Article: Topiramate

Topiramate
Systematic (IUPAC) name
2,3:4,5-Bis-O-(1-methylethylidene)-beta-D-fructopyranose sulfamate
Identifiers
CAS number	97240-79-4
ATC code	N03AX11
PubChem	5284627
DrugBank	APRD00237
Chemical data
Formula	C12H21NO8S
Mol. weight	339.363 g/mol
SMILES	NS(=O)(=O)OC[C@@]13OC(C)(C)O[C@H]3C2OC(C)(C)OC2CO1
Pharmacokinetic data
Bioavailability	80%
Metabolism	30% hepatic, 70% is excreted unchanged
Half life	19 to 23 hours
Excretion	70% renal (in urine) in unchanged form
Therapeutic considerations
Pregnancy cat.	?
Legal status	℞ Prescription only
Routes	oral

Topiramate (brand name: Topamax^®) is an anticonvulsant drug produced by Ortho-McNeil, a division of Johnson & Johnson. 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 Food and Drug Administration (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. In May 2006 the U.S. National Institutes of Health web site clinicaltrials.gov listed several studies sponsored by Ortho-McNeil which propose to examine the use of topiramate on migraine, cluster, and severe headaches within various demographics.

Pharmacodynamics

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 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, particularly 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 secondarily generalized tonic-clonic seizures in the kindling model, findings predictive 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 possible. 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.
• Alcohol may cause increased sedation or drowsiness, and increase the risk of having a seizure.

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 maintenance treatment are 100 to 200mg 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.