Absorption/Bioavailability
The absolute bioavailability of divalproex sodium extended-release tablets administered as a single dose after a meal was approximately 90% relative to intravenous infusion.
When given in equal total daily doses, the bioavailability of divalproex sodium extended-release tablets is less than that of divalproex sodium delayed-release tablets. In five multiple-dose studies in healthy subjects (N=82) and in subjects with epilepsy (N=86), when administered under fasting and nonfasting conditions, divalproex sodium extended-release tablets given once daily produced an average bioavailability of 89% relative to an equal total daily dose of divalproex sodium delayed-release tablets given BID, TID, or QID. The median time to maximum plasma valproate concentrations (C
max) after divalproex sodium extended-release tablets administration ranged from 4 to 17 hours. After multiple once-daily dosing of divalproex sodium extended-release tablets, the peak-to-trough fluctuation in plasma valproate concentrations was 10-20% lower than that of regular divalproex sodium delayed-release tablets given BID, TID, or QID.
Conversion from Divalproex Sodium Delayed-Release Tablets To Divalproex Sodium Extended-Release Tablets
When divalproex sodium extended-release tablets are given in doses 8 to 20% higher than the total daily dose of divalproex sodium delayed-release tablets, the two formulations are bioequivalent. In two randomized, crossover studies, multiple daily doses of divalproex sodium delayed-release tablets were compared to 8 to 20% higher once-daily doses of divalproex sodium extended-release tablets. In these two studies, divalproex sodium extended-release tablets and divalproex sodium delayed-release tablets regimens were equivalent with respect to area under the curve (AUC; a measure of the extent of bioavailability). Additionally, valproate C
maxwas lower, and C
minwas either higher or not different, for divalproex sodium extended-release tablets relative to divalproex sodium delayed-release tablets regimens (see Table 8).
Table 8. Bioavailability of Divalproex Sodium Extended-Release Tablets Relative to Divalproex Sodium Delayed-release Tablets When Divalproex Sodium Extended-Release Tablets Dose is 8 To 20 % Higher
|
Study Population
| Regimens
| Relative Bioavailability
|
| Divalproex sodium extended-release tablets vs divalproex sodium delayed-release tablets
| AUC
24
| C
max
| C
min
|
Healthy Volunteers
(N=35)
| 1,000 & 1,500 mg divalproex sodium extended-release tablets vs 875 & 1,250 mg divalproex sodium delayed-release tablets
| 1.059
| 0.882
| 1.173
|
Patients with epilepsy on concomitant enzyme-inducing antiepilepsy drugs
(N = 64)
| 1,000 to 5,000 mg divalproex sodium extended-release tablets vs 875 to 4,250 mg divalproex sodium delayed-release tablets
| 1.008
| 0.899
| 1.022
|
Concomitant antiepilepsy drugs (topiramate, phenobarbital, carbamazepine, phenytoin, and lamotrigine were evaluated) that induce the cytochrome P450 isozyme system did not significantly alter valproate bioavailability when converting between divalproex sodium delayed-release tablets and divalproex sodium extended-release tablets.
Distribution
Protein Binding
The plasma protein binding of valproate is concentration dependent and the free fraction increases from approximately 10% at 40 mcg/mL to 18.5% at 130 mcg/mL. Protein binding of valproate is reduced in the elderly, in patients with chronic hepatic diseases, in patients with renal impairment, and in the presence of other drugs (e.g., aspirin). Conversely, valproate may displace certain protein-bound drugs (e.g., phenytoin, carbamazepine, warfarin, and tolbutamide)
[see Drug Interactions (
7.2)
for more detailed information on the pharmacokinetic interactions of valproate with other drugs
].
CNS Distribution
Valproate concentrations in cerebrospinal fluid (CSF) approximate unbound concentrations in plasma (about 10% of total concentration).
Metabolism
Valproate is metabolized almost entirely by the liver. In adult patients on monotherapy, 30-50% of an administered dose appears in urine as a glucuronide conjugate. Mitochondrial β-oxidation is the other major metabolic pathway, typically accounting for over 40% of the dose. Usually, less than 15-20% of the dose is eliminated by other oxidative mechanisms. Less than 3% of an administered dose is excreted unchanged in urine.
The relationship between dose and total valproate concentration is nonlinear; concentration does not increase proportionally with the dose, but rather, increases to a lesser extent due to saturable plasma protein binding. The kinetics of unbound drug are linear.
Elimination
Mean plasma clearance and volume of distribution for total valproate are 0.56 L/hr/1.73 m
2and 11 L/1.73 m
2, respectively. Mean plasma clearance and volume of distribution for free valproate are 4.6 L/hr/1.73 m
2and 92 L/1.73 m
2. Mean terminal half-life for valproate monotherapy ranged from 9 to 16 hours following oral dosing regimens of 250 to 1,000 mg.
The estimates cited apply primarily to patients who are not taking drugs that affect hepatic metabolizing enzyme systems. For example, patients taking enzyme-inducing antiepileptic drugs (carbamazepine, phenytoin, and phenobarbital) will clear valproate more rapidly. Because of these changes in valproate clearance, monitoring of antiepileptic concentrations should be intensified whenever concomitant antiepileptics are introduced or withdrawn.
Specific Populations
Effect of Age
Pediatric
The valproate pharmacokinetic profile following administration of divalproex sodium extended-release tablets was characterized in a multiple-dose, non-fasting, open label, multi-center study in children and adolescents. Divalproex sodium extended-release tablets once daily doses ranged from 250-1,750 mg. Once daily administration of divalproex sodium extended-release tablets in pediatric patients (10-17 years) produced plasma VPA concentration-time profiles similar to those that have been observed in adults.
Elderly
The capacity of elderly patients (age range: 68 to 89 years) to eliminate valproate has been shown to be reduced compared to younger adults (age range: 22 to 26 years). Intrinsic clearance is reduced by 39%; the free fraction is increased by 44%. Accordingly, the initial dosage should be reduced in the elderly
[see Dosage and Administration (
2.4)]
.
Effect of Sex
There are no differences in the body surface area adjusted unbound clearance between males and females (4.8±0.17 and 4.7±0.07 L/hr per 1.73 m
2, respectively).
Effect of Race
The effects of race on the kinetics of valproate have not been studied.
Effect of Disease
Liver Disease
Liver disease impairs the capacity to eliminate valproate. In one study, the clearance of free valproate was decreased by 50% in 7 patients with cirrhosis and by 16% in 4 patients with acute hepatitis, compared with 6 healthy subjects. In that study, the half-life of valproate was increased from 12 to 18 hours. Liver disease is also associated with decreased albumin concentrations and larger unbound fractions (2 to 2.6 fold increase) of valproate. Accordingly, monitoring of total concentrations may be misleading since free concentrations may be substantially elevated in patients with hepatic disease whereas total concentrations may appear to be normal
[see
Boxed Warning, Contraindications (
4), and Warnings and Precautions (
5.1)]
.
Renal Disease
A slight reduction (27%) in the unbound clearance of valproate has been reported in patients with renal failure (creatinine clearance < 10 mL/minute); however, hemodialysis typically reduces valproate concentrations by about 20%. Therefore, no dosage adjustment appears to be necessary in patients with renal failure. Protein binding in these patients is substantially reduced; thus, monitoring total concentrations may be misleading.
Drug Interaction Studies with No Interaction or Likely Clinically Unimportant Interaction
Antacids
A study involving the co-administration of valproate 500 mg with commonly administered antacids (Maalox, Trisogel, and Titralac - 160 mEq doses) did not reveal any effect on the extent of absorption of valproate.
Chlorpromazine
A study involving the administration of 100 to 300 mg/day of chlorpromazine to schizophrenic patients already receiving valproate (200 mg BID) revealed a 15% increase in trough plasma levels of valproate.
Haloperidol
A study involving the administration of 6 to 10 mg/day of haloperidol to schizophrenic patients already receiving valproate (200 mg BID) revealed no significant changes in valproate trough plasma levels.
Cimetidine and Ranitidine
Cimetidine and ranitidine do not affect the clearance of valproate.
Acetaminophen
Valproate had no effect on any of the pharmacokinetic parameters of acetaminophen when it was concurrently administered to three epileptic patients.
Clozapine
In psychotic patients (n=11), no interaction was observed when valproate was co-administered with clozapine.
Lithium
Co-administration of valproate (500 mg BID) and lithium carbonate (300 mg TID) to normal male volunteers (n=16) had no effect on the steady-state kinetics of lithium.
Lorazepam
Concomitant administration of valproate (500 mg BID) and lorazepam (1 mg BID) in normal male volunteers (n=9) was accompanied by a 17% decrease in the plasma clearance of lorazepam.
Olanzapine
No dose adjustment for olanzapine is necessary when olanzapine is administered concomitantly with valproate. Co-administration of valproate (500 mg BID) and olanzapine (5 mg) to healthy adults (n=10) caused 15% reduction in C
maxand 35% reduction in AUC of olanzapine.
Oral Contraceptive Steroids
Administration of a single-dose of ethinyloestradiol (50 mcg)/levonorgestrel (250 mcg) to 6 women on valproate (200 mg BID) therapy for 2 months did not reveal any pharmacokinetic interaction