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Ketamine is the first truly new pharmacological approach to treating depression in the past 50 years and could herald a new generation of rapid acting antidepressants, researchers have predicted.
“We haven’t had anything really new for about 50 or 60 years,” said Allan Young, professor of mood disorders at the Institute of Psychiatry, Psychology and Neuroscience at King’s College, London, at a briefing on 12 July at London’s Science Media Centre.
Most of the new launches have been “tinkering with drugs which were really discovered in the ’50s and ’60s,” he explained. “Even the famous Prozac, which came in in the late ’80s, is really just a refinement of the tricyclic antidepressants that came in the ’50s. People say we are still in the age of steam, and we need to go to the next technological advance.”
In the past few years the focus has fallen on ketamine, which is used for pain relief and anaesthesia but is better known for being a horse sedative and a “club drug” that can induce hallucinations and calmness. It has been found to have rapid antidepressant effects and to be effective in many patients with treatment resistant depression.
US clinics increasingly offer IV infusions of ketamine off label, and in March esketamine, a nasal ketamine based drug, was approved by the US Food and Drug Administration for treatment resistant depression,1 at a cost of £32 400 (€36 060; $40 615) per patient per year.
Carlos Zarate, chief of the Experimental Therapeutics and Pathophysiology Branch at the US National Institute of Mental Health, who has been a key figure in the discovery and evaluation of ketamine as an antidepressant, said that one of the main problems with current antidepressants was their speed of onset in terms of antidepressant and anti-suicidal effects.
He explained that it took 10-14 weeks to see significant improvement with monoaminergic based antidepressants. “In my mind that is too slow,” he said. “We are focusing on treatments that can produce results within hours. That is where we are heading with the next generation of antidepressant, and ketamine is now the prototype for future generation antidepressants which will have rapid, robust antidepressant effects—rapid within a few hours.”
Efficacy and tolerability
Zarate said that, besides correcting chemical imbalances of serotonin and norepinephrine, the new generation of ketamine based antidepressants had other effects such as enhancing plasticity and restoring the synapses and dendrite circuits that shrivel in depression.
When ketamine is given to patients it binds to the N-methyl-D-aspartate (NMDA) receptor, causing a series of transient side effects including decreased awareness of the environment, vivid dreams, and problems in communicating. But the half life of ketamine is only two to three hours, so these side effects quickly subside, whereas the therapeutic effects of the drug last seven days or longer.
Zarate’s team is now focusing on the 24 metabolites of ketamine to hone the drug’s efficacy and tolerability. One of these, hydroxynorketamine, has already been shown to have similar antidepressive effects to ketamine in animals, without the side effects, and it is due to be tested in patients this autumn.
“Ketamine may actually be a prodrug for hydroxynorketamine,” said Zarate.
A few dozen patients with treatment resistant depression have been treated with ketamine in UK trials, and the European Medicines Agency and the Medicines and Healthcare Products Regulatory Agency are due to reach a decision on authorising esketamine for marketing in October. If the drug is approved private clinics will be able to provide it. But it would be unlikely to be available through the NHS until at least 2020, if at all, as the National Institute for Health and Care Excellence would need to deem it cost effective.
Rupert McShane, consultant psychiatrist and associate professor at the University of Oxford, said that, as well as the likely high cost of esketamine, patients treated with it must be observed in a clinic for two hours after each administration. This would require substantial clinical time, as esketamine is given twice a week for the first month, once a week for the second month, and once a week or once a fortnight from then on.
McShane also recommended that, if approved, a multidrug registry should be set up to monitor the long term safety and effectiveness of ketamine based drugs. Patients would be asked to input their use of any prescribed ketamine, esketamine, or any other future ketamine based product, as well as any self medication with illicit ketamine.