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

Abstract

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.kg(-1) were given either as an intravenous injection (1.0 mL.kg(-1)) or as an oral solution (5.0 mL.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-[(3)H]proline uptake in Caco-2 cell monolayers with an inhibition constant K(i) of 6.6 mmol.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.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.

Figure 2

Transepithelial transport of 0.34, 3.5 and 7 mmol·L⁻¹ gaboxadol across Caco-2 cell monolayers. (A) Apparent permeability coefficients, P_app, calculated from flux data (A–B) shown as a function of apical gaboxadol concentration. The apical pH was 6.0 and basolateral pH was 7.4. Statistically significant difference from the P_app of 0.34 mmol·L⁻¹ gaboxadol observed by Student's t-test, *P < 0.05. (B) P_app of 3.5 mmol·L⁻¹ gaboxadol at various conditions. P_app of 3.5 mmol·L⁻¹ gaboxadol was measured in absence or presence of 35 mmol·L⁻¹ tryptophan (Trp) at an apical pH of 6.0 and basolateral pH of 7.4 or at pH 7.4 at both sides. Each data point represents the mean ± standard error of the mean obtained in 3–4 independent cell passages. Statistically significant difference observed by Student's t-test: difference from 3.5 mmol·L⁻¹ gaboxadol (A-B), **P < 0.005 or difference from 3.5 mmol·L⁻¹ gaboxadol in presence of tryptophan (Trp; A–B), ##P < 0.005.

Figure 1

Inhibition of apical L-[³H]proline uptake via hPAT1 by 0.015–30 mmol·L⁻¹ gaboxadol (Gbx) and 0.03–40 mmol·L⁻¹ tryptophan (Trp) in Caco-2 cell monolayers at days 25–28 after seeding. The apical concentration of L-[³H]proline was 12.5 nmol·L⁻¹ (1 µCi·mL⁻¹). Uptake of L-proline was measured for 5 min with an apical pH of 6.0 and a basolateral pH of 7.4. Values are means ± standard error of the mean (SEM) of 3–4 independent cell passages. Fifty percent inhibitory concentration (IC₅₀) values (pIC₅₀± SEM) gaboxadol 6.6 mmol·L⁻¹ (2.18 ± 0.08), tryptophan 7.7 mmol·L⁻¹ (2.11 ± 0.08).

Figure 4

Dose-response curves for the maximal plasma concentration (C_max) of gaboxadol after co-administration of an oral solution of 2.5 mg·kg⁻¹ gaboxadol and increasing concentrations of tryptophan. Maximum of the curve is the mean C_max (2502 ng·mL⁻¹) of the control group that was dosed with gaboxadol alone. Each point is determined separately as mean ± standard error of the mean in six dogs.

Figure 3

Plasma concentration versus time profiles after intravenous (insert) or oral dosing of gaboxadol to male beagle dogs during 10 h. The dogs received 2.5 mg·kg⁻¹ gaboxadol intravenously or orally (2.5 mg·kg⁻¹; 3.5 mmol·L⁻¹) with tryptophan. Tryptophan was given as a co-administration together with gaboxadol. Data shown are means ± standard error of the mean (n= 6).

Figure 6

(A) Dose-response curves for the estimated absorption rate constant (k_a) of gaboxadol after co-administration of an oral solution of 2.5 mg·kg⁻¹ gaboxadol with increasing concentrations of tryptophan. The maximum of the fitted curve is the mean k_a at zero tryptophan concentration, 2.87%·h⁻¹. (B) Absorption rate constants, k_a, of gaboxadol and paracetamol in the absence or presence of 150 mg·kg⁻¹ tryptophan. Each point is determined separately as mean ± standard error of the mean in six dogs.

Figure 5

Deconvolution profiles of gaboxadol absorption (A) and paracetamol absorption (B; gastric emptying) following oral administration. (A) The dogs received orally gaboxadol (5.0 mL·kg⁻¹, 2.5 mg·kg⁻¹) together with 0–150 mg·kg⁻¹ tryptophan given as a co-administration. (B) The dogs received 2.5 mg·kg⁻¹ of gaboxadol with 50 mg·kg⁻¹ of paracetamol with or without tryptophan. Data points are mean ± standard error of the mean (n= 6). Significantly different *, P < 0.05 using Student's t-test.