Novel Organophosphate Ligand O-(2-Fluoroethyl)-O-(p-Nitrophenyl)Methylphosphonate: Synthesis, Hydrolytic Stability and Analysis of the Inhibition and Reactivation of Cholinesterases
Year: 2016
Type: article
Abstract: The organophosphate O -(2-fluoroethyl)- O -( p -nitrophenyl) methyphosphonate 1 is the first-in-class, fluorine-18 radiolabeled organophosphate inhibitor ([ 18 F] 1 ) of acetylcholinesterase (AChE). In rats, [ 18 F] 1 localizes in AChE rich regions of the brain and other tissues where it likely exists as the (CH 3 )( 18 FCH 2 CH 2 O)P(O)-AChE adduct (ChE- 1 ). Characterization of this adduct would define the inhibition mechanism and subsequent postinhibitory pathways and reactivation rates. To validate this adduct, the stability (hydrolysis) of 1 and ChE- 1 reactivation rates were determined. Base hydrolysis of 1 yields p -nitrophenol and (CH 3 ) (FCH 2 CH 2 O)P(O)OH with pseudo first order rate constants ( k obsd ) at pH 7.4 (PBS) of 3.25 × 10 –4 min –1 ( t 1/2 = 35.5 h) at 25 °C and 8.70 × 10 –4 min –1 ( t 1/2 = 13.3 h) at 37 °C. Compound 1 was a potent inhibitor of human acetylcholinesterase (HuAChE; k i = 7.5 × 10 5 M –1 min –1 ), electric eel acetylcholinesterase (EEAChE) ( k i = 3.0 × 10 6 M –1 min –1 ), and human serum butyrylcholinesterase (HuBChE; 1.95 × 10 5 M –1 min –1 ). Spontaneous and oxime-mediated reactivation rates for the (CH 3 ) (FCH 2 CH 2 O)P(O)-serine ChE adducts using 2-PAM (10 μM) were (a) HuAChE 8.8 × 10 –5 min –1 ( t 1/2 = 131.2 h) and 2.41 × 10 –2 min –1 ( t 1/2 = 0.48 h), (b) EEAChE 9.32 × 10 –3 min –1 ( t 1/2 = 1.24 h) and 3.33 × 10 –2 min –1 ( t 1/2 = 0.35 h), and (c) HuBChE 1.16 × 10 –4 min –1 ( t 1/2 = 99.6 h) and 4.19 × 10 –2 min –1 ( t 1/2 = 0.27 h). All ChE- 1 adducts undergo rapid and near complete restoration of enzyme activity following addition of 2-PAM (30 min), and no aging was observed for either reactivation process. The fast reactivation rates and absence of aging of ChE- 1 adducts are explained on the basis of the electron-withdrawing fluorine group that favors the nucleophilic reactivation processes but disfavors cation-based dealkylation aging mechanisms. Therefore, the likely fate of radiolabeled compound 1 in vivo is the formation of (CH 3 )(FCH 2 CH 2 O)P(O)-serine adducts and monoacid (CH 3 )(FCH 2 CH 2 O)P(O)OH from hydrolysis and reactivation.
Source: Chemical Research in Toxicology
Institutions: University of Montana, Southern Research Institute
Language: English
Subfield: Pharmacology
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W2511101415