Synthesis and evaluation of novel radioligands for positron emission tomography imaging of the orexin-2 receptor

Orexin receptors (OXRs) in the brain have been implicated in diverse physiological and neuropsychiatric conditions. Here we describe the design, synthesis, and evaluation of OXR ligands related to (1R,2S)-2-(((2-methyl-4- methoxymethylpyrimidin-5-yl)oxy)methyl)-N-(5-fluoropyridin-2-yl) -2-(3-fluorophenyl)cyclopropanecarboxamide (9a) applicable to positron emission tomography (PET) imaging. Structural features were incorporated to increase binding affinity for OXRs, to enable carbon-11 radiolabeling, and to adjust lipophilicity considered optimal for brain penetration and low nonspecific binding. 9a displayed nanomolar affinity for OXRs, and autoradiography using rat brain sections showed that specific binding of [¹¹C]9a was distributed primarily to neocortical layer VI and hypothalamus, consistent with reported localizations of orexin-2 receptors (OX₂Rs). In vivo PET study of [¹¹C]9a demonstrated moderate uptake of radioactivity into rat and monkey brains under deficiency or blockade of P-glycoprotein, and distribution of PET signals in the brain was in agreement with autoradiographic data. Our approach and findings have provided significant information for development of OX₂R PET tracers.