Abstract
Gabapentin, [1-(aminomethyl) cyclohexaneacetic acid], a structural analog of $\gamma$-aminobutyric acid (GABA), is being developed for the treatment of epilepsy. Unlike GABA, gabapentin crosses the blood-brain barrier after systemic administration. Gabapentin is an effective antiepileptic drug in patients with partial and secondarily generalized seizures who are uncontrolled with use of existing anticonvulsant drug therapy. The purpose of the present study was to evaluate the bioequivalence of two gabapentin 400 mg capsules, $Neurontin^{(R)}$ capsule 400 mg (Pfizer Inc.) and Gabatin capsule 400 mg (Korean Drug Co. Ltd), according to the guidelines of the Korea Food and Drug Administration (KFDA). The release of gabapentin from the two gabapentin formulations in vitro was tested using KP VIII Apparatus II method with various dissolution media (pH 1.2, 4.0, 6.8 buffer solution and water). Twenty six healthy male subjects, 23.58$\pm$1.50 years in age and 66.74$\pm$8.31 kg in body weight, were divided into two groups and a randomized 2$\times$2 cross-over study was employed. After one capsule containing 400 mg as gabapentin were orally administered, blood was taken at predetermined time intervals and the concentrations of gabapentin in serum were determined using HPLC with fluorescence detector. The dissolution profiles of two formulations were similar at all dissolution media. In addition, the pharmacokinetic parameters such as $AUC_t$, $C_{max}$ and $T_{max}$ were calculated and ANOVA test was utilized for the statistical analysis of the parameters using logarithmically transformed $AUC_t$, $C_{max}$ and untransformed $T_{max}$. The results showed that the differences between two formulations based on the reference drug, $Neurontin^{(R)}$ capsule 400 mg, were 2.04, -3.68 and 16.79% for $AUC_t$, $C_{max}$ and $T_{max}$, respectively. There were no sequence effects between two formulations in these parameters. The 90% confidence intervals using logarithmically transformed data were within the acceptance range of log 0.8 to log 1.25 (e.g., log 0.91$\sim$log 1.16 and log 0.87$\sim$log 1.11 for $AUC_t$ and $C_{max}$, respectively). Thus, the criteria of the KFDA bioequivalence guideline were satisfied, indicating Gabatin capsule 400 mg was bioequivalent to $Neurontin^{(R)}$ capsule 400 mg.
