Efficacy Assessment of an Uncharged Reactivator of NOP-Inhibited Acetylcholinesterase Based on Tetrahydroacridine Pyridine-Aldoxime Hybrid in Mouse Compared to Pralidoxime.

(1) Background: Human exposure to organophosphorus compounds employed as pesticides or as chemical warfare agents induces deleterious effects due to cholinesterase inhibition. One therapeutic approach is the reactivation of inhibited acetylcholinesterase by oximes. While currently available oximes are unable to reach the central nervous system to reactivate cholinesterases or to display a wide spectrum of action against the variety of organophosphorus compounds, we aim to identify new reactivators without such drawbacks. (2) Methods: This study gathers an exhaustive work to assess in vitro and in vivo efficacy, and toxicity of a hybrid tetrahydroacridine pyridinaldoxime reactivator, KM297, compared to pralidoxime. (3) Results: Blood-brain barrier crossing assay carried out on a human in vitro model established that KM297 has an endothelial permeability coefficient twice that of pralidoxime. It also presents higher cytotoxicity, particularly on bone marrow-derived cells. Its strong cholinesterase inhibition potency seems to be correlated to its low protective efficacy in mice exposed to paraoxon. Ventilatory monitoring of KM297-treated mice by double-chamber plethysmography shows toxic effects at the selected therapeutic dose. This breathing assessment could help define the No Observed Adverse Effect Level (NOAEL) dose of new oximes which would have a maximum therapeutic effect without any toxic side effects.

Respiratory failure triggered by cholinesterase inhibitors may involve activation of a reflex sensory pathway by acetylcholine spillover.

Respiration failure during exposure by cholinesterase inhibitors has been widely assumed to be due to inhibition of cholinesterase in the brain. Using a double chamber plethysmograph to measure various respiratory parameters, we observed long "end inspiratory pauses" (EIP) during most exposure that depressed breathing. Surprisingly, Colq KO mice that have a normal level of acetylcholinesterase (AChE) in the brain but a severe deficit in muscles and other peripheral tissues do not pause the breathing by long EIP. In mice, long EIP can be triggered by a nasal irritant. Eucalyptol, an agonist of cold receptor (TRPM8) acting on afferent sensory neurons and known to reduce the EIP triggered by such irritants, strongly reduced the EIP induced by cholinesterase inhibitor. These results suggest that acetylcholine (ACh) spillover from the neuromuscular junction, which is unchanged in Colq KO mice, may activate afferent sensory systems and trigger sensory reflexes, as reversed by eucalyptol. Indeed, the role of AChE at the cholinergic synapses is not only to accurately control the synaptic transmission but also to prevent the spillover of ACh. In the peripheral tissues, the ACh flood induced by cholinesterase inhibition may be very toxic due to interaction with non-neuronal cells that use ACh at low levels to communicate with afferent sensory neurons.