Synthesis, radiolabeling and in vivo evaluation of [11C](R)-1-[4-[2-(4-methoxyphenyl)phenyl]piperazin-1-yl]-3-(2-pyrazinyloxy)-2-propanol, a potential PET radioligand for the 5-HT7 receptor

In the search for a novel serotonin 7 (5-HT7) receptor PET radioligand we synthesized and evaluated a new series of biphenylpiperazine derivatives in vitro. Among the studied compounds, (R)-1-[4-[2-(4-methoxyphenyl)phenyl]piperazin-1-yl]-3-(2-pyrazinyloxy)-2-propanol ((R)-16), showed the best combination of affinity, selectivity, and lipophilicity, and was thus chosen for carbon-11 labelling and evaluation in pigs. After intravenous injection, [(11)C](R)-16 entered the pig brain and displayed reversible tracer kinetics. Pretreatment with the 5-HT7 receptor selective antagonist SB-269970 (1) resulted in limited decrease in the binding of [(11)C](R)-16, suggesting that this radioligand is not optimal for imaging the brain 5-HT7 receptor in vivo but it may serve as a lead compound for the design of novel 5-HT7 receptor PET radioligands.