Imperial College London, UK

An organic chemist highlights an ingenious way to make radiotracers.

In diagnostic medicine, radiotracers are used to image molecules in the body by methods such as positron emission tomography. Radionuclides such as carbon-11, nitrogen-13 or fluorine-18 are incorporated into molecules normally used by the body, such as glucose, or into designer organic molecules that bind to receptor sites within the body and provide valuable information about how they function. But the short half-lives of these radionuclides — 20, 10 and 110 minutes respectively — mean that the chemical reactions used to make the tracers must be speedy.

One step that is often time-consuming is separating out synthesized radiotracers from the large quantities of unreacted precursor molecules used to ensure efficient use of the radionuclide.

A new approach to this problem has been reported that exploits fluorous solid-phase extraction, a technique that separates molecules on the basis of their fluorine content (R. Bejot et al. Angew. Chem. Int. Edn 48, 586–589; 2009). Veronique Gouverneur at the University of Oxford, UK, and her team designed heavily fluorinated precursor molecules — typically containing 13–25 fluorine atoms — that release their fluorine-rich section on reaction with a radionuclide. This generates a mixture of the desired defluorinated radiotracer and undesired fluorine-rich molecules. The mixture is then passed through a bed of fluorinated silica, which retains the heavily fluorinated compounds, leaving the radiotracer to emerge free of contaminants and ready for clinical use.

Gouverneur's work opens the door to the development of a plethora of new synthetic chemistry methods and the creation of new radiotracers for use in areas including tumour imaging and neuroimaging.