Background: ( R,S)-ketamine has gained attention for its rapid-acting antidepressant actions in patients with treatment-resistant depression. However, widespread use of ketamine is limited by its side effects, abuse potential, and poor oral bioavailability. The ketamine metabolite, ( 2R,6R)-hydroxynorketamine, exerts rapid antidepressant effects, without ketamine’s adverse effects and abuse potential, in rodents. Methods: We evaluated the oral bioavailability of ( 2R,6R)-hydroxynorketamine in three species (mice, rats, and dogs) and also evaluated five candidate prodrug modifications for their capacity to enhance the oral bioavailability of ( 2R,6R)-hydroxynorketamine in mice. Oral administration of ( 2R,6R)-hydroxynorketamine was assessed for adverse behavioral effects and for antidepressant efficacy in the mouse forced-swim and learned helplessness tests. Results: ( 2R,6R)-hydroxynorketamine had absolute bioavailability between 46–52% in mice, 42% in rats, and 58% in dogs. Compared to intraperitoneal injection in mice, the relative oral bioavailability of ( 2R,6R)-hydroxynorketamine was 62%, which was not improved by any of the candidate prodrugs tested. Following oral administration, ( 2R,6R)-hydroxynorketamine readily penetrated the brain, with brain to plasma ratios between 0.67–1.2 in mice and rats. Oral administration of ( 2R,6R)-hydroxynorketamine to mice did not alter locomotor activity or precipitate behaviors associated with discomfort, sickness, or stereotypy up to a dose of 450 mg/kg. Oral ( 2R,6R)-hydroxynorketamine reduced forced-swim test immobility time (15–150 mg/kg) and reversed learned helplessness (50–150 mg/kg) in mice. Conclusions: These results demonstrate that ( 2R,6R)-hydroxynorketamine has favorable oral bioavailability in three species and exhibits antidepressant efficacy following oral administration in mice.
Task-Modulated Brain Activity Predicts Antidepressant Responses of Prefrontal Repetitive Transcranial Magnetic Stimulation: A Randomized Sham-Control Study
