A Novel Way To Radiolabel Human Butyrylcholinesterase for Positron Emission Tomography through Irreversible Transfer of the Radiolabeled Moiety

Abstract

The enzyme butyrylcholinesterase (BChE) is known to be involved in the detoxification of xenobiotics in blood plasma and is associated with the progress of neurodegenerative disorders, diabetes type 2, obesity, and diseases of the cardiovascular system. In the present study, we developed carbamate-based inhibitors serving as positron emission tomography (PET) radiotracers with (18) F and (11) C as radioisotopes to visualize BChE distribution. These inhibitors are radiolabeled at the carbamate site and transfer this moiety onto BChE, which thus results in covalent and permanent radiolabeling of the enzyme. There are no comparable radiotracers for cholinesterases described to date. By ex vivo autoradiography experiments on mice brain slices and kinetic investigations, selective and covalent transfer of the radiolabeled carbamate moiety onto BChE was proven. These tracers might provide high resolution of BChE distribution in vivo to enable investigations into the pathophysiological mechanisms of diseases associated with alterations in BChE occurrence.

Molecular Imaging of Hydrolytic Enzymes Using PET and SPECT

“… 48 Specific imaging of BChE is a new area of interest, and recently agents specifically targeting BChE have been reported. 49 , 50 …”

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

“…In consequence, they may not reflect the regional enzyme distribution, but rather plasma delivery rate of the tracer [25]. Irreversible inhibitor-derived tracers can have a complex mechanism of enzyme inactivation and it is often complicated to accurately determine in vivo kinetics and distribution [24,26]. In preliminary in vivo studies, the tracer showed high initial uptake in rat cerebral cortex after intravenous injection, but the hydrolysis rate of these ester is presumably still too high [28].…”

Synthesis and Initial Characterization of a Selective, Pseudo‐irreversible Inhibitor of Human Butyrylcholinesterase as PET Tracer

“…In case of BChE, one can design either an inhibitor‐ or substrate‐based compound for radiolabeling, while the former can be classified in reversibly or irreversibly acting inhibitors. Even though substrate analogs as BChE PET tracers can usually pass the blood–brain barrier (BBB), their hydrolysis rate is often too high to achieve an accurate estimation of the actual enzyme distribution in the brain [6a,19] . 1‐ 11 C‐Methyl‐4‐piperidinyl‐ n ‐butyrate ( 11 C‐MP4B) and N ‐[ 18 F]fluoroethylpiperidin‐4‐ylmethyl butyrate are examples of such tracers with a pronounced rate of hydrolysis in brain and plasma.…”

“…Other irreversible ChE inhibitors are organophosphates and a respective PET tracer was designed to target central nervous system AChE in rats, but these compounds are highly toxic nerve agents [24] . However, due to the lack of results concerning irreversibly acting and selective BChE PET tracers as potential in vivo AD diagnostics, we continued our previous studies, in which we had investigated enzyme kinetics and binding, radiolabeling and preliminary ex vivo brain tissue binding of pseudo‐irreversible, selective BChE inhibitors with high affinity [12,19,25] . Starting point of our investigations was heptylcarbamate 1a (Figure 1) bearing a tetracyclic carrier scaffold.…”

“… Design of radiotracer 2 based on potent, selective BChE inhibitors 1a , 1b and 1c with (pseudo‐)irreversible binding mode. The mechanism of enzyme‐inactivation is presented below [12,19] …”