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Topiramate - A Monosaccharide Anticonvulsant

Author: wikipedia, Posted on Thursday, November 03 @ 06:32:13 IST by RxPG  

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Neurology

Topiramate is an anticonvulsant drug. It is used to treat epilepsy in both children and adults. In children it is also indicated for treatment of Lennox-Gastaut syndrome (a disorder that causes seizures and developmental delays). It is also FDA approved for, and now most frequently prescribed for, the prevention of migraines. It has been used by psychiatrists to treat bipolar disorder, although it is not FDA approved for this purpose and such use is somewhat controversial. This drug has been investigated for use in treatment of obesity, especially to aid in the reduction of binge eating, and also as a possible treatment for alcoholism. However, these uses are not actively promoted by the manufacturer, and like its use for bipolar disorder, are "off-label" uses. The drug is also used in clinical trials to treat Post Traumatic Stress Disorder. A pilot study suggests that Topiramate is possibly effective against infantile spasm.

Pharmacodynamics

Chemically, Topiramate is a sulfamate-substituted monosaccharide, related to fructose, a rather unusal chemical structure for an anticonvulsant. Topiramate is quickly absorbed after oral use. Most of the drug (70%) is excreted in the urine as unchanged drug. 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. 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.

Side effects

The most common side effects include a change in taste (carbonated beverages, especially diet sodas and beer, taste particularly bad) and feelings of pins and needles in the head and extremities. Less common side effects include cognitive deficiency (particularly word-finding difficulty); grogginess; lethargy; renal stones, impairment of fine motor skills; vision abnormality and transient or permanent vision loss (see below for FDA warning); weight loss; breast pain; abdominal pain; menstrual disorder; taste changes; pharyngitis; sinusitis; diplopia; rash; leukopenia; fatigue; dizziness; insomnia; anxiety; depression; paresthesia; diarrhea; nausea; dyspepsia; constipation; dry-mouth; dysmenorrhea.

Rarely, the inhibition of carbonic anhydrase may be strong enough to cause metabolic acidosis of clinical importance.

The side-effects most frequently leading to discontinuation of therapy with topiramate were :

* psychomotor slowing (4.1%)
* memory problems (3.3%)
* fatigue (3.3%)
* confusion (3.2%)
* somnolence (3.2%).

The Food and Drug Administration (FDA) has issued a notification alerting physicians who prescribe topiramate, and their patients, to the risk of vision loss (blindness). Acute myopia and secondary angle closure glaucoma, in a small subset of patients who take topiramate regularly, may cause transient (reversible), or permanent, loss of vision. The symptoms, which typically begin in the first month of use, include blurred vision and eye pain. If addressed early in its course, discontinuation of topiramate, along with other measures deemed prudent by the prescribing physician and/or ophthalmologist, may halt the progression of the ocular damage, and may reverse the visual impairment. Patients who take topiramate and who feel pain in or around their eyes, or notice a loss of vision, visual acuity, or blurred vision, are advised to seek consultation with their physician as soon as reasonably possibly. According to the FDA: "in more than 825,000 patients...As of August 17, 2001 there have been 23 reported cases: 22 in adults and 1 in pediatric patients. It is generally recognized that postmarketing data are subject to substantial under-reporting."

Another serious side-effect is the development of osteoporosis in adults and children (bones affected break more easily) and rickets (abnormal, deformed growth of bones) in children. Topiramate may also slow the growth of children. All of these conditions should be detected early by performing regular clinical examinations of the patients.

In other postmarketing research, a risk of decreased sweating and hyperthermia was discovered. Pediatric patients (children) are especially prone to this side-effect. It is recommended that children treated with Topiramate should be monitored closely for evidence of decreased sweating and increased body temperature, especially in hot weather. All patients, particularly those with other predisposing factors, should be instructed to maintain an adequate fluid intake in order to minimize the risk of kidney stone formation.

Interactions

* As Topiramate inhibits carbonic anhydrase, the concomittant use of other inhibitors of carbonic anhydrase (e.g. Acetazolamide) may lead to an increased risk of renal stones.
* enzyme inductors (e.g. Carbamazepine) : The elimination of Topiramate may be increased, possibly requiring dose escalations of Topiramate.
* Phenytoin : Topiramate may increase the plasma-levels of Phenytoin.
* Topiramate itself is a weak inhibitor of CYP 2C19 and induces CYP 3A4. Under Topiramate a decrease of plasma-levels of estrogens (e.g. 'the pill') and Digoxin have been noted.

Doses

In order to avoid early side-effects (e.g. cognitive dysfunction) the initial dose normally is low and increased in slow steps. The usual initial dose is 25 to 50mg daily in 2 single doses. Recommended increments are 25 to 50mg every 1 or 2 weeks. Common doses for maintainance treatment are 200 to 400mg daily. The highest dose possible is 1,000mg daily in divided doses.

Overdose

In case of overdose, call your local poison control center at 1-800-222-1222. If the victim has collapsed or is not breathing, call local emergency services at 911.

Symptoms of overdose may include but are not limited to:

* seizures (!)
* dizziness, drowsiness, tiredness
* agitation
* depression
* speech problems
* blurred vision, double vision
* troubled thinking
* loss of coordination
* inability to respond to things around you
* loss of consciousness
* confusion and coma
* fainting
* upset stomach and stomach pain
* loss of appetite and vomiting
* excessive hunger
* shortness of breath; fast, shallow breathing
* pounding or irregular heartbeat
* muscle weakness
* bone pain

A specific antidote is not available. Treatment is entirely symptomatic.

References

* Goodman & Gilman's : The Pharmaceutical Basis of Therapeutics, 10th. edition, 2001
* Benkert, Hippius : Kompendium der Psychiatrischen Pharmakotherapie (German), 4th. edition, 2003
* E. Mutschler, G. Geisslinger, H. K. Kroemer and M. Schδfer-Korting : Arzneimittelwirkungen (German), 8th edition, 2001


Note: This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Topiramate".



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