Chem 2601/2012

Molecular Imaging

Lecture 5 and 6: Radiotracers and applications for diagnostic imaging

Dr. Erik Årstad, KLB room 2.11 (e.arstad@ucl.ac.uk)

Overview (lecture 5 and 6):

1) Radiotracers – key principles

2) Properties of radiotracers

3) Interactions of tracers with biological targets

4) Nuclear imaging of cancer

5) Nuclear imaging of brain diseases

6) Nuclear imaging in research and drug discovery

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Radiotracers – key principles:

1) Principles of imaging with radiotracers 2) Properties of radiotracers 3) Interactions of radiotracers with biological targets

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After two years on non-successful work: “. . . .I failed completely. In order to make the best of this depressing situation, I decided to use Radium-D as an indicator of lead. . . “ Later it was found that Radium-D is 210Pb. A radiotracer (also known as a tracer or radiopharmaceutical) is a chemical compound that allows investigation of a biochemical process without influencing the process.

George De Hevesy (1911): “My boy, if you are worth your salt, you try to separate Radium-D from all that lead.”

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Basic principles of radiotracers: To image a biological target or process, the following criteria must be fulfilled: - The radiotracer must be able to reach its intended target in vivo - The radiotracer must interact with its intended target so that its distribution changes in a concentration dependent manner - The radiotracer must not influence the process it measures (= high specific binding)

Properties of radiotracers:

1) Affinity and binding potential

2) Lipophilicity

3) Pharmacokinetics and time-activity curves

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Tracer interaction with target depends on binding affinity:

Affinity (Kd) = [Ligand]∗[Receptor] [Ligand-Receptor complex]

and target concentration: Binding potential (BP) =

[Receptor] Kd

Affinity ideally between 0.5-5 nM

Binding Potential > 2

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The ability of a tracer to reach its target depends on: Physiochemical properties (e.g. lipophilicity) Metabolic stability Ability to cross biological barriers (membranes etc.)

Lipophilicity and Log P

Lipophilicity is a measure of the polarity of a compound (water vs. fat solubility)

Lipophilicity is measured as the logarithm of the partition coefficient between water and octanol

Log P = log

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[organic phase] [aqueous phase]

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Brain uptake as function of Log P:

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Question: A compound is shaken with equal amounts of water and octanol. You find that the aqueous phase contains 0.1% of the compound and the organic phase contains 99.9%. What is the Log P? What would be the measured Log P if the compound would contain 1% of a water soluble impurity?

Pharmacokinetics is the distribution of a tracer over time (referred to as time-activity curves) Increased biological half-life (retention) in target tissue is the basis of the signal

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time-activity curve

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Interactions of radiotracers with biological targets 1) Imaging of receptor binding 2) Imaging of transporters 3) Imaging of enzymes 4) Imaging metabolic pathways

Imaging of receptors

Receptor-radiotracer complex Receptor

Radiotracer

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= retention of tracer = washout of tracer

D2 Receptor imaging with [123I]IBF SPECT:

123I

O

NHH

[123I]IBF

Uptake of [123I]IBF over 2.5 h in humans 15

in blood

Time-activity curves:

Basis of the signal

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in target (striatum) and non-target (frontal cortex)

Imaging transporter function

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Transporter

Tracer substrate transporter function

Transporter

Tracer inhibitor Transporter-radiotracer complex

And transporter expression

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Imaging of transporter function in brain tumour: [18F]FET

ONH2

O-

O

18F

[18F]FET Time-activity curve

Imaging enzyme function

Enzyme

Tracer substrate Multiple tracer molecules trapped

Imaging enzyme expression

Enzyme

Tracer inhibitor 1 Tracer is trapped per enzyme

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Enzyme

Tracer substrate Multiple tracer molecules trapped

Imaging Metabolic Processes

= transporter function + enzyme function

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Applications in biomedical research - oncology 1) Imaging glucose metabolism with [18F]FDG

2) Imaging membrane synthesis with Choline 3) Imaging protein synthesis with amino acids

4) Imaging of cell proliferation with [18F]FLT

Imaging of glucose metabolism with FDG

Glucose Transporters

Fluoride blocks further metabolism.

Hexokinase

Cancer cells have increased rate of proliferation, and tend to be in an anaerobic environment = increased glucose metabolism

Phosphate group prevents leakage from the cell

[18F]FDG:

OHO

HO

OH

18FOH

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OHO

HO18F

O

OH

PO32-

1 2

FDG for diagnostic imaging of cancer

Healthy control B-cell lymphoma 23

Recap: imaging of Glut5 transport with fluorescently labelled fructose

Cells lacking Glut5, cells with Glut5, after addition of fructose

NBD derivative

Cy5.5. derivative

Question: Why is imaging of glucose metabolism so much more successful with nuclear imaging than with fluorescence? 24

FDG PET: Limitations for diagnostic applications

Healthy control

High background in the brain and bladder Low uptake in slow growing tumours (e.g. prostate) FDG relates to metabolic activity only - limited biochemical information

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68Ga-DOTANOC PET 18F-FDG PET

Imaging metabolism vs. receptor expression

Metabolic activity SSR receptor expression 26

Imaging of Choline metabolism

Choline Transporters

Cancer cells have increased rate of proliferation = increased membrane lipid synthesis = increased uptake of Choline

Phosphate group prevents leakage from the cell

Choline

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1 2

NOH

Choline kinase

NO

PO32-

+

+

Imaging prostate cancer with choline tracers:

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Imaging cell proliferation with [18F]FLT

Allows assessment of tumour aggressiveness Can predict response to chemotherapy

Can distinguish cancer from infection However, much lower sensitivity than FDG

Salskaov et al., Semin Nucl Med. 2007, 37(6):429-439

N

NH

O

OO

HO

18F 29

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Imaging response to chemotherapy with [18F]FLT: Before chemotherapy 7 days after chemotherapy

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Imaging of transporter function/protein synthesis in brain tumours: [11C]methionine

H2N C

H311CS

O

OH

Nuclear imaging of brain diseases

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Nuclear imaging of brain diseases 1) Imaging of Amyloid plaque in Alzheimer’s disease 2) Imaging of movement disorders (Parkinson disease)

Alzheimer’s disease is a form of dementia Disease frequency increases rapidly with age 15% of population aged >80 years have AD No known treatments – but several therapies in clinical trials Need for imaging to enable diagnosis and to evaluate treatments

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Alzheimer’s disease is associated with Amyloid plaques:

N

SHO

NH

11CH3

[11C] PIB

Amyloid plaque

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[11C]PIB vs. [18F]FDG

[18F]FDG

[11C]PIB

Healthy (67)

AD patient (79)

Klunk, Engler, Nordberg, et al Review: A Nordberg; The Lancet Neurology Annal Neurol 2004, 55, 306-319 Vol 3, September 2004, 519-527 36

78% 8% 9% 5%

PIB PET scans in healthy elderly volunteers (n=32)

Healthy controls with +ve scans had memory Z-scores0.5 lower than controls with –ve scans

-ve scan +ve scans

[11C]PIB Scans in healthy elderly volunteers

In memory tests subjects with amyloid plaque scored lower on average than subjects without amyloid plaque 37

Parkinson’s disease (PD) A class of movement disorders Caused by neurodegeneration (death of nerve cells) Selective loss of dopamine producing cells = classical PD Wider loss of nerve cells, including dopamine producing cells = atypical PD

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Imaging Parkinson’s disease with [123I]FP-CIT SPECT

Healthy subject

Parkinson’s disease

Dopamine transporter

NF

I

O

OMe

[123I]FP-CIT 39

Imaging PD – D2 receptor expression with

[123I]IBZM

Classical PD

Atypical PD

Dopamine D2 receptor

I

OH

OMe O

HN

N

[123I]IBZM 40

Nuclear imaging in research and drug discovery 1) Use of PET to study receptor occupancy 2) Use of PET to study drug behaviour

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Imaging of drug receptor occupancy (and release of endogenous neurotransmitters)

Receptors at synapse

Tracer

Receptors at synapse

Tracer

Drug

= Drug competes with tracer for binding

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Imaging of drug receptor occupancy: time-activity curves

Effect of Smoking on nicotinic acetylcholine receptors

Conclusion: Smoking results in complete block of nAChRs

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Profiling drugs in man: What is the optimal dose?

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Early clinical evaluation using radiolabelled drug in tracer amounts

Drug concentration too low to cause pharmacological or toxic effects

Early ADME data at relatively low cost

Drug development: The microdosing concept

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Microdosing concept: brain uptake and drug metabolism

Does the drug penetrate the blood-brain barrier?

No hmmm Yes!

What is the metabolic profile?? What is the biological half-life?

Lappin and Garner (2003). Nature reviews, Drug Discovery. Vol 2; 233-240. 47

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Synopsis: Radiotracers

A radiotracer is a chemical compound that allow investigation of a biochemical process without influencing the process.

Tracers can interact with different biological targets, e.g. receptors, transporters, enzymes and metabolic processes. The information obtained depends on the mechanism for tracer accumulation.

Important properties of tracers are: affinity, lipophilicity, selectivity, and pharmacokinetics. Also important with choice of radionuclide, labelling chemistry and specific activity.

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Synopsis: Applications of nuclear imaging

FDG PET is limited by high background in brain and bladder, low uptake in slow growing tumours and because it does not provide specific biochemical information.

For diagnostic imaging of cancer, imaging of receptor expression can provide increased sensitivity, FLT can enable imaging of proliferation, choline can allow diagnosis of prostate cancer, FET can enable imaging of brain tumours.

For brain imaging, PIB enables detection of amyloid plaque as a marker of Alzheimer’s disease, and a combination of imaging of dopamine transporters with FP CIT and imaging of D2 receptors with IBZM allows accurate diagnosis of Parkinson’s disease.

For drug development, microdosing studies allows early evaluation of ADME properties, and drug occupancy studies allows determination of the effective dose.

PET in drug development:

Microdosing: Radiolabelled drug in tracer amount Good for: ADME data Not good for: Dose optimisation Mechanism of action Pharmacological effect Toxicity

Receptor occupancy studies: Radiotracer + drug Good for: Optimising dose Mechanistic studies

Microdosing + receptor occupancy: Very few subjects needed + big cost savings! NB: Large scale trials still needed but much lower risk

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Learning outcomes - you should understand: The limitations of FDG, and how the use of other tracers, including FLT, Choline, FET and RGD peptides can provide additional information. The principle of imaging amyloid plaque with PIB and how this relates to Alzheimer’s disease. The principle of imaging Dopamine transporters and D2 receptors and how this enables diagnosis of Parkinson’s disease. What microdosing studies are and how they can be used in drug development The principles of drug occupancy studies and how these can be used for drug development

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Assessment – you should be able to apply your knowledge of radioactivity and nuclear imaging to explain underlying principles, solve practical problems and provide rationale explanations related to: Imaging with FDG and its limitations , and how the use of other tracers, including FLT, Choline, and amino acids can provide additional information. The principle of imaging amyloid plaque with PIB and how this relates to Alzheimer’s disease. The principle of imaging Dopamine transporters and D2 receptors and how this enables diagnosis of Parkinson’s disease. What microdosing studies are and how they can be used in drug development The principles of drug occupancy studies and how these can be used for drug development

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Further reading:

PET in Oncology: Ashley M Groves AM, Win T, Haim SB, Ell PJ (2007). Lancet Oncol., 8: 822–30 Buerkle A, Weber WA (2208), Cancer Metastasis Rev (2008) 27:545–554 Salskaov et al., Semin Nucl Med. 2007, 37(6):429-439 PET in microdosing and drug development: Lappin G, Garner RC (2003). Nat Rev Drug Discov. 2(3):233-40. Brooks DJ (2005). NeuroRx., 2; 226–236. Mamo D et al. (2007). Am J Psychiatry, 164: 1411–1417. Brain imaging: Thobois S et al. (2001). Neurophysiol Clin., 31:321-40 Tasch K, Ell PJ (2006). Clin Med., 6:259–62 Henriksen G, Willoch F (2007). Brain., 131(Pt 5):1171-96.

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