May 6, 2014. Psychiatric illnesses exert one of the greatest health burdens in developed countries, and most drug therapies for these disorders are based on discoveries made decades ago. Yet drug companies are leaving psychiatry. This is, in part, because of a lack of biomarkers that could help determine quickly and easily if therapies and drugs actually work in the brain, according to Siva Digavalli of Bristol-Myers Squibb, who helped organize a conference on April 8 on “Translational Neuroscience in Psychiatry: Light at the End of the Tunnel” at the New York Academy of Sciences. Digavalli said one main goal of the gathering was to review the state of research on biomarkers in schizophrenia and depression.
Considering the many requirements biomarkers have to fulfill, it is perhaps not surprising that they are hard to find: They should be reliable, noninvasive, and ”translatable,” or consistently show the same differences in animal models as in humans.
But if the findings presented at the meeting are any indication, there is indeed light at the end of the tunnel, as the meeting’s title suggests. Speakers described several promising electrophysiological biomarkers for schizophrenia and depression that are based on using electroencephalography (EEG) to measure the tiny electric currents in the cortex with surface electrodes applied to the scalp. One of them, mismatch negativity (MMN), may be robust enough to be used in clinical trials for schizophrenia drug candidates. Speakers also discussed ATP levels in the brain and genes whose expression can be measured in blood as new potential depression biomarkers, and the anesthetic ketamine and similarly acting compounds as an emerging new class of antidepressants.
Electrophysiological biomarkers in schizophrenia
Perhaps the best-established class of biomarkers involves EEG. One of the most extensively validated EEG markers is MMN, a response by many brain areas, including the auditory cortex, to an infrequent "oddball" sound after a series of expected sounds. Because MMN doesn’t require attention, it’s easy to measure and very reproducible, said Daniel Javitt of Columbia University, who led one of the two studies that first showed more than 20 years ago that MMN is impaired in schizophrenia patients (Javitt et al., 1993).
Since then, MMN has emerged as the biomarker with one of the largest effect sizes in schizophrenia patients, said Gregory Light of the University of California in San Diego. “It’s among the most widely replicated, highly significant deficits in all of schizophrenia research,” he said, adding that MMN can predict whether someone who is at risk will eventually develop schizophrenia with about 60 percent accuracy—much higher than the 2-25 percent accuracy of other measures (Perez et al., 2014). Light said he recently found that large MMN deficits can even predict whether schizophrenia patients will respond to some forms of cognitive and social skill training. What’s more, he said, the MMN is strongly linked to clinical symptoms, and to cognitive and real-world abilities to navigate through life in schizophrenia patients and healthy people (Light and Braff, 2005). “[MMN has] been described as a breakthrough biomarker—the one that we have been waiting for in psychiatry,” he said.
MMN is also translatable: Light said there are now reliable measurements for MMN in monkeys and rats, and drugs seem to affect the MMN in animals the same way they do in humans. One such drug is ketamine, which inhibits NMDA-type receptors for the neurotransmitter glutamate. It is used to create animal models of schizophrenia, because it causes symptoms that mimic the disorder in humans. It is hypothesized that schizophrenia is caused, at least in part, by less active glutamate signaling due to impaired NMDA receptors. Ketamine has been shown to impair the MMN in healthy humans, mice, rats, and monkeys, Light said. At the meeting, Digavalli reported that in rats, a chemical compound that inhibits an NMDA receptor subtype that’s primarily found in the cortex impairs the MMN as well.
Javitt said he tracks the human MMN in response to drugs that do the opposite—improve the MMN by further activating the NMDA receptor—which would be expected to alleviate schizophrenia-related symptoms. He found, for example, that the same dose of the NMDA agonist D-serine that improved MMN in chronic schizophrenia patients also improved the MMN in adolescents who were at risk to develop the condition. This was accompanied by an improvement in their negative symptoms, such as number of social interactions. Researchers have even started to use MMN as a biomarker to track the response to schizophrenia candidate drugs in early clinical trials, Javitt said. If the results hold up in later trials, companies may finally be willing to get back into the field of psychiatry, he added.
One obstacle to using the MMN in larger, especially multicenter clinical trials, however, is the belief that only highly trained technicians in specialized hospital EEG labs can measure it, Light said. “For biomarkers to take off, we need to get out of specialty laboratories and into real-world clinical settings,” said Light, adding that he has been involved in efforts to achieve just that: He has been advising the company San Diego Instruments in their development of a device that anyone can use (after a few hours of training) to measure MMN in about 20 minutes with just a few sensors placed on the scalp.
So far, the results look promising: Light’s analysis of data collected from nearly 2,000 people (half of them schizophrenia patients) at several research centers across the U.S. shows that the device accurately measures MMN deficits in patients and healthy volunteers. Light and colleagues now plan to use these data for gene association studies to identify genetic markers that correlate with MMN deficits. Light has also tested if he can accurately measure the MMN in schizophrenia patients and healthy volunteers with an even simpler single-electrode, wireless device made by the company Neuroverse (a start-up that Light also advises) that can be used with an iPhone or an iPad. The results are promising: "We are very encouraged by the fidelity of the data," Light said.
Another, more recently identified EEG biomarker candidate is a 40-Hz oscillation of the auditory cortex in response to a 40-Hz tone. The response has been found to be reliably impaired in schizophrenia patients, said Digavalli, who now has data showing that in rats, the NMDA receptor inhibitor MK-801 impairs the 40-Hertz oscillation as well. His team found that other compounds that inhibit the NMDA receptor can also modulate the 40-Hz signal, suggesting that the signal could indeed be used as a translatable functional biomarker for the development of drugs that target the NMDA receptor.
Javitt discussed yet another potential EEG biomarker for schizophrenia: an alpha rhythm in the visual cortex of around 10 Hz. The rhythm is strongest when the eyes are closed, but suppressed when the eyes are open to allow processing of incoming stimuli. To see if this is also the case in schizophrenia patients, Javitt and colleagues asked volunteers to press a button when they saw the letter A followed by X on a computer screen, but not if they saw the letter B followed by X.
Schizophrenia patients couldn’t distinguish very well between the two and pressed the button in either case; what’s more, their visual cortex failed to inhibit the alpha activity when they performed the task (Dias et al., 2013). The schizophrenia patients “didn’t put their brain into gear” to prepare for the task and process information, Javitt said. Alpha inhibition could be used not only as a biomarker, he added, but also as a biofeedback readout to train schizophrenia patients or people at risk to develop it to improve their brain activity.
Too much noise
Failure to suppress the alpha rhythm in the visual cortex isn’t the only way that brains of people with schizophrenia can show too much activity. Steven Siegel of the University of Pennsylvania discussed the observation that certain areas of the subjects' cortex also have increased gamma activity at frequencies above 30 Hz. For a long time, he said, this “noise” has been dismissed as an artifact, but when he and his colleagues took a closer look in mice, they found evidence that it might be real.
They engineered mice with impairments in a subunit of the NMDA receptor, modeling the putative glutamate transmission deficit of schizophrenia. They then measured the EEG response of their auditory cortex to a tone. The cortex of the engineered mice showed less of a signal in response to the tone than normal mice, but it was also more active, even in the absence of the tone. This increased activity was in the frequency range above 30 Hz, consistent with the noise that’s been observed in the brains of schizophrenia patients.
Because the noise seems to come from pyramidal neurons in humans, Siegel and colleagues also engineered mice whose NMDA receptors were only impaired in these neurons. The EEG noise was still present, and when they injected current into single neurons of brain slices taken from these mice, the pyramidal neurons in the slices consistently fired too much. To Siegel, this suggests that impaired NMDA receptors on pyramidal neurons could be responsible for the noise, perhaps by changing the membrane channels to make the neurons fire more easily. Indeed, a substance that inhibits neuronal activity by opening chloride channels reduced the noise in the brain slices from the mice.
If the findings hold up in humans, Siegel said, then one way to treat schizophrenia could be to develop drugs that can reduce the noise in the pyramidal neurons.
Depression: ketamine and beyond
Inhibiting NMDA-type glutamate receptors with ketamine is one of the major ways to mimic schizophrenia-like symptoms in animals and humans. But 14 years ago, John Krystal of Yale University and colleagues found that ketamine can alleviate depression in humans, suggesting that glutamate is also involved in depression (Berman et al., 2000).
At the time, Krystal was surprised that ketamine alleviates depression within 24 and 48 hours—much faster than traditional antidepressants, which can take weeks to show an effect. But by now, many studies have confirmed this, suggesting that ketamine is a good drug in crisis situations, for example, for patients with suicidal thoughts, said Krystal. In fact, he added, one dose starts to reduce symptoms within four hours, its effects last for about three weeks, and weekly or biweekly administration has been shown to have antidepressive effects for up to 18 months.
A single dose of ketamine is effective in 40-60 percent of treatment-resistant cases of depression, whereas traditional drugs only improve symptoms in about 10-15 percent of such cases, Krystal said. In light of these promising findings, there is now “a great deal of interest and enthusiasm” to turn the observations with ketamine into a treatment for patients with depression, he said.
But that’s easier said than done. For one thing, ketamine is usually given as an infusion, and the FDA has only approved it as an anesthetic. This means that doctors must prescribe it off label, and insurance companies are unlikely to pay for it. And because the patent for traditional ketamine (which is a mix of two isoforms) has expired, companies aren’t very interested in financing the clinical trials required to get FDA approval for depression, Krystal said, though he added that the company Johnson & Johnson is currently testing its own patented version of a nasal spray containing S-ketamine, the more effective isoform, in humans.
Meanwhile, researchers are also developing alternative drugs that affect glutamate levels in a similar way to ketamine. In part, that’s because of concerns that long-term ketamine use could cause psychosis, and people could abuse ketamine because it induces euphoria, says Javitt, who is working on an NMDA receptor antagonist called D-cycloserine, which also increases glutamate levels but doesn’t cause euphoria and could be taken as a pill. Dan Iosifescu of the Icahn School of Medicine at Mount Sinai, New York City, reported that he found another novel compound that may act as an antidepressant in humans: the antibiotic minocycline. He found that it not only changes glutamate levels, but also increases the levels of glutathione, the most important antioxidant in the brain.
Biomarkers for depression
Traditional antidepressants typically take weeks to show effects and sometimes don’t work at all, which is why psychiatrists need time and a lot of trial and error to find a drug that works. As a result, about half of depressed patients stop coming to their psychiatrists after a few weeks, said Andrew Leuchter of the Semel Institute at the University of California in Los Angeles. Researchers are therefore trying to identify biomarkers that can help doctors find out sooner whether an antidepressant has any effect on the brain.
Depression is hypothesized to stem partly from impaired communication between limbic structures, which generate emotions, and the prefrontal cortex, which normally controls them. As a result, negative emotions often get out of control, Iosifescu said. He found that depressed patients have lower levels of the energy carrier ATP in the anterior cingulate, which processes the information exchange between these two regions, and ATP levels reached normal levels only in the patients whose depressive symptoms improved after treatment with thyroid hormone. Lower ATP levels, and compensatory increases in phosphocreatine (a longer-term form of energy storage), in the anterior cingulate could therefore be used as an early biomarker to find out which patients will respond to antidepressants, Iosifescu said.
Oscillations in the prefrontal cortex could also be used as a biomarker, said Leuchter, who found that in depressed people, this area tends to oscillate more slowly. But when he gave them an SSRI antidepressant called escitalopram, patients whose oscillations accelerated a week after they started taking the drug were at least five times more likely to completely recover from depression after taking the drug for seven weeks than patients who didn’t show any acceleration.
Many depressed patients first report their condition to general practitioners, half of whom fail to correctly diagnose it, said Eva Redei of Northwestern University. To address this issue, Redei is trying to identify blood-based biomarkers, which she said would enable even non-specialist doctors to correctly diagnose the condition.
Because depression is thought to be caused in large part by genetic factors and stress, Redei and her colleagues looked for genes that are expressed at abnormal levels in an inbred rat strain that inherited symptoms of depression, and in normal rat strains that were exposed to stress. This led to the identification of 11 related human genes that are expressed at abnormal levels in the blood of depressed adolescents compared with healthy controls (Pajer et al., 2012). Results of a similar, bigger study in adults are promising, said Redei, who now plans to obtain large enough numbers to get FDA approval for the test. If successful, this would be the first FDA-approved blood-based diagnostic test for depression that could be done in any laboratory, she said.
The many promising biomarkers discussed at the meeting are a reason for hope that companies will want to get back into the field of psychiatry, Digavalli said after the meeting. “That hopefully will encourage people to not quit the area and [instead] capitalize on these kinds of discoveries,” he said.—Andreas von Bubnoff.