Topiramate (Topamax)

Generic Name: Topiramate

Brand Name: Topamax, Topiragen, Topamac, Topimax

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Topiramate (Topamax) is a sulfamate substituted monosaccharide anticonvulsant. Its indications include:

• Monotherapy for partial onset or primary generalized tonic-clonic seizures.
• Adjunctive therapy for partial onset seizures, primary generalized tonic-clonic seizures, and seizures associated with Lennox-Gastaut syndrome.
• Prophylaxis of migraine headache.

Topiramate also shows promise in pediatric migraine prophylaxis, weight loss, and perhaps angry/aggressive behaviour.

Topiramate dosage:

Epilepsy, adjunctive therapy
Adults: 200 to 400 mg daily in 2 divided doses in adults with partial seizures and 400 mg daily in 2 divided doses in adults with primary generalized tonic–clonic seizures. Initiate therapy at 25 to 50 mg daily and titrate to an effective dose in increments of 25 to 50 mg weekly. Doses over 400 mg do not improve response.

Children 2 to 16 yr of age: 5 to 9mg/kg/day in 2 divided doses. Initiate therapy at 25 mg or less (based on range of 1 to 3mg/kg/day) nightly for first wk and titrate to an effective dose at 1- to 2-wk intervals by increments of 1 to 3mg/kg/day in 2 divided doses.

Epilepsy, monotherapy
Adults and children 10 yr of age and older: 400 mg daily in 2 divided doses. Initiate therapy at 50 mg daily and titrate to an effective dose in increments of 25 to 50 mg weekly.

Migraine
Adults: 50 mg in the morning and evening.
Topamax begins reducing the frequency of migraines within 1 month.

Dosage in renal failure: The major route of elimination of unchanged topiramate and its metabolites is via the kidney.
If Clcr is < 70 mL/minute per 1.73 m2, decrease dosage by 50%.

Note: Do not abruptly discontinue Topiramate. Topiramate may be discontinued by dose decreasing in weekly intervals of 50-100 mg/day gradually over 2-8 weeks for seizures, and by decreasing in weekly intervals by 25-50 mg/day for migraine prophylaxis.

Side effects:

Central nervous system: paresthesia (51%); dizziness (32%); fatigue (30%); somnolence (29%); psychomotor slowing (21%); nervousness (19%); ataxia (16%); concentration and memory difficulties, confusion (14%); depression, speech or behavior problems, nystagmus (11%); language problems (10%); aggressive reaction, insomnia, tremor (9%); abnormal gait, hypoaesthesia (8%); cognitive problems (7%); anxiety (6%); hyperkinesia (5%).

Gastrointestinal: anorexia (24%); nausea (14%); diarrhea (11%); abdominal cramps, dyspepsia (7%); increased saliva (6%).

Other: weight decrease (21%); taste perversion (15%); abnormal vision (13%); diplopia (10%); alopecia (5%).

Precautions:

Metabolic acidosis (condition associated with excessive acid in the blood). Hyperchloremic, nonanion gap, metabolic acidosis (i.e., decreased serum bicarbonate below the normal reference range in the absence of chronic respiratory alkalosis) is associated with topiramate. This metabolic acidosis is caused by renal bicarbonate loss due to the inhibitory effect of topiramate on carbonic anhydrase.

CNS Effects: High rate of unwelcome neurocognitive, psychiatric, and mood disturbances. The incidence is greater with rapid titration and higher doses. Topiramate may also cause paresthesia and ataxia.

Glaucoma. Has been associated with myopia and secondary angle-closure glaucoma in adults and children, typically within 1 month of initiation. Stop immediately in case of acute onset of decreased visual acuity.

Hyperthermia and oligohidrosis (decreased sweating). May be associated (rarely) with severe oligohydrosis and hyperthermia, most frequently in children. Monitor closely during strenuous exercise, during exposure to high environmental temperature, or in patients receiving anticholinergic drugs.

Kidney stones. The risk of kidney stones is about 2-4 times that of the untreated population. Maintain adequate fluid intake to decrease stone formation.

Paresthesia. Paresthesia (usually tingling or pricking of the extremities) appears to be frequent with Topamax. Paresthesia was more frequently reported in the monotherapy epilepsy trials and migraine prophylaxis trials versus the adjunctive therapy epilepsy trials.

Children. Safety and efficacy have not been established in children < 2 years of age for adjunctive therapy and < 10 years of age for monotherapy.

Topamax/Toprol confusion: similarity in spelling between Topamax (topiramate) and Toprol (metoprolol succinate) may result in prescribing and dispensing errors.

Drug Interactions:

Topiramate has minimal hepatic drug interactions. Inhibits carbonic anhydrase, CYP2C19 (weak). Induces CYP3A4 (weak).

• Acetazolamide, dichlorphenamide: increased risk of nephrolithiasis and hyperthermia.
• Carbamazepine: may reduce topiramate levels by 40%.
• Hydrochlorothiazide: increased topiramate levels.
• Oral contraceptives (estrogens): blood levels of estrogens are decreased when coadministered with topiramate, this may lead to a loss of efficacy.
• Phenytoin: May decrease topiramate levels by as much as 48%; topiramate may increase phenytoin concentration by 25%
• Valproic acid: increased risk of hyperammonemia.

Pregnancy

Pregnancy Category D.

An increased risk of cleft lip and cleft palate (oral clefts) has been observed in infants born to women treated with topiramate (Topamax) during pregnancy.

Nursing Mothers
Based on limited data, topiramate was found in breast milk; low concentrations were detected in nursing infants.

References:

• 1. U.S. FDA Topamax (Topiramate) Information