Cyclic nucleotide phosphodiesterases (PDEs) are a class of intracellular enzymes that inactivate the secondary messenger molecules, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Thus, PDEs regulate the signaling cascades mediated by these cyclic nucleotides and affect fundamental intracellular processes. Pharmacological inhibition of PDE activity is a promising strategy for treatment of several diseases. However, the role of the different PDEs in related pathologies is not completely clarified yet. PDE-specific radioligands enable non-invasive visualization and quantification of these enzymes by positron emission tomography (PET) in vivo and provide an important translational tool for elucidation of the relationship between altered expression of PDEs and pathophysiological effects as well as (pre-)clinical evaluation of novel PDE inhibitors developed as therapeutics. Herein we present an overview of novel PDE radioligands for PET published since 2012.

Purpose: 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. Methods: [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. Results: [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. Conclusions: 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.