Improved one-pot synthesis of 5-substituted 2’-deoxy-2’-[¹⁸F]fluoro-arabinofuranosyluracil derivatives

Objectives: Thymidine analogues (5-substituted 2’-deoxy-2’-[¹⁸F]fluoro-arabinofuranosyluracil derivatives) are promising PET tracers being evaluated for imaging tumor cell proliferation and/or reporter gene expression. Their conventional synthesis is rather tedious involving multi-step procedures resulting in low radiochemical yield of desired products and inconvenient applications for clinical use. We have previously reported the use of Friedel-Crafts catalysts as an alternative synthesis approach. In this study, we investigated the feasibility of further improving their radiosynthesis for the purpose of facilitating clinical translation.

Methods: A simplified one-pot synthesis of thymidine analogues, including [¹⁸F]FAU, [¹⁸F]FMAU, [¹⁸F]FEAU, [¹⁸F]FFAU, [¹⁸F]FCAU, [¹⁸F]FBAU, and [¹⁸F]FIAU, was developed. Various polar and nonpolar solvents were explored to improve the coupling condition of 2-deoxy-2-[¹⁸F]fluoro-1,3,5-tri-O-benzoyl-d-arabinofuranose ([¹⁸F]-sugar) and substituted uracil in the presence of trimethylsilyl trifluoromethanesulfonate and hexamethyldisilazane.

Results: Using a one-pot synthesis approach, 5-substituted 2’-deoxy-2’-[¹⁸F]fluoro-arabino-furanosyluracil derivatives were successfully synthesized in high radiochemical purity ((≥ 99%). As compared to polar solvents, nonpolar solvents are more suitable for the coupling of [¹⁸F]-sugar and substituted uracil in terms of providing a higher yield of the conjugation and a greater ratio of the desired product (β-anomer) to the side product (α-anomer). Among all investigated solvents, 1,4-dioxane was identified as the best. For instance, as compared to the previously reported solvent (1,2-dichloroethane), the reaction using 1,4-dioxane achieved better β/α anomer ratio (1.74 : 1 vs. 1.23 : 1) in the case of 2′-deoxy-2′-[¹⁸F]fluoro-5-methyl-1-β-d-arabinofuranosyluracil ([¹⁸F]-FMAU) synthesis. In addition, 1,4-dioxane is listed as a Class 2 residual solvent in the US Pharmacopeia with a concentration limit of 380 ppm, which is less toxic than 1,2-dichloroethane (Class 1 residual solvent with a concentration limit of 5 ppm).

Conclusions: We have successfully synthesized 5-substituted 2’-deoxy-2’-[¹⁸F]fluoro-arabino-furanosyluracil derivatives in excellent radiochemical purity using an improved one-pot synthesis method. As compared to 1,2-dichloroethane, 1,4-dioxane appears to be a better solvent in terms of synthesis yield and anomer selectivity. The employment of a less toxic solvent permits the translation of the improved approach to clinical production. This new method is versatile, which warrants a wide range of applications for ¹⁸F-labeling of other nucleoside analogues.