Radiofluorinated histamine H₃ receptor antagonist as a potential probe for in vivo PET imaging: radiosynthesis and pharmacological evaluation.

The histamine H(3) receptor (H(3)R) plays a role in cognition and memory processes and is implicated in different neurological disorders, including Alzheimer's disease, schizophrenia, and narcolepsy. In vivo studies of the H(3)R occupancy using a radiolabeled PET tracer would be very useful for CNS drug discovery and development. We report here the radiosynthesis, in vitro and in vivo evaluation of a novel (18)F-labeled high-affinity H(3)R antagonist (18)F-ST889. The radiosynthesis was accomplished via nucleophilic substitution of the mesylate leaving group with a radiochemical yield of 8-20%, radiochemical purity >99%, and specific radioactivity > 65 GBq/μmol. (18)F-ST889 exhibited high in vivo stability and rather low lipophilicity (logD(7.4)=0.35 ± 0.09). In vitro autoradiography showed specific binding in H(3)R-rich brain regions such as striatum and cortex. However, in vivo PET imaging of the rat brain with (18)F-ST889 was not successful. Possible reasons are discussed.