A Comparison of 11C-Labeled l-DOPA and l-Fluorodopa as Positron Emission Tomography Tracers for the Presynaptic Dopaminergic System

11C-labeled 3,4-Dihydroxy-phenyl-L-alanine (L-DOPA) and L-fluorodopa were used as tracers for the functional state of the presynaptic dopamine system in anesthetized monkeys with positron emission tomography. The radiotracer disposition in brain tissue and plasma were studied and effects induced by pharmacologic challenges were evaluated. 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (6R-BH4) increased the striatal influx rate constant, e.g., striatal Ki for L-[β-11C]DOPA, but it induced no effect on the Ki-value using L-[β-11C]-6-fluorodopa. Studies of radiolabeled tracer and metabolites in plasma showed substantial differences between the two tracers. At baseline conditions, 60% unchanged L-[β-11C]DOPA was detected in plasma 50 minutes after tracer injection and the 3-O-methylated fraction accounted for 25% of total radioactivity. For L-[β-11C]-6-fluorodopa, the relation was inverse; about 25% unchanged tracer and 60% 3-O-methyl metabolite were present in plasma after 50 minutes. A site-specific 11C-labeling in the carboxylic position in the molecules revealed a significant specific retention of radioactivity in striatum with L-[carboxy-11C]-6-fluorodopa but not with L-[carboxy-11C]DOPA. The 3-O-methyl metabolite of L-DOPA is known to pass the blood-brain barrier and may interfere with the calculation of the Ki value using a brain reference region. Thus, extensive 3-O-methylation in circulation of the fluorinated analog could obscure the detectability of potential functional change in striatal Ki of the tracer when using a reference tissue model for calculation.