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

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 A_β 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.