Absorption
Following a 100 mg ZONISADE dose in normal volunteers, the time to maximum plasma concentrations (T
max) occurred within 0.5–5 hours.
Zonisamide pharmacokinetics are dose-proportional in the range of 200 to 400 mg. Once a stable dose is reached, steady state is achieved within 14 days.
Effect of Food
When ZONISADE is administered with food, the zonisamide T
maxis delayed, occurring at 3.5–7.5 hours, but food has no effect on the bioavailability of zonisamide.
Distribution
The apparent volume of distribution (V/F) of zonisamide is about 1.45 L/kg following a 400 mg oral dose. Zonisamide, at concentrations of 1.0–7.0 mcg/mL, is approximately 40% bound to human plasma proteins. Zonisamide extensively binds to erythrocytes, resulting in an eight-fold higher concentration of zonisamide in red blood cells than in plasma. Protein binding of zonisamide is unaffected in the presence of therapeutic concentrations of phenytoin, phenobarbital, or carbamazepine.
Elimination
The plasma clearance of oral zonisamide is approximately 0.30–0.35 mL/min/kg in patients not receiving enzyme-inducing antiepileptic drugs (AEDs). The clearance of zonisamide is increased to 0.5 mL/min/kg in patients concurrently on enzyme-inducing AEDs
(see Potential for Other Drugs to Affect ZONISADE). After a single-dose administration, renal clearance of zonisamide is approximately 3.5 mL/min.
Metabolism
Zonisamide is metabolized by N-acetyl-transferases to form N-acetyl zonisamide and by CYP3A4 to form 2–sulfamoylacetylphenol (SMAP).
Excretion
The elimination half-life of zonisamide in plasma is approximately 63 hours. The elimination half-life of zonisamide in red blood cells is approximately 105 hours. Zonisamide is excreted primarily in urine as parent drug and as the glucuronide of a metabolite. Following multiple dosing, 62% of the radiolabeled dose was recovered in the urine, with 3% in the feces by day 10. Of the excreted dose, 35% was recovered as zonisamide, 15% as N-acetyl zonisamide, and 50% as the glucuronide of SMAP.
Specific Populations
Patients with Renal Impairment
Single 300 mg zonisamide doses were administered to three groups of volunteers. Group 1 was a healthy group with a creatinine clearance ranging from 70–152 mL/min. Group 2 and Group 3 had creatinine clearances ranging from 14.5–59 mL/min and 10–20 mL/min, respectively. Zonisamide renal clearance decreased with decreasing renal function (3.42, 2.50, and 2.23 mL/min, respectively). Marked renal impairment (creatinine clearance < 20 mL/min) was associated with an increase in zonisamide AUC of 35%
[see Use in Specific Populations (
8.6)]
.
Patients with Hepatic Impairment
The pharmacokinetics of zonisamide in patients with impaired liver function have not been studied
.
Age
The pharmacokinetics of a 300 mg single dose of zonisamide were similar in young (mean age 28 years) and elderly subjects (mean age 69 years).
Drug Interaction Studies
In-Vitro Studies
Enzymes
In vitro studies using human liver microsomes show insignificant (<25%) inhibition of cytochrome P450 isozymes 1A2, 2A6, 2C9, 2C19, 2D6, 2E1, 3A4, 2B6, or 2C8 at zonisamide levels approximately two-fold or greater than clinically relevant unbound serum concentrations. Therefore, ZONISADE is not expected to affect the pharmacokinetics of other drugs via cytochrome P450-mediated mechanisms.
Transporters
An
in-vitrostudy showed that zonisamide is a weak inhibitor of P-gp (MDR1).
In-Vivo Studies
Potential for Zonisamide to Affect Other Drugs
Antiepileptic Drugs
In epileptic patients, steady state dosing with zonisamide capsules resulted in no clinically relevant pharmacokinetic effects on carbamazepine, lamotrigine, phenytoin, or sodium valproate.
Oral Contraceptives
In healthy subjects, steady state dosing with zonisamide capsules did not affect serum concentrations of ethinylestradiol or norethisterone in a combined oral contraceptive.
CYP2D6 Substrates
Coadministration of multiple dosing of zonisamide up to 400 mg/day with single 50-mg doses of desipramine did not significantly affect the pharmacokinetic parameters of desipramine, a probe drug for CYP2D6 activity.
Potential for Other Drugs to Affect ZONISADE
CYP3A4 Inducers
The half-life of zonisamide following a 400 mg dose in patients concurrently on enzyme-inducing AEDs such as phenytoin, carbamazepine, or phenobarbital, was between 27-38 hours; the half-life of zonisamide in patients concurrently on the non-enzyme inducing AED, valproate, was 46 hours.
These effects are unlikely to be of clinical significance when ZONISADE is added to existing therapy; however, changes in zonisamide concentrations may occur if concomitant CYP3A4 inducing antiepileptic or other drugs are withdrawn, dose adjusted or introduced, an adjustment of the ZONISADE dose may be required
[see Drug Interactions (
7.3)]
.
CYP3A4 Inhibitors
Steady-state dosing of either ketoconazole (400 mg/day) or cimetidine (1200 mg/day) had no clinically relevant effects on the single dose pharmacokinetics of zonisamide given to healthy subjects.