Chemistry:Gaboxadol

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Gaboxadol, also known as 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP) and by its former developmental code names Lu-2-030, MK-0928, and OV101, is a GABAA receptor agonist related to muscimol which was investigated for the treatment of insomnia and other conditions like Angelman syndrome but was never marketed.[1][2][3][4][5] At lower doses, the drug has sedative and hypnotic effects, and at higher doses, it produces hallucinogenic effects.[3][6][5] It is taken orally.[3][1]

The drug acts as a potent and selective partial agonist of the GABAA receptor, the major signaling receptor of the inhibitory endogenous neurotransmitter γ-aminobutyric acid (GABA).[3][2] However, it acts as a preferential supra-maximal agonist at extrasynaptic δ subunit-containing GABAA receptors.[7][2] In contrast to GABAA receptor positive allosteric modulators like benzodiazepines and Z drugs, gaboxadol is an orthosteric agonist of the GABAA receptor, acting on the same site as GABA rather than at an allosteric regulatory site.[8][3][2] As a result, gaboxadol has differing effects from benzodiazepines and related drugs.[8][3][2][7] Gaboxadol is a conformationally constrained synthetic analogue of GABA and of muscimol, an alkaloid and hallucinogen found in Amanita muscaria (fly agaric) mushrooms.[3][2][9][10] It has greatly improved drug-like properties compared to these compounds.[2][9][11][5]

Gaboxadol was first described by Povl Krogsgaard-Larsen and colleagues in 1977.[3][5][12] It was assessed in clinical studies for various uses in the 1980s, but was not found to be useful.[2][7][6][5] In the 1990s and 2000s, gaboxadol was repurposed for treatment of insomnia and completed phase 3 clinical trials for this indication.[3][6][7][13] However, development was discontinued for safety and effectiveness reasons in 2007.[1][14][7][5] Subsequently, gaboxadol was repurposed again for treatment of Angelman syndrome and fragile X syndrome, but development for these uses was discontinued as well.[1][4][15][16]

Use and effects

Gaboxadol produces sedative and hypnotic effects at lower doses and hallucinogenic effects at higher doses.[3][6][5] It has also been reported to produce mood elevation[17] and sometimes euphoria.[6][18]

Hypnotic effects

Gaboxadol has been assessed in clinical studies at doses ranging from 10 to 160 mg.[2][5] It was studied in clinical trials for treatment of insomnia specifically at doses of 5 to 20 mg.[3][8] The drug's effects at a dose of 10 mg were anecdotally described by Povl Krogsgaard-Larsen as similar to having drunk two or three beers.[5] It was found to be limitedly effective for improving sleep at doses of 5 and 10 mg, but was more effective at doses of 15 to 20 mg.[8][7][3][19] Higher doses for insomnia were precluded by a narrow therapeutic index and high rates of psychiatric adverse effects at such doses.[19][20]

Gaboxadol has been found to decrease sleep onset latency, increase sleep duration, increase slow wave sleep (SWS) and slow wave activity (SWA), preserve sleep architecture, not affect REM sleep, and improve subjective sleep quality and daytime functioning.[2][3][21][22] The drug was found to allow people to fall asleep and stay asleep whilst exposed to continuous recorded stream of road traffic noise, a model of transient insomnia.[5][23] Gaboxadol's hypnotic effects have been found to be stronger in women than in men.[21][13] On the other hand, SWS decreases with age, especially in men, and gaboxadol was found to substantially compensate for the reduction in SWS in elderly men.[2][24][5][25] The drug was also studied in experimental sleep restriction and was found to increase SWS and improve daytime functioning, for instance symptoms of sleepiness and fatigue, despite equal total sleep durations.[22][26]

There was no tolerance to the hypnotic effects of gaboxadol after 5 days of repeated administration in animals.[2][27] Similarly, it maintained effectiveness in short-term clinical studies in humans.[13] However, gaboxadol was subsequently found to be initially effective in improving sleep in insomnia but to not maintain its benefits after 1 month.[8][28] In addition, gaboxadol showed mixed effectiveness at the assessed doses of 10 to 15 mg in two large 3-month clinical trials for insomnia.[14][13]

The effects of gaboxadol on sleep differ from those of widely used GABAA receptor positive allosteric modulators like benzodiazepines and Z drugs, which have been found to disrupt rather than enhance SWS and SWA despite improving sleep onset and duration.[11][2][29][9] In addition, unlike such agents, gaboxadol caused no rebound insomnia on discontinuation and produced no next-day residual symptoms.[8][7][30] While dissimilar from GABAA receptor positive allosteric modulators, the effects of gaboxadol on sleep are similar to those of the related GABAA receptor agonist muscimol and of the GABA reuptake inhibitor tiagabine.[11][24][9][2][31]

Although gaboxadol was found to be effective in the treatment of insomnia and uniquely able to improve SWS, it was found to have less robust effects on traditional hypnotic effectiveness measures like sleep onset and duration at the evaluated doses compared to zolpidem.[22][32][33] In addition, it was more effective for improving sleep maintenance than for improving sleep onset.[13]

Gaboxadol was developed for the treatment of insomnia, in which disruption of SWS is not the main feature.[8][32] The effects of gaboxadol in people with sleeping problems specifically involving impaired SWS have largely not been studied and are unknown.[8][33]

Hallucinogenic effects

Gaboxadol was assessed at supratherapeutic doses of 30 to 45 mg and compared to the Z drug zolpidem in drug users during its development for treatment of insomnia.[5][13][18] At these doses, gaboxadol produced euphoria and hallucinogenic effects such as dissociation, perceptual changes, and hallucinations.[5][13][18][19] The rates of such psychiatric adverse effects were 15% with placebo, 38% with 15 mg, 72% with 30 mg, and 88% with 45 mg gaboxadol.[20] It showed less euphoria and misuse potential, more negative and dissociative effects, and fewer sedative effects than zolpidem in these individuals.[18] At a dose of 60 mg twice daily in an early study, gaboxadol was described as producing effects including dizziness, vomiting, somnolence, and strong sedation.[2] High doses of gaboxadol have also been reported to produce delirium, amnesia, and loss of consciousness.[5]

According to journalist and scientist Hamilton Morris, the drug can produce strong hallucinogenic effects at high doses similarly to muscimol, with hallucinogenic effects starting at around doses of 30 or 40 mg and powerful hallucinogenic effects occurring at a dose of about 65 mg of the zwitterion.[34][35][36][37][38] Morris has described hallucinogenic effects he experienced with gaboxadol as follows:[39][5]

"The next night I increased the dose to 35mg sublingually, and it was then that gaboxadol's relationship to muscimol became manifest. In my darkened bedroom I could hear otherworldly music emanating from the motor of a box fan, the white-noise buzzing slowing, taking on the character of an electric viola, the room’s various shadows animated by strange movements, as if cast by a flickering candle — but none of this proved distracting. Once again I fell into an all-consuming slumber."[39][5]

He has also reported other qualitative accounts of the hallucinogenic effects of gaboxadol.[36][37][38] Morris has stated that gaboxadol is every bit as powerful as a hallucinogen as serotonergic psychedelics like ayahuasca, but is qualitatively completely different.[38][37]

Side effects

Side effects of gaboxadol include dizziness, sedation, somnolence, headache, nausea, vomiting, and tachycardia, among others.[2][40][20][41][19][42] It has also been reported to produce giddiness, depersonalization, impaired concentration, and bradycardia.[42] In clinical studies for insomnia, gaboxadol has been found to be generally well-tolerated for up to 12 months.[13] At high doses, it can produce hallucinogenic effects and delirium.[5][13][18][42]

Interactions

Gaboxadol is metabolized exclusively via glucuronidation and is not appreciated metabolized by cytochrome P450 enzymes, and hence would not be expected to interact with cytochrome P450 inhibitors or inducers.[43]

In contrast to the case of γ-aminobutyric acid (GABA) and muscimol, the binding of gaboxadol to the GABAA receptor does not appear to be stimulated by the benzodiazepine and GABAA receptor positive allosteric modulator diazepam in vitro.[9][44] In addition, gaboxadol did not show synergistic effects in combination with alcohol or benzodiazepines in vitro or in vivo in animals.[45][46][47]

Pharmacology

Pharmacodynamics

Gaboxadol acts as a potent and selective GABAA receptor partial agonist.[3][2] 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.[3][2] 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.[9][48] Unlike muscimol, it is not also a GABA reuptake inhibitor to any extent, and it does not inhibit the enzyme GABA transaminase (GABA-T).[49]

The drug shows functional selectivity at the GABAA receptor relative to GABA itself, activating GABAA receptors of different α subunit compositions with varying efficacies.[50][51] Its Emax 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.[50][51] 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.[52][53][54] Its affinity for extrasynaptic α4β3δ subunit-containing GABAA receptors is 10-fold greater than for other subtypes.[55] 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.[56] 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.[54] Mice with the GABAA receptor δ subunit knocked out are unresponsive to the hypnotic effects of gaboxadol.[7][57] Because of its preferential agonism of extrasynaptic GABAA receptors, gaboxadol has been referred to as a "selective extrasynaptic GABAA agonist" or "SEGA".[58][31] In contrast to gaboxadol, benzodiazepines and nonbenzodiazepines do not activate δ subunit-containing GABAA receptors.[7][43] On the other hand, alcohol is known to selectively potentiate δ subunit-containing extrasynaptic GABAA receptors analogously to gaboxadol.[59][60][61] In addition, neurosteroids and propofol act on extrasynaptic δ subunit-containing GABAA receptors.[7][62][2]

Gaboxadol shows 25- to 40-fold lower potency as a GABAA receptor agonist than muscimol in in vitro studies.[63] Compared to muscimol, gaboxadol binds less potently to α4β3δ subunit-containing GABAA receptors (EC50 = 0.2 μM vs. 13 μM), but is capable of evoking a greater maximum response (Emax = 120% vs. 224%).[54] Although gaboxadol is far less potent than muscimol in vitro, it is only about 3 times less potency than muscimol in rodents in vivo.[64][63] This is attributed mainly to gaboxadol's much greater ability to cross the blood–brain barrier than muscimol.[63] 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.[65] Gaboxadol is also more selective than muscimol and has been said by Povl Krogsgaard-Larsen to be much less toxic in comparison.[50][11][66][5]

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.[3][31][49][67] In rodent drug discrimination studies, gaboxadol has been found to fully generalize with muscimol.[31][68] 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.[8][2][31] Similarly, gaboxadol did not generalize with the neurosteroid pregnanolone.[31] On the other hand, gaboxadol has shown partial generalization with the barbiturate pentobarbital.[31] 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.[69][70] This is in contrast to benzodiazepines like diazepam.[69][70]

Pharmacokinetics

Absorption

The absorption of gaboxadol is rapid and almost complete with oral administration (83–96%).[14][71][72][73] 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).[14][74] 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%.[14][75][76] However, they may simply delay the absorption of gaboxadol and decrease peak levels.[14] In contrast to the case of the PAT-1, the drug is not a substrate of the proton-coupled di-/tripeptide transporter (PepT-1).[14] Peak levels of gaboxadol are reached 15 to 60 minutes after an oral dose.[14][43][77]

Distribution

The distribution of gaboxadol has been studied in rodents.[65] It penetrates the blood–brain barrier and hence is centrally active unlike γ-aminobutyric acid (GABA).[2][50][42] The drug enters the brain in amounts that are 30 to 100 times higher than those of muscimol given at the same dose in rodents and hence shows greater blood–brain barrier permeability in comparison.[65] 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.[65] 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.[14][78] The drug is distributed unevenly in the brain in rodents.[65] The plasma protein binding of gaboxadol in humans is very low at less than 2%.[14][71]

Metabolism

Gaboxadol is metabolized by O-glucuronidation mainly via the enzyme UGT1A9 into gaboxadol-O-glucuronide.[14] To a lesser extent, UGT1A6, UGT1A7, and UGT1A8 also catalyze the formation of this metabolite.[71] Unlike muscimol, gaboxadol is not a substrate for GABA transaminase (GABA-T) and does not undergo metabolic transamination.[65] It is said to be more resistant to metabolism than muscimol.[50][66] Gaboxadol-O-glucuronide is the only metabolite of gaboxadol formed in significant amounts.[43] Gaboxadol is not metabolized by the cytochrome P450 system.[43]

Elimination

Gaboxadol is excreted in urine (83–94%) mainly unchanged and partially as gaboxadol-O-glucuronide (34%).[14][71][43][77][73] 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).[14][71] The drug has an elimination half-life in humans of 1.5 to 2.0 hours.[72][42][79] Two hours following attainment of peak concentrations, levels of gaboxadol are reduced by about 50% in humans.[77] In rodents, the half-life of gaboxadol was about twice as long as that of muscimol.[65] 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%.[71]

Chemistry

Gaboxadol, also known by its chemical name 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP), is a conformationally constrained synthetic analogue of the major inhibitory neurotransmitter γ-aminobutyric acid (GABA) and of the Amanita alkaloid muscimol.[6]

Properties

Gaboxadol is a zwitterion, with pKa values of 4.3 (acidic) and 8.3 (basic) and a log P value of –0.61.[50][80] It was formulated pharmaceutically as the hydrochloride salt.[80] The compound's solubility is greater than 30 mg/mL at physiological pH.[80]

Synthesis

The chemical synthesis of gaboxadol has been described.[3][81][82] Its synthesis has been described as tedious, starting with a commercially unavailable precursor, requiring at least 6 synthetic steps, and having very low yields.[5] This has limited the affordability and availability of gaboxadol.[5]

Analogues

Analogues of gaboxadol (THIP) include γ-aminobutyric acid (GABA), muscimol, 4-AHP, thio-THIP, aza-THIP, iso-THIP, THAZ, THPO, piperidine-4-sulfonic acid (P4S), isonipecotic acid, and isoguvacine, among others.[11][24][50][83] Numerous attempts to develop pharmacologically interesting analogues of gaboxadol have failed over the decades.[84] This can be attributed to the very strict structural requirements for GABAA receptor binding and activation.[6] As such, gaboxadol has been described as a unique compound and GABAA receptor agonist.[84]

History

Gaboxadol was first synthesized and described by the Danish chemist Povl Krogsgaard-Larsen in 1977.[3][5][84][12] It was developed via structural modification of muscimol, a constituent of Amanita muscaria mushrooms.[11][9][10] In the early 1980s, the drug was the subject of a series of small pilot clinical studies that evaluated it in the treatment of various medical conditions, but it was not found to be useful.[5]

In 1996, a somnologist named Marike Lancel at the Max Planck Institute for Psychiatry studied the effects of gaboxadol on sleep in rodents and found that it had unique positive effects on sleep, such as increased slow wave sleep.[5][84][45][85] In 1997, Lancel and colleagues published the first clinical study of the effects of gaboxadol on sleep in humans and found similar sleep improvements as in rodents.[2][5][86] Subsequently, gaboxadol underwent formal clinical development for treatment of insomnia by Lundbeck and Merck.[1][5][3][82] It reached phase 3 trials for this indication by at least 2004.[3] The drug was expected to be a blockbuster drug for its pharmaceutical developers.[87][45][19]

In 2007, the development of gaboxadol was terminated by Lundbeck and Merck.[1][19][41] They cited lack of effectiveness in a large 3-month clinical trial, the occurrence of high rates of psychiatric adverse effects at supratherapeutic doses in a misuse liability study with drug users, a frequent incidence of tachycardia at therapeutic doses, and other reasons.[5][19][14][20] Moreover, there was anxiety in the pharmaceutical industry concerning hypnotics at the time owing to bizarre reports of zolpidem (Ambien)-induced delirium that had emerged in the media in 2006.[5] This may have resulted in greater concern about potential liability issues.[5] Merck was also struggling with recent litigation from its drug rofecoxib (Vioxx), which may have made it further averse to liability.[5][45] When presented with the data on the hallucinogenic effects of high doses of gaboxadol, a Merck executive remarked "looks like LSD to me!"[45] A New Drug Application (NDA) was ultimately never submitted to the United States Food and Drug Administration (FDA).[45][41][19] Many of the companies' employees were said to have been surprised and confused by the discontinuation[34] and the decision is still critically debated.[84]

Journalist and scientist Hamilton Morris wrote and published a notable exposé on gaboxadol in Harper's Magazine in 2013, including his self-experimentation with the drug.[4][7][5] According to Morris, the discontinuation of gaboxadol's late-stage development may have deprived people with insomnia access to an effective, safe, and non-addictive treatment.[5] In addition, Morris has critiqued the pharmaceutical industry as being more interested in selling minimally effective drugs devoid of side effects instead of medications with real therapeutic effects but a higher risk of litigation.[5]

In 2015, Lundbeck sold its rights to the molecule to Ovid Therapeutics, whose plan was to develop it for Angelman syndrome (AS) and fragile X syndrome (FXS).[1][88] It was known internally at Ovid Therapeutics under the developmental code name OV101.[1] In 2021, development of gaboxadol for Angelman syndrome and fragile X syndrome was discontinued due to lack of effectiveness.[1][16][89]

Society and culture

Names

Gaboxadol is the generic name of the drug and its INN and USAN.[3][90] It is also known by its former developmental code names Lu-2-030 or Lu-02-030 (Lundbeck), MK-0928 (Merck), and OV101 (Ovid Therapeutics).[3][1] In addition, gaboxadol is well-known in the scientific literature by its chemical name 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP).[6][49]

Media coverage

Gaboxadol was covered, along with muscimol and Amanita muscaria, in an episode of Hamilton Morris's Hamilton's Pharmacopeia.[35][91]

Notable individuals

Povl Krogsgaard-Larsen and Hamilton Morris have both self-experimented with gaboxadol.[7][5][35][36][37] Morris has described gaboxadol as the "perfect hypnotic" and as the "best hypnotic" he'd ever tried, but also found that it produced strong hallucinogenic effects at high doses.[7][34][5][35][36][37]

Grey market use

Gaboxadol has been obtained rarely from the grey market, for instance from China, for hypnotic and hallucinogenic purposes.[5][34][35][36][37]

The closely related GABAA receptor agonist muscimol, found in Amanita muscaria mushrooms, has been reported to induce sleep in humans similarly to gaboxadol, in addition to its well-known hallucinogenic effects that occur at higher doses.[10][92] While gaboxadol was never approved for medical use, informal microdosing of muscimol and Amanita mushrooms for improvement of sleep has become increasingly prevalent by the mid-2020s.[10][93][94] However, muscimol is far less-researched compared to gaboxadol,[10] and is less selective and said to be much more toxic in comparison.[50][11][66][5] In addition, Amanita mushrooms contain other pharmacologically active compounds besides muscimol, such as the glutamate receptor agonist and neurotoxin ibotenic acid and the muscarinic acetylcholine receptor agonist and parasympathomimetic muscarine, which are liable to pose toxicity risks as well.[95][35] Povl Krogsgaard-Larsen has warned about safety concerns with regard to medicinal use of Amanita mushrooms.[35]

Gaboxadol is not a controlled substance anywhere in the world as of October 10, 2025.[45][19][20]

Research

Gaboxadol was studied in the 1980s by Lundbeck and others in the treatment of a variety of medical conditions,[84][2][7][6][5] including pain,[96] anxiety,[42] mania,[97] schizophrenia and tardive dyskinesia,[98][99] epilepsy,[100] Huntington's disease,[101] and Alzheimer's disease.[102] It showed poor clinical effectiveness as an anticonvulsant, in accordance with prior animal studies.[84][100] In addition, it had only weak anxiolytic effects in humans and at doses that were accompanied by substantial side effects.[84][42] On the other hand, gaboxadol was found to be an effective analgesic in some patients and was equipotent to morphine in these individuals.[84] Moreover, it lacked the respiratory depression and other characteristic adverse effects of morphine.[84] However, gaboxadol was ultimately not further developed due to its pronounced sedative and other side effects.[84][42]

Later on, in the 1990s and 2000s, gaboxadol was developed for the treatment of insomnia and reached phase 3 clinical trials for this indication.[2][3][86][5] However, development was discontinued in 2007 for safety and effectiveness reasons.[1][14][7][5] Multiple large phase 3 trials were completed and published.[7][28][13][20] As a result, gaboxadol was not approved and will likely never be used as a hypnotic commercially.[7] There has been some further study of gaboxadol as a hypnotic by David Nutt and colleagues after the discontinuation of its development.[103][104] The drug was also studied for treatment of major depressive disorder in combination with escitalopram in a phase 2 trial, but was ineffective.[105][5][106]

Following discontinuation of its development for insomnia, gaboxadol was repurposed by Ovid Therapeutics for treatment of the Angelman syndrome and fragile X syndrome.[1][4][107][108][109] It reached phase 3 and phase 2 clinical trials for these conditions, respectively.[1][109][107] However, development was discontinued for these uses as well in 2021.[1][16][89] The drug is no longer under development for any indication.[1]

See also

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 "Gaboxadol - Lundbeck A/S". 15 March 2023. https://adisinsight.springer.com/drugs/800014543. 
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 2.22 2.23 "Gaboxadol--a new awakening in sleep". Current Opinion in Pharmacology 6 (1): 30–36. February 2006. doi:10.1016/j.coph.2005.10.004. PMID 16368265. 
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21 3.22 "Gaboxadol". Drugs of the Future 29 (5): 0449. 2004. doi:10.1358/dof.2004.029.05.803754. http://access.portico.org/stable?au=pjbf78x8shw. Retrieved 19 June 2025. 
  4. ↑ 4.0 4.1 4.2 4.3 "Modulation of GABA A receptor function and sleep". Current Opinion in Physiology 2: 51–57. 2018. doi:10.1016/j.cophys.2017.12.011. http://spiral.imperial.ac.uk/bitstream/10044/1/55616/2/Brickley%20et%20al.Curr%20Opin%20Physiol%20revised.Dec%202017.pdf. Retrieved 5 October 2025. 
  5. ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17 5.18 5.19 5.20 5.21 5.22 5.23 5.24 5.25 5.26 5.27 5.28 5.29 5.30 5.31 5.32 5.33 5.34 5.35 5.36 5.37 5.38 5.39 "Gaboxadol". Harper's Magazine. August 2013. http://harpers.org/archive/2013/08/gaboxadol/. Retrieved 2014-11-20. 
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 "GABAA Agonists and Partial Agonists: THIP (Gaboxadol) as a Non-Opioid Analgesic and a Novel Type of Hypnotic1". GABA(A) agonists and partial agonists: THIP (Gaboxadol) as a non-opioid analgesic and a novel type of hypnotic. Adv Pharmacol. 54. 2006. pp. 53–71. doi:10.1016/s1054-3589(06)54003-7. ISBN 978-0-12-032957-1. "In cancer patients and also in patients with chronic anxiety (Hoehn‐Saric, 1983) the desired effects of Gaboxadol were accompanied by side effects, notably sedation, nausea, and in a few cases euphoria. The side effects of Gaboxadol have, however, been described as mild and similar in quality to those of other GABA‐mimetics (Hoehn‐Saric, 1983). This combination of analgesic and anxiolytic effects of THIP obviously has therapeutic prospects. [...]" 
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 7.12 7.13 7.14 7.15 7.16 "GABA Receptors and the Pharmacology of Sleep". Handbook of Experimental Pharmacology 253: 279–304. 2019. doi:10.1007/164_2017_56. ISBN 978-3-030-11270-7. PMID 28993837. 
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 "Development of Subtype-Selective GABAA Receptor Compounds for the Treatment of Anxiety, Sleep Disorders and Epilepsy". GABA and Sleep. Basel: Springer Basel. 2010. pp. 25–72. doi:10.1007/978-3-0346-0226-6_2. ISBN 978-3-0346-0225-9. http://link.springer.com/10.1007/978-3-0346-0226-6_2. Retrieved 4 October 2025. 
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 "Muscimol as an ionotropic GABA receptor agonist". Neurochemical Research 39 (10): 1942–1947. October 2014. doi:10.1007/s11064-014-1245-y. PMID 24473816. "The effects of THIP on sleep resembled those reported earlier for muscimol and were dissimilar from those induced by benzodiazepine modulators of GABAA receptors [45].". 
  10. ↑ 10.0 10.1 10.2 10.3 10.4 "Classics in Chemical Neuroscience: Muscimol". ACS Chemical Neuroscience 15 (18): 3257–3269. September 2024. doi:10.1021/acschemneuro.4c00304. PMID 39254100. 
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 "GABA(A) agonists and partial agonists: THIP (Gaboxadol) as a non-opioid analgesic and a novel type of hypnotic". Biochemical Pharmacology 68 (8): 1573–1580. October 2004. doi:10.1016/j.bcp.2004.06.040. PMID 15451401. "Similar [sleep] results [relative to gaboxadol] have been obtained with muscimol, with the GABA uptake inhibitor Tiagabine [65], and with the glia-selective GABA uptake inhibitor, THPO (Fig. 2) [66] [...]". 
  12. ↑ 12.0 12.1 "A new class of GABA agonist". Nature 268 (5615): 53–55. July 1977. doi:10.1038/268053a0. PMID 196200. Bibcode: 1977Natur.268...53K. 
  13. ↑ 13.00 13.01 13.02 13.03 13.04 13.05 13.06 13.07 13.08 13.09 "Effect of gaboxadol on patient-reported measures of sleep and waking function in patients with Primary Insomnia: results from two randomized, controlled, 3-month studies". Journal of Clinical Sleep Medicine 6 (1): 30–39. February 2010. doi:10.5664/jcsm.27707. PMID 20191935. "Gaboxadol is no longer in clinical development for the treatment of insomnia based on an assessment of its overall clinical profile in phase 3 trials, including those reported here, which suggested limited or variable efficacy, and also the occurrence of psychiatric side effects at supra-therapeutic doses in an abuse liability study involving drug abusers.11,12 [...] 11. Lundbeck. Discontinuation of development program for gaboxadol in insomnia. H. Lundbeck website. [...] March 27, 2007. Accessed May 26, 2009. 12. Schoedel KA, Rosen LB, Alexander R, et al. A single-dose randomized, double-blind, crossover abuse liability study to evaluate the subjective and objective effects of gaboxadol and zolpidem in recreational drug users. Clin Pharmacol Ther 2009; 85 (Suppl 1):S22. Abstract PI-44.". 
  14. ↑ 14.00 14.01 14.02 14.03 14.04 14.05 14.06 14.07 14.08 14.09 14.10 14.11 14.12 14.13 14.14 "Potential involvement of the proton-coupled amino acid transporter PAT1 (SLC36A1) in the delivery of pharmaceutical agents". Journal of Drug Delivery Science and Technology 23 (4): 293–306. 2013. doi:10.1016/S1773-2247(13)50046-3. https://linkinghub.elsevier.com/retrieve/pii/S1773224713500463. Retrieved 4 October 2025. "Gaboxadol is a bicyclic analogue of the neurotransmitter GABA. Pharmacologically, gaboxadol acts as a selective extra-synaptic GABAA receptor agonists (SEGA) and was the first compound identified in a novel class of sleep agents [68]. The drug development of gaboxadol, with the indication for treatment of primary insomnia, was discontinued in 2007, partly due to the lack of efficacy observed in a large 3-month efficacy and safety study conducted in the United States [65], and partly due to the occurrence of psychiatric side effect at supra-therapeutic doses in an abuse liability study involving drug abusers [69, 70]. Early preclinical studies in rat, mouse, and human have shown that the absorption of gaboxadol is fast and almost complete (84-96 %) [71, 72]. As gaboxadol is a zwitterionic compound with pKa values of 4.31 and 8.13 [46] and a logDpH 7.4 value of -2.37 (unpublished data), the physicochemical data of the compound indicates that the intestinal transport may require the action of one or more membrane transporters. Also, the plasma protein binding of gaboxadol is low (< 15 %) in rodents [73] and less than 2 % in humans [74].". 
  15. ↑ "Emerging Therapies and challenges for individuals with Angelman syndrome". Current Opinion in Psychiatry 34 (2): 123–128. March 2021. doi:10.1097/YCO.0000000000000674. PMID 33395098. 
  16. ↑ 16.0 16.1 16.2 "Final trial data support Ovid's decision to stop OV101 program in...". 23 August 2023. https://angelmansyndromenews.com/news/final-trial-data-support-ovid-decision-stop-ov101-clinical/. 
  17. ↑ "The potential use of GABA agonists in psychiatric disorders: evidence from studies with progabide in animal models and clinical trials". Pharmacology, Biochemistry, and Behavior 18 (6): 957–966. June 1983. doi:10.1016/s0091-3057(83)80021-5. PMID 6351106. "Recently interest has taken hold on the possibility that GABA systems may play a role in affective disorders. The major impetus for this effort has been the demonstration that one GABA agonist (progabide) has antidepressant qualities (see below) and that another GABA agonist (THIP) is mood elevating (Krogsgaard-Larsen, personal communication).". 
  18. ↑ 18.0 18.1 18.2 18.3 18.4 "Poster Session I (PI 1-89): PI-44: A single-dose randomized, double-blind, crossover abuse liability study to evaluate the subjective and objective effects of gaboxadol and zolpidem in recreational drug users". Clinical Pharmacology & Therapeutics 85 (S1 [Supplement: Abstracts of the 2009 Annual Meeting of the American Society for Clinical Pharmacology and Therapeutics. National Harbor, Maryland, USA. March 18–21, 2009]): S9–S36 (S22–S22). 16 January 2009. doi:10.1038/sj.clpt.2008.283. ISSN 0009-9236. 
  19. ↑ 19.0 19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 "Merck, Lundbeck scrap insomnia drug after trials". 9 August 2007. https://www.reuters.com/article/business/merck-lundbeck-scrap-insomnia-drug-after-trials-idUSN28285490/. 
  20. ↑ 20.0 20.1 20.2 20.3 20.4 20.5 "Discontinuation of development program for gaboxadol in insomnia: Teleconference 28 March 2007". 28 March 2007. https://www.lundbeck.com/investor/Presentations/Teleconference/Teleconference_gaboxadol_20070328.pdf. 
  21. ↑ 21.0 21.1 "Sleep Physiology, Circadian Rhythms, Waking Performance and the Development of Sleep-Wake Therapeutics". Handbook of Experimental Pharmacology 253: 441–481. 2019. doi:10.1007/164_2019_243. ISBN 978-3-030-11270-7. PMID 31254050. "Agonists of the extra-synaptic GABAA receptor such as gaboxadol, also known as THIP, reliably induce SWS and SWA in healthy participants at baseline, in a model of transient insomnia (traffic noise, Dijk et al. 2012), a model of sleep onset insomnia (Mathias et al. 2001), a circadian phase advance model (Walsh et al. 2007), older participants (Lancel et al. 2001) and insomnia patients (Lankford et al. 2008). Interestingly, the effects of gaboxadol on sleep are much stronger in women than in men (Dijk et al. 2010b; Ma et al. 2011; Roth et al. 2010).". 
  22. ↑ 22.0 22.1 22.2 "Enhancement of slow wave sleep: implications for insomnia". Journal of Clinical Sleep Medicine 5 (2 Suppl): S27–S32. April 2009. doi:10.5664/jcsm.5.2S.S27. PMID 19998872. 
  23. ↑ "Enhanced slow wave sleep and improved sleep maintenance after gaboxadol administration during seven nights of exposure to a traffic noise model of transient insomnia". Journal of Psychopharmacology 26 (8): 1096–1107. August 2012. doi:10.1177/0269881111421971. PMID 22002961. 
  24. ↑ 24.0 24.1 24.2 "Specific GABA(A) agonists and partial agonists". Chemical Record 2 (6): 419–430. 2002. doi:10.1002/tcr.10040. PMID 12469353. 
  25. ↑ "Effect of repeated gaboxadol administration on night sleep and next-day performance in healthy elderly subjects". Neuropsychopharmacology 30 (4): 833–841. April 2005. doi:10.1038/sj.npp.1300641. PMID 15602499. 
  26. ↑ "Slow wave sleep enhancement with gaboxadol reduces daytime sleepiness during sleep restriction". Sleep 31 (5): 659–672. May 2008. doi:10.1093/sleep/31.5.659. PMID 18517036. 
  27. ↑ "gamma-aminobutyric Acid(A) (GABA(A)) agonist 4,5,6, 7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol persistently increases sleep maintenance and intensity during chronic administration to rats". The Journal of Pharmacology and Experimental Therapeutics 293 (3): 1084–1090. June 2000. doi:10.1016/S0022-3565(24)39335-8. PMID 10869413. 
  28. ↑ 28.0 28.1 "Effect of gaboxadol on sleep in adult and elderly patients with primary insomnia: results from two randomized, placebo-controlled, 30-night polysomnography studies". Sleep 31 (10): 1359–1370. October 2008. PMID 18853933. 
  29. ↑ "Slow-wave sleep deficiency and enhancement: implications for insomnia and its management". The World Journal of Biological Psychiatry 11 (Suppl 1): 22–28. June 2010. doi:10.3109/15622971003637645. PMID 20509829. 
  30. ↑ "Next-day residual effects of gaboxadol and flurazepam administered at bedtime: a randomized double-blind study in healthy elderly subjects". Human Psychopharmacology 24 (1): 61–71. January 2009. doi:10.1002/hup.986. PMID 18985628. 
  31. ↑ 31.0 31.1 31.2 31.3 31.4 31.5 31.6 "Gaboxadol, a selective extrasynaptic GABA(A) agonist, does not generalise to other sleep-enhancing drugs: a rat drug discrimination study". Neuropharmacology 52 (3): 844–853. March 2007. doi:10.1016/j.neuropharm.2006.10.009. PMID 17196996. "In studies from other laboratories, gaboxadol (5.6 mg/kg i.p. training dose) did not generalise to midazolam (Ator, 1991), and rats trained to discriminate lorazepam (1 mg/kg i.p.), midazolam (0.4 mg/kg s.c.) or diazepam (2.5 mg/kg i.p.) from vehicle did not generalise to gaboxadol (Nielsen et al., 1983; Ator and Griffiths, 1986; Rauch and Stolerman, 1987). Gaboxadol showed partial generalisation to pentobarbital (5 or 10 mg/kg i.p. training dose) in two studies (Ator and Griffiths, 1986; Grech and Balster, 1993). The only compound to which gaboxadol has fully generalised is the GABAA agonist, muscimol (1 mg/kg i.p. training dose; Grech and Balster, 1997; Jones and Balster, 1998). [...] For example, zolpidem, indiplon, RS-zopiclone and S-zopiclone were all reported to enhance sleep onset and increase the total duration of sleep (Nakajima et al., 2000; Zammit et al., 2004; Swainston Harrison and Keating, 2005; Thomson Scientific, 2006). Gaboxadol did not affect sleep onset and had no effect on rapid eye movement (REM) sleep, but increased the total duration of slow-wave sleep in rats (Lancel and Faulhaber, 1996), which resembled the changes it induces in human sleep (Faulhaber et al., 1997). Muscimol had similar effects to gaboxadol on sleep in rats, although it also increased REM sleep (Lancel et al., 1996).". 
  32. ↑ 32.0 32.1 "Discontinued drugs 2007: central and peripheral nervous system drugs". Expert Opinion on Investigational Drugs 18 (2): 109–123. February 2009. doi:10.1517/13543780802687371. PMID 19236259. 
  33. ↑ 33.0 33.1 "Emerging anti-insomnia drugs: tackling sleeplessness and the quality of wake time". Nature Reviews. Drug Discovery 7 (6): 530–540. June 2008. doi:10.1038/nrd2464. PMID 18511929. 
  34. ↑ 34.0 34.1 34.2 34.3 "Speaking with Psychonaut Hamilton Morris about sleep". 7 August 2013. https://www.nydailynews.com/2013/08/07/speaking-with-psychonaut-hamilton-morris-about-sleep/. 
  35. ↑ 35.0 35.1 35.2 35.3 35.4 35.5 35.6 Morris H (9 January 2018). "A Fungal Fairy Tale". Hamilton's Pharmacopeia. Season 2. Episode 7. Vice Media. Viceland.
  36. ↑ 36.0 36.1 36.2 36.3 36.4 Hamilton Morris (25 December 2021). "PODCAST 36: An Amanita Christmas with Dr. Povl Krogsgaard-Larsen". The Hamilton Morris Podcast (Podcast). Patreon. Event occurs at ~48:00, ~2:07:30. Retrieved 14 February 2025. [Morris:] [...] they did produce enough [gaboxadol] [...] to conduct a number of self-experiments, some at very high doses. He experienced extremely dramatic psychedelic effects at those high doses. [...] I have a written report—I mentioned that I had a friend [...] [a]nd he took a very, very large dose of [gaboxadol] [...] it was 63 mg of the zwitterion. [...] It's you know very, very dramatic hallucinogenic effects. He describes his entire reality being fragmented. [...]
  37. ↑ 37.0 37.1 37.2 37.3 37.4 37.5 Hamilton Morris (29 December 2022). "POD 65: Dr. Andrew Gallimore on DMTx and Reality Switch Technologies". The Hamilton Morris Podcast (Podcast). Patreon. Event occurs at 1:02:16–1:04:33. Retrieved 21 March 2025. [Morris:] I've used high doses of gaboxadol and that is as psychedelic as anything else. It's different, of course. It's a different type of experience entirely. But that same sort of proliferation of ideas and perceptual disturbances is very much present. It is not in any way analogous to a benzodiazepine. It's something that is visionary and completely alien and strange. [...] It's worth trying. Not because it's enjoyable or good. It's also not bad either. [...] the most intense gaboxadol experience of my life was in Japan. [...] [I was like] at night I'm gonna take gaboxadol at a high dose to knock myself out. And I'd taken gaboxadol at lower doses many many times before and I'd had mild effects from it. [...] [Due to jet lag] I was much more awake and alert and unintentionally had one of the most intense psychedelic experiences of my life taking this stuff that I was thinking was just going to knock me out. [...] I spent the entire night in this visionary state trying to figure out a way to lose consciousness but instead I was hyperconscious [...]
  38. ↑ 38.0 38.1 38.2 Joe Rogan (26 June 2018). "Joe Rogan Experience #1136". YouTube (Podcast). The Joe Rogan Experience. Archived from the original on 26 June 2018. [Morris:] [Gaboxadol is] every bit as powerful as ayahuasca or something like that, but completely different. [...] it's something that's been experienced by relatively few people, so you don't even have this spiritual or metaphorical vocabulary for it. [...] So I took a high dose I believe it was 45 [mg], but don't quote me on that, of [gaboxadol]. [...] And it was unbelievable. I mean, I couldn't fathom the intensity of what I experienced. It was, you know, just this rushing sense of becoming a passive observer in my own consciousness and seeing all of my thoughts produced by someone else that were racing at a speed that was so fast that I found it physically dizzying and had to lay down. And I felt as if the acceleration was pushing me toward an ultimate state that was sleep and that sleep and death represented the ultimate state of consciousness. [...] [I had this] transformative existential trip accidentally [...] [Rogan:] Did you ever recreate that kind of experience? [Morris:] No, because it was, it was a bit much, I would say. And I know people that have taken even more, and it turns into just your entire visual field transforming into rotating cubes where each face of the cube represents a different aspect of your life, your future, your past, your present, you know, really dramatic stuff. [...]
  39. ↑ 39.0 39.1 "Writers Go in Search of a Good Night's Sleep, by Harper's Magazine". 23 July 2013. https://harpers.org/2013/07/writers-go-in-search-of-a-good-nights-sleep/. "“I heard about gaboxadol and decided I had to try it,” writes Hamilton Morris of a rare chemical remedy for insomnia that, though it is nowhere near being approved by the U.S. Food and Drug Administration, could be an improvement over Ambien, Valium, and Xanax. But what really appeals to Morris is the hallucinogenic delirium gaboxadol is said to induce. The intrepid reporter scores some: “In my darkened bedroom,” he writes, “I could hear otherworldly music emanating from the motor of a box fan, the white-noise buzzing slowing, taking on the character of an electric viola, the room’s various shadows animated by strange movements as if cast by a flickering candle—but none of this proved distracting.” Morris finds that gaboxadol is indeed the perfect hypnotic." 
  40. ↑ "Adverse events of pharmacological interventions for insomnia disorder in adults: a systematic review and network meta-analysis". Frontiers in Psychiatry 16. 2025. doi:10.3389/fpsyt.2025.1461166. PMID 40704033. 
  41. ↑ 41.0 41.1 41.2 "Merck & Co and Lundbeck's sleep drug terminated in Phase III". 29 March 2007. https://pharmatimes.com/news/merck_and_co_and_lundbecks_sleep_drug_terminated_in_phase_iii_989624/. "The firms said they are discontinuing studies of the because data from recently-completed Phase III studies suggest that the overall clinical profile for gaboxadol in insomnia does not support further development. As a result of this new information, Merck and Lundbeck added that they will not file gaboxadol with the US Food and Drug Administration, or any other regulatory agencies worldwide, and are terminating the project. [...] “new safety data showed a dramatic increase in psychiatric adverse events at doses as low as twice the recommended dose, raising the possibility of real safety issues in sleep-drug abusers.” Although earlier trials showed effectiveness in sleep onset and maintenance, the drug failed to do either in the latest trials, Mr Tooley wrote, noting that a recent sleep lab study failed to show sufficient effects at lower doses." 
  42. ↑ 42.0 42.1 42.2 42.3 42.4 42.5 42.6 42.7 "Effects of THIP on chronic anxiety". Psychopharmacology 80 (4): 338–341. 1983. doi:10.1007/BF00432116. PMID 6414002. "THIP, a 4,5,6,7-tetrahydroisoxazolo(5,4-C)pyridin-3-ol, is a muscimol analog which exhibits specific GABA-agonists properties without affecting enzymes involved in the synthesis or the catabolism of the neurotransmitter. It is 5–15-times weaker than muscimol and substantially less toxic. THIP penetrates the blood–brain barrier and has a half-life of 1.5–2 h (H Lundbeck and Company 1981).". 
  43. ↑ 43.0 43.1 43.2 43.3 43.4 43.5 "Effect of short-term treatment with gaboxadol on sleep maintenance and initiation in patients with primary insomnia". Sleep 30 (3): 281–287. March 2007. doi:10.1093/sleep/30.3.281. PMID 17425224. "When given orally in healthy subjects, gaboxadol is rapidly absorbed (tmax of 30-60 min) and eliminated (t½ of 1.5 h). More than 95% of the dose is excreted in the urine, mostly unchanged. A glucoronide conjugate is the only metabolite formed in significant amounts. Hence the CYP450 system does not have significant involvement in the metabolism of gaboxadol.". 
  44. ↑ "Diazepam stimulates the binding of GABA and muscimol but not THIP to rat brain membranes". Neuroscience Letters 38 (3): 315–320. August 1983. doi:10.1016/0304-3940(83)90388-9. PMID 6314189. 
  45. ↑ 45.0 45.1 45.2 45.3 45.4 45.5 45.6 Drugged: The Science and Culture Behind Psychotropic Drugs. EBL ebooks online. Oxford University Press. 2013. ISBN 978-0-19-995798-9. https://books.google.com/books?id=k808BAAAQBAJ. Retrieved 5 October 2025. 
  46. ↑ "Gaboxadol: in vitro interaction studies with benzodiazepines and ethanol suggest functional selectivity". European Journal of Pharmacology 467 (1–3): 49–56. April 2003. doi:10.1016/s0014-2999(03)01603-0. PMID 12706454. 
  47. ↑ "Rotarod studies in the rat of the GABAA receptor agonist gaboxadol: lack of ethanol potentiation and benzodiazepine cross-tolerance". European Journal of Pharmacology 482 (1–3): 215–222. December 2003. doi:10.1016/j.ejphar.2003.10.007. PMID 14660025. 
  48. ↑ "GABA(A) receptor channel pharmacology". Current Pharmaceutical Design 11 (15): 1867–1885. 2005. doi:10.2174/1381612054021024. PMID 15974965. 
  49. ↑ 49.0 49.1 49.2 "Pharmacodynamic effects and possible therapeutic uses of THIP, a specific GABA-agonist". Pharmaceutisch Weekblad. Scientific Edition 4 (5): 145–153. October 1982. doi:10.1007/BF01959034. PMID 6292818. 
  50. ↑ 50.0 50.1 50.2 50.3 50.4 50.5 50.6 50.7 "GABA(A) receptor ligands and their therapeutic potentials". Current Topics in Medicinal Chemistry 2 (8): 817–832. August 2002. doi:10.2174/1568026023393525. PMID 12171573. "The fact that muscimol is a non-specific GABAA receptor agonist [38, 39], a substrate for the GABA-metabolizing enzyme, GABA transaminase [40], and moreover a neurotoxin, makes the compound therapeutically less valuable. [...] Further conformational restriction of the GABA structural element in muscimol has been achieved by incorporating the amino group into a piperidine ring leading to the bicyclic analogue, THIP, a specific GABAA agonist [11]. THIP has been shown to be devoid of the neurotoxic properties of muscimol and, in contrast to muscimol, is metabolically stable.". 
  51. ↑ 51.0 51.1 "Bioisosteric determinants for subtype selectivity of ligands for heteromeric GABA(A) receptors". Bioorganic & Medicinal Chemistry Letters 11 (12): 1573–1577. June 2001. doi:10.1016/s0960-894x(01)00184-6. PMID 11412984. 
  52. ↑ "Pharmacological characterization of a novel cell line expressing human alpha(4)beta(3)delta GABA(A) receptors". British Journal of Pharmacology 136 (7): 965–974. August 2002. doi:10.1038/sj.bjp.0704795. PMID 12145096. 
  53. ↑ "Extrasynaptic GABAA Receptors Are Critical Targets for Sedative-Hypnotic Drugs" (in en). Journal of Clinical Sleep Medicine 02 (2). 2006-04-15. doi:10.5664/jcsm.26526. ISSN 1550-9389. 
  54. ↑ 54.0 54.1 54.2 "Muscimol as an ionotropic GABA receptor agonist". Neurochemical Research 39 (10): 1942–1947. October 2014. doi:10.1007/s11064-014-1245-y. PMID 24473816. 
  55. ↑ "Beyond classical benzodiazepines: novel therapeutic potential of GABAA receptor subtypes". Nature Reviews. Drug Discovery 10 (9): 685–697. July 2011. doi:10.1038/nrd3502. PMID 21799515. 
  56. ↑ "Distinct activities of GABA agonists at synaptic- and extrasynaptic-type GABAA receptors". The Journal of Physiology 588 (Pt 8): 1251–1268. April 2010. doi:10.1113/jphysiol.2009.182444. PMID 20176630. 
  57. ↑ "The EEG effects of THIP (Gaboxadol) on sleep and waking are mediated by the GABA(A)delta-subunit-containing receptors". The European Journal of Neuroscience 25 (6): 1893–1899. March 2007. doi:10.1111/j.1460-9568.2007.05455.x. PMID 17408425. 
  58. ↑ "Efficacy of the selective extrasynaptic GABA A agonist, gaboxadol, in a model of transient insomnia: a randomized, controlled clinical trial". Sleep Medicine 9 (4): 393–402. May 2008. doi:10.1016/j.sleep.2007.06.006. PMID 17765013. 
  59. ↑ "GABA(A) receptors and alcohol". Pharmacology, Biochemistry, and Behavior 90 (1): 90–94. July 2008. doi:10.1016/j.pbb.2008.03.006. PMID 18423561. 
  60. ↑ "Ethanol acts directly on extrasynaptic subtypes of GABAA receptors to increase tonic inhibition". Alcohol 41 (3): 211–221. May 2007. doi:10.1016/j.alcohol.2007.04.011. PMID 17591544. 
  61. ↑ "Physiology and pharmacology of alcohol: the imidazobenzodiazepine alcohol antagonist site on subtypes of GABAA receptors as an opportunity for drug development?". British Journal of Pharmacology 154 (2): 288–298. May 2008. doi:10.1038/bjp.2008.32. PMID 18278063. 
  62. ↑ "Are extrasynaptic GABAA receptors important targets for sedative/hypnotic drugs?". The Journal of Neuroscience 32 (11): 3887–3897. March 2012. doi:10.1523/JNEUROSCI.5406-11.2012. PMID 22423109. 
  63. ↑ 63.0 63.1 63.2 "GABAergic actions of THIP in vivo and vitro: a comparison with muscimol and GABA". European Journal of Pharmacology 65 (1): 21–29. July 1980. doi:10.1016/0014-2999(80)90204-6. PMID 7398775. "The magnitude of the differences between drug potencies in iontophoretic studies closely paralleled their relative potencies in binding studies, with muscimol approximately 3 times more potent than GABA and 25-40 times more potent than THIP. After systemic (i.v.) administration, however, muscimot was only 3 times more potent than THIP in inhibiting reticulata cell firing, possibly because THIP passes the blood-brain barrier more readily.". 
  64. ↑ "GABA-A agonists and GABA uptake inhibitors: Structure-activity relationships". Drug Development Research 21 (3): 169–188. 1990. doi:10.1002/ddr.430210304. ISSN 0272-4391. https://onlinelibrary.wiley.com/doi/10.1002/ddr.430210304. Retrieved 4 October 2025. "The anticonvulsant effects of THIP and muscimol have been compared in a variety of animal models. THIP typically is two to five times weaker than muscimol in suppressing seizure activities.". 
  65. ↑ 65.0 65.1 65.2 65.3 65.4 65.5 65.6 "Relative disposition of the GABA agonists THIP and muscimol in the brain of the rat". The Journal of Pharmacy and Pharmacology 34 (10): 676–678. October 1982. doi:10.1111/j.2042-7158.1982.tb04702.x. PMID 6128395. 
  66. ↑ 66.0 66.1 66.2 "Muscimol, a psychoactive constituent of Amanita muscaria, as a medicinal chemical model structure". Acta Chemica Scandinavica. Series B 35 (5): 311–324. 1981. doi:10.3891/acta.chem.scand.35b-0311. PMID 6274117. 
  67. ↑ "Synergistic antidepressant-like action of gaboxadol and escitalopram". European Neuropsychopharmacology 22 (10): 751–760. October 2012. doi:10.1016/j.euroneuro.2012.02.001. PMID 22406239. 
  68. ↑ "The discriminative stimulus effects of muscimol in rats". Psychopharmacology 129 (4): 339–347. February 1997. PMID 9085403. 
  69. ↑ 69.0 69.1 "GABAA receptor drugs and neuronal plasticity in reward and aversion: focus on the ventral tegmental area". Frontiers in Pharmacology 5: 256. 2014. doi:10.3389/fphar.2014.00256. PMID 25505414. 
  70. ↑ 70.0 70.1 "GABA site agonist gaboxadol induces addiction-predicting persistent changes in ventral tegmental area dopamine neurons but is not rewarding in mice or baboons". The Journal of Neuroscience 32 (15): 5310–5320. April 2012. doi:10.1523/JNEUROSCI.4697-11.2012. PMID 22496576. 
  71. ↑ 71.0 71.1 71.2 71.3 71.4 71.5 "Metabolism and renal elimination of gaboxadol in humans: role of UDP-glucuronosyltransferases and transporters". Pharmaceutical Research 26 (2): 459–468. February 2009. doi:10.1007/s11095-008-9799-5. PMID 19082692. 
  72. ↑ 72.0 72.1 Lund, J., Helboe, T., & Mengel, H. (2006, January). Absorption, metabolism and excretion profile of gaboxadol in humans. In Sleep (Vol. 29, pp. A41-A41). https://scholar.google.com/scholar?cluster=17960150700023416661
  73. ↑ 73.0 73.1 "Preliminary studies on the absorption, distribution, metabolism, and excretion of THIP in animal and man using 14C-labelled compound". Acta Pharmacologica et Toxicologica 49 (2): 116–124. August 1981. doi:10.1111/j.1600-0773.1981.tb00879.x. PMID 7336969. 
  74. ↑ "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". European Journal of Pharmaceutical Sciences 42 (3): 192–198. February 2011. doi:10.1016/j.ejps.2010.11.009. PMID 21112392. 
  75. ↑ "Intestinal gaboxadol absorption via PAT1 (SLC36A1): modified absorption in vivo following co-administration of L-tryptophan". British Journal of Pharmacology 157 (8): 1380–1389. August 2009. doi:10.1111/j.1476-5381.2009.00253.x. PMID 19594759. 
  76. ↑ "5-Hydroxy-L-tryptophan alters gaboxadol pharmacokinetics in rats: involvement of PAT1 and rOat1 in gaboxadol absorption and elimination". European Journal of Pharmaceutical Sciences 39 (1–3): 68–75. January 2010. doi:10.1016/j.ejps.2009.10.013. PMID 19900542. 
  77. ↑ 77.0 77.1 77.2 "THIP, a specific and clinically active GABA agonist". Neuropharmacology 23 (7): 837–838. 1984. doi:10.1016/0028-3908(84)90272-7. https://linkinghub.elsevier.com/retrieve/pii/0028390884902727. Retrieved 22 September 2025. 
  78. ↑ "Plasma and CNS concentrations of Gaboxadol in rats following subcutaneous administration". European Journal of Pharmacology 562 (1–2): 47–52. May 2007. doi:10.1016/j.ejphar.2007.01.017. PMID 17362924. "Using both methods, we observed that Gaboxadol penetrates the brain extensively. After the initial redistribution of Gaboxadol in plasma and CNS, the concentrations and elimination from these two compartments seemed to follow similar parameters. Since no particular transporters of Gaboxadol over the blood brain barrier have been identified, it is likely that passive diffusion alone can account for the CNS pharmacokinetics. The protein binding that was observed in the present study was below 15%. This is similar to the binding in humans (Lund et al., 2006). When plasma levels are corrected for protein binding, the data further confirm the passive penetration of Gaboxadol in the brain, which is apparent when free plasma and brain levels reach unity after an initial equilibration stage (De Lange et al., 2000). In all, these data, indicate that Gaboxadol readily penetrates the brain and suggest that the concentration determined in the brain is a direct reflection of the concentration available for receptor interaction.". 
  79. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Madsen_1983
  80. ↑ 80.0 80.1 80.2 "Utility of PBPK Absorption Modeling to Guide Modified Release Formulation Development of Gaboxadol, a Highly Soluble Compound With Region-Dependent Absorption". Journal of Pharmaceutical Sciences 105 (2): 722–728. February 2016. doi:10.1002/jps.24674. PMID 26457884. Bibcode: 2016JPhmS.105..722K. "[Gaboxadol] is a zwitterion with pKa values of 4.3 (acidic) and 8.3 (basic) and log P of –0.61. It is dosed as the hydrochloride (HCl) salt. The compound solubility is more than 30 mg/mL in the physiological pH range.". 
  81. ↑ Rong, L., & Chang, D. (2007). Synthesis of a novel hypnotic, gaboxadol. Chinese Journal of Medicinal Chemistry, 17(3), 166–. https://scholar.google.com/scholar?q=intitle%3A%22Synthesis+of+a+novel+hypnotic%2C+gaboxadol%22
  82. ↑ 82.0 82.1 Krogsgaard-Larsen P, "Heterocyclic compounds", US patent, issued 14 July 1981
  83. ↑ "Synthesis and Pharmacological Evaluation of Amidine Containing GABAA Receptor Agonists". EFMC International Symposium on Medicinal Chemistry Manchester, UK Aug. 28 - Sept. 1, 2016. 2016. pp. P278. https://istina.msu.ru/media/publications/article/6f5/7ea/31004876/ISMC-Book-web-1209.pdf#page=231. 
  84. ↑ 84.00 84.01 84.02 84.03 84.04 84.05 84.06 84.07 84.08 84.09 84.10 Krogsgaard-Larsen, Povl (2018). "THIP/Gaboxadol, a Unique GABA Agonist". Reference Module in Biomedical Sciences. Elsevier. doi:10.1016/b978-0-12-801238-3.97290-8. ISBN 978-0-12-801238-3. https://linkinghub.elsevier.com/retrieve/pii/B9780128012383972908. Retrieved 7 October 2025. 
  85. ↑ "The GABA(A) agonist THIP increases non-REM sleep and enhances non-REM sleep-specific delta activity in the rat during the dark period". Sleep 20 (12): 1099–1104. December 1997. doi:10.1093/sleep/20.12.1099. PMID 9493918. 
  86. ↑ 86.0 86.1 "The GABAA agonist THIP produces slow wave sleep and reduces spindling activity in NREM sleep in humans". Psychopharmacology 130 (3): 285–291. April 1997. doi:10.1007/s002130050241. PMID 9151364. 
  87. ↑ "Merck Cancels Work on a New Insomnia Medication". 29 March 2007. https://www.nytimes.com/2007/03/29/business/29sleep.html. 
  88. ↑ "Former Teva CEO's new gig at Ovid Therapeutics". CNBC. 16 April 2015. https://www.cnbc.com/2015/04/16/former-teva-ceos-new-gig-at-ovid-therapeutics.html. 
  89. ↑ 89.0 89.1 "Ovid Stops Development and Testing of OV101 for Fragile X". 23 April 2021. https://fragilexnewstoday.com/news/ovid-stops-development-testing-ov101/. 
  90. ↑ "GABOXADOL". 15 February 2008. https://drugs.ncats.io/substance/K1M5RVL18S. 
  91. ↑ "The Trippy Truth About Amanita muscaria, the World's Most Famous Mushroom". 8 October 2022. https://doubleblindmag.com/amanita-muscaria/. "Also of interest is Hamilton Morris’ Pharmacopeia episode on Amanita and his Harper’s article on one of its constituents I didn’t cover here, gaboxadol." 
  92. ↑ "Fungal hallucinogens psilocin, ibotenic acid, and muscimol: analytical methods and biologic activities". Therapeutic Drug Monitoring 35 (4): 420–442. August 2013. doi:10.1097/FTD.0b013e31828741a5. PMID 23851905. 
  93. ↑ "Emerging Risks of Amanita Muscaria: Case Reports on Increasing Consumption and Health Risks". Acta Medica Lituanic 32 (1): 182–189. 2025. doi:10.15388/Amed.2025.32.1.23. PMID 40641545. 
  94. ↑ "Exploring User Experiences with Amanita muscaria: A Thematic Analysis of Reddit Online Forum Discussions". Substance Use & Misuse 60 (7): 952–961. 2025. doi:10.1080/10826084.2025.2476141. PMID 40057818. "The commonly reported reason for Amanita muscaria use was to improve sleep.". 
  95. ↑ "Amanita muscaria: chemistry, biology, toxicology, and ethnomycology". Mycol Res 107 (Pt 2): 131–146. February 2003. doi:10.1017/s0953756203007305. PMID 12747324. https://www.davidmoore.org.uk/21st_Century_Guidebook_to_Fungi_PLATINUM/REPRINT_collection/Michelot_etal_A.muscaria_chemistry_biology_toxicology.pdf. 
  96. ↑ "Analgesic from mushrooms begins clinical trials". Science 212 (4493): 431. April 1981. doi:10.1126/science.212.4493.431.c. PMID 7010604. 
  97. ↑ "Therapeutic effects of GABA-ergic drugs in affective disorders. A preliminary report". Pharmacology, Biochemistry, and Behavior 19 (2): 369–372. August 1983. doi:10.1016/0091-3057(83)90067-9. PMID 6415677. 
  98. ↑ "The effect of tetrahydroisoxazolopyridinol (THIP) in tardive dyskinesia: a new gamma-aminobutyric acid agonist". Archives of General Psychiatry 39 (9): 1017–1021. September 1982. doi:10.1001/archpsyc.1982.04290090021005. PMID 6126170. 
  99. ↑ "Brain gamma-aminobutyric acid abnormality in tardive dyskinesia. Reduction in cerebrospinal fluid GABA levels and therapeutic response to GABA agonist treatment". Archives of General Psychiatry 44 (6): 522–529. June 1987. doi:10.1001/archpsyc.1987.01800180032006. PMID 3034188. 
  100. ↑ 100.0 100.1 "THIP: a single-blind controlled trial in patients with epilepsy". Acta Neurologica Scandinavica 67 (2): 114–117. February 1983. doi:10.1111/j.1600-0404.1983.tb04552.x. PMID 6845976. 
  101. ↑ "THIP treatment of Huntington's disease". Neurology 33 (5): 637–639. May 1983. doi:10.1212/wnl.33.5.637. PMID 6221200. 
  102. ↑ "GABA-agonist therapy for Alzheimer's disease". Clinical Neuropharmacology 9 (3): 257–263. 1986. doi:10.1097/00002826-198606000-00004. PMID 2872956. 
  103. ↑ "Drug development in psychiatry: 50 years of failure and how to resuscitate it". The Lancet. Psychiatry 12 (3): 228–238. March 2025. doi:10.1016/S2215-0366(24)00370-5. PMID 39952266. 
  104. ↑ "Differences between magnetoencephalographic (MEG) spectral profiles of drugs acting on GABA at synaptic and extrasynaptic sites: a study in healthy volunteers". Neuropharmacology 88: 155–163. January 2015. doi:10.1016/j.neuropharm.2014.08.017. PMID 25195191. 
  105. ↑ "Altered γ-aminobutyric acid neurotransmission in major depressive disorder: a critical review of the supporting evidence and the influence of serotonergic antidepressants". Drug Design, Development and Therapy 9: 603–624. 2015. doi:10.2147/DDDT.S62912. PMID 25653499. "Another line of inquiry may be drugs that act as agonists at the GABAA-receptor orthosteric binding site. Although preclinical data suggested that the combination of gaboxadol and escitalopram had synergistic antidepressant-like effects in nonclinical models,90 in a clinical trial 5 and 10 mg gaboxadol did not add any benefit over escitalopram treatment alone.91". 
  106. ↑ "Combining escitalopram with gaboxadol provides no additional benefit in the treatment of patients with severe major depressive disorder". The International Journal of Neuropsychopharmacology 15 (6): 715–725. July 2012. doi:10.1017/S146114571100112X. PMID 22008735. 
  107. ↑ 107.0 107.1 "Gaboxadol in angelman syndrome: A double-blind, parallel-group, randomized placebo-controlled phase 3 study". European Journal of Paediatric Neurology 47: 6–12. November 2023. doi:10.1016/j.ejpn.2023.07.008. PMID 37639777. 
  108. ↑ "The STARS Phase 2 Study: A Randomized Controlled Trial of Gaboxadol in Angelman Syndrome". Neurology 96 (7): e1024–e1035. February 2021. doi:10.1212/WNL.0000000000011409. PMID 33443117. 
  109. ↑ 109.0 109.1 "Gaboxadol in Fragile X Syndrome: A 12-Week Randomized, Double-Blind, Parallel-Group, Phase 2a Study". Frontiers in Pharmacology 12. 2021. doi:10.3389/fphar.2021.757825. PMID 34690787. 

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See also
Receptor/signaling modulators
GABAA receptor positive modulators
GABA metabolism/transport modulators

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