Positron Emission Tomography: Tool to Facilitate Drug Development and

to Study Pharmacokinetics

Robert B. Innis, MD, PhDMolecular Imaging BranchNational Institute Mental Health

Outline of Talk

1. PET has high sensitivity and specificity

2. PET used in therapeutic drug development

3. Pharmacokinetic modeling of plasma concentration and tissue uptake can measure receptor density

4. Study drug distribution: “peripheral” benzodiazepine receptor

5. Study drug metabolism: inhibit defluorination

Positron Emission Tomography

PET vs. MRI

PET MRI

Spatial Resolution

2 – 6 mm << 1 mm

Sensitivity 10-12 M 10-4 M

Temporal Resolution

minutes <1 sec

Radionuclide (11C): high sensitivityLigand (raclopride): high selectivityRadioligand [11C]raclopride: high sensitivity

& selectivity

Radioligand = Drug + Radioactivity

1. Drug administered at tracer doses

a) No pharm effects

b) Labels <1% receptors

c) Labeled subset reflects entire population

2. Radioligand disposed like all drugs

a) Metabolism & distribution

3. Radiation exposure

NIH Rodent PET Camera18F bone uptake rat

Developed By: Mike Green & Jurgen Seidel

PET: Tool in TherapeuticDrug Development

• Determine dose and dosing interval

• Identify homogeneous group

• Biomarker for drug efficacy

• Monitor gene or stem cell therapy

Lazabemide blocks [11C]deprenyl binding to monoamine-oxidase-B (MAO-B)

Selegilene is more potent and longer acting than lazabemide

PET: Tool in TherapeuticDrug Development

• Determine dose and dosing interval

• Identify homogeneous group

• Biomarker for drug efficacy

• Monitor gene or stem cell therapy

Dopamine Transporter: Located on DA TerminalsRemoves DA from Synapse

SPECT Imaging of Dopamine Transporter in Caudate and Putamen of Human Brain

Healthy ParkinsonStage 1

123I-β-CIT Dopamine Transporter SPECT:Decreased in Parkinson’s Disease

PET: Tool in TherapeuticDrug Development

• Determine dose and dosing interval

• Identify homogeneous group

• Biomarker for drug efficacy

• Monitor gene or stem cell therapy

Serial Dopamine Transporter Imaging in a Parkinson Patient

Institute for Neurodegenerative Disorders

PET Imaging of Amyloid: Biomarker for Alzheimer’s Disease

PET: Tool in TherapeuticDrug Development

• Determine dose and dosing interval

• Identify homogeneous group

• Biomarker for drug efficacy

• Monitor gene or stem cell therapy

Viral vectors deliver genethat synthesizes dopamine (DA)

Infuse virus into striatum (target cells)

Target cells express the DA gene

Gene Therapy Using Viral Vectors

Control Gene:Lac-Z

postpreDA Synthesis Gene:

AADC

pre post

PET Dopamine Imaging in Hemi-Parkinson Monkey:

Monitors gene for DA synthesis in right striatum

Normal Unilateral Lesion

Embryonic Stem Cells PET & MRI

PET Imaging to Monitor Embryonic Stem Cell Treatment of “Parkinson Disease” in Rats

Outline of Talk

1. PET has high sensitivity and specificity

2. PET used in therapeutic drug development

3. Pharmacokinetic modeling: plasma concentration and tissue uptake

4. Study drug distribution: “peripheral” benzodiazepine receptor

5. Study drug metabolism: inhibit defluorination

AUC=32 AUC=16

Bra

in D

rug

TimeTime

Brain Uptake of [18F]Fluoxetine:Measures Density of Serotonin Transporters &

Affinity of Fluoxetine

Patient Healthy

AUC=32 AUC=16

Bra

in D

rug

TimeTime

Brain Uptake of [18F]Fluoxetine:Measures Density of Serotonin Transporters &

Affinity of Fluoxetine

Patient Healthy

Inject Activity 20 mCi 10 mCi

Patient Healthy

AUC=32 AUC=16

Bra

in D

rug

TimeTime

Brain Uptake of [18F]Fluoxetine:Measures Density of Serotonin Transporters &

Affinity of Fluoxetine

Patient Healthy

Inject Activity 20 mCi 20 mCi

Patient Healthy

AUC=32 AUC=16

Bra

in D

rug

TimeTime

Brain Uptake of [18F]Fluoxetine:Measures Density of Serotonin Transporters &

Affinity of Fluoxetine

Patient Healthy

Inject Activity 20 mCi 20 mCi

Weight 50 kg 100 kg

Patient Healthy

AUC=32 AUC=16

Bra

in D

rug

TimeTime

Brain Uptake of [18F]Fluoxetine:Measures Density of Serotonin Transporters &

Affinity of Fluoxetine

Patient Healthy

Inject Activity 20 mCi 20 mCi

Weight 100 kg 100 kg

Patient Healthy

AUC=32 AUC=16

Bra

in D

rug

TimeTime

Brain Uptake of [18F]Fluoxetine:Measures Density of Serotonin Transporters

Patient Healthy

Inject Activity 40 mCi 20 mCi

Weight 100 kg 100 kg

Liver disease Yes No

Patient Healthy

Binding Potential (BP)

BP equals uptake in brain relative to how much activity is delivered in arterial plasma

AUC=2Pla

sma

Dru

g

Time

Bra

in D

rug

AUC=16

Area Brain Curve

Area Plasma CurveBP =

BP = 162

= 8

Bra

in D

rug

Time

Binding Potential: Independent of Injected Dose*

Double Plasma Input =>Double Brain Response

AUC=32

AUC=16 BP 1st Time = 324 = 8

BP 2nd Time = 162 = 8

Pla

sma

Dru

g

AUC=2

AUC=4

*If ligand does not saturate receptors - i.e. if tracer doses used

What’s So difficult?Limited, noisy data.

Pla

sma

Par

ent

A

ctiv

ity

Bra

in A

ctiv

ity

Time

AUC = ?

AUC = ?

Tissue uptake is proportional to density of receptors and the affinity of the drug

BindingPotential

affinity1

maxD

maxD

max BK

BK

BBP

drugaffinitybinding1

constant bindingon dissociati

densityreceptor

D

D

max

K

K

B

Plasma Brain

K1

k2

Outline of Talk

1. PET has high sensitivity and specificity

2. PET used in therapeutic drug development

3. Pharmacokinetic modeling: plasma concentration and tissue uptake

4. Study drug distribution: “peripheral” benzodiazepine receptor

5. Study drug metabolism: inhibit defluorination

“Peripheral” Benzodiazepine Receptor

1. Mitochondrial protein highly expressed in macrophages and activated microglia

2. Exists in periphery and brain

3. Multiple potential functions: steroid synthesis, nucleotide transport

4. Distinct from typical benzodiazepine GABAA receptor in brain

5. Marker for cellular inflammation

Old and New PBR PET Ligands

[11C](R)-PK11195

IC50 = 0.8 nM* ; cLogP = 5.3

[11C]PBR28

IC50 = 0.6 nM* ; cLogP = 3.0

New LigandAryloxyanilide StructureHigher specific receptor signalLower lipophilicity

PBR Imaging in Cerebral Ischemia

Cerebral ischemia (stroke) consists of a necrotic core surrounded by a penumbra with salvageable tissue.

Penumbra accumulates a large number of activated microglia.

21

9

5

1

2

4

14

(nCi/mg)B

CA(%SUV)

0

300

100

200

[11C]PBR28 PET summation image

In vitro [3H]PK11195 autoradiography

Cresyl violet staining

Contralateral

Peri-ischemic area

Central ischemic region

Radioactivity accumulates in the peri-ischemic area and correlates with PBR receptor autoradiography.

500

250

%SUV

0

[11C]PBR28 Monkey Brain: Total Uptake

Nonspecific Uptake: Preblocked with PK11195

[11C]PBR01 Time-activity Curves

0 25 50 75 100 125 1500

100

200

300

400

500

Time (min)

%SU

V

0 25 50 75 100 1250

250

500

750

1000

1250 Frontal Cortex

Cerebel larPutamen

4th Ventricle

Parent in plasma

Time (min)%

SUV

Non-radiolabeled PBR01 1 mg/kg i.v.

Baseline scan Blocking scan

% standard uptake value (%SUV)100 %SUV = Average of activity in the whole body

High levels of specific binding

2 min 25 min 115 min

Brain

Lungs

Kidneys

Heart

LiverGall bladder

Urinary bladder

Spleen

Baseline

BlockedPK11195 10 mg/kg

Receptor Blockade Displaces from Lung & KidneyDrives More Radioligand to Brain

Brain Time Activity Curves

0 20 40 60 80 100 1200

50

100

150

200

250Front. ctx

Thalamus

White matter

Time (min)

Act

ivit

y (%

SU

V)

Lines are unconstrained2-compartment fits

Imaging Peripheral Type Benzodiazepine ReceptorsUsing [11C]PBR28 in Human

PET average all frames

Subject #1

Subject #2

Total binding Non-specific binding

[3H]PK 11195 Receptor Autoradiography:Human Carotid Artery with Plaque

Outline of Talk

1. PET has high sensitivity and specificity

2. PET used in therapeutic drug development

3. Pharmacokinetic modeling: plasma concentration and tissue uptake

4. Study drug distribution: “peripheral” benzodiazepine receptor

5. Study drug metabolism: inhibit defluorination

[18F]FCWAY: Defluorination Bone uptake: human skull at 2 h

[18F]Fluoride

Brain

Skull

Miconazole Inhibits Defluorination & Bone Uptake

Temp CtxSkull

[18F]FCWAY: Miconazole

Baseline 15 mg/kg 30 mg/kg 60 mg/kg

Disulfiram: Decreases Skull Activity &Increases Brain Uptake

Baseline DisulfiramImages at 2 h in same subject. Disulfiram 500 mg PO prior night

Disulfiram: Decreases skull uptake of fluoride &Increases brain uptake of [18F]FCWAY

0 25 50 75 100 1250

100

200

300

400

500

Baseline

Disulfiram

Time (min)

Me

an

%

SU

V

0 25 50 75 100 1250

100

200

300

400Baseline

Disulfiram

Time (min)

Me

an

%

SU

V

Skull Temporal Cortex

0 25 50 75 100 1250

250

500

750

1000

1250

1500

1750

2000

Baseline

Disulfiram

Time (min)

Ac

tiv

ity

(B

q/m

L)

Disulfiram: Decreases plasma fluoride &Increases plasma radiotracer [18F]FCWAY

[18F]fluoride[18F]FCWAY

(parent tracer)

0 25 50 75 100 1250

200

400

600

800

1000

1200

1400

1600

Baseline

Disulfiram

Time (min)

Ac

tiv

ity

(B

q/m

L)

Summary of Talk

1. PET has high sensitivity and specificity

2. PET used in therapeutic drug development

3. Pharmacokinetic modeling: plasma concentration and tissue uptake

4. Study drug distribution: “peripheral” benzodiazepine receptor

5. Study drug metabolism: inhibit defluorination

Self-Assessment Quiz: True or False?

• Positron emission tomography (PET) studies involve the injection of a radioactively labeled drug that emits a particle called a positron.

• PET shows the location of radioactivity in a cross section (or tomograph) of the body.

• PET can be used to quantify the density of specific proteins in the body.

• Compartmental modeling of PET data typically uses measurements over time of 1) PET images of the target tissue and 2) concentrations of unchanged parent radioligand in plasma.