NEWS & ANALYSIS Nature Reviews Drug Discovery | Published online 30 Oct 2018; doi:10.1038/nrd.2018.187 ‘Party drug’ turned anti depressant approaches approval Johnson & Johnson has submitted its esketamine for regulatory approval, but researchers still don’t understand how the fast-acting antidepressant lifts moods. DrAfter123/DigitalVision Vectors Sara Reardon Monteggia, a neuroscientist at Vanderbilt later learned that the antibiotic, at low doses, University. blocks the NMDA receptor, a glutamate When researchers showed in 2006 that the Yet it is far from clear how this work will receptor. Then in the late 1990s,Nature when Reviews | Drug Discovery anaesthetic ketamine — also known as the play out. Whereas early evidence suggested psychiatrist John Krystal of Yale University club drug Special K — was a rapid and potent that ketamine acted through the NMDA was curious about whether the antidepressant, big pharmaceutical receptor, many of the first-generation neurotransmitter glutamate contributed to companies quickly jumped into the game. ketamine mimetics that were designed to act schizophrenia, he decided to test the known Extensive efforts to improve on decades-old on this target failed in clinical trials (TABLE 1). NMDA receptor antagonist ketamine in nine antidepressants had floundered, but ketamine Accumulating evidence now suggests that depressed patients. finally promised a novel mechanism of action ketamine’s antidepressant activity may be At the time, glutamate had mostly been and the potential to help treatment-resistant more complicated. studied for its role in learning and memory. But patients. As a result, some companies are quietly Krystal’s group found that ketamine induced a Because ketamine is an old drug and going back to the drawing board. “My sense is rapid improvement in mood in patients. difficult to commercialize for a new that NMDA-receptor blocking studies are Zarate and Husseini Manji, who is now head indication, early entrants into this space set diminishing quite quickly and people are neuroscience researcher at J&J, set out to out to build ketamine mimetics that could looking at other mechanisms,” says replicate the surprising findings at the NIMH in replicate the anaesthetic’s effect, ideally psychiatrist Carlos Zarate at the National a larger trial, enrolling 18 subjects with major without its hallucinatory side effects. A few Institute of Mental Health (NIMH). While depression. The results from this small study of these ketamine-inspired drugs are now NMDA blockers haven’t been abandoned, suggested that ketamine was a miracle drug — nearing the finish line (TABLE 1). In September, he says, “companies are just giving a second lifting a person’s mood almost immediately. Johnson & Johnson (J&J) filed for FDA approval thought to whether they want to continue Reporting in the Archives of General Psychiatry, of a nasal spray containing esketamine — an pursuing these programmes.” Until a clearer they showed that 70% of depressed patients isomer of ketamine that the company has picture of the mechanism is worked out, the responded to ketamine within 24 hours. By patented. Despite some lingering questions field may be doomed to a trial and error hunt contrast, in one of the largest studies of people about its efficacy compared with ketamine, for better-than-ketamine mimetics. with depression, only one-third of patients experts in the field expect the drug will be responded to selective serotonin reuptake approved, providing the first antidepressant Novel antidepressant activity inhibitors (SSRIs) after 8 weeks. breakthrough in decades. The most commonly used antidepressants Ketamine also appears to reduce suicidal “What’s exciting is not that there’s going target signalling by the monoamine thoughts — something that no other drug is to be a new drug approved, but that we’re neurotransmitters serotonin, dopamine and known to do — and its effects last for weeks going to have a whole new class of drug noradrenaline. But starting in the 1950s, to months. approved,” says psychiatrist James Murrough researchers using the antibiotic D-cycloserine “Ketamine works so well it would be hard at Mount Sinai Hospital. “Everyone’s waiting to treat tuberculosis found that the drug to do better,” says neuroscientist Todd Gould with bated breath.” alleviated patient melancholy. Researchers of the University of Maryland. Some clinics This is fostering high hopes that have taken this conclusion to heart, and are psychiatric drug development — which has already offering ketamine to depressed seen an exodus of major pharma companies patients on an off-label basis (BOX 1). Drug owing to continuing failures — could be developers have meanwhile been working poised for a renaissance. The number of What’s exciting is not that hard to make next-generation alternatives, ketamine trials has skyrocketed, not only in armed with a preliminary hypothesis for how depression but also for obsessive–compulsive there’s going to be a new the drug lifts moods. disorder, post-traumatic stress disorder drug approved, but that we’re When ketamine is used as an anaesthetic and even chronic pain. “If ketamine works and going to have a whole new or a hallucinogen, it blocks the NMDA we understand the effects of ketamine on class of drug approved receptor. This in turn stimulates the release these different disorders, it could really open of a glutamate burst, which is believed to be the way for drug discovery,” says Lisa responsible for the drug’s hallucinatory NATURE REVIEWS | DRUG DISCOVERY VOLUME 17 | NOVEMBER 2018 | 773 ©2018 Spri nger Nature Li mited. All ri ghts reserved. NEWS & ANALYSIS Table 1 | Select list of ketamine mimetics as antidepressants Drug Company Mechanism Status Esketamine Janssen/J&J NMDAR antagonist NDA Rapastinel (formerly GLYX-13) Allergan NMDAR partial agonist Phase III AV-101 VistaGen Therapeutics NMDAR antagonist Phase II NRX-101 (D-cycloserine plus lurasidone) NeuroRx Pharma NMDAR modulator plus 5-HT2A receptor antagonist Phase II AGN -241751 Allergan NMDAR modulator or partial agonist Phase II AXS-05 (dextromethorphan plus bupropion) Axsome Therapeutics NMDAR antagonist plus norepinephrine and Phase I dopamine reuptake inhibitor Traxoprodil Pfizer NMDAR antagonist Discontinued Lanicemine AstraZeneca NMDAR antagonist Discontinued Decoglurant and basimglurant Roche mGlu modulators Discontinued Rislenemdaz Cerecor/Merck & Co. NMDAR antagonist Discontinued 5-HT2A, 5-hydroxytryptamine receptor 2A; J&J, Johnson & Johnson, mGlu, metabotropic glutamate receptor; NDA, new drug application; NMDAR, N-methyl-D- aspartate receptor. effects. The neurotransmitter then stimulates side effects, researchers hope that success for Others aren’t ready to give up on NMDA other receptors that control gene J&J will lift all boats. inhibition just yet. Monteggia reported earlier transcription to enable rapid rewiring of brain “I think once esketamine is approved, and this year in Neuropsychopharmacology that circuits. This rewiring, or plasticity, is thought it becomes likely a multibillion-dollar drug, when she repeated a similar experiment, she to cause the antidepressant effect. you’ll see big pharma coming back,” says drug found that very high levels of HNK could When developing a pharmaceutical researcher Ronald Duman at Yale University. indirectly block the NMDA receptor through version of ketamine, companies have an as-yet-unknown mechanism. generally decided to target the start of this Upping the AMPA? J&J’s Manji is also skeptical about reading pathway. J&J, for instance, chose to develop Basic research on ketamine’s mechanism of too much into the effect of HNK in mice. If the the S-enantiomer of ketamine because it is action complicates future ketamine-mimetic NMDA receptor is uninvolved, the company’s four times as potent at blocking the NMDA discovery plans, however. In 2016, Gould and esketamine nasal spray should not work as receptor as regular ketamine, which is a mix Zarate published a startling paper in Nature, well as it has, Manji says. He suspects that of R and S-enantiomers. J&J’s Manji says that proposing that a metabolic byproduct previous NMDA antagonist failures can the company has no plans to compare its of ketamine — not the drug itself — was largely be chalked up to dosing problems and product directly with ketamine in a clinical responsible for the mood altering side effect profiles, rather than a problem trial. But overall, esketamine’s side effects — activity in mice. The metabolite (2R,6R)- with the target itself. including hallucinations — seem similar to hydroxynorketamine, or HNK, didn’t seem to Researchers are trying to reconcile these the original drug. interact with the NMDA receptor at all. Nor various results. For instance, ketamine might The company recently published results did it appear to cause the hallucinatory side quickly reverse depression by blocking the from two phase III studies on depression, effects of esketamine, even at doses nearly NMDA receptor, but perhaps HNK is and will conclude a suicidal ideation trial 40 times greater than the normal dose of responsible for maintaining the effect over next year. Clinical trial results were mixed, ketamine. time, says Monteggia. Zarate and Gould are however. In one study of 223 participants, The result suggested that drug developers planning to file for FDA permission later this esketamine significantly reduced depression may have been going after the wrong target year to start clinical trials with HNK in 2019, at 28 days. But the results