Insights into Elution of Anion Exchange Cartridges: Opening the Path towards Aliphatic 18F-Radiolabeling of Base-Sensitive Tracers

Abstract

Aliphatic nucleophilic substitution (S<sub>N</sub>2) with [<sup>18</sup>F]fluoride is the most widely applied method to prepare <sup>18</sup>F-labeled positron emission tomography (PET) tracers. Strongly basic conditions commonly used during <sup>18</sup>F-labeling procedures inherently limit or prohibit labeling of base-sensitive scaffolds. The high basicity stems from the tradition to trap [<sup>18</sup>F]fluoride on anion exchange cartridges and elute it afterwards with basic anions. This sequence is used to facilitate the transfer of [<sup>18</sup>F]fluoride from an aqueous to an aprotic organic, polar reaction medium, which is beneficial for S<sub>N</sub>2 reactions. Furthermore, this sequence also removes cationic radioactive contaminations from cyclotron-irradiated [<sup>18</sup>O]water from which [<sup>18</sup>F]fluoride is produced. In this study, we developed an efficient elution procedure resulting in low basicity that permits S<sub>N</sub>2 <sup>18</sup>F-labeling of base-sensitive scaffolds. Extensive screening of trapping and elution conditions (>1000 experiments) and studying their influence on the radiochemical yield (RCY) allowed us to identify a suitable procedure for this. Four PET tracers and three synthons could be radiolabeled in substantially higher RCYs (up to 2.5-fold), even from lower precursor amounts, using this procedure. Encouraged by these results, we applied our low basicity method to the radiolabeling of highly base-sensitive tetrazines, which cannot be labeled using state-of-art direct aliphatic <sup>18</sup>F-labeling procedures. Labeling succeeded in RCYs of up to 20%. We believe that our findings facilitate PET tracer development by opening the path towards simple and direct S<sub>N</sub>2 <sup>18</sup>F-fluorination of base-sensitive substrates.

Fully Automated GMP-Compliant Synthesis of [18F]FE-PE2I

“…Whilst the RCYs are acceptable, an increasing clinical demand for [ 18 F]FE-PE2I prompted us to try to improve the yield of the radiolabeling reaction, also in light of the increased radiolysis with increased starting activities. Recently, we published a study focusing on optimizing the preconditioning and eluting anions of the QMA cartridge to allow improved aliphatic labeling of base-sensitive tracers [ 11 ].…”

“…Due to increasing demand and to minimize radiation exposure to the personnel, the process was transferred to an automated cassette-based radiosynthesis system (Synthera ® +, IBA) with a preparative HPLC module. Furthermore, following an intensive investigation of conditions for elution of the [ 18 F]fluoride-trapping cartridge, radiochemical yields (RCYs) were improved significantly [ 11 ]. Herein, we report data on the optimization of the synthesis, stabilization of the product along with a presentation of the validation of the production process and quality control according to the good manufacturing practice (GMP).…”

Development of the First Aliphatic 18F-Labeled Tetrazine Suitable for Pretargeted PET Imaging – Expanding the Bioorthogonal Tool Box

“…In order to circumvent these challenges, we want to exploit our latest labeling strategy, which permits aliphatic labeling of highly reactive H-Tz derivatives. 27 For this reason, we evaluated how to introduce a fluoroethyl moiety into the base structure of 1, while retaining the necessary key properties for in vivo chemistry. We explored how a) the removal of one acetic acid chain, b) the introduction of a fluoroethyl moiety, c) the removal of the -F atom on the aromatic ring and d) varying the substitution pattern of the phenyl ring, influences the possibility of tetrazines to ligate in vivo (Figure 2).…”

“…We decided to use a nosylate leaving group since we have recently reported -for a similar scaffold -that increased RCYs up 10-fold were accessible using this leaving group compared to mesylate or tosylate leaving groups. 27 Radiolabeling of [ 18 F]15 was carried out in a one-pot, two-step reaction sequence (Scheme 3). A protection/deprotection strategy was chosen since unprotected carboxylic acids prevent 18 Ffluorinations.…”

“…24 Radiolabeling was only possible using low basicity labeling conditions. 27 3-Substituted 1,2,4,5-tetrazines are reported to be too sensitive for standard 18 F-fluorination approaches. 20 [ 18 F]15 was labeled in a radiochemical yield (RCY) of 13 ± 2 % (n = 3) with a radiochemical purity (RCP) of >98% and a molar activity (AM) of 55 ± 5 GBq/µmol.…”