Edited by Solomon H. Snyder, Johns Hopkins University School of Medicine, Baltimore, MD, and approved January 24, 2019 (received for review September 19, 2018)
Standard antidepressant treatments require weeks to show effectiveness. A single subanesthetic dose of ketamine rapidly attenuates many clinical signs and symptoms of depression; however, ketamine treatment also has many adverse effects, including dissociation and potential for abuse, which are mediated by NMDA glutamate receptor (NMDAR) inhibition. Previous work has revealed that the ketamine metabolite (2R,6R)-hydroxynorketamine (HNK) induces antidepressant-like responses in rodents while minimizing the adverse effects observed with ketamine. The results of this study, using a multitude of experimental approaches, confirm that antidepressant-relevant concentrations of (2R,6R)-HNK are not sufficient to block NMDARs. This provides a basis for work directed at alternative molecular targets and toward novel drugs that exert rapid antidepressant effects independent of NMDAR inhibition and NMDAR-mediated adverse effects.
Preclinical studies indicate that (2R,6R)-hydroxynorketamine (HNK) is a putative fast-acting antidepressant candidate. Although inhibition of NMDA-type glutamate receptors (NMDARs) is one mechanism proposed to underlie ketamine’s antidepressant and adverse effects, the potency of (2R,6R)-HNK to inhibit NMDARs has not been established. We used a multidisciplinary approach to determine the effects of (2R,6R)-HNK on NMDAR function. Antidepressant-relevant behavioral responses and (2R,6R)-HNK levels in the extracellular compartment of the hippocampus were measured following systemic (2R,6R)-HNK administration in mice. The effects of ketamine, (2R,6R)-HNK, and, in some cases, the (2S,6S)-HNK stereoisomer were evaluated on the following: (i) NMDA-induced lethality in mice, (ii) NMDAR-mediated field excitatory postsynaptic potentials (fEPSPs) in the CA1 field of mouse hippocampal slices, (iii) NMDAR-mediated miniature excitatory postsynaptic currents (mEPSCs) and NMDA-evoked currents in CA1 pyramidal neurons of rat hippocampal slices, and (iv) recombinant NMDARs expressed in Xenopus oocytes. While a single i.p. injection of 10 mg/kg (2R,6R)-HNK exerted antidepressant-related behavioral and cellular responses in mice, the ED50 of (2R,6R)-HNK to prevent NMDA-induced lethality was found to be 228 mg/kg, compared with 6.4 mg/kg for ketamine. The 10 mg/kg (2R,6R)-HNK dose generated maximal hippocampal extracellular concentrations of ∼8 µM, which were well below concentrations required to inhibit synaptic and extrasynaptic NMDARs in vitro. (2S,6S)-HNK was more potent than (2R,6R)-HNK, but less potent than ketamine at inhibiting NMDARs. These data demonstrate the stereoselectivity of NMDAR inhibition by (2R,6R;2S,6S)-HNK and support the conclusion that direct NMDAR inhibition does not contribute to antidepressant-relevant effects of (2R,6R)-HNK.
↵1E.W.L. and T.A.T. contributed equally to this work.
↵2S.F.T., E.F.R.P., S.M.T., E.X.A., and T.D.G. contributed equally to this work.
Author contributions: E.W.L., T.A.T., S.J.M., P.Z., Y.A., J.K., S.F.T., E.F.R.P., S.M.T., E.X.A., and T.D.G. designed research; E.W.L., T.A.T., S.J.M., P.Z., Y.A., J.K., J.L., S.K., F.-H.W., S.S., C.E.J., P.Y., P.J.M., C.J.T., C.A.Z., and R.M. performed research; E.W.L., T.A.T., S.J.M., P.Z., Y.A., J.K., C.E.J., and S.F.T. analyzed data; and E.W.L., T.A.T., P.Z., E.F.R.P., S.M.T., E.X.A., and T.D.G. wrote the paper.
Conflict of interest statement: T.D.G. has received research funding from Janssen, Roche, and Allergan Pharmaceuticals, and served as a consultant for FSV7 LLC during the preceding 3 years. The authors declare competing financial interests: T.D.G., P.Z., R.M., P.J.M., C.J.T., and C.A.Z. are listed as co-inventors on a patent application for the use of (2R,6R)-hydroxynorketamine and (2S,6S)-hydroxynorketamine in the treatment of depression, anxiety, anhedonia, suicidal ideation, and post-traumatic stress disorders. C.A.Z. and R.M. are listed as co-inventors on a patent for the use of (2R,6R)-hydroxynorketamine, (S)-dehydronorketamine, and other stereoisomeric dehydro- and hydroxylated metabolites of ketamine metabolites in the treatment of depression and neuropathic pain. R.M., P.J.M., C.J.T., and C.A.Z. have assigned patent rights to the US Government but will share a percentage of any royalties that may be received by the Government. P.Z. and T.D.G. have assigned their patent rights to the University of Maryland, Baltimore, but will share a percentage of any royalties that may be received by the University of Maryland, Baltimore. S.F.T. received research support from Janssen, is a consultant for Janssen, is a member of the Scientific Advisory Board for Sage Therapeutics, is a co-founder of NeurOp, Inc., and co-inventor on Emory-owned IP. S.J.M. owns stock in NeurOp, Inc., which is developing NMDAR inhibitors for use in treating neurological disease and disorders. All other authors declare no competing interests.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1816071116/-/DCSupplemental.
Published under the PNAS license.
