Modification of a commercial synthesizer for concentrated preparation of [F-18]SFB

Objectives N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB) is one of the most widely used labeling moieties for the incorporation of fluorine-18 into biomolecules. We have recently been asked to provide [F-18]SFB in high activity to volume concentrations, thus we modified our synthesis process to include an SPE concentration step.

Methods The Neptis Synthesizer system provides GMP qualified methods and cassette based kits for many fluorine-18 based radiopharmaceuticals and labeling moieties including [F-18]SFB. In addition, the flexibility of a development mode is available which allows the user to create their own sequences and modify the company provided chemistries. The [18F]SFB synthesis is conducted in a single reaction vessel and consists of 3 distinct chemical steps: aromatic nucleophilic substitution with [F-18]fluoride on the triflate salt of ethyl 4-TMA-benzoate, hydrolysis with tetramethylammonium hydroxide, and activation using TSTU. Following dilution with 4% aqueous acetic acid, the crude reaction mixture is SPE purified through Waters Oasis WCX and HLB Sep Paks in series. [F-18]SFB is then eluted with 5 mL of 80% aqueous acetonitrile. To provide a more concentrated radiolabeling agent, modifications were made to the synthesis program and cassette plumbing to afford dilution of the purified [F-18]SFB solution and trapping on Sep Pak C18 cartridge, providing the flexibility to elute [F-18]SFB with a solvent of choice for bioconjugation. The chemistry was conducted using both commercially available kits as well as cassettes with in-house prepared reagents.

Results [F-18]SFB was prepared in 35-45% dcRCY, n=10 in 60 min. The radiochemical purity was >95% as determined by HPLC.

Conclusions Using the Neptis Synthesizer, we have modified the cartridge-based method to include a final concentration step to allow for high activity to volume ratios with reproducible yields and radiochemical and chemical purity suitable for labeling biomolecules.