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Chapter 8

MEDRADSC 2Z03 Chapter Notes - Chapter 8: Nucleophilic Substitution, Nucleophile, Leaving Group


Department
Medical Radiation Sciences
Course Code
MEDRADSC 2Z03
Professor
Dawn Danko
Chapter
8

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PET RADIOPHARMACEUTICAL CHEMISTRY - FLUORINATIONS
Over the last few decades, many methods have been developed to prepare PET
radiopharmaceuticals. The majority of PET radiopharmaceuticals are small molecules
labelled with fluorine-18 and carbon-11. More recently, greater attention has been given
to the labelling of larger molecules. For example, peptides with radioisotopes such as
zirconium-89, copper-64 and gallium-68.
In this course we will focus on fluorine-18 and carbon-11 labelling using 2 versatile and
reliable methods:
1) Nucleophilic substitution with [18F]fluoride and,
2) [11C]methylation.
[18F]Fluorination Using Nucleophilic Substitution
In this section, the synthesis of [18F]FDG will be used as an example. However, a range
of PET radiopharmaceuticals can be synthesised using this method.
The first step in any synthesis using fluorine-18 is separation of [18F]fluoride from the
cyclotron target [18O]water. This is usually achieved by passing the solution through an
ion exchange cartridge e.g. QMA Sep-Pak® (Waters) where the [
18
F]fluoride is trapped on the cartridge. The [
18
F]fluoride is then eluted from the cartridge into a reaction vessel typically using a
solution containing a mixture of potassium carbonate (K
2
CO
3
), and Kryptofix®. The acetonitrile and water are then removed using heat and inert gas
flow. The [
18
F]fluoride, in the presence of Kryptofix®, can now be re-dissolved in a solvent such as
acetonitrile and is ready for the nucleophilic substitution reaction. In this reaction, the
nucleophile is [
18
F]fluoride.
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