In vivo and in vitro evaluations of intestinal gabapentin absorption: effect of dose and inhibitors on carrier-mediated transport.

PURPOSE: Gabapentin exhibits saturable absorption kinetics, however, it remains unclear which transporters that are involved in the intestinal transport of gabapentin. Thus, the aim of the current study was to explore the mechanistic influence of transporters on the intestinal absorption of gabapentin by both in vivo and in vitro investigations METHODS: Pharmacokinetic parameters were determined following a range of intravenous (5-100 mg/kg) and oral doses (10-200 mg/kg) in rats. Transepithelial transport (50 µM-50 mM) and apical uptake of gabapentin (0.01-50 mM) were investigated in Caco-2 cells. The effect of co-application of the LAT-inhibitor, BCH, and the b(0,+)-substrate, L-lysine, on intestinal transport of gabapentin was evaluated in vivo and in vitro. RESULTS: Gabapentin showed dose-dependent oral absorption kinetics and dose-independent disposition kinetics. Co-application of BCH inhibited intestinal absorption in vivo and apical uptake in vitro, whereas no effect was observed following co-application of L-lysine. CONCLUSIONS: The present study shows for the first time that BCH was capable of inhibiting intestinal absorption of gabapentin in vivo. Furthermore, in Caco-2 cell experiments BCH inhibited apical uptake of gabapentin. These findings may imply that a BCH-sensitive transport-system was involved in the apical and possibly the basolateral transport of gabapentin across the intestinal wall.

Intestinal drug transport via the proton-coupled amino acid transporter PAT1 (SLC36A1) is inhibited by Gly-X(aa) dipeptides.

The oral absorption of some drug substances is mediated by nutrient transporters. As a consequence, nutrients and drugs may compete for available transporters, and interactions at the level of intestinal absorption are possible. Recently, we have identified δ-aminolevulinic acid, Gly-Gly, and Gly-Sar as substrates of the amino acid transporter PAT1. The aim of the present study is to investigate if other Gly-containing dipeptides interact with PAT1, and whether they can inhibit PAT1 mediated drug absorption, in vitro and in vivo. The in vitro methods included two-electrode voltage clamp measurements on hPAT1 expressing Xenopus laevis oocytes, which were used to investigate the PAT1-mediated transport of 17 different Gly-containing dipeptides (Gly-X(aa) or X(aa)-Gly). Also, the transepithelial transport of the PAT1 substrate gaboxadol was investigated across Caco-2 cell monolayers in the presence of different dipeptides. The in vivo part consisted of a pharmacokinetic study in rats following oral administration of gaboxadol and preadministration of 200 mg/kg dipeptide. The results showed that in hPAT1 expressing oocytes Gly-Tyr, Gly-Pro, and Gly-Phe inhibited currents induced by drug substances. In Caco-2 cell monolayers, Gly-Gly, Gly-Sar, and Gly-Pro significantly inhibited the PAT1 mediated absorptive transepithelial transport of gaboxadol; however, when orally administered to rats, Gly-Gly, Gly-Sar, Gly-Pro, or Gly-Tyr did not alter the pharmacokinetic profile of gaboxadol. In conclusion, the present study identifies selected dipeptides as inhibitors of PAT1 mediated drug absorption in various in vitro models.

Rectal absorption of vigabatrin, a substrate of the proton coupled amino acid transporter (PAT1, Slc36a1), in rats.

PURPOSE: To investigate the rectal absorption of vigabatrin in rats, based on the hypothesis that PAT1 (Slc36a1) is involved. METHODS: Male Sprague-Dawley rats were dosed rectally with five different gels, varying in buffer capacity, the amount of vigabatrin, and co-administration of proline or tryptophan. Western blotting was used to detect rPAT1 in rat rectal epithelium. X. Laevis oocytes were injected with SLC36A1 cRNA for the expression of hPAT1, prior to two-electrode voltage clamp measurements. RESULTS: rPAT1 protein was present in rat rectal epithelium. Approximately 7%-9% of a 1 mg/kg vigabatrin dose was absorbed after rectal administration, regardless of the formulation used. Increasing the dose of vigabatrin 10-fold decreased the absolute bioavailability to 4.2%. Co-administration of proline or tryptophan changed the pharmacokinetic profile, indicating a role of PAT1 in the rectal absorption of vigabatrin. Transport of vigabatrin via hPAT1 expressed in X. Laevis oocytes had a K(m) of 5.2 ± 0.6 mM and was almost completely inhibited by tryptophan. CONCLUSIONS: Although vigabatrin is a PAT1 substrate and the rPAT1 protein is expressed in the rectum epithelium, vigabatrin has low rectal absorption in rats.

Synthesis and biological activity of argiotoxin 636 and analogues: selective antagonists for ionotropic glutamate receptors.

More discerning than the parent: Analogues of the polyamine toxin argiotoxin 636 (shown docked in the ion channel of an ionotropic glutamate (iGlu) receptor; N blue, O red) distinguish subtypes of iGlu receptors. Depending on which of the two internal amine groups is replaced with a methylene group, the analogue inhibits one or other of two receptor subtypes as potently as the natural compound, which itself inhibits both subtypes nonselectively.

Pharmacokinetic aspects of the anti-epileptic drug substance vigabatrin: focus on transporter interactions.

Drug transporters in various tissues, such as intestine, kidney, liver and brain, are recognized as important mediators of absorption, distribution, metabolism and excretion of drug substances. This review gives a current status on the transporter(s) mediating the absorption, distribution, metabolism and excretion properties of the anti-epileptic drug substance vigabatrin. For orally administered drugs, like vigabatrin, the absorption from the intestine is a prerequisite for the bioavailability. Therefore, transporter(s) involved in the intestinal absorption of vigabatrin in vitro and in vivo are discussed in detail. Special focus is on the contribution of the proton-coupled amino acid transporter 1 (PAT1) for intestinal vigabatrin absorption. Furthermore, the review gives an overview of the pharmacokinetic parameters of vigabatrin across different species and drug-food and drug-drug interactions involving vigabatrin.

Gaboxadol has affinity for the proton-coupled amino acid transporter 1, SLC36A1 (hPAT1)--A modelling approach to determine IC(50) values of the three ionic species of gaboxadol.

The human proton-coupled amino acid transporter, SLC36A1 (hPAT1), is situated in the apical membrane of small intestinal epithelium. It is involved in cellular uptake of amino acids and orally administered drug substances such as δ-aminolevulinic acid, vigabatrin and gaboxadol. Gaboxadol (Gbx) is a selective extrasynaptic GABA(A) receptor agonist with high oral bioavailability in rat, dog and human. It is a zwitterionic compound with pK(a) values of 4.3 and 8.1. Dependent on the pH of the solution Gbx will be present as three individual ionic species, i.e. cationic (Gbx(+)), zwitterionic (Gbx(+/-)) and anionic (Gbx(-)). The aim of the present study was to elucidate the individual affinities of Gbx(+), Gbx(+/-) and Gbx(-) for SLC36A1. The ability of Gbx to concentration-dependently inhibit a SLC36A1 mediated l-[(3)H]proline uptake was investigated in Caco-2 cell monolayers at apical pH 5.0-6.8. The IC(50) values were computed using an in silico model relying on a genetic algorithm. The IC(50) values of Gbx(+), Gbx(+/-) and Gbx(-) were estimated to 2.6mM, 16mM and >1000mM. This indicates that the positive charge is essential for Gbx binding to SLC36A1. The negative charge is tolerated in the zwitterionic form, whereas no affinity is observed for the anionic form.

Assessment of structurally diverse philanthotoxin analogues for inhibitory activity on ionotropic glutamate receptor subtypes: discovery of nanomolar, nonselective, and use-dependent antagonists.

An array of analogues of the wasp toxin philanthotoxin-433, in which the asymmetric polyamine moiety was exchanged for spermine and the headgroup replaced with a variety of structurally diverse moieties, was prepared using parallel solid-phase synthesis approaches. In three analogues, the spermine moiety was extended with an amino acid tail, six compounds contained an N-acylated cyclohexylalanine, and four analogues were based on a novel diamino acid design with systematically changed spacer length between N-cyclohexylcarbonyl and N-phenylacetyl substituents. The analogues were studied using two-electrode voltage-clamp electrophysiology employing Xenopus laevis oocytes expressing GluA1(i) AMPA or GluN1/2A NMDA receptors. Several of the analogues showed significantly increased inhibition of the GluN1/2A NMDA receptor. Thus, an analogue containing N-(1-naphtyl)acetyl group showed an IC(50) value of 47 nM. For the diamino acid-based analogues, the optimal spacer length between two N-acyl groups was determined, resulting in an analogue with an IC(50) value of 106 nM.