Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Ziel/Aim:

Because specific subtypes of nicotinic acetylcholine receptors (nAChR) are involved in various neurodegenerative diseases, there is a continuing need for their visualization by PET. Compared to epibatidine and the 2-[F-18]F-A-85380, the novel homoepibatidine derivative [F-18]flubatine (formerly named as [F-18]NCFHEB) showed better pharmacological properties, promoting first studies in humans. The former radiosynthesis starting from a bromo precursor was hampered by very low radiochemical yields (RCY ~3%). Therefore, a highly improved approach, transferable to an automated module, was necessary. Herein, we report on comprehensive experiments to improve all steps of radiosynthesis.

Methodik/Methods:

The preparation of [F-18]flubatine is based on a two-step radiosynthesis: The nucleophilic aromatic radiofluorination followed by the cleavage of a protecting group. A variety of new precursors with various LGs and PGs was synthesized and tested under different labelling and purification conditions (such as precursor amount, solvent, temperature, thermal/microwave heating, SPE techniques). As LGs, -Cl, -NO2 and the -N(Me3)3 + group with iodide, triflate and other counter anions were tested. The protective function of ethoxycarbonyl, Fmoc, trityl und Boc at the NH-group was studied as well as their quantitative removal. Both SPE and ion exchange cartridges were tested for purification. Analytical and semi-preparative HPLC separation protocols were developed using RP18-AQ phases (isocratic/gradient method; aqueous MeCN as eluent). Furthermore, the stability of [F-18]flubatine was investigated under chemical and physiological conditions. For optimization, all radiochemical steps were controlled by radio-HPLC and radio-TLC.

Ergebnisse/Results:

Application of the -N(Me3)3 iodide precursor with a Boc-PG (0.5-1.0 mg, MeCN, 83°C) provided by far the best radiochemical results. For the final product labelling efficiencies of 90±5%, RCY 70±5% (n=25), spec. activity of >350 GBq/μmol were achieved within a total time of 2.0 h. Complete deprotection of the Boc-PG succeeded with 1M HCl (90°C, 5 min). Compared to results with other PGs, this enabled high labelling efficiencies without by-products, and thus, an easy, fast and quantitative separation. Prior to the deprotection step, the purification of the crude reaction mixture with a C18 cartridge and elution with 1% HOAc/MeCN increased the RCYs, too. The final product was isolated in very high purity by semi-preparative HPLC (tR ~30 min). [F-18]flubatine was stable in diluted HCl, NaOH and K2CO3 solutions as well as under physiological conditions.Furthermore, a chiral HPLC method was developed to prepare enantiomerically pure flubatine.

Schlussfolgerungen/Conclusions:

Various procedures were tested to get a validated [F-18]flubatine radiosynthesis as a prerequisite for an automated radiotracer production, applicable for human studies.

Literatur/References: