[18F]F13640, a 5-HT1A Receptor Radiopharmaceutical Sensitive to Brain Serotonin Fluctuations

Abstract

Introduction: Serotonin is involved in a variety of physiological functions and brain disorders. In this context, efforts have been made to investigate the in vivo fluctuations of this neurotransmitter using positron emission tomography (PET) imaging paradigms. Since serotonin is a full agonist, it binds preferentially to G-protein coupled receptors. In contrast, antagonist PET ligands additionally interact with uncoupled receptors. This could explain the lack of sensitivity to serotonin fluctuations of current 5-HT_1A radiopharmaceuticals which are mainly antagonists and suggests that agonist radiotracers would be more appropriate to measure changes in neurotransmitter release. The present study evaluated the sensitivity to endogenous serotonin release of a recently developed, selective 5-HT_1A receptor PET radiopharmaceutical, the agonist [¹⁸F]F13640 (a.k.a. befiradol or NLX-112).

Materials and methods: Four cats each underwent three PET scans with [¹⁸F]F13640, i.e., a control PET scan of 90 min, a PET scan preceded 30 min before by an intravenous injection 1 mg/kg of d-fenfluramine, a serotonin releaser (blocking challenge), and a PET scan comprising the intravenous injection of 1 mg/kg of d-fenfluramine 30 min after the radiotracer injection (displacement challenge). Data were analyzed with regions of interest and voxel-based approaches. A lp-ntPET model approach was implemented to determine the dynamic of serotonin release during the challenge study.

Results: D-fenfluramine pretreatment elicited a massive inhibition of [¹⁸F]F13640 labeling in regions known to express 5-HT_1A receptors, e.g., raphe nuclei, hippocampus, thalamus, anterior cingulate cortex, caudate putamen, occipital, frontal and parietal cortices, and gray matter of cerebellum. Administration of d-fenfluramine during PET acquisition indicates changes in occupancy from 10% (thalamus) to 31% (gray matter of cerebellum) even though the dissociation rate of [¹⁸F]F13640 over the 90 min acquisition time was modest. The lp-ntPET simulation succeeded in differentiating the control and challenge conditions.

Conclusion: The present findings demonstrate that labeling of 5-HT_1A receptors with [¹⁸F]F13640 is sensitive to serotonin concentration fluctuations in vivo. Although the data underline the need to perform longer PET scan to ensure accurate measure of displacement, they support clinical development of [¹⁸F]F13640 as a tool to explore experimental paradigms involving physiological or pathological (neurological or neuropsychiatric pathologies) fluctuations of extracellular serotonin.

Keywords: 5-HT1A receptors; PET imaging; [18F]F13640; agonist; fenfluramine; serotonin release.

FIGURE 1

(A) [¹⁸F]F13640 in vitro binding in cat brain with increasing concentrations of serotonin (Front Cx, frontal cortex; Cing, cingulate cortex; Septum, lateral septum; Cereb, cerebellum). (B) [¹⁸F]F13640 in vitro binding in cat brain with addition of 10 μM of Gpp(NH)p (Front Cx, frontal cortex; Cing Cx, cingulate cortex; Septum, lateral septum; Cereb, cerebellum).

FIGURE 2

SUVr PET images of [¹⁸F]F13640 binding in the three experimental conditions for one cat, i.e., (A) control conditions, (B) pre-injection of d-fenfluramine 30 min before PET acquisition, using the centrum semiovale as a reference region (D). SUVr images are averaged from 32 to 90 min time of acquisition; (C) injection of d-fenfluramine 30 min after PET acquisition beginning (Post cing, posterior cingulate cortex; Front Cx, frontal cortex; Caudate, caudate putamen; Ent Cx, entorhinal cortex; DRN, dorsal raphe nucleus; PN, pontine nucleus; Thal, thalamus; Coll, colliculi; CS, centrum semiovale).

FIGURE 3

(A) Averaged time-activity curves of [¹⁸F]F13640 ± SEM (circles, control conditions; triangles, pretreatment study with a pre-injection of 1 mg/kg of d-fenfluramine, 30 min before PET acquisition). (B) Averaged time-activity curves of [¹⁸F]F13640 ± SEM (circles, control conditions; triangles, challenge study with a post-injection of 1 mg/kg of d-fenfluramine, 30 min after beginning of PET acquisition). (C) Averaged SUVr of [¹⁸F]F13640 ± SEM (circles, control conditions; triangles, pretreatment study with a pre-injection of 1 mg/kg of d-fenfluramine, 30 min before PET acquisition). (D) Averaged SUVr of [¹⁸F]F13640 ± SEM (circles, control conditions; triangles, challenge study with a post-injection of 1 mg/kg of d-fenfluramine, 30 min after beginning of PET acquisition) Caudate, caudate putamen; Hip, hippocampus; Cereb, cerebellum; Ant Cing, anterior cingulate cortex; DRN, dorsal raphe nucleus; CS, centrum semiovale.

FIGURE 4

(A) Averaged binding ratios of [¹⁸F]F13640 calculated in control (NaCl) and pretreatment (d-fenfluramine) conditions in the four cats (^∗p < 0.05; ^∗∗p < 0.01; ^*⁣*⁣**p < 0.0001 paired t-test, non-corrected). (B) Occupancy (%) of serotonin calculated in control (NaCl) and challenge (d-fenfluramine) conditions in the four cats (^∗p < 0.05; ^∗∗p < 0.01; ^*⁣*⁣**p < 0.0001). For values see Table 1.

FIGURE 5

Voxel-to-voxel analysis from 30 to 90 min PET acquisition, showing the significant decreases of [¹⁸F]F13640 binding after d-fenfluramine challenge compared to control scans (p < 0.01, non-corrected). Occ Ctx, Occipital Cortex; Par Ctx, Parietal Cortex; Cing gyr, Cingulate gyrus; Coll, Colliculi; DRN, Dorsal raphe Nuclei; Cereb, Cerebellum; Prc, precuneus.

FIGURE 6

Simulation of the kinetics of serotonin release illustrated by variation (%) of the efflux rate k2at_b of four regions of interest (Ant Cing, anterior cingulate cortex; Caudate, caudate putamen; Cereb, cerebellum; Hip, hippocampus).