1 Name of Medicine
Gabapentin.
2 Qualitative and Quantitative Composition
600 mg.
Each tablet contains the active ingredient gabapentin 600 mg.
For the full list of excipients, see Section 6.1 List of Excipients.3 Pharmaceutical Form
Gabapentin tablets 600 mg.
White to off white, oval, biconvex, film coated tablets approximately 18.00 mm long and 9.20 mm wide, engraved with '600' on one face and plain on the other.4.1 Therapeutic Indications
WP Gabapentin is indicated for the treatment of partial seizures, including secondarily generalised tonic-clonic seizures, initially as add-on therapy in adults and children age 3 years and above who have not achieved adequate control with standard antiepileptic drugs.
WP Gabapentin is indicated for the treatment of neuropathic pain.
4.2 Dose and Method of Administration
Dosage.
Epilepsy dosage for adults and children older than 12 years of age.
Initiation of treatment should be as add-on therapy. Gabapentin can be given orally with or without food.
In controlled clinical trials, the effective dose range was 900 mg/day to 1800 mg/day given in divided doses (three times a day).
Therapy may be initiated by administering 300 mg of gabapentin three times a day on Day 1, as 300 mg capsules (available in other brands) or half 600 mg tablets, or by titrating the dose as described below.
Titration to an effective dose can take place rapidly, over a few days, giving 300 mg gabapentin on Day 1, 300 mg gabapentin twice a day on Day 2, 300 mg gabapentin three times a day on Day 3, as 300 mg capsules (available in other brands) or half 600 mg tablets. Titration may be preferable for patients with renal impairment, patients with encephalopathy, patients on more than 2 other antiepileptic drugs and patients with multiple other medical problems.
To minimise potential side effects, especially somnolence, dizziness, fatigue and ataxia, the first dose on Day 1 may be administered at bedtime. If necessary, the dose may be increased using 300 mg or 400 mg capsules (available in other brands) or 600 mg or 800 mg (available in other brands) tablets three times a day up to 2400 mg/day. Dosages up to 2400 mg/day have been well tolerated in long-term open-label clinical studies. The maximum time between doses in the three times a day schedule should not exceed 12 hours.
Neuropathic pain in adults older than 18 years of age.
The starting dose is 900 mg/day given in three daily divided doses, and titrated if necessary, based on response, up to a maximum dose of 3600 mg/day.
Dose adjustment in impaired renal function in patients with neuropathic pain or epilepsy.
Dose adjustment is recommended in patients with compromised renal function and/or in those undergoing haemodialysis (see Table 1).
For patients undergoing haemodialysis who have never received gabapentin, a loading dose of 300 mg to 400 mg is recommended, and then 200 mg to 300 mg of gabapentin following each 4 hours of haemodialysis.
Dosage for children aged 3 to 12 years of age.
The effective dose of gabapentin is 25 mg/kg/day to 35 mg/kg/day given in three divided doses (3 times a day). Titration to an effective dose can take place over 3 days by giving 10 mg/kg/day on Day 1, 20 mg/kg/day on Day 2, and 30 mg/kg/day on Day 3. Dosages up to 40 mg/kg/day to 50 mg/kg/day have been well tolerated in a long-term clinical study. Doses of 60 mg/kg/day have also been administered to a small number of children.
Unlike other agents in this class, it is not necessary to monitor gabapentin plasma concentrations to optimise gabapentin therapy. Further, gabapentin may be used in combination with other antiepileptic drugs without concern for alteration of the plasma concentrations of gabapentin or serum concentrations of other antiepileptic drugs.
Discontinuation of gabapentin.
If gabapentin is discontinued and/or an alternate anticonvulsant medication is added to the therapy, this should be done gradually over a minimum of 1 week (see Section 4.4 Special Warnings and Precautions for Use, Discontinuation).4.3 Contraindications
WP Gabapentin is contraindicated in patients who have demonstrated hypersensitivity to gabapentin or the inactive ingredients in the tablets.
4.4 Special Warnings and Precautions for Use
General.
Although there is no evidence of rebound seizures with gabapentin, abrupt withdrawal of anticonvulsants in epileptic patients may precipitate status epilepticus. When in the judgement of the clinician there is a need for dose reduction, discontinuation, or substitution of alternative anticonvulsant medication, this should be done gradually over a minimum of one week.
Gabapentin is generally not considered effective in the treatment of absence seizures and may exacerbate these seizures in some patients. Consequently, WP Gabapentin should be used with caution in patients who have mixed seizure disorders that include absence seizures.
Gabapentin treatment has been associated with dizziness and somnolence, which could increase the occurrence of accidental injury (fall). There have also been post-marketing reports of confusion, loss of consciousness and mental impairment. Therefore, patients should be advised to exercise caution until they are familiar with the potential effects of the medication.
Central nervous system depression.
Respiratory depression.
Gabapentin has been associated with central nervous system (CNS) depression including sedation, somnolence, loss of consciousness as well as serious cases of respiratory depression. This may occur without concomitant opioid use. Patients with compromised respiratory function, respiratory or neurological disease, renal impairment and the elderly are at higher risk of experiencing these severe adverse effects. Concomitant use of CNS depressants including opioids with gabapentin increases the risk of respiratory depression.
Concomitant use with opioids and other CNS depressants.
Patients who require concomitant treatment with opioids may experience increases in gabapentin concentrations. Concomitant use of opioids may result in severe sedation, respiratory depression, coma, and death. Limit dosages and durations of gabapentin to the minimum required to achieve desired therapeutic effect. Patients who require concomitant treatment with CNS depressants, including opioids should be carefully observed for signs of central nervous system (CNS) depression, such as somnolence, sedation and respiratory depression; and the dose of gabapentin or concomitant treatment with CNS depressants including opioids should be reduced appropriately (see Section 4.5 Interactions with Other Medicines and Other Forms of Interactions, Concomitant use with opioids).
Suicidal behaviour and ideation.
Antiepileptic drugs (AED), including gabapentin, increase the risk of suicidal thoughts or behaviour in patients taking these drugs for any indication. Patients treated with any AED for any indication should be monitored for the emergence or worsening of depression, suicidal thoughts or behaviour, and/or any unusual changes in mood or behaviour.
Pooled analyses of 199 placebo-controlled clinical trials (mono- and adjunctive therapy) of 11 different AEDs showed that patients randomised to one of the AEDs had approximately twice the risk (adjusted Relative Risk 1.8, 95% CI: 1.2, 2.7) of suicidal thinking or behaviour compared to patients randomised to placebo. In these trials, which had a median treatment duration of 12 weeks, the estimated incidence rate of suicidal behaviour or ideation among 27,863 AED-treated patients was 0.43%, compared to 0.24% among 16,029 placebo-treated patients, representing an increase of approximately one case of suicidal thinking or behaviour for every 530 patients treated. There were four suicides in drug-treated patients in the trials and none in placebo-treated patients, but the number is too small to allow any conclusion about drug effect on suicide.
The increased risk of suicidal thoughts or behaviour with AEDs was observed as early as one week after starting drug treatment with AEDs and persisted for the duration of treatment assessed. Because most trials included in the analysis did not extend beyond 24 weeks, the risk of suicidal thoughts or behaviour beyond 24 weeks could not be assessed.
The risk of suicidal thoughts or behaviour was generally consistent among drugs in the data analysed. The finding of increased risk with AEDs of varying mechanisms of action and across a range of indications suggests that the risk applies to all AEDs used for any indication. The risk did not vary substantially by age (5-100 years) in the clinical trials analysed. Table 2 shows absolute and relative risk by indication for all evaluated AEDs.
The relative risk for suicidal thoughts or behaviour was higher in clinical trials for epilepsy than in clinical trials for psychiatric or other conditions, but the absolute risk differences were similar for the epilepsy and psychiatric indications.
Anyone considering prescribing gabapentin or any other AED must balance this risk with the risk of untreated illness. Epilepsy and many other illnesses for which AEDs are prescribed are themselves associated with morbidity and mortality and an increased risk of suicidal thoughts and behaviour. Should suicidal thoughts and behaviour emerge during treatment, the prescriber needs to consider whether the emergence of these symptoms in any given patient may be related to the illness being treated.
Patients, their caregivers, and families should be informed that AEDs increase the risk of suicidal thoughts and behaviour and should be advised of the need to be alert for the emergence or worsening of the signs and symptoms of depression, any unusual changes in mood or behaviour, or the emergence of suicidal thoughts, behaviour, or thoughts about self-harm. Behaviours of concern should be reported immediately to the treating doctor.
Drug rash with eosinophilia and systemic symptoms.
Severe, life-threatening, systemic hypersensitivity reactions such as drug rash with eosinophilia and systemic symptoms (DRESS) have been reported in patients taking antiepileptic drugs including gabapentin.
It is important to note that early manifestations of hypersensitivity, such as fever or lymphadenopathy, may be present even though rash is not evident. If such signs or symptoms are present, the patient should be evaluated immediately. WP Gabapentin should be discontinued if an alternative aetiology for the signs or symptoms cannot be established.
Anaphylaxis.
WP Gabapentin can cause anaphylaxis. Signs and symptoms in reported cases have included difficulty breathing, swelling of the lips, throat, and tongue, and hypotension requiring emergency treatment. Patients should be instructed to discontinue WP Gabapentin and seek immediate medical care should they experience signs or symptoms of anaphylaxis.
Abuse potential or dependence.
Gabapentin is a potential drug of abuse and dependence.
There have been post-market reports of overdose and deaths among users of gabapentin, particularly with concomitant use of other sedating medicines, such as opioids and/or benzodiazepines.
The risk of abuse of gabapentin should particularly be monitored among current or past opioid and/or benzodiazepine users.
Patients should be carefully evaluated for a history of substance abuse and/or psychiatric disorders prior to being prescribed gabapentin and observed for signs of gabapentin abuse (e.g. development of tolerance, increase in dose, drug seeking behaviour).
Discontinuation.
Withdrawal symptoms have been observed in some patients after discontinuation of gabapentin, including severe symptoms in patients taking high doses. Withdrawal symptoms after discontinuation of both short-term and long-term treatment with gabapentin have been observed in some patients. The following events have been mentioned: insomnia, headache, nausea, anxiety, hyperhidrosis and diarrhoea. Discontinuation should be done gradually over a minimum of one week (see Section 4.2 Dose and Method of Administration, Discontinuation of gabapentin).
Information for patients.
To assure safe and effective use of WP Gabapentin, the following information and instructions should be given to patients:
1. You should inform your physician about any prescription or non-prescription medications, alcohol, or drugs you are now taking or are planning to take during your treatment with gabapentin.
2. No teratogenic effects have been found in animals. However, the risk to the human fetus cannot be dismissed. Therefore you should inform your physician if you are pregnant, or if you are planning to become pregnant, or if you become pregnant while you are taking gabapentin.
3. Gabapentin is excreted in human milk, and the effect on the nursing infant is unknown. You should inform your physician if you are breast-feeding an infant.
4. Gabapentin may impair your ability to drive a car or operate potentially dangerous machinery. Until it is known that this medication does not affect your ability to engage in these activities, do not drive a car or operate potentially dangerous machinery.
5. You should not allow more than 12 hours between gabapentin doses. If you have missed a dose by not more than 4 hours, take the dose as soon as you remember. However, if you have missed a dose by more than 4 hours, you should skip the dose and continue taking following doses as usual.
6. Prior to initiation of treatment with gabapentin, the patient should be instructed that a rash or other signs or symptoms of hypersensitivity such as fever or lymphadenopathy may herald a serious medical event and that the patient should report any such occurrence to a physician immediately.
Use in renal impairment.
See Section 5.2 Pharmacokinetic Properties; Section 4.2 Dose and Method of Administration.
Use in the elderly.
See Section 5.2 Pharmacokinetic Properties.
Paediatric use.
Epilepsy.
Safety and effectiveness in children below the age of 3 years have not been established.
Neuropathic pain.
Safety and effectiveness in children below the age of 18 years have not been established.
Effects on laboratory tests.
False positive readings were reported with the Ames N-Multistix SG dipstick test when gabapentin was added to other anticonvulsant drugs. To determine urinary protein, the more specific sulfosalicylic acid precipitation procedure is recommended.4.5 Interactions with Other Medicines and Other Forms of Interactions
There are spontaneous and literature case reports of respiratory depression, sedation and death associated with gabapentin. When co-administered with CNS depressants, including opioids. In some of these reports, the authors considered the combination of gabapentin with opioids to be a particular concern in frail patients, in the elderly, in patients with serious underlying respiratory disease, with polypharmacy, and in those patients with substance abuse disorders.
Anticonvulsants.
In pharmacokinetic studies, no interactions were observed between gabapentin and phenobarbital (number of subjects, N = 12), phenytoin (N = 8), valproic acid (N = 17), or carbamazepine (N = 12).
Oral contraceptives.
Gabapentin did not influence the steady-state pharmacokinetics of norethindrone and ethinylestradiol when administered concomitantly with an oral contraceptive containing these two drugs (N = 13).
Antacid.
Coadministration of gabapentin with large dose antacid (aluminium hydroxide 3600 mg, magnesium hydroxide 1800 mg) reduced gabapentin bioavailability (AUC) by about 20% (N = 16). Although the difference was not expected to be clinically significant it is recommended that gabapentin should be taken about 2 hours following antacid administration, when the interaction has been shown to be diminished.
Cimetidine.
In the presence of cimetidine at 300 mg four times a day (QID), the mean apparent oral clearance of gabapentin fell by 14% and creatinine clearance by 10% (N = 12). Thus cimetidine appeared to alter the renal excretion of both gabapentin and creatinine, an endogenous marker of renal function.
Probenecid.
Renal excretion of gabapentin was unaltered by probenecid, a blocker of renal tubular secretion.
Concomitant use with opioids.
In post-marketing experience, there are reports of respiratory failure, coma and deaths in patients taking gabapentin and other CNS depressant medications including opioids, and in patients who have a history of substance abuse (see Section 4.4 Special Warnings and Precautions for Use, Concomitant use with opioids and other CNS depressants).
Morphine.
A literature article reported that when a 60 mg controlled-release morphine capsule was administered 2 hours prior to a 600 mg gabapentin capsule (N = 12), mean gabapentin AUC increased by 44% compared to gabapentin administered without morphine (see Section 4.4 Special Warnings and Precautions for Use). Morphine pharmacokinetic parameter values were not affected by administration of gabapentin 2 hours after morphine. The magnitude of interaction at other doses is not known.4.6 Fertility, Pregnancy and Lactation
Effects on fertility.
No adverse effects on fertility or reproduction were observed in rats at doses up to 2000 mg/kg/day administered in the diet, with estimated exposure (plasma AUC) 14 times clinical exposure at the MRHD of 2400 mg/day.
(Category B3)
Gabapentin crosses the human placenta.
Congenital malformations and adverse pregnancy outcomes have been reported with gabapentin use, however there are no adequate and well-controlled studies in pregnant women and no definite conclusions can be made as to whether gabapentin is causally associated with an increased risk of congenital malformations or other adverse developmental outcomes when taken during pregnancy. The risk of birth defects is increased by a factor of 2-3 in the offspring of mothers treated with an antiepileptic medicinal product.
Gabapentin should be used during pregnancy only if the potential benefit to the mother clearly outweighs the potential risk to the fetus.
The risk of having a child with a congenital defect as a result of antiepileptic medication is far outweighed by the dangers to the mother and fetus of uncontrolled epilepsy.
It is recommended that:
women on antiepileptic drugs (AEDs) receive pre-pregnancy counselling with regard to the risk of fetal abnormalities;
AEDs should be continued during pregnancy and monotherapy should be used if possible at the lowest effective dose as risk of abnormality is greater in women taking combined medication;
folic acid supplementation (5 mg) should be commenced four weeks prior to and continue for twelve weeks after conception;
specialist prenatal diagnosis including detailed mid-trimester ultrasound should be offered.
Studies in animals have shown reproductive toxicity. The potential risk for humans is unknown.
Embryofetal development studies with gabapentin in mice at oral doses up to 3000 mg/kg/day and in rats at oral doses up to 1500 mg/kg/day revealed no evidence of fetal malformations. Delayed ossification in the skull, vertebrae and limbs, indicative of fetal growth retardation, was reported in the offspring of mice administered gabapentin at oral doses of 1000 and 3000 mg/kg/day during organogenesis, and rats administered gabapentin 2000 mg/kg/day in the diet during mating and throughout gestation. An increased incidence of hydroureter and/or hydronephrosis was observed in rats treated with dietary gabapentin from late gestation to weaning (see Section 4.6 Fertility, Pregnancy and Lactation, Use in lactation). In these studies, exposure to gabapentin (based on areas under the concentration time curve) was up to 5 times higher in the mouse, and up to 14 times higher in the rat, than in humans at the recommended maximum dose of 2400 mg/day.
In female rabbits given 60, 300 or 1500 mg/kg/day gabapentin orally during the period of organogenesis, maternal toxicity and abortion were observed at the high dose, but at the low and mid doses, no evidence of harm to the fetus was observed.
Gabapentin is excreted in human milk.
In a peri-postnatal study in rats administered gabapentin in the diet at doses of 500, 1000 and 2000 mg/kg/day from late gestation to weaning, there was a dose related reversible increase in the incidence of hydroureter and hydronephrosis in 21 day-old pups. Because the effect on the nursing infant is unknown, and because of the potential for serious adverse reactions in nursing infant from gabapentin, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother. Gabapentin should be used in nursing mothers only if the benefits clearly outweigh the risks.4.7 Effects on Ability to Drive and Use Machines
Patients should be advised not to drive a car or operate potentially dangerous machinery until it is known that this medication does not affect their ability to engage in these activities.
4.8 Adverse Effects (Undesirable Effects)
Adults and children older than 12 years of age with epilepsy.
Gabapentin has been evaluated for safety in approximately 2000 subjects and patients and was well tolerated. Of these, 543 patients participated in controlled clinical trials.
The most commonly observed adverse events associated with the use of gabapentin in combination with other antiepileptic drugs, not seen in an equivalent frequency among placebo-treated patients, were somnolence, dizziness, ataxia, fatigue, and nystagmus.
Approximately 7% of the 2074 individuals who received gabapentin in the premarketing clinical trials discontinued treatment because of an adverse event. The adverse events most commonly associated with withdrawal were somnolence, ataxia, fatigue, nausea and/or vomiting, and dizziness.
Incidence in controlled epilepsy clinical trials.
Table 3 lists the treatment-emergent signs and symptoms that occurred in at least 1% of gabapentin-treated patients with epilepsy participating in gabapentin placebo-controlled trials. In these studies, either gabapentin or placebo was added to the patient's current antiepileptic drug therapy. Adverse events were usually mild to moderate in intensity.
Other adverse events observed during all epilepsy clinical studies.
Those events that occurred in at least 1% of the study participants with epilepsy who received gabapentin as adjunctive therapy in any clinical study and that are not described in the previous section as frequently occurring treatment-emergent signs and symptoms during placebo-controlled studies are summarised below.
Body as a whole.
Asthenia, malaise, facial oedema.
Cardiovascular system.
Hypertension.
Digestive system.
Flatulence, anorexia, gingivitis.
Haematologic and lymphatic systems.
Purpura most often described as bruises resulting from physical trauma.
Musculoskeletal system.
Arthralgia.
Nervous system.
Vertigo, hyperkinesia, increased, decreased or absent reflexes, paraesthesia, anxiety, hostility.
Respiratory system.
Pneumonia.
Urogenital system.
Urinary tract infection.
Special senses.
Abnormal vision, most often described as a visual disturbance.
Children from 3 to 12 years of age with epilepsy.
The most commonly observed adverse events reported with the use of gabapentin in combination with other antiepileptic drugs in children 3 to 12 years of age, not seen in equal frequency among placebo-treated patients, were viral infection, fever, nausea and/or vomiting, and somnolence.
Approximately 8% of the 292 children aged 3 to 12 years who received gabapentin in preapproval clinical trials discontinued treatment because of an adverse event. The adverse events most commonly associated with withdrawal in children were somnolence (1.4%), hyperkinesia (1.0%), and hostility (1.0%). See Table 4.
Other events in more than 2% of children but equally or more frequent in the placebo group included: pharyngitis, upper respiratory infection, headache, rhinitis, convulsions, diarrhoea, anorexia, coughing, and otitis media.
Adverse events occurring during clinical trials in children treated with gabapentin that were not reported in adjunctive therapy trials in adults are:
Body as a whole.
Dehydration, infectious mononucleosis.
Digestive system.
Hepatitis, oral moniliasis.
Haematologic and lymphatic systems.
Coagulation defect.
Nervous system.
Aura disappeared, occipital neuralgia.
Psychobiologic function.
Sleepwalking.
Respiratory system.
Pseudo-croup, hoarseness.
Adults older than 18 years of age with neuropathic pain.
The most commonly observed adverse events reported with the use of gabapentin in adults older than 18 years of age with neuropathic pain, seen in at least twice the frequency among placebo-treated patients, were dry mouth, peripheral oedema, weight gain, abnormal gait, amnesia, ataxia, confusion, dizziness, hypoaesthesia, somnolence, thinking abnormal, vertigo, rash and amblyopia.
Of the 821 adults who received gabapentin in the painful diabetic peripheral neuropathy and post-herpetic neuralgia trials, 13.2% discontinued treatment because of an adverse event. The adverse events most commonly associated with withdrawal were dizziness (4.4%), somnolence (2.9%), and nausea (1.3%). See Table 5.
Post-marketing experience.
The following adverse events have been reported in patients receiving gabapentin post-marketing, however, the data are insufficient to support an estimate of their incidence or to establish causation.
Sudden, unexplained deaths have been reported where a causal relationship to treatment with gabapentin has not been established. Additional post-marketing adverse events reported include blood creatine phosphokinase increased, rhabdomyolysis, abnormal liver function, acute kidney failure, agitation, allergic reaction including urticaria, alopecia, anaphylaxis, anaemia, angioedema, hyperglycemia and hypoglycemia (most often observed in patients with diabetes), breast hypertrophy, gynaecomastia, cardiac arrest, chest pain, convulsions, depersonalisation, drug rash with eosinophilia and systemic symptoms, erythema multiforme, fall, hypersensitivity including systemic reactions, hyponatraemia, jaundice, loss of consciousness, movement disorders such as choreoathetosis, dyskinesia and dystonia, myoclonus, palpitation, pancreatitis, renal impairment, speech disorder, sexual dysfunction (including changes in libido, ejaculation disorders and anorgasmia), Stevens-Johnson syndrome, tachycardia, thrombocytopenia, tinnitus, urinary incontinence and symptoms of psychosis such as delusions, hallucinations, and thinking abnormal.
Generalised oedema, hepatitis, hypotension, neuropathy/peripheral neuropathy and syncope have been rarely reported.
Adverse events following the abrupt discontinuation of gabapentin have also been reported. The most frequently reported events were anxiety, insomnia, nausea, pain, and sweating.
Some cases of hypomania have been reported after commencement of gabapentin. In each case, other anticonvulsants had been used concurrently, and symptoms of hypomania resolved following a reduction in dosage or cessation of the drug.
Reporting suspected adverse effects.
Reporting suspected adverse reactions after registration of the medicinal product is important. It allows continued monitoring of the benefit-risk balance of the medicinal product. Healthcare professionals are asked to report any suspected adverse reactions at www.tga.gov.au/reporting-problems.4.9 Overdose
Signs and symptoms.
Symptoms of an overdose included somnolence, ataxia, dizziness, double vision, nystagmus, slurred speech, drowsiness, loss of consciousness, lethargy, mild hypotension and gastrointestinal symptoms including diarrhoea. Gabapentin overdose alone has not been reported to produce significant cardiotoxicity.
Overdoses as high as 108 g have been reported with full recovery following symptomatic therapy. Reduced absorption of gabapentin at higher doses may limit drug absorption at the time of overdosing and, hence, minimise toxicity from overdoses.
Treatment of overdosage.
There is no specific antidote for gabapentin; treatment is symptomatic. The patient should be monitored closely and given supportive care where necessary to maintain vital functions.
Overdoses may involve other concurrent medications and should be treated accordingly.
Activated charcoal may reduce absorption of the drug if given within one hour after ingestion. In patients who are not fully conscious or have impaired gag reflex, consideration should be given to administering activated charcoal via nasogastric tube once the airway is protected.
Gabapentin can be removed by haemodialysis. Although haemodialysis has not been performed in the few overdose cases reported, it may be indicated by the patient's clinical state or in patients with significant renal impairment.
Ipecac-induced emesis is not recommended because of the potential for CNS depression.
For information on the management of overdose, contact the Poisons Information Centre on 13 11 26 (Australia).5 Pharmacological Properties
5.1 Pharmacodynamic Properties
Mechanism of action.
The mechanism by which gabapentin exerts its anticonvulsant action is unknown. Gabapentin is structurally related to the neurotransmitter GABA (gamma-aminobutyric acid) but its mechanism of action is different from that of several other drugs that interact with GABA synapses including valproate, barbiturates, benzodiazepines, GABA transaminase inhibitors, GABA uptake inhibitors, GABA agonists, and GABA prodrugs. In vitro studies with radiolabelled gabapentin have characterised a novel peptide binding site in rat brain tissues including neocortex and hippocampus that may relate to anticonvulsant activity of gabapentin and its structural derivatives. However, the identification and function of the gabapentin binding site remains to be elucidated. Gabapentin at relevant clinical concentrations does not bind to other common drug or neurotransmitter receptors of the brain including GABAA, GABAB, benzodiazepine, glutamate, glycine or N-methyl-d-aspartate receptors.
Gabapentin does not interact with sodium channels in vitro and so differs from phenytoin and carbamazepine. Several test systems ordinarily used to assess activity at the NMDA receptor complex have been examined. Results are contradictory. Accordingly no general statement about the effects, if any, of gabapentin at the NMDA receptor can be made. Gabapentin slightly reduces the release of monoamine neurotransmitters in vitro. Gabapentin administration to rats increases GABA turnover in several brain regions in a manner similar to valproate sodium, although in different regions of brain. The relevance of these various actions of gabapentin to the anticonvulsant effects remains to be established. In animals, gabapentin readily enters the brain and shows efficacy in some, but not all, seizure models.
These animal models included genetic models of seizures, and seizures induced by maximal electroshock, from chemical convulsants including inhibitors of GABA synthesis.
Clinical trials.
Partial seizures. Adults.
The effectiveness of gabapentin as adjunctive therapy was established in three multi-centre, placebo-controlled, double-blind, parallel-group clinical trials in 705 adults with refractory partial seizures. The patients enrolled had a history of at least 4 partial seizures per month in spite of receiving one of more antiepileptic drugs at therapeutic levels and were observed on their established antiepileptic drug regimen during a 12-week baseline period. In patients continuing to have at least 2 (or 4 in some studies) seizures per month, gabapentin or placebo was then added on to the existing therapy during a 12-week treatment period. Effectiveness was assessed primarily on the basis of the percent of patients with a 50% or greater reduction in seizure frequency from baseline to treatment (the "responder rate") and a derived measure called response ratio, a measure of change defined as (T - B)/(T + B), where B is the patient's baseline seizure frequency and T is the patient's seizure frequency during treatment. Response ratio is distributed within the range -1 to +1. A zero value indicates no change while complete elimination of seizures would give a value of -1. Increased seizure rates would give positive values. A response ratio of -0.33 corresponds to a 50% reduction in seizure frequency. The results given below are for all partial seizures in the intent-to-treat (all patients who received any doses of treatment) population in each study, unless otherwise indicated.
One study compared gabapentin 1200 mg/day, given as three divided doses (divided TID) with placebo. Responder rate was 23% (14/61) in the gabapentin group and 9% (6/66) in the placebo group; the difference between groups was statistically significant. Response ratio was also better in the gabapentin group (-0.199) than in the placebo group (-0.044), a difference that also achieved statistical significance.
A second study compared primarily 1200 mg/day gabapentin (N = 101), given as three divided doses, with placebo (N = 98). Additional smaller gabapentin dosage groups (600 mg/day, N = 53; 1800 mg/day, N = 54) were also studied for information regarding dose response. Responder rate was higher in the gabapentin 1200 mg/day group (16%) than in the placebo group (8%), but the difference was not statistically significant. The responder rate at 600 mg (17%) was also not significantly higher than in the placebo, but the responder rate in the 1800 mg group (26%) was statistically significantly superior to the placebo rate. Response ratio was better in the gabapentin 1200 mg/day group (-0.103) than in the placebo group (-0.022); but this difference was also not statistically significant (p = 0.224). A better response was seen in the gabapentin 600 mg/day group (-0.105) and 1800 mg/day group (-0.222) than in the 1200 mg/day group, with the 1800 mg/day group achieving statistical significance compared to the placebo group.
A third study compared gabapentin 900 mg/day, given as three divided doses (N = 111) and placebo (N = 109). An additional gabapentin 1200 mg/day dosage group (N = 52) provided dose-response data. A statistically significant difference in responder rate was seen in the gabapentin 900 mg/day group (22%) compared to that in the placebo group (10%). Response ratio was also statistically significantly superior in the gabapentin 900 mg/day group (-0.119) compared to that in the placebo group (-0.027), as was response ratio in 1200 mg/day gabapentin (-0.184) compared to placebo.
A one week, prospective, multi-centre, randomised, double-blind, placebo lead-in, parallel-group study compared the tolerability of gabapentin administered as an initial dosage of 900 mg/day versus a dosage titrated to 900 mg/day over three days (i.e. 300 mg on Day 1, 600 mg on Day 2, 900 mg on Day 3). 781 patients (titrated = 383, non-titrated = 388) involved in the study had partial seizures which were not adequately controlled with one or two other antiepileptic drugs. For the MITT population, on both the first day of active medication, and all 5 days of active medication, there were no clinically meaningful treatment group differences in the incidences of fatigue, ataxia, and somnolence (i.e. the upper 95% confidence limit for the difference < 7.5%). Only the difference in dizziness exceeded this upper confidence limit (upper confidence limit = 10.7% for the first day and 11.3% for all 5 days), with the non-titrated group reporting the higher incidence, however, it did not lead to increased discontinuation in this group.
Paediatric patients.
The safety and efficacy of gabapentin administered as adjunctive therapy for the treatment of partial seizures in paediatric patients aged 3 to 12 years were assessed in two randomised, double-blind, parallel-group, placebo-controlled, multicentre clinical studies. The studies were conducted in 247 children who had refractory partial seizures and were receiving 1 to 3 standard antiepileptic drugs. After a 6-week baseline phase, during which patients received their prescribed antiepileptic drugs, there was a 12-week double-blind treatment phase. Patients who had experienced a minimum of 4 seizures during baseline were randomised and had either gabapentin (25 to 35 mg/kg/day) or placebo added to their baseline AEDs. The primary analysis of RRatio (MITT population) demonstrated that gabapentin was significantly better than placebo in controlling partial seizures (p = 0.04). Results for the ITT population did not show a significant difference in RRatio between the treatment groups. Further analysis using rank-transformed data was performed as the data showed evidence of non-normality of distribution. Results of this analysis showed that mean RRatio was significantly lower (better) for the gabapentin treatment group than for the placebo group in both the MITT (p = 0.01) and ITT (p = 0.03) populations.
Neuropathic pain. Adults.
The efficacy and safety of gabapentin for the treatment of neuropathic pain in adults older than 18 years of age were assessed in two randomised, double-blind, parallel-group, placebo-controlled, multicentre studies. One study examined the efficacy and safety of gabapentin in the treatment of painful diabetic peripheral neuropathy and the other study was conducted in patients with post-herpetic neuralgia. The studies were of a similar design. Following a baseline screening week and randomisation, gabapentin was titrated from 900 mg/day to 1800 mg/day, 2400 mg/day and 3600 mg/day divided into three times a day dosing consecutively over the first four weeks of the study. Patients were then maintained at the maximum dose that was tolerated for the remaining four weeks. The primary efficacy measure used in both studies was change from baseline to the final week in mean pain score obtained from daily pain diaries (pain was measured using an 11-point Likert scale). Several secondary outcomes were also assessed including: the Short-Form McGill Pain Questionnaire (SF-MPQ) (sensory, affective and total pain scores), SF-MPQ visual analogue scale (VAS) and present pain intensity scale (PPI), mean sleep interference score, Patient and Clinical Global Impression of Change (PGIC and CGIC), and the quality of life measures SF-36 Quality of Life Questionnaire (QOL) and Profile of Mood States (POMS).
Results from both studies demonstrated that gabapentin provided statistically significantly greater improvement in relief of neuropathic pain than placebo. In patients with painful diabetic peripheral neuropathy, mean pain score decreased by 2.6 in patients receiving gabapentin and 1.4 in patients receiving placebo (p < 0.001). In the post-herpetic neuralgia study, mean pain score decreased by 2.1 in patients receiving gabapentin and 0.5 in patients receiving placebo (p < 0.001). Gabapentin was significantly better than placebo in controlling pain from week two of both studies (p < 0.001). Sleep interference scores, Short-Form McGill sensory, affective and total pain scores, VAS and PPI scale as well as PGIC, CGIC and some of the quality of life measures showed significant differences in favour of gabapentin.
5.2 Pharmacokinetic Properties
All pharmacological actions following gabapentin administration are due to the activity of the parent compound; gabapentin is not appreciably metabolised in humans.
Absorption.
Gabapentin bioavailability is not dose proportional, i.e. as dose is increased, bioavailability decreases. A 400 mg dose, for example, is about 25% less bioavailable than a 100 mg dose.
Over the recommended dose range of 300 mg to 600 mg three times a day, however, the differences in bioavailability are not large, and bioavailability is about 60%. The bioavailability of the 800 mg dose was found to be approximately 35% in single and multiple dose studies. The absolute bioavailability of gabapentin following daily doses of 1200 mg/day, 2400 mg/day, 3600 mg/day, and 4800 mg/day averaged 47%, 34%, 33%, and 27% respectively. Food has no effect on the rate and extent of absorption of gabapentin.
Distribution.
Gabapentin circulates largely unbound (< 3%) to plasma proteins. The apparent volume of distribution of gabapentin after 150 mg intravenous administration is 58 ± 6 L (Mean ± SD). In patients with epilepsy, steady-state pre-dose (Cmin) concentrations of gabapentin in the cerebrospinal fluid were approximately 20% of the corresponding plasma concentrations.
Metabolism and excretion.
Gabapentin is eliminated from the systemic circulation by renal excretion as unchanged drug. Gabapentin is not appreciably metabolised in humans.
The elimination half-life of gabapentin is 5 to 7 hours and is unaltered by dose or following multiple dosing. Gabapentin elimination rate constant, plasma clearance, and renal clearance are directly proportional to creatinine clearance. In elderly patients, and in patients with impaired renal function, gabapentin plasma clearance is reduced. Gabapentin can be removed by haemodialysis.
Dose adjustment in patients with compromised renal function or in those undergoing haemodialysis is recommended (see Section 4.2 Dose and Method of Administration).
Special populations.
Patients with renal insufficiency.
Subjects with renal insufficiency (mean creatinine clearance ranging from 13 mL/min to 114 mL/min) were administered 400 mg oral dose of gabapentin. The mean gabapentin half-life ranged from about 6.5 hours (patients with creatinine clearance (CLcr) > 60 mL/min) to 52 hours (CLcr < 30 mL/min) and gabapentin renal clearance ranged from about 90 mL/min (CLcr > 60 mL/min) to about 10 mL/min (CLcr < 30 mL/min). Gabapentin dosage should be adjusted in patients with compromised renal function (see Section 4.2 Dose and Method of Administration).
Patients on haemodialysis.
In a study in anuric patients, the elimination half-life of gabapentin on non-dialysis day was about 132 hours; dialysis three times a week (4 hours duration) lowered the apparent half-life of gabapentin by about 60%, from 132 hours to 51 hours. Gabapentin dosage should be adjusted in patients undergoing haemodialysis (see Section 4.2 Dose and Method of Administration).
Elderly (≥ 65 years).
In a study examining the effect of age on the elimination of gabapentin, apparent oral clearance (CL/F) of gabapentin decreased as age increased, from about 225 mL/min in those younger than 30 years of age to about 125 mL/min in those older than 70 years of age. Renal clearance also declined with age; however, the decline in the renal clearance of gabapentin can largely be explained by the decline in renal function. Reduction of gabapentin dose may be required in patients who have age-related compromised renal function.
Children and adolescents.
Gabapentin pharmacokinetics were determined in 24 healthy paediatric subjects between the ages of 4 and 12 years. In general, gabapentin plasma concentrations in these children are similar to those in adults.
5.3 Preclinical Safety Data
Genotoxicity.
There is no evidence that gabapentin has genotoxic potential. It was not mutagenic in vitro in standard assays using bacterial or mammalian cells. Gabapentin did not induce structural chromosome aberrations in mammalian cells in vitro or in vivo, and did not induce micronucleus formation in the bone marrow of hamsters.
Carcinogenicity.
Gabapentin was given in the diet to mice at 200, 600, and 2000 mg/kg/day and to rats at 250, 1000 and 2000 mg/kg/day for 2 years. A statistically significant increase in the incidence of pancreatic acinar cell adenoma and carcinoma was found only in male rats at the highest dose. Peak plasma gabapentin concentrations and areas under the concentration time curve in rats at 2000 mg/kg/day were 14 times higher than plasma concentrations in humans given the recommended maximum tolerated dose of 2400 mg/day. The pancreatic acinar cell tumours in male rats were low grade malignancies, which did not metastasise or invade surrounding tissue, and were similar to those seen in concurrent controls. The relevance of these pancreatic acinar cell tumours in male rats to carcinogenic risk in human is unclear.6 Pharmaceutical Particulars
6.1 List of Excipients
Hyprolose, magnesium stearate and Opadry white YS-1-18111 (ARPING No. 3289).
6.2 Incompatibilities
Incompatibilities were either not assessed or not identified as part of the registration of this medicine.
6.3 Shelf Life
In Australia, information on the shelf life can be found on the public summary of the Australian Register of Therapeutic Goods (ARTG). The expiry date can be found on the packaging.
6.4 Special Precautions for Storage
Store below 25°C.
6.5 Nature and Contents of Container
Gabapentin tablets 600 mg.
Available in blister (PVC/PE/PVDC/Al) packs of 10, 20, 30, 45, 50, 60, 84, 90, 100 tablets.
Not all packs are available.
6.6 Special Precautions for Disposal
In Australia, any unused medicine or waste material should be disposed of in accordance with local requirements.
6.7 Physicochemical Properties
Gabapentin is a white to off-white crystalline powder. It is freely soluble in water and both basic and acidic aqueous solutions. It has values for LogP of -1.10, and values for pKa of 3.68 and 10.70.
Chemical name: 1-(aminomethyl) cyclohexaneacetic acid.
Molecular formula: C9H17NO2. Molecular weight: 171.24.
Chemical structure.
CAS number.
60142-96-3.7 Medicine Schedule (Poisons Standard)
S4 - Prescription Only Medicine.
Summary Table of Changes
