PET imaging evaluation of [(18)F]DBT-10, a novel radioligand specific to α7 nicotinic acetylcholine receptors, in nonhuman primates

Abstract

Purpose: Positron emission tomography (PET) radioligands specific to α7 nicotinic acetylcholine receptors (nAChRs) afford in vivo imaging of this receptor for neuropathologies such as Alzheimer's disease, schizophrenia, and substance abuse. This work aims to characterize the kinetic properties of an α7-nAChR-specific radioligand, 7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-[(18)F]-fluorodibenzo[b,d]thiophene 5,5-dioxide ([(18)F]DBT-10), in nonhuman primates.

Methods: [(18)F]DBT-10 was produced via nucleophilic substitution of the nitro-precursor. Four Macaca mulatta subjects were imaged with [(18)F]DBT-10 PET, with measurement of [(18)F]DBT-10 parent concentrations and metabolism in arterial plasma. Baseline PET scans were acquired for all subjects. Following one scan, ex vivo analysis of brain tissue was performed to inspect for radiolabeled metabolites in brain. Three blocking scans with 0.69 and 1.24 mg/kg of the α7-nAChR-specific ligand ASEM were also acquired to assess dose-dependent blockade of [(18)F]DBT-10 binding. Kinetic analysis of PET data was performed using the metabolite-corrected input function to calculate the parent fraction corrected total distribution volume (V T/f P).

Results: [(18)F]DBT-10 was produced within 90 min at high specific activities of 428 ± 436 GBq/μmol at end of synthesis. Metabolism of [(18)F]DBT-10 varied across subjects, stabilizing by 120 min post-injection at parent fractions of 15-55%. Uptake of [(18)F]DBT-10 in brain occurred rapidly, reaching peak standardized uptake values (SUVs) of 2.9-3.7 within 30 min. The plasma-free fraction was 18.8 ± 3.4%. No evidence for radiolabeled [(18)F]DBT-10 metabolites was found in ex vivo brain tissue samples. Kinetic analysis of PET data was best described by the two-tissue compartment model. Estimated V T/f P values were 193-376 ml/cm(3) across regions, with regional rank order of thalamus > frontal cortex > striatum > hippocampus > occipital cortex > cerebellum > pons. Dose-dependent blockade of [(18)F]DBT-10 binding by structural analog ASEM was observed throughout the brain, and occupancy plots yielded a V ND/f P estimate of 20 ± 16 ml/cm(3).

Conclusion: These results demonstrate suitable kinetic properties of [(18)F]DBT-10 for in vivo quantification of α7-nAChR binding in nonhuman primates.

Keywords: Alpha 7; Nicotine; Nicotinic acetylcholine receptor; PET.

Figure 1

Chemical structures and in vitro binding profiles of α₇-specific PET radioligands. ^aHuman α₇ nAChR in stably transfected SH-SY5Y cells, with [³H]methyllycaconitine as radioligand. ^bHuman α₄β₂ and α₃β₄ nAChR in stably transfected HEK-293 cells, with [³H]epibatidine as radioligand. ^cRat α₆β₂* nAChR obtained from rat striatum by immunoimmobilization using anti-rα₆ nAChR antibody, with [³H]epibatidine as radioligand. ^dPercentage of inhibition at 0.1 μM concentration of test compound. ^eTaken from [28]. ^fTaken from [29].

Figure 2

Radiosynthesis of the α₇-specific compound [¹⁸F]DBT-10. A. Radiolabeling conditions. B. Semi-preparative HPLC traces showing purification of [¹⁸F]DBT-10, which elutes at ~17 min. C. Analytical HPLC traces from an injection of [¹⁸F]DBT-10 product solution. The UV peak at ~1.4 min is from the presence of ascorbic acid in the product solution.

Figure 3

[¹⁸F]DBT-10 in the arterial plasma. A. Radio-HPLC traces for subject M1’s baseline scan, with [¹⁸F]DBT-10 at ~11 min and metabolites at ~1 min and ~7 min. B. Measured [¹⁸F]DBT-10 parent fractions for all acquired scans. The three subjects are identified by separate symbols; M1: ⋄, M2: △,M3:○, M4:×. C. [¹⁸F]DBT-10 input functions for all baseline scans, in units of SUV (activity normalized by injected dose and subject weight). The three subjects are identified by separate lines: M1, solid; M2, dashed; M3, dash-dot, red; M4, dash-dot, black.

Figure 4

Ex vivo metabolite analysis of [¹⁸F]DBT-10 in brain and blood showing typical HPLC traces of radioactivity from extracted brain tissue (hippocampus) (top), and from an arterial plasma sample drawn immediately prior to animal sacrifice (bottom). For the plasma sample, t=0 represents the beginning of back-flush from the capture column.

Figure 5

[¹⁸F]DBT-10 time activity curves. Values are expressed in SUV (radioactivity concentration/i.d. × weight × 1,000). Open symbols are the measured concentrations, while solid lines show the preferred 2TCM fit. Regions shown include thalamus (▲), frontal cortex (□), hippocampus (○), occipital cortex (⋄), and cerebellum (▼).

Figure 6

[¹⁸F]DBT-10 uptake in the brain of a rhesus monkey. Top: images of [¹⁸F]DBT-10 V_T/f_P calculated on a voxel-wise basis using MA1 (t* = 60 min); Bottom: corresponding MRI images with red crosshairs for orientation.

Figure 7

[¹⁸F]DBT-10 Lassen plots with cold ASEM blocking. Optimal linear fit r² and parameters are shown, where the slope represents fractional occupancy and x-intercept corresponds to V_ND/f_P. ASEM dose-dependently blocked [¹⁸F]DBT-10 binding. The blocking drug ASEM was administered in doses of 0.69 mg/kg (◆) and 1.24 mg/kg (▲) for M1 and 1.24 mg/kg (○) for M2.