Clinical implications of serum protein binding in epileptic children during sodium valproate maintenance therapy

Clinical Pharmacokinetic Monitoring of Free Valproic Acid Levels: A Systematic Review

BACKGROUND

Current guidelines recommend therapeutic drug monitoring as a critical component of valproic acid (VPA) therapy. Due to high protein binding, the active unbound (free) portion of VPA can be misrepresented by total VPA serum levels in certain clinical scenarios. Monitoring free VPA serum levels may be warranted when assessing the clinical response to VPA therapy.

OBJECTIVES

The aims were to conduct a systematic review to identify a therapeutic range for free VPA serum levels; to explore the correlation of free VPA serum levels with clinical toxicity and therapeutic benefit; and to examine predictors of discordance between free and total VPA levels.

METHODS

Medline, EMBASE, Cochrane Central Register of Controlled Trials (CENTRAL), PsycINFO, BIOSIS Previews, and Cumulative Index to Nursing and Allied Health Literature (CINAHL) were searched from the time of database inception to June 20, 2021. Randomized controlled trials and observational studies that evaluated any patient receiving VPA with free VPA level monitoring were included.

RESULTS

Of 189 citations, we identified 27 relevant studies, which included 14 observational studies, two case series, and 11 case reports. Three studies provided a therapeutic range for free VPA levels between 20 and 410 μmol/L. Two studies suggested the occurrence of hyperammonemia and thrombocytopenia at free VPA serum levels above 60 µmol/L and 103.3 µmol/L, respectively. Two studies suggested an upper limit for neurotoxicity at free VPA serum levels of 70 µmol/L and 207.9 µmol/L. Hypoalbuminemia was identified as a predictor of therapeutic discordance.

CONCLUSIONS

This review demonstrates a paucity of data informing the clinical utility of free VPA serum levels. Further high-quality trials are needed to validate an optimal therapeutic range for free VPA levels.

Intractable epilepsy in adults

Drug-resistant or "intractable" epilepsy presents a major challenge for physicians, epilepsy associations and researchers. Dr. Leppik discusses the evaluation of patients who may be intractable, the nature of intractable epilepsy and the factors which contribute to intractability.

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.

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.

Glucuronidation metabolic kinetics of valproate in guinea pigs: nonlinear at clinical concentration levels

PURPOSE

Nonlinear conjugation metabolic rate of valproic acid (VPA) has been speculated previously from plasma elimination and liver concentration of VPA in guinea pigs. The purposes of the present study were to assess our speculation by direct measurement of VPA glucuronidation rate in vitro.

METHODS

VPA at various concentrations (10-200 micrograms/ml) was incubated with guinea-pig liver-homogenate, mitochondria or microsome in the presence of cofactor, uridine 5'-diphosphoglucuronic acid (UDPGA). The maximum glucuronidation rate (Vmax) and Michaelis-Menten constant (Km) of VPA were determined.

RESULTS

On a body weight basis, the Vmax and the Km values of VPA glucuronidation estimated from liver homogenate were 1.8 mumol/min/kg and 0.3 mumol/ml, respectively; and that from microsome suspension were 1.2 mumol/min/kg and 0.16 mumol/ml, respectively. These data are comparable with the primary metabolic parameters observed from previous in vivo study. The glucuronidation clearance calculated from these parameters was 0.10-0.48 fraction of total clearance, which was in agreement with the reported data observed from clinical and animal urinary recoveries of VPA-G. The glucuronidation reaction was not detectable in mitochondria suspension.

CONCLUSIONS

The glucuronidation kinetics of VPA is nonlinear and saturable within clinical concentration range. Estimation of in vivo VPA glucuronidation kinetics from in vitro kinetic parameters is feasible.

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.

In vivo determinations of carbamazepine and carbamazepine-10, 11-epoxide binding parameters to serum proteins in monotherapy patients

The in vivo serum protein binding parameters of carbamazepine (CBZ) and carbamazepine-10, 11-epoxide (CBZ-E), which was the main metabolite of CBZ in plasma, were determined in sera from 27 patients on CBZ monotherapy. Based on the results by recent studies, the authors assumed that CBZ and CBZ-E binding to serum proteins were composed of specific binding sites on alpha 1-acid glycoprotein (AAG) and albumin. Therefore, the authors determined the binding parameters of each compound by specific binding equation for two proteins. Association constants for drug-AAG binding were .071 L/mumol for CBZ and .016 L/mumol for CBZ-E. Conversely, those for drug-albumin binding were .00052 L/mumol for CBZ and .00072 L/mumol for CBZ-E. Within the investigated total concentration ranges in each compound, the AAG binding contributes largely to the drug-serum protein interactions. Furthermore, our results indicate that the albumin binding contributes to the nonsaturable serum protein binding of these compounds in the therapeutic range.

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.

Apparent valproic acid neurotoxicity in a hypoalbuminemic patient

OBJECTIVE

To report a case of possible neurotoxicity caused by markedly elevated free valproic acid (VPA) plasma concentrations.

CASE SUMMARY

A patient with a history of a mixed-type seizure disorder that had been treated with oral VPA 1000 mg four times daily for the previous two years was admitted to the neurology service with the chief complaint of increasing difficulty in walking and involuntary muscle jerks that were new in onset. The patient was hypersomnolent and dysarthric. The total plasma VPA concentration was 103 micrograms/mL, which was only slightly above the recommended therapeutic range (50-100 micrograms/mL). VPA free fraction and free plasma concentrations, however, were unexpectedly elevated (26 percent, 26.8 micrograms/mL, respectively). Further laboratory evaluation revealed a serum albumin concentration of 33 g/L. The neurologic symptoms resolved upon VPA dosage reduction.

DISCUSSION

VPA displays concentration-dependent protein binding, resulting in disproportionate increases in drug free fraction with increasing drug concentration. This effect may be magnified in patients with decreased plasma protein-binding capacity. The plasma protein-binding kinetics of VPA are reviewed and the implications for therapeutic drug monitoring are discussed.

CONCLUSIONS

It is likely that the markedly elevated free VPA plasma concentrations contributed to the neurologic symptoms displayed in this patient. In patients with decreased albumin concentrations, failure to recognize concentration-dependent protein binding, as well as exclusive reliance upon total drug concentrations, may lead to erroneous pharmacokinetic and therapeutic interpretations.

Dose-dependent inhibition in plasma protein binding of valproic acid during continued treatment in guinea-pigs

Plasma protein binding of valproic acid over a wide range of steady-state plasma concentration (11.3 +/- 2.6-1303.0 +/- 122.9 micrograms mL-1: s.e.m., n = 5) in guinea-pigs has been studied. Valproic acid was given by intravenous constant infusion. At steady-state the plasma protein binding of valproic acid was analysed. Nonlinear binding was observed. Unbound fraction (fu) of valproic acid increased from 25 to 95% with the increase of steady-state plasma concentration (Css). The plasma protein-bound drug concentration (Cb) of valproic acid increased initially with Css but decreased after the Css exceeded 345.0 micrograms mL-1, where the Cb was 152.5 +/- 26.8 micrograms mL-1. At a Css of 1303.3 +/- 122.9 micrograms mL-1 the Cb was significantly (P less than 0.05) decreased to 72.8 +/- 20.2 micrograms mL-1. Binding characteristics of valproic acid in-vitro were studied using drug-free guinea-pig plasma with added valproic acid (10-1000 micrograms mL-1). The binding behaviour was also nonlinear in-vitro. The fu increased from 14 to 79% with the increase of valproate concentrations. No decrease in Cb was observed throughout the range. The study demonstrated that binding characteristics of valproic acid in-vivo and in-vitro are not parallel. The results suggest that valproic acid may produce or induce plasma protein binding competitors; metabolites of valproic acid may be implicated.

Valproic acid treatment of experimental status epilepticus

The efficacy of valproic acid (VPA) in control of generalized convulsive status epilepticus was tested in a rat model. Rats with cortical cobalt lesions were injected with homocysteine thiolactone to induce secondarily generalized tonic-clonic seizures (GTCS). The median effective dose (ED50) for control of GTCS was 211.9 mg/kg (270 micrograms/ml in serum 30 min post dose) when treatment was given intraperitoneally after the second GTCS. VPA entered both serum and brain very rapidly after injection, with little change in concentration from 5 to 30 min post dose. In earlier experiments with phenytoin, phenobarbital, diazepam and lorazepam in this model, we found that the serum concentrations produced by the ED50s versus GTCS were very similar to those which have been reported to be effective in treating human status epilepticus. If this same relationship holds true for VPA, we would predict that a serum concentration of around 270 micrograms/ml VPA would be required for control of generalized convulsive status epilepticus in human patients. The safety of this high a concentration of VPA has not been tested.

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.

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

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