Open Access Research article

Synthesis of two potential NK1-receptor ligands using [1-11C]ethyl iodide and [1-11C]propyl iodide and initial PET-imaging

Stina Syvänen12, Jonas Eriksson13, Tove Genchel2, Örjan Lindhe1, Gunnar Antoni1 and Bengt Långström13*

Author Affiliations

1 Uppsala Imanet, GE Healthcare, Box 967, 751 09 Uppsala, Sweden

2 Department of Pharmaceutical Biosciences, Uppsala University, Box 591, 751 24 Uppsala, Sweden

3 Department of Biochemistry and Organic Chemistry, Box 576, Uppsala University, 751 23 Uppsala, Sweden

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BMC Medical Imaging 2007, 7:6  doi:10.1186/1471-2342-7-6

Published: 30 July 2007

Abstract

Background

The previously validated NK1-receptor ligand [O-methyl-11C]GR205171 binds with a high affinity to the NK1-receptor and displays a slow dissociation from the receptor. Hence, it cannot be used in vivo for detecting concentration changes in substance P, the endogenous ligand for the NK1-receptor. A radioligand used for monitoring these changes has to enable displacement by the endogenous ligand and thus bind reversibly to the receptor. Small changes in the structure of a receptor ligand can lead to changes in binding characteristics and also in the ability to penetrate the blood-brain barrier. The aim of this study was to use carbon-11 labelled ethyl and propyl iodide with high specific radioactivity in the synthesis of two new and potentially reversible NK1-receptor ligands with chemical structures based on [O-methyl-11C]GR205171.

Methods

[1-11C]Ethyl and [1-11C]propyl iodide with specific radioactivities of 90 GBq/μmol and 270 GBq/μmol, respectively, were used in the synthesis of [O-methyl-11C]GR205171 analogues by alkylation of O-desmethyl GR205171. The brain uptake of the obtained (2S,3S)-N-(1-(2- [1-11C]ethoxy-5-(3-(trifluoromethyl)-4H-1,2,4-triazol-4-yl)phenyl)ethyl)-2-phenylpiperidin-3-amine (I) and (2S,3S)-2-phenyl-N-(1-(2- [1-11C]propoxy-5-(3-(trifluoromethyl)-4H-1,2,4-triazol-4-yl)phenyl)ethyl)piperidin-3-amine (II) was studied with PET in guinea pigs and rhesus monkeys and compared to the uptake of [O-methyl-11C]GR205171.

Results

All ligands had similar uptake distribution in the guinea pig brain. The PET-studies in rhesus monkeys showed that (II) had no specific binding in striatum. Ligand (I) had moderate specific binding compared to the [O-methyl-11C]GR205171. The ethyl analogue (I) displayed reversible binding characteristics contrary to the slow dissociation rate shown by [O-methyl-11C]GR205171.

Conclusion

The propyl-analogue (II) cannot be used for detecting changes in NK1-ligand levels, while further studies should be performed with the ethyl-analogue (I).