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We have synthesized N-[3-(2',4'-dichlorophenoxy)-2-18F-fluoropropyl]-N-methylpropar gylamine (18F-fluoroclorgyline) as a potential positron emission tomography (PET) radiotracer for monoamine oxidase A (MAO-A). The radiosynthesis was carried out by a 18F-fluoride-for-mesylate substitution reaction in approximately 20% radiochemical yield in specific activities of 1-2 Ci/micromol. Selectivity for MAO-A was demonstrated by the high affinity of clorgyline (IC50 = 39 nM) and lower affinity of (R)-deprenyl (IC50 > or = 100 microM) for the inhibition of 18F-fluoroclorgyline binding in vitro in rat brain membranes. The uptake of 18F-fluoroclorgyline in the rat brains was high (> 1.0% injected dose/g). The binding of 18F-fluoroclorgyline in the rat brain correlated with the distribution of MAO-A and was inhibited by preadministration of MAO-A inhibitors, clorgyline, and Ro 41-1049, whereas (R)-deprenyl, a MAO-B blocker, had no inhibitory effect. These results suggest that 18F-fluoroclorgyline is a potential PET radiotracer for MAO-A.

We have identified the value of 18F-fallypride [(S)-N-[(1-allyl-2-pyrrolidinyl)methyl]-5-(3-[18F]fluoropropyl)-2, 3-dimethoxybenzamide], as a dopamine D-2 receptor radiotracer for the study of striatal and extrastriatal receptors. Fallypride exhibits high affinities for D-2 and D-3 subtypes and low affinity for D-4 (3H-spiperone IC50s: D-2 = 0.05 nM [rat striata], D-3 = 0.30 nM [SF9 cell lines, rat recombinant], and D-4 = 240 nM [CHO cell lines, human recombinant]). Biodistribution in the rat brain showed localization of 18F-fallypride in striata and extrastriatal regions such as the frontal cortex, parietal cortex, amygdala, hippocampus, thalamus, and hypothalamus. In vitro autoradiographic studies in sagittal slices of the rat brain showed localization of 18F-fallypride in striatal and several extrastriatal regions, including the medulla. Positron emission tomography (PET) experiments with 18F-fallypride in male rhesus monkeys were carried out in a PET VI scanner. In several PET experiments, apart from the specific binding seen in the striatum, specific binding of 18F-fallypride was also identified in extracellular regions (in a lower brain slice, possibly the thalamus). Specific binding in the extrastriata was, however, significantly lower compared with that observed in the striata of the monkeys (extrastriata/cerebellum = 2, striata/cerebellum = 10). Postmortem analysis of the monkey brain revealed significant 18F-fallypride binding in the striata, whereas binding was also observed in extrastriatal regions such as the thalamus, cortical areas, and brain stem.

We have developed 18F-fluoxetine as a radiotracer analog of the antidepressant drug fluoxetine (Prozac). In vitro saturation experiments of 18F-fluoxetine were carried out on rat midbrain tissue and citalopram was used for measuring nonspecific binding. A saturation curve for the binding of 18F-fluoxetine was not obtained. Even when fluoxetine (10 microM) was used for measurements of nonspecific binding, a saturation curve was difficult to obtain. Other compounds, such as deprenyl, clorgyline, amphetamine, and reserpine were also not able to reduce the binding of 18F-fluoxetine. Ex vivo autoradiographic experiments with 18F-fluoxetine did not reveal any specific uptake in various brain regions. In vivo administration of 18F-fluoxetine in rats showed similar uptake in all the brain regions with little regional selectivity. A subcellular analysis of rat brain tissue after intravenous (IV) administration of 18F-fluoxetine indicated significant amounts of binding in mitochondria and synaptosomes. In summary, in vitro experiments with 18F-fluoxetine indicate little specific binding. Binding to the serotonin transporter was not identifiable. High nonspecific binding of the tracer resulting from its subcellular nature in the brain masks the ability to detect binding to the serotonin uptake sites in vivo. These findings indicate that a large portion of the binding of 18F-fluoxetine in rat brains is subcellular and clears slowly out of the cells. Other sites, such as monoamine oxidase, may also play a significant role in the action of fluoxetine.

We have prepared N-alkyl, aryl, fluoroalkyl, fluoroaryl and iodoaryl derivatives of 7-chloro-8-hydroxy-3-methyl-1-(3'-aminophenyl)-2,3,4,5-tetrahydro-1 H-3-benzazepine (SCH 38548) as high-affinity ligands for the dopamine D1 receptor. Binding affinities of the compounds for dopamine D1, D2, and serotonin 5-HT2 receptor sites in rat brain homogenates were measured. The affinity of SCH 38548 for dopamine D1 receptors was found to be 0.53 +/- 0.46 nM, whereas lower affinities (in the micromolar range) for dopamine D2 and serotonin 5-HT2 receptors were found. Alkylation (ethyl, n-propyl and benzyl) and acylation (benzoyl) of the amino group of SCH 38548 did not decrease affinities for the D1 receptors significantly. The fluoroethyl, fluoropropyl, and fluorobenzyl derivatives showed approximately an 8-fold, 9-fold, and 3-fold decrease in affinity for the D1 sites compared to SCH 38548. The N-4-fluorobenzoyl derivative, however, showed a similar affinity for the D1 sites as for SCH 38548. All four fluorinated derivatives exhibited weak binding at D2 and serotonin 5-HT2 receptors. The N-(4-18F-fluorobenzoyl)SCH 38548 was prepared by reacting SCH 38548 with 4-18F-fluorobenzoyl fluoride in 2-5% radiochemical yield with a specific radioactivity of approximately 600-700 Ci/mmol. The N-(3-18F-fluoropropyl)SCH 38548 was prepared by reacting SCH 38548 with 18F-fluoropropyl iodide in 2-5% radiochemical yield with a specific radioactivity of approximately 600-700 Ci/mmol. N-(4-18F-fluorobenzoyl)SCH 38548 failed to localize in the dopaminergic sites in the rat and rhesus monkey brain. Biodistribution of N-(3-18F-fluoropropyl)SCH 38548 in rats showed specific uptake and retention (0.64% injected dose/g at 30 min) of the radiotracer in the striata, with striata-to-cerebellum ratios reaching 12 at 2 h postinjection (p.i.). Positron emission tomography scans in rheusus monkeys indicate selective uptake of the radiotracer in the striata. After IV injection of N-(3-18F-fluoropropyl)SCH 38548, a rapid brain uptake of the tracer from blood was observed. Initial uptake in the striata and cerebellum was approximately 0.02% of injected dose/cc. Nonspecific uptake from the tissue surrounding the striata cleared slowly. The striata-to-cerebellum ratio increased from 1.20 to 3.5 min postinjection to approximately 2.5 at 120 min p.i. The specific uptake of N-(3-18F-fluoropropyl)SCH 38548 in the striata was displaced by IV administration of SCH 24518 (2 mg/kg).

Various thienylcyclohexyl piperidine (TCP) derivatives were synthesized and evaluated as potential candidates for use as radiotracers for the in vivo study of the NMDA receptor ion-channel. Modification of the thienylcyclohexylpiperidine was accomplished by substituting its piperidine ring either with other cycloamine rings or N-alkyl-substituted amines, and these two classes of TCP derivatives were synthesized using two different methods. The compounds exhibited affinities ranging from 65 nM up to micromolar in competition assays for the receptor ion-channel labeled with 3H-(+)-MK-801 in rat brain homogenates. Radiosynthesis of 1-[1-(2-thienyl)-4-([18F]fluoro)-cyclohexyl]-1,2,5, 6-tetrahydropyridine was carried out by nucleophilic substitution reaction of 1-[1-(2-thienyl)-4-tosyloxycyclohexyl]-1,2,5,6-tetrahydropyridine with no carrier added 18F-, and the yield was approximately 5-10% (decay corrected) in specific activities of 500-1000 Ci/mmol after reverse-phase HPLC purification. The tracer showed good uptake in rat brains after i.v. injection (approx. 0.10% injected dose/g at 30 min. p.i.). However, the specific uptake in receptor-rich regions (striata, hippocampus, frontal cortex, and parietal cortex) improved only marginally with time compared to cerebellum. Three hours postinjection, parietal cortex showed a maximum ratio of 1.9. Preliminary PET experiment with this radiotracer in a rhesus monkey showed good uptake in the brain regions. However, little retention of the radiotracer was observed in the receptor-rich regions.

We have developed (S)-N-[(1-allyl-2-pyrrolidinyl)methyl]-5-(3-18F-fluoropropyl)-2- methoxybenzamide (18F-desmethoxyfallypride) as a fluorine-18 radiotracer with properties analogous to that of 11C-raclopride. In vitro experiments in rat brain homogenates showed an association rate constant of 2.16 x 10(8) M(-1)min(-1) and a dissociation rate constant of 0.073 min(-1). High striatal uptake (up to 0.08% injected dose/cc) of 18F-desmethoxyfallypride in rhesus monkeys was observed in PET experiments. The radiotracer cleared from the striata with a dissociation rate of 1.80 x 10(-2) min(-1). Striatum to cerebellum ratios peaked at 3.0 in 30 min after which they decreased steadily. Intravenously administered haloperidol displaced specifically bound 18F-desmethoxyfallypride with a koff of 0.058 min(-1). Synaptic dopamine released by the treatment of the monkeys with d-amphetamine increased the dissociation rate of 18F-desmethoxyfallypride to 0.83 min(-1) thus reducing specifically bound 18F-desmethoxyfallypride by 56% over a period of 42 mins compared to a reduction of only 20% in controls during this time period. The sensitivity of 18F-desmethoxyfallypride towards competition with dopamine should make this radiotracer useful in PET studies to evaluate in vivo pharmacological effects of various agents that alter levels of endogenous dopamine.

High affinity and selective fluorinated derivatives of 7-chloro-8-hydroxy-3-methyl-1-(3'-aminophenyl)-2,3,4,5-tetrahydro-1H-3- benzazepine (SCH 38548) for dopamine D1 receptors have been prepared. The derivatives exhibit subnanomolar to nanomolar affinities for dopamine D1 receptors and micromolar to millimolar affinities for dopamine D2 and 5-HT2 receptors. Biodistribution of N-(3-[18F]fluoropropyl)SCH 38548 in rats showed specific uptake and retention (0.64% injected dose/g at 30 min) of the radiotracer in the striata with striata to cerebellum ratios reaching 12 at 2 h post-injection. Preliminary positron emission tomography scans in rhesus monkeys indicate selective uptake of the radiotracer in the striata.