Abstract
This chapter performs fluorodopa (FDOPA)/positron emission tomography (PET) to assess the effects of entacapone on the systemic metabolism and striatal uptake of FDOPA using a comprehensive kinetic model. These results are compared with those obtained through other proposed models to evaluate the sensitivity of each to the plasma 3-O-methyl-FDOPA (3-O-MFD) fraction. This study also presents the measurement of the time course of the plasma FDOPA and 3-O-MFD fractions using high pressure liquid chromatography (HPLC). The striato-occipital ratios (SOR) have been calculated that estimate the striatal FDOPA uptake rate constant graphically using the plasma FDOPA and occipital tissue time–activity curves. The striatal dihydroxyphenylalanine (DOPA) decarboxylase (DDC) activity has also been estimated using a model incorporating independent measurements of 3-O-MFD transport kinetic rate constants. With the preadministration of entacapone, the pharmacological efficiency in plasma prolonged significantly. It has been concluded that entacapone prolongs the circulation time of FDOPA in the plasma, but does not alter rate constants for striatal FDOPA uptake or decarboxylation.
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