Heteroaryl derivatives of suvorexant as OX1R selective PET ligand candidates: Cu-mediated 18F-fluorination of boroxines, in vitro and initial in vivo evaluation.

Background: The orexin receptor (OXR) plays a role in drug addiction and is aberrantly expressed in colorectal tumors. Subtype-selective OXR PET ligands suitable for in vivo use have not yet been reported. This work reports the development of ¹⁸F-labeled OXR PET ligand candidates derived from the OXR antagonist suvorexant and the OX1R-selective antagonist JH112.

Results: Computational analysis predicted that fluorine substitution (1e) and introduction of the fluorobenzothiazole scaffold (1f) would be suitable for maintaining high OX1R affinity. After multi-step synthesis of 1a-1f, in vitro OXR binding studies confirmed the molecular dynamics calculations and revealed single-digit nanomolar OX1R affinities for 1a-f, ranging from 0.69 to 2.5 nM. The benzothiazole 1f showed high OX1R affinity (K_i = 0.69 nM), along with 77-fold subtype selectivity over OX2R. Cu-mediated ¹⁸F-fluorination of boroxine precursors allowed for a shortened reaction time of 5 min to provide the non-selective OXR ligand [¹⁸F]1c and its selective OX1R congener [¹⁸F]1f in activity yields of 14% and 22%, respectively, within a total synthesis time of 52-76 min. [¹⁸F]1c and [¹⁸F]1f were stable in plasma and serum in vitro, with logD_7.4 of 2.28 ([¹⁸F]1c) and 2.37 ([¹⁸F]1f), and high plasma protein binding of 66% and 77%, respectively. Dynamic PET imaging in rats showed similar brain uptake of [¹⁸F]1c (0.17%ID/g) and [¹⁸F]1f (0.15%ID/g). However, preinjection of suvorexant did not significantly block [¹⁸F]1c or [¹⁸F]1f uptake in the rat brain. Pretreatment with cyclosporine A to study the role of P-glycoprotein (P-gp) in limiting brain accumulation moderately increased brain uptake of [¹⁸F]1c and [¹⁸F]1f. Accordingly, in vitro experiments demonstrated that the P-gp inhibitor zosuquidar only moderately inhibited polarized, basal to apical transport of 1c (p < 0.05) and had no effect on the transport of 1f, indicating that P-gp does not play a relevant role in brain accumulation of [¹⁸F]1c and [¹⁸F]1f in vivo.

Conclusions: The in vitro and in vivo results of [¹⁸F]1c and [¹⁸F]1f provide a solid basis for further development of suitable OXR PET ligands for brain imaging.