Introduction
Sigma-1 receptors are expressed throughout the CNS and are implicated in schizophrenia and depression. We have evaluated recently the high affinity (KD = 0.5 ± 0.2 nM, log P = 2.8) sigma-1 receptor radiotracer [18F]2-Flouropropyl-[(4-cyanophenoxy)methyl]piperidine ([18F]FPS) in rodents, baboons and performed the first brain imaging study and safety evaluation of this tracer in healthy humans. In contrast to suitable kinetics in the baboon brain (equilibrium was reached by 60–80 min), in the human CNS [18F]FPS does not reach equilibrium by 4 hr, supporting the development of a lower affinity tracer. Studies in rats also indicated very slow clearance of [18F]FPS from the brain, suggesting that the awake rat may be a better model than baboons for predicting the regional brain kinetics of these sigma-1 receptor tracers in humans. We describe here the synthesis and in vivo evalution in rats of [18F]WLS1.002 (Ki = 5 nM, log P = 2.42), a structurally similar, lower affinity sigma-1 receptor tracer compared to [18F]FPS. A comparison is made between the brain uptake and clearance of [18F]WLS1.002 and [18F]FPS.
Methods
[18F]WLS1. 002 was synthesized by heating the corresponding N-ethylmesylate precursor in anhydrous [18F]flouide/K222/acetonitrile for 15 min. Purification was accomplished by reversed phase HPLC, and solutions of [18F]WLS1.002 were formulated in sterile saline. Regional brain biodistribution (5–90 min; eight regions), metabolism and pharmacologic blocking studies of [18F]WLS1.002 (25 μCi, 100 μl saline, iv) were carried out in awake adult male rats. The drugs used in blocking studies included WLS1.002, FPS, BD1008, and SM-21. All were administered (1 mg/kg, 100 μl saline, iv) 5 min prior to tracer injection. The data obtained from these studies were compared to that collected previously for [18F]FPS.
Results
[18F]WLS1. 002 was synthesized (n = 6) in good yeild (58 ± 8% EOB), and high specific activity (2.89 ± 0.80 Ci/μmol EOS) and radiochemical purity (98.3 ± 2.1%). Whole brain activity (n = 4) for [18F]WLS1.002 was 1.41 ± 0.06 %ID/g at 5 min and was reduced by 33% to 0.96 ± 0.30 %ID/g by 90 min, whereas for [18F]FPS no significant reduction was noted over this same time period. Examination of regional brain distributon revealed a localization for [18F]WLS1.002 like that of [18F]FPS. At 15 min, highest %ID/g of [18F]WLS1.002 was in occiptial cortex (1.86 ± 0.06 %ID/g), Frontal Cortex (1.76 ± 0.38 %ID/g), Striatum (1.44 ± 0.25 %ID/g), and lowest in the hippocampus (1.01 ± 0.02 %ID/g). In all regions examined, peak uptake occurred by 5 min, followed by clearance of [18F]WLS1.002 that was significantly faster than [18F]FPS. Blood activity from 15–90 min was low (<0.07%ID/g). Metabolite analysis (1 hr) revealed that [18F]WLS1.002 remained highly intact in the brain. Blocking studies (1 hr) showed a >90% reduction in [18F]WLS1.002 brain uptake by sigma-1 binding drugs but not by the sigma-2 selective compound SM-21.
Conclusion
[18F]WLS1. 002 exhibits excellent characteristics in vivo and may provide a superior brain imaging radiotracer for human PET studies due to its lower sigma-1 receptor affinity and faster CNS clearance compared to [18F]FPS.