Measuring the in Vivo Binding Parameters of [18F]-Fallypride in Monkeys Using a PET Multiple-Injection Protocol

The goal of this work was to quantify the in vivo transport and binding parameters of [F-18]fallypride and the D2/D3 receptor density (B′max) in both the striatal (putamen, caudate, ventral striatum) and extrastriatal regions (thalamus, amygdala, cerebellum, temporal and frontal cortices) of the rhesus monkey brain. Multiple-injection PET experimental protocols with injections of radiolabeled and unlabeled doses of fallypride were used to estimate the K1, k2, kon/VR, koff and B′max kinetic parameters. The experimental design was chosen using the D-optimal criterion to maximize the precision of the estimated binding parameters for the various brain regions. There was a significant range in B′max for the putamen (27pmol/mL), caudate (23pmol/mL), ventral striatum (14pmol/mL), thalamus (1.8pmol/mL) and amygdala (0.9pmol/mL). Significant receptor binding was also found in the cortical regions. Knowledge of these in vivo rate constants serves as a necessary step in using [F-18]fallypride PET to measure D2/D3 receptor density and drug occupancy in clinical research applications. We believe the precise parameter estimates derived from these complicated experimental protocols are necessary for proper application of drug occupancy and clinical research studies with [F-18]fallypride, which often rely on the validity of assumptions regarding the model parameters.