[Current data on gabapentin]

OBJECTIVE

To review the current data on gabapentin (GBP) with regard to its mechanism of action, pharmacokinetic characteristics, clinical efficacy, tolerance and mode of administration.

DEVELOPMENT

GBP has a multiple mechanism of action which justifies its use in many clinical conditions besides epilepsy. These include neuropathic pain, psychiatric disorders, disorders of movement, and migraine. Its pharmacokinetic characteristics are nearly those of an ideal drug, with no interactions. Regarding clinical efficacy, there is much evidence of this in polytherapy and in monotherapy in adults and children with partial crises, with rare adverse effects and the exceptional possibility to establish the total dose of GBP in only a few days.

CONCLUSIONS

GBP is an effective drug in patients of all ages with partial crises. Because of its excellent tolerance and decreasing dose-dependent kinetics, conditioned by less absorption of the drug, we recommend the administration of GBP at doses higher than those normally used. Thus we suggest a dose of 900 to 2,400 mg/day in patients with a new diagnosis, of 1,800 to 3,600 mg/day as a second therapeutic alternative and even 2,400 to 4,800 mg/day when other antiepileptic drugs have failed to be of use.

Use of gabapentin in the treatment of neuropathic pain

OBJECTIVE

To evaluate the role of gabapentin for the treatment of neuropathic pain.

DATA SOURCES

Clinical literature was identified through MEDLINE (from 1990 to October 1999). Key search terms were gabapentin and pain.

DATA SYNTHESIS

Neuropathic pain can be a problematic, chronic syndrome that is frequently refractory to current drug treatments. Gabapentin is a newer generation antiepileptic drug that is commonly used in treatment of neuropathic pain. An evaluation of clinical trials using gabapentin to treat neuropathic pain was performed.

CONCLUSIONS

Gabapentin appears to be effective in treating various neuropathic pain disorders. Gabapentin may have advantages over current therapies, such as a favorable safety profile and lack of drug interactions; however, cost issues and limited experience may limit the use of gabapentin as a first-line option.

[GABA-ergic system and antiepileptic drugs]

gamma-Aminobutyric acid (GABA) belongs to main inhibitory neurotransmitters in the central nervous system and activates three types of specific receptors--GABAA, GABAB i GABAC. At present, little is known about GABAC-mediated events. GABAB receptors are metabotropic, whilst stimulation of ionotropic GABAA receptors results in opening the chloride channel, followed by influx of chloride ions and hyperpolarization. The GABAA receptor possesses also binding sites for benzodiazepines and barbiturates which, via these sites, enhance GABAA-mediated events. Another antiepileptic drug potentiating GABA-ergic inhibition is valproate, which increases synthesis of GABA and reduces its metabolism. Among new antiepileptic drugs associated with the GABA-ergic system are tiagabine, vigabatrin, and to a certain degree--gabapentin. Tiagabine blocks neuronal and glial uptake of GABA whilst vigabatrin increases the synaptic concentration of GABA by inhibition of GABA aminotransferase. Gabapentin, probably through the activation of glutamic acid decarboxylase, leads to the increase in synaptic GABA. However, this antiepileptic drugs is also binds to specific sites within voltage-dependent calcium channels, which results in the reduced intraneuronal concentration of calcium ions. Presumably, tiagabine and vigabatrin possess only one mechanism of action, associated with the increased GABA-ergic inhibition. Although topiramate and felbamate were shown to enhance GABA-mediated events, they have additional mechanisms of action, including blockade of voltage-dependent sodium channels and inhibition of glutamatergic neurotransmission.

Changes in use of valproate and other mood stabilizers for patients with schizophrenia from 1994 to 1998

OBJECTIVE

The study describes changes over time in the adjunctive use of valproate and other mood stabilizers-lithium, carbamazepine, and gabapentin--among hospitalized psychiatric patients with a diagnosis of schizophrenia.

METHODS

For each calendar year from 1994 through 1998, data were drawn from a database containing clinical and drug prescription information for every inpatient in the adult civil facilities of the New York State Office of Mental Health.

RESULTS

In 1994 a total of 26.2 percent of inpatients diagnosed as having schizophrenia received a mood stabilizer, compared with 43.4 percent in 1998. In 1994 lithium was the most commonly prescribed mood stabilizer, for 13.2 percent of patients, followed by valproate, for 12.3 percent. In 1998 valproate was the most commonly prescribed, for 35 percent of patients, followed by lithium, for 11.3 percent. On average, patients received valproate for about two-thirds of their hospital stay, at a mean dose of 1,520 mg per day.

CONCLUSIONS

The adjunctive use of valproate nearly tripled from 1994 to 1998 among patients with a diagnosis of schizophrenia. Valproate has become the most commonly prescribed mood stabilizer for this population, despite the paucity of evidence in the literature for efficacy in this use. Controlled clinical trials are needed to examine the adjunctive use of mood stabilizers, in particular valproate, among patients with schizophrenia.

Novel anticonvulsants: a new generation of mood stabilizers?

Accumulating evidence suggests that at least some novel anticonvulsants may have mood-stabilizing properties. This paper reviews the literature for empirical studies of this topic. Lamotrigine has the most evidence in favor of its efficacy, with two double-blind studies in which it was more efficacious than placebo in the treatment of bipolar depression. However, it is associated with a 1/1000 risk of potentially fatal Stevens-Johnson syndrome. Gabapentin, although safe and well-tolerated, has been found in two double-blind studies not to be efficacious in treatment-refractory mania or refractory bipolar depression. Topiramate is currently supported only by naturalistic evidence of mild to moderate mood-stabilizing efficacy, but it has the advantage of often producing weight loss. Based on these data, lamotrigine may be effective, in monotherapy or as an adjunct, for treating depression in type I bipolar disorder, but suggestions regarding gabapentin and topiramate await further efficacy data. Most of the current findings derive from small, non-double-blind studies, and further research is required before clinicians can consider any of these agents to be mood stabilizers.

Gabapentin in the prophylaxis of migraine: a double-blind randomized placebo-controlled study

OBJECTIVE

Gabapentin is an antiepileptic drug of new generation that increases brain GABA levels. We report the results of a three-month randomised double-blind placebo-controlled study on the effects of gabapentin in the prophylaxis of patients with migraine meeting the IHS criteria.

PATIENTS AND METHODS

We treated 63 patients suffering from migraine with or without aura. Patients treated their attack at home using symptomatic drugs and clinical assessment was recorded on a diary. After a washout of 8 week from any other prophylactic treatment, all patients were treated with 1200 mg/day of gabapentin; this is our therapeutic plan: 400 mg/day from 1st to 3rd day, 800 mg/day from 4th to 6th day and 1200 mg/day from 7th day.

RESULTS

No patients withdrew, gabapentin was well tolerated; adverse events (somnolence, dizziness, tremor, fatigue and ataxia) generally were transient and mild to moderate in severity and in 13 patients (27%) only occurred. At the end of treatment, in such case, we reported a significative reduction of frequency and intensity of migraine in 30 patients treated with gabapentin.

DISCUSSION

Our observations indicate that gabapentin is well tolerated by patients and that reduces headache frequency and use of symptomatic drugs in both groups. Gabapentin shows to have an effective therapeutic action in the prophylactic treatment of migraine.

Gabapentin-induced mitogenic activity in rat pancreatic acinar cells

Gabapentin induces pancreatic acinar cell tumors in rats through unknown, yet apparently nongenotoxic mechanisms. The primary objective of this study was to determine whether gabapentin acts as a tumor promoter by stimulating acinar cell proliferation in rat pancreas. To this end, indices of pancreatic growth, including increased pancreatic weight, stimulation of acinar cell proliferation, and/or enhanced expression of immediate-early oncogenes were monitored in rats given gabapentin in the diet at 2 g/kg/day for up to 12 months. Rats fed raw soy flour (RSF), a known inducer of pancreatic acinar cell tumors through cholecystokinin-mediated mitogenic stimulation, were used throughout as positive controls. In addition, recent data suggests that gabapentin binds to the alpha(2)delta subunit of a voltage-gated, L-type calcium channel. Because signaling pathways for proliferative processes in pancreatic acinar cells involve intracellular calcium mobilization, the effects of gabapentin on intracellular calcium mobilization ([Ca(2+)](i)) and (3)H-thymidine incorporation were investigated in pancreatic acinar cells isolated from normal rat pancreas and in the AR42J rat pancreatic tumor cell line. As indicated by BrdU labeling indices, acinar cell proliferation increased 3-fold by Day 3 of RSF treatment and remained slightly greater than controls throughout the experiment. Pancreatic weights of RSF-fed rats were 32 to 56% greater than controls throughout the experiment. In contrast, gabapentin had no effect on pancreatic weight or acinar cell labeling index, and therefore had no apparent effect on pancreatic growth. In isolated pancreatic acinar cells, however, gabapentin induced mobilization of intracellular calcium and caused a slight increase in (3)H-thymidine incorporation. The data suggest that gabapentin may possess low level mitogenic activity, which is not easily detectable in in vivo assays.

Allosteric modulation of [(3)H]gabapentin binding by ruthenium red

Gabapentin is an anticonvulsant with an unknown mechanism of action. However, it has been proposed that gabapentin acts by binding to voltage-gated calcium channels. To further characterize the interaction of gabapentin with its endogenous binding site in cerebral cortex, we tested for competitive and allosteric interactions between [(3)H]gabapentin and a variety of calcium channel binding ligands. Most ligands for voltage- or ligand-gated calcium channels (verapamil, the omega-conotoxins MVIIC and GVIA, ryanodine, caffeine, capsaicin, MK-801) had no significant effect on [(3)H]gabapentin binding. However, ruthenium red, a relatively nonselective calcium channel ligand, was found to robustly modulate [(3)H]gabapentin binding. Ruthenium red slowed the association and dissociation kinetics of [(3)H]gabapentin while increasing the number of detectable binding sites. Spermine and MgCl(2), which also bind to calcium channels and modulate [(3)H]gabapentin binding, were found to act in a similar manner. These findings support the contention that the principal endogenous binding site for gabapentin is a calcium channel; they characterize the nature of the allosteric interaction of spermine, MgCl(2) and ruthenium red with this binding site; and they suggest possible mechanisms by which gabapentin may modulate calcium channel function and ultimately produce therapeutic actions.

Recurrent and migratory reflex sympathetic dystrophy in children

Reflex sympathetic dystrophy is a syndrome characterized by superficial pain and tenderness associated with swelling, vasomotor instability, and dystrophic changes of the skin. In children, it is rarely reported and is felt to have a more benign and self-limited course. This case illustrates that, in children, reflex sympathetic dystrophy can occur without any previous history of trauma, and may be recurrent and migratory. A review of the literature is included. An 11-year-old girl, with no history of trauma, presented in 1992 with spontaneous onset of right leg pain. She was diagnosed with reflex sympathetic dystrophy, and she was treated unsuccessfully with oral medications. Her symptoms then resolved in 2 weeks after receiving epidural anaesthesia and aggressive physical therapy. Over the next 5 years, she presented to the paediatric rehabilitation clinic three times with recurrent RSD in her bilateral arms. The first two times were refractory to conservative management and resolved with four stellate ganglion blocks. The third recurrence persisted with three stellate ganglion blocks and resolved with gabapentin.

Myoclonus associated with the use of gabapentin

PURPOSE

To report on the occurrence of myoclonus in patients receiving gabapentin (GBP) for the treatment of epilepsy.

METHODS

Clinic charts of 104 consecutive patients started on GBP were reviewed. All patients were treated by the same physician, and most were specifically asked about the presence of myoclonus.

RESULTS

We found 13 cases of myoclonus. All patients had refractory epilepsy and were taking other antiepileptic drugs (AEDs). Six patients had a severe chronic static encephalopathy; five patients had no medical diagnosis other than seizures. Ten patients developed multifocal myoclonus. Three patients developed focal myoclonus, contralateral to their epileptic focus. Two patients had an exacerbation of preexistent myoclonus. An EEG performed during myoclonus on three patients showed no correlate. The myoclonus tended to persist as long as GBP was maintained, whereas discontinuance of GBP resulted in rapid cessation of the myoclonus. In all cases the myoclonus was subtle and did not significantly interfere with daily activities.

CONCLUSIONS

GBP-associated myoclonus appears to be relatively frequent. It is usually mild and can easily be overlooked. Discontinuation of therapy is not necessary in most cases.

The anticonvulsant compound gabapentin possesses anxiolytic but not amnesic effects in rats

This report describes the effects of the antiepileptic agent gabapentin on anxiety and memory. Male Wistar rats received intraperitoneal administrations of gabapentin (10, 30 and 100mg/kg), diazepam (1 mg/kg), saline or diazepam vehicle 30 minutes prior to experimental procedures. Animals were: (1) tested on step-down inhibitory avoidance (footshock 0.3 mA) and habituation to an open-field for memory assessment; and (2) submitted to the elevated plus-maze to evaluate the potential anxiolytic effects of gabapentin. Animals treated with gabapentin showed a reduction in anxiety similar to that observed in animals treated with diazepam. Memory was not affected by gabapentin in any of the tests, but was impaired by diazepam. The lack of effects of gabapentin on memory suggest a potential advantage of this drug over compounds with previously known anxiolytic property, which have amnesic effects at doses used for the treatment of anxiety disorders.

Tremor-predominant Parkinson's disease. Approaches to treatment

Parkinson's disease is a neurodegenerative disorder that manifests clinically with variable degrees of tremor, muscle rigidity, bradykinesia and postural instability. Tremor-predominant Parkinson's disease is characterised by prominent tremor of one or more limbs with a relative lack of significant rigidity and bradykinesia. Despite the lack of other disabling motor symptoms, the tremor of tremor-predominant Parkinson's disease can be very disabling, especially if a postural and kinetic component exists. A wide variety of treatments for Parkinson's disease tremor are currently available and include use of oral medications, injections with botulinum toxin and neurosurgical procedures. Some of the first line medications (levodopa, dopamine agonists, anticholinergics) are very effective in controlling tremor. However, some patients with Parkinson's disease tremors are unresponsive to first line drugs and treatment with second line medications (clozapine, amantadine, clonazepam, propranolol, neurontin) should be attempted. In the small number of patients with disabling tremor that is refractory to all medications, neurosurgical intervention should be considered. Both thermocoagulation and deep brain stimulation at several different neuroanatomical sites (thalamus, globus pallidus, subthalamic nucleus) offer good to excellent tremor control with relatively low risk to the patient.

Gabapentin affects glutamatergic excitatory neurotransmission in the rat dorsal horn

We investigated the effects of gabapentin (GBP) on glutamatergic synaptic transmission in the dorsal horn of the rat spinal cord. Patch clamp whole cell recordings were made from superficial and deep dorsal horn neurons of rat spinal cord slices. In the majority of neurons in the superficial lamina, GBP decreased the amplitudes of evoked excitatory postsynaptic currents (evoked EPSCs) mediated by either non-NMDA or NMDA receptors. In contrast, neurons in the deep lamina showed variable effects, with a lower incidence of decrease in amplitude of evoked EPSCs and a subset of neurons showing an increase in amplitude of evoked NMDA receptor-mediated EPSCs. No enhancement of evoked non-NMDA receptor-mediated EPSCs was observed in either lamina. To determine whether the observed effects of GBP are presynaptic and/or postsynaptic, spontaneous miniature excitatory postsynaptic currents (mEPSCs) were studied. In neurons that showed a decrease in its frequency of mEPSCs by GBP, no change in the amplitude or shape accompanied the effect. On the other hand, in neurons that showed an increase in the frequency of NMDA receptor-mediated mEPSCs, the effect accompanied an increase in amplitude. These results suggest that GBP presynaptically inhibits glutamatergic synaptic transmission predominantly in the superficial lamina, while postsynaptically enhancing NMDA receptor-mediated transmission in some neurons of the deep lamina. The antinociceptive effects of GBP may involve the inhibition of the release of excitatory amino acids from presynaptic terminals.

Gabapentin "add-on therapy" for idiopathic chronic hiccup (ICH)

The efficacy of cisapride, omeprazole, and baclofen (COB) for treatment of idiopathic chronic hiccup (ICH). was proven in several studies. The combination is considered, at present, to be "therapy of choice" for this condition. Substituting gabapentin for baclofen in baclofen resistant ICH cases can occasionally be successful. We present here cases where gabapentin was used successfully in combination with cisapride and omeprazole (COG/one patient) or as an "add-on" with cisapride, omeprazole, and baclofen (COBG/three patients). We conclude that, with baclofen and gabapentin, we are in possession of two substances that are, as a part of a combination therapy, quite effective for ICH. Because of the far more extensive experience with baclofen, we use it as a first-line therapy, together with omeprazole and cisapride. In cases where the results are not entirely satisfactory, the addition of gabapentin should be considered.

Antiallodynic effect of intrathecal gabapentin and its interaction with clonidine in a rat model of postoperative pain

BACKGROUND

Systemic administration of gabapentin was shown previously to attenuate mechanical allodynia in a rat model of postoperative pain. Because intrathecal administration of gabapentin is effective in other hypersensitivity states, the authors tested its effect in the postoperative model, its interaction with another antiallodynic agent (clonidine), and a possible mechanism of gabapentin action (entry into sites of action via an L-amino acid transporter).

METHODS

Male Sprague-Dawley rats were anesthetized with halothane, and an incision of the plantaris muscle of right hind paw induced punctate mechanical allodynia. Withdrawal threshold to von Frey filament application near the incision site was determined before and 2 h after surgery. Then, an intrathecal injection was performed and thresholds were determined every 30 min for 3 h thereafter.

RESULTS

Paw incision induced a mechanical hypersensitivity (mechanical threshold > 25 g before incision and < 5 g after). Intrathecal gabapentin dose-dependently (10-100 microg) reduced mechanical allodynia. Intrathecal injection of an inhibitor of L-amino acid transporters or a competitor for this transporter, L-leucine, did not reverse the intrathecal effect of gabapentin. The ED50 of intrathecal gabapentin, clonidine, and their combination were 51, 31, and 9 microg, respectively, and isobolographic analysis showed synergy between gabapentin and clonidine.

CONCLUSIONS

Intrathecal gabapentin is effective against tactile allodynia that occurs after paw incision, and interacts synergistically with clonidine. Unlike results in vitro, gabapentin does not obligatorily need to enter cells via the L-amino acid transporter mechanism to achieve its effects in vivo.

Dystonia in a patient treated with propranolol and gabapentin

We present a 68-year-old patient with essential tremor who was treated with propranolol hydrochloride (80 mg daily) and gabapentin (900 mg daily) after a history of mild success of gabapentin alone in relieving his symptoms. The patient had several daily episodes of paroxysmal dystonic movements in both hands. After reducing the propranolol dose to 40 mg daily, the dystonic movements resolved. This case suggests a synergistic effect between propranolol and gabapentin.

Effect of anticonvulsants on nocturnal sleep in epilepsy

Our objective was to determine, in three separate studies, the effects of controlled-release carbamazepine (CBZ-CR), lamotrigine (LTG), and gabapentin (GBP) on nocturnal sleep in epilepsy. Antiepileptic drugs (AEDs) control seizures and also modify hypnic structure. Despite widespread clinical use, their effects on sleep are not well known. PSG was performed in all three studies as follows: CBZ-CR: at baseline, after initial administration of CBZ-CR 400 mg, and after 1 month of CBZ-CR treatment (400 mg BID) in a sample of seven temporal lobe epileptic (TLE) patients. Results were compared with those of nine healthy volunteers; LTG: at baseline, after 3 months of stable treatment with LTG (300 mg/day); GBP: at baseline, after 3 months of stable treatment with GBP (1800 mg/day). Significant findings are as follows for each study. The acute administration of CBZ-CR increased number of stage shifts, reduced REM sleep, and increased REM sleep fragmentation. In the TLE group, these effects were almost completely reversed after chronic treatment. LTG increased REM sleep, reduced number of entries into REM sleep, decreased number of phase shifts, and decreased percentage of slow-wave sleep. GBP increased REM sleep percentage, increased mean duration of REM periods, reduced number of awakenings, and reduced stage 1 sleep percentage. We conclude that CBZ-CR disrupts REM sleep, but only during acute administration. LTG and GBP improve sleep stability while reducing seizures.

Treatment of dementia-associated agitation with gabapentin

The authors describe the use of gabapentin in the treatment of 4 outpatients with dementia-associated agitation. On the basis of clinical case reports and the Overt Agitation Severity Scale, all 4 patients had reduced agitation with gabapentin. Three of 4 patients were successfully titrated to a full dose of 2,400mg/day. These findings suggest a possible role for gabapentin in the behavioral management of patients with dementia.

Paroxysmal dystonia induced by exercise and acetazolamide

We report a case of a 40-year-old woman with dystonic attacks precipitated by slight exercise. Episodes lasted 2-5 min and were not precipitated by sudden movements or by being startled, drinking alcohol, coffee or tea, or by stress. Secondary dystonia was ruled out and brain magnetic resonance imaging (MRI) was unremarkable. Routine and video electroencephalogram (EEG) during and between attacks were normal. Acetazolamide greatly worsened her condition, whereas gabapentin [1-(aminomethyl) cyclohexaneacetic acid] treatment markedly reduced the frequency and severity of the episodes.