and VPA by AGM, lack of any pharmacokinetic interactions between drugs and no acute adverse effects, make the combinations of AGM with PB or VPA of pivotal importance for epileptic patients. It seems that modulation of AGM concentration in the brain may occur favorable in further clinical practice.

Valproate unbound fraction and distribution volume following rapid infusions in patients with epilepsy

The availability of an intravenous formulation now makes possible rapid administration of valproate (VPA) loading doses, but estimates of key VPA pharmacokinetic parameters in patients have limited the use of this approach. VPA disposition was characterized in 112 epilepsy patients, with or without enzyme inducing comedications, randomized to either 3.0 or 1.5 mg/kg/min infusions of valproate sodium injection. Maximum dose was ≤15 mg/kg per infusion. Total and unbound plasma VPA concentrations were determined from blood samples obtained prior to and for 6 h following the infusion. Analyses of covariance assessed the effect of induction, weight, age, gender, albumin, creatinine, and infusion rate on pharmacokinetics. Maximum total and unbound VPA concentrations were 94 and 14 mg/l, respectively. Total concentration fell below 50 mg/l within 3 h in induced and 6 h in uninduced patients. VPA unbound fraction decreased from 15% at maximum concentration to 9% at 45 mg/l. The mean (S.D.) distribution volume was 0.21 (0.044) l/kg. Induction status, albumin concentration, and infusion rate significantly affected pharmacokinetics. Measurement of unbound VPA may be useful when alterations in binding are suspected. Infusions up to 3 mg/kg/min produce predictable total VPA concentrations when induction status and albumin levels are considered.

Effect of Felbamate on the Plasma Protein Binding of Valproate

Coadministration of felbamate is known to raise plasma valproate concentrations. In an attempt to explain this interaction, the effect of felbamate (FBM) on the plasma protein binding of valproate (VPA) was investigated in vitro and in healthy subjects. In fresh drug-free human plasma spiked with 20.0, 60.0 and 100.0 mg NaVPA/L without and with 75.5 mg FBM/L, the overall mean (± SD) unbound VPA fraction was 10.3 ± 1.6% without and 11.7 ± 2.1% with FBM. Regression analysis, used to distinguish FBM effects on plasma protein binding from effects due to altered total VPA concentration (VPA shows concentrationdependent protein binding), indicated that the mean percentage unbound VPA rose by 1.44% (95% CI = 0.59 to 2.30%). In 12 subjects the mean (± SD) trough concentrations of total VPA in plasma rose from 21.0 ± 6.1 mg/L to 45.1 ± 9.5 mg/L (p < 0.01) when VPA was taken, and the overall mean (± SD) unbound VPA fraction increased from 10.5 ± 0.77% to 11.7 ± 1.17%. Regression analysis indicated that there was a mean increase of 0.68% (95% CI = 0.13 to 1.23%) in plasma-unbound VPA because of a direct effect of FBM on plasma protein binding, the remaining 0.5% increase being due to the altered VPA concentration. While FBM appears to displace VPA from plasma proteins, this interaction is small and unlikely to make any important contribution to the overall effect of adding FBM to VPA, although the total interaction between the two may be large enough to necessitate a lowering of VPA dose.

Decreased plasma protein binding of valproate in patients with acute head trauma.

1. One hundred and ten plasma samples were obtained from 50 patients treated with valproate for prophylaxis of post-traumatic head injuries. The samples were selected to include a wide range of albumin concentrations and were assayed for free and total valproate concentrations. Valproate binding parameters were determined from the Scatchard equation for one binding site using reweighted least squares analysis. 2. Plasma albumin concentrations were measured in 130 patients with head trauma. They started to decrease immediately after trauma, reaching a minimum at 5-7 days of approximately 24% of baseline value and did not return to normal until 1 month. 3. The free fraction of valproate varied six to seven-fold as albumin concentration ranged from 1.5 to 4.8 g 100 ml-1 (218-696 mumol l-1). The mean association constant for binding (Ka) was 0.008 mumol l(-1) and the mean number of binding sites (N) was 2.0. There values were similar to those reported for valproate in otherwise healthy patients with epilepsy. 4. Because of saturable protein binding of valproate, hypoalbuminaemia may necessitate the monitoring of free valproate concentrations to avoid toxicity when valproate is used in patients with acute head injury.

The disposition of valproate and its metabolites in the late first trimester and early second trimester of pregnancy in maternal serum, urine, and amniotic fluid: effect of dose, co-medication, and the presence of spina bifida.

We have studied 52 pregnancies in epileptic women taking long-term valproate and have measured the concentrations of the parent compound and 13 of its metabolites by gas chromatography-mass spectrometry in amniotic fluid, maternal serum, and 24 h maternal urine samples. All metabolites of valproate present in the serum could also be detected in the amniotic fluid, although at much lower concentrations. Amniotic fluid concentrations of valproate and several of its metabolites ((E) delta 2-valproate, (2E,3'E) delta 2,3'-valproate, and 3-keto-valproate) correlated with total valproate concentrations as well as with unbound valproate concentrations in maternal serum. We suggest that the amniotic fluid acts as a deep compartment, with slow appearance and disappearance of valproate and its main metabolites. The data further suggest that during the first and early second trimesters of pregnancy the beta-oxidation of valproate decreases. In pregnancies associated with fetal neural tube defects (n = 5) significantly higher daily doses of valproate were used compared with normal pregnancies (n = 47). This resulted in higher concentrations of valproate in maternal serum. However, the metabolite patterns in maternal serum, 24 h urine samples, and amniotic fluid did not show any significant differences in pregnancies with neural tube defects.

Cognitive function in adult epileptic patients on long-term sodium valproate

A battery of psychometric tests was administered to 102 adult patients with epilepsy. Forty-eight were receiving long-term treatment with sodium valproate (VPA) as monotherapy, and 28 took a combination of VPA with either carbamazepine or phenytoin. An additional 26 patients were untreated, and the tests were also administered to 24 nonepileptic controls. The polypharmacy group performed significantly less well than the other groups in 6 of the 11 tests. Overall, there were no differences in performance between the VPA monotherapy patients and both the drug-free groups. However, patients with higher total and free VPA concentrations reported more sedation and scored less well on the threshold detection task. Patients were divided into those “tolerant” and “intolerant” to the drug in terms of these two tests. The “intolerant” group had a significantly earlier age of onset of epilepsy and a lower nonverbal IQ. Monotherapy with VPA is unlikely to impair cognitive function and may be a suitable choice if cognitive function is an important consideration.

Protein Binding of Antiepileptic Drugs During Pregnancy, Labor, and Puerperium

Twenty-four epileptic women were followed-up during late pregnancy, labor, and early puerperium in order to detect possible alterations in serum protein binding of antiepileptic drugs (AEDs). The total and free concentrations of carbamazepine (CBZ), phenytoin (PHT), and valproate (VPA) in maternal serum were measured. In addition, the concentrations of albumin, alpha 1-acid glycoprotein (AGP), and free fatty acids (FFA) were also measured. Total AED concentrations during labor were influenced by changes in drug dosages; total PHT increased during the first puerperal weeks. During labor the free fraction of CBZ remained stable, whereas PHT and particularly VPA free fractions increased. This phenomenon was parallel to the increase in FFA concentration; FFA concentrations decreased again during the first days postpartum. Albumin and AGP concentrations were low during pregnancy and labor, and increased after delivery. The total umbilical CBZ and PHT concentrations were not significantly different from maternal concentrations. The total VPA concentration in umbilical serum was significantly higher than that in maternal serum. The free fraction of CBZ was higher and that of PHT and VPA lower in umbilical than in maternal serum at delivery. Umbilical cord serum had a higher albumin but a lower AGP and FFA concentration than maternal serum. The changes in PHT and particularly VPA free fraction associated with changes in FFA concentration should be considered when assessing the total concentration of these drugs in maternal and umbilical serum.

Free and total serum valproate concentrations: their relationship to seizure control, liver enzymes and plasma ammonia in children.

The relationships between total and free serum valproate (VPA) concentrations and seizure control, serum liver enzyme activity and plasma ammonia concentration were studied in 61 epileptic children. Enzyme-immunoassay (EMIT)R methods gave higher values of total VPA concentration than gas-liquid chromatography (GLC) methods. In over 80% of children with complete seizure control the ranges of total VPA concentration were 140-420 mumol/L with GLC methods and 210-560 mumol/L with EMIT methods. The range of free VPA concentrations in 78% of children with complete seizure control was 8.8-26.4 mumol/L. Increased liver enzyme activity was observed in 6 of the 61 children and raised plasma ammonia concentration in 11 of 50 children. Plasma ammonia concentration was related to total serum VPA but was not related to free serum VPA. Increased serum liver enzyme activity was related to VPA dose per kg but not to free or total serum VPA concentration. Thus free VPA concentrations do not appear to be more useful than total VPA concentrations in predicting seizure control and do not correlate with liver enzyme activity or plasma ammonia concentration.

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.

The influence of free fatty acids on valproic acid plasma protein binding during fasting in normal humans.

The effect of physiologic variations of free fatty acid levels on in vivo valproic acid plasma protein binding was studied in 6 healthy adult subjects. 14 blood samples were taken during a 12-h dosing interval at steady state while in a fed condition and also during a 27 h fast. Free fraction and total valproate concentration were determined by equilibrium dialysis and GLC, respectively. Free fatty acid levels were determined from both fresh samples and samples incubated at 37 degrees C for 12 h, the latter in order to simulate equilibrium dialysis conditions. Fasting resulted in increased serum free fatty acid levels in all subjects, ranging from 34-182% (p less than 0.01). Incubation also caused free fatty acid levels to rise, more so in fed samples (50-87%, p less than 0.01) than in fasting samples (10-50%, p less than 0.01). Fasting resulted in a 9% increase in the mean free fraction for all subjects combined (P less than 0.01). Regression analysis of 180 sets of values for free fraction, total valproate concentration and free fatty acid level suggested that valproate concentration accounts for 17% and free fatty acid level for 37% of the variation in free fraction. Mean clearance was unchanged by fasting despite an increased free fraction suggesting decreased intrinsic clearance (i.e. decreased metabolism) of valproate under these conditions.