Comparison of methodologies for the assessment of dopamine receptor binding in

subregions of the striatum

Functional Neuroimaging Lab School of Psychology

University of NewcastleSchizophrenia Research Institute

Darlinghurst, Australia

Sharna JamadarMentor: Julie Price

PET Modality Coordinator: Jonathan Carney

Project Aims• Become familiar with

the basics of PET radioligand methods– Compartmental

models– Logan graphical

method (arterial input function)

– Logan graphical method (cerebellum reference tissue)

• Become familiar with two types of dopamine radioligands– Raclopride– CFT– How do these differ?

Understand utility of PET radiotracer methods for the study of behaviour

PET Methodology• Compartmental models

– Model parameters determined by iterative non-linear least-squares fitting, used to obtain receptor binding measures, use arterial blood as model input

• Outcome measures:– Distribution volume (VT) the ratio of the concentration of radioligand in a region of tissue to

that in plasma (at equilibrium)– Distribution volume ratio (DVR) is related to receptor density x affinity, and equal to VT / VND

– Binding potential (BP) proportional to receptor density x affinity

1T

2T

VT = free + nonspecific + specific

VND = free + nonspecific(non-displaceable reference uptake)

PET Methodology• Logan Graphical Methods

– Linear alternative, use arterial blood as input (VT), possible to use reference-tissue (DVR)

– Transforms multiple time measurements of plasma and tissue uptake into a linear plot, the slope related to receptor binding measures

– Appropriate for radiotracers for which a constant relationship between blood and brain tissue radioactivity is established during the study (steady-state)

– Advantages• Simpler calculations (non-iterative)• Not reliant upon definition of underlying compartments

Radiotracers for Dopamine• [C-11]Raclopride

– D2/3 receptors– Benzamide that shows selective and moderate affinity

for D2 receptors and binds reversibly to postsynaptic D2 receptors

• [C-11]CFT 2-carbomethoxy-3-(4-[18F]-fluorophenyl)tropane – Dopamine transporter– Cocaine analogue that shows good selectivity for the

dopamine transporter over other transporters and shows little non-specific binding in the brain

– Dopamine transporter is present exclusively in dopamine-synthesising neurons, thus is an index of presynaptic dopaminergic function.

– (Almost) irreversible binding

Project

• N=4

• Assess compartmental modeling and graphical methods for [C-11]raclopride & [C-11]CFT

• Estimate binding potential: BP (VT/VND) – 1– Cerebellum used as reference tissue to estimate VND

• Free of dopamine receptors, good estimate of non-displaceable (i.e., free + non-specific) uptake

– Which method(s) are acceptable?• 1 tissue compartment model

• 2 tissue compartment model

• Logan (arterial)

• Logan (reference tissue)

Project

•Dorsal caudate (DCA)•Anteroventral striatum (AVS)

•accumbens, ventromedial caudate + anteroventral putamen

•Middle caudate (MCA)•Dorsal putamen (DPU)•Ventral putamen (VPU)

First defined in Drevets et al. (1999) in baboon, used in humans Drevets et al. (2001)

CER 1 Tissue Compartment

[C-11]RAC

0.1

0.35

K1 = 0.116 Err = 0.0134 %Err = 11.5

k2 = 0.300 Err = 0.0307 %Err = 10.2

VT = 0.387 = VND SS=0.057

Receptor free region= free + unspecific binding

µC

i/mL

obs-

fit

Time (min)

CER 1 Tissue Compartment

CER2 Tissue Compartment

[C-11]RAC

0.1

0.35

0.35

0.1

K1 = 0.130 Err = 0.0221 %Err = 17.0

k2 = 0.383 Err = 0.612 %Err = 42.4

k3 = 0.036 Err = 0.130 %Err >100

k4 = 0.232 Err = 0.397 %Err > 100

VT = 0.39 SS=0.055

K1 = 0.116 Err = 0.0134 %Err = 11.5

k2 = 0.300 Err = 0.0307 %Err = 10.2

VT = 0.387 = VND SS=0.057

Receptor free region= free + unspecific binding

Similar VTSlightly lower SS in 2TBetter k estimation in 1T

µC

i/mL

obs-

fit

Time (min)

CER 1 Tissue Compartment

CER2 Tissue Compartment

[C-11]RAC

0.1

0.35

K1 = 0.116 Err = 0.0134 %Err = 11.5

k2 = 0.300 Err = 0.0307 %Err = 10.2

VT = 0.387 SS = 0.057

0.35

0.1

K1 = 0.130 Err = 0.0221 %Err = 17.0

k2 = 0.383 Err = 0.612 %Err = 42.4

k3 = 0.036 Err = 0.130 %Err >100

k4 = 0.232 Err = 0.397 %Err > 100

VT = 0.39 SS=0.055

DPU

K1 = 0.097 Err = 0.0040 %Err = 4.1

k2 = 0.055 Err = 0.0027 %Err = 4.9

VT = 1.77 SS = 0.087

0.5

0.14

Receptor-rich region= free + unspecific + specific binding

Differences in curve shapes= differences in clearance &specific binding

CER 1 Tissue Compartment

CER2 Tissue Compartment

[C-11]RAC

0.1

K1 = 0.116 Err = 0.0134 %Err = 11.5

k2 = 0.300 Err = 0.0307 %Err = 10.2

VT = 0.387 SS = 0.057

0.1

K1 = 0.130 Err = 0.0221 %Err = 17.0

k2 = 0.383 Err = 0.612 %Err = 42.4

k3 = 0.036 Err = 0.130 %Err >100

k4 = 0.232 Err = 0.397 %Err > 100

VT = 0.39 SS=0.055

DPU

K1 = 0.097 Err = 0.0040 %Err = 4.1

k2 = 0.055 Err = 0.0027 %Err = 4.9

VT = 1.77 SS = 0.087

0.5

0.14

DPU

0.5

0.14

K1 = 0.121 Err = 0.091 %Err = 15.6

k2 = 0.143 Err = 0.120 %Err = 84.1

k3 = 0.139 Err = 0.218 %Err >100

k4 = 0.123 Err = 0.051 %Err = 41.1

VT = 1.81 SS=0.072

Differences in curve shapes= differences in clearance &specific binding

Similar VTLower SS in 2TBetter k estimation in 1T

0.35

0.35

DPU CER 1 Tissue Compartment

DPU CER2 Tissue Compartment

[C-11]RAC

µC

i/mL

obs-

fit

Time (min)

DPU CER 1 Tissue Compartment

DPU CER2 Tissue Compartment

DPU CERLogan Arterial

DPULogan Cerebellum

[C-11]RAC

VT = 1.87 VT = 0.459

VT = 1.81 VT = 0.39 DVR = 4.02

DVR = 1.87/0.459 = 4.08

DVR=4.64

µC

i/mL

obs-

fit

Time (min)

Cp/ROI

RO

I/R

OI

CER 1 Tissue Compartment

[C-11]CFT

K1 = 0.347 Err = 0.009 %Err = 2.6

k2 = 0.042 Err = 0.001 %Err = 2.8

VT = 8.34 SS = 0.092

0.6

0.08

µC

i/mL

obs-

fit

Time (min)

CER 1 Tissue Compartment

[C-11]CFT

K1 = 0.347 Err = 0.009 %Err = 2.6

k2 = 0.042 Err = 0.001 %Err = 2.8

VT = 8.34 SS = 0.092

CER2 Tissue Compartment

0.6

0.08

0.6

0.14

K1 = 0.401 Err = 0.012 %Err = 3.1

k2 = 0.056 Err = 0.040 %Err = 8.0

k3 = 0.005 Err = 0.030 %Err = 69.2

k4 = 0.012 Err = 0.014 %Err > 100

VT = 10.36 SS=0.063

VT 1T < 2TLower SS in 2TBetter k estimation in 1T

µC

i/mL

obs-

fit

Time (min)

CER 1 Tissue Compartment

[C-11]CFT

K1 = 0.347 Err = 0.009 %Err = 2.6

k2 = 0.042 Err = 0.001 %Err = 2.8

VT = 8.34 SS = 0.092

CER2 Tissue Compartment

0.6

0.08

0.6

0.14

K1 = 0.401 Err = 0.012 %Err = 3.1

k2 = 0.056 Err = 0.040 %Err = 8.0

k3 = 0.005 Err = 0.030 %Err = 69.2

k4 = 0.012 Err = 0.014 %Err > 100

VT = 10.36 SS=0.063

DPU

1.0

0.06

K1 = 0.307 Err = 0.003 %Err = 1.0

k2 = 0.005 Err = 0.0002 %Err = 5.0

VT = 68.0 SS = 0.037

DPU

1.0

0.12

K1 = 0.275 Err = NA %Err < -8000

k2 < 0 Err = 0.002 %Err < -100

k3 = 0.051 Err = 0.021 %Err = 40.7

k4 < 0 Err = 0.069 %Err < -100

VT < 0 SS=0.194

Irreversible binding

CER 1 Tissue Compartment

[C-11]CFT

CER2 Tissue Compartment

0.6

0.08

0.6

0.14

DPU

1.0

0.06

DPU

1.0

0.12

VT = 68 VT = 8.34

µC

i/mL

obs-

fit

Time (min)

CER 1 Tissue Compartment

[C-11]CFT

CER2 Tissue Compartment

0.6

0.08

0.6

0.14

DPU

1.0

0.06

DPU

1.0

0.12

DPU CER

Logan Arterial

DPULogan Cerebellum

VT = 68VT = 8.34

VT = 65 VT = 8.84

DVR = 3.03

DVR = 65/8.84 = 7.45

DVR=8.14

Cp/ROI

RO

I/R

OI

Interim Summary• [C-11]Raclopride 2Tcomp better fit• [C-11]CFT 1Tcomp better fit • Conclusions consistent with known properties of the

radiotracers:– [C-11]Raclopride shows reversible binding during the PET study.

Thus k3 and k4 can be determined– [C-11]CFT shows irreversible binding in receptor-rich regions

during the PET study. Thus k4 cannot be accurately determined

RAC - DPU CFT - DPU

Comparison of binding potential between methods

Simplified methods are appropriate for raclopride

Comparison of binding potential between methods

Utility of PET radioligand methods for the study of behaviour

• Or, I’m a psychologist, why do I care?

Utility of PET radioligand methods for the study of behaviour

• Or, I’m a psychologist, why do I care?

Utility of PET radioligand methods for the study of

behaviour• Sequential motor learning

– [C-11]Raclopride BP in dorsal striatum decreases during finger sequence learning task

– Both implicit & implicit learning of complex motor sequences increase [C-11]raclopride displacement in the caudate & putamen

• Reward-related processes– Decreased striatal [C-11]raclopride BP during an active but not

passive reward task

• Cognition– Decreases in [C-11]raclopride BP when planning a set shift, during

spatial planning and spatial working memory

Variability in BP outcomes are related to behaviour

Utility of PET radioligand methods for the study of behaviour

• Variability in BP outcomes are related to behaviour

• Variability in BP outcomesare related to EEG synchrony

Acknowledgements• PET Facility

– Julie Price– Jonathan (Eoin) Carney– Carl Becker– Amy Wagner

• MNTP– Seong-Gi Kim– Bill Eddy– Tomika Cohen– Rebecca Clark

• Schizophrenia Research Institute, Australia

• University of Newcastle, Australia

Comparison of binding potential between methods

Comparison of binding potential between methods

Comparison of binding potential between methods

BP = K3/k4

BP=VTROI/VTCER-1 = K1/k2(1+k3/k4)-1

Interim Summary

• Logan– Susceptible to bias– Bias is worse in CFT

because of slower reference tissue clearance relative to plasma

– Bias not so bad in RAC because of similar clearance in reference tissue relative to plasma