Abstract View

Introduction: Carbon-11 labeled carfentanil ([¹¹C]CFN) is a routinely employed high-affinity positron emission tomography (PET) agent that agonistically images the µ-opioid receptor, which plays a role in an array of pathological and physiological processes, including pain and addiction. Despite its high clinical utility, [¹¹C]CFN requires stringent quality assurance measures to mitigate its extreme potency, which can introduce the risk of respiratory depression even at PET dosing concentrations. Accordingly, scaffolds based on µ-opioid antagonists labeled with fluorine-18 would carry marked benefits over [¹¹C]CFN by offering a safer pharmacodynamic profile and improved distribution capability owing to the longer half-life of fluorine-18 (109.8 min) vs. carbon-11 (20.4 min). Herein, we describe the radiosynthesis of two new fluorine-18 µ-opioid receptor imaging agents based on fluorinated, non-radioactive MOR antagonists that were initially developed as therapeutics by Lilly and GSK.^1,2

Methods: [¹⁸F]Fluoride was produced via the ¹⁸O(p,n)¹⁸F nuclear reaction in a cyclotron (55 μA beam for 15 min generated ca. 900 mCi of [¹⁸F]fluoride). The [¹⁸F]fluoride was transferred to the synthesis module and trapped on a QMA-light Sep-Pak preconditioned with 0.5M aqueous KOTf. [¹⁸F]Fluoride was eluted into the reaction vessel of a Trasis AIO synthesis module using KOTf/K₂CO₃ (5.0 mg/0.5 mg) in 1.0 mL 4:1 MeCN:H₂O. An additional 1 mL of MeCN was added to the reactor, and the fluoride was dried azeotropically under vacuum at 110 °C. The reaction vial was then cooled to 25 °C before sequential addition of the substrate (1.0 equiv.) in DMF and [Cu] (1.0 equiv.) in DMF and nBuOH. The mixture was then heated to 120 °C for 20 min, then cooled to 25 °C. An aqueous solution of HCl (1M, 1 mL) was added, and the reaction vial was heated to 100 °C for 5 min. The solution was then cooled to room temperature before quenching with 1 equiv. of NaOH in 6 mL of HPLC eluent. The mixture was then passed through an Al₂O₃ cartridge to remove unreacted ¹⁸F^- and loaded onto a semi-prep column for separation and reformulation into an injectable dose.

Results: The requisite boronate Lilly precursor was synthesized in good yield via an SNAr/reductive amination/Miyaura borylation sequence. The targeted imaging agent was successfully labeled manually using a modified method reported by Gouverneur³, achieving 40% radiochemical conversion (RCC) and >85% radiochemical purity (RCP) without purification. Translation to automated synthesis proved challenging; however, optimization revealed that elevated substrate loading and copper concentration were required to achieve efficient labeling, providing sufficient isolated yields and purities (>99%) for preclinical studies (Figure 1, a).

The protected GSK precursor was synthesized via a similar route and has also been successfully labeled manually. Efforts are currently underway to deprotect the labeled intermediate and fully automate the synthesis. (Figure 1, b)

Conclusions and Future Work: Two new µ-opioid receptor antagonists were successfully labeled via copper-mediated radiofluorination manually using the corresponding boronate precursor. In particular, the Lily antagonist has been successfully synthesized on a remote-controlled module and obtained in RCYs and in molar quantities sufficient for non-human primate preclinical studies. These PET imaging studies, including blocking studies with opioid ligands like naloxone, are currently underway to assess opioid receptor selectivity, tissue binding, and brain subregion radiotracer localization. These studies are ongoing, and results will be presented at the upcoming WMIC meeting.

References:

1. Siegel, M.G., Stucky, R.D., and Takeuchi, K. 6-substituted nicotinamide derivatives as opioid receptor antagonists. 77.

2.Plakas, K., Hsieh, C.-J., Saturnino Guarino, D., Hou, C., Chia, W.-K., Young, A., Schmitz, A., Ho, Y.-P., Weng, C.-C., Lee, H., et al. (2025). Toward a Small-Molecule Antagonist Radioligand for Positron Emission Tomography Imaging of the Mu Opioid Receptor. ACS Chem. Neurosci. 16, 1592–1603.

3. Taylor, N. J.; Emer, E.; Preshlock, S.; Schedler, M.; Tredwell, M.; Verhoog, S.; Mercier, J.; Genicot, C.; Gouverneur, V. Derisking the Cu-Mediated ¹⁸F-Fluorination of Heterocyclic Positron Emission Tomography Radioligands. J. Am. Chem. Soc. 2017, 139 (24), 8267– 8276