Characterization of Ketamine’s (2,6)-Hydroxynorketamine Metabolites: Pharmacokinetic and Behavioral Considerations for Antidepressant Applications

Abstract

Despite numerous available treatments for depression, most are not only slow to take effect but also ineffective in many patients, highlighting the need for novel and effective antidepressant treatments. While (R,S)-ketamine (ketamine) has gained attention for its rapid-acting antidepressant effects in previously treatment-resistant patients, its widespread antidepressant use is limited by adverse effects and abuse potential. The ketamine metabolite (2R,6R;2S,6S)-hydroxynorketamine (HNK), and most potently the (2R,6R)-HNK stereoisomer, shares the antidepressant-like behavioral effectiveness, but lacks the adverse effect burden and abuse potential, of ketamine in rodents, suggesting that (2R,6R)-HNK, and possibly other HNKs (12 have been identified, but only (2R,6R)- and (2S,6S)-HNK were previously studied), may be favorable antidepressant drug candidates. However, several important considerations for the development of HNKs as antidepressants were not previously evaluated. Such considerations were addressed in the present study. First, the pharmacokinetic profiles of the 12 HNKs were characterized, revealing robust differences in plasma and brain concentrations among the various HNKs, despite similar brain-to-plasma ratios and rapid elimination profiles. Second, it was demonstrated that (2R,6R)-HNK has favorable oral bioavailability (45-52%) and orally administered (2R,6R)-HNK exerts antidepressant-relevant behavioral effects, but not overt adverse effects, in mice. Third, the sex-dependent differences in the metabolism of ketamine and (2R,6R)-HNK in mice were characterized. It was demonstrated that, following ketamine administration, female mice have lower levels of ketamine and higher levels of HNK than male mice. In addition, following direct dosing, female mice have higher levels of (2R,6R)-HNK than males. Male gonadal hormones at least partly mediate these differences. Further, the relative effectiveness of the four (2,6)-HNKs in the mouse forced swim test were compared, revealing that both (2R,6S)- and (2S,6R)-HNK exert effects at lower doses compared to either (2R,6R)- or (2S,6S)-HNK, and establishing a relative rank-order of effectiveness of (2R,6R)- > (2S,6R)- > (2R,6R)- > (2S,6S)-HNK. Finally, it was identified that the novel compound, (5R)-methyl-(2R,6R)-HNK recapitulates the antidepressant dose-response relationship of (2R,6S)-HNK, with which it shares a similar three-dimensional structure, suggesting a critical role of three-dimensional structure in mediating antidepressant-relevant effectiveness. Altogether, these studies represent several important insights which may support the development of (2,6)-HNKs as antidepressant treatments.