Simple preparation and purification of ethanol-free solutions of 3′-deoxy-3′-[18F]fluorothymidine by means of disposable solid-phase extraction cartridges

Introduction: 3′-Deoxy-3′-[18F]fluorothymidine ([18F]FLT) shows great potential as a tracer for proliferative studies with PET. However, its regular application is often limited by low radiochemical yields and the use of a troublesome HPLC separation. Moreover, a high content of ethanol, at least one-fold higher than the European Pharmacopoeia and US Pharmacopoeia's established limit, is always present in the final product. The present study reports an optimization of the reaction conditions and a simple and straightforward purification step which affords a solution of [18F]FLT suitable for human use. Methods: Several conditions and materials were tested for both the nucleophilic substitution and purification step. The latter was achieved by means of a series of commercial solid-phase extraction cartridges. Very conveniently, the whole one-pot synthesis was carried out on commercial automated modules using basically the same setup employed for the synthesis of [18F]FDG. Results: Under routine conditions, radiochemical yields of 37% [decay-corrected to start of synthesis (SOS)] were achieved in ca. 39 min from SOS, with radiochemical purities >98% (usually >99%). The negligible radiolysis observed could be easily suppressed by adding 0.5% of EtOH. Typical unlabelled chemical impurities detected were thymidine (0.15 ppm), thymine (0.28 ppm) and stavudine (0.05 ppm). Conclusions: A reliable, simple and efficient preparation of [18F]FLT has been developed, able to afford an ethanol-free solution of the tracer with no need for any HPLC purification. Because of its similarity to the [18F]FDG synthesis, the method can be readily implemented on basically all the commercial modules developed for this common radiotracer.