Improved in vivo PET imaging of the adenosine A_2A receptor in the brain using [¹⁸F]FLUDA, a deuterated radiotracer with high metabolic stability.

Abstract

The adenosine A_2A receptor has emerged as a therapeutic target for multiple diseases, and thus the non-invasive imaging of the expression or occupancy of the A_2A receptor has potential to contribute to diagnosis and drug development. We aimed at the development of a metabolically stable A_2A receptor radiotracer and report herein the preclinical evaluation of [¹⁸F]FLUDA, a deuterated isotopologue of [¹⁸F]FESCH. [¹⁸F]FLUDA was synthesized by a two-step one-pot approach and evaluated in vitro by autoradiographic studies as well as in vivo by metabolism and dynamic PET/MRI studies in mice and piglets under baseline and blocking conditions. A single-dose toxicity study was performed in rats. [¹⁸F]FLUDA was obtained with a radiochemical yield of 19% and molar activities of 72-180 GBq/μmol. Autoradiography proved A_2A receptor-specific accumulation of [¹⁸F]FLUDA in the striatum of a mouse and pig brain. In vivo evaluation in mice revealed improved stability of [¹⁸F]FLUDA compared to that of [¹⁸F]FESCH, resulting in the absence of brain-penetrant radiometabolites. Furthermore, the radiometabolites detected in piglets are expected to have a low tendency for brain penetration. PET/MRI studies confirmed high specific binding of [¹⁸F]FLUDA towards striatal A_2A receptor with a maximum specific-to-non-specific binding ratio in mice of 8.3. The toxicity study revealed no adverse effects of FLUDA up to 30 μg/kg, ~ 4000-fold the dose applied in human PET studies using [¹⁸F]FLUDA. The new radiotracer [¹⁸F]FLUDA is suitable to detect the availability of the A_2A receptor in the brain with high target specificity. It is regarded ready for human application.