Arketamine

Arketamine, also written (R)-ketamine, is one of the two mirror-image forms in racemic ketamine. Esketamine, the other, was the one taken to approval because it binds the NMDA receptor roughly four times more tightly ^[7].

The preclinical case for arketamine is genuinely strong and consistently reproduced by the group that built it. Across rodent models, arketamine has shown antidepressant-like effects that are more potent and longer lasting than esketamine's, alongside less dissociation-like behaviour, weaker reward-related responses and fewer effects suggesting abuse liability. The paradox that it does this while binding NMDA receptors more weakly is the strongest single argument in the literature that NMDA blockade is not the antidepressant mechanism ^[1]. Arketamine has also been proposed specifically for cognitive impairment in psychiatric illness, where it improved deficits in rodent models that esketamine did not ^[4].

The human record does not yet support any of it. A randomized, double-blind, placebo-controlled crossover pilot in treatment-resistant depression found arketamine no better than saline, though it was extremely well tolerated with minimal dissociation ^[2]. A 2023 review from the NIMH group summarising the enantiomer literature concluded that recent controlled trials found no significant antidepressant efficacy versus placebo and that caution is warranted in interpreting the compound's potential ^[3]. The pilot was small and underpowered, which is the standard and reasonable objection; it is also the only kind of evidence available so far.

Tolerability is the part of the human record that looks good. In the controlled pilot in treatment-resistant depression, dissociation and other adverse events were minimal and the compound was described as extremely safe at the dose tested, which matches the rodent finding of weaker dissociation-like and reward-related responses than esketamine produces ^[2][1].

That is a narrow claim and should be read narrowly. It rests on a single small crossover trial plus animal work, not on a safety database. There is no long-term human exposure data, no information on repeated dosing over months, and nothing on whether the bladder and cognitive harms associated with heavy racemic ketamine use would appear with sustained arketamine use; since it is still an NMDA antagonist, the absence of evidence there is not evidence of absence. It is not approved anywhere, not available as a medicine, and not something to source.

The two enantiomers of ketamine were separated and characterised decades before either was proposed as an antidepressant, and the pharmacology of the period pointed clearly at the S-form: it bound NMDA receptors harder, anesthetised better and was the obvious development candidate ^[7]. Arketamine was, in effect, the discarded half.

Interest reversed from around 2015 onward, driven largely by work from Kenji Hashimoto's group at Chiba University reporting that arketamine outperformed esketamine on antidepressant-like measures in rodents while producing fewer of the unwanted behavioural effects ^[1]. That body of work ran alongside, and partly against, the American finding that ketamine's effect depended on its conversion to (2R,6R)-hydroxynorketamine; a 2017 study from the same Japanese group reported that the metabolite did nothing in a rat learned-helplessness model where arketamine itself worked ^[5].

Clinical development followed, and has been disappointing so far. A placebo-controlled crossover pilot published in 2023 found no separation from saline in treatment-resistant depression ^[2], and a review the same year from the NIMH group concluded that controlled trials had not shown efficacy against placebo ^[3]. The compound is still in development rather than abandoned, and its advocates argue the trials have been underpowered.

Arketamine has an unusually polarised standing. Within one research tradition it is treated as the compound that should have been developed, and the argument is made in strong terms: better antidepressant effect in animals, less dissociation, less abuse potential, and a mechanism that exposes the NMDA story as incomplete. Outside that tradition it is increasingly cited as a cautionary example of how far rodent behavioural pharmacology can travel without a human result behind it ^[3].

Among readers who follow nootropics and mood compounds, it tends to be discussed as though the preclinical claims were established findings about people, which they are not. It is also occasionally confused with the racemate or assumed to be obtainable; it is neither approved nor sold, and material offered under this name has no legitimate source. The fairest summary is that the disagreement is real, both sides are arguing from evidence, and the human trials that would settle it have not been run at adequate size.