Routine determination of [18F]-L-6-fluorodopa and its metabolites in blood plasma is essential for accurate positron emission tomography studies.

A batch-contact alumina-extraction method has been used to separate 18F]-L-6-fluorodopa (FD) from its principal metabolite, 3-O-methyl-18F]-6-fluorodopa (3-OMe-FD), in arterial blood plasma samples collected from subjects pretreated with carbidopa during positron emission tomography (PET) scans. The time course of the metabolite-corrected blood plasma activity is then used as an input function for kinetic analysis of striatal FD uptake. Results obtained from using the batch-contact alumina-extraction method were compared with those from high performance liquid chromatography, and also with those from a chromatographic alumina cartridge technique developed in this laboratory. In 60 human subjects including normal healthy volunteers and patients diagnosed as having a movement disorder, arterial blood plasma samples were collected after FD injection during a two-hour PET scan and analyzed by the batch-contact alumina-extraction method. The activity ratio (metabolites/FD) increased linearly with time for all subjects. However, there was a wide variation in the slope of the plot of the activity ratio (metabolites/FD) versus time among the subjects. No significant linear or curved relationship was observed between the slope and the age of the subject. Separation of FD from its metabolites is therefore necessary for each PET-FD study conducted.