Plasma protein binding of valproic acid in healthy subjects and in patients with renal disease

Valproic Acid for Hyperactive Delirium and Agitation in Critically Ill Patients

BACKGROUND

Delirium and agitation are common syndromes in critically ill patients. Valproic acid (VPA) has shown benefit in intensive care unit (ICU)-associated delirium and agitation, but further evaluation is needed.

OBJECTIVE

The purpose of this study was to evaluate the effectiveness and safety of VPA for hyperactive delirium and agitation in critically ill adult patients.

METHODS

A retrospective cohort study at NYU Langone Health was conducted in critically ill patients treated with VPA for hyperactive delirium or agitation from October 1, 2017 to October 1, 2022. The primary outcome was effectiveness of VPA, defined as a reduction in the total number of any concomitant psychoactive medication by day 3 of VPA treatment. Secondary outcomes included the effect of VPA on the doses of concomitant medications and adverse events.

RESULTS

A total of 87 patients were included in the final analysis. By day 3 of VPA treatment, a 33% reduction (P < .001) in the total number of concomitant psychoactive medications was observed. VPA decreased the need for sedatives, as assessed by midazolam equivalents, but no significant changes were seen with dexmedetomidine alone, opioids, or antipsychotics. A 10 mg/kg loading dose was utilized in 36% of the cohort and its use decreased the risk for initiating additional psychoactive medications by day 3 of therapy (OR 2.8, 95% CI 1.0-7.8, P = .047), with benefits noted as early as 48 h after initiation. Adverse events were low in the total cohort (10.3%).

CONCLUSION AND RELEVANCE

The addition of VPA to a complex pharmacologic regimen for hyperactive delirium and agitation is safe and can assist in the prevention of polypharmacy and overall workload in critically ill patients admitted primarily for cardiogenic shock and respiratory failure requiring mechanical ventilation.

A novel method for predicting the unbound valproic acid concentration

In this study, we constructed a prediction formula for unbound valproic acid (VPA) concentration that was more accurate and widely applicable than previously reported formulae. A total of 136 datasets from 75 patients were analyzed retrospectively. The median of free fraction of VPA was 0.16 (interquartile range: 0.07; range: 0.07-0.45). The parameter that combined total VPA concentration (C_tVPA) and serum albumin (SA), (C_tVPA [μM] - 2 × SA [μM]), was significantly related to the free fraction of VPA (r = 0.76, p < 0.001). We constructed a combined parameter-based prediction formula for unbound VPA concentration. Analysis using external datasets from patients without severe renal failure showed that the prediction errors of the unbound VPA concentration were lower than those of previously reported formulae. Although the previous formulae showed large prediction errors, especially in the specific range of C_tVPA values, the constructed formula showed a weak trend with C_tVPA or SA. The formula based on (C_tVPA [μM] - 2 × SA [μM]) had high prediction accuracy and wide applicability in predicting the unbound VPA concentration in patients without severe renal failure.

Developmental changes in the extent of drug binding to rat plasma proteins

Binding of therapeutics to proteins in blood plasma is important in influencing their distribution as it is their free (unbound) form that is able to cross cellular membranes to enter tissues and exert their actions. The concentration and composition of plasma proteins vary during pregnancy and development, resulting in potential changes to drug protein binding. Here, we describe an ultrafiltration method to investigate the extent of protein binding of six drugs (digoxin, paracetamol, olanzapine, ivacaftor, valproate and lamotrigine) and two water soluble inert markers (sucrose and glycerol) to plasma proteins from pregnant and developing rats. Results showed that the free fraction of most drugs was lower in the non-pregnant adult plasma where protein concentration is the highest. However, plasma of equivalent protein concentration to younger pups obtained by diluting adult plasma did not always exhibit the same extent of drug binding, reinforcing the likelihood that both concentration and composition of proteins in plasma influence drug binding. Comparison between protein binding and brain drug accumulation in vivo revealed a correlation for some drugs, but not others. Results suggests that plasma protein concentration should be considered when using medications in pregnant and paediatric patients to minimise potential for fetal and neonatal drug exposure.

Evaluation of Free Valproate Concentration in Critically Ill Patients

Protein binding of valproate is variable in ICU patients, and the total valproate concentration does not predict the free valproate concentration, even when correcting for albumin. We sought to quantify valproate free concentration among ICU patients, identify risk factors associated with an increasing free valproate concentration, and evaluate the association between free valproate concentration with potential adverse drug effect.

DESIGN

Retrospective multicenter cohort study.

SETTING

Two academic medical centers.

PATIENTS

Patients greater than or equal to 18 years of age with concomitant free and total valproate concentrations collected in the ICU.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

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.

CONCLUSIONS

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.

Zinc Deficiency and Oxidative Stress Involved in Valproic Acid Induced Hepatotoxicity: Protection by Zinc and Selenium Supplementation

Valproic acid (VPA) is an antiepileptic drug, which its usage is limited due to its hepatotoxicity. The present study was conducted to investigate the efficacy of zinc (Zn) and selenium (Se), necessary trace elements, against VPA-induced hepatotoxicity in Wistar rats. The animals were divided into five groups: control, VPA 200 mg/kg, VPA + Zn (100 mg/kg), VPA + Se (100 mg/kg), and VPA + Zn + Se. The administration of VPA for four consecutive weeks resulted in decrease in serum level of Zn in rats. Also, an increase in liver marker enzymes (ALT and AST) and also histological changes in liver tissue were shown after VPA administration. Oxidative stress was evident in VPA group by increased lipid peroxidation (LPO), protein carbonyl (PCO), glutathione (GSH) oxidation, and reducing total antioxidant capacity. Zn and Se (100 mg/kg) administration was able to protect against deterioration in liver enzyme, abrogated the histological change in liver tissue, and suppressed the increase in oxidative stress markers. Zn and combination of Zn plus Se treatment showed more protective effects than Se alone. These results imply that Zn and Se should be suggested as effective supplement products for the prevention of VPA-induced hepatotoxicity.

Valproate Protein Binding Is Highly Variable in ICU Patients and Not Predicted by Total Serum Concentrations: A Case Series and Literature Review

STUDY OBJECTIVE

The free fraction of valproate (the pharmacologically active moiety, normally 5-10%) may vary significantly in critically ill patients, but this topic is understudied, with only four prior intensive care unit (ICU) case reports. The objective of this study was to evaluate the range of valproate plasma protein binding in ICU patients.

DESIGN

Observational study of consecutive ICU patients.

SETTING

Neurocritical and medical critical care services in a nonuniversity academic medical center.

PATIENTS

Consecutive ICU patients treated with valproate with serum albumin less than 4 g/dl.

MEASUREMENTS AND MAIN RESULTS

Simultaneous total and free trough serum valproate concentrations were measured as were serum creatinine, blood urea nitrogen, albumin, platelets, and transaminase values. The reference concentration range was 50-125 mg/L (total) and 5-17 mg/L (free). Valproate concentrations were categorized as within reference range, low, or high, and as concordant if both concentrations were in the same category. Data are reported as median (interquartile range). Fifteen patients (nine men) were evaluated. The median age was 63 (34-70) years. The valproate dose was 3 g/day (35 mg/kg/day). No patient had a valproate free fraction of 5-10%; the median was 48%, and the range was 15-89%. Total and free concentrations showed poor correlation (0.43) and were concordant in only two patients (both in the reference range). Free valproate concentration was poorly predicted by an equation correcting for albumin (r = 0.45). Suspected adverse drug events occurred in 10 patients: hyperammonemia in 7 of 12 tested (58%), elevated transaminases in 2 of 15 (13%), and thrombocytopenia in 5 of 15 (33%).

CONCLUSIONS

Protein binding of valproate was highly inconsistent in this cohort of ICU patients, and total valproate concentrations did not predict free concentrations, even when correcting for albumin. Additional research to define best practice for dosing and monitoring valproate and the relationship between free valproate concentrations and clinical or adverse effects in ICU patients is needed.

Impact of hypoalbuminemia on voriconazole pharmacokinetics in critically ill adult patients

Setting the adequate dose for voriconazole is challenging due to its variable pharmacokinetics. We investigated the impact of hypoalbuminemia (<35 g/liter) on voriconazole pharmacokinetics in adult intensive care unit (ICU) patients treated with voriconazole (20 samples in 13 patients) as well as in plasma samples from ICU patients that had been spiked with voriconazole at concentrations of 1.5 mg/liter, 2.9 mg/liter, and 9.0 mg/liter (66 samples from 22 patients). Plasma albumin concentrations ranged from 13.8 to 38.7 g/liter. Total voriconazole concentrations in adult ICU patients treated with voriconazole ranged from 0.5 to 8.7 mg/liter. Unbound and bound voriconazole concentrations were separated using high-throughput equilibrium dialysis followed by liquid chromatography-tandem mass spectrometry (LC-MSMS). Multivariate analysis revealed a positive relationship between voriconazole plasma protein binding and plasma albumin concentrations (P < 0.001), indicating higher unbound voriconazole concentrations with decreasing albumin concentrations. The correlation is more pronounced in the presence of elevated bilirubin concentrations (P = 0.05). We therefore propose to adjust the measured total voriconazole concentrations in patients with abnormal plasma albumin and total serum bilirubin plasma concentrations who show adverse events potentially related to voriconazole via a formula that we developed. Assuming 50% protein binding on average and an upper limit of 5.5 mg/liter for total voriconazole concentrations, the upper limit for unbound voriconazole concentrations is 2.75 mg/liter. Alterations in voriconazole unbound concentrations caused by hypoalbuminemia and/or elevated bilirubin plasma concentrations cannot be countered immediately, due to the adult saturated hepatic metabolism. Consequently, increased unbound voriconazole concentrations can possibly cause adverse events, even when total voriconazole concentrations are within the reference range.

Peritoneal Dialysis Alters Tolbutamide Pharmacokinetics in Rats with Experimental Acute Renal Failure

The plasma concentration profile of the antidiabetic agent tolbutamide was investigated in glycerol-induced acute renal failure (ARF) rats receiving or not receiving peritoneal dialysis (PD) to assess the impact of performing dialysis on tolbutamide pharmacokinetics. It was revealed that the plasma concentration of tolbutamide was decreased by 23.4% by performing PD in ARF rats, while it was not changed by PD in normal rats. The decrease in the plasma concentration of tolbutamide was nearly proportional to the increase in its volume of distribution. To clarify the mechanisms responsible for the decreased tolbutamide concentration caused by PD, the plasma protein binding of tolbutamide was examined in normal and ARF rats. The plasma unbound fraction of tolbutamide was higher in ARF rats than in normal rats, and the dissociation constants were 3.5+/-0.7 and 5.5+/-0.2 microg in normal and ARF rats, respectively. These results indicated that the unbound fraction of tolbutamide was increased in ARF rats because of its protein binding being suppressed. It is therefore likely that since a measurable amount of tolbutamide can distribute in peritoneal dialysate in ARF rats, but not in normal rats, the plasma concentration of tolbutamide was decreased by performing PD only in ARF rats. These findings suggest that diabetes medication with tolbutamide should be carefully performed in patients receiving dialysis treatment.

Hypouricemia in severely disabled children: influence of valproic acid and bed-ridden state

Although hypouricemia does not directly elicit clinical symptoms, it is a sensitive indicator for detecting renal tubular involvement. To determine the influence of valproic acid (VPA) and a non-ambulatory state on the serum uric acid level in epileptic children, we performed a cross-sectional study of laboratory data including serum and urinary uric acid levels and renal tubular function levels in epileptic children. We studied 93 patients in our outpatient clinic. They were divided into four groups according to two factors; VPA administration and the ambulatory state: non-ambulatory patients taking VPA (24 cases), non-ambulatory patients not taking VPA (18 cases), ambulatory patients taking VPA (29 cases), and ambulatory patients not taking VPA (22 cases). The laboratory data including uric acid levels and renal tubular function in each group were analyzed statistically using analysis of variance. Both VPA (P<0.05) and a non-ambulatory state (P<0.01) significantly decreased the serum uric acid levels with its increased urinary excretion of uric acid. However, in ambulatory patients, the uric acid level was not decreased. Serum uric acid levels was significantly decreased in non-ambulatory, severely disabled children treated with VPA. It should be borne in mind that VPA-induced renal tubular dysfunction may be present in severely disabled children. However, further investigation is necessary to determine the factor in severely disabled children that causes hypouricemia

Assessment of tear concentrations on therapeutic drug monitoring. II. Pharmacokinetic analysis of valproic acid in guinea pig serum, cerebrospinal fluid, and tears

PURPOSE

To quantitatively describe the pharmacokinetics of valproic acid (VPA) in guinea pig serum (total [Cf+b] and free [Cf]), cerebrospinal fluid (CSF) [C]CSF and tears [C]T using a simple kinetic model, and to examine whether [Cf] and [C]CSF can be predicted by [C]T using the resulting pharmacokinetic parameters.

METHODS

[Cf+b], [Cf], [C]CSF and [C]T were determined after bolus i.v. injection of 10 or 20 mg/kg VPA using GC/ECNCI/MS.

RESULTS

[Cf+b] could be quantitatively described by a two compartment model with linear elimination kinetics. [Cf] was separately analyzed using multi-exponential equations. [C]CSF was analyzed using a simple kinetic model in which the CSF compartment is independently connected with the serum compartment by the apparent diffusion constants (KINCSF and KOUTCSF). [C]T was analyzed using the same simple kinetic model used for [C]CSF. The values of [C]CSF and [Cf] in the steady state can be represented by the following equations; [C]CSF = KINCSF/KOUTCSF x [Cf], [Cf] = KOUT/KINT x [C]T, and indicating that [Cf] and [C]CSF can be predicted by [C]T using the resulting pharmacokinetic parameters.

CONCLUSIONS

The measurement of [C]T which can be collected non-invasively and estimated the pharmacokinetic parameters for [Cf], [C]CSF, and [C]T might be a very useful method for TDM of VPA.

Comparison of the pharmacokinetics of lamotrigine in patients with chronic renal failure and healthy volunteers

AIMS

The aim of this study was to compare the pharmacokinetics of the anti-epileptic agent, lamotrigine, in patients with chronic renal failure and healthy volunteers.

METHODS

Non-compartmental pharmacokinetics of a single oral dose (200 mg) of the anti-epileptic agent, lamotrigine, and its main metabolite, lamotrigine N2-glucuronide, were determined for 10 patients with chronic renal failure of mean estimated creatinine clearance 18 ml min-1 and a control group of 11 healthy volunteers, matched for age and gender.

RESULTS

For lamotrigine, there were no significant differences in Cmax, tmax, AUC, t1/2,z, CL/F and amount excreted in urine although t1/2,z tended to be longer for the renal failure group with a mean (+/-s.d.) of 35.9 +/- 10.7 h vs 27.8 +/- 4.3 h for the control group. For the renal failure group. VZ/F was 18% higher (95% CI 1% to 39%) compared with controls and CLR was reduced to 61% (95% CI 46% to 80%) of the control group value. For lamotrigine glucuronide, Cmax was increased 4-fold (95% CI 3.1 to 5.3) and AUC 7.8-fold (95% CI 6.0 to 10.1) in the renal failure group compared with controls. CLR was approximately 9-fold lower and apparent t1/2 was increased by 53% (95% CI 27% to 84%). Concentrations of an N2-methylated cardio-active metabolite, previously observed in dogs, were below the limit of detection (2 ng ml-1) of the ASTED/h.p.l.c. assay in the renal failure group as well as controls.

CONCLUSIONS

These results indicate that impaired renal function will have little effect on the plasma concentrations of lamotrigine achieved for a given dosing regimen.

Comparison of two binding equations for prediction of the concentration of unbound valproic acid in the serum of adult epileptic polytherapy patients

Because binding of valproic acid to plasma proteins affects the efficacy of the drug in the treatment of epilepsy (only the unbound fraction of the drug is effective) we have compared two methods which use different binding parameters to predict the in-vivo concentration of unbound valproic acid in serum. The study was performed on 46 serum samples from 29 polytherapy adult patients with epilepsy. Mean prediction error, mean absolute prediction error and root mean squared error were calculated for each method; these values served as a measure of prediction bias and precision. The mean absolute prediction errors and root mean squared errors for the two methods were similar in magnitude (Method 1, mean absolute prediction error = 10.0 microM, root mean squared error = 15.0 microM; Method 2, mean absolute prediction error = 10.3 microM, root mean squared error = 13.5 microM). Method 2 had a general tendency to over-predict unbound valproic acid; both methods had a tendency to over-prediction for total concentrations above 500 microM. Method 1 had a tendency to under-prediction at total concentrations below 250 microM. Within the total concentration range of valproic acid investigated, Method 1 was superior to Method 2 for prediction of unbound serum valproic acid. Our approach using Method 1 may be useful for prediction of unbound serum valproic acid concentration in patients with total valproic acid concentrations ranging from 250 to 500 microM; Method 2 may be useful for patients with total valproic acid below 500 microM. Our results suggest that there is wide and unpredictable variability in valproic acid binding to serum proteins among study populations.

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.

Plasma protein binding kinetics of valproic acid over a broad dosage range: therapeutic implications

The aim of the study was to characterize, from the relationship between total and free serum levels of valproic acid obtained over a broad dosage range (10-50 mg/kg), the parameters defining the in-vivo kinetic behaviour of the binding of valproic acid to plasma proteins, their pharmacokinetic and clinical repercussions, and their application to therapeutic drug monitoring (TDM). The study was performed in nine healthy adults (20-35 years) who were given doses of 1000 (group A), 2000 (group B) and 3000 mg (group C) of sodium valproate according to a compensated cross-over design, simultaneously determining the total and free serum levels of valproic acid over a 24-h period. The mean free fraction increases with dose, although this increase is only significant (P < 0.05) for the highest dose (3000 mg). The variation in the free fraction of valproic acid begins to become significant (P < 0.05) at a total drug concentration above 100 mg/l. The mean values of the dissociation constant (K) and binding sites (n) were 460 mumol/l and 1.79, respectively, showing a variability of 86.6 and 38.7%, respectively, and a residual variability of 13.0%. Significant differences (P < 0.05) were found for the total plasma clearance (Cl) but not for the intrinsic plasma clearance (Clu) values, despite their tendency to decrease with the dose. If TDM is to be used for valproic acid, it is the free serum levels that should be determined, especially if high doses are administered, because the total serum levels are not a true reflection of the free ones, as is the case of other anti-epileptic drugs.

Nonlinear protein binding and enzyme heterogeneity: effects on hepatic drug removal

The kinetics of substrate removal by the liver and the resulting nonlinear changes in unbound fraction along the flow path at varying input drug concentrations were examined by a model simulation study. Specifically, we varied the binding association constant, KA, and the Michaelis-Menten constants (Km and Vmax) to examine the steady state drug removal (expressed as hepatic extraction ratio E) and changes in drug binding for (i) unienzyme systems and (ii) simple, parallel metabolic pathways; zonal metabolic heterogeneity was also added as a variable. At low KA, E declined with increasing input drug concentration, due primarily to saturation of enzymes; only small differences in binding were present across the liver. At high KA, a parabolic profile for E with concentration was observed; changes in unbound fraction between the inlet and the outlet of the liver followed in parallel fashion. Protein binding was the rate-determining step at low input drug concentrations, whereas enzyme saturation was the rate-controlling factor at high input drug concentration. Heterogeneous enzymic distribution modulated changes in unbound fraction within the liver and at the outlet. Despite marked changes in unbound fraction occurring within the liver for different enzymic distributions, the overall transhepatic differences were relatively small. We then investigated the logarithmic average unbound concentration and the length averaged concentration as estimates of substrate concentration in liver in the presence of nonlinear drug binding. Fitting of simulated data, with and without assigned random error (10%), to the Michaelis-Menten equation was performed; fitting was repeated for simulated data obtained with presence of a specific inhibitor of the high-affinity, anteriorly distributed pathway. Results were similar for both concentration terms: accurate estimates were obtained for anterior, high affinity pathways; an overestimation of parameters was observed for the lower affinity posteriorly distributed pathways. Improved estimations were found for posteriorly distributed pathways upon inhibition with specific inhibitors; with added random error, however, the improvement was much decreased. We applied the method for fitting of several sets of metabolic data obtained from rat liver perfusion studies performed with salicylamide (SAM) (i) without and (ii) with the presence of 2,6-dichloro-4-nitrophenol (DCNP), a SAM sulfation inhibitor. The fitted results showed that SAM sulfation was a high-affinity high-capacity pathway; SAM glucuronidation was of lower affinity but comparable capacity as the sulfation pathway, whereas SAM hydroxylation was of lower affinity and lower capacity.

Rational use of antiepileptic drug levels

Antiepileptic drug (AED) levels are obtained frequently in clinical practice, but their complex relation to seizures or drug toxicity often makes interpretation of the results difficult. Research studies have not always taken into account clinical, as well as pharmacokinetic and pharmacodynamic, factors which may influence the drug level-effect relationship. AED levels should be drawn at an appropriate time in relation to drug ingestion and clinical symptoms. Systematic investigations in selected patients, during which several levels are obtained, may be more rewarding than routine measurements in a large clinic population.

Pharmacokinetics of total and free valproic acid during monotherapy in infants

The pharmacokinetics of free and total valproic acid (VPA) in plasma and whole blood after oral administration during steady state was investigated in seven infants (mean age 10.7 months) receiving monotherapy. The VPA concentrations in whole blood closely followed those in plasma but at a reduced level. A positive correlation was found between dose and mean plasma concentration (r = 0.71). Mean terminal half-lives were similar in plasma and whole blood (12.5 and 15.5 h, respectively), but were considerably longer than for free VPA (6.4 and 6.5 h, respectively; P less than 0.01). There was a significant decrease in half-lives with increasing age (P less than 0.05). Plasma and whole blood clearance for total VPA was higher than reported in older infants and adults (17.8 and 28.9 ml/kg per hour) and was considerably higher for free VPA (127.6 and 188.8 ml/kg per hour, respectively). The increase in clearance compared with that in older subjects is well in concordance with a lower protein binding of VPA (mean 85.3%). Of special importance is that the percentage of unbound VPA increased with increasing concentrations of total VPA. The fraction of unbound VPA in plasma increased even more in subjects with low albumin concentrations (P less than 0.01).

Comparison of valproate concentrations in human plasma, CSF and brain tissue after administration of different formulations of valproate or valpromide

The concentration of valproate was measured in plasma, CSF and brain tissue of patients who underwent resective surgical treatment because of severe temporal lobe epilepsy after pretreatment with either a sustained release formulation of valproate (Depakine Chrono; 5 patients), the conventional formulation of valproate (Depakine; 6 patients) or valpromide (Depamide; 2 patients). With a mean serum value for all 13 patients of 32.3 micrograms/g valproate, the mean brain/serum ratio was 15.1% (SD 6.1%). The valproate concentration of the hippocampus was significantly higher than that of the amygdala and patients who had the sustained release formulation had significantly higher valproate concentration in the CSF and in the hippocampal formation than those patients who had the conventional valproate. Since a few patients had tumors, whereas others had varying degrees of gliosis, it cannot be ruled out that these differences are the result of different histopathological conditions with related differences in blood-brain barrier functions.

Pitfalls of pharmacokinetic dosage guidelines in renal insufficiency

As the renal elimination of most drugs is closely correlated with the endogenous creatinine clearance, it is possible to use this parameter of kidney function to adjust drug dosage in renal failure. However, this simple procedure neglects possible changes in the volume of distribution, plasma protein binding, drug metabolism, intestinal absorption, and pharmacodynamics in renal insufficiency, as well as the occurrence of biologically active drug metabolites. Because of these uncertainties in critical cases the validity of the dosage calculated using the creatinine clearance should be checked by clinical surveillance and measurements of drug blood concentrations. Further, pharmacokinetic dosage guidelines based on the individual creatinine clearance may not be applicable to diuretics and drugs which have markedly differing kinetics of pharmacodynamic effects and blood levels.

Analysis of the factors influencing anti-epileptic drug concentrations--valproic acid

The factors that influence valproic acid (VPA) serum concentrations and level:dose ratios were evaluated, retrospectively, on 51 consecutive routine VPA determinations from 50 chronically treated epileptic patients. The influence of co-medicated anti-epileptic drugs (phenytoin, phenobarbital, carbamazepine), alone or in combination, on total and free levels of VPA was studied. Furthermore, the possible influence of certain physiological and/or pathophysiological factors (age, weight, sex and clinical laboratory data) was considered. The total level:dose ratio was lower when VPA was given in combination with phenytoin or with carbamazepine than when VPA was given alone. The free level:dose ratio also decreased during concomitant treatment with phenytoin. The free fraction of VPA was unaltered when in combination with phenytoin or with carbamazepine, whereas it was decreased by a combination with phenytoin plus carbamazepine. As a whole, strong, positive, correlations existed between the VPA dose (mg/kg/day) and the total and free serum levels of VPA in the range of less than 15 mg/kg/day, but both levels of VPA tended to flatten out at the range of more than 15 mg/kg/day. These findings should therefore be considered when defining dosage regimens or interpreting serum drug concentrations. Stepwise multivariate regression analysis (MVR) showed that the VPA dose, simultaneous carbamazepine intake, serum glutamic oxalacetic transaminase (SGOT) and serum albumin concentration were important determinants of VPA serum concentrations.

Disease-induced variations in plasma protein levels. Implications for drug dosage regimens (Part II)

Part I of this article, which appeared in the previous issue of the Journal, discussed the implications of variations in plasma protein levels in a number of diseases: hepatic and renal disease, acute myocardial infarction, burns, cancer, diabetes mellitus, hyperlipidaemia and inflammatory diseases. In Part II the authors continue their review with a further range of disease states, and consider their import for drug dosages.

Pharmacokinetics of free and total sodium valproate in adolescents and young adults during maintenance therapy

The pharmacokinetics of total and free valproic acid (VPA) in plasma and whole blood was investigated in seven adolescents and young adults (mean age 17.3 years) during a dosage interval at steady state. The concentration curves of VPA in whole blood after an oral morning dose (mean 8.2 mg/kg body wt.) closely followed those in plasma but at a reduced level. The apparent volume of distribution (Vd) of total VPA was 0.150-0.197 l/kg body wt. and of free VPA 0.911-1.58 l/kg body wt., which indicates considerable distribution of unbound VPA as well as drug binding to extravascular proteins. The terminal half-life of free VPA (6.4-6.7 h) was significantly shorter (P less than 0.05) than the half-life of total VPA (10.4-11.9 h). The binding of VPA in plasma was concentration dependent and fluctuated considerably within the individual dosage intervals. Concentrations of unbound VPA in plasma water of whole blood varied to a corresponding degree, since distribution to blood cells was low (mean 2.2%). It is concluded that there are substantial differences in the pharmacokinetics of free and total VPA. This may contribute to the well-known poor correlation between dose, plasma concentrations and effect of VPA.

Experimental studies and controlled clinical testing of valproate and vigabatrin

Gamma-aminobutyric acid (GABA) is the most important inhibitory transmitter, quantitatively, in the CNS. Evidence exists that decreased GABAergic neurotransmission may play a role in some forms of epilepsy. Consequently, manipulating the GABA system may be a therapeutic possibility in the treatment of this disease. Inhibition of the major GABA degrading enzyme, GABA-transaminase (GABA-T), seems to be the most promising approach. Currently, 2 antiepileptic drugs, valproate (VPA) and vigabatrin, gamma-vinyl GABA (GVG), are available, which are supposed to inhibit the degradation of GABA. Both drugs cause an increase in the total concentration of GABA in the brain, but to a different extent. VPA produces a moderate elevation, which seems to be the result of a marked increase in the transmitter-related GABA pool, while the pronounced elevation in GABA concentration observed during treatment with GVG seems to be caused mainly by an increase in the non-transmitter-related (glial) GABA pool. In order to investigate this apparently differential influence of VPA and GVG on the GABA system, a number of studies were undertaken in selectively cultured astrocytes and neurons from mice. For both drugs neuronal GABA-T proved far more sensitive with regard to inhibition than glial GABA-T. In order to obtain a more direct measure of a potential GABAergic mechanism of action of VPA and GVG, synaptic release of endogenous GABA was determined after culturing neurons in the presence of clinically relevant concentrations of the drugs. GVG caused a significant increase in GABA release, even at concentrations as low as 25 microM. For VPA only the highest of the investigated concentrations (300 microM) augmented GABA release. It is concluded that the antiepileptic effect of GVG seems to be caused by a direct GABAergic mechanism of action. For VPA an influence on the GABA system may play a role in the antiepileptic effect of the drug. However, the lack of definite data on human brain levels of VPA after chronic treatment, combined with evidence that VPA exhibits a number of other effects that may be relevant for its antiepileptic properties, makes the interpretation of a GABAergic mechanism of action difficult. Controlled clinical trials have been increasingly applied within all areas of medicine. In 1982 a survey of the literature identified 29 studies of antiepileptic drugs, where the design involved randomization, the double-blind principle and a statistical analysis of the results.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Plasma protein binding of digitoxin and some other drugs in renal disease

Plasma protein binding of most acidic drugs is decreased in uraemia, whereas the binding of basic drugs is usually unchanged or decreased. Decreased protein binding in patients with renal disease mainly relates to drugs binding to albumin. Digitoxin binds to a specific site on the albumin molecule. Conflicting reports exist on digitoxin-protein binding in patients with renal disease. In ten patients with end-stage renal disease treated with haemodialysis we found only a slightly increased free fraction of digitoxin. A heparin-induced increase of the free fraction of digitoxin during haemodialysis has been reported. However, this increase was caused by the generation of non-esterified fatty acids in vitro. If this in vitro lipolysis was blocked, no increase of free digitoxin could be detected. Alterations of digitoxin-protein binding in uraemic patients during haemodialysis and during the intervals between haemodialysis treatments are small.

Valproate: an updated review

Valproate in all its aspects is comprehensively surveyed. Previous reviews covering various aspects such as mechanism of action, clinical pharmacology, clinical efficacy in epilepsy, febrile convulsions and other neurological disorders, side effects, teratogenicity and intoxications are discussed and updated (161 references).

Blood-brain barrier transport of valproic acid

Valproic acid distribution in brain is less than that of other anticonvulsants such as phenytoin or phenobarbital. Possible mechanisms for this decreased distribution space in brain include (a) increased plasma protein binding of valproate relative to the other anticonvulsants and (b) asymmetric blood-brain barrier (BBB) transport of valproate such that the brain-to-blood flux exceeds the blood-to-brain flux. These mechanisms are investigated in the present studies using the intracarotid injection technique in rats and rabbits. In the rat, the brain uptake index (BUI) of [14C]valproate relative to [3H]water is 51 +/- 6%, indicating the blood-to-brain transport of water is twofold greater than that of valproate. However, the BUI of [14C]valproate relative to [3H]water decreased with time after carotid injection during a 4-min washout period, which indicates that brain-to-blood transport of valproate is greater than that of water. This suggests that the permeability of the BBB to valproate is polarized, with antiluminal permeability being much greater than luminal permeability. In rabbits, the BUI of [14C]valproate is 47 +/- 7% in newborns and 17 +/- 6% in adult animals. However, the high drug extraction in newborns may be attributed to decreased cerebral blood flow in the neonate as the BBB permeability-surface area (PS) products are unchanged (e.g., PS = 0.13 and 0.11 ml min-1 X g-1 in the newborn and adult rabbit, respectively). With regard to plasma protein binding effects on valproate transport, brain valproate uptake was also measured in the presence of human, lamb, pig, rat, horse, goat, hamster, dog, and mouse sera. Higher brain uptakes were observed when the unbound fraction of drug increased. However, our data indicate that a fraction of the valproic acid entering the capillaries bound to plasma proteins had the capacity to equilibrate with brain because of enhanced drug dissociation from albumin in the brain microcirculation. Since plasma protein-bound valproate is available for uptake by brain, the major factor underlying the diminished distribution of the drug in brain appears to be the asymmetric transport properties of the BBB to valproic acid.

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.

Concentration-effect relationships of valproic acid

Valproic acid is an effective broad spectrum anticonvulsant drug. It has a relatively short half-life, and large diurnal fluctuations in serum concentrations occur, thus making it difficult to define clear relationships between individual serum concentrations and either therapeutic or adverse effects. The value of routine 'one-off' measurements of valproate in clinical practice are further reduced by the absence of a clearly defined dose-related neurotoxicity syndrome. The often quoted therapeutic range for valproate of 50 to 100 mg/L has therefore to be regarded with some circumspection, although available data does suggest an increased incidence of adverse reactions with serum concentrations above 100 mg/L.

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

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

Effect of serum albumin on free fractions of phenobarbital and valproic acid in patients with convulsive seizures

The free fractions of phenobarbital and valproic acid were assayed with Free-Level system I (Syva) and by an enzymatic immunoassay technique (EMIT) in 186 patients under chronic anti-convulsant therapy at precisely 2 hrs after they had taken the medicine at breakfast. The free fractions of PB ranged from 49 to 53% in monopharmacy and from 50 to 55% in polypharmacy. Those of VPA ranged from 10 to 12% in monopharmacy and from 8 to 11% in polypharmacy. The regression lines of the free fractions in PB monopharmacy and PB polypharmacy against serum albumin concentration indicated a negative correlation. There was no difference between the free fractions in PB monopharmacy and polypharmacy at the same albumin concentration. The regression lines of the free fractions in VPA monopharmacy and polypharmacy against serum albumin concentration indicated a negative correlation. The free fractions of VPA did not vary under a total VPA concentration of 80 micrograms/ml, while those of VPA above a total VPA concentration of 90 micrograms/ml increased with the total concentration, in the monopharmacy groups. On the other hand, free fractions of VPA in the polypharmacy groups did not vary with the total VPA concentration.

Effect of valproic acid on zinc metabolism in the rat

Previous studies have suggested that the mechanism of valproic acid (VPA) hepatotoxicity may involve a drug-induced Zn deficiency. To test this hypothesis, the uptake of 65Zn or tissue Zn concentration was determined in plasma, liver, bone, kidney, and brain of adult male rats, administered parenteral VPA according to one of 3 schedules: 750 mg/kg; 500 mg/kg; and 100 mg/kg/day X 7 days. Histopathological changes in liver and weight loss were observed in rats 5 days after administration of VPA (750 mg/kg). The plasma Zn level in VPA toxic rats was significantly depressed compared to saline-injected controls, although the Zn content of liver and bone was unaffected. Furthermore, tissue uptake of 65Zn was not altered in rats 6 h after receiving VPA 500 mg/kg or after chronic administration at 100 mg/kg/day. On the basis of the present study, there is no evidence that Zn deficiency is induced by hepatotoxic doses of VPA in rats.

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.

Anticonvulsant drugs. An update

A considerable amount of information is now available concerning the clinical pharmacology of the anticonvulsant drugs. Some of the more important data are reviewed in this article. In recent years, valproic acid (or sodium valproate) has found a place as a major anticonvulsant agent, while older drugs such as troxidone and sulthiame seem to be disappearing from use. Although much information is available, the essential mechanisms of action of the anticonvulsant drugs are still not understood, either at a molecular or at an electrophysiological level. The pharmacokinetics of the anticonvulsants in common use are now reasonably well documented, though some minor questions are still to be answered. Numerous interactions between anticonvulsants and endogenous substances or other drugs administered concurrently (including other anticonvulsants) have been recorded, but much work still needs to be done to elucidate the frequency and mechanisms of the various interactions. Many adverse effects of the anticonvulsants are known, but further unwanted effects of long-established drugs continue to emerge from time to time, including the still somewhat controversial matter of anticonvulsant-related dysmorphogenesis. The use of valproic acid and its sodium salt has been associated with a worrying incidence of serious liver and pancreatic toxicity. Adequate basic data are now available to put the clinical use of anticonvulsants on a rational basis, but much work remains to be done in this area. In particular, the question of 'therapeutic ranges' of plasma concentrations of the various drugs needs to be reinvestigated in a rigorous statistical fashion, and in relation to different clinical types of epilepsy. The usefulness of monitoring free rather than total drug concentrations also needs further investigation. The ultimate test of the validity of all background scientific pharmacological information about anticonvulsants is its usefulness in the treatment of patients with epilepsy.

Alteration of drug-protein binding in renal disease

The protein binding of acidic drugs but not basic drugs is decreased in serum from patients with poor renal function. This decreased binding is due to the retention of compounds that displace drugs from their binding sites on albumin. Phenytoin and valproic acid are the 2 drugs that require a change in the values for therapeutic levels to allow for this decreased binding.

Free level monitoring of antiepileptic drugs. Clinical usefulness and case studies

The free fraction of phenytoin, carbamazepine and valproic acid shows considerable interindividual variability, especially in the presence of associated disease or drug interactions. When binding is altered, the total concentration no longer reflects the amount of pharmacologically active drug in the plasma: this may mislead the clinician into making inappropriate dosage adjustments. Measuring the free drug concentration eliminates a potential source of interpretative errors and may be preferentially used to monitor therapy in selected patients.

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.

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

The effect of sample incubation and storage on the protein binding of the antiepileptic drug valproic acid (VPA) and on the concentration of free fatty acids (FFA) was investigated in serum and plasma collected from four normal volunteers. Both the free fraction of VPA and the concentration of FFA increased progressively with time when samples were incubated at 4 to 37 degrees C. These effects occurred to the same extent in both serum and heparinized plasma. At 4 degrees C and at room temperature, the increase in free drug fraction was relatively small (18 and 25% respectively at 24 h), whereas at 37 degrees C it was quite considerable (24% at 8 h and 40% at 24 h). At room temperature, FFA rose on average by 22% at 4 h, 34% at 8 h, and 86% at 24 h, whereas at 37 degrees C the increases at the same incubation times were 59, 90, and 160%, respectively. There was a strong positive relationship between changes in free VPA fraction and FFA content of the samples. The time-dependent changes in VPA binding capacity described in this study may lead to overestimation of the actual free concentration in vivo, especially when this is estimated by equilibrium dialysis or ultracentrifugation techniques requiring long incubation (centrifugation) times at 37 degrees C.