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.