Tiagabine
Tiagabine, sold under the brand name Gabitril, is an anticonvulsant medication which is used in the treatment of epilepsy. It is also used off-label in the treatment of insomnia and anxiety disorders. However, off-label use is discouraged as the drug has been associated with new-onset seizures in people without epilepsy. Tiagabine is taken orally.
Side effects of tiagabine include dizziness, asthenia, non-specific nervousness, muscle tremors, diarrhea, depression, and emotional lability. The drug acts as a selective GABA transporter 1 blocker or GABA reuptake inhibitor, and hence acts as an indirect GABA receptor agonist, increasing GABAergic signaling in the brain. It may increase activation of both GABAA and GABAB receptors. The effects of tiagabine on sleep resemble those of GABAA receptor agonists like gaboxadol and muscimol, primarily enhancing slow wave sleep, and differ from those of GABAA receptor positive allosteric modulators like benzodiazepines and Z drugs. The drug's elimination half-life is 4.5 to 9hours, but can be shorter in people taking enzyme-inducing anticonvulsants.
Tiagabine was discovered in 1988 and was introduced for medical use in 1997. Generic formulations have become available. The drug is not a controlled substance in the United States.
Medical uses
Epilepsy
Tiagabine is approved by the United States Food and Drug Administration as an adjunctive treatment for partial seizures in epilepsy in individuals of age 12 and up. It is effective as monotherapy and combination therapy with other anticonvulsant drugs in the treatment of partial seizure.Other uses
Insomnia
Tiagabine is used in the treatment of insomnia. Lower doses than those used in epilepsy, in the range of 2 to 16mg, are used to treat insomnia.The drug has been found to enhance slow wave sleep in the context of insomnia. Its effects on SWS are dose dependent, with a 2- to 4-fold increase in SWS at doses of 8 to 16mg but mixed findings for a dose of 4mg. Findings are mixed in terms of the influence of tiagabine on sleep onset, sleep duration, nighttime awakenings, self-reported sleep ratings, and ratings of restorative or refreshing sleep. Tiagabine has been found to decrease the cognitive impairment and high cortisol levels caused by sleep restriction, with this being related to the drug's SWS improvement. On the other hand, despite increasing SWS, tiagabine did not improve memory consolidation.
The effects of tiagabine on sleep, for instance primarily increasing SWS, resemble those of gaboxadol and muscimol but are very different from those of conventional GABAA receptor positive allosteric modulators like benzodiazepines and Z drugs.
The American Academy of Sleep Medicine's 2017 clinical practice guidelines recommended against the use of tiagabine in the treatment of insomnia due to limited effectiveness and very low quality of evidence.
Anxiety disorders
Tiagabine may be prescribed off-label to treat certain anxiety disorders, such as panic disorder and social anxiety disorder. Tiagabine may be used alongside selective serotonin reuptake inhibitors, serotonin–norepinephrine reuptake inhibitors, or benzodiazepines for anxiety. The drug was ineffective for generalized anxiety disorder.Neuropathic pain
Tiagabine can be used in the treatment of neuropathic pain. It can be used alongside antidepressants, gabapentin, other anticonvulsants, or opioids for neuropathic pain.Available forms
Tiagabine is available in the form of 2, 4, 5, 10, 12, 15, and 16mg oral tablets. The drug is taken 1 to 4times per day due to its short elimination half-life. A sustained-release formulation would be advantageous but has not been developed or marketed.Contraindications
s of tiagabine include hypersensitivity to tiagabine or its ingredients and severe hepatic impairment. The drug should be avoided in pregnant and nursing women.Side effects
s of tiagabine are dose-related. The most common side effect of tiagabine is dizziness. Other side effects that have been observed with a rate of statistical significance relative to placebo include asthenia, somnolence, nervousness, memory impairment, tremor, headache, diarrhea, and depression. Adverse effects such as confusion, aphasia, stuttering, and paresthesia may occur at higher dosages of the drug. Tiagabine has been associated with new-onset seizures and status epilepticus in people without epilepsy in post-marketing surveillance. This may be dose-related, although it has been reported at doses of as low as 4mg/day, and may also be related to concomitant use of other medications that lower the seizure threshold. Some of these seizures occurred around the time of dose increases. There may be an increased risk of psychosis with tiagabine treatment, although data is mixed and inconclusive. Tiagabine can also reportedly interfere with visual color perception. It has not been found to cause psychomotor, cognitive, or memory impairment. Unlike certain other GABAergic drugs like muscimol, gaboxadol, and CI-966, tiagabine has not been associated with hallucinogenic effects.Overdose
Tiagabine overdose can produce neurological symptoms such as lethargy, single or multiple seizures, status epilepticus, coma, confusion, agitation, tremors, dizziness, dystonias, abnormal posturing, and hallucinations, as well as respiratory depression, tachycardia, and hypertension or hypotension. Overdose may be fatal especially if the victim presents with severe respiratory depression or unresponsiveness.Interactions
Combination of tiagabine with enzyme-inducing anticonvulsants like carbamazepine, phenytoin, primidone, and phenobarbital can decrease the elimination half-life of tiagabine to as low as 2 to 3hours. Conversely, tiagabine does not significantly affect the hepatic metabolism of other anticonvulsants such as carbamazepine, phenytoin, and valproic acid. Other interactions have also been reviewed.Pharmacology
Pharmacodynamics
Tiagabine acts a selective GABA transporter 1 blocker and hence as a GABA reuptake inhibitor. The GAT-1 is one of at least four distinct GABA transporters, with the GAT-1 being the predominant subtype in the brain, accounting for 85% of GATs in this part of the body, and thought to be responsible for most γ-aminobutyric acid reuptake in synapses. The drug has more than 1,000-fold selectivity for the GAT-1 over the GABA transporter 2, GABA transporter 3, and betaine/GABA transporter. It also shows no significant affinity for GABA receptors or numerous other targets. In addition, it does not affect key cardiac ion channels. Through GAT-1 blockade, tiagabine increases levels of GABA, the major inhibitory neurotransmitter in the central nervous system, and consequently increases GABA receptor activation and GABAergic signaling, including of both GABAA and GABAB receptors. The drug has been found to increase GABAergic signaling in the hippocampus, globus pallidus, ventral pallidum, and substantia nigra in animals. It produces anticonvulsant, neuroprotective, hypnotic, analgesic, and anxiolytic-like effects in animals.In rodent drug discrimination tests, tiagabine partially substituted for muscimol and diazepam but did not substitute for gaboxadol, phenobarbitol, or zolpidem. When tiagabine was used as the training drug however, gaboxadol near-fully substituted for tiagabine. Similarly, indiplon partially substituted for tiagabine. On the other hand, zolpidem, eszopiclone, baclofen, and gabapentin all did not substitute for tiagabine. The GABAA receptor antagonist -bicuculline at non-convulsant doses partially antagonized tiagabine's interoceptive effects, whereas higher doses that might more fully antagonize its cue were not assessed due to risk of convulsions. These findings suggest involvement of the GABAA receptor in the subjective effects of tiagabine, at least in rodents. Conversely, the GABAB receptor does not appear to be involved.
Tiagabine increases benzodiazepines' affinity to cortical and limbic GABAA receptors and influences electroencephalography measurements by increasing frontal activity and reducing posterior activity in the brain.
With regard to pharmacophore, the most stable binding mode of tiagabine in the GAT-1 is that where the nipecotic acid fragment is located in the main ligand binding site, and aromatic thiophene rings are arranged within the allosteric site, which yields GAT-1 in an outward-open state. This interaction is mediated through GAT-1's sodium ion mimicry, hydrogen bonding and hydrophobic interactions.
File:MEG power change induced by tiagabine in 14 healthy volunteers..png|thumb|right|250px|class=skin-invert-image|Tiagabine enhances MEG delta power in healthy volunteers.
Tiagabine enhances the power of cortical delta oscillations up to 1,000% relative to placebo, which may result in an EEG or MEG signature resembling non-rapid eye movement sleep even while the person who has taken tiagabine is awake and conscious. This demonstrates that cortical delta activity and wakeful consciousness are not mutually exclusive, i.e., high amplitude delta oscillations are not always a reliable indicator of unconsciousness.