R-Ketamine

R-Ketamine, also known as arketamine or (R)-ketamine, is one of the two enantiomers of ketamine. It has the same molecular formula as S-ketamine but a different three-dimensional arrangement of atoms. Arketamine has a lower affinity for the NMDA receptor than S-ketamine, yet preclinical studies have reported strong and sometimes longer-lasting antidepressant-like effects. Its therapeutic potential remains under investigation.

Chemistry and stereochemistry

Ketamine contains a chiral centre and exists as two mirror-image forms: R-(−)-ketamine and S-(+)-ketamine. A racemic preparation contains equal amounts of both. Arketamine refers specifically to the R enantiomer, while esketamine is the S enantiomer.

The two forms cannot be treated as pharmacologically identical. Their spatial arrangement affects receptor affinity, metabolism and some of their observed effects. This is one reason why studies of racemic ketamine do not always predict the results of studies using a single enantiomer.

Pharmacology

R-ketamine is an NMDA receptor antagonist, but its affinity for the receptor is lower than that of S-ketamine. Reviews commonly report approximately four- to five-fold greater NMDA receptor affinity for S-ketamine. Arketamine also produces metabolites, including hydroxynorketamine, that have been investigated for possible roles in its biological effects.

Preclinical research has found that R-ketamine can produce antidepressant-like effects in rodents that last longer than those observed with S-ketamine in some experimental models. Proposed mechanisms extend beyond direct NMDA receptor blockade and include downstream changes in synaptic signalling. These findings remain an active area of research (Molecular mechanisms underlying the antidepressant actions of arketamine, 2022).

Clinical research

Arketamine has been investigated as a possible treatment for depression, particularly because preclinical studies suggest a different balance of antidepressant and dissociative effects from S-ketamine. However, results in humans have been less consistent than those in animal models.

A review of clinical and preclinical evidence noted that some early clinical findings were encouraging, but randomised controlled trials had not demonstrated a significant antidepressant effect over placebo. The authors therefore cautioned against treating the preclinical results as proof of clinical efficacy (The antidepressant actions of ketamine and its enantiomers, 2023).

Unlike esketamine, arketamine is not an approved medicine for depression. Its clinical use remains investigational, and further controlled trials are needed to establish efficacy, safety and the best treatment protocol.

Subjective and dissociative effects

The broader dissociative effects of ketamine are described in the PsychonautWiki ketamine profile, but most user reports do not distinguish R-ketamine from the racemic product. This makes it difficult to distinguish the subjective effects of R-ketamine reliably.

As one component of racemic ketamine, R-ketamine contributes to the overall dissociative and anaesthetic profile. Laboratory and animal studies suggest that it may produce fewer psychotomimetic effects than S-ketamine under some conditions, but this has not been established as a predictable difference in all human settings.

Safety and research limitations

Arketamine’s safety profile is not fully established as a standalone medicine. Findings from racemic ketamine cannot be attributed entirely to the R enantiomer, and results from animal experiments do not always translate to human treatment. Its possible advantages in mood research remain hypotheses under clinical evaluation.

As with other ketamine preparations, dissociation, changes in perception and impairment can occur. The extent to which these effects differ from S-ketamine in clinical practice requires further direct comparison.

Related compounds

R-ketamine is the enantiomeric counterpart of S-Ketamine (esketamine). Together, they make up racemic ketamine. Their molecular composition is the same, but their stereochemistry and pharmacological properties differ.