Zonisamide

ZONISAMIDE- zonisamide capsule
BluePoint Laboratories

Rx Only

DESCRIPTION

Zonisamide, USP is an antiseizure drug chemically classified as a sulfonamide and unrelated to other antiseizure agents. The active ingredient is zonisamide, USP, 1,2-benzisoxazole-3-methanesulfonamide. The empirical formula is C8 H8 N2 O3 S with a molecular weight of 212.23 g/mol. Zonisamide, USP is a white powder, pKa = 10.2, and is moderately soluble in water (0.80 mg/mL) and 0.1 N HCl (0.50 mg/mL).

The chemical structure is:

Chemical Formula of Zonisamide
(click image for full-size original)

Zonisamide, USP is supplied for oral administration as capsules containing 25 mg, 50 mg or 100 mg zonisamide, USP. Each capsule contains the labeled amount of zonisamide, USP plus the following inactive ingredients: hydrogenated vegetable oil, microcrystalline cellulose, and sodium lauryl sulfate. The printed capsule shell of the different strengths is made from the following ingredients:

25 mg – D&C Red #28, FD&C Blue #1, gelatin and titanium dioxide

50 mg – D&C Yellow #10, FD&C Blue #1, FD&C Red #40, gelatin and titanium dioxide

100 mg — D&C Yellow #10, FD&C Blue #1, FD&C Yellow #6, gelatin and titanium dioxide

The dyes used in the printing ink are FD&C Blue #1, FD&C Blue #2, FD&C Red #40, D&C Yellow #10 aluminum lake and iron oxide black. Additionally, the printing ink also contains propylene glycol and shellac.

CLINICAL PHARMACOLOGY

Mechanism of Action:

The precise mechanism(s) by which zonisamide exerts its antiseizure effect is unknown. Zonisamide demonstrated anticonvulsant activity in several experimental models. In animals, zonisamide was effective against tonic extension seizures induced by maximal electroshock but ineffective against clonic seizures induced by subcutaneous pentylenetetrazol. Zonisamide raised the threshold for generalized seizures in the kindled rat model and reduced the duration of cortical focal seizures induced by electrical stimulation of the visual cortex in cats. Furthermore, zonisamide suppressed both interictal spikes and the secondarily generalized seizures produced by cortical application of tungstic acid gel in rats or by cortical freezing in cats. The relevance of these models to human epilepsy is unknown.

Zonisamide may produce these effects through action at sodium and calcium channels. In vitro pharmacological studies suggest that zonisamide blocks sodium channels and reduces voltage-dependent, transient inward currents (T-type Ca2+ currents), consequently stabilizing neuronal membranes and suppressing neuronal hypersynchronization. In vitro binding studies have demonstrated that zonisamide binds to the GABA/benzodiazepine receptor ionophore complex in an allosteric fashion which does not produce changes in chloride flux. Other in vitro studies have demonstrated that zonisamide (10 to 30 mcg/mL) suppresses synaptically-driven electrical activity without affecting postsynaptic GABA or glutamate responses (cultured mouse spinal cord neurons) or neuronal or glial uptake of [3 H]-GABA (rat hippocampal slices). Thus, zonisamide does not appear to potentiate the synaptic activity of GABA. In vivo microdialysis studies demonstrated that zonisamide facilitates both dopaminergic and serotonergic neurotransmission. Zonisamide is a carbonic anhydrase inhibitor. The contribution of this pharmacological action to the therapeutic effects of zonisamide is unknown. However, as a carbonic anhydrase inhibitor, zonisamide may cause metabolic acidosis (see WARNINGS, Metabolic Acidosis subsection).

Pharmacokinetics: Absorption

Following a 200 to 400 mg oral zonisamide dose, peak plasma concentrations (range: 2 to 5 mcg/mL) in normal volunteers occur within 2 to 6 hours. In the presence of food, the time to maximum concentration is delayed; occurring at 4 to 6 hours, but food has no effect on the bioavailability of zonisamide.

Zonisamide absorption is dose-proportional in the range of 200 to 400 mg. Cmax and AUC, however , increase disproportionately at 800 mg, possibly due to saturable binding of zonisamide to red blood cells. Once a stable dose is reached, steady state is achieved within 14 days.

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 to 7 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.

Metabolism and Elimination

Following oral administration of 14 C-zonisamide to healthy volunteers, only zonisamide was detected in plasma. 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. Zonisamide undergoes acetylation by N-acetyl-transferases to form N-acetyl zonisamide and reduction to form the open ring metabolite, 2–sulfamoylacetyl phenol (SMAP). Of the excreted dose, 35% was recovered as zonisamide, 15% as N-acetyl zonisamide, and 50% as the glucuronide of SMAP. Reduction of zonisamide to SMAP is mediated by cytochrome P450 isozyme 3A4 (CYP3A4). Zonisamide does not induce its own metabolism. The plasma clearance of oral zonisamide is approximately 0.30 to 0.35 mL/min/kg in patients not receiving enzyme-inducing antiepilepsy drugs (AEDs). The clearance of zonisamide is increased to 0.5 mL/min/kg in patients concurrently on enzyme-inducing AEDs.

After a single-dose administration, renal clearance of zonisamide is approximately 3.5 mL/min. The clearance of an oral dose of zonisamide from red blood cells is 2 mL/min. 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.

Specific Populations:

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 to 152 mL/min. Group 2 and Group 3 had creatinine clearances ranging from 14.5 to 59 mL/min and 10 to 20 mL/min, respectively. Zonisamide renal clearance decreased with decreasing renal function (3.42, 2.50, 2.23 mL/min, respectively).

Marked renal impairment (creatinine clearance < 20 mL/min) was associated with an increase in zonisamide AUC of 35% (see DOSAGE AND ADMINISTRATIONsection).

Hepatic Impairment:

The pharmacokinetics of zonisamide in patients with impaired liver function have not been studied (see DOSAGE AND ADMINISTRATION section).

Age:

The pharmacokinetics of a 300 mg single dose of zonisamide was similar in young (mean age 28 years) and elderly subjects (mean age 69 years).

Gender and Race:

Information on the effect of gender and race on the pharmacokinetics of zonisamide is not available.

Effects of zonisamide on cytochrome P450 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, zonisamide is not expected to affect the pharmacokinetics of other drugs via cytochrome P450-mediated mechanisms.

Potential for zonisamide to affect other drugs

Anti-epileptic drugs

In epileptic patients, steady state dosing with zonisamide resulted in no clinically relevant pharmacokinetic effects on carbamazepine, lamotrigine, phenytoin, or sodium valproate.

Oral contraceptives

In healthy subjects, steady state dosing with zonisamide 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.

P-gp substrate

An in vitro study showed that zonisamide is a weak inhibitor of P-gp (MDR1) with an IC50 of 267 μmol/L. There is a theoretical potential for zonisamide to affect the pharmacokinetics of drugs which are P-gp substrates. Caution is advised when starting or stopping zonisamide or changing the zonisamide dose in patients who are also receiving drugs which are P-gp substrates (e.g., digoxin, quinidine).

Potential for Medicinal Products to Affect Zonisamide

Concomitant medications that can induce or inhibit CYP3A4 or N-acetyl-transferases may affect the pharmacokinetics of zonisamide. Drugs which inhibit or induce glucuronide conjugation are not expected to influence the pharmacokinetics of zonisamide.

The absence of a clinically significant pharmacokinetic interaction between zonisamide and lamotrigine indicates a low potential for zonisamide to interact with substances which are metabolized by UDP-GT.

CYP3A4 Induction: Drugs that induce liver enzymes increase the metabolism and clearance of zonisamide and decrease its half-life. 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 to 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 zonisamide is added to existing therapy; however, changes in zonisamide concentrations may occur if concomitant CYP3A4 inducing anti-epileptic or other drugs are withdrawn, dose adjusted or introduced, an adjustment of the zonisamide dose may be required. If coadministration with a potent CYP3A4 inducer (e.g. rifampicin) is necessary, the patient should be closely monitored and the dose of zonisamide and other drugs that are CYP3A4 substrate may need to be adjusted.

CYP3A4 Inhibition: 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. Therefore, modifications of zonisamide dosing is not necessary when coadministered with known CYP3A4 inhibitors.

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