Separation of a potential VAChT radioligand from its nitro precursor - an HPLC study for optimization of F-18-radiosynthesis


Separation of a potential VAChT radioligand from its nitro precursor - an HPLC study for optimization of F-18-radiosynthesis

Wenzel, B.; Fischer, S.; Hiller, A.; Brust, P.; Steinbach, J.

1. Introduction
The F-18-benzoyl group is a useful functionality for the development of PET-radiotracers in order to introduce F-18 in a simple one step radiosynthesis. Since nitro leaving groups are common for the nucleophilic F-18-substitution, we utilized a nitro precursor for F-18-labeling of a vesamicol analog to evaluate it as new radioligand for the vesicular acetylcholine transporter (VAChT), which is an interesting target for imaging of presynaptic cholinergic deficiencies using PET.
The separation of a F-18-labeled compound from its nitro precursor in the radiosynthesis is often challenging, accordingly we firstly used the approach of reduction of the remaining nitro precursor to its amine, because this amine is normally easier to isolate from the radiotracer by semi-preparative HPLC than the nitro precursor itself. However, in order to avoid this additional step during the radiosynthetic procedure, we were interested to develop a suitable HPLC method to separate directly the radiotracer from its nitro precursor. Several analytical HPLC columns and eluent systems were investigated.

2. Materials & Methods
Because of the basic character of the analytes (tertiary amines) several columns were selected which are specially designated for the separation of basic analytes due to their potential to reduce peak tailing (LiChrospher Select B, Nucleosil AB, Reprosil Gold, Reprosil-Pur Basic HD). To study the influence of possible pi-pi-interactions, especially of the nitro precursor, we also used different phenyl and cyano columns (Nucleodur Sphinx, Fortis Phenyl, Reprosil Phenyl, LiChrospher CN). For comparison also a bare silica phase and commonly used AQ phases were investigated.
Aqueous acetonitrile and methanol mixtures with ammonium acetate, triethylamine/acetic acid, and trifluoroacetic acid were used as typical eluent systems.

3. Results
The aims of this HPLC study were: (i) to find a suitable separation method in which the fluoro compound clearly elutes in front of the nitro precursor, and (ii) to get more information about the suitability of different column types regarding separation data such as retention, peak tailing and resolution.
In general, with almost all columns the two compounds could be separated, however, the quality of separations was different and surprisingly, changes in elution order were observed. For example, using aqueous acetonitrile/ammonium acetate as eluent system, the fluoro compound elutes in front of the nitro precursor at low content of organic modifier. In contrast, a reversed retention behavior was observed at high content of acetonitrile which was also found by using methanol as organic modifier. Additionally, we observed that not all columns, designated to be especially appropriate for basic analytes, showed satisfying separation properties. Finally, a phenyl column was selected for the semi-preparative separation of the 18-F-labeled radiotracer from its nitro precursor and a reduction of the total radiosynthesis time by one hour could be achieved .

4. Conclusion
Due to this intensive analytical HPLC study, we were able to select a suitable column for an improved purification of a radiotracer from its remaining nitro precursor, which resulted in a considerable shortening of the radiosynthetic procedure.

  • Poster
    16th European Symposium on Radiopharmacy and Radiopharmaceuticals, 26.-29.04.2012, Nantes, France

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Publ.-Id: 16742