[18F]F13640: a selective agonist PET radiopharmaceutical for imaging functional 5-HT1A receptors in humans

Abstract

Purpose: F13640 (a.k.a. befiradol, NLX-112) is a highly selective 5-HT_1A receptor ligand that was selected as a PET radiopharmaceutical-candidate based on animal studies. Due to its high efficacy agonist properties, [¹⁸F]F13640 binds preferentially to functional 5-HT_1A receptors, which are coupled to intracellular G-proteins. Here, we characterize brain labeling of 5-HT_1A receptors by [¹⁸F]F13640 in humans and describe a simplified model for its quantification.

Methods: PET/CT and PET-MRI scans were conducted in a total of 13 healthy male volunteers (29 ± 9 years old), with arterial input functions (AIF) (n = 9) and test-retest protocol (n = 8). Several kinetic models were compared (one tissue compartment model, two-tissue compartment model, and Logan); two models with reference region were also evaluated: simplified reference tissue model (SRTM) and the logan reference model (LREF).

Results: [¹⁸F]F13640 showed high uptake values in raphe nuclei and cortical regions. SRTM and LREF models showed a very high correlation with kinetic models using AIF. As concerns test-retest parameters and the prolonged binding kinetics of [¹⁸F]F13640, better reproducibility, and reliability were found with the LREF method. Cerebellum white matter and frontal lobe white matter stand out as suitable reference regions.

Conclusion: The favorable brain labeling and kinetic profile of [¹⁸F]F13640, its high receptor specificity and its high efficacy agonist properties open new perspectives for studying functionally active 5-HT_1A receptors, unlike previous radiopharmaceuticals that act as antagonists. [¹⁸F]F13640's kinetic properties allow injection outside of the PET scanner with delayed acquisitions, facilitating the design of innovative longitudinal protocols in neurology and psychiatry.

Trial registration: Trial Registration EudraCT 2017-002,722-21.

Keywords: 5-HT1A receptors; Brain; Functional receptor; Modeling study; PET imaging; [18F]F13640.

Fig. 1

Summary flow-chart of healthy subjects’ inclusion for the pilot study and main test–retest study. Analysis focused only on the 8 PET-MRI subjects and 4 PET-MRI subjects when AIF is considering

Fig. 2

Example of an arterial input function (AIF) for a subject with whole blood (Cwb) and uncorrected plasma curve (Cp). A Logarithmic time scale. B Double-logarithmic scale for the same subject

Fig. 3

Parametric image of BP values estimated with LREF_CERWM. BP image is overlaid on the T1 MRI of the participant (CERWM = cerebellum white matter; CERGM = cerebellum grey matter)