Investigation of the Antidepressant Effects of (2R,6R)-HNK, an Enhancer of Synaptic Glutamate Release, in Treatment-Resistant Depression

Primary: Change from baseline on Montgomery-Asberg Depression Rating Scale total scores

Secondary: Change from baseline on Montgomery-Asberg Depression Rating Scale total scores, Change from baseline on item 10 (suicidality) of the Montgomery-Asberg Depression Rating Scale and total score on the Columbia Suicide Severity Rating Scale and the Scale for Suicide Ideation., Change from baseline on the Hamilton Depression Rating Scale, Hamilton Anxiety Rating Scale, Positive and Negative Affect Schedule, Snaith-Hamilton Pleasure Scale, Temporal Experience of Pleasure Scale, and cognitive tasks., Incidence of AEs and total scores using the Clinician Administered Dissociative States Scale, Young Mania Rating Scale, Brief Psychiatric Rating Scale, vital signs, changes in clinical laboratory evaluations, and electrocardiograms., Pain thresholds and tolerance during quantitative sensory testing while undergoing fMRI., Pain thresholds and tolerance during quantitative sensory testing., Proportion of participants achieving response (defined as a >/=50% reduction from baseline in Montgomery-Asberg Depression Rating Scale total score), Proportion of participants in remission (defined as Montgomery-Asberg Depression Rating Scale total score =10)