Streamlined Radiosynthesis of [18F]Fluproxadine (AF78): An Unprotected Guanidine Precursor Enables Efficient One-Step, Automation-Ready Labeling for Clinical Use

Background/Objectives: [18F]Fluproxadine (formerly [18F]AF78) is a PET radiotracer targeting the norepinephrine transporter (NET) with potential applications in cardiac, neurological, and oncological imaging. Its guanidine moiety, while essential for NET binding, presents major radiosynthetic challenges due to high basicity and the harsh deprotection conditions required for protected precursors. Previous methods relied on multistep procedures, strong acids, and complex purification, limiting clinical translation. This study aimed to develop a practical one-step radiosynthesis suitable for routine and automated production. Methods: A direct SN2-type nucleophilic [18F]fluorination was performed using an unprotected guanidine precursor to eliminate deprotection steps. Reaction parameters, including the base system, solvent composition, precursor concentration, and temperature, were optimized under conventional and microwave heating. Radiochemical conversion (RCC) and operational robustness were evaluated, and purification strategies were assessed for automation compatibility. Results: Direct [18F]fluorination using the unprotected precursor reduced the total synthesis time to 60–70 min. Optimal conditions employed a tert-butanol/acetonitrile (4:1) solvent system with K2CO3/Kryptofix222, affording RCC up to 33% under conventional heating. Microwave irradiation further improved efficiency, achieving RCC of up to 64% within 1.5 min at 140 °C. The method showed broad tolerance to variations in the base molar ratio and precursor concentration and enabled isocratic HPLC purification. Conclusions: This one-step radiosynthesis overcomes longstanding challenges in [18F]fluproxadine production by eliminating harsh deprotection and enabling high-yield, automation-ready synthesis, thereby improving clinical feasibility.