Synthesis and Initial Characterization of a Reversible, Selective 18F-Labeled Radiotracer for Human Butyrylcholinesterase

Abstract

Purpose A neuropathological hallmark of Alzheimer’s disease (AD) is the presence of amyloid-β (Aβ) plaques in the brain, which are observed in a significant number of cognitively normal, older adults as well. In AD, butyrylcholinesterase (BChE) becomes associated with Aβ aggregates, making it a promising target for imaging probes to support diagnosis of AD. In this study, we present the synthesis, radiochemistry, in vitro and preliminary ex and in vivo investigations of a selective, reversible BChE inhibitor as PET-tracer for evaluation as an AD diagnostic. Procedures Radiolabeling of the inhibitor was achieved by fluorination of a respective tosylated precursor using K[18F]. IC50 values of the fluorinated compound were obtained in a colorimetric assay using recombinant, human (h) BChE. Dissociation constants were determined by measuring hBChE activity in the presence of different concentrations of inhibitor. Results Radiofluorination of the tosylate precursor gave the desired radiotracer in an average radiochemical yield of 20 ± 3 %. Identity and > 95.5 % radiochemical purity were confirmed by HPLC and TLC autoradiography. The inhibitory potency determined in Ellman’s assay gave an IC50 value of 118.3 ± 19.6 nM. Dissociation constants measured in kinetic experiments revealed lower affinity of the inhibitor for binding to the acylated enzyme (K2 = 68.0 nM) in comparison to the free enzyme (K1 = 32.9 nM). Conclusions The reversibly acting, selective radiotracer is synthetically easily accessible and retains promising activity and binding potential on hBChE. Radiosynthesis with 18F labeling of tosylates was feasible in a reasonable time frame and good radiochemical yield.

Dual-Acting Small Molecules: Subtype-Selective Cannabinoid Receptor 2 Agonist/Butyrylcholinesterase Inhibitor Hybrids Show Neuroprotection in an Alzheimer’s Disease Mouse Model

“…All procedures used to determine the binding properties of the inhibitors on ChEs have been established before. ,,,,,,− …”

Radiosynthesis and whole‐body distribution in mice of a 18F‐labeled azepino[4,3‐b]indole‐1‐one derivative with multimodal activity for the treatment of Alzheimer's disease

“…Whole physicochemical and neurochemical data, as well as the recent interest devoted to identification of PET tracers based on competitive inhibitors to study the BChE role in AD progression, [ 32,33 ] prompted us to investigate the in vivo ability of 2a to cross BBB and penetrate CNS, and its whole‐body distribution. To do this, we undertook the radiosynthesis of [ 18 F] 2a to study the distribution into the brain and the peripheral organs in mice.…”

“…[30] It has been reported that the BChE distribution is involved in the coregulation of cholinergic and noncholinergic neurotransmission at the hippocampal level in humans; therefore, the area-specific effect retrieved might overlap BChE distribution and specific function in that area. [31] Whole physicochemical and neurochemical data, as well as the recent interest devoted to identification of PET tracers based on competitive inhibitors to study the BChE role in AD progression, [32,33] prompted us to investigate the in vivo ability of 2a to cross BBB and penetrate CNS, and its whole-body distribution. To do this, we undertook the radiosynthesis of [ 18 F]2a to study the distribution into the brain and the peripheral organs in mice.…”

Novel benzimidazole-based pseudo-irreversible butyrylcholinesterase inhibitors with neuroprotective activity in an Alzheimer's disease mouse model

“…All procedures used to determine the binding properties of the inhibitors on ChEs have been established before. 19,20,24,26,27,37,[43][44][45][46][47][48]…”

“…52 The maximum and minimum number of operations were set to 10 6 and 10 5 with an autoscale factor of 5. As the demethylated carbamate function is supposed to be transferred to the catalytic Ser198 as seen and suggested by several studies, 23,46,53 a constraint was set for the distance between the carbonyl-carbon of the carbamate function and the sidechain oxygen of Ser198 of 2.5-4.0 Å (spring constant = 5.0). The binding site was defined by a radius of 20 Å around the γ-oxygen of Ser198.…”