469180

Drug Discovery Pathways in Depression: Findings

from KetamineExtended Essay

FHS of Medical and Physiological Sciences2013

Word Count: 2990Topic: A8 – Neuroscience, The Biology of Brain Disorders

1

Introduction

The first evidence of a drug to combat depression was suggested 60 years ago in Selikoff,

Robitzek and Ornstein's study on anti-tubercular agents1, however, pharmacological

intervention in the disorder is still severely limited. Perhaps the most disappointing result of

over half a century of research is that the most widely used class of antidepressants, selective

serotonin reuptake inhibitors (SSRIs), take over 2 months to produce remission in the

majority of responders, with 15% of responsive patients requiring more than 13 weeks of

treatment2. Recently, however, studies into the effectiveness of ketamine in relieving the

symptoms of depression have shown hope that targeting the glutamatergic system could yield

rapidly-acting treatments.

Depression is a prevalent and crippling mental health disorder that is one of the leading causes

of disability, affecting roughly 16% of the world's population3. It is characterised primarily by

depressed mood or loss of interest and pleasure and, though it can bipolar (with manic

episodes between periods of depression), it is in the more common unipolar variant that most

findings from ketamine have occurred, which are reported here. In patients, depression is

tested by scales in an interview whilst in animals a variety of behavioural tests that model

depression are used.

Two such models are the forced swim and tail suspension test used by Trullas and Skolnick4

to generate the first prediction that N-methyl-D-aspartate (NMDA) antagonists could produce

antidepressant effects in 1990. After this, it was almost a decade until Berman et al. provided

the first clinical evidence that ketamine could significantly reduce the symptoms of

depression as measured by the Hamilton Depression Rating Scale (HDRS) after 72 hours

following double blind infusions in seven subjects5. As intravenous infusions of tricyclic

antidepressants had previously6 been shown to produce effects on a similar time-scale, it was

not until a later study by Zarate et al. in 2006 that the speed of ketamine's action was truly

appreciated7. Significance in HDRS scores between the placebo and active drug groups was

achieved just 110 minutes after the infusion.

Should ketamine (or derivatives) be introduced to the clinic, this reduction in latency would

have many advantages beyond reducing hours lost to disability. Such a treatment could be

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used to prevent self harm and suicides if applied during a distressing episode8. Studies have

shown that in children and young adults, starting a course of treatment with traditional

antidepressants seems to increase the risk of suicide attempt9. Therefore, drugs which can

exert a rapid antidepressant action could be used during the latency period of the serotonergic

antidepressants to improve the safety of starting a new course of treatment. Furthermore,

outside the clinic, such drugs could prove useful in researching the pathophysiology of

depression as the changes underlying the antidepressant effects in humans would be expected

to occur on a time-scale more suitable for study than the weeks needed by traditional

antidepressants.

However, before drugs acting via related mechanisms can be approved, there are several

hurdles that must be overcome. Primarily, ketamine does not provide a sustained treatment for

depression. In Zarate's 2006 study, after 71% of patients met the response criteria, only 35%

maintained the response for a week. In a more recent study, Aan het Rot administered

ketamine to depressed patients on days 1, 3, 5, 8, 10 and 12 of his study10, leading to an 85%

mean reduction in Montgomery-Åsberg Depression Rating Scale scores, and 8 of the 9

patients lasting on average 19 days before relapsing. Ketamine is not suitable for continued

administration though, for two mains reasons. Firstly, rapid development of tolerance to

ketamine was demonstrated by Pouget in 2010, in a study involving five species of non-

human primates being presented with saccade tasks11. Secondly, there have been concerns

about the safety profile of ketamine (and NMDA antagonists in general). Apart from the

general psychomotor retardation, Krystal et al. showed that a sub-anaesthetic dose of

ketamine (0.5mg/kg) could elicit emotional blunting and withdrawal as well as impairing

memory in the short term, delaying word recall12, but more worrying are findings that

blockade of NMDA receptors in adult13-14 and developing15 rats is neurotoxic, whilst in

humans administration of ketamine can lead to psychosis16. Furthermore, clinical use of

ketamine may be further limited by its potential for abuse17 and addiction18.

Mechanisms of Ketamine's Antidepressant Action

Isolating the mechanisms by which ketamine produces its antidepressant effect is crucial in

attempting to remove the undesirable aspects of its application. Though several classes of

receptor have been shown to be implicated in its effects19, Harrison20 was the first to show

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ketamine non-competitive antagonist effects on the NMDA channel, which is thought to be

responsible for antidepressant action of the drug. Evidence for this hypothesis initially comes

from Paul's findings that standard antidepressant treatment modulates the NMDA receptor as

a final common pathway21. More recently, the glutamatergic system has been shown to be

dysfunctional, leading to elevated occipital and prefrontal and decreased anterior cingulate

cortex glutamate levels in patients with depression using proton magnetic resonance

spectroscopy22 and post-mortem studies23.

Though ketamine does result in NMDA antagonism, a recent study by Cornwell24 et al.

showed that this was not in fact necessary to produce antidepressant behavioural effects in

mice. Instead, synaptic potentiation via an increased α-amino-3-hydroxy-5-methyl-4-

isoxazolepropionic acid (AMPA) receptor throughput has been hypothesised as leading to the

rapid amelioration of the symptoms of depression via synaptic potentiation. Maeng et al.

tested this theory using NBQX (an AMPA antagonist), showing that it only attenuated a

decrease in tail suspension test immobility time when given prior to ketamine (and not on its

own or prior to imipramine treatment)25. Supporting the role of AMPA potentiation as an

antidepressant mechanism, Chourbaji et al. created a knockout mouse with the AMPA GluR-1

subunit deleted26. These knockout mice showed increased learned helplessness, as well as

neurochemical signatures of depression (such as reduced serotonin and norepinephrine

levels). The traditional antidepressant imipramine has been shown to increase the synaptic

expression of this subunit27 whilst another tricyclic antidepressant (desipramine) and the SSRI

fluoxetine have been found to reduce GluR-3 levels28 in rats, suggesting that AMPA receptor

modulation is involved in their antidepressant action.

Unlike traditional antidepressants, however, ketamine may activate the mammalian target of

rapamycin (mTOR). Li et al. used western blot analysis to show that ketamine administration

increased levels of phosphorylated and activated forms of both mTOR and growth factors

linked to mTOR activation in rats as well as raising levels of postsynaptic proteins, indicating

increased synapse and spine formation29. Moreover, ketamine was also shown to produce

rapid antidepressant effects in forced swim and learned helplessness tests, which crucially was

blocked by infusion of rapamycin (an mTOR inhibitor) into the prefrontal cortex. Since these

findings, Jernigan et al. have studied post mortem brain samples of 12 patients with

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depression against 12 controls30. Their western blot analysis showed significant reductions not

only in mTOR, but also in p70S6K, eIF4E, and eIF4B (core downstream signalling targets of

the pathway) in the prefrontal cortex of those subjects that had suffered from depression.

However, the role of mTOR in depression is not settled as rapamycin has been reported to

have antidepressant properties itself31 (though mTOR inhibition is yet to be shown as the

mechanism) and as in Autry's study into behavioural response to ketamine administration in

mice, pre-treatment with rapamycin had no effect after 30 minutes32. It may therefore be

possible that the action of the mTOR pathway is one of maintenance, as indicated in Li's study

which tested for symptomatic relief 24 hours after administration of ketamine.

The study by Autry suggested that changes in the level of Brain Derived Neurotrophic Factor

(BDNF) might be responsible for the rapid-acting antidepressant properties of ketamine. After

half an hour, ketamine administered to wild-type mice produced antidepressant effects in

forced swim and novelty feeding tests but no effect was observed when ketamine was given to

inducible BDNF knockout mice. Western blot and ELISA analysis also showed increases in

BDNF levels, though no changes in the levels of mRNA were observed at either 30 minutes or

24 hours after ketamine infusion, leading the authors to propose that increases in BDNF

translation (and not transcription) account for the effects of ketamine. This built on previous

work by Garcia that had shown that whilst both ketamine and imipramine reduced tail

suspension immobility, only ketamine increased the levels of BDNF in the hippocampus of

rats33. More evidence for the role of increased BDNF relieving depression was demonstrated

by Hoshaw et al. who showed that a single introcerebroventricular infusion of BDNF into a

rat could decrease immobility in the tail suspension test for up to 6 days34. BDNF dysfunction

has also been implicated in post mortem studies of 30 humans who committed suicide35. The

study by Karege et al. showed significant reductions of BDNF in the prefrontal cortex and in

the hippocampus compared to non-suicide controls, regardless of the diagnostic status of the

suicide victims. However, as with mTOR, there is still controversy as to the importance of

BDNF in the action of ketamine. After ketamine administration, no changes in the level of

BDNF in the blood of 23 patients of depression were seen in a study by Machado-Vieira36.

Potential Clinical Drugs

In spite of Machado-Vieira's findings, drugs that target BDNF have still been shown to elicit

5

antidepressant behavioural changes in mice. Drugs that act as antagonists for the group I

metabotropic glutamate receptors (mGluR1 and mGluR5) are known to increase hippocampal

BDNF mRNA37 and produce antidepressant effects when applied to rodents. When applied to

mice, EMQMCM and MTEP (antagonists for mGluR1 and mGluR5 respectively) decreased

tail suspension test immobility and also caused increased escape duration in the force swim

test38 with effects of a similar magnitude to those seen after imipramine administration.

Promisingly, Inta showed via c-Fos mapping that MPEP (a mGluR5 antagonist) activated

similar stress-related brain areas as traditional antidepressants, but even at very high doses did

not induce the neurotoxicity or schizophrenia-like behavioural changes that are characteristic

of NMDA antagonists39. In addition, Hoffmann-La Roche have completed a phase 2 trial for

an mGluR5 antagonist, RO4917523 as an adjunctive therapy40 with another still recruiting

participants41, though a recent trial by AstraZeneca for their mGluR5 antagonist, AZD2066,

was terminated as the company withdrew from drug discovery in depression42.

The group II metabotropic glutamate receptors have also been targeted in novel

antidepressants via antagonism of mGluR2 and mGluR3. Studies have shown that mGluR2/3

antagonists LY34149543 and MGS003944 have rapid antidepressant activity, and that, in line

with findings from ketamine, pretreatment with rapamycin blocked the sustained (but not

acute) antidepressant effects of these drugs45-46. Hoffmann-La Roche also have a phase 2 trial

for this class of drugs with RO4995819 that is still recruiting47 and BrainCells Inc. have

recently completed two loading dose studies into MGS0039 (under the name BCI-632)48-49 in

healthy volunteers. However, unlike with the group I antagonists, there have been concerns

over the safety profile of these drugs. Any drug that activates the mTOR pathway could

accelerate tumour growth, making it unsuitable for use in cancer patients50, but also as

sustained induction of mTOR (due to genetic mutations) has been shown to be associated with

diseases such as autism and Fragile X syndrome51.

In addition to targeting the mTOR pathway via mGluR2/3 receptors, MGS0039 was shown to

activate AMPA receptors in studies where NBQX attenuated its effect on the release of

serotonin and the tail suspension test52. The potential antidepressant action AMPA potentiators

was shown by Li who reported a reduction in the immobility of rats given LY392098 in the

tail suspension test which was blocked by NBQX53. This drug was later found to also enhance

6

the effect of traditional antidepressants such as fluoxetine, citalopram and imipramine, with

greater than ten-fold shifts in their dose-effect functions54. Another AMPAkine, LY451646,

was also shown to have antidepressant activity in BDNF+/−heterozygous null mice55 in the

forced swim test, producing effects 45 minutes after infusion. As with other targets for

glutamatergic antidepressants, potential clinical drugs are in clinical trials, with Zarate and the

National Institute for Mental Health recently completing a trial into the effectiveness of ORG

24448 to treat depression56. In addition, drugs that regulate AMPA receptor membrane

localisation, such as lamotrigine and riluzole, have been suggested as pathways for the

development of novel antidepressants57 and have been shown to be clinically useful as an

adjunctive therapy in depression58-59.

Finally, novel antidepressants that act as NMDA antagonists are also in development. In their

study on the actions of ketamine, Maeng and Zarate25 showed that Ro25-6981, an NR2B

subunit specific antagonist of the NMDA receptor, also produced antidepressant effects in

mice, though this was not as long-lasting as that produced by ketamine. Traxoprodil (CP-

101,606) is also an NR2B specific antagonist of NMDA that has been shown in humans to

produce an antidepressant response. In their study60, Preskorn first administered paroxetine for

6 weeks with a placebo infusion at 3 weeks. Those that did not respond to this treatment were

then randomised to receive a second infusion of either traxoprodil or a placebo whilst

continuing with paroxetine treatment. Those receiving the active drug in the second infusion

showed greater reductions, with a higher response rate to treatment, in the Montgomery

Asberg Depression Rating Scale than those who received a second placebo infusion.

Surprisingly traxoprodil produced dissociative symptoms in 40% of patients, though this

effect seems separate to the drug's antidepressant action as some subjects that did not

experience this effect and not all of those that did met response criteria. These side effects

have been hypothesised to be absent in the NR2B antagonist MK-0657 which completed it's

phase 1 study this month (June 2012)61 Disappointingly, however, Evotech recently

terminated its phase 2 trial of their NR2B antagonist EVT 101 due to problems with

participant recruitment62.

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Drug Name Proposed Mechanism(s) Clinical Status in depression

RO4917523 BDNF increases via mGluR5 antagonism36 Ongoing phase 2 studies40,41

AZD2066 BDNF increases via mGluR5 antagonism36 Terminated phase 2 study42

MGS0039 mTOR induction via mGluR2/3 antagonism44 Two phase 1 studies complete48.49

RO4995819 MTOR induction via mGluR2/3 antagonism Phase 2 study recruiting47

ORG 24448 AMPA potentiation Completed phase 2 study56

Lamotrigine AMPA receptor membrane localisation57 Used as an adjunctive therapy58

Riluzole AMPA receptor membrane localisation57 Used as an adjunctive therapy59

CP101,606 NR2B antagonist70 No current studies, shown to produce antidepressant effects in humans60

MK-0657 NR2B antagonist71 Completed phase 1 study61

EVT101 NR2B antagonist Cancelled due to subject recruitment62

Table 1: Showing the proposed mechanisms and clinical status of promising drugs acting on the glutamatergic system to treat depression

Future Prospects for Glutamatergic Antidepressants

Whilst incomplete, the hypothesised mechanism of action has yielded potential targets for

depression, and drugs that act on these have shown promising results in rodents and are

completing clinical trials in humans (table 1). Therefore, whilst there have only been small

gains in clinical treatment for depression from this research so far, there is still hope that the

pathways outlined above could be targeted by novel antidepressants which could become

primary treatments for depression. Perhaps more excitingly still, the rapid effects of ketamine

might allow it to be used as a short term treatment to prevent suicides7 or during a particularly

distressing episode to relieve the symptoms of depression, which is not possible with current

medications.

These compounds have been shown to be potential treatments for depression as, aside from

their potential clinical benefit, they are predicted to lack the undesirable features of ketamine

treatment. It is unsurprising, given their novelty, that none of the drugs listed in table 1 have

been reported to have abuse potential, but promising that the group I metabotropic

glutamatergic receptor antagonists were shown to lack the neurotoxicity of ketamine39.

8

Though it was surprising that traxoprodil produced psychomimetic effects, these were less

common than in ketamine treatment and were unrelated to its antidepressant effects. However,

as table 1 also makes clear, many of these drugs are yet to enter the clinic and so judgement

on side effects is limited.

Aside from potential clinical benefits, the shifting focus of antidepressant research away from

the monoamine based therapies might well be a positive in itself. Half a century after the

discovery of the first antidepressants1, almost all still act on the serotonergic system and most

have the same mechanism of action as the tricyclic antidepressants, discovered in 1957 by

Kuhn63. In addition, over the last decade very few new antidepressants have been marketed

and the large pharmaceutical companies GlaxoSmithKline and AstraZeneca have withdrawn

from drug discovery research in depression. Even though it has yet to pay off in the form of a

novel treatment, research into the glutamatergic system in depression might prompt interest in

atypical pathways and potential antidepressants rather than variants on serotonergic agents.

Although caution over the potential of these agents must still be exercised, with very few

potential drugs making it through clinical trials, the range of novel targets currently

undergoing clinical trials is promising – even if it may still be some years before these drugs

can be used to treat patients. Much current evidence is from studies in rodents, the use of

which in modelling depression has raised concerns64. Additionally, though there is much

evidence to suggest that glutamatergic agents can have antidepressant effects, none of these

have been tested against active placebos and this may have inflated their antidepressant

effect65. The selective NMDA antagonists especially have clear side effects that may unblind

subjects to the drug they are receiving, inflating the antidepressant effect. Even ketamine is

only just entering clinical trials with an active placebo (midazolam)66-67 and the results of these

trials may show ketamine to be a less effective antidepressant than is currently believed.

Though clearly the research into glutamatergic antidepressants has yet to come to fruition in

treating patients, just over a decade since Bergman's initial study4, a putative pathway for the

action of ketamine has been described and drugs to target these pathways without the

undesirable general effects of ketamine have entered clinical trials. Given that the past decade

was particularly poor for the discovery of novel antidepressants68, and that the World Health

9

Organisation predicts that by 2030 unipolar depression will be the leading cause of burden

due to disease in the world69, it is crucial that this work is continued. If the drugs outlined here

can pass phase 3 trials and be approved for the clinic, then not only will these offer hope for

sufferers of depression resistant to treatment by serotonergic drugs and potentially be the first

drugs able to give rapid-acting relief from depression, but may also shift the focus of drug

discovery in depression to the glutamatergic system via the multiple plausible targets outlined

here.

10

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