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Gabapentin

Gabapentin (brand name: Neurontin^®) is an anticonvulsant medication indicated in the treatment of epilepsy and neuropathic pain. Gabapentin is known for having a relatively mild side-effect profile, and passes through the body unmetabolized. more...

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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 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.

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

Marketing of gabapentin

Gabapentin is best known under the brand name Neurontin manufactured by Pfizer . In October 2004, FDA approved 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 Pfizer 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. While off-label prescriptions are common for a number of drugs and are perfectly legal (if not always appropriate), marketing for off-label uses of a drug is strictly illegal. In 2004, Pfizer agreed to pay a $430 million settlement for the illegal marketing of Neurontin for off-label purposes, and further legal action is pending.

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

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Gabapentin helpful for hot flushes in postmenopausal women - Patient Oriented Evidence That Matters: practice recommendations from key studies
From Journal of Family Practice, 7/1/03 by Christopher Haines

Guttuso T, Kurlan R, McDermott MP, Kieburtz K. Gabapentin's effects on hot flashes in postmenopausal women: A randomized controlled trial. Obstet Gynecol 2003; 101:337-345.

* PRACTICE RECOMMENDATIONS

Gabapentin effectively decreases both the frequency and severity of hot flushes in postmenopausal women who report 7 or more hot flushes per day. Gabapentin, an anticonvulsant, is indicated by the US Food and Drug Administration as adjunct therapy in the treatment of partial seizures in epilepsy.

Although somnolence, dizziness, and peripheral edema are commonly experienced by patients taking this medication, gabapentin provides an effective treatment for reducing the number of hot flushes in women for whom hormone replacement therapy (HRT) is not recommended or desired.

* BACKGROUND

Although HRT is effective in reducing hot flushes, recent data from the Women's Health Initiative and other concerns about estrogen use have limited its use in postmenopausal women. Safe, effective, and well-tolerated therapies for hot flushes would be welcomed.

* POPULATION STUDIED

The investigators enrolled 59 postmenopausal women who reported a daily average of 7 or more hot flushes. Study participants were recruited by advertisements and from a local news program about complementary hot flush therapies.

Hot flushes were defined as the sudden onset of heat sensation accompanied with sweating that spontaneously resolved within 1 hour. At least 1 daytime hot flush per day was required. Patients were excluded if they had recent surgical menopause, recent treatment with HRT, or impaired renal function.

* STUDY DESIGN AND VALIDITY

In this randomized, double-blind trial, subjects were assigned to receive 12 weeks of therapy with gabapentin 300 mg capsules 3 times daily (900 mg/d), or matching placebo.

Subjects recorded frequency and severity of hot flushes in a diary, rating each on a scale of 1 to 7. Daily frequency was calculated by adding the number of hot flushes recorded in a week and dividing by the number of days in that week on which hot flushes were recorded. A daily hot flush composite score was calculated by adding the severity scores over 1 week and dividing by the number of days in that week for which hot flushes were recorded.

This randomized, double-blind prospective study used concealed allocation, and analysis was by intention-to-treat. Eighty-seven percent of the patients receiving gabapentin completed the study, as did 96% of those receiving placebo.

Relative weaknesses include some missing data for certain weeks (the authors used the last observation carried forward) and a reported financial interest that the principal investigator had in the use of gabapentin for hot flushes.

A major limitation of this study is that there may have been a profound placebo response in the patients treated with gabapentin. Half of the patients receiving the drug experienced a side effect, and these side effects could have augmented the already profound placebo response (29% in the control group). A longer study (6-12 months) would have helped determine whether any placebo effect waned, giving us a truer approximation of the effect of gabapentin.

* OUTCOMES MEASURED

The primary outcome measure for the double-blinded portion of the study was the percentage change in hot flush frequency from baseline to treatment week 12. Secondary outcome measures included changes in the hot flush composite scores from baseline to week 12.

* RESULTS

The gabapentin group experienced a 45% decrease in mean hot flush frequency, compared with a 29% decrease in the placebo group (from 10.8 to 5.9 per day in the gabapentin group and from 10.3 to 7.3 per day in the placebo group; P=.02). The gabapentin group experienced a 54% decrease in the mean hot flush composite score, compared with a 31% decrease in the placebo group (44.9 to 20.5 vs. 39.5 to 27.3; P=.01). This is a significant change, but certainly less than the 90% efficacy that has been observed with HRT.

Fifty percent of patients in the gabapentin group reported at least 1 adverse event, compared with 28% in the placebo group. The most common adverse effects reported with gabapentin were somnolence (20% of patients) and dizziness (13%). Thirteen percent of patients in the gabapentin group withdrew due to adverse effects, compared with 3% of patients in the placebo group. Measured serum protein levels decreased significantly in the gabapentin group compared with the placebo group (P=.05).

Christopher Haines, MD, MA, and Marc I. Harwood, MD, Department of Family Medicine, Thomas Jefferson University Hospital, Philadelphia, Pa. E-mail: christopherhaines@comcast.net.

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