Some of the views expressed in the following notes on newly approved products should be regarded as preliminary, as there may have been limited published data at the time of publication, and little experience in Australia of their safety or efficacy. However, the Editorial Executive Committee believes that comments made in good faith at an early stage may still be of value. Before new drugs are prescribed, the Committee believes it is important that more detailed information is obtained from the manufacturer's approved product information, a drug information centre or some other appropriate source.
300 mg and 400 mg capsules
Gammaaminobutyric acid (GABA) has anticonvulsant effects in animals. Although the precise mechanism of action is unclear, gabapentin, an analogue of GABA, can reduce the frequency of seizures in patients whose partial seizures are incompletely controlled by other drugs.
The bioavailability of gabapentin decreases as the dose is increased. Food does not significantly affect absorption. The drug penetrates the blood brain barrier and concentrations in the cerebrospinal fluid are approximately 20% of the plasma concentration. Gabapentin is not metabolised. It is mainly excreted in the urine and the dose should be adjusted in patients with renal impairment. The elimination hal flife is 5-7 hours. It is not necessary to monitor the plasma concentration.
A multi centre, double blind study of gabapentin as an adjunctive therapy found that a dose of 1200 mg/day reduced the number of partial seizures more than placebo.1 In other trials, the effective dose range was 900-1800 mg/day. The drug is given in divided doses and is introduced gradually over a few days.
As gabapentin is only approved for use in combination with other antiepileptic drugs, attributing drug related adverse reactions can be problematic. In clinical studies, approximately 7% of the trial population withdrew due to adverse effects. Many of the adverse effects involved the nervous system, the most common effects being somnolence, dizziness, ataxia and fatigue. Pancreatic tumours have occurred in rodents given gabapentin, but the relevance of this finding to clinical practice is unclear.
- U.K. gabapentin study group. Gabapentin in partial epilepsy. Lancet 1990;335:1114-7.