MicroPET imaging of the brain serotonin transporter with [¹⁸F]βFEmZIENT

Abstract

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Objectives: The CNS serotonin transporter (SERT) resides on presynaptic serotonergic neurons and functions to remove serotonin from the synapse. A reduction in SERT density has been attributed to neuropsychiatric disorders such as depression and suicide and, therefore, the SERT is the target of the SSRI class of antidepressants. Because CNS SERT serves as a marker for serotonergic neuronal anatomy and integrity, the ability to image CNS SERT with PET may aid in the diagnosis and maintenance of depression by allowing in vivo measurement of SERT density in specific brain regions. As part of an ongoing research project in our laboratories to develop nortropane-based SERT imaging agents we have been investigating the monoamine transporter binding of 2β-carbo(2-fluoroethoxy)-3β-(3’-((Z)-2-iodoethenyl)phenyl)nortropane (βFEmZIENT) and the microPET imaging properties of [¹⁸F]βFEmZIENT.

Methods: βFEmZIENT was synthesized by Pd⁰-catalyzed coupling of 2β-carbo(2-fluoroethoxy)-3β-(3’-bromophenyl)nortropane with (Z)-1,2-bis(trimethylstannyl)ethylene followed by iododestannylation with ICl. The radiolabeling precursor was prepared by hydrolysis of the methyl ester analog (mZIENT) in refluxing H₂O/1,4-dioxane followed by N-Boc protection. Radiolabeling was achieved by deprotonation of the N-Boc acid in DMF with 0.1 M Bu₄NOH_(aq) and then O-alkylation of the carboxylate salt with [¹⁸F]fluoroethylbrosylate at 90 ^oC. Acid-catalyzed cleavage of the N-Boc group at 90 ^oC, neutralization, and semi-prep HPLC purification (Waters XTerra RP₁₈ 5 μm, 19 x 100 mm, 60:40:0.1 v/v/v MeOH/H₂0/NEt₃, t_R = 15-19 min) afforded [¹⁸F]βFEmZIENT with an average decay corrected yield of 8% (from the end of [¹⁸F]fluoroethylbrosylate synthesis) and an average radiochemical purity of 98%. Brain imaging was performed in anesthetized cynomolgus monkeys with a Concorde microPET P4.

Results: In vitro competition binding assays with HEK cells stably expressing the SERT, dopamine transporter (DAT), or norepinephrine transporter (NET) afforded the following inhibition constants (K_i, nM): SERT 0.43, DAT 87.6, NET 109.9. The octanol/H₂O partition coefficient of [¹⁸F]βFEmZIENT was determined to be logP_7.4 = 1.69. Analysis of the microPET images indicated a high uptake in SERT-rich brain regions with peak uptake achieved at 50 min post-injection. Comparison of uptake in these brain regions with cerebellum uptake afforded the following ratios at 75 and 195 min p.i., respectively: caudate = 1.34, 1.69, putamen = 1.56, 2.08, thalamus = 1.50, 1.93, midbrain = 1.44, 2.23, pons = 1.14, 1.58, medulla = 1.26, 1.81.

Conclusions: βFEmZIENT is a high affinity ligand for the SERT and is selective for the SERT over the DAT and NET. MicroPET imaging with [¹⁸F]βFEmZIENT showed high uptake in the SERT-rich regions of the brain with peak uptake achieved at 50 min p.i. Supported by NIMH.