PET imaging of beta-adrenoceptors in human brain: a realistic goal or a mirage?

Beta-adrenoceptors are predominantly located in the cerebral cortex, nucleus accumbens and striatum. At lower densities, they are also present in amygdala, hippocampus and cerebellum. Beta-2 sites regulate glial proliferation during ontogenic development, after trauma and in neurodegenerative diseases. The densities of beta-1 adrenoceptors are changed by stress, in several mood disorders (depression, excessive hostility, schizophrenia) and during treatment of patients with antidepressants. A technique for beta-adrenoceptor imaging in the human brain is not yet available. Although 24 (ant)agonists have been labeled with either (11)C or (18)F and some of these are successful myocardial imaging agents, only two (S-1'-(18)F-fluorocarazolol and S-1'-(18)F-fluoroethylcarazolol) could actually visualize beta-adrenoceptors within the central nervous system. Unfortunately, these radiopharmaceuticals showed a positive Ames test. They may be mutagenic and cannot be employed for human studies. Screening of more than 150 beta-blockers described in the literature yields only two compounds (exaprolol and L643,717) which can still be radiolabeled and evaluated for beta-adenoceptor imaging. However, other imaging techniques could be examined. Cerebral beta-adrenoceptors might be labeled after temporary opening of the blood-brain barrier (BBB) and simultaneous administration of a hydrophilic ligand such as S-(11)C-CGP12388. Another approach to target beta-adrenoceptor ligands to the CNS is esterification of a myocardial imaging agent (such as (11)C-CGP12177), resulting in a lipophilic prodrug which can cross the BBB and is split by tissue esterases. BBB opening is not feasible in healthy subjects, but the prodrug approach may be successful and deserves to be explored.