Topiramate (brand name Topamax) is an anticonvulsant (antiepilepsy) drug. In late 2012, topiramate was approved by the United States Food and Drug Administration (FDA) in combination with phentermine for weight loss. The drug had previously been used off-label for this purpose. Topiramate was originally produced by Ortho-McNeil Neurologics and Noramco, Inc., both divisions of the Johnson & Johnson Corporation. This medication was discovered in 1979 by Bruce E. Maryanoff and Joseph F. Gardocki during their research work at McNeil Pharmaceuticals.
Topiramate came into commercial use in 1996. Generic versions are available in Canada and these were approved by the FDA in September 2006. Mylan Pharmaceuticals was granted final approval for generic topiramate by the FDA for sale in the United States. The last patent for topiramate in the U.S. was for use in children and expired on February 28, 2009.
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Medical uses
Topiramate is used to treat epilepsy in children and adults, and it was originally used as an anticonvulsant. In children, it is indicated for the treatment of Lennox-Gastaut syndrome, a disorder that causes seizures and developmental delay. It is also Food and Drug Administration (FDA) approved for, and most frequently prescribed for, the prevention of migraines. The drug is also used to treat migraines due to the effect it has on the blood vessels in the brain. It has been found to be increasingly effective for migraine sufferers with limited side effects.
Research and off-label
Psychiatrists have used topiramate to treat bipolar disorder, although the available evidence does not support its use in any phase of bipolar disorder treatment. A more recent review, published in 2010, suggested a benefit of topiramate in the treatment of symptoms of borderline personality disorder, however the authors note that this was based only on one randomized controlled trial and requires replication. Also the authors noted that the long-term effects have not been studied.
This drug has been used as a treatment for alcoholism, obesity and antipsychotic-induced weight gain.
The drug is also used in clinical trials to treat post traumatic stress disorder. A pilot study suggested that topiramate is effective against infantile spasms. Another study recommends topiramate as an effective treatment in the prevention of periventricular leukomalacia in preterm infants after an hypoxic-ischemic injury.
Recent clinical reports indicate that it may have mood stabilizing properties. Other off-label and investigational uses of topiramate include the treatment of essential tremor, bulimia nervosa, obsessive-compulsive disorder, idiopathic intracranial hypertension and cluster headache. Topiramate has not been shown to work as a pain medicine in diabetic neuropathy, the only neuropathic condition in which it has been adequately tested. Dispensing errors have been reported between Topamax (topiramate) tablets and Toprol-XL (metoprolol succinate) extended-release tablets thereby requiring extra care to ensure the correct medication is being taken.[2]
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Warnings and contraindications
People taking topiramate should be aware of the following risks:
- Avoid activities requiring mental alertness and coordination until drug effects are realized.
- Topiramate may impair heat regulation, especially in children. Use caution with activities leading to an increased core temperature, such as strenuous exercise, exposure to extreme heat, or dehydration.
- Topiramate may cause visual field defects.
- Topiramate may decrease effectiveness of estrogen-containing oral contraceptives.
- Taking topiramate in the 1st trimester of pregnancy may increase risk of cleft lip/cleft palate in infant.
- As is the case for all antiepileptic drugs, it is advisable not to suddenly discontinue topiramate as there is a theoretical risk of rebound seizures.
Adverse effects
Adverse effects by incidence:
Very common (>10% incidence) adverse effects include:
Common (1-10% incidence) adverse effects include:
Rarely, the inhibition of carbonic anhydrase may be strong enough to cause metabolic acidosis of clinical importance.
The U.S. Food and Drug Administration (FDA) has notified prescribers that topiramate can cause acute myopia and secondary angle closure glaucoma in a small subset of people who take topiramate regularly. The symptoms, which typically begin in the first month of use, include blurred vision and eye pain. Discontinuation of topiramate may halt the progression of the ocular damage, and may reverse the visual impairment.
Preliminary data suggests that, as with several other anti-epileptic drugs, topiramate carries an increased risk of congenital malformations. This might be particularly important for women who take topiramate to prevent migraine attacks. In March 2011 the FDA notified healthcare professionals and patients of an increased risk of development of cleft lip and/or cleft palate (oral clefts) in infants born to women treated with Topamax (topiramate) during pregnancy and placed it in Pregnancy Category D.
Topiramate has been associated with a statistically significant increase in suicidality, and "suicidal thoughts or actions" is now listed as one of the possible side effects of the drug "in a very small number of people, about 1 in 500."
Overdose
Symptoms of acute and acute on chronic exposure to topiramate range from asymptomatic to status epilepticus, including in patients with no seizure history. In children, overdose may also result in hallucinations. Topiramate has been deemed the primary substance that led to fatal overdoses in cases that were complicated by polydrug exposure. The most common signs of overdose are dilated pupils, somnolence, dizziness, psychomotor agitation, and abnormal, uncoordinated body movements.
Symptoms of overdose may include but are not limited to:
A specific antidote is not available. Treatment is entirely supportive.
Interactions
Topiramate has many drug-drug interactions. Some of the most common are listed below:
- As topiramate inhibits carbonic anhydrase, use with other inhibitors of carbonic anhydrase (e.g. acetazolamide) increases the risk of kidney stones.
- Enzyme inducers (e.g. carbamazepine) can increase the elimination of topiramate, possibly necessitating dose escalations of topiramate.
- Topiramate may increase the plasma-levels of phenytoin.
- Topiramate itself is a weak inhibitor of CYP2C19 and induces CYP3A4; a decrease in plasma levels of estrogens and digoxin has been noted during topiramate therapy. This can reduce the effectiveness of oral contraceptives (birth control pills); use of alternative birth control methods is recommended. Neither intrauterine devices (IUDs) nor Depo-Provera are affected by topiramate.
- Alcohol may cause increased sedation or drowsiness, and increase the risk of having a seizure.
- As listed in the 06/29/2005 label posted at the Drugs@FDA website page 14,'conditions or therapies that predispose to acidosis may be additive to the bicarbonate lowering effects of Topiramate'.
- Oligohidrosis and hyperthermia were reported in post-marketing reports about topiramate; antimuscarinic drugs (like trospium) can aggravate these disorders.
Pharmacology
Chemically, topiramate is a sulfamate-substituted monosaccharide, related to fructose, a rather unusual chemical structure for an anticonvulsant.
Topiramate is quickly absorbed after oral use. Most of the drug (70%) is excreted in the urine unchanged. The remainder is extensively metabolized by hydroxylation, hydrolysis, and glucuronidation. Six metabolites have been identified in humans, none of which constitutes more than 5% of an administered dose.
Several cellular targets have been proposed to be relevant to the therapeutic activity of topiramate. These include (1) voltage-gated sodium channels; (2) high-voltage-activated calcium channels; (3) GABA-A receptors; (4) AMPA/kainate receptors; and (5) carbonic anhydrase isoenzymes. There is evidence that topiramate may alter the activity of its targets by modifying their phosphorylation state instead of by a direct action. The effect on sodium channels could be of particular relevance for seizure protection. Although topiramate does inhibit high-voltage-activated calcium channels, the relevance to clinical activity is uncertain. Effects on specific GABA-A receptor isoforms could also contribute to the antiseizure activity of the drug. Topiramate selectively inhibits cytosolic (type II) and membrane associated (type IV) forms of carbonic anhydrase. The action on carbonic anhydrase isoenzymes may contribute to the drug's side-effects, including its propensity to cause metabolic acidosis and calcium phosphate kidney stones.
Topiramate inhibits maximal electroshock and pentylenetetrazol-induced seizures as well as partial and secondarily generalized tonic-clonic seizures in the kindling model, findings predictive of a broad spectrum of activities clinically. Its action on mitochondrial permeability transition pores has been proposed as a mechanism.
While many anticonvulsants have been associated with apoptosis in young animals, animal experiments have found that topiramate is one of the very few anticonvulsants [see: levetiracetam, carbamazepine, lamotrigine] that do not induce apoptosis in young animals at doses needed to produce an anticonvulsant effect.
Detection in body fluids
Blood, serum or plasma topiramate concentrations may be measured using immunoassay or chromatographic methods to monitor therapy, confirm a diagnosis of poisoning in hospitalized patients or to assist in a medicolegal death investigation. Plasma levels are usually less than 10 mg/L during therapeutic administration, but can range from 10-150 mg/L in overdose victims.
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