Proof-of-concept optimization of a copper-mediated ¹⁸F-radiosynthesis of a novel MAGL PET tracer on a high-throughput microdroplet platform and its macroscale translation.

Abstract

Copper-mediated radiofluorination has demonstrated remarkable potential in forming aromatic C-¹⁸F bonds of radioligands for positron emission tomography (PET). Achieving optimal results often requires optimization efforts, requiring a substantial amount of radiolabeling precursor and time, severely limiting the experimental throughput. Recently, we successfully showcased the feasibility of performing and optimizing Cu-mediated radiosynthesis on a high-throughput microdroplet platform using the well-known and clinically used radioligand [¹⁸F]FDOPA as an illustrative example. In our current work, we optimized the Cu-mediated synthesis of a novel monoacylglycerol lipase (MAGL) PET tracer ([¹⁸F]YH149), showing the versatility of droplet-based techniques for early stage tracer development. Across 5 days, we conducted a total of 117 experiments, studying 36 distinct conditions, while utilizing <15 mg of total organoboron precursor. Compared to the original report in which the radiochemical yield (RCY) was 4.4 ± 0.5% (<i>n</i> = 5), the optimized droplet condition provided a substantial improvement in RCY (52 ± 8%, <i>n</i> = 4) and showed excellent radiochemical purity (100%) and molar activity (77-854 GBq μmol^-1), using a starting activity of 0.2-1.45 GBq. Furthermore, we showed for the first time a translation of the optimized microscale conditions to a vial-based method. With similar starting activity (0.2-1.44 GBq), the translated synthesis exhibited a comparable RCY of 50 ± 10% (<i>n</i> = 4) while maintaining excellent radiochemical purity (100%) and acceptable molar activity (20-46 GBq μmol^-1). The successful translation to vial-based reactions ensures wider applicability of the optimized synthesis by leveraging widely available commercial vial-based synthesis modules.