Pharmacokinetics. Following a 300 mg oral dose in healthy volunteers, the mean maximum plasma concentration of gabapentin is reached in 2 to 3 hours. Absorption kinetics of gabapentin are dose dependent. The bioavailability of a single oral dose of gabapentin is about 60%, but decreases with increasing dose, suggesting a saturable transport system. Unlike GABA, gabapentin has some lipophilicity, enabling it to readily cross the blood-brain barrier. At therapeutic concentrations, gabapentin is a substrate for the large neutral amino acid transporter (LAT1) across the blood-brain barrier (09). Gabapentin is extensively distributed in body tissues and is not bound to human plasma proteins.
Gabapentin is eliminated almost completely by renal clearance. Renal impairment reduces drug clearance and raises plasma gabapentin concentrations.
The pharmacokinetic parameters of gabapentin are not altered by repeated administration. Clinical effects are seen over the dose range of 600 to 1800 mg. Blood concentrations of gabapentin greater than 2 mg/mL are associated with a clinical response in patients with epilepsy. The dosage of gabapentin, however, should be adjusted according to clinical response. The half-life is 5 to 7 hours, meaning the drug needs to be administered 3 times a day to maintain optimal plasma concentrations.
In healthy subjects, the daily exposure provided by less frequent gabapentin extended-release dosing is not significantly different from same daily dose with gabapentin immediate-release, administered more frequently.
Therapeutic drug monitoring. Because of pharmacokinetic variability, therapeutic drug monitoring is useful for patients under treatment with gabapentin, but challenges in application for indications such as restless legs syndrome include variable dosage regimens that lead to differences in interpretation of serum concentrations (23). A rapid and validated isocratic fluorometric high performance liquid chromatography method is available for the determination of gabapentin in human plasma as well as urine for clinical application and can be used to evaluate pharmacokinetics of gabapentin in humans (39).
Pharmacodynamics. Gabapentin slightly reduces the release of several monoamine neurotransmitters (norepinephrine and serotonin), but not acetylcholine, from mammalian brain tissue. Gabapentin protects against convulsions in standard animal seizure models, and this predicts the clinical efficacy of gabapentin in patients with partial seizures and secondarily generalized tonic-clonic seizures. No tolerance has been noted with respect to the anticonvulsant effects of gabapentin.
Even though it is structurally like GABA, gabapentin does not act through mechanisms related to this neurotransmitter, but rather by events modulated through its interaction with a receptor thought to be present only in the brain. However, some neurons that respond to gabapentin are GABAergic. The specific binding site is a protein of voltage-gated calcium channels and, as a result, modulates the action of calcium channels and neurotransmitter release. Gabapentin is like carbamazepine and phenytoin with respect to effects on excitatory mechanisms and segmental inhibition in the trigeminal complex, but it differs in its effects on inhibitory pathways descending from the reticular formation.
Gabapentin is a neuroprotective agent by inhibition of glutamate synthesis. The mechanism of relief of neuropathic pain is not clear, but gabapentin does not appear to affect the same pathways as opioids or tricyclic antidepressants. Current evidence indicates that it affects a voltage-gated calcium channel in the pain-transmitting nerve cells of the spinal cord.
Gabapentin enacarbil, approved for restless legs syndrome, is an actively transported prodrug of gabapentin that provides sustained dose-proportional exposure to gabapentin and predictable bioavailability (22). In a population pharmacodynamics study, response increased with increasing dose as assessed by Clinical Global Impression of Improvement, whereas the total score on International Restless Legs Scale was similar at all exposures tested.
Mirogabalin besylate (DS-5565) shows greater sustained analgesia due to a high affinity to, and slow dissociation from, the α2δ-1 subunits in the dorsal root ganglion (21). Additionally, it produces a lower level of central nervous systemspecific adverse drug reactions due to a low affinity to, and rapid dissociation from, the α2δ-2 subunits in the cerebellum. Maximum plasma concentration is achieved in less than 1 hour, compared to 1 hour for pregabalin and 3 hours for gabapentin. The plasma protein binding is relatively low, at less than 25%. As with all gabapentinoids, it is also largely excreted via the kidneys in an unchanged form, so the administration dose should also be adjusted according to renal function. The equianalgesic daily dose for 30 mg of mirogabalin is 600 mg of pregabalin and over 1200 mg of gabapentin. The initial adult dose starts at 5 mg, given orally twice a day, and is gradually increased by 5 mg at an interval of at least a week, to 15 mg. In conclusion, mirogabalin is anticipated to be a novel, safe gabapentinoid anticonvulsant with a greater therapeutic effect for neuropathic pain in the dorsal root ganglia and lower adverse reactions in the cerebellum.