Fluorinated benzazepines: 1. Synthesis, radiosynthesis and biological evaluation of a series of substituted benzazepines as potential radiotracers for positron emission tomographic studies of dopamine D-1 receptors
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).