Development of Novel ¹⁸F‑Labeled Selective Orexin‑2 Receptor Radioligands for Positron Emission Tomography

The orexin-2 receptor (OX₂R), a G protein-coupled receptor activated by the neuropeptides, orexin A and B, plays an integral role in orchestrating motivation, feeding behavior, and the sleep-wake cycle. Pharmacological modulation of OX₂R has shown therapeutic potential for a variety of central nervous system (CNS) diseases, most notably narcolepsy and insomnia. Noninvasive imaging of OX₂R could enable the visualization of its regional distribution, facilitate assessments of target engagement, and support the development of OX₂R-directed therapies. Nonetheless, there are currently no suitable radioligands available for imaging OX₂R with positron emission tomography (PET). Herein, we report the design and evaluation of two novel PET ligand candidates, [¹⁸F]1 ([¹⁸F]-OX₂-2303) and [¹⁸F]2 ([¹⁸F]-OX₂-2304), as potential imaging probes for OX₂R. Both candidates exhibit excellent OX₂R binding affinity (K _i = 0.1 and 1 nM, respectively) and remarkable selectivity over OX₁R (>600-fold). In vitro autoradiography confirmed robust and selective binding to OX₂R in rat brain sections. In vivo PET imaging revealed low brain uptake at baseline, attributed to active efflux by P-glycoprotein (P-gp) and/or breast cancer resistance protein (BCRP). Furthermore, pharmacological inhibition of these efflux transporters markedly enhanced brain penetration and OX₂R antagonists demonstrated notable blocking effects to OX₂R tracers during these conditions. Collectively, [¹⁸F]1 ([¹⁸F]-OX₂-2303) and [¹⁸F]2 ([¹⁸F]-OX ₂ -2304) constitute promising chemical starting points for the development of OX₂R PET radioligands, although further medicinal chemistry optimization will be required to overcome transporter-mediated efflux from the brain.