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

## TL;DR

This paper introduces two new radioligands for PET imaging of the orexin-2 receptor, which could help in developing therapies for CNS disorders like narcolepsy.

## Contribution

The development of two novel 18F-labeled radioligands with high selectivity and affinity for the orexin-2 receptor.

## Key findings

- Both radioligands show excellent OX2R binding affinity and selectivity over OX1R.
- In vivo PET imaging revealed low brain uptake due to efflux transporters, but this was improved with pharmacological inhibition.
- The compounds represent promising starting points for OX2R PET radioligand development.

## Abstract

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.