Pharmacodynamics
Gaboxadol acts as a potent and selective GABAA receptor partial agonist. In contrast to GABAA receptor positive allosteric modulators like benzodiazepines, Z drugs, barbiturates, and alcohol, gaboxadol is an agonist of the orthosteric site of the GABAA receptor and the same site that the neurotransmitter γ-aminobutyric acid binds to and activates. Whereas the related GABAA receptor agonist muscimol is a highly potent partial agonist of the GABAA-ρ receptor (GABAC receptor), gaboxadol is a moderately potent antagonist of this receptor. Unlike muscimol, it is not also a GABA reuptake inhibitor to any extent, and it does not inhibit the enzyme GABA transaminase (GABA-T).
The drug shows functional selectivity at the GABAA receptor relative to GABA itself, activating GABAA receptors of different α subunit compositions with varying efficacies. Its EmaxTooltip maximal efficacy values at GABAA receptors were approximately 71% at α1 subunit-containing receptors, 98% at α2 subunit-containing receptors, 54% at α3 subunit-containing receptors, 40% at α4 subunit-containing receptors, 99% at α5 subunit-containing receptors, and 96% at α6 subunit-containing receptors. Moreover, gaboxadol has been found to act as a supra-maximal agonist at α4β3δ subunit-containing GABAA receptors, low-potency agonist at α1β3γ2 subunit-containing receptors, and partial agonist at α4β3γ subunit-containing receptors. Its affinity for extrasynaptic α4β3δ subunit-containing GABAA receptors is 10-fold greater than for other subtypes. Gaboxadol has a unique affinity for extrasynaptic α4β3δ subunit-containing GABAA receptors, which mediate tonic inhibition and are typically activated by ambient, low levels of GABA in the extrasynaptic space. The supra-maximal efficacy of gabaxadol at α4β3δ subunit-containing GABAA receptors has been attributed to an increase in the duration and frequency of channel openings relative to GABA. Mice with the GABAA receptor δ subunit knocked out are unresponsive to the hypnotic effects of gaboxadol. Because of its preferential agonism of extrasynaptic GABAA receptors, gaboxadol has been referred to as a "selective extrasynaptic GABAA agonist" or "SEGA". In contrast to gaboxadol, benzodiazepines and nonbenzodiazepines do not activate δ subunit-containing GABAA receptors. On the other hand, alcohol is known to selectively potentiate δ subunit-containing extrasynaptic GABAA receptors analogously to gaboxadol. In addition, neurosteroids and propofol act on extrasynaptic δ subunit-containing GABAA receptors.
Gaboxadol shows 25- to 40-fold lower potency as a GABAA receptor agonist than muscimol in in vitro studies. Compared to muscimol, gaboxadol binds less potently to α4β3δ subunit-containing GABAA receptors (EC50Tooltip half-maximal effective concentration = 0.2μM vs. 13μM), but is capable of evoking a greater maximum response (EmaxTooltip maximal efficacy = 120% vs. 224%). Although gaboxadol is far less potent than muscimol in vitro, it is only about 3times less potency than muscimol in rodents in vivo. This is attributed mainly to gaboxadol's much greater ability to cross the blood–brain barrier than muscimol. However, it appears to be due to gaboxadol levels being several-fold higher than levels of muscimol with systemic administration of the same doses as well. Gaboxadol is also more selective than muscimol and has been said by Povl Krogsgaard-Larsen to be much less toxic in comparison.
In animals, gaboxadol has been found to produce sedation, hypnotic effects, motor impairment, muscle relaxation, hypolocomotion, anxiolytic-like effects, antidepressant-like effects, analgesic effects, and anticonvulsant effects. In rodent drug discrimination studies, gaboxadol has been found to fully generalize with muscimol. However, gaboxadol, GABAA receptor positive allosteric modulators like benzodiazepines and Z drugs, and the GABA reuptake inhibitor tiagabine all do not generalize between each other, suggesting that their interoceptive effects are different. Similarly, gaboxadol did not generalize with the neurosteroid pregnanolone. On the other hand, gaboxadol has shown partial generalization with the barbiturate pentobarbital. Gaboxadol does not produce self-administration or conditioned place preference in rodents or baboons, suggesting that it lacks rewarding or reinforcing effects and has low addictive potential. This is in contrast to benzodiazepines like diazepam.
Pharmacokinetics
Absorption
The absorption of gaboxadol is rapid and almost complete with oral administration (83–96%). It is a zwitterionic compound and its absorption involves active transport via intestinal transporters such as the proton-coupled amino acid transporter 1 (PAT-1). Coadministration of PAT-1 inhibitors like tryptophan or 5-hydroxytryptophan (5-HTP) has been found to decrease the absorptive permeability of gaboxadol by 53 to 89%. However, they may simply delay the absorption of gaboxadol and decrease peak levels. In contrast to the case of the PAT-1, the drug is not a substrate of the proton-coupled di-/tripeptide transporter (PepT-1). Peak levels of gaboxadol are reached 15 to 60minutes after an oral dose.
Distribution
The distribution of gaboxadol has been studied in rodents. It penetrates the blood–brain barrier and hence is centrally active unlike γ-aminobutyric acid (GABA). The drug enters the brain in amounts that are 30 to 100times higher than those of muscimol given at the same dose in rodents and hence shows greater blood–brain barrier permeability in comparison. In addition, whereas 90% of the muscimol in the brain is in the form of metabolites in rodents, 80% of the gaboxadol in the brain is in unchanged form. It is unknown which transporters are involved in the transport of gaboxadol across the blood–brain barrier or if it simply crosses into the brain via passive diffusion, although the latter may be more likely. The drug is distributed unevenly in the brain in rodents. The plasma protein binding of gaboxadol in humans is very low at less than 2%.
Metabolism
Gaboxadol is metabolized by O-glucuronidation mainly via the enzyme UGT1A9 into gaboxadol-O-glucuronide. To a lesser extent, UGT1A6, UGT1A7, and UGT1A8 also catalyze the formation of this metabolite. Unlike muscimol, gaboxadol is not a substrate for GABA transaminase (GABA-T) and does not undergo metabolic transamination. It is said to be more resistant to metabolism than muscimol. Gaboxadol-O-glucuronide is the only metabolite of gaboxadol formed in significant amounts. Gaboxadol is not metabolized by the cytochrome P450 system.
Elimination
Gaboxadol is excreted in urine (83–94%) mainly unchanged and partially as gaboxadol-O-glucuronide (34%). It is taken up from blood into the kidneys via the organic anion transporter OAT1 (SLC22A6), while the glucuronide is effluxed into urine via the multidrug resistance protein MRP4 (ABCC4). The drug has an elimination half-life in humans of 1.5 to 2.0hours. Twohours following attainment of peak concentrations, levels of gaboxadol are reduced by about 50% in humans. In rodents, the half-life of gaboxadol was about twice as long as that of muscimol. In people with severe renal impairment, circulating levels of gaboxadol were increased by 5-fold, and the renal clearance of gaboxadol was decreased by 34% while that of gaboxadol-O-glucuronide was decreased by 50%.
Gaboxadol can be administered via oral. The route of administration can influence both the onset and intensity of nausea.
