Synthesis and biological characterization of a promising F-18 PET tracer for vesicular acetylcholine transporter

Abstract

Nine fluorine-containing vesicular acetylcholine transporter (VAChT) inhibitors were synthesized and screened as potential PET tracers for imaging the VAChT. Compound 18a was one of the most promising carbonyl-containing benzovesamicol analogues; the minus enantiomer, (-)-18a displayed high potency (VAChT Ki = 0.59 ± 0.06 nM) and high selectivity for VAChT versus receptors (> 10,000-fold). The radiosynthesis of (-)-[18F]18a was accomplished by a two-step procedure with 30 – 40% radiochemical yield. Preliminary biodistribution studies of (-)-[18F]18a in adult male Sprague–Dawley rats at 5, 30, 60 and 120 min post-injection (p.i.) were promising. The total brain uptake of (-)-[18F]18a was 0.684 ID%/g at 5 min p.i. and by 120 min p.i. slowly washed out to 0.409 %ID/g.; evaluation of regional brain uptake showed stable levels of ~0.800 %ID/g from 5 to 120 min p.i in the VAChT-enriched striatal tissue of rats, indicating the tracer had crossed the blood brain barrier and was retained in the striatum. Subsequent microPET brain imaging studies of (-)-[18F]18a in nonhuman primates (NHPs) showed high striatal accumulation in the NHP brain; the standardized uptake value (SUV) for striatum reached a maximum value of 5.1 at 15 min p.i. The time-activity curve for the target striatal region displayed a slow and gradual decreasing trend 15 min after injection, while clearance of the radioactivity from the cerebellar reference region was much more rapid. Pretreatment of NHPs with 0.25 mg/kg of the VAChT inhibitor (-)-vesamicol resulted in a ~90% decrease of striatal uptake compared to baseline studies. HPLC metabolite analysis of NHP plasma revealed that (-)-[18F]18a had a good in vivo stability. Together, these preliminary results suggest (-)-[18F]18a is a promising PET tracer candidate for imaging VAChT in the brain of living subjects.

Kinetics modeling and occupancy studies of a novel C-11 PET tracer for VAChT in nonhuman primates

“…PET scans of the NHP brain were carried out on two adult male cynomolgus macaques weighing 5.5–7.5 kg following established procedures. [26, 29–31] Subject A underwent four baseline scans (including two scans with two arterial blood sampling) and seven scans following pharmacological intervention. Subject B underwent two baseline scans (including one scan with arterial blood sampling).…”

Synthesis, resolution, and in vitro evaluation of three vesicular acetylcholine transporter ligands and evaluation of the lead fluorine-18 radioligand in a nonhuman primate

“…Racemic compounds (4-(2-fluoroethoxy)phenyl)(1-(3-hydroxy-1,2,3,4-tetrahydronaphthalen-2-yl)piperidin-4-yl)methanone, (±)- 5 and (6-(2-fluoroethoxy)pyridin-3-yl)(1-(3-hydroxy-1,2,3,4-tetrahydronaphthalen-2-yl)piperidin-4-yl)methanone, (±)- 6 were synthesized by following our published procedures 18, 20, 22, 23, 27 and resolved by a Chiralcel OD column.…”

“…20, 21, 24 To the dried the solution of 2-[ 18 F]fluoroethyl tosylate (2.6 – 3.0 GBq) was added (−)- 18 (2 – 4 mg) in anhydrous DMF (200 µL), then a suspension of 95% sodium hydride in anhydrous DMF (4 – 5 equivalent, 100 µL) was added in one portion. The vessel was capped, vortexed, and then heated at 85 °C for 15 min.…”

“…NHP scans and image analysis were conducted according to our published methods. 15, 20, 34, 37–39 Two PET studies were performed on an adult male cynomolgus monkey (7.3 kg) with a microPET Focus 220 scanner (Concorde/CTI/Siemens Microsystems, Knoxville, TN). The animal was fasted for 12 h prior to each PET study and a 2 h dynamic emission scan was acquired after administration of 145 – 190 MBq of (−)-[ 18 F] 7 via the venous catheter.…”

“… a K i values (mean ± SEM) were determined in three independent experiments. b Calculated value at pH 7.4 with ACD/ILab, version 7.0 (Advanced Chemistry Development, Inc., Canada). c Reference 20, d Reference 23. …”