1. The Role of Neuroimaging in Clinical Trials and Drug Discovery In Psychiatry
Joseph C. Masdeu, MD, PhD
Section on Integrative Neuroimaging
National Institutes of Health

2. Neuroimaging in Drug Discovery and Development
Preclinical models
Early clinical experiments to demonstrate 'proof of biology'
Linking target engagement (TE) to
drug-induced biological changes expected to give clinical benefit
Clinical trials to demonstrate proof of concept (PoC),
Engaging a particular target is linked to
a meaningful change in a clinical end point
thus demonstrating a new avenue to treat a condition in patients

3. The Glycine Transporter 1 and Schizophrenia
In schizophrenia NMDA function is impaired
Lisman JE et al. Trends Neurosci. 2008;31:234
GlyT-1 regulates glycine levels at NMDA sites
Glycine is an essential co-agonist of NMDA receptors
GlyT-1 inhibitors enhance NMDA function (preclinical studies)
Selective GlyT-1 inhibitors are being studied as drug candidates for schizophrenia
Catafau A et al. Neuroreceptor Meeting. Pittsburgh 2008

4. [11C]GSK931145: A New PET Ligand for Glycine Transporter 1
Transporter characteristics
Autoradiography
Physiology
Pharmacology

5. [11C]GSK931145: A New PET Ligand for Glycine Transporter 1
Pig
Primate
Human
[11C]GSK distribution in a healthy volunteer

6. [11C]GSK931145: A New PET Ligand for Glycine Transporter 1
This new GlyT-1 PET ligand is being applied to:
Understand the role of GlyT-1 in Neuropsychiatric disorders
Drug development
pharmacokinetics (PK)
receptor occupancy (RO)
pharmacodynamics (PD)
dose estimations  

7. New Drug Characteristics

8. Uses of PET in Drug Development
Using established or newly developed PET radiotracers for a particular target
E.g., dopamine (DA) or serotonin receptors and transporters
Determining the degree of target engagement needed to exert therapeutic effects
Norepinephrine transporter occupancy in ADHD
Cannabinoid CB1 occupancy in obesity and other psychiatric disorders
To study an enzyme effect
Second messenger systems as phosphodiesterases I–V in depression

9. Properties of Drug Candidate Determined by Neuroimaging
Whether it crosses the blood–brain barrier
And, thus, it is delivered to the target compartment (ie the brain)
Whether it engages the appropriate target
Receptor
Transporter
Enzyme
in a dose/exposure-related manner

10. Uses of PET in Drug Development
Two main approaches
Radiolabel the new drug
Estimate target occupancy
By the new drug

11. Drug Candidate Biodistribution and Kinetics
Usual Radioisotopes:
PET: [11C], [18F]
SPECT: [123I], [99mTc]
Ideally, a new lead drug
candidate should be isotopically radiolabeled with a PET or SPECT radioisotope
To determine brain distribution of the compound
Some of its washout characteristics
Whether it is a substrate for BBB pumps
More likely to induce multidrug resistance
Also helpful in animal studies

12. Limitations to Radiolabeling Candidate Drugs
Primary quantitative radiotracers label only a very small number of potential drugs
Brain exposure for compounds that cross the BBB slowly may be underestimated
Given the short acquisition times possible with these isotopes
90 min 11C
8 h for 18F
Higher brain concentrations after chronic dosing
True target engagement may be overestimated
Total regional brain activity is recorded, not distinguishing between
binding to the test drug
brain-penetrated radiolabeled metabolites of the test drug
free tracer
tracer nonspecific binding

13. Instead of Radiolabeling Candidate Drug: Label Drug Target
Use existing PET/SPECT tracers
E.g., to the serotonin transporter
to determine the target occupancy of the new drug
in displacement studies
More active drug
Less labeled target available

14. Serotonin Trasporter Occupancy After Rx with Paroxetine
Studied with 123I-ADAM SPECT
Decrease in midbrain uptake (71% serotonin transporter occupancy)
Baseline
After 6-week paroxetine 20 mg/day
Catafau AM et al. Psychopharmacology 2006;189:145

15. Serotonin Trasporter Occupancy After Rx with Paroxetine
But baseline was similar
Healthy controls
Depressed patients
Catafau AM et al. Psychopharmacology 2006;189:145

16. PET versus SPECT for the Study of D2 Receptor Occupancy
Comparing:
[123I]IBZM SPECT
[11C]Raclopride PET
In the same subjects
Occupancy values measured by SPECT were lower than those measured with PET
12.4% (occipital cortex as reference region)
13.8% (cerebellum as reference region)
But the correlation between D2 occupancy using either method approximates 1.0
Catafau A et al. Neuroimage 2009, Epub ahead of print

17. PET: Better Anatomical Definition than SPECT (D2 Receptor Occupancy Study)

18. Functional MRI (fMRI) Arterial spin labeling (ASL)
To study
brain systems responses to external stimuli
the modification of these responses by drug treatment
In normal healthy volunteers
In patients
Complement neuroreceptor imaging by revealing the neurocircuitry involved in behavior and responses

19. fMRI as Pre-Biomarker The SSRI antidepressant citalopram
Fearful
Happy
Left
Right
fMRI as Pre-Biomarker
The SSRI antidepressant citalopram
reduced amygdala activation in response to fearful faces in normal volunteers
The amygdala response to fearful stimuli
could develop into a pre-biomarker for antidepressant effects
Harmer CJ et al. Biol Psychiatry 2006;59:816

20. Role of MRI in Drug Distribution Proton (1H) Spectroscopy (MRS)
Assessment of glutamate and GABA amino acids
Psychotropic drugs with GABAergic or glutamatergic mechanisms of action
eg acamprosate, for the Rx of alcohol dependence
7Li-MRS has been used to measure lithium concentrations in patients' brains
and to relate it to plasma concentrations (Plenge et al, 1994)
19F-MRS has been used to determine concentrations of fluorinated antipsychotics
fluphenazine and trifluoperazine (Durst et al, 1990)
And antidepressants such as fluoxetine (Karson et al, 1993)
Also contain a fluorine atom
perphenazine, risperidone, most of the butyrophenone antipsychotics,
the SSRIs fluvoxamine and paroxetine
several benzodiazepines and the benzodiazepine antagonist flumazenil

21. Effect of Cytidine on Bipolar Depression and on Brain Glutamate Levels
Region Studied
Anterior Cingulate Gyrus
MR spectrum
Yoon SJ et al. Neuropsychopharmacology 2009 [Epub ahead of print]

22. Cytidine Effect Linked to  Glutamate
Studied with
MR spectroscopy
Scores on the Hamilton Depression Rating Score
Yoon SJ et al. Neuropsychopharmacology 2009 [Epub ahead of print]

23. Biomarker Characteristic that is objectively measured and evaluated
as an indicator of
normal biologic processes,
pathogenic processes, or
pharmacological responses to a therapeutic intervention
Frank R, Hargreaves R. Nat Rev Drug Discov 2003;2: 566

24. Types of Biomarkers Type 0 Type 1 Type 2
Tracks the natural course of the disease
Type 1
Examines the effects of intervention along with the known mechanism of action of the drug
But without strict relationship to clinical outcome
Type 2
Change in the biomarker is predictive of clinical outcome
At present, most imaging methods in psychiatry do not meet the biomarker status

25. ‘Emerging’ Biomarker or ‘Pre-biomarker’
With dopamine D2 receptor PET imaging
A link was found between
Useful to
fortify Proof of Concept for D2 DA antagonists
establish a dose range to test for new, pure D2 antagonists
Dopamine D2 receptor occupancy
efficacy to treat delusions, hallucinations

26. Dopamine D2 Receptor Binding
D2 binding in a healthy control
Measured with
[11C]raclopride
Most widely used
[11C]FLB 457
Better for cortical D2
But still a [11C] compound
[18F]fallypride
Better for cortex, [18F]
But scanning time about 4 hours
[11C]FLB 457 PET
Takahashi H et al. Biol Psychiatry 2006;59:919

27. Dopamine D2 Receptor Binding in Schizophrenia
Decreased D2 receptor binding in untreated schizophrenia
Thalamus
Dorsomedial nucleus
Striatum
Anterior cingulate
Amygdala
Temporal cortex
Patients < Controls
Buchsbaum MS et al. Schizophr Res 2006;85:232
ACNP 2008, Phoenix Az

28. Reduced D2 Receptor Binding: Greater Receptor Occupancy by Endogenous DA
Striatal dopamine (DA) production is increased in schizophrenia
Measured with
[18F]fluoro-DOPA PET
 dopamine causes
down-regulation of the D2 receptors
 stimulation of the D2 receptors
Hallucinations
Treated by D2 antagonists
18FDOPA striatal Ki values
Controls
Patients
Meyer-Lindenberg A et al. Nat Neurosci 2002;5:267

29. DA D2 Receptor Antagonists in Schizophrenia
Drug: Haloperidol
D2 binding studied with [11C]raclopride
Effective: <80% occupancy
>80% occupancy: parkinsonism or akathisia
Kapur S et al. Am J Psychiatry 2000;157: 514

30. PET* to Assess Drug Behavior
Straightforward to assess:
Neutral orthosteric site antagonists
Haloperidol
Considerable challenges in the assessment of target engagement (TE) and linking TE to efficacy for:
Agonists
Partial agonists
Aripiprazole
Inverse agonists
Allosteric modulators
Many new therapeutic approaches to psychiatric disorders use positive allosteric modulators
As a means to fine-tune the primary excitatory (Glu) and inhibitory (GABA) systems
*In combination with structural MRI

31. Partial Agonists: Occupancy Studies of the Antipsychotic Aripiprazole
Conventional wisdom:
within a 'therapeutic window' of 65–80% striatal D2 receptor occupancy
D2 DA antagonists have antipsychotic efficacy
with minimal EPS side effects
But for aripiprazole, occupancies closer to 90 or 95% were needed for the therapeutic range of the drug
Because the likely mechanism of action of aripiprazole is partial agonist at D2 receptors
The original 'therapeutic window' of 65–80% receptor occupancy is valid for D2 DA antagonists only

32. Antidepressants and dosing
SSRIs have been shown to occupy 80% or more of the serotonin transporter (SERT)
at clinically used doses
Within this class of drugs, this seems to be independent of the specific SSRI
Meyer JH, et al. Am J Psychiatry 2001;158:1843

33. Antidepressant Dosing and PET: The Clomipramine Paradox
The tricyclic antidepressant (TCA) clomipramine
occupies 80% of the SERT
at doses as low as 10 mg
at a plasma concentration of 1.42 ng/ml
Yet, clinically used doses are 50–150 mg per day
Therapeutic plasma concentrations range 175–450 ng/ml*
Is blockade of the SERT and the norepinephrine transporter (NET) the therapeutic principle of clomipramine (and of the TCAs in general)?
Or do TCAs behave completely different from SSRIs, due to their broad pharmacological actions at many different molecular targets?
How valid are the studies upon which therapeutic doses and plasma concentrations have been determined for clinical use of the TCAs over decades?
*Baumann P et al. Pharmacopsychiatry 2004;37:243

34. Agonists and Partial Agonists: Complex Pharmacology for PET Occupancy Studies
Agonists exert their pharmacological effects with only a few percent occupancy
For instance, occupancy of the GABAA–benzodiazepine receptor site by benzodiazepines is only 15–25% at full pharmacological effect
These compounds act with a very high receptor reserve

35. Dosing and PET
A radiotracer/pre-biomarker that has been demonstrated to predict the biological effects of a certain class of compounds
might lose its validity for a drug
with a slight modification of its mechanism of action
even if it binds to the same target molecule

36. Imaging and Dose Finding for Clinical Trials: Rejecting New Drugs
If a dose that demonstrated adequate target engagement in humans
With a high degree of confidence
does not have efficacy in the clinical trial
Proof of concept can be rejected, and a drug target can be abandoned more quickly

37. Rejecting a Disease Mechanism: Role of PET
Aprepitant blocked the neurokinin 1 receptor
Visualized with [18F]SPA-RQ PET
But did not improve depression
No better than placebo on the Hamilton Depression Scale
Aprepitant Rx
Baseline
After 160mg for 40 days r

38. Rational Drug Dosing Using Imaging as a Tool
Determination of receptor (or transporter) occupancy
increasingly being used for various classes of psychotropic drugs to guide clinical drug dosing
For the FDA registration of new antipsychotics acting on D2 DA receptors
it is now almost a routine or recommended requirement
Less well established are the relationships between
occupancy of the SERT and other transporter molecules by antidepressants
clinical variables

39. Rational Drug Dosing Using Imaging as a Tool
This approach cannot be used where such radiotracers do not exist
For example, for the recent first phase II clinical trial of the novel mGluR2/3 agonist LY in patients with schizophrenia*
a dose had been chosen
without the benefit of a PET radiotracer for occupancy studies
because all efforts to develop such a PET ligand failed
*Patil ST et al. Nat Med 2007;13:1102

40. Summary Conclusions Neuroimaging in Drug Development*
Justification/rationale for a specific neurotransmitter receptor system as a target for therapeutic intervention
E.g., dopamine, serotonin, etc
Radiolabeling the potential therapeutic compound of interest to examine biodistribution and BBB penetration
Rational therapeutic dosing
to test efficacy efficiently in the target patient population
Mechanism of pharmacological action
how does the efficacious action might occur
during therapeutic doses
in the target patient populations
*Especially as pertains to neurotransmitter and neuroreceptor imaging

41. Thank you!

42. Thank you!

43. Imaging to Determine Level of Target Engagement (TE)
With isotopically labeled drug candidates to estimate total brain exposure
generally using either C-11 or F-18 tracers
Functional studies such as measurement of test drug effects on regional cerebral blood flow
(PET/SPECT or MRI)
A direct measure of TE employing a PET radioligand that can be used for measuring occupancy
E.g., DA D2/D3 receptor occupancy studies with [11C]raclopride.

44. Imaging to Determine Level of Target Engagement (TE)
It is challenging to determine what level of TE is needed for a novel mechanism for a class of compounds
It is typically guided by efficacy studies in appropriate preclinical models

45. Target Identification and Therapeutic Rationale
Example: Hyperdopaminergic state associated with the positive symptoms of schizophrenia
Studies of presynaptic DA neuron function measuring dopa decarboxylase with [18F]fluorodopa
Studies demonstrating elevations in amphetamine-induced intrasynaptic DA release
DA occupancy of D2/3 receptors by endogenous DA
Studies showing elevation of D2 receptor density or binding potential

46. Serotonin Trasporter Occupancy After Rx with Paroxetine
Relationship between
Paroxetine plasma concentrations
Serotonin transporter occupancy (%SERTocc)
Measured by means of 123I-ADAM SPECT
Paroxetine plasma levels (ng/mL)
Catafau AM et al. Psychopharmacology 2006;189:145

47. Dopamine antagonists (antipsychotics)
It has been known for more than 30 years that these compounds exert their effects on the positive symptoms of schizophrenia by antagonizing DA D2 receptors (Seeman et al, 1976). It was later discovered with PET that clinically effective doses of typical neuroleptics occupy D2-like DA receptors in the human striatum in the range between 65 and 90% (Farde et al, 1992). The suggestion of a 'therapeutic window' between 60 and 80% striatal D2 RO for sufficient treatment response and a 'ceiling' of around 80% occupancy, above which extrapyramidal side effects (EPS) are likely, was later confirmed by several other researchers (eg Kapur et al, 2000). Although clozapine and quetiapine seem to be exceptions, this rule also applies for most of the second-generation, 'atypical' antipsychotics (Nyberg et al, 1999). When their doses are raised above a certain threshold, striatal (and potentially extrastriatal) D2 DA occupancy increases to levels that are associated with a higher incidence of EPS. As described above the relationship between doses of antipsychotic drugs and their (striatal) D2-like DA RO has almost approached the status of a biomarker,

48. Dopamine antagonists (antipsychotics)
But long-term functional outcome in schizophrenia is determined by
improvement in cognitive function
rather than control of positive symptoms (Bowie et al, 2006)
And the available antipsychotics have limited activity against cognitive symptoms (Keefe et al, 2007)
Thus, D2/D3 receptor occupancy is an incomplete marker of disease control or progression
Bowie CR et al. Am J Psychiatry 2006;163:425
Keefe RD et al. Arch Gen Psychiatry 2007;64:633

49. Target Identification and Therapeutic Rationale
Example: Hypodopaminergic state associated with the abuse of cocaine, alcohol, ecstasy…
Furthermore, other evidences from studies in, as well as methamphetamine and ecstasy users, have demonstrated reductions of DA and some serotonin chemical markers supporting a concept of reduced DA function. **

50. Single vs Multiple Dosing of the Candidate Drug
In many cases single-dose studies very accurately predict receptor occupancies achieved with multiple dosing
But this might not be the situation for certain drugs
E.g., ziprasidone
Single-dose occupancy studies should be supplemented by subchronic studies
to avoid an incorrect dose selection for large efficacy trials

51. Single vs Multiple Dosing of Ziprasidone
For occupancy studies, ziprasidone was given to healthy volunteers in single oral doses of mg
Causing a striatal D2 occupancy of 67%-85%
It was concluded from these studies that effective antipsychotic ziprasidone doses should be around 40 mg
But phase II clinical efficacy studies demonstrated 40 mg to be not better than placebo
Ziprasidone has potent antipsychotic efficacy at 120–160 mg
PET studies in patients after subchronic treatment demonstrated that the striatal D2 occupancy was markedly lower than was predicted by the early single-dose studies

52. Single vs Multiple Dosing of Ziprasidone
Vernaleken I et al. J Clin Psychopharmacol 2008;28:608

53. Striatal vs Extrastriatal brain Occupancy
The selection of the correct brain region for determination of receptor occupancy
Is an increasingly crucial issue
Conventional belief of a 'therapeutic window' in the range of 60–80% striatal D2 occupancy
is true for most antipsychotics
But, low-affinity D2 antagonists
such as clozapine or quetiapine
occupy striatal D2 receptors (<50%)
to a lesser extent than cortical D2 receptors (>50%)
(Gründer et al, 2006; Kessler et al, 2006).
A likely basis for their beneficial extrapyramidal side effect profile

54. Mechanism of Action of Candidate Therapeutic Drugs
PET imaging can provide actual empirical evidence for a drug candidate's effect or potential effect
E.g., potential use of DA transporter (DAT) inhibitors in the treatment of stimulant abuse
In nonhuman primates, the preadministration of the DAT inhibitor, GBR12909, blocked the amphetamine-induced DA release substantially
Before the amphetamine challenge, there was an increase of basal intrasynaptic DA, which could be useful in reducing, for example, cocaine craving