2-(2′-((Dimethylamino)methyl)-4′-(3-[¹⁸F]fluoropropoxy)-phenylthio)benzenamine for positron emission tomography imaging of serotonin transporters

Abstract 

Introduction

A new ¹⁸F ligand, 2-(2′-((dimethylamino)methyl)-4′-(3-[¹⁸F]fluoropropoxy)-phenylthio)benzenamine ([¹⁸F]1), for positron emission tomography (PET) imaging of serotonin transporters (SERT) was evaluated.

Methods

Binding affinity was determined through in vitro binding assays with LLC-PK1 cells overexpressing SERT, NET or DAT (LLC-SERT, LLC-NET and LLC-DAT) and with rat cortical homogenates. Localization and selectivity of [¹⁸F]1 binding in vivo were evaluated by biodistribution, autoradiography and A-PET imaging studies in rats.

Results

This compound displayed excellent binding affinity for SERT in vitro with K_i=0.33 and 0.24 nM in LLC-SERT and rat cortical homogenates, respectively. Biodistribution studies with [¹⁸F]1 showed good brain uptake (1.61% dose/g at 2 min postinjection), high uptake into the hypothalamus (1.22% dose/g at 30 min) and a high target-to-nontarget (hypothalamus to cerebellum) ratio of 9.66 at 180 min postinjection. Pretreatment with a SERT selective inhibitor considerably inhibited [¹⁸F]1 binding in biodistribution studies. Ex vivo autoradiography reveals [¹⁸F]1 localization to brain regions with high SERT density, and this binding was blocked by pretreatment with SERT selective inhibitors. Small animal PET (A-PET) imaging in rats provided clear images of tracer localization in the thalamus, midbrain and striatum. In A-PET chasing experiments, injecting a SERT selective inhibitor 75 min post-tracer injection causes a dramatic reduction in regional radioactivity and the target-to-nontarget ratio.

Conclusion

The results of the biological studies and the ease of radiosynthesis with moderately good radiochemical yield (RCY=10–35%) make [¹⁸F]1 an excellent candidate for SERT PET imaging.

Abbreviations: 1, 2-(2′-((dimethylamino)methyl)-4′-(3-fluoropropoxy)-phenylthio)benzenamine, 2, 2-(2′-((dimethylamino)methyl)-4′-(2-fluoroethoxy)-phenylthio)benzenamine, 2-INXT, (R)-N-methyl-(2-iodo-phenoxy)-3-phenylpropylamine, (+)-McN5652, (+)-trans-1,2,3,5,6,10-beta-hexahydro-6-[4-(methylthio)phenyl]pyrrolo-[2,1-a]isoquinoline, ACF, 2-[(2-amino-4-chloro-5-fluorophenyl)thio]-N,N-dimethyl-benzenmethanamine, ADAM, 5-iodo-2′-(2-((dimethylamino)methyl)phenylthio)benzenamine, AFE, 2-[2-[[(dimethylamino)methyl]phenyl]thio]-5-(2-fluoroethyl)phenylamine, AFM, 2-[(2-amino-4-fluoro methylphenyl)thio]-N,N-dimethylbenzenemethanamine, A-PET, animal positron emission tomography, BSA, bovine serum albumin, CB, cerebellum, CX, cortex, DASB, 3-amino-4-(2-dimethylaminomethylphenylsulfanyl)benzonitrile, DAT, dopamine transporter, FADAM, N,N-dimethyl-2-(2-amino-4-fluorophenylthio)benzylamine, GBR12909, 1-(2-[bis(4-fluorophenyl)-[methoxy]ethyl)-4-(3-phenylpropyl) piperazine, HP, hippocampus, HY, hypothalamus, IDAM, (5-iodo-2-((2-(dimethylaminomethyl)-phenylthio)benzylalcohol), 5-HT, serotonin, IPT, N-(3-iodopropen-2-yl)-2beta-carbomethoxy-3beta-(4-chlorophenyl) tropane, LLC-PK₁, Hampshire pig kidney cells, NET, norepinephrine transporter, nisoxetine, (±)-N-methyl-3-(2′-methoxyphenoxy)-3-phenylpropylamine, PET, positron emission tomography, ROI, region of interest, SERT, serotonin transporter, ST, striatum, TAC, time activity curve

Keywords: Rat brain, Autoradiography, Binding affinity, 5-HTT, F-18, Radioligand