PET radiotracers

A PET radiotracer (also known as PET tracer) is a positron-emitting radiopharmaceutical used in positron emission tomography. Each tracer consists of a positron-emitting isotope (radioactive tag) bound to an organic ligand (targeting agent). The ligand component of each tracer interacts with a protein target, resulting in a characteristic distribution of the tracer throughout the tissues. The ideal PET radiopharmaceutical should only interact with the protein target and not give accumulation phenomena.²

For example, the pre-eminent PET radiotracer fluorodeoxyglucose (FDG) is comprised of F-18 isotope bound to 2-deoxy-2-glucose, an analogue to glucose. The 2-deoxy-2-glucose ligand is a substrate for the hexokinase/glucokinase enzymes involved in early carbohydrate metabolism; thus, FDG is chemically linked to cellular metabolic activity ¹. It serves as a particularly good tracer agent because it tends to stay "trapped" within metabolically active cells due to the absence of the hydroxide group ("deoxy-").

There is an increasing list of chemical compounds which are being used for PET imaging. A list of commonly used compounds includes:

• acetate (C-11)
• choline (C-11)
• fludeoxyglucose (F-18)
• sodium fluoride (F-18)
• methionine (C-11)
• prostate-specific membrane antigen (PSMA) (Ga-68)
• DOTATOC, DOTANOC, DOTATATE (Ga-68)
• florbetaben, florbetapir (F-18)
• rubidium (Rb-82) chloride

The synthesis of PET tracers begins in cyclotrons with the formation of small molecules called precursors. For example the isotope carbon-11 can be produced both as carbon dioxide (C-11 CO₂) and as methane (C-11 CH₄) ³, respectively the most oxidized and the most reduced chemical form.