| Antiepileptic drugs: gabapentin |
Gabapentin is structurally related to gamma-aminobutyric acid (GABA) and there is now evidence that it inhibits glutamate synthesis and elevates brain GABA levels. Its exact mechanism of action is unknown, but is thought to involve interaction with a subset of voltage-gated calcium channels. Gabapentin is also used in the management of neuropathic pain. It is known to interact with neuronal calcium channels in the spinal cord to reduce release of neurotransmitters, as well as increasing GABA synthesis and antagonising non-NMDA (N-methyl-D-aspartate) receptors. It has been shown to be efficacious in the treatment of diabetic neuropathy and postherpetic neuralgia, with around 30% of patients expecting to achieve more than 50% reduction in pain.
Gabapentin does not bind to plasma proteins or induce hepatic enzymes. It has a half-life of 5 to 7 hours, hence the need for 3 times daily dosing.
Gabapentin is absorbed by a saturable amino acid uptake system in the gastrointestinal tract, so that as the dose is increased, bioavailability decreases. Antacids bind with gabapentin and reduce its bioavailability by approximately 20%, so they should not be given together. Gabapentin has few other drug interactions. Gabapentin is excreted unchanged in the urine, therefore the dose must be reduced in renal impairment.
Common adverse effects are somnolence, dizziness and ataxia.
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