Combination of positron emission tomography with liquid chromatography in neuropharmacologic research

Positron emission tomography (PET) is an in vivo autoradiographic technique that determines the radioactive distribution and kinetics of a radiolabelled tracer in a tissue. By choice of tracer, it is possible to study physiological processes in living animals and man non-invasively. PET has certain disadvantages such as limited spatial resolution and simultaneous measurement of radiolabelled tracer with the metabolites formed. For an adequate interpretation of the data obtained, complementary techniques such as column liquid chromatography of radioactive composition in blood, plasma, urine and tissue samples have to be used. The prerequisites for any chromatographic technique used for the radioanalysis of substances are speed, high selectivity and high separation efficiency. Examples from PET studies in combination with chromatographic analysis will be given. The utilization of L-dopa in the brain constitutes several steps. Analysis by column liquid chromatography of metabolites in plasma and in monkey brain tissue will make it possible to elucidate different utilization processes of the tracer. Kinetic studies of 11C-labelled neuropeptides such as methionine-enkephalin and substance P revealed high radioactivities in the brain of monkeys. However, simultaneous determination plasma and urine radioactivities using liquid chromatography with radiochemical and photometric detection both indicated that the brain radioactivities emanated to a large extent from 11C-labelled metabolites formed in vivo. Studies with PET using radiotracers having a rapid and extensive metabolism require complementary techniques in the evaluation. High detection selectivity, by combination of photometric and radiochemical detection and rapid and efficient separation, will make liquid chromatography a most important complement in the analysis.