Rapid, efficient, and economical synthesis of PET tracers in a droplet microreactor: application to O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET)

Abstract

Conventional scale production of small batches of PET tracers (e.g. for preclinical imaging) is an inefficient use of resources. Using O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET), we demonstrate that simple microvolume radiosynthesis techniques can improve the efficiency of production by consuming tiny amounts of precursor, and maintaining high molar activity of the tracers even with low starting activity. The synthesis was carried out in microvolume droplets manipulated on a disposable patterned silicon “chip” affixed to a heater. A droplet of [18F]fluoride containing TBAHCO3 was first deposited onto a chip and dried at 100\u2009°C. Subsequently, a droplet containing 60\u2009nmol of precursor was added to the chip and the fluorination reaction was performed at 90\u2009°C for 5\u2009min. Removal of protecting groups was accomplished with a droplet of HCl heated at 90\u2009°C for 3\u2009min. Finally, the crude product was collected in a methanol-water mixture, purified via analytical-scale radio-HPLC and formulated in saline. As a demonstration, using [18F]FET produced on the chip, we prepared aliquots with different molar activities to explore the impact on preclinical PET imaging of tumor-bearing mice. The microdroplet synthesis exhibited an overall decay-corrected radiochemical yield of 55\u2009±\u20097% (n\u2009=\u20094) after purification and formulation. When automated, the synthesis could be completed in 35\u2009min. Starting with <\u2009370\u2009MBq of activity, ~\u2009150\u2009MBq of [18F]FET could be produced, sufficient for multiple in vivo experiments, with high molar activities (48–119\u2009GBq/μmol). The demonstration imaging study revealed the uptake of [18F]FET in subcutaneous tumors, but no significant differences in tumor uptake as a result of molar activity differences (ranging 0.37–48\u2009GBq/μmol) were observed. A microdroplet synthesis of [18F]FET was developed demonstrating low reagent consumption, high yield, and high molar activity. The approach can be expanded to tracers other than [18F]FET, and adapted to produce higher quantities of the tracer sufficient for clinical PET imaging.