Is oral absorption of vigabatrin carrier-mediated?

The aim of the study was to investigate the intestinal transport mechanisms responsible for vigabatrin absorption in rats by developing a population pharmacokinetic (PK) model of vigabatrin oral absorption. The PK model was used to investigate whether vigabatrin absorption was carrier-mediated and if the proton-coupled amino acid transporter 1 (PAT1) was involved in the absorption processes. Vigabatrin (0.3-300 mg/kg) was administered orally or intravenously to Sprague Dawley rats in the absence or presence of PAT1-ligands l-proline, l-tryptophan or sarcosine. The PK profiles of vigabatrin were described by mechanistic non-linear mixed effects modelling, evaluating PAT1-ligands as covariates on the PK parameters with a full covariate modelling approach. The oral absorption of vigabatrin was adequately described by a Michaelis-Menten type saturable absorption. Using a Michaelis constant of 32.8 mM, the model estimated a maximal oral absorption rate (V_max) of 64.6 mmol/min and dose-dependent bioavailability with a maximum of 60.9%. Bioavailability was 58.5-60.8% at 0.3-30 mg/kg doses, but decreased to 46.8% at 300 mg/kg. Changes in oral vigabatrin PK after co-administration with PAT1-ligands was explained by significant increases in the apparent Michaelis constant. Based on the mechanistic model, a high capacity low affinity carrier is proposed to be involved in intestinal vigabatrin absorption. PAT1-ligands increased the Michaelis constant of vigabatrin after oral co-administration indicating that this carrier could be PAT1.