In Search of an Effective in vivo Reactivator for Organophosphorus Nerve Agent-Inhibited Acetylcholinesterase in the Central Nervous System

Organophosphorus nerve agents irreversibly inhibit the enzyme acetylcholinesterase (AChE), leading to excessive cholinergic neurotransmission and causing toxic lethal consequences. Current nerve agent therapies in the U.S. include 2-pralidoxime to reactivate inhibited AChE. Due to its quaternary structure, this oxime does not readily cross the blood brain barrier (BBB) to reactivate brain AChE and to mitigate CNS toxicity. We have shown earlier that the tertiary oxime monoisonitrosoacetone (MINA) crossed BBB, provided some degree of CNS AChE reactivation, enhanced survival, and mitigated the seizure activity following nerve agent exposure. In this study, the in vivo reactivating capabilities of several tertiary oximes, diacylmonoxime (DAM), N,N-diethyl-3-(2-(hydroxyimino)acetoxy)propan1-aminium chloride (DHAP), RS194B, JK-3-38, SWRI48A, SWRI53A, pro-2-PAM, and diethyxime, were compared to each other and to MINA, following subcutaneous exposure of guinea pigs to 1.0 x LD50 dose of sarin (GB), cyclosarin (GF) or VX. Four to eight doses (ranging from 5.6 to 240.0 mg/kg, i.m.) of each oxime were treated 15 min after the nerve agent exposure. Animals were euthanized 45 min after oxime treatment; blood and target tissues (brain regions, diaphragm, heart, skeletal muscle) were collected. AChE activity was measured using the Ellman assay. In GB exposure, pro-2-PAM and JK-3-38 enhanced the toxicity at doses above 25 and 35.5 mg/kg, respectively. In peripheral tissues pro-2-PAM provided the greatest AChE reactivation (21-68%), while SWRI53A showed reactivation (4%) only at the highest dose (180 mg/kg), with the rank order of pro-2-PAM > RS194B = JK-3-38 > DHAP > MINA > diethyxime > DAM = SWRI48A = SWRI53A. In the CNS tissues, MINA displayed the highest capacity to reactivate AChE (15-31%), while DAM and JK-3-38 provided no reactivation even at the highest dose tested; the rank order was MINA > pro-2-PAM > DHAP > SWRI48A = SWRI53A > RS194B = diethyxime > JK-3-38 = DAM. In GF exposure, only MINA and SWRI48A showed brain AChE reactivation (8-17%), while only MINA and pro-2-PAM reactivated VX-inhibited brain AChE (8-23%). Thus, the findings suggest that, like the quaternary oximes, tertiary oximes exhibit different reactivation capacities that depend on the nerve agent inhibiting AChE. So far, MINA is the only tertiary oxime capable of reactivating brain AChE inhibited by these three nerve agents.