[CARBONYL-C-11]DESMETHYL-WAY-100635 (DWAY) IS A POTENT AND SELECTIVE RADIOLIGAND FOR CENTRAL 5-HT1A RECEPTORS IN-VITRO AND IN-VIVO

[carbonyl-C-11]Desmethyl-WAY-100635 (DWAY) is possibly a low-level met abolite appearing in plasma after intravenous administration of [carbo nyl(11)C]WAY-100635 to human subjects for positron emission tomographi c (PET) imaging of brain 5-HT1A receptors. In this study we set out to assess the ability of DWAY to enter brain in vivo and to elucidate it s possible interaction with 5-HT1A receptors. Desmethyl-WAY-100635 was labelled efficiently with carbon-11 (t(1/2) = 20.4 min) in high speci fic radioactivity by reaction of its descyclohexanecarbonyl analogue w ith [carbonyl-C-11] cyclohexanecarbonyl chloride. The product was sepa rated in high radiochemical purity by high-performance liquid chromato graphy (HPLC) and formulated for intravenous injection. Rats were inje cted intravenously with DWAY, sacrificed at known times and dissected to establish radioactivity content in brain tissues. At 60 min after i njection, the ratios of radioactivity concentration in each brain regi on to that in cerebellum correlated with previous in vitro and in vivo measures of 5-HT1A receptor density. The highest ratio was about 22 i n hippocampus. Radioactivity cleared rapidly from plasma; HPLC analysi s revealed that DWAY represented 55% of the radioactivity in plasma at 5 min and 33% at 30 min. Only polar radioactive metabolites were dete cted. Subsequently, a cynomolgus monkey was injected intravenously wit h DWAY and examined by PET. Maximal whole brain uptake of radioactivit y was 5.7% of the administered dose at 5 min after injection. The imag e acquired between 9 and 90 min showed high radioactivity uptake in br ain regions rich in 5-HT1A receptors (e.g. frontal cortex and neocorte x), moderate uptake in raphe nuclei and low uptake in cerebellum. A tr ansient equilibrium was achieved in cortical regions at about 60 min, when the ratio of radioactivity concentration in frontal cortex to tha t in cerebellum reached 6. The corresponding ratio for raphe nuclei wa s about 3. Radioactive metabolites appeared rapidly in plasma, but the se were all more polar than DWAY, which represented 52% of the radioac tivity in plasma at 4 min and 20% at 55 min. In a second PET experimen t, in which a cynomolgus monkey was pretreated with the selective 5-HT 1A receptor antagonist, WAY-100635, at 25 min before DWAY injection, r adioactivity in all brain regions was reduced to that in cerebellum. A utoradiography of post mortem human brain cryosections after incubatio n with DWAY successfully delineated 5-HT1A receptor distribution, Rece ptor-specific binding was eliminated in the presence of the selective 5-HT1A receptor agonist, 8-OH-DPAT [(+/-)-8-hydroxy-2-dipropylaminotet ralin]. These findings show that: (a) intravenously administered DWAY is well able to penetrate brain in rat and monkey, (b) DWAY is a highl y effective radioligand for brain 5-HT1A receptors in rat and monkey i n vivo and for human brain in vitro, and (c) the metabolism and kineti cs of DWAY appear favourable to successful biomathematical modelling o f acquired PET data. Thus, DWAY warrants further evaluation as a radio ligand for PET studies of 5-HT1A receptors in human brain.