Development and Preclinical Evaluation of PET Radiotracers Targeting Adenosine A₁ Receptors

Several adenosine A₁ receptor (A₁R) radiotracers for positron emission tomography (PET) have been developed to study their neuromodulatory functions and role in brain disorders. While two xanthine-based radiotracers ([¹¹C]-MPDX and [¹⁸F]-CPFPX) have been used in humans, we aimed to improve the metabolic stability and specific binding. Guided by structure-activity relationship (SAR) studies, 10 derivatives were synthesized with binding affinities up to 0.12 nM. Three subnanomolar candidates (3, 8, 9) were radiolabeled with C-11 (t _1/2 = 20.4 min) for evaluation using in vivo PET imaging and ex vivo rodent brain biodistribution. Although [¹¹C]8 demonstrated a higher blood-brain barrier (BBB) permeability, negligible in vivo specific binding was observed. Ex vivo studies indicated that all three compounds are substrates for brain efflux pumps. Despite optimized affinity, BBB permeability and in vivo binding specificity remain challenges. These findings inform development of nonxanthine A₁R radiotracers and highly potent CNS A₁R drugs.