Abstract View

Introduction: In the synthesis of fluorine-18 labeled PET radiotracers it is desirable to incorporate the radiolabel in the final step of synthesis, an approach termed “late-stage” radiofluorination. These single-step ¹⁸F-fluorination strategies, where chemically possible, attempt to maximize radiochemical yields by minimizing radiosynthetic inefficiencies and losses due to radioactive decay during extended synthetic protocols. This approach has been used for decades for the SN2 fluorination of aliphatic precursors, and with the development of new methodologies, many electron-rich aromatic substrates can now be radiolabeled in a single step. The use of the cassette based Trasis AllInOne (AIO) automated synthesizer minimizes limitations associated with conventional synthesis modules such as the risk of cross-contamination and the time, training needed for extensive cleaning of the synthesis units before and after synthesis, thereby reducing radiotracer production time and improving production reliability. In our facility, we hoped to streamline the development and validation of new radiotracer production protocols, to quickly bring a number of ¹⁸F-radiotracers on-line for routine manufacture. Although the conditions and reagents required for these late-stage fluorination methodologies differ significantly, we sought to develop a universal cassette and synthesizer time-list for single-step radiofluorination that needs only minor modifications of reaction times and temperatures for different tracers, while maintaining cGMP compliance, simplicity, and reproducibility for routine clinical production.

Methods: A universal cassette design for nucleophilic single-step radiofluorination, and reformulation was developed on the Trasis AIO system which is versatile across multiple radiotracers labeled at both aliphatic and aromatic positions, including copper-mediated radiofluorination. The cassette was designed with an emphasis on “dirty” and “clean” zones. The left side of the cassette (positions 1–10) constitute the dirty zone which contains the reagents involved in the synthesis. The right side of the cassette (positions 11–18) is considered “clean” and contains the product after semi-preparative HPLC purification thereby minimizing the potential cross-contamination between reagents and purified product. Additionally, the design reduces the complexity of individualized semi-preparative HPLC eluent preparation by allowing the on-line mixing of mobile-phase components from universal stock components, according to the purification needs of each radiotracer. The single-step radiofluorination involves the following steps, with each of the corresponding cassette positions highlighted: [¹⁸F]fluoride drying (1–6, reactor), radiolabeling of the precursor (7–8, reactor), semi-preparative HPLC purification (9–10, 12), reformulation (12–18), and sterile filtration (transfer to clean room, 11).

Results: The cassette was successfully adapted for cGMP production of three radiotracers now in clinical production: [¹⁸F]BCPP-EF, [¹⁸F]FEOBV, and [¹⁸F]CNY-07. We have recently validated a fourth, copper mediated single-step radiofluorination of [¹⁸F]SynVesT-1 using the universal cassette design and time list. The fully automated process yields [¹⁸F]SynVesT-1 with a radiochemical yield of 6.5–16.6% (non-decay corrected), starting from 1950–2050 mCi of fluorine-18. The product has excellent radiochemical purity (>99%) and complies with all quality parameters detailed in USP <823>.

Conclusion: The universal cassette design reduces the development burden and facilitates the rapid translation of new radiotracers into routine clinical production. The design offers a reliable, reproducible and cGMP approach for the single-step radiofluorination of ¹⁸F-radiotracers.