indication

Used for the treatment and control of partial seizures and severe tonic-clonic (grand mal) seizures and also for the prevention of migraine headaches. In children it is also used for treatment of Lennox-Gastaut syndrome.

pharmacology

Topiramate is an anticonvulsant indicated in the treatment of epilepsy and migraine. Topiramate enhances GABA-activated chloride channels. In addition, topiramate inhibits excitatory neurotransmission, through actions on kainate and AMPA receptors. There is evidence that topiramate has a specific effect on GluR5 kainate receptors. It is also an inhibitor of carbonic anhydrase, particular subtypes II and IV, but this action is weak and unlikely to be related to its anticonvulsant actions, but may account for the bad taste and the development of renal stones seen during treatment. Its possible effect as a mood stabilizer seems to occur before anticonvulsant qualities at lower dosages. Topiramate inhibits maximal electroshock and pentylenetetrazol-induced seizures as well as partial and secundarily generalized tonic-clonic seizures in the kindling model, findings predective of a broad spectrum of antiseizure activities clinically.

mechanism of action

The precise mechanism of action of topiramate is not known. However, studies have shown that topiramate blocks the action potentials elicited repetitively by a sustained depolarization of the neurons in a time-dependent manner, suggesting a state-dependent sodium channel blocking action. Topiramate also augments the activity of the neurotransmitter gamma-aminobutyrate (GABA) at some subtypes of the GABA_A receptor (controls an integral chloride channel), indicating a possible mechanism through potentiation of the activity of GABA. Topiramate also demonstrates antagonism of the AMPA/kainate subtype of the glutamate excitatory amino acid receptor. It also inhibits carbonic anhydrase (particularly isozymes II and IV), but this action is weak and unlikely to be related to its anticonvulsant actions.

toxicity

Symptoms of overdose include abdominal pain, agitation, blurred vision, convulsions, depression, dizziness, double vision, drowsiness, impaired coordination, impaired mental activity, low blood pressure, reduced consciousness, severe diarrhea, sluggishness, and speech problems.

biotransformation

Not extensively metabolized, 70% of the dose is eliminated unchanged in the urine. The other 30% is metabolized hepatically to six metabolites (formed by hydroxylation, hydrolysis, and glucuronidation), none of which constitute more than 5% of an administered dose.

absorption

Rapid with pleak plasma concentrations occurring after 2 hours and a bioavailability of 80%.

half life

19 to 23 hours

route of elimination

Topiramate is not extensively metabolized and is primarily eliminated unchanged in the urine (approximately 70% of an administered dose).

drug interactions

Acetazolamide: Additive renal carbonic anhydrase inhibition may occur increasing the risk of crystaluria and renal calculi. Increased risk of nephrolithiasis. Consider altnerate therapy.

Brinzolamide: As both brinzolamide and topiramate are carbonic anhydrase inhibitors, there is an increased risk of adverse effects.The development of acid-base disorders with concurrent use of ophthalmic and oral carbonic anhydrase inhibitors has been reported. Avoid concurrent use of different carbonic anhydrase inhibitors when possible.

Carbamazepine: Carbamazepine may decrease the effectiveness of Topiramate by increase its clearance. Monitor for changes in the therapeutic and adverse effects of Topiramate if Carbamazepine is initiated, discontinued or dose changed. Adverse effects related to CNS depression have also been observed during concomitant therapy.

Chlorothiazide: Thiazide diuretics such as chlorothiazide may enhance the hypokalemic effect of topiramate. Thiazide diuretics may increase the serum concentration of topiramate. Monitor for increased topiramate concentrations/adverse effects (e.g., hypokalemia) with initiation/dose increase of a thiazide diuretic. Closely monitor serum potassium concentrations with concomitant therapy. Topiramate dose reductions may be necessary.

Ethinyl Estradiol: Topiramate may decrease the effect of the oral contraceptive, Ethinyl estradiol. An alternate form of contraception should be used during concomitant therapy.

Fosphenytoin: Increased phenytoin/decreased topiramate

Lithium: Topiramate could modify lithium levels

Maraviroc: Topiramate, a CYP3A4 inducer, may decrease the serum concentration of Maraviroc by increasing Maraviroc metabolism and clearance. A dose adjustment of Maraviroc may be required. Monitor for changes in Maraviroc therapeutic and adverse effects if Topiramate is initiated, discontinued or dose changed.

Mestranol: Topiramate may decrease the effect of the oral contraceptive, Mestranol. An alternate form of contraception should be used during concomitant therapy.

Phenytoin: Increased phenytoin/decreased topiramate

Tobramycin: Increased risk of nephrotoxicity

Triprolidine: The CNS depressants, Triprolidine and Topiramate, may increase adverse/toxic effects due to additivity. Monitor for increased CNS depressant effects during concomitant therapy.