Improved in vivo PET imaging of the adenosine A2A receptor in the brain using [18F]FLUDA, a deuterated radiotracer with high metabolic stability

Abstract

Purpose The adenosine A2A receptor has emerged as a therapeutic target for multiple diseases, and thus the non-invasive imaging of the expression or occupancy of the A2A receptor has potential to contribute to diagnosis and drug development. We aimed at the development of a metabolically stable A2A 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 A2A 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 A2A 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 A2A receptor in the brain with high target specificity. It is regarded ready for human application.

Non-Invasive Assessment of Locally Overexpressed Human Adenosine 2A Receptors in the Heart of Transgenic Mice

“…[ 11 C]TMSX was later used in subsequent studies to compare the A 2A -AR density in hearts between endurance athletes and untrained men [22,23]. The non-xanthine derivative [ 18 F]FESCH was one of the first 18 F-labelled A 2A -AR PET-imaging probes [24,25], and its deuterated isotopologue [ 18 F]FLUDA was recently developed and evaluated in vitro as well as in vivo by our group [18]. In the present study, we could show a reduction of the FOC in atrial preparations (stimulated with CGS 21680) from transgenic, but not from wild type mice, after adding FLUDA (Figure 2).…”

“…In a former study with [ 18 F]FLUDA, we determined a K D value of 4.30 ± 0.73 nM and a B max value of 556 ± 143 fmol/mg wet weight in the striatum of healthy CD-1 mice and a K D value of 0.68 nM, as well as a B max value of 218 fmol/mg wet weight in the striatum of piglets [18], a brain region with a high A 2A -AR density. In the human heart, the A 2A -AR is localised at the level of the Z-line of atrial myocytes, where it is co-expressed with α-actinin and the ryanodine receptor [26].…”

“…Based on the promising in vitro observations, we performed PET studies over 60 min under baseline and blocking conditions to determine the biodistribution of [ 18…”

“…A 2A -AR imaging has successfully been performed in neurological disorders using positron emission tomography (PET) with [ 11 C]preladenant and, recently, [ 18 F]MNI-444 in a clinical phase III study [16,17]. The K i of [ 18 F]MNI-444 towards human A 2A -AR is comparably high (2.8 nM) [17]. Hence, a striatum-to-cerebellum ratio (measure of specific binding) of about 12 was found at 60-70 min after injection in humans [17].…”

“…Hence, a striatum-to-cerebellum ratio (measure of specific binding) of about 12 was found at 60-70 min after injection in humans [17]. Our recently developed A 2A -AR-specific PET radiotracer [ 18 F]FLUDA (human A 2A -AR: K i = 0.6 nM; human A 1 -AR: K i = 767 nM) appears highly suitable for the non-invasive A 2A -AR imaging of mice and piglet brains [18]. In terms of assessing the A 2A -AR availability in the heart, aiming at the visualisation of pathologically relevant changes, earlier attempts were performed by Ishiwata and colleagues using the A 2A -AR radioligands [ 11 C]KF17837 and [ 11 C]TMSX (Figure 1) in rabbits and humans, whereat [ 11 C]TMSX revealed good properties for PET imaging and high plasma stability [19,20].…”