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

Background: Major depressive disorder (MDD) is a serious mental illness that can put people at risk of self-harm and death. Many drugs are used to treat MDD, but it can take a long time for them to be effective. Researchers want to know if a faster-acting drug, (2R,6R)-hydroxynorketamine (HNK), can better treat the symptoms of MDD. Objective: To test a study drug (HNK) in people with MDD. Eligibility: People aged 18 to 70 years with MDD. They must have had a screening assessment under protocol 01-M-0254. Design: Participants will be tapered off their current MDD drugs over 2 to 5 weeks. They will stay off of the drugs for up to 2 weeks prior to starting the study medication and procedures. They will have a physical exam with blood tests. They will have tests of their heart function, mood, and thinking. They will answer questions about their symptoms. They may choose to have imaging scans and scans of their brain activity. HNK is given through a tube attached to a needle inserted into a vein. Participants will receive infusions on this schedule: They will receive 4 infusions over 2 weeks. They will stay in the clinical center overnight after each infusion or for the duration of the study. They will receive no drugs for 2 to 3 weeks. They will have 4 more infusions over 2 weeks, with overnight stays after each or for the duration of the study. One set of 4 infusions will be the HNK. The other set of 4 infusions will be a placebo. A placebo looks just like the real drug but contains no medicine. Participants will not know when they are getting the HNK or placebo. ...

* INCLUSION CRITERIA: In order to be eligible to participate in this study, an individual must meet all of the following criteria: 1. Ability of participant to understand and willingness to sign a written informed consent document. To verify this, participants must score \>= 80% on the consent quiz. 2. Stated willingness to comply with all study procedures and availability for the duration of the study. 3. 18 to 70 years of age. 4. All participants must have undergone a screening assessment under protocol 01-M-0254. 5. Participants must fulfill DSM-IV or DSM-5 criteria for MDD, single episode or recurrent without psychotic features, based on clinical assessment and confirmed by a structured diagnostic interview (SCID-P). Participants must be experiencing a current major depressive episode lasting at least two weeks. 6. Participants must have an initial score of \>= 20 on the MADRS and a YMRS score of \