FROM KETAMINE METABOLITE TO CANDIDATE THERAPEUTIC: PRECLINICAL DISCOVERY OF (2R,6R)-HNK RELEVANCE TO DEPRESSION TREATMENT

Rapid-acting ketamine-based therapies have reshaped antidepressant drug development, yet their dissociative side effects, abuse liability, and challenges with repeat dosing underscore the need for pharmacologically distinct alternatives. Hydroxynorketamines (HNKs) are formed in vivo after ketamine administration and the 12 HNK stereoisomers are distinguished by the position of cyclohexyl ring hydroxylation (at the 4, 5, or 6 position) and their unique stereochemistry at two stereocenters. Although HNKs were initially classified as inactive metabolites because of their lack of anesthetic effects, a series of studies have identified (2R,6R)-HNK (2R6R) as a potential mediator of the antidepressant actions of ketamine. 2R6R preserves the therapeutic potential of ketamine while exhibiting minimal NMDAR antagonism, no dissociative or anesthetic activity, and no measurable abuse liability in preclinical models. I will discuss how these preclinical data— spanning behavior, synaptic physiology, pharmacology, and translational biomarkers—led to the selection of 2R6R as a drug development candidate supporting the ongoing clinical program of 2R6R in treatment-resistant depression and other neuropsychiatric indications. Using rodent behavioral paradigms of stress-and depression-relevant phenotypes, we demonstrated that 2R6R produces rapid and sustained antidepressant-like responses at doses devoid of the locomotor, dissociative-like or anesthetic and intravenous self-administration effects typical of ketamine. Mechanistic studies revealed that 2R6R enhances AMPARmediated synaptic transmission and up-regulates synaptic plasticity biomarkers in hippocampal and prefrontal regions. Hippocampal Schaffer-collateral CA1 synapses are rapidly potentiated in vitro following 2R6R application, and are persistently primed to support enhanced long-term potentiation long after drug wash-out both in vivo and in vitro mediated in a mTORC1, BDNF/TrkB, IP3R, and L-type calcium channel dependent manner. Stereoisomer-specific pharmacology—including the 2R6R isomer rather than the 2S6S or racemic HNK was guided by differential efficacy in rodent models and a more favorable safety/tolerability profile. Our recent unpublished studies have assessed stereoisomer-specific synaptic effects of all 12 naturally produced HNKs identifying robust differences in pharmacodynamic properties indicating how HNK structure mediates pharmacological activity. Overall, this talk will illustrate how mechanistic neuroscience, translational pharmacology and biomarker-driven decision-making converged to move forward 2R6R from bench to clinic. Attendees will gain insight into how academic-NIH collaborations can chart a path for novel molecules in neuropsychiatric indications — expanding the therapeutic toolkit beyond traditional monoaminergic treatments and ketamine itself.

References

Riggs LM, Aronson S, Mou TCM, Pereira EFR, Thompson SM, Gould TD (2025). Bioactive ketamine metabolite exerts in vivo neuroplastogenic effects to improve hippocampal function in a treatment-resistant depression model. Cell Reports. 44, 115743. (2025) Rapid hippocampal synaptic potentiation induced by ketamine metabolite (2R,6R)hydroxynorketamine persistently primes synaptic plasticity. Neuropsychopharmacology. 50, 928-940.