Intestinal gaboxadol absorption via PAT1 (SLC36A1): modified absorption in vivo following co-administration of L-tryptophan

Background and purpose: Gaboxadol has been in development for treatment of chronic pain and insomnia. The clinical use of gaboxadol has revealed that adverse effects seem related to peak serum concentrations. The aim of this study was to investigate the mechanism of intestinal absorption of gaboxadol in vitro and in vivo. Experimental approach: In vitro transport investigations were performed in Caco-2 cell monolayers. In vivo pharmacokinetic investigations were conducted in beagle dogs. Gaboxadol doses of 2.5 mg center dot kg(-1) were given either as an intravenous injection (1.0 mL center dot kg(-1)) or as an oral solution (5.0 mL center dot kg(-1)). Key results: Gaboxadol may be a substrate of the human proton-coupled amino acid transporter, hPAT1 and it inhibited the hPAT1-mediated L-[H-3]proline uptake in Caco-2 cell monolayers with an inhibition constant K-i of 6.6 mmol center dot L-1. The transepithelial transport of gaboxadol was polarized in the apical to basolateral direction, and was dependent on gaboxadol concentration and pH of the apical buffer solution. In beagle dogs, the absorption of gaboxadol was almost complete (absolute bioavailability, F-a, of 85.3%) and T-max was 0.46 h. Oral co-administration with 2.5-150 mg center dot kg(-1) of the PAT1 inhibitor, L-tryptophan, significantly decreased the absorption rate constant, k(a), and C-max, and increased T-max of gaboxadol, whereas the area under the curve and clearance of gaboxadol were constant. Conclusions and implications: The absorption of gaboxadol across the luminal membrane of the small intestinal enterocytes is probably mediated by PAT1. This knowledge is useful for reducing gaboxadol absorption rates in order to decrease peak plasma concentrations.