Synthesis and evaluation of THIQ based indoles as potential PET radioligands for imaging the serotonin transporter

Objectives: The serotonin transporter (SERT) is crucial for the regulation of the synaptic concentration of serotonin and is a primary target in the development of antidepressants. To provide access to new efficient ligands for the SERT binding site for PET imaging, N-substituted tetrahydroisoquinoline (THIQ) derivatives both of electron-deficient 3-cyclohexyl and 3-propylindole were selected as lead. Thus, cis-5-cyanoindole-3-yl cyclohexylamine (CFPI, K_i,hSERT = 6.2 nM) labelled with fluorine-18 [1] and the 5-fluoroindole-3-yl propylamine (FMI, K_i,hSERT = 4.1 nM) labelled with carbon-11 [2] were developed as new PET-agents. Here we report on the labelling of CFPI and FMI, and the evaluation of both radiotracers in vivo.
Methods: [¹⁸F]CFPI was synthesized via nucleophilic etherification of the corresponding tetrahydroisoquinolin-6-ol with 1-[¹⁸F]fluoro-2-tosyloxyethane ([¹⁸F]FETos) as secondary labelling agent. It was obtained by a two-step process followed by semipreparative HPLC purification with an overall RCY of 13±7% (decay corrected EOB, total synthesis time 180 min). [¹¹C]FMI was synthesized similarly by an etherification process of the corresponding tetrahydroisoquinolin-6-ol with [¹¹C]CH₃I to yield the product with RCY 10±4% (decay corrected EOB, total synthesis time 27 min). The brain uptake kinetics and the target specicficity of [¹⁸F]CFPI was investigated in female CD-1 mice by organ distribution at 5, 30, 60, and 120 min p.i., and blocking studies at 60 min p.i. (n=4 per time point). The distribution of [11C]FMI in the brain of juvenile pig was assessed by dynamic PET imaging under baseline and consecutively blocking conditions (n=2). Pre-treatment with citalopram (5 mg/kg) as selective SERT ligand was used to asses the specificity of the binding of [¹¹C]FMI. For comparison, [¹¹C]DASB was investigated in an additional animal.


Results: Radiotracers were obtained in radiochemical purity of ≥99%, with specific activity of 1500 GBq/µmol for [¹¹C]FMI and 150 GBq/µmol for [¹⁸F]CFPI. PET scans were performed after i.v. injection of 0.5 – 1 GBq [¹¹C]FMI. In comparison to [¹¹C]DASB with a midbrain-to-cerebellum ratio of 2 at 120 min p.i., [¹¹C]FMI displayed no specific accumulation in SERT-relevant regions (midbrain-to-cerebellum ratio ~1 at 120 min p.i.). Furthermore, pre-tretament with citalopram did not affect the uptake of [¹¹C]FMI in different brain regions as observed by the time activity curves. Comparable results were obtained in biodistribution studies on [18F]CFPI. After i.v. injection of ~ 300 kBq [¹⁸F]CFPI, brain-to-plasma ratios of <1 were determined at each time up to 120 min p.i. Low clearance from the brain and other organs implies a high non-specific binding. Pre-treatment with citalopram was without effect on the activity distribution or elimination route of [¹⁸F]CFPI.
Conclusions: The in vivo data obtained for [¹¹C]FMI in pig brain and [¹⁸F]CFPI in mice indicate a high non-specific binding of the two radiolabelled N-substituted THIQ derivatives. Therefore, both [¹¹C]FMI and [¹⁸F]CFPI are not suitable as SERT-selective PET radioligands.
Research Support: Work was supported by DFG, BMBF and Joint German-Israeli Research Program (MOST #1888).
References: [1] Funke, U. et al. (2008), Biorg. Med. Chem. Lett. 18, 4727-3047, [2] Ben-Daniel, R. et al. (2008) Biorg. Med. Chem. 16, 6364-6370.