Conference Coverage

Ketamine may rely on opioid receptors for antidepressive effect



– Ketamine and the more recently Food and Drug Administration–approved esketamine have generated a great deal of excitement in psychiatry over their potential to treat depression and other psychiatric disorders. However, the mechanism of action is not completely understood, and efforts are underway to better understand the drugs.

Much has been made of ketamine’s interaction with the N-methyl-D-aspartate (NMDA) receptors. One belief is that chronic stress leads to the accumulation of extracellular glutamate, which leads to a range of negative consequences. Ketamine blocks excess glutamate in the synapse and may thus reverse these downstream effects.

But it may not be so clear cut, according to Nolan Williams, MD. During a session on ketamine’s mechanism of action at the annual meeting of the American Psychiatric Association, Dr. Williams, assistant professor of psychiatry and behavioral sciences at Stanford (Calif.) University, noted that ketamine is “one of the most dirty drugs we know. It affects most neurotransmitter systems and has all sorts of downstream effects.”

Pain research has already shown that ketamine’s mechanisms can be complex. It affects the opioid system, both directly and indirectly, and an opioid receptor antagonist blocks ketamine’s pain-relieving effect.

Since opioids are also known to have antidepressant effects, it’s natural to wonder if ketamine’s mechanism also involves the opioid system. The opioid antagonist naltrexone provides a tool to study the problem. Dr. Williams reasoned that if ketamine relies in whole or in part on the opioid system for its antidepressive effect, then administering naltrexone ahead of ketamine should blunt or even eliminate its efficacy.

To examine the question, the team conducted a study of 11 patients with treatment-resistant depression. Right away, it was clear that the participants had some bias toward the treatment. “They all said the same thing to me: ‘My psychiatrist said that your study doesn’t make any sense because this is an NMDA antagonist. I’m going to get two free ketamine infusions.’ So they were preloaded with this idea that they were going to get very potent antidepressant effects (even with naltrexone),” said Dr. Williams.

The study had a crossover design and included two ketamine infusions. Participants were randomized to get either naltrexone or placebo 1 hour before the first ketamine infusion, and then were allowed to relapse back to within 20% of their baseline depression score before receiving the second ketamine infusion, when they received naltrexone or placebo, whichever hadn’t been given in the first treatment.

Of 12 who completed the study, seven responded to placebo plus ketamine treatment, and six remitted. But when they crossed over to the naltrexone arm, the result was very different: In three of four measures, there was no significant difference between the pretreatment and posttreatment results. “The same people who were almost exclusively in remission during the ketamine plus placebo condition got nothing out of ketamine plus naltrexone,” said Dr. Williams. Naltrexone had a similar negating effect on the positive response to ketamine on the suicide item of the 17-item Hamilton Rating Scale for Depression.

The researchers also looked at whether ketamine’s dissociative effect could be responsible for its activity through altering the participant’s mental state, but determined that it may be necessary for the antidepressant effect – but it is not sufficient.

“What we concluded pretty strongly was that the opioid properties of ketamine are necessary to have the antidepressive effect. We’re not saying it’s sufficient, but it appears to be quite necessary,” Dr. Williams said. He added that the results don’t mean that ketamine’s effect on NMDA is unimportant. In fact, “it’s probable that the ketamine is driving the mood in a direction, and these glutamine-related changes are probably what’s maintaining the mood in that direction,” he added.

As to the clinical impacts of his findings, Dr. Williams emphasized the need to understand the mechanisms behind the ketamine class. He pointed out that in pivotal trials of esketamine, there were six deaths, three by suicide, all of them in the esketamine group. Surprisingly, two of those who took their own lives had scored a 0 on the Columbia-Suicide Severity Scale, both at baseline and at the visit immediately preceding their deaths. The score wasn’t available for the third. “These weren’t people who were particularly dangerously suicidal. So understanding the mechanism is really important to understanding the safety of this drug, who we should give it to, and what sorts of things we should watch out for,” Dr. Williams said.

The study was funded by the National Institutes of Health, Spectrum, the Brain and Behavior Research Foundation, and Stanford Bio-X. Dr. Williams had no disclosures.

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