(-)-N-(11)C-propyl-norapomorphine ((11)C-NPA) is a new dopamine agonist PET radiotracer that holds potential for imaging the high-affinity states of dopamine D(2)-like receptors in the living brain. The goal of this study was to develop and evaluate analytic strategies to derive in vivo (11)C-NPA binding parameters.
Methods: Two baboons were scanned 4 times after (11)C-NPA injections. The metabolite-corrected arterial input functions were measured. Regional brain time-activity curves were analyzed with kinetic and graphical analyses, using the arterial time-activity curve as the input function. Data were also analyzed with the simplified reference-tissue model (SRTM) and graphical analysis with reference-region input.
Results: (11)C-NPA exhibited moderately fast metabolism, with 31% +/- 5% of arterial plasma concentration corresponding to the parent compound at 40 min after injection. Plasma clearance was 29 +/- 1 L/h, and plasma free fraction (f(1)) was 5% +/- 1%. For kinetic analysis, a 1-tissue compartment model (1TCM) provided a good fit to the data and more robust derivations of the tissue distribution volumes (V(T), in mL/g) than a 2-tissue compartment model (2TCM). Using 1TCM, V(T)s in the cerebellum and striatum were 3.4 +/- 0.4 and 7.5 +/- 2 mL/g, respectively, which led to estimates of striatal binding potential (BP) of 4.0 +/- 1.1 mL/g and striatal equilibrium specific-to-nonspecific partition coefficient (V(3)") of 1.2 +/- 0.2. V(T) values derived with graphical analysis were well correlated with but slightly lower than V(T) values derived with kinetic analysis. V(3)" values derived with SRTM were well correlated with but slightly higher than V(3)" values derived with kinetic analysis. Using any method, a significant difference was detected in BP and V(3)" values between the 2 animals. It was determined that 30 min of scanning data were sufficient to derive V(3)" values using kinetic, graphical (arterial input and reference-region input), and SRTM analyses.
Conclusion: This study indicates that (11)C-NPA is a suitable PET tracer to quantify the agonist high-affinity sites of D(2)-like receptors.