The Role of Neuroimaging in Clinical Trials and Drug Discovery In Psychiatry

Joseph C. Masdeu, MD, PhDSection on Integrative NeuroimagingNational Institutes of Health

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

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

[11C]GSK931145: A New PET Ligand for Glycine Transporter 1

Transporter characteristics Autoradiography Physiology Pharmacology

[11C]GSK931145: A New PET Ligand for Glycine Transporter 1

[11C]GSK931145 distribution in a healthy volunteer

Pig Primate Human

[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

New Drug Characteristics

Properties of Drug CandidateDetermined 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

Uses of PET in Drug Development

Two main approaches Radiolabel the new drug Estimate target occupancy

By the new drug

Drug Candidate Biodistribution and Kinetics

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

Usual Radioisotopes:

PET: [11C], [18F]

SPECT: [123I], [99mTc]Ideally, a new lead drugcandidate should be isotopically radiolabeled with a PET or SPECT radioisotope

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

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

Serotonin Trasporter Occupancy After Rx with Paroxetine

Studied with 123I-ADAM SPECT

Decrease in midbrain uptake (71% serotonin transporter

occupancy)

After 6-week paroxetine 20 mg/day

Baseline

Catafau AM et al. Psychopharmacology 2006;189:145

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

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

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

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

Fearful

Happy

Left

Left Right

Right

Role of MRI in Drug DistributionProton (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

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]

Cytidine Effect Linked to Glutamate

Studied with MR spectroscopy

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Yoon SJ et al. Neuropsychopharmacology 2009 [Epub ahead of print]

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

Types of Biomarkers Type 0

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

‘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

Dopamine D2 Receptor Binding

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

D2 binding in a healthy control

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

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

Meyer-Lindenberg A et al. Nat Neurosci 2002;5:267

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Controls Patients

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

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

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

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

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

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

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

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

r

After 160mg for 40 days

Aprepitant Rx

Baseline

Summary ConclusionsNeuroimaging 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

Thank you!

Thank you!

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.

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

Target Identification and Therapeutic Rationale

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

Example: Hyperdopaminergic state associated with the positive symptoms of schizophrenia

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

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,

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;163:425Keefe RD et al. Arch Gen Psychiatry 2007;64:633

Target Identification and Therapeutic Rationale

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. **

Example: Hypodopaminergic state associated with the abuse of cocaine, alcohol, ecstasy…

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

Single vs Multiple Dosing of Ziprasidone For occupancy studies, ziprasidone was given to

healthy volunteers in single oral doses of 40-60 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

Single vs Multiple Dosing of Ziprasidone

Vernaleken I et al. J Clin Psychopharmacol 2008;28:608

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

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