First results of the one-step one-pot radiosynthesis of the selective alpha7 nicotinic receptor radioligand [¹⁸F]NS10743 using a microwave-based automated system

Objectives: Alpha7 nicotinic acetylcholine receptors (α7 nAChRs) play an important role in many neuropsychiatric diseases and various types of cancer [1]. [¹⁸F]NS10743, a promising and highly competetive α7 nAChR radioligand [1], has been successfully radiolabeled with high specific activity and radiochemical purity using a manual single-mode microwave device [2]. However, the palladium-catalyzed reduction of the remaining aromatic nitro-precursor (NS10796) to amine raises pharmaceutical restrictions and impedes the development of an automated radiosynthesis system. Here we report first results of a modified Discover CEM^® microwave-based automated radiosynthesis of [¹⁸F]NS10743, which is followed by a direct RP-HPLC separation of the product from the respective nitro precursor.
Methods: The reference compound NS10743 and the corresponding nitro-precursor NS10796 were separated by different analytical RP-HPLC conditions (Multospher 120 RP 18 AQ, ReproSil-Pur 120 CN, Lichrospher 60 RP Select B, ReproSil Gold 120 C18) using (26-95%) ACN with 10 mM NH4OAc in water. F-18 microwave-assisted nucleophilic substitution of the nitro-precursor (3.0-4.0 mg) was conducted using Kryptofix^® 2.2.2 (K222) as phase transfer catalyst. Reaction variables such as solvent system, reaction temperature, and time as well as microwave settings such as synthesis mode, applied power and cooling cycles were systematically optimized. Labelling yields were calculated based on radio-TLC and used to estimate the reproducibility of the microwave-based automated reactor. Semi-preparative RP-HPLC columns were chosen based on previous analytical tests, the desired product [¹⁸F]NS10743 separated from the precursor, purified with a Sep-Pak C18 cartridge and eluted with MeOH. Radiochemical purity was determined by radio-TLC and analytical radio-HPLC.

Figure. Analytical RP-HPLC separation of NS10743 from the corresponding nitro-precursor NS10796 using Reprosil-Pur 120 CN column (left) and one-step one-pot radiosynthesis of [¹⁸F]NS10743 (right).
Results: Optimal separation of NS10743 from the nitro-precursor NS10796 was obtained using the RP-HPLC Reprosil-Pur CN column and the related semi-preparative column using 26% ACN with 10 mM NH4OAc in water. NS10743 was eluted in front of NS10796 at low flow rates of 0.8 and 1.2 mL/min in analytical and semi-preparative RP-HPLC systems, respectively. Radiofluorination of NS10743 was successfully achieved using power cycling synthesis mode with 24 cooling cycles in DMF (140-158 °C) at a microwave radiation of 75 W for 15 min (Figure). Labelling efficiencies of ≥54% and radiochemical purity of ≥99% were achieved under these conditions, rendering a moderate-to-good reproducibility of the microwave cavity.
Conclusions: We achieved a promising one-step one-pot radiosynthetic route for the radioligand [¹⁸F]NS10743, avoiding the palladium-catalyzed reduction step. The microwave-based automated system module showed a moderate-to-good reproducibility, which encourages the further coupling with the automated system.
Acknowledgements: The work was financially supported by DFG (DE 1165/2-1).
References: [1] Brust P, Deuther-Conrad W(2012) Neuroimage Clinical Applications (Bright P., ed.) InTech - Open Access Publisher, 533-558. [2] Deuther-Conrad W, et al. (2011) Eur J Nucl Med Mol Imaging, 38, 1541–49.