PET radiotracer development for imaging high-affinity state of dopamine D2 and D3 receptors: Binding studies of fluorine-18 labeled aminotetralins in rodents

Imaging the high-affinity, functional state (HA) of dopamine D2 and D3 receptors has been pursued in PET imaging studies of various brain functions. We report further evaluation of F-18-5-OH-FPPAT, and the newer F-18-5-OH-FHXPAT and F-18-7-OH-FHXPAT. Syntheses of F-18-5-OHFHXPAT and 18F-7-OH-FHXPAT were improved by modifications of our previously reported procedures. Brain slices and brain homogenates from male Sprague-Dawley rats were used with the 3 radiotracers (74-111 kBq/cc). Competition with dopamine (1-100 nM) and Gpp(NH) p (10-50 mu M) were carried out to demonstrate binding to dopamine D2 and D3 HA-states and binding kinetics of 18F-5-OH-FPPAT measured. Ex vivo brain slice autoradiography was carried out on rats administered with 18F-5-OH-FHXPAT to ascertain HA-state binding. PET/CT imaging in rats and wild type (WT) and D2 knock-out mice were carried out using 18F-7-OH-FHXPAT (2-37 MBq). Striatum was clearly visualized by the three radiotracers in brain slices and dopamine displaced more than 80% of binding, with dissociation rate in homogenates of 2.2 x 10(-2) min(-1) for F-18-5-OH- FPPAT. Treatment with Gpp(NH) p significantly reduced 50-80% striatal binding with faster dissociation rates (5.0 x 10(-2) min(-1)), suggesting HA-state binding of F-18-5-OH-FPPAT and F-18-5- OH-FHXPAT. Striatal binding of F-18-5-OH-FHXPAT in ex vivo brain slices were sensitive to Gpp(NH) p, suggesting HA-state binding in vivo. PET binding ratios of F-18-7-OH-FHXPAT in rat brain were ventral striatum/cerebellum = 2.09 and dorsal striatum/cerebellum = 1.65; similar binding ratios were found in the D2 WT mice. These results suggest that in vivo PET measures of agonists in the brain at least in part reflect binding to the membrane-bound HA-state of the dopamine receptor.