Evaluation of 18F-CPFPX, a novel adenosine A1 receptor ligand: in vitro autoradiography and high-resolution small animal PET.

UNLABELLED: Adenosine modulates brain activity through 4 G protein-coupled receptors, primarily adenosine A(1) receptors (A(1)ARs). A(1)ARs are heterogeneously distributed throughout the brain and participate in many physiologic processes-for example, the induction of sleep and feedback inhibition of excitatory neurotransmission. There is also evidence that A(1)ARs are involved in brain pathologies, including cerebral ischemia, epilepsy, and neurodegeneration. Therefore, measuring A(1)ARs in the living brain has been a long-standing goal. This report describes the preclinical evaluation of (18)F-8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ((18)F-CPFPX), a novel A(1)AR PET ligand. METHODS: CPFPX, a xanthine-based A(1)AR antagonist, was labeled with either (18)F or (3)H, maintaining identical chemical structures, and evaluated in rats as a putative radioligand for in vivo or in vitro imaging of brain A(1)ARs by quantitative receptor autoradiography and the combination of high-resolution small animal PET and MRI. RESULTS: (3)H-CPFPX bound with nanomolar affinity (K(d), 4.4 nmol/L) to A(1)ARs and showed a distribution typical of cerebral A(1)ARs. In extensive in vitro competition studies, (3)H-CPFPX proved to be a highly selective and specific A(1)AR radioligand. Neither the nonxanthine-type adenosine A(2A) receptor antagonist ZM 241385 nor multiple cholinergic, serotoninergic, and glutamatergic receptor compounds competed for (3)H-CPFPX below the micromolar level. In vivo animal PET and ex vivo autoradiographic experiments measured radioactivity in discrete brain regions after intravenous injection of (18)F-CPFPX. (18)F-CPFPX had excellent in vivo stability and penetrated the blood-brain barrier immediately after injection due to its high lipophilicity. Brain uptake was rapid and particularly high in gray matter regions. Retention of (18)F-CPFPX was highest in the cerebellum, thalamus, and neocortex with evidence of saturable binding. Low binding potentials were found in the midbrain. In vivo displacement PET experiments with the A(1)AR antagonist 8-cyclopentyl-1,3-dipropylxanthine showed a 72% +/- 8% displacement of (18)F-CPFPX. CONCLUSION: (18)F-CPFPX is a highly selective and specific ligand for A(1)ARs and a suitable radioligand for noninvasive PET imaging of A(1)ARs in the living brain. These studies also support the application of high-resolution animal PET as an effective in vivo imaging tool in the evaluation process of new radioligands.