Transcript Tracer Development for Molecular Imaging

Molecular Imaging Concept
Biochemical
Target
Imaging
Device
Probe
Time
Administer probe
Image probe
Diagnostic Utility
Sensitivity Ability to either detect the probe signal
at the target (direct response) or change in a
signal that is dependent on the quantity of
probe at the target (indirect response)
Specificity Ability to distinguish the target from
nontarget processes or tissues
Molecular Imaging Modalities
• Gamma ray emission
•Positron emission – annihilation photons (PET)
•Single photon emission (SPECT)
•Direct signal from tissue in response to probe concentration
• Gamma ray absorption
•Used in CT scans
• Contrast with high atomic number nuclei that absorb gamma rays (Iodine)
• Magnetic resonance
• Protons provide signal in clinical MRI scans
• Perturb proton relaxation with Gd contrast agents
• Use paramagnetic nuclei (e.g., 13C, 19F) labeled probes
•Optical
•Fluorescent molecules (luciferase/luciferin; GFP)
•Visible wavelengths have limited depth of detectability
•Infrared extends depth a bit
•Ultrasound
•Acoustic absorption/modification (microbubbles)
•Thermoacoustic stimulation (absorption probes)
CT Contrast Agents
• High atomic number – x-ray absorption
– Ba, I, Gd, Au
• Intraintestinal or intravascular (extracellular)
Examples:
Barium sulfate (oral, rectal admin.)
iopromide (iodinated IV contrast agent)
Radioapague Contrast Agent Enhance the
Organ Delineation
Stomach
liver
Kidneys
Ovarian
Without contrast administration
With contrast administration
Opportunities in microCT:Anatomy and
physiology
Catheter
700
600
vena cava
Enhancement in HU
500
400
kidney
300
input (heart)
muscle
200
100
kidne
y
spine
0
0
50
100
150
200
250
-100
heart
vena cava
Time in seconds
The representative CT images of a dynamic sequence with iodinated contrast
enhancement. From top to the bottom, one can see the flowing path of the contrast
medium within the blood stream: from tail vein to vena cava, to heart, and then to kidney.
These three images are associated with the first 8 seconds of the sequence.
Proton MR Contrast Agents
• Positive contrast agents (appearing bright on MRI)
Small molecular weight compounds containing as their active
element Gadolinium (Gd), Manganese or Iron
Unpaired electron spins in their outer shells and long
relaxivities, which make them good T1 relaxation agents.
Examples:
GD-DTPA, Gadopentetate dimeglumine, gadoteridol, and
gadoterate meglumine are utilized for the central nervous
system and whole body
Mangafodipir trisodium for lesions of the liver
Gadodiamide for the central nervous system.
Proton MR Contrast Agents
• Negative contrast agents (appearing predominantly dark on
MRI) are small particulate aggregates often termed
superparamagnetic iron oxide (SPIO). These agents produce
predominantly spin-spin relaxation effects, but very small
particles smaller than 300 nm also produce substantial T1
relaxation.
• A special group of negative contrast agents (appearing
dark on MRI) are perfluorocarbons because their presence
excludes the hydrogen atoms responsible for the signal in
MR imaging.
MR Molecular Probes
• Some MR contrast agents require biocompatible
carriers/capsules
– Reduce toxicity – hide the bad guy inside
– Target specific cells/proteins/processes
Examples:
Ferumoxide – SPIO core particles (~150nm), dextran T10 covering
Nanomag – SPIO particles (50nm) with cross-linked
dextran and amino acid sequences to form bonds to
organic compounds
P7228 – SPIO, anionic dextran layer – can be
encapsulated by positively charged liposomes
Biologically Important NMR Nuclei
1H
-
Wall thickness, ejection fraction, wall motion,
perfusion, coronary artery angiography. (large signal
from ~50M concentration in tissues)
31P – ATP, PCr, P , PDE, PME, pH , [Mg2+], kinetics of
i
i
creatine kinase and ATP hydrolysis.
23Na – Transmembrane Na+ gradient, tissue and cartilage
structure.
13C – Glycogen, metabolic rates, substrate preference, drug
metabolism, etc.
19F – Drug metabolism, pH, Ca2+ and other metal ion
concentration, pO2, temperature, etc.
2H –
Perfusion, drug metabolism, tissue and cartilage
structure.
In vivo detection sensitivity limits use of C-13 and F-19 molecular probes
(C-13 requires >0.1mM, F-19 >5 mM)
Advantages of PET
• PET has high sensitivity (~pmol of probe can be detected)
• PET images biochemistry. Small radionuclides (C-11,
F-18) label small biological molecules with retention
of biological specificity.
• PET images are quantitative
General Aspects of
PET Tracers
• Understanding of targeted biochemical process
• Practical synthesis: sufficient yield and purity, automated
• Tissue uptake and kinetics are specific to targeted process
• Fate of radiolabel understood for metabolized tracers
• Tracer distribution is sensitive to answer clinical questions
relevant to diagnosis, prognosis or monitoring of therapy
• Tissue kinetics amenable to mathematical modeling to give
quantitative indices
Positron Decay
A
A
Z X N  Z1Y N1
nuclide
C-11
N-13
O-15
F-18
I-124
+
e 
half-life
20.3 min
10 min
124 sec
110 min
4.2 d (+ high Energy photon)
e.g., 18F  18O + e+ + 
Biochemical/Physicological Targets for PET Imaging
•
•
•
•
•
•
•
•
•
•
Substrate metabolism
carbohydrates, fatty acids, amino acids, oxygen, nucleosides,
oligonucleotides
Receptor binding
adrenergics, cholinergics, neurotransmitters, hormone
receptors, growth factor receptors
Ionic transport
Na, K, Ca, F, Cl, I
Perfusion
water, ammonia, butanol
pH
Blood volume (11CO, C15O)
Hypoxia (misonidazoles)
Redox potentials
Protein-protein interactions
monoclonal antibodies
Gene expression
reporter genes and probes
Challenge #1:
Radiochemical limitations
•Short radionuclide half-life (<2 hr)
•Limited radionuclide availability
•Radiation exposure to chemist
>90% of PET probes are synthesize by simple 1 or 2 step labeling
followed by purification and formulation
Synthesis times typically under 45 min for C-11 (t 1/2 = 20 min)
and under 2 hr for F-18 (t 1/2 = 110 min)
Challenge #2:
Biochemical Complexity
TARGET
Challenge #3:
The radioimaging signal is chemically nonspecific
*Probe
*(Intermediates)
*TARGET
*Nonspecific binding to proteins or membranes
*alternative binding or metabolic products
*systemic metabolites
(i.e. hepatic)
Specificity! Specificity! Specificity!
Challenge #4:
Physiological Barriers To Delivery of Probe to the Target
Arterial
Blood
TARGET
(Subcellular compartmentation may also limit delivery)
Transport of probe to target should not be rate-limiting
Limits utility of technique in poorly perfused tissues
Challenge #4B:
Subcellular Barriers of Delivery
TARGET
Again, transport of probe to target should not be rate-limiting,
And probe must be able to leave cell if not acted upon.
11C-Acetylene
•
as PET Probe and Labeling
Intermediate
11C-acetylene
(C2H2) may be useful as a radiolabeling intermediate for organic molecules in
physiology studies
• [11C]C2H2, by itself, can be used in perfusion
studies (i.e. brain)
Comparison of PET Tracers for Measuring Tissue Perfusion
Tracer
Physical
half-life
Octanol/Water
partition coeff.
(log P)
[15O]water
[13N]ammonia
[11C]CH3F
[18F]CH3F
[11C]C2H2
[11C]butanol
[15O]butanol
2.01
9.97
20.4
109.8
20.4
20.4
2.01
-1.38
-1.38
0.51
0.51
0.37
0.88
0.88
Solubility
in water (g/
100 ml)
34
0.23
0.23
0.106
7.8
7.8
Cost per
synthesis*
Ease of
Synthesis
High
Low
Moderate
Moderate
Low
Moderate
Moderate
Simple
Simple
Difficult
Moderate
Simple
Moderate
Difficult
* Using proton accelerator and most common nuclear reaction for production
11
CO2 + 12CO2
1) Trap on Ba
2) 900 C / H2
Ba*CC
(Madsen et al., 1981)
*CCH
2
O
O
*
N
O
Bu-Li
O
O
O
O
H*CCLi
+
O
O
*
O
O
NaOH
OH
*
O
O
*
+
OH
O
[3,4-11C]-2-oxo-butynoic acid (COBA)
Computer-controlled
Apparatus for synthesis of
C-11 Acetylene
Thermocouple
Wires
Trap Outlet
Trap inlet
Sieve Trap
Dose Calibrator
C
0
C-11 CO2
V1a
0
V1b
1
0
0
V2
1
C
Recirculation
Pump
1
1
Waste
C
Waste
Quartz Rxn
Vessel
V4
a
b
d
Furnace
c
V3
Soda Lime
Trap
Helium Hydrogen
Sampling Bag
0
V5
C
1
Draw Sample
for Analysis
Dose Calibrator
Number of Ions Collected (Millions)
Gas Chromatography/ Mass Spec of C-11 Acetylene Product
Standard
Product
4
3
2
1
0
0
1
2
3
4
5
6
Time (min)
7
8
9
Radioactivity (Arbitrary Units)
Gas Chromatography / Rad. Detection of C-11 Acetylene Product
5
4
3
2
1
0
0 1 2 3 4 5 6 7 8 9 10 11 12
Time (min)
Target: Myocardial Fatty Acid Oxidation
• Long-chain fatty acids are the predominant
substrates for production of ATP in heart.
• Abnormalities of fatty acid oxidation by the
myocardium are associated with ischemic heart
disease, congestive heart failure,
cardiomyopathies, and deficiencies of carriers,
enzymes or co-factors required for fatty acid
transport or oxidation.
• The lack of a specific radiolabeled probe of
fatty acid oxidation has impeded the
development of a non-invasive technique for
assessment of fatty acid oxidation.
Myocardial Metabolism of Fatty Acids
Myocyte
Lipids
LCFA
FATr
lipase
LCFA-CoA
hyd.
LCFA
CPT-I
LCFA-carn
ACS
VLAD
CPT-II
LCFA-CoA b-ox. MTP
CAT LCFA-carn
MCFA-CoA
b-ox.
Acetyl-CoA
Mitochondrion
6-Thia Analogs
4-Thia Analog
18
F
S
OH
O
14-[18F]fluoro-6-thia-heptadecanoic
acid (14F6THA)
O
18
F
S
OH
17-[18F]fluoro-6-thia-heptadecanoic
acid (17F6THA)
O
18
F
S
OH
16-[18F]fluoro-4-thia-hexadecanoic
acid (FTP)
O
18
F
O
18
F
Myocyte
O
S
(LC-AcylCoA synthetase)
(LC-AcylCoA hydrolase)
O
18
F
S
Outer Membrane
S
CoA
Complex Lipids
(acyl transferase)
(CPT-I)
O
18
F
S
O
Mitochondrion
Carn
(Translocase, CPT-II)
Inner Membrane
O
18
F
S
(VLC-acylCoA dehydrogenase)
O
18
F
Plasma
O
S
S
S
S
CoA
(Mit. Trifunctional Protein)
CoA slow18
F
OH O
S
S
CoA
(spontaneous)
Protein Binding
18
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dC1(t)/dt = k1 Cp(t) - (k2 + k3) C1(t)
dC2(t)/dt = k3 C1(t) - k4 C2 (t)
Ctot(t) = (1-BV) (C1+ C2) + BV Cp
(1)
(2)
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2-Compartment Model Fit to FTP Kinetics in Isolated Rat
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F-18 FTP in Normal Human Subject
Short-axis Images of Heart at 50-55 min p.i.
Duke University Medical Center
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Diabetic Cardiomyopathy Patient
F-18 FTP (Fatty Acid Oxidation)
SPECT Tc-99m Myoview Perfusion Scan
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