Ketamine/esketamine: Putative mechanisms of action

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A model of how ketamine works

Numerous publications from preclinical and clinical research collectively have woven a putative model of how K/ESK may rapidly improve depression by ultimately increasing synaptogenesis in the human prefrontal cortex—a part of the brain known to atrophy in states of chronic stress and depression.4 What is well established is the noncompetitive antagonism of K/ESK at the N-methyl-D-aspartate (NMDA) glutamate receptor, but this pharmacodynamic property may or may not be responsible, or even required, for the ultimate antidepressant effect 4 hours after administration. It has been shown that unlike anesthetic doses of K/ESK that inhibit glutamate, subanesthetic doses activate neuronal glutamate transmission in the prefrontal cortex.5

A significant body of evidence supports agonism of the glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor as an important step in the cascade of events that ultimately increases levels of the mammalian target of rapamycin (mTOR), which unleashes protein synthesis in synapses facilitating synaptogenesis. Pretreatment with AMPA receptor antagonists blocks the downstream effect of synaptogenesis.6,7 In support of this putative mechanism, hydroxynorketamine, a metabolite of racemic ketamine that has also demonstrated RAAD activity in a ketamine-like manner, is dependent upon AMPA glutamate receptor upregulation and activation, while not requiring activity at the NMDA-glutamate receptor.8,9

A comprehensive model on the putative molecular cascade of events contributing to the antidepressant effect of ketamine has recently been published10 and mirrors the excellent previous review by Abdallah et al.11 Hirota and Lambert10 propose that antagonism of interneuronal NMDA-glutamate receptors on GABAergic interneurons may result in a prefrontal cortex surge of glutamate, which increases agonism of the AMPA-glutamate receptor. This AMPA-glutamate receptor agonism has been shown to increase expression of brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF),12 both of which converge on increasing levels of mTOR, and the subsequent activation of mTOR, which putatively plays a role in increased production of scaffolding proteins and increased synaptogenesis, especially in the prefrontal cortex. In support of this model, during infusion and at 24 hours after a single ketamine infusion in individuals with MDD, functional connectivity MRI demonstrated an increase in global brain connectivity in the prefrontal cortex.13,14 The demonstration of increased global connectivity in the prefrontal cortex of patients with MDD, both during ketamine infusion and at 24 hours post-infusion, supports the clinical observations in clinics treating patients with K/ESK.

Opioid receptors and ketamine

During the past year, there has been significant discussion in psychiatry about the possible role of the mu opioid receptor and opioid system activation in ketamine’s RAAD effect. Remarkably, the literature supporting this hypothesis in humans is based on a single study by Williams et al.15 The authors’ claim: “We now present the first evidence in humans that opioid receptors are necessary for ketamine’s acute antidepressant effect.” In fact, in my opinion, this single study, which has not been replicated, is highly flawed. It included 30 adults with TRD, but only 12 of the 14 participants who qualified for the planned interim analysis completed the double-blind crossover. The population studied was quite treatment-refractory; the average duration of MDD was 24.1 years, the average age at onset was 17.3 years, and the duration of the current depressive episode at the time of the study was 8.6 years. Most significant to me was the reason the study was terminated: “At the interim analysis, given the finding that the combination of ketamine and naltrexone was not only ineffective but also noxious for many participants, we decided to stop enrolling patients in the study.” A distinct possibility is that the noxious adverse effects from the naltrexone impacted the participants’ experience in a negative manner, dampening down any antidepressant effect from ketamine.

In the August 2019 issue of Molecular Psychiatry, these same authors published a second article16 with conclusions based solely on “a secondary analysis of” the data from the same 12 participants in their first publication. Williams et al16 concluded that naltrexone also decreases the anti-suicidality effects of ketamine. Without any additional data or clinical research, these same authors extrapolated their hypothesized opioid receptor activity of ketamine to include it being responsible for ketamine’s established anti-suicidal effects.

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