Evaluation of d-amphetamine effects on the binding of dopamine D-2 receptor radioligand, 18F-fallypride in nonhuman primates using positron emission tomography

We have investigated the ability of dopamine to compete with the binding of the high affinity dopamine D2 receptor positron emission tomography (PET) radioligand, 18 F-fallypride. In vitro dissociation of 18 F-fallypride with dopamine in rat striatal homogenates exhibited a dissociation rate, koff, of 1.76 3 10 22 min 21 while the association rate constant, kon, was found to be 5.30 3 10 8 M 21 min 21 . This resulted in a dissociation constant, KD of 33 pM for 18 F-fallypride. For in vivo studies, we investigated the effects of reserpine and d-amphetamine treatment on 18 F-fallypride in an attempt to study competition of endogenous dopamine with the radioligand at the receptor sites in rats and monkeys. PET experiments with 18 F-fallypride in two male rhesus monkeys were carried out in a PETT VI scanner. In control experiments, rapid specific uptake of 18 F-fallypride in the striata was observed (0.05-0.06% injected dose (ID)/g) while nonspecifically bound tracer cleared from other parts of the brain. Striata/cerebellum ratios for 18 F-fallypride were approximately 8 at 80 min postinjection, respectively. The monkeys received various doses (0.25to1.50mg/kg)ofd-amphetamine(AMPH)pre-andpostinjectionoftheradioligand.There wasadecreaseofspecificallybound 18 F-fallyprideaswellasevidenceofanenhancedclearance of specifically bound 18 F-fallypride after administering AMPH in the two monkeys. The dissociation rates, koff ,o f 18 F-fallypride without AMPH was ,10 24 min 21 but after 25 min preadministration of AMPH (1 mg/kg), it was 4.1 3 10 23 min 21 and after 17, 45 and 90 min postadministration of AMPH (1 mg/kg) it was 3.6 3 10 23 to 4.0 3 10 23 min 21 . Lower doses of AMPH (0.25 mg/kg) had a reduced effect on the binding of 18 F-fallypride. No effect was seen until about 30 minutes after the injection of AMPH. Studies with various doses indicated that 18 F-fallypride has a maximum response at doses of 0.75-1.50 mg/kg, with an approximately 16%/hour reduction in binding. These results indicate thatAMPH stimulated release of endogenous dopamine reduces the specific binding of 18 F-fallypride.