Relationship between valproic acid dosage, plasma concentration and clearance in adult monotherapy patients with epilepsy

Effect of patients in-use and accelerated stability conditions on quality attributes and pharmacokinetic profile of four FDA approved extended-release anti-epileptic-drug products

Divalproex (DVS) is a popular drug widely used in various neurological and psychiatric disorders. Commercially, it is a multisource-drug available in different generic equivalents. Incidents of (class II)-recalls have been repeated over the last years due to failure to consistently meet dissolution specifications. Class II recalls are known to be associated with temporary or medically reversible adverse health consequences. This study aimed to evaluate the dissolution profiles, among other quality attributes, of select FDA-approved extended-release DVS products before and after exposure to conditions usually seen as short-lived and insignificant on product stability, such as pharmacy dispensing and patients' in-use conditions to assess their possible role in the failures observed. Products were stored for 6 weeks in pharmacy vials at 30 °C/75 % RH to simulate patient in-use conditions, for 12 weeks in unsealed HPDE bottles at 25 °C/65 % RH to simulate the pharmacy storage conditions, and for 3 days in open containers at 40 °C/75 % RH for accelerated stability studies. Physicochemical changes were detected by near infrared imaging, Fourier transformed infrared, X-ray powder diffraction and differential scanning calorimetry. All samples were analyzed for in vitro dissolution. Two products were further selected for in vivo study on Beagle dogs before and after storage. The physicochemical characterization tests revealed changes in tablets' composition and drug crystallinity over time. An improved discriminatory dissolution test was developed and used in this study. The in vitro release testing revealed that short-lived environmental changes at 30 or 25 °C could fail some unit doses and significantly lower the drug release (average reduction among all products was 12.97 ± 11.3 % and 27.48 ± 10.26 %, respectively). Some extended-release products showed a significant increase in the amount of drug dissolved in the first 6 h (early burst) owing to changes in tablet surface morphology and enhanced drug dissolution. In vivo studies showed a decrease in the AUC0-t by overall average of 21.1 % using the non-transformed data, a decrease that mirrored the dissolution results. The study shows that significant changes can occur during routine drug dispensing and patients' use that might variably impact the stability and quality of commercial bioequivalent unit doses. It is possible that these changes may also contribute to the adverse effects reported on DVS or upon drug switches that were previously attributed to the intersubject variability. The study findings are encouraging to further investigate the effect of such minor excursions on the drug effectiveness during products' shelf lives especially for narrow therapeutic index drugs.

The role of efflux transporters and metabolizing enzymes in brain and peripheral organs to explain drug-resistant epilepsy

Drug‐resistant epilepsy has been explained by different mechanisms. The most accepted one involves overexpression of multidrug transporters proteins at the blood brain barrier and brain metabolizing enzymes. This hypothesis is one of the main pharmacokinetic reasons that lead to the lack of response of some antiseizure drug substrates of these transporters and enzymes due to their limited entrance into the brain and limited stay at the sites of actions. Although uncontrolled seizures can be the cause of the overexpression, some antiseizure medications themselves can cause such overexpression leading to treatment failure and thus refractoriness. However, it has to be taken into account that the inductive effect of some drugs such as carbamazepine or phenytoin not only impacts on the brain but also on the rest of the body with different intensity, influencing the amount of drug available for the central nervous system. Such induction is not only local drug concentration but also time dependent. In the case of valproic acid, the deficient disposition of ammonia due to a malfunction of the urea cycle, which would have its origin in an intrinsic deficiency of L‐carnitine levels in the patient or by its depletion caused by the action of this antiseizure drug, could lead to drug‐resistant epilepsy. Many efforts have been made to change this situation. In order to name some, the administration of once‐daily dosing of phenytoin or the coadministration of carnitine with valproic acid would be preferable to avoid iatrogenic refractoriness. Another could be the use of an adjuvant drug that down‐regulates the expression of transporters. In this case, the use of cannabidiol with antiseizure properties itself and able to diminish the overexpression of these transporters in the brain could be a novel therapy in order to allow penetration of other antiseizure medications into the brain.

Ataluren for drug-resistant epilepsy in nonsense variant-mediated Dravet syndrome and CDKL5 deficiency disorder

Objective

Ataluren is a compound that reads through premature stop codons and increases protein expression by increasing translation without modifying transcription or mRNA stability. We investigated the safety and efficacy of ataluren in children with nonsense variants causing Dravet Syndrome (DS) and CDKL5 Deficiency Syndrome (CDD).

Methods

This single‐center double‐blind, placebo‐controlled crossover trial randomized subjects to receive ataluren or placebo for 12 weeks (period 1), a 4‐week washout, then another 12‐week treatment (period 2). The primary outcome was ataluren’s safety profile. The secondary outcome measures were (1) changes in convulsive and/or drop seizure frequency and (2) changes in minor seizure types during ataluren treatment compared to placebo. Exploratory objectives assessed changes in cognitive, motor, and behavioral function as well as quality of life during ataluren therapy.

Results

We enrolled seven subjects with DS and eight subjects with CDD. Three treatment‐related adverse events (AE) occurred during the blinded phases. Two subjects withdrew due to AE. Ataluren was not effective in reducing seizure frequency or improving cognitive, motor, or behavioral function or quality of life in subjects with either DS or CDD due to nonsense variants. Limitations included a small sample size and 12‐week treatment phase, possibly too short to identify a disease‐modifying effect.

Significance

There was no difference between ataluren and placebo; ataluren is not an effective therapy for seizures or other disorders in children with DS or CDD due to nonsense variants. There were no drug‐related serious AE during the double‐blind period, consistent with ataluren’s favorable safety profile in larger studies. (Funded by Epilepsy Foundation, Dravet Syndrome Foundation, Finding A Cure for Seizures and Epilepsy and PTC Therapeutics, Inc.; ClinicalTrials.gov number, NCT02758626).

Estimation of apparent clearance of valproic acid in adult Saudi patients

Background Valproic acid is one of several antiepileptic medications requiring therapeutic drug monitoring due to its complex and wide pharmacokinetic interindividual variability. Objective The objective of this study was to determine the population pharmacokinetics of valproic acid in adult Saudi patients and to identify factors that explain its pharmacokinetic variability. Setting Tertiary referral teaching hospital, Riyadh, Saudi Arabia. Method A retrospective chart review was performed at King Saud University Medical City of patients who received oral valproic acid. The population pharmacokinetic models were developed using Monolix 4.4. After development of the base model, we investigated several covariates including age, sex, weight, total daily dose, and cotherapy with carbamazepine and phenytoin. Main outcome measures the pharmacokinetic parameters of valproic acid and the variables that contributing towards its inter-individual variability. Results The analysis included a total of 54 valproic acid plasma concentrations from 54 patients (42.5% male). The data were sufficiently described by a one-compartment model with linear absorption and elimination processes. Average parameter estimates for valproic acid apparent clearance (CL/F) and apparent volume of distribution (V/F) were 0.14 L/h and 37.7 L (fixed), respectively. The inter-individual variability (coefficients of variation) in CL/F was 12%. The most significant covariates for valproic acid CL/F were age, body weight, total daily dose, and cotherapy with carbamazepine and phenytoin. Conclusion This model showed significant inter-individual variability between subjects. Our findings showed that patient age, body weight, total daily dose, and cotherapy with carbamazepine and phenytoin are the most significant covariates of valproic acid clearance. Collectively, healthcare providers should take these factors in consideration for optimal valproic acid dosage regimen.

Quality, origins and limitations of common therapeutic drug reference intervals

Therapeutic drug monitoring (TDM) is used to manage drugs with a narrow window between effective and toxic concentrations. TDM involves measuring blood concentrations of drugs to ensure effective therapy, avoid toxicity and monitor compliance. Common drugs for which TDM is used include aminoglycosides for infections, anticonvulsants to treat seizures, immunosuppressants for transplant patients and cardiac glycosides to regulate cardiac output and heart rate. An essential element of TDM is the provision of accurate and clinically relevant reference intervals. Unlike most laboratory reference intervals, which are derived from a healthy population, TDM reference intervals need to relate to clinical outcomes in the form of efficacy and toxicity. This makes TDM inherently more difficult to develop as healthy individuals are not on therapy, so there is no "normal value". In addition, many of the aforementioned drugs are old and much of the information regarding reference intervals is based on small trials using methods that have changed. Furthermore, individuals have different pharmacokinetics and drug responses, particularly in the context of combined therapies, which exacerbates the challenge of universal TDM targets. This focused review examines the origins and limitations of existing TDM reference intervals for common drugs, providing targets where possible based on available guidelines.

Efficacy of valproic acid for retinitis pigmentosa patients: a pilot study

Purpose

The purpose of this study was to examine the efficacy and safety of valproic acid (VPA) use in patients with retinitis pigmentosa (RP).

Patients and methods

This was a prospective, interventional, noncomparative case study. In total, 29 eyes from 29 patients with RP whose best-corrected visual acuities (BCVAs) in logarithm of the minimum angle of resolution (logMAR) ranged from 1.0 to 0.16 with visual fields (VFs) of ≤10° (measured using Goldmann perimeter with I4) were recruited. The patients received oral supplementation with 400 mg of VPA daily for 6 months and were followed for an additional 6 months. BCVAs, VFs (measured with the Humphrey field analyzer central 10-2 program), and subjective questionnaires were examined before, during, and after the cessation of VPA supplementation.

Results

The changes in BCVA and VF showed statistically significant differences during the internal use of VPA, compared with after cessation (P=0.001). With VPA intake, BCVA in logMAR significantly improved from baseline to 6 months (P=0.006). The mean deviation value of the VF significantly improved from baseline to 1 month (P=0.001), 3 months (P=0.004), and 6 months (P=0.004). These efficacies, however, were reversed to the baseline levels after the cessation of VPA intake. There were no significant relations between the mean blood VPA concentrations of each patient and the changes in BCVA and VF. During the internal use of VPA, 15 of 29 patients answered “easier to see”, whereas blurred vision was registered in 21 of 29 patients on cessation. No systemic drug-related adverse events were observed.

Conclusion

While in use, oral intake of VPA indicated a short-term benefit to patients with RP. It is necessary to examine the effect of a longer VPA supplementation in a controlled study design.

Effects of antiepileptic drugs on hippocampal neurons coupled to micro-electrode arrays

Hippocampal networks exhibit spontaneous electrophysiological activity that can be modulated by pharmacological manipulation and can be monitored over time using Micro-Electrode Arrays (MEAs), devices composed by a glass substrate and metal electrodes. The typical mode of activity of these dissociated cultures is the network-wide bursting pattern, which, if properly chemically modulated, can recall the ictal events of the epileptic phenotypes and is well-suited to study the effects of antiepileptic compounds. In this paper, we analyzed the changes induced by Carbamazepine (CBZ) and Valproate (VPA) on mature networks of hippocampal neurons in “control” condition (i.e., in the culturing medium) and upon treatment with the pro-convulsant bicuculline (BIC). We found that, in both control and BIC—treated networks, high doses (100 μM–1 mM) of CBZ almost completely suppressed the spiking and bursting activity of hippocampal neurons. On the contrary, VPA never completely abolish the electrophysiological activity in both experimental designs. Interestingly, VPA cultures pre-treated with BIC showed dual effects. In fact, in some cultures, at low VPA concentrations (100 nM–1 μM), we observed decreased firing/bursting levels, which returned to values comparable to BIC-evoked activity at high VPA concentrations (100 μM–1 mM). In other cultures, VPA reduced BIC-evoked activity in a concentration-independent manner. In conclusion, our study demonstrates that MEA-coupled hippocampal networks are responsive to chemical manipulations and, upon proper pharmacological modulation, might provide model systems to detect acute pharmacological effects of antiepileptic drugs.

Modern methods for analysis of antiepileptic drugs in the biological fluids for pharmacokinetics, bioequivalence and therapeutic drug monitoring

Epilepsy is a chronic disease occurring in approximately 1.0% of the world's population. About 30% of the epileptic patients treated with availably antiepileptic drugs (AEDs) continue to have seizures and are considered therapy-resistant or refractory patients. The ultimate goal for the use of AEDs is complete cessation of seizures without side effects. Because of a narrow therapeutic index of AEDs, a complete understanding of its clinical pharmacokinetics is essential for understanding of the pharmacodynamics of these drugs. These drug concentrations in biological fluids serve as surrogate markers and can be used to guide or target drug dosing. Because early studies demonstrated clinical and/or electroencephalographic correlations with serum concentrations of several AEDs, It has been almost 50 years since clinicians started using plasma concentrations of AEDs to optimize pharmacotherapy in patients with epilepsy. Therefore, validated analytical method for concentrations of AEDs in biological fluids is a necessity in order to explore pharmacokinetics, bioequivalence and TDM in various clinical situations. There are hundreds of published articles on the analysis of specific AEDs by a wide variety of analytical methods in biological samples have appears over the past decade. This review intends to provide an updated, concise overview on the modern method development for monitoring AEDs for pharmacokinetic studies, bioequivalence and therapeutic drug monitoring.

The impact of dosage of sustained-release formulation on valproate clearance and plasma concentration in psychiatric patients: analysis based on routine therapeutic drug monitoring data

Pharmacokinetic parameters were compared between 2 dosages of valproic acid (VPA) sustained-release (SR) formulation in psychiatric patients. A total of 66 psychiatric patients (21 women and 45 men; age range, 18-60 years) receiving 500 mg/d (n = 28) or 1000 mg/d (n = 38) of VPA SR for at least 4 weeks were recruited into the study. A 5-mL blood sample was collected into a plain tube and immediately centrifuged for plasma. Separation of free VPA was further done using Centrifree micropartition devices. Both total and free VPA concentrations (C(total) and C(free)) were analyzed by TDx analyzer with fluorescence polarization immunoassay technique. The patients' characteristics and pharmacokinetic parameters were compared between the 2 dosage groups using independent t test or χ² test where appropriate. The results show that the increment in C(total) (mg/mL) for every milligram-per-kilogram increment in dosage was decreased from 7 ± 3 to 4 ± 1 (mg/L) / (mg/kg) when the total VPA clearance (CL(total)) increased from 6.1 ± 2.6 to 9.0 ± 3.1 mL/kg per hour with the increasing dose in the 500 mg/d and 1000 mg/d groups, respectively (P < 0.05). The increment in C(free) [0.6 ± 0.3 vs 0.5 ± 0.2 (mg/L) / (mg/kg)] and CL(free) (80.4 ± 41.5 vs 92.2 ± 47.6 ml/kg per hour) were not significantly different. Owing to the saturation of protein binding, percent free VPA was significantly increased from 8 ± 3 to 11 ± 3 when the dose was increased from 500 to 1000 mg/d (P < 0.05). In conclusion, an increase in the VPA dose resulted in a disproportional increase between dosage and C(total), whereas a proportional increase between dosage and C(free) still existed. Therefore, our study suggests that the therapeutic range of C(free) is 4 to 12 mg/L based on the therapeutic range of C(total) (45-100 mg/L) for general psychiatric conditions.

[Therapeutic drug monitoring of valproate]

Valproic acid is an anticonvulsant drug available in France since 1967. It is a broad spectrum molecule indicated in various forms of epilepsy of the adult and the child, but it is also prescribed in the treatment of different other pathologies of nervous system. The divalproate sodium is indicated in the treatment of bipolar disorders. The valproic acid is marketed under various pharmaceutical forms, with different corresponding tmax values. But, whatever the administered preparation, the circulating active molecule is the ion valproate. Elimination half-life is from 11 to 20 h. Metabolization of valproate is important and represents its main route of elimination. Valpromide is comparable to a prodrug which metabolizes in valproate. The inter and intraindividual variability of the plasma concentrations are important. Several studies show a concentration-effect relationship, but two interventional trials ended in the lack of interest of the Therapeutic Drug Monitoring (TDM), although it is of current practice. However, numerous drug interactions may modify the plasma concentrations of valproate. The therapeutic range is from 50 to 100 mg/L (346-693 micromol/L). The level of proof of the interest of the TDM for this molecule was estimated in: recommended.

Improvement of pentylenetetrazol-induced learning deficits by valproic acid in the adult zebrafish

Pentylenetetrazol (PTZ) has been shown to induce seizure-like behavior, learning deficits in passive avoidance response test, and an increase in hsp70 (heat shock protein 70) mRNA expression in the adult zebrafish; PTZ has been increasingly appreciated as an excellent model system for the study of seizures. In this study, we demonstrate that valproic acid (VPA), an antiepileptic drug, suppresses seizure-like behavior and improves learning ability in adult zebrafish treated with PTZ. Pretreatment with VPA significantly reduces rapid involuntary movement and abrupt changes in moving direction in the PTZ-treated zebrafish. PTZ-induced learning impairments were also improved in the zebrafish pretreated with 200 or 500 microM VPA. However, the scopolamine-induced impairments of learning ability were not improved by VPA pretreatment. It is worth noting that while the zebrafish treated with 500 microM VPA for 1-3 weeks learned the passive avoidance response, those treated with 1 or 2mM VPA for 3h didn't. Furthermore, the increased level of hsp70 expression induced by PTZ, a stress marker protein, was significantly reduced in the VPA-pretreated zebrafish brains. Collectively, our data show the antiepileptic effects of VPA in the adult zebrafish, which coincides with reduced hsp70 mRNA expression, rescued learning impairment under PTZ-treated conditions.

Once-daily dosing is appropriate for extended-release divalproex over a wide dose range, but not for enteric-coated, delayed-release divalproex: evidence via computer simulations and implications for epilepsy therapy

Divalproex sodium extended-release (divalproex-ER), administered once-daily, maintains plasma valproic acid (VPA) concentrations for 24h, whereas enteric-coated, delayed-release divalproex sodium (divalproex) requires multiple-daily doses to do the same. We hypothesize that a once-daily divalproex regimen should not be administered to epilepsy patients requiring high total daily doses, e.g., 35.6-56 mg/kg/day, due to the potential for high (>125 mg/L) maximum VPA concentrations (C(max)). We examined the impact of once-daily dosing, divalproex vs. divalproex-ER, on steady-state plasma VPA concentration-time profiles at commonly used doses in monotherapy (uninduced) and polytherapy (hepatic enzyme-induced) virtual adult patients. Only the 1125 mg once-daily divalproex dose had mean C(max)<100mg/L; >or=2000 mg produced mean C(max)>or=125 mg/L. Mean divalproex C(min) was approximately 50 mg/L at two of four doses tested, whereas mean ER C(min) was >73 mg/L at all doses tested. Once-daily divalproex peak-trough fluctuation was 4.4-6.2-fold greater than once-daily divalproex-ER. We predict that excursions beyond the conventional recommended VPA plasma concentration range will commonly occur with high total mg daily doses (>or=2000 mg) of enteric-coated divalproex, if dosed once-daily, potentially producing clinical toxicity. This divalproex formulation should not be dosed once-daily at high total mg daily doses due to this risk. Divalproex-ER is the appropriate formulation for administration on a once-daily basis, especially if large total mg/day doses are required for the control of seizure activity.

Valproate activates the Notch3/c-FLIP signaling cascade: a strategy to attenuate white matter hyperintensities in bipolar disorder in late life?

OBJECTIVES

Increased prevalence of deep white matter hyperintensities (DWMHs) has been consistently observed in patients with geriatric depression and bipolar disorder. DMWHs are associated with chronicity, disability, and poor quality of life. They are thought to be ischemic in their etiology and may be related to the underlying pathophysiology of mood disorders in the elderly. Notably, these lesions strikingly resemble radiological findings related to the cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephelopathy (CADASIL) syndrome. CADASIL arises from mutations in Notch3, resulting in impaired signaling via cellular Fas-associated death domain-like interleukin-1-beta-converting enzyme-inhibitory protein (c-FLIP) through an extracellular signal-regulated kinase (ERK)-dependent pathway. These signaling abnormalities have been postulated to underlie the progressive degeneration of vascular smooth muscle cells (VSMC). This study investigates the possibility that the anticonvulsant valproate (VPA), which robustly activates the ERK mitogen-activated protein kinase (MAPK) cascade, may exert cytoprotective effects on VSMC through the Notch3/c-FLIP pathway.

METHODS

Human VSMC were treated with therapeutic concentrations of VPA subchronically. c-FLIP was knocked down via small interfering ribonucleic acid transfection. Cell survival, apoptosis, and protein levels were measured.

RESULTS

VPA increased c-FLIP levels dose- and time-dependently and promoted VSMC survival in response to Fas ligand-induced apoptosis in VSMC. The anti-apoptotic effect of VPA was abolished by c-FLIP knockdown. VPA also produced similar in vivo effects in rat brain.

CONCLUSIONS

These results raise the intriguing possibility that VPA may be a novel therapeutic agent for the treatment of CADASIL and related disorders. They also suggest that VPA might decrease the liability of patients with late-life mood disorders to develop DWMHs.

Dose-related pharmacokinetics and pharmacodynamics of valproate in the elderly

Valproate exhibits a complex pharmacokinetic profile due to concentration-dependent protein binding and clearance. It has been shown that the protein binding of valproate decreases as the serum concentration increases in a young adult population. Furthermore, the percentage of protein binding is lower in the elderly compared with young adults at comparable low therapeutic serum concentrations. The extent of valproate protein binding at higher concentrations in the elderly has not been described. Studies conducted in the elderly have found unbound valproate clearance to be decreased compared with younger adults, although these changes in clearance have not been evaluated at higher therapeutic serum concentrations. We evaluated the pharmacokinetics of valproate (protein binding and clearance) across a wide dosage range in the elderly and measured the impact of this on drug-related side effects using a single-blind within-subject study design in 6 healthy elderly volunteers (aged 65-76 years). Steady-state total and unbound serum valproate concentrations were assessed at 3 doses: 500, 1000, and 1500 mg/d. As doses and valproate serum concentrations increased, the unbound fraction (10.0%, 13.0%, 17.4%) and total clearance (4.8, 6.0, 6.7 mL/h/kg) increased, respectively. Unbound clearance decreased (49.4, 45.8, 39.4 mL/h/kg) with increasing valproate serum concentrations. Drug-induced CNS effects and nausea severity scores correlated with total and unbound serum valproate concentrations. Significant dose-dependent changes in valproate pharmacokinetics were observed in the elderly.

Neural crest cell motility in valproic acid

Neural crest cells (NCCs) exit the dorsal neural tube and migrate to sites where they form diverse tissues. Valproic acid (VPA) is an anticonvulsant drug that induces neural tube and related defects. Altered NCC migration and proliferation have been proposed as mechanisms of teratogenicity. We cultured neural tube segments from chick embryos in 0.75-3.0mM VPA. We used image analysis, proliferation assays, and fluorescence localization to investigate NCCs during VPA exposure. VPA inhibited attachment of explants and the number that produced migrating cells. VPA markedly decreased the proportion of cells migrating individually, promoting migration as epithelial sheets. VPA at 3mM decreased cellular spreading. Area and perimeter change per minute were reduced, but migration velocity was not. VPA at 2mM reduced proliferation 11% and 3mM arrested proliferation. Immunostaining of VPA-exposed explants revealed N-cadherin-positive cell boundaries within sheets, but independent NCCs did not stain. F-actin staining was reduced in independent NCCs. The data support a VPA mechanism involving interference with epithelial-mesenchymal transition.

Elevated free fractions of valproic acid in a heart transplant patient with hypoalbuminemia

OBJECTIVE

To report a case demonstrating the importance of monitoring unbound valproic acid (VPA) serum concentrations in a patient with hypoalbuminemia.

CASE SUMMARY

A 53-year-old white woman status-post heart transplantation was admitted to the hospital for declining cardiac function, possible rejection, and increased lethargy requiring intubation. An extensive workup of the patient's profound lethargy was initiated, including an evaluation of her VPA regimen. Initially, VPA dosages were adjusted based on the total serum concentration of VPA. Hypoalbuminemia compounded with increased lethargy prompted the measurement of unbound serum concentrations of VPA. The VPA dosage was then adjusted based on the unbound rather than the total VPA serum concentration; the patient eventually improved and was discharged from the hospital.

DISCUSSION

Lethargy is a concentration-related adverse effect of VPA. The nonlinear pharmacokinetic and protein saturation characteristics of VPA may result in nonproportional elevations in unbound drug, and subsequent increases in adverse effects, when dosage adjustments are based solely on measurement of total VPA serum concentrations in patients with hypoalbuminemia.

CONCLUSIONS

This case report suggests that appropriate monitoring of unbound drug concentrations of VPA may prevent unrecognized concentration-related adverse effects. Awareness of the pharmacokinetic relationship and adverse effects of VPA will aid clinicians in identifying the etiology of symptoms.

Valproate population pharmacokinetics in children

OBJECTIVE

A population analysis of the kinetics of valproic acid (VPA) in children with epilepsy was performed in order to characterize the covariates which influence VPA clearance (CL).

METHODS

A total of 770 steady-state serum concentration samples was analysed. These were collected during VPA therapy from 255 children, aged 0.1-14 years and weighting 4-74 kg. Age, total body weight (TBW), VPA daily dose, sex and comedication with carbamazepine (CBZ) were considered as covariates. Population analysis was made with NONMEM program, assuming a one-compartment model, fixing the VPA absorption rate, bioavailability and distribution volume at values found in the literature. The results of the population pharmacokinetics analysis were validated in a group of 45 epileptic patients.

RESULTS

The final regression model for VPA clearance, that included TBW (kg), daily dose (mg/kg) and CBZ comedication as covariates with a significant influence on this parameter, was as follows: CL (L/h) = 0.012 TBW0.715 DOSE0.306(1.359 CBZ). The coefficient of variation for interpatient variability in CL was 21.4% and the residual variability estimated was 23.9% for a concentration of 65 mg/l. In order to estimate the predictive performance of the selected final model, predictions of the VPA serum concentrations were calculated and compared with VPA measured concentrations in the validation group. This assessment revealed an important improvement in the predictive performance of VPA concentrations in comparison with the basic model that did not include any covariates (root squared mean error: 19.50 vs. 39.73 mg/l).

CONCLUSION

A population pharmacokinetic model is proposed to estimate the individual CL for paediatric patients receiving VPA in terms of patient's dose, weight and concomitant CBZ, in order to establish a priori dosage regimens.