Article: Gabapentin

Gabapentin
Systematic (IUPAC) name
2-[1-(aminomethyl)cyclohexyl]acetic acid
Identifiers
CAS number	60142-96-3
ATC code	N03AX12
PubChem	3446
DrugBank	APRD00015
Chemical data
Formula	C9H17NO2
Mol. weight	171.237 g/mol
Pharmacokinetic data
Bioavailability	Rapid, in part by saturatble carrier-mediated L-amino acid transport system 60% for 0.9g daily to 27% for 4.8g daily dose Food increases absorption 14%
Protein binding	Less than 3%
Metabolism	not appreciably metabolized
Half life	5-7 hours
Excretion	renal
Therapeutic considerations
Pregnancy cat.	Benefit of treatment outweighs risk to fetus. Risk teratogenicity greater if more than one drug used[1]
Legal status	PoM (UK)
Routes	oral

Gabapentin (brand name: Neurontin^®) was initially synthesized to mimic the structure of GABA for the treatment of epilepsy. Nowadays, gabapentin has been widely used as a medication to relieve pain, especially neuropathic pain. Gabapentin is well tolerated in most patients, has a relatively mild side-effect profile, and passes through the body unmetabolized.

Gabapentin is similar in structure to the neurotransmitter GABA but is not believed to act on the same brain receptors. Its exact mechanism of action is unknown, but its therapeutic action on neuropathic pain is thought to involve voltage-gated calcium ion channels.

Clinical uses

Gabapentin was originally approved by the FDA as an antiseizure medication. It is not effective for patients with absence seizures.

Gabapentin has also been used in the treatment of bipolar disorder. However, its off-label use for this purpose is increasingly controversial. Some claim gabapentin acts as a mood stabilizer and has the advantage of having fewer side-effects than more conventional bipolar drugs such as lithium and valproic acid. Some small, non-controlled studies in the 1990s, most sponsored by gabapentin's manufacturer, suggested that gabapentin treatment for bipolar disorder may be promising. However, more recently, several larger, controlled, and double-blind studies have found that gabapentin was no more effective than (and in one study, slightly less effective than) placebo, and the manufacturer has even halted its own studies regarding gabapentin and bipolar disorder. Despite this scientific evidence against the efficacy of gabapentin in the treatment of bipolar disorder, many psychiatrists continue to prescribe it for this purpose.

Gabapentin has also been used off label in the treatment of anxiety disorders such as social anxiety disorder and obsessive-compulsive disorder, in treatment-resistant depression, and for insomnia. Gabapentin may be effective in reducing pain and spasticity in multiple sclerosis. It has been used off-label to treat neuropathic pain.

In addition to its use in mood disorders, gabapentin was approved by the FDA for treating postherpetic neuralgia (neuropathic pain following shingles) and other painful neuropathies.

Gabapentin has also been found to help patients with post-operative chronic pain (usually caused by nerves that have been severed accidentally in an operation and when grown back, have reconnected wrongly) Symptoms of this is a tingling sensation near or around the area where the operation was performed, sharp shooting pains, severe aches after much movement, constant 'low ache' all day and sometimes a general 'weak' feeling. These symptoms can appear many months after an operation, and therefore the condition can go un-noticed.

Marketing of gabapentin

Gabapentin is best known under the brand name Neurontin manufactured by Pfizer subsidiary Parke-Davis. In December 2004, FDA granted final approval to a generic equivalent to Neurontin made by Israeli firm Teva. Neurontin is one of Pfizer’s best selling drugs, and was one of the 50 most prescribed drugs in the United States in 2003. However, in recent years Pfizer has come under heavy criticism for its marketing of Neurontin, facing allegations that behind the scenes Parke-Davis marketed the drug for at least a dozen supposed uses for which the drug had not been FDA approved. By some estimates, so-called off-label prescriptions account for roughly 90% of Neurontin sales^[2]. While off-label prescriptions are common for a number of drugs and are perfectly legal (if not always appropriate), marketing of off-label uses of a drug is strictly illegal. In 2004, Pfizer agreed to pay $430 million in fines for the illegal marketing of Neurontin for off-label purposes, and further legal action is pending. However, Pfizer maintains that the illegal activity originated in 1996, well before it accquired Parke-Davis (through its acquisition of Warner-Lambert) in 2000. Several lawsuits are underway after people prescribed gabapentin for off-label treatment of bipolar disorder attempted or committed suicide.

Pfizer has developed a successor to gabapentin, called pregabalin (being marketed as Lyrica®). Structurally related to gabapentin, Pregabalin is effective for neuropathic pain associated with diabetes and shingles, and for the treatment of epilepsy and seizures.

Side effects

Gabapentin's most common side effects in adult patients include dizziness, drowsiness, and peripheral edema (swelling of extremities)^[3] Children 3-12 years of age were also observed to be susceptible to mild-to-moderate mood swings, hostility, concentration problems, and hyperactivity. An increase in formation of adenocarcinomas was observed in rats during preclinical trials, however the clinical significance of these results remains undetermined.

Abuse Potential

Though Gabapentin is not a controlled substance, it does produce psychoactive effects that could lead to abuse of the drug. However, it is widely regarded as having little or no abuse potential. As to why, it is unknown. Pregabalin, a Gabapentinoid with higher potency marketed for neuropathic pain, is a controlled substance, under the DEA schedule 5; akin to codeine-based cough syrup.