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Mechanism Of Action
Divalproex sodium dissociates to the valproate ion in the gastrointestinal tract. The mechanisms by which valproate exerts its therapeutic effects have not been established. It has been suggested that its activity in epilepsy is related to increased brain concentrations of gamma-aminobutyric acid (GABA).
The relationship between plasma concentration and clinical response is not well documented. One contributing factor is the nonlinear, concentration dependent protein binding of valproate which affects the clearance of the drug. Thus, monitoring of total serum valproate may not provide a reliable index of the bioactive valproate species.
For example, because the plasma protein binding of valproate is concentration dependent, the free fraction increases from approximately 10% at 40 mcg/mL to 18.5% at 130 mcg/mL. Higher than expected free fractions occur in the elderly, in hyperlipidemic patients, and in patients with hepatic and renal diseases.
The therapeutic range in epilepsy is commonly considered to be 50 to 100 mcg/mL of total valproate, although some patients may be controlled with lower or higher plasma concentrations.
In placebo-controlled clinical trials of acute mania, patients were dosed to clinical response with trough plasma concentrations between 85 and 125 mcg/mL [see DOSAGE AND ADMINISTRATION].
The absolute bioavailability of Depakote ER 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 Depakote ER is less than that of Depakote (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, Depakote ER given once daily produced an average bioavailability of 89% relative to an equal total daily dose of Depakote given BID, TID, or QID. The median time to maximum plasma valproate concentrations (Cmax) after Depakote ER administration ranged from 4 to 17 hours. After multiple once-daily dosing of Depakote ER, the peak-to-trough fluctuation in plasma valproate concentrations was 10-20% lower than that of regular Depakote given BID, TID, or QID.
Conversion from Depakote to Depakote ER
When Depakote ER is given in doses 8 to 20% higher than the total daily dose of Depakote, the two formulations are bioequivalent. In two randomized, crossover studies, multiple daily doses of Depakote were compared to 8 to 20% higher once-daily doses of Depakote ER. In these two studies, Depakote ER and Depakote regimens were equivalent with respect to area under the curve (AUC; a measure of the extent of bioavailability). Additionally, valproate Cmax was lower, and Cmin was either higher or not different, for Depakote ER relative to Depakote regimens (see Table 8).
Table 8: Bioavailability of Depakote ER Tablets
Relative to Depakote When Depakote ER Dose is 8 to 20% Higher
|Study Population||Regimens||Relative Bioavailability|
|Depakote ER vs. Depakote||AUC24||Cmax||Cmin|
|Healthy Volunteers (N=35)||1000 & 1500 mg Depakote ER vs.875 & 1250 mg Depakote||1.059||0.882||1.173|
|Patients with epilepsy on concomitant enzyme-inducing antiepilepsy drugs (N = 64)||1000 to 5000 mg Depakote ER vs. 875 to 4250 mg Depakote||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 Depakote and Depakote ER.
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 for more detailed information on the pharmacokinetic interactions of valproate with other drugs].
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.
Mean plasma clearance and volume of distribution for total valproate are 0.56 L/hr/1.73 m² and 11 L/1.73 m², respectively. Mean plasma clearance and volume of distribution for free valproate are 4.6 L/hr/1.73 m² and 92 L/1.73 m². Mean terminal half-life for valproate monotherapy ranged from 9 to 16 hours following oral dosing regimens of 250 to 1000 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.
Effect of Age
The valproate pharmacokinetic profile following administration of Depakote ER was characterized in a multiple-dose, non-fasting, open label, multi-center study in children and adolescents. Depakote ER once daily doses ranged from 250-1750 mg. Once daily administration of Depakote ER in pediatric patients (10-17 years) produced plasma VPA concentration-time profiles similar to those that have been observed in adults.
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). 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].
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 m², respectively).
Effect of Race
The effects of race on the kinetics of valproate have not been studied.
Effect of 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, and WARNINGS AND PRECAUTIONS].
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.
The effectiveness of Depakote ER for the treatment of acute mania is based in part on studies establishing the effectiveness of Depakote (divalproex sodium delayed release tablets) for this indication. Depakote ER's effectiveness was confirmed in one randomized, double-blind, placebo-controlled, parallel group, 3-week, multicenter study. The study was designed to evaluate the safety and efficacy of Depakote ER in the treatment of bipolar I disorder, manic or mixed type, in adults. Adult male and female patients who had a current DSM-IV TR primary diagnosis of bipolar I disorder, manic or mixed type, and who were hospitalized for acute mania, were enrolled into this study. Depakote ER was initiated at a dose of 25 mg/kg/day given once daily, increased by 500 mg/day on Day 3, then adjusted to achieve plasma valproate concentrations in the range of 85-125 mcg/mL. Mean daily Depakote ER doses for observed cases were 2362 mg (range: 500-4000), 2874 mg (range: 1500-4500), 2993 mg (range: 15004500), 3181 mg (range: 1500-5000), and 3353 mg (range: 1500-5500) at Days 1, 5, 10, 15, and 21, respectively. Mean valproate concentrations were 96.5 mcg/mL, 102.1 mcg/mL, 98.5 mcg/mL, 89.5 mcg/mL at Days 5, 10, 15 and 21, respectively. Patients were assessed on the Mania Rating Scale (MRS; score ranges from 0-52).
Depakote ER was significantly more effective than placebo in reduction of the MRS total score.
The efficacy of valproate in reducing the incidence of complex partial seizures (CPS) that occur in isolation or in association with other seizure types was established in two controlled trials.
In one, multi-clinic, placebo controlled study employing an add-on design, (adjunctive therapy) 144 patients who continued to suffer eight or more CPS per 8 weeks during an 8 week period of monotherapy with doses of either carbamazepine or phenytoin sufficient to assure plasma concentrations within the “therapeutic range” were randomized to receive, in addition to their original antiepilepsy drug (AED), either Depakote or placebo. Randomized patients were to be followed for a total of 16 weeks. The following Table presents the findings.
Table 9: Adjunctive Therapy Study Median Incidence of
CPS per 8 Weeks
|Add-on Treatment||Number of Patients||Baseline Incidence||Experimental Incidence|
|* Reduction from baseline statistically significantly greater for valproate than placebo at p ≤ 0.05 level.|
Figure 1 presents the proportion of patients (X axis) whose percentage reduction from baseline in complex partial seizure rates was at least as great as that indicated on the Y axis in the adjunctive therapy study. A positive percent reduction indicates an improvement (i.e., a decrease in seizure frequency), while a negative percent reduction indicates worsening. Thus, in a display of this type, the curve for an effective treatment is shifted to the left of the curve for placebo. This Figure shows that the proportion of patients achieving any particular level of improvement was consistently higher for valproate than for placebo. For example, 45% of patients treated with valproate had a ≥ 50% reduction in complex partial seizure rate compared to 23% of patients treated with placebo.
The second study assessed the capacity of valproate to reduce the incidence of CPS when administered as the sole AED. The study compared the incidence of CPS among patients randomized to either a high or low dose treatment arm. Patients qualified for entry into the randomized comparison phase of this study only if 1) they continued to experience 2 or more CPS per 4 weeks during an 8 to 12 week long period of monotherapy with adequate doses of an AED (i.e., phenytoin, carbamazepine, phenobarbital, or primidone) and 2) they made a successful transition over a two week interval to valproate. Patients entering the randomized phase were then brought to their assigned target dose, gradually tapered off their concomitant AED and followed for an interval as long as 22 weeks. Less than 50% of the patients randomized, however, completed the study. In patients converted to Depakote monotherapy, the mean total valproate concentrations during monotherapy were 71 and 123 mcg/mL in the low dose and high dose groups, respectively.
The following Table presents the findings for all patients randomized who had at least one post-randomization assessment.
Table 10: Monotherapy Study Median Incidence of CPS
per 8 Weeks
|Treatment||Number of Patients||Baseline Incidence||Randomized Phase Incidence|
|High dose Valproate||131||13.2||10.7*|
|Low dose Valproate||134||14.2||13.8|
|* Reduction from baseline statistically significantly greater for high dose than low dose at p ≤ 0.05 level.|
Figure 2 presents the proportion of patients (X axis) whose percentage reduction from baseline in complex partial seizure rates was at least as great as that indicated on the Y axis in the monotherapy study. A positive percent reduction indicates an improvement (i.e., a decrease in seizure frequency), while a negative percent reduction indicates worsening. Thus, in a display of this type, the curve for a more effective treatment is shifted to the left of the curve for a less effective treatment. This Figure shows that the proportion of patients achieving any particular level of reduction was consistently higher for high dose valproate than for low dose valproate. For example, when switching from carbamazepine, phenytoin, phenobarbital or primidone monotherapy to high dose valproate monotherapy, 63% of patients experienced no change or a reduction in complex partial seizure rates compared to 54% of patients receiving low dose valproate.
Information on pediatric studies are presented in section 8.
The results of a multicenter, randomized, double-blind, placebo-controlled, parallel-group clinical trial demonstrated the effectiveness of Depakote ER in the prophylactic treatment of migraine headache. This trial recruited patients with a history of migraine headaches with or without aura occurring on average twice or more a month for the preceding three months. Patients with cluster or chronic daily headaches were excluded. Women of childbearing potential were allowed in the trial if they were deemed to be practicing an effective method of contraception.
Patients who experienced ≥ 2 migraine headaches in the 4-week baseline period were randomized in a 1:1 ratio to Depakote ER or placebo and treated for 12 weeks. Patients initiated treatment on 500 mg once daily for one week, and were then increased to 1000 mg once daily with an option to permanently decrease the dose back to 500 mg once daily during the second week of treatment if intolerance occurred. Ninety-eight of 114 Depakote ER-treated patients (86%) and 100 of 110 placebo-treated patients (91%) treated at least two weeks maintained the 1000 mg once daily dose for the duration of their treatment periods. Treatment outcome was assessed on the basis of reduction in 4-week migraine headache rate in the treatment period compared to the baseline period.
Patients (50 male, 187 female) ranging in age from 16 to 69 were treated with Depakote ER (N=122) or placebo (N=115). Four patients were below the age of 18 and 3 were above the age of 65. Two hundred and two patients (101 in each treatment group) completed the treatment period. The mean reduction in 4-week migraine headache rate was 1.2 from a baseline mean of 4.4 in the Depakote ER group, versus 0.6 from a baseline mean of 4.2 in the placebo group. The treatment difference was statistically significant (see Figure 3).
Figure 3 : Mean Reduction In 4-Week Migraine Headache
1. Meador KJ, Baker GA, Browning N, et al. Fetal antiepileptic drug exposure and cognitive outcomes at age 6 years (NEAD study): a prospective observational study. Lancet Neurology 2013; 12 (3):244-252.
Last reviewed on RxList: 3/4/2016
This monograph has been modified to include the generic and brand name in many instances.
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